Arran Field: a complex heterolithic reservoir on the margins of the Forties Fan System

Abstract

The Arran Field contains gas-condensate accumulated within the Paleocene Forties Sandstone Member of the Sele Formation. It is located along the margin of the medial Forties turbidite depositional system, on the eastern flank of the Central Graben's Eastern Trough. The field comprises a low-relief southern extension (Arran South) from a high-relief northern closure (Arran North) around a salt diapir. The eastern margin of the field represents the pinch-out of the Forties Sandstone Member against the Jaeren High. As part of the field development planning, a comprehensive re-evaluation of subsurface data was undertaken. A thorough understanding of the reservoir distribution and turbidite architectures was vital to ensure that the appropriate elements were captured within the reservoir model. This was achieved through a thorough integration and multidisciplinary interpretation of all available data including seismic, core, petrophysical and analogue data. These data indicate that the best quality Forties Sandstone Member reservoir consists of stacked, elongate, amalgamated and non-amalgamated fairway sandstone bodies. These thick-bedded and sand-dominated reservoir units pass laterally into, and are extensively interbedded with, linked debrites, heterolithic low-density turbidite lobe fringe deposits, slumps, and debris flows, along with hemipelagic and turbiditic shales. A seismic shale volume (Vshale), derived from inverted pre-SDM data, together with reflection seismic data, were used to identify and map intra-reservoir depositional lobe geometries. These show large-scale, lobe-like depositional bodies which migrated laterally over time and onlapped on to the Jaeren High to the east. Within these, smaller-scale elongated lobe bodies, generally derived from the NW, are interpreted from layer-parallel extractions of the seismic Vshale volume. Possible slump units were also identified, predominantly derived from the edges of the Arran North salt diapir, suggesting that the basin floor topography was mobile during deposition. The seismic Vshale volume was used to condition the static facies model, utilizing probability relationships between the seismic data and core facies at the wells, providing a soft linkage between the data and models developed. Core, analogue data and facies interpretations from the seismic data were utilized to ensure that appropriate reservoir body geometries and spatial relationships were maintained in the static model and allowed key reservoir heterogeneities to be captured. This integrated approach also supported analysis of reservoir uncertainties, with specific focus on the vertical and lateral reservoir connectivity within this lobe-dominated reservoir.

Hierarchical parameterization and compression-based object modelling of high net:gross but poorly amalgamated deep-water lobe deposits

“…Overall, the proportions and distribution of facies deduced from the compression-based object model (Figure 17c) are quite similar to those present in facies models of these kinds of sequences (e.g. Collins et al 2015, Jones et al 2015), but with one crucial distinction: in our model all finegrained units are represented explicitly in the model, rather than forming part of the facies definitions. Therefore, while it is possible to derive facies identifiers from our model as we have done in Figure 17c, it is impossible from a facies model to derive the detailed network of finegrained units that represent the main heterogeneities in this type of reservoir.…”

“…Examination of highly vertically exaggerated images are required to see the complex architectures of these models of hierarchical lobe deposits, but views without vertical exaggeration are needed to compare the models with outcrop photos. In Figure 16, a portion of the model is compared to a cliff section of the Annot sandstone lobe deposits at the same scale (Collins et al 2015). There is a close qualitative correspondence between the outcrop and the model.…”

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

“…Frigg: Heritier et al 1979); lateral fan pinch-out against basin-margin highs (e.g. Everest and Arran fields: O'Connor & Walker 1993;Collins et al 2015); compressional inversion TERTIARY DEEP-MARINE RESERVOIRS: INTRODUCTIONstructures, and fields where hydrodynamic flow has a significant impact on the trap (e.g. Ormen Lange: Grecula et al 2014).…”

“…In addition, local bathymetric effects appear to have modified the facies trends and architecture that would have been predicted from regional trends. Collins et al (2015) also describe a lobedominated Forties Sandstone Member reservoir located on the margin of the Eastern Trough of the Central Graben. The lobes are sand-rich, and were deposited in a basin-margin, fan-fringe setting with common scouring and localized channelization.…”

“…Stacking and connectivity in lobe-dominated reservoirs are not as well understood, as highlighted by the uncertainty range inherent in the architecture and connectivity of the lobe-dominated Fram and Arran reservoirs (Collins et al 2015;Jones et al 2015). Conceptual models are emerging with which to identify the nature of reservoir-quality variation and sand-body stacking in these settings (Prélat et al 2009;Grundvåg et al 2014), and their potential impact (Amy et al 2013), but further quantitative calibration from outcrops, particularly in relation to the impact of seafloor bathymetry (cf.…”