Organic inclusions in salt. Part 2: oil, gas and ammonium in inclusions from the Gabon margin

Organic inclusions have been found in a salt diapir south of Port-Gentil in the North Subbasin of the Gabon margin. Two main types of organic inclusions are described. Type 1 gas-rich (CH4 and CO2) oil inclusions located in recrystallized halite, associated with a complex mineral daughter paragenesis: anhydrite, quartz, K-feldspar, dolomite, ammonium sylvite and carnallite (1 and 2.5% molar NH4Cl, respectively). The oil is essentially aliphatic with some aromatic content, the composition being characteristic of mature oil. The different methane/C2 + ratios in the gas and oil phases indicate an internal pressure close to 200 bars, which is compatible with hydrostatic pressure at sampling depth. Homogenization temperatures of two-phase hydrocarbon inclusions inside dolomite daughter crystals have been measured (Th = 113°C). Type 2 organic inclusions are located in healed fracture planes in recrystallized halite and contain aliphatic oil; the brine content is high, and no gases have been detected. Different alkane compositions occur in different fracture planes. Type 2 inclusions correspond to injections of mature oil into halite fracture systems at low temperature (≈60°C). Comparisons between fluid inclusions in halite occurring in the Bresse area (France), and in the North Subbasin of the Gabon margin, help one to follow carbon and nitrogen cycles from diagenesis to catagenesis. The organic inclusions hosted in salt rocks reflect the geochemical reactions that have occurred in the salt over time.