An investigation of subfacies and representative microfacies of the Cretaceous Chishan Group in the Wubao fault zone of Gaoyou sag, Subei basin, China

This study aims to identify favorable oil–gas reservoir facies in the Chishan group of the Wubao fault zone (Gaoyou sag, Subei basin, China) using the methods of outcrops and cores observation, granularity analysis, scanning electron microscope, log data, etc. The results suggest that the Chishan group in the studied area mainly develops desert sedimentary system, and contains five kinds of facies from bottom to top, namely dry salt lake, aeolian sand, intermittent river, fan delta, and salt lake facies; the Chishan group was divided two members according to the lithology and a sedimentary cycle of base-level. The lower member of the Chishan group in the Wubao fault zone contained dry salt lake, intermittent river, and aeolian sand facies. The upper member of the Chishan group in the Chenbao area consisted mainly of aeolian sand. The southwestern Zhousong area contained (from east to west) a succession of aeolian sand, intermittent river, fan delta, and salt lake facies. Among these facies, aeolian sand was divided into aeolian sand dune, interdune, and aeolian sand sheet three sub-lithofacies. In the aeolian sand dune, the sand dune was a typical micro-lithofacies and had a scattered distribution. The fan delta had a bead-like distribution along the main fault, in which mainly underwater distributary channel and sand bank were developed. The sand bodies of the sand dune, underwater distributary channel, and sand bank were all well developed, and had average porosity values of >20%, meaning that they were favorable oil–gas reservoirs. The interdune sediments were fine-grained and had low porosity and permeability. Hence the reservoir properties of the interdune sediments were poor, and they could represent either fluid interlayers of reservoirs or source beds.