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The Controlling Factors of Fractured Limestone Reservoir-Forming of Upper Cretaceous in North Kairouan Region, Pelagian Basin, North Africa

Deng, Yilin *1; Yang, Chunlin 2
(1) Research Institute of Petroleum Exploration & Development-Northwest (NWGI), PetroChina, Lanzhou, China.
(2) China National Oil and Gas Exploration and Development Corporation, Beijing, China.

The Pelagian Basin in North Africa is one of the important oil and gas basins. So far, forty exploration wells are drilled among the twenty structures in North Kairouan Region. There are signs of oil and gas in most exploration wells, but commercial reservoir is discovered only in one structure, which definitely demonstrates that the forming conditions of the oil and gas reservoir in this region are complicated. Based on the seismic, drilling, analytic and testing data, the comprehensive analysis of the forming conditions of the Upper Cretaceous fractured limestone reservoir reveals the following points: the formation of the oil and gas reservoir in this region is mainly controlled by oil resource conditions, the strike and abundance of the fracture and fault, as well as the distribition of effective transport layer and thick cap rock; the structural areas developed northwest faults & fractures, composite passage system and thick cap rock are favorable areas of hydrocarbon migration and accumulation. Such viewpoints are based on the following factors: The main structures in the studied area can’t generate hydrocarbon because they are located above the hydrocarbon-generation threshold, so no oil and gas can accumulate in the trap unless the hydrocarbon migrates from the oil generation center to the upper part of the structure. In addition, the conditions of long distance transportation of hydrocarbon are not available. Affected by the Northern Atlas fold belts, the studied area suffered from the northwest compression stress since late Cretaceous. The faults and fractures developing in the direction of northeast are closed, which is unfavorable for the hydrocarbon migration and accumulation. However, the faults and fractures developing in the direction of northeast are unsealed, which is beneficial for the lateral and vertical migration of the hydrocarbon and for the improvement of the limestone reservoir property. The movement of salt in the late Palaeocene makes the mudstone of the Haria formation, which is the regional covering strata, partially eroded, resulting in the maldistribution and the change of the thickness of the cap rock in the region, which can greatly influence the height of hydrocarbon column of reservoir that can be sealed.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California