塔中西北部奥陶系碳酸盐岩埋藏溶蚀作用

General characteristics of burial dissolution for Ordovician carbonate reservoirs in the northwest of Tazhong area

对塔中西北部奥陶系碳酸盐岩埋藏溶蚀发育的地质背景、充填矿物（岩石）学-地球化学特征进行了研究,埋藏溶蚀作用在碳酸盐岩储层发育中有重要作用.塔中西北部多期次构造活动和中-晚二叠世为主的岩浆-热液活动是埋藏溶蚀产生的基本条件;深埋或热液作用是以黄铁矿、绿泥石、石英、铁方解石、萤石等标型矿物和具有能发出明亮桔黄色光的富锰贫铁方解石胶结物、生长环带、铁白云石溶蚀等特征.方解石充填物中富含铁、锰、镁、锶等元素,δ18OPDB平均值为-6.6‰,δ13CPDB平均值为-2.0‰;流体包裹体均一温度为115～130℃和170～210℃,盐度为5%～7%、8%～9%和12%～13%,反映了埋藏成岩作用中有深部流体或循环流体参与.

The geological background and rock geochemistry characteristics of carbonate burial dissolution in the Ordovician system of the northwestern Tazhong area were analyzed. The results show that the burial dissolution has an important role in the development of carbonate rock reservoir. The magmation and thermal fluid alternation in the Middle-Late Permian and the multistage tectonic movement are the basic conditions for developing burial dissolution. The hypogenic diagenesis is characterized by the minor volumes of critical fingerprints of pyrite, chlorite, quartz, fluorite, ferrodolomite, solid bitumen and the bright orange-yellow luminescent calcites with rather low contents of iron elements and high richness of manganese contents existing in fracture-filling oscillatory-zoned calcites. The chemical analysis also indicates that the fracture-filling calcites have very high contents of manganese, strontium and magnesium elements. The isotope data show that the mean value of δ^18 OPDB is - 6.6‰ and δ^13 CPDB is - 2.0‰. In addition, the most striking feature in histograms of micro-thermometric measurements for aqueous inclusion in the Ordovician reservoirs is characterized by homogenization temperature from 115℃ to 130℃ and from 170℃ to 210℃. The calculated salinity of the inclusion ranges from 5% to 7%, from 8% to 9% and from 12% to 13%. It is concluded that the multistage and original hypersaline formation water overlapped by the magmatic-hydrothermal fluid in a large scale of strong epigenic dissolution exists in the northwestern Tazhong area. Key words.. Tazhong area; Ordovician; carbonate rocks; buried dissolution; geochemical characteristics