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High-resolution geochemistry in the Lucaogou Formation, Junggar Basin: Climate fluctuation and organic matter enrichment
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The Permian Lucaogou Formation in the Junggar Basin, NW China is the target layer for shale oil exploration, but its hydrocarbon precursors have remained the focus of debate. In this study, we investigated the Lucaogou source rocks throughout Well J10025 by conducting detailed petrological, paleontological, and geochemical analyses for the purpose of revealing the occurrence of cyanobacterial blooms as specific hydrocarbon events in the upper Lucaogou Formation. The morphological characteristics of the microfossils and the geochemical signatures of the microfossil-bearing layers support a biological affinity with Microcystis, a kind of cyanobacteria. Microcystis observed as colonial forms embedded in the upper Lucaogou Formation are of great abundance, indicating the presence of cyanobacterial blooms. They were further evidenced by cyanobacteria-derived biomarkers including low terrestrial/aquatic ratio, high 2α-methylhopane index values, and high abundance of 7- and 8-monomethyl heptadecanes. The blooms occurred in a semiarid and brackish paleoenvironment with anoxic to suboxic water conditions and intermittent volcanic eruptions. Permian Microcystis blooms contributed to the enrichment of organic matter in the upper Lucaogou Formation in two main ways: by directly promoting the accumulation of algal biomass and by creating an oxygen-depleted environment for better preservation of organic matter. This study adds a new record to the geological occurrences of cyanobacterial blooms in the Permian, and provides unique insight into the hydrocarbon generation of Jimsar shale oil in the Junggar Basin.
Lakes are a major emitter of the atmospheric greenhouse gas methane (CH4); however, their roles in past climate warming episodes remain poorly understood owing to a scarcity of geological records. Here we report the occurrence of sustained and intensified microbial CH4 cycling in paleo-Lake Junggar in northwestern China, one of the largest known Phanerozoic lakes, during Early Permian climate warming. High-precision U-Pb geochronology refines the age of the upper Lucaogou Formation to the Artinskian, which marks a major glacial-to-postglacial climate transition. The 13C-enriched authigenic dolomites indicate active methanogenesis in the anoxic lake sediments, and 13C-depleted hopanes suggest vigorous methanotrophy in the water column. The intensification of CH4 cycling coincided with increasing global temperature, as evidenced from elevated continental chemical weathering. Our results suggest that the lacustrine CH4 emissions acted as a positive feedback to global warming and contributed to the demise of the Late Paleozoic Ice Age.
Objectives: The Carboniferous and Permian systems of the Junggar Basin recorded the crucial process of the evolution of the Paleo-Asian Ocean, and possess one of the most important oil and gas resources in China. However, the stratigraphic framework and correlation among different parts of the basin remain highly controversial, which led to poor understanding of the geological events and exploration of spatial and temporal distributions of the source rocks.
Methods: In this paper, we provided a state-of-the-art review of the Carboniferous and Permian stratigraphic framework and re-evaluated the correlation based on the latest U-Pb geochronologic data in combination with lithostratigraphic and biostratigraphic data in the Junggar Basin, Xinjiang, Northwest China.
Results: In light of the foregoing analysis, several new conclusions archived: 1) The source rocks of the Lucaogou, Pingdiquan, and Lower Wuerhe formations were very likely to be deposited consistently during the Sakmarian-Artinskian ages. 2) The Fengcheng Formation may range from the late Pennsylvanian to the early Cisuralian. 3) The Shirenzigou, Tashenkula, and lower part of Wulabo formations were deposited during the Pennsylvanian. 4) A long gap probably existed between the Hongyanchi and Quanzijie formations and led to no deposits from the Kungurian to late Guadalupian in the Junggar Basin.
Conclusions: The revised stratigraphic framework has greatly changed the previous age assignments of the Carboniferous-Permian strata in the Junggar basin, which accordingly improved the understanding of the sedimentary models and paleoclimate evolutionary history for the entire basin in the Paleo-Asian Ocean.
准噶尔盆地的石炭—二叠系记录了古亚洲洋演化的关键阶段，是重要的烃源岩层位，但其时代划分及地层对比存在极大的不确定性，限制了对地质事件和烃源岩成因及分布规律的认识。本文系统总结了目前已经发表的年代地层和生物地层工作，以碎屑锆石年龄和生物地层数据为依据，结合沉积演化特征，厘定并提出了准噶尔盆地石炭—二叠系划分和对比的新方案。芦草沟组、平地泉组和下乌尔禾组烃源岩均形成于乌拉尔世（Cisuralian，早二叠世）萨克马尔期(Sakmarian)—亚丁斯克期(Artinskian)；风城组时代跨越石炭系—二叠系界线；石人子沟组、塔什库拉组和乌拉泊组下部形成于晚石炭世；盆地内很可能缺失空谷阶(Kungurian)至瓜达鲁普统(Guadalupian)大部分沉积。新的地层对比框架极大地改变了以往对准噶尔盆地石炭—二叠纪地层时代归属的认识，为建立统一的全盆地沉积演化模式提供了新的依据和方案。
Authigenic albites occur widely in clastic reservoirs with important implications for diagenesis and reservoir formation. The middle Permian Lucaogou Formation in the Jimusaer Sag (Junggar Basin, NW China), where major exploration breakthroughs in shale oil have been achieved, reveals a new phenomenon that authigenic albites are abundant in unique mixed carbonate–volcanic–clastic sequences. This has not been reported in the literatures. To fill the knowledge gap, the origin of these authigenic albites and their relationship with dissolution pores (i.e. diagenesis implications) were investigated. Results show that two types (I and II) of authigenic albite were identified within the shale oil reservoirs. Euhedral Type I authigenic albites with 3–10 μm only occur in dolarenite intraclasts and are symbiotic with amorphous dolomite minerals with a pure chemical composition of >99% albite-end-member content. Larger Type II authigenic albites with 10–50 μm are widely distributed in reservoirs, primarily in dissolution pores, and coexist with authigenic dolomite minerals or dolomite overgrowths. Their chemical composition is less pure with anorthite-end-member contents that range from undetectable to 9.77%, with an average of 1.34%. A symbiotic relationship, pure chemical composition, size, and euhedral morphology indicate that Type I authigenic albites precipitated during syngenetic hydrothermal action. However, the morphology of dissolution pores, residual symbiotic “orthoclase”, impure chemical composition and carbon–oxygen isotope indicate that Type II were the products of the dissolution and reprecipitation of “perthite” crystal pyroclasts influenced by acid organic fluids in latter diagenesis. The differential dissolution of “orthoclase” and “albite” components in “perthite” crystal pyroclasts formed enormous intergranular secondary pores in the presence of dolomite minerals in the shale oil reservoirs.
Paleogeography is the study of the changing surface of Earth through time. Driven by plate tectonics, the configuration of the continents and ocean basins has been in constant flux. Plate tectonics pushes the land surface upward or pulls it apart, causing its collapse. All the while, the unrelenting forces of climate and weather slowly reduce mountains to sand and mud and redistribute these sediments to the sea. This article reviews the changing paleogeography of the past 750 million years. It describes the broad patterns of Phanerozoic paleogeography as well as many of the specific paleogeographic events that have shaped the modern continents and ocean basins. The focus is on the changing latitudinal distribution of the continents, fluctuations in sea level, the opening and closing of oceanic seaways, mountain building, and how these paleogeographic changes have affected global climate, ocean circulation, and the evolution of life. This review presents an atlas of 114 paleogeographic maps that illustrate how Earth's surface has evolved during the past 750 million years. During that time interval, Earth has witnessed the formation and breakup of two supercontinents: Pannotia and Pangea. The continents have been transformed from low-lying flooded platforms to high-standing land areas crisscrossed by the scars of past continental collisions. Oceans have opened and closed, and then opened again in a seemingly never-ending cycle. ▪ The changing configuration of the continents and ocean basins during the past 750 million years is illustrated in 114 paleogeographic maps. ▪ These maps describe how the surface of Earth has been continually modified by mountain building and erosion. ▪ The changing paleogeography has affected global climate, ocean circulation, and the evolution of life. ▪ The data and methods used to produce the maps are described in detail.
