Comprehensive Research On Reef Characteristics And Heterogeneity Of Reef Reservoir Of Upper Permian Changxing Formation In Northeastern Sichuan Basin

This research is mainly based on theories of carbonate sedimentology, carbonate diagenesis, structural geology, paleobiology, as well as the applications of seismic, logging, core and outcrop data. After integrated study and analysis, advances obtained are as below:1. At changing of late Permian, depositional framework of northeastern Sichuan basin extended from northeast to southwest. West part of it was deep water shelf depositional environment, east part was shallow water platform environment. Drowned and exposured disconformities developed, dividing Changxing formation into two 3-order sequence stratigraphies, in whose high stand system tracts, reef reservoirs are located.2. Reef building organisms are mainly Sponge, Hydroid, Bryozoan, and Coral, but sponge was the dominant contributor. Rock types include sponge bafflestone, sponge bindstone and sponge framestone. And reef building model has been established. The formation of wave resisting structure is complicated, mainly via inner reef sedimentation, precipitation cement and organism agglutination.3. After initial depositional and diagenetic alterations, primary porosities of reef reservoir were partly preserved, serving as pathways for fluids, then they experienced meteoric leaching,,dissolution and dolomitization. Under deep burial conditions, it was modified by calcite cement, filling, burial compaction, dolomitization and multi-stage dissolution. At late stage, unselective dissolution did much more good to reservoir properties, organic acid, CO2 and H2S created by TSR reaction eroded carbonate rock severely, accelerating the dissolution of limestone and dolostone, so properties of reservoir was well improved.4. Heterogeneity of reef reservoir is determined by reef facies, depositional fabric, depositional background and complexity of diagenesis. Vertically, because of diagenetic modifications in vadose, phreatic and mixing zone, reservoir properties of the upper part of reef are constructively altered. On the other hand, the lower part of the reef nearly has no improvement. Laterally, the windward side of reef reservoir is destructively modified for the calcite cementation by interaction of seewater and reef sediment, but the properties of lee side reef are well improved by the replacement of evaporite backflow with high ratio of Mg and Ca. In one word, heterogeneity of reef reservoir is dominantly related with sediment diagenesis.