Factors controlling tight oil and gas reservoir development in the Jurassic siliciclastic-carbonate rocks in Sichuan Basin, China

Abstract

The prediction of “sweet spot” of multi-lithology composite tight oil and gas reservoirs within mixed siliciclastic-carbonate sequences is a hot topic in oil and gas exploration. There are mixed lacustrine carbonate-siliciclastic rocks in the Da'anzhai Member of Ziliujing Formation in the Eastern Slope of western Sichuan Depression. The reservoir lithology is complex, and the main factors controlling the development of high-quality reservoirs are yet to be known. Based on a large number of drilling, core and thin section observation, well log and seismic data, this paper systematically studies the characteristics and factors controlling the development of the mixed siliciclastic-carbonate reservoir. The results show that the reservoir is composed primarily of coquina, sandstone, breccia and shale in lithology, and its development is mainly controlled by sedimentary microfacies, diagenesis, tectonism (fracturing) and paleogeomorphology during the sedimentary period. The effective fractures in reservoirs of the low-energy shell shoal facies are well developed with relatively good physical properties. With the change of sedimentary microfacies from low-energy shoal to high-energy shoal to arenaceous shoal, the hydrocarbon production capacity worsens step by step. Diagenesis, such as dissolution and fracturing, has a certain effect on reservoir physical properties. The palaeogeomorphic highs and slopes had well-developed fractures during the sedimentary period which are conducive to constructive dissolution, and thus they are favorable zones for reservoir development. The development of structural fractures further amplifies the influence of dissolution; thus the fracture zones are also favorable for the development of high-quality reservoirs. The favorable sedimentary facies zone and fracture development degree serve as the core factors for the formation of high-quality mixed reservoirs. The muddy limestone with certain dissolved pores and structural fractures deposited under the low-energy shell shoal setting is of relatively high-quality reservoir in the Da'anzhai Member.