Print this pageIron Oxide Reduction as Evidence for Hydrocarbon Migration through the Entrada Sandstone of the Moab Anticline, Utah
GUSCOTT, S.C., I. R. GARDEN, K. A. FOXFORD, S. D. BURLEY, J. J. WALSH and J. WATTERSON
The Moab Anticline, south-east Utah, is a small, exhumed, hydrocarbon palaeo-reservoir. Reduction of iron oxides in the Middle Jurassic, eolian-dominated Entrada Sandstone provides a record of hydrocarbon migration pathways through the anticline. The distribution of the reduction patterns across the flanks and the crest of the anticline reflects the effects of structural and sedimentological heterogeneity on hydrocarbon migration in the subsurface. Evidently, reducing fluids were not affected by either aeolian stratification, which results in 1-2 orders of magnitude variation in matrix permeability, or by slip bands which have permeabilities up to 3 orders of magnitude below that of the matrix sandstone. Neither is the geometry of reduction fronts significantly affected by faults with sand:sand juxtapositions. Faults with shale-smeared slip surfaces do, however, influence the reduction patterns.
The spatial distribution of the reduction fronts indicates that the hydrocarbons entered the anticline down-dip on its south-west limb and migrated towards the structural crest of the anticline. Each successive up-dip trap was filled to spill point as the fluid migrated towards the anticline crest. At the crest the hydrocarbons ponded at structural highs where top seal was provided by overlying mudstones of the basal Morrison Formation, whilst fault seal was facilitated by either impermeable membrane seals or mudstone juxtaposition.