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Coke structures present in solid bitumen hosted by Dengying dolomite (most photographs were imaged under oil-immersion cross-polarized incident light; Fig. 4E and I were imaged under plane-polarized incident light; FM = fine-grained mosaic; MM = medium-grained mosaic; CM = coarse-grained mosaic; CFM = coarse flow mosaic; Fi = fibrous; Do = domain; FL = finger-like): (A) Two types of coke structure hosted within SB; MM-CM on the left and FM on the right. Small areas of CFM are found between MM-CM and FM; the boundary of these two generations appears to be discontinuous and irregular (GK1-31); (B) SB displaying granular mosaic structures of various sizes. Highly anisotropic CM
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Reservoir solid bitumen is pervasive throughout the Sinian Dengying Formation dolomites of the Sichuan Basin and neighboring areas, yet its origin remains controversial. Optical and scanning electron microscope (SEM) studies of the Dengying Formation were carried out to elucidate the nature and origin of the solid bitumen. Solid bitumen in dissolut...
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... al., 1993). Five types of previously described coke structures (Grint and Marsh, 1981;White, 1976) are recognized in Dengying reservoir solid bitumen, including fine-grained mosaic, medium-grained mosaic, coarse-grained mosaic, coarse flow mosaic, and domain and fibrous textures ( (i.e., mesophase; White, 1976). The presence of cross-extinction ( Fig. 4C) is reflective of (1) a Taylor-Brooks globe structure, the basic struc- tural mode of the solid bitumen mesophase, and (2) clustered lami- nae composed of polycyclic aromatic compounds ( Brooks and Taylor, 1965). The coarse flow mosaic texture is characterized by >5 μm wide and > 20 μm long anisotropic bitumen units (Fig. 4C, D). ...
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... of cross-extinction ( Fig. 4C) is reflective of (1) a Taylor-Brooks globe structure, the basic struc- tural mode of the solid bitumen mesophase, and (2) clustered lami- nae composed of polycyclic aromatic compounds ( Brooks and Taylor, 1965). The coarse flow mosaic texture is characterized by >5 μm wide and > 20 μm long anisotropic bitumen units (Fig. 4C, D). Domain solid bitumen shows larger anisotropic areas (>20 μm long × >20 μm wide; Fig. 4D, E). Finally, fibrous solid bitumen displays alternating dark and bright bands ( Fig. 4E-I). It is the rule rather than the exception that solid bitumen of the studied samples displays multiple types of coke struc- ...
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... struc- tural mode of the solid bitumen mesophase, and (2) clustered lami- nae composed of polycyclic aromatic compounds ( Brooks and Taylor, 1965). The coarse flow mosaic texture is characterized by >5 μm wide and > 20 μm long anisotropic bitumen units (Fig. 4C, D). Domain solid bitumen shows larger anisotropic areas (>20 μm long × >20 μm wide; Fig. 4D, E). Finally, fibrous solid bitumen displays alternating dark and bright bands ( Fig. 4E-I). It is the rule rather than the exception that solid bitumen of the studied samples displays multiple types of coke struc- ...
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... of polycyclic aromatic compounds ( Brooks and Taylor, 1965). The coarse flow mosaic texture is characterized by >5 μm wide and > 20 μm long anisotropic bitumen units (Fig. 4C, D). Domain solid bitumen shows larger anisotropic areas (>20 μm long × >20 μm wide; Fig. 4D, E). Finally, fibrous solid bitumen displays alternating dark and bright bands ( Fig. 4E-I). It is the rule rather than the exception that solid bitumen of the studied samples displays multiple types of coke struc- ...
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... oil. Gaseous constituents may solid- ify upon cooling to a pitch-like solid substance, perhaps pyrolytic car- bon, during the heating of organic matter that preferentially fills frac- tures and vugs ( Goodarzi et al., 1993;Rimmer et al., 2015;Kwiecińska and Pusz, 2016). Thin-skinned globular solid bitumen displays the mor- phology of a gas bubble (Figs. 4J and 6F, G). Stacked shards of broken bitumen spheres appear quite similar to stacked sheet-like solid bitumen (Fig. 4J). It can be inferred that sheet-like and thin skinned globular solid bitumen may have precipitated from wet gas ...
