Page 106 - Petroleum Geology

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sentially because of a restraint on further migration that is due to stratigraphic
causes. Of the many possibilities, the two most important are organic reefs
enclosed above by mudstones, and unconformities where the subcrop of
the permeable formations is sealed by mudstone on the unconformity.
Some very large fields are palaeogeomorphic traps, where transgression has
led to porous and permeable beds against a contemporary topographic
feature that was covered by mudstone as the transgression proceeded. Wed-
ging, lensing, and convergence of rock units also cause stratigraphic traps.
In practice it is not always possible to assign an accumulation exclusively
to a single class of trap. Many anticlinal traps are faulted, and are classed as
structural traps. Most unconformity traps are also faulted, some also folded.
Organic reefs and palaeogeomorphic traps are usually not folded, but may
be faulted, and they are usually purely stratigraphic traps.

GEOPHYSICS

It is not my purpose here to review the role of geophysics in petroleum
geology, but rather to pick out some salient points to illustrate the impor-
tance of the topic in petroleum geology.
Geophysics is the study of the physical properties of the Earth and the
study of the Earth through its physical properties. It is concerned with the
distribution of matter in and around the earth, particularly that within the
Earth to depths to 10 km or so. It is concerned with the force of gravity and
its variation from place to place; the strength and direction of the magnetic
field and their variations from place to place; the occurrence of earthquakes
and the transmission of their energy, and the propagation of artificially gen-
erated elastic (seismic) waves through the earth.
As applied to economic geology, geophysics consists of measuring the
physical properties of the rocks beneath an area (gravity, magnetism, radio-
activity) and measuring the response of the rocks to artificially generated
energy fields (seismic, electric or electro-magnetic). The scale of such investi-
gations varies from the global to the very local (such as the measurement of
the velocity of sound in the wall of a borehole). The application of geophys-
ical techniques to the search for petroleum has not only been of immense
importance to the petroleum industry, it has also provided an important stim-
ulus to the broader development of the science of geophysics. It is the
practical application, largely, that has kept geophysics closely related to
geology.
Although many of tne early regional exploratory surveys were based on
magnetic and gravity methods, and gravity surveys were used to detect salt
domes and other structures, their importance in petroleum exploration has
declined. The enormous improvements in seismic technology since 1960,
largely due to improvements in computer technology, in turn, often stimulated