Off Group Gypsum Causes And Distribution Of Reservoir Development. The Northeastern Sichuan Basin

Based on the principle and methods of carbonate sedimentology and sedimentary geochemistry, and guided by the theories of carbonate reservoir geology, the sedimentation, diagenesis, the distribution of reservoir and gypsum, the relationship of gypsum and reservoir of Feixianguan Formation in northeast Sichuan Basin have been comprehensively studied. Multiple methods from different branches of geology have been used in this research. The main contents and conclusions of this dissertation are as follows:Feixianguan Formation was formed under lowering of a three-graded sea level which was composed by five four-graded sequences. Feixianguan Formation is grouped into five sections according to the principle of stratigraphic correlation for the same era. The spread of sedimentary facies was mainly in control of the evolution of the Kaijiang-Liangping Trough. The platform facies can be further classified into subfacies as oolitic dam, oolitic beach, tidal flat and lagoon from margin to interior of the platform.Besides of the compaction, there are many diagenesis impacting the reservoir such as dolomitization, ablation, infilling of minerals, cementation of calcite and gypsum. In which, the cementation and the infilling lead to the reduction of porosity and permeability values while the dolomitization and the four times ablation increased porosity and permeability values.Based on the analysis of a large number of thin section and the distribution characteristics of sulfur isotope, the conclusion is drawn that the gypsum in Feixianguan Formation is mainly formed in supratidal sabkha or in intertidal lagoon. The sedimentation mechanism of the gypsum is that the SO₄²- which come from the PTB convulsion and the Ca²+ which come from seawater are combined in CaSO₄ if their concentration product overruns its state of equilibrium.The gypsum of Feixianguan Formation is mainly distributed in the first segment and the fifth segment, while the reservoir is mainly distributed in the first segment and the second segment. The depth of gypsum is gradually reduced and the horizon is gradually elevated under the control of sedimentary facies from the inner platform to the marginal platform, while thereservoir's depth is opposite.By study of the depth of the gypsum and reservoir, the conclusion is drawn that the relationship of the distribution of gypsum and reservoir is growth and decline as a whole.