Formation and Distribution of Deep High Quality Reservoirs of Ordovician Yingshan Formation in the Northern Slope of the Tazhong Area in Tarim Basin

Tazhong is one key exploration area in Tarim basin because the Ordovician Yingshan Formation hosts abundant oil and gas resources. There are a large set of platform facies carbonate rocks developed in the Yingshan Fm., dominated by highe-nergy deposition. The Yingshan is buried deeply, but karst reservoir occurs as thick layers distributing in a large area. Early Ordovician fracture Tazhong I resulted in uplift of the Tazhong uplift and was subjected to intense erosion to form a weathering crust at top of the Lower Ordovician Yingshan Fm. The reservoirs is dominated by vuggy and fracture-vuggy type. The paleo-landscape restoration, seismic attribute analysis, seismic acoustic impedance inversion joint logging techniques and methods were employed to identify cave development zone and the fault fracture zone to determine the scope of the distribution of favorable reservoirs. Studies suggest that lithology and petrography is the important basis for development of karst-type reservoir. Interlayer karst controls lamination and distribution scale of reservoir and faulting and fracture are major permeability channel. Hidden karst forms a large amount of available fractured-vuggy space, which greatly improves reservoir properties. The study found that high-quality reservoirs are distributed mainly with the zone between top and 120 m below and form a quasi-layer reservoir with a certain horizontal connectivity linked by faults, structural fissures and dissolution.