Geology Reference
In-Depth Information
Prospect, well 1). For the trough on the whole this value appears to be too
conservative as the Burunnaya Prospect area gravitates to the stabilized
platform portion. There, a depositional gap occurred between the Triassic
and overlying Jurassic formations (Kissin, 1964). The gap apparently facili-
tated the paleo-discharge for the Triassic waterhead complex. This suggests
possibility of a higher abnormality factor in the Triassic mobile belt part
of the trough. And that in turn may result in a higher average value of this
parameter for the entire Triassic-Lower Cretaceous interval.
Thus, we have relatively moderate AHFP in the lower intervals of the
penetrated section. Its extreme manifestations (abnormality factors of up
to 2.20) are mostly associated with overlying waterhead Upper Cretaceous
complex and its overlying thick (up to 3,000 m) clayey Maikopian Series.
The Maikopian series includes lens-shaped, spatially drastically variable
reservoirs where the role of syngenetic AHFP and AHPP (caused by the
consolidation of the interval's clays) is obvious. These give reason to sug-
gest that the main cause for such distribution of the formation pressure
abnormality was accumulation of epigenetic energy vertically-introduced
(from the lower stages of the sediment cover) under a thick plastic regional
barrier of the Maikopian Formation. This barrier, in turn, has syngenetic
abnormality.
We believe that this concept is supported by the proportional associa-
tion between abnormality factor values in the Upper Cretaceous carbon-
ate reservoirs, the extent of its fracturing and the seal's thickness. At the
prospects with decreased Maikopian thickness, closeness of the Upper
Cretaceous reservoir to the surface (discharge zone) and elevated fracturing
(Zamankul and Staro-Groznenskoye fields) formation pressure abnormal-
ity is minimal, 1.29-1.33. On the structures where the Maikopian thick-
ness is substantially increased and the slightly fractured Upper Cretaceous
reservoir is at a significant depth, the abnormality reaches maximum val-
ues of 1.70-2.20 (West Gudermes, North-Malgobek, Andreyevskoye and
other fields) (see Figure 3.11).
The Triassic- Lower Cretaceous interval is the charge zone of the
entire overlying section with high-pressure fluids and epigenetic energy.
The generation of syngenetic AHFP in that interval appears to be a final
result of two major mechanisms. These mechanisms are the difference
in heat expansion between the fluids and the enclosing rocks in the high
temperature (great depth) zone and elastic stress effects of the neotec-
tonic nature.
The latter mechanism is common for all intervals of the sedimentary
section in the region. This, in combination with the other stated causes,
predetermines in general stressed status of the geo-fluid-dynamic system
