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A comparison of clay diagenesis data obtained from a study of Tertiary shales from 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 diagenesis occurs. 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 may be totally or partially trapped and released slowly through time. In either situation, the water can act as a vehicle for hydrocarbon migration p ovided 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 related folds in basins where shale structures are the dominant type formed.
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 beddingplane types at depths where the temperature is near that required for thermal generation of petroleum. Although these faults may be important for fluid redistribution in shallow sandstone-shale sections, they are a minor factor in moving hydrocarbons out of shale below the faults in much of the Texas offshore area.
Fluid movement upward through microfracture systems in the deeply buried overpressured section overlying and extending upward from 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. This flushing process would be enhanced by smectite diagenesis because water derived from smectite that was trapped during basin subsidence would cause the flushing process 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.
Shale tectonism is also the primary mechanism for structural development in the Tertiary section of the Niger delta; however, seismic data indicate that the rate of dip change of seaward-dipping listric growth faults is commonly less than that observed in Texas where dips as low as 10°-15° can occur at depths as shallow as 10,000-15,000 ft (3,048-4,572 m). Syndepositional faults in Nigeria are formed in sandstone-shale sections where the clay composition of shale is primarily kaolinite and where little water of smectite diagenesis has been added to the pore system of the host sedimentary section. Subtle differences in structural styles in the Tertiary sections of Texas and Nigeria are probably the result of differences in clay composition of the shaly sections being deformed
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