Palaeotemperatures and Diagenetic Phases of the Upper Triassic Oil—bearing Sandstones in the Eastern Part of the Ordos Basin

The maximum palaeotemperature of oil–bearing sandstones in the Upper Triassic in the eastern Ordos basin has been determined by using many methods including the vitrinite reflectance, fluid inclusion, apatite fission track, illite crystallinity, chlorite polytype and diagenetic change of authigenic minerals. The thermal gradient in the Late Mesozoic was about 2.9–3.0°C/100 m. The Upper Triassic was in a mature stage of organic matter and hydrocarbon began to be generated and migrated during this period. The palaeotemperatures of oil–bearing sandstones were in the range of 88–110°C; those for the generation and migration of oil ranged from 112 to 122°C. The thickness of the denuded strata overlying the Upper Triassic was 2465–2750 m. The present burial depth of oil–bearing sandstones is generally from 400 to 1200 m. At a depth of ca. 1900 m, the temperature may reach 140°C. Bebw this depth, organic matter was supermature and mainly generated gas.