Shale characteristics in the southeastern Ordos Basin, China: Implications for hydrocarbon accumulation conditions and the potential of continental shales

Abstract

Continental shales in China are generally characterised by low thermal maturity and high clay content, characteristics that are significantly different from those of marine shales documented in the USA and elsewhere. Whether such continental shales have commercial hydrocarbon potential as marine shale in the USA is a question for the China petroleum industry. Recently, vertical drills in the lacustrine shales in the Yanchang Formation produced an average of two to five tons of oil and 1000–3000m3 of gas per day by reservoir hydraulic fracturing, which demonstrates that the continental shales can also achieve viable hydrocarbon production. Shale and gas samples collected from the Chang 7 (C7) and Chang 9 (C9) Members, Yanchang Formation in the southeastern Ordos Basin were examined for geochemical, petrologic and gas content analysis. The results show that the C7 and C9 Members developed a huge volume of organic-rich shales (with 2–4wt.% TOC) in the deep or semideep lacustrine, characterised primarily by type-II kerogen, with a relatively low thermal maturity and a vitrinite reflectance ranging from 0.5 to 1.5% Ro, which decreases from west to east. The hydrocarbon product varies with the thermal maturity. In the southwestern corner, the gas content measured by canister desorption equals 1.15–3.49m3/t rock, and the gas-absorption capacity ranges from 3 to 5m3/t rock, whereas in the eastern part, the gas content is low, and oil production is 1–4m3/day on average. Nanometre-scale pores and micro-fractures are well developed. All of this indicates that the continental shales in the southeastern Ordos Basin might have huge shale hydrocarbon resources. However, the clay content of the continental shales of C7 and C9, ranging from 40 to 60% of the bulk mineral content, are much higher than for gas produced in marine shales, which might lead to significant challenges for successful development.