Paleo-weathering crust at the top of Sinian Dengying Formation in northern Guizhou and its petroleum exploration significance

Abstract：The paleo-weathering crust is important for karst reservoir formation and hydrocarbon accumulation. Field geological survey reveals that there exists a paleo-weathering crust at the top of Sinian Dengying Formation in northern Guizhou, which was formed by Tongwan tectonic movement and is composed of yellowish brown scoriaceous clay rock. Its geochemistry is characterized by depletion of MgO and relative enrichment of SiO2, Al2O3, Fe2O3, Fe2O, P2O5 and TiO2. The paleo-weathering crust is somewhat different in different facies, which results in the difference of underlying karst reservoirs. The platform edge shallow facie was uplifted strongly, and its primary pores were favorable for the meteorological water eluviation, and also favorable for karstification to improve the dolomite reservoir, thus forming lots of reservoir spaces, such as?cavern, dissolved seam, intragranular pore and intergranular pore. On the contrary, the other facies were affected relatively weakly by karstification，and the reservoir quality was unfavorable. It is concluded that the buried paleo-weathering crust of the shallow facie at the top of Dengying Formation may become an important exploration area.