热流体对深埋白云岩储集性影响及其油气勘探意义——塔里木盆地柯坪露头区研究

Influence of hot fluids on reservoir property of deep buried dolomite strata and its significance for petroleum exploration: A case study of Keping outcrop in Tarim basin

深埋条件下白云岩能否形成有效储层是塔里木盆地深层油气勘探面临的主要挑战之一,塔深1井大于8000m深度白云岩岩芯仍具有多孔洞带发育,并具有形成储层的孔渗性,揭示了深部白云岩有效储层的存在。研究证实,该类孔洞的形成与热流体活动相关,但其在地层中分布的特征和广泛性有待揭示。通过野外露头观察及室内测试分析,在柯坪地区经历过深埋条件的上震旦统-下奥陶统白云岩中发现了与塔深1井相似的溶蚀孔洞的普遍存在现象。孔洞内充填的石英、自形白云石以及方解石矿物原生盐水溶液包裹体均一温度依次可达到368℃、314℃和303℃,远高于相应地层最大埋深(约6000m)条件下推测的正常地层温度范围(120～240℃),盐度分布范围依次为3.39%～9.86%NaCleqv、1.05%～18.13%NaCleqv和4.34%～9.98%NaCleqv。同时,研究也发现了黄铁矿、萤石、重晶石、石英、菱铁矿、毒砂和鞍形白云石等热流体相关矿物组合,并在相应流体包裹体内发现了CO2、H2S和烃类气体等对白云岩具有溶蚀性的气体存在。综合以上测试成果,证实柯坪露头区白云岩地层内存在大规模的异常热流体活动,推断可能与盆地深部大规模岩浆热事件相关。研究揭示热流体活动产生了顺层溶蚀、冷缩裂缝、差异性溶蚀以及热流体再作用的大型溶塌等多种孔隙空间结构,孔隙类型主要为缝-洞复合型。揭示热流体溶蚀改造作用主要受构造裂缝和地层界面等因素控制,其对白云岩地层孔隙空间的改造主要表现为建设性与破坏性共存,研究区则以建设性为主。

Whether deep buried dolomite can form effective reservoirs is the main challenge for petroleum exploration in deep area of Tarim Basin.The cores sampled from Tashen-1 well at the burial-depth of 8000m have revealed that the dolomites are well-developed with vuggy zones,have high porosity and permealbity favorable for forming effective petroleum reservoirs,and revealing the existence of effective dolomite reservoirs in deep part.It has been confirmed vuggy zones are of hot fluid origin.However,the detailed characteristics of vuggy zones including their origin and their distribution in strata need to be disclosed by further studies.Based on field outcrop observation as well as experimental analyses,it was found that solution pores similar with those in Tashen-1 well are widely spread in Upper Sinian and Lower Ordovician dolomite （which have experienced deep-buried conditions） in Keping area.It was also found that primary saline water inclusions of vug-filling quartz,automorphic dolomite and calcite have homogenization temperature of up to 368℃,314℃ and 303℃ respectively,which are much higher than normal formation temperature range （120~240℃） of corresponding host strata at the maximum burial depth of about 6000m.The salinity of inclusions is respectively 3.39%~9.86% NaCleqv,1.05%~18.13% NaCleqv and 4.34%~9.98% NaCleqv.Moreover,a variety of hydrothermal mineral associations related to pyrite,fluorite,barite,quartz,siderite,dalarnite and saddle dolomite are identified in dolomite samples.In the inclusions of these hydrothermal minerals,CO2,H2S,hydrocarbon gases which may dissolve dolomite are also recognized.It was proposed that dolomite strata in Keping outcrop area experienced large-scaled abnormal hot fluid activities which might be related to extensive magmatic activities in deep part of the basin.It created abundant pore space types,such as bedding dissolution,differential dissolution,cooling fissure and dissolution-slump belt caused by hot fluid.Pore types are mainly characterized by combined type of solution enhanced fracture and vug.Hot fluid activity is mainly controlled by structural fractures and stratigraphical boundary.In this study area,the modification of hot fluid to dolomite reservoirs is mainly represented by the coexistence of constructive and destructive effects,and it is characterized by constructive effect.