Basement highs: Definitions, characterisation and origins

A glossary of commonly used terms related to the geometric forms and geological settings of basement highs is presented to assist cross‐disciplinary understanding, qualifying prefixes for the term basement are discussed and a scheme for characterising basement highs is presented. This scheme is designed to standardise, and to add rigour to, description of basement highs. It will thereby enhance basement high comparisons and assist understanding of basement highs across technical disciplines. The scheme enables systematic characterisation of: the geometry of a basement high; the lithologic units and structures in, above and around it; timings; tectonics and origins of the basement high and play elements relating to resource prospectivity. Use of this scheme is demonstrated using the southern Rona Ridge (West of Shetland, UK Continental Shelf). The tectonic, isostatic, erosional and stratigraphic processes that form basement highs are also discussed, and examples in proven petroleum systems are presented.


| INTRODUCTION
Basement highs currently receive much attention from the petroleum industry because of recent reservoir discoveries in basement highs, such as on the Utsira High, Norwegian Continental Shelf (e.g. Olsen, Briedis, & Renshaw, 2017;Riber, Dypvik, & Sørlie, 2015) and the Rona Ridge, UK Continental Shelf (e.g. Trice, 2014). Petroleum is currently being produced from basement reservoirs, including from the Bach Ho "buried hill", offshore SE Vietnam (e.g. Cuong & Warren, 2009) and the Zeit Bay field fractured basement, Egypt (El Sharawy, 2015). Although it is possible that basement rocks may form reservoirs that are not in basement highs, petroleum exploration of basement rocks has focussed on highs. Basement highs can be provenance for basinal sediments, influence sediment and petroleum migration pathways, form fluid traps (petroleum, potable water and geothermal water) and act as nucleation points for carbonate build-ups (e.g. Trice, 2014). Basement highs can also influence migration and precipitation of fracturehosted mineralisation and base metal sulphides (e.g. Garbarino, Naitza, Tocco, Farci, & Rayner, 2003;Hitzman & Valenta, 2005). We use basement high to refer to an area in which the basement rocks are significantly higher than in the surrounding areas (Figure 1; e.g. Landes, Amoruso, Charlesworth, Heany, & Lesperance, 1960). We use the term significantly to mean the magnitude is sufficient to strongly influence the petroleum system.
Basement highs may or may not be: (a) above present-day sea level; (b) present-day topographic or bathymetric features; and (c) partly or completely covered by younger rocks or sediments. Basement highs occur across a wide range of scales and in wide variety of tectonic settings. Basement highs generally, however, typically share various common characteristics. They are commonly unconformably overlain by younger rock units, often with condensed depositional sequences. They are typically fringed by younger rocks or sediments and are commonly bounded on at least one side by a fault system. Also, basement highs generally show evidence of either uplift or relatively less subsidence than the surrounding younger sediments or rocks. Basement highs may occur within or adjacent to basins.
A petroleum play is a group of fields and prospects having a chance for charge, reservoir and trap and belonging to a geologically related stratigraphic unit (e.g. Royal Dutch Shell, 2013). A mineral play is a group of geologically related mineral deposits and prospects within a chronostratigraphically bound unit (Banks, Walter, et al., 2019). Several questions about basement highs should be answered to model the evolution of a basin or the prospectivity of a petroleum or mineral play, including the following. What type of basement is being characterised? What effects did the basement high have on the extent and quality of play elements? Can the basement high be a reservoir? Can a commodity occur within, adjacent to or above the basement high? How did the basement high evolve and how did this evolution relate to petroleum or mineral commodity generation, migration pathways and entrapment? What information is needed to improve the model? Answering these questions requires careful analysis and description of the basement high.
Modifying a classic quotation about petroleum in basement reservoirs by Landes et al. (1960), resources in basement highs are not geological "accidents" but are accumulations that obey all the rules of sourcing, migration and entrapment, so basement highs should be examined with the same professional skill and zeal that is applied to deposits in the surrounding sedimentary rocks. The study therefore does the following: 1. A glossary of topographic and structural terms related to basement high geometries and geological settings is presented. This is designed to help geoscientists better communicate and integrate these terms across technical disciplines. 2. The various uses of basement are summarised, and it is recommended that qualifying prefixes are used to explain what is meant by the term. 3. A characterisation scheme is presented with the aim of standardising the description of basement highs, which should then make it easier to compare different basement highs. Use of this scheme is demonstrated with the southern Rona Ridge (UK Continental Shelf).
4. Different origins of basement highs are listed because this can be helpful in evaluating petroleum source, migration, reservoir and trap.
We focus on basement highs that range from petroleum field to regional scales (i.e. more than circa 100 km 2 ). Although this study concentrates on the relationships between basement highs and petroleum systems, the characterisation scheme can readily be modified for use on basement highs that host mineral deposits, groundwater aquifers or geothermal reservoirs. This study is, therefore, aimed at geoscientists in petroleum, minerals, groundwater and geothermal resource industries. Those geoscientists can include geophysicists, seismic interpreters, basin and reservoir modellers, petroleum geologists, sedimentologists, hydrologists and structural geologists.