The eastern Junggar Basin, controlled by continental extension (rift), was deposited by lacustrine dominated sediments during middle Permian Lucaogou period. An unusual porphyritic-like texture was observed in Lucaogou/Pingdiquan dark fine-grained organic-rich sediments in two sub-tectonic units in the basin. The “phenocrysts” are composed of two types of mineral assemblages. The first is a coarse euhedral calcite assemblage in the Jimusar Sag, and the second consists of dolomite, analcime, and pyrite in the Shishugou Sag. The lithological and mineralogical features indicate a hydrothermal origin for these phenocryst-like minerals. The chondrite-normalized rare earth element patterns show flat or positive Ce anomalies and negative Eu anomalies, which reflect a suboxic to anoxic, off-axis site from the center of the fault system, where the temperature of the hydrothermal fluid might be less than 250 °C. The high ratios of BaN/LaN at 1.6–65.5, strongly positive Sr anomalies at Sr/Sr* = 5.54–39.9, and relatively low ⁸⁷Sr/⁸⁶Sr isotopes at 0.705002–0.705776 in the coarse calcite suggest an origin of mixed sources of lake water, underlying biogenetic sediments, and deep magmatic water. However, the low ⁸⁷Sr/ ⁸⁶Sr ratios of 0.705321–0.705968 in the dolomite and δ³⁴SV-CDT of 10.8‰–12.3‰ in the pyrite indicate that water–underlying-rock interaction and the abiotic thermochemical sulfate reduction of lake water or organic matter might have participated together resulting in the precipitation of the dolomite–analcime–pyrite assemblages in the Shishugou Sag.
The AAPG Super Basin Initiative creates an action plan to help
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many of the world’s greatest onshore and offshore
basins. Workflows that caused production peaks to reach new
heights included horizontal drilling and hydraulic fracturing in
unconventional onshore reservoirs and enhanced seismic imaging
in conventional offshore reservoirs. Improved understanding of
source rocks, petroleum systems, oil habitats, stratigraphy, rock
properties, and clinoform architecture enhances the exploration
and development toolkit. Reduced costs, improved processes, and
multidisciplinary teams created a new golden age in many super
basins.
Super basin thinking forms a new paradigm useful in directing
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they possess key geologic factors and a basin-level economy of
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Geological records of Milankovitch cycles provide a temporal framework for reconstructing Earth climate evolution and mapping out the history of the ancient Solar System. Understanding the fundamental celestial mechanics driving astronomical rhythms during the Permian Period from stratigraphic record is challenging because of the inherent limitations of astronomical solutions, the chaotic nature of Solar System motion, and the paucity of cyclostratigraphic studies of geological records. The Junggar and adjacent basins of northwestern China constituted a large tectonic lake in the Middle Permian, which deposited one of the thickest and richest petroleum source rock intervals in the world. Constructing an accurate timescale for the Permian Junggar Basin is critical to understand the processes that controlled organic carbon burial as well as paleoclimatic and paleoenvironmental background conditions. In this study, detailed spectral analyses of natural gamma ray (NGR) data were performed on the lacustrine muddy-dominated Lucaogou Formation from five exploitation wells. The results reveal significant meter-scale sedimentary cycles of 37.8 m, 10.7–10 m, 3.85–3.2 m, and 2.0–1.58 m, reflecting oscillations in lithologies (variations in changing clay content within the mudstone, sandstone and dolomite). These cycle wavelength ratios match well with those of the Milankovitch cycles predicted for Middle Permian Period. The depositional duration of the Lucaogou Formation was estimated at ~3 Myr. Correlation of the ~1.2 Myr obliquity modulation cycles among the NGR logs, sedimentation rates, and lake levels of the Junggar Basin, as well as with the Middle Permian global sea level, suggests that long-term astronomical “grand cycles” tightly forced climatic and sedimentary processes throughout the Lucaogou Formation. According to two independent approaches, we reconstruct Middle Permian astronomical parameters (Earth precession constant: k and fundamental frequencies terms: gi; i = 1, 2, 3, 4, 5) and calculate astronomical periods (eccentricity, obliquity and precession). These results document astronomical forcing of Paleozoic lake systems, constrain Earth-Moon orbital evolution, and refine the fundamental astronomical frequencies of the Solar System during the Middle Permian.
Shale oil exploration has been a key area of onshore oil and gas exploration in China in recent years. In this study, organic geochemistry and element geochemistry are united to study the shale oil and source rock in the Lucaogou formation of Jimusar sag, in order to reveal the paleoclimate, paleoenvironment, source of organic matter, and factors affecting organic matter accumulation and shale oil generation. The shale oil in the study area is mainly accumulated in two strata with good reservoir properties and oiliness, known as the upper sweet spot and lower sweet spot. Indexes of biomarkers and sensitive elements revealed the warm and semi-arid paleoclimate during Lucaogou formation, and the water column was brackish to salty. Water stratification caused a suboxic to anoxic environment in the deep-water column and coincided with the anoxic photic zone phenomenon. Compared with the lower sweet spot, the more humid climate, deeper and fresher water, and stronger water stratification characterize the upper sweet spot during sedimentation. This made the photic zone with freshwater more suitable for the reproduction of algae in the upper sweet spot. Meanwhile, the organic matter was well-preserved in the anoxic zone. Volcanic ash caused algae bloom, which promoted primary productivity and ensured the supply of organic matter. The composition and distribution pattern of biomarkers prove that phytoplankton is the most important source of organic matter in the study area and the contribution of higher plants is insignificant. The relationship between parameters of paleoproductivity and the redox condition versus total organic carbon (TOC) suggests that compared with the preservation conditions, the input of organic carbon is the most important controlling factor of organic matter accumulation in the study area.
The nature of the rocks exposed to weathering and erosion on continents exerts an important control on weathering feedbacks and the supply of nutrients to the oceans. It also reflects the prevailing tectonic regime responsible for the formation of continents. How the chemical and lithological compositions of the continents evolved through time is, however, still a matter of debate. We use an extensive compilation of terrigenous sediment compositions to better constrain the nature of rocks at the surface of continents at 3.25 Gyr and 250 Myr ago. Specifically, we use geochemical ratios that are sensitive indicators of komatiite, mafic, and felsic rocks in the provenance of the sediments. Our results show that the average Al 2 O 3 /TiO 2 ratio of fine-grained terrigenous sediments decreased slightly over time from 26.2 ± 1.3 in the Archean to 22.1 ± 1.1 (2SE)in the Phanerozoic. In contrast, in the same time interval, the average Zr/TiO 2 ratio stayed nearly constant at ∼245. Considering the distinct behaviors of Al, Ti and Zr during sedimentary processes, we find that hydrodynamic mineral sorting had a minor effect on the chemical composition of Archean fine-grained sediments, but could have been more effective during periods of supercontinents. We show that the compositions of Phanerozoic sediments (Al 2 O 3 /TiO 2 , Zr/TiO 2 , La/Sc, Th/Sc, Ni/Co, Cr/Sc)are best explained with igneous rocks at the surface of continents consisting of 76 ± 8 wt% felsic, 14 ± 6 wt% Arc-basalts and 10 ± 2 wt% within-plate basalts, most likely in the form of continental flood basalts. Applying the same mass-balance calculations to the Paleoarchean suggests continental landmasses with 65 ± 7 wt% felsic, 25 ± 6 wt% mafic and 11 ± 3 wt% ultramafic rocks (all 2SE), likely in the form of komatiites. The presence of volumetrically abundant felsic rocks at the surface of continents (as evident from the sediment record)as well as at mid-crustal levels (as evident from presently exposed igneous rock record)in Paleoarchean cratons is currently best explained with the onset of subduction magmatism before 3.25 Gyr.
