An approach to the palaeokarsts and pool accumulation in the Dengying Formation (Sinian) of the Weiyuan gas field, Sichuan

The Dengying Formation (Sinian) in Weiyuan, Sichuan is mostly built up of algal dolostones deposited in the shallow-marine tidal-flat environments, where the palaeokarsts are well developed. The palaeokarsts may be divided, according to their features, into three types: intraformational, weathering crust and porous palaeokarsts. The intraformational karsts resulted from the karstification produced by the leaching of the uplifted carbonate rocks during deposition and early diagenesis by meteoric water. The weathering crust karsts are associated with the surficial karst-type landform and weathering crust generated by the permenant penetrating and leaching of the carbonate rocks by meteoric water. When carbonate rocks are buried, the interstial water was steadily released with increasing depths, thus resulting in the formation of the porous palaeokarsts. The intraformational karsts and weathering crust karsts at the uppermost parts of Z4¹ and Z4², respectively are believed to be the most important reservoirs in the gas field. In particular, the weathering crust karsts developed as a porous buried hill at the uppermost part of Z4² may facilitate the early accumulation of oil and gas in this region.