Geochemistry of Gas Emanations from Mud Volcanoes and Pockmarks from Offshore Egypt

Prinzhofer, Alain A.¹, Eric Deville¹, Jean Paul Foucher², Jean Claude Caprais², Jean Mascle³ (1) Institut Francais du Petrole, Rueil-Malmaison Cedex, France (2) Ifremer (3) Geosciences Azur, Villefranche-sur-Mer, France

The Nautinil cruise allowed submersible sampling of gas macro-seeps, solubilized in water or as gas bubbles (North Alex mud volcano). Their chemistry, carbon isotopes and noble gas isotopes define three genetic families:

- bacterial dry gases, with isotopically very light methane (Amon volcano, Menes Caldera). They are probably generated in shallow horizons in the sediments,

-thermogenic gases, without any post-genetic fractionation (Napoli, Isis, Osiris andNorth Alex volcanoes). They are generated in deep petroleum systems, and are collected after their upwelling to the surface,

-gases with intermediate compositions, corresponding to either a mixing betweenthermogenic and bacterial gas, or to gases fractionated during their uplift (Kheops and Kephren volcanoes).

For the thermogenic gases, genetic parameters as source-rocks and maturity could be characterized: Osiris gas is clearly more mature than Isis. North Alex corresponds to a gas of low maturity, generated from a different source-rock. Napoli, (south of Crete) is asso­ciated to a different source-rock. No evidence of any biodegradation could be observed (lack of fractionation for propane and normal-butane), proof that gas seepage is quick enough to prevent any alteration.

For the first time, noble gas isotopic patterns were performed in offshore gas seeps. The Kephren and North Alex gases show a longer residence time, indicating an important thermogenic contribution. The Isis gases present a “residual” signature, indicative of an important leakage of the associated deep hydrocarbon reservoir.

These geochemical results on gas seeps give valuable information for oil and gas exploration in poorly explored deep offshore plays.