Tectonically driven organic fluid migration in the Dabashan Foreland Belt: Evidenced by geochemistry and geothermometry of vein-filling fibrous calcite with organic inclusions

摘要

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). δ~(13)C_(PDB) and δ~(18)O_(PDB) values of the fibrous calcite range from -4.8‰ to -1.9‰ and -12.8‰ to -8.4‰ respectively, which is lighter than that of associated carbonate host rocks ranging from -1.7‰ to +3.1‰ and -8.7‰ to -4.5‰. A linear relationship between δ~(13)C_(PDB) and δ~(18)O_(PDB) 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-bearing 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.