Page 199 - Geology and Geochemistry of Oil and Gas

168 FORMATION OF HYDROCARBON ACCUMULATIONS

within the metamorphic basement, which is fractured and possibly connected at a
distance with the surface (through an erosion surface or fractures), AHFP can
disappear.
Hydrocarbon accumulation begins to form at a location in the reservoir where
resistance to the fluid movement becomes greater than the forces causing this
movement. What is an oil (and/or gas) accumulation? The ‘‘Petroleum Reference
Book’’ (1984, p. 205–206) gives the following definition: ‘‘All writers define an oil
and gas accumulation as a single aggregation of these mineral resources. Sometimes
such accumulation is called elementary, local, isolated, or restricted from all sidesy
The major accumulation parameters are the quality and amount of oil, gas and
condensate. The shape and habitat of an accumulation are determined by the type of
reservoir and trap, temperature, pressure and natural drive (energy)’’. What is not
clear in this definition is the expression ‘‘single aggregation’’ and the ensuing idea of
the outline (contours) of the accumulation.
It is well established that all hydrocarbon accumulations contain some water, the
content of which may be 2% or ‘‘greater’’. What does ‘‘greater’’ mean? When
evaluating the hydrocarbon reserves, it is common to determine the oil–water
contact from the depth of perforations that produce a water-free oil, or (in the case
of log determinations) of the calculated oil saturation in excess of 50%. This is
actually quite definite. All oil accumulations contain at least 0.2–0.5% of water,
which is in no way a reflection of its true content, due to the relative permeability.
Even a reservoir with the oil saturation below 50% can yield the water-free oil
(depending on the fluid properties). The definition ‘‘log-calculated saturation of 50%
‘‘is also not clear. A flow of water-free oil as well as the flow of water alone can be
obtained from such a reservoir.
The issue of the contacts and fluid separations (w-o-g) in the reservoirs requires
further studies. It is especially important in the case of presence of emulsions in the
accumulations. This is observed in many small and medium size accumulations in the
Western Siberia and in the Eastern Pre-Caucasus (Sudarikov, 1978). How does one
interpret the concepts of ‘‘oil–water contact’’ and the fluid (w-o-g) ‘‘separation’’
surfaces, which are crucial in the appraisal, development and reserve calculation? Is
it possible that by changing the production technology we will be able to recover
commercial hydrocarbons outside the ‘‘contours’’?
A special category among the accumulations, the conditions of which deviate
from the average, are hydrocarbon accumulations at ‘‘great’’ depths with
temperatures above 120–1501C. Changes in the physical conditions at great depths
create a totally different environment.
Attempts to determine the speed of migration in the process of oil and gas
accumulation, assuming a uniform filling, were not successful. For example, during
the formation of the Anastasyev-Troitsk Field (North Caucasus) the length of
migration was 25 km, and the life of the structure was 15 MMY. Thus, the speed of
migration was 1.7 mm/year (Maksimov et al., 1977).
Based on all the aforementioned, it can be stated that the fluids move non-
uniformly, and their speed may reach up to tens and even hundreds of meters per year
(established for water). According to Beletskaya (1990, p. 281), the most intensive