Sedimentary Environment And Reservoir Characteristics Of Mixed Siliciclastic-Carbonate Sediments In Funing 2^nd Member Of Gaoyou Sag And Jinhu Sag,Subei Basin

Mixed siliciclastic-carbonate sediments（MSCSs）is not only an important issue in petroleum exploration and development,but also the focus of sedimentary geology.The Funing 2^ndMember is important source rock and oil reservoir in Gaoyou and Jinhu Sag,with mixed deposits of clastic rocks and carbonate rocks developed simultaneously.The characteristics of mixed sedimentary reservoir control the efficient exploration and development of oil and gas.Systematic analysis of mixed sedimentary environment,mixed sedimentary model and mixed rock reservoir characteristics are of great significance for conventional and unconventional oil and gas production in the study area,as well as for the study of MSCSs in continental lacutural basin.Based on modern sedimentology and reservoir geology theory,a large amount of geological and geophysical data,and on the basis of establishing fine stratigraphic framework,this paper carried out a comprehensive study on sedimentary environment,sedimentary facies and reservoir characteristics of mixed sedimentary rocks,and achieved the following results.The filling development model of Funing 2^ndMember is established.It is clear that Funing 2^ndMember covers the whole study area and develops three kinds of boundaries:cutting,fault and overlapping.The Funing 2^ndMember is subdivided into 5 small layers and a unified stratigraphic framework spanning the two depressions is established.The thickness of each small layer is thin in the west and thick in the east of Jinhu Sag,and thin in the north and thick in the south of Gaoyou Sag.The paleosedimentary environment of Funing 2^ndMember is established.It is clear that during the sedimentary period of Funing 2^ndMember,the geomorphic feature was undulating in the early and flat in the late,the climate was dry in the early and wet in the late,the lake level was low in the early and high in the late,the salinity was high in the early and low in the late,and non-sulfide anoxic water body was developed in the study area.The sedimentary background of Funing 2^ndMember can be divided into the early saline-hypersaline water environment,the middle brackish-saline water environment and the late brackish water environment.Both the mixed sedimentary series and sand-grade mixed sedimentary rocks are found in the middle of Funing 2^ndMember in Jinhu sag and the bottom of Funing 2^ndMember in Gaoyou Sag,among which sand-grade mixed sedimentary rocks develop 5types of main lithofacies.Sand-grade mixed sedimentary rocks in Jinhu Sag are mainly distributed in beach-bar deposits with corrugated cross bedding and are composed of ooid and siliciclastics of similar size.Sand-grade mixed sedimentary rocks in Gaoyou sag are mainly distributed in deep water gravity flow deposits,develop massive bedding and graded bedding,and are composed of different sizes of ooid,intraclast and siliciclastics with clay and marl.It is clear that the mud-grade sedimentary rocks of Funing 2^ndMember in the study area are mainly composed of micrite carbonate minerals,quarts-feldspar grains and clay minerals.The sedimentary structures such as fine/thin/thick lamina,thick block laminar and microscopic lamina are developed.It can be divided into 9 lithofacies groups and15 subtypes,among which 12 subtypes belong to mixed sedimentary rocks.The lower part of Funing 2^ndMember mainly develops mud-grade sedimentary rocks with thick layered/thick lamina,the upper part mainly develops mud-grade sedimentary rocks with fine/thin lamina,and the top part develops mud-grade sedimentary rocks with microscopic lamina.The GR-△SP logging identification chart of sand-grade mixed sedimentary rock and GR-AC logging identification chart of mud-grade mixed sedimentary rock in Funing 2^ndMember are compiled.The spatial distribution of the mixed sediments is defined and the main controlling factors are discussed.For the Funing 2^ndMember sand-grade mixed sedimentary rocks,the tectonic setting controlled the stable and differential supply of siliciclastics,the paleoclimate and the provenance lithology were the main factors affecting the carbonate precipitation,and the turbulent lake water and seasonal flood were the hydrodynamic conditions for the formation of MSCSs.For the Funing 2^ndMember mud-grade mixed sedimentary rocks,it is pointed out that the seasonal strong stratification of water controlled the formation process of mixed deposition,and the water depth affected the preservation of mixed deposition.Two types of mixed sedimentary models are established in the study area,and it is believed that the palaeogeomorphic characteristics and corresponding water depth in different stages are the main factors leading to the different distribution of mixed sedimentary rocks.The Funing 2^ndMember sand-grade mixed sedimentary rocks in the study area are in intermediate lithogenic stage A.It is clear that the porosity of sand-grade mixed sedimentary rock caused by traction flow in Jinhu Sag is about 3%～22%,early carbonate cementation and later secondary dissolution are the most important diagenesis,and secondary dissolution pores are the main reservoir space.In Gaoyou Sag,the sand grade mixed sedimentary rock caused by gravity flow was seriously damaged by compaction and its porosity is less than 5%.It is pointed out that Gaoji and Tanggang areas of Jinhu Sag are the advantageous areas of Funing 2^ndMember sand-grade mixed sedimentary rock reservoir.The nano-micron intercrystalline pores in the Funing 2^ndMember mud-grade mixed sedimentary rocks have good storage performance,but organic pores have poor storage performance.It is found that the biological core-pores are a special kind of high quality reservoir space.Fine/thin lamina mixed sedimentary rocks dominated by carbonate and fine lamina mixed sedimentary rocks enriched by biological shell are the most favorable lithofacies for unconventional oil and gas exploration.Based on the lithofacies evaluation of mud-grade mixed sedimentary rocks and the maturity characteristics of Funing 2^ndMember source rocks,the favorable area classification is completed,and it is pointed out that the Funing 2^ndMember"Qijianfeng"section in the north of Jinhu Sag Longgang sub-sag is the most favorable area and section for unconventional oil and gas exploration.