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Abstract
The largest Ordovician reef-type oil and gas reservoir discovered in China is distributed along the Upper Ordovician carbonate platform margin in the northern of central uplift of Tarim Basin. It is an unconventional hydrocarbon reservoir with complicated oil-gas-water distribution and without obvious edge water and bottom water. The analyses on forming condition and enrichment law of the reservoir showed that a large rimed carbonate platform margin system was formed, and multi-phases depositions of reefs, banks and limemud mounds were developed in the early stage of the Late Ordovician along the northern margin of central uplift. The vertically superimposed and laterally connected reef-type reservoir was distributed widely along the platform margin of Tazhong No.1 structural belt and has many kinds of secondary corrosion voids characterized by low porosity and low permeability. Three major stages of hydrocarbon activities were accountable for the abundant hydrocarbon accumulation. The low-abundance reef-bank type lithologic reservoirs were distributed along the margin tectonic zone. The carbonate platform margin controlled the hydrocarbon migration and accumulation, and a large hydrocarbon-enriched belt was formed. The oil-gas enrichment was mainly controlled by reservoirs. The difference of oil and gas occurrence was caused by multi-stage hydrocarbon accumulation and heterogeneity of reservoirs.
... The Tazhong uplift, located in the central uplift of the platform in the Tarim Basin (Figure 1), is one of the most hydrocarbon-rich areas in the basin. The Ordovician hydrocarbon reserves in this area are estimated to be 340 billion bbl (Yang et al., 2007). However, the area has undergone multiple tectonic movements, resulting in complicated hydrocarbon traps (Yang et al., 2007), which makes the analysis of hydrocarbon migration history extremely challenging. ...
... The Ordovician hydrocarbon reserves in this area are estimated to be 340 billion bbl (Yang et al., 2007). However, the area has undergone multiple tectonic movements, resulting in complicated hydrocarbon traps (Yang et al., 2007), which makes the analysis of hydrocarbon migration history extremely challenging. At present, insufficient information exists on the hydrocarbon charge process in the Tazhong area. ...
... The Tazhong uplift can be further subdivided into several structural belts from north to south as the Tazhong 1 slope belt, the Tazhong 10 structural belt, the central horst belt, and the Tazhong 1-8 buried hill belt (Figure 1). Their structural positions are relatively higher in elevation in the north than in the south (Yang et al., 2007). Recent exploration suggests that the potential oil reserves in the Ordovician carbonate reservoirs of this area amount to 22 billion bbl, and the gas reserves are more than 1.78 tcf (Yang et al., 2007). ...
The process and mechanisms of secondary hydrocarbon migration in the Tazhong uplift, Tarim Basin, were investigated based on the analysis of the regional structure and by integrating geologic, hydrodynamic, and geochemical parameters. Parameters successfully analyzed included the fluid potential, fluid properties, production outputs, and diamantane index. The results indicated that hydrocarbons migrated into the Tazhong uplift from the northern part of the Manjiaer depression through a series of injection points (IPs) during four orogenies, that is, the early Caledonian (510 Ma), the late Caledonian (439 Ma], the late Hercynian-Indosinian (290 Ma], and the Yanshanian-Himalayan (208 Ma]. A total of six IPs were identified at the intersections of the northeast-trending faults and the northwest-trending flower strike faults. The hydrocarbons migrated from the IPs into traps along regional trends from northwest to southeast and from northeast to southwest. The hydrocarbon migration process and patterns determined the distribution of hydrocarbon properties and production rates in the Tazhong uplift. With increasing distance from the IPs, daily hydrocarbon production decreases, and the hydrocarbons become progressively heavier and display lower gas: oil ratios. Copyright © 2013, The American Association of Petroleum Geologists. All rights reserved.
... Despite some success in the exploration of these reef-shoal reservoirs, low production from low poro-perm (<6%; <1 mD) rocks has often inhibited commercial exploitation of these deep reservoirs. As most reef-shoal reservoirs cannot be exploited economically by conventional technologies, they are commonly indicated to be unconventional tight reservoirs [13,23,24]. Horizontal well drilling and large acid fracturing technologies have been carried out to enhance production [13]. ...
The largest carbonate condensate field in China has been found in the central Tarim Basin. Ordovician carbonate reservoirs are generally attributed to reef-shoal microfacies along a platform margin. However, recent production success has been achieved along the NE-trending strike-slip fault zones that intersect at the platform margin. For this contribution, we analyzed the strike-slip fault effects on the reef-shoal reservoirs by using new geological, geophysical, and production data. Seismic data shows that some NE-trending strike-slip faults intersected the NW-trending platform margin in multiple segments. The research indicated that the development of strike-slip faults has affected prepositional landforms and the subsequent segmentation of varied microfacies along the platform margin. In addition, the strike-slip fault compartmentalized the reef-shoal reservoirs into multiple segments along the extent of the platform margin. We show that fractured reef-shoal complexes are favorable for the development of dissolution porosity along strike-slip fault damage zones. In the tight matrix reservoirs (porosity < 6%, permeability < 0.5 mD), the porosity and permeability could be increased by more than 2–5 times and to 1–2 orders of magnitude in the fault damage zone, respectively. This suggests that high production wells are correlated with “sweet spots” of fractured reservoirs along the strike-slip fault damage zones, and that the fractured reservoirs in the proximity of strike-slip fault activity might be a major target for commercial exploitation of the deep Ordovician tight carbonates.
... Based on the analysis, the ancient city-low uplift reservoir matrix porosity is poorer, matrix reservoir space for oil and gas reservoir and the contribution of production is not big, reservoir storage capacity and is closely related to the development degree of reservoir cracks and holes (9)(10)(11)(12)(13). On the basis of existing research results, comprehensive reservoir development characteristics of the ancient city area, carbonate reservoir region can be divided into three categories (table 1). ...
