Petrological Characteristics And Reservoir Evaluation Of Carbonate Conglomerate Of Shahejie Formation In Shulu Sag

Conglomerate body,as a special oil and gas reservoir,is gradually gaining attention worldwide and has achieved remarkable exploration results.In recent years,huge hydrocarbon geological reserves have been discovered in the carbonate conglomerate reservoir of the Shahejie Formation in the Shulu sag of the southwestern part of the Bohai Bay Basin,which indicates that the carbonate conglomerate reservoir has great exploration potential.In this thesis,we analyze the lithological classification of carbonate conglomerates in the Shahejie Formation of the Shulu sag,analyze the lithogenesis of these rocks in the reservoir,and then evaluate the reservoir classification and predict the distribution of favorable reservoirs,provide geological parameters for reservoir evaluation and development,and provide a geological basis for the selection of favorable exploration areas in the study area.(1)From macroscopic to microscopic observations in cores and microscopic thin sections,it is concluded that the composition of the gravels of the carbonate conglomerates of the Shahejie Formation in the Sulu sag is mainly limestone and dolomite in composition.The conglomerates are mainly mud-crystal tuffs/dolomites and silt-crystal tuffs/dolomites.The source of the conglomerates is derived from the carbonate strata of the Ordovician Liangjiashan Formations,based on the sedimentary background and palaeomorphological features of the study area,combined with the seismic phase features and lithological evidence within the layers.(2)According to the international common grain-size classification scheme,the conglomerates of the Shahejie Formation in the Shulu sag can be divided into three categories: fine conglomerate(-1<φ<-2),medium conglomerate(-2<φ<-6)and coarse conglomerate(-6<φ<-8).The overall composition of conglomerate fillings includes two major categories: calcareous and muddy.The sorting is moderate to poor,with two types of support,granular support and miscellaneous base support.The rounding of grains can also be divided into two types,one in which large grains are rounded-subrounded and small grains are angular-sub-angular,and the other in which both large and small grains are rounded-sub-rounded.(3)The carbonate conglomerates in the study area are classified into seven types according to the structure of the conglomerates: Type I-a-1(grain-supported-sorted medium-sub-rounded),Type I-a-2(grain-supported-sorted medium-sub-angularangular),Type I-b-1(grain-supported-sorted poor-sorted-sub-rounded),Type II-a-1(miscellaneous-based supported-sorted medium-sub-rounded),Type II-a-2(miscellaneous-based supported-sorted moderate-sub-angular-angular),II-b-1(miscellaneous base support-poorly sorted-sub-rounded),II-b-2(miscellaneous base support-poorly sorted-sub-angular-angular).The overall structural maturity of the conglomerate is low,and heterogeneous-base supported conglomerates predominate.It is mainly concentrated in the Sand Section 1 and Sand Section 3 formations.(4)The diagenesis of each type of conglomerate in the study area is dominated by compaction,cementation and dissolution,and the reservoir space includes four types of intergranular pores,intragranular pores,dissolution micropores and fractures.The reservoir in the study area is generally of fracture-pore type.According to the classification of conglomerates in the study area,the composition,tectonic position and physical properties of conglomerates are combined to classify various types of conglomerate reservoirs,among which,Ⅰ-a-1,Ⅰ-a-2,Ⅰ-b-1 is a favorable reservoir,Ⅱ-a-1,Ⅱ-a-2,Ⅱ-b-1 are less favorable reservoirs,and Ⅱ-b-2 is a poor reservoir.Based on the lithology,tectonic position,pore development status and oil test of the reservoirs,typical conglomerate reservoirs in the study area are identified.Based on the lithology and tectonic position of the reservoir,the reservoir characteristics were judged and the pore development status and logging data were used to verify that the typical well conglomerates in the study area were type I and II reservoirs,respectively.