The Phanerozoic history of the Paleo-Asian, Tethyan, and Pacific oceanic domains is important for unraveling the tectonic evolution of the Eurasian and Laurentian continents. The validity of existing models that account for the development and closure of the Paleo-Asian and Tethyan Oceans critically depends on the assumed initial configuration and relative positions of the Precambrian cratons that separate the two oceanic domains, including the North China, Tarim, Karakum, Turan, and southern Baltica cratons. Existing studies largely neglect the Phanerozoic tectonic modification of these Precambrian cratons (e.g., the effects of India-Arabia-Eurasia convergence and post-Rodinia rifting). In this work we systematically restore these effects and evaluate the tectonic relationships among these cratons to test the hypothesis that the Baltica, Turan, Karakum, Tarim, and North China cratons were linked in the Neoproterozoic as a single continental strip, with variable along-strike widths. Because most of the tectonic boundaries currently separating these cratons postdate the closure of the Paleo-Asian and Tethyan Oceans, we are able to establish a >6000-km-long Neoproterozoic contiguous continent referred to here as Balkatach (named from the Baltica–Karakum–Tarim–North China connection). By focusing on the regional geologic history of Balkatach’s continental margins, we propose the following tectonic model for the initiation and evolution of the Paleo-Asian, Tethyan, and Pacific oceanic domains and the protracted amalgamation and growth history of the Eurasian continent. (1) The early Neoproterozoic collision of the combined Baltica–Turan–Karakum–South Tarim continents with the linked North Tarim–North China cratons led to the formation of a coherent Balkatach continent. (2) Rifting along Balkatach’s margins in the late Neoproterozoic resulted in the opening of the Tethyan Ocean to the south and unified Paleo-Asian and Pacific Oceans to the north (present-day coordinates). This process led to the detachment of Balkatach-derived microcontinents that drifted into the newly formed Paleo-Asian Ocean. (3) The rifted microcontinents acted as nuclei for subduction systems whose development led to the eventual demise of the Paleo-Asian Ocean during the formation of the Central Asian Orogenic System (CAOS). Closure of this ocean within an archipelago-arc subduction system was accommodated by counterclockwise rotation of the Balkatach continental strip around the CAOS. (4) Initial collision of central Balkatach and the amalgamated arcs and microcontinents of the CAOS in the mid-Carboniferous was followed by a bidirectional propagation of westward and eastward suturing. (5) The closure of the Paleo-Asian Ocean in the early Permian was accompanied by a widespread magmatic flare up, which may have been related to the avalanche of the subducted oceanic slabs of the Paleo-Asian Ocean across the 660 km phase boundary in the mantle. (6) The closure of the Paleo-Tethys against the southern margin of Balkatach proceeded diachronously, from west to east, in the Triassic–Jurassic.
The Permian Lucaogou oil shales in the eastern Junggar Basin have long been regarded as important source rocks and reservoirs for tight oil exploration. The oil shales can also be used to assess the provenance and tectonic setting of the East Junggar region using major, trace and rare earth element geochemistry. The low chemical index of alteration ratios (37.36–64.18, 45.54 on average), low plagioclase index of alteration ratios (26.99–62.95, 42.99 on average), and high index of compositional variability ratios (0.64–1.66, 1.10 on average) suggest that the Permian Lucaogou oil shales mostly consist of immature sediments that have experienced a weak weathering intensity. The samples have low TiO2/Al2O3 and TiO2/Zr ratios that indicate a felsic origin with minor intermediate igneous rocks. The high Th/Sc and Zr/Sc ratios suggest a felsic origin without sediment recycling. The samples show fractionated light rare earth elements and relatively flat heavy rare earth elements patterns with weak negative Eu anomalies, which implies that the main provenance is lower Carboniferous intermediate–acid volcanic rocks. In addition, multiple major- and trace-element‐based discrimination diagrams show that the parent rock of clastic sediments formed mainly in a continental island arc and active continental margin environment, consistent with previous studies on the tectonic background of the East Junggar region during the late early Carboniferous.
• Key Points
• Immature sediments are present in the Permian Lucaogou oil shales and have a weak weathering intensity.
• The provenance of the clastic sediments of the Permian Lucaogou oil shales was predominantly early Carboniferous intermediate–acid volcanic rocks.
• The clastic sediments of the Permian Lucaogou oil shales were developed in a continental island arc and active continental margin environments.
• The Permian Lucaogou oil shales provide crucial information for tectonic setting and evolution of the northern Xinjiang during the Carboniferous–Permian.
Organic matter (OM) accumulation in terrestrial sediments shows not only a significant carbon sink in the Earth's carbon cycle but an important origin of fossil fuels, which is closely associated with the complex and diverse depositional environments and climate conditions. The Junggar Basin developed the most laterally extensive and thickest alkaline lacustrine sediments as well as the richest hydrocarbon source rock/oil shale interval of all the world during the deposition of the Lucaogou Formation and equivalent units. In this study, the sedimentary succession of 564 m with a mudstone and dolomite matrix of the Lucaogou oil shale from the Qi 1 well located in the southern Junggar Basin was investigated, which can be divided into two members based on lithology. This paper is a synthetic use of major element oxides, trace, and rare-earth elements as well as organic petrography data as proxies to evaluate the provenance, paleotectonic setting, paleoclimate and weathering conditions, paleoenvironment, and paleoproductivity as well as origin and accumulation of OM of the Lucaogou sediments.
It is suggested that no recycling sediments in all the studied samples, based on parameters including the Th/Sc vs. Zr/Sc plot, the Chemical Index of Alteration (CIA) being linearly dependent on Weathering Index of Parker (WIP), and the high Index of Compositional Variability. The provenance mainly from intermediate-felsic volcanic rocks (e.g., granodiorite, andesite and dacite) is supported by the bivariate plots of Euanom vs. Th/Sc, La/Th vs. Hf, La/Sc vs. Co/Th, Cr/Th vs. Sc/Th, Y/Ni vs. Cr/V, and Nb/Y vs. Zr/TiO2 as well as triangular diagrams of mafic–felsic–weathering, M+–4Si–R2+ and Rb/V–Zr/Zn–Sc/Nb. The continental island arc is inferred from paleotectonic setting discrimination diagrams (i.e., the La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10 ternary as well as Ti/Zr vs. La/Sc binary diagrams). The chemical weathering indices (i.e., the CIA, WIP, weathering index (W), Weathering Intensity Scale, Sodium Depletion Index, and Ga/Rb vs. K2O/Al2O3 binary diagram) indicate the paleoclimate conditions were cold/arid with weak chemical weathering in a warming and/or enhanced continental weathering episode. Besides, the Sr/Ba ratios and gallium concentrations indicate the paleosalinity was brackish to hypersaline and was higher in the lower member in relation to that in the upper member. However, the sub-oxic environment of the benthic water (inferred from the MoEF vs. UEF co-variations and Corg/P ratios as well as large sizes of framboidal pyrites) and the moderate paleoproductivity (evaluated via the Sibio, Babio, and Pbio values, as well as Ni/Al and Cu/Al ratios) caused by predominate algae and bacteria (inferred from the organic petrography) kept stable in the paleolake during this period. The OM accumulation was mainly controlled by the preservation conditions rather than the paleoproductivity and limited detrital inputs/low deposition rates, which was indicated by the plot of Co × Mn values vs. Cd/Mo ratios and the total organic carbon contents being negatively related to stable carbon isotope compositions of OM.
