Paleoenvironmental conditions and organic matter accumulation in Upper Paleozoic organic-rich rocks in the east margin of the Ordos Basin, China

Abstract

Marine-continental transitional shale gas contains significant resources, but scant areas have been developed economically. The evolution of paleoenvironmental and potential of high-quality shales generation from the Carboniferous to Permian (mainly of Benxi, Taiyuan and Shanxi formations) in the east margin of Ordos Basin, China, was systematically studied by integrated analysis including rock-mineral composition, trace elements and organic geochemical testing. The results show that the Benxi Formation consisted principally by marine deposits, while the co-existence of marine and marine-continental transitional deposits are found in the Taiyuan Formation. The Shanxi and its upon Xiashihezi formations are mainly constituted by continental deposits. The Taiyuan Formation has been affected by frequent transgression and regression processes, while the sedimentary environment of the Shanxi Formation was relatively stable. The degree of oxidation gradually increased from the Benxi to the Xiashihezi Formation, and the paleoclimate transformed from humid and warm climate to aridity. The paleosalinity decreased gradually, even though the environment was generally maintained in a salt water environment with considerable rainfall and relatively active hydrodynamic conditions. The paleoproductivity increased gradually, and a generally anoxic environment in the surface water contributed to the enrichment of organic matter. The rock-mineral compositions of the shales are dominated by clay minerals and quartz, and the organic matter content of the shales is relatively high (averaging of 1.28%). The shales primarily contain type III kerogen, and the shales have generally entered a high maturity stage with good gas generation potential. Furthermore, the gas generated from the interbedded coal seams in the Upper Paleozoic is a stable gas supply for interbedded shales. The results provide a geochemical basis for further study of marine-continental transitional shales in the Ordos Basin and supply recommendations for the optimization of high-quality shales production in similar basins around the world.