Reservoir geological models of reef complexes in the Middle Ordovician Yijianfang Formation in the Bachu area, Tarim basin, and its implications for hydrocarbon exploration in the Tazhong area, Xinjiang, China

More than 90 reef complexes were found in Member Ⅱ of the Middle Ordovician Yijianfang Formation in the Bachu area, Tarim basin, Xinjiang, by 1∶50000 geological mapping of reef complexes. Reef complexes are large and closely spaced at platform margins and are small and occur sparsely in the platform interior. Based on an analysis of six typical reef complexes, the types, associations, characteristics and organism communities of various microfacies, as well as the growth stages of the reef complexes were studied in detail. Furthermore, based on the identification of plentiful mold thin-sections and study of the porosity, permeability and stable isotopes of the aforesaid typical reef complexes and inter-reef areas, the authors analyzed the characters, origin and heterogeneity of carbonate reservoirs, and two of reservoir geological models were established, one for platform-margin reef complexes and inter-reef areas, and the other for platform-interior reef complexes and inter-reef areas. The reservoir geological models of reef complexes exposed in the Yijianfang Formation have important implications for guiding hydrocarbon exploration in reef complexes of the Upper Ordovician Lianglitag Formation, Tazhong area.