Search

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]

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]

Determining the place of paleo-oil cracking is crucial to understanding the formation mechanism of the gas reservoirs in the Precambrian–Cambrian strata. For the old strata the attainable information is limited because most paleo-oil have been cracked to gas at high thermal maturity, such as the Anyue giant gas field in the Moxi–Gaoshiti Bulge of Sichuan Basin, Southwestern China. In this study, the place of paleo-oil cracking was discussed based on solid bitumen found in the reservoirs. Numerous solid bitumen from the deep boreholes in the Precambrian Dengying (Z 2 dn 2 and Z 2 dn 4 ) and Lower Cambrian Longwangmiao (Є 1 l) formations of the Moxi–Gaoshiti Bulge were analyzed. Elemental and carbon isotope analyses and optical microscopic observations, such as Raman spectroscopy, scanning electron microscopy, and electronic probe, were performed on these solid bitumen samples. The obtained parameters, including the reflectance of the solid bitumen, parameters I(D)/I(G) and S(D)/S(G) from the Raman spectra, the compositions and carbon isotope of gas, burial history, and the predicted "equivalent vitrinite reflectance Ro" (EqVRo) for reservoirs, jointly support that solid bitumen are epigenetic bitumen in reservoirs other than primary bitumen in source rocks. This study provides a relatively solid evidence of in situ cracking of paleo-oil and the thermochemical sulfate reduction reaction between paleo-oil and dolomite reservoirs. Pyrite were found both in carbonate rocks and argillaceous rock, and the sulfur in pyrite is perceived as a pathway of H 2 S formation. The results provide a basis for understanding the formation mechanism of the deep huge gas field that was derived from paleo-oil cracking, and may contribute to future petroleum exploration in deep Precambrian–Lower Cambrian systems. • A vitrinite reflectance profile of the extensive shale form one well was provided. • The solid bitumen was confirmed to be from in situ oil cracking at high thermal maturity. • Evidences proved that the TSR reaction was involved in this crude oil cracking. • The reaction of pyrite with hydrocarbons was considered as a pathway of H 2 S formation. • Maturities" of solid bitumen and reservoirs were quantitative calculated and compared. [ABSTRACT FROM AUTHOR]