The Jurassic upper Smackover ooid-lime-grainstone of the central Gulf Coast has been a major hydrocarbon exploration target for nearly half a century. Study of the geologic and sedimentologic framework, petrography, two-phase fluid inclusions, and trace element and stable isotope chemistry indicates the following diagenetic history.

The upper Smackover is a blanket ooid grainstone ranging in thickness from 300 to 400 ft (91 to 122 m), and covering at least 4,000 sq mi (10,400 sq km). In the northern part of the Arkansas shelf, a regional meteoric water system was established in the grainstone shortly after deposition, and prior to significant burial. This meteoric water system gave way to conditions which were increasingly more marine toward the south near the Louisiana-Arkansas state line. The fresh-to-marine hydrologic gradient is responsible for the strong precompaction regional diagenetic overprint that eventually controlled available diagenetic pathways for the unit during burial. Three diagenetic zones are present: (1) a northern zone, dominated by oomoldic porosity and preburial spar cementation; (2) a sou hern zone, characterized by compaction effects, late subsurface cement, primary porosity preservation, and late secondary subsurface porosity generation; and (3) a transitional zone having characteristics common to both the northern and southern zones.

During the shallow burial phase, compaction became increasingly more important to the south in zones dominated by marine connate fluid, whereas rock integrity and a secondary oomoldic porosity system had already been established in the northern zone. As burial increased, brine originating in the underlying Louann salt reached the shelf grainstone and began to displace the original connate fluid. The brine triggered the inversion to calcite of remaining grains and cement composed of unstable mineralogy. When the Louann brine had displaced the original connate fluid, CaSO₄ replacement as gypsum and anhydrite was common across all three diagenetic zones. Also, precompaction fine cement associated with oomoldic porosity was recrystallized and reequilibrated with the concentrate brine. Localized replacement dolomitization may well have developed throughout the burial history of the sequence, but was most commonly postcompactional. Postcompaction poikilitic calcite cement and baroque dolomite were precipitated from the Louann brine under conditions of elevated temperature and pressure.

As subsidence continued, organic material from the basinward equivalents of the upper Smackover lime-grainstone arrived on the shelf and catalyzed SO₄ reduction, resulting in pyrite and galena replacement in the southern and transitional zones. As hydrocarbon maturation continued in the fine-grained equivalents of the upper Smackover, H₂S and CO₂ by-products of these processes began to increase in the brine of the southern and transitional zones, ultimately causing regional dissolution of existing carbonate, and significantly enhancing preserved porosity.

Porosity in the northern zone was established early, and preserved through moderate burial. Although porosity is well developed, permeability has been largely destroyed by recrystallization. Dolomitization is a prerequisite for economic reservoir development in the northern zone.

Porosity in the southern zone is solution-enhanced, preserved, primary, intergranular porosity. Compaction is the most important porosity destruction process. Compaction in the absence of an early diagenetic overprint is affected primarily by grain type, and secondarily by size and sorting. Pellets and oncolites compact easily, yet destroy porosity during burial. Ooids and rhodolites resist compaction during burial, therefore are related to porosity preservation. Grain-type distribution, as well as size and sorting, are controlled by depositional environment processes, with the most favorable areas for porosity development (concentrations of ooids and rhodolites) associated with early salt-related structural movement. Porosity distribution in the southern zone, therefore, can be outli ed with the aid of geophysical

End_Page 597------------------------------

techniques.

Porosity in the transitional zone is early secondary, or preserved primary intergranular, or a combination. Porosity in this zone, therefore, is highly unpredictable, and a strong potential exists for diagenetic traps unrelated to structure or original depositional environment.

Documentation of significant deep-subsurface secondary porosity possibly associated with hydrocarbon maturation, opens the potential for hydrocarbon exploration in deeply buried, carbonate-dominated basins. This would include carbonate rocks heretofore thought to be well below the depth of carbonate porosity maintenance because of lithostatic load.