Reservoir controlling factors and geological prediction models of the 4th member of the Xujiahe Formation in Xiaoquan-Xinchang area, the western Sichuan Basin

A study is carried out on the controlling factors and geological prediction models in the 4th member of the Xujiahe Formation in Xiaoquan-Xinchang area, western Sichuan Basin. Technical methods or tools such as thin section observation, SEM and cathodoluminescence spectroscope had been employed and data like reservoir physical parameters, sedimentary facies and so on had been utilized. The results show that the reservoirs in the formation are mainly sandstones with high lithic content, such as lithiarenite, sublitharenite and calcarenaceous sandstone. These sandstones have high compositional maturity and medium textural maturity. Reservoir space is mainly intergranular dissolved pore, intragranular dissolved pore and moldic pore. The reservoir are mainly of porous type and fractured-porous type with poor reservoir quality, and are typical tight sandstone reservoirs. Compaction and cementation caused densification of the sandstone while dissolution and fracturing played key roles in the formation of reservoirs with relatively good quality. Factor as depositional microfacies, the position of sandstone in the Paleostructures during early burial process and dissolution period, and fracture growth (mainly at the lower section) are considered to control the formation of reservoirs with relatively good quality in the 4thmember of the Xujiahe Formation. Hence, two geological prediction models are established for the upper and lower section of the 4th member respectively: they are delta front distributary channel microfacies and/or mouth bar microfacies plus paleostructural high during early burial process, sandy braided distributary channel microfacies and/or mouth bar microfacies plus paleostructural high during dissolution period plus fracture zone.