Page 218 - Origin and Prediction of Abnormal Formation Pressures

TECTONICS AND OVERPRESSURED FORMATIONS 193
Basinward

B \\

D
E \ "
F g

E
F

K~:\~ Prograding sand series
Fig. 8-2. Schematic section showing stratigraphic rise of overpressures in relation to prograding sedimenta-
tion modified by growth faults. (Modified after Harkins and Baugher, 1969, in Fertl, 1976, fig. 1.17, p. 24.
Courtesy of Society of Petroleum Engineers of A.I.M.E.)

The role of growth faults in the structural deformation of geo-pressured sediments has
also been described by Ocamb (1961) and Thorsen (1963).
The Oligocene and Miocene sediments in southwestern Louisiana consist of three
facies: (1) continental and deltaic facies consisting of massive sands; (2) neritic facies
composed of alternating sands and shales; and (3) shale facies consisting of argillaceous
sediments deposited on the outer shelf and slope. Shallow water and continental
sediments overlap the marine sequence deposited earlier in deeper water (Dickey et
al., 1968). Abnormally high pressures are first encountered in the neritic facies directly
beneath the base of the more massive and continuous deltaic sands. Harkins and
Baugher (1969) stated that in order for abnormally high pressures to develop, the shales
usually must be over 200 ft in thickness. The intertonguing sand-shale facies forms
down-slope from the deltaic facies and, therefore, as a prograding sequence, tends
to rise stratigraphically in a basinward direction (Harkins and Baugher, 1969). The
stratigraphic units thicken seaward.
The geologic structure associated with this sedimentation pattern is dominated
by growth faults lying roughly parallel to the coast between salt domes (Fig. 8-3).
Embayments are areas where salt domes are scarce and growth faulting has caused some
stratigraphic units to be abnormally thick. Abnormal pressures are usually found at
depths of 10,000-11,000 ft. Dickey et al. (1968) pointed out that the stratigraphic units
are thicker on the downthrown side of the growth faults than they are on the upthrown
side (Fig. 8-4). Their explanation for this thickening of the sediments is that movement
along the fault plane was continuous during sedimentation. The fault planes cut the
seafloor while sediments were being swept over it, so that the downthrown block was
covered with a thicker layer of sediment. As shown in Fig. 8-4, grabens also commonly
occur. Abnormal pressures are associated with this structure-facies relationship and rise