Sedimentary Facies Of The Cambrian In Jiulongshan And Surrounding Areas In Northern Sichuan And Its Relationship With Source, Reservoir And Cap

Through the observation and description of several key field profiles and drilling cores in the Jiulong mountain and peripheral areas of Northern Sichuan basin,Collating and analyzing the drilling and logging data.Summarizing the sedimentary,tectonic and lithofacies palaeogeography background.Studying the Cambrian stratigraphic characteristics,sedimentary facies types and characteristics and the relationship between sedimentary facies and source reservoir cap.First,according to the color,thickness and sedimentary structure of the outcrop,Combined with the electrical characteristics of the existing exploratory wells,dividing the Cambrian strata in the study area.It is clear that the important geological boundary characteristics and contact relationship between the upper and lower strata in the Cambrian strata.Furthermore,the stratigraphic characteristics,vertical and horizontal distribution and plane distribution of Cambrian in the study area are studied.On the basis of fully understanding the sedimentary background of the Cambrian in the North Sichuan,the sedimentary facies of the Cambrian section in Jiulong Mountain and its peripheral areas are fine depicted,and the sedimentary facies and microfacies types are divided according to their facies markers.As a whole,in the Cambrian period,Jiulong Mountain and adjacent areas experienced two semi sea level elevations in the slope-shelf-shore-bank-delta and the limited platform-tidal flat.It can be divided into 6 phases,8 subphases and 17 microphases.In the Early Cambrian Qiangzhusi to the Canglangpu early stage,the continental shelf facies in Jiulong Mountain and its adjacent areas can be further divided into deep water shelf and shallow shelf subfacies.The middle and late period of the Canglangpu together with the development of riparian delta facies and further divided into the foreshore,the front Delta,the delta front and the delta plain.During the period from the Longwangmiao to the Xixiangchi,the platform and tidal flat sedimentary facies are restricted,and the lagoon,platform beach and mixed tidal flat subfacies can be further divided.The sedimentary facies of the Longwangmiao group and the Xixiangchi experienced the transition from the mixed tidal flat to the restricted platform from the north to the south and from west to east in the plane.The sedimentary facies belt,which is favorable for the formation and evolution of source rocks,is continental shelf facies belt;The sedimentary facies favorable for the formation and evolution of reservoirs is the facies belt in the continental shelf margin of the beach,shore,delta and restricted platform;The sedimentary facies belt favorable for the formation and evolution of the cap rocks is the mixed tidal flat facies belt.Overall,during the Cambrian period,Jiulong Mountain and its adjacent areas experienced two cycles of half sea level rise and fall(Three stage cycle).Based on the study of strata and sedimentary facies,the relationship between sedimentary facies and source reservoir cap is analyzed.The source rocks in the study area are developed in the Qiongzhusi group,belonging to the continental shelf sedimentary environment.Its source rocks are characterized by high TOC,high maturity,large thickness and stable distribution,and the source rocks are of good quality.The reservoir developed in the restricted platform,the lagoon environment of the Longwangmiao and the xixiangchi formation and the shore delta depositional environment of the Canglangpu formation.But because the study area is close to the paleocontinent of Northwest Sichuan and the ancient land of the Hannan,the late destructive diagenesis is strong.In general,the reservoir is low pore,low permeability pore type and pore fractured reservoir.The cover layer of the study area is mainly the Douposi formation,which is developed in the sedimentary environment of the mixed tidal flat.A set of cloudy sandstone,sandy dolomite and muddy dolomite are deposited,and the lithology is dense and a good direct cover layer.In the Cambrian overlying strata,the Ordovician dense limestone and the Lower Silurian Long maxi formation are more than 300-400m thick,which can be used as regional caprocks and have strong sealing ability to oil and gas.In conclusion,the Cambrian system has good source rocks and caprock,but poor reservoir quality.