Page 189 - Origin and Prediction of Abnormal Formation Pressures

164 W.H. FERTL, G.V. CHILINGAR AND J.O. ROBERTSON JR.

is then plotted on a linear or logarithmic scale versus borehole depth, thus establishing
normal compaction trend lines. Inasmuch as shale porosity commonly increases in
overpressured zones, any decrease in bulk specific weight may indicate the presence
of overpressured environments. Quantitative pressure evaluation is then possible by the
equivalent depth method (Fertl, 1976) or from empirical curves established for a given
area (Boatman, 1967).
Major limitations are (1) examination of cavings and/or recirculated cuttings which
constitute the contaminating part of drill cuttings being investigated, and (2) limited care
taken by rigsite personnel to collect and analyze samples. Several other factors, which
may greatly affect the measured bulk specific weight values of drill cuttings, include:
(1) presence of shale gas which decreases the apparent bulk specific weight values;
(2) presence of organic-rich shales, which results in lower bulk specific weight values;
(3) lithologic variations, e.g., presence of silty or sandy shales, mudstones, and marls;
variation in carbonate content of shales also affects bulk specific weight; (4) presence
of heavy minerals, such as pyrite (Permian Basin, U.S.A.; offshore Cameroon, Africa;
South China Sea area), siderite (South China Sea area; Mackenzie Delta, Canada), and
mica (biotite and muscovite; North Sea area), will increase the bulk specific weight
values; (5) age boundaries, unconformities, differential compaction, structural effects,
and position within the clastic basin may affect the normal compaction trendline (Fertl,
1977).

Shale factor

The shale factor can be successfully measured by the methylene blue test (Nevins
and Weintritt, 1967). This shale formation factor method (Gill, 1968; Mondshine,
1969; Gill and Weintritt, 1970) may be equated with the cation exchange capacity
(CEC) of solids carried by the drilling fluid out of the wellbore. This CEC value, in
turn, can be related to the water-holding capacity of drill cuttings or montmorillonite
content. The shale factor also appears to be a supplementary and useful indicator for
the detection of impermeable pressure seals (caprocks) on top of the overpressured
zones.

Volume of shale cuttings

During drilling, entry into overpressured environments is characterized by an increase
in the penetration rate, which gives rise to an increase in volume of cuttings over the
shale shaker.

Shape and size of shale cuttings

In pressure transition zones, the shape of drill cuttings is angular and sharp, rather
than rounded as found in normal, hydrostatic pressure environments. Furthermore,
cuttings from high-pressure formations are unusually large and splintery in appearance.