Graptolite-rich shale is the main layer of shale gas resources in the southern marine sedimentary basin. Recently, shale gas resources were discovered in the Ordovician marine graptolite-rich strata in the Ordos Basin. The graptolite shale in the study area is different from the marine graptolite shale in the Yangtze plate in southern China, and further exploration is needed. This paper presents core samples of the graptolite-rich shale of the Pingliang Formation in the southwest Ordos Basin as research objects. The graptolite genus and graptolite shale characteristics were studied using core observation, electron microscopy, scanning electron microscopy, and geochemical analysis. We determined the role of the sedimentary environment and thermal maturation of graptolite shale in hydrocarbon formation and explored the possibility of hydrocarbon generation. Many graptolite epidermises provide buried organic matter. The quiet sea and low-energy marine environment create favorable conditions for preserving organic matter. The tectonic process resulted in the evolution stage in the oil generation window. Different types of pores formed the spaces of hydrocarbons. Therefore, the shale of the Pingliang Formation has shale oil exploration potential, which complements the shale gas in the northwestern margin of the basin, and provides new venues for shale oil and gas exploration in northern China.

Since the 1950s, major breakthroughs have been made in the field of carbonate oil and gas exploration, and large reservoirs have been found in carbonate strata in many countries; however, the validity of highly evolved carbonate source rocks has been controversial. Because of the loss of organic carbon during acid-solution processing in conventional total organic carbon (TOC) determination, we decided to use a new method of reducing acid-solution losses in order to more effectively quantify acid-soluble organic matter in carbonate rock samples. Different levels of acid-soluble organic matter were present in different types of samples, and there was no positive correlation between the content of acid-soluble organic matter and conventionally measured TOC values. However, the thermal evolutionary maturity of samples with high acid-soluble organic matter content is relatively low, and the high hydrocarbon generation conversion rate of acid-soluble organic matter and conventional TOC in carbonate rocks is an important reason for the low residual TOC in highly evolved carbonate rocks. The new method is helpful in re-evaluating the abundance of organic matter in carbonate rocks and in providing evidence for studying hydrocarbon generation and the hydrocarbon capacity of carbonate source rocks.

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.