The exposed and unexposed shoals and their bearings on the carbonate reservoirs in the Feixianguan Formation in the northeastern part of the Sichuan Basin

The shoal deposits crop out over a wide area of the Feixianguan Formation in the northeastern part of the Sichuan Basin. The palaeogeographic settings and sea-level changes during the shoal formation are interpreted to be major controls on the differences of lithology and reservoir quality in different parts of the basin. The shoals in the Guangyuan-Tongjiang-Nanjiang-Bazhong zone are confined to the third member of the Lower (Triassic) Feixianguan Formation, and formed during the rising phases of sea level. The sediments were always drowned by sea water, and even never cropped out on the surface. The lithologies contain grey oosparite and sparry calcarenite completely filled by calcite crystals. These rocks are characterized by low porosity and permeability and poor reservoir quality. The shoals in the Daxian-Xuanhan zone appear in the second member of the Feixianguan Formation. The sedimentary basement is represented by a subaqueous rise created during the falling phases of sea level. The sediments commonly cropped out in the supratidal zone have been subjected to the dolomitization and dissolution of meteoric water. After burial, the rocks have also been subjected to the (recrystallization) and further dissolution. The lithologies include dolarenite, porous dolostone and residual oolitic dolostone intercalated with minor amount of limestone and oolitic limestone, which are characterized by abundant intercrystal porosity, interparticle porosity and dissolution openings, high porosity, good permeability and reservoir quality. This kind of reservoir rocks in the second member of the Feixianguan Formation in the Daxian-(Xuanhan) zone may provide a good example for the future petroleum exploration.