Sequence Stratigraphic Characterization And Lithologic Trap Distribution In Large Depression Lake Basin

The continental depression type lake basin has stable provenance,thin sedimentary reservoirs,wide distribution range,obviously controlled by lacustrine fluctuation,strong reservoir heterogeneity and the lithologic trap has a complex origin and distribution.These characteristics affect the exploration and development of lithologic reservoirs.How to establish sequence stratigraphic framework of a large depression-type lake basin,determine the spatial distribution pattern of the sedimentary system and sand body under sequence framework,and determine the spatial distribution pattern of different genetic lithologic traps has become an important problem for the exploration and development of massive lithologic oil and gas accumulation in the depression type basin.This thesis is based on a large number of core and outcrop,logging data and predecessors'studying results.Comprehensive research is carried out in Yanchang Formation in the Longdong Area of Ordos Basin about the sequence stratigraphy,genetic types of sand body,sand body spatial structure and sequence stratigraphic framework of lithologic reservoir genetic types,lithologic trap formation and spatial distribution characteristics and so on.Based on the identification of the accumulation pattern and 11 time-significant interfaces of the Yanchang Formation in the Longdong Area of Ordos Basin,Yanchang Formation is divided into 1 super-long-term cycle,5 long-term cycles(third-order sequence)and 17 medium-term cycles(fourth-order sequence).LSC?is mainly composed of Chang 10 oil member,which contains three of the middle-term cycles.LSC?is composed of Chang 9₂,Chang 9₁,Chang 8₂ oil members,included three middle-term cycles;LSC?consists of Chang 8₁-6₃ oil members,contains three middle-term cycles;LSC?consists of Chang 6₂-3₃ oil members,included three middle-term cycles;LSC?consists of Chsng32-1 oil members,contain the five middle-term cycles.Based on sand and mud superimposed style,and controlling factors and symmetry characteristics of the process of the base-level cycle.The long-term base-level cycle could be divided into low water level slowly rising cycle structure(LSC?-?),high water level rapidly rising and slowly falling cycle structure(LSC?-?),high water level decline slowly cycle structure(LSC?).In the middle-term cycle structure,asymmetrical cycle or nearly symmetrical cycle structure is mostly developed in base level rising semi-cycle structure,while the semi-symmetrical cycle structure dominated by base-level falling is not developed in various sedimentary environments.The short-term base-level cycle can be divided into three types,asymmetric cycle with upward"deepen",asymmetric cycle with upward"shallower",and symmetric cycle with 7 types.The study area is dominated by river controlled braided river delta,gravity flow subaqueous fan deposition and lacustrine deposition developed in the southwest provenance area,among which the delta front subaqueous distributary channel sand body,mouth bar sand body,channel-mouth bar composite sand body and deepwater turbidite fan sandstone are the most important reservoir sand bodies of Yanchang Formation.The Yanchang Formation has complex sand and mud assemblage styles and spatial distribution characteristics.According to the vertical interbedded features of sand and mud,it is divided into six assemblage styles:Sand in the mud,embedded mudstone,interactive sand and mudstone,mud in the sand,sandstone finger type and pure mudstone.According to logging and outcrop survey results,the sandstone can be divided into four vertical superposition styles:independent,superposition,cut and superposition and compound.The plane contact relationship of sandstone in different cycles is classified into 7 types:Integral(isolated type),overflow(levee)contact,distributary bay contact,joint type,lateral cut,substitute,and mouth bar joint.Current exploration results and typical reservoir analyses show that the Yanchang Formation in Longdong Area is mainly composed of lithologic and structural lithologic reservoirs.The lithologic reservoir is divided into 3 classes:Sandstone up-dip pinch out,sandstone lens and the heterogeneity(property)block,can be further subdivided into 10kinds of reservoir types:Sandstone up-dip pinch out reservoir,the sandstone lateral up-dip pinch out,mouth bar lenticular sandstone reservoir,turbidity fan lenticular sandstone reservoir,the thin layer of channel sandstone physical property lens reservoir,thick layer of the channel-mouth bar complex sandstone lens physical property reservoir,substitutional heterogeneity(lithology)lateral block reservoir,lateral cutting type lateral heterogeneity block reservoir,destructive diagenesis physical property sealing reservoir,constructive diagenesis physical property sealing reservoir types.Therefore,lithologic traps are divided into 3 types,6 subtypes and 10 genetic types based on factors such as the origin of traps,sand body types and spatial distribution characteristics.This study holds that the genesis and spatial distribution of the lithologic traps in depression type lake basins are not only related to the rise and fall of long-term cycle base level but also closely related to the sedimentary period of paleo-lake basin bottom type and the spatial distribution and vertical superposition style of sandstone caused by paleo-lake bottom type.The sandstone lens reservoir is mainly an in-source reservoir,which is mainly distributed in the delta front mouth bar and deep-water turbidity fan sand body near the flood surface FS2,FS3 and FS4(high-quality hydrocarbon source rock).The up-dip pinch out and(physical property)heterogeneous barrier reservoirs of sandstone are mostly off-source reservoirs,mainly composed of subaqueous distributary channels and channel-mouth bar superposed sand bodies,which are widely distributed in each layer of Yanchang Formation in Longdong Area.Among them,in the system tract of the long-term slow rising base level of low water level,it is related to the retrogressive accumulation sand body of the delta front,and the sandstone pinch out lithologic traps are formed along the slope break zone of the ancient lake basin successively.In the system tract with low water level and long-term base level slow decline,the topmost overlying sandstone with strong progradational delta formed dipping pinch out lithologic trap distributed continuously into the basin.There are various types of lithologic traps in the subaqueous distributary channel,mouth bar and channel-mouth bar complex in the delta front,mainly physical sealing and heterogeneous blocking lithologic traps.In the system tract of high water level falling slow and low water level falling fast,a large number of lithologic traps are formed in the deepwater area of the basin,which are interbedded with hydrocarbon source rocks.To sum up,the formation of lithologic traps in depressed-type lake basin is characterized by complexity,diversity of types and spatial distribution.The paleogeomorphology and the bottom type of lake basin determine the dispersal path and pattern of sediments and the spatial distribution of the lithologic traps in the model.The base-level cycle process controls the spatial and temporal disposition of sand and mud(reservoir-source)rocks,and the matching of paleogeomorphology in the sedimentary period controls the types and spatial distribution of lithologic traps.The genesis,morphology and spatial distribution of lithologic traps are adjusted by the genetic types and structural characteristics of sand bodies under sequence lattice.Also,the spatial and temporal disposition of source,reservoir and oil source faults determines the effectiveness of lithologic traps.