Types, characteristics, origin and exploration significance of reef-shoal reservoirs: A case study of Tarim Basin, NW China and Sichuan Basin, SW China

Taking reef-shoal bodies developed in multiple formations in the Tarim and Sichuan Basins as examples, this paper examines the origin of pores and heterogeneity of different types of reef-shoal reservoirs, and tries to find out the distribution rules of various reef-shoal reservoirs. Reef-shoal reservoirs can be classified into two main types, i.e. biohermal reservoirs and grain shoal reservoirs. The former, related to reef framework, mostly in the form of frilled platform margins, can be subdivided into two categories; interior platform reef complex and margin reef complex (sedimentary or tectonic origin). The latter, unrelated to the reef framework, is controlled by the fluctuation of sea level on gentle slope carbonate platforms, are widely developed on carbonate ramps due to the migration of shoals, and can be subdivided into interior platform shoals and platform-margin shoals on steep slopes. Reservoir space within reef-shoal carbonates was primarily formed during syngenetic and burial stages. Syngenetic porosity is controlled by the original rock fabric and dissolution related to the exposure of sequence boundaries, while porosity development and distribution during the burial stage is closely related to that in the syngenetic stage, showing inherited features. Reef-shoal reservoirs are strongly heterogeneous, and complicated with respect to effective reservoir distribution. Platform margin reefs and biohermal shoals coexisting with reefs on the top of shallowing upward sequences are the main reservoirs and rich in porosity. The grain shoals deposited on top of shallowing upward cycles of isolated reefs are also the main zones rich in pores, and usually interbed with tight reservoirs. Overall, grain shoal reservoirs are larger in scale and better in physical properties than biohermal reservoirs.