ABSTRACT

Recent petrographic, trace element, isotopic, and fluid inclusion work has necessitated a major revision of the diagenetic model for the Smackover Formation at Walker Creek field, southern Arkansas. Prior studies concluded that all diagenetic changes occurred within a few tens of feet of the earth's surface. This investigation has shown that near-surface diagenesis is confined to very minor marine cementation. The equant calcspar and baroque dolomite cementation occurred much later at temperatures between 60° and 120°C during the initial stages of hydrocarbon migration in the deep subsurface.

Earlier workers also concluded that the equant cementation is unrelated to depositional facies. They attributed the layered permeable and impermeable zones to permeability preservation in a meteoric vadose zone and occlusion in a meteoric phreatic zone. Because the equant cement is of a late origin, it can no longer be attributed to a meteoric zone.

The equant cement does occur in all depositional facies, but the effect of this cement upon the permeability of the rock is related to the original depositional facies. Permeability has been preserved in the coarser grained ooid and ooid-intraclast grainstones. This may, in turn, be related to the size of the pore throats, because cementation could block the pore throats completely in the finer grained material only. The coarser rock remained permeable and was made more permeable by late-stage dissolution.

This new-found relationship of permeability and original depositional facies has several implications regarding future hydrocarbon exploration in the southern Arkansas-northern Louisiana area.