Study On Sedimentary Facies And Reservoir Charateristics Of The Upper Fourth Member Of Shahejie Formation In Dongying Sag,Bohai Bay Basin

The Bohai Bay Basin is assumed to be the most oil and gas bearing basin in China.The Paleogene half graben Dongying Depression which is a suborder structural component situated in the Jiyang Sub-basin of the Bohai Bay Basin,which is considered by its distinctive petroleum geology.In this study,two typical blocks of Li567 and Tuo764 in the north of Dongying Sag in Bohai Bay Basin are taken as the research objects.The sedimentary environment and reservoir characteristics of sedimentary gravity flow and turbidites deposits are studied,which provides guidance for the exploration and development of low-permeability reservoirs in the study area.This study will help to understand the nature of the deposition and reservoir characteristics of the upper Es4 s sandstone which is assumed to be potential reservoir in the study area.Facies analysis,grain size distribution,petrographic and well log interpretations have been conducted during this study.Massive sandstone,parallel laminated mudstone,interbeded sandstone and mudstone,contorted(deformed)sandstone and mudstone,parallel laminated sandstone facies of upper fourth member of Shahejie Formation in Tuo764 area suggested to have deposited in distal lobes and deep lakes influenced by turbidity.Compaction is the main cause of reduction of porosity in Tuo764 area.Most of the primary porosity has been destroyed by compaction and cementation.Secondary porosity,dissolution porosity and fracture porosity is dominant in Tuo764 area.The particle contact type is mainly point-line contact,followed by line contact and point contact.The dissolution pores are long and narrow,with varying in sizes and connectivity with the pore throats.Planar bedded mudstone,planar bedded sandstone,matrix supported conglomerate,gradded and pebbly sandstone,parallel bedded silty mudstone refers to be deposited in near shore,sub-aqueous fans of fan delta system.The particles in the rock are mainly line-point contact,while the line contact and point contact are the second.It has mechanical compaction,metasomatism,cementation and dissolution.Cementation is the main reason for the reduction of primary porosity in Li567 area.Most of the potassium felpdspar concentration in the samples of li567 has been replaced by quartz calcite and other cementing minerals.Calcite is the main cementing mineral in the study area which has filled almost all the primary porosity.Quartz overgrowth has also destroyed much of the primary porosity in Li567.It is quite obvious that the upper fourth member of Shahjie Formation in Li567 area has went under strong diagenesis processes as compared to Tuo764 area.The Structureless or massive sandstone facies(fss)of distal lobes and deep lake subfacies constructs a reservoir in the upper fourth member of Shahejie Formation Tuo764 area.This sedimentary facies is comprised of coarse to fine grained,well sorted and subangular in nature.It has less intergranular pores but more of fracture porosity constructs a reservoir.The reservoir zone 1 in Tuo764 area ranges from 3949-3962 meters having total thickness of 11 meters and containg 11.95% average porosity.The planar bedded sandstone facies(fpbs)of inner fan deposited in underwater distributary channel;having medium to fine,well sorted and rounded to subangular grains;graded and pebbly sandstone(fgps)medium to coarse grained,moderately sorted and sub rounded in nature construct a reservoir with in the upper fourth member of Shahejie Formation in Li567 area.These facies mainly belong the the fan delta front and were deposited by under water distrbutray channels of the fan delta and dissolution porosity is dominant in these facies.The depth of the zone 2 in the upper fourth member of the Shahjie formation in Li567 starts from apprtoximately 2919-3119 meters having,total thickness about 200 meters bearing average porosity of 11%.The well logs showed that porous resevoir zone 2 in Li567 has bigger thickness.The average porosity in zone two of Li567 having thickness of 200 meters was recorded as 11%but as compared to reservoir zone 1 in Tuo764 bearing thickness of 11(m)has higher average porosity of 11.95%.