乍得Bongor盆地基岩潜山储层特征与影响因素研究

Characteristics and influence factors of basement buried-hill reservoir in Bongor Basin,Chad

基岩潜山油气藏是油气勘探的重要领域。本文以覆盖Bongor盆地主要潜山构造带的高精度三维地震资料,近百口基岩钻井的岩心资料和系统解释成果为基础,对基岩储层的特征、展布规律及影响因素进行精细研究。通过开展岩心观察和镜下分析,明确了Bongor盆地基岩以花岗岩、正长岩和二长岩等岩浆岩和混合花岗岩、片麻岩等变质岩为主,基岩储集空间以破碎的粒间孔、构造裂缝和溶蚀孔洞为主。主要潜山带上的基岩钻井揭示潜山储层纵向上具有分带性,位于潜山顶部的风化壳和破碎带储集条件好,向下进入潜山内部储集条件逐步变差,平面上基岩储层分布广泛,但非均质性强,不同构造部位储层厚度和品质差异较大。通过对潜山构造带的构造恢复和地震、测井、岩心综合研究,明确Bongor盆地潜山储层主要受古构造位置、基岩岩性与矿物成分及构造活动的影响。位于构造高部位的潜山较低部位潜山基岩储层更为发育。长英质矿物较铁镁质矿物脆性强,应力作用下容易产生裂缝,沿着裂缝发生溶蚀而形成孔洞,富长英质矿物层段比铁镁质矿物层段储层品质更好。受构造活动的影响,主要断裂带附近构造裂缝发育,现今最大主应力方向与主要裂缝方向接近,有利于裂缝的张开与保存。基岩储层特征与影响因素的研究为潜山目标评价和钻前储层预测提供了可能。

Basement reserve is an important target in oil and gas exploration.Basing on high resolution 3 D seismic data that cover main buried-hill structures,the core data and petrophysical interpretation of nearly one hundred basement wells,this paper aims to study basement reservoir characteristics,vertical and lateral distribution and influence factors of basement reservoir property.The direct basement core observation and thin section microscopic study show that the basement is a complex of igneous and metamorphic rocks which are mainly composed of granite,syenite,monzonitic granite,migmatitic granite and gneiss.The main spaces in basement reservoir are inter-grain pore,cracks and cavities.The reservoir study of the wells drilled in the main buried-hill structures indicate that basement reservoirs are vertically zoning.The reservoir property in crust and broken zone on top of buried-hill is better than that in the bottom and it gets worse as going down into the buried-hill.The basement reservoir is widely and heterogeneously distributed on the buried-hills.The thickness and property are different at different structural areas.Integrated study of structural restoration,seismic,well logging and core data indicate that the basement reservoir property is influenced by paleostructural location,lithology and mineral component and structural movements.The buried-hills which located at high paleotectonic location usually have thicker basement reservoir than the buried-hills located in low paleotectonic location.Felsic minerals are more brittle than mafic minerals and easier to develop fractures under pressure.Dissolutions usually happen with pores and caves developing along the fractures.Comparing to the mafic minerals dominant intervals,felsic minerals dominant intervals usually have better reservoirs.The fractures are well developed nearby the main faults because of structural movements.The directions of present maximum principle stress field and main fractures are very close and it is favorable for the fracture opening and preservation.The study of characteristics and influence factors of basement reservoir provides good possibility for the buried-hill target evaluation and basement reservoir pre-drilling prediction.