Potential for deep geothermal energy in Scotland: study volume 2

3 The geological context

3.1 Some terminology and concepts

This section contains a brief introduction to some of the geological terminology and concepts that are relevant to this assessment of geothermal energy potential. The glossary also contains definitions for some terms.

The Earth has a core, a mantle, and a crust which form approximately 15%, 84% and less than 0.1% respectively of its volume. The crust is effectively the thin 'skin' of cool, solid, brittle rocks forming the outermost layer or shell of the Earth. In the continents the crust is typically approximately 30 km thick, but it can be significantly thicker where two continents have collided. Beneath the oceans, the crust is substantially thinner, typically just 5-10 km thick. The deepest borehole drilled anywhere in the world to date, on the Kola Peninsula in Russia, reached a maximum depth of 12.26 km. For comparison, most of the boreholes that have so far been drilled in onshore parts of Scotland are less than 1 km deep, and most boreholes drilled offshore in the hunt for hydrocarbons do not exceed 5 km.

The crust consists of three main classes of rocks:

• igneous rocks form by solidification of magma (molten rock); they can solidify within the crust (forming intrusions) or they can erupt onto Earth's surface (forming lava flows and pyroclastic deposits); common types of igneous rock include granite and gabbro (which always occur as intrusions), and basalt and andesite (which are commonly erupted, but can form intrusions).
• sedimentary rocks form by deposition of particulate matter at Earth's surface; common types of sedimentary rock include sandstone, conglomerate, siltstone and limestone.
• metamorphic rocks are former igneous rocks or sedimentary rocks that have been subjected to heat and pressure within Earth's crust, with the result that the original textures and mineral assemblages have changed significantly. Common types of strongly metamorphosed rock include gneiss and schist.

Rocks can be either crystalline (formed entirely of interlocking crystals) or granular (formed of adhering particulate matter, such as sand grains). Metamorphic rocks and most igneous rocks are crystalline. Most sedimentary rocks are granular.

Most rocks are brittle, and when they are subjected to strain they react by breaking along fractures. There are two common types of fracture: joints, which are cracks formed by simple opening; and faults, which develop when the rock breaks under shearing strain and the opposing sides of the break move relative to each other and parallel to the fracture.

Fluids (including water, oil, natural gas and air) pass through rocks in pore spaces. There are two main types of pore space: fracture pore space, in which fluid is held within fractures; and intergranular (or matrix) pore space, in which fluid is held within the spaces between grains or particles. Water can only move through crystalline rocks via connected open fractures, whereas in granular rocks water can move through intergranular pores, or through fractures, or a combination of both.

The position of rocks within the crust can change significantly over geological time. On a continental scale, this happens as tectonic plates move on top of the mantle. On a smaller scale, movements on geological faults cause rocks to change their positions relative to each other. Rocks also move vertically within the crust: those that were formerly at Earth's surface can become buried beneath younger deposits, and rocks that were originally at considerable depth can rise to shallow depths as a result of uplift and erosion. In this way, rocks that formed at Earth's surface become buried and metamorphosed, and rocks that formed or were modified deep in the crust can become exposed at Earth's surface.

In many parts of the crust, older rocks that were formed or affected by one set of events are overlain by younger rocks that were formed or affected by a different set of events. In such settings, the older rocks are referred to as basement and the younger, overlying rocks, as cover. Typically, the cover will consist of non-metamorphosed sedimentary (granular) rocks, and the basement will be metamorphosed (crystalline) rocks. Basement-cover relationships are potentially important in geothermal energy, as rock units with contrasting characteristics ( e.g. permeability, heat production capacity and thermal conductivity) can be juxtaposed across the boundary.

Thermal conductivity varies from rock type to rock type. Typical values reported for some common rock types are presented in Table 1. However, the thermal conductivity of rock can change with changing temperature and pressure. For example, compaction (which increases with depth and results in loss of pore space) can increase thermal conductivity. This has little effect in crystalline rocks such as granite (which typically has a very low proportion of pore space at all depths) but can make a significant difference in rocks that are typically porous at shallow depths, like sandstone. The thermal conductivity of rocks also typically decreases slightly with increasing temperature. Thermal conductivity values are usually reported for rocks at the surface, or in the near-surface zone, but it must be borne in mind that for some rock types thermal conductivity is likely to change significantly with depth.

