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

Two areas of shale diapirism in a deltaic setting were examined using 3D seismic data from offshore Brunei to determine the origin, evolution and geometry of shale diapirs. The study areas cover parts of the Egret and Ampa fields within the Baram Delta province. The Ampa diapir consists of a small bulge of the mobile Setap shale around which lies a zone of chaotically disrupted 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 methane 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 reflections 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 differential loading. The initial reactive diapir shape was modified by lateral or oblique upward migration of fluidized shales into the hanging wall of the fault controlling the reactive diapir. This probably occurred as a network of dykes and sills. The presence of some preserved bedding is indicated by weak but coherent reflectivity from the area initially defined as chaotic shale. The spreading of overpressure and the injection of shale into the overburden is thought to be a late phenomenon, possibly Late Pliocene or Early Pleistocene 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 the overburden. Gas migration, perhaps ahead of the diapir intrusion, probably 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 the diapiric mud.