Classification of Complex Reservoirs in Superimposed Basins of Western China

Abstract:Many of the sedimentary basins in western China were formed through the superposition and compounding of at least two previously developed sedimentary basins and in general they can be termed as complex superimposed basins. The distinct differences between these basins and monotype basins are their discontinuous stratigraphic sedimentation, stratigraphic structure and stratigraphic stress‐strain action over geological history. Based on the correlation of chronological age on structural sections, superimposed basins can be divided into five types in this study: (1) continuous sedimentation type superimposed basins, (2) middle and late stratigraphic superimposed basins, (3) early and late stratigraphic superimposed basins, (4) early and middle stratigraphic superimposed basins, and (5) long‐term exposed superimposed basins. Multiple source‐reservoir‐caprock assemblages have developed in such basins. In addition, multi‐stage hydrocarbon generation and expulsion, multiple sources, polycyclic hydrocarbon accumulation and multiple‐type hydrocarbon reservoirs adjustment, reformation and destruction have occurred in these basins. The complex reservoirs that have been discovered widely in the superimposed basins to date have remarkably different geologic features from primary reservoirs, and the root causes of this are folding, denudation and the fracture effect caused by multiphase tectonic events in the superimposed basins as well as associated seepage, diffusion, spilling, oxidation, degradation and cracking. Based on their genesis characteristics, complex reservoirs are divided into five categories: (1) primary reservoirs, (2) trap adjustment type reservoirs, (3) component variant reservoirs, (4) phase conversion type reservoirs and (5) scale‐reformed reservoirs.