Characteristics and significance of the burial dissolution of dolomite reservoirs: Taking the Lower Paleozoic in eastern Tarim Basin as an example

Abstract

Based on the characteristics of core, thin-section, physical properties and carbon, oxygen and strontium isotope, the characteristics of Lower Paleozoic dolomite reservoirs, dissolution features and mechanism in eastern Tarim Basin were studied. Massive dissolution pore-cave dolomite is the main reservoir type in eastern Tarim Basin. The dissolution pores and caves are mainly medium to small with dense distribution. The fillings in dissolution pores and cracks commonly show negative δ18O, generally far lower than the surrounding rock; slightly negative δ13C, partially positive; slightly high 87Sr/86Sr. Proportion of CH4 and CO2 is high in fluid inclusions and some of them contain high proportion of H2S, which demonstrated that the organic matter was at the high evolution stage when the inclusions were captured, suggesting deep burial circumstances. Silicification and high homogeneous temperature also indicate the presence of hydrothermal fluids. Dissolution includes organic acid dissolution and hydrothermal water dissolution. Acid fluid mainly includes organic acid, CO2 and H2S, which were mainly generated in the process of evolution of organic matter, with a little H2S from sulfate reduction, a little CO2 from volcanic activity. As the carrier of acid fluid, hydrothermal fluids overcome kinetic obstruction of dissolution and dissolve the deep dolomite. Major faults and associated faults provide channels for hydrothermal fluids, the acid fluid was further dispersed in dolomites through numerous inter-crystal pores caused by dolomitization and micro-cracks caused by compaction to complete dissolution for dolomite reservoirs. 摘要: ：根据岩心、薄片、物性特征及碳、氧、锶同位素组成特征，对研究塔里木盆地东部下古生界白云岩储集层特征、溶蚀作用特征及机理进行了研究。研究表明，块状溶蚀孔洞型白云岩为塔东地区最主要的储集岩类型，溶蚀孔洞以中小孔为主，分布较密集。溶蚀孔洞及裂缝充填物δ18O值偏负，一般远低于围岩；δ13C值略偏负，部分正值；87Sr/86Sr值略偏高。包裹体成分中CH4、CO2含量较高，部分包裹体H2S含量也较高，包裹体形成时有机质已经进入高演化阶段，反映深埋藏环境。大量硅化作用以及较高的均一温度均反映热液活动。溶蚀作用主要包括有机酸溶蚀、热水溶蚀；酸性流体主要有有机质演化过程中产生的有机酸、CO2、H2S，还有以及硫酸盐还原产生的H2S以及和深部火山活动带来的CO2；热流体作为酸性流体的载体，并克服溶蚀作用动力学障碍，完成了对深层白云岩的溶蚀。深大断裂以及其伴生的断裂为酸性热流体流体的运移提供通道，白云岩白云石化作用形成的晶间孔以及压实作用下形成的微裂缝使得酸性流体呈弥散状扩散并对白云岩储集层进行整体溶蚀。图7表2参25 Key words: eastern Tarim Basin, Lower Paleozoic, dolomite, dissolution pore, burial dissolution, acidic fluid, hydrothermal fluid