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... the heating of organic matter that preferentially fills frac- tures and vugs ( Goodarzi et al., 1993;Rimmer et al., 2015;Kwiecińska and Pusz, 2016). Thin-skinned globular solid bitumen displays the mor- phology of a gas bubble (Figs. 4J and 6F, G). Stacked shards of broken bitumen spheres appear quite similar to stacked sheet-like solid bitumen (Fig. 4J). It can be inferred that sheet-like and thin skinned globular solid bitumen may have precipitated from wet gas ...
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... our study, S/C atomic ratios of medium-grained mosaic bitumen are slightly less than those of the matrix (Fig. 9A). It is plausible that the presence of various types of coke structures within a single solid bi- tumen fragment (Fig. 4) may reflect slight differences of chemical com- positions among bitumen precursors. Fine-grained mosaic solid bitumen appears to have been derived from asphaltene-rich heavy oil of high molecular weight, low plasticity, and low mobility, whereas solid bi- tumen displaying the coarse mosaic structure, including flow and do- main, appear ...
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... Some have argued that coke struc- tures like those described in the present study form by physical coa- lescence of mesophase spheres (Marsh and Cornford, 1976;Goodarzi et al., 1993;Stasiuk, 1997). Specifically, the merging of two or more coarse-grained mesophase spheres can yield larger anisotopic units, in- cluding coarse flow mosaics (Fig. 4B, C), a process documented by SEM observations (Fig. 9A, B). The mesophase spheres are composed of lam- inated carbon substances (Fig. 9A, B), most likely polycyclic aromatic compounds ( Brooks and Taylor, 1965;Marsh and Cornford, 1976). The 12 Fig. 7. Semi-quantitative EDS spectra of solid bitumen and associated minerals of the Dengying ...
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... mineral. Strong Ba and S peaks suggest that the mineral is barite (BaSO 4 ) (refer to Fig. 6L). coalescence of mesospheres resulted in the parallel arrangement of lam- inated aromatic sheets at the contacts of mosaic units (Fig. 9A, B). More- over, the consolidation of coarse flow mesophase spheres can further evolve into fibrous mosaics (Fig. 4G, H). The diminished spacing of in- dividual fibers proximal to gas vacuoles (Fig. 4H, I) may be linked to de- velopment of overpressure in association with gas generation (Stasiuk, 1997;Tian et al., 2008). It is worth considering the possibility that the co-existence of different coke structure reflects the coalescence of mesophase spheres ...
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... (refer to Fig. 6L). coalescence of mesospheres resulted in the parallel arrangement of lam- inated aromatic sheets at the contacts of mosaic units (Fig. 9A, B). More- over, the consolidation of coarse flow mesophase spheres can further evolve into fibrous mosaics (Fig. 4G, H). The diminished spacing of in- dividual fibers proximal to gas vacuoles (Fig. 4H, I) may be linked to de- velopment of overpressure in association with gas generation (Stasiuk, 1997;Tian et al., 2008). It is worth considering the possibility that the co-existence of different coke structure reflects the coalescence of mesophase spheres during the aforementioned compositional differenti- ation. In summary, solid ...
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... ation. In summary, solid bitumen displaying the granular mosaic structure likely originated from asphaltene-rich petroleum precursors, whereas domain and fibrous solid bitumen was sourced from aromatic-rich pe- troleum precursors. The very small finger-like solid bitumen (~1 μm) precludes easy microscopic recognition of its coke structure (Fig. 4D). Porous chicken drumstick-shaped solid bitumen comprised of finger-like solid bitumen (Fig. 6D) likely reflects greater plasticity and robust gas generation potential of its precursor, perhaps aromatic-rich liquid pe- troleum, which also has been evidenced by the carbonization of aro- matic compounds (Marsh et al., 1973;Marsh and ...
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... Rogers et al., 1974;Jacob, 1993;Wilson, 2000). The presence of vesicles implies an episode of substantial pressure reduction that re- sulted in degassing of the reservoir ( Rogers et al., 1974;Wilson, 2000). Vesicles produced in association with degassing should not be confused with rare large isolated vesicles within massive solid bitumen masses (Figs. 4I and 6B) that likely formed in association with oil ...
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... lower mobility hydrocarbon is relatively enriched in as- phaltenes, which are readily adsorbed onto pore walls. Compositional differentiation perhaps induced by gravity may occur within the solid phase of bitumen during rapid heating (Fig. 4C). Asphaltene-enriched liquid will be characterized by high molecular weight and a fine-grained mosaic coke structure, whereas the aromatic-enriched phase is defined by relatively low molecular weight and medium-and coarse-grained mosaics. Coarse-flow and domain structures were formed by the coales- cence of granular mosaics of smaller ...