The oil and gas exploration has been broken through in ordovician-Yingshan formation of the lower Paleozoic in ancient city-low uplift, in Tarim Basin. The preliminary study found that the rich oil and gas resources are mainly in karst cave of the dolomite reservoir. It is revealed that the exploration potential of the carbonate reservoir in Yingshan formation of the ordovician. So, the systematic evaluation and study on the carbonate reservoir for Yingshan formation of the Ordovician in ancient city-low uplift have been done. Based on the analysis of drilling, logging, well logging and testing data, it is found that: The reservoir spaces of carbonate reservoir of ancient city area are mainly intergranular pore, intergranular dissolved pore, karst cave, structural fracture and diagenetic fracture. The reservoir of Yingshan formation in the ancient city area is mainly crack pore-cave reservoir, secondly fracture type reservoir, and, the porosity for the third section of Yingshan formation is relative development with large thickness which should be the key exploration target. The dolomite reservoir of Yingshan formation has the characters of multi layers, thin layers vertically and the different comparison in horizon. The lower part of high-quality dolomite reservoir in Yingshan formation is an important exploration area of carbonate reservoir in ancient city-low uplift. The studies, on the micro and macro development characteristics of the carbonate reservoir of ordovician-Yingshan formation in ancient city-low uplift, provide ideas and guidance for the exploration and deployment of the carbonate reservoir.
... In the Tarim Basin, the lower Cambrian gypsum salt layer, which is widely developed in the central uplift, serves as the regional decollement layer and exerts considerable control over the structural deformation of the overlying formations. Extensive previous studies have mainly focused on geometric styles (Li et al., 2008;Li et al., 2013;Wu et al., 2012;Yang et al., 2013) and structural evolution (Zhang et al., 2002;Li et al., 2009;Yu et al., 2010;Guan et al., 2011), as well as hydrocarbon accumulation (Yang et al., 2007;Xiang et al., 2010;Pang et al., 2013, Liu et al., 2015, among other aspects. However, few systematic studies have focused on the characteristics and mechanisms of deformation controlled by the deep gypsum salt layer in the Tazhong area. ...
... The formation is divided into 4 members: the 1st member is primarily composed of intrasparite, the 2nd member is primarily composed of intramicrite and micrite, the 3rd member is primarily composed of dolomitic limestone interbedded with micrite, and the 4th member is primarily composed of dolomitic limestone, limy dolomite, and dolomite. The Yingshan Formation is a large-scale gas-condensate reservoir that is quasi-layer distributed and controlled by weathering crust (Zhou et al., 2006;Yang et al., 2007;Han et al., 2008;Lin et al., 2012). As the direct cap rock, the 3rd to 5th members of the Lianglitag Formation directly control the oil/gas distribution in the Yingshan Formation. ...
The Yingshan Formation, located on the Tazhong Northern Slope, contains oil-and gas-rich layers with the reserves of about 700 x 106 TOE. The high-resistivity inner layers isolate the hydrocarbon bearing zones and form the sequential sets of reservoir bed-seal assemblages in a vertical direction within the Yingshan Formation, which is directly bound above by a micritic carbonate cap rock that is overlain by the 3rd to 5th members of the Lianglitag Formation. The sealing capability of the cap rock and inner barrier layers was evaluated macroscopically and microscopically in terms of the core breakthrough pressure and thin-section identification. The evaluation parameters were extracted from the statistical analysis of drilling and logging data. The 3rd to 5th members of the Lianglitag Formation are more shaly, but the inner barrier layers in the Yingshan Formation are more dolomitic. Argillaceous limestone is more capable of sealing oil and gas zones than micritic limestone. The 3rd to 5th members of the Lianglitag Formation, of which the gamma ray response and core displacement pressure are greater than 20 API and 14 MPa, respectively, provide good sealing with thicknesses of more than 100 m and have better sealing with thickness of more than 200 m. For the same porosity, dolomite has lower coreflood displacement pressure than limestone. The difference in coreflood displacement pressure between the barrier layers and the underlying reservoir bed is 6 MPa, the cutoff value for sealing capability. Carbonate sealing was controlled by early sedimentation and was influenced by late diagenesis. The direct cap rock is dense and has cement content of more than 10%, up to 31%. The reservoir bed has cement content of less than 10%. Generally, the direct cap rock and the inner barrier layers are relatively stable on the lateral distribution.
... The Tazhong area of the Tarim Basin, an inherited palaeohigh zone, is situated at a favorable destination of oil and gas migration, where abundant hydrocarbon resources are contained. Because several sets of source rocks developed in the Tarim Basin (Zhang et al., 2005), multiple stages of hydrocarbon charge processes occurred, and frequent adjustment, reformation or even destruction took place at late stage, the phase of marine hydrocarbons in the platform basin zone of the Tarim Basin is diverse and its genesis is complex Lü et al., 2009;Yang et al., 2007;Zhao et al., 2009;Zhu et al., 2011b;Zhang et al., 2007;2011c;Zhou et al., 2010;Yang et al., 2010;Gao et al., 2012). Bitumen, extra heavy oil, heavy oil, light oil, condensate oil and natural gas coexist, leading to the mixed color of crude oil (Zhu et al., 2012d). ...
The Tazhong area is located in the hinterland of the Taklimakan Desert, where abundant hydrocarbon resources are contained in the deep Ordovician carbonate rocks. Great exploration results have been obtained in the Middle and Lower Ordovician Yingshan formation over recent years, and a large layered oil/gas field has been proven. The fluid properties of this reservoir are complicated, and it can be divided into condensate gas reservoir, volatile oil reservoir and oil reservoir in accordance with fluid component and PVT phase behavior. These reservoirs distribute in alternation horizontally, without apparent boundaries vertically. Based on the study and analysis on test, exploration and production data, it is believed that the diversity of fluid properties results from the multiphase uneven infusion action of oil and gas, especially the severity of gas cut at late stage: the reservoir suffered from severe gas cut became condensate gas reservoir, the one suffered from weak gas cut became volatile oil reservoir, and the one free from gas cut still keeps the original oil reservoir state. It is discovered through study that the strike-slip fault plays an important role in the occurrence of gas cut at late stage: it not only acted as the pathway of gas migration at late stage, but also had an important impact on the reformation of reservoir. The impact of reservoir nature on gas cut mainly finds expression in the difference of combination of reservoir space: the mutually connected large pore/cavity and fracture system was favorable for gas charge at late stage, where condensate gas reservoir was mostly formed; while the isolated pore/cavity where the tectoclase developed weakly was relatively sealed, was unfavorable for the sufficient infusion of natural gas at late stage, and usually keeps the original oil reservoir state or formed volatile oil reservoir. Based on the discussion to the factors that affect the severity of gas cut in the paleo-reservoir at late stage, this paper clarifies the accumulation mechanisms and controlling factors of different kinds of reservoirs, establishes the accumulation mode of 3 types of reservoirs, and this is very significant to the effective prediction of hydrocarbon distribution in the deep carbonate rocks of the Tazhong area.