The lacustrine organic -rich shale in the Permian Lucaogou (LCG) Formation of the Jimsar Sag, Junggar Basin, is one of the main shale oil plays in China. In this paper, geological and geochemical research techniques were employed to evaluate the geochemical variability of the lacustrine shale and the production of organic matter and its preservation conditions. The LCG Formation is characterized by its complex mineral compositions and a wide range of organic matter richness and quality. The presence of high proportions of β-carotane and C29 steranes, indicates that the organic matter mainly originated from phytoplankton and aquatic algal-bacterial organisms, especially cyanobacteria. This study found that the productivity of the Lower LCG Member (P2l1) was highest, and the Middle LCG Member (P2l2) was the lowest. During the deposition of the Lower LCG Member, the lake's bottom water was predominantly a reducing environment, and the degradation of organic matter was largely a result of bacterial sulfate reduction. During the deposition of the Middle and Upper LCG members, the lake's bottom water was mainly oxidizing, and the degradation of organic matter was likely to be caused by aerobic processes. Based on a comprehensive analysis of the origin and production of organic matter, as well as its depositional environment and preservation conditions, two organic matter accumulation models were proposed to explain the distribution of the organic-rich shale. In model A, the high influx of volcanic ash released nutrients and brought abundant sulfate into the water, the accumulation of organic matter was mainly controlled by the preservation of organic matter, which was mainly controlled by BSR. In the model B, the influx of volcanic ash was small, organic matter was mainly degraded by oxygen and the accumulation of organic matter is mainly determined by the production of organic matter.
The Early-Middle Permian is regarded as a critical period for the Junggar Basin in marine to continental transition. Lacustrine organic-rich shale successions were deposited in the Lucaogou Formation in the southern Junggar Basin (SJB) at that time. Although the paleoenvironment conditions of the Lucaogou Formation have been discussed from diverse perspectives, their spatial and temporal evolution remains controversial. This study systematically investigated 11 outcrop sections and cores in three sags from the SJB, namely the Chaiwopu Sag, the Fukang Sag, and the Jimsar Sag, respectively. Paleoclimatic indicators (i.e., CIA values and Sr/Cu ratios) show that the Lucaogou Formation was generally deposited under semiarid to arid conditions, among which the Jimsar Sag was more arid than the other two sags. Meanwhile, paleoredox indices (V/Cr, V/(V + Ni) ratios, and MoEF, UEF values) illustrate dysoxic-anoxic conditions prevailing during its deposition. Paleosalinity indicators (Sr/Ba and B/Ga ratios) suggest high salinity levels in the three sags, among which the Jimsar Sag exhibited the highest salinity. Our results suggest that, relative to the Chaiwopu and Fukang sags, the Jimsar Sag experienced a more arid, more restricted, and saltier condition during deposition of the Lucaogou Formation. There is a generally lake transgression trend, with relatively less arid climate and less salty water columns in the upper Lucaogou in all the three sags. This transgression may be related to the deglaciation of Gondwanaland, which cause an increase in global temperature and more rainfall. The SJB was composed of a cluster of lakes that were relatively independent and rarely connected with each other. However, lake level rising may have coalesced to form a giant paleo-lake when lake transgression took place.
The Middle Permian Lucaogou Formation is the source rock and the main oil shale producing formation in the southeastern Junggar Basin. This study focused on the Lucaogou Formation exposed in two outcrop sections on the northern flank of the Bogda Mountain, namely the Jingjingzigou and Dalongkou sections. Here, we present integrated analysis of the sedimentology, major and trace elements, mineral components and total organic carbon contents. The paleo-environment was reconstructed including provenance, redox conditions, paleo-salinity, chemical weathering intensity and primary organic matter productivity. The results showed that the upper and lower units were deposited in distinct depositional environments with different organic matter accumulation mechanisms. The lower unit was characterized by low lake level, dry climate, fresh-brackish and well-oxygenated water. While during the deposition of the upper unit the lake level rose, climate turned wetter and the bottom water became less oxidized and much saltier. The mechanism of the organic matter accumulation is different for these two units. The preserved organic matters were mainly controlled by the primary productivity in the lower unit and by the redox conditions in the upper unit.
Organic matter enrichment in black shales has frequently been linked with hydrothermal activity. However, it is poorly understood how hydrothermal activity affected paleo-environments and the enrichment of organic matter in ancient lacustrine sediments. The hydrothermal activity recorded in the Middle Permian Lucaogou Formation (LCGF) in the Jimusar Sag, Junggar Basin, Northwest China, provides an ideal sedimentary archive to explore this issue. The mineralogical composition, major and trace element, total organic carbon, and sulfur content of forty-three shale samples of the LCGF were investigated to better understand how hydrothermal activity influences organic matter enrichment in lacustrine shales. The terrigenous influx, redox conditions, paleoproductivity, hydrothermal effects, and organic matter enrichment mechanisms are discussed. The results show that the terrigenous influx is insufficient overall, and the lower section of the LCGF has less detrital materials than the upper section. The redox conditions of the bottom water of the lake in the Jimusar Sag during deposition of the LCGF were suboxic to anoxic, which, combined with relatively high paleoproductivity, is favorable for organic matter enrichment. Hydrothermal activity occurred more frequently during deposition of the lower section compared with the upper section of LCGF. Hydrothermal fluids provided sufficient nutrients to enhance organic matter production during deposition of the LCGF and resulted in organic matter enrichment.
Sediments originated from volcanic-hydrothermal activities show clear relationship with hydrocarbon generation and accumulation in fine-grained sedimentary rocks at a regional scale. However, the fundamental processes and mechanisms of this relationship are still poorly understood at a microscopic scale to hamper efficient hydrocarbon exploration. In addition, coarse grains common in volcanic lacustrine fine-grained sediments have been traditionally interpreted as diagenetic products during early and late burial. Our study, however, suggests that coarse calcite grains are not diagenetic, but formed by subaqueous explosion. The calcite grains occur in lacustrine fine-grained sedimentary rocks in the middle Permian Lucaogou Formation in the Santanghu intracontinental rift basin, NW China. They are sand sized, angular, mono- and polycrystalline, and discretely scatter in silt to clay-sized non-calcite pyroclastic matrix. The calcite grains and the mud matrix are interpreted as a pyroclastic debris flow deposit. In-situ ⁸⁷Sr/⁸⁶Sr ratios (0.704932–0.705840, and 0.705137 on average) of the calcite grains are similar to those of mantle rocks, indicating an origin closely related to mantle-derived fluids. The in-situ δ¹³C values (4.24‰ to 5.06‰, and 4.57‰ on average) and extremly negative δ¹⁸O values (−21.70‰ to −21.18‰, and − 21.49‰ on average) indicate that extensive hydrothermal alteration and microbial activities influenced the formation of the calcite. Element composition and homogenizaiton temperature of fluid incusions of calcite and biomarkers of the mudstone all suggest a relatively low-temperature hydrothermal condition in a saline and reducing environment. Furthermore, hydrocarbon-rich inclusions are rich in the calcite grains and have two modes of occurrence and maturity. They indicate that the organic matter was generated from algae bloom, which was caused by volcanic-hydrothermal activities. Alternatively, the hydrocarbon may have an inorganic origin originated from a mantle source. Our findings provide an alternative interpretation on the origin of coarse calcite grains in fine-grained sedimentary rocks and demonstrate a microscopic relationship between volcanic-hydrothermal activities and hydrocarbon generation and accumulation in a tectonic active rift basin.
