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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 exploration of high production, natural gas wells (>106 m3/d) has been extremely successful in the Gaoshiti and Moxi areas of the Leshan–Longnvsi paleo‐uplift, in the west‐central Sichuan Basin. This paper focuses on the Sinian and Cambrian reservoirs from the Gaoshiti and Moxi areas, where a dataset of new trace elements in bitumen and potential source rock is reported and where hydrocarbon‐source correlation is attempted based on trace elements characteristics of the 12 potential source rocks and 22 bitumen samples. Results show that the bitumen from the three reservoirs can be distinguished from each other using trace element distribution patterns and redox‐ and provenance‐sensitive elemental ratios. Bitumen in the second and fourth members of the Sinian Dengying Formation were derived primarily from the third member of the source rocks of the Dengying Formation, which represents a self‐sourced paleo‐petroleum system. In contrast, bitumen in the reservoirs of the Cambrian Longwangmiao Formation is derived from the Cambrian Qiongzhusi Formation and third member of Dengying Formation. The trace element method used in this study improves our understanding of the complex hydrocarbon–source relation and confirm the potential target for petroleum exploration in the study area. [ABSTRACT FROM AUTHOR]

Pyrobitumen has been discovered in the dolostone reservoir of the Central Sichuan Basin, southwest China, accumulates coevally with hydrothermal minerals in pores. The pyrobitumen has strong anisotropy with an evolutionary sequence from mosaic to a fibrous texture. A similar sequence of anisotropy has also been observed in mesophase pitch during the process of coking coal. Reflectance of the pyrobitumen shows a positive correlation with anisotropic evolution and increasing depth. Hopane and sterane biomarkers are abundant in extracts of highly anisotropic pyrobitumen with smooth, rough, hyperplastic surface and honeycomb micropores. Methylpyrene and methylchrysene maturity indicators and the reflectance values of pyrobitumen show a positive linear correlation in Cambrian samples, but show random correlation in Sinian samples. Fluid inclusion microthermometry of hydrothermal minerals, together with the burial and heat flow history of the reservoir indicate an abrupt thermal event that may have impacted the generation and evolution of pyrobitumen. This appears to be confirmed by the reflectance values of pyrobitumen in the source rock and reservoir. The pyrobitumen in source rock was less heated by the hydrothermal fluid due to its poor porosity and have lower reflectance values. However, the pyrobitumen in weathering crust reservoir was more heated by the hydrothermal fluid and have higher reflectance values, because the reservoir rocks have higher porosity. The occurrence and characteristics of pyrobitumen indicate an in-situ pyrolysis of oil, which was triggered by an abrupt thermal event in the reservoir. The reconstruction of burial history and heat flow, as well as the hydrothermal fluid minerals of the dolomite reservoir all indicate the thermal event of the Sichuan Basin might correlate with the Emei mantle plume in Late Permian. [ABSTRACT FROM AUTHOR]

Highlights • Unique geochemistry of high mature reservoir pyrobitumens in Lower Cambrian. • Parallel application and cross-validation of REE, carbon isotopes and biomarkers. • Multiproxy study of oceanographic conditions in the Lower Cambrian. • Variations of inorganic element ratios in probitumens along migration pathways. • A potential means through analysis of bitumen to study paleo-oil reservoirs. Abstract The Moxi–Gaoshiti Bulge of the Sichuan Basin in southwestern China is unique because it provides insights into the geochemistry of Lower Cambrian reservoir bitumens and their geological significance for paleo-oil reservoirs. Analyses of biomarkers, carbon isotopes, trace elements (TE) and rare earth elements (REE) were conducted on reservoir bitumens and the associated source rocks to constrain sedimentary environments and bitumen/paleo-oil sources. Based on biomarkers, carbon isotopes, TE ratios and REE distribution patterns, both of the Cambrian Qiongzhisi (Є 1 q) and Sinian Dengying (Z 2 dn) source rocks can be hydrocarbon sources for the Cambrian Longwangmiao (Є 1 l) bitumens in the MG Bulge (Lower Cambrian Doushantuo Formation not discussed here). Abundant n -alkanes relative to isoprenoids, significant C 21 and C 22 steranes, predominance of dibenzothiophene (DBT), and low ratios of biomarkers, such as Pr/Ph, DBT/P, C 22 TT/C 21 TT and C 35 H(22S)/C 34 H(22S) indicated that the source rock was characterized as shale from an overall highly reducing marine depositional environment. The average δ13C values from the compositions of the solid bitumens show an abnormal order: saturate hydrocarbons < asphaltenes < N-S-O compounds < aromatic hydrocarbons, suggesting that high temperature might affect the normal fractionation among different organic matter fractions. The reservoir bitumens are enriched in Mo, Tl, Pb and Ba, strongly depleted in Bi Compared to PAAS, and mostly well correlated in the REE distribution patterns. In the primary migration pathways suggested by the DBT concentration, the parameters ∑REE and Zr/Cu show an overall increase, and the ratios V/Ni, Ni/Co, V/Zr and V/Cr have decreasing trends. The comprehensive analyses of molecular biomarkers, carbon isotopes, TE and REE in pyrobitumens of the highly mature reservoirs in this study suggest a potential means to study paleo-oil reservoirs, and can thus provide valuable information for deep oil and gas exploration in Precambrian – Lower Cambrian petroleum systems. [ABSTRACT FROM AUTHOR]

