Characteristics Of Sequence Stratigraphy Of Depression Basin And Distribution Of Litho-strati-Reservoir

The Cretaceous Songliao Lake was developed in a humid climatic zone. During the period of maximum flooding, the lake suffered at least 3 times of sea invading.which brought abundant of organic materials.The basin was in the phase of rifting from the late Jurassic to early Cretaceous.and in a stage of depression from the mid cretaceous to the late cretaceous.present a depositional characteristics of a huge lake. Stratigraphic/lithologic traps play a very important role in reserves and production increase in Chinese onshore exploration.it is practically proved that sequence stratigraphy is one of the key exploration technologies.The purpose of sequence stratigraphic analysis is recognize and trace the key interface, by identify sequence boundary,original flooding surface, maximum flooding surface.integrate with the analysis of sedimentary system, Lithofacies ,summarize the principle of distribution of strati-sequence,constitute sequence stratigraphy framework. Lowstand system tract(LST) is a suit of depositional system which developed on the surface of sequence interface and beneath original flooding surface. Transgressive systems tract (TST) is restricted between the original flooding surface and maximum flooding surface Highstand system tract(HST) is developed on the original flooding surface and beneath a new sequence boundary. Analysis on the information of the characteristics of shallow structure, thickness of stratum and the development of structure and sediment showed:there are obvious sructural slope-break zone on the Longhupao-Da-an step. and sructural slope-break zones in the area can be divided into 2 ranks.The role of tectonic, eroding and sediment are the 3 geological forces which controlled the formation of sructural slope-break zone.Theoretically, every geological forces has its own special type of sructural slope-break zone.In fact the sructural break-slope zone are effected by the different geological forces during different epoches of development in the basin,so the types of sructural slope-break zone are always in a transitional genre. Oberservation of rock indicate that the sedimentary system style include lake,delta and turbidity fans.The analysis of sedimentary system showed: Continent-scouce rock materials in the west area are abundant,the major deposit system is composed mainly of fluvial-delta deposit system. Delta system are fully developed.from quantou formation to yaojia formation,there were several times of lake Transgression and regression. Under the background of sructural slope-break zone, the inner architecture of system tract can be identified. DuringLST,lake shore is below the low sructrual break-slope zone. continent-scouce clastic rock is transmited into basin which the lake lever is under the base level,from lake to bank the constitution of LST are half-deep mudstone ,shallow lacustrine sediment, turbidity fans , incised channel filling and flood plain.During TST, as the accommodation space constantly increased,sediments are almost hold on the bank where the lake shore is constantly transgressing, from lake to bank the constitution are deep mudstone,oil shale,delta front delts ,distal bar. and mouth bar of fan delta, shallow lacustrine of transgressive lake.During Hst,while the rising speed of base-level decreased gradually,the increasing speed of accommodation space decreased. Sediments are constantly put forward. The constitution of HST is similar with TST. But the delta is developed in the environment of lake regression. Based on comprehensive research of the sedimentary system the distribution of sands bodies are obvious, subfans are well developed inLST and are mainly distributed beneath the slope feet. Low sructrual break-slope controlled the distribution of turbidity fans, Lake shore that migrated many times in the history of geology which controlled the bank sediment,along the different Lake shore,subtle traps possessed the trait of groupful and scopeful. sructrual slope-break zone controlled distribution of delta front delts. river channel on the sructrual break-slope zone controlled the thickness of incised channel filling. The forming mechanism of litho-strati traps is:During TST, incised channel are well developed,which will forming incised channel filling of mid porosity with mid permeability Fan delta, turbidity fans beneath slope are located near the oil production center, provide suitable conditions for different kinds of litho-stratigraphic traps,are the most favorable target.During TST ,distal bar and subaqueous channel deposit are paned frequently,forming sand reservoir of mid porosity with mid permeability cover by lake mud.During HST,sand bodies are piled up toward lake due to the role of advancement, mudstone upon sand bodies can be served as regional seal bed it is difficult for hydrocarbon migrate horizontally inside the sand body,it is the fault that make oil move upward to the reservoir. Traps in this area are diversified as a result of the special physiognomy. plays cotrolled by low-level sructrual slope-break zone are mainly reserval lithologic-structural traps, lithologic traps, lithologic-stratigraphic traps.Play between sructrual break-slope zones are mainly fault-lithology play and sratum ontapping play. Play controlled by high level sructrual slope-break zone, are mainly stratigraphic onlap reservoir. The consept of sructrual slope-break zone in lake are similar to Vail`passive continental marginal basin, so the oil exploration offshore can take a reference onshore and there also have great difference...