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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. [ABSTRACT FROM AUTHOR]

The Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin is regarded as an important reservoir with great potential for hydrocarbon exploration. It is previously indicated that the Xixiangchi carbonates have experienced extensive dolomitization, however, the origin of dolomitizing fluids and the dolomitization mechanism still remain uncertain. In this study, a set of petrographic and geochemical examinations, including rare earth elements (REE) and isotopic (C, O, and Sr) compositions were used to trace the origins of dolomitizing fluids and associated diagenetic processes. The petrographic examination revealed three types of matrix dolomites (D1, D2, D3) and one cement saddle dolomite (SD). These phases have crystal size ranges of less than 30 μm (very fine to fine crystals, D1), 30–100 μm (fine to medium crystals, D2), 100–300 μm (medium to coarsely crystalline dolomite, D3), and 0.3–4 mm (fracture filling cements, SD), respectively. D1 is characterized by non to very weak luminescence, weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), and high 87Sr/86Sr ratios (0.71062 ± 0.00122). In combination with δ13C (−1.5‰ ± 0.2‰) and δ18O (−9.7‰ ± 0.5‰) compositions, D1 is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. In contrast, D2 is characterized by intercrystalline pores, dirty crystal surfaces, similar δ13C (−1.4‰ ± 0.4‰) compositions but higher δ18O (−8.9‰ ± 0.7‰) compositions, and lower 87Sr/86Sr ratios (0.70992 ± 0.00035), similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04). The coarser D2 is regarded to be formed by the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment. D3 is distinguished by a cloudy core with clear rims, showing slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02) and similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) than those of D1 and D2. Combined with the δ18O compositions (−10.4‰ ± 0.4‰) and 87Sr/86Sr ratios (0.70989 ± 0.00048), D3 is thought to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment. The fractures filling SD dolomite consists of nonplanar and much coarser crystals with undulatory extinctions and brighter red luminescence. The lower δ18O (−11.1‰ ± 0.3‰) compositions, lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01) of SD than the matrix dolomites, and similar Ce anomalies (Ce/Ce* = 0.83 ± 0.01) are indicative of hydrothermal dolomitization, with possible fluids associated with the magma during the period of Emei taphrogenic movement. In addition, the 87Sr/86Sr ratios (0.70941 ± 0.00003) of SD suggest probable origin from the coeval seawater partially. Therefore, SD dolomite is interpreted to be formed by hydrothermal dolomitization with mixed dolomitizing fluid of seawater and hydrothermal fluids. In summary, all the matrix dolomites have almost the same ΣREE concentrations and exhibit similar near-flat REE partition patterns with weak LREE enrichments, weakly negative Ce anomalies, and negative Eu anomalies. Such characteristics of REE compositions are indicative of similar evolved dolomitizing fluid, such as seawater or seawater- derived fluids. By contrast, SD dolomites have a different REE partition pattern with left-leaning characteristics, LREE depletions, and negative Eu anomalies, thus suggesting a different dolomitizing fluid source from the matrix dolomites. In addition, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir, indicating that the dolomitizing history of carbonate has a strong influence on the quality of potential dolomite reservoirs. [ABSTRACT FROM AUTHOR]

