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Based on theoretical considerations, diffusion of low molecular weight hydrocarbons through the water-saturated pore system of sedimentary rocks can be expected as a common and ubiquitous process in the subsurface. Wherever concentration gradients develop (e.g., with the onset of hydrocarbon generation near the contact between organic-rich and organic-lean strata) diffusion of mobile components should occur. Diffusion processes play a dual role in the subsurface: as an initial step for transportation of hydrocarbons from source rocks toward carrier rocks, and at a later stage, when a reservoir accumulation has formed, as a destructive process by light hydrocarbon dissipation through the cap rock.
Geochemical evidence to illustrate the role and the effects of diffusion in both processes generally is represented by characteristic relationships between concentration depth trends and the molecular size and structure of the various hydrocarbon species in the transported mixture. Also, shale cap rocks of productive reservoir hydrocarbon accumulations are permeable, and diffusive loss of light hydrocarbons is significant. For individual light hydrocarbons, diffusive halos can be recognized in the cap rock above the reservoir accumulation.
Based on newly determined effective diffusion coefficients, model calculations have been made for quantitation of the outlined observations. In this way it was possible to demonstrate that molecular diffusion through the water-saturated pore space of shale source rocks represents an effective process for primary migration of gas and can account for transportation of such.
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