Page 217 - Origin and Prediction of Abnormal Formation Pressures

'oi,i A
192 G.V. CHILINGAR, W. FERTL, H. RIEKE AND J.O. ROBERTSON JR.

es ma p essured '

High pressure formation

B

_~ Main sand series ..... ,, ,, ,, , ,, ,,, ,, ,~
oo, o, 0 os,o,,c

..... P essure

......... 'ouy-high pressure
Fig. 8-1. (A,B) Types of fault seals necessary to preserve abnormal-pressure environments. (Modified after
Dickinson, 1953, in Fertl, 1976, fig. 1.16, p. 24.) Sand to sand contact across a fault results in normal
pressures, whereas isolated sands (i.e., sand to shale contact across a fault) are overpressured. (Courtesy of
the American Association of Petroleum Geologists.)

Dickey et al. (1968) offered an explanation of how abnormally high formation
pressures originated in the Gulf Coast sediments. According to Dickey et al., in
southwestern Louisiana, the pattern of abnormally high pressure zones appears to be
related to the patterns of faulting contemporaneous with sedimentation and compaction.
The process creating these faults (growth faults) prevents the expulsion of water from
the pores of argillaceous sediments during compaction and diagenesis. The abnormally
high pore pressures might have facilitated sliding and slumping of the sediments at the
edge of continental shelf. Dickey et al. (1968) noted that growth faults have many of the
characteristics associated with slump-type landslides and that they may indeed be the
result of old slides that have ceased their activity and were later buried by sedimentation.