Reservoir-forming conditions of Ordovician weathering crust in the Maigaiti slope, Tarim Basin, NW China

摘要

Based on comprehensive geophysics and geology data, the characteristics and reservoir controlling factors, as well as oil and gas distribution law and exploration direction, are analyzed for the Ordovician weathered crust in the Maigaiti slope and its periphery with paleo-uplift background in the Tarim Basin. Structural mapping and analysis reveal that the largest carbonate weathering crust is developed around the Maigaiti slope which forms a favorable reservoir-seal assemblage in combination with the overlying Silurian/Carboniferous mudstone. The Ordovician carbonates experienced multi-stages of karstification and are dominated by small-size fracture-vug reservoirs which are controlled by the karst paleogeomorphology, tectonism and lithology. Drilled wells are located mainly in the northern karst depression which has weak karstification and heavy filling. Paleogeomorphology reconstruction and seismic reservoir prediction suggest that reservoirs are well developed in the southern karst slope. The comprehensive analysis of structural revolution and hydrocarbon generation history demonstrates that the hydrocarbon migration and accumulation are controlled by the evolution and migration of the paleo-uplift. There are two charging stages: large scale oil accumulation in the late Hercynian period and adjustment and re-accumulation of oil cracked gas in the late Himalayan period. The Manan and Maixi slopes in the eastern and western sides of the paleo-uplift are the favorable areas for hydrocarbon accumulation and adjustment over a long period of time, have the conditions of forming large-scale carbonate fracture-vug type pools, and are important strategic relay exploration areas in the Tarim Basin.