AND STRUCTURAL FEATURES RELATED TO BASEMENT HIGHS
A wide range of terms are used in both academic literature and the natural resources industries to describe topographic or structural features within and around basins (Table 1; Figure 2). Although some glossaries have been published that include terms relating to basement highs (e.g. Nystuen, 1989;Peacock, Knipe, & Sanderson, 2000), basement terms are commonly used loosely and interchangeably. Although Nystuen (1989) provides a useful classification scheme for many types of structures within and around basins, there is a need for more rigorous definitions of basement highs terms, to enable consistent characterisation. We, therefore, provide definitions of numerous terms that are commonly used to describe the forms and geological settings of basement highs (Table 1). These definitions are kept simple, nonrestrictive and generic to accommodate overlap and ambiguity of the literature's engrained terms. We use the term significantly in these definitions to mean that the feature strongly influences the petroleum system.

PEACOCK And BAnKS
We acknowledge two outstanding issues relating to the definitions given in Table 1 that should serve as discussion topics during case-specific basement high interpretations. Firstly, some definitions may need to change when the scale or resolution of observation changes. For example, a ridge may become a horst when faults are resolved by better seismic data. Similarly, a horst may be better defined as an anticline if it is established that fault throw is significantly smaller than the amplitude of the fold. Table 2 shows examples of basement highs across a wide range of sizes. Some basement high terms should be scale dependent. For example, it would not be useful to include every bump along a Top Basement seismic reflector as a basement high. F I G U R E 1 Examples of basement highs. (a) The subaerial Liverpool Land Basement High is a volume of pre-Permian "basement" rock (naturally fractured crystalline basement) that is significantly higher than the surrounding areas of basement rocks. The section is based on field data onshore and seismic data offshore (Banks, Bernstein, et al., 2019;Figure 2a). The basement rocks are buried below the Jameson Land Basin onshore, and by the Liverpool Land Basin on the western North Atlantic margin. (b) Interpreted seismic section across the Frøya High, offshore Mid-Norway, which is a submarine basement high covered by younger sedimentary rocks and sediments (modified from Muñoz-Barrera, Henstra, Kristensen, Gawthorpe, & Rotevatn, in review; Figure 5c). The Frøya High is bounded to the west by the Klakk Fault complex, which separates the basement high from the Rås Basin, with the Froan Basin unconformably overlying basement rocks to the east T A B L E 1 List of topographic and structural terms commonly used to describe geometric forms and geological settings of basement highs, with definitions, and examples with known petroleum systems and key references. These terms are illustrated in Figure 2. Regional is used here to mean of a scale larger than a petroleum field

