Abstract: Hydrocarbon and Non-Hydrocarbon Gas in Salt Environments, A Contribution to Gas Genesis Understanding

J. Pironon, S. Grishina

The analysis of organic inclusions is a contribution to the understanding of the thermal and biochemical history of the evaporite basins by the in-situ observation of organic evolution products which are preserved by entrapment in salt. Recent applications of microanalytical techniques (FT- infrared, Raman, FT-Raman, UV-fluorescence) have confirmed and elucidated the nature of the organic phases trapped in inclusions.

Three geological environments were studied corresponding to different levels of organic maturation: 1) the Bresse salt deposit (France) where the presence of solid organic matter, immature oil, carbon dioxide, and ammonium is detected, is characteristic of the early diagenesis of the massive salt series deposited at the Eocene-Oligocene period in the West European continental rift, 2) the Gabon margin where oil, methane, carbon dioxide and ammonium correspond to catagenetic products trapped in a salt diapir of the Cretaceous salt series of the Ogooue delta, and 3) the Lena-Tungusska oil-bearing region of Siberian platform where graphite, carbonaceous material, sulphur, carbon dioxide, methane, nitrogen, and oils correspond to the evolution of the evaporite-brine-organic matter system nder influence of basalt intrusions.

The organic history of salt rocks can be followed from Bresse to Siberian examples. Proteins and carbohydrates are rapidly destroyed during early diagenesis (Bresse-France) : ammonium and carbon dioxide are produced by deamination and decarboxylation respectively, the complex organic molecular association is simplified and geopolymers (kerogen precursors) are produced. With increasing maturity (Gabon), kerogen is transformed in aliphatic compounds which are progressively cracked to produce short alkane chains and finally methane. Residual carbon dioxide is also produced. In Siberia, the organic matter evolution is linked to the d/h ratio, where d is the distance to the dolerite sill and h the thickness of the sill. Inclusions of oil, methane and nitrogen are associated to K-chloride a d ferric oxide zones. Carbon dioxide density increases when the d/h ratio decreases. Carbonaceous materials with uncommon shapes are observed at d/h less than 2. In the three environments secondary oil inclusions, free of gas, are systematically observed in fracture planes.

Three origins for carbon dioxide are suspected: 1) decarboxylation of biopolymers during early diagenesis in Bresse, 2) catagenesis of kerogen in Gabon, 3) carbonate dissolution during magmatism in Siberia. The origin of nitrogen species is clarified : 1) ammonium ions are produced in reducing environments during early diagenesis and stored in brines and K-minerals (Bresse and Gabon), and 2) ammonium ions are oxidized and molecular nitrogen is released from K-minerals (Siberia). The hypothesis of nitrogen release from organic matter by ⁴⁰K radiolysis is also proposed for Siberia.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France