The Research Of Microfacies And Genetic Mechanisms Of Beach And Bar Sandy Body In 2^nd Member Of Shahejie Formation, Dawangbei Sag In Chezhen Depression

The research of sedimentary facies is an important work,which is the groundwork in the exploration of oil and gas.The distribution of reservoir body and the establishment of play have important connection with the types and distribution of sedimentary facies in the basin.So in basin the elaborate analyse of the sedimentary facies is an important working in the exploration of oil and gas.Dawangbei Sag lies in the middle part of the Chezhen depression,which is the sub-sedimentary cell and important play of the Shengli oil field.In the depression the Shahejie 2^nd formation formed fault-depressed period,which is the important exploration target.The exploration area is about 200 km².There are fifty exploratory wells,and there is some core in these wells.3D seismic have worked in the depression and the quality is very fine.The main depositional system of the Shahejie 2^nd formation is the beach and bar,the character of which is laminal sandstone and mudstone alternately appear.And we found the oil and gas in them.The types of sandstone contain:fine sandstone,silt sandstone and calcite sandstone.And the percent of the sandstone is 20～60%in the formation.The thickness of the sandstone is thin,universal thickness is 2～3m,the most thickness is 5～7m.The Shahejie 2^nd formation was divided into 6 sandstone groups,and the oil and gas was found in them.In the different part of the depression, the main oil-bearing series is different sandstone groups.In the gentle slope the main oil-bearing series is 1-3 sandstone groups,and near the central sag is 4-6 sandstone groups.The types of oil pool which we have found,is structural-lithologic oil pool,stratified-lithologic oil pool.But the beach-bar sandy body,the thickness of which is thin,change very quickly in the landscape orientation,distribution of which is complicated.The exploration of the beach-bar sandy body is very hard.At present,the research of inner structure and void distribution on beach and bar sandy body is not consummate all over world and is unsubstantial in research area.In this paper base on the data of core,logging,3D seismic,etc,we analyze the sedimentary facies of beach and bar in the research area. The beach and bar sandy body change very quickly in landscape orientation and vertical direction.In this paper we first constitute the high-resolution stratigraphy trellis.Base on the strata Classification and correlation,the Shahejie 2^nd formation was divided into 9 sub-squences. This working is the base of research of sedimentary microfacies and void distribution of the beach and bar.Beach and bar form in the shallow water—mere and coastal lake.In this paper we analyze the palaeodepth with sedimentary lithofacies and ripple index.We found the palaeodepth of beach and bar sedimentary environment is about 1-7m in Shahejie 2^nd formation sag.So the main rock is fine sandstone,siltstone and muddy siltstone,they were affected by the wave,lake current and windstorm.There are a lot of swash cross bedding,swash cross lineation,wavilness bedding and mud gravel in these sandstone.The sandstone mainly is offwhite,gray,sage green and variedness,and the mudstone,mainly gray-green,gray-black and purple-brown.Because the water is very shallow,the beach and bar easily was affected by the windstorm wave.Mud gravel continually appear.But storm current sediments is different classical tempestite facies.In this area,some mud gravel appear after the windstorm,and the environment still keep natural shallow water sedimentary environment.The storm current only faintly reconstruct the beach and bar sandy body.Base on the working of other researchers,the beach and bar face was divide into key bar, side-portion of bar,between- portion of bars and beach with the different characters of them in this paper.We analyze these characters among different microfacies with sedimentary lithofacies, vertical sedimentary sequence,logging facies and so on.In the key bar sand body,its thickness is big,a little of mud in it,its sorting is fine.The sequence of side-portion of bar is the key bar,but the percentage composition and of sandstone is litter than the key bar.The beach sandy body is very thin,but its acreage is very big.In between-portion of bars,mud and laminal sandstone is main sediments,it is the bad reservoir.We analyze the distribution of beach and bar,base on the characters of each microfacies and the high-resolution stratigraphy trellis.In the S9 to S6 period,the lake level mainly fall down and partial short-lived raise,and in this fault-depressed period,the main the tectonism of sag is taphrogeny.The stage is deeper than the S5 to S1.The Yihezhuang bulge perennial exist and gently shrinks.In the S5 to S1 period,the lake level mainly raise and partial short-lived fall down,in the S1 period the lake area is the biggest,and in this fault-depressed period,the main the tectonism of sag is depression.The structural relief is acetabuliform.The stage become shallow in the whole sag,and the sedimentary area become big than big with the lake level raise. The Yihezhuang bulge perennial gently disappear.At last,a lot of sandy body developed in the whole shallow water over-lake in Dawangbei Sag.In the Shahejie 2^nd formation fault-depressed period,taphrogeny or depression is the main tectonism in different period,but the taphrogeny is weak in the whole period.The whole structural relief is acetabuliform.Base on the analysis of all kind sedimentary sandy body in the sag,we found that a spot of rivers breathe into the sag,the delta sandy body is not flourishing,a little of onshore sediments is bring into sag.And the sag is an classical shallow water over-lake. The wave,lake current and windstorm erode the delta sandy body and lake shore,and a lot of sediments was produced.These sediments was continuously accommodate and times without number carried,redeposit.At last these sediments form the beach and bar sandy body which connect together,and developed in the whole shallow water over-lake in Dawangbei Sag.Base on the developed scope,types,arrangement and other characters of the sedimentary facies in the sag,we construct the sandy beach and bar sedimentary facies model of shallow water over-lake of the Shahejie 2^nd formation in Dawangbei sag.And we contrast this model with the sandy beach and bar sedimentary facies model of the Shahejie 2^nd formation in Gubei Sag and the sandy beach and bar sedimentary facies model of the Shahejie 4^th formation in Gubei Sag,the downfaulted basin.