... In particular, a group of large carbonate oil and gas fields have been proved consecutively at the Lianglitage reef flats of the Ordovician carbonates and the Yijianfang-Yingshan karstic reservoirs in the Tarim Basin (Zhang et al., 2007;Lü et al., 2009;Zhao et al., 2009;Zhu et al., 2011a;Zhou et al., 2010). The Yingshan karstic reservoirs in the Tazhong area are buried at 5000~6500m generally, covering a continuous area of approximately 1.0×10 4 km 2 (Yang et al., 2007a;; with 3P reserves up to 7×10 8 t, they are characterized by large low-abundance accumulation and large-span quasi-layered distribution. Therefore, it is greatly potential to explore the deep carbonate reservoirs. ...
Weathered crust reservoirs of marine carbonate rocks and large-Area quasi-layered carbonate reservoirs controlled by massive unconformity surfaces have become an important play for exploration of marine hydrocarbons across China. In the Tazhong area, oil and gas originating from the marine facies of the Ordovician Yingshan formation, present large-scale integrated continuous accumulation along massive structures. In particular, they are accumulated continuously in layered form in carbonate fracture-cavity reservoir system, and not controlled by tertiary structural belts. Moreover, the Yingshan reservoirs are complex in types, comprising cross distribution of high gas/oil ratio (GOR) condensate gas reservoirs, volatile oil reservoirs and normal oil reservoirs, and large burial depth span and high heterogeneity of quasi-layered reservoirs. The complexity in diagenesis and accumulation lead to the unique distribution and enrichment of carbonate reservoirs in the Tazhong area. The analysis of blow-down and leakage depths, heterogeneity, and physical properties of pores, caves and fractures of the Yingshan formation carbonate reservoirs indicates that the interstratal karsts have superimposed with faults and fractures to form fracture zones, so that the reservoir bodies present integrated continuous distribution characteristics spatially, providing favorable conditions for quasi-layered continuous enrichment of oil and gas. During late periods, a massive amount of natural gas had been filled along the gas-source faults. Based on the variations of relative contents of light hydrocarbons and aromatic naphthalene homologs in crude oil, it is found that the quasi-layered continuous carbonate reservoirs in the Tazhong area are different in terms of the properties of oil and gas and the types of oil reservoirs as a result of the formation of high GOR high-yield condensate gas reservoirs near the oil/gas-source faults due to gas washing, migration and fractionation. Late gas charge and transformation process acting on earlier oil reservoirs is an important geochemical action in the formation mechanism of carbonate reservoirs in the Tazhong area.
... For example, in the Tazhong (Central Tarim) area, the large-scale basement faulting and strike-slip extrusion activities on the northern slope of the Tazhong uplift led to fracture growth and broken zone in carbonate rock near the fault zone (Han et al., 2007;Luo et al., 2007Luo et al., , 2006. Faulting and fractures themselves can play a positive role in the modification of the reservoir bed to a cer-tain extent (Zhao et al., 2007), while the multi-level, multi-directional communication of the fractures undoubtedly improved the quality of carbonate reservoir bed markedly (Yang et al., 2007b). (2) Faulting also caused hydrothermal dolomitization. ...
Ordovician carbonate buried-hill reservoir beds in the Hetianhe (和田河) gas field, lo-cated in the Mazhatage (玛扎塔格) structural belt on the southern margin of the Bachu (巴楚) faulted uplift, southwestern Tarim basin, were studied. Based on field survey, core and slice observation, the general characteristics of carbonate buried-hill reservoir beds and specifically Ordovician carbonate buried-hill reservoir beds in the Hetianhe gas field were discussed. The karst zone of the reservoir beds in Hetianhe gas field was divided into superficial karst zone, vertical infiltration karst zone, lower sub-surface flow karst zone, and deep sluggish flow zone from top to bottom. The effects of faulting on Or-dovician carbonate buried-hill reservoir beds in the Hetianhe gas field were obvious. The faulting in-tensified the karstification and increased the depth of denudation. Faulting and subsequent fracture growth modified the reservoir beds and improved the physical property and quality of the reservoir beds. Moreover, faulting enhanced the development of the dissolution holes and fractures and increased the thickness of the effective reservoir beds. Meanwhile, faulting made the high porosity-permeability carbonate belts, which created conditions for the hydrocarbon accumulation, develop near the fault zone. KEY WORDS: Tarim basin, Hetianhe gas field, carbonate buried hill, faulting, reservoir bed. INTRODUCTION Fine or poor characteristics of carbonate bed play an important role in the oil and gas production.
The Ordovician of the Tazhong area in the Tarim Basin has suffered multi-cyclic hydrocarbon charging, making Tazhong a typical condensate gas district. In this paper, production and test data were gathered and a detailed comparison was conducted on the geology and the fluid distribution and characteristics between the eastern and western Tazhong area. Eastern and western regions exhibit significant differences in tectonic structure, fluid distribution, and physical-chemical properties of oil and gas. Compared with the eastern region, the western part has a greater development of discordogenic gas associated with strike-slip faults which, combined with the Tazhong No. 1 fault zone, control the fluid distribution. The eastern region is mainly controlled by the Tazhong No. 1 fault zone. Fluid have markedly homogeneous properties in the east, but are heterogeneous in the west. The origins of oil and gas are different between the east and the west. In the east, hydrocarbons are mainly from Ordovician source rocks and natural gas is mostly derived from kerogen pyrolysis. In the west, the hydrocarbons mainly originated from Cambrian source rocks, and the gas was mostly generated by crude oil cracking. In sum, the east region is dominated by primary condensate gas reservoirs, and the western region is dominated by secondary condensate gas reservoirs. Because of the different geological settings and fluid physical properties, differences in the condensate gas reservoirs in the eastern and the western Tazhong area have been analyzed, and appropriate formation mechanisms for condensate gas origins are established.
