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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]

Two mudstone samples in low-maturity from the E 2 s 3 source rock interval were used to hydrous pyrolysis experiments of gold tube with isothermal heating in order to better understand natural gas origins of Langgu Sag. The results displayed that the E 2 s source rocks in Langgu Sag attained the peak yield of nearly 350 mg/g TOC at about 0.9% of Ro for C 6+ compounds due to thermal cracking occurrence of them if Ro higher than 0.9%, and C 6-14 compounds would reach the maximum yield at about 1.3% of Ro owed to C 14+ compounds preferentially cracking to them. Hydrocarbon gases were dominated by kerogen cracking gases at Ro lower than about 1.3% (referred to oil associated gases) and higher than about 1.8% (interpreted to residual kerogen cracking gases) but by secondary gases (belonged to early-generated liquid hydrocarbon cracking gases) if Ro at about 1.3%–1.8% for E 2 s source rocks. Furthermore, the yield of kerogen cracking gases would reach 90–100 ml/g TOC at 1.3% of Ro, far greater than 20 ml/g TOC as the lower limit standard of an effective gas source rock, even the kerogen cracking gas yield could reach 20 ml/g TOC if Ro at about 0.8%, therefore, as long as the E 2 s source rocks in Langgu Sag reach mature level, they can become effective gas source rocks. Considering their calculated Ro mainly distributing between 1.0% and 1.3% except Ordovician buried-hill reservoirs of the D2 oil-gas field, the proved natural gases in Langgu Sag mainly originated from kerogen cracking gases at Ro lower than 1.3%. And the gases from Ordovician buried-hill reservoirs of the D2 oil-gas field, with calculated Ro ranging about 1.3%–1.6%, essentially derived from residual liquid hydrocarbon cracking gases of the E 2 s 4 source rocks, because the Ro of E 2 s 3 source rock is mainly lower than 1.1% but it reaches about 1.8% for E 2 s 4 source rock at present. The natural gases in shallow reservoirs experienced different degrees of bacterial degradation and formed methane-dominated gas reservoirs, consequently, the C 1 /C 1-5 values displayed a range of 0.96–0.99 for bacterial degradation gas reservoirs and of 0.67–0.89 for natural gas reservoirs without bacterial reworking. • Hydrocarbon potentials of the E 2 s source rocks in Langgu Sag were studied by gold tube pyrolysis. • The E 2 s source rocks attained the peak oil yield at about 0.9% of Ro and gas is dominated by primary gases at Ro <1.3%. • The E 2 s intervals in Langgu Sag can be effective gas source rocks at Ro >0.8% based on the standard of 20 ml/g TOC. • Natural gases in Langgu Sag are dominated by primary gases generated at Ro <1.3% but some experienced bacterial degradation. [ABSTRACT FROM AUTHOR]

A physical model was developed to quantify 4 He accumulation in gas reservoirs. Both radiogenic 4 He from external crustal flux and in situ 4 He production were calculated with this model. Considering the time-accumulation effect of 4 He, the formation time of a gas reservoir can be estimated. The proposed method was applied to Puguang gas reservoir in southwest China's Sichuan Basin, the formation time of which was estimated as 32.5 Ma, which corresponds to the Oligocene epoch and to the mid-Himalayan movement, and which is consistent with gas formation and accumulation history. This result indicates that the proposed model based on 4 He accumulation can be a useful tool for studying the formation time of gas reservoirs. [ABSTRACT FROM AUTHOR]

Variations in the stable carbon isotope signature of marine carbonate rocks (δ13C carb) are utilized as an indicator for changes in depositional environment and tracers of diagenetic alteration. Various petrologic (textural changes) and geochemical indicators (trace elements) have been used to evaluate the effect of diagenetic alteration on the δ13C carb signature. However, how diagenetic processes involving interactions between organic matter and minerals impact the δ13C carb records of marine carbonate rocks has not been investigated thoroughly. In this study, systematic stable carbon (δ13C carb) and oxygen (δ18O carb) isotope analyses were conducted on marine carbonates from the Middle Ordovician Majiagou Formation in the Ordos Basin, China to evaluate secondary overprint of the primary δ13C carb record. 496 carbonate samples from the gas reservoir (potential gas source rock) were selected from one well located in the natural gas exploration target—mid-east part of the Ordos Basin. A depleted δ13C carb excursion (i.e., a shift to more negative values) occurred in the 100 m-thick anhydrite-bearing carbonate sequence of this formation. The δ13C carb and δ18O carb values for the whole rock vary from −10.9‰ to 1.8‰ and −11.5‰ to −4.0‰, respectively. The calcite veins and vug-filling calcites also have depleted carbon isotope signature ranging between −21.1‰ and −4.4‰. The secondary calcites observed under the microscope were generated by the conversion from isotopically depleted organic matter and resulted in the depleted δ13C carb signature. The commonly occurring thermochemical sulfate reduction in the study area and the degradation of potentially enriched carboxylic acid salts in anhydrite-bearing carbonate setting are major factors contributing to the depleted δ13C carb values. The conversion from organic carbon to inorganic carbon through organic matter-involved diagenetic processes plays a significant role in explaining the genesis of depleted δ13C carb excursions in deeply buried marine carbonate sequences. [Display omitted] • High-amplitude depleted δ13C carb signatures were first found to occur in the Middle Ordovician carbonate sequence. • Diagenetic processes involving organic matter impacted the original δ13C carb record. • Organic-origin calcites generated by TSR resulted in the depleted δ13C carb signatures. [ABSTRACT FROM AUTHOR]