The Characteristic And Evolution Of Early Paleozoic Dolomite Reservoir In Gucheng Area Of Terim Basin

In recent years,the dolomite reservoir in Ordovician in Gucheng area,obtain oil and gas industry breakthrough and become the important exploration field in Tadong area.Due to the low degree of previous exploration,less drilling,weak basic geological research,especially no further research on origin of dolomite reservoir space,diagenesis evolution and development regularity of favorable dolomite reservoir,the progress in carbonate rock exploration in Gucheng area have been restricted.By means of experimental analysis techniques of carbonate reservoir in this paper,integrated metrological characteristics with geochemistry characteristics,microcosmic structure and macro geological genesis,reservoirs characteristics and evolution,the formation,evolution and development regularity of dolomite reservoir in Lower Paleozoic are studied.Achievements obtained are as follows:By establishing the petrography and geochemical identification marks,six kinds of three-phase four-type dolomitize dolomite are developed and summarized in Gushing area: powder-microcrystal dolomite caused by the syngenetic evaporating pump dolomitization in the first period,anhedral powder and microcrystalline dolomite resulted from the penecontemporaneous seepage refulx dolomitization,idiomorphic powder and medium crystalline dolomite made from the shallow burial dolomitization,dirty and clean medium coarse crystalline and saddle dolomites because of late hydrothermal dolomitization.The medium-coarse crystalline dolomites develop secondary dissolution pores and show better reservoir properties.Three phases of hydrothermal process are identified for the first time in this region.Fully employed with microanalysis in situ,six identification marks,such as hydrothermal minerals,cathodoluminescent color,order degree of the dolomite,rare earth element,,strontium isotope,carbon and oxygen isotopes,are established.The mechanism and the model of the three phases of hydrothermal process and their implications for the formation of reservoir permeability have been further studied.The first phase of the albitization in the area has little effect on reservoir reconstruction.The intergranular pore,intergranular dissolved pore,dissolved fissure and vugs formed by the second-third phase of dolomitization and hydrothermal dissolution are the main reservoir spaces of dolomite.The second phase hydrotermal process in Late Ordovician shows the dolomization to Low Ordovician,and dissolution to the dolomites of Cambrian.The third phase hydrothermal process in Late Permian,showing dissolution characteristic,is the key process to form the dolomite reservoirs in this area.The recovery of diagenesis process of dolomite and evolution history of reservoir space shows that the reservoir spaces caused by the shallow burial dolomitization during early diagenetic period,the second phase of hydrothermal dissolution during middle-late diagenetic period,which match well with the time that a large amount of oil were generated,are the main reservoir spaces of paleo-reservoir.The dissolved pore and dissolved fissure-pore-vugs type reservoir caused by the third phase of hydrothermal dissolution during late diagenetic period are the effective accumulating spaces for gas reservoir.Therefore,the superposition areas where the dolomite and fault structure are developed are the favorable areas that develop high-quality reservoirs.The dolomite reservoirs in the lower Yingshan formation and in reef flat complex of Cambrian are practical exploration fields.