Reservoir depositional system analysis of Ordovician carbonate platform margin in Yijianfang outcrops of western Tarim Basin
塔里木盆地西部一间房露头区奥陶系台缘储层沉积体系分析

High-quality reservoir phase belts prediction of carbonate platform margin is the focus that petroleum explorationists have always been concerning. Fine structure analysis and reservoir property study of outcrop depositional system is the key to summarizing prediction models. Based on detailed field geological investigation, lateral tracing and correlation of lithosomic bodies, profile survey about typical depositional systems and systematic sampling and lab analysis, the composition of genetic facies and reservoir property have been studied about platform margin reef-shoal depositional system of Yijianfang Formation in the Middle Ordovician and platform margin slope-slope fan depositional system of Lianglitake Formation in the Upper Ordovician in Yijianfang outcrop area of western Tarim Basin. The study suggested that platform margin depositional system had complex internal structure, whose genetic facies have relatively fixed space framework and different reservoir properties. Reef-shoal depositional system always developed on the platform margin near the carbonate platform, which consists of reef, bioclastic beach and background deposit of platform margin. The bioclastic beach developed around reefs, in front of which was special gravel beach. From inside to outside of bioclastic beach, the sediment size becomes finer, sorting gets worse, bioclast content decreases and calcsparite cement reduces. But on the platform margin near the basin, slope-slope fan depositional system often developed, which mainly consists of slope, slope fan, fan channel and slip block. Slope fans were usually cut by fan channels and small slope fans develop around the slip block. From proximal part to distal part of the slope fans and fan channels, sediments gradually changed from rudaceous to arenaceous. In the above-mentioned two kinds of platform margin depositional systems, the reef-front interior arenaceous beaches, reef-back interior arenaceous beaches, and distal arenaceous slope fans have good reservoir quality. In comparison, the potential reservoir quality of reef-shoal depositional system is better than slope-slope fan depositional system on the platform margin overall.