2002 GCSSEPM Foundation Ed Picou Fellowship Grant for Graduate Studies in the Earth Sciences Recipient. David Pyles

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1 2002 GCSSEPM Foundation Ed Picou Fellowship Grant for Graduate Studies in the Earth Sciences Recipient David Pyles David Pyles is a Ph.D. student in the Energy and Minerals Applied Research Center at the University of Colorado at Boulder. He received his M.S. in geology from the Colorado School of Mines (2000) and his B.S. at California State University, Chico (1997). His research focuses on submarine-fan architectures and processes utilizing outcrop and subsurface data sets. To date, his research has resulted in the publication of three papers (including two in GCSSEPM volumes), one field guide (co-author), and 12 abstracts. He is also a co-editor of one book (GCSSEPM Deep-Water Core Workshop, Northern Gulf of Mexico). The Temporal and Spatial Evolution of Architectural Elements and Facies Associations in the Carboniferous Ross Sandstone, Western Ireland Applications to the Construction of Reservoir Models in Structurally Confined Basins, Northern Gulf of Mexico David R. Pyles: Energy and Minerals Applied Research Center, Department of Geological Sciences, University of Colorado at Boulder, Boulder, Colorado dpyles@emarc.colorado.edu

2 INTRODUCTION The most active exploration area in the United States is northern deep water Gulf of Mexico where approximately 12 BBOE have been discovered (Pettingill and Weimer, 2001). In this area, intraslope minibasins associated with mobile substrate are commonly the locus of stacked turbidite systems with high net pay per unit area. Many of the fields that produce in these basins can have high flow rates and high ultimate recoveries compared to unconfined submarine-fan reservoirs. In spite of the large amount of money invested in the deep water Gulf of Mexico, there are still many problems in developing these reservoirs. A commonly reported problem for maturing deepwater fields is that the production history turned out to be considerably different than was originally predicted from early, simplistic reservoir models (Weimer et al., 2000a). Predicting reservoir performance requires fluid-flow simulations from wellconstrained reservoir models. The construction of these reservoir models requires a detailed understanding of stratal attributes in the reservoir. Presently, most reservoir models are constructed with interpretations made from 3D seismic data, wireline logs, cores, pressure tests, and, in some cases stratal attributes from outcrop analogs and shallow seismic studies of modern seafloors. Most of the problems experienced in the production of turbidite reservoirs occur at a sub-seismic scale. Observations and interpretations made at outcrop analogs can be used to address these small-scale architectural features. For example, the size, shape, vertical, and lateral distributions of architectural elements and the vertical and lateral distributions of architectural elements in space, and the distribution of petrophysical properties within architectural elements can be measured and described at well-exposed outcrops. These are among the many input parameters, which may be below seismic resolution that must be entered in the construction of object-based, stochastic reservoir models. There are about seventeen outcrops of submarine-fan strata presently recognized worldwide that are of significant lateral and vertical size to be described and documented in a semiquantitative fashion for reservoir models (Figure 1). Presentations at the December, 2000, GCSSEPM Bob F. Perkins Research Conference highlighted most of these outcrops (Weimer et al., 2000b). Only three of these outcrops contain strata that are fine-grained, contain high net to gross, and were deposited in structurally-confined basins similar to northern Gulf of Mexico intraslope minibasins. They are the Cretaceous Lewis Shale of Wyoming, the Eocene central basin clinoforms of Spitzbergen, and the Ross Sandstone of Ireland (Figure 1). The purpose of this research is to study the Upper Carboniferous Ross Sandstone of County Clare, western Ireland (Figure 2) as an analog for northern Gulf of Mexico reservoirs. Three of the papers at the 2000 GCSSEPM Research Conference (Weimer et al., 2000b) addressed the general setting of the Ross Formation (Elliott, 2000; Martinsen et al., 2000; and Sullivan et al., 2000). Our research will build upon this foundation by collecting more detailed information and analyzing the information in a quantitative fashion that can be readily used to address the stratigraphic evolution of northern Gulf of Mexico intraslope minibasins, and aid in the construction reservoir models within these minibasins.

