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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 39 (1989), Pages 187-194

The Norphlet Reservoir in Mobile Bay; Origins of Deep Porosity

Brian E. Lock (1), Samuel W. Broussard (1)

ABSTRACT

We have applied thin section and scanning electron microscope petrographic techniques in an attempt to better understand the erratic distribution of highly porous, reservoir quality sands (the "Reservoir Zone') and overlying tight, thoroughly cemented sands (the "Tight Cap Zone') that together constitute the arkosic 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 early cement may have been a carbonate, similar to modern very early, fine grained calcite cements being studied at the Gulf of California margin of the Gran Desierto, or it may have been an evaporite; both halite and anhydrite cements are well recorded from the Norphlet. Either these cements were never present in the upper part of the formation, or they were removed during the marine transgression that ended Norphlet sedimentation. The uppermost Norphlet sands were reworked during the transgression, and it is primarily these marine modified sands which constitute the Tight Cap Zone.

Early oil staining, seen on the original detrital surfaces of sand grains, is common in the upper part of the formation, commonly but not exclusively within the Tight Cap Zone. This oil is an early feature, preceded only by small, localized patches of silica cement and by some authigenic chlorite rim development. The oil stains are not common in the lower part of the formation, probably because the carbonate/evaporite cement was already in place and occluding most porosity. Oil was not permanently trapped in the Norphlet in Mobile Bay.

After oil passed through, quartz and feldspar overgrowth cementation occluded remaining porosity in the formation. These silicate cements occur in those parts of the formation which were not already tightly cemented with evaporites or carbonates, and define the present tight zone.

Subsequent carbonate dissolution by acidic waters or evaporite flushing by relatively low salinity waters removed most of the early cements, probably at a relatively late diagenetic stage. Microcrystalline alkali feldspars were then nucleated on the surfaces of feldspar sand grains, causing a moderate reduction in the new, secondary porosity. Since some of these small crystals appear not to be attached to a feldspar (or other) substrate surfaces, it is possible that they may have partly replaced the early cements before the removal of the latter; this may account for the unusual morphology of these crystals.

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 can not be ruled out. It is not unlikely that there are organic-rich shales or carbonate muds within the Pine Hill member in down-dip areas not yet penetrated by the drill bit. Our knowledge of the marine Norphlet shales and carbonates, located only a few miles south of Mobile Bay, is also very limited. Source rock potential in the down-dip Norphlet can not be ruled out at this time.

The scenario described above differs from those suggested by other authors in that it suggests a realistic mechanism for deep porosity in the Norphlet, and emphasizes that the gas is not a product of cracking of the Smackover-derived Norphlet oil. The distribution of porous and tight diagenetic facies poses problems in the development of the deep gas trend, and a fuller understanding of the controls on porosity is critical.

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