Geobiological evaluation of hydrocarbon-generating organisms and source rocks in the Ordovician Majiagou Formation, east-central Ordos Basin

Over recent years, breakthroughs have been achieved in the pre-salt natural gas exploration in the Ordovician Majiagou Formation, east-central Ordos Basin. Given the generally low total organic carbon (TOC) content in the carbonate-evaporite assemblages in the formation, there is an urgent need to seek high-quality source rocks and investigate their formation conditions. Using core samples from the Majiagou Formation in the east-central Ordos Basin, we analyze the formation’s paleoecology, paleoenvironment, and hydrocarbon-generating organisms, delving into the characteristics of hydrocarbon-generating organism assemblages and their changes with the paleoenvironment. Furthermore, we perform geobiological grading evaluation using habitat types and biogenic Ba as biological indices, alongside the V/(V+Ni) ratio and mineral types in evaporites as geological indices. Key findings are as follows: (1)The hydrocarbon-generating organisms in the Majiagou Formation encompass planktonic algae, benthic red algae, benthic brown algae, and benthic blue-green algae. (2)During the deposition of the Majiagou Formation, water bodies with high biological productivity created conditions for the formation of source rocks. Specifically, the high-frequency sealevel fluctuations and the alternating dry and wet climates induced frequent changes in the salinity and redox conditionsof water bodies. Therefore, the favorable conditions for organic matter preservation serve as the key to the formation of source rocks in the study area. (3)Geobiological facies favorable to source rock formation primarily include the marginal facies of gypsum-dolomite lagoons with halophilic algae. The mud-dolomite flat facies and gypsum-mud lagoon facies found in the shallow subtidal zones of planktonic algae, are to follow. (4)Compared with the results of residual organic carbon abundance evaluation, those of source rock geobiology are more optimistic. The effective sources rocks mainly include are gypsiferous argillaceous dolomites, argillaceous dolomites, gypsum mudstones, and dolomitic mudstones, all present as dark shaly laminae or bands.