Abstract:

The carbonate epigenic karstification controls the development and distribution of fracture-cave system in the Ordovician reservoir of Tahe Oilfield. Ordovician reservoir of Tahe Oilfield has experienced two periods of karstification in the Caledonian and Early Hercynian, composing a highly heterogeneous reservoir. Based on the karst origin, evolution and growth mechanism of Tahe Oilfield, the Ordovician reservoir was vertically divided into surface karst zone, vertical percolation karst zone and horizontal subsurface flow karst zone, as well as three karst paleotopography units as highlands, slopes and depressions horizontally. In this paper, outcrop observations, cores, drilling, well logging, seismic and production performance are used to affirm that karst cavities, dissolved fractures and honeycomb cavities are effective reservoir space in the region, and the matrix as the impermeable layer can block the fracture-cave system to a certain extent. Combined with the epigenic karstification theory and the influences on reservoir space, the reservoir space of the Tahe Oilfield are divided into sinkhole, undercurrent cavity and small cave according to their different causes, and the cavities can also be divided into collapse filling cavty, mechanical filling cavity and chemical filling cavity according to the filling patterns. Meanwhile, the geological model of different types of pore-cave systems was built up. Taking advantage of the development characteristics of the carbonate reservoir of Tahe Oilfield and production practice, the Tahe Oilfield karst reservoir space distribution model was eventually built up.

Key words: epigenic karstification (EK), Ordovician carbonate, reservoir space, geological model