Key geological controls on migration and accumulation for hydrocarbons derived from mature source rocks in Qaidam Basin

Abstract

The mature oil/gas source rocks exist mainly in Jurassic strata in the northwest and in Tertiary strata in the west of Qaidam Basin. Oil and gas generated from the mature hydrocarbon source rocks in the Jurassic and Tertiary mainly accumulate in Tertiary formations in the Qaidam Basin. The main sedimentary facies in Tertiary formations include alluvial plain, lakeshore, shallow lake, semi-deep lake, alluvial fans, fan delta and sublacustrine fan. However, there were no fluvial facies. Most of the alluvial plain facies and lakeshore facies are very narrow and they become wide only where basin-bounding faults are present. Most parts of the basin are filled with shallow lake sediments such as pelitic siltstone and silty sandstone. The deposition systems have determined the hydrocarbon migration and accumulation patterns in Tertiary formations in different parts of the basin. In the inner basin where continuous permeable formations are absent, faults and fractures are the main pathways. Fractured rocks are the main reservoir types. Along the basin margins where continuous permeable formations occur in abundance, faults and fractures are also the main pathways. Therefore, the faults and fractures are very important for the hydrocarbon migration and accumulation. Four migration and accumulation models have been developed based on this study: (1) Migration and accumulation model through thrust faults. This model is mainly for migration and accumulation at the basin margin. The mudrock associated with the major fault is about 10 m thick so that there exist the hanging wall and footwall migration and accumulation zones. Where the major fault extends into source rocks areas, hydrocarbon migrated to and accumulated in the reservoir in the hanging wall. Otherwise, the faults just seal the reservoir in the footwall. (2) Migration and accumulation model through combined syn-depositional reverse faults and strike-slip faults. This model is also mainly for migration and accumulation at basin margins in which the strike-slip faults control different kinds of sedimentary fans and connections with hydrocarbon source areas; they are very suitable for hydrocarbon accumulation. (3) Migration and accumulation model through recoil faults. This model is mainly for migration and accumulation in fractured reservoirs in the inner basin region. (4) Migration and accumulation model through superficial thrust fault and lower strike-slip fault complex. This is mainly for migration and accumulation in the north of Qaidam Basin where the reservoirs are at the footwall of the superficial thrust faults.