Abstract:

Many deep wells in the Bozhong sag reveal oil and gas reservoirs in Mesozoic volcanic rocks, with most well sections having unsatisfactory production capacity and a few wells having higher production capacity. Reservoir heterogeneity is a core issue that restricts exploration. By means of casting thin sections, scanning electron microscopy, geophysics, and physical simulation experiments, the formation mechanism of Mesozoic volcanic rock reservoirs in the Bozhong sag was studied. The results indicate that the degree of development of structural fractures determines the differences in reservoir physical properties. Fractured reservoirs with a majority of structural fractures can break through the constraints of conventional volcanic edifices, lithology, and other primary factors, forming large-scale high-quality reservoirs. The development degree of structural fractures is influenced by the strike-slip activity during the Himalayan period and the pre-existing faults in the basement. The tensile faults formed by strike-slip during the Himalayan period mainly control the development of fractures in the reservoir section within 200 m of the top of the volcanic rock buried hill. The upward activation and connection of the pre-existing basement faults play an important role in controlling the development of fractures in the entire volcanic rock body. The physical simulation experiment shows that the single strike-slip shear zone has a narrow range of fractures, which are only concentrated near the fracture zone, but the right lateral left step strike-slip transition overlap area formed by the segmented growth of multiple strike-slip faults has large compressive torsional deformation and a wide range, which can form large-scale fractures; Under the background of forward compression, the strength of rock fracture is mainly limited to near fault zones, with a smaller scale. Based on this, this article proposes for the first time that the Himalayan period is a critical period for the development of structural fractures in volcanic rocks. The strike-slip overlay area of the Bozhong sag during the Himalayan period is a favorable zone for searching for large-scale fractured reservoirs.

Key words: Bozhong sag, Mesozoic, volcanics, structural fracture, pre-existing fracture, strike-slip overlap