THE UPPER SMACKOVER OF THE GULF RIM: DEPOSITIONAL SYSTEMS, DIAGENESIS, POROSITY EVOLUTION AND HYDROCARBON PRODUCTION

Abstract

Abstract A generalized stratigraphic framework for the upper Jurassic is suggested, in which the name Haynesville Formation is utilized, and the name Lower Cotton Valley Lime is suppressed in favor of the Gilmer, where appropriate. Eustatic sea-level fluctuations have resulted in patterns of sedimentation common to upper Jurassic sequences across the entire northern Gulf of Mexico region and hence indicate the wide applicability of this generalized stratigraphic framework. The presently accepted model of the Smackover-Haynesville sedimentation must be modified to take into account sea-level fluctuations, subsidence, and sediment availability. The model developed here is simply one of lower Smackover basin fill during a rapid transgressive phase and upper Smackover regional shoaling during a sea-level stillstand in which sedimentation was in equilibrium with subsidence. The lower and upper Smackover are not necessarily time equivalent, but represent two separate sedimentologic sea-level regimens. The Haynesville Formation is thought to be a separate sedimentologic package that was deposited during the next sea-level rise; the Gilmer Limestone formed a shelf-margin barrier behind which the lagoonal Buckner evaporites were deposited. The evaporites graded landward into quartzose clastics. Predictable regional porosity patterns have developed in the Smackover-Haynesville, in response to early diagenetic overprints, controlled largely by eustatic sea-level subsidence interactions. These patterns include: updip oomoldic porosity in a regional meteoric-water system developed during the upper Smackover sea-level stillstand; downdip porosity preservation under marine conditions along the shelf margin; regional dolomitization associated with reflux of evaporitive waters from the Buckner lagoon behind the Gilmer shelf-margin barrier. Structural hydrocarbon traps associated with salt movement are the most common type of Late Jurassic trap. Buckner evaporites or Haynesville shales usually form the seals in the Late Jurassic reservoirs. Jurassic source rocks are probably lower Smackover limestones and Norphlet shales. Sourcing is generally local with migration into updip areas, particularly where regional dolomitization has occurred. The time of migration, which is a key factor in a viable Smackover exploration strategy, varies across the Gulf in response to the subsidence history of each individual basin. Future Jurassic exploration will center on south Texas, the Gilmer shelf margin, and the updip Smackover along the bounding graben fault systems. Most production will be gas, but some oil should occur along the updip faulted fairways.