乍得Bongor盆地花岗质基岩潜山储层特征

乍得Bongor盆地基底是由早寒武世及更老的花岗岩、混合花岗岩和片麻岩等构成,经历了古生代-侏罗纪长期的风化剥蚀夷平作用。早白垩世受中非剪切带走滑-拉张作用影响形成拉分盆地,同时在基底形成大量的构造裂缝;晚白垩世强烈反转,古近纪发育成为统一的盆地。2007年以来实施多层系立体勘探,不仅在下白垩统沉积地层发现一系列大中型油气田,而且还在花岗质基岩潜山获得高产油气流,证实了5个潜山油藏带。根据储集空间的特征将基岩储层划分为孔隙型和裂缝型两类。综合地震、测井、地层成像、元素测井和岩心分析等资料,垂向自上而下将潜山的储层序列划分为风化淋滤带、缝洞发育带、半充填裂缝发育带和致密带。潜山所处的构造位置和埋藏深度决定了潜山储层序列发育的完整性和物性的好坏。基岩储层的垂向分带性为钻前储层的地震横向预测和评价提供了可能。

Characteristics of granitic basement rock buried-hill reservoir in Bongor Basin,Chad

The basement rocks of Bongor Basin are mainly composed of the Early Cambrian and older granite, magmatic granite, gneiss, etc., which underwent long period of weathering/leaching, erosion and planation from Paleozoic to Jurassic. The basin is a pull-apart basin resulted from the strike-slip and extension of the Central African Shear Zone during the Early Cretaceous which created fracture meshwork in the basement. Then the basin was strongly overturned and uplifted during the Late Cretaceous, and united to be a unique basin during the Paleogene. The multi-storey and three-dimensional exploration resulted in the discoveries of several large-medium oil and gas fields in sandstones, but also of high DST oil flows from the granitic basement, confirming 5 oil-bearing buried-hill play fairways in the basin. Based on the storage space composition characteristics, the granitic basement reservoirs can be divided into two types: pore-type and fracture-type. Combining all seismic, logging, FMI, ECS and core data, the reservoir series of buried-hills can be divided vertically into weathering/leaching zone, fractured-cave zone, half-filled-fractured zone, and tight zone. The location and depth of buried-hills decided the development completion and petrophysical quality of reservoir series. The vertical zonation of granitic basement reservoir provides good possibility for the prediction and evaluation of buried-hill reservoirs before drilling.