Page 326 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 11

ABNORMALLY LOW FORMATION PRESSURES

V.A. SEREBRYAKOV, G.V. CHILINGAR and J.O. ROBERTSON JR.

INTRODUCTION

The existence of underpressured fluid chambers has enormous significance to oil and
gas exploration and production in the world. Such fluid compartments are determinative
elements for undetected hydrocarbon traps (i.e., so-called subtle traps). Traditionally,
most hydrocarbon production in the world has been from conventional structural and
stratigraphic traps. Traps of a newly identified type, the underpressured hydrocarbon
traps, may evolve from conventional traps as a result of changes in temperature and
pressure.
This kind of underpressured traps can be created by considerable overburden removal
and local temperature change due to uplift and erosion giving rise to decreased pore
pressure. Chapter 11 is an outline of a theoretical basis for an investigation of the
validity of this concept. Examples of these kinds of traps can be found in the Denver
and Oklahoma basins (Russell, 1972), the Alberta Basin (Hitchon, 1969), and the
Volga-Ural and Middle Kura basins (Dobrynin and Serebryakov, 1989).
It is important to construct a model for potential hydrocarbon sources in geological
sections with abnormally low pressure. This model can be constructed using the change
of rock temperature in local zones with significant uplift and erosion. For estimating
the thicknesses of eroded deposits the authors used the method of compression curves,
that indicate the presence of unconformities and the thickness of eroded deposits not
only near the surface, but also deep in the geologic section. The modeling of subsurface
underpressured zones may indicate a technique for making underpressured compartment
traps viable exploration targets, because exploration strategies can be made significantly
more effective if the mechanism of their formation is well understood.
Subnormal pressures were discussed in three very important books: Gurevich et
al. (1987), Dobrynin and Serebryakov (1989) and Dobrynin and Kuznetsov (1993).
Gurevich et al. (1987) briefly analyzed all possible mechanisms of pressure subnormal-
ity, including permafrost degradation, fast leakage of gas from gas pools, decrease in
temperature, formation of gas hydrates, expansion of rocks owing to the reduction in
the overburden caused by erosion and disappearance of ice cover, etc. These authors
believed that temperature decrease is possibly the most common cause of subnormally
low pressures and occurs in formations overlain by permafrost. It does not include cases
of pressure subnormality caused by distribution of the piezometric head in deep layers
under the highest portions of the Earth's surface and by fluid withdrawal. Subnormal
pressure distributions in several regions were analyzed.
Dobrynin and Serebryakov (1989) believed that the major origin of subnormally low
pressures is associated with permafrost and temperature changes. Subnormal pressures