Structural evolution of shale diapirs from reactive rise to mud volcanism:3D seismic data from the Baram delta, offshore Brunei Darussalam

Citation
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
Citations number
39
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF THE GEOLOGICAL SOCIETY
ISSN journal
00167649 → ACNP
Volume
156
Year of publication
1999
Part
3
Pages
633 - 650
Database
ISI
SICI code
0016-7649(199905)156:<633:SEOSDF>2.0.ZU;2-7
Abstract
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.