Identification of fracture-vug complex from karsted carbonates and its significance in petroleum geology

Key words:  carbonate  karstification  reservoir space  fracture-vug complex  Tahe Oilfield

Abstract:

Medium to large scale fractures and caves are important reservoir spaces in karsted oilfields, which has been proven by petroleum exploration and production in Tahe Oilfield. However, a great number of small vugs or caves distributed along fractures have been found in outcrops of the Ordovician carbonates and the cores from Tahe Oilfield. These small caves are connected by fractures to form very good hydrocarbon storages and seepage channels, which are referred to as fracture-vug complex. By investigations on outcrops, drilling-cores and well-loggings, our study shows that the fracture-vug complexes are distributed widely in karsted carbonates including surface karst zone, vadose karst zone and runoff karst zone, as well as among the fault induced fractures. Four genetic types of fracture-vug complexes can be recognized. Formation of the fracture-vug complexes are recognized that carbonate fractures or joints were enlarged by atmospheric water leaching, and caves were formed at cross of fractures or joints and at easy corroding lithologies so that resulted in small caves distributing along fractures and along easy corroding beddings. Identification of fracture-vug complex helps to predict and evaluate reservoir spaces in karsted carbonate rocks.