The hydrocarbon potential of an area is often judged by the existing production and reserve estimates of the area. For a new exploration program, all available geophysical, geologic, and engineering data must be interpreted. The results of the new wells must be integrated for subsequent development operations. Such exploration studies normally include an understanding of the regional geology, geologic and/or geophysical maps, show maps, cores, tests, mud logs, and wire line logs.
Cores and wire line logs are among the most important data to a formation evaluationist, whether he is a geologist or an engineer. Petrophysicists integrate these data to derive indirectly many basic reservoir parameters, such as porosity and fluid saturation, and to determine volumetrically the reservoir oil in place. There are many current logging techniques which can be combined for this purpose.
A well log is a graphical presentation of a physico-chemical characteristic of the geologic formations measured in a borehole as a function of depth. The logs determine lithologic characteristics of the sedimentary rocks and pinpoint hydrocarbon-bearing strata. Before the invention of well logs, such geologic variables were ascertained by (1) inspection and analysis of drill cuttings and cores and (2) formation tests. Many of these operations are being used less today because the necessary subsurface information can be obtained from well logs at lower costs and with sufficient resolution.
The first well log was designed and recorded in 1927 by the Schlumberger brothers in Pechelbronn, Alsace, France (Allaud and Martin, 1976).
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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).