塔里木盆地奥陶系萤石成因及其油气地质意义

Genesis of the fluorite in the Ordovician and its significance to the petroleum geology of Tarim basin

塔里木盆地塔中45井上奥陶统发现以萤石层为储层的特殊类型油气藏。为了明确塔里木盆地奥陶系萤石成因及热液溶蚀储层形成时期,本文通过电子自旋共振测年、稀土元素、微量元素、激光拉曼、包裹体氢氧同位素、硫同位素等多种分析手段,对塔中45井区及巴楚-柯坪露头区奥陶系内萤石发育段进行了热液成矿、热液溶蚀与油气成藏关系的综合研究。目前,多数学者认为萤石形成于晚二叠世的岩浆期后热液,并相应存在一期海西期的热液溶蚀,此次研究表明,塔中45井区及巴楚-柯坪露头区的萤石成矿流体可能为低温大气淡水循环热液。塔中45井区断裂溶扩带充填萤石形成于晚燕山-喜马拉雅期,与海西期岩浆期后热液无必然联系;巴楚-柯坪露头区风化壳溶洞充填萤石形成于海西期,萤石成矿期与海西期火山活动热事件影响期匹配关系良好。由此推断,塔中45井区萤石成矿热液溶蚀作用发生于晚燕山-喜马拉雅期。

A special type of oil pool, with fluorite bed as reservoir, has been found from the upper Odovician of TZ45 well in central Tarim Basin. To determine the genesis of the fluorite and the formation age of the hydrothermal - dissolved carbonate reservoir in the Ordovician of central Tarim Basin and other places, the relationship among hydrothermal mineralization, hydrothermal dissolution and hydrocarbon accumulation has been probed synthetically through different analyses, including electron spin resonance （ESR） dating, REE, trace element, fluid inclusion laser raman, fluid inclusion hydrogen and oxygen isotope, and sulfur isotope analysis. At present, most scholars consider that the fluorite was formed in late Permian by the magmatic fluids associated with the volcanic activities in Tarim Basin, and concurrently there was an episode of hydrothermal dissolution. This study proved that the fluorite, developed in the Odovician carbonate in Tz45 well and Bachu-Kerpin area, was probably formed by the low - temperature meteoric circular hydrothermal fluids. Filled in the dissolution-enlarged space along the fault belt, the fluorite in Tz45 well deposited in the late Yanshanian-Himalayan and has no necessary tie with the Hercynian magmatic fluids. However, filled in the caves along the weathering crust of the Ordovician earbonate outcrops, the fluorite in Bachu-Kerpin area was formed in Hercynian, and there is a good correlation in age between the mineralization and the volcanic thermal activity. So the dissolution of the Odovician carbonate by the fluorite hydrothermal fluids in TzA-5 well occurred in late Yanshanian - Himalayan.