ABSTRACT

The geology of the Mary Ann Field is best understood in light of regional studies, which help to establish a depositional model based on both facies distribution and thickness variation. These studies also show major differences between onshore and offshore Norphlet deposits in diagenesis, hydrocarbon trapping and migration.

The Jurassic Norphlet sandstone was deposited in an arid basin extending from East Texas to Florida by a fluvial-eolian depositional system, prior to a marine transgression represented by the Smackover Formation. Until discovery of the Mary Ann Field in 1979, Norphlet production was restricted to onshore areas, mostly along the Pickens-Pollard fault system in Mississippi, Alabama and Florida. The Mary Ann Field is a Norphlet dry gas accumulation, and it was the first offshore field in the Gulf of Mexico to establish economic reserves in the Jurassic. The field is located in Mobile Bay, approximately 25 mi to the south of Mobile, Alabama. Formed by a deep-seated (20,000 + ft) faulted salt pillow, Mary Ann Field produces from a series of stacked eolian dune sands. situated near the Norphlet paleocoastline. Five lithofacies have been recognized in cores from the Mobil 76 #2 well. Each lithofacies has a distinct reservoir quality. Optimum reservoir facies are dune and sheet sands. Non-reservoir facies are interdune (wet and dry), marine reworked and evaporitic sands. Following deposition, these sediments have undergone varying degrees of diagenesis. Early cements in well-sorted sands supported the pore system during compaction. However, later cementation by chlorite, silica and alteration of liquid hydrocarbons to an asphaltic residue, have completely occluded the pore system in parts of the reservoir.