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Ahead of Print Abstract
DOI:10.1306/11152322091
Karst topography paces the deposition of lower Permian, organic-rich, marine–continental transitional shales in the southeastern Ordos Basin, northwestern China
Zhang Xi12 , Zhao Xiaoming12 , Ge Jiawang12 , Li Shuxin3 , and Zhang Tingshan12
1 School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China
2 Southwest Petroleum University Natural Gas Geology Key Laboratory of Sichuan Province, Chengdu 610500, China
3 PetroChina Coalbed Methane Company Limited, Beijing 100028, China
Ahead of Print Abstract
Organic-rich continental and marine–continental (i.e., transitional) shales are characterized by numerous hydrocarbon production layers having an uneven horizontal distribution, which are challenging to locate and exploit. We examined the effects of karst topography on organic carbon accumulation during the early Permian in the southeastern Ordos Basin, northwest China, using outcrop and well data. Our study involved geomorphological, sedimentological, petrological, and geochemical methods. We identified a regional unconformity on the Dongdayao Limestone (DDYL) that formed in the early Permian (Asselian; i.e., in the Shanxi Formation) in the study area, based on: (1) cave, pore, and breccia development in outcrops and drill cores; (2) high Mn–Fe and low Sr contents associated with negative δ18O and normal δ13C values, which are indicative of strong leaching by meteoric waters; and (3) the irregular thickness of the DDYL that is indicative of differential karstification, resulting in the formation of horizontal gullies. The karst topography of the DDYL was identified based on the moldic and residual thickness methods, including karst highland, gentle slope, and microbasin geomorphic units. We propose that the karst topography controlled the redox environment and led to enrichment of the organic-rich transitional shales in the selected submember of the Shanxi Formation. The U/Th, V/Cr, and V/(V+Ni) ratios exhibit a linear relationship with geomorphic unit types. The karst microbasins had a weakly oxic environment, which widely preserved thick, organic-rich, transitional shales having high total organic carbon content and gas-bearing potential.
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