Shale
Compacted mud, and the commonest sedimentary rock on Earth — the source rock for nearly all the world's oil and gas, and the seal that traps it.
Shale is mud that has been buried and compressed until it became rock, and it is more abundant than every other sedimentary rock put together. Its defining feature is fissility: it splits into thin sheets along the bedding, because the flat clay particles it is made of settled parallel to one another and stayed that way.
That same fabric makes it almost impermeable across the bedding, and this is why shale matters so disproportionately. It is simultaneously the rock in which almost all petroleum forms, and the rock that stops petroleum escaping from the reservoirs beneath it. An oil field is usually a permeable sandstone or limestone with a shale lid.
How it forms
Mud accumulates where water is still enough for clay-sized particles to settle — deep ocean floors, lake bottoms, lagoons, floodplains. Burial then expels the water and rotates the clay flakes into alignment, and what was sixty per cent water by volume becomes rock.
Where the bottom water was oxygen-poor, organic matter settled with the mud and was not eaten or oxidised. Those black shales are the petroleum source rocks, and they preserve a second chemistry too: with no oxygen present, iron and sulfur combine into pyrite, which is why fossils in black shale are so often replaced by it.
Compressed further, and heated, shale recrystallises into slate — the clay minerals rebuilt as mica, aligned by pressure rather than by settling, and cleaving across the original bedding rather than along it.
Economic significance
Shale spent most of industrial history as the rock that got in the way — too weak to build with, prone to swelling and landslides, and a nuisance to tunnel through. It is quarried in quantity for brick clay and as an ingredient of cement, and that was the whole of its direct economic role.
Horizontal drilling and hydraulic fracturing changed that within about a decade, by making it possible to produce oil and gas from the source rock itself rather than waiting for it to migrate into a reservoir. The consequences have been geopolitical rather than merely industrial, and the environmental argument about the method is unresolved.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Clay — material · 40–75% · the clay minerals that make up most of the rock, and whose flat particles give it its splitting
- Quartz — mineral · 15–40% · as silt-sized grains carried in with the mud
- Pyrite — mineral · in the black shales, where oxygen-poor bottom water let iron and sulfur combine instead of oxidising — and where fossils are so often replaced by it
- Calcite — mineral · as a cement between the grains, and abundant enough in the calcareous shales to grade into marl
- Baryte — mineral · as bedded barite, precipitated on the sea floor rather than deposited from a vein — a distinct and commercially important mode of occurrence
is a source for
- Slate — rock · slate is shale taken further: the clay rebuilt as mica and aligned by pressure, cleaving across the original bedding rather than along it
- Schist — rock · the ordinary metamorphic path — mud to slate to phyllite to schist, each step hotter and coarser than the last
- Jet — material · the Toarcian shales of the Yorkshire coast, where it occurs as flattened lenses within the black organic-rich mudstone that records the same anoxic seafloor
contains
- Carbon — element · as organic matter in the black shales, where oxygen-poor bottom water preserved it — the same preservation that made coal, in a different setting
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing