Study On The Sedimentary System Of The Paleogene Shahejie Formation In Southern Coast Of Laizhouwan Sag,Bohai Bay Basin

The study area is located in Laizhou Bay depression in the South of Bohai Sea.The study is sandwiched between the east and west branches of the Tanlu strike-slip fault in the southern part of the Bohai Sea.Since 1989,several hydrocarbon-bearing structures have been discovered,and three levels have been discovered.The petroleum geological reserves are about 1.3×108t,and the exploration potential is huge.But so far,the existing research on the Shahejie Formation in the southern part of the Laizhou Bay Sag still has the following problems:?1?The structure and genetic evolution of the southern slope belt and the eastern strike-slip belt are very complex.?2?The sequence stratigraphic framework of the Shahejie Formation of the Paleogene in the study area cannot meet the needs of high-precision research,and the drilling and seismic stratification in some areas do not match.?3?The study of sedimentary system and facies is relatively low,and the main controlling factors and evolution rules of sedimentary facies are still unclear.?4?The study area is far from the main hydrocarbon-generating depression,and the scale of the internal hydrocarbon-generating depression is small,and the hydrocarbon-generating potential of source rocks is still unclear.?5?Differential enrichment patterns and main controlling factors of oil and gas in Lainan slope zone and Laidong strike-slip zone need to be put forward.This study is directed by the theory of sedimentology and sequence stratigraphy,aiming at the study of the main controlling factors of sedimentary system distribution and sedimentary evolution.Using regional geology,three-dimensional seismic data,core and logging data,the isochronal sequence stratigraphic framework of Paleogene Shahejie Formation in the southern part of Laizhou Bay Depression is established.Under the constraints of isochronal sequence stratigraphic framework,the types of sedimentary systems are divided,the distribution boundaries of various sedimentary facies belts are accurately depicted,the spatial distribution characteristics of sedimentary systems and the controlling factors of sedimentary facies evolution are discussed,and the sedimentary facies model is established.The main achievements and understandings are as follows:?1?The sequence stratigraphic framework of Paleogene Shahejie Formation in the study area has been established.The Paleogene Shahejie Formation in the study area has identified nine isochronal interfaces.It includes three secondary interfaces:SB₅?T₅??SB₆?T₆??SB₇?T₇?,four three-level interfaces:SB₃?T₃??SB₆¹?T₆¹??SB₆²?T₆²??SB₇¹?T₇¹?,two maximum flooding surfaces:MFS₆^2'(T₆^2')?MFS₆^1'(T₆^1').?2?The sedimentary system and distribution of the Paleogene Shahejie Formation in the study area are clarified.Braided River delta,fan delta,beach bar and shore-shallow lake depositional systems are mainly developed in the study area.The braided river delta depositional system developed in the western part of the study area.Fan delta depositional systems are developed in the East and Braided River Delta and fan delta composite deposits are developed in the southern slope zone.The beach-bar sedimentary system is developed in the local uplift area of the front of the braided river delta in Western China.There are two-way sediment supply systems in southwest and Southeast China.?3?The evolution law of sedimentary system in the study area is summarized.The development type and scale evolution law of the sedimentary system of Shahejie Formation in the study area can be divided into two stages.?1?From the fourth to third member of Shahejie Formation,braided river delta,fan delta,saline lake basin and beach bar system developed,and the delta scale was small.?2?The braided river delta,fan delta and lake sedimentary system are the main sedimentary systems in the third member of Shahejie Formation,the first member and the second member of Shahejie Formation.Braided river deltas in early Western and southwestern China were small in scale,and sandy and calcareous beaches and dams were well developed.Late braided river deltas are well developed.From bottom to top,the braided river delta in Western China is smaller and then larger,and the scale of fan delta varies similarly.The lakes gradually transited from early salinization to brackish water and freshwater-brackish water.?4?The sedimentary model of the study area and four sand control models of secondary tectonic belts in the study area have been established.?1?The Braided River Delta is the most developed slope zone in the western part of the study area,and the direction of provenance is parallel to the strike of the main fault,which is a sediment control model with parallel point provenance.?2?The strike-slip zone in the northeastern part of the study area develops a series of secondary echelon faults with nearly north-south strike.Its strike obliquely intersects with the direction of provenance,which provides migration channels for clastic materials and controls the size and shape of sediment deposits,as well as the sediment control model for the provenance of the echelon oblique intersection point.?3?The main faults in the southeastern part of the study area have small scale but large drop,most of them are distributed in the North-South direction,perpendicular to the source direction,and are the sediment control model of the vertical line of fault step.?4?Faults in the northern steep slope zone of the study area mostly consist of fault terraces,which spread eastward and westward,and are the sediment control model of the relay oblique intersection point.Based on the study of sedimentary system,sedimentary facies and sand control models of secondary tectonic belts in the study area,the sedimentary model of Paleogene Shahejie Formation in southern Laizhou Bay Depression is established by considering synthetically the controlling effects of faults on the strike of sand bodies,slope on the scale of sand bodies and water depth on the types of sand bodies.?5?The favorable facies zones of source,reservoir and caprock in the study area are predicted.The study area has good hydrocarbon generating capacity,including two main hydrocarbon generating sags,the South sub-depression and the North sub-depression.The deep-semi-deep lacustrine mudstone has the largest thickness and high hydrocarbon generating potential in the middle and lower sub-member of Sha 3and Sha 4.The best reservoir facies belts should be braided river delta,fan delta estuary bar and underwater distributary channel microfacies.Generally,the distribution scale of the first,second,upper and middle sub-sections of Shahejie Formation is relatively large.A set of shore-shallow lake mudstones are widely developed in the first,second,upper and middle sub-member of Shahejie Formation,with high continuity and large thickness,which can play a role of regional caprock and provide favorable conditions for the preservation of Paleogene Shahejie Formation reservoirs.