Page 88 - Petroleum Geology

CHAPTER 4

A SYNOPSIS OF PETROLEUM GEOLOGY

INTRODUCTION

Geology is concerned with observing facts about the Earth, studying
the relationships between these facts, and interpreting them in terms of
natural processess in space and time. It is a descriptive science, in the first
place, because the diverse data of geology cannot be interpreted until they
have been accurately described and represented on a scale that the human
mind can embrace. It is an electric science because it must borrow from
many disciplines, notably biology, chemistry and physics. It is a speculative
science in part (like archaeology and astronomy) because hypotheses based
on geological data are rarely verifiable; but speculation is a valid scientific
activity provided it is based ,on reliable observations. So the geologist’s work
falls into two main activities: the description of present geology and the
interpretation of that geology in terms of past geology and geography. Geol-
ogists tend to specialize in such topics as palaeontology, sedimentology, geo-
chemistry, structural geology and geophysics, and seek to advance our knowl-
edge by concentrating on a special facet of the science.
Petroleum geology is general geology with a specific aim, and all these
things apply to petroleum geology. The petroleum geologist’s work also has
its descriptive and interpretive aspects, but the emphasis tends to linger on
the descriptive because the goal of petroleum geology is a deterministic
model of the area under study - ultimately, the oil or gas field. To achieve
this goal, the specialists of petroleum geology tend to work in teams (which
also broadens their minds).
Petroleum geologists, whatever their speciality, tend to become either
exploration geologists or development geologists. The difference is not only
a matter of scale, but also of outlook that can be so different that there is
danger of the one not understanding the other properly. Petroleum explora-
tion, in its simplest terms, consists of studying large regions that do or could
contain petroleum, identifying progressively smaller areas of progressively
greater interest in these until a prospect worth drilling has been identified,
and discovering oil or gas in one or more of these. The development geologist
starts with the discovery well and a detailed seismic survey, and locates ap-
praisal wells to assess the size and nature of the accumulation or accumula-
tions. If petroleum is found to be in commercially viable quantities, the
development geologist seeks to obtain an accurate model of the accumula-
tion on maps and cross-sections that can be used for estimating the recover-