柴达木盆地狮子沟构造下干柴沟组上段有效裂缝与地应力关系及其意义

Relation between Effective Fractures and In-Situ Stress as well as Its Significance in Upper Xiaganchaigou Formation in Shizigou Structure,Qaidam Basin

柴达木盆地西南部狮子沟构造下干柴沟组上段致密湖相碳酸盐岩中普遍发育构造裂缝,是控制油气高产的关键因素.研究旨在建立该段现今地应力状态及裂缝产状分布规律,这对在致密储层中圈定有效裂缝和考虑压裂缝分布的水平井方向设计具有重要意义.本文以岩心、薄片揭示裂缝特征,从三维地震数据体提取断裂与A、B、C井钻井轨迹的空间关系;选取A、B两口井水平段建立地应力剖面;选取A、C两口井成像测井段统计裂缝产状并进一步以极射赤平投影、玫瑰花图和倾角分布图展现产状规律.3口井水平段走向近平行于北西向走滑正花状断裂系.地应力测井显示下干柴沟组上段测量段为走滑断层地应力状态.未充填的高导缝和充填的高阻缝主体走向均为北东,倾角为高角度至近竖直,其形成受控于早中新世以来的最大水平主应力方向为北东向的走滑断层地应力状态.未充填裂缝或发生溶蚀的充填裂缝可作为油气储集空间和运移通道.北东走向未充填的竖直裂缝为最优的有效裂缝.靠近北西向主断裂的北东向水平井将钻遇更多天然裂缝,压裂形成北东向延伸的竖直有效裂缝.在北西向水平井产量不佳情况下,靠近断裂的北东向水平井设计值得考虑.

In the SW Qaidam Basin,fractures are widely developed in tight lacustrine carbonate rocks of the Upper Xiagnchaigou Formation in the Shizigou structure,which is the key factor controlling the high production of hydrocarbon.The purpose of this study is to establish the present-day in-situ stress state and distribution patterns of fracture attitudes,which is significant to determine effective fractures and to design horizontal well orientations considering hydraulic fracture distribution.In this study,fracture characteristics were revealed in cores and thin sections.The spatial relation between deep faults and trajectories of Well A,Well B and Well C was extracted from 3D seismic data.We selected horizontal intervals of Well A and Well B to construct in-situ stress profiles and image log intervals of Well A and Well C to count fracture attitudes which were further shown by stereonets,rose diagrams and fracture dip histograms.Horizontal intervals of three selected wells are nearly parallel to the NW-trending faults belonging to the strike-slip positive flower structure.Geostress logging shows that the measuring intervals in the Upper Xiaganchaigou Formation are under the strike-slip faulting stress state.Most unfilled conductive fractures and filled resistive fractures have NE strikes and high to near-vertical dips,and they were dominated by the strike-slip faulting stress state whose maximum horizontal principal stress is in the NE direction since the Early Miocene.Unfilled fractures and dissolved fractures which were previously filled can act as hydrocarbon reservoirs and migration pathways.NE-trending unfilled fractures with vertical dips are the most effective fractures.If NE-trending horizontal wells are drilled near NW-trending main faults,more natural fractures will be met,and NE-trending hydraulic fractures with vertical dips are effective.The design of NE-trending horizontal wells is worth considering due to the poor production of NW-trending horizontal wells.