塔里木盆地塔河北部“过溶蚀残留型”断溶体发育特征及其成因

Characteristics and origin of over-dissolution residual fault-karst reservoirs in the northern Tahe oilfield,Tarim Basin

为了研究塔里木盆地塔河北部强剥蚀区海西早期古岩溶洞穴发育特征,用古地貌恢复、岩溶水系统分析、测井-岩心洞穴识别和地震属性刻画等方法进行了YQ5井区的洞穴类型样式识别、缝洞结构刻画和洞穴成因演化研究。研究结果表明:YQ5井区在塔河油田Ⅱ号和Ⅲ号古岩溶台地北部的地势平缓区,总体为多期次岩溶叠加改造后的残留地貌,主要发育幅差较小的溶峰洼地、溶丘洼地和溶丘平原,南部发育NE向展布的峰丛垄脊沟谷。与塔河油田主体区及斜坡区不同,YQ5井区地下和地表水系的流向与地貌趋势不一致,岩溶水系统遭受构造作用破坏,导致补给、径流和排泄的岩溶水循环过程不完整。YQ5井区主要发育暗河型洞穴和“过溶蚀残留型”断溶体。暗河型洞穴充填较为严重,洞穴的有效储集空间受到破坏,影响油气开发效果。岩溶台地的构造抬升造成区域侵蚀基准面的下降,顺走滑断裂的垂向侵蚀作用有利于“过溶蚀残留型”断溶体的持续发育和保存,油气开发效果好。“过溶蚀残留型”断溶体的发育主控因素为走滑断裂、地层剥蚀强度和负向地貌。与塔河古岩溶台地演化过程一致,YQ5井区的岩溶演化经历深切曲流期、岩溶改造期和下渗断溶期3个阶段。暗河型洞穴被持续改造破坏,断溶体则持续建造。

This study aims to investigate the characteristics of the Early Hercynian paleokarst caves in the extensively eroded area of the northern Tahe oilfield in the Tarim Basin.Using methods such as paleogeomorphic reconstruction,karst water system analysis,logging-core observation-based cave identification,and seismic attribute characterization,we identify the cave types and styles in well block YQ5,characterize fracture-cave structures in this area,and explore the origin of the caves.The results show that the well block YQ5,located in the region with flat and gentle terrain to the north of Nos.ⅡandⅢpaleokarst platforms of the Tahe oilfield,generally exhibits residual landforms after modification by multi-phase superimposed karstification.Karst peak-depression,karst hill-depression,and karst hillplain zones of low relief are primarily observed in the well block,with peak cluster-ridge/valley zones extending in the NE direction in the south.Different from the main body and slope area of the Tahe oilfield,the well block YQ5 displays a flow direction of the underground and surface water systems inconsistent with the geomorphic trend.Furthermore,the disrupted karst water system therein by tectonism,leads to an incomplete karst water cycle of recharge,runoff,and discharge.Caves of underground river type and over-dissolution residual fault-karst reservoirs are primarily found in the well block.The caves of underground river type are characterized by significant filling,which compromises the effectiveness of reservoir spaces,thus adversely affecting oil and gas exploitation.In contrast,for the over-dissolution residual fault-karst reservoirs,the vertical erosion along strike-slip faults,resulting from a decline in regional erosion base level due to the tectonic uplift of the karst platforms,is beneficial to the continuous development and preservation of these reservoirs,thus contributing to effective oil and gas exploitation.Overall,the dominant factors for the over-dissolution residual fault-karst reservoirs encompass the development of strike-slip faults,the denudation intensity of strata,and negative landforms.Consistent with the paleokarst platforms observed in the Tahe oilfield,karst in the well block YQ5 also experienced three evolutionary stages:the deeply incised meandering river stage,the karstification-induced modification stage,and the infiltrated fault-karst stage.During these stages,caves of underground river type underwent continuous transformation and destruction,while the fault-karst reservoirs experienced ongoing construction.