沾化凹陷桩西油田古近系东营组重力流水道的沉积特征及形成条件

Depositional Features and Controls on Gravity Flow Channel of Dongying Formation of Paleogene System,Zhanhua Sag,Jiyang Depression

济阳坳陷沾化凹陷桩西油田古近系东营组下段砂体平面呈条带状展布,与曲流河单河道沉积砂体形态类似。砂岩的沉积成因是建立高精度时间地层格架、预测储集层属性参数空间分布及挖潜剩余油的关键基础之一。综合分析区域地质背景、地层沉积序列、岩石学性质、沉积组合以及砂体几何形态等方面,认为桩西油田东下段砂岩是在半深湖—深湖环境沿湖盆长轴方向延伸的远源滑塌型重力流水道—浊流水道沉积;沉积物源主要来自北部埕北低凸起附近的扇三角洲和水下扇。根据重力流水道的成因机制,将东下段浊流水道划分为水道中心微相、水道边缘微相和水下漫溢微相。单一水道中心微相沉积序列相当于鲍玛序列的AB、ABD或AC组合,主要沉积中厚层正递变层理含砾细砂岩相和块状层理混合碎屑岩相,属中孔中低渗储集层。研究区滑塌型浊流水道形成的外部控制因素是火山喷发及控盆断层的幕式活动所导致的长期基准面下降;内部控制因素是充足的物源、垂直湖盆长轴方向的阶梯状断裂以及负向湖盆底形等。建立了反映该区浊流水道的沉积特征及发育条件的沉积模式。

Sandstones of the lower member of Dongying formation of paleogene system in Zhuangxi oilfield,Zhanhua sag,Jiyang depression are of string-belt shaped extension,which are extremely similar to the geometry of sandstones deposited by meandering fluvial. Depositional genesis of the sandstone is one of the key basis for high resolution stratigraphic framework construction,reservoir properties distribution prediction and remaining-oil redevelopment. Combined with regional geological setting,stratigraphic succession,petrological features,depositional features association,and sand body geometries of this member,it is considered that the sandstone of lower member of Dongying formation,which extends along the long axis of faulted basin,was deposited by slumping-typed gravity flow-turbidity current channel of long distance sediments source in the semi-deep to deep lacustrine environment with the confine of lacustrine bottom configuration. The sediment sources were from the fan-deltas,sub-lacustrine fans in Chengdao area in the northern part. In terms of the depositonal regime of gravity flow channel,the turbidity channel was subdivided into three types of microfacies：central channel,channel margin,and subaqueous overflow. Sedimentary succession of individual central channel equals to members association of AB,ABD and AC of Bouma sequence. Reservoir rocks in the channel center are dominated by gravelly sandstone with normal graded bedding and hybrid sandstone with massive bedding,which are medium porosity and permeability. The external controls on the slumping-typed turbidity current channel in study area is of the long term base level fall caused by volcanic eruption,episodic activity of faults controlled basin,the internal controls of the abundant sediments source,stepped faults perpendicular to the long axis of faulted basin,and negative lacustrine bottom configurations. A depositional model of tubidity current channel is established based on the depositional features and conditions of sediments.