Controlling Effects Of Contemporaneous Normal Fault In Different Scale On Deltaic Depositional Architecture

In recent years,the research on sedimentary response of basin tectonic activity has received extensive attention.Previous studies mainly focused on the control of large-scale basin level contemporaneous faults on the distribution of depositional system.In recent years,scholars around the world have found that there are many small-scale contemporaneous normal faults in rift basins,which have small scale,limited control range,complex distribution shape and large amount.However,at present,the research on the control effect of such locally developed small-scale contemporaneous faults on reservoir architecture is still relatively poor.Deltaic sandstones are important oil and gas reservoirs in rift basin.At present,the control effect of contemporaneous normal faults with different scale on deltaic depositional architecture still needs to be studied.The target layer of this dissertation is the lacustrine delta deposition of the second member of Shahejie Formation of the Wang43 complex fault block in Wangjiagang Oilfield,Dongying Depression.This dissertation comprehensively applies core,wire-line log curves and 3D seismic data to explore the sedimentary architecture characteristics of deltaic sand bodies controlled by contemporaneous faults in complex fault block area of rift basin from the perspective of sedimentary response to tectonic activity.Based on the high frequency sequence stratigraphic framework of different orders in the second member of Shahejie formation,the temporal and spatial distribution of contemporaneous faults and their control effect on stratigraphic development styles and paleogeomorphic units was clarified.There are 9 contemporaneous faults in the study area,one of which is 3^rd order,and the others are 4^th order faults.The distribution of contemporaneous normal faults is dense and has different dip ditrection.There are four contemporaneous normal faults assemblages:step style,domino style,graben style and horst style.The distribution of fault activity along the strike of contemporaneous normal faults is uneven.The contemporaneous normal faults have the characteristics of segmented and continuous growth.Three types of high-frequency sequence structural styles under the control of contemporaneous normal faults are proposed.They are uniform subsidence development style,differential subsidence development style and filling and leveling development style.Due to the different activity of different parts of a single contemporaneous normal fault,micro paleogeomorphic units related to fault segment weak fault activity and micro paleogeomorphic units related to fault segment with strong fault activity were formed.The contemporaneous normal fault assemblages with different tendencies controlled the development of fault related paleogeomorphic units,forming several micro horst belts,graben belts and terraces controlled by fault activities.Due to the scale difference of the contemporaneous faults,the third-order contemporaneous faults experienced a long time of activity and a large range of influence on the related paleogeomorphology,while the fourth-order contemporaneous faults formed during the deposition of the second member of Shahejie Formation controlled a small influence range on paleogeomorphology.The development characteristics and distribution of shallow water deltaic depositional facies controlled by contemporaneous normal faults in different scales wereclarified,and the sedimentary model of shallow water delta controlled by contemporaneous normal faults in different scales in complex fault block area was proposed.The deposits formed in the second member of Shahejie Formation in the study area were developed in the seasonal shallow water delta under semi-arid paleoclimate.Affected by the seasonal sediment supply,the thin sand bodies in distributary channels were developed,and the vertical interbedding with red mudstone was developed.Contemporaneous normal faults control the delta depositional process in high frequency cycles,and then affect the planar distribution of sedimentary facies near contemporaneous normal faults.Influenced by dip direction and fault activity of the contemporaneous faults,the deltaic distributary channels are more likely to pass through the position with weak fault activity.In the area with strong activity of north dipping contemporaneous normal fault,the distributary channels are deflected or limited at the position with strong fault activity.The distribution range of the distributary channels are increased.The depositional facies types,and distribution on both sides of the contemporaneous faults are different.In the area with strong activity of south dipping contemporaneous normal fault,the distributary channel is restricted in the process of deposition,and mainly deposited in the downthrow side.The distribution of shallow water deltaic single sand bodies controlled by contemporaneous normal faults in different scales was clarified,and the control effect of contemporaneous normal faults on the distribution of oil layers in complex fault block area was summarized.Further analysis shows that contemporaneous normal faults can also be used as boundary of the deltaic single sandbodies,and control the scale and superimposition style of single sand bodies.The flow direction and sand body deposition model are controlled by the micro geomorphic unit related to single contemporaneous normal fault.The scale and vertical superposition style of single sand body are controlled by the micro horst belt and micro graben belt of contemporaneous normal fault combination.The thickness and scale of sand bodies in the downthrow side of contemporaneous normal faults are positively correlated with the activity of contemporaneous normal faults.In the fault terrace zone and graben area formed by contemporaneous normal fault combination,the thickness of sand body is increased,and multi-stage continuous superimposed sand bodies are developed vertically.In the micro horst area formed by south dipping and north dipping contemporaneous normal fault,the thickness of sand body is small,and isolated single sand body sand body is developed vertically.The oil reservoirs in the study area are mainly located in the distributary channel sand bodies,and the distribution range is affected by faults and sedimentary facies boundaries.The contemporaneous normal fault controls the oil reservoir boundary and development scale.In the hanging wall area of contemporaneous fault with strong activity and the footwall area of contemporaneous fault with weak activity,continuous fault related lithologic oil reservoirs could be formed vertically.