The origin of Precambrian dolomites has attracted research interest as a result of their association with petroleum resources. The long diagenetic history of these dolomites and multiple periods of hydrothermal activity mean that dating and identifying different periods of dolomite formation is challenging. This study focuses on dolomites in the Precambrian Dengying Formation of the Sichuan Basin, central China, and presents new petrographic, in situ laser ablation inductively coupled plasma mass spectrometry U–Pb and trace element, and Sr–C–O isotopic data to characterize three types of dolomite and constrain their age and origin. In situ U–Pb dating of type I dolomite yielded a lower intercept age of 637 ± 35 Ma. The rare earth element (REE) patterns, 87Sr/86Sr ratios (0.70827–0.70903), and δ13C VPDB (0.8‰–1.0‰) and δ18O VPDB (−1.5‰ to −1.0‰) values of type I dolomite indicate their derivation from seawater or a precursory source with seawater characteristics. Type II dolomite yielded an age of 636 ± 15 Ma and has lower ΣREE and slightly higher δ13C VPDB (1.2‰–1.5‰) and δ18O VPDB (−0.9‰ to −0.3‰) values than type I, as well as similar 87Sr/86Sr ratios and parallel REE patterns that are indicative of formation by reflux seepage dolomitization. The type III dolomite is characterized by large crystals, unique REE patterns, high 87Sr/86Sr values (0.70942–0.70975), and negative δ18O VPDB values (−6.4‰ to −3.1‰) that indicate a hydrothermal origin. The relationships between hydrothermal dolomite and bitumen indicate that the paleo-oil charging within the Dengying Formation reservoir occurred after the hydrothermal activity at 516 ± 49 Ma. The early hydrothermal activity provided space for the subsequent fluid movement, and oil and gas charging. Our integrated approach, incorporating petrographic, geochemical, isotopic, and geochronological data, constrains the ages of different dolomite phases and an oil-charging event within the Dengying Formation reservoir and provides new insights into the evolution of petroleum systems in general. • We identified three types of dolomites of Dengying Formation matching with the age of diagenesis and fluid activity. • The LA–ICP–MS in situ U–Pb dating of Dengying Formation dolomite yields a lower intercept age of 637 ± 35 Ma. • The dating results suggest an extensional event existed at 516 ± 49 Ma, and accounted for the deposition of hydrocarbon source rocks. • This study outlines potential approaches for furthering our understanding of dolomites and ancient petroleum systems. [ABSTRACT FROM AUTHOR]

The Sinian Dengying Formation is a key target for deep oil and gas exploration in the Sichuan Basin, China. Natural gas was mainly distributed in the Ziyang–Weiyuan area (ZWA) and the Gaoshiti–Moxi area (GMA). At present, the genetic types and sources of natural gas are still unclear and controversial. The chemical composition, stable carbon isotopes, and biomarkers were used to analyze geochemical characteristics, genetic types, and gas–source relations. The results showed that the composition and content of hydrocarbon gas in the ZWA and GMA were similar. Natural gas was a typical dry gas with an average dry coefficient (C 1 /C 1 +) of 0.9991, which reflected higher methane (CH 4) content and lower heavy hydrocarbon gas (C 2 +) content. For nonhydrocarbon gas, CO 2 (0.01%–26.34%), N 2 (0.03%–24.95%), H 2 S (0.10%–3.19%), and He (0.01%–0.37%) were mainly distributed. Compared with the GMA, the ZWA was characterized by higher N 2 content and lower CO 2 content. Although the carbon isotopes had not been reversed, the values of δ13C 2 –δ13C 1 were different between the ZWA and GMA, with the GMA having the higher value. The assemblage relation between the natural gas composition and carbon isotopes indicated that the natural gas type was dominated by oil-cracking gas. Mud-shale in the high maturity stage of thermal evolution greatly influenced on the content of N 2 and He. H 2 S mainly belonged to thermochemical sulfate reduction (TSR), and most of CO 2 was formed by deep carbonate metamorphism and TSR. The gas–source relation showed that natural gas in the ZWA was derived from the Cambrian Qiongzhusi Formation and that the gas in the GMA mainly originated from the Sinian Dengying Formation and the Cambrian Qiongzhusi Formation. The established hydrocarbon generation model indicated that the Sinian Dengying Formation is potentially gas-rich. • The Sinian Dengying Formation natural gas was a typical dry gas, dominated by oil-cracking gas. • Source rocks of the Qiongzhusi Formation and Dengying Formation were the mainly gas source for gas reservoirs. • The resource potential was qualitatively evaluated by the established hydrocarbon generation model. [ABSTRACT FROM AUTHOR]

