Page 33 - Origin and Prediction of Abnormal Formation Pressures

16 E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV

of 0.85 psi/ft (19.6 kPa/m) exceed the FPGs of most sandstone reservoirs. Thus, if
hydrocarbons are present initially, they probably would have escaped through fractures.
(For a detailed discussion see Law and Spencer, 1998.)
The writers, however, would like to point out that well testing is a sword of two edges
in overpressured formations. As soon as some fluids are produced, the effective pressure
increases to a critical limit and closes down the pores (irreversible compaction). Thus
many overpressured reservoirs have been condemned in the belief that the reservoir
cannot produce fluids (see Belonin et al., 2002).

SUMMARY

Abnormal subsurface formation pressures are encountered throughout the world and
are produced by many different causes that may be physical, chemical, or a combination
of the two. Many reasons for the formation of subsurface abnormal fluid pressures
have been postulated and discovered. There is some disagreement among engineers
and geologists regarding some of the mechanisms that have been proposed for the
origin of abnormal pressures; however, developments in drilling, seismic technology
and well-logging are resolving many of the disputes. This book presents analyses of the
theories for the creation and maintenance of abnormal fluid pressures in sedimentary
rock environments and their prediction.
Clark (1961) coined the term 'tectonic overpressure' during his discussion of tectonic
compression. Dickey et al. (1968) developed theories based on faulting. Rieke and
Chilingarian (1974), Magara (1975) and Plumley (1980) discussed compaction as a
'disequilibrium mechanism' causing abnormal fluid pressures. According to Gilreath
(1968) and Johnson and Bredeson (1971), diapirism of salt and shale was responsible
for the creation of some abnormal pressure environments. The influence of abnormal
formation temperature on the maintenance of abnormally high fluid pressures (especially
along the Gulf Coast of the United States) was discussed by several authors: Harkins and
Baugher (1969), Kharaka et al. (1977), Donaldson (1980), and others. Phase changes
of minerals during diagenesis and catagenesis was investigated by Powers (1967) and
Hanshaw and Bredehoeft (1968). Osmotic pressures were investigated by McKelvey
and Milne (1962), Hanshaw and Zen (1965), Swarbrick and Osborne (1998), and
others.
The two mechanisms of formation of overpressures which have been underestimated
in the past are (1) hydrocarbon (both liquid and gas) generation (e.g., Hunt et al., 1998),
and (2) upward gas migration along faults from lower to upper horizons, resulting in the
overpressures in the upper horizons (Khilyuk et al., 2000). (See Chapter 2.)
The chapters that follow are devoted to more detailed analyses of the origins of
abnormal subsurface pressures, their prediction and distribution, the effects of diagenetic
and catagenetic changes, and mathematical models.