Hydrocarbon migration in fracture-cave systems of carbonate reservoirs under tectonic stresses: A modeling study

The Tahe oilfield, located in the southwest of the Akekule nosing structure, northern Tarim basin, was the most prolific oilfield targeting at the Ordovician carbonate reservoirs in China. The reservoir space was dominant with fracture-cave systems commonly induced by tectonics and karstification. Although hydrocarbon production had proceeded for two decades in the Tahe oilfiled, the control of oil and gas accumulations was still doubtful. In this work, the periodic fluid flow induced by cyclic tectonic stresses was proposed as the mechanism of hydrocarbon migration in the fracture-cave systems of carbonate reservoirs. The fracture networks formed conduits for fluid flow, and the fluid pressure in caves transmitted from stress field provided the driving force. The constitutive equations were established among stresses, fracture densities and flow velocities. Four quasi-3D geological models were constructed to simulate the flow velocities on the Ordovician surface of Akekule nosing structure in the critical tectonic stages. The simulated results supplied indicative information on oil and gas migration and accumulation in the tectonic stages. Combining with the oil and gas charge history, a conceptual model was built to reveal the multi-stage oil and gas charge and accumulation in the Ordovician of Akekule nosing structure.