渤中凹陷奥陶系深埋环境下碳酸盐岩溶蚀成因分析

Dissolution mechanism analysis of Ordovician carbonates under burial environment of Bozhong Sag,Bohai Sea area

渤中凹陷奥陶系碳酸盐岩具有明显深埋环境下溶蚀特征,溶蚀形成的次生孔隙有效改善了储层物性。通过岩石薄片鉴定、扫描电镜观察、阴极发光观察、流体包裹体分析及天然气碳同位素分析等手段对研究区溶蚀成因进行了系统分析。结果显示:研究区奥陶系储层以灰岩、白云岩及灰质白云岩为主,储集空间以多期溶蚀形成的次生孔隙及裂缝为主。该储层经历了正常海水、大气淡水及深埋藏3个成岩环境,其成岩作用则以胶结及溶蚀作用交互为特征。深埋藏环境下碳酸盐矿物溶蚀主要受控于沿深大断裂注入的幔源CO_2、H_2S及硫酸盐热化学反应形成的酸性流体和晚期成藏过程中有机质成熟产生的有机酸和酸性气体。综合分析认为由于更易受到深大断裂及表生溶蚀作用影响,研究区东南侧高部位是奥陶系碳酸盐岩优质储层有利发育区。

Obvious corrosion characteristics under deep burial environment can be found in the Ordovician carbonate rocks of Bozhong Sag,and secondary pores which were caused by corrosion improve the reservoir property.Systematic analysis of dissolution mechanism in research area were studied using normal thin-sections,cast thin-sections,scanning electron microscope,cathodoluminescence analysis,fluid inclusion analysis and natural gas carbon isotope analysis.The results show the Ordovician reservoir in the study area was dominated by limestone,dolomite and calcite dolomite,the secondary pores formed by multiphase corrosion and fractures are mainly reservoir space.The reservoir has experienced three diagenetic environments including normal sea water,atmospheric fresh water and deep burial environment.The diagenesis is characterized by interaction between cementation and dissolution.Carbonate mineral dissolution in the deep burial environment are mainly controlled by mantle sourced CO2,H2S injected along the deep fault,acidic fluid formed by sulfate thermochemical reaction and organic acids and acidic gas produced in the process of late hydrocarbon accumulation.Comprehensive analysis shows that the southeast side of the study area where is more susceptible to deep fracture and surface dissolution effect is Ordovician carbonate reservoir favorable development area.