Gravitational And Magnetic Fields And Structural Features In Shiwu Fault Depression

Shiwu Fault Depression constitutes a superimposed one with a break-sinking characteristic developing in the Early Cretaceous Epoch.Located in the southeast of the Song Liao Basin,the Shiwu Fault Depression is one of the most important regions for petroleum geology research and petroleum prospecting.For years of exploration,many achievements have been made in the shallow strata of Shiwu Fault Depression.However,the research of deep formation is still at a low level,the exploration of magma and basal structure is still in the initial phase.The fault structural framework is not clear yet.After a systematic processing of gravity and magnetic data of Shiwu Fault Depression,analytical continuation figure,moving average filtering figure,horizontal direction derivative,basic datum level total gradient figure and other base maps have been got.On this basis,combined with seismic,geological and drilling data,this paper concluded the faults structure,explained the cause of local gravity and magnetic anomaly,determined the distribution characteristics of the magnetic body.And then,the basement lithology distribution was predicted,the geological tectonic units were divided.The main achievements and conclusions are as follows:(1)The residual gravity anomaly in research area distributed zonally high and low,the trend is NE.The local gravity anomalies are caused by intrusive igneous rocks and depth of the basement.Basement uplift tends to cause local high gravity anomaly,whereas basal sag zone cause local gravity low anomaly;accordance with its surrounding formation for basal or cover,intrusive igneous rocks cause low or high gravity anomalies.(2)Fractures are developmental in the study area.The trends of faults structure are mainly NE,NW and EW.The extension scale of NE faults is large,and the distance is long too.NE faults are the main faults of the basin,and controlled the distribution of stratigraphic and structural framework.(3)The cretaceous is widely distributed in research area,and the lower boundary of the depth is different.In west sub-sag,the thickness of sedimentary formation is larger,the biggest can reach 8000 m.From west to east,sedimentary formation grows thinning,which shows aobvious basket shape fault characteristics.(4)Magnetic body are widely distributed in the research area,and obviously controlled by fractures,towards the same direction of the fractures.Local magnetic anomaly is mainly composed of magnetic basement uplift,the acidic and basic magmatic rocks.In the research area,Deep metamorphic rock cause lower positive magnetic anomalies;Acidic magmatic rocks cause negative magnetic anomaly;Basic magmatic rocks cause high magnetic anomaly.(5)In the research area,basement lithology can be divided into six classes: the deep metamorphic rock,shallow metamorphic rock,acidic magmatic rocks,neutral magmatic rocks,basic magmatic rock and mixed rocks which contains shallow metamorphic rock and acid magmatic rocks.Deep metamorphic rocks are distributed in the basement uplift of Sangshutai;magmatic rocks are usually distributed along with the fracture;shallow metamorphic rock are distributed throughout the research area.(6)The research area can be divided into 5 secondary tectonic units: Sangshutai sag,Sujiatun sag,Shuanglong sag,central uplift and southeast slope.