Characteristics And Main Controlling Factors Of Dolomite Reservoirs In The Middle Permian Qixia Formation In Northwest Sichuan

Guided by theories of sedimentology,reservoir geology,mineral petrology and geophysics,by the means of outcrop measurement,core observation,laboratory analysis and other means,combined with drilling logging data,to research the characteristics of stratum,sedimentary facies and reservoirs in Qixia Formation.Define the main controlling factors and developmental patterns of high-quality reservoir development.According to the top and bottom boundary marks of the Qixia Formation,the formations were divided and compared with the field profile data and drilling log data.The results show that the Qixia Formation are unconformity contacting with Liangshan Formation,but conformity contacting with the overlying Maokou Formation.In the NE-SW direction,the thickness of the Qixia Formation is along the front line of the Longmenshan Mountain,which has the characteristics of increasing first,then decreasing,and changing smoothly.Based on the stratigraphic division and contrast,combined with the tectonic evolution background,the sedimentary facies analysis of the Qixia Formation in the area is carried out and it is pointed out that the Qixia period is entirely in the sedimentary environment of the carbonate platform,which can be further divided into two types:the platform edge and the open platform phase.The basin-slope phase,platform marginal facies and open platform facies are mainly developed from west to east.Combined with various indoor testing and analysis methods,the work is carried out from the aspects of petrological characteristics,reservoir space type,reservoir physical characteristics,reservoir pore structure characteristics,reservoir type division and reservoir distribution characteristics.Conduct a classification evaluation.The Qixia Formation reservoir is dominated by grain dolomite,and the reservoir space is most developed with intercrystalline dissolved pores.The caves and cracks are generally developed in the study area;the porosity frequency distribution is mainly concentrated between 1.0%and 4.0%,and the permeability is mainly distributed between 0.001 and 10mD.Fine-medium crystal dolomite face rate is most developed.The pore structure of the reservoir is complex and has strong heterogeneity.According to the mercury intrusion curve,it can be divided into four types:?,?,?and ?.The ? type is most developed.The reservoir type of Qixia Formation is mainly crack-pore(hole)type,followed by pore(hole)type.According to the reservoir evaluation,the reservoir types are divided into class ? low-permeability reservoirs,class ? ultra-low permeability reservoirs and class ? non-reservoirs.Class ? reservoirs are the main reservoir types in the study area.The reservoir is mainly developed in the middle and upper part of the Qixia Formation in the longitudinal direction,and the reservoir is stably distributed in the Shuangyushi structural belt in the horizontal direction,which can be continuously tracked and contrasted.The diagenesis of carbonate rocks in the Qixia Formation of the study area mainly includes:dolomitization,dissolution,rupture,cementation,compaction and pressure dissolution.By the means of X-ray diffraction order analysis,fluid inclusion uniform temperature and salinity determination,carbon and oxygen isotope analysis,cathodoluminescence analysis,trace elements and rare earth element analysis,define the formation process of the Middle Permian Qixia Formation dolomite in the northwestern Sichuan Basin:early high-energy shoal facies-Penecontemporaneous atmospheric water eluviation-shallow burial period dolomitization-late hydrothermal fluid reform.The development and distribution of reservoirs are controlled by sedimentary facies,sedimentary paleogeomorphology,diagenesis,and late tectonic fracture.Sedimentation provides the material basis for reservoir development and sedimentary highlands of paleogeomorphology control the distribution of high-energy shoal area;Penecontemporaneous atmospheric water eluviation not only forms a large number of secondary dissolution pores(vugs),but also provides migration pathway for the fluid of dolomitization;The early pores(vugs)are maximally inherited and preserved during the stage of the shallow burial dolomitization;The cracks formed by the late tectonic fracture further connect the pores(vugs)system,Which are of great significance for the hydrocarbon migration,accumulation and production of the Qixia Formation reservoirs of Shuangyushi structure in the NW Sichuan Basin.