3 The Ross Sandstone is an ideal outcrop to address these objectives for many reasons. First, like many minibasins in northern Gulf of Mexico, the Ross is a fine-grained (fine sand), high net to gross (circa 0.65) submarine-fan system that was deposited in a structurally confined basin, similar in scale to intraslope basins in northern Gulf of Mexico (Figure 2b), and supplied via shelf-margin deltas and an unstable slope during high-frequency, high-amplitude, glacio-eustatic sea-level cycles. Second, the architectural elements (channel-form, lobe-form, sheet-form, and chaotic/contorted bodies) of the Ross Formation are similar in dimension and stacking patterns to the producing turbidite systems of the intraslope basins of the northern Gulf of Mexico. Third, a vertical profile through the Ross records early, middle, and late confined basinfill deposits, allowing us to analyze the stratigraphic evolution of a confined minibasin.

4 Three exposures of the Ross Sandstone, corresponding to early, middle, and late confined basin-fill deposits respectively, will be analyzed in detail. They are the Dunmore exposure of the lower Ross, the Kilbaha Bay exposure of the middle Ross, and the Rinevella Point exposure of the upper Ross (Figure 2). The Dunmore exposure is located on the southwestern end of the Loop Head peninsula (Figs. 2 and 3). The lower Ross Sandstone, which represents early basin-fill deposits, crops out here. The exposure is up to 85 meters thick and continuous for more than 2500 meters in the depositional-dip direction. This exposure has the highest net to gross of all the exposures that will be studied. The strata here stack to form sheet-form architectural elements that are composed of sandstone and mudstone. Sheet-form architectural elements compose 100% of this exposure. The rocks at this exposure were deposited when this site of the basin was dominantly depositional, with little bypass.

5 The Kilbaha Bay exposure is located at the small village of Kilbaha (Figs. 2 and 4). The middle Ross Sandstone, which represents medial submarine-fan deposits, crops out here. The exposure is located within and above the present foreshore and is easily accessible. The exposure is up to 20 meters thick and continuous for more than 1400 meters in the depositional-dip direction. This exposure has an intermediate net to gross compared to the other exposures that will be studied. The strata here stack to form sheet-form and channel-form architectural elements that are composed of sandstone and mudstone. Sheet-form architectural elements compose 85 % of this exposure, whereas channel-form architectural elements compose 15 % of this exposure. The rocks at this exposure were deposited when this site of the basin was transitional between erosional and depositional. The Rinevella Point exposure is located on the east side of Rinevella Bay (Figs. 2 and 5). The upper Ross Sandstone, which represents late basin fill deposits, crops out here. The exposure is located within and above the present foreshore and is easily accessible. The exposure is up to 25 meters thick and continuous for more than 450 meters in the depositional-dip direction. This exposure has the lowest net to gross of all the exposures that will be described. The strata here stack to form channel-form, sheetform, and chaotic/contorted architectural elements that are composed of sandstone and mudstone. Channel-form architectural elements compose 60% of the strata at this exposure, sheets compose 30%, and chaotic/contorted bodies compose 10%. The rocks at this exposure were deposited when this site of the basin was dominantly a site of bypass, with minor deposition.

