Abstract:

The Yingshan Formation is one of the most important oil and gas reservoirs for Tarim Basin due to the karstification developed in upper part of the formation. However, the detailed characteristics of reservoir can not be precisely unraveled on the basis of limited well data. The Xiker outcrop area, located in the west of Tarim Basin, has undergone multiple tectonic movements since Ordovician. Karstification developed at these tectonic unconformities. Therefore, the exposed Yingshan Formation shows varied paleokarst characteristics. By field and thin-section observation and description, diverse karst-characteristics have been identified, including large caves, cave breccias, vugs, fractures and dissolution channels. Caves and breccias, produced by intense dissolution and collapse, can directly form reservoir. Dissolution vugs are widely developed and usually generated with dissolution channels and fractures. Multiple tectonic fractures can significantly increase the permeability of the karst reservoir. Combining with the lithofacies of the Yingshan Formation, the grain-dominated limestones have greater density of dissolution vugs than lime mud dominated limestones have. In addition, dissolution channels and tectonic fractures are more likely to be generated in limestones with higher grains ratio. As a result, the high-quality fracture reservoir and compound reservoir with vug, cave and fracture are produced.

Key words: unconformity paleokarst, paleokarst reservoir, carbonate reservoir, Yingshan Formation, Xiker, Tarim Basin