The Petroleum System consists of four main Geologic components, source, reservoir, seal and trap plus additional processes necessary to generate and store hydrocarbons in the subsurface. Their presence is required to generate a viable EXPLORATION TARGET.

In and around Orkney all the major elements of the Petroleum System are present.


  • Organic Rich, Mature, Lake Laminites – Mudstone


  • Aeolian Desert Dunes
  • River Sands


  • Lake Laminites – Mudstone, Flagstones and Volcanics


Petroleum System

The Petroleum System consists of a mature source rock, migration pathway, reservoir rock, trap and seal. Appropriate relative timing of formation of these elements and the processes of generation, migration and accumulation are necessary for hydrocarbons to accumulate and be preserved.

The components and critical timing relationships of a petroleum system can be displayed in a chart that shows geologic time along the horizontal axis and the petroleum system elements along the vertical axis.

Exploration plays and prospects are typically developed in basins or regions in which a complete petroleum system has some likelihood of existing.

Source Rock Hydrocarbon Generation

The formation of hydrocarbon liquids from an organic rich source rock with kerogen and bitumen to accumulates as oil or gas.

Generation depends on three main factors:

  • the presence of organic matter rich enough to yield hydrocarbons,
  •  adequate temperature,
  • and sufficient time to bring the source rock to maturity.
  • Pressure and the presence of bacteria and catalysts also affect generation.
  • Generation is a critical phase in the development of a petroleum system.


The movement of hydrocarbons from their source into reservoir rocks.

  • The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion.
  • The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration.
  • Migration typically occurs from a structurally low area to a higher area in the subsurface because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock.
  • Migration can be local or can occur along distances of hundreds of kilometres in large sedimentary basins, and is
  • critical to the formation of a viable petroleum system.


The phase in the development of a petroleum system during which hydrocarbons migrate into and remain trapped in a reservoir.


A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids.

  • Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and
  • they form under temperature conditions at which hydrocarbons can be preserved.
  • A reservoir is a critical component of a complete petroleum system.

Seal (cap rock)

An impermeable rock that acts as a barrier to further migration of hydrocarbon liquids.

Rocks that forms a barrier or cap above and around reservoir rock  forming a trap such that fluids cannot migrate beyond the reservoir. The permeability of a seal capable of retaining fluids through geologic time is   ~  10-6 to 10-8 darcies.  commonly

  • shale, mudstone
  • anhydrite
  • salt,
  • A seal is a critical component of a complete petroleum system


A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate.

Traps are described as

  • structural traps
    • Hydrocarbon traps that form in geologic structures such as folds and faults
  • stratigraphic traps
    • Hydrocarbon traps that result from changes in rock type or pinch-outs, unconformities, or other sedimentary features such as reefs or buildups
  • A trap is an essential component of a petroleum system.
  • The Orkney Islands consist almost entirely of sedimentary rocks and subordinate lavas and tuffs of Middle and Upper Old Red Sandstone age. A Basement Complex composed of metamorphic rocks of Moinian type and Caledonian granites forms a number of small inliers near Yesnaby and Stromness in West Mainland and on the island of Graemsay.

  • During the Devonian Period, North America and Northern Europe were joined together forming Euramerica, one of the three major continental masses on the Devonian globe, with Orkney placed south of the equator. