MODERN OIL GENERATION AND PYROLYSIS AT >800ºC FROM SPONTANEOUS COMBUSTION IN A LANDSLIDE OF MIOCENE SHALE, CALIFORNIA

A natural landslide in an organic-rich shale (2-6% TOC) within the Miocene Puente Shale occurred in Spring 2008. In early June 2009, smoke was observed emanating from the slide deposit. Gas analyses from other studies indicate the presence of C2 through C4 compounds, which demonstrate the combustion of hydrocarbons. Our investigation using field sampling and lab studies indicates that oxidation of iron sulfides has generated heat sufficient to crack hydrocarbons from the kerogen in the shale. Thick, tar-like coatings can be observed surrounding some of the heat vents. The generated hydrocarbons were then burned to produce local hot spots in the shale exceeding 800ºC. The slide is adjacent to a landfill in Orange County California, but the landslide and combustion is unrelated to the landfill activities.

Analyses of the shale indicate sulfur content of 2.3 to 4 wt% and SEM confirms both cubic (pyrite) and orthorhombic (marcasite) forms of iron sulfides. Kerogen extracts indicate a Type II kerogen with relatively high sulfur (2 to 8.5 wt %) and nitrogen (about 2 wt %) content. Hydrous pyrolysis of the kerogen indicates hydrocarbon generation at relatively low temperature (peak generation of hydrocarbons at 380º C). All of these aspects of the shale, combined with the introduction of oxygen and moisture from the slide process, promoted the rapid build-up of heat and resulting combustion. By-products from pyrolysis of the organics at the heat vents include cadmium sulfide, selenium compounds, ammonium sulfate, native sulfur, and gypsum. The hot landslide has been mediated as an interim measure by covering the slide with about one half meter of a clay-bearing soil. This apparently has removed the available oxygen.

The slide is similar to other hot landslides in which heat is triggered from oxidation of iron sulfides, but this one is different in that the generation of hydrocarbons resulted in temperatures more than 500ºC higher than in cases of kerogen poor shales where no hydrocarbons were generated.