Characteristics and controlling factors of beach-controlled karst reservoirs in Cambrian Longwangmiao Formation, Moxi-Gaoshiti area, Sichuan Basin, NW China

Abstract

Based on data from boreholes, cores and lab analysis, the characteristics, genesis and controlling factors of different types of reservoirs in Cambrian Longwangmiao Formation, Moxi-Gaoshiti area, Sichuan Basin, are examined, and the distribution of favorable reservoir zones is predicted. The reservoirs can be subdivided into four types according to the different types of reservoir space and their combination with the “piebald” karst system: pinhole, “piebald” pinhole, cave, and “piebald” cave. Among them, the “piebald” cave reservoir is the best in quality, followed by the cave and “piebald” pinhole reservoirs, and the pinhole reservoir is the worst in quality. The genesis and controlling factors of Longwangmiao Formation reservoir are that the regional shoal deposition gave rise to a large area of grain dolomite, the layers with intergranular pores and small amount of intragranular dissolution pores of shoal facies provide a material base for later karst reformation. During the Caledonian period, the karst water flowing and corroding along the porous bed formed previously played a key role in the formation of premium reservoirs. During the period of the Caledonian-Hercynian, the tectonic paleogeomorphology controlled the fluid potential of karst water, which in turn decided the development of reservoirs. Karst is most developed on the slope of the paleotopography (along the well line of Moxi 201-Moxi 9-Moxi 12), where “piebald” cave or cave reservoirs usually occur, which are the most favorable reservoir zones. Key words: grain dolomite, reservoir characteristics, reservoir genesis, “piebald” karst system, Cambrian Longwangmiao Formation, Sichuan Basin, Moxi-Gaoshiti area