The Neoproterozoic provides an important record of glacial–postglacial paleoenvironments, organic carbon cycling, and hydrocarbon accumulation. Here, to better understand the environmental, sedimentary, and petroleum geological potential of the Neoproterozoic, we summarize and review research on paleoenvironment and hydrocarbon prospectivity during the era for regions worldwide and also present a case study of the Marinoan postglacial Ediacaran Doushantuo Formation in the Sichuan Basin, upper Yangtze region, China. Sediments that were deposited during the Neoproterozoic postglaciations generally have hydrocarbon-generating potential. In some places, the sediments have reached stages of burial and thermal evolution appropriate for hydrocarbon generation, with some sites having been exploited for commercial production. In the Sichuan Basin, the distribution and thickness of Doushantuo Formation sediments were controlled by glacially eroded topography and/or extension-related subsidence. Abundant organic matter was produced through paleo-productivity (Ba/Al and Cu/Al proxies can reach up to 181.08 and 8.94, respectively) and anoxia (U-EF and Mo-EF proxies can reach up to 5.45 and 14.00, respectively) governed by marine transgression. In South China, many depocenters of the Doushantuo Formation sediments have a strip-shaped geometry in the plane and are expected to be prospective for oil and gas. The reported findings can be used as a reference for further studies of glacial and postglacial deposits with organic matter accumulation of the Neoproterozoic and other periods. [ABSTRACT FROM AUTHOR]

The paleo-environment and organic matter accumulation after the Marinoan "Snowball Earth" event is of great significance to understand the Earth system and petroleum geology during this critical period. However, the present studies in South China are not well constrained due to the lack of continuous sedimentary records. Here, we present an integrated petrological and geochemical study of a unique, continuous, and complete outcrop in the northeastern Sichuan Basin, SW China. Results show that the postglacial Doushantuo shales in the study area can be divided into three intervals (from bottom to top: Unit1–Unit2–Unit3) according to geochemistry evolutionary trend. Multiple redox proxies (V EF , U EF , Mo EF , C org /P, Th/U, and U Mo covariation pattern) indicate the bottom redox condition changed from oxic to suboxic, and back to weakly oxic stratigraphically. Ba bio and Ba/Al paleoproductivity proxies suggest that the primary productivity was middle–high and it was slightly higher in Unit 2. The CIA corr values of Doushantuo shales range from 72.2 to 86.8 and gradually increase from Unit 1 to Unit 3, indicating relatively intense weathering conditions and global warm and humid paleoclimatic conditions. The Doushantuo shales in our studied outcrop is approximately 100 m in thickness. They are at over mature stage with average random reflectance of 1.86–2.14%, which makes the recovered original organic matter abundance up to ~2.0 wt%. The kerogen of the shales is type II. The accumulation of organic matter was mainly controlled by the preservation environments, with the primary productivity level as a secondary control. These data were further compared with other outcrops reported in previous studies for a comprehensive understanding. The variation of organic carbon contents in the Doushantuo shales hold similar features across the entire South China, with the middle part producing the highest-quality source rocks with conventional and unconventional hydrocarbon geological significance. This suggests that changes in sea level and paleo-weathering after the Marinoan glaciation controlled the enrichment of organic carbon in shales. The enrichment is a large-scale geological event across basins rather than a local one. This might be general and referenced for the postglacial and/or interglacial shales. [ABSTRACT FROM AUTHOR]

• Major elements trace hydrocarbon sources in over-mature petroleum systems. • The Sinian Dengying Formation gas systems in the Sichuan Basin were investigated. • Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P have variable occurrence states. • Mn/Fe ratio and ternary 10Mn–Fe–P diagram are used to trace hydrocarbon sources. • Lower Cambrian shales are the main sources for natural gases. Identifying hydrocarbon sources in over-mature petroleum systems is important and challenging. Here we present a pilot study using inorganic geochemistry to trace hydrocarbon sources in the giant natural gas accumulations within the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin, SW China. We investigated the major elements (Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) as tracers of hydrocarbon sources for the first time. Our results show that the over-mature solid bitumen in the reservoirs contains parts per million amounts of major elements in addition to carbon (i.e., organic matter). Na and K in the bitumen occur mainly in the V–Ni porphyrin form. Al, Fe, and Mn are present in the Fe porphyrin form. The behavior of P is relatively complex, and it may reside mainly in a non-porphyrin organic form. The other elements (Ti, Mg, and Ca) may be present as emulsified "invisible" nano-minerals. The Mn/Fe ratio and ternary 10Mn–Fe–P diagram can be used for hydrocarbon–source rock correlations. Based on the proxies, it is suggested that the lower Cambrian Qiongzhusi Formation was the main source of hydrocarbons in the Dengying Formation. Our data suggest that inorganic geochemistry is an effective method for the study of over-mature hydrocarbons and complements traditional organic geochemical approaches. [ABSTRACT FROM AUTHOR]

