塔北南缘中奥陶统顺层岩溶储层特征及成因

Characteristics and Origin of Middle Ordovician Karst Reservoirs in South Margin of Northern Tarim Basin

塔北南缘中奥陶统地层序列完整,其上无长期暴露的不整合面,但是钻井揭示其中岩溶储层发育。以上奥陶统桑塔木组剥蚀线为界,塔北隆起可划分为潜山区和内幕区两部分,潜山区以洞穴型、裂缝+孔洞型储层为主;内幕区以裂缝+孔隙+孔洞型储层为主,孔隙+孔洞型储层和洞穴型储层也有发育。岩溶储层总体上呈层状分布,集中发育于中奥陶统顶面以下100 m范围内;平面上自潜山区向内幕区储层发育程度逐渐减弱,局部沿断裂集中分布。通过岩心、薄片、成像测井、水化学等资料和地质背景的分析认为,内幕区上奥陶统富泥质地层隔挡了潜流带地下水外泄地表,致使潜水面处于非完全稳定状态,其下"缓流带"具较高水势,为内幕区地下水运移提供了动力,从而在中奥陶统发生顺层岩溶作用。古隆起及斜坡背景为顺层岩溶储层的发育提供了地质背景,渗透性好的颗粒灰岩和裂缝系统为顺层岩溶作用提供了有效的流体通道,顺层岩溶作用则是各类储集空间形成的关键。

The Middle Ordovician strata are well deposited in the south margin of northern Tarim Basin.Drill data reveale that no long-term exposed unconformity surface exists on the Middle Ordovician strata and the karst reservoirs develop in the strata.Bordered by the pinchout line of upper Ordovician Shangtamu Fm.,a burial hill area in the north and an inner area in the south can be divided.Cavernous and fractured＋vuggy reservoirs are predominant in the burial hill area.Fractured＋porous＋vuggy reservoirs are dominant while porous＋vuggy and cavernous reservoirs are also common in the inner area.Generally,the development of karst reservoirs exhibits bed-liked in lateral and focuses vertically within the interval of 100 meters below the top surface of Yijianfang Fm.,whereas the reservoirs get weak in deveploping extent from north to south and distribute locally along faults.By means of the analysis of cores,thinsections,FMI,hydrologic data and geologic setting,it is shown that the Middle Ordovician karst reservoirs originate considerably from karstification taking place along rock layers.The fact that the overlying upper Ordovician mud-rich strata in the inner area prevented phreatic underground water from flowing back to the surface brings in an unstable phreatic water table under which the slow flow zone provides high potential energy for underground water to flow.It is suggested that the geological setting incluidng paleouplift and slope is the prerequisite of developing karstifiaction,and permeable grainstone and fracture system may form available pathways for diagenetic fluids.The karstification along layers advances a variety of reservoir spaces.