Tectonically Driven Organic Fluid Flow in Dabashan Foreland Belt: Recorded by fibrous calcite veins contained hydrocarbon bearing inclusions

Key words: 
• fibrous calcite veins /
• fluid inclusion /
• overpressure fluid /
• fluid flow expelling /
• Dabashan orogenic belt

Abstract: Fibrous calcite veins with organic inclusions were considered as indication to oil/gas generation and expelling under overpressure. Abundant fibrous calcite veins contained hydrocarbonbearing inclusions filled in fractures were found in lower Paleozoic hydrocarbon source of black mudstone, clay shale and limestone as well as some faults in Dabashan Foreland Belt. Composition analysis shows that fibrous calcite veins are kind of low magnesium calcite which formed during later diagenesis rather than sedimentary period. 13CVPDB and 18OVPDB of fibrous calcite veins, ranging from -1.9‰ to -4.8‰ and -8.4‰ to -12.8, respectively, are lighter than that of surrounding carbonate rocks, ranging from -1.7‰ to +3.1‰ and -8.7‰ to -4.5‰,  show obvious linear relationship, which indicates that fibrous calcite veins were mixture fluid from different source fluids. Fibrous calcite veins contain immiscible fluid inclusions such as solid bitumen, methane bearing liquid fluid inclusion and vaporliquid aqueous inclusions. Bitumen inclusions are residue organic during oil/gas migration. Homogeneous temperatures of vaporliquid aqueous inclusions are from 140 to 196 ℃ with peak of 179℃. Salinities of vaporliquid aqueous inclusions are high with average of 9.7 wt% NaCl. Mehtods of IsoPT phase diagrams of aqueous fluid inclusions and methane inclusions were used to get fluid pressure when fibrous calcite veins formed. The pressure of fibrous calcite vein with hydrocarbonbearing inclusions in Dabashan Foreland Belt is from 150 to 200 MPa, which indicates of abnormal overpressure fluid. Geology and geochemistry features indicate that fibrous calcite veins contained hydrocarbonbearing inclusions are not fluid formed by overpressure of mudrocks source rocks at shallow buried phase. However, combined with the regional sedimentary characteristics and tectonic evolution, it indicates that fibrous calcite veins contained hydrocarbonbearing inclusions were fluid of natural gas drove to expel under overpressure of tectonic stress during InidanChina tectonic movement and Yanshan Foreland tectonics.