The potential of tight oil resource in Jurassic Da’anzhai Formation of the Gongshanmiao oil field, central Sichuan Basin

Abstract：The Da’anzhai reservoir in the Gongshanmiao oil field belongs to tight oil type, and its physical properties are extremely tight with porosity less than 2% and permeability below 0.05mD. The oil reservoir in Da’anzhai extends extensively and has relatively oil enriched places called “sweet spots”. Vertically, the oil layers in Da’anzhai are strongly heterogeneous. The top and bottom segments of the section are composed of thick shelly limestone and are main producing parts in Da’anzhai section; the middle segment composed of thin shelly limestone interbedded with shale is the secondary pay zone. The structure and the thickness of shelly limestone and dark shale are not the main mechanism of oil enrichment. "Sweet spots" are located in the sloping area on the western side of Gongshanmiao. Based on geological information and production data of wells, the authors calculated the recoverable resource using the volumetric method and production decline fitting method, and the result is 3.38 million tons and 1.56 million tons, respectively.