The coupling relationship between volcanic-hydrothermal activities and hydrocarbon generation has been a common understanding in recent years. However, there is a lack of theoretical and practical guidance in the application hydrocarbon production. The fine-grained sedimentary rocks of the middle Permian Lucaogou Formation in the Jimsar Sag in the Junggar Basin, NW China, are critical source rocks and have been simplify regarded as a mudrocks for target for tight oil exploration. Recently studies show that rocks were deposited in an intracontinental lacustrine environment with multiple volcanic-hydrothermal activities. Their complex micro-scale lithological characteristics restrict the understanding and further exploration and development of tight oil. Hence, this study focuses on the detailed lithological and their organic geochemical characteristics of the Lucaogou Formation using microscopic thin-section observation, scanning electron microscopy, X-ray diffraction, Rock-Eval, saturated hydrocarbon chromatography and mass spectrometry. The following key conclusions were obtained. First, the massive mudrocks are sub-divided into lithofacies for hydrocarbon generation, including tuffaceous shale, calcareous shale and dolomitic shale, and, microinterval reservoirs, including dolostone and sedimentary tuff. Second, the origins of organic matter in Lucaogou Formation were mainly plankton, bacteria with minor of land plants. The depositional environment is high salinity and anoxic conditions. Materials derived from volcanic-hydrothermal activities, lacustrine precipitation, and terrestrial transportation mixed with various proportions to form this formation. Third, a mixture of carbonate minerals, felsic minerals and clay minerals is the best source rock, and the higher the mixture, the better. This research not only provides insight into the influence of volcanic-hydrothermal activities on lithology and organic matter enrichment, but also providing a potential scheme for the tight oil exploration and development in similar rifted geological settings.
β-Carotene, the precursor of β-carotane, is commonly rich in brackish sediments and would become unstable theoretically under the exposure to high temperatures, bright light conditions, and acidic or oxdizing environments. However, the specific process and mechanism of the absence of such compounds are still not clear sufficiently. To fill the knowledge gap, in this study we explore the origin of the absence of β-carotane in a 50-m-thick section of the Middle Permian Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwest China, based on a combined mineralogical, petrographic, organic and inorganic geochemical study. Biomarker analysis indicated that the β-carotane-free section was deposited in a brackish, reduced, and semi-deep lacustrine sedimentary environment. Hydrothermal fluid effects during sediment deposition are inferred to have caused the breakdown of β-carotene. The hydrothermal (<100 °C) effects are evidenced by mineralogy and geochemistry, e.g., the hydrothermal-indicative minerals (reedmergnerit, tuff and pyrite), high Fe/Ti and low Al/(Al + Fe) ratios, rare earth elements (REEs) distribution. Associated biomarker variations were found, e.g., low i-alkanes/n-alkanes, high C24Tetracyclic/C26Tricyclic terpanes, high (C19 + C20)/(C21 + C23) tricyclic terpanes, high C29/C30 hopanes, and very low gammacerane/C30 hopane ratios. Our data suggest that hydrothermal fluids can prevent the preservation of β-carotene, and the absence of β-carotane indicates a period of hydrothermal activity during deposition of the Lucaogou Formation in a 50-meter-thick sequence.
The Junggar Basin is one of the very large, long-lived, Late Paleozoic lacustrine basins of China, developed by the closure of the early Paleozoic Junggar Ocean. The novelty of the reported research is outcrop measurements through the mid-Permian Jingzingzigou and Lucaogou formations, allowing reconstruction of interacting very coarse and very fine-grained alluvial-to-lacustrine depositional cycles within a km-thick succession of organic-rich mudstones. The deposits are interpreted as coarse grained fan deltas that extend subaqueously into Junggar lacustrine basin, depositing fine-grained muddy, sediment gravity flow deposits. The coarseness and frequency of both alluvial and subaqueous sediment gravity flows that thin and pinch out over short distances suggests that the margin of the basin was likely steep and fringed by relatively deep lake waters, but was not the main margin of sediment supply. The proposed depositional model highlights the process response to climate-driven lake-level changes around the steep lake margin. However, the coarse-grained alluvial to fan-delta cycles and the muddy lacustrine cycles occur at different thickness scales. The former (5–13 m thick) contain internal erosion surfaces and are dominated by river sheet flood and debris flow deposits and are likely incomplete due to frequent shifting, erosion and reworking of the sediments. The lacustrine cycles (few meters thick) are dominated by current-driven, very fine sandstone, siltstone and organic-rich mudstone beds with ripple lamination and graded beds, as well as dolomites during times or places where subaqueous currents were lacking. The age dating control is not detailed but we propose that depositional cycles were likely a response to Milankovitch orbital cyclicity, also suggested by previous Permian climate modeling, that cause alternating wet and dry periods and trigger tens of meters lake level changes with abrupt grainsize changes in the shallow proximal areas and more subtle changes in sediments of distal/deep lacustrine regions. The climate changes therefore cause both sediment supply changes subaerially and lake accommodation changes subaqueously, though these would also be modulated to a lesser extent in the resultant stratigraphy by autogenic responses and tectonic forcing of sediment yield.
In recent 30 years, Th/U ratio has been using widely in evaluating the paleoredox conditions during the depositions of source rocks. However, there are few studies on the preconditions of the use of Th/U ratio for evaluating the paleoredox conditions. In many areas, the evaluation results of redox conditions in paleoenvironment obtained by Th/U ratio is different from that reflected by other parameters. The accuracy of judgment of paleoredox conditions by Th/U ratio is questioned. In this paper, the weathering degree of source rocks during the deposition are determined by chemical index of alteration (CIA) and Al2O3-CaO, NaO-K2O (A-CN-K) ternary diagram, whereas the lithologies of parent rocks during source rocks depositions are determined by A-CN-K ternary diagram, Al2O3/TiO2 ratio and REEs. Furthermore, the effects of the weathering degree, the properties of parent rocks and sedimentation rate on Th/U ratio to evaluate the accuracy of paleoredox conditions are analyzed. The results show that in lacustrine basins, although the content of uranium (U) gradually increased with the decrease of oxygen content in the water column, the obvious enrichment of thorium (Th) in sediments because of parent rock undergoing strong weathering and relative fast sedimentation rate influences the accuracy of Th/U ratio for evaluating the paleoredox conditions. Additionally, the different initial content of U, Th and Th/U ratio in different lithologies also influence the accuracy of Th/U ratio for evaluating the paleoredox conditions. The distribution characteristics of Th/U ratio are the result of the comprehensive influence of various conditions, not just paleoredox condition. When evaluating the redox condition of paleolake water by the enrichment factors of redox sensitive elements (RSEs) and bimetal ratio, the effect of weathering, lithologies of parent rocks and sedimentation rate should be taken into consideration on distributions of parameters.
Reservoir quality is the key parameter controlling shale oil enrichment. However, the complicated lithofacies, which control diagenesis, result in reservoir heterogeneity in the shale strata, creating challenges for reservoir prediction. Combining with the detail analysis of inorganic and organic geochemistry, characteristics of the main lithofacies and diagenetic alteration were investigated in Permian Lucaogou Formation, Jimusar sag, Junggar Basin. Laminated rocks with laminae combinations containing pyroclastic and terrigenous clastic laminae, terrigenous clastic and dolomite laminae, and pyroclastic and carbonate laminae were identified. Massive rocks mainly includes dolomicrite, dolomitic tuff, silt dolostone, dolomitic siltstone, and tuffaceous siltstone. Feldspar dissolution and carbonate cementation were the two dominant diagenetic types, further causing heterogeneity of the shale oil reservoirs. Feldspar dissolution generally occurred in laminated rocks containing pyroclastic laminae with low content of carbonate minerals (PL1), as well as in their adjacent interbedded siltstones. Carbonate cementation generally occurred in laminated rocks containing carbonate laminae, as well as in their adjacent interbedded siltstones. Whether in laminated and massive rocks, sparry carbonate crystals were also precipitated adjacent to organic matter and in microfractures. In the burial processes, enriched transition metal elements of V and Ti in pyroclastic laminae promote catalytic pyrolysis of organic matter. Abundant CO2 produced during the catalytic pyrolysis promoted feldspar dissolution. Dissolution of sedimentary carbonate minerals by organic acids and CO2 provided the material sources for secondary carbonate cements. It promoted the recrystallization and re-precipitation of sparry carbonate crystals adjacent to organic matter and in the terrigenous clastic laminae or interbedded siltstones. Ca²⁺, Mg²⁺ Sr²⁺, Fe²⁺, and Mn²⁺ produced by the transformation of smectite to illite in the pyroclastic laminae also provided Ca source for secondary carbonate cements. External diagenetic fluids providing materials along microfractures is another genesis of carbonate cements in the Permian Lucaogou Formation.
