The Comprehensive Study Of Reservoir Of Feixianguan Formation In Lower Triassic Series, Northeast Sichuan

Based on the principle and methods of carbonate sedimentology and reservoir diagenesis, and guided by the theories of carbonate reservoir geology, the sedimentation, diagenesis, controlling factors for reservoir and distribution of reservoir of Feixianguan Formation in northeast Sichuan have been comprehensively studied. Multiple methods from different branches of geology and advanced analyze and test technology 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 accompanied by two four-graded eustasy. Feixianguan Formation is grouped into four sections acc6rding to eustasy, lithology and elec-log characteristics. They are the first section, the second section, the third section and the fourth section. The reservoir distributes major in the first and second sections, minor in third section.The formation was formed in an isolated carbonate platform, which was surrounded by deep-water Kaijiang-Liangping Trough, South Qinling Ocean and Exi Trough (or basin). The isolated carbonate platform environment (facies) can be further classified into subenvironments: tidal flat, lagoon, marginal platform, continental shelf and slope. The sedimentation of the initial period of Feixianguan in Northeast Sichuan succeeded the sedimentary framework of the early period of Late Permian. Subsequently, the isolated platform accreted and the trough (or basin) contracted. The platform evolved into homogeneous tidal flat-lagoon in the fourth period of Feixianguan. The oolitic bank formed in marginal platform is most advantageous to form reservoir. As time going by, the bank peripheral moved toward deep-water and the bank layers rose.The characteristics and distribution of sedimentary facies are controlled by structure and eustasy, besides sedimentary environment itself. Regional basement fault and syngenetic fault which are connected with extension andcontraction of the South Qinling Ocean control the fundamental framework of sedimentary facies. The marginal area located in uplifted wall of fault is most advantageous to form marginal platform bars. Eustasy influences distribution of sedimentary facies in vertical and horizontal direction. Each sedimentary facies moved toward deep-water area with the lowering of sea level.There are mainly three times calcite and once gypsum cementation, infilling of seven minerals, compaction and pressolution, degypsum, solution and dolomitization in diagenesis. Cementation, infilling and compaction lead to the reduction of porosity, while solution and dolomitization increase porosity.Four times of solution took place, once in contemporaneous period, three in burial period. Contemporaneous solution was caused by the meteoric water when platform edge oolitic beach was subaerial exposure soon after its sedimentation. Such solution was weak, while infilling strong, so its effectiveness was poor. It formed some moldic pores. The first burial solution was linked to organic acid generated when mother rock was mature in Leping Formation (Longtan Formation and Changxing Formation). The solution was strong, while infilling weak, so a lot of intergranular, intercrystalline and intragranular dissolution pores were formed. These pores were usually half filled by the bitumen. Acid fluid was transferred to the reservoir of platform edge oolitic beach though contemporaneous fault at ocean trough edge. The second burial solution was connected with H2S generated by TSR (Thermochemical Sulfate Reduction). The solution was also strong, while infilling weak, so it formed a large number of moldic and intragranular dissolution pores. It could also enlarge the pores, which were formed in the first burial solution. There was no bitumen in these kinds of pores. The third burial solution was linked to tectonic activity in Xishan Period. The solution took place mainly along fractures and generated soirie dissolution fractures and vugs, but most of them were filled with coarse-crystal calcite, sulphur, gypsum and quartz.There are three types of dolomitization in Feixianguan Formationaccording to detailed analysis of petrologic feature, sedimentary characteristics and geochemical feature. Mixing-water dolomitization existed in platform edge oolitic beach. Dolomite is fine crystal and pores are abundant. Seepage-reflux dolomitization occurred in lagoons and point bars. Dolomite is microcrystal and pores are less abundant and strongly filled with gypsum. The burial dolomitization is weak. Dolomite is mainly medium crystal and euhedral. It distributed along fracture and stylolite or replaced oolith selectively. The dolomitic layers rose from lagoons to marginal platform as a result of eustasy, and the thickness of oolitic dolostone increased in the same direction. Dolomitization models which can exactly explain the formation of dolomite in the research area are put forward.Pore evolution of the reservoir is studied in detail. "Four History Evolutionary Model" is put forward, by which we can combine occurrence, development and disappearance of pores in reservoir with diagenesis, the generation of petroleum and the tectonic conditions. This model is very important to the research of the petroleum pools. Six kinds of diagenetic evolutionary series are put forward, indicating that early mixing-water dolomitization of platform edge oolitic beach and burial dissolution are the basic factors for high-quality reservoir.Petrophysical parameters and lithologic and pore texture characteristics of reservoir are comprehensively studied, displaying that oolitic dolomite and silt-crystal dolomite are the basic rocks for reservoir. Three types of reserve and permeable space, eight kinds of pore, three kinds of throat and three types of pore-throat texture are identified. Connecting factor is put forward, which can fully reflect pore texture. Oolitic exterior loop pore is recognized for the first time, and its forming mechanism is explained correctly. Systematic classification and evaluation of reservoir has been made. The reservoir layers rise from interior platform to marginal platform. What's more, the nearer the marginal platform is, the better the reservoir physical characteristics, and the thicker the reservoir.The analysis of factors which control the formation of reservoir indicates that sediments of platform edge oolitic beach is the basis for high-quality reservoir, while mixing-water dolomitization is essential condition and burial dissolution is the key. Regional distribution of reservoir has been predicted according to these controlling factors.