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... and medium-and coarse-grained mosaics. Coarse-flow and domain structures were formed by the coales- cence of granular mosaics of smaller size (Fig. 9B). Overpressurization of the reservoir related to extensive oil cracking to gas may have been responsible for the transformation of coarse-flow and domain solid bitu- men into fibrous solid bitumen (Fig. 4G, H). Indeed, as postulated ear- lier, the spacing of individual fibers may be linked to overpressurization (Stasiuk, 1997; Fig. 4H, I). Thus, massive solid bitumen of the Dengying reservoir was likely derived from the solid phase of hydrocarbon (Fig. 12C2 C4). Exsolution of gas dissolved in asphaltene-enriched heavy oil induced by Early ...
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... size (Fig. 9B). Overpressurization of the reservoir related to extensive oil cracking to gas may have been responsible for the transformation of coarse-flow and domain solid bitu- men into fibrous solid bitumen (Fig. 4G, H). Indeed, as postulated ear- lier, the spacing of individual fibers may be linked to overpressurization (Stasiuk, 1997; Fig. 4H, I). Thus, massive solid bitumen of the Dengying reservoir was likely derived from the solid phase of hydrocarbon (Fig. 12C2 C4). Exsolution of gas dissolved in asphaltene-enriched heavy oil induced by Early Triassic uplift and related reservoir paleo-pressure reduction ( Liu et al., 2014;Figs. 10 and 12C2, C3) may have been re- sponsible ...
Citations
... The Deng-4 Member reservoir of the Central Sichuan Basin has experienced a very high thermal evolution process, with a burial depth of more than 6000 m in geological history, some of which could exceed 8000 m, and the EqRo is approximately 3.0%-4.0% (Gao et al., 2018). ...
... Member is mainly composed of argillaceous dolomite and thin mudstone at the bottom, while the other three members of the group are dolomite deposits (Gao et al., 2018(Gao et al., , 2021. Owing to the dissolution pores produced by the Tongwan Movement, high-quality reservoirs have developed in the Deng-4 Member. ...
Ultra‐deep Sinian carbonate rocks are rich in natural gas, and the Deng‐4 Member of the Central Sichuan Basin has been shown to have an extremely high degree of thermal evolution. They are characterized by dry gas produced due to the thermal cracking of palaeo‐oil reservoirs (PORs). The distribution of PORs as materials for the formation of cracked gas reservoirs remains unclear. The complex multistage evolution process and limited drilling data make it challenging to precisely and systematically recover a range of PORs. Based on basin and petroleum system modelling (BPSM), the bottom‐up migration and accumulation method was used to investigate the distribution of PORs. Research shows that the PORs in the Sinian Deng‐4 Member were mainly formed in the Early–Middle Triassic under the mixed contribution of Cambrian and Sinian source rocks, and the coupling of palaeostructural and trap settings controlled its accumulation. Generally, the distribution of PORs can be divided into two categories: one is located at the core of the uplift zone and is controlled by structural‐stratigraphic composite traps, whereas the other is controlled by a series of lithologic traps formed in the slope area. Sensitivity analysis showed that the distribution patterns of the PORs were mostly related to the facies conditions (physical characteristics) of the reservoirs. The widely distributed PORs provide necessary and sufficient material conditions for the formation of cracked Sinian gas reservoirs in the Central Sichuan Basin, and the North Slope area has great potential for natural gas exploration in the future.
... Generally, the source-rock pyrobitumen has relatively weak optical anisotropy, while the reservoir pyrobitumen has obviously strong optical anisotropy, especially at the overmature stage (Yang et al., 2018;Luo et al., 2018Luo et al., , 2021Wang et al., 2019Wang et al., , 2021Yao et al., 2021;Zuo et al., 2022). Previous studies found that reservoir pyrobitumen with sphere, flow and fibrous textures had strong optical anisotropy, and the random optical reflectance frequency distribution of it is bimodal and improperly tends to lower the pyrobitumen reflectance (PBR o ) values (Mastalerz et al., 2018;Gao et al., 2018;Yang et al., 2018;Zuo et al., 2022). However, source-rock pyrobitumen and reservoir pyrobitumen with slight, fine and irregular mosaic textures are relatively weak and their random optical reflectance frequency distribution is unimodal Sanei, 2020;Zuo et al., 2022). ...
