Experiment on microfractures of Mesozoic volcanic rock reservoirs in the Bozhong Sag: A case of andesite reservoir

The Mesozoic volcanic rocks are widely developed in the Bohai Bay Basin, North China. However, the regional oil-gas reserves vary greatly in lateral direction, and many wells frequently failed and exploration is facing high risk. Understanding the development of internal fractures of volcanic rocks can provide a theoretical basis for the exploration of Mesozoic volcanic buried-hill. We selected the Mesozoic andesite to carry out microfractures experiments under the action of tensile stress and compressive stress, in which the development ability and spatial distribution of microfractures in andesite rocks were analyzed. Results show that andesite generated microfractures in both compressive and tensile stress. Many microfractures occur under low compressive stress, and the size of microfractures does not increase with the increase of compressive stress. The reason may be that andesite contains many pre-existing structural fracture, dissolution-assisted fractures, and/or dissolution pores, which are “reactivated” to generate microfractures after small stress. Considering the tectonic evolution of the Bozhong Sag, we believed that andesite had experienced NE compression-shearing in early Yanshanian, extension in middle Yanshanian, dextral compression-shearing in late Yanshanian, and extension in the Himalayan, resulting in relatively well-developed internal fractures and improved the transportation and storage capacity, which have met the basic conditions of forming a high-quality reservoir.