Karst landforms and reservoir characteristics in the Ordovician strata of Lungu 7,Tabei uplift

The Tabei uplift of the Tarim basin is one of the three largest ancient uplifts. Lungu7 well block is located in the northern uplift in Lunnan buried hill in the primary geomorphic unit karst highland, high area of Lunnan low raise. It is rich in oil and gas resources, and the Ordovician carbonates are one of the richest oil-bearing series, and one of the main formations for exploration and development. The different geomorphic units control the ancient hydrodynamic conditions, and further determine the diversity of karst development. Therefore, the precise shape of the karst paleogeomorphology is of great significance for the reasonable arrangement of high production and efficient wells. This study uses the relative residual thickness method to characterize ancient karst landforms and ancient hydrodynamic conditions. Two types of secondary geomorphic unit are classified, karst highland and karst slope. Furthermore, six types of tertiary geomorphic unit are classified after the precise description of the secondary geomorphic unit on the basis of combinations of micro-geomorphologic landforms. Through further research on the karst reservoir distribution pattern, this study considered that the reservoir volume is mainly comprised of secondary solution pores, corrosion cracks and cave systems, formed by later structure and karstification. According to the combined characteristics of the holes, caves and seams, as well as the relationship between fracture porosity and hole porosity, the Ordovician Yingshan karst reservoirs are divided into four types, fracture, hole, crack-hole, and cave. Compared to karst slopes, karst highlands more easily preserve karst reservoir space due to stronger denudation. The karst slope zone easily forms a large fracture-cave cube because of it strong hydrodynamic conditions, well-developed karst piping system and good connectivity. It can be treated as karst cave reservoir, which provides the basis for future exploration and development.