Page 216 - Origin and Prediction of Abnormal Formation Pressures

191

Chapter 8

TECTONICS AND OVERPRESSURED FORMATIONS

G.V. CHILINGAR, W. FERTL, H. RIEKE and J.O. ROBERTSON JR.

INTRODUCTION

Abnormally high pore fluid pressures may result from local and regional tectonics.
The movement of the Earth's crustal plates, faulting, folding, lateral sliding and
slipping, squeezing caused by downdropping of fault blocks, diapiric salt and/or shale
movements, earthquakes, etc. can affect formation pore pressures.
Due to the movement of sedimentary rocks after lithification, changes can occur
in the skeletal rock structure and interstitial fluids. A fault may vertically displace a
fluid-bearing layer and either create new conduits for the migration of fluids giving
rise to pressure changes or create up-dip barriers giving rise to isolation of fluids and
preservation of the original pressure at the time of tectonic movement. Sahay (1994)
noted that this barrier may be created by either the fault itself or by bringing the
impermeable layer in contact with the permeable layer up-dip.
In strongly folded formations there is a reduction in pore volume (due to compres-
sion) along with an attenuation of competent layers (in limbs) and accumulation in the
cores of anticlinal folds. An additional rupturing of layers of formations also takes place
due to squeezing of and stretching of the skeletal rock structure beyond its elastic limit.
Thus, there is a development of high fluid pressure in isolated blocks.
According to Sahay (1999), in the Surnimastagarh anticline of Jammu, India (Siwalik
Belt), overpressures up to 2.38 times hydrostatic have been encountered during drilling.
In the outer folded belt of the Assam-Arakan system, pressures 1.8 to 2 times the
hydrostatic have been encountered while drilling at the Masimpur area of Assam. In
the Balh well of Punjab (Himalayan Foothills), the formation pressure encountered was
2.14 times the hydrostatic.
For details on abnormal pressures in India apparently caused by tectonic activity, one
can consult Sahay and Fertl (1988).

FAULTING AS A CAUSE OF OVERPRESSURED FORMATIONS

Dickinson (1953), Murray (1961), Carver (1968), Classen (1968), Dickey et al.
(1968), Meyers (1968), Harkins and Baugher (1969), Jones (1969), and Fowler (1970)
stressed the importance of various types of faults in developing the abnormal-pressure
environments. Fig. 8-1 illustrates abnormal pressures as related to faults, whereas
Fig. 8-2 is a schematic diagram showing the stratigraphic rise of abnormal pressure as
related to prograding sedimentation modified by growth fault.