The Mechanism Of Carbonate Reservoir Formation Under The Background Of Mixed Sedimentation

There was mixed siliciclastic-carbonate sediment in Carboniferous at Bachu-Makit area in Tarim Basin. Not only the sedimentary sequence was interactivedeposition of clastic rock and carbonate, but also there was siliciclastic-carbonate onthe micro-scale. Through research of the petrologic feature about mixing sediment,the taxonomy on composition of mixed rock was made again, and described thedistribution of mixed rock in Carboniferous in study area. Base on the taxonomy onorigin of mixed rock reported by Mount, the origin of mixed rocks in study area wereanalyzed. With the characteristics of mixed rocks, the stratigraphic sequence andsedimentary facies of Carboniferous at Bachu-Makit area was studied. According tothe method of Vail, there were4third-sequences in Carboniferous in the study area,and then discussed the distribution of mixed rocks and origin of mixing sediment.During early TST and late HST the main origins of mixed sediment are facies mixing,while during late TST and early HST the main origins are punctuated mixing andfacies mixing. Actually the relationship of eustatic sea level change and mixingsediment is the amount of terrigenous detrital inputting to mixing area. Therefore,HST is the most probability period ofmixing sediment. The study area developed thesedimentary facies system of near-shore to carbonate platform. Through studying ofthe characteristic of sediment, three mixing sediment model were established,including mixing tidal flat, mixing lagoon and mixing platform. It’s suggested thatpunctuated mixing in platform was related to early diagenesis. The main controllingfactors of mixing sediment in Carboniferous were tectonic and eustatic sea levelchange, because there were many denudation areas during middle-late Hercynianorogeny, and there were4changes of eustatic sea level. This thesis is focus on the relationship of mixing sediment and carbonate oil/gasreservoir. Both of carbonate reservoirs of Xiaohaizi Formation and Bioclasticlimestone segment have the background of mixing sediment. Through the research ofthe four-property relationship of reservoir, the effectivereservoir rocks of XiaohaiziFormation are residual grainlimy dolostone/dolostone and sparry grainlimydolostone/dolostone, while that of Bioclastic limestone segment is powder limydolostone/dolostone with gypsum. All of these reservoir rocks are mixed rocks withsome terrigenous detrital. With lithologic and geochemical method, the diagenesis ofthese main reservoir rocks was studied. The main influencing factors on reservoirproperty are dolomitization, dissolution, cementation, and precipitation of autogeneticminerals. The special diagenesis of mixed carbonate rocks are quartz overgrowth andprecipitation of dickite/kaolinite. The mixing sediment has effect on platform mainlythrough punctuated mixing. It’s suggested that the punctuated mixing inCarboniferous was controlled by offshore current, relating to giantmonsoon. Thepunctuated mixing might influence the early diagenesis on the sediment of shoal, notonly the seawater dolomitization was promoted, but also can lead to lack cementamong carbonate grains, because of relativesmall fall of sea levelassociated with thestrongest offshore current. As a result, this mechanism can explain the origin ofresidual grainlimy dolostone/dolostone. This is the main formation mechanism ofmixed carbonate reservoir. The most important influence factor of reservoir propertyis dolomitization. There were four models of dolomitization in mixed carbonatereservoirs, including reflux dolomitization, seawater dolomitization, shallow burialdolomitization and middle-deep burial dolomitization. It’s suggested that seawaterdolomitization might be the base of formation of reservoir of Xiaohaizi Formation,and shallow burial dolomitization and recrystallization can further modification theproperty of reservoir. And the reflux dolomitization and shallow burialrecrystallization might be the base of formation of reservoir of Bioclastic limestonesegment. The dissolution mechanism is the key facor of reservoir origin for the twocarbonate reservoirs, especially burial dissolution. By thin section, cathodeluminescence, XRD, electronic probe, minor element, carbon and oxygen isotope, it’sshowed that the carbonate reservoir of Xiaohaizi Formation had experiencehydrothermal dissolution, and carbonate reservoir of Bioclastic limestone segmenthad experience dissolution associate with thermochemical sulfate reduction(TSR).Combining with drill and well log materials, the region of influence of thesedissolution was determined. The hydrothermal dissolution can influence the reservoir property of Xiaohaizi Formation in Yasongdi area and Bashituo area, while TSRmainly influence Bashituo-Xianbazha area and north of it.