Table 1 Thermal conductivity values reported for common rock types.

Rock type	Rock character	Mean thermal conductivity ( W m -1 K -1)	Reported in
sandstone	sedimentary	3.3 to 4.9	Lee et al., 1984
mudstone	sedimentary	1.5 to 2.2	Lee et al., 1984
limestone	sedimentary	2.8 to 3.0	Lee et al., 1984
coal	sedimentary	0.31	Lee et al., 1984
slate (metamudstone)	metamorphic	2.7	Lee et al., 1984
basement metasediment	metamorphic	3.51	Lee et al., 1984
basement metasediment	metamorphic	3.1 to 3.5	Wheildon et al., 1984
granite	igneous, silica-rich	3.0 to 3.5	Wheildon et al., 1984
granodiorite & tonalite	igneous, silica-rich	2.5 to 2.9	Wheildon et al., 1984
dolerite (and basalt)	igneous, silica-poor	2.2	Lee et al., 1984
peridotite	igneous, silica-poor	2.2	Wheildon et al., 1984

3.2 Bedrock geology of Scotland

Scotland has a complex and diverse bedrock geology, which is the product of more than three billion years of Earth processes ( Figure 2). The country sits on the edge of the European continent and has been geologically 'quiet' for more than 50 million years; there is no evidence at the surface or in explored parts of the subsurface for ongoing volcanic or hydrothermal activity.

For the purposes of this assessment of geothermal energy potential, the numerous units that make up the bedrock geology of the country have been simplified into seven 'categories', each of which has a distinct character in terms of component rocks, permeability and thermal conductivity ( Figure 3 and Table 2).

3.2.1 Strongly metamorphosed igneous and sedimentary rocks

The Western Isles and Northwest Highlands are underlain mainly by thickly banded, strongly metamorphosed, crystalline rocks (gneiss), much of which consisted originally of intrusions of granitic (granite and similar) and basaltic (basalt and similar) rock. These are the oldest rocks in Scotland, and rocks with similar characteristics probably occur at depth beneath most (possibly all) other parts of the country. Sequences of non-metamorphosed sandstone and metamorphosed sandstone and limestone overlie the gneisses in parts of the mainland. The area is bounded to the east by the outcrop of the Moine Thrust Zone, a major fault which dips east at a low angle and therefore underlies a large area of ground to the east of its position at surface.

3.2.2 Metamorphosed sedimentary rocks

Much of the Shetland Islands and most of the Northern Highlands and Grampian Highlands are underlain by metamorphosed and folded sedimentary rocks. These originally formed a very thick sequence of interbedded sandstone and mudstone units, but metamorphism has given these rocks a crystalline character. A major fault, the Great Glen Fault, separates the Northern Highlands from the Grampian Highlands. The fault is near-vertical and associated with a substantial topographic feature (the Great Glen). The Great Glen Fault may extend to the base of the crust, and possibly into the top part of the mantle. The large mainland outcrop of this category of rocks is bounded to the north-west by the Moine Thrust Zone and to the south-east by the Highland Boundary Fault. The orientation of the latter structure is not well understood, but it is likely to dip towards the north-west.

3.2.3 Weakly metamorphosed sedimentary rocks

Much of the Southern Uplands area is underlain by weakly metamorphosed sandstone and mudstone (shale). The sandstone is typically poorly sorted (wacke), and would have been relatively impermeable prior to metamorphism. Following weak metamorphism, the original sedimentary rocks now have a partly crystalline character. The originally gently dipping beds of sedimentary rock have been much disrupted by folding, faulting and tilting. The area is bounded on its north-west side by the Southern Upland Fault. The border with England defines part of the south-east boundary.

3.2.4 Large intrusions of basic igneous rock

Several large intrusions of silica-poor (basic) igneous rock (gabbro and peridotite) crop out in Aberdeenshire, including the Morvern-Cabrach, Huntly-Knock and Insch masses. The intrusions are thought to be broadly flat-lying and saucer-shaped (laccoliths).

