The Geological Evidences and Impacts of Deep Thermal Fluid on Lacustrine Carbonate Reservoir in the Actic Area of the North Part of Bozhong Depression, Bohai Bay Basin
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摘要: 渤海海域渤中凹陷北陡坡带沙一段发育湖相碳酸盐岩及相关的混积岩类油藏,探明储量超亿方,产量高,埋深超过3 200~3 800 m.分析表明:湖相碳酸盐岩段及混积岩段发生了强烈的白云岩化作用,溶蚀作用强,储层物性好,孔隙度可超过25%.早期研究认为生屑的溶蚀是储层物性改善的重要因素.进一步研究表明,除了上述因素外,储层还受到了深部热流体的影响.通过包裹体激光拉曼分析证实,储层中存在大量含CO2、N2、H2S、H2的包裹体,CO2最高含量超过70%;研究区多个构造富含CO2,碳同位素及氦同位素显示其为幔源成因;对储层中大量的自生黄铁矿分析表明,大部分黄铁矿Co/Ni比值大于1,显示了热液成因;在缝洞及大孔隙内沉淀典型微晶鞍形白云石,具弧形晶面,波状消光,扫描电镜下鞍形白云石内见密集的生长纹.综合分析表明,富CO2深部幔源热液侵位时间为3 Ma以来,侵位后造成了白云石晶间残余方解石的大量选择性溶解;酸性流体对粒内长石也产生了强烈的溶蚀作用,形成了大量砾石及岩屑的铸模孔并沉淀高岭石,流体上述溶解作用对储层段物性改善起到了决定性影响.随着方解石及长石的溶解,流体PH值的升高,盐度的增加,储层出现明显的白云岩化作用,晚期白云石的较高的均一温度及鞍形白云石证实了深部热液对白云岩化的贡献.幔源流体主要来自深大断裂及早期火山后期的排气、排液作用.Abstract: Several high-yielding lacustrine carbonate rock and mix-deposit reservoirs with the total proven reserves of more than 1×108 cubic metres have been found in the actic area of the north part of Bozhong depressoin, with the reservoir depth of 3 200~3 800 meters. The research shows that there are strong dolomitization and dissolution in the lacustrine carbonate rock and mix-deposit rock, and the porosity can be over 25%. Early research suggests that the dissolution of biodetritus is the decisive factor to improve reservoir physical property. But the further research shows obvious deep thermal fluid activity evidences in this belt, and deep thermal fluid plays a decisive role on forming the high quality reservoirs. Based on the Laser Raman analysis, there are a large number of inclusions which contain CO2, N2, H2S and H2, and the high content of CO2 can be over 70%; There are 3 CO2-bearing structures found at the Actic Area of Shijiutuo Uplift, the δ13C CO2 values, R/Ra values values show that CO2 was derived from the mantle. There are a lot of pyrite in the lacustrine carbonate rock, and the Co/Ni ratio of most pyrite is greater than 1, which indicates most of the pyrite derived from the thermal fluid. Saddle dolomite are found at the fractures and big pores, which have the typical arc-shaped crystal and wavy extinction characteristics, and the crystal size is about 0.05~0.10 mm. Dense growth striations are observed under the SEM. According to the thermal history and inclusion homogenization temperature drop points, the thermal fluid activity time is 3~0 Ma. The CO2 thermal fluid activities produced strong dissolution of calcite cements and feldspars in reservoir, which played an important role on the increasing porosity. With the dissolution of the calcite cements and the volcano lithic fragments and gravels, the PH value and salinity of fluid increased, which resulted in strong dolomitization. The higher inclusion homogenization temperature of late stage dolomite and the saddle-like dolomite show that the thermal fluid promote the dolomitization. The mentle-derived thermal fluid is from the deep fault and the exhaust and drainage of the early volcano.
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图 8 湖相碳酸盐岩储层的溶蚀作用
a.方解石溶蚀残余,白云石泥晶套保存完整,QHD36-3-2,铸体薄片,3 763.83 m;b.方解石溶蚀残余,周边孔隙发育,QHD29-2E-5井,铸体薄片,3 362.00 m;c.薄片下未见方解石胶结物,孔隙发育,泥晶套保留,岩屑和砾石铸模孔发育,QHD29-2E-5井, 铸体奥片,3 376.55 m;d.大量泥晶套保留并搭成格架,砾石铸模孔发育,砾石大部分被溶蚀,未见方解石,QHD29-2E-5井,铸体薄片,3 377.49 m;e.长石的强烈溶解,仅残余部分矿物格架,QHD29-2E-5井,扫描电镜,3 380.35 m;f.白云石环边的残余孔隙内,书页状自生高岭石发育,QHD29-2E-5井,扫描电镜,3 381.26 m.
Fig. 8. The dissolution of lacustrine carbonate rock reservoir
表 1 渤中凹陷北侧陡坡带各构造CO2含量统计表
Table 1. CO2 content of the Actic area of the north part of Bozhong depression
井号 BD2 QHD29-2-3 QHD29-2-1 BZ15 BZ17 BZ18 BZ3-1-1 BZ3-2-1 CO2含量 71.3% 89.1% 42.2% 22.8% 17.1% 16.2% 19.6% 19.5% 表 2 渤中凹陷北侧储层中激光拉曼包裹体成分
Table 2. The content of reservoir inclusions of the Actic area of the north part of Bozhong depression
井号 深度(m) CO2 H2S CH4 N2 H2 宿主矿物 QHD29-2E-5 3 370.05 16.1 20.8 63.1 鲕粒内充填的连晶白云石 QHD29-2E-5 3 382.10 35.7 64.3 砾石填隙物中的第三世代衬垫白云石 QHD29-2E-5 3 383.10 78.1 21.9 粒间孔隙充填白云石胶结物 QHD35-2-3 3 327.93 72.9 3.8 23.3 交代颗粒的方解石胶结物 QHD35-2-3 3 320.80 51.4 26.9 21.7 石英加大 QHD35-2-3 3 478.18 76.3 8.4 15.4 石英加大 QHD29-2-2 3 321.40 23.0 77.0 石英加大 QHD29-2-2 3 322.40 57.0 43.0 石英加大 QHD29-2-2 3 328.00 80.8 19.2 石英加大 QHD29-2E-4 3 454.98 51.2 7.3 41.5 石英加大 -
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