| Weixinan Sag is a secondary depression in the north of Beibuwan Basin, which proved to be a hydrocarbon-rich depression. WZ10-3Oilfield is located in Weixinan Sag, with abundant oil and gas resources, and great exploration potential. The study of sedimentary facies of the Member3of Liushagang Formation in WZ10-3Oilfield and surrounding areas has taken great achievements in the discover of sedimentary facies, sedimentary facies types, reservoir classification and evaluation, and so on. However, the division of sedimentary microfacies is not proposed, the summary of sedimentary facies distribution and sedimentary evolution rules is not deep enough, and lack of systematic analysis of reservoir. Therefore, conducting a comprehensive, systematic study of sedimentary facies and reservoir characteristics of the Member3of Liushagang Formation in WZ10-3Oilfield and surrounding areas is very necessary. In this way, the detailed basic geological data can be provided for the oil and gas exploration. It also can guide exploration and development of other oil and gas fields in Beibuwan sag, Weixinan Basin.Associated to the project "Comprehensive study on reservoir of the Member3of Liushagang Formation in WZ10-3Oilfield and surrounding areas", this paper selectes the interval Ⅰ and interval Ⅱ of the Member3of Liushagang Formation as research object. Guided by the high resolution sequence stratigrapy theory, the group of L3Ⅰ+Ⅱ has been divided. Then, the sedimentary facies and reservoir characteristics were systematically studied in middle-term base level cycle, finally, the distribution of favorable reservoir of L3Ⅰ+Ⅱ have been predicted.For the development features of L3Ⅰ+Ⅱ in study area, using the theory of high resolution sequence stratigrapy theory, combining with logging, log and analysis assay,3different levels sequence interfaces and maximum lake flooding surface have been recognized. Based on this, the L3Ⅰ+Ⅱ was divided into one long-term base level cyclical sequence and four middle-term base level cyclical sequence. Then, the high resolution sequence stratigraphy was stablished, which is the basis for the next sedimentary study.On the basis of core observation and discription, in combination with logging, log, analysis assay, the sedimentary facies of L3Ⅰ+Ⅱ in study region has been analysed. The result shows that most of rock colours of L3Ⅰ+Ⅱ are gray, gray-green and brown, some are dark gray and gray black, little of them are red-brown, mottled and gray. This reflects the depositional environment was weak reducing-reducing environment. Rock types of L3Ⅰ+Ⅱ are mainly clastic rocks, with wide range of grain size, middle sorting, poor roundness, pore cementitious and medium-low textural maturity. Granularity-accumulated curves of the L3Ⅰ+Ⅱ in study area can be classified into5patterns, e.g."uphill arched arc" pattern representing debris flow deposition, one segment suspension pattern representing turbidity deposition, typical bi-segment and tri-segment patterns representing traction current deposition. Sedimentary structures are abundant, including gravity flow causes sedimenrary structures of massive bedding, grading bedding, traction flow causes sedimenrary structures of cross bedding, parallel bedding, lenticular bedding, and so on. According to this sedimentological markers, combining with sedimentary background, fan delta-lacustrine depositional system were identified in the L3Ⅰ+Ⅱ, and further divided into4sub-facies of fan delta plain, fan delta front, pro-fan delta and shallow lake, and some microfacies.By analyses of individual well sedimentary facies, section sedimenrary facies and plane sedimentary facies, sedimentary facies distribution and sedimentary evolution rule in middle-term base level cycle were discussed. The results indicate that sedimentary facies distribution was regular during the depositional period of the L3Ⅰ+Ⅱ. Fan delta was developed in the northwest part of the study area, while the shore shallow lacustrine sub-facies was developed in the southeast part of the study area. From the depositional period of MSC1cycle to MSC4cycle, the distribution range of fan delta was continuously decreasing with the lacustrine level gradually rising.Based on the study of sedimentary facies, the petrological feature, characteristics, controlling factors and heterogeneity of reservoir of L3Ⅰ+Ⅱ were systematically studied by comprehensive utilization of core physical properties, log physical properties and other analysis assay data. The sandstones types of the L3Ⅰ+Ⅱ were mainly quartz sandstone and feldspar quartz sandstone. The dominant reservoir space was primary intergranular pores and intergranular dissolved pores, the next is intragranular dissolve pores. The L3Ⅰ+Ⅱ has both ultra-low porosity and ultra-low permeability reservoir, and middle porosity and middle permeability reservoir. Lithology, sedimentary facies and diagenesis are the factors affecting reservoir property. The L3Ⅰ+Ⅱ reservoir has strong heterogeneity.Finally, by comprehensive analysis of sedimentary facies and reservoir of the L3Ⅰ+Ⅱ in study area, it was discovered that subaqueous distributary channel sandbody which was most developed and has good physical properties was the favorable oil and gas reservoir facies belt. Especially the front of wells areas of A1—C1—C4—A3. |
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