Page 200 - Geology and Geochemistry of Oil and Gas

TIME OF FORMATION OF HYDROCARBON ACCUMULATIONS 169
hydrocarbon expulsion stage coincides in time with the maximum generation. The
following question arises here: When was it happening?

9.4. TIME OF FORMATION OF HYDROCARBON ACCUMULATIONS
There are many methods of determining the time of formation of hydrocarbon
accumulation (Mikhaylov, 1984):

9.4.1. Paleogeologic Method

The paleogeologic techniques enable the scientists to determine the time when an
accumulation began to form. Two seemingly clear-cut premises are valid here: (1) the
accumulations could not form prior to the rocks in which the accumulation is
located and (2) the accumulations began forming only after the formation of trap.
The latter premise, apparently clear, turns out to be indistinct when the time of
trap formation is determined. The most common way of determining the time of
formation of structures is based on the environment of lithofacies formation, as
proposed by Belousov in 1942. Unfortunately, the restrictions of the paleotectonic
technique proposed by Belousov (1942) are usually neglected. They include, in
particular, the limitations of the lateral size and the nature of facies. As a result, the
paleotectonic analysis of the formation of structures is reduced to the comparison of
thicknesses, which does not reflect either the timing or conditions of the structure
formation. As a consequence, different geologists arrive at quite different
conclusions regarding the same region.
The time of formation of a fault trap is bracketed by the time of the fault
formation. The formation time of a stratigraphic trap is determined by the age of
immediately overlying sediments.
An accumulation is formed as a result of migration of hydrocarbons. Migration
by buoyant force can begin only if there is a regional tilt. The steeper the slope, the
stronger the buoyant force. Could this be the reason for an early formation of oil
accumulations over the geosynclinal flank of the foredeeps?

9.4.2. Mineralogic Technique

The essence of the mineralogic technique is determining the sequence of post-
depositional alterations within the oil-saturated reservoir and within the same
reservoir outside the OWC. Liquid hydrocarbons cause retardation of catagenic
processes within the accumulation of hydrocarbons, compared with those outside of it.
Important in the application of this technique is to determine the morphologic
relations between the oil and authigenic minerals, as well as to find oil in the voids
having resulted from the dissolution of carbonates and sulfates at various stages of
catagenesis. The position of an old oil–water contact may be identified from the
mineralogical data (Sakhibgareyev, 1985), in order to determine the history of
accumulation.