Page 223 - Origin and Prediction of Abnormal Formation Pressures

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

subsidence and tilting was not dependent on differential compaction and differential
loading, as described for faults formed during regressive and still-stand phases of
deposition. Instead it was controlled by forces below or outside the area of deposition.
These forces may owe their origin to either salt movement or basement tectonics." Some
manifestations of contemporaneous faulting can be explained when seafloor inclination
and basinward formational dips are compared with rates of deposition. Gravity-slide
faults are the most significant of these. "Many of them become bedding-plane types at
depth" (Bruce, 1973).
According to Hospers (1971), the Niger Delta area in Nigeria, Africa, has a
clay-shale base of considerable thickness. The subsurface structure of this delta is
characterized by typical growth faults with associated rollover structures, which are
interpreted as being caused by gravity. Overpressures are encountered in the delta area.
Roberts (1972) developed several tectonic concepts based upon the pore fluid
pressure hypothesis of Hubbert and Rubey (1959). According to him, overthrusts cannot
develop unless the thrust sheet is underlain by a weaker layer or unless abnormal
pore fluid pressures are restricted to this layer. The conditions of failure implied by
the Hubbert-Rubey hypothesis indicate that both requirements are met if the ready
ingress or egress of pore fluid is prevented during impending shear failure. Under
these circumstances, sediments capable of further compaction undergo an increase in
formation pressure so that the effective value of ~ (i.e., pore pressure: total overburden
pressure ratio) at failure is unity. This type of behavior is typical of shale horizons,
which act as the locus of overthrust faults. Dilation hardening affects the intervening
sandstone or limestone horizons, which in turn form the overthrust sheets. Once shear
failure is initiated, movement is essentially frictionless as long as excess pore pressures
(overpressures) are maintained (Roberts, 1972).

SHALE DIAPIRISM (MUD LUMPS, MUD VOLCANOES)
Fertl (1976) noted that conditions necessary for diapirism are a density inversion in-
cluding a material of low shear strength. This may be produced when a low-permeability
formation is rapidly loaded and depocenters are rapidly shifted (Gretener, 1969). Such
conditions are found in delta areas of major rivers, such as the Mississippi, Niger, Nile,
Danube, and Amazon. Mud lumps (small shale diapirs) are formed by shales having
high water content (high porosity) and low shear strength that have been rapidly loaded
by sands (Morgan, 1952; Murray, 1961).
On a small scale, diapirism produces mud lumps, whereas on a large scale mud
volcanoes are formed. Mud volcanoes represent an overpressure phenomenon caused
by an intrusion at depth of mud and/or a mixture of mud and solid rocks (Fig. 8-7).
It is necessary to distinguish material that has been extruded over the ground surface
from material that has been intruded diapirically (now exposed as a result of erosion
of the older enveloping rock). Suter (1960) has called both "diapiric rocks" and
assigned the term "sedimentary volcanism", as did Kugler (1933, 1938), to emphasize
the non-igneous nature of the phenomenon. One must also distinguish between (1)
accretionary cones of gently extruded mud accompanied by gas and water, and (2) gas