Page 162 - Petroleum Geology

CHAPTER 7

THE NATURE OF OIL AND GAS FIELDS

SUMMARY

(1) Oil and gas fields are not purely geological entities, because they involve
engineering, economics and finance, and the normal range of goods and ser-
vices for the people who operate them.
(2) Current known reserves recoverable by present technology at present
prices are dominated by relatively few very large fields. At the end of 1977,
about 85% was in only 288 fields.
(3) The volumetric size distribution of accumulations, rather than fields,
appears to follow Zipf’s law in which the relative sizes when ranked are ap-
proximated by the successive terms of the harmonic series, 1,1/2,1/3,1/4,
. . . , l/n, so that the product of rank number and size is approximately con-
stant. For the world, this constant appears to be at least 120 X 10’ barrels of
oil (19 X lo9 m3).
(4) The sum of this harmonic series suggests that the total ultimate recover-
able reserves of oil, by present technology at present prices, will be about
1,800 X lo9 bbl (286 X lo9 m3), which is comparable with some other esti-
mates.
(5) The as-yet-undiscovered recoverable reserves are probably in relatively
few very large fields, but the proportion is probably less than that of the dis-
covered fields because the very large fields are easier to find.
(6) Increasing geological, geophysical, geochemical, engineering and finan-
cial skills will be required to find the remaining reserves.

Our purpose here is to seek the wider context of petroleum geology as a
sort of intermezzo and, after reviewing the present distribution of oil fields
by size, venture into the speculative domain of the total world resource of
oil. It must be realized that reliable statistics of the world’s oil fields are never
up-to-date because it takes some years for new discoveries to be accurately
assessed, and more before the figures are published. Perhaps Hedberg was
right when he wrote that ultimate reserve estimation was a topic that belong-
ed more to the after-dinner speech than serious writing!
An oil or gas field is a petroleum accumulation that has been discovered
and found to contain enough petroleum of sufficient quality to be worth
more in the market place than the total cost of getting it there. It remains an
oil or gas field until the value of the production in the market place no longer
exceeds adequately the cost of getting it there. It is then abandoned. It is not