Differential enrichment mechanism of organic matters in the marine-continental transitional shale in northeastern Ordos Basin, China: Control of sedimentary environments

The organic-rich shales deposited in marine-continental transitional environments are well developed in China. Previous studies mainly focused on the petrology and geochemical characteristics of the transitional shale. However, the factor controlling organic matter (OM) enrichment in the shale has not been studied adequately, limiting the optimal reservoir selection in shale gas exploration. Based on laboratory analyses and field investigations, this paper documents the geochemical features of the transitional shales in Upper Paleozoic (the Benxi, Taiyuan and Shanxi Formations) of the northeastern Ordos Basin, China. The sedimentary environment evolution and the relevant influence on OM enrichment are discussed to reveal the factor controlling the interlayer difference of the OM enrichment in the shale. The results show that the shale in the Taiyuan Formation hosts the highest OM content in the Upper Paleozoic. The sedimentary setting of the study area transited from barrier-flats to fluvial deltas, accompanied by gradually arid climates, decreasing degree of reduction and salinity of waters, and increasing clastic influx. During the middle stage of the marine-continental transition (the depositional stage of the Taiyuan Formation), the warm and humid climate was beneficial to the OM production, and the anoxic condition was conducive to the OM preservation. In addition, the increasing clastic influx ensured the OM input, and the moderate depositional rate favored the OM preservation. All the factors above commonly established an excellent environment for the OM enrichment. Our results reveal the OM enrichment mechanism in transitional shales, which is helpful to the optimal reservoir selection in the shale gas exploration in the Ordos Basin as well as the similar basins.