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Variation in Diapiric Structure Development and Productivity, Northern Gulf Coast Basin
Thomas G. Fails
Paleo-geologic (palinspastic) reconstructions of salt dome development are used to compare growth patterns and processes characteristic of different salt dome classes, to understand diapiric processes, and for salt structure evaluation prior to drilling.
Coastal Salt Basin diapiric structures (salt domes and clay ["shale"] domes) can be classified by the diapir top/objective section relationship (continental shelf sands and shales and sandy upper slope strata comprise the objective section) as: Penetrant (or shallow piercement) salt domes where salt pierces the entire objective section and the overlying non-objective alluvium; semi-penetrant (or intermediate piercement) salt domes where salt pierces part of the objective section and arches the overlying beds; and non-penetrant (or deep-seated) diapiric structures where the diapir (salt or clay) is buried in upper slope shales beneath the arched objective section base.
Cross-sections from 44 published salt dome field studies were palinspastically restored to illustrate developmental patterns. They represent 15 penetrants, 8 semi-penetrants and 21 non-penetrants. All fields are in the Eocene, Oligocene, Miocene or Pliocene producing trends of southeast Texas and south Louisiana.
Penetrant salt diapirs maintain their crests close to the depositional surface. Internal convergence (thinning of flanking strata toward the diapir) is very common, as are major growth faults and deep flank unconformities. Traps form early and are preserved.
Semi-penetrant diapirs include more diapiric clay than penetrants. All were buried before alluvial deposition commenced. Major, downthrown-away, diapir-peripheral faults and unconformities are characteristic, and internal convergence is common at depth. Traps form early, but some lose critical dips later.
Non-penetrant diapiric structures develop much differently. The diapirs were already buried and inactive in slope facies shales when objective section deposition commenced. Differential flank subsidence, modest internal convergence and growth faulting characterize development. Effective traps sometimes form late and critical dips may be reduced or lost, reducing productivity.
Salt dome development in the Mesozoic-filled interior salt basins is very different. Penetrant diapirs are common. Most are slim salt cylinders, some with circumdomal overhangs. Many exhibit internal divergence (thickening of strata toward the diapir) in part of the section, and productivity is limited or absent. Evidence of semi-penetrants and non-penetrants is limited. Most salt-associated production in the interior basins occurs on non-diapiric salt pillows or on salt-flow-related turtle structures.
Differences in salt dome development patterns between the coastal and interior basins relate to basin type and depth, depositional rate, the presence or absence of thick marine shales, abnormal pressures, and large growth faults.
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