Lacustrine organic-rich fine-grained rocks are widespread in the second member of the Permian Lucaogou Formation (P2l²) in the Malang Sag, Santanghu Basin, with complex lithology and significant changes of lithofacies due to frequent volcanic activity. In this study, the mineralogical and petrological characteristics, types of organic matter (OM), and effect of volcanism on the sedimentary environment and OM accumulation were analyzed using organic, carbon, and oxygen isotopes, as well as major and trace element data. The results revealed that the P2l² was characterized by mixed deposition of volcanic ash and carbonate minerals, with four main rock types: tuff, dolomitic tuff, tuffaceous dolomite, and dolomite. The fine-grained rocks dominantly contain type I and II1 kerogens, which were in a low-mature stage and exhibited excellent original hydrocarbon generative potential. When volcanic activity was strong, large amounts of nutrients were released by the volcanic ash, which then entered the lake, in turn promoting the blooming of phytoplankton, and causing the δ¹³C values in the tuff to decrease. In addition, the paleotemperature decreased for a short time, and the suboxic-euxinic environment was conducive to the preservation of OM, resulting in high total organic carbon (TOC) content in the tuff. In contrast, during intermittent or weak periods of volcanic activity, the climate was hotter and drier, and evaporation resulted in a sustained increase of salinity. Consequently, the growth of algae was restricted, which brought about an increase in the δ¹³C values and relatively low TOC content in the dolomite. The relationships among primary productivity, redox conditions, clastic influx proxies, and TOC content illustrate that primary productivity was the most vital control on the accumulation of OM. High primary productivity was mainly driven by volcanic ash falls. Meanwhile, high salinity and suboxic-euxinic conditions were beneficial to the preservation of OM, giving rise to a high degree of OM enrichment.
Although redox conditions are the dominant control on authigenic enrichment of trace metals in marine sediments, other factors may be important within environments having relatively uniform redox characteristics, such as some anoxic silled basins. Notably, watermass chemistry (specifically, aqueous trace-metal concentrations) and sedimentation rate can also influence the authigenic accumulation of redox-sensitive trace metals such as molybdenum (Mo) and uranium (U) in the sediment, although these effects have received less attention than redox controls to date. Here, we (1) utilize a diffusion-reaction model to evaluate the effects of variations in watermass chemistry and sedimentation rate on authigenic trace-metal enrichment, and (2) present case studies of Mo and U enrichment in modern Black Sea sediments and North American Devonian-Carboniferous boundary (DCB) black shales that illustrate these influences. In both case studies, redox conditions were assessed using non-trace-metal-based proxies (i.e., C-S-Fe, FeT/Al, and Corg:P). Stations 6 and 7 of the modern Black Sea, at water depths of 380 and 1176 m, respectively, exhibit marked differences in authigenic Mo and U enrichment: median Mo/TOC is 13.2 at Station 6 (range 11.5-14.8) versus 5.7 at Station 7 (range 3.7-7.6), and median U/TOC is 2.6 at Station 6 (range 1.5-3.0) versus 1.3 at Station 7 (range 0.7-1.9) (note: units are ppm/% or 10⁻⁴, and ranges are 16th-84th percentiles). Given the nearly identical redox conditions and sedimentation rates at these two sites, the most likely cause of the >2× enrichment of Mo and U at Station 6 relative to Station 7 is differences in aqueous Mo and U concentrations, which decline steeply through the upper part of the Black Sea water column, demonstrating the influence of watermass chemistry on patterns of authigenic trace-metal enrichment in the sediment. For the DCB black shales, the median Mo/TOC is 22.2 in the Lower and Upper Bakken (range 15.8-27.0), 28.8 in the Sunbury (range 19.2-37.2), and 14.3 in the Cleveland (range 9.3-22.5), and the median U/TOC is 6.2 in the Lower and Upper Bakken (range 3.5-9.6), 2.6 in the Sunbury (range 1.7-3.4), and 1.2 in the Cleveland (range 0.7-1.7). Differences in Mo and U concentrations between these formations show no relationship to inferred aqueous trace-metal concentrations, Mn-Fe particulate shuttles, or paleoenvironmental redox conditions, but they broadly correlate with variation in sedimentation rates, providing evidence that sedimentation rates can measurably influence the degree of authigenic trace-metal enrichment of marine sediments.
Paleoredox conditions are commonly evaluated based on elemental proxies but, despite their frequency of use, most of these proxies have received little comparative evaluation or assessment of their range of applicability to paleomarine systems. Here, we evaluate 21 elemental proxies, including six proxies based on the C-S-Fe-P system (TOC, S, TOC/S, DOPT, Fe/Al, Corg/P), nine proxies based on trace-metal enrichment factors (CoEF, CrEF, CuEF, MoEF, NiEF, PbEF, UEF, VEF, ZnEF), and six additional proxies from Jones and Manning (1994) (U/Th, Uauth, V/Cr, Ni/Co, Ni/V, (Cu+Mo)/Zn), in 55 Phanerozoic marine formations. We used principal components analysis (PCA) to determine relationships among these 21 proxies in each formation and then sought to identify patterns across the full database. The first principal component (PC1) accounted for 40.1% of total dataset variance on average, with the highest median loadings on trace-metal enrichment factors (NiEF 0.82, MoEF 0.76, all nine >0.50). The next highest median loadings are on C-S-Fe-P proxies (TOC 0.58, DOPT 0.30, Corg/P 0.28), with bimetal proxies yielding uniformly lower loadings (Ni/Co 0.18, V/Cr 0.13). PCA of the factor loadings for the 55 study formations demonstrated associations among the 21 elemental proxies linked to specific sediment host phases: (1) an organic cluster associated with TOC, Mo, V, and Zn, (2) a uranium cluster associated with all U-based proxies, and (3) a sulfide cluster associated with S and Fe as well as the trace metals Co, Cu, Ni, and Pb (i.e., the major and typical minor constituents of diagenetic pyrite).
The findings of the present study have important ramifications for use of elemental proxies for paleoredox analysis. First, all of the proxies examined here are influenced by environmental redox conditions to some degree, although the degree of redox influence on any given proxy can vary considerably from one formation to the next. Second, sedimentary enrichment of most proxies depends on the presence of specific mineral and organic host phases, and evaluation of elemental redox proxy data requires an understanding of how elements are partitioned among those phases. Third, no single proxy is a universally reliable redox indicator, although some are more consistently useful than others—notably, TOC and trace-metal EFs. Fourth, because of this inherent variability in proxy response, adoption of redox proxy thresholds established in earlier published studies is discouraged. Instead, we recommend that future redox studies establish redox thresholds on a formation-specific basis through internal cross-calibration of multiple redox proxies.
Existing redox classifications and the calibrations of elemental proxies to modern environmental redox scales are in need of re-evaluation. Here, we review environmental redox classifications, commonly used elemental redox proxies, and their intercalibration, and we propose a novel approach to improve the calibration of such proxies, using datasets from the modern Black Sea, Saanich Inlet, and California Margin as examples. Our approach is based on recognition of compound covariation patterns among pairs of elemental redox proxies within a redox framework based on three key thresholds: (1) the Re⁴⁺/Re³⁻ couple near the suboxidized/subreduced boundary of the suboxic zone, (2) the U⁶⁺/U⁴⁺ couple in the middle of the subreduced zone, and (3) the SO4²⁻/H2S couple at the suboxic/euxinic boundary. Within this framework, it is possible to determine the relative timing of onset and the degree of enrichment of other elemental redox proxies. Our analysis demonstrates that, even though some elements exhibit limited enrichment within the suboxic zone, the bulk of authigenic enrichment of the redox-sensitive elements considered in this study occurs within the euxinic zone. One important finding of our study is that the threshold value associated with a given elemental proxy can vary considerably between depositional systems. For this reason, it is inadvisable to transfer published threshold values (i.e., from earlier paleoredox studies) to completely different formations, and redox proxies must be internally calibrated for each individual paleodepositional system under investigation.