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7 RESEARCH OBJECTIVES AND METHODS The objectives of the research are threefold and are described in detail below. 1. The first research objective is to collect detailed measurements of lower, middle, and upper Ross Sandstone exposures, which correspond to early, middle, and late confined basin-fill deposits (Figures 2, 3, 4, and 5). The measurements that will be collected include detailed, closely-spaced stratigraphic columns, correlations panels, and photopanels for each outcrop. We will use these data to quantitatively distinguish the stratal characteristics of these different exposures. These data will also allow us to evaluate the stratigraphic evolution of the Ross confined basin-fill deposits. This objective will not only allow us to increase on the large foundation of knowledge in the Ross Sandstone, but it will also allow us to address many unanswered questions about the architectures and facies of submarine-fan strata that fill confined basins. Understanding the evolution of the Ross confined basin will undoubtedly provide insight into the stratigraphic evolution of confined minibasins in northern Gulf of Mexico and other similar basins around the world. This is especially true since most studies of confined minibasins in northern Gulf of Mexico focused on the development of depositional concepts using shallow 3D seismic data, conventional 3D seismic data and high-frequency 2D seismic data through confined basin-fill sediments (for example Prather et al., 1998; Badalini et al., 2000; Beaubouef and Friedmann, 2000). Outcrop observations provide insight at a much smaller scale than do seismic data and will therefore provide insight that seismic studies cannot. Additionally, the qualitative and quantitative techniques that we will employ will allow for very precise measurements and trends to be accounted for. 2. The second research objective is to utilize the quantified stratal attributes to constrain two-dimensional, object-based, stochastic reservoir models of the three different exposures studied in detail. The reservoir models will be constructed using ROXAR RMS software. This will allow us to develop lists of critical input parameters that are necessary to stochastically reproduce the distribution of architectural elements and the distribution of petrophysical properties within the architectural elements for early, middle, and late confined basin-fill reservoirs. The reservoir models will utilize petrophysical properties from northern Gulf of Mexico and other confined basin-fill reservoirs. This objective will provide the geological community with input data that can be used to help constrain reservoir models for some confined basin reservoirs, most notably in the northern Gulf of Mexico. 3. The third research objective is to construct two-dimensional synthetic seismic profiles from the reservoir models. Several synthetic seismic profiles will be constructed for each outcrop utilizing a variety of frequencies, rock properties, and fluid properties. The profiles will be used to analyze the seismic facies of these types of reservoirs. The synthetic seismic profiles constructed in this study will be useful for both students and oil-company geologists to learn to recognize different stratal architectures on seismic profiles. REFERENCES CITED Badalini, G., B. Kneller, and C.D. Winker, 2000, Architecture and processes in the late Pleistocene Brazos-Trinity turbidite system, Gulf of Mexico continental slope: GCSSEPM Foundation 20 th Annual Bob F. Perkins Research Conference, p

8 Beaubouef, R.T., and S.J. Friedmann, 2000, High resolution seismic/sequence stratigraphic framework for the evolution of Pleistocene intra slope basins, western Gulf of Mexico: Depositional models and reservoir analogs: GCSSEPM Foundation 20 th Annual Bob F. Perkins Research Conference, p Elliott, T., 2000, Depositional architecture of a sand-rich, channelized turbidite system: the upper Carboniferous Ross Sandstone Formation, western Ireland: GCSSEPM Foundation 20 th Annual Bob F. Perkins Research Conference, p Martinsen, O.J., T. Lien, and R.G. Walker, 2000, Upper Carboniferous deep water sediments, western Ireland: analogues for passive margin turbidite plays: GCSSEPM Foundation 20 th Annual Bob F. Perkins Research Conference, p Pettingill, H.S., and P. Weimer, 2001, World-wide deep water exploration and production: past present and future: GCSSEPM Foundation 21 st Annual Bob F. Perkins Research Conference, p Prather, B.E., J.R. Booth, G.S. Steffens, and P.A. Craig, 1998, Classification, lithologic calibration, and stratigraphic succession of seismic facies of intraslope basins, deepwater Gulf of Mexico: AAPG Bull., v. 82, p Sullivan, M., G. Jensen, F. Goulding, D. Jennette, L. Foreman, D. Stern, 2000, Architectural analysis of deep-water outcrops: implications for exploration and development of the Diana sub-basin, western Gulf of Mexico: GCSSEPM Foundation 20 th Annual Bob F. Perkins Research Conference, p Weimer, P., R.M. Slatt, P. Dromgoole, M. Bowman, and A. Leonard, 2000a, Developing and managing turbidite reservoirs: case histories and experiences: results of the 1998 EAGE/AAPG Research Conference: AAPG Bull., v. 84, p Weimer, P., R.M. Slatt, J. Coleman, N.C. Rosen, H. Nelson, A.H. Bouma, M.J. Styzen, and D.T. Lawrence eds., 2000b, Deep-water reservoirs of the world: GCSSEPM Foundation 20 th Annual Bob F. Perkins Research Conference, 1105 p.