Storage space, pore–throat structure of igneous rocks and the significance to petroleum accumulation: An example from Junggar Basin, western China

Abstract

Igneous rocks in deep basins are important unconventional petroleum reservoirs. Storage space and pore–throat structure of igneous rocks are crucial to be considered as effective reservoirs. There developed large-scale petroleum reservoirs in the Carboniferous igneous rocks of Hongshan-Zhongguai Bulge in the Junggar Basin, western China. The storage space and pore–throat structures of various types of igneous rocks and their significance to petroleum accumulation were studied based on the analysis of core samples. The methods include macroscopic core observation, microscopic observation under microscopes, scanning electron microscopy, micro-resistivity imaging logging and mercury injection experiments. Results show that the storage space is made up of primary gas pores, secondary dissolution pores and various types of fractures. The fillings of fractures are mainly calcite, followed by asphalt, chlorite and gypsum. The igneous rocks have different pore-throat structures, which govern the reservoir properties. Four types of igneous rock reservoirs were recognized, and the most favorable type is volcanic breccia and part of andesite. Dissolution plays an important role in remodeling the pore-throat structure and storage space. Tuff is the most susceptible to dissolution and present greatest alteration in storage space among the studied igneous rocks. An igneous rock weathering crust with high porosity formed near the unconformity capping the Carboniferous rocks, and oil is most abundant within 150 m across the unconformity surface. Faults and the weathering crust jointly control the formation of igneous rock reservoirs. The fault-controlled reservoirs commonly form in structural high position, and stratigraphic reservoirs generally developed in slope area.