Sequence stratigraphy and sedimentary history of the Humber Group (Late Jurassic-Ryazanian) in the Outer Moray Firth (UKCS, North Sea)

Abstract

The Humber Group in the Outer Moray Firth is an onlapping sedimentary succession of Late Oxfordian to Ryazanian age, recording a trend of continuously rising relative sea-level. Nineteen subordinate transgressive-regressive cycles ('sequences') have been identified in the numerous wells in the area. The identification of these sedimentary units throughout the study area is indispensable for the interpretation of its sedimentary history, and hence for the ability to predict the possibility of subtle stratigraphic traps. Transgression began during the Late Oxfordian in the North Buchan Graben and culminated in the Early Ryazanian, when only parts of the Halibut Horst and the Fladen Ground Spur remained exposed. Mapping of the onlap reveals the drowning from the south of a significant topographic relief, which was further enhanced by syndepositional rifting. Three structural grains affected sedimentation: (1) E-W (possibly a repeated reactivation of pre-Late Jurassic faults, e.g. North Halibut Graben: pre-Late Oxfordian to Middle Volgian); (2) NE-SW (e.g. Theta Graben: Late Oxfordian to Early Kimmeridgian; Highlander-Piper Ridge: Kimmeridgian to Mid-Volgian); (3) NW-SE (e.g. Witch Ground Graben: Early Volgian to Ryazanian). Sedimentation was initially dominated by coastal plain and shoreface facies of marine-dominated deltas and estuaries (Piper Formation s.l.), sourced mainly from the Fladen Ground Spur. From deposition of the upper Piper Formation onwards, intra-rift horsts, particularly the Halibut Horst, were also important sand sources. Deltaic systems back-stepped toward the basin flanks when accommodation space generation began to exceed sediment supply. Increased rifting in Early Volgian times triggered large-scale mass flow deposition from the Halibut Horst and from the East Piper High. Back-stepping shoreface systems continued their progressive onlap of the basin margins. Lowstand systems tracts in the individual sequences were muted by the overpowering long-term transgression, and significant mass flow deposits were not primarily controlled by the collapse of constructional highstand shelf margins. Instead, long-term tectonic uplift of specific areas sourced basinmargin submarine fan systems, typically confined to half graben on the flanks of the basin (e.g. Claymore sands), and basin-floor submarine channel systems in the core of the Witch Ground Graben (e.g. Galley sands). The potential for subtle stratigraphic traps is greatest in these turbiditic sandbodies where their boundaries are abrupt, as a result of channeling in self-constructed levees, or confinement by fault-induced seafloor topography.

Distribution of discontinuous mudstone beds within wave-dominated shallow-marine deposits: Star Point Sandstone and Blackhawk Formation, Eastern Utah

“…Examples include the Brent and Humber Group deposits of the North Sea (e.g., Richards and Brown, 1986;Howell et al, 1996;Kadolsky et al, 1999;Husmo et al, 2003); the Wilcox Formation onshore Gulf of Mexico (e.g., Galloway et al, 2000); and numerous Tertiary delta systems of Southeast Asia (e.g., Hodgetts et al, 2001;Ainsworth et al, 2005). These reservoirs typically have simpler depositional architectures and are therefore easier to produce than reservoirs within fluvial and tide-dominated shoreline systems (Ainsworth et al, 2005;Martinius et al, 2005;Howell et al, 2008aHowell et al, , 2008b.…”

A Stratigraphic Review of the Late Callovian to Oxfordian Interval, Fisher Bank Basin Area (Uk Sector, Central North Sea)

“…Well 22/5b-A10 has what appears to be the most complete section across the J40/J50 (Rattey and Hayward, 1993) sequence boundary in the present study, with all the Early to Late Oxfordian palyzones between UJP1 and UJP5 represented. Although only cuttings samples were available, this well clearly shows that the base J50 transgression is at least as old as earliest Late Oxfordian Palyzone UJP4, a similar age to the onset of Humber Group deposition in the Outer Moray Firth as described by Duxbury et al (1999) and Kadolsky et al (1999).…”

“…), which is of a Palyzone UJP5 age. The Piper I-Shale represents the UJ5 (Redcliffense) MFS of and Kadolsky et al (1999).…”

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

“…Although fraught with difficulty (see Hesketh & Underhill 2002 for discussion), recent advances in biostratigraphic control in the Outer Moray Firth Late Jurassic has afforded a greater level of correlation and the construction of accurate palaeogeographic and play fairway maps (Davies et al 1996;Kadolsky et al 1999). These studies help to demonstrate that each of the major sandstone bodies within the Piper Formation suggests that they represent individual progradational cycles that probably represent short-lived regressive pulses within an overall transgressive phase (e.g.…”