Wiggins arch may hold Haynesville potential

Scott L. Montgomery
Petroleum Consultant
Seattle

Don C. Mozynski
Petroleum Geologist
Dallas

The Wiggins arch is a major, east-west trending basement feature in southern Mississippi whose petroleum potential remains largely unknown.

Buried to depths of 18,000-21,000 ft, the arch is overlain by middle-upper Jurassic deposits (Fig. 1 [216,721 bytes]). These deposits include such regionally significant reservoir and source intervals as the Norphlet, Smackover, and Haynesville formations. Evaluation of these formations has been based on sparse penetration, averaging only two wells per township.

To date, attention has focused almost exclusively on possible accumulations in the Smackover. Lack of success in this interval has been used to support the conclusion that the arch is barren of hydrocarbons. Such a conclusion, based as it is on very limited data, appears premature.

Little or no analysis has been devoted to the Haynesville formation on the Wiggins arch, despite significant production from the lower portion of this formation (Frisco City sandstone) to the northeast, along another basement uplift of similar scale (Conecuh ridge).¹ In addition, oolite shoal and pinnacle reef reservoirs widely productive in the East Texas basin and identified seismically in North Louisiana suggest the need to consider such facies development in similar paleo-settings elsewhere in the northern gulf region, particularly where abundant source rocks exist.

These considerations provide context and incentive for new evaluations of the Haynesville on the Wiggins arch. Recent analyses of sample, log, and seismic data suggest that Haynesville facies trends included a south-facing carbonate shelf with oolite shoal and patch/pinnacle reef development concentrated above, or along the margins of, local basement highs. Similarity of seismic anomalies to those observed in the East Texas Cotton Valley lime (Haynesville) reef play is notable and indicates significant reservoir and trapping potential along the southern flank of the arch.

Setting, background

The Wiggins arch is part of a series of basement highs that includes, to the east, the Baldwin high (Baldwin County, Ala.) and Conecuh ridge. The arch forms the southern boundary to the Mississippi Interior salt basin.

Regional geophysical studies confirm that the Wiggins arch is underlain by thickened continental crust and may represent a partially rifted segment of the pre-existing North American craton.^2-3 Drilling has shown the upper portion of the arch to be mainly composed of igneous and metamorphic rocks of Paleozoic age.⁴

The arch was created in Late Triassic-Early Jurassic time and remained elevated during the Late Jurassic, alternately separated from the mainland to the north by shallow marine, tidal flat, and lagoonal environments.

Long-term exposure resulted in a highly irregular erosional surface characterized by local fault-controlled highs with up to 500 ft or more of relief.⁵ Subsequent burial involved several transgressive-regressive marine cycles during the Late Jurassic. Progressive updip onlap, thinning, pinchout, and facies changes in the Norphlet-Smackover-Haynesville succession have been identified and mapped at a reconnaissance level.^5-6 Salt of the Louann Group is absent over much of the arch, grading rapidly updip to crystalline anhydrite along the flanks before pinching out altogether.

During Norphlet and lowermost Haynesville deposition, local highs developed coarse-grained detrital aprons and were partially or completely buried in alluvium. Smackover and middle-upper Haynesville deposits include high-energy shelf facies, notably oolite shoal and possibly reefal buildups. Haynesville strata are overlain by a thin transgressive cover of Bossier shale, in turn buried by up to 1,500 ft of fluvial-deltaic and nonmarine Cotton Valley sandstones and shales. Both the Haynesville and Cotton Valley intervals show thinning of 500-800 ft between the Mississippi Interior Salt basin and Wiggins arch, indicating that the latter remained a positive element into latest Jurassic time.

Rhodes and Maxwell⁵ indicate the existence of pre-Jurassic, northeast-trending faults on the arch with up to 2,000 ft or more of vertical offset. These faults control the location of local highs on the arch and may have served as important pathways for vertical migration into the lower-middle Haynesville.

