ABSTRACT

The structural framework for late Mesozoic sedimentation in the northern Gulf of Mexico Basin includes a series of regional positive and negative warpings oriented perpendicular to depositional strike. The result is a series of embayments and uplifts. Hydrocarbon production occurs in both settings, but two of the uplifts (Sabine and Monroe) are directly responsible for giant hydrocarbon traps on their crests and flanks. In contrast, one uplift (the Peninsular Arch-Ocala Uplift) is totally barren. The Wiggins Arch is an interesting combination; significant production occurs on the flanks, but the crest is barren.

By analogy with the Sabine and Monroe uplifts, the crest of the Wiggins Arch should be a high-priority exploration area, yet it remains virtually untested. This condition has led to a paucity of well control, especially deep well control. The magnitude of this paradox becomes evident by a simple look at the distribution of oil and gas fields in this area. In all of south Mississippi, only three counties do not produce: George, Jackson and Harrison.

The discovery of major Smackover and Norphlet production at Chunchula and Hatters Pond fields (Mobile County, Alabama) in 1974 and 1975 did spur exploration in the area. The discovery of major Norphlet production at Lower Mobile Bay Field (Alabama) in 1979 has perpetuated this exploration effort.

Four wells have now penetrated the entire sedimentary sequence (20,000 feet) on the Wiggins Arch and reached basement rock. Basement in two of these wells is granite, and in the other two phyllite. In all four wells, the basement rocks have been age-dated between 275 and 300 m.y. (Early Permian-Late Pennsylvanian).

Three of these deep tests are recorded as penetrating an abbreviated stratigraphic sequence in which the Jurassic Haynesville

Figure 1. Index map of major positive structural elements of northern Gulf of Mexico Basin.

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Formation lies directly on basement rock. Thus, the Smackover, Norphlet, and Louann are missing. The absence of Louann Salt from the crest of the Wiggins Arch comes as no surprise. The arch is recorded as a pronounced minimum on regional gravity maps, and evidence of salt tectonics is clearly absent.

The absence of the primary target formations, Smackover and Norphlet, may not have been a surprise, but it is certainly disappointing, because it implies that the crest of the Wiggins Arch is baldheaded. This paper presents a seismic-stratigraphic interpretation which proposes that the three wells are isolated occurrences and that the target formations are present on the crest of the Wiggins Arch. If this is true, a huge exploratory opportunity can be envisioned.

In order to take advantage of this opportunity, Conoco has recorded a sizable seismic grid over much of the area. The following seismic-stratigraphic interpretation of this grid is one solution, but it is not a unique solution. The interpretation is hampered greatly by the absence of any well on the south flank of the Wiggins Arch that has penetrated the target formations.

Three of the four basement penetrations are located on small, isolated basement highs. These highs and others in the area are probably the result of differential erosion of the basement block. These highs are shown seismically by subtle structural relief and, more markedly, by reflection character. The stratigraphic sequence of Haynesville carbonate-evaporite facies overlying granitic basement does not produce significant seismic reflections. In contrast, the Norphlet and/or Louann produces a strong set of reflections. This seismic-stratigraphic evidence is the main criterion for postulating the basement highs. Surrounding each of the highs is a set of strong reflections interpreted to indicate the presence of Norphlet and/or Louann. Thus, only the isolated basement highs are baldheaded, not the entire Wiggins Arch.

The Norphlet of southern Alabama and Mississippi was deposited as a large clastic tongue which exited the Appalachian drainage and prograded across the Louann salt plain. The tongue is 150 miles long, 75 miles wide, and 1,000 feet thick. Depositional environments grade from alluvial in the north to fluvial-lacustrine in the center, to eolian in the south (against the Wiggins Arch). The upper portion of the eolian facies was reworked by transgressing seas which later deposited the Smackover. This is the facies that probably surrounds the basement islands, and it is the reservoir at Hatters Pond Field.

Smackover stratigraphy of the northern Gulf of Mexico Basin is well documented in the literature. Despite intraformational and interformational facies variations, the sequence of Smackover-Buckner-Haynesville is remarkably similar throughout the area. However, in the area of the thick Norphlet tongue, some very strange relationships exist: the Smackover is abnormally thin (less than 100 feet in places), and the Haynesville contains many salt layers in zones up to 400 feet thick.

In the Wiggins Arch area, thickness comparisons of the Smackover-Haynesville are so anomalous that they strongly imply that the lower "Haynesville" was deposited contemporaneously with the Smackover. This implication is supported by palynological interpretation. Thus, the published sequence of Haynesville lying on basement in the three subject wells is only partially correct.

This sequence is actually a Smackover facies lying on basement, but the Smackover was deposited in a Haynesville-like sabkha which was perched on top of the basement highs. If this interpretation is correct, these sabkhas are likely to be flanked by high-energy

Figure 2. Jurassic stratigraphic nomenclature of Wiggins Arch area.

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reefs on the seaward side and oolite bars in tidal passes.

The regional position of this shoal area should be noted. Recent deep drilling at Bogalusa Field (Washington Parish, Louisiana) and Catahoula Creek Field (Hancock County, Mississippi) indicates that the Smackover-Haynesville shelf edge follows the western end of the Wiggins Arch. In fact, the shelf edge appears to bulge southward around the Wiggins salient. Thus, the proposed reefoolite-sabkha shoals are located far out on the Smackover carbonate shelf near the shelf edge, and about 75 miles from the continental shoreline.

An analogous shoal-island complex has been described along the projection of the Tamaulipas Peninsula of northeastern Mexico. Deposition of the Zuloaga Formation (Smackover equivalent) in this area was partially controlled by a series of basement islands. The Zuloaga displays varied lithofacies which represent deposition in diverse carbonate environments, from lagoon to high-energy shoal to low-energy shelf.

A modern analogy is deposition along the Trucial Coast of the southeastern Persian Gulf. Piercement salt domes form pre-Holocene outcrops and shoals far out into the gulf (50-100 kilometers). These shoals and islands are the sites of complex carbonate sedimentation which includes carbonate sands and muds, reefs, ooid shoals, and sabkhas.

The Smackover and Norphlet formations are major producing units in the area of southern Mississippi and Alabama. If the Smackover-Norphlet stratigraphy around the Wiggins Arch is similar to that proposed herein, a significant hydrocarbon trend exists. The stratigraphy proposed should offer both reservoir rock and hydrocarbon traps. The problem is how to get exploratory wells drilled in an area that lacks significant structural closure (except the baldheaded basement highs). Exploratory prospects based solely on seismic stratigraphy are difficult to pinpoint, especially where control wells do not exist. Add to these problems the 20,000-foot drilling depth, and a real challenge emerges. We believe that the magnitude of the potential will justify the risk.