The Geochemical Study On The Carbonate Resevior Of The Maokou Formation In Southeastern Sichuan Province

The Maokou carbonate reservoir is a solution-fracture reservoir in SoutheasternSichuan Province, mainly controlled by paleokarst, so it will take important value andhuge economic significance to research on the paleokarst reservoir by geochemicalmethods, such as dividing into ancient karst type, distinguishing diageneticenvironment, and fluid properties and source.On the basis of making full use of previous research achievements, focusing onthe six sections in study area as my important analysis objects. Applying a variety oftest and analysis methods, such as rock and mineral test, carbon and oxygen isotopeanalysis, strontium isotope analysis, cathode luminescence, trace element and fluidinclusion analysis, the petrological and geochemical characteristics of the reservoirhave been obtained by the detailed research. Author has gotten the followingunderstanding:(1) Rock types of carbonates on the Maokou Formation are micrite limestone,micrite bioclast limestone, dolomitic limestone, sand gravel limestone, calcsparitebioclastic limestone, etc. micrite limestone or micrite bioclast limestone mainlydistributed in all the Maokou Formation. Karst breccia is found at the top of theMaokou Formation and paleokarst is relatively common, which can be found onalmost every field outcrop and core. Dissolution fractures and solution pores are theimportant reservoir space types.(2) Carbonate diagenesis environment of the Maokou Formation in study area isdivided into atmospheric water, atmospheric water mixed with seawater and marinediagenetic environment. The compaction-pressure solution and cementation are thedestructive diagenesis for the reservoir transformation, dolomitization and karstdissolution can reform porosity and permeability of the reservoir better.Thekarstification is the largest contribution to the development of the reservoir on theMaokou Formation, dolomitization is not universal.(3) Fluid inclusions are scattered or zonational, what consist of the liquid phase,gas phase water inclusions, and a small amount of organic inclusions. Thecharacteristic on the inclusions shows four different temperature ranges, that impliesfour possible fluids flow on the Maokou Formation. According tocathode luminescence image and trace elements analysis under under the differentluminous intensity, there are at least three fluids exist different properties and source.(4)Based on carbon, oxygen and strontium isotopic characteristics analysis onmicrite limestone of the Maokou Formation, it suggests that carbon, oxygen andstrontium isotopic composition remain unchanged, which are consistent with theisotopic compositions from the normal carbonates at the global and common times. However, for the late sedimentary period of the Maokou Formation, the fractionationof oxygen isotope is diversified and has not obvious numerical change, carbon isotopedecreases continuously, strontium isotope ratio fall apparently. These characteristicsshows that the Emeishan taphrogenic movement and Dongwu structural movementare the most intense at late Maokou, the isotopic compositions are strongly affectedby terrigenous matters, carbon isotope in the sea water reduces. As a large number ofbasalts erupt into the water, the strontium isotope ratios in the sea water drop andcause the cracking of the organic matter, strontium isotope ratio is low and the carbonisotope continue to reduce in micritic limestone at late Maokou.(5) According to carbon and oxygen isotope characteristics analysis on differentgenerations of calcite in the different diagenetic environment, in addition to the lateMaokou Formation directly affected by the atmospheric water, carbon isotopes arebelow the average normal value of marine isotopic composition, the change of carbonisotope is small, but oxygen isotopic values change relatively obvious. Based on thechange of oxygen isotope, the mixed diagenetic environment may distribute between-7.45‰and-10.79‰.(6) The paleokarst types of the Maokou Formation are divided into sedimentaryperiod (layers) karst, weathering period (supergene) karst, burial karst and fold karst.These types could be well distinguished from the petrological and geochemicalcharacteristics of carbonate and calcite cement.