Research On The Main Controlling Factors Of Microbial Dolomite Reservoir Development In The Upper Deng Fourth Member Of Sinian In Moxi Area

Microbial dolomite plays an important role in China's deep marine carbonate strata,and it is a hot research topic in the world,too.The ancient uplift of central Sichuan is one of the most enriched and oil-rich structures of the Cambrian microbial carbonate rocks,and the Moxi area is located in the oblique gentle zone in the ancient uplift of central Sichuan,a group of algal dolomite-based strata is mainly deposited in the area.According to the actual situation of the study area,this paper is under the guidance of oil and gas geology,petrology,sedimentology,logging geology,tectonic geology,reservoir geology,organic geochemistry,and uses the existing drilling,logging,logging,core and geochemical data in the research area.By combining macroscopic and microscopic research methods,research the development characteristics and developmental control factors of microbial dolomite reservoirs in the upper submember of fourth member of Dengying Formation in Gaoshiti-Moxi area.In the study area,the stratum of the upper submember of fourth member of Dengying Formation thinned from the middle to the sides,the Formation thickness of the upper submember of fourth member of Dengying Formation in the bore-field of MX52-MX111 can be greater than 180m.It is clear that the types of microbial dolomite in the upper submember of fourth member of Dengying Formation in the study area mainly laminar algal dolomite,algal stromatolite dolomite,algal clot dolomite and algal-sand-clastic dolomite.The reservoir space of the upper submember of fourth member of Dengying Formation in the study area is mainly the secondary pore,hole and crack,which generally has the traces of the transformation of the later diagenesis,and the internal reservoir space is mostly filled with saddle dolomite or euhedral quartz.The average porosity of microbial dolomite reservoir Rocks is about 4.22%,the average permeability is about 0.85 mD,and the physical condition is better than that of non-microbial dolomite.Based on the collocation of various reservoir spaces and combined with core observation and conventional logging,directional identification of imaging logging,physical characteristics,etc.,this paper divides the microbial dolomite reservoirs in the upper submember of fourth member of Dengying Formation of Moxi area into the following three categories:pore type,pore-hole type and crack-hole type reservoirs,combined with single well test yield analysis,pore-holes type and crack-hole type reservoirs can be identified as high-quality reservoirs and they are centrally distributed in the upper submember of fourth member of Dengying Formation.The reservoir space of microbial dolomite is obviously controlled by microbial morphological structure,algal stromatolite dolomite and algal clot dolomite are the main reservoir rocks that constitute high-quality reservoirs.Sedimentation mainly controls reservoir distribution through the distribution of sedimentary facies,high-quality reservoirs developed more in the developed area and the secondary developed area of mouds and shoals.The diagenesis changes the original physical properties of the rock and redistributes the pore space.Through a large number of petrological analyses,the evolulion pattern of microbial dolomite reservoirs in the study area can be summarized as:Strong cementation after sedimentation,porosity as low as 3%or less;Tongwan ? episode uplifted the formation to undergo denudation and karstification,and the porosity increased to more than 10%;siliceous cement is produced in shallow burial stage and filled with pores;hydrothermal dissolution produces a large amount of reservoir space,but hydrothermal minerals fill the reservoir space,maintaining porosity at around 4%;during the burial diagenesis stage,the mineral filling causes the porosity to drop to 3%;dissolution during burial periods causes a slight increase in porosity by 1%;the hydrothermal activity at the end of the Late Permian is also constructive and destructive,then the diagenetic evolution of the study area is in a relatively closed system,and the reservoir space does not change much.Due to the low paleo-geomorphology and serious silicaization of the restricted platform,it suffered by alteration of diagenesis rarely.The main control factors of microbial dolomite reservoir development in the upper submember of fourth member of Dengying Formation of the study area can be summarized as:microbial structure,sedimentation,silicification,karstification and hydrothermal action.