Abstract
The Lower Cretaceous Baxigai Formation (K]b) is reservoir for large accumulations of hydrocarbons in the western Yingmaili Area of the Tabei Uplift, Tarim Basin, China. Porosity evolution is a vital influence on reservoir quality. In this study, based on core observation, physical properties, casting thin section, cathodoluminescence (CL), scanning electron microscopy (SEM) and grain-size analysis (GSA), combined with the burial history-thermal history of the Baxigai Formation, the porosity evolution of the K1b sandstone reservoir was quantitatively restored. The K1b experienced early stage of slow shallow burial (with burial depths less than 2400 m) and later stage of rapid deep burial, the diagenetic evolution sequence;; chlorite film→slow shallow burial and compaction→early calcite cementation/anhydrite cementation→ econdary quartz enlargement→late rapid deep burial and compaction→dissolution of feldspar and carbonate cement→kaolinite precipitation→late ferriferous calcite cementation. Based on the pore structure characteristics, the porosity contribution of each diagenetic can be quantitatively calculated, to restore the actual porosity evolution history. The results indicate that two types of reservoirs exist in the Kib. Due to the influence of depositional environments and later diagenetic (for type I reservoirs, Φ_0 = 37.38, moderate compaction and weak cementation;; for type II reservoirs, Φ_0 = 35.95, moderate compaction and medium cementation), the current reservoirs have differential distribution characteristics. Effective porosity (greater than 15%) of the reservoirs during the two hydrocarbon accumulation periods promoted the formation of oil and gas reservoirs.
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