The Lucaogou Formation contains significant amount of shale oil and tight oil resources in China, and the target strata studied here is located in the Jimsar Sag, Junggar Basin. A combination of mudstone, silty mudstone, dolomite mudstone and limy mudstone from this formation was investigated for the organic petrology and organic geochemistry and thus organic matter origin, thermal maturity and depositional condition were evaluated. Source rocks indicate good potential in hydrocarbon generation, which were featured by high TOC, S2 and HI. Tmax and biomarker ratios reveal that source rocks are mature and have enter oil generation window. Microscopical analyses reveal that liptinitic organic matter are dominant components, especially the amorphous components, which are commonly accompanied with advanced plant fragments. Organic matter assemblages indicate organic materials originated from aquatic organisms and terrestrial plants, which is evidenced from the discrimination diagrams of Pr/nC17-Ph/nC18 and C27–C29 regular steranes. Vertical variations in the abundance of organic matter from the bottom up reflect the changes of the depositional conditions, which are consistent with the indications of organic geochemical parameters. In the lower Lucaogou (LLF), the depositional conditions varied from the relatively shallow water, moderate-energy, proximal suboxic/dysoxic-anoxic condition to the deeper, low-energy, distal suboxic-anoxic condition, as endorsed by the enhanced OM degradation and the preservation of palynomorphs. In the upper Lucaogou (ULF), the conditions changed gradually from a brackish, stratified, suboxic-anoxic condition to fresh, aerobic condition, followed by a stratified oxygen-depleted setting with a relatively higher saline upwards, as supported by the variation in the organic components. Overall, the conditions of LLF are more reducing than that of ULF, which is demonstrated by Pr/Ph ratios, gammacerane index and ETR. The understanding of the redox conditions and their evolution of the Lucaogou Formation is crucial to expound the formation of source rocks and evaluate the hydrocarbon-generating potential.
Salinity is a fundamental property of watermasses that is useful in paleoenvironmental and paleoecological studies, yet the theory of application of geochemical proxies to paleosalinity reconstruction is underdeveloped. Here, we explore the use of three elemental ratios for paleosalinity reconstruction: boron/gallium (B/Ga), strontium/barium (Sr/Ba), and sulfur/total organic carbon (S/TOC) ratios. We compiled a large set of modern aqueous and sedimentary chemical data representing a range of salinity facies (i.e., freshwater, brackish, marine) in order to test the relationships of these proxies to ambient watermass salinity and to determine their viability for paleosalinity analysis. Sediment data were limited to fine-grained siliciclastic units (muds/shales/mudstones) without significant carbonate content, in which the elements of interest were mainly acquired through adsorption of dissolved species, forging a connection between elemental proxy values and watermass chemistry. In modern systems, watermass salinity is correlated with these proxies, yielding r of +0.99 and +0.76 for aqueous and sediment B/Ga, +0.66 and +0.54 for aqueous and sediment Sr/Ba, and +0.98 for aqueous sulfate and +0.66 for sediment S/TOC (all significant at p(α) < 0.01). These relationships establish the basis for use of these elemental ratios as paleosalinity proxies. Elemental crossplots permitted estimation of approximate salinity thresholds for each proxy: (1) B/Ga is <3 in freshwater, 3–6 in brackish, and >6 in marine facies; (2) Sr/Ba is <0.2 in freshwater, 0.2–0.5 in brackish, and >0.5 in marine facies; and (3) S/TOC is <0.1 in freshwater and >0.1 in brackish and marine facies. S/TOC did not discriminate effectively between brackish and marine facies, probably because microbial sulfate reduction (MSR) is generally Corg-limited rather than sulfate-limited in both facies. The accuracies of these thresholds for prediction of the salinity facies of sediments are ∼88% for B/Ga, ∼66% for Sr/Ba, and ∼91% for S/TOC. Although the Sr/Ba proxy is slightly less robust owing to difficulty in removing all carbonate Sr influence and/or to greater mobility of Sr and Ba in the burial environment, we strongly advocate use of multiple proxies in order to support paleosalinity interpretations. Finally, we illustrate the application of these proxies with case studies of (1) the Ordos Basin in North China, which contains Ordovician marine shales and Triassic terrestrial mudstones, and (2) the mid-Eocene Bohai Bay Basin in NE China, which accumulated brackish to marine mudstones.
The extent of marine influence on the Paleogene Bohai Bay Basin in northeastern China, which accumulated mainly continental (e.g., fluvial and lacustrine) facies, remains under debate. Evidence from geochemical, paleontological, and lithological data have documented several marine incursions, but no systematic study has been conducted to investigate their timing and frequency and their significance for understanding the Paleogene tectonic evolution of northeastern China. In order to address the marine influence on sediment accumulation and to identify variations in paleosalinity during deposition of the mid-Eocene lower Es3 (Es3L) Member of the Shahejie Formation, this study analyzed multiple paleosalinity proxies (B/Ga, Sr/Ba, and S/TOC) in fine-grained siliciclastic (argillaceous) sediments of the Luo-69 drillcore from the Dongying Depression. All three proxies yielded similar paleosalinity interpretations, with peak salinity and marine influence (i.e., maximum B/Ga, Sr/Ba, and S/TOC values) occurring at the base of the Es3L Member, and progressively declining salinities, representing a shift toward brackish conditions, upsection over an interval of 185.5 m representing ~2.2 Myr of sedimentation. This paleosalinity record suggests gradual long-term variations that were probably due to changes in tectonically controlled gateways linking the Bohai Bay Basin to the Yellow Sea. In addition to analysis of the mid-Eocene Es3L Member, published data on the stratigraphic distribution of marine fossils, glauconite, and elemental paleosalinity proxies are applied to evaluate the history of marine incursions into the Bohai Bay lake system throughout the mid to late Paleogene (~50–24 Ma). This analysis revealed four intervals of probable major marine incursions dating to the early Eocene, the middle Eocene, the early Oligocene, and the late Oligocene.
The middle Permian Lucaogou Formation (P2l) in the Jimusaer Sag of the southeastern Junggar Basin, NW China hosts China’s first commercial tight (shale) oil production. Two tight-oil sweet spot intervals have been identified within the P2l formation. Coupled chemostratigraphic and sedimentary facies analysis reveals that the sweet spot intervals were deposited in deep–shallow saline lacustrine to nearshore environments under an overall dry climate setting. The sweet spot intervals in the P2l formation comprises several chemostratigraphically and lithologically distinct units deposited under the influence of subtle climatic and environmental changes that have previously not been recognized. A total of 11 depositional units have been identified within the two sweet spot intervals based on an Integrated Prediction Error Filter Analysis (INFEFA) of Gamma Ray logs and environmental parameters derived from chemostratigraphic data. The tight oil reservoir sweet spot intervals were found to be controlled by the spatial and temporal distribution of total organic carbon (TOC) and reservoir properties (e.g., porosity and permeability) and source-reservoir coupling. Two potential tight oil exploration plays are recognised, including those depositional units with porous reservoir beds interbedded with high-TOC source beds (a self-generation play), and those units with porous reservoir beds adjacent to high-TOC source beds (a near-source play).
