In November of 1979 the Society of Exploration Geophysicists held its 49th Annual International Meeting in New Orleans. The technical sessions which ran in parallel embodied a curious mix of technical applications and theory. Among the sessions were offerings on general and nuclear waste disposal, deconvolution and filtering, modeling, migration, 3-D seismic, potential field methods, and a symposium on pattern recognition. While some of these titles are self-descriptive, it is particularly revealing to note these session contents and observe their relationships to other and allied disciplines.
Techniques for waste disposal represent a general awareness of the environmental repercussions of virtually all human activities. Knowledge of the subsurface gained by geophysical means can be translated into sites which would be particularly tolerant for the safe repose of harmful waste products. Deconvolution operations and filtering are standard signal processing procedures which permeate systems and control theory, speech processing, radar, sonar and communication.
Modeling in the exploration context encompasses one of the more sophisticated computer simulation tasks which is currently addressed the description of wave propagation in complex multimedia environments representing the earth's near subsurface. Migration and 3-D seismic purely in technical terms can be described as imaging methods again addressed to a most difficult problem. Of course, the principles and objectives are quite analogous to those of computer tomography (CT scanning) the role of which in medical imaging was recently honored with a Nobel Prize (Cormack and Hounsfield, 1979).
Potential field methods draw on the most elegant formalisms of classical
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Concepts and Techniques in Oil and Gas Exploration
Exploration for oil and gas has witnessed dramatic changes in its nearly 120 year history. Initially, prospects were located by surface shows or seeps; random drilling was predominant. In the early twentieth century, the anticlinal theory became a dominant element in locating traps. Direct mapping of structures by using magnetic, gravity, and seismic data began in the mid 1920s.
In the last 10 years a true revolution has occurred in the use of seismic data in exploration. To a minor degree, the first use of seismic data to locate reefs and carbonate buildups took place in about 1950, but the main era of more quantitative stratigraphic trap detection began in the late 1960s when direct hydrocarbon detection by the so-called “bright spot” concept was first used in the Gulf Coast Cenozoic offshore. In a very short time since then, an increasingly sophisticated seismic mapping approach has swept through the exploration industry. It is now possible to map seismically subsurface stratigraphy, model stratigraphic analogs, and make comparisons of the recorded data with known analogs in order to “read the subsurface.”
We are now on the verge of an era of synergism in revolution where exploration techniques are being integrated with reservoir delineation and production engineering methods. Synergism, according to Webster, is “the joint action of agents…, which when taken together increase each other's effectiveness.” Recognizing this need as a key to success, several companies have begun to integrate the know-how from geology, petrophysics, and reservoir engineering in developing plays such as the Ozona-Sonora gas play in West Texas.
Predicting accurately all the variables in the subsurface requires the solution of an extremely complex equation. This is because there are so many parameters which cannot be scientifically measured adequately ahead of (or, for that matter, after) the drilling bit. Thus, our exploration efforts are designed to reduce the risk of being wrong in our solution of the “subsurface equation.”
Risk reduction in exploration can be greatly facilitated by bringing all the necessary technical expertise to bear on the problem. The exploration hexagon in Figure 1 illustrates the interrelation among six broad technologies that can be used to minimize the risk of drilling a dry hole. Frequently, there are insufficient data available to make use of all disciplines and often only one or two create a play.
A play is an exploration activity involving a geographically designated and geologically definable volume of rock in which one or more targets for hydrocarbons can be described (see Figure 2).