Low-temperature gas from marine shales: wet gas to dry gas over experimental time

摘要

Marine shales exhibit unusual behavior at low temperatures under anoxic gas flow. They generatecatalytic gas 300° below thermal cracking temperatures, discontinuously in aperiodic episodes, andlose these properties on exposure to trace amounts of oxygen. Here we report a surprisingreversal in hydrocarbon generation. Heavy hydrocarbons are formed before light hydrocarbonsresulting in wet gas at the onset of generation grading to dryer gas over time. The effect ismoderate under gas flow and substantial in closed reactions. In sequential closed reactions at100°C, gas from a Cretaceous Mowry shale progresses from predominately heavy hydrocarbons(66% C5, 2% C1) to predominantly light hydrocarbons (56% C1, 8% C5), the opposite of thatexpected from desorption of preexisting hydrocarbons. Differences in catalyst substratecomposition explain these dynamics. Gas flow should carry heavier hydrocarbons to catalytic sites,in contrast to static conditions where catalytic sites are limited to in-place hydrocarbons. In-placehydrocarbons and their products should become lighter with conversion thus generating lighterhydrocarbon over time, consistent with our experimental results.We recognize the similarities between low-temperature gas generation reported here and thenatural progression of wet gas to dry gas over geologic time. There is now substantial evidence fornatural catalytic activity in source rocks. Natural gas at thermodynamic equilibrium and the resultsreported here add to that evidence. Natural catalysis provides a plausible and unique explanationfor the origin and evolution of gas in sedimentary basins.