Organic inclusions in salt. Part I: Solid and liquid organic matter, carbon dioxide and nitrogen species in fluid inclusions from the Bresse basin (France)

Abstract

Organic inclusions from the Etrez and Cormoz areas of the Paleogene Bresse salt basin (France) have been studied. Different organic fractions are trapped with brine solutions in fluid inclusions in halite. Isolated inclusions from both areas are brine-bearing, and organic matter is sometimes associated with daughter mineral phases (carbonate, sulfate, clay). Structured and unstructured solid organic matter is observed both in the Etrez area (central basin) and in the Cormoz area (Northern ridge), while liquid organic droplets are only observed in fluid inclusions from the Cormoz area. FTIR microspectroscopic measurements show a low aliphatic contribution, and the presence of aromatic and oxygen double bonds for the solid organic matter. Low to high aliphatic contents (C5 and C7+, deduced from CH 2 CH 3 ratio measurements), low to high CC and CO contributions, and the presence of characteristic CN functions of amino-acids are detected in liquid droplets by FTIR spectroscopy. Carbon dioxide, originating from the thermal degradation of solid organic matter, is generally detected. Organic two-phase inclusions (brine + liquid organic matter) in planes are located in clear halite crystals from the Cormoz area. High aliphatic contributions, and variable aromatic and oxygen contents are observed. Ammonium ions (NH 4 +) are detected in the brine solution, and no carbon dioxide contribution is recorded. Organic matter trapped in the halite of the Bresse basin is immature to early mature. In the deeper part of the basin (Etrez, 800–1000 m), the organic matter in the inclusions is slightly thermally degraded, with little or no generation of liquid organic compounds. This agrees with the data on organic matter previously published from this area. In the Northern ridge (Cormoz, 500–800 m), liquid heteroatomic compounds (C, H, O, N) were produced, migrated through the salt and were trapped without gas phases. This implies a slightly lower temperature in the deeper part (Etrez) of the Bresse basin. Differences in thermal gradients between the Northern ridge, and the deeper part of the Bresse basin, must be invoked to explain differences in organic matter maturation. The analysis of organic inclusions is a contribution to understanding the thermal and biochemical history of the Bresse basin by the in situ observation of organic evolution products which are preserved by entrapment in salt.