Formation Mechanism of Deeply Buried High-quality Reservoir: Example from Clastic Reservoir of Sangonghe Formation in Fudong Slope Zone, Junggar Basin

A high-quality clastic reservoir exists at the depth range of 3 600~4 300m in Sangonghe Formation of Fudong slope zone, Junggar Basin, having an average porosity of 12.7% and average permeability of 12.2mD. In order to explore its formation mechanism, 1 232 reservoir samples have been tested and analyzed through core observation, thin identification, X-ray diffraction and scanning electron microscope, and then their sedimentary environment, rock and pore types have been determined. And the geochemical characteristics, organic matter types and maturity of 13 source rock samples have been analyzed by GC-MS method. The results reveal that the reservoir of the research area is developed in the braided river delta-lake sedimentary system, which mainly consists of braided river delta frontier and shore-shallow lake subfacies. The lithology is mainly composed of medium sandstone, coarse sandstone and fine sandstone, which mostly belong to feldspar sandstone and feldspar lithic sandstone with low texture maturity and component maturity. Pore types primarily contain intergranular pore, intergranular solution pore and solution pore in grains. The kerogen of source rocks belongs to type Ⅱ₂-Ⅲ and has a wide range of maturity, which produces abundance of acetic acid and oxalic acid. Dissolution is the main cause for the development of deep high-quality reservoirs the pyrolysis. The organic matter in source rock released organic acid to corrode feldspar particles through fractures and unconformities into reservoir, and then the secondary pores was formed, which improves the reservoir properties. The kaolinite, served as the dissolution product, was developed mostly in deep high-quality reservoirs, providing a favorable evidence for the mechanism of dissolution.