A developmental model of lacustrine shale gas genesis: A case from T3y7 shale in the Ordos Basin, China

Exploration for gas producing lacustrine shales in the Ordos Basin has identified considerable capacity in T3y7 shale in the Upper Triassic. The T3y7 shale is a black color, siliclastic organic rich lacustrine mudrock. The mineral components primarily include clay minerals, quartz and feldspar, with the brittle mineral (quartz, feldspar and carbonate) content accounting for 56 wt. %. The presence of type II kerogens, the high TOC contents (most 2%–4%), thermal maturation reaching the oil/gas window and constant organic-rich shale thickness indicate that optimum conditions for shale gas have been reached. The maximum adsorption gas content of T3y7 shale reaches 1 cm3/g at 0.5 MPa, and the value of most samples was more than 2 cm3/g and, at times, even reached 4.6 cm3/g at 5 MPa pressure, which is higher than the main gas shale in the U.S. Organic carbon isotope suggests that the natural gas in T3y7 shale is associated with oil-prone and thermally mature gases. According to the simulation product, three stages can be identified in the shale thermal maturation and hydrocarbon expulsion process: the oil window (0.5% < Ro < 1.1%), wet gas window (1.1% < Ro < 1.8%) and the dry gas window (Ro > 1.8%). These results indicate that the natural gas in T3y7 shale is characterized as wet gas. The relationship between the gas/oil ratio and the conversion degree of organic matter (TRHI) was established in this study. Comprehensive analysis of basin evolution, the development of organic-rich shale and burial history, shale gas – shale oil/gas – shale oil – shale gas was regularly distributed from the bottom to the upper strata. Simultaneously, shale oil – wet gas – shale gas developed from the depression center to the basin slope and, subsequently, to the basin margin. In the lake, the zonation of organic types was obvious, which was manifested as Type I-prone in the semi-deep and deep lake while Type III-prone in the shallow lake. The organic matter of target shale was primarily Type-II. The shale gas of T3y7 shale was mainly pyrolysis yield, which was most associated with oil, and the wet gas appears broadly. Compared with the marine-continent transition and marine shale, T3y7 shale is characterized by a moderate burial depth and thickness, high TOC content, low porosity, high gas content, mostly adsorbed gas and wet gas.