Diagenesis And Reservoir Characteristics Of The Upper Part Of The Fourth Member Of The Shahejie Formation In The Southern Slope Belt Of The Bonan Sag,Jiyang Depression

The upper fourth member of Shahejie Formation in the southern slope belt of the Bonan sag in Jiyang Depression is one of the important target layers of oil and gas exploration,where interactively develops various sedimentary systems such as terrigenous clastic rocks,carbonate rocks,mixed siliciclastic-carbonate sediments and gypsum rocks.Therefore,the study of diagenesis and reservoir characteristics is particularly important for the oil and gas exploration of this complex sedimentary system.Studies have shown that twelve types of lithofacies are developed in the upper fourth member of Shahejie Formation: gray massive conglomerate facies with matrix supported,gray massive pebbly sandstone facies,gray medium-coarse sandstone facies with cross beddings,gray silt-fine sandstone facies with parallel beddings,gray massive calcareous sandstone facies,gray massive sandy limestone facies,dark gray massive grainstone facies,gray massive organic framework limestone facies,dark gray massive mudstone facies,offwhite massive dolomite facies,white massive gypsum facies and dark gray massive mudstone facies.The diagenesis types of the upper fourth member of Shahejie Formation mainly consist of cementation and dissolution with overall characteristics of strong cementation and weak compaction.According to the diagenetic stage division specification,the terrigenous clastic rocks in the study area are mainly in the sub-stage B of the early diagenesis and sub-stage A of the middle diagenesis,while the carbonate rocks are mainly in the eogenetic stage.The diagenetic sequences of terrigenous clastic rocks and carbonate rocks are slightly different,but they overall go through the following stages: compaction ? early pyrite cementation/early calcite cementation ? early dissolution(calcite cement,feldspar,etc.)/quartz overgrowth of?stage ? late calcite cementation/calcite metasomasis of feldspar,cuttings,dolomite/iron dolomite metasomasis of dolomite ? late dissolution,resulting in forming secondary pores.The sedimentary diagenetic environment mainly undergoes the multi-stages of acid – alkaline – acid – alkaline – acidity.There are ten types of pores in the upper fourth member of Shahejie Formation:intergranular pores,organic framework,intercrystal pores,obscured pores,interparticle dissolution pores,intraparticle dissolution pores,molded pores,interbedded fissures,structural fractures,dissolved fractures,where interparticle dissolution pores and intraparticle dissolution pores are the best developed lithofacies.The high-quality reservoirs are mainly distributed in the depth range of 2000 m to 2650 m,where the lithofacies types are mainly gray silt-fine sandstone facies with parallel beddings,gray massive medium-coarse sandstone facies with cross beddings,some gray massive conglomerate facies with matrix supported,dark gray massive grainstone facies and gray organic framework limestone facies.There are a total of six diagenetic facies types identified in the upper fourth member of Shahejie Formation: moderately dissolved facies with weak compaction and weak cementation,strongly dissolved facies with medium compaction and medium cementation,moderately dissolved facies with medium compaction and medium cementation,moderate dissolved facies with weak compaction and strong cementation,moderate dissolved facies with medium compaction and strong cementation,weakly dissolved facies with strong compaction and strong cementation.The first four types of diagenetic facies can be used as high-quality reservoirs.Based on the results of comprehensive facies,diagenetic facies and sedimentary facies,it is believed that high-quality reservoirs are mainly developed in the following sedimentary facies: carbonate rock dam microfacies,fan-delta plain distributary channel microfacies,fan delta front subaqueous distributary channel microfacies,and relatively weak energy on both sides carbonate sedimentary zone.