Page 245 - Origin and Prediction of Abnormal Formation Pressures

PREDICTION OF ABNORMALLY HIGH PRESSURES IN PETROLIFEROUS SALT-BEARING SECTIONS 217

leaving the salt plug, which is trapping a massive gas accumulation. A great number
of unfavorably located wells exit the salt plug below the gas-water contact (GWC) or
penetrate only the lowermost portion of the productive horizons.
For a reliable AHFP zone forecast in evaporites it is necessary to accurately delineate
the boundaries of salt plugs. Zilberman et al. (2000) proposed to utilize the thickness of
sediments synchronous with the formation of the salt overhang as a reference thickness.
The vertical cross-section of a salt plug resembles a mushroom. A salt plug has (1) a
'stem', formed due to piercing the overlying sediments along the faults by Devonian
salt, and (2) a 'cap' comprising the overhanging fragments that remain after regional
and local erosion events. Salt plug overhangs are indications of synchronous evolution
of the salt bodies and of the sediment cover portion subsequent to the salt break-through
(Fig. 9-3). In the Dnepr-Don Basin, for instance, after the first portions of the Devonian
salt reached the surface or the seafloor during the Carboniferous, they were destroyed.
Some traces of the event are preserved, in the form of peculiar sediments composed of
the Devonian clastic and igneous rock fragments called deluvial talus. This talus indi-
cates that the salt was dissolving faster than it was fed through the salt-supplying channel
(the stem). The process continued through early Permian time when the volume of salt
deposits in the sedimentary basin increased. This resulted in the prevalence of salt sup-
ply compared to its destruction. Salt overhangs began to form around the salt stems. As
the distance to the stem increases, the overhangs lie over younger Lower Permian sed-
iments and gradually pinch out. After a regional erosion, the Upper Permian sediments
overlie the Lower Permian deposits with the included upper portions of the salt plugs.
Thus, the sediments from the Svyatogor Rhythm through the base of the Lower
Permian accumulated simultaneously with the overhang development. The thickness
of this interval is assumed to indicate a standard of the maximum overhang thickness
(Fig. 9-3). The location where the salt thickness is greater than the standard one is
the salt plug proper. Seismic surveys delineate the areal extent of the plug and several
vertical cross-sections. Thus, one can identify points (locations) with the standard
thickness. The required outline is obtained by connecting these points (Zilberman et
al., 1971). Applications of this technique in the DDB gave favorable results for a
preliminary and operational AHFP forecast.

QUANTITATIVE AHFP FORECAST
Analysis of the thickness of the indicator sequences and of sediments synchronous
with the salt overhang development provides a way to derive a qualitative reservoir pres-
sure distribution and an areal outline of the AHFP zones. This, however, is insufficient to
prevent dangerous pressure surges (kicks) while drilling. What is needed is to determine
dangerous intervals in the section as well as the magnitude of expected overpressures.
As an example, overpressured gas accumulations in the Bakhmut evaporite series of
the DDB are associated with clastic and carbonate components of the rhythms. Such
gas accumulations are most common in the Podbryantsev Formation. The quantitative
AHFP forecast in the evaporite sealing sequences is best analyzed using pore pressure
in the reservoirs sealed by these sequences.