鄂尔多斯盆地富县地区马家沟组热液改造特征与研究意义^*

Mechanism and significance of hydrothermal activity in the Majiagou Formation,Fuxian area, Ordos Basin

摘要

构造—热事件对油气生成、储集层演化和成藏有重要的控制作用,但是鄂尔多斯盆地构造—热事件对碳酸盐岩的影响研究较少。以富县地区奥陶系马家沟组储集层为例,通过岩心及薄片观察,利用阴极发光、包裹体测温、碳氧稳定同位素、锶同位素、电子探针微区元素分析、稀土元素分析,岩石学与地球化学研究充分结合,识别出区内存在2期与燕山期热事件相关的胶结作用: 第Ⅰ期以缝洞内鞍形白云石发育为特征,其 δ¹³C值和 δ¹⁸O 值均较偏负,锶同位素值较正常海相灰岩高,稀土元素Eu和La负异常,元素分析显示FeO含量异常高、MnO含量较高、SrO含量较高、K₂O及Na₂O含量低,胶结物包裹体均一温度高; 第Ⅱ期以缝孔洞内石英充填及交代和切割鞍形白云石的裂缝方解石发育为特征,其 δ¹³C值与奥陶系碳酸盐岩背景值接近、δ¹⁸O值总体偏低,锶同位素值更高,稀土元素展现出特别的La负异常和Eu正异常,元素分析MnO及FeO含量较高,部分K₂O及Na₂O含量较高,石英包裹体均一温度高。第Ⅰ期流体为早白垩世拉张断裂作用下上覆石炭—二叠系地层有机流体、奥陶系地层内部流体、少量深部流体三者联合作用形成; 第Ⅱ期流体为早白垩世拉张断裂作用下深部流体与少量奥陶系地层内部流体两者联合作用形成,深部流体穿越了下伏马家沟组内蒸发盐岩地层。早白垩世热事件对奥陶系碳酸盐岩储集层的影响具有双向性: 第Ⅰ期热流体活动多为建设性,与大规模油气生成的时间一致,裂缝和溶蚀孔洞可构成油气有效输导和储集空间;第Ⅱ期热流体活动以孔隙和裂缝充填为主,多为破坏性,但规模较小。

Abstract

It is of great significance to study the mechanism of hydrothermal activity,related to the Yanshanian thermal events in Majiagou Formation,Fuxian area of the Ordos Basin,to reveal the origin of reservoirs. The two stages of hydrothermal activity related to the Yanshanian thermal events in Majiagou Formation,Fuxian area are identified,based on a comprehensive analysis of petrology and geochemistry,including core and thin section observation and cathode luminescence,homogenization temperature of fluid inclusions,carbon and oxygen stable isotopes,strontium isotope,electron probe microanalysis and rare earth element analyses. The results show that the stage-Ⅰhydrothermal activity is characterized by the development of saddle dolomite in the fissure-cavern systems,with low and negative δ¹³C and δ¹⁸O values,higher strontium isotope value than normal marine limestone,negative abnormal Eu and La in rare earth elements,rich content of MnO,FeO,SrO and poor content of K₂O and Na₂O in trace elements,and high homogenization temperature of fluid inclusions. The geochemical characteristics reveal that the saddle dolomite was formed by the combined action of internal fluids in the Ordovician strata,shallow fluids in the Carboniferous-Permian strata,and deep hydrothermal fluid under Early Cretaceous extensional fault activity,in which the deep hydrothermal fluid is post-magmatic hydrothermal fluid related to basalt eruption. The stage-Ⅱhydrothermal activity is characterized by quartz precipitation in fracture-vug systems,and the development of fractures calcite that cut saddle dolomite,with δ¹³C values close to the background value of Ordovician carbonate rocks,low δ¹⁸O value,higher strontium isotope value than saddle dolomite,negative abnormal La and positive abnormal Eu in rare earth elements,rich content of MnO,FeO,K₂O,Na₂O in trace elements,and high homogenization temperature of fluid inclusions in quartz. The geochemical characteristics reveal that the calcite and quartz were formed by the combined action of internal fluids in the Ordovician strata and deep hydrothermal fluid under Early Cretaceous extensional fault activity,in which the deep hydrothermal fluid is the post-magmatic hydrothermal fluid related to the activities of the Early Cretaceous monzonite and monzonitic diorite magmatic intrusion. Hydrothermal activity related to Yanshanian thermal events has both constructive and destructive effects on the formation and preservation of reservoir space in the Majiagou Formation,but the overall sphere of influence is limited.

关键词

鄂尔多斯盆地 / 富县地区 / 马家沟组 / 碳酸盐岩 / 热液作用

Key words

Ordos Basin / Fuxian area / Majiagou Formation / carbonate rock / hydrothermal activity

基金资助

*中国石化科技部项目(编号:P22179)资助