Fluid features during the initial stage of hydrocarbon generation and expulsion in salt-water lacustrine laminated source rocks of Dongying Depression

Key words:  Dongying Depression  mature shales  bedding-parallel fibrous veins  initial stage of hydrocarbon generation and expulsion  low mature hydrocarbon

Abstract:

The bedding-parallel fibrous veins (Beef) in salt-water lacustrine mature shales of the Eocene Dongying Depression are studied in this paper. Based on the study of vein petrography and fluid inclusions features, we demonstrate that the Beef was the outcome during the initial stage of hydrocarbon generation and expulsion. We determine the fluid features during this stage by means of the micro-spectrofluorimetry and the laser Raman spectrum. The results show that the Beefs are one-stage fluid products, and the filling crystals are all ferroan-calcites. The hydrocarbon fluids during the Beef formation are characterized by low mature fluids with lots of bitumen. The total contents of the aromatic hydrocarbon (aver. 38.8％) and NSO compounds (aver. 6.8％) are high, as well as the API degree of the oil (aver. 32.9). The average equivalent Ro of the bitumen-bearing inclusions calculated based on the Raman spectra is 0.59％. As a consequence, the Beef is a symbol of fluid overpressure caused by hydrocarbon generation and expulsion during the initial stage in mature shales of Dongying Depression. During this stage, the salt-water lacustrine source rocks had high degradation efficiency of organic matters, so the horizontal fractures caused by overpressure can provide important channels for the expulsion of the abundant low mature hydrocarbon fluids in the early stage. The obvious fractionation processes occurred during the horizontal migration of hydrocarbon fluids.