Term Definition Example References
Fault zone Defined by Hills (1940) as the zone of disturbed rocks between faulted blocks. Fault zone is commonly used for a system of related fault segments that interact and link, and are restricted to a relatively narrow band or volume (Nevin, 1931) San Andreas Fault (Sylvester & Smith, 1976) Gibson (1994) Flexural uplift or subsidence Buoyancy-induced vertical (isostatic) deformation that decreases in magnitude away from a fault (Egan, 1992) commonly modelled as an elastic response to fault slip (Roberts & Yielding, 1991) Central Greece (Poulimenos & Doutsos, 1997) Weissel and Karner (1989) Footwall uplift Uplift that occurs below a fault (in the footwall of a normal fault) Northern North Sea (Yielding, 1990) Jackson and McKenzie (1983) Growth fault A normal fault that is characterised by an increase in displacement down the dip of the fault, and by an increase in sediment thickness in the hanging-wall towards the fault plane, with older beds commonly having steeper dips than younger beds. This implies that the fault was active and cut the Earth's surface during sedimentation Offshore Louisiana (Losh, Eglinton, Schoell, & Wood, 1999) Ocamb (1961) Half-graben Asymmetric area of subsidence controlled by hangingwall subsidence above a controlling (basin-margin) fault (Barr, 1987 Roberts and Yielding (1991) High A general term for topographic, bathymetric and/or geological feature within which some or all of the rocks are higher than those of the same age in the surrounding areas (Blake et al., 1978). This may be used in preference to either basement high or basin high to avoid having to specify basement involvement, and without the need for the feature to be entirely within a basin Utsira High (Wild & Briedis, 2010) Dickinson (1979) Horst Elongate area of relative uplift mostly bounded by sub-parallel normal fault zones that dip away from the area of uplift (Reid, Davis, Lawson, & Ransome, 1913). Horsts are commonly bounded by grabens or half-grabens Auk Field, central North Sea (Trewin, Fryberger, & Kreutz, 2003) Dennis (1967) Intrabasinal high See basin high Montepetra intrabasinal high, northern Apennines, Italy Conti, Fontana, Mecozzi, Panieri, and Pini (2010) Massif A high of regional size, and usually consists of crystalline rocks Frøya High (Hinz, 1972) Ryan, Calder, Donohoe, andNaylor (1987) Metamorphic core complex A generally dome-or arch-like uplift of metamorphic or plutonic rocks overlain by tectonically detached and relatively unmetamorphosed cover rocks (Coney, 1980a;1980b). The faults that cause exhumation may be normal faults (Crittenden, Coney, & Davis, 1980) or thrusts (Dallmeyer, Johansson, & Möller, 1992) Rhodope metamorphic core complex, Greece (Dinter & Royden, 1993) Dewey (1988) Plateau An elevated tract of comparatively flat or level land; a tableland (Simpson, & Weiner, 1989 Secondly, some terms remain imprecise and have overlaps, and may need case-specific definition. For example, what should be the boundary between definitions of anticline, arch and dome? Should a fault-bound spur be called a horst? Such structures as domes, fault blocks, ridges and spurs can, for example, all be defined simply as basement highs until further data are available. Careful definition and explanation are important because nonexperts, or experts who have not previously worked on a particular basement high, can be confused or misled by imprecise terminology. Some basement highs will be a combination of various other types, such as a structure that is a combination of fault block and anticline. This suggests that the certainty of the interpretation should be qualified when assigning a geometric term to a particular basement high. We suggest indicating the level if certainty in the data and interpretation in the characterisation scheme presented in Section 4.
Despite these issues, Table 1 should add clarity to terms that are deeply engrained yet typically insufficiently defined in the literature.

