Deep Porosity Prediction in the Norphlet Formation, Alabama: Linking Early Grain Coat Development to Paleo Groundwater Regimes

Joanna M. Ajdukiewicz¹, Paul H. Nicholson², Jill Pachell³, and Abbas Seyedolali^4
1 ExxonMobil Upstream Research Company, Houston, TX
² ExxonMobil International Limited, London, United Kingdom
³ ExxonMobil Production Company, U.S. Operation, Houston, TX
⁴ Consultant, Houston, TX

The Jurassic eolian Norphlet formation in Mobile Bay, Alabama, is a major gas reservoir with up to 18% intergranular porosity despite burial depths to 22,000 ft and temperatures to 420°F. To optimize well placement for development, a study was undertaken to improve understanding of a quartz-cemented tight zone with <7% porosity that occurs everywhere at the top of the Norphlet, overprinting all facies and varying from 10 to 180 ft in thickness.

Analyses indicate that the distribution of the late, high-temperature tight-zone quartz cement is controlled by early grain-coat diagenesis in late-Norphlet groundwater regimes. Best reservoir quality in Mobile Bay is preserved below a paleo water table, where highly continuous diagenetic chlorite coats formed and later inhibited quartz cement. The tight zone formed in the paleo vadose zone, above the water table, where chlorite coats are less complete. Discontinuous chlorite and early-infiltrated illitic coats allowed late quartz cementation and enhanced intergranular pressure-solution. Chlorite abundance increases downward from the Norphlet top, until, at the tight-zone base, coats become continuous enough to preserve porosity at high thermal exposures. A water-table control on cementation is consistent with the tight-zone configuration observed in Mobile Bay. Observations do not support other proposed tight zone controls such as marine reworking, decementation, paleo-oil overmaturation, and localized strain.

In the onshore-Alabama area, coats of diagenetic illite also preserve porosity. Diagenetic illite formed where an inferred fresh-water aquifer penetrated the updip Norphlet during early burial, dissolving feldspars and forming kaolinite. Meteoric diagenesis in this area overprints parts of the paleo-vadose zone, reducing upper-tight-zone thickness.