Abstract

The geologic evolution of the Upper Jurassic strata of the northern deepwater Gulf of Mexico was analyzed using the logs from 25 wells, mud logs, and biostratigraphic data, and 500 km of 2D seismic data. These strata record the initial transgression in the Gulf of Mexico after final continental breakup and early ocean spreading. After deposition of the Louann salt (Callovian), the Norphlet eolian system (early Oxfordian) was deposited on the salt (thickness ranges from 300–1200 feet). These strata were then transgressed by the Smackover Formation, a shallow marine limestone comprising five distinct lithofacies (thickness ranges in feet): Red Shale (0–34), Pyritic-Dolomite (55–225), Basal Carbonate (95–275), Middle Marl (95–495), and Upper Carbonate (55–165).

The Haynesville Formation (Kimmeridgian–early Tithonian) overlies the Smackover, and consists of shallow marine marls and limestones (thickness ranges from 135 to 2550 feet). With continued water deepening, the Cotton Valley Formation was deposited, primarily as a shallow-marine shale unit but with occasional storm and thin progradational deposits (thickness ranges from 220 to 2075 feet).

Deposition of these Upper Jurassic strata was strongly affected by active salt tectonism. The Middle Ground Arch, a northeast–trending basement high, segmented the Norphlet into an eolian facies to the north, and a mixed eolian-fluvial-overbank setting to the south. During the Kimmeridgian, salt rafts developed and began to translate basinward (southwest) creating differential accommodation for the Haynesville and Cotton Valley strata, as recorded by the widely varying thickness in the wells. As a result, expulsion rollovers developed, creating basinward thickening wedges that terminate onto the decollement within the fault gaps of the rafts. Today, a series of isolated rafts area present; the traps for all four discoveries developed associated with the formation of the rafts.