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uAbstract

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uMethodology

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uAbstract

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uMethodology

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Figure Captions

	Figure 1. Map showing the studied field and well locations.
	Table 1. Results of porosity and permeability values obtained from studied core.
	Table 2. Summary of the shoreline facies and petrophysical results for the studied core.

 

Introduction

Ichnology, the study of the ‘tracks, trails and burrows’ made by animals in various substrates, has grown tremendously in the last five decades as researchers have continued to expand its importance in sedimentologic studies. The utility of trace fossils is not limited to the interpretation of paleoenvironments, paleobathymetry, and paleoecological stress. Recent works have shown that ichnology can be applied to allostratigraphy (Pemberton et. al., 1992a) and sequence stratigraphy for delineation of key stratal surfaces (Bockelie, 1991; Pemberton et. al., 1992a; Pemberton et. al., 1992b; MacEachern et. al., 1992; Taylor and Gawthorpe, 1993; Pemberton and MacEachern, 1995). Furthermore, trace-fossils can impact reservoir quality. Researchers, such as Gingras et al (2002), Gingras et al (2004a), and Gingras et al (2005), have applied ichnology to reservoir fluid flow using advanced methods and techniques. Gingras et al (2004b) demonstrated that burrowed dolomitic limestone of the Yeoman Formation in the Williston Basin possess good flow conduits because of the pathways created by the burrows.

In this study an attempt is made to illustrate the characteristics of the shoreline facies in the Coastal Swamp Depobelt of the Niger Delta, using ichnology and to assess the effect of bioturbation on reservoir quality of the shoreline sediments. The study also addresses the relationship between trace-fossil assemblages and depositional facies in the shoreline deposits. It also documents the pattern and style of bioturbation within the shoreline deposits and assesses the effect of horizontal burrows and intense bioturbation on the lower and middle shoreface.

Methodology

This study focuses on core samples of 7.5cm in diameter and about 53m long covering depth intervals of 2637.1-2620m; 3302.5-3284m; and 3581-3563m. The samples were obtained from well 13 of the Beta Field in the Coastal Swamp Depobelt (Figure 1). Each of the core samples was slabbed into one-third and two-thirds sections, with the one-third section mounted permanently with epoxy resin on core boxes (0.9m long in each box) and studied for sedimentological and paleontological description while the two-thirds section was digitally photographed under white and ultra-violet light prior to description to obtain improved visualization of chosen ichnological features.

Porosity and permeability results from core samples were obtained from the Location Sample Services (LSS) laboratory where the study was done. Each of the core samples was also calibrated to the wireline logs to establish characteristic wireline signatures.

Results

Foreshore, Shoreface and Offshore Facies Associations

Six facies associations of the shoreline deposits were observed based on the lithology, physical and biogenic sedimentary structures. The facies associations include foreshore, upper shoreface, proximal and distal middle shoreface, lower shoreface and offshore facies associations.

Permeability and Porosity Results

The porosity and permeability results from core samples incorporated into the ichnofacies studies to assess the effect of burrowing on reservoir quality are shown in Table 1 .

Interpretation and Discussion

A summary interpretation of the shoreline facies associations and the petrophysical results based on Dresser Atlas (1985) and Wichtl (1990) is given in Table 2 . The information in Table 2 shows that although the foreshore and upper shoreface are poorly burrowed, they have very good to excellent porosity and permeability. These high petrophysical values are thought to be due to the effect of the grain texture such as sorting and low clay content. Wave action is significant within these intervals resulting in textural maturity, good sorting and well rounded grains (Tucker, 1991). The porosity and permeability values for the middle shoreface vary from fair to very good.

The sparsely burrowed proximal middle shoreface has lower porosity and permeability values when compared with the well burrowed distal middle shoreface, even though the latter has mudstone lenses which should lower vertical permeability. This result suggests that the increase in bioturbation may have increased the porosity and permeability level in the distal middle shoreface. Generally, clay content in shoreline sands increases with depth as the clay minerals act as buffers to fluid flow. The interconnectivity between burrows in this case has enhanced permeability. Gingras et al. (1999) demonstrated that higher degrees of bioturbation can increase effective permeability in a reservoir. Table 2 further shows that low permeability values are observed in the least burrowed proximal lower shoreface when compared with the intensely burrowed distal lower shoreface.

Conclusion

The summary of petrophysical results presented in Table 2 shows that highly burrowed horizons possess higher porosity and permeability values than sparsely burrowed intervals of the same lithologic unit. This suggests that the act of bioturbation, which can be referred to as ‘sediment-mixing activities’, enhances the permeability and porosity of a sediment by creating flow conduits and increasing the surface area of the sediment, thereby enhancing reservoir homogeneity.

References

Bockelie, J.F., 1991, Ichnofabric mapping and interpretation of Jurassic reservoir rocks of the Norwegian North Sea, Palaios, v. 6, p. 206-215.

Bromley, R. G., 1996, Trace Fossils, biology, taphonomy and applications: Chapman and Hall, London, 360 p.

Dresser Atlas, 1982, Well logging and interpretation techniques: The course for home study, Dresser Atlas publication.

Egbu, O.C., G.C. Obi, and C.O. Okogbue, 2005, Recognition of incised valley-fills: examples from ‘Beta’ field in the Coastal swamp depobelt, Niger Delta, PTDF Workshop Abstract, v. 1, p. 7.

Ekdale, A.A., R.G. Bromley, and S.G. Pemberton, 1984, Ichnology: Trace fossil in sedimentology and  stratigraphy, Society of Economic Palaeontologists and Mineralogists, Short Course, v. 15, p. 54 - 60.

Gingras, M.K., S.G. Pemberton, C.A. Mendoza, and F. Henk, 1999, Modeling fluid flow in trace fossils: Assessing the anisotropic permeability of Glossifungites surfaces, Petroleum Geoscience, v.5, p. 349-357.

Gingras, M.K., C. A. Mendoza, and S. G. Pemberton, 2004a, Fossilized worm burrows influence the resource quality of porous media: AAPG Bulletin, v. 88, v. 875-883.

Gingras, M.K., S.G. Pemberton, K. Muelenbachs, and H. Machel, 2004b, Conceptual models for burrow-related, selective dolomitization with textural and isotopic evidence from the Tyndall Stone, Canada, Geobiology, v. 2, v. 21-30. 

MacEachern, J.A., I. Raychaudhuri, and S.G. Pemberton, 1992, Stratigraphic applications of the Glossifungites ichnofacies: Delineating discontinuities in the rock record, in S. G. Pemberton, ed., Application of Ichnology to Petroleum Exploration, Society of Economic Palaeontologists and Mineralogists, Core Workshop, v. 17, p. 169-198.

Pemberton, S.G., J.C. van Wagoner, and  G.D. Wach, 1992a, Ichnofacies of a wave-dominated shoreline, in S.G. Pemberton, ed., Application of ichnology to petroleum exploration, Society of Economic Palaeontologists and Mineralogists, Core Workshop, v. 17, p. 339-382.