Norphlet Reservoir in Mobile Bay: Origins of Deep Porosity

Brian E. Lock, Samuel W. Broussard

We have applied thin section and SEM petrographic techniques in an attempt to understand better the erratic distribution of highly porous, reservoir quality sands ("reservoir zone"), and overlying tight, thoroughly cemented sands ("tight zone") that together constitute the Norphlet Formation of Mary Ann field. Our conclusions are summarized as follows.

The deep (> 20,000 ft) Norphlet porosity is secondary, resulting from dissolution of an early cement. This cement was not, primarily, a carbonate--spotty carbonate cements are still present, but show little evidence of ever having been significantly more extensive. Both halite and anhydrite cements are well recorded from the Norphlet, and we believe that early evaporite cementation took place either as a result of saline ground waters migrating up from the underlying Louann, or during the pre-Smackover marine transgression. These cements either were never present in the top part of the formation, or were dissolved out during the transgression and related reworking of the upper part of the Norphlet sand pile. It may have been that evaporite cementation placed limits on the marine re orking; certainly, there is an approximate equivalence between the present tight zone and the reworked portion of the sand.

Oil staining is common, primarily seen on the surfaces of the original clastic grains in the upper zone. This oil is an early feature, preceded only by small, localized patches of silica cement and by some chlorite rim development. The oil stains are not common in the lower part of the formation, either because evaporite cements were already in place, or (less probably) because an oil-water contact was present and the oil limited the extent of subsequent evaporite cements.

Oil did not remain in the Norphlet, other than as grain surface staining; it presumably leaked out of the Norphlet into overlying units.

After the oil passed through, quartz and alkali feldspar overgrowth cementation occluded remaining porosity in the arkosic sands of the formation. These silicate cements occurred in those parts of the formation that were not already tightly cemented with evaporites, and define the present tight zone.

Subsequent flushing by undersaturated formation waters removed most of the evaporite cements, probably at a much later diagenetic stage. Microcrystalline alkali feldspars were then nucleated on the surfaces of feldspar sand grains, causing a very moderate reduction in the new, secondary porosity. As some of these small crystals appear not to be attached to feldspar surfaces, it is possible that they may have been partly replacive within the evaporites before extensive evaporite removal.

The final diagenetic event of significance was the introduction of gas. It is generally believed that the gas originated within the basal Smackover, although the evidence is inconclusive. A source within the Louann Salt or, more realistically, the Pine Hill anhydrites between the salt and the Norphlet cannot be ruled out. It is not unlikely that there are organic-rich shales or carbonate muds within the Pine Hill member in downdip areas not yet penetrated by the drill bit.

The scenario described above differs from those suggested by other authors in that it suggests that the Norphlet gas is not a product of cracking of the Norphlet oil residues. The distribution of porous and tight diagenetic facies poses problems in the development of the trend and is not yet predictable. The recognition that porosity is linked to early evaporite cement distribution may prove to be the key to porosity prediction.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.