Types And Depositional Characteristics Of 'source To Sink' Systems Of The Strike-slip And Extensional Composite Basin In Oligocene,liaodong Bay Depression,China

Tectonic styles and sand-control factors of continental rift basins with narrow sedimentary facies belt are extremely complicated and their reservoir prediction is also difficult.The application of traditional sand-control theories encountered numerous problems in continental rift basins of Bohai Bay.Meanwhile,modifying on depositional basins formed by strike-slip faults increaseed more difficulties on reservoir prediction.The analysis on 'source to sink' has become a hot issue toward basin analysis and reservoir prediction.Numerous types of 'source to sink' systems were developed in Oligocene of Liaodong Bay depression in Bohai Bay basin,which controlled the spatial stratigraphic distribution of sedimentary sands in depositional basins.Based on the newly 3D seismic data covering Liaodong Bay,drill data and logging data combined with sedimentary geology and sequence stratigraphy theories,palaeogeographic patterns could be restored in Oligocene of Liaodong Bay depression.Characteristics,distribution laws and coupling relationships of provenance,transport and depositional systems have been specified from the points of provenance areas,transport areas and depositional areas.Types and depositional characteristics of 'source to sink' systems under strike-slip and extension tectonic activities have been established based on the quantitative analysis on units and control factors of 'source to sink' system.Not only that,the depositional processes in Oligocene of Liaodong Bay depression can also be restored to designate the favorable 'source to sink' areas.The research achievements of this article could not only enrich the 'source to sink' system theories of continental rift basins,but also instruct oil-gas exploration in Bohai oil field.The main research achievements of the article are as follows:(1)On the basis of sequence stratigraphic theories,the Oligocene depositional stratas were divided into five 3rd-order sequences(SQs2,SQs1,SQd3,SQd2,SQd1)with combination of well log and seismic data.The Oligocene deposits were classified into 5 depositional systems including braid river delta,meandering river delta,fan delta,sublacustrine fan and lacustrine facies by using well logging data,well cores and 3D seismic data.The depositional systematic distribution of Liaodong Bay depression can be revealed as follows according to the combination of seismic facies and well logging facies.SQs2 stage: Braid river delta and fan delta facies were developed together and no obvious deep lacustrine facies were observed;Liaoxi Uplift provided clastics alone with minitype fans around it.SQs1 stage: Shallow lacustrine facies with abundant muddy deposits were developed;carbonate deposits were accumulated on Liaoxi Uplift.SQd3 stage: More braid river delta and deep lacustrine faces were developed in Liaodong Bay,but sandy deposits were poor in Lizhong Sag.SQd2 stage: Large-scale deltas and numerous sublacustrine fans were developed in this stage;superposition delta fans were well developed in axis direction owing to strike-slip faults in Liaozhong Sag.SQd1 stage: Large-scale braid river delta and meandering river delta in axis direction were developed well in this stage with low accommodation and sand-rich in basin.(2)Types of provenances were indicated by light and heavy minerals,lithic fragments,and stratigraphic characteristics in convergent areas to reveal the differences between extrabasinal regional provenances and intrabasinal partial provenances.Extrabasinal regional provenances include Dashi river,Liugu river,Lin river,Daqing river and Fuzhou river systems supplied clastics steadily with little variation during SQs2 to SQd1 of Oligocene.The clastic supply capabilities of Dashi river,Liugu river systems are strong,while it was moderate in Lin river,Daqing river systems and weak in Fuzhou river systems.The supply capability of JZ25 source to sink system was strong,JZ20 and LD16 source to sink systems was moderate and LD5 and LD9 source to sink systems was weak,which formed intrabasinal partial provenances.(3)Sediment transport pathways were divided into erosional valleys,depositional valleys and depositional slopes of east and west sides in Liaodong Bay.The transportation capability of sediment transport pathways was under the control of strike-slip activities and tectonic uplift and is intense in west but weak in east during same depositional stage.The scales of intrabasinal sediment transport pathways were smaller than extrabasinal ones.Minitype half-depositional valleys were developed on long axis and gentle slope of short axis directions of intrabasinal uplifts,and minitype erosional valleys were developed on steep slope of short axis directions of intrabasinal uplifts.Types of sediment transport pathways could transform from erosional valleys to depositional valleys,and finally to depositional slope during the later period of rift lake basin with the rising of base level,which is owing to filling and leveling up and alleviation of tectonic activities.(4)Slope-break belts under the control of paleogeomorphology of convergent areas were divided into 9 types.The developing and combinational characteristics of depositional systems on different slope-break belts were analyzed through their spatial stratigraphic distributions.Depositional sandy deposits in Oligocene of Liaodong Bay depression were described as 15 extra-basinal and 5 intra-basinal 'source to sink' systems in the light of paleogeomorphology.The scales of depositional sands and the terrain elevation is thought to have the highest correlation according to the quantitative analysis on parent rocks,slope-break belts,faults activities,depositional thickness and so on.These 'sour to sink' systems were classified as 6 types extra-basinal and 3 types intra-basinal 'source to sink' systems containing 1)(extra-basinal source)geomorphology gentle slope 'source to sink' system,depositional gentle slope 'source to sink' system,fault-step gentle slope in same direction 'source to sink' system,fault-step gentle slope in opposite direction 'source to sink' system,faulted steep slope 'source to sink' system and strike-slip fault 'source to sink' system;2)(intra-basinal source)normal slope 'source to sink' system in long axis,minitype steep slope 'source to sink' system in short axis,and minitype gentle slope 'source to sink' system in short axis.(5)Based on the spatial stratigraphic distribution of 'source to sink' systems,the 9 types of 'source to sink' systems are supposed to be obvious regionalization,which were under the control of tectonic activities and paleogeomorphology,so the 'source to sink' processes in Oligocene of Liaodong Bay depression could be restored in consideration of depositional system analysis.The scales of depositional sands and fault activity rates in different 'source to sink' system during each Oligocene sequence were quantitatively analyzed to ascertain favorable 'source to sink' areas,so that the superimposing maps of favorable 'source to sink' areas in Oligocene of Liaodong Bay depression could be indicated.The area is mainly distributed in the middle and northeast of Liaodong Bay depression,which is under the control of clastic supply capabilities of provenance,accommodation and paleogeomorphology formed by strike-slip fault activities.