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.
Highlights
The remarkable success of shale gas development in North America has led to the vigorous development of shale gas exploration and increased the investigation of global shale gas potential [1,2,3,4,5,6]
The chemical index of alteration (CIA) of Shanxi Formation is positively correlated with TOC (R2 = 0:44, Figure 4), which indicates that climate has a great impact on total organic carbon and that warm and humid climate is conducive to the growth of the advanced plants
There are positive correlations between the TOC and U, V, and V/Cr (R2 = 0:65, 0.87, and 0.62, respectively). These results indicate that the dysoxic conditions of the bottom water preservation environment were the key factor that contributed to the accumulation of organic matter for the Taiyuan Formation shale
Summary
The remarkable success of shale gas development in North America has led to the vigorous development of shale gas exploration and increased the investigation of global shale gas potential [1,2,3,4,5,6]. According to the sedimentary environment, China’s organic rich shale can be divided into three types: marine shale, marine-continental transitional shale, and continental shale [5, 8]. Geofluids organic-rich shales in the marine-continental transitional facies are mainly black coal-bearing shales with high total organic carbon (TOC) content and large accumulated thickness, which are the best characteristics and quality for developing shale gas [11]. The Carboniferous-Permian is a key period for the change of sedimentary environment from marine to continental facies in China. Shales rich in marine-continental transitional organic matter are widely deposited, including Northern China, the Tarim Basin and the Junggar Basin of the Carboniferous-Permian, the Middle Carboniferous Benxi Formation, the Upper Carboniferous Taiyuan Formation, and the Lower Permian Shanxi Formation in the Ordos Basin [12, 13]. In order to better understand the potential of transitional shale gas, it is necessary to further study the reservoir characteristics and their relationship with sedimentary environment
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