This paper was prepared for the Abnormal Subsurface Pressure Symposium of the Society of Petroleum Engineers of AIME, to be held in Baton Route, La., May 15–16, 1972. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made.

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Abstract

Following the general discussion of The Worldwide Occurrence of Abnormal Pressures Part I, Part II discusses some specific type Part I, Part II discusses some specific type examples in the United States, the North Sea, Africa and South East Asia.

Introduction

The following examples are selected as the general type example for the area under discussion. In each case the general trend of the pore pressure profile is plotted versus depth. pore pressure profile is plotted versus depth. Naturally, since each area is covered in a very general manner the type example is only a general example. When drilling, variations in the depositional pattern and in the position in a basin may cause an entirely different pressure profile. So these profiles are not to be profile. So these profiles are not to be considered as a drilling guide but rather as a general description of the conditions that have already been found in these areas.

In all of the examples given the general mechanism is compression of the shales with the formation of a cap structure. As a guide, the cap structure and the general lithology of the area is indicated in the text.

TYPICAL PRESSURE PROFILES CALIFORNIA - SANTA BARBARA CHANNEL (Figure 1)

The Santa Barbara Channel of California is a deep trench, heavily faulted, that has received rapid deposition from the Coast Range mountains to the east. The normal sequence in the Santa Barbara Channel is sands and shales with sands predominating. Some deep drilling indicates that at 10,000' to 14,000' a massive shale forms a seal on a pressured section. While it has not been extensively penetrated, seismic records indicate that the pressure below the seal should rise over several thousand feet to an equivalent overburden value of 17 ppg. The section, as noted, is badly faulted, ppg. The section, as noted, is badly faulted, and in some cases fault blocks cause pressures to appear several thousands feet higher or lower than expected.

CALIFORNIA - CALUSA COUNTY (Figure 2)

In the typical profile, the section is a sand and shale sequence with a change to a massive limey shale at-about 4,000'. Pressures increase: to a pore pressure equivalent of about 121/2 ppg at 8,000'. At that point a limey sequence in the shale causes an abrupt increase in pore pressure to a probable overburden gradient at about 9,000' to 10,000'.

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