Peridotites, Serpentinization, and Hydrocarbons

Stanley B. Keith and Monte M. Swan
MagmaChem, L.L.C, Sonoita, AZ

Serpentinization of peridotites by oceanic or metamorphic sourced brines under strongly reduced conditions and temperatures of 200-500 C produces hydrocarbon-rich, chloride and/or bicarbonate metal-bearing brines. Serpentinization is common on the ocean floor along fracture zones (Lost City), beneath conventional petroleum in rifts due to sedimentary burial (Gulf of Mexico) or thrust loading (Roan Trough), and at the top of flat subducting oceanic crust (Eocene beneath UT, CO, WY). Peridotites exhibit high-gravity, low-magnetic signatures. Serpentinized peridotites exhibit high-magnetic, low-gravity signatures. Volume expansion during serpentinization of up to 8X causes diapiric doming and induces expulsion of hydrocarbon-stable brines. There are 2 major types of peridotites: 1) magnesian dunitic peridotite with low V/Ni, high Au-Mg-Cu-Cr-Na/K, up to1400 ppm C (lithosphere source 51-130 km), 2) quartz alkalic aluminum-spinel peridotite with high V/Ni, high S-Mo-Ti-Al-Mn-Fe-U-K/Na up to 800 ppm C (athenosphere source 360-420 km). If hydrogen-stable (mainly thermogenic methane) peridotite-sourced brines rise into shelf carbonate sequence, they may form magnesian or quartz alkalic hydrothermal dolomite (HTD) and thermogenic gas. If the brines breech the hydrosphere they may produce "white smokers" (tuffa vent mounds/pinnacle reefs) along faults and enrich shales with exhalative metal and hydrocarbon. Petroleum condensate typically forms in reservoirs between the HTD zone and seep sites at the top of the lithosphere. Type I kerogen in black shale vents from Mg peridotite-sourced brines whereas Type II kerogen in black shale vents from quartz alkalic peridotite-sourced brines. Correspondingly hydrocarbon chemistry divides oil and gas into 2 major types: 1) magnesian sweet, low-sulfur paraffinic-naphtheric, 2) quartz alkalic sour, high-sulfur aromatic asphaltic. Geochemical markers that tie oil and gas to specific peridotite hydrothermal sources include nano-particle native metals and diamonds, and V-Ni porphyrins.