Depositional and diagenetic history and petroleum geology of the Jurassic Norphlet Formation of the Alabama coastal waters area and adjacent federal waters area

The discovery of deep (> 20,000 ft) gas reservoirs in eolian sandstone of t he Upper Jurassic Norphlet Formation in Mobile Bay and offshore Alabama in the late 1970s represents one of the most significant hydrocarbon discoveri es in the nation during the past several decades. Estimated original proved gas from Norphlet reservoirs in the Alabama coastal waters and adjacent fe deral waters is 7.462 trillion ft(3) (Tcf) (75% recovery factor). Fifteen f ields have been established in the offshore Alabama area. Norphlet sediment was deposited in art arid environment in alluvial fans, a lluvial plains, and wadis in updip areas. In downdip areas, the Norphlet wa s deposited in a broad desert plain, with erg development in some areas. Ma rine transgression, near the end of Norphlet deposition, resulted in rework ing of the upper part of the Norphlet Formation. Norphlet reservoir sandstone is arkose and subarkose, consisting of a simpl e assemblage of three minerals quartz, albite, and K-feldspar. The present framework grain assemblage of the Norphlet is dominantly diagenetic, owing to albitization and dissolution of feldspar. Despite the simple framework c omposition, the diagenetic character of the Norphlet is complex. Important authigenic minerals include carbonate phases (calcite, dolomite, Fe-dolomit e, and breunnerite), feldspar (albite and K-feldspar), evaporite minerals ( anhydrite and halite), clay minerals (illite and chlorite), quartz, and pyr obitumen. The abundance and distribution of these minerals varies significa ntly between onshore and offshore regions of Norphlet production. The lack of sufficient internal sources of components for authigenic minerals, combi ned with unusual chemical compositions of chlorite (Mg-rich), breunnerite, and some minor authigenic minerals, suggests that Louann-derived fluids inf luenced Norphlet diagenesis. In offshore Alabama reservoirs porosity is dominantly modified primary poro sity. Preservation of porosity in deep Norphlet reservoirs is due to a comb ination of factors, including a lack of sources of cement components and la ck of pervasive early cement, so that fluid-flow pathways remained open dur ing burial. Below the dominantly quartz-cemented tight zone near the top of the Norphle t, pyrobitumen is a major contributor to reduction in reservoir quality in offshore Alabama. The highest reservoir quality occurs in those wells where the present gas-water contact is below the paleohydrocarbon-water contact. This zone of highest reservoir quality is between the lowermost occurrence of pyrobitumen and the present gas-water contact.