P. Van Rensbergen et al., Structural evolution of shale diapirs from reactive rise to mud volcanism:3D seismic data from the Baram delta, offshore Brunei Darussalam, J GEOL SOC, 156, 1999, pp. 633-650
Two areas of shale diapirism in a deltaic setting were examined using 3D se
ismic data from offshore Brunei to determine the origin, evolution and geom
etry of shale diapirs. The study areas cover parts of the Egret and Ampa fi
elds within the Baram Delta province. The Ampa diapir consists of a small b
ulge of the mobile Setap shale around which lies a zone of chaotically disr
upted country-rock reflections. At the crest of this chaotic zone, vertical
, cylindrical shale pipes arise that end in a wider head. The diapir heads
wear halos of high amplitude reflections, probably due to the presence of m
ethane gas. The Egret data show the same features, except that the bulge of
the source layer is not detected due to low reflectivity and chaotic refle
ctions at the base of the overburden. The bulging of the Setap shale on the
Ampa data is interpreted as modified reactive diapirism in response to dif
ferential loading. The initial reactive diapir shape was modified by latera
l or oblique upward migration of fluidized shales into the hanging wall of
the fault controlling the reactive diapir. This probably occurred as a netw
ork of dykes and sills. The presence of some preserved bedding is indicated
by weak but coherent reflectivity from the area initially defined as chaot
ic shale.
The spreading of overpressure and the injection of shale into the overburde
n is thought to be a late phenomenon, possibly Late Pliocene or Early Pleis
tocene and the mobility of the shale is attributed to overpressure increase
due to gas generation within the Setap shale and maybe the lower part of t
he overburden. Gas migration, perhaps ahead of the diapir intrusion, probab
ly facilitated the mud injection. The Ampa and Egret diapirs formed in two
diapiric phases, an early reactive diapirism and a later active diapirism.
But the phases have little in common; they are seperated by about 9 million
years of inactivity and differ in process and probably differ in degree of
overpressure, in overpressure generating mechanism, and in properties of t
he diapiric mud.