3.2.5 Intrusions of granite and related rocks

Intrusions of igneous rock are common in many parts of Scotland. The intrusions span a wide range of compositions, including silica-rich (granite, granodiorite, tonalite), silica-poor (gabbro and peridotite), and intermediate (diorite, syenite) variants. Many thousands of small intrusions ('minor intrusions') typically have sheet-like and pipe-like forms. Smaller numbers of large intrusions typically have broadly oval form at outcrop; the largest can be 30 km across and crop out over an area in excess of 300 km ². The largest intrusions typically consist of granite and/or granodiorite (which is closely related to granite). The Grampian Highlands, in particular the East Grampians region and Argyll, contains the greatest concentration of intrusions, large and small. Concentrations of large intrusions also occur in the Northern Highlands, and the western part of the Southern Uplands. Large granite intrusions also crop out on Arran, Skye and the Shetland Islands. The Midland Valley has only a handful of relatively small granite intrusions at outcrop but is dissected by numerous minor intrusions, mainly of basaltic and andesitic composition. The granitic intrusions of Scotland are described in more detail in section 8.2.1

3.2.6 Thick and extensive lava flows

Thick sequences of mainly basaltic (silica-poor) and andesitic (intermediate silica content) lava underlie several parts of Scotland. In the Lorne area of Argyll a pile of lava flows crops out across an area of approximately 300 km ² and has a maximum preserved thickness of c. 800 metres. In the Midland Valley, sequences of lava (and pyroclastic rocks) up to several kilometres thick underlie nearly all of the upland areas. Andesitic lavas crop out across an area of around 600 km ² around Cheviot, astride the Scotland-England border, and have a preserved thickness of about 500 metres. Extensive fields of basaltic and andesitic lava on Skye, Mull, and in Morvern may attain a vertical thickness in excess of two thousand metres.

3.2.7 Non-metamorphosed sedimentary rocks

Thick sequences of sedimentary rock that have never been metamorphosed to any significant degree underlie large parts of Scotland, including most of the Midland Valley, the Orkney Islands, Caithness and fringes of the Moray Firth to the north and east of Inverness.

In geological terms, the Midland Valley is a fault-bounded block of relatively young, non-metamorphosed rocks that has been downthrown relative to the blocks of older, metamorphosed rocks on either side (forming the Grampian Highlands and Southern Uplands). At outcrop, the area consists very largely of sedimentary rocks and lavas of Devonian and Carboniferous age. The Carboniferous strata of the Midland Valley include the only significant occurrences of coal in Scotland. The underlying crystalline basement rocks are not exposed other than in small areas adjacent to the bounding faults, and their nature is very poorly constrained. Based on geophysical survey data, the basement-cover boundary is interpreted to be at a depth of approximately 8 km beneath much of the central part of the Midland Valley, though it is likely to be significantly shallower in places.

The Orkney Islands, Caithness and Moray Firth fringes (and the offshore areas between them) are underlain by sedimentary rocks that were deposited on the floor of, and adjacent to, a large shallow lake (the Orcadian Lake). The sedimentary succession is characterised by alternating beds of conglomerate and sandstone in some areas, and siltstone and sandstone in others.

In south-west Scotland, sedimentary rocks of mainly Permian and Triassic age are preserved in several places, filling ancient valleys and downthrown blocks. These are described in more detail in section 7.3.1.1.

Figure 2 Cross-sections of Scotland's bedrock geology, constructed down to 15 km. The coloured polygons represent different geological units (colours are not the same as those used in Figure 3). A detailed description of the cross-sections is beyond the scope of this report, but they give a sense of the complexity of the subsurface geology.

Figure 3 Simplified BGS bedrock geology map of Scotland. MT - Moine Thrust; GGF - Great Glen Fault; HBF - Highland Boundary Fault; SUF - Southern Upland Fault.

Table 2 Summary information for the seven bedrock categories described in section 3.2.

* where: very low = <2.0; low = 2.0-2.5; moderate = 2.5-3.0; high = 3.0-3.5; very high = >3.5; units are W m ^-1 K ^-1