Origin and distribution of Cambrian to Ordovician weathering crust dolostone reservoirs in the Ordos Basin, NW China

Abstract

Recent advances in natural gas exploration have led to notable breakthroughs in weathering crust dolostone reservoirs. The Huaiyuan Epeirogeny, which occurred from the Cambrian to Ordovician periods, contributed to the formation of weathering crust dolostones in the southwestern Ordos Basin, NW China. A combination of core analysis, thin section microscopy, scanning electron microscopy, and conventional and image logs has been employed to characterize the karst features present in these reservoirs. Lithologies are classified based on crystalline size, with pore spaces including isolated pores, cavities, and fractures. Five distinct image log assemblages have been identified to describe the karst features. Vertically, the weathering crust is divided into four zones: the surface karst zone, the vertical vadose zone, the horizontal underflow zone, and the deep slow flow zone. These vertical zonations are predicted using image logs and conventional well logs. The majority of gas-producing intervals are located in the vertical vadose zone. The cavity and fracture systems in the surface zone are distributed locally, while fractures in the vertical vadose zone, especially those connected to vugs, considerably enhance hydrocarbon productivity. Based on these findings, a karstification model for the weathering crust has been developed, offering valuable insights into the karst features of these reservoirs. This research provides a reference for future oil and gas exploration in weathering crust dolostone reservoirs in similar basins.