断陷湖盆浅水三角洲的形成过程与发育模式——以苏丹Muglad盆地Fula凹陷Jake地区AG组为例

The forming process and development pattern of shallow water delta in fault depression lacustrian basin:a case study of AG Formation in the Jake area in Fula sag,Muglad Basin,Sudan

以苏丹Muglad盆地Fula凹陷Jake地区为例,利用岩心、测井和三维地震资料,结合水槽模拟实验展开湖盆浅水三角洲沉积学特征的研究,总结浅水三角洲形成过程和发育模式。Jake地区白垩系Abu Gabra组上部发育浅水三角洲,随着水体变浅,其单一朵叶规模和沉积物厚度逐渐减小。在垂向上分别出现小型叠瓦状、波状、薄片状和发散状4种前积反射样式,单一同相轴长宽比依次增大。而在平面上,朵叶中水道的频繁分叉和改道使分流河道呈鳞片状展布。浅水三角洲的发育期受季节性水位升降和物源供给影响较大,一个沉积周期中可分为洪水期、间洪期和枯水期。洪水期物源供给充足,其前中期水位高,水道在水下被破坏,朵叶快速向前呈席状沉积,后期水位下降,朵叶露出水面,新水道出现;间洪期水位较高,物源供给较充足,主流线频繁变化,水道宽而浅,朵体扩大明显;枯水期物源供给匮乏,水位低,水道窄而深并下切朵体,朵叶建设作用弱。

Taking the Jake area in Fula Depression,Muglad Basin,Sudan as an example,the sedimentology characteristics of shallow water delta in lacustrian basins were studied using core,logging and 3D seismic data in combination with flume simulation experiment,so as to summarize the formation process and development pattern of shallow water delta.Shallow water deltas are developed in the upper part of the Cretaceous Abu Gabra Formation in the Jake area,where the scale of single lobe and sediment thickness gradually decrease with decreasing water depth.In the vertical direction,there are small imbricate,wavy,flaky and divergent progradational reflection patterns;the length-width ratio of single event increases successively.On the horizontal level,frequent bifurcation and divagation of channels in lobes result in the scale-like distribution of distributary channels.The development period of shallow water delta is greatly influenced by seasonal fluctuation of water level and sediment supply.A cycle of sedimentation can be divided into flood period,interflood period and drought period.In the flood period,sediment supply is adequate.In the early-middle stage,water level is high and channels are destroyed under water;lobes move forward quickly,showing blanket deposit.In the late stage,water level decreases and lobes break the surface of water.Thus,new channels are formed.During the interflood period,water level is relatively high and sediment supply is adequate;the mainstream line frequently changes and channels are wide and shallow;lobes expand significantly.In the drought period,sediment supply is deficient and water level is low;channels are narrow and deep,undercutting lobes;the construction effect of lobe is weak.