Understanding hydrocarbon migration mechanism is a key scientific problem that needs to be resolved for effective hydrocarbon exploration in the Ordovician rocks of the Tazhong area, Tarim Basin. Based on analyses of geological setting and reservoir distribution characteristics, hydrocarbon migration process, and mechanisms in the Ordovician, Tazhong area, were investigated by integrating geological, geophysical, and geochemical parameters. Successfully analysed parameters included logging reservoir properties, seismic attributes, production outputs, fluid properties, and hydrocarbon maturity parameters. Results indicate that hydrocarbons were introduced into the Ordovician through a series of charging positions during 3 accumulation periods — middle Caledonian, late Hercynian, and Himalayan. In total 11 hydrocarbon charging positions were identified at intersection zones of the northeast-trending faults and northwest-trending flower strike faults. There are anomalies in all parameters on hydrocarbon charging positions. With increasing distance from the charging positions, anomalies of oil density, wax content, dry gas coefficient, hydrogen sulphide content, gas/oil ratio, well production, 4-/1-MDBT ratio of crude oil weakened, and natural gas' carbon isotope composition becoming lighter along a northwest to southeast direction are present. Evidence from main hydrocarbon pathway systems theory, formation mechanisms of fault intersection zones, and obvious improvement in reservoir properties at intersection zones supports this viewpoint. Further exploration activities in the Ordovician of the Tazhong area should be conducted in the North Slope's west area, and intersection zones in tectonic up-dip direction should be emphasized. Copyright
The hydrocarbon accumulation model of Ordovician reservoirs in the Tazhong area, Tarim Basin, was investigated based on the analysis of the key factors controlling hydrocarbon generation, migration, and enrichment process and history. The results show that the hydrocarbon generation processes are controlled by Cambrian–Lower Ordovician and Upper Ordovician source rocks distributed in the Tazhong area and the Manjiaer Sag. Vertical hydrocarbon migration distance is controlled by the vertical source-reservoir distribution, faults, and caprocks. Structure played a major role in the lateral hydrocarbon migration along NW-SE direction. Lateral migration pathways were governed by unconformities, transport faults, and reef-beach body reservoirs. A part from the sedimentary control, hydrocarbon enrichment is controlled by recent tectonic events. The hydrocarbon charging points refer to the intersection of northeast- and northwest-trending faults. Two types of hydrocarbon accumulation models could be retained, (a) mixed-sourced area model, in which the hydrocarbons are sourced from both the Tazhong area and the Majiaer Sag, and (b) single-sourced area model, in which the hydrocarbons are only sourced from the Tazhong area itself.
With a lot of drilling coring and core thin section observations, imaging logging and logging data, as well as 3D seismic data interpretation and on the basis of determining the seismic sequence stratigraphic framework of Yingshan Formation and Penglaiba Formation in the Mid-Lower Ordovician in Tanzhong northern slope of Tarim Basin, we conduct sedimentary paleogeomorphology restoration and reservoir research to further establish corresponding platform evolution model, reservoir evolution model and geological model. Penglaiba depositional period in the region is of gentle slope platform deposition model and Yingshan depositional period is of weak rimmed platform deposition model; reservoir development is under strong control of sedimentation, which is closely related to level 3 or level 4 sequence boundary exposure and corrosion as well as burial fluid corrosion and features quasi-layered and pored reservoir development, small depositional landform differentiation; the laminated or block dolomite reservoir in Penglaiba Formation will be the important exploration area in the next step.
Palaeozoic central Tarim basin which is complex in geology and rich in hydrocarbon is a typical composite reservoir area. Based on the production and research data obtained by now, by multidisciplinary analyses on the hydrocarbon source rocks, reservoir types, cap rock, and migration pathway of Tazhong area, the following conclusions have reached. Multiple phase hydrocarbon charging occurred in Tazhong area with multiple source rocks, and the reservoir types are complicated with multiple reservoir-cap groups. The traps include anticline, fault depression, reef flat, lithologic trap, magmatic trap and so on. Migration pathway includes fault, unconformity, fracture-hole, sandstone formation and so on, among which fault is mainly a vertical pathway, and the unconformity, fracture-hole and sandstone formation are mainly lateral pathways. Studies suggest that multiple tectonic movements and sedimentary evolution are a fundamental cause for the composite reservoir formation; multiple source rocks and multiple hydrocarbon chargings are the base of complex reservoir; inhomogeneous geology condition, especially faults and reservoir, is the direct cause for multiple types of reservoirs and its complex distribution. This research result is significant for the composite reservoir theory's advancement and for the hydrocarbon exploration in Tazhong area.
In this study, a detailed analysis of the field outcrop profiles, drilling cores and well logging data showed that the Sichuan Basin in Cambrian and Ordovician mainly developed restricted platform facies, open platform facies, and platform marginal facies. Combined with seismic data, the Cambrian and Ordovician were respectively divided into five and six 3rd order sequences in accord with the theory and method of sequence stratigraphy. With the 3rd-order sequence as the unit, the map of sequence lithofacies paleogeography was prepared for Sichuan Basin and adjacent areas in Cambrian and Ordovician using the single factor analysis and multifactor comprehensive mapping method. In the Cambrian and Ordovician, terrigenous clastic rocks mainly deposited in the west, and carbonate rocks dominated the east. Regarding the evolution of carbonate platform, clastics-hybrid shelf in the early stage of Early Cambrian (Meishucun Age and Qiongzhusi Age) and carbonate ramp in the middle stage of Early Cambrian (Canglangpu Age) evolved to rimmed platform from the late Early Cambrian (Longwangmiao Age) to the Middle Ordovician. In the Late Ordovician, the combined effects of global sea level rise and tectonic subsidence of Yangtze plate southeast passive continental margin subducted to Cathaysia led to rapid rise of sea level and increase in deposition water depth. As a result, the upper Yangtze (Sichuan) platform was submerged, further developing into a deep-water ramp-basin facies. In the Early Cambrian, terrigenous clastic rocks and carbonate hybrids mainly deposited in the early stage, whereas carbonate rocks mainly deposited in clear water areas in the late stage. A large area of favorable reservoirs (i.e. , intra-platform shoals and intra-platform dolomites) of restricted platform facies was developed in the basin. As affected by the Ordovician transgression, the basin mainly developed open platform facies with intra-platform shoal facies in banded distribution. In the Late Ordovician, the platform was submerged and deposited with micritic limestone, argillaceous limestone, mudstone of Baota Formation, Linxiang Formation, and Wufeng Formation, which became the regional cap rocks throughout the entire basin. Argillaceous limestone and mudstone of Wufeng Formation are the important high quality source rock in Sichuan Basin. The favorable reservoirs of natural gas consist of dolarenite, oolitic dolomite, and silty-fine crystal dolomite, with well-developed dissolution pores (average porosity 2%~6%), large monolayer thickness, and good spatial continuity. Based on the evaluation of favorable reservoirs, four favorable exploration zones of the Cambrian and Ordovician were selected through comprehensive analysis of the hydrocarbon accumulation conditions, and the favorable exploration zones of Leshan-Longnüsi Palaeouplift and Luzhou Palaeouplift have developed large-scale structural traps and widely distributed favorable reservoirs, thus are of great exploration potential.
