Petrographic and geochemical characteristics of deep-lacustrine organic-rich mudstone and shale of the Upper Triassic Chang 7 member in the southern Ordos Basin, northern China: Implications for shale oil exploration

Abstract

Organic-rich lacustrine mudstones and shales in Chang 7 member of the Upper Triassic Yanchang Formation are potential shale oil reservoirs. Petrographic and geochemical data were used to study their lithologic and geochemical characteristics, mineral composition and sedimentary environments. Three different types of deep lacustrine deposits are identified, including unstratified massive mudstone (muddy debrite), normally graded mudstone (muddy turbidite) and laminated shale. Mudstones and shales contain relatively abundant brittle minerals (quartz, feldspar, pyrite and carbonates), which are favorable for the hydraulic fracturing. Terrigenous input and diagenesis conversion process are the main sources of felsic minerals. Laminated shales yield the highest TOC values (average of 7.47%) as well as S1 and S2 values. Most of the mudstones and shales contain Type II organic matter, while contain little Type I organic matter. The Ro and Tmax values combined with OEP versus CPI and C29ββ/(ββ + αα)sterane versus αααC29sterane20S/(20S + 20R) discrimination diagrams and Ts/(Ts + Tm) ratios, indicate that the mudstones and shales are mature. Ph/C18 versus Pr/C17 and Pr/Ph versus gammacerane/C30-hopane discrimination diagrams together with U/Th, V/(V + Ni) and Ce/La ratios show that mudstones and shales reflect a reducing depositional environment. N-alkanes of ΣnC21-/ΣnC22 + values and C27-C29 regular sterane distributions suggest that plankton and terrestrial plants are the parent materials of organic matter. In addition, small amounts of extended homohopanes (>C31) and gammacerane combined with Sr/Ba values support a fresh water depositional environment. These conditions have resulted in high quality and widespread distribution of organic mudstones and shale, which are favorable for shale oil exploration.