BASEMENT
Basement is commonly used loosely in the geosciences, and different definitions for it are given across the literature in a range of contexts ( Figure 3). Basement rock can mean a variety of things, depending on the region being discussed and the perspective of the geoscientist (Koning, 2003). A rigid definition of basement is not possible because of entrenchment of various basement terms in the literature, and because the term must be broad enough to cover a wide range of data types, locations and geological ideas (Koning, 2003). For example, some geoscientists use basement to refer to nonsedimentary rocks, regardless of age, if they are unconformably overlain by a sedimentary rock or sediment (e.g. Garbarino et al., 2003;Jordan & Allmendinger, 1986;Landes et al., 1960;Lu, Zhao, Wang, & Hao, 2008). In contrast, P'An (1990) gives a definition of basement that includes rocks with a sedimentary origin, providing they have little or no matrix porosity.
We recommend that, to avoid potential confusion and misunderstanding between geoscientists, the term basement should not be used by itself wherever possible, but use one or more prefixes that denote(s) the basis on which that basement type is defined. Table 3 shows examples of recommended prefixes for the range of basement types. Geoscientists should explain the basis of their basement prefix. The questions "what is the basement type?" and "how is top basement defined?" should be answered for each study, location and data type. Note that we use the general term basement high in this study because we are not discussing a particular basement type or implying how it was defined.

SCHEME FOR BASEMENT HIGHS
Here, we present a systematic scheme for characterising basement highs. The approach is similar to the scheme for characterising fracture networks presented by Peacock and Sanderson (2018) because it identifies distinct analysis types, and because it is structured such that characterisation of a basement high progresses from descriptive, to quantitative and to genetic. The characterisation scheme presented in Table 4 is demonstrated using the southern Rona Ridge, offshore UK. This example is used because it has a proven petroleum system and enough data are available in the public domain and peer-reviewed papers to enable a detailed characterisation by a third party.
We recommend that the scheme presented in Table 4 should be sequentially populated using all available data and interpretation types, which may include published literature, fieldwork, rock and fluid samples, gravity and magnetic data, seismic surveys and mineral production information. We also recommend that the analyst states their degree of certainty for each part of the scheme (i.e. high, moderate, low and no information) to indicate strength of models and gaps in knowledge, even if such statements are qualitative. It is important to properly reference credible publications that are available to the reader (see Santini, 2018). In our analysis of the Rona Ridge, however, we have at times had to use such sources such as company reports or presentations, some of which are only available online (e.g. Hurricane Energy, 2019a-c).
F I G U R E 2 Schematic illustration of topographic and structural features related to basement highs, as defined in Although we have developed this characterisation scheme (Table 4) primarily for the petroleum industry, it requires only few modifications or additional criteria to be usable for other commodities (Section 4.7).

| Basic description of a basement high
Characterisation of a basement high should commence by providing geographic information and the geological setting. This information should include what would be included, for example, in a field description of an outcrop or in a geological setting chapter of a thesis or report. This would include geographical details about the location of the basement high, the types of data available and observational information about the geology of the area. These fundamental descriptions for the southern Rona Ridge are shown in Table 4 (Section A) and

| Geometry of a basement high
Geometric information about a basement high should enable readers to visualise its shape, and such information would also

| Size
This should include the area of a basement high in map view, or the long and short axes of the basement high. It may be difficult or ambiguous to define the exact size of a basement high, especially because the stated area covered depends upon the depth slice at which the area is displayed. Also, data coverage may not be consistent over the area of the basement high. Table 2 shows examples of basement highs across a range of scales.

