Migration of Fluids in Sedimentary Basins

The basic mechanisms governing the migration of water and hydrocarbons in sedimentary basins are related ultimately to basinal structural history, rock properties, compaction, and temperature regime, which themselves are interdependent. Compaction of the sedimentary rocks promotes the expulsion of water during the progressive burial of the sediments. The physical-chemical properties of the rocks are responsible for the migration of the expelled water toward the surface and for the distribution in the basin of various parameters such as hydraulic potential, salinity, heat flow, temperature, and cementation. Primary migration is used herein to refer to migration of hydrocarbons soon after their generation; secondary migration refers to their delayed movement. Oil has difficulty migrating in shaly rocks. A possible way of removing oil from a source rock, in addition to the colloidal or true solubilization in water or the flow in a three-dimensional oil-wet kerogen network, is through its solubilization in high-pressure gas generated in the deeper part of the basin and coming up through preferential paths such as faults and fractures. Molecular distillation of the oil occurs because its vapor pressure in the liquid phase is higher than the vapor pressure in the gaseous phase. In the upper part of a lithologic column, retrograde condensation occurs and gas and condensate accumulations start to form below little compacted plastic caprocks. Owing to the high displacement pressures of the gaseous column, gas can pass through the caprock and disperse to the surface while the condensate remains in place. The pore-size distribution of the little compacted shaly caprock is responsible for the chemical and isotopic fractionation of the rising gas; almost pure and isotopically light methane thus is found in the upper layers of a stratigraphic column. High-volatile oil can be easily degraded by circulating water, especially in the presence of sulfate ions, hydrogen sulfide, and bacteria, and transformed progressively to heavy oil. Geochemical tools which can be used by explorationists in prospecting for new oil and gas fields include isotope analysis, hydraulic potential, salinity, temperature, cementation, maturity, and maps of shows.

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