Sequence stratigraphy, sedimentary systems and implications for hydrocarbon exploration in the Qaidam Basin, Northern China

Abstract The Qaidam Basin is endowed with abundant Tertiary hydrocarbon resources and possesses major exploration potential. Lacking an overall understanding on sequence stratigraphy and sedimentary facies of the basin is not favorable for making new progress in hydrocarbon exploration. In this study, using outcrop, core, well logging and seismic data, the Tertiary strata of the Qaidam Basin was divided into 7 second-order sequences, 12 third-order sequences and 24 system tracts. In addition, 7 sedimentary facies types were identified, i.e., diluvial fan, braided river, flood plain, fan delta, braided river delta, lacustrine deposit and turbidite fan. Six single sedimentary factors were selected for analysis, and with third-order sequences as the unit, the distribution and evolution of sedimentary systems were defined. For the controlling factors of lithofacies paleogeography, “snow mountains controlling lakes” and “fault steps control basins” were proposed. It is believed that under a compressional tectonic setting, lake level fluctuation is closely related to elevations of peripheral mountains, and lake water is sourced principally from mountain snowmelt. A higher structural uplifting amplitude will result in a larger area of mountain snow, a greater water supply and a larger lake area. Boundary faults of the basin retreated towards the mountains in a step way, resulting in an expanded basin. However, the faults advancing towards the center of the basin aroused basin shrinkage. From a sedimentary view, the distribution characteristics of Tertiary source rocks and reservoirs of the Qaidam Basin were analyzed; it is proposed that the periphery of the hydrocarbon generation center of the Shizigou–Mangya sag is the present-day optimum exploration zone, and the periphery of the hydrocarbon generation center of the Yiliping depression center is a potential favorable exploration zone.