Page 220 - Origin and Prediction of Abnormal Formation Pressures

TECTONICS AND OVERPRESSURED FORMATIONS 195
Vermilion Area
A A'
North South
BIk. 14 BIk. 46 BIk. 76 BIk. 96 BIk. 115
I I I I I
- 4000'

- 8000'
Vermilion Area
I 9 1000'
oo,
- 10,000'

12,000'

14 ooo' ~//2;
~ /~normally-High Pressure
Fig. 8-4. Schematic section illustrating stratigraphic rise of abnormally high formation pressures (AHFP)
across growth faults in the Vermilion area of southern Louisiana. (Modified after Harkins and Baugher,
1969, fig. 4, p. 963, in Rieke and Chilingarian, 1974, fig. 172, p. 322. Courtesy of Society of Petroleum
Engineers of A.I.M.E.)

Contemporaneous faults are those whose movements occur during sedimentation
(Hardin and Hardin, 1961). There can be several causes for regional contemporaneous
faults (Carver, 1968), the most significant of which are basement tectonics, deep salt or
shale movement, slump across flexures, slump at the shelf edge, differential compaction,
response to crustal loading, or a combination of these factors. Bishop (1973) studied
this type of faults in North Louisiana and South Arkansas and concluded that Jurassic
contemporaneous faults generally parallel regional structural and depositional strike
and tend to be slightly younger basinward (south). Most of these faults appear to be
downthrown toward the basin. Beds on downthrown sides are greatly thickened and
throws increase with depth. Correlation of individual units is very difficult to impossible.
Because the downthrown blocks are tilted, sediments are thickest adjacent to the fault.
The fault planes appear to be curved, and although having a high angle (60-70 ~ near the
top, they may flatten with depth. As a result of this flattening, together with towage of
underlying Jurassic salt away from downthrown blocks, faults are not known to extend
below the salt. "They do not cut beds younger than Jurassic and die out upward in a
conformable section" (Bishop, 1973).
Bruce (1973) summarized the mechanism for development of regional contempora-
neous faults corresponding to overpressured shales and related sediment deformation
as follows: "Regional contemporaneous faults of the Texas coastal area are formed
on the seaward flanks of deeply buried linear shale masses characterized by low bulk
density and high fluid pressure." According to him, the seismic data show that these