Based on the seismic reflection, logging, core and slice microscopic, three kinds of sedimentary facies were identified in Gucheng area in the middle-lower Ordovician Yingshan formation from west to east, including open platform, platform edge and platform-margin slope. Drilling GC6, GC7, GC8 and GC4 are located in the platform margin, development of sand-chip shoal and interbank sea. The paleogeomorphy plays an important role in controlling distribution and sedimentary facies types of platform. The change of Sea level controls the position of the platform margin in the Yingshan deposition period.
The depositional microfacies of carbonate illustrate the feature of the depositional water energy and environment. Additionally, microfacies are the fundamental of pore space development and facilitate the formation of favorable reservoirs. Ten depositional microfacies (Mf1-Mf10) and four microfacies patterns (MA1-MA4) are recognized in Yingshan Formation, the northern slope of central Tarim Basin, through a large number of thin-sections and cores. The microfacies patterns are distributed horizontally along with the Tazhong No. 1 fault zone. High-energy to medium-to-high-energy depositional patterns developed beside the boundary of platform (i.e. MA1 and MA2). However, the microfacies patterns in the inner platform indicate the decrease of water-energy to the central basin. Two microfacies patterns, including medium-to-low-energy tidal flat (MA3) and low-energy-dolomitic tidal flat (MA4), have been suggested in the inner platform. Medium-to-low-energy microfacies patterns transform to medium-to-high-energy microfacies patterns from Yingshan 2nd to Yingshan 1st. During the late stage of Yingshan 1st, which is the main stage for forming carbonate shoals, the depositional water energy gradually increased from the bottom to the top. The high energy microfacies which were on the top of Yingshan Formation, were the primitive material for favorable reservoir, because they formed several types of pores, and developed structural fractures and karstification more easily.
The Tazhong area, at the centre of the Tarim Basin, northwestern China, contains abundant hydrocarbon resources and features complex geological setting, principally in the Lower Ordovician carbonate reservoirs. Despite years of exploration and development, the hydrocarbon accumulation mechanisms and the key factors controlling hydrocarbon accumulation are still unclear. This lack of knowledge is the critical factor restricting hydrocarbon exploration in the Lower Ordovician Yingshan Formation. An analysis of the geological setting and reservoir distribution characteristics, the accumulation mechanisms and the key factors are investigated to predict the target exploration area. Our results show that the hydrocarbons, which experienced four stages of accumulation, originated from mixed sources including Cambrian–Lower Ordovician and Middle–Upper Ordovician source rocks within the Tazhong area and beyond. Hydrocarbons, by point charging, migrated into weathering crust karst reservoirs along 10 intersections of northeast and northwest fault sets, and then laterally migrated, principally in the NW–SE direction. The Yingshan Formation carbonate reservoirs are typically lithologically defined and primarily controlled by differential capillary forces. The accumulation and distribution of hydrocarbons in the Yingshan Formation were controlled by excellent reservoirs and hydrocarbon charge energy. The physical properties of excellent reservoirs control the petroliferous features and the hydrocarbon accumulation threshold: these reservoirs have a threshold with the minimum porosity and permeability, approximately 1.8% and 0.1 mD, and have a threshold with the maximum capillary forces, approximately 2.5 MPa. Hydrocarbon charge energy controls the hydrocarbon enrichment range: hydrocarbons enriched in areas near injection points and the hydrocarbon charge distance threshold of 20 km. Excellent reservoirs and hydrocarbon charge energy coupled index (RSI) controls hydrocarbon accumulation and distribution. Accumulation does not occur when the value of RSI is Keywords: Tazhong area; favourable accumulation target prediction; hydrocarbon accumulation mechanism; key factors; quantitative characterisation Document Type: Research Article DOI: http://dx.doi.org/10.1080/08120099.2015.1038730 Affiliations: 1: State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, 102249, PR China 2: Research Institute of Exploration and Development, Tarim Oilfield Company, PetroChina, Korla, Xinjiang, 84100, PR China Publication date: May 19, 2015 $(document).ready(function() { var shortdescription = $(".originaldescription").text().replace(/\\&/g, '&').replace(/\\, '<').replace(/\\>/g, '>').replace(/\\t/g, ' ').replace(/\\n/g, ''); if (shortdescription.length > 350){ shortdescription = "" + shortdescription.substring(0,250) + "... more"; } $(".descriptionitem").prepend(shortdescription); $(".shortdescription a").click(function() { $(".shortdescription").hide(); $(".originaldescription").slideDown(); return false; }); }); tandf/aes/2015/00000062/00000004/art00007 dcterms_title,dcterms_description,pub_keyword 6 5 20 40 5 GA_googleFillSlot("Horizontal_banner_bottom");
Abundant oil and natural gas is embedded in Palaeozoic strata of the Tarim Basin in which reservoirs are buried at the depth about 5500 ∼ 10000m. The phase states of hydrocarbon fluids in depth or the depth where mass liquid oil lost is a concerned theoretical question, and the question that marine facies in the Tarim Basin rich in gas or oil related to the future deliverability plan of Tarim oil field. We studied the thermal stability of oil in the Tarim Basin, especially researched the compensating effect of low geothermal gradient and quick deep burial process, considered that the depth where oil cracking began is at 9000 ∼ 10000m or deeper, and the correspondent reservoir temperature is higher than 210°C, a mount of oil may exist above this depth. Though the research of key geosciences problem of hydrocarbon migration and accumulation, it demonstrated that hydrocarbon mainly accumulated in Late Hercynian stage in the platform area, and hydrocarbon generated from source rocks are primarily charged into steadily existed paleo-uplifts and the slope area; during Late Hercynian stage, those carbonate reservoirs were buried in the depth of 800 ∼ 2500m and karst reservoirs developed, which is the base of blanket hydrocarbon accumulation and consequently implied that reservoirs located in the slope area buried in depth of 7000 ∼ 9000m will be the important prospecting region of black oil and condensate oil.
