A comparison of clay diagenesis data obtained from a study of Tertiary shales from wells drilled in the Brazos-Colorado River system of Texas, the Mississippi River system of Louisiana, and the Niger River system of Nigeria illustrates significant differences in temperature intervals over which smectite (expandable clay) diagenesis occurs.

The age of the shales studied ranges from 1 to 50 m.y., and the threshold temperature required to initiate diagenesis ranges from about 160°F (71°C) in Mississippi River sediments to more than 300°F (150°C) in the Niger delta. Water expelled from smectite into the pore system of the host shale during the process of diagenesis may migrate out of the shale early, or it may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration provided hydrocarbons are present in a form and in sufficient quantities to be transported.

Observations from the northern Gulf of Mexico basin indicate a close relation between buildup of high fluid pressure and the smectite-illite transformation process. Abnormal pressures exert partial control on the type and quantity of hydrocarbons accumulated because pressure potential determines the direction of fluid flow, and overpressuring partly controls the geometry of growth faults and other related faults and folds in the basin.

The depths to which growth faults can penetrate and the angle of dip that these faults assume at depth are largely dependent on fluid pressure in the sedimentary section at the time of faulting. Dips of some faults in Texas have been observed to change abruptly within the interval of smectite diagenesis, and some faults formed in the overpressured Miocene and younger sections become bedding-plane types at depths above the temperature level required for thermal generation of petroleum. Although these faults are important for fluid redistribution in the shallow sandstone-shale section, they play a minor role in moving hydrocarbons out of shales below the faults in much of the Texas offshore area.

Fluid movement upward along fault systems in the lower Tertiary section, which overlies fault trends in the sub-Tertiary section, is proposed as a mechanism for flushing hydrocarbons from the deeper portion of the northern Gulf of Mexico basin. These fault systems would have maximum development immediately above the "basement faults," with displacement decreasing progressively upward. Seismic data indicate that, in the upper (younger) Tertiary section, these deep fault systems are represented by near-vertical (high-angle) fracture systems that cut across the low-angle growth faults. Fluid movement within these deep fault and fracture systems would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing pr cess to continue for longer periods of time and to extend to greater depths than could be attained if only remnants of original pore water were present.

Based on data obtained from both the Brazos-Colorado and Mississippi River systems, it is concluded that smectite dehydration in shale is a major factor in both hydrocarbon migration and accumulation in basins where expandable clays are present. Concepts developed here can be applied to any basin that has had or now contains expandable clay shales. The effect of smectite diagenesis and the time of fluid release out of the shales must be considered with all other stratigraphic, structural, and geochemical parameters considered in basin evaluation.

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