Reservoir Characteristics And Main Controlling Factors Of Lower Ordovician Penglaiba Formation In Yubei Area

This paper is based on sedimentology, reservoir geology, geochemistry as the theoretical basis, studies the sedimentary facies, reservoir characteristics and diagenesis of the Penglaiba Formation in the Yubei area of the Tarim Basin through the analysis of core, thin section, scanning electron microscope, cathode luminescence and so on.This paper has achieved the following results:(1)The sedimentary period of the Penglaiba Formation is restricted platform,and it can be further divided into the lagoon, tidal flat and intra-platform bank. The appearance of Caliche, Vadose pisolite,Ruiniform horizon and Tepee structure in the core of Yubei 5 well indicate that Penglaiba Formation once had short-term exposure.They also proved that T78 interface is an unconformity between Yingshan Formation and Penglaiba Formation.(2)Dolomite is the main rock types of Penglaiba Formation.According to the characteristics of rock structure and special structure, dolomite can be divided into fine-crystalline dolomite,medium dolomite, coarse-grain dolomite,(residual) grain dolomite and laminar algal dolomite etc,and the reservoir quality of medium dolomite is the best. The main reservoir space is intercrystalline pore,intercrystal solution openings,dissolved pore and tectonic fissure. Micritization, dissolution, pressolution,recrystallization,fracturing and dolomitization have positive influence on the reservoir.However,cementation,compaction,filling and hydrothermal processes have negative impact.(3)Analysis of the carbon and oxygen isotope in calcite fillings, fracture cave dolomite fillings and calcite fillings in the Penglaiba Formation shows that the carbon isotope value in 9 samples of 14 is lower than the minimum value of the early Ordovician,which is-1.5‰.And in the 11 samples of fracture cave dolomite fillings,10 of the oxygen isotope value δ18O is-10.1‰~-14.4‰,the average is-12.9‰,much lower than the value of the normal marine carbonate rocks in the early Ordovician. At the same time, it is also lower than the oxygen isotopic composition of saddle dolomite,so it has high negative features.And the result indicates that the meteoric water is an important fluid in the diagenesis.(4)High quality reservoir is controlled by sedimentary facies,diagenesis and tectonic movement.Intra-platform bank. of the restricted platform can provide good material basis, the role of atmospheric water associated with the T78 interface is the key factor for the formation of high quality reservoir,and dolomitization is beneficial to the preservation of porosity.In addition,in development of Hetian Uplift can provide a good condition for the atmospheric fresh water leaching. Structural fractures formed by tectonic movement can increase the contact area between water and carbonate rocks,which increase the dissolution range.In addition, it is a good fluid percolation channel.