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

Abstract

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.

Significance of Mud Volcanism

“…Resurgence of activity correlates with local earthquakes. Recently, two geophysical studies have shown MVism onshore [Morley et al, 1998] and offshore of Brunei [Van Rensbergen et al, 1999]. Regarding the first, multiphase intrusion of shale, indicated from dyke formation patterns, affected mid-Miocene anticlinal sediments [Morley et al, 1998].…”

“…More recently, subsequent erosion and uplift remobilized the shale bodies and gave way to MV formation in the Jerudong anticline [Morley et al, 1998]. In the Ampa and Egret areas offshore in the Baram delta, the situation is more complex, with an intrusive phase followed by MVism, which in turn got buried, before migrating gas and water may have helped a renewal of ascent and extrusion [Van Rensbergen et al, 1999]. High-resolution seismic data across the structures suggest an interfingering of shales (showing a chaotic texture with incorporated country rock) and the wall rock (well stratified sediment).…”

“…Such MVs got either eroded or buried, leaving an irregularity in the cylindrical dyke, which in the future may act as a trap for fluids (the latter possibly triggering the next eruptive phase). Dating of the stratigraphic position of the Setap Shale parent bed (early to middle Miocene) and of the MV extrusion (latest Pliocene) allows the duration of inactivity of the undercompacted sediment to be dated as 9 Myr [Van Rensbergen et al, 1999].…”

Birth of a mud volcano: East Java, 29 May 2006

“…The model proposed by Brown (1990), Van Rensbergen et al (1999), Davies and Stewart (2005), and Stewart and Davies (2006) is that hydrofractures can penetrate several kilometers of the crust and transport a fluid-sediment mix that erupts to form a pioneer volcano. Because in this case we know that the mud-water mix has been transported ~2 km through the overburden, through new or reactivated fractures, the Lusi eruption supports the models proposed by these authors.…”

Seismic Interpretation and Classification of Mud Volcanoes of the South Caspian Basin, Offshore Azerbaijan

“…Luo et al (1994) conclude that most of the pore-fluid generating mechanisms (illite to smectite transforma tion, aquathermal expansion, kerogen maturation) contribute little to overpressure increase. Kerogen to gas or oil to gas transition produce significant volume changes (Rensbergen et al, 1999). In addition, Forbes et al (1992) and Sandal (1996) report that even thin units of a rock can form effective permeability barriers.…”