Fluid features during hydrocarbon generation and expulsion from laminated source rocks

Bed-parallel, fibrous calcite veins are widespread within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression of Bohai Bay Basin, East China. Such veins mainly occur in the horizontal organic-rich laminae (composed of horizontal banded lamalginites) of mature source rocks, with higher TOC and lower carbonate contents than the adjacent non-vein sections vertically. They are products of diagenesis accompanying burial, and their formation was coeval with hydrocarbon generation and expulsion from the initial fracture opening and subsequent dilation, which was evinced by the occurrence of medium plane (calcite+bitumen) and primary hydrocarbon inclusions in the fibrous calcites. Petrographically primary two-phase (oil+gas) hydrocarbon inclusions, and coeval aqueous inclusions are distributed either within individual fibrous calcite grains or between two adjacent fibres. In rare cases, inclusions with only liquid hydrocarbons were observation. The hydrocarbon liquid shows various UV-fluorescence colours (brownish, yellow, yellow-green, green), indicating different composition of oil from less mature to relatively high maturity. The micro-spectrofluorimetry of yellow-green fluid inclusions contains the two dramatic peaks (511 nm and 568 nm) of yellow and green fluorescence, suggesting that it was the by-product by fractionation. In addition, many bitumen-bearing oil inclusions could also be observed in the fibrous calcite veins. Consequently, we surmise that immiscibility and heterogeneous trapping of liquid hydrocarbon, bitumen and aqueous solution during the fibrous calcite growth are the best explanation for these above features. The PVT calculation by use of isochores intersection of oil inclusions and aqueous inclusions, combined with study of the burial history show that veins were formed during the Oligocene Dongying sedimentary stage (32.8-24.6Ma). We conclude the fluid overpressure up to approximately twice (2x) the hydrostatic value (i.e., ~0.5–0.6x lithostatic) are the most common during the hydrocarbon generation and primary migration. The highest degrees of overpressure are recorded by the rare monophase petroleum inclusions. The resulting isochores of these highest density inclusions project to pressures that overlap with the lithostatic gradient. Thus, we contend that vein dilation in the absence of lithostatic fluid pressure suggested a primarily steady-state process, in which dilation was accommodated and offset by concomitant narrowing of the adjacent wall rock laminae. And this process was likely driven by dissolution of CaCO₃ from adjacent wall rocks, coupled with the later reprecipitation of calcite into the nascent horizontal fractures.