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 semiregional 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 floodb...

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

“…4), and an average grain size of 0.131 mm. Compositionally, the Judy field samples show a similar narrow range from arkosic to lithic-arkosic arenites (McKie et al 2010;Nguyen et al 2013).…”

“…5b). This may in part reflect the less-well-developed chlorite grain coatings and occurrence in the Judy compared with the Heron fields (McKie et al 2010;Nguyen et al 2013). …”

Exceptional reservoir quality in HPHT reservoir settings: Examples from the Skagerrak Formation of the Heron Cluster, North Sea, UK

“…Carbonate cements are very localised throughout the Skagerrak Formation and sourced by syn-depositional calcitic palaeosols in very fine grained floodplain deposits or by calcrete and dolocrete rip-up clast at channel bases (McKie et al, 2010). The scarce carbonate cements in the three data sets are mainly present as deformed carbonate-clay rip-up clasts (Fig.…”

“…The occurrence and importance of grain coating chlorite in the Skagerrak sandstone reservoirs has been reported from the J-ridge area (Nguyen et al, 2013;Grant et al, 2014) and the Heron Cluster (McKie et al, 2010;Taylor et al, 2015). However, the mixed chloriteillite coatings of the Egret samples show a tendency to bridge between grain coatings and thereby fill the pore space and block pore throats.…”

A classification scheme for fluvial–aeolian system interaction in desert-margin settings

“…In cases where interdune corridors terminate in closed depressions, they typically host ponded flood waters, the suspended-load deposits of which commonly form mudstone or salt layers that are relatively resistant to erosion due to their cohesive nature (Loope et al, 1995;Bloomfield et al, 2006McKie et al, 2010Höyng et al, 2014;Figure 15b). This has an important impact on sediment preservation potential.…”