Effects of the matrix on the oil production of supertight limestone in a lacustrine mixed sedimentary environment: The case of the Jurassic Da'anzhai member in the central Sichuan Basin, China

摘要

Lacustrine limestone always develops multiple matrices, and the matrices characteristics and effects on oil production have not been carried out effectively. Although the lacustrine limestone of the Jurassic Da'anzhai Member in the central Sichuan Basin is considered supertight, it still produces relatively large amounts of oil. To explain this yield characteristic, typical matrix samples were analyzed to determine the classification and effects on tight oil production, and identify the geological causes of different matrices. (1) The matrix types can be divided into calcareous matrix, argillaceous matrix, siliceous matrix and mixed matrix, and their micro pore-crack systems are well developed. (2) The argillaceous matrix samples are not as tight as the calcareous matrix samples with relatively low discharge pressure and high maximum mercury saturation. The mercury withdrawal efficiency and NMR mobile water saturation of the calcareous matrix samples are higher than those of the argillaceous matrix samples. The isotherms of the nitrogen adsorption and the adsorption loop indicate a wedge-shaped pore type and a slit-shaped pore type. The nanoscale storage space gradually increases from the calcareous matrix to the siliceous matrix to the mixed matrix and finally the argillaceous matrix. (3) The matrix pore-crack system effectively controls tight oil production. The simultaneous development of multiscale pores and cracks in the limestone matrix is the key factor in tight oil production. (4) Different matrix types are finally formed by the original sediments determined by the lacustrine mixed sedimentary after diagenetic evolution processes. Calcareous, siliceous, argillaceous and mixed matrices are mainly formed in the shell beach core, sandy shoal, shell beach slope, and shell beach edge, respectively. The shell beach edge and shell beach slope are associated with favorable matrix types.