In recent years, hydrocarbon prospecting in China marine basin progressed quickly and diseoverd series of marine oil filed, marine oil and gas is now playing a more important role. Especially, under the guide of new hydrocarbon geological theory of marine basin, exploration of marine oil and gas will enter a high-speeding development period, by which the pressure of eastern terrestrial facies basins prospecting and exploitition will be eased greatly. New hydrocarbon geological theory of marine basin contains three aspects. In the aspect of hydrocarbon productivity, China marine sedimentary basins are not lack of high-TOC argillaceous source rocks but the hydrocarbon productivity of pure carbonate is limited: marine gases mainly originated from oil-cracking gas in deep reservoirs under high temperature and coal formations of transitional facies under high evolutionary phase: under low geothermal gradient and deepseated condition, crude oil of Tarim basin can exist in the reservoirs buried below 9000m, the prospecting potentiality of Tarim Basin is great. In the aspect of reservoirs, intensely alterlating by TSR in depth and the development of interstratal karst and veneering kaist expanded the prospecting of carbonate reservoirs. According to the aspect of hydrocarbon accumulation, the proposal of the large area, quasi-layerd, sequential distributed and fracture-cavern accumulation models expanded the hydrocarbon exploration area and decreases the exploration cost: the discovery- of series of old aged reservoirs has enhanced geologists' faith to seek primary type oil reservoirs in complex structure area. Our studies suggest that the main exploration area can extend to the depth of 9000m around the uplift slope.
Chinese marine carbonate oil and gas exploration has shown rapid development momentum in recent years, and a number of large oil and gas fields have been discovered. These oil and gas fields have the following characteristics: the oil and gas are distributed over a large area and in quasi-layered form; reserves are of large scale but low abundance; the reservoir scale is greater than 1000 meters; there is no unified gas-water interface, but a uniform temperature and pressure system; reservoirs are mainly low-porosity, permeable vuggy reservoirs with strong heterogeneity. The fracture-cavity carbonate reservoir space types include primary dissolved pores, incompletely filled underground rivers, unconformities, fractures formed by multi-stage faults, and others. The fluid states include Darcy flow, non-Darcy flow, pipe flow, and various forms of seepage. Due to poor connectivity between the carbonate spaces, oil and gas column heights are usually within 50 m, and their buoyancy is not sufficient to overcome capillary resistance, resulting in obvious oil, gas, and water gravity segregation and an inconsistent gas-water interface. These heterogeneous carbonate large-area and large-cavity oil and gas fields consist of a series of small composite contiguous oil and gas reservoirs, which means that the hydrocarbon-water distribution is not completely controlled by local structures and that reservoirs lack a clear boundary. Because of void layers formed by weathered dripping in syngenetic periods, most of the caves or holes in deeply buried layers are difficult to compact. In addition, early oil and gas injection has an important role in preserving reservoir effective porosity. Hence, an effective reservoir can be maintained at a depth of 10,000 m. Therefore, there may be great potential in uplift slope areas (7,000 to 10,000 m) for exploration of vuggy carbonate oil and gas.
Dibenzothiophene series is one of the most important compositions in crude oil, which generated under multiple geological and geochemical processes. The relationships between dibenzothiophene series and other biomarkers (C29 αα sterane 20R, and C28</ sub> triaromatic sterane 20R), combined with the research of thermochemical sulfate reduction (TSR), biodegradation, geological ages, and oil source in Paleozoic oil in Tazhong area, indicated that there are three control factors for high concentration of dibenzothiophene series. First, both middle-upper Ordovician and lower Ordovician-Cambrian source rocks are marine carbonate sedimentary rocks which could produce abundant organic sulfur compounds including dibenzothiophene series. Second, biodegradation could cause the enrichment of organic sulfur compounds. In addition, sulfate reducing bacteria was able to transfer hydrocarbons and S in SO4</ sub>2- in oil-bearing reservoir water into organic sulfur compound. It might be the main path to produce dibenzothiophene series. Third, sulfur compounds might have been formed by TSR, because S of SO42- in oil-bearing reservoir water could also be transferred to H2S and organic sulfur compounds under high temperature, which might result in increase of dibenzothiophene series in crude oil.
The Ordovician carbonate platform at the Yijianfang outcrop of the Bachu uplift region in the western Tarim Basin contains four types of genetic facies associations developed in the calciclastic slope-fan depositional system: an olistostrome zone, fan channels, lobes, and a marginal slope. The olistostrome zone is characterized by olistoliths and slump fans, whereas the fan channels and lobes are further divided into proximal and distal fades. The marginal slope deposits constitute the background sedimentation in which the calciclastic slope fans are intercalated. From proximal to distal parts of the fan channels and lobes, their scale gradually becomes smaller, and the size and sorting of grains become finer and better, respectively. Analysis of the stratigraphic framework indicates that the fans formed in the lower strata of the Upper Ordovician Lianglitage Formation in four high-frequency sequences (i.e., Pss1-Pss4). Field paleocurrent measurements indicate northeast-southwest depositional strike for the early platform margin of the Lianglitage Formation. Sediments in the calciclastic slope fans were derived from the platform margin, and evolution of the calcidastic slope fans was generally progradational from Pss1 to Pss2 and then continuously retrogradational from Pss2 to Pss4. The calciclastic slope fans in the outcrop area are not reservoir-prone rocks, but interpretation on these fans can provide useful information about potential hydrocarbon reservoirs along the platform margin. The P-wave velocity, S-wave velocity, and density variations in each genetic facies may be used to identify the subsurface calcidastic slope-fan depositional system.
Marine carbonate reservoirs, as a focus of petroleum exploration and development all over the world, are involved with high exploration risk and prediction difficulty owing to high heterogeneity and diversity of reservoir beds. In the Tarim Basin, NW China, carbonate reservoirs with resources about 38 % of the whole basin in a large prospecting area are mainly distributed in the Cambrian and Ordovician in central (Tazhong) and northern (Tabei) Tarim. Recently on the northern slope, Tazhong Low Rise, Central Uplift, Tarim Basin, a breakthrough has been made in the karsted weathering crust of Lower Ordovician Yingshan Formation and reef-flat reservoir of Upper Ordovician Lianglitag Formation. As a new frontier of exploration, oil/gas distribution and controlling factors of carbonate reservoirs in the Yingshan Formation are not clearly understood. In this work, play elements of the Yingshan Formation, such as seal-reservoir bed assemblage, oil/gas properties, and faulting, were studied by core and slice observation and field investigation. High-quality reservoir beds of Yingshan Formation are quasi-layer distributed in the interstratal karst belt about 250 m below the unconformity. The reservoir beds of fracture-void and void are formed by faulting, associated fracturing, and karstification. The Yingshan Formation is a large-scale condensate gas reservoir with partly oil. Owing to different oilgas infilling periods, isolated pools far from the faults are primarily oil in the Hercynian; oppositely, condensate gas reservoirs near the faults are intensely influenced by gas invasion during the Himalayan movement. Laterally, oil/gas distribution is controlled by stratal pinch-out and strike-slip faults. Vertically, cap rock of the third to fifth members of the Lianglitag Formation and Yingshan interior high resistivity layers are superimposed with Yingshan reservoir beds to form several seal-reservoir bed assemblages. Oil and gas are superimposed and affected by gas invasion with characteristics of oil in the upper horizon and gas in the lower horizon.