| Shape
The shape of a basement high should be described or quantified at least in map view and in one cross-section, but ideally also in 3D. It is common in geology to assign a simple descriptive term to the outline geometries of features. Simple descriptive shapes that could be used to describe the map view (i.e. 2D) geometries of basement highs include circular, oval, triangular, square, rectangular, rhombic, etc. Simple descriptive shapes that could be used to describe the 3D geometries of basement highs include cuboid, wedge, flat-top dome, etc. The assigned shape could then be used in basement high volumetric calculations (e.g. Belaidi, Bonter, Slightam, & Trice, 2016). Note that many natural features tend to have fractal geometries (Mandelbrot, 1982), so shapes tend to become more elaborate as resolution increases.  Kauffman and Steidtmann (1981), Salah and Alsharan (1998) Structural Igneous and metamorphic rocks that are overlain by a deformed sedimentary cover, with deformation in the sedimentary typically uncoupled with deformation in the structural basement Sylvester and Smith (1976), Vendeville, Ge, and Jackson (1995), McQuarrie (2004) Orogenic b Rocks deformed during an orogenic event that are subsequently partly or completely covered by younger sediments Gessner, Collins, Ring, and Güngör (2004) Weathered Regolith and saprock units above the fresh bedrock of an already defined basement type Wright (1992) Geophysics Gravity Region of the subsurface showing a "strong" gravity response Nunziata and Rapolla (1987) Magnetic Region of the subsurface showing a "strong" magnetic response. It may refer to either: (a) the rocks below a magnetic response; or (b) the rock unit causing the magnetic response Behrendt and Wotorson (1970), Skilbrei et al. (2002), Treitel, Clement, and Kaul (1971) Acoustic/seismic Region of the subsurface showing a "strong" response to a passing seismic wave in the subsurface. Typically used for the region beneath the deepest coherent or continuous seismic reflector of a stratified sedimentary succession Allaby (2013), Bruvoll et al. (2012), Cooper, Davey, and Cochrane (1987) Fluid flow Porosity Rocks with matrix porosity and permeability that is too low for them to store or produce an economically viable hydrocarbons Hayes (1991) Naturally fractured crystalline Igneous or metamorphic rocks that produce fluids from fractures Trice (2014) Industrial Economic Typically used for the subsurface region beneath the rocks that contain commercial oil or gas, but we suggest it could be broadened to mean rocks below the depth at which economic mineral resources may be exploited Burgess (1974), Selley (1978) a Other ages of rock have been used to describe basement, including, for example, Silurian (Himmerkus, Reischmann, & Kostopoulos, 2009) and even Miocene (Woodside, Mascle, Huguen, & Volkonskaia, 2000). b The names of orogenic events are commonly used as prefixes to basement to describe the rocks deformed during that orogen. Examples include Caledonian basement (Ritzmann & Faleide, 2007) and Variscan basement (Maluski, Rajlich, & Matte, 1993).

High
Recognition criteria (data used to identify the basement high, such as fieldwork, seismic data and gravity data)

2D and 3D reflection seismic b High
Other available data (e.g. geophysical, bathymetry, air photograph, satellite imagery, lithology) Offset seismic and well data, regional geological analysis a High 2. Geometry of the basement high ( Figure 5) Size (area covered in map view, or lengths of long and short axes, to shallowest saddle of regional basement level)~2 Origin of the basement high (processes that created the basement high;

| Depth or altitude of the crest
Information should be given about the shallowest point (apex) below mean sea level for the top of a submarine basement high, or the altitude of the highest point above mean sea level of a subaerial basement high.

| Depth or altitude of the base
Information is needed about the depth or height relative to mean sea level where the flank of a basement high becomes part of the regional basement elevation.

| Height
An estimate should be given of the vertical distance between the apex and the base of a basement high.

| Topography of the upper surface
The topography of the upper surface of a basement high should be described. This would include, for example, the maximum and average slope or the cross-sectional geometry (e.g. horizontal, planar sloping, undulating). Table 4 (Section B) and Figure 5 give information about the geometric features of the southern Rona Ridge.

| Lithologies related to a basement high
This should include information about the known or inferred lithologies that comprise a basement high, which could be igneous, meta-igneous, meta-sedimentary and/or sedimentary. The description should also include the known or inferred lithologies around and above a basement high. For sediment or sedimentary rock units around or above a basement high, information should include such details as their ages, thicknesses,

| Basement high structures
The structures within, around and above a basement high should be described, and this can be done using a variety of data types ( Figure 6). Initial focus would be on structures that define the boundaries, flanks and segmentation of a basement high, including basin-bounding faults. Such description would help identify features that could have been major fluid flow conduits or barriers, or have formed traps. This description would also help for selecting appropriate analogues for a basement high. Structures within, around or above the basement high to be described include faults, fracture systems, folds, gravity-collapse structures and erosional features. Kinematic data, if available, should be presented as evidence of the displacement directions of faults. Figures 5 and 6, and

