Pressure Seals—Interactions with Organic Matter, Experimental Observations, and Relation to a “Hydrocarbon Plugging” Hypothesis for Pressure Seal Formation
Jean K. Whelan, Lorraine Buxton Eglinton, Lawrence M. Cathles III, 1994. "Pressure Seals—Interactions with Organic Matter, Experimental Observations, and Relation to a “Hydrocarbon Plugging” Hypothesis for Pressure Seal Formation", Basin Compartments and Seals, Peter J. Ortoleva
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Organic geochemical characteristics diagnostic of pressure seals have been determined for two wells in the Moore-Sams field of the Tuscaloosa trend, Louisiana Gulf Coast (Mix and Bizette wells) and one well penetrating a much weaker pressure transition zone of the Anadarko basin, Oklahoma (Weaver well). Preliminary data suggest these characteristics of organic matter in zones of pressure seals: a rapid increase in vitrinite reflectance near the top of the pressure seal; fractionation of bitumens through the pressure seal with a gradual change from lighter to heavier n-alkanes with increasing depth in the pressure seal; a buildup of hydrocarbons just beneath the pressure seal; and an enhancement of asphalt (or asphaltene) throughout the general zone of the pressure seal. For all three wells, very tight associations of carbonate cements, fine pyrite, asphaltenes, and micrinite (generally considered to be a residual product of hydrocarbon generation) were observed in the general zone of pressure seals, suggesting that interactions of organic and inorganic materials may be required for pressure seal formation and maintenance, even in fairly organic lean wells such as Weaver. A sharp jump in thermal maturity, as measured by vitrinite reflectance, occurs at the top of the Mix pressure transition zone. Maturity levels below the seal reach gas thermal window levels, suggesting that gas formation within and below the (seal) zone is contributing both to overpressuring and sealing of pressure seals investigated here. It is proposed that all these observations can be accommodated if the pressure drop across the seal pressure transition zone causes separation of oil and gas and deposition of asphalt from the upward streaming oil/gas that carried them from sources at greater depths. Permeability is reduced through a combination of asphalt hydrocarbon plugging, inorganic alteration, and (most important) gas-water capillary effects.
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Basins worldwide exhibit an unexpected degree of hydrologic segregation. There can be regions of a sedimentary basin that are isolated from their surroundings by a relatively thin envelope of low-permeability rock with an interior of sufficiently high permeability to maintain a consistent internal hydrostatic fluid pressure gradient. These have been named pressure compartments. Presure compartments have several remarkable features, just one of which is that internal fluid pressures can greatly exceed or be significantly less than any regional topographically controlled hydrologic head or drain. This publication contains 30 chapters that take detailed looks at pressure compartments in general, and detail case studies of these compartments in specific basins, such as the Anadarko and Gulf of Mexico. The volume also looks at other considerations in sedimentary basins such as hydrodynamic and thermal characteristics, and mechanical properties of rock.