The unique Moxi–Gaoshiti Bulge of Sichuan Basin in southwestern China provides insights into the biomarker signatures of bitumens and their geological significance for paleo-oil reservoirs in Precambrian strata. Solid bitumens with various shapes occur widely in the voids, solution pores, cracks, stylolites, and veins of reservoir rocks. Biomarker analysis was conducted on bitumens and the associated source rocks by quantitative detection with gas chromatography–mass spectrometry (GC–MS) to constrain thermal maturity, sedimentary environments, bitumen/paleo-oil sources, and the possible paleo-oil filling directions, which are dominant geological factors that control paleo-oil accumulations. Quantitative analysis of biomarkers showed that the bitumens in the Sinian dolomite of the Moxi–Gaoshiti Bulge belong to the same group. The bitumens are highly matured, and their reflectance values (%R b ) are greater than 1.5 with an average of 2.96. These bitumens originated from thermal cracking of oil (pyrobitmen) via a high-temperature process. The biomarker characteristics revealed that the bitumens and related paleo-oil reservoirs originated from a highly reduced but moderately saline marine shale environment. The marine shales of the Sinian Dengying and lower Cambrian Qiongzhusi formations are their major source rocks. The directions of the primary oil-filling pathways in the Gaoshiti and Moxi sub-bulges are from west to east and from northwest to southeast, respectively. These directions indicate that the Deyang–Anyue sag west of the Moxi–Gaoshiti Bulge is the major hydrocarbon source kitchen. [ABSTRACT FROM AUTHOR]

The Moxi–Gaoshiti (MG) Bulge of the Sichuan Basin in southwestern China is unique because of its enormous gas field in old Cambrian and Proterozoic marine strata, where oils in carbonate reservoirs have been nearly completely cracked to solid bitumen and natural gas because of high to post-mature thermal maturity levels. Numerous solid bitumen-bearing dolomites obtained from boreholes in the MG Bulge provide an ideal opportunity to trace the major filling orientations for the Lower Cambrian paleo-oil reservoirs. Biomarker analysis was conducted on solid bitumens through quantitative detection with gas chromatography–mass spectrometry. Results show that all solid bitumen samples in the MG Bulge exhibit similar compositions of molecular markers, thereby implying that they belong to the same oil population and that paleo-oils should be derived from the same source kitchen/bed. Low pristane/phytane and dibenzothiophene (DBT)/phenanthrene ratios, as well as the predominance of DBT among DBT, dibenzofuran, and fluorene, are observed in the solid bitumen samples, which suggest that related paleo-oil reservoirs likely originated from a highly reducing marine shale environment. All isopleth maps of the DBT concentrations, 4-methylDBT/1-methylDBT, 4,6-dimethylDBT/(1,4 + 1,6)-dimethylDBT (DMDBT), and (2,6 + 3,6)-DMDBT/(1,4 + 1,6)-DMDBT ratios consistently indicate that the overall orientation of oil migration in the MG Bulge is primarily from the west-to-east direction, with the strongest filling point located at approximately the Mx12 well. The north-to-west direction is an additional filling orientation. These indicators show that those paleo-oils that had been cracked to pyrobitumens mainly originated from a source kitchen in the western part of the MG Bulge. A hydrocarbon source kitchen in the northern part of the MG Bulge may also have developed but further research on this is needed. The recognition of these solid bitumen molecular signatures, particularly the consistent results using DBT concentrations and alkyl DBT thermal maturity indicators, suggests a potential approach that can be applied to the study of paleo-oil reservoirs. [ABSTRACT FROM AUTHOR]