| Timing of events
The absolute ages of rock units or deformation events (e.g. from sediment growth packages with constrained biostratigraphy or radiometric dating) in, around and above a basement high should be listed. The relative ages of rocks and structures (e.g. from seismic reflectors of known or inferred ages, and from cutting and abutting relationships of faults) should be stated if absolute age data are unavailable. It should be determined whether the basement high developed before, during or after deposition of the surrounding rocks. The sequence of events that have modified the basement high, including ages of relative uplift, needs to be determined. It may also be possible to comment on the style of relative uplift of a high. For example, a basement high may have risen relative to a fixed datum while the surrounding basins subsided, or a basement high may have undergone subsidence but at a  slower rate than the surrounding basins. Table 4 (Section E) and Figure 7 show information about the timing of events on and around the southern Rona Ridge.

| Origins and tectonic settings
Basement highs can occur in a range of tectonic or erosional settings and can be caused by a range of processes.
Description of any basement high should include an interpretation of its origin and originating tectonic processes (Table 5). A basement high may be the product of more than one geological process (i.e. a combination basement high). For example, a particular basement high might have formed as a horst, influenced both by isostatic behaviour of the basement rocks and by erosion. This aspect of basement high characterisation should be incorporated into basin evolution and play assessments because the process(es) that created a basement high may have influenced other geological processes, including those that control petroleum system and petroleum play elements. Table 4 (Section F) and Figure 7 show information about the tectonics of the southern Rona Ridge. We suggest that the Rona Ridge formed because of uplift of the flank of the Faroe-Shetland Basin, which is a Mesozoic rift system.

| Influences on prospectivity
This section of the characterisation scheme (Table 4, Section G) is designed primarily for the petroleum industry, although petroleum play elements can be easily modified for use as mineral play elements (Section 4.7; e.g. Banks, Walter, et al., 2019;McCuaig, Scarselli, O'Connor, Busuttil, & McCormack, 2018). For petroleum play analysis (e.g. Grant, Milton, & Thompson, 1996), information or prediction is needed about the influence of the basement high on migration pathways, reservoir, trap and seal elements, and the timings of each of these. As with analysis of other play types, basement high play characterisation should include probabilistic assessment of the uncertainty of the interpretation relating to each play element (e.g. Roy, 1979). Figure 8 shows these play components using the southern Rona Ridge. If petroleum or other minerals have been discovered in or around a basement high, then its geometry, lithology, structures, origin and evolution (Sections 4.2-4.6) are crucial inputs to estimate possible gross rock volume and fluid column heights. Knowledge of lithologies, porosity-permeability ranges and fault-fracture systems in and around a basement high is also required to consider possible fluid leakage that could influence petroleum volumes. The depths of contacts between fluid types are also crucial information. (Table 4, sections A and B), illustrated using the south Rona Ridge 3D Top Basement depth structure map (Hurricane Energy, 2019b; see Figure 4 for location). A = Lancaster Field oil-water contact at 1678 m true vertical depth sub-sea level (TVDSS; Hurricane Energy, 2019c). B = Lincoln oil discovery "oil down to" at 2,258 m TVDSS (Hurricane Energy, 2019c). C = Whirlwind Discovery "oil down to" at TVDSS ( RPS Energy Consultants, 2017b). The depth of the apex is from RPS Energy Consultants (2017b)  The characterisation scheme shown in Table 4 is designed principally for evaluation of basement highs in the petroleum industry. It is, however, modifiable to basement high characterisation for other purposes and industries. For mineral exploration in and around basement highs, for example, mineral assemblages could be inserted into the exploration and production summary of Table 4, and deposit types could replace trap types. Basement high characterisation for groundwater, geothermal and contaminant transport evaluations could include such categories as climate, rainfall, surface drainage and subsurface fluid flow pathways.