... The dating results revealed that hydrothermal activity occurred at 259.4 Ma, highly consistent with the occurring time of the Emeishan mantle plume (259.1 Ma) (Su et al., 2020). The burial history of strata showed that the DY Formation temperature at 259.4 Ma was only about 120-145 • C (Gao et al., 2018;Yang et al., 2018), significantly lower than the temperature limit of crude oil cracking (>160 • C). However, the hydrothermal temperature in the DY Formation was higher than 200 • C (Fig. 5) and even reached 300 • C in certain areas (Zhang, 2019), providing a thermal condition for crude oil cracking. ...
... Gas deasphalting occurs as a result of gas injection, forming solid bitumen while changing the composition of crude oils. 73 The existence of solid bitumen and single-phase gas inclusion 74 also supports the explanation of the genesis of type A HFP crude oils in the slope zone. In addition to the formation of HFP crude oils with high wax contents in the slope zone that far away from the source kitchen in the depression zone, the continuous expulsion of light components also leads to the enrichment of relatively long chain waxes, resins, and asphaltenes in the depression zone. ...
The freezing point of crude oils is an important factor affecting the process of exploration, development, and transportation of crude oils. In the Dongying Sag, the freezing point of crude oils varies greatly in space, and high-freezing-point crude oils have been found in many structural belts. The freezing point of crude oils in the Dongying Sag is generally high, ranging from −40 to 80 °C. The distribution of high-freezing-point crude oils in the Dongying Sag is scattered, and they have been found at different depths, layers, and structural belts, with relatively more occurrences in specific areas. Based on the relationship between the wax content and the resin + asphaltene content, the high-freezing-point crude oils in the Dongying Sag can be classified into three types (type A, type B, and type C). Type A, type B, and type C high-freezing-point crude oils are mainly found in the gentle slope zone, depression zone, and uplift zone, respectively. The contents of wax and resins + asphaltenes in crude oils are the key factors controlling the freezing point of crude oils in the Dongying Sag. Oil source correlation using biomarkers and n-alkane carbon isotopes demonstrates that different types of high-freezing-point crude oils are generated from different source rocks, indicating that the source of crude oils is not the only factor controlling the freezing point of crude oils. The hydrocarbon-generating parent material of hydrocarbon source rocks developed in different depositional environments of the Dongying Sag is generally characterized by the dominance of bacteria and algae and is supplemented by terrestrial higher plants. This lays the material foundation for the formation of waxes during the maturation of hydrocarbon source rocks. The change of temperature and pressure conditions during the process of oil expulsion from source rocks and migration in the carrier bed will lead to phase fractionation and composition changes of oils, thus forming type A freezing point crude oils in the slope zone and type B freezing point oils in the depression zone. The increase in resin and asphaltene content caused by biodegradation and water washing in shallower buried oil reservoirs is the reason for the formation of type C high-freezing-point crude oils. The research results indicate that the organic matter composition and hydrocarbon alteration during migration and accumulation jointly control the formation of high-freezing-point crude oils in lacustrine basins. The extensive input of terrestrial organic matter into the source rocks of the lacustrine basin laid the foundation for the formation of high-freezing-point crude oils, and the migration fractionation and biodegradation of the crude oils further promoted the formation of high-freezing-point crude oils.
... Considering that the samples were cleared by deionized water and the barium-enriched minerals are found filling the pyrobitumen pores without other symbiotic clastic materials contained in the mud, these barium-enriched minerals are suggested to be authigenic in the pores. Gao et al. [11] found some barium-enriched minerals in the DY pyrobitumen pores and considered them authigenic barite related to hydrothermal activity. The apparent decrease in Ba concentrations after the treatment indicates that the Ba enrichment in pyrobitumen is in barite and other barium-enriched minerals. ...
... Combining the previous study results and the precipitation environment of illite and salt, these minerals that filled the pyrobitumen pores are of hydrothermal origin. The related hydrothermal fluids have high salinity (>18 wt% NaCl) and could pass through deep clastic strata during their upward migration [11,61] (Fig. 1b and c). Therefore, the hydrothermal fluids with high salinity and rich terrigenous materials provide precipitation conditions for salt and illite. ...
... Identifying hydrothermally altered pyrobitumen has not attracted much attention, and most scholars only refer to the reflectance and optical texture of pyrobitumen for preliminary judgment [11]: [50]. Goodarzi (1993) first proposed this method when studying the pyrobitumen in the Baffin Island Nanisivik mine, Canada [63]. ...
