Modeling—the Forward Method
Oil and gas deposits are found in porous formations in sedimentary basins. Under normal conditions, the reservoirs occur at locations where the appropriate porous formation is at a higher elevation than the surrounding region. The task of the exploration geophysicist is to locate such occurrences. The most common means of doing this is the proper interpretation of seismic data recorded for the region of interest. As with any physical procedure of this nature, it becomes highly desirable to simulate the data collection process in the laboratory to gain insight by the examination of known situations. This process forms the concept behind seismic modeling for exploration purposes.
The results from modeling of seismic wave propagation are used in many ways by the geophysicist to predict and understand seismic wave phenomena. Uses fall in the broad categories of (1) demonstrating the consistency of interpretation of real data; (2) providing synthetic data sets for testing processing techniques, acquisition parameters, and new ideas; and (3) educating the geophysicist in wave propagation.
Perhaps the most obvious use of modeling is in the generation of synthetic data sections for a proposed model for comparison with the actual data. Although it would be unusual for the synthetic data to match the real data precisely, the interpreter can, by the degree of likeness, increase his confidence in the proposed geologic cross-section. This comparison of model data with field data can be performed at various stages (e.g., common source records, stacked sections, migrated sections) by applying similar processing to
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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).