Study on Petroleum Accumulation of Carboniferous Carbonate in the Eastern Pre-Caspian Basin

The Pre-Caspian Basin is mainly located in the northwest of Kazakhstan. PetroChina currently has three exploration blocks (Middle Block, T-I Block and T-II Block) in the eastern edge of the basin, and has discovered many oil fields or oil-bearing structural belts. Studying the oil and gas accumulation pattern in the eastern margin of the basin and clarifying the oil and gas distribution law will help guide the next exploration direction. On the basis of previous studies, combined with the latest thin section, seismic, logging and other data, the petroleum geological conditions are comprehensively analyzed, and the oil and gas reservoir forming model in this area is summarized, and the following conclusions are drawn: 1) The “double platform margin” sedimentary model can be established in the study area. The reservoir is mainly composed of bright-crystal bio-granular limestone, as well as micritic grainstone, algal boundstone, microcrystalline-silty dolomite and bioclastic dolomite. The porosity distribution range is 1%–25%. Bright grain limestone and bioclastic dolomite developed in the inner beach of the open platform are favorable reservoirs. 2) Four sets of II or III type marine mudstone and shale source rocks are mainly developed in the Middle Devonian and Lower Permian in the work area at the eastern edge of the basin. Two sets of source-reservoir-cap assemblages are developed in the Carboniferous system, which are Visean mudstone - KT-II carbonate reservoir - MKT mudstone cap, MKT mudstone - KT-I carbonate reservoir - Permian mudstone and salt rock cap. There are three stages of oil and gas filling in the early Carboniferous, early Permian and Jurassic. 3) This area mainly develops carbonate structure-lithologic oil and gas accumulation model, which has the following characteristics: Dual-side supply of hydrocarbons in three phases; beach-phase controlled storage, interface-structure controlled superiority; lateral transport, interface-structure controlled enrichment; multi-phase accumulation with late stage setting. It is speculated that there are oil sources in the west and east and three stages of filling. The Carboniferous KT-II and KT-I carbonate rocks are controlled by the sedimentary facies belt (beach facies), and high-quality reservoirs are mostly developed at the places where the structural fractures are developed. The areas near the unconformity surface are strongly affected by the post-secondary transformation (such as dissolution, dolomitization, etc.). There are multiple stages of reservoir formation in the study area. Due to the structural inversion of high parts in the north of the study area in late Triassic, oil and gas migrated to the study area for secondary migration to form oil reservoirs. Based on the latest geological data, this paper systematically analyses the conditions of oil and gas accumulation in the eastern edge of the basin, summarizes the oil and gas accumulation model in the eastern edge of the Caspian Basin, and provides ideas for the next exploration deployment.