Regional facies trends

A recent sequence stratigraphic analysis of Late Jurassic deposits to the northeast (Conecuh embayment-Conecuh ridge, southwest Alabama) interprets the upper Smackover as a highstand systems tract (HST) overlain by southward-prograding, stacked shelf-margin wedges of the Buckner and Haynesville intervals.⁷ In this area, the Haynesville consists dominantly of marginal marine and nonmarine clastics and thin evaporites (anhydrite) associated with a broad platform (Chunchula platform).⁷

To the southwest, however, the Middle-Upper Haynesville undergoes gradation first to anhydritic shale and carbonate wackestone/packstone (north flank, Wiggins arch), then to carbonate grainstone and packstone (south flank), and finally to carbonate mudstone. South of the arch, the interval grades into marine shales of the Bossier shale. These relationships are interpreted to reflect a southwestern progression from restricted lagoonal settings on the distal Chunchula platform to a relatively high-energy carbonate shelf and, farther south, subtidal and offshore marine environments (Fig. 2a) [166,833 bytes] and (Fig.2b [136,895 bytes]).

New rock properties

Analysis of Haynesville samples from several key wells on the Wiggins arch provides significant evidence for the existence of reservoir quality rocks and associated hydrocarbons. These wells are shown on the structure map of Fig. 3 [173,875 bytes].

The Mobil 1 Lula Anderson (28-5s-9w) and 1 USA (1-6s-10w) wells, located respectively on the central portion and northern flank of the of the arch, exhibit the following upward progression in lithofacies:

1. gray dolomitic mudstone with interbedded anhydrite;
2. dolomitic mudstone with traces of ooid shoal material;
3. conglomerate consisting of micritized, dolomitic ooid grainstone lithoclasts;
4. fine-medium grained, argillaceous, glauconitic, micaceous quartz sandstone.

This progression indicates a transgressive succession, involving change in depositional setting from peritidal/sabkha, to low-energy shelf, to higher energy foreshelf in which oolitic material was introduced from nearby grainstone shoals. The Mobil 1 Anderson well is structurally high to all other wells on the arch, being located on the flank of a seismically defined inselberg. The well penetrated 70 ft of Frisco City (lower Haynesville) alluvial fan and fan delta material immediately above basement (no Smackover or Buckner present), containing scattered oolites indicative of proximal grainstone shoal development.⁵

In the General Crude International Paper 5-10 (5-4s-12w), sited roughly 20 miles to the northwest and approximately 1,000 ft downdip to the 1 Anderson, a similar progression is observed, with the foreshelf conglomerate facies consisting of a greater array of lithoclasts, including argillaceous packstones, nonfossiliferous mudstone, frequent ooid grainstones, and coralline algal wackestone (one example) in an argillaceous, bioclastic packstone matrix. In this well, as in the Mobil 1 Anderson, the foreshelf conglomerate facies is 40-50 ft thick and significantly dolomitized.

Thin section photomicrographs showing micritized and dolomitized ooid grainstone facies are given in Fig. 4 [324,965 bytes]. Significantly, the example from the Mobil 1 Anderson contains large amounts of bitumen and dead oil in abundant interparticle pore areas (black mineral in Fig. 4b), providing evidence that this facies possessed excellent reservoir characteristics allowing for a high degree of oil charging.

Two wells drilled by Shell along the northern flank of the arch, the 1 Dantzler (13-2s-9w) and 1 University of Mississippi Unit (11-3s-9w) each penetrated a thin (10-25 ft) interval of dark gray dolomitic, argillaceous mudstone containing lenses of micritized ooid grainstone. These intervals are encased in anhydrite-rich sands (above) and mudstones (below) and are interpreted to represent material shed northward into back-shoal, restricted lagoonal settings.

Reservoir model

A potential analog for Haynesville reservoir development on the Wiggins arch may be the deeply buried Upper Smackover oolite shoal productive zones at Chunchula field.

