The Sedimentary Geochemical Characteristics and Geological Significance of the Wufeng-Longmaxi Formation Accumulation of Organic Matter Black Shale on the Southeastern Sichuan Basin, China

Abstract

At present, the understanding of the sedimentary paleoenvironment and organic matter enrichment law of black shale in the Wufeng-Longmaxi Formation is still insufficient. This time, we investigated the total organic carbon (TOC) content, X-ray diffraction mineralogical composition, and the major and trace element abundances of the newly recovered cores of three shale gas exploration wells in Changning, Qianjiang, and Xiushan areas which were, respectively, selected to discuss and compare the varying paleoenvironmental conditions and the factors that control organic matter accumulation, such as terrigenous input, redox conditions, primary productivity, and the degree of water retention in the basin. The results show that black siliceous shale lithofacies, gray-black shale lithofacies, gray-black silty shale lithofacies, gray argillaceous siltstone lithofacies, and gray shell marl lithofacies are mainly deposited in the Wufeng-Longmaxi Formation in the southeastern Sichuan Basin. Organic-rich black shale is mainly concentrated in the lower part of the Wufeng Formation and Longmaxi Formation. The continuous thickness of organic carbon content greater than 2% in the Changning area is 32 m, the continuous thickness in the Qianjiang area is 22 m, and the continuous thickness in the Xiushan area is 11 m. The hydrocarbon generation potential gradually weakens from west to east. The identification of the geochemical behavior characteristics of major and trace elements shows that enrichment of the organic matter black shale section in the Changning-Qianjiang-Xiushan areas is mainly formation in the anoxic-moderate water mass restriction environment with high primary productivity and less terrigenous detritus input under the low hydrodynamic conditions. However, the correlations of terrigenous detrital proxies Ti/Al, redox proxies V/Cr, Ni/Co, and paleoproductivity proxies Babio with the TOC contents show that the Changning area is a favorable area for the formation of organic-rich black shale and the Qianjiang area is affected by the Wuling underwater uplift; the thickness of the black shale deposits becomes thinner; the Xiushan area is adjacent to the Xuefeng uplift, and the deposition conditions of the accumulation of organic matter black shale deteriorate, which is unfavorable for shale gas exploration.

Variational Reservoir Characteristics of the Wufeng–Longmaxi Formation from Different Sedimentary Region and Its Implications in Southeastern Sichuan Basin, China

“…The analysis of paleo-oceanic redox conditions is of great significance for evaluating the accumulation of organic matter . Basically, the redox-sensitive trace elements such as U, Th, V, Ni, Cr, and Co were usually selected for reconstructing paleo-ocean environments, enabling many achievements. ,,, In this work, the ratio of V/(V + Ni) is adopted to investigate the redox conditions associated with deposition of the Wufeng–Longmaxi formation in the southeastern Sichuan Basin. Usually, when V/(V + Ni) is greater than 0.54, the anoxic condition is judged for the sedimentary environment …”

“…Furthermore, in the early Ordovician, the sedimentary environment in the southeastern Sichuan Basin is deep-water shelf . During the late Ordovician Wufeng period, the Chuanzhong uplift expanded and the Xuefeng area uplifted with the Upper Yangtze region affected by the tectonic compression of the Duyun movement in Guangxi .…”

“…Basically, shale is simultaneously treated as a hydrocarbon source rock and a hydrocarbon reservoir formation . When shale is regarded as a hydrocarbon source rock, it can be demonstrated that the higher the organic matter content of hydrocarbon source rocks, the higher the natural gas content generated. , Therefore, when evaluating the hydrocarbon generation capacity of shale, that is, a kind of hydrocarbon source rock, the geochemical characteristics of the hydrocarbon source rock including organic matter abundance, organic matter type, and degree of organic matter evolution need to be involved . Switching to the perspective of being a hydrocarbon reservoir formation, with the successful exploitation of shale gas in North America, such as the Barnet, Marcellus, and Woodford shales, , shale gas in China also has received much attention from sedimentologists. − Especially, the Wufeng–Longmaxi shale (i.e., from the upper Ordovician Wufeng to the lower Silurian Longmaxi formation) is currently the key stratum for commercial shale gas exploration and development in the whole Sichuan Basin and its periphery, represented by the southeast Sichuan Basin. ,− …”

“…Additionally, the high content of brittle minerals in shale is more propitious for the gas flow to be stimulated from the shale reservoir through hydraulic fracturing. , According to a study of the organic geochemical characteristics of well Yucan-6 situated in the southeastern Sichuan Basin, the TOC content varied between 1.84 and 4.79% (average = 3.04%) with good hydrocarbon generation condition, the vitrinite reflectance ( R o ) value ranged from 1.73 to 2.03% (average = 1.94%), which is in a postmature thermal evolutionary stage, and the organic matter type is characterized as kerogen I or II 2 . However, with the vigorous progress of shale gas exploration and development, breakthroughs still have not been made in the southeastern Sichuan Basin except the Fuling Jiaoshiba region where shale gas exploration and development on a large scale has been accomplished, ,, impeding the shale gas exploitation process in the southeastern Sichuan Basin.…”

Reservoir Characteristics and Gas Content of Black Shale of Wufeng-Longmaxi Formation in Qianjiang Area, Southeast Chongqing, China

“…During the Late Ordovician Wufeng-Early Silurian Longmaxi sedimentation period, affected by the Chuanzhong-Qianzhong-Xuefeng Mountain uplift, the Sichuan Basin was a restricted shallow sea shelf sedimentary environment [16][17][18][19][20][21][22][23]. The sedimentary water body gradually became shallow from the Fuling Jiaoshiba in the basin to the southeast of Chongqing in the basin margin.…”