Tertiary structures and hydrocarbon entrapment in the Weald Basin of southern England

Abstract

The Weald Basin of southeast England was formed by rapid subsidence associated with thermal relaxation following early Mesozoic extensional block faulting. The basin appears initially to have taken the form of an easterly extension of the Wessex Basin but became the major depoccntre during the Upper Jurassic and Lower Cretaceous, with associated active faulting. These movements appear to have ceased prior to Albian times and a full Upper Cretaceous cover is believed to have bccn deposited in a gentle downwarp which extended far beyond the confines of the Weald and Wessex Basins. Major inversion of the Weald Basin took place in the Tertiary, with both gentlc regional uplift, which in the eastern part of the basin is estimated to have cxceedcd 5000 fcct (1525 metres), and intense local uplift along pre-existing zones of weakness, which Icd to the formation of compressional features such as tight folds and reverse faults. Zones of Tertiary deformation appear to have been strongly influenced by underlying, particularly Hercynian, structural trends.Lower Jurassic source rocks reached maturity in the early Cretaceous and initial migration occurred at this time, often over long distances, into traps closed by prc-Aptian faults. Tertiary tilting and uplift led to the breaching of many of these pre-existing traps and the formation of large folded closures. A second phase of hydrocarbon migration, particularly of gas, took place at this time, with significant vertical migration along fault zones. Major reservoirs located to date occur in Middle Jurassic carbonates and Upper Jurassic sandstones, but deep burial in the basin has caused considerable destruction of primary reservoir characteristics: changes in the temperature and pressure regimes and the mobilization of fluids within the basin resulting from the Tertiary uplift caused further diagenetic changes, particularly in the carbonate reserwfirs.Exploration of the Weald Basin remains at a very early stage, with a low drilling density to date. The more recent drilling has focussed on earlier structures, but traps formed or modified during the Tertiary movements represent important exploration objectives, although general deterioration in reservoir quality towards the centre and cast of the basin makes large fold closures in these areas less attractive. o~ I-Z W m • ..I m "lr'."--q O ,.d ,d at University of Tokyo on June 1, 2015 http://sp.lyellcollection.org/ Downloaded from HYDROCARBON ENTRAPMENT IN THE WEALD BASIN 373 restricted to the areas of thick Triassic and Lower Jurassic rocks underlying Dorset, Somerset and parts of Wiltshire). It is bounded to the north and east by the London-Brabant Massif and is separated from the Central Channel Basin and the Paris Basin by a regional arch, the Portsdown-Paris Plage ridge. The majority of outcrop within the basin area is of Lower Cretaceous age, but Upper Cretaceous Chalk outcrops around the margins and there are extensive areas of Tertiary outcrop in the London Basin, to the north; and the Hampshire Basin, to the South. Fig...

UK data and analysis for shale gas prospectivity

“…Liassic shales form the main source rocks for the Wessex and Weald oilfields (Ebukanson & Kinghorn 1986). These shales are mature for gas generation and small gas accumulations (dubbed 'enigmatic' by Butler & Pullan 1990) occur on the margins of the Weald Basin and associated with oil at Wytch Farm in the Wessex Basin (Fig. 1).…”

Cenozoic uplift of Variscan Massifs in the Alpine foreland: Timing and controlling mechanisms

“…This motion, which was initiated already during the late Eocene, was compensated by inversion of Mesozoic extensional basins in the Channel-Western Approaches and Celtic Sea areas, by up warping of the Weald-Artois axis, resulting in intermittent opening and closure of the Dover Strait, and by deformation of the Armorican Massif (Figs. 6 and 8;Ziegler, 1987Ziegler, , 1990Butler and Pullan, 1990;van Vliet-Lanoe et al, 1998a,b;Mansy et al, 2003;Gibbard and Lewin, 2003). However, as the main inversion phases of these basins are not fully synchronous with the main subsidence phases of ECRIS, far-field Alpine collision-related stresses presumably played a contributing role (Ziegler, 1990;.…”

The tectonic and stratigraphic framework of the United Kingdom's oil and gas fields

“…Creation of the ocean helped initiate intraplate compression, which together with the far-field compressional effects of Alpine collision led to the tectonic inversion of former sedimentary basins across N W Europe (e.g the Wessex Basin, Fig. 15; Colter & Harvard 1981;Underhill & Paterson 1998;Underhill & Stoneley 1998), the Weald Basin (Butler & Pullan 1990) and in the Southern North Sea (e.g. Glennie & Boegner 1981;Van Hoorn 1987;Ziegler 1987;Badley et al 1989).…”

“…In both the Weald and the Wessex Basins the main charge was provided by the Lower Jurassic Lias Group source rocks rather than the Kimmeridge Clay Formation. Modified after Butler & Pullan (1990). oil has been produced from a structurally-elevated horst block in the Claymore Field, which contains a Namurian, coal-bearing fluvial reservoir succession beneath the main productive Jurassic and Cretaceous horizons .…”