Exploration geophysics is a young, hybrid science developed during the last 60 years to aid the geologist. Geophysical methods include measurements of the earth's magnetic, gravitational, and electrical fields; however, seismic recording of propagating elastic waves constitutes the principal method for mapping hydrocarbon deposits, accounting for more than 94 percent of all expenditures in geophysical activity (cf., Oil and Gas J., September 14, 1981). Modern exploration seismology uses advanced technology from many diverse fields such as communications engineering, electronics, information retrieval, telemetry, and optics.
Seismic profiles provide a cross-section of the subsurface revealing structural details such as faults, folds, anticlines, and synclines. In some cases compositional information such as porosity, permeability, water saturation, and hydrocarbon content can also be predicted. Presently, seismic data provide the most reliable means of acquiring subsurface structural and stratigraphic information without drilling wells.
The combined use of geology and seismology greatly reduces the risks involved in oil and gas exploration. The success rate of new-field wildcats in the United States was much higher for the years 1979–1980 than ever before, as shown in Figure 1. These statistics correlate directly with major advances in seismic technology (Crook, 1979), underlining the importance of improved geophysical methods.
The seismic crew count has historically been a leading indicator of oil and gas activity. The crews record and process massive amounts of seismic data on a routine basis for the oil industry. Seismic surveys are carried out on a surface grid in order to build a
Figures & Tables
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).