ABSTRACT

Regional structure and stratigraphic research of the Frio Formation of the eastern Gulf Coast suggests that southeastern Louisiana was an overall positive, stable area typified by the growth of several outer shelf lepidocyclinid packestone banks (resistive electric log zones) at 30° North latitude (Figs. 1 and 2). Petrographic and isotopic data from conventional cores indicates that the marine banks were cemented early and modified the geometry of post-depositional reservoir sandstones at the shelf edge, and assisted in the regional development of the Florida Parishes Frio embayment (Fig. 3). The sandstones are funnelled through breaks in the carbonate banks at the shelf edge as defined by mapping both carbonate and sand distribution and analyzing superimposed patterns. A direct relationship with oil and gas field distribution is inferred from the maps (Fig. 4).

The transgressive carbonate banks are underlain by lowstand systems tract sandstones that harbor all oil and gas production in the Frio Formation in the eastern Gulf Coast. Vertical changes in carbonate mineralogies, clay minerals, and calcareous nannoplankton from conventional cores illustrate a shift in carbonate lysoclines during early relative sea-level rise. The transgressive banks are subsequently overlain by a condensed section comprised of cold water nannoplankton. The condensed section occurs at the top of the resistive zone (bank) and defines a chronostratigraphic horizon useful for rigid depth-time correlations.

The chronostratigraphic datums of four condensed sections (tops of resistive zones) in the upper Frio Formation and 170 other biostratigraphic events were compiled from six paleontologic reports using the graphic correlation method to prepare a subsurface composite model. By input of data from Poag (1966) and Butler (1963), the upper Frio Formation appears correlative to a minimum of 3.5 feet (1.4 m) of the Lower Paynes Hammock Formation outcrop in Wayne County, Mississippi (Fig. 5). Published age ranges suggest that this interval was formed between the youngest age of 27.7 Ma and latest of 27.985 or 28.83 Ma (Bray, 1989). A survey of the outcrop was then made to assess regional outcrop relationships in the Gulf Coast.

The type locality of the Chickasawhay and Paynes Hammock Formations in Wayne County, Mississippi (Blanpied et al., 1934) are no longer accessible and a new locality was surveyed in the nearby vicinity. The new measured section spans the Lower Catahoula, Paynes Hammock, and Chickasawhay Formations, and shows that the Lower Catahoula and Paynes Hammock Formations are conformable (Fig. 6). Several diastems are indicated in the lower Paynes Hammock or upper Chickasawhay verifying sequence stratigraphy data from Marlin (1991). The regional outcrop study suggests that the presently labeled Lower Catahoula Formation contact becomes diachronous to the west. The Lower Catahoula Formation lies conformably over the equivalent of the subsurface upper Frio Formation (Paynes Hammock Formation) in Wayne County, Mississippi and further eastward (within the Suwanee Formation), and over the Vicksburg Formation in southwestern Louisiana and Texas (Fig. 7).

Use of chronostratigraphic horizons in the graphic correlation model, as identified by conventional core analyses, aid correlation of the subsurface sediments to a surface exposure. Analysis of regional outcrop relationships suggests that the facies changes from east to west and lithofacies labeling of the "Catahoula Formation" are prime suspect for historical outcrop correlation errors. Until radiometric age-dating of the Catahoula Formation is done in southwestern Louisiana and Texas, the exact placement of the Frio Formation within the Catahoula Formation in the western Gulf Coast will not be known. However, early Arikareean vertebrate fossil discoveries near the Toledo Bend reservoir in western Louisiana may be of assistance (Albright, 1991).

Additional outcrop sample studies and more detailed graphic correlation modeling of the Chickasawhay Group or equivalent sediments along strike across the Gulf Coast will lead to a better understanding of the economic sediments found downdip in the subsurface.

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Figure 1. Structure map on the top of the Oligocene Frio Formation (Marlin, 1991). Structural dip is to the south-southwest with the onset of growth faults approximating the paleo-shelf edge near 30° north latitude. North of 31° north latitude the Frio Formation is masked by surface casing and freshwater resistivity responses or cannot be differentiated on well logs.

Figure 2. Isopach map of the Oligocene Frio Formation suggesting the shelf-edge is marked by an increased thickening, and where lepidocyclinid wackestone-packstone banks occur as stacked resistive intervals on induction-electric logs (small, dotted outline). The Hackberry embayment and newly defined Florida Parishes Frio embayment are marked by Isopach thicks (negaments) and are common to oil and gas deposits.

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FIGURE 3. [Grey Scale] Typical photomicrograph of a foraminiferal-rich bank common to the late Oligocene Frio Formation shelf deposits. These Lepidocyclina sp. rich zones have low porosity (5-8%) and permeability (less than 10 mD), and have calcite cements with a range from -1 to -5 O¹⁸ (PDB) and 1 to -2.3 C¹³ (PDB) or normal marine signatures (Marlin, 1991). Vertical profiles of typical banks illustrate dolomitization of muds at the base overlain by a zone of aragonite dissolution, and dissolution or shelter voids in-filled with calcite spar suggesting a relative rise In carbonate lysoclines. Banks are commonly overlain by a pelagic drape.

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Figure 4. Relationship of sandstone, carbonate and shale facies at the shelf edge. Informal division of the upper Frio Formation into members A B, and C and mapping the percentage of facies within each suggests that carbonate banks affect the distribution of overlying or post-depositional sandstone trends. The relationship illustrated is made by superimposition of facies percent maps.

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Figure 5. Graphic correlation plot of the subsurface upper Frio Formation composite to the Paynes Hammock Formation in Wayne County, Mississippi. Circles and crosses are first and last occurrence events, respectively. Data from Poag (1966) entered in to the Y-axis of the model suggests that a minimum of 3.5 feet (1.3 m) of the Paynes Hammock Formation correlates to the subsurface upper Frio Formation (dotted lines). An alternative line of correlation (line 2) alternately suggests that the upper contact of the Frio Formation is biostratigraphically similar to the lower Catahoula Formation.

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FIGURE 6. [Grey Scale] Photograph of the newly rediscovered contacts between the Paynes Hammock and Catahoula Formation outcrops illustrating a conformity between the two formations in Wayne County, Mississippi.

Figure 7. Inferred basal contact of the Catahoula Formation with underlying sediments of the outcrop or subsurface across the Gulf Coast. The graphic correlation to outcrop and this diagram suggests that the lower Catahoula Formation is diachronous in most areas except eastward of east-central Mississippi.

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