The Lower to Middle Permian terrestrial successions in the southern Junggar and Turpan basins provide a wealth of information regarding the paleoclimatic and paleogeographic evolution of mid-latitude NE Pangea. Two sections, namely the Xiaolongkou section in the southern Junggar Basin and the southwest (SW) Tarlong section in the Turpan Basin, were systematically studied using detrital zircon geochronology and whole-rock geochemical analysis. Stratigraphic studies suggest that, in both sections, the Lucaogou and Hongyanchi formations are dominated by lacustrine deposits, while the Quanzijie Formation represents mixed fluvial-lacustrine and loess environments. The Lucaogou Formation displays low CIA (Chemical Index of Alteration) values and Th/U ratios, suggesting semiarid to arid conditions in both sections. The Hongyanchi Formation shows low to intermittently moderate CIA values and Th/U ratios, indicating variably semiarid-arid conditions alternating with subhumid-semiarid episodes. The lower part of the Quanzijie Formation reflects subhumid-semiarid to semiarid-arid conditions, while the upper part of the Quanzijie Formation mainly records semiarid-arid and shifts into subhumid conditions toward the top. Detrital zircon and element data indicate that, within each section, all three formations display similar zircon age distributions and geochemical characteristics, suggesting no significant changes in provenance during deposition. However, the provenance characteristics of the two sections differ from each other; the Xiaolongkou section is rich in felsic material, with two detrital zircon age peaks, while the SW Tarlong section contains more mafic components, with a single peak in detrital zircon age spectrum. This suggests that the greater Junggar-Turpan Basin was highly partitioned with several independent provenance systems active during the Early-Middle Permian. The abrupt change in depositional environments in both sections may have resulted from uplift of the North Tian Shan, which induced rapid regression of the lakeshore and uplift of sediment source and catchment areas.
Trace metal enrichments in sedimentary deposits are of prime interest because they are governed by processes that also control the production and preservation of organic matter. Consequently, trace metals have been used in reconstructions of the (palaeo)depositional environment of organic-rich deposits, but most of these studies have primarily focused on hydrographically restricted basins and the response of trace metals to changing redox conditions whereas the role of trace metals in the nutrient cycle and primary productivity in upwelling settings remains relatively unexplored.
In this study we present a comprehensive database of published trace metal concentration data in modern organic-rich deposits from a variety of marine settings. Scrutiny of the compiled dataset has resulted in the development of novel trace metal based proxies that allow the distinction between two marine end-member depositional settings that are associated with enhanced organic carbon burial: open marine settings on the continental margin associated with upwelling (e.g. Namibian Margin) and hydrographically restricted marginal marine basins (e.g. Black Sea).
It is shown that high Cd/Mo values are typical for sediments from continental margin upwelling settings whereas Co and Mn concentrations (expressed as Co ∗ Mn values) are high in samples from hydrographically restricted marine basins. The Cd/Mo ratios are thought to track the relative importance of productivity versus preservation with high values in productivity driven systems attributed to the transfer of Cd to the sediments from re-mineralised plankton biomass. Co ∗ Mn values, on the other hand, are believed to reflect the supply and reactive behaviour of Co and Mn and can be used to assess circulation patterns in the water column and the relative contribution of deep versus surface/river water influx to the basin. It is demonstrated that the combined use of the Cd/Mo and Co ∗ Mn proxies provides a highly effective way to distinguish modern/recent marine sedimentary environments, which holds promise for its use in palaeo-environmental reconstructions.
The Shichanggou oil shale, located in the northern Bogda Mountain, on the southern margin of the Junggar Basin, was deposited in a Late Permian lacustrine environment. The concentrations of minerals and trace elements in selected oil shale samples from the Shichanggou area were measured by X-ray fluorescence (XRF) and inductively-coupled plasma mass spectrometer (ICP-MS). Analyzed oil shale samples are characterized by high total organic carbon (TOC) contents (7.76–22.15%) and ash yields (49.35–70.73%) with oil yields from 3.92% to 13.8%.
Shichanggou oil shale samples are rich in SiO2 (64.77%), followed by Al2O3 (9.14%) and Fe2O3 (4.19%). Compared with average shale composition (ASC) from Clarke and North American Shale Composite (NASC), analyzed oil shale samples are enriched in Ca (4.04%) and Mg (3.14%), especially P (0.69%). Al2O3 correlates quite well with Fe2O3, K2O, Na2O, MnO, TiO2, Cu, Ba, Co, and Ni for their association with clay minerals. Besides, the significant correlations between Fe2O3 and MnO, Co, Ni, Ti, Cr are considered to result from their similarity on geochemical behavior. All selected oil shales are characterized by distinctly sloping light rare earth elements (LREE) trends ((LaN/SmN =2.33–3.61) accompanied by flat heavy rare earth elements (HREE) trends, with distinct Eu negative anomalies (0.58–0.74).
Discrimination diagrams based on geochemistry and some ratios of elements indicate that Shichanggou oil shales were deposited in deep water environments with warm-humid climates and anoxic conditions, and we believe the original sediments were consisted of felsic rocks from the west orogenic belt. Geochemical data for oil shales suggest a continental island arc setting. Together with regional geological evidence, the Shichanggou oil shales were deposited in a back-arc intracontinental basin.
The middle Permian Lucaogou Formation in the Jimusaer Sag of the southeastern Junggar Basin, NW China, was the site of a recent discovery of a giant tight oil reservoir. This reservoir is unusual as it is hosted by lacustrine mixed dolomitic-clastic rocks, significantly differing from other tight reservoirs that are generally hosted by marine/lacustrine siliciclastic–calcitic sequences. Here, we improve our understanding of this relatively new type of tight oil reservoir by presenting the results of a preliminarily investigation into the basic characteristics and origin of this reservoir using field, petrological, geophysical (including seismic and logging), and geochemical data. Field and well core observations indicate that the Lucaogou Formation is a sequence of mixed carbonate (mainly dolomites) and terrigenous clastic (mainly feldspars) sediments that were deposited in a highly saline environment. The formation is divided into upper and lower cycles based on lithological variations between coarse- and fine-grained rocks; in particular, dolomites and siltstones are interbedded with organic-rich mudstones in the lower part of each cycle, whereas the upper part of each cycle contains few dolomites and siltstones. Tight oil accumulations are generally present in the lower part of each cycle, and dolomites and dolomite-bearing rocks are the main reservoir rocks in these cycles, including sandy dolomite, dolarenite, dolomicrite, and a few dolomitic siltstones. Optical microscope, back scattered electron, and scanning electron microscope imaging indicate that the main oil reservoir spaces are secondary pores that were generated by the dissolution of clastics and dolomite by highly acidic and corrosive hydrocarbon-related fluids.
The mineralogy and geochemistry of the Middle Permian Lucaogou shale provide information regarding tectonic setting, depositional environment, sedimentary provenance, and chemical properties. The Lucaogou shale consists predominately of quartz and clay minerals with subsidiary feldspar. The mineralogical composition is enriched in SiO2, P2O5, CaO, and Na2O relative to post-Archean Australian shales (PAAS). The abundant free SiO2 and elevated CaO relative to PAAS have led to depletions of most of trace elements due to dilution effects. The chemical index of alteration (CIA) indicates that sediment-source region has undergone low to moderate chemical weathering. The index of compositional variability (ICV) suggests that the shales have low compositional maturity and are most likely dominated by first-cycle deposits. The dominance of plagioclase over K-feldspar in amounts and extrapolation of the analyzed shales back to the plagioclase-alkali feldspar line in the A-CN-K diagram indicate that the shales could have been derived from plagioclase-rich source rocks. The Al2O3/TiO2 ratio and chondrite-normalized rare earth element (REE) patterns display a derivation from felsic rocks. Tectonic setting discriminant diagrams and the chondrite-normalized REE patterns infer a continental island arc setting for sediment-source region. The redox-sensitive elemental ratios and paleosalinity indices reflect an anoxic mildly brackish-influenced to brackish-influenced paleoenvironment.
The Middle Permian Lucaogou Formation in the Jimusar Sag, Eastern Junggar Basin, is one of the highlights for unconventional petroleum exploration and development in China. Approximately 580 m-long cores from 18 wells in the Sag were carefully examined, and 500 core samples were collected. These samples were analysed by combined methods including X-Ray diffraction (XRD), thin-section observation and organic geochemical analyses.
