Abstract:

Xujiaweizi fault depression is a Late Jurassic-Early Cretaceous deep half-graben type extensional fault depression in northern Songliao basin, and volcanic rocks developed. The article is mainly based on macro and micro studies on outcrops, cores and thin sections. Tectonic fractures and corrosion fractures are well developed, and show apparent regularity. Most of primary fractures are transformed into secondary fractures by later tectonic stress or dissolution. Fractures formation are controlled by several factors, for instance, regional tectonic stress and its evolution, structure, lithology, lithofacies, dissolution, weathering. Among these, tectonic stress field evolution, lithology and lithofacies, weathering and dissolution are dominant factors. Most of tectonic fractures are characterized by high angle, tensional and torsional, part filled or unfilled, multi-period, and multi-directional, which is the results of the superposition of various geological and tectonic role from Late Jurassic to Neogene. Local development of fractures are controlled by local stress field distribution. In general, since obvious stress concentration, fault systems are fracture development zone. Local stress are especially concentrated in the hanging wall of normal fault, the end of normal and reverse fault, and anticline axis, where tectonic fractures are abundant. The effective hydrocarbon reservoirs in volcanic rocks are controlled by combination of primary pore spaces and structural fractures. The fracture development shows vertical cyclicity because of multi-eruption, weathering and unconformable surface. The dissolution fractures develop near unconformity. In plane, the fractures mainly developed in fault-concentrated area, faults intersection, anticline and syncline. Pores and fractures is the most developed in explosive and effusive facies, especially in effusive rhyolite. The rhyolite is the most advantageous reservoir and high yield gas production area.

Key words: volcanic rocks, fracture, tectonic stress field, volcanic facies, formation mechanism, Xujiaweizi fault depression, Songliao basin