(Table 1) Geochemical and geothermometry data from fibrous calcite vein fills in the Dabashan Fold Belt, China

Fibrous calcite veins with organic inclusions have been widely considered as indicators of oil and gas generation and migration under overpressure. Abundant fibrous calcite veins containing organic-bearing inclusions occur in faulted Lower Paleozoic through Triassic hydrocarbon source rocks in the Dabashan Foreland Belt (DBF). d13CPDB and d18OPDB values of the fibrous calcite range from - 4.8 to -1.9 to per mil and - 12.8 to - 8.4 per mil respectively, which is lighter than that of associated carbonate host rocks ranging from - 1.7 to + 3.1 per mil and - 8.7 to - 4.5 per mil. A linear relationship between d13CPDB and d18OPDB indicates that the calcite veins were precipitated from a mixture of basinal and surface fluids. The fibrous calcite contains a variety of inclusions, such as solid bitumen, methane bearing all-liquid inclusions, and vapor-liquid aqueous inclusions. Homogenization temperatures of aqueous inclusions range from 140 to 196° with an average of 179°. Salinities of aqueous inclusions average 9.7 wt% NaCl. Independent temperatures from bitumen reflectance and inclusion phase relationships of aqueous and methane inclusions were used to determine fluid pressures. Results indicate high pressures, elevated above typical lithostatic confining pressure, from 150 to 200 MPa. The elevated salinity and high temperature and pressure conditions of the fibrous calcite veins argue against an origin solely from burial overpressure resulting from clay transformation and dehydration reactions. Instead fluid inclusion P-T data and geochemistry results and regional geology indicate abnormally high pressures during fluid migration. These findings indicate that tectonic stress generated fracture and fault fluid pathways and caused migration of organic bearing fluids from the DBF during the Yanshan orogeny.

含有机包裹体的纤维状方解石脉（fibrous calcite veins）已被广泛视为超压环境下油气生成与运移的指示标志。大巴山前陆盆地带（Dabashan Foreland Belt, DBF）内的下古生界至三叠系断裂烃源岩中，产出大量含有机包裹体的纤维状方解石脉。该类方解石的δ¹³C_(PDB)与δ¹⁸O_(PDB)值分别为-4.8‰~-1.9‰与-12.8‰~-8.4‰，轻于伴生碳酸盐围岩的δ¹³C_(PDB)值（-1.7‰~+3.1‰）与δ¹⁸O_(PDB)值（-8.7‰~-4.5‰）。δ¹³C_(PDB)与δ¹⁸O_(PDB)之间的线性关系表明，该方解石脉由盆地流体与地表流体混合沉淀形成。纤维状方解石内发育多种包裹体，包括固态沥青、含甲烷全液态包裹体及气液液态水包裹体。水包裹体的均一温度为140~196℃，平均179℃；水包裹体的盐度平均为9.7 wt%氯化钠。研究借助沥青反射率以及水包裹体与甲烷包裹体的相态关系获取的独立温度数据，限定了流体压力：结果显示流体压力介于150~200 MPa之间，显著高于典型静岩围压。该纤维状方解石脉所具备的高盐度、高温高压特征，排除了其仅由黏土转化脱水作用引发的埋藏超压成因。综合流体包裹体温压数据、地球化学结果与区域地质背景，研究认为流体运移过程中存在异常高压。上述发现表明，燕山运动（Yanshan orogeny）期间构造应力产生了裂缝与断裂流体通道，驱动含烃流体从大巴山前陆盆地带内运移。