The hydrothermal activity in the Ediacaran Dengying (DY) Formation in central Sichuan has modified the reservoirs and paleo-oil reservoirs; however, these modifications are not specific due to the challenges in recovering the hydrothermal scale. This paper classified pyrobitumen in the DY Formation into five categories by optical textures and analyzed the physical structure and trace element composition of the different pyrobitumen using scanning electron microscope, reflectance, and trace elements. Finally, the responses of the trace element concentrations and optical textures of the pyrobitumen to hydrothermal alteration were discussed and used to restore the hydrothermal scale in the central Sichuan Basin. The results show that isotropic, fine-grained, medium -grained, coarse-grained mosaic, and fibrous pyrobitumen develop in the DY Formation. As the optical texture of the pyrobitumen changes from isotropic to mosaic to fibrous, the order degree of aromatic lamella in pyrobitumen gradually increases, the maturity, the maximum reflectance, and the bireflectance of pyrobitumen gradually increase. Except for isotropic pyrobitumen, anisotropic pyrobitumen is subject to varying degrees of hydrothermal alteration. The hydrothermal alteration on pyrobitumen is primarily in the form of material exchange , precipitating illite, salt, and barium-enriched minerals in pyrobitumen pores and enriching some elements (Ba, Cr, Nb, and Sr) in the pyrobitumen. As the optical texture changes from isotropic to mosaic to fibrous, the element enrichment degree gradually increases. Medium-grained mosaic, coarse-grained mosaic, and fibrous pyrobitumen are widely observed in central Sichuan. Therefore, hydrothermal modification on the DY reservoirs and paleo-oil reservoirs is not limited to the deep faults but widely develop in central Sichuan, and should be an essential part of the further study of hydrocarbon accumulation.
... Bitumen is a common byproduct within matured or over-matured oil/gas reservoirs. The bitumen in the Dengying Formation is pyrobitumen [41,[44][45][46][47], and it formed during in situ transformation from paleo-oil reservoirs to paleo-gas reservoirs; therefore, the location of the bitumen retained today is equivalent to the location of the paleo-oil/gas reservoirs. According to the abundance and distribution of bitumen in dissolved pores and karst cavities, two relict paleo-oil/gas reservoirs are preserved along the paleokarst interface ( Figure 2). ...
... Bitumen is a common byproduct within matured or over-matured oil/gas reservoirs. The bitumen in the Dengying Formation is pyrobitumen [41,[44][45][46][47], and it formed during in situ transformation from paleo-oil reservoirs to paleo-gas reservoirs; therefore, the loca- ...
... The Ro value of the reservoir bitumen in Dengying Formation is 2.62%-4.05% (average 3.47%) [52], which belongs to over-mature pyrobitumen [43,46,47]. ...
The coexistence of numerous Mississippi Valley-type (MVT) Pb–Zn deposits and oil/gas reservoirs in the world suggests a close genetic relationship between mineralization and hydrocarbon accumulation. The Wusihe MVT Pb–Zn deposits are located along the southwestern margin of the Sichuan Basin. Based on the spatiotemporal relation between Pb–Zn deposits and paleo-oil/gas reservoirs, ore material sources, and processes of mineralization and hydrocarbon accumulation, a new genetic relationship between mineralization and hydrocarbon accumulation is suggested for these deposits. The Wusihe Pb–Zn deposits are hosted in the Ediacaran Dengying Formation dolostone, accompanied by a large amount of thermally cracked bitumen in the ore bodies. The Pb–Zn deposits and paleo-oil/gas reservoirs are distributed along the paleokarst interface; they overlap spatially, and the ore body occupies the upper part of the paleo-oil/gas reservoirs. Both the Pb–Zn ore and sphalerite are rich in thermally cracked bitumen, in which µm sized galena and sphalerite may be observed, and the contents of lead and zinc in the bitumen are higher than those required for Pb–Zn mineralization. The paleo-oil/gas reservoirs experienced paleo-oil reservoir formation, paleo-gas reservoir generation, and paleo-gas reservoir destruction. The generation time of the paleo-gas reservoirs is similar to the metallogenic time. The source rocks from the Cambrian Qiongzhusi Formation not only provided oil sources for paleo-oil reservoirs but also provided ore-forming metal elements for mineralization. Liquid oil with abundant ore-forming metals accumulated to form paleo-oil reservoirs with mature organic matter in source rocks. As paleo-oil reservoirs were buried, the oil underwent in situ thermal cracking to form overpressure paleo-gas reservoirs and a large amount of bitumen. Along with the thermal cracking of the oil, the metal elements decoupled from organic matter and H2S formed by thermochemical sulfate reduction (TSR) and minor decomposition of the organic matter dissolved in oilfield brine to form the ore fluid. The large-scale Pb–Zn mineralization is mainly related to the destruction of the overpressured paleo-gas reservoir; the sudden pressure relief caused the ore fluid around the gas–water interface to migrate upward into the paleo-gas reservoirs and induced extensive metal sulfide precipitation in the ore fluid, resulting in special spatiotemporal associated or paragenetic relations of galena, sphalerite, and bitumen.
