塔河油田中--下奥陶统古岩溶洞穴巨晶方解石成因及演化

Genesis and Evolution of Gigantic Calcites in Paleokarstic Caves of Middle-Lower Ordovician in Tahe Oilfield

根据碳、氧、锶同位素和包裹体的系统分析,结合区域地质背景,探讨了塔河油田中—下奥陶统岩溶洞穴中巨晶方解石的成因和演化特征。巨晶方解石的δ(13C)和δ(18O)与中—下奥陶统背景值相比,两者均较低,表明巨晶方解石具有大气水岩溶特征;其N(87Sr)/N(86Sr)比值高于背景值,说明受到壳源锶的影响,海西早期是壳源锶的主要来源时期;巨晶方解石中出现单相水溶液包裹体和淡水盐度也表明其形成于地表岩溶环境中。根据碳、氧、锶同位素和包裹体的综合分析,洞穴巨晶方解石形成于海西早期潜流岩溶带环境中。巨晶方解石形成后,在海西晚期构造运动中受到洞穴塌陷作用改造,塔河油田东部的岩溶洞穴塌陷现象比西部严重,东部地区的巨晶方解石因上覆地层压力形成了缝合线。印支—喜山期构造运动在巨晶方解石中形成了裂缝和微裂缝;而且,巨晶方解石还经历了埋藏溶蚀作用,溶蚀作用与喜山晚期天然气的生成、运移关系较密切,且东部地区比西部地区强烈。

This paper discusses the genesis and evolution of gigantic calcites filled in paleokarstic caves of Middle-Lower Ordovician in Tahe Oilfield according to systematic analysis of fluid inclusions and carbon, oxygen, strontium isotopes, combined with regional geologic background. The lower δ（13 C） and δ（18O） of gigantic calcites than carbonate rocks background of Middle-Lower Ordovician indicate that the gigantic calcites are characteristics of meteoric water karst. While the higher N（87 Sr）/N（86 Sr） ratios than the background reflect that the terrestrial strontium plays an important role and the Early Herlynian paleokarst is the main source periods of terrestrial strontium. Single-phase aqueous inclusions and freshwater salinity in gigantic calcites also prove its formation in the condition near surface karst. The comprehensive analysis of carbon, oxygen, strontium isotopes and inclusions reveals that the gigantic calcites are generated in the phreatic zone conditions of Early Herlynian paleokarst. After the formation of gigantic calcites, the paleokarst caves collapsed in the Late Herlynian movement, causing the breaking down of paleokarst caves in the east areas greater than that in the west areas of Tahe Oilfield. Consequently, stylolites are produced in the gigantic calcites due to the pressures from overlying formations. Fissures and micro-fissures are engendered during Indosinian-Himalayan tectonic movement. Futhermore, gigantic calcites, which have experienced burial dissolution, have close relationship with the generation, migration of natural gas in Late Himalayan with a greater effect on the east areas than the west areas.