The genesis and prediction of dolomite reservoir in reef-shoal of Changxing Formation-Feixianguan Formation in Sichuan Basin

Abstract

In the past 20 years, great exploration achievements and new progresses have been made in Permian Changxing Formation(abbreviated as C.F.) and Triassic Feixianguan Formation(abbreviated as F.F.)in Sichuan Basin. The main results and progress include: (1)several gas fields, such as Puguang gasfield, Longgang gasfield, and so on are found; (2)C.F. and F.F. contain 3 third order sequence; (3)the reservoir mainly develop in reef shoal microfacies of platform margin; (4)the gasfields are lithologic pools distributed along platform margin. Although many common views have been achieved, the divergence of reservoir formation is still remained. Based on the study of petrology and diagenesis combined with the analysis datas, the basic reservoir characteristics, pore types and their genesis, dolomization and its influence of the C.F. and F.F. have been clarified in the paper. It is pointed out that the main lithology of the reef shoal dolomite reservoir in C. F. is residual bioclastic dolostone, residual reef framework dolostone and fine-medium crystalline dolostone. The reservoir spaces are dissloved pores and vugs,which formation is closely related to early meteoric water dissolution and shallow burial dolomization. The main lithology of oolitic dolomite reservoir in F.F. is residual oolitic micrite dolostone, residual oolitic silty-fine dolostone and fine crystal dolostone. The storege is mainly intergranular pore, oolitic moldic and their enlarged pore. The mineral composition of oolitic, early freshwater dissolution and dolomization are the main controlling factors of reservoir formation. It is predicted that the platform edge is the favorable distribution area of reef shoal and oolitic shoal dolomite reservoir.