A several stage diagenetic process involving early marine cementation (calcite), following by meteoric dissolution, fabric-selective dolomitization, and non-selective dolomitization during deeper burial is proposed. In this case, intercrystalline and interparticle porosity resulted from removal of original calcium carbonate cement in shoal facies rocks. The presence of anhydrite cement in some samples is possible evidence of a late stage of fresh-water leaching.

Kopaska-Merkel et al.⁸ noted a strong association between unusually porous upper Smackover reservoirs and proximity to the Wiggins arch. This is interpreted as a possible result of the arch acting as a positive element to increase and focus the flow of subsurface brines along its margins. It is also significant, as these authors point out, that over 35% of all liquid hydrocarbons produced from the Smackover in Alabama come from reservoirs located along the northern portion of the arch-often, as in the case of Chunchula field, from depths below 18,000 ft where porosities in the range of 15-20% exist.

Rhodes and Maxwell,⁵ meanwhile, indicate good porosity preservation in Smackover oolite shoal facies even at 20,000 ft along the southern portion of the Wiggins arch. Similar facies in the Haynesville may therefore be expected to retain significant reservoir quality.

Seismic data

Evaluation of approximately 1,800 line miles of 2D seismic data over the Wiggins arch in Mississippi (Harrison, Jackson counties) indicates the presence of numerous anomalies in the upper Haynesville (Fig. 5 [117,190 bytes]). These anomalies occur as loss of amplitude in the Haynesville reflector, with occasional suggestion of draping in the overlying Bossier shale and Cotton Valley Group.

In a number of cases, anomalies are associated with the flanks of local basement highs (Fig. 5b, uppermost [340,483 bytes]) or with local changes in slope (upper left, right). Generally speaking, seismic anomalies in the Haynesville of the Wiggins arch are similar in overall attributes but larger in scale than those typical of the East Texas Haynesville reef play.¹

It has been observed, with regard to the Smackover on the arch, that seismic facies mapping and related paleogeographic reconstructions can be used to outline detailed shelf morphology, including paleo-island distribution and geometry.⁵

Similar capabilities should be considered for the Haynesville. It is apparent from the existing data, for example, that a sizable proportion of anomalies are grouped into several parallel NW-SE trends, each 1-3 miles in width and corresponding to distinct breaks in paleo-slope. One such trend crosses near the Mobil 1 USA well; another is proximal to the Mobil 1 Anderson well. Such trends, in conjunction with lithologic and facies data, suggest a series of oolite shoals (with possible local patch reef development) that migrated updip in two or more stages during the noted late Haynesville transgressive episode.

Source rock data

Geochemical analyses of Upper Haynesville samples (cuttings) from the General Crude International Paper 5-10 well provide important new information of relevance to exploration in this area.

Relevant data indicate that this portion of the Haynesville contains significant total organic carbon (up to 1.01 wt %), but very low quantities of pyrolizable hydrocarbons (S₂ < 0.5 mg/g) and low hydrogen index values (5-17), suggesting relatively poor source capability. vitrinite reflectance values, on the other hand, vary from 1.79-2.26 (average 2.05) within the foreshelf conglomerate facies and thus correlated dominantly with the lower portion of the wet gas field.

These values are considered highly significant, as they overturn the common assumption that rocks at this depth (~19,100 ft) on the Wiggins arch are overmature. In updip areas, for example in the Mobil USA well, potential Smackover source rocks occur at roughly equivalent depths (19,300-19,500 ft).

Conclusions

During deposition of the Upper Haynesville, the southern portion of the Wiggins arch was a shallow, low-angle carbonate shelf rimmed by oolite shoals and interrupted along its northern portion by local, shoal-flanked islands.

As during Smackover time, back-shoal areas were the site of restricted deposition, possibly aiding early, fabric-selective dolomitization.⁵ Porosity preserved in shoal complexes may be sufficient to account for seismic anomalies identified at the top of the Haynesville and characterized by significant loss of amplitude.

Smackover source rocks are present along the southern flanks of the Wiggins arch and occur at depths that correspond to the wet-dry gas transition.

