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This paper, prepared primarily for the inexperienced geologist, describes the subsurface structural map with stratigraphic additions and modifications, its preparation, and its uses in the search for petroleum. An accurate, relatively uncluttered base map with the wells spotted correctly on it, is a prerequisite. Well logs provide the bulk of the basic data required, and the most useful of these logs is the electric log. Electric-log correlation is based on electrical characteristics of the datum and its position in a sequence of correlative events. Correlation difficulty is a function of lateral stratigraphic variation and of structural complication.
The mapping datum should reflect the structure of prospective producing zones, should occur over a wide area, should be encountered by most wells, and the geologist must be able to recognize it and correlate it.
Mapping should begin in areas of greatest control, and should reveal producing characteristics and trapping elements of the fields already productive. The most reliable geological interpretations are based on production information, as well as on the correlation of well logs. When production information is sketchy and the geology complicated, mapping on multiple horizons is a useful technique to determine the correct geological solution.
The contour interval selected for regional mapping will be based on studies of fields that will reveal closure necessary for accumulation.
In exploratory mapping, the geologist will direct his primary search for traps already known in the area, but he should not overlook the possibility that traps not previously recognized may occur in the area studied.
The occurrence of oil or gas shows in a non-productive well is direct evidence of a trap, and their presence is an important aid in interpretation.
Fault-throw computations can give additional control on the opposite side of the fault plane, but as they are subject to many errors, they should be used with caution. Determination of the dip and strike of faults is best determined by contouring on the fault plane.
The reflection seismograph is the geologist's most important source of additional information, but its limitations should be recognized.
The modification of the basic subsurface structural map by the addition of boundary lines of permeability of the various prospective zones, will make it useful in exploration for combination structural and stratigraphic traps.
Current maintenance of exploratory maps, made necessary by intense competition, will also enable the geologist to evaluate best his previous mapping efforts and those of his predecessors.
The exploration geologist's proper attitude is described as reasonable optimism. This leads to the conclusion that exploration for large reserves in the older producing districts should be concentrated in areas where they are known to exist. In areas previously explored unsuccessfully, new basic approaches and techniques would seem most likely of success.
The geologist must actively "sell" his maps and its prospects to his management, since all of his previous efforts are useless unless his work is used in acquiring land and drilling wells.
Barring the discovery of a direct method of finding oil, the various subsurface methods based on well control will eventually tend to supplant all other methods in the search for petroleum.
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