Control of Facies and Diagenesis on Porosity of Deeply Buried Dolomitized Reef and Oolite in Upper Permian Changxing and Lower Triassic Feixianguan Formations in NE Sichuan Basin, China

摘要

The dolomltized reef and oolites in the Upper Permian Changing and Lower Trlassic Feixianguan formations respectively are prolific gas reservoirs in NE Sichuan Basin, China. At the Paguang field of the basin, the present depth of the reservoirs ranges from 5 000 to 6 500 m. The reef locates on the platform margin, and the oolites on the same site and otherwise in the platform interior. On the seismic profile, the reef and oolites are identified as mounded blank and chaotic reflections respectively. Several diagenesls events are crucial for porosity development in the reservoirs. The penecontemporaneous and burial dolomitization produced intercrystalline porosity as channel for the organic/inorganic acid fluid whereby the acid dissolution occurred and the expanded vug formed. In addition, the dissolution from the products of thermochemical sulphate reduction (TSR) further enhanced the vug during the tectonic uplifting. The porous reservoirs preferentially located at the upper part of depositional cycles shallowing upwards.