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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 935

Last Page: 935

Title: Upper Jurassic Evaporites and Carbonates of Gulf Coast: ABSTRACT

Author(s): Paul M. Harris

Article Type: Meeting abstract


Shallow-water carbonate rocks of the Upper Jurassic Smackover Formation contain some of the most promising exploration targets of the Gulf Coast. Evaporites of the Louann, the Buckner, and within the Smackover have been important factors in forming the reservoirs.

The Louann, a thick salt lying stratigraphically below the Smackover, localized production and influenced facies patterns and diagenesis. The updip limit of the Louann coincides with peripheral basin faults, so fault traps in the Smackover arr to some degree related to salt movement. Salt flowage created highs on the sea floor that localized high-energy oolite bars and, in some places, reef development. Some Smackover porosity was created secondarily during early freshwater diagenesis in association with the highs. Late-stage burial diagenesis in the Smackover, which is also important in forming reservoir porosity, may have been caused by interstitial fluids that were expelled from the Louann during salt compaction.

Anhydrite occurs in the Buckner and Smackover in several forms: (1) nodules, (2) microcrystalline replacement, (3) laminations of blocky crystals, (4) poikilitic cement, (5) poikilitic replacement, (6) fracture fill of blocky or lath-shaped crystals, (7) lath-shaped replacement crystals, and (8) plugging of oomoldic porosity. The Buckner anhydrites and shales are important seals for Smackover reservoirs. Commonly the Buckner interfingers with the reservoir at the updip limit of a field and as a result of basinward progradation forms an impermeable cap. The Buckner may have influenced early diagenesis of the uppermost Smackover by introducing brines into the lime sands. Thickness maps of the Buckner serve as a valuable exploration tool for the Smackover, because porous facies are assoc ated with Smackover highs underlying the Buckner thins. Anhydrite within Smackover grainstones can locally reduce porosity to form internal seals. The presence of minor amounts of anhydrite in Smackover carbonate rocks can confuse log interpretations on neutron and density log crossplots by causing overestimates of the dolomite percentage and underestimating of porosity.

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