Geology Reference
In-Depth Information
prevents the dissipation of accumulations in the underlying deposits. The
same as in the previous case, descending vertical fluid migration and sub-
sequent redistribution are provided by pressure gradients directed into
the discharge zones.
Of special interest under the latter option is a case wherein clayey
sequences overlie a carbonate reservoir with fracture capacity. Within
this combination of competent and incompetent intervals the descend-
ing fluid flows; therefore, its accumulation may occur primarily in the
elevated fractured zones. Such zones are usually associated with crestal,
most faulted areas of local structures. It is clear that such a geologic envi-
ronment prevents both massive lateral fluid migration and the formation
of regional oil and gas accumulation zones. Such a geologic situation is
commonly accompanied by zonal hydrocarbon saturation, which is spo-
radic in the regional plan. Accumulations of this type are commonly
found in the contact zone between the thick Paleogene-Miocene clays and
the underlying Upper Cretaceous carbonate reservoirs (Indolo-Kuban,
Tersk-Caspian Troughs), Maracaibo Depression: (Young
et al
., 1959;
Durmishyan, Muradian and Rachinsky, 1976). The range of the descend-
ing fluid flow is defined by the penetration depth of the fracture system
and by the extent of its areal development within a specific stratigraphic
interval. In cases when fracturing involves the entire carbonate sequence,
such migration may occur on a grand scale causing the formation of rela-
tively large hydrocarbon accumulations and regional oil and gas accumu-
lation zones.
In all reviewed cases, high-pressure fluids, upon entering permeable
intervals, cause an increase in the formation pressure. The possibility of
its preservation in time is defined by the discharge conditions of the geo-
hydrodynamical system. In limited fracturing zones, AHFP of such gen-
eration is intense; in wider zones, its manifestations are more modest.
Patterns in spatial distribution of AHFP's modern and paleo-gradients
may serve as diagnostic attributes for the reservoir members' saturation
options with fluids. In cases where they decrease up the stratigraphic sec-
tion (or decreased in the geologic past), the assumption is legitimate of a
probable ascending vertical fluid migration and of the overlying complexes
being the discharge zones. Based on these data, it may be suggested that
a thick clayey or chemical sequence with AHPP (a peculiar injector) is
present in the not yet penetrated section. The inverse gradient distribu-
tion may be interpreted as a manifestation of the vertical fluid migration
in conditions of hydrodynamical isolation of the upper reservoir-receptor
positioned directly under the seal. It also may be interpreted as a result of
the descending migration and of the underlying permeable beds being the
