Geophysical Reserve Classification Proved, Probable, and Possible

The development of 3-D seismic is the single most important technological breakthrough in many years.1 Some guidelines to separate reserves into the traditional classification of proved, probable, and possible using seismic methods have been developed and refinements to the current definitions proposed. These guidelines supplement the SPE reserve definitions and the SPEE (Society of Petroleum Evaluation Engineers) guidelines. which were published in the 80's and do not detail the use of seismic data in classification of reserves. Since the oil and gas industry is increasing the use of 3-D seismic. it is time for the SPE to recognize the usefulness of 3-D seismic data in classifying reserves. Use of 3-D seismic data is especially important in classifying reserves in complex structural and stratigraphic traps with limited well and production data. The seismic methodology and thought process and typical interpretation steps ate illustrated in two simulated case studies. Smarter. faster. cheaper. more profitable oil and gas development and reserve evaluation result from the use of 3-D seismic. INTRODUCTION Recent advances in seismic technology. computing power. and software provide engineers and geophysicists with the tools to make quantitative measurements of reservoir parameter. These measurements can be made with very dense spatial sampling over an entire reservoir. The oil and gas industry frequently relies on the use of seismic data to determine reservoir parameters in areas which are a significant distance from the wellbore seismic imaging methods are emerging as promising new measurement tools which can be to bridge the information gap between wells and help constrain stochastic models.2 The seismic method can be used to define the limits of a reservoir and to characterize the rock properties within the reservoir. These properties include thickness, continuity, lithology, net/gross ratio, porosity, permeability, fracturing, and water saturation. The SPE reserve definitions and the SPEE guidelines are generally designed to classify reserves based on well data and production history. The area of a reservoir considered proved is generally limited to:Producing wells.Their immediate offsets.Locations where interpretations of well data indicate lateral continuity of the producing formation.The area above the lowest known hydrocarbons. In 1981 the SPE permitted assignment of proved reserves on the basis of core or log data by wing the analogous reservoir concept (the data must indicate the reservoir is hydrocarbon bearing and analogous to other productive reservoirs in the same area). This paper extends the analogous reservoir concept to include application of modem seismic techniques. Once the seismic attributes of the productive formations have been recognized and verified by drilling. how should the remaining undrilled areas identified by 3-D seismic interpretation be classified If there are proved reserves associated with any wells that lie within a 3 D seismic survey. and if the reservoir characterization process is performed with due diligence by a team of geologists. geophysicists, and engineers, then a larger spatial area of the reservoir than that defined by well control should be classifiable as proved. In the "gap" areas between wells, and in areas extending beyond well control, current SPE/SPEE classification methods do not consider the accuracy of parameter determination that is available through the use of 3-D seismic data. Table 13 and Figure 14 illustrate the success of predictions from 3 D seismic interpretations. One of the most important reasons for determining reserve classification is to justify investment. which is especially important early in the life of the field. The analogous reservoir concept is especially applicable to reserve classification in areas where large capital investments must be justified before production can start (i.e. offshore). In the modem offshore industry. detailed facilities are designed, and multi-million dollar expenditures are customarily made, on the basis of data from a small number of wells integrated with 3-D seismic data onshore fields may also need to show sufficient reserves to satisfy building pipelines and surface facilities. The difficulties encountered without 3 D seismic data are illustrated by comparing Figures 2 and 3.5 The 3-D structure is more complex and less continuous than the simplified 2-D structure. The methods discussed here may better reflect the current thought process in reserve classification when integrating 3-D seismic data with limited well and production data than the current SPE/SPEE classification methods. The authors recognize the tremendous difficulties in classifying reserves. especially at early stages of development. The intent of this paper is to promote greater discussion on the use of the seismic method in classifying reserves. As technology advances the existing SPE/SPEE classification methods can run into difficulties. When a horizontal well is drilled the seismic method may be the only way the reservoir thickness and hydrocarbon water contact can be determined. There are as many variables associated with reservoir parameters as industry experts who are attempting to provide a satisfactory resultant reserve classification. It is the authors' Intent to present reservoir classification techniques that will increase the contribution offered by seismic data.