Sedimentary Characteristics Of Beach-bar Sandbodies And Distribution Of High-quality Reservoirs Of The Upper Fourth Member Of The Shahejie Formation Of Gao 892 Block In The Dongying Depression

Gao 892 block is located in the north part of Boxing Sag,the Dongying Depression,beach-bar sandbody reservoirs of the upper fourth member of the Shahejie Formation are the main reservoirs.With combination of drilling core,well-log datas as well as associated analytical testing,the sedimentary characteristics and distribution of beach-bar sandbody reservoirs in Gao 892 block are analyzed.By analyzing the controlling mechanism of the changes of sediment supply and accommodation space(A/S)to the beach-bar sandbodies in different types of base-level cycles,the characteristics of the reservoirs are as follows:Combing core observation and analysis,two types of short-term cycles of the upper fourth member of the Shahejie Formation in Gao 892 block are established: shallowing upward nonsymmetric and symmetric short-term base-level cycles,the former is more developed.On the basis of different stacking patterns of short-term cycles,the upper fourth member of the Shahejie Formation is divided into 6 middle-term and 30 short-term cycle sequences in this paper.Then the high-resolution sequence stratigraphic framework is established with several sections parallel and vertical to shoreline.By summarizing the well-logging parameters of sedimentary characteristics of beach-bar sand bodies,logging precise recognition is established to identify beach and bar sand bodies of non-coring intervals,then beach and bar sand body units are recognized according to different sedimentary characteristics of interval zone of bar deposits.The paper analyzes the effect of A/S to the sedimentary of beach-bar sand bodies in short-term cycles,and the middle-term cycle sequences which are stacked by different types of short-term cycles also follow the regularity.The distribution of beach-bar sandbodies controlled by various grades of base-level cycles is summarized: The beach-bar sandbodies all deposit in the short-term base-level falling cycle;the beach sand is evenly distributed in every middle-term cycle,and the bar sandbodies are mainly distributed in the early middle-term rising cycle or in the middle of later middle-term falling cycle because of the moderate A/S values,sediments are easy to deposit.In each middle-term cycle,the beach-bar sandbodies are characterized by progradation sedimentation,but they are characterized by retrogradation sedimentation from the entire target strata,in some local deposition the migration characterization is obvious.All the characteristics reflect the beach-bar sandbodies are controlled by A/S values.On the basis of core observation with thin section analysis,the influence of different factors to the reservoirs is considered,and then RQI,lithofacies,thickness of sandbodies are chosen for the controlling factors to evaluate the reservoirs.The beach-bar sandbody reservoirs are classified: type I and type II are better,type III is poor.The sedimentary hydrodynamic force of Type I is strong enough that the grading is better and the shale content is low,then the reservoir physical property is better;the A/S values of sedimentary environment are usually higher in type II and type III,the shale content is high,so the pore-throat structure is poorer as well as the cementation,then the physical property is worse.The high-quality reservoirs of beach-bar sandbodies are mainly deposited in the middle-term falling cycle,and quantities in the latter sedimentary are more than the earlier.