ORGANIC INCLUSIONS IN SALT .2. OIL, GAS AND AMMONIUM IN INCLUSIONS FROM THE GABON MARGIN

Organic inclusions have been found in a salt diapir south of Port-Gent il in the North Subbasin of the Gabon margin. Two main types of organi c inclusions are described. Type 1 gas-rich (CH, and CO,) oil inclusio ns located in recrystallized halite, associated with a complex mineral daughter paragenesis: anhydrite, quartz, K-feldspar, dolomite, ammoni um sylvite and carnallite (1 and 2.5% molar NH4Cl, respectively). The oil is essentially aliphatic with some aromatic content, the compositi on being characteristic of mature oil. The different methane/C2 + rati os in the gas and oil phases indicate an internal pressure close to 20 0 bars, which is compatible with hydrostatic pressure at sampling dept h. Homogenization temperatures of two-phase hydrocarbon inclusions ins ide dolomite daughter crystals have been measured (Th = 113 degrees C) . Type 2 organic inclusions are located in healed fracture planes in r ecrystallized halite and contain aliphatic oil; the brine content is h igh, and no gases have been detected. Different alkane compositions oc cur in different fracture planes. Type 2 inclusions correspond to inje ctions of mature oil into halite fracture systems at low temperature ( approximate to 60 degrees C). Comparisons between fluid inclusions in halite occurring in the Bresse area (France), and in the North Subbasi n of the Gabon margin, help one to follow carbon and nitrogen cycles f rom diagenesis to catagenesis. The organic inclusions hosted in salt r ocks reflect the geochemical reactions that have occurred in the salt over time.