FOR BASEMENT HIGH ANALYSIS
Characterising basement highs is an important aspect of basin and basement analysis, and petroleum, groundwater, geothermal and mineral resource evaluations. Researchers and economic geologists conducting screening assessments are likely to have little corporate data available to them, and so will be heavily reliant upon public domain and internet searches for basement high interpretations and schematic figures. Data sources will include peer-reviewed publications and corporate reports, some of which are independently  Energy (2015). (e) Centimetre-scale "microfracture" aperture on a sidewall core plug. Height of core plug is 45 mm. Figure 6c,e courtesy of Clare Slightam, Hurricane Energy. (f) Information can also be obtained about basement highs from outcrop analogues . Photograph taken from an unmanned aerial vehicle ("drone") of fractured granite at Sennen Cove, Cornwall, UK. The cliff is ~30 m high audited. We have written this study to help clarify the basement high terminology and to suggest a thorough basement highs characterisation scheme. We suggest that the descriptions and illustrations of basement highs are commonly insufficient, and often fall short of what would be included in a routine description of, for example, a mountain range or a nonbasement petroleum reservoir. Even the terminology used can be vague or misleading. For example, although such terms as the Bach Ho Field "buried hill" (Cuong & Warren, 2009) and Zeit Bay "fractured basement" (El Sharawy, 2015) give some information about a basement petroleum field, these phrases can lack rigour. They do not enable readers to envisage the basement high, assess its prospectivity or use it as an analogue for another basement high. We hope this study will help geoscientists to more systematically describe and report their basement high information, and build 4D models of these structures.

| CONCLUSIONS
A glossary of terms to describe and define the geometries of basement highs is presented (Table 1), with the aims of clarifying the terminology and improving cross-disciplinary understanding. Basement has a broad range of meanings and uses in the geosciences, so we suggest that a qualifying prefix should be used, and succinct description of it be stated in reports and figures, to make it clear what type of basement is being described and how it was F I G U R E 7 Example of the origins and timing of events related to a basement high illustrated using a tectonostratigraphic evolution chart, with fluid charge events and periods of uplift erosion and subsidence burial shown. This example shows how the Lancaster Field fractured crystalline basement reservoir (southern Rona Ridge) originated through several processes (Table 5) during its multi-event deformation history, and was consequently an uplifted, erosional, rotated faulted block that has most recently been affected by glacial rebound then subsidence. Note that the geological time axis is not to scale. Modified from Hurricane Energy (2018 (Table 3). We define basement high in this study to mean an area in which the basement rocks are significantly higher than in the surrounding areas, significantly being used to mean that they influence the petroleum system. Note that we use the general term basement high here because we are not specifying a particular basement type, dataset or identification criterion. A scheme is presented to systematically and thoroughly characterise basement highs (Table 4). This includes description of the location and geometry of the basement high, related lithologies and structures, the tectonics and origins of the basement high, the timings of modifying events and the influence on commodity resources and prospectivity. Use of this scheme is demonstrated using the southern Rona Ridge (West of Shetland petroleum province, UK). The scheme can easily be modified for use in the mineral, geothermal and groundwater resource sectors. The characterisation scheme presented in Table 4 is, therefore, an expandable guide for describing basement highs systematically and consistently for different purposes across the geosciences.

PEACOCK And BAnKS
A range of processes can create basement highs, as listed in Table 5. We suggest that knowledge of, and models for, the origins of basement highs is likely to improve understanding of other geological processes related to basement highs, and will improve understanding of their influence on commodity prospectivity. For example, knowledge of the processes that created and modified a basement high may enhance 4D understanding of the petroleum system affected by that basement high.

ACKNOWLEDGEMENTS
Steve Laubach, Chris Morley and Taija Torvela are thanked for their helpful reviews. We thank Rob Gawthorpe (University of Bergen), Thomas Kristensen (Equinor), John Hopper (GEUS) and Paul Knutz (GEUS) for helpful discussions. The following are thanked for their help and generosity with the figures: Dan Bonter, Clare Slightam and Robert Trice (Hurricane Energy); Jonas Del Pin Hamilton (GEUS); Jhon Meyer Muñoz-Barrera and Fabian Tillmans (University of Bergen). We thank GEUS for permission to republish Figure 1a. We have no conflicts of interest. Data sharing is not applicable to this article because no new data were created or analysed in this study.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article because no new data were created or analysed in this study.