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Reservoir Compartmentalization -the segregation of a petroleum accumulation into a number of individual fluid/pressure compartments -occurs when flow is prevented across 'sealed' boundaries in the reservoir. These boundaries are caused by a variety of geological and fluid dynamic factors, but there are two basic types: 'static seals' that are completely sealed and capable of withholding (trapping) petroleum columns over geological time; and 'dynamic seals' that are low to very low permeability flow baffles that reduce petroleum crossflow to infinitesimally slow rates. The latter allow fluids and pressures to equilibrate across a boundary over geological time-scales, but act as seals over production time-scales, because they prevent crossflow at normal production rates -such that fluid contacts, saturations and pressures progressively segregate into 'dynamic' compartments.

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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...

Structurally complex reservoirs form a distinct class of reservoir, in which fault arrays and fracture networks, in particular, exert an over-riding control on petroleum trapping and production behaviour. With modern exploration and production portfolios commonly held in geologically complex settings, there is an increasing technical challenge to find new prospects and to extract remaining hydrocarbons from these more structurally complex reservoirs. Improved analytical and modelling techniques will enhance our ability to locate connected hydrocarbon volumes and unswept sections of reservoir, and thus help optimize field development, production rates and ultimate recovery. This volume reviews our current understanding and ability to model the complex distribution and behaviour of fault and fracture networks, highlighting their fluid compartmentalizing effects and storage-transmissivity characteristics, and outlining approaches for predicting the dynamic fluid flow and geomechanical behaviour of structurally complex reservoirs. This introductory paper provides an overview of the research status on structurally complex reservoirs and aims to create a context for the collection of papers presented in this volume and, in doing so, an entry point for the reader into the subject. We have focused on the recent progress and outstanding issues in the areas of: (i) structural complexity and fault geometry; (ii) the detection and prediction of faults and fractures; (iii) the compartmentalizing effects of fault systems and complex siliciclastic reservoirs; and (iv) the critical controls that affect fractured reservoirs.Structurally complex reservoirs form a distinct class of reservoir in which fault arrays and fracture networks, in particular, exert an over-riding control on petroleum trapping and production behaviour

The Pierce Field in the Central UK North Sea is a twin diapir structure that produces from the Paleocene Forties Sandstone Member (Forties Sandstone). Different hydrocarbon–water contacts encountered in the wells around both diapirs have been variously ascribed to a hydrodynamically tilted oil–water contact or else some form of stepped (compartmentalized) contact. Recent reinterpretation of the structure, sedimentology and fluid geochemistry has indicated that the stratigraphic architecture of the reservoir is the prime control on fluid flow over both geological and production time-scales. These depositional architectures deflect the hydrodynamic flow of aquifer water around the field, resulting in a modified-tilted-contact. The same depositional architectures control the flow of fluids under production. The Forties Sandstone was emplaced by turbidity flows influenced by pre-existing seafloor topography that funneled the flows into discrete sediment corridors and into the Pierce area. The rising twin diapirs further influenced the flows by forming: (a) a small salt withdrawal basin between the diapirs that captured sediment; and (b) enough seafloor topography to prevent the bulk of the flows from depositing significant amounts of sand over the crest of the diapirs. Because the bulk of the high permeability sands were deposited in a rim around the diapirs, the aquifer and injected water does not always flow to structurally higher elevations, but follows the geometry of the channelized sands. While faults are present on both South and North Pierce, they are not extensive and do not appear to play a major role in the compartmentalization of the field. From production data, pressure communication can be inferred around North Pierce and around the majority of South Pierce, the main exception being a block bound by large throw faults in the SE of the southern diapir. Geochemical fingerprinting of the hydrocarbons in Pierce shows families of oils that suggest that the northern and southern parts of the reservoir are separate oil compartments, which is a result of the interaction of the filling history and the stratigraphic and structural architecture of the reservoir.

This paper describes basic 'rules-of-thumb' that offer an indication of common uncertainties and pitfalls, as well as the analytical methods, data requirements and work elements required to replicate the impact of faults on fluid flow in production simulation models successfully. The first, and most important, stage in this modelling process is to ensure that an accurate structural interpretation is incorporated into the simulation model. In particular, that all fault linkages and cross-fault juxtapositions are taken from the seismic interpretation into the simulation grid. Fault rocks sometimes reduce the rate of cross-fault flow in which case it is important to account for this reduction in flow within simulation models. This is best achieved if databases of fault rock properties, measured from the field of interest or nearby similar reservoirs, are up-scaled to calculate fault transmissibility multipliers. It is sometimes necessary to consider not just the single-phase permeability but also the capillary pressure and relative permeability characteristics of the fault rocks present. Finally, all the relevant static and dynamic data must be appraised critically. However, the interpretation of such data is usually non-unique and misinterpretations can create errors in the production-related fault seal analysis. Where these basic guidelines are followed, it has been our experience that the project time required to achieve a history match of production data is dramatically reduced. In addition, as the history match is more geologically reasonable, the model is often more reliable for predicting the long-term behaviour of the reservoir. This gives confidence in the model's forecast to guide development planning and day-to-day field management decisions.

This paper examines the geometry and genesis of a mineralized thrust-related fracture system, developed at a major rheological interface between Witwatersrand sediments and Ventersdorp volcanics in the Witwatersrand Basin of South Africa. At the Elandsrand Gold Mine, the conglomeratic Ventersdorp Contact Reef, which separates these successions, is imbricated by kilometre-wide zones of minor thrusts containing ultracataclastic fault rocks associated with hydrothermal fluid flow. Several sets of fractures are intimately associated with the imbricates, which link to underlying kilometre-scale thrusts. The fractures provided a linked fluid flow pathway and structural sites for precipitation of gold within the Ventersdorp Contact Reef, carrying >90% of observed gold, with the remaining 10% occupying post-diagenetic matrix corrosion spaces within a few 100#m of fractures. The relatively rigid volcanics deformed into low-amplitude kilometre-wavelength flexures, centred on the imbricate zones and underlying kilometre-scale thrusts. These major structures controlled the thrust-fracture system formation along the Ventersdorp Contact Reef-lava contact. Three progressive stages of structural evolution were identified: (1) layerparallel shear across the contact, with stratigraphy-focused shallow fracture formation;(2) propagation of a kilometre-scale thrust towards the contact, leading to kilometre-scale flexure of the volcanics, with layer-parallel shearing in the limbs producing steep fractures and minor thrusts; (3) propagation of the major thrust along the contact, imbricating the sediments with shallow fracture formation in ramps.