According to the present research, the cage structure makes adamantane very stable to resist cracking even at a higher temperature, so its concentration will be increased with temperature rising. But adamantane quantitative analysis of oil in Lunnan area shows a poor correlation between its concentration and geotemperature. Instead, it has a good relationship with maturation parameters such as the content of C29ααα 20R sterane and C28 triaromatic steroid 20R as well as Ts/Tm. The reason is that the oil and cracked products generated in deep zone upwards charged in oil layers in low maturity and then turned into mixed oil which had higher concentration of Ts, adamantine etc. So it makes the parameters such as Ts/Tm ratio and adamantine concentration elevate. The incorporation a great amount of low molecular hydrocarbons diluted C29 ααα 20R sterane and C28 triaromatic steroid 20R, which resulted in the parameters of the mixed oil to indicate a higher maturation. Oils with higher adamantane concentration are mainly distributed on Lunnan fault-host zone, Sangtamu fault-host zone and Jilake structure zone. Cracked oil generated in deeper bed migrated along fractures which acted as channels upwards into the reservoir, and so adamantane content of the mixed oil increased. The source of Paleozoic oils in Tarim basin is involved only in two beds: lower Ordovician-Cambrian and middle-upper Ordovician. But according to the data in this paper, there is no obviously difference in adamantane concentration of oils from the two layers.
The combination patterns and depositional characteristics of the carbonate banks are investigated based on outcrop sections, thin sections, and carbon isotopes of Ordovician in the western Tarim Basin, China. Four carbonate bank combination patterns are deposited in the Ordovician, western Tarim Basin, including: Reef‐Bank Complex (RBC), Algae‐Reef‐Bank Interbed (ARBI), Thick‐Layer Cake Aggradation Bank (TLCAB), and Thin‐Layer Cake Retrogradation Bank (TLCRB). All combination patterns show clear periods vertically. The RBC is mainly composed of reefs and bioclastic banks, and the dimension of the RBC depends on the scale of the reefs. Bioclastic banks deposits surround the reefs. The range of the ARBI is determined by the scale of algae‐reefs, algae peloid dolomite microfacies and algal dolomite microfacies deposit alternating vertically. TLCAB and TLCRB are deposited as layer‐cakes stacking in cycles and extending widely with cross bedding developed. The grains of TLCAB and TLCRB are diverse and multi‐source. With the impacting of relative sea level change, biological development and geomorphology, the ARBI, TLCAB or TLCRB, RBC are successively developed from the Lower Ordovician Penglaiba Formation to the Middle Ordovician Yijianfang Formation. The depositional environment analysis of Ordovician indicates that the RBC and ARBI can form effective oil and gas reservoirs, and the TLCAB and TLCRB have the potential to form the huge scale oil and gas reservoirs and to be the crucial targets of exploration for the Ordovician carbonate banks in the future.
: Complex hydrocarbon reservoirs developed widely in the superimposed basins of China formed from multiple structural alterations, reformation and destruction of hydrocarbon reservoirs formed at early stages. They are characterized currently by trap adjustment, component variation, phase conversion, and scale reformation. This is significant for guiding current hydrocarbon exploration by revealing evolution mechanisms after hydrocarbon reservoir formation and for predicting remaining potential resources. Based on the analysis of a number of complex hydrocarbon reservoirs, there are four geologic features controlling the degree of destruction of hydrocarbon reservoirs formed at early stages: tectonic event intensity, frequency, time and caprock sealing for oil and gas during tectonic evolution. Research shows that the larger the tectonic event intensity, the more frequent the tectonic event, the later the last tectonic event, the weaker the caprock sealing for oil and gas, and the greater the volume of destroyed hydrocarbons in the early stages. Based on research on the main controlling factors of hydrocarbon reservoir destruction mechanisms, a geological model of tectonic superimposition and a mathematical model evaluating potential remaining complex hydrocarbon reservoirs have been established. The predication method and technical procedures were applied in the Tazhong area of Tarim Basin, where four stages of hydrocarbon accumulation and three stages of hydrocarbon alteration occurred. Geohistorical hydrocarbon accumulation reached 3.184 billion tons, of which 1.271 billion tons were destroyed. The total volume of remaining resources available for exploration is ∼1.9 billion tons.
The Late Ordovician Lianglitage Formation comprises 13 microfacies (Mf1-Mf13) that were deposited on a carbonate platform at the Tazhong Uplift of the Tarim Basin in Northwest China. Each type of microfacies indicates a specific depositional environment with a certain level of wave energy. Four primary groups of microfacies associations (MA1-MA4) were determined. These associations represent different depositional facies, including reef-shoal facies in the platform margin (MA1), carbonate sand shoal facies (MA2) and oncoid shoal (MA3) on open platforms, and lagoon and tidal flat facies (MA4) in the platform interior. Each microfacies association was generated in a fourth-order sedimentary sequence developing within third-order sequences (SQ1, SQ2, and SQ3, from bottom to top), showing a shallowing-upward trend. High-frequency sequences and facies correlation between wells suggests that the reef-shoal facies more successively developed in the southeastern part of the platform margin, and high-energy microfacies were more strictly confined by the top boundary of fourth-order sequences in the northwestern part of the platform. The highstand systems tract (HST) of the SQ2 is characterized by reef-shoals that developed along the platform margin and tidal flats and lagoons that developed in the platform interior, while the SQ3 is characterized by the oncoid shoal facies that generally developed on the uplift due to a regionally extensive transgression that occurred during the latter part of the Late Ordovician. The results of this study can be used for investigating the development and distribution of potential reservoirs; the reservoirs in southeastern part of the platform margin may be of premium quality because the high-energy microfacies were best preserved there.
