Progress in Stratigraphic Seismic Exploration and the Definition of Reservoirs

Abstract

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering. Summary The blending of exploration tools calling upon geology and geophysics has produced an approach to stratigraphic seismic exploration of great power in the identification and definition of hydrocarbon reservoirs. From the geologicside, facies analysis used conventionally and with color-derived seismic displays scaled in velocity can suggest the most likely depositional models and the associated lithology. Next, very detailed and precise velocity measurements from the seismic data by means of the moveout relationship and independently from the reflection strength or seismic amplitude measure can confirm orcontradict a proposed model. When full consistency is established between these components and all other available data, then the exploration data have been utilized fully. Furthermore, maps of velocity variation within proposed reservoir units can be developed to guide initial drilling, and after calibration with wells, to develop and operate the field. The full procedure also surmounts certain fundamental limitations inherent in conventional exploration technology and leads to new and exciting exploration plays. This paper illustrates all concepts and procedures with case studies and examples. Understanding Seismic Stratigraphic Exploration Stratigraphic and Structural Traps. Many knowledgeable explorationists have recognized the current shift in exploration emphasis from the quest for the simple structural trap to stratigraphic traps. The special volume put togetherby Halbouty exemplifies the collective current view. The difference between stratigraphic and structural traps can be as clear as the difference between ananticline and a pinchout, or it may be quite subtle where both elements play arole. Structural traps may be complex in geometric terms or of gentle expression. and so can become difficult to recognize and find. Stratigraphictraps, on the other hand, with or without the role of structure, by definitionentail greater subtlety. We can understand this point best by looking moreclosely at some of the mechanisms by which such traps are formed. Changes inlithology and porosity development by fracturing or diagenetic activity arejust some of the ways stratigraphic traps may develop. Conventional explorationtools often are taxed beyond their capability by the task of defining certainstratigraphic traps. The following discussion explains the nature of suchlimitations and formulates tools and procedures that can be used effectively todefine these exploration objectives. We also note that the technology used toattain this goal also has bearing on the delineation of hydrocarbon reservoirs. Limits of Seismic Visibility. In recent times it has been fashionable todiscuss how thin a bed may be measured by the seismic method . More recently ina rather comprehensive discussion. Neidell showed that very thin beds indeedmay be defined with conventional seismic data, and several case studies havesince been published. JPT P. 709^