Ordovician carbonate rock matrix fractured-porous reservoirs in Tahe Oilfield, Tarim Basin, NW China

Abstract Features of porphyritic limestone matrix fracture-vug reservoirs in Tahe Oilfield, Tarim Basin, NW China was examined by core observation, thin section identification, cathodoluminescence, microscopic fluorescence, scanning electron microscope and energy spectrum analysis. Thick and widespread in the Lower-Middle Ordovician Yijianfang and Yingshan Formations, sand-clastic masses in the porphyritic limestone are rich in oil, quite a proportion of oil and gas produced currently is from their matrix rather than completely from the fracture cave system in the past understanding. In irregular spot or ribbon shape on cores, dolomitic sand-clastic masses usually account for about 40% of the total surface of core in many layers, arenite section has a surface porosity of around 39%. The arenite section is well-crystallized dolomite, the dolomite crystals are mainly 100−350 μm in diameter, equivalent to medium-fine sand; high-pressure mercury injection experiment results show that the reservoir physical property of sand-clastic masses is much better than that of micrite limestone, with a porosity of 12.57%−36.39%. There developed abundant stylolites and microcracks around the sand-clastic masses, which connect the oil bearing units of dolomitc sand-clastic masses, making micropores and microcracks communicate and become effective reservoir space.