Study On The Difference Of Oolitic Beach Reservoirs In Feixianguan Formation In Tieshan Area, Northeast Sichuan

The oolitic shoal gas reservoir of Feixianguan formation at the platform margin of Kaijiang-Liangping trough in northeastern Sichuan is a major breakthrough field of carbonate gas exploration in Sichuan Basin.At present,the exploration and development of oolitic shoal gas reservoirs are focused on the east side of the trough,but the research degree is relatively low on the west side of the trough,where the study area is located.Therefore,this paper takes the oolitic shoal of Feixianguan formation in Tieshan area of northeast Sichuan as the research object,and uses the data of drilling,logging,core,thin section and geochemistry analysis.Based on the study of the basic characteristics of strata,sedimentary facies and the evolution characteristics of oolitic shoal,the difference of the characteristics of two types of oolitic shoal reservoirs(oolitic dolomite and oolitic limestone)are compared,and the causes of the difference of oolitic shoal reservoirs are also researched,which is expected to provide theoretical guidance for the later exploration and development of oolitic shoal gas reservoirs of Feixianguan formation in the area.Based on the types of rock association and the characteristics of well-logging response,the top-bottom boundary of the Feixianguan formation in the area are determined and the internal stratum is divided from bottom to top into T1f1,T1f2,T1f3,T1f4.The thickness of the Feixianguan formation(367m-860m)gradually thickens from the southwest to the northeast.The Feixianguan formation in the area is divided into 4 major facies by sedimentary background and facies marker:slope-trough facies,platform margin facies,open platform facies and evaporative platform facies.Using the comparison of the adjacent wells and the study of the plane distribution characteristics of oolitic shoal,it is considered that there are three periods of oolitic shoal development in the area,which are the evolution from the middle-upper part of T1f1 to the T1f3 in the longitudinal direction,and the gradual migration from the inner platform facies to the slope-trough facies in the lateral direction.According to the lithology,the oolitic shoals in the area can be divided into two types:oolitic dolomite shoal and oolitic limestone shoal.The oolitic dolomite shoal,which moves with the migration of the platform margin to the sea trough,is mainly distributed along the platform margin and relatively limited with small thickness(11m?57m).It is mainly distributed from the upper part of T1f1 to the middle part of the in the longitudinal direction.The oolitic limestone shoal is deposited in both the open platform facies and the platform margin facies,and widely distributed with great thickness(15m-103m).The oolitic limestone shoal is distributed in the middle-upper part from the T1f1 to the T1f3 in the longitudinal direction.The pore types of the two types of oolitic shoal reservoirs(oolitic dolomite and oolitic limestone)are mainly intergranular-dissolved pore,but the pore connectivity of oolitic dolomite is better and the pore diameter is larger than oolitic limestone.The average porosity and permeability of the former are 7.54%and 39.25×10-3?m2,the relationship between porosity and permeability is generally in positive correlation,and the mercury injection curve shows the feature of coarse distortion.The average porosity and permeability of the later are 1.52%and 0.797×10-3?m2,with low correlation between porosity and permeability,and its mercury injection curve is characterized by thin distortion.The development of the karst caves in the region is generally poor,and the development of oolitic dolomite is better than oolitic limestone.The study reveals that the oolitic dolomite reservoir mainly consists of the type ? and ? reservoir,and the oolitic limestone reservoir mainly consists of type ? reservoir.The study shows that the diversity of oolitic shoal reservoirs is mainly controlled by sedimentary facies,diagenesis and tectonism.Controlled by sedimentary facies,oolitic shoal developing in high-energy environment is the material basis for the formation of oolitic shoal reservoirs.Oolitic limestone in platform margin facies is easily transformed into oolitic dolomite by dolomitization.In diagenesis,dolomitization and dissolution play constructive roles in the oolitic shoal reservoir.The formation of oolitic dolomite reservoir is mainly controlled by the isseepage-reflux dolomitization,which is further reformed by the hydrothermal fluid.The preexisting pores in its early stage are further expanded and dissolved by dissolution.Burial dissolution is of great significance to the formation of oolitic limestone reservoirs.On one hand,the fractures formed by tectonism are favorable for the migration of acidic fluids causing dissolution.On the other hand,the reservoir and permeability conditions of the two types of oolitic shoal reservoirs are improved by structural fracture,especially the permeability of oolitic limestone reservoirs is remarkably increased.