Enrichment Mechanism of the Upper Carboniferous-Lower Permian Transitional Shale in the East Margin of the Ordos Basin, China: Evidence from Geochemical Proxies

Abstract

The organic-rich shale of the Upper Carboniferous-Lower Permian transition period in the eastern margin of the Ordos Basin, China, was formed in a marine-continental facies sedimentary environment. With a high content of total organic carbon (TOC) and a large cumulative thickness, it is considered a good source rock for shale gas development. The sedimentary environment of marine-continental transitional shale is obviously different from that of marine shale, which leads to different enrichment characteristics of organic matter. In this paper, shale samples were collected from XX# well of the Taiyuan and Shanxi Formations across the Upper Carboniferous-Lower Permian, which is typical marine-continental transitional shale. The TOC, major elements, and trace elements were measured, and the formation and preservation conditions were investigated using multiple geochemical proxies, including paleoclimate, redox parameters, paleoproductivity, and controls on the accumulation of organic matter. The TOC of Shanxi Formation is higher than that of Taiyuan Formation. In the Taiyuan Formation, TOC is positively related to the redox index (V, U, and V/Cr), indicating that the dysoxic bottom water environment is the key factor controlling organic matter accumulation. For Shanxi Formation, there is a positive correlation between TOC and paleoclimate, which indicates that the enrichment of organic matter is affected by warm and humid paleoclimate and oxic environment. In addition, the paleoproductivity is lower with a positive correlation with TOC for the marine-continental transitional organic-rich shale, suggesting that it was inferior to the gathering of organic matter.

Discussion on Transitional Shale Gas Accumulation Conditions from the Perspective of Source-Reservoir-Caprock Controlling Hydrocarbon: Examples from Permian Shanxi Formation and Taiyuan Formation in the Eastern Margin of Ordos Basin, NW China

“…The current research focuses on transitional shale gas, mainly focusing on the reservoir characteristics of the Shanxi Formation [11][12][13][14][15][16]. Systematic comparative studies of the Shanxi Formation and Taiyuan Formation are mostly performed with respect to sedimentary evolution, organic matter enrichment, and pore size distribution [17][18][19][20][21][22][23]. Specific research on the Taiyuan Formation has rarely been reported.…”

Jurassic Terrestrial Shale Gas Potential of the Northern Kashi Sag in the Tarim Basin, Northwestern China

“…The continental basins in China are widely distributed, and the continental shale is developed in many strata such as Mesozoic-Cenozoic [8][9][10][11], which is one of the required replacement fields for shale gas exploration. Industrial discoveries of continental shale gas are found in several Triassic stratigraphic wells in Ordos Basin and Jurassic strata in Sichuan Basin [12,13]. However, compared with marine shale, the output of continental shale gas is lower.…”