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the Ba N –Ni N –Cu N ternary diagram, V N –Ni N –Mo N ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE) N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the V N –Ni N –Mo N ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the V N –Ni N –Mo N ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions. [ABSTRACT FROM AUTHOR]

The Upper Triassic Xujiahe (须家河) Formation in the Sichuan (四川) Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by oldlands in multiple directions including the north, east and south. Thus, the basin was generally characterized by coastal and widely open and shallow lacustrine deposition during the Late Triassic Xujiahe period. This is similar to the modern well-known Poyang (鄱阳) Lake. Therefore, we investigated the framework and conceptual model of the Sichuan Basin during the Xujiahe period with an analogue to the Poyang Lake. Results show that the conceptual model of the deposition can be divided into transgressive and regressive stages. The first, third and fifth members of the formation are in transgressive stage and the deposits are dominated by shore and shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have implications for understanding the large-scale sand deposition elsewhere. [ABSTRACT FROM AUTHOR]

Carbonate petroliferous basins usually develop fault-reservoir composite migration pathway systems. Accurate estimates of transport efficiency are important for locating oil and gas prospects, which is of considerable interest to oil explorers. The Cambrian Longwangmiao Formation in the Moxi–Gaoshiti area in the Sichuan Basin, the largest single monoblock gas accumulation in China with complicated migration pathway systems, was selected as an example to demonstrate the method. Using a combination of geologic, statistical, experimental data analyses, and quantitative characterization, a quantitative method for calculating the fault-reservoir composite transport efficiency is established for the first time. The results show that fault-reservoir composite migration pathway systems control the hydrocarbon accumulation. The analysis of data from 17 drilling wells using this method indicated a strong correlation between well productivity and composite transport efficiency, and the matching degree of actual and predicted results was as high as 83%. The best locations for oil and gas exploration are near areas with a high composite transport efficiency. Project planners can quickly find oil and gas prospects by matching calculated composite transport efficiencies to observed petroleum occurrences in a basin to avoid costly drilling mistakes. The method will have wide application in other carbonate basins around the world. • The fault-reservoir composite migration pathway controls hydrocarbon accumulation. • Quantitatively method for the composite transport efficiency is first established. • Composite transport efficiency is useful for location of oil and gas prospects. • Prospects for future drilling in the Moxi-Gaoshiti area of China are recommended. [ABSTRACT FROM AUTHOR]

Rhenium–osmium (Re–Os) geochronology is a robust technique for constraining the accurate timing of hydrocarbon evolution, but its effectiveness in over-mature petroleum systems has not been well constrained. Here we conduct a case study in the Neoproterozoic Sichuan Basin, China, i.e., the Sinian Dengying sequence (635–542 Ma). Re–Os isotopic data from this highly mature hydrocarbon reservoir yield a Re–Os isochron ages of 414 ± 44 Ma with an initial 187Os/188Os (Os i) of 1.36 ± 0.19 for bitumen and a 78 ± 37 Ma age with an Os i of 3.21 ± 0.17 for pyrobitumen. Coupled with thermal evolution of possible source rocks and fluid-inclusion homogenization temperatures, the 414 Ma age marks the timing of oil generation, and the 78 Ma age indicates the timing of gas generation associated with the thermal cracking of oil, and the formation of pyrobitumen in the reservoir. A comparison of the Os i of the 414 Ma bitumen and that of possible source rocks at the time of bitumen formation reveal that it was primarily sourced from shale of the lower Cambrian Qiongzhusi Formation. The results of this study provide new insights to better understand the over-mature oil–gas system in the Sinian Sichuan Basin, and show that Dengying Formation reservoirs in the depocenter areas of the basin are potentially favorable for oil and gas exploration. The results further highlight the utility of the Re–Os geochronometer to constrain the accurate age and source of hydrocarbon evolution. • Hydrocarbon evolution in Sinian Sichuan Basin was accurately dated by Re–Os geochronology. • Two Re–Os isochron ages were obtained at 414 Ma and 78 Ma. • The two ages mark the timing of oil generation and thermal cracking of oil, respectively. • The reservoir bitumens were sourced mainly from the lower Cambrian Qiongzhusi shales. • The Dengying Formation in the depocenter areas are potential reservoirs. [ABSTRACT FROM AUTHOR]