Page 163 - Petroleum Geology

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therefore a purely geological entity, but one that involves engineering, trans-
port, marketing, finance and economics. During its life it involves people
with a great range of skills in the various activities required. These people re-
quire the same varieties of goods and services as people in other activities,
and so oil and gas fields include houses, shops, schools, recreation, and the
power to run these. The only difference between an oil-field community and
others is, perhaps, that the average age is rather younger.
Oil and gas fields, like people, come in all shapes and sizes, some onshore,
some offshore. In plan, they may be long and narrow, or nearly circu1ar;in
section, they can be thick or thin, deep or shallow. In size, they can be very
large or rather small - but they must all contain some minimum quantity of
recoverable oil or gas of marketable quality. This quantity may be quite small
near markets or near other fields, or rather large if offshore at some distance
from markets. There are very many accumulations that are too small to be-
come fields. Oil and gas fields are commonly in trends (that are geological
trends) or groups or groups of trends, and these collectively form petroleum
provinces.
An oil or gas field begins as an anomaly on a map, revealed by regional
geological and geophysical surveys. The anomaly is investigated with more
detailed mapping, and if this shows features that could trap petroleum, the
company must decide whether it is sufficiently promising to drill and if they
can obtain the finance to drill it (or seek partners). The drilling site is chosen
on the basis of a detailed seismic reflection survey. Such is the precision of
seismic surveys that there will be great confidence in the geometry of the
anomaly and in the general nature of the sedimentary rocks in it, and, if there
is some local knowledge, also the ages of the rocks. There may even be direct
indications of gas (“bright spots”, “flat spots”), but the survey will not gen-
erally provide any information on whether oil was actually generated and
came to accumulate there. That can only be found out by drilling.
The first well drilled to a prospect is an exploration well, sometimes called
a wildcat. It is designed to acquire information, not necessarily to produce
what it finds on a permanent basis. This exploration borehole will be designed
and drilled to the greatest depth of practical interest, and the geological results
- the stratigraphy, ages of the rocks, dips, sonic velocity characteristics,
maturation levels of organic matter in the rocks, and fossils recovered in cores
and samples - all contribute to a refinement of the original model that was
based on the seismic surveys. More seismic work may be done. If petroleum
is not found, the information gained may lead to the drilling of further ex-
ploratory wells in the area. If petroleum is found, it will be tested and ana-
lysed, and the pressures and flow rates will be measured.
On the basis of the refined model, appraisal wells will be sited and drilled
to establish as quickly and as economically as possible the minimum size of
the accumulation. This is not just a matter of drilling elsewhere on the anomaly
but rather of deciding how big such an accumulation must be if it is to be