: The Tazhong reef-flat oil-gas field is the first large-scale Ordovician organic reef type oil-gas field found in China. Its organic reefs were developed in the early Late Ordovician Lianglitag Formation, and are the first large reefs of the coral-stromatoporoid hermatypic community found in China. The organic reefs and platform-margin grain banks constitute a reef-flat complex, mainly consisting of biolithites and grainstones. The biolithites can be classified into the framestone, bafflestone, bindstone etc. The main body of the complex lies around the wells from Tazhong-24 to Tazhong-82, trending northwest, with the thickness from 100 to 300 m, length about 220 km and width 5–10 km. It is a reef-flat lithologic hydrocarbon reservoir, with a very complex hydrocarbon distribution: being a gas condensate reservoir as a whole with local oil reservoirs. The hydrocarbon distribution is controlled by the reef complex, generally located in the upper 100–200 m part of the complex, and largely in a banded shape along the complex. On the profile, the reservoir shows a stratified feature, with an altitude difference of almost 2200 m from southeast to northwest. The petroleum accumulation is controlled by karst reservoir beds and the northeast strike-slip fault belt. The total geologic reserves had reached 297.667 Mt by 2007.
The oil, gas and water volumes revealed by the productivity of exploratory wells do not reflect the actual underground situations. Under the geologic conditions, a certain amount of dissolved natural gas is stored in oil or water. Based on the production test data of exploratory wells in the Tazhong uplift of the Tarim basin, this paper discusses in detail the differences in occurrence and distribution featrues between the surface and underground natural gases; presents a restoration of the surface gas occurrence to actual underground geologic conditions according to the dissolubility of natural gas under different temperature, pressure and medium conditions; and classifies the natural gas into three states, i.e. the oversaturated, saturated and undersaturated, according to its relative content underground. Through a comparative analysis of the differences in surface and underground occurrences of natural gas, it discusses the hydrocarbon reservoir formation mechanism and distribution rules, thereby providing guidances as new methods and technologies for the prediction of potential natural gas reservoir distribution in the study area.
The Tarim Basin is the largest petroliferous basin in the northwest of China, and is composed of a Paleozoic marine craton
basin and a Meso-Cenozoic continental foreland basin. It is of great significance in exploration of Ordovician. In over 50
years of exploration, oil and gas totaling over 1.6 billion tonnes oil-equivalent has been discovered in the Ordovician carbonate
formation. The accumulation mechanisms and distribution rules are quite complicated because of the burial depth more than
3,500 m, multi-source, and multi-stage accumulation, adjustment, reconstruction and re-enrichment in Ordovician. In this paper,
we summarized four major advances in the hydrocarbon accumulation mechanisms of Ordovician carbonate reservoirs. First, oil
came from Cambrian and Ordovician source rocks separately and as a mixture, while natural gas was mainly cracked gas generated
from the Cambrian-Lower Ordovician crude oil. Second, most hydrocarbon migrated along unconformities and faults, with different
directions in different regions. Third, hydrocarbon migration and accumulation had four periods: Caledonian, early Hercynian,
late Hercynian and Himalayan, and the latter two were the most important for oil and gas exploration. Fourth, hydrocarbon
accumulation and evolution can be generally divided into four stages: Caledonian (the period of hydrocarbon accumulation),
early Hercynian (the period of destruction), late Hercynian (the period of hydrocarbon reconstruction and re-accumulation),
and Himalayan (the period of hydrocarbon adjustment and re-accumulation). Source rocks (S), combinations of reservoir-seal
(C), paleo-uplifts (M), structure balance belt (B) matched in the same time (T) control the hydrocarbon accumulation and distribution
in the Ordovician formations. Reservoir adjustment and reconstruction can be classified into two modes of physical adjustment
and variation of chemical compositions and five mechanisms. These mechanisms are occurrence displacement, biodegradation,
multi-source mixing, high-temperature cracking and late gas invasion. Late hydrocarbon accumulation effects controlled the
distribution of current hydrocarbon. The T-BCMS model is a basic geological model to help understanding the control of reservoirs.
At present, the main problems of hydrocarbon accumulation focus on two aspects, dynamic mechanisms of hydrocarbon accumulation
and the quantitative models of oil-bearing in traps, which need further systemic research.
Key wordsTarim Basin-carbonate reservoirs-oil and gas exploration of the Ordovician-hydrocarbon accumulation mechanism-hydrocarbon enrichment rule
Located in the middle of the Tarim Basin, Tazhong is a typical area of compound reservoirs rich in oil and gas found in the
Carboniferous, Silurian and Ordovician strata. The proved, probable and possible reserves (3P reserves) in the area amount
to 5×108 tons, so it is of great significance to study the advances and problems in hydrocarbon exploration in the Tazhong area. On
the basis of exploration history and analysis of scientific problems, we outline eight achievements: distribution characteristics
of reservoirs, stages of reservoir formation, different sources of oil and gas and their respective contributions, the effective
regional caprock and reservoir-caprock combinations dominating the vertical distribution of hydrocarbon reservoirs, the control
of the Tazhong Palaeo-uplift on reservoir formation and establishing geologic models, structure balance belts influencing
the reconstruction and residual potential of reservoirs after accumulation, the rules and mechanisms of fractures controlling
oil and gas, and the types of favorable reservoirs and their characteristics of controlling oil and gas distribution. We further
point out the main problems about the oil and gas exploration in the Tazhong area and put forward some relevant proposals.
Key wordsTarim Basin-petroleum geology and exploration-controlling factors of hydrocarbon accumulation-rules of hydrocarbon accumulation
The geological background and rock geochemistry characteristics of carbonate burial dissolution in the Ordovician system of the northwestern Tazhong area were analyzed. The results show that the burial dissolution has an important role in the development of carbonate rock reservoir. The magmation and thermal fluid alternation in the Middle-Late Permian and the multistage tectonic movement are the basic conditions for developing burial dissolution. The hypogenic diagenesis is characterized by the minor volumes of critical fingerprints of pyrite, chlorite, quartz, fluorite, ferrodolomite, solid bitumen and the bright orange-yellow luminescent calcites with rather low contents of iron elements and high richness of manganese contents existing in fracture-filling oscillatory-zoned calcites. The chemical analysis also indicates that the fracture-filling calcites have very high contents of manganese, strontium and magnesium elements. The isotope data show that the mean value of δ18 OPDB is -6.6‰ and δ13CPDB is -2.0‰. In addition, the most striking feature in histograms of micro-thermometric measurements for aqueous inclusion in the Ordovician reservoirs is characterized by homogenization temperature from 115°C to 130°C and from 170°C to 210°C. The calculated salinity of the inclusion ranges from 5% to 7%, from 8% to 9% and from 12% to 13%. It is concluded that the multistage and original hypersaline formation water overlapped by the magmatic-hydrothermal fluid in a large scale of strong epigenic dissolution exists in the northwestern Tazhong area.