Characteristics and controlling factors of the dolostone reservoirs in the Buqu Formation, southern Qiangtang depression

The dolostone reservoirs are interpreted as the "sweet spot" reservoirs in the Buqu Formation, southern Qiangtang depression. The study of the controlling factors of the dolostone reservoirs may be helpful to the prediction of the favourable exploration areas. The results of research in field investigation and thin section examination in this study have disclosed that the dolostone reservoirs in the Buqu Formation, southern Qiangtang depression are mostly developed in the intraplatform shoals of the carbonate platforms. The restricted waters in the intershoal sags in the later stages of the highstand systems tract may provide dolomitized fluids. The dolostone reservoirs have well-developed intracrystal solution openings, intercrystal pores, intercrystal solution openings and dissolved pore spaces. The controlling factors are represented by depositional, diagenetic and tectonic processes. The interconnected pores are well preserved in the grainstones widespread in the intraplatform shoals, and thus may be served as the migration passages for the reworking of fluids. The dolomitization during the shallow burial diagenesis may be most favourable for the development of the reservoir spaces of the dolostones. The tectonic compression during the second episode of the Yanshanian movement may provide the thermal fluids for the generation of the saddle dolomite. The acidic fluids produced by the destruction of the fossil oil reservoirs and meteoric fresh water may play an important part in the dissolution and reworking of the dolostone reservoirs. The key factors include dolomitization in the shallow burial diagenesis and organic acidic dissolution before the first hydrocarbon generation.