... Therefore, the anisotropy increase and the frequency distribution of reflectance become bimodal (Patrick et al., 1989;Schoenherr et al., 2007;Yang et al., 2018). Other studies have attempted to use the maximum and minimum reflectance for maturity characterization, but they found that the maximum and minimum reflectance differ by an order of magnitude and vary with the polarization angle, which makes it difficult to characterize the maturity Stasiuk, 1991, 1992;Stasiuk, 1997;Gao et al., 2018;Zhang et al., 2019). Raman spectral parameters have achieved promising results in this field of research, and pyrobitumens with different optical textures have the same Raman spectral parameters, which can then be used to characterize the maturity of strongly anisotropic pyrobitumen (Liu et al., 2012). ...
... Previous studies have used the random and maximum-minimum reflectance of pyrobitumen under nonpolarized and plane-polarized light, respectively, to characterize the maturity. However, even in the same sample, the range of reflectance is large, reflecting the strong anisotropy of pyrobitumen and/or different thermal evolution degrees (Gao et al., 2018;Yang et al., 2018;Zhang et al., 2019). As such, these rocks are ideal for investigating the reflectance and maturity characterization using strongly anisotropic and highly mature pyrobitumen. ...
... In particular, our method has advantages in characterizing the maturity of highly anisotropic pyrobitumen, with reflectance in the range of 2%-10%, and it is also suitable for isotropic to slightly anisotropic pyrobitumen, with reflectance in the range of 0.7%-2% (Jacob, 1989). Representative examples include the overmature, exhumed conventional oil reservoirs of sandstone in the Lower Pennsylvanian Jackfork Formation in Oklahoma; carbonate reservoirs of the Upper Devonian Leduc Formation in the Western Canada Basin; the lower Cambrian Ara Formation in the South Oman Salt Basin; and the Lower Triassic Feixianguan Formation, the lower Permian Qixia Formation, and the Sinian Dengying Formation in the Sichuan Basin, China (Cardott, 1991;Stasiuk, 1997;Schoenherr et al., 2007;Liu et al., 2008;Gao et al., 2018). In addition, our method can also apply to some small-scale pyrobitumen in the unconventional reservoirs or source rocks, such as the lower Cambrian Qiongzhusi Formation in the Sichuan Basin. ...
Maturity evaluation of highly evolved organic matter is an important problem in oil and gas geochemistry, because conventional organic geochemical proxies are not applicable due to hydrocarbon generation and expulsion. In this study, we investigated the pyrobitumen in the reservoirs of the Sinian Dengying Formation in the Sichuan Basin, China. We focused on aromatic hydrocarbon biomarkers, and the calibration and application of aromatic maturity proxies using pyrobitumen reflectance and Raman spectral parameters. Our results show that only the thiophene compounds are effective maturity indicators, such as MDR (4-/1-MDBT), MDR′ (4-MDBT/(1-MDBT+4-MDBT)), MDR1 (1-MDBT/DBT), 4,6-/(1,4+1,6)-DMDBT, (2,6 +3,6)-/(1,4+1,6)-DMDBT, and [2,1]BNT/([2,1]BNT+[1,2]BNT), as these are largely independent of the optical anisotropy of pyrobitumen The condensation and incorporation of thiophene compounds can lead to an increase in the reflectance of pyrobitumen, which means the thiophene compound parameters are positively or negatively correlated with pyrobitumen reflectance New potential maturity parameters were proposed, including DMDBTs/MDBTs, TeMDBTs/(DBT+MDBTs +DMDBTs+TMDBTs+TeMDBTs), 1-MDBT/MDBTs, and [2,1]BNT/BNTs. This method and these parameters are universally applicable to maturity characterization, especially in rocks that lack vitrinite, contain strongly anisotropic organic matter, and have high contents of thiophene compounds.
