Controlling factors for effective reservoir development of Ordovician Kelimoli Formation in western Ordos Basin

Abstract

In recent years, some progress has been made in the exploration of Ordovician natural gas in the western Ordos Basin, among which, the Ordovician Kelimoli Formation has the highest degree of natural gas accumulation. However, the controlling factors for effective reservoir development are complicated, leading to unsatisfactory exploration effect. Based on the data of core observation, thin section identification, drilling and logging, the development types and controlling factors of effective reservoir in the Kelimoli Formation were systematically studied, and a development model of effective reservoir was established. The results show that the effective reservoir types of Kelimoli Formation in the western Ordos Basin mainly include dolomite intercrystal (dissolved) pore, limestone karst fracture and structural micro-fracture, and marginal platform granular pore type reservoir is locally developed. The development of effective reservoir was mainly controlled by sedimentary facies, diagenesis, paleokarst weathering and tectonic movement. The tidal flat facies and marginal platform facies provided the material basis for the original reservoir of the Kelimoli Formation, and on this basis, the dolomite intercrystal (dissolved) pore and marginal platform granular pore reservoirs were formed in the early diagenesis. In the stage of burial diagenesis, recrystallization obviously reformed the original dolomite reservoir, and the crystal framework became more stable. New intercrystal (dissolved) pores were formed or underwent secondary recrystallization. At the same time, the original structure of marginal platform granular pores was damaged, filled or improved, and the present marginal platform residual granular pores were formed. Paleo-karst weathering controlled the development of fracture-vuggy reservoirs in the middle and upper parts of the Kelimoli Formation, and high-yield wells were mainly drilled in the upper groove direction of the karst slope groove zone and karst highlands. The multi-stage tectonic movement continuously transformed and controlled the development of micro-fracture system, forming porous media reservoirs distributed around the fault zone. Therefore, it is a favorable direction for further exploration to find favorable facies belts of tidal flat facies and marginal platform facies, updip positions of karst paleo-geomorphology or high positions of fault zone.

Keywords

• effective reservoir type
• paleokarst
• weathering
• diagenesis
• kelimoli formation
• ordovician
• western ordos basin