Lithofacies characteristics and their controlling effects on reservoirs in buried hills of metamorphic rocks: A case study of late Paleozoic units in the Aryskum depression, South Turgay Basin, Kazakhstan

摘要

Metamorphic rock reservoirs are special reservoirs that are quite different from traditional sedimentary rock reservoirs and contain rich hydrocarbon reserves. However, their lithofacies characteristics and reservoir properties are relatively underexplored, and the key factor in reservoirs is lithofacies. Based on core samples, thin sections, whole-rock analysis and well-log and seismic data, we evaluated the lithologies, petrological characteristics and reservoir properties of metamorphic rocks and then explored the main factors controlling reservoir formation and metamorphic rock quality in the Aryskum depression, South Turgay Basin, Kazakhstan. In this area, four types of lithofacies are identified: granitic gneiss (high DEN and CN, low GR), leptite (high GR and CN), gabbro (low GR and CN) and granite (high GR and low CN); among these rocks, granitic gneiss is widely distributed. The reservoir spaces of metamorphic rocks are divided into fractures and pores produced via metamorphism and structural movements. Fractures can be subdivided into structural fractures (SFs), dissolution fractures (DFs), cleavage microfissures (CMfs) and microfissures (Mfs), and pores include dissolved intergranular pores (DInterPs), dissolved intragranular pores (DIntrPs) and vugs (Vgs). Numerous SFs and DFs provide the main reservoir spaces for hydrocarbons. The reservoir quality of metamorphic rocks is primarily controlled by faults, fractures, lithofacies, minerals, dissolution and paleogeomorphology. Lithofacies and minerals are the internal factors that control dissolution and fragmentation, while paleogeomorphology and faults are the external factors that control fracture development. Leptite and granitic gneiss are high-quality reservoir lithofacies, and high paleogeomorphology areas and fault zones are the optimal locations for fractures and pores. Reservoir spaces undergo a series of repeated changes from formation to development, filling and reformation, and a comprehensive reservoir evolution model is proposed to illustrate the distribution of high-quality reservoirs. This study has scientific significance for investigating the reservoir-forming mechanisms of metamorphic rocks and practical applications for exploring and developing oil and gas fields in areas with metamorphic rocks.