鄂尔多斯盆地西南缘延长组断缝体特征及对油藏的调控作用

Fault-fracture body characteristics and their effect on hydrocarbon distribution of Yanchang Formation in southwestern margin of Ordos Basin

鄂尔多斯盆地西南缘断缝体发育,针对断缝体对延长组油藏甜点分布控制作用的研究有待深化。利用大量的地震及测井解释结果,从断缝体演化机制角度对断缝体特征进行精细描述,进而系统探讨断缝体对油气的调控作用。研究结果表明,鄂尔多斯盆地西南缘发育直立走滑断裂,多具有“Y”字型、花状及负花状结构,且断裂常穿过白垩系底、延安组底、长7油层组底等界面,向下则插入基底。部分断裂仍具有早期逆断性质,表明断裂在后期反转程度不彻底。主断裂在不同部位具有不同的形态及偏移量,剖面上表现为张扭及压扭性质的循环转变,平面上则表现为不同类型断裂组合形式的交替出现。构建了断缝体中走滑断裂的发育模式,走滑断裂具有典型的多期活动、继承发育的特征。长8油层组主要发育垂直缝及水平层理缝,水平层理缝的发育频率为62.5%,垂直缝的发育频率为37.5%,且垂直缝含油级别相对较高。裂缝主要发育于分流河道细砂岩。当距主断裂距离大于1.25~1.5 km时,裂缝发育程度急剧降低,存在断缝体边界;在该边界范围内,厚度小于6 m的单砂体裂缝较为发育,当单砂体厚度超过6m,裂缝发育程度急剧降低。所构建的基于沉积(基础)、构造(主导)及裂缝(见效)的指标体系可以有效预测研究区长8油层组断缝体油藏有利区。

Fault-fracture bodies are developed in the southwestern margin of Ordos Basin,and thus,the study on the controlling ef-fect of fracture bodies on sweet spot distribution of Yanchang Formation reservoirs needs to be deepened.In this paper,based on the extensive seismic and logging interpretation results,the characteristics of fault-fracture bodies are described from the point of view of their evolution mechanism,and then the controlling effect of fault-fracture bodies on oil and gas is systematically discussed.The results show that vertical strike-slip faults are developed in the southwest margin of Ordos Basin,with Y-shaped,flower-like,and negative flower-like structures.The faults usually pass through the bottom of Chang 7 Member,the bottom of Yan’an Formation,the bottom of the Cretaceous series,and the basement.Some of the faults retain the early reverse fault properties,indicating that the late inversion degree is not complete.The main fault has different shapes and migrations in different parts,and the section shows the cyclic transformation of tension-torsion and compression-torsion properties.On the plane,different types of fracture combinations appear alternately.The development model of strike-slip faults is established,and the strike-slip faults have typical characteristics of“multi-stage activity and inherited development”.Chang 8 Member mainly develops vertical fractures and horizontal bedding frac-tures.The development frequency of horizontal bedding fractures is 62.5%,while that of vertical fractures is 37.5%.The oil level of vertical fractures is relatively higher.Fractures are mainly developed in fine sandstone in distributary channels.When the distance from the main fault is greater than 1.25-1.5 km,the degree of fracture development decreases sharply,showing a fault-fracture body boundary.In addition,fractures are relatively developed in the single sand body within 6 m from the main fault.When the thickness of the single sand body exceeds 6 m,the degree of fracture development decreases sharply.The study shows that the index system based on sedimentation(foundation),structure(dominant),and fracture(effective)can effectively predict the favorable zones of Chang 8 fault-fracture body reservoir.