... Previous studies have described mesophase in reservoirs in detail, with the microtextures appearing to differ from those in mesophase produced under experimental conditions (Yang et al., 2018a, Gao et al., 2018. Generation of mesophase in the reservoir of the Anyue gas field occurred under formation pressure (Yang et al., 2018a, Gao et al., 2018, with the cracking of the oil leading to a momentary high pressure surge in the pore system (Zou et al., 2014;Zhu et al., 2015a;Yang et al., 2018b). ...
... Previous studies have described mesophase in reservoirs in detail, with the microtextures appearing to differ from those in mesophase produced under experimental conditions (Yang et al., 2018a, Gao et al., 2018. Generation of mesophase in the reservoir of the Anyue gas field occurred under formation pressure (Yang et al., 2018a, Gao et al., 2018, with the cracking of the oil leading to a momentary high pressure surge in the pore system (Zou et al., 2014;Zhu et al., 2015a;Yang et al., 2018b). ...
... However, faults connect the reservoirs, serving as pathways for oil migration and creating a unified petroleum system (Zou et al., 2014;Liu et al., 2018;Yang et al., 2018aYang et al., , 2018b. Mesophase has been found in the Z2dn2, Z2dn4, and Є1l formations throughout the Chuanzhong Uplift (Yang et al., 2018a, Gao et al., 2018. ...
The Neoproterozoic-Lower Paleozoic dolostone gas reservoirs in the Chuanzhong Uplift in Southwestern China contain the mesophase pyrobitumen (pyrobitumen with mesophase). The mesophase in the pyrobitumen is mostly composed of a mixture of condensed macromolecule polycyclic aromatic hydrocarbons (PAH), and has been mostly converted to various grain, flow, and domain textures. The volcanic activity nearby the Chuanzhong Uplift may have generated very hot (over 300 °C) hydrothermal fluid, with migration of the fluid into the dolostone reservoir transforming the hydrocarbons in the reservoir into an anisotropic carbon by-product, which formed under high temperature and pressure conditions; over 300 °C and 200 MPa according to fluid inclusion analysis. The high temperature-pressure in reservoir was caused by sudden devolatilization of the hydrocarbons due to hydrothermal heating, which formed the unusual texture of the mesophase. The elliptical mesophase grains (EG), the honeycomb structure of pyrobitumen, and the occurrence of polarized classes of mesophase in single pyrobitumen deposits are all unusual textures. This study investigates the texture development of this pressure-affected pyrobitumen. Observation and study of the mesophase suggest that the class of mesophase in pyrobitumen is determined by temperature, while pressure (assessed from associated fluid inclusions) significantly affected the texture. Analysis of the texture of the pyrobitumen in conjunction with previous thermometry results from methane inclusions suggests that the high class mesophase of the pyrobitumen could be applied as a temperature indicator for geological conditions. Furthermore, the unusual textures of the pyrobitumen including elliptical mesophase grains (EG), the honeycomb structure of pyrobitumen, and the occurrence of polarized classes of mesophase in single pyrobitumen could reflect the abnormal high formation pressure.
... The central and eastern Sichuan Basin are located on the different structural units of the Sichuan Basin. Solid bitumen is pervasive in the Sinian carbonate reservoirs of the Sichuan Basin (Gao et al., 2018). ...
... Optical characteristics, thermal maturity levels, and geochemical compositions of solid bitumen in the central Sichuan Basin have been extensively studied, yet its thermal maturity remains controversial (Chen et al., 2017;Gao et al., 2018;Liu et al., 2018;Shi et al., 2018;Zhang et al., 2019). The application of solid bitumen reflectance plays an important role in assessing thermal maturity due to the intensified exploitation of deep carbonate hydrocarbon systems in the central Sichuan Basin (Gao et al., 2018;Liu et al., 2018;Shi et al., 2018;Zhang et al., 2019). ...
... Optical characteristics, thermal maturity levels, and geochemical compositions of solid bitumen in the central Sichuan Basin have been extensively studied, yet its thermal maturity remains controversial (Chen et al., 2017;Gao et al., 2018;Liu et al., 2018;Shi et al., 2018;Zhang et al., 2019). The application of solid bitumen reflectance plays an important role in assessing thermal maturity due to the intensified exploitation of deep carbonate hydrocarbon systems in the central Sichuan Basin (Gao et al., 2018;Liu et al., 2018;Shi et al., 2018;Zhang et al., 2019). However, it is difficult to interpret the reflectance values of solid bitumen debris in the Sinian carbonate reservoirs according to the established empirical thresholds because of their extensive values (0.12%-9.35%) which are not common in ordinary reservoirs (Gao et al., 2018;Zhang et al., 2019). ...
