Tom McKie

Stratigraphic and structural compartmentalization of dryland fluvial reservoirs: Triassic Heron Cluster, Central North Sea

Abstract This paper describes the nature and relative significance of stratigraphic and structural compartmentalization in dryland fluvial reservoirs using data drawn from the Heron Cluster (Heron, Egret and Skua) oil fields in the UK Central North Sea. The Triassic Skagerrak Formation reservoir in these fields was deposited in a variety of dryland terminal fluvial settings, ranging from relatively arid terminal splay and playa to more vegetated, channel-confined systems with associated floodplain and palustrine facies. Laterally extensive floodbasin shales punctuate this terminal fluvial architecture. Static and dynamic data indicate that these fields are compartmentalized: geochemical data indicate significant fluid variations both between wells and vertically within individual wells; material balance calculations suggest production from restricted connected volumes, locally from a subset of the range of oils present; and re-perforation across significant shale boundaries access undepleted reservoir with different fluid compositions. Lateral variations could be ascribed to prominent structuration within these fields, but in general these high net:gross reservoirs do not have a viable fault seal mechanism. Early (syn-halokinetic) grounding of Triassic ‘pods’ between salt swells during salt withdrawal has resulted in zones of intense faulting along the zone of contact of the pod and the underlying basement, and also on the flanks of pods as the margins collapsed under further salt withdrawal. This deformation occurred under relatively shallow burial depths and is largely expressed by disaggregation zones and phyllosilicate fault rocks. Fault property averaging algorithms (e.g. shale gouge ratio), indicate that the sands should communicate across the juxtapositions, implying that the fluids and pressures should equilibrate between reservoir sands. However, the stratigraphic differences across major shales in both fluid geochemistry and pressure caused by draw-down are preserved despite the presence of these faults. The preservation of stratigraphic compartments indicates that for these faults the deformation mechanism was probably dominated by clay smear, in which the shale-prone sequence was smeared down the fault planes without losing its coherence. This style of stratigraphic compartmentalization occurs across several shale-prone intervals that are correlatable across the region. In some cases these mark the boundaries to major changes in fluvial depositional character, provenance and floodplain drainage, suggesting an extrinsic control that led to shale packages defining consistent barriers in all the fields. Other shale barriers do not show major changes in depositional character and, although correlatable, appear to be the product of semi-regional advance and retreat of the fluvial systems, possibly combined with nodal avulsion. In contrast to reservoirs deposited by large exorheic rivers, the terminal nature of these dryland fluvial systems appears to have resulted in the repeated interfingering of fluvial and floodbasin facies over a scale of many tens of kilometres. As a result such terminal fluvial reservoirs are prone to stratigraphic compartmentalization. However, thinner shales are prone to breaching by fluvial erosion and as a result not all correlatable shale events form barriers and only a subset will compartmentalize. Mitigation against this compartmentalization requires a development strategy where well trajectory and perforation maximizes stratigraphic exposure.

The application of sequence stratigraphy to the understanding of Late Jurassic turbidite plays in the Central North Sea, UKCS

Abstract The identification and correlation of genetic sequences has significantly enhanced our understanding of the Late Jurassic stratigraphy in the Central North Sea. The integration of genetic sequences with an appreciation of the basinal mudrock stratigraphy, and the identification of depositional sequence boundaries provides a powerful tool to understand and potentially predict the distribution of subtle stratigraphic turbidite plays. The largely syn-rift Late Oxfordian-Kimmeridgian was deposited during a low frequency, second-order sea-level rise, culminating in the eudoxus condensed section. Deposition was dominated by shallow marine shelfal sandstones and offshore mudrocks. Sandy turbidites are rare. Whilst the requisite depositional slopes and deep water areas are likely to have existed, the effects of high frequency relative sea-level falls are likely to have been suppressed. As the second-order sea-level rise accelerated towards the eudoxus condensed section, the supply of sand into the basin was reduced such that the potential for turbidite deposits was minimal. The largely post-rift Kimmeridgian-Volgian section is dominated by basinal mudrocks. However, regionally correlatable ‘hot’ and ‘cool’ units provide information on sediment flux into the basin which may be linked to relative sea-level changes on the basin margin, with cool units interpreted to be the product of enhanced depositional rates during lowstand. Turbidites commonly occur within the ‘cool’ mudrock units. A Middle Volgian ‘cool’ unit is likely to represent a second-order sea-level fall, within which are composite turbidite packages which are considered to be the product of higher frequency lowstands (lowstand sequence set).

Sedimentological evolution of Sele Formation deep-marine depositional systems of the Central North Sea

Abstract The Paleocene–Eocene-aged Sele Formation is developed across the basinal region of the Central North Sea. The section comprises a number of deep-marine fan systems that expanded and contracted across the basin floor in response to relative sea-level changes on the basin margin and fluctuating sediment yield off the Scottish landmass modulated by climate and hinterland uplift. Persistent sediment entry points to the basin resulted in the development of discrete axial and transverse fan fairways with a geometry dictated by an irregular bathymetry sculpted by differential compaction across Mesozoic faults, halokinesis and antecedent fan systems. A high-resolution biostratigraphic framework has allowed the evolution of fan-dispersal systems in response to these effects to be tracked across the basin within four genetic sequences. The proximal parts of the fans comprised channel complexes of low sinuosity, high lateral offset, and low aggradation. The development of these systems in a bathymetrically confined corridor of the Central Graben (c. 65 km wide), combined with high sediment supply, resulted in the eventual burial of any underlying relief. The behaviour of sand-rich reservoirs in this region is dominated by the permeability contrast between high-quality channel fairways and more heterolithic overbank regions, with the potential for early water breakthrough and aquifer coning in the channel fairways, and unswept volumes in overbank locations. Compartmentalization of compensationally stacked channel bodies occurs locally, with stratigraphic trapping caused by lateral channel pinch-outs, channel-base debrites, mud-rich drapes and abandonment fines. Towards the southern part of Quadrant 22, approximately 150 km down-palaeoflow, the systems became less confined and in this region are dominated by channel–lobe complexes, which continued to interact with an irregular bathymetry controlled by antecedent fans, mass-transport complexes and halokinesis in the form of rising salt diapirs. Reservoirs in this region are inherently stratigraphically compartmentalized by their heterolithic lithology and compensational stacking of lobes, and further complicated by structuration and instability induced by the diapiric or basement structures needed to generate a trapping structure in these settings.

Tertiary deep-marine reservoirs of the North Sea region: an introduction

T. MCKIE1*, P. T. S. ROSE2, A. J. HARTLEY3, D. W. JONES1 & T. L. ARMSTRONG4 Shell UK Ltd, Altens Farm Road, Nigg, Aberdeen AB12 3FY, UK Apache North Sea, Caledonia House, Prime Four Business Park, Kingswells Causeway, Aberdeen AB15 8PU, UK Department of Geology & Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK Talisman Sinopec Energy UK Ltd, Talisman House, 163 Holburn Street, Aberdeen AB10 6BZ, UK