中扬子北缘冲断构造带的古流体记录及其对页岩气保存意义——以保地1井为例

为研究中扬子北缘大洪山冲断带古流体特征及其对页岩气保存的指示意义，以该区保地1井为例，系统采集志留系裂缝脉体样品并开展流体地球化学与包裹体分析。测试结果显示，方解石脉δ13C值变化范围为-8.19‰~0.16‰，碳同位素最小值远小于志留系灰岩夹层，也小于海相碳酸盐岩，结合脉体包裹体群脱硫系数高达23.53~87.90且自下而上具有降低的趋势，表明古流体中有下伏寒武系膏岩层卤水混入，导致古流体SO₄2-增高，以及寒武系TSR产生的H₂S、CO₂沿着裂缝系统进入志留系，致使方解石脉δ13C负偏和钻遇地层气显发现H₂S。志留系龙马溪组底部包裹体类型以纯水溶液包裹体为主，最大均一温度峰值约为110~120℃，最小均一温度峰值为60~80℃；结合区域构造演化，大洪山冲断带较川东地区志留系页岩抬升剥蚀时间早，古流体形成阶段埋深浅、温度低、含气饱和度低。滑脱带附近包裹体均一温度变化范围大，最低均一温度发育于该段，表明滑脱带构造活动期次更多、持续时间更长，特别是晚期构造活动更强，成为页岩气逸散的长期通道。综合分析表明，大洪山冲断带发育多个滑脱层的冲断、变形，对多层系页岩气藏和常规气藏破坏较大。 

Geochemical characteristics of paleo-fluids in thrust belt in the northern Middle Yangtze and its significance for shale gas preservation: a case study of well Baodi 1

To study the geochemical characteristics of paleo-fluids in the Dahongshan thrust belt of Middle Yangtze and its significance for shale gas preservation, in this study, Silurian vein samples were collected from well Baodi 1 and fluid geochemistry and inclusions were systematically analyzed. Results show that the variation range of δ13C of calcite veins is -8.19‰ to 0.16‰, with the minimum value much lower than that of Silurian limestone interlayer and marine carbonate. The desulfurization coefficient of vein inclusion group is 23.53 to 87.90, with downward trend from bottom to top. It is indicated that the SO₄2- in paleo-fluid of Silurian increases because of the mixing of underlying Cambrian gypsum brine, as well as CO₂ and H₂S entering into Silurian along fracture system from Cambrian TSR productions, resulting in negative δ13C of calcite veins and H₂S show in drilled formations. The inclusions at the bottom of Silurian Longmaxi Formation are mainly pure aqueous solution inclusions. The maximum homogenization temperature peak is about 110 to 120℃, and the minimum homogenization temperature distribute from 60 to 80℃. Compared with those in the eastern Sichuan Basin, the shale in Dahongshan thrust belt entered uplifting and denudation stage earlier, and the paleo-fluid formation stage had shallower burial depth, low temperature and low gas saturation. The range of homogenization temperature of inclusions near the detachment zone is wider, and the lowest homogenization temperature is developed in this section, indicating the detachment zone has more periods of tectonic activity and longer duration time, especially in the later stage, which has become a long-term channel for shale gas escape. Comprehensive analysis shows that there are many detachment layers in the Dahongshan thrust belt, and the thrust and deformation destroyed multi-layer shale gas reservoirs and conventional gas reservoirs.