The potential problem of gas destruction by thermochemical sulfate reduction, often mentioned in connection with Smackover reservoirs, does not appear an important factor in the case of the Haynesville, as evidenced by the lack of H₂S in samples, the small amount of observed pyrite, and the local abundance of iron staining in clastic samples.

Future discussions of hydrocarbon potential on the Wiggins arch need to consider the Upper Haynesville as a potential reservoir target. In addition, though not discussed here, the basal Haynesville Frisco City sandstones comprise a possible objective along the margins of the arch, proximal to Smackover source rocks.¹ Production from the Frisco City to the north, in Alabama, is confined to specific sandstone facies (mid-fan, wadi, eolian, beach); it is likely that the most immature of these have suffered diagenetic alteration and porosity loss on the Wiggins arch due to deeper burial.

Traps for Haynesville shoal facies reservoirs should include stratigraphic and combination types. Lateral porosity pinchout, local fault truncation, and differential compaction of overlying Bossier shales should be considered. In cases where shoals are associated with faulting, the possibility of fracturing also exists, as it appears some structures have undergone post-Haynesville movement.

References

1. Montgomery, S.L., Baria, L.R., and Handford, C.R., Frisco City sandstone: Upper Jurassic play in southern Alabama, AAPG Bull., Vol. 81, No. 10, 1997, pp. 1,595-1,611.
2. Pindell, J.L., and Dewey, J.F., Permo-Triassic reconstruction of western pangea and the evolution of the Gulf of Mexico/Caribbean Region, Tectonics, Vol. 1, 1982, pp. 179-211.
3. Hutley, J.K., Triassic/Jurassic faulting patterns of Conecuh ridge, southwest Alabama (abs.), AAPG Bull., Vol. 69, No. 2, 1985, p. 286.
4. Dallmeyer, R.D., 40AR/39AR ages from subsurface crystalline basement of the Wiggins arch and southwesternmost Appalachian Piedmont: Implications for late Paleozoic terrane accretion during assembly of Pangea, American Journal of Science, Vol. 289, 1989, pp. 812-828.
5. Rhodes, J.A., and Maxwell, G.B., Jurassic stratigraphy of the Wiggins arch, Mississippi., GCAGS Transactions, Vol. 43, 1993, pp. 333-344.
6. Cagle, J.W., and A* Khan, M., Smackover-Norphlet stratigraphy, south Wiggins arch, Mississippi and Alabama, GCAGS Transactions, Vol. 33, 1983, pp. 23-29.
7. Prather, B.E., Evolution of a Late Jurassic carbonate/evaporite platform, Conecuh embayment, northeastern Gulf Coast, U.S.A., AAPG Bull., Vol. 76, No. 2, 1992, pp. 164-190.
8. Kopaska-Merkel, D.C., Mann, S.D., and Schmoker, J.W., Controls on reservoir development in a shelf carbonate: Upper Jurassic Smackover formation of Alabama, AAPG Bull., Vol. 78, No. 6, 1994, pp. 938-959.

The Authors

Scott L. Montgomery is a petroleum consultant and author residing in Seattle. He is lead author of the "E&P Notes" series in the AAPG Bulletin and the quarterly monograph series "Petroleum Frontiers" published by Petroleum Information/Dwights LLC. His current research interests include frontier plays and field re-development in North America. He holds a BA degree in English from Knox College and an MS degree in geological sciences from Cornell University.

Don C. Mozynski is a carbonate stratigrapher with Rand Paulson Oil & Gas specializing in paleoenvironmental reconstructions and interpretation of carbonate depositional environments. He has 23 years' experience in the petroleum industry and has performed exploratory work for Texaco, Deminex U.S., Paladin Exploration, and other companies. His expertise extends to carbonate provinces throughout the U.S., including the East Texas, Anadarko, and Permian basins and the Wiggins arch in Mississippi. He holds a BA degree in geological sciences from Case Western Reserve University.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.