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Interactive depositional systems of marine carbonates and gypsum salt rocks are closely related to natural gas reservoirs. Despite continuous progress in the exploration of new areas of marine carbonate genesis within the Ordos Basin, the source and mechanism of "sub-salt" natural gas genesis remains controversial. In this study, we investigated natural gas genesis through geochemical analysis of Lower Paleozoic natural gas samples from the mid-eastern Ordos Basin, obtaining natural gas composition data and carbon/hydrogen isotope compositions. We found evident differences between the geochemical characteristics of "sub-salt" and "post-salt" natural gas; the methane carbon isotope signature of "sub-salt" natural gas was lighter overall than that of "post-salt" natural gas, while the ethane carbon isotope composition of the former was more widely distributed and partially lighter than that of the latter. Combining these data with the regional geological background and existing geochemical data, it is evident that Ordovician "post-salt" natural gas comprises a composite of Upper Paleozoic coal-type gas and Lower Paleozoic oil-type gas, with the oil-type gas accounting for the largest proportion. In contrast, the "sub-salt" natural gas was formed and preserved within the Ordovician marine carbonates or sourced from deeper and more ancient hydrocarbon source rocks. Geochemical anomalies, including light methane carbon isotopes and ethane carbon isotopes with coal-type gas characteristics, are closely related to the prevalence of thermochemical sulfate reduction during hydrocarbon formation and reservoir formation of natural gas in "sub-salt" strata. [ABSTRACT FROM AUTHOR]

Original organisms are the biological precursors of organic matter in source rocks. Original organisms in source rocks are informative for oil‐source rock correlation and hydrocarbon potential evaluation, especially for source rocks which have high‐over level of thermal maturity. Systematic identification of original organism assemblages of the Lower Paleozoic potential source rocks and detailed carbon isotopic composition of kerogen analyses were conducted for four outcrop sections in the Tarim basin. Results indicated that the original organism assemblages of the lower part of the Lower Cambrian were composed mainly of benthic algae, whereas those of the Upper Cambrian and the Ordovician were characterized by planktonic algae. Kerogen carbon isotopic data demonstrated that the δ13Ckerogen values of source rocks dominated by benthic algae are lower than –34‰, whereas the δ13Ckerogen values of source rocks dominated by planktonic algae are higher than –30‰ in general. We tentatively suggested that the carbon species those are utilized by algae and the carbon isotopic fractionation during photosynthesis are the major controls for the δ13Ckerogen values in the Lower Paleozoic source rocks in the Tarim basin. Correlating the δ13C values of oils exploited in the Tarim basin, the original organism assemblages, and δ13Ckerogen values of source rocks, it implied that the Lower Paleozoic oils exploited in the Tarim basin should be sourced from the source rocks with original organism assemblages dominated by planktonic algae, and the hydrocarbon sourced from the Cambrian benthic algae should be of great exploration potential in future. Original organism assemblages in source rocks can provide important clues for oil‐source rocks correlation, especially for the source rocks with high thermal maturity. [ABSTRACT FROM AUTHOR]

The article offers a study related to the way of predicting production state by carbon isotope fractionation in the fuling shale gas field, Southeast Sichuan Basin. Topics discussed include well gas samples and shale core samples of the Longmaxi Formation were collected from the well and field analysis, gas composition and carbon isotope test were completed. Article concludes that desorption gas in Fuling area has a favorable indication effect on the enrichment of shale gas.

• The carbon isotope curve index (CICI) summarizes the isotope ratios of C 1 - C 3. • Carbon-isotope curves can be divided into Convex, Linear, Concave and Reversed. • In Normal-type gas, leakage and preferential propane biodegradation increase CICI. • In Normal-type gas, migration and mixing with microbial methane decrease CICI. The carbon isotope curve (δ13C n versus 1/n) is often used to evaluate the origin and secondary alteration of hydrocarbon gases, but the factors that influence the carbon isotope curve are not fully understood, limiting its practical application. We define a carbon isotope curve index (CICI) to quantitatively characterize the curve type. Four general curve types are defined: a Normal (δ13C 1 < δ13C 2 < δ13C 3) type and three Reversed types consisting of R 1 (δ13C 1 > δ13C 2 > δ13C 3), R 2 (δ13C 1 < δ13C 2 > δ13C 3), and R 3 (δ13C 1 > δ13C 2 < δ13C 3). CICI is defined as arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for Normal and R 2 , –180 + arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for R 1 , and 180 + arctan (3Δ13C 3-2 /Δ13C 2-1) × 180/π for R 3. CICI ranges from 0 to 90 for Normal, –180 to –90 for R 1 , –90 to 0 for R 2 , and 90 to 180 for R 3. Normal gases can be divided into three categories: Linear (CICI = 45), Convex (CICI 0 to < 45), and Concave (CICI > 45 to 90). The influence of kerogen type and maturity on CICI is investigated by comparing thermogenic gases generated from different kerogens in both sedimentary basins and pyrolysis experiments. Curves for low-maturity lacustrine and low-to-high-maturity coal gases are mainly Convex. In contrast, curves for high maturity to overmature marine gases are more Concave. The curve type of thermogenic gas changes from Convex to Concave with increasing maturity, and the mixing of gases generated from kerogen-cracking and oil-cracking results in the Concave type. The effect of post-generation alteration processes on CICI was elucidated using data from three field studies. In Normal-type gas, preferential biodegradation of propane and leakage increase CICI, while migration and mixing with microbial methane decrease CICI. Preferential biodegradation of propane and leakage are two other reasons for the Concave type. [ABSTRACT FROM AUTHOR]

Fourteen gases and their associated oils were collected at the surface from production wells in the Shahejie Formation, Dongying Formation, and Archean in six zones around the Qingshui Sub-sag of the Liaohe Depression, Bohai Bay Basin. The molecular compositions and carbon and hydrogen isotopic compositions of natural gas were measured. The origin, migration, accumulation, and secondary alteration of natural gas were studied based on the lateral and vertical comparison of natural gas geochemical characteristics. The natural gases include humic-type (δ13C 2 > −29.0‰, δ13C 1 > −43.0‰), sapropelic-type (δ13C 2 < −29.0‰, δ13C 1 > −43.0‰), and secondary microbial (δ13C 1 of −48.5‰, C 1 /(C 2 +C 3) of 46.3, δ13CO 2 of −1.5‰) gases. The humic-type and sapropelic-type gases were derived from the source rocks in the third and fourth members of the Shahejie Formation, respectively. The humic-type gases dominate. The biodegradation level of natural gas in the study area varies, and biodegraded gases are characterized by high Δ13C(C 3 –C 2), low C 3 /C 2 , and low C 3 /iC 4 and are associated with biodegraded oils. The vertical migration of natural gas in the west and east of the Qingshui Sub-sag is limited. There are good additional exploration prospects for humic-type gas, derived from high-maturity source rocks of the third member of the Shahejie Formation in the deep layers (depth >4.5 km) of the Qingshui Sub-sag. Secondary microbial gas is a potential exploration target in the shallow layers (depth <2.5 km). • Humic-type gas is from the source rock of the 3rd member of Shahejie Formation. • Sapropelic-type gas is from the source rock of the 4th member of Shahejie Formation. • Biodegraded gas: 13C-rich propane, depletion of propane and n-butane. • Secondary microbial gas: 13C-rich carbon dioxide and 13C-depleted methane. • Natural gas mainly accumulates near the source rock. [ABSTRACT FROM AUTHOR]