Preliminary Discussion of Net/Gross Distribution in Gulf of Mexico Deep Water Reservoirs
Published:December 01, 2000
Keith W. Shanley, Ciaran J. O’Byrne, Ramsey R. Fisher, Tad M. Smith, 2000. "Preliminary Discussion of Net/Gross Distribution in Gulf of Mexico Deep Water Reservoirs", Deep-Water Reservoirs of the World, Paul Weimer
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In recent years, exploration and development portfolios have increasingly shifted towards deep water reservoir systems, the majority of which are physically located in present-day deep water settings. Because of their deep water setting, decisions regarding substantial capital expenditure are often required on the basis of 3D seismic coverage and extremely limited well data. The drilling of multiple appraisal wells to delimit accurately reservoirs often results in unacceptable erosion of financial value. These operational and financial constraints make early probabilistic assessments of project value increasingly important in decision analysis.
A critical component of the analysis of project value is an understanding of the distribution of reservoir parameters including reservoir net/gross. Cumulative probability distributions of reservoir net/gross, based on well penetrations, have been calibrated to a suite of deep water seismic facies in an attempt to quantify variations in reservoir quality and to supplement seismic attribute analyses. This approach of calibrating distinctive seismic facies provides a useful tool for risk assessment in both early phases of exploration as well as development planning. We recognize 10 seismic facies comprising 6 major facies classes in deep water deposits in the Gulf of Mexico. These seismic facies represent discrete elements that share similar, observable (on seismic data) geometric characteristics. Similar to rock-based facies, these seismic facies elements form the building blocks of larger facies assemblages or associations.
In this paper we present preliminary net/gross distributions for two common seismic facies – “Facies C-Sheets” and “Facies B-Concave-erosional.” Each of these facies can be subdivided into two sub-facies, C1/C2 and B1/B2. Examination of the distributions of these data suggest that minimum estimates of net/gross (P10 value; i.e.; 90% of all data have a net/gross in excess of P10) range from approximately 25% (C2) to 55% (C1), most likely estimates of net/gross (P50 value; i.e., 50% of all data have a net/gross in excess of P50) range from approximately 72% (C1) to 86% (B1), and maximum estimates of net/gross (P90 value; i.e., 10% of all data have a net/gross in excess of P90) range from approximately 92% (C2 and B2) to 97% (C1). These values vary somewhat from other data sources, notably Deepstar (1996) and Prather et al. (1998). Possible reasons for the variations from these datasets are discussed in this paper.