Shallow-lacustrine sand-rich deltaic depositional cycles and sequence stratigraphy of the Upper Triassic Yanchang Formation, Ordos Basin, China

摘要

Shallow-lacustrine delta deposits of the Upper Triassic Yanchang Formation form the most important petroleum reservoir sandstone units in the Ordos Basin, China. Recent hydrocarbon exploration and outcrop studies demonstrated that shallow-lacustrine sand-rich deltas developed extensively along the gentle flanks and central part of the basin. The vertical succession of facies indicates that the Yanchang Formation records the entire lacustrine life cycle of the Late Triassic Ordos Basin. Four third-order transgressive-regressive cycles and two larger shallow-lacustrine deltaic cycles have been recognized. A process-based depositional model, derived in part from the modern Ganjiang delta in Poyang Lake, China, is used to infer the origins of the sand-rich lacustrine delta deposits. Slow basin subsidence, abundant sediment supply, autocyclic processes and a hydrologically open (overfilled) lake setting were the four main controls on the development of basin-wide sand-rich deltaic sequences. Recognition that inter-connected delta-plain sand bodies lying above the lower sequence boundary of the Yanchang Formation act as important migration pathways for oil, gas and other basin fluids, aids in exploration for stratigraphic and diagenetic traps in the delta plain area.