Page 201 - Geology and Geochemistry of Oil and Gas

170 FORMATION OF HYDROCARBON ACCUMULATIONS

A negative aspect of this technique is usually obtaining overblown results due to
the incomplete conservation of the catagenetic processes in the presence of liquid
hydrocarbons.

9.4.3. Helium– Argon Technique

The helium–argon technique was developed by Savchenko in 1935 and published
in 1977. The technique is based on the ratio of the radioactive helium and argon of
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cosmic origin of gases dissolved in water: t ¼ 77.1 He/Ar 10 , where t is the time
the water resides in rocks and He/Ar is the ratio of their contents in water in %, by
vol. The technique was improved in 1950 by Kozlov (taking the solubility into
account) and in 1955 by Cherepennikov (in: Eremenko and Chilingar, 1996)
(introduction of correction coefficients). Critical analysis by V. P. Yakutsina and V.
V. Tikhomirov (in: Eremenko and Chilingar, 1996) showed that helium and argon in
gases and water have several origins. Besides, the error in determining the He/Ar
ratio can exceed 60%.

9.4.4. Determination Based on the Composition of Oil Fractions with Boiling Point
Below 2001C

The technique based on the composition of oil fractions with boiling point below
2001C was introduced by A. N. Reznikov (1967, in: Eremenko and Chilingar, 1996)
and is based on the catagenetic transformation of oil:
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C oil ¼ f AP =N 4 (9.3)
where C oil is the degree of catagenic transformation of oil; A, P, and N are the
contents of aromatic, paraphinic, and naphthenic hydrocarbons (in %). C oil ranges
from fractions of 1% in the Cenozoic oils to a few dozens of percent in the oils from
Paleozoic deposits.
The age of oil as determined by this technique reflects the time duration that the
oil resided within an environment at certain temperature interval. The effects on the
results obtained by the source organic matter and its catagenetic alteration, as well as
the effects of weathering on the generated oil, are unknown.
A number of techniques (mostly physical) were developed to help determine the
time of formation of oil and gas accumulations.

9.4.5. Volumetric Technique

The volumetric technique is based on the Boyle-Marriotte and Clapeyron
equations and has been proposed by A. Levorsen in 1958. Inasmuch as the reservoir
pressure by the end of formation of accumulation is assumed to be hydrostatic,
H f =g ¼ ðP 0 V 0 =P f V f ÞðT f Z f =T 0 Z 0 Þ (9.4)
where H f is the thickness of overburden, P f and P 0 are the reservoir pressures at the
time of formation of accumulation and at present, T f and T 0 are the temperatures at