The Diagenesis Research To The Reservoir In The Triassic Feixianguan Formation Of Dawan Area,Puguang Gas Field

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 the Bay Area Feixianguan Formation sedimentary in Puguang gas field have been comprehensively studied. Multiple methods from different branches of geology and advanced analysis have been used in this research. The main contents and conclusions of this dissertation are as follows:Feixianguan Formation and upper&lower of formation lithologic、electrical characteristics was very obvious. The formation can be divided into four sections from lower to upper. They are the first section, the second section, the third section, the fourth section. The second section can be divided into three sections. They are fei2-3, fei2-2, fei2-1. The reservoir distributes major in fei2-3, fei2-2, minor in fei2-1, the third section is poor reservoir or non-reservoir.The formation was formed in an isolated carbonate platform, which was surrounded by deep-water Kaijiang-Liangping Trough, South Qinling Ocean and Exi Trough. The isolated carbonate platform environment can be further classified into the carbonate platform and edge of the carbonate platform. The carbonate platform can also be divided into evaporation platform, restricted platform, and open platform according to the sedimentary characteristics. The sedimentation of the initial period of Feixianguan in Dawan Area succeeded the sedimentary framework of the early period of Late Permian. Subsequently, the isolated platform accreted and the trough 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 towards deep-water and the bank layers rose.The mainly diagenesis:micritizatiom compaction and pressolution、infilling of some minerals、dolomitization、degypsum、recrystallization、solution、tectonic disruption and so on; Cementation、infilling and compaction lead to the reduction of porosity, which is the destructive diagenesis; While solution and dolomitization increase porosity, which is the constructive diagenesis.Three times of solution took place, once in contemporaneous period, two in burial period. Contemporaneous solution was caused by the meteoric water when oolitic beach of platform edge was subaerial exposure soon after its sedimentation. Such solution was strong, the area was wide. It formed some moldic pores. The first burial solution was linked to organic acid generated when mother rock was mature. The solution was strong, while infilling weak, so a lot of intergranular, intercrystalline and intergranular 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 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.There are two types of dolomitization in Feixianguan Formation according to detailed analysis of petrologic feature, sedimentary characteristics and geochemical feature. Mixing-water dolomitization existed in oolitic beach of platform edge. Dolomite is fine-grained and pores are abundant. Seepage-reflux dolomitization occurred in lagoons and point beachs. Dolomite is microcrystal and pores are less abundant and strongly filled with gypsum. 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.Pore evolution of the reservoir studied in detail. We can combine occurrence, development and disappearance of pores in reservoir with diagenesis. This model is very important to the research of the petroleum depositions. Six kinds of diagenesis 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.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.