Maturity assessment of solid bitumen is significant for the reconstruction of the thermal evolution history and obtaining the key periods of hydrocarbon generation for the Qiongzhusi source rocks in the eastern and central Sichuan Basin because of the absence of vitrinite in the marine mudstones. Based on the Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy analyses of solid bitumen, the thermal maturity of solid bitumen in the Sinian reservoir was evaluated, which was used to calibrate the thermal maturation history modelling results of the Qiongzhusi source rocks. The FTIR spectrums of solid bitumen are characterized by depleted aliphatic carbon and CO group, and enrichment of aromatic carbon, which indicates that the vitrinite reflectance equivalent (VReq) values of solid bitumen in the Sichuan Basin exceed at least 1.3%. The calculated VReq values of solid bitumen inferred by Raman spectroscopy analysis are 3.8%–4.09% and 3.51%–3.77% in the eastern and central Sichuan Basin, respectively. The calculated VReq values of bitumen in fluorite and calcite inclusions of the central Sichuan Basin vary from 3.54% to 3.64%. The thermal evolution history of the Qiongzhusi source rocks can be divided into two heating–cooling stages. At the end of the second heating stage, which is the Late Cretaceous, the Qiongzhusi source rocks reached the highest temperature of 250 and 225°C in the eastern and central Sichuan Basin, respectively. The different burial and thermal history of the Qiongzhusi source rocks controls their key periods of hydrocarbon generation in the eastern and central Sichuan Basin. Two episodes of intense oil generation of the Qiongzhusi source rocks were determined at 440–380 Ma and 290–260 Ma, and one phase of gas generation from oil cracking was from 260 to 85 Ma in the eastern Sichuan Basin. The intense oil generation of the Qiongzhusi source rocks occurred at 240–185 Ma and the oil cracking phase was from 185 to 90 Ma in the central Sichuan Basin. We obtained the key periods of hydrocarbon generation for the Qiongzhusi source rocks in the eastern and central Sichuan Basin.
... Dengying Formation developed a large suit of dolomite sediments, which can be subdivided into four members from bottom to top based on the enrichment degree of algae and structural characteristics: Z 2 dn 1 , Z 2 dn 2 , Z 2 dn 3 , and Z 2 dn 4 . Reservoir solid bitumens are widely developed in the Dengying dolomite in Sichuan Basin and its peripheral areas [39][40][41]. Reservoir solid bitumens in this study were collected from Z 2 dn 4 dolomites. The black mudstones and shales of the Lower Cambrian Qiongzhusi formation (Є 1 q) were widely developed in Sichuan Basin and its peripheral areas. ...
... It had large thickness (100-400 m) [42] and was enriched in organic matter, which was considered major source rocks in Sichuan Basin [38,39,[42][43][44]. Previous studies indicated that the Upper Ediacaran natural gas and reservoir solid bitumens chiefly originated from the Lower Cambrian mudstones and shales [31,37,40,44]. ...
In order to investigate trace and rare earth element partitioning in organic fractions of mudstones, this study isolated organic fractions of mudstones, including insoluble organic fraction (kerogen), soluble organic fraction (extract), or expelled hydrocarbon (reservoir solid bitumen), and the isolated organic fractions and their corresponding whole rocks for trace and rare earth element compositions were measured. Analysis of trace and rare earth element compositions in organic fractions of lacustrine and marine mudstones revealed that mudstone kerogens were more enriched in rare earth elements (REE) and redox-sensitive trace elements (e.g., U, Mo, and Ni) relative to corresponding whole rocks. During the oil generation, middle rare earth elements (Sm-Ho), especially Eu, migrated from kerogen to extract more easily than the rest REE. The Eu was easily transferred to soluble hydrocarbon under the acidic and reducing environments formed by oil generation, resulting in the higher concentrations of Eu relative to its neighboring REE (Sm and Gd) and the pronounced positive Eu anomalies. Transition metal elements (e.g., Mo, V, Cr, Co, Ni, Cu, and Zn) also more easily released from kerogen than the rest elements, especially V and Ni. The enrichment and mobilization of trace elements in organic fractions of mudstones, such as Mo, U, V, Ni, and Ba, are closely related with their geochemical behaviors during the depositional and early digenetic processes, providing the potential information for predicting the distribution characteristics of trace elements in the expelled hydrocarbons of mudstones (e.g., crude oil and solid bitumen) and fingerprinting of oil to source.
