Low-temperature gas from marine shales: wet gas to dry gas over experimental time |
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Authors: | Frank D Mango Daniel M Jarvie |
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Institution: | (1) Petroleum Habitats, 806 Soboda Ct, Houston, Texas 77079, USA;(2) Worldwide Geochemistry, 218 Higgins Street, Humble, Texas 77338, USA |
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Abstract: | Marine shales exhibit unusual behavior at low temperatures under anoxic gas flow. They generate catalytic gas 300° below thermal
cracking temperatures, discontinuously in aperiodic episodes, and lose these properties on exposure to trace amounts of oxygen.
Here we report a surprising reversal in hydrocarbon generation. Heavy hydrocarbons are formed before light hydrocarbons resulting
in wet gas at the onset of generation grading to dryer gas over time. The effect is moderate under gas flow and substantial
in closed reactions. In sequential closed reactions at 100°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 that expected from desorption of preexisting hydrocarbons. Differences in catalyst substrate composition
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-place hydrocarbons and their products should become lighter with
conversion thus generating lighter hydrocarbon over time, consistent with our experimental results. |
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