塔北地区奥陶系地下河溶洞充填规律与储集性能

Filling patterns and reservoir property of the Ordovician buried-river karst caves in the Tabei area,Tarim Basin

基于野外露头、油田钻井、测井及地震资料,从溶洞充填物类型及旋回组合分析入手,开展古岩溶地下河溶洞充填序列及规律研究,同时探讨其油气地质意义。研究结果表明,塔河油田奥陶系岩溶地下河溶洞充填率达89.9%,且以沉积砂泥岩和垮塌角砾岩充填为主,发育下粗上细多个组合旋回,构成多旋回沉积充填组合和多旋回垮塌-沉积充填组合2种类型。前者分布于岩溶斜坡下游地貌平坦区,放空漏失井比例小、低产井比例高;后者分布于地貌落差较大的岩溶斜坡上游区,放空漏失井比例大,出现大量高产井。分析认为,地下河迂回曲折的空间结构及其径流强搬运能力,使大量岩溶碎屑物质充填其中,导致其超高充填率;潜水面季节性升降使其充填物具旋回性及可对比性,加之水流冲蚀和构造活动,导致溶洞多期垮塌,构成了上游区多旋回垮塌-沉积充填组合,未充填空间发育;地下相对封闭的碳酸钙过饱和环境,使其充填物钙质胶结严重,孔隙度降低;未充填空间将是油气挖潜的重点靶区。

An integration of outcrop observations,as well as data from drilling,logging,and seismic surveys in an oilfield is applied to analyze the filling types and filling cycle assemblages of karst caves associated with paleokarst buried rivers;accordingly,the filling sequences and patterns of the paleokarst buried rivers,as well as the discussion on their petroleum geological implications.The results show that the Ordovician buried-river karst caves with a filling rate of 89.9%in the Tahe oilfield,are predominantly filled with sandy mudstones and collapse breccias.These karst caves host multiple combination cycles featuring coarse-grained lower parts and fine-grained upper parts,which can be classified into polycyclic sedimentary assemblages and polycyclic collapse-sedimentary assemblages for filling.The former is distributed in the karst slope’s lower reaches of flat landform,where wells with lost circulation and stringers account for small and high proportions,respectively.In contrast,the latter is situated in the karst slope’s upper reaches featuring landform of great drops,where wells with lost circulation are of high proportion together with multiple highyielding wells.The following conclusions can be reached through analysis:(1)The tortuous spatial structure of buried rivers,combined with their strong runoff transport capacity,facilitate the filling of large amounts of karst detrital materials,resulting in an extremely high filling rate;(2)The seasonal fluctuations in the phreatic surface lead to the formation of cyclic and comparable fillings.This,coupled with water erosion and tectonic activities,gives rise to multiphase collapses of karst caves.Consequently,polycyclic collapse-sedimentary filling assemblages are formed in the upper reaches,with unfilled spaces developed;(3)The relatively closed underground environment supersaturated with calcium carbonate,results in severe calcareous cementation of fillings,decreasing the intergranular porosity;(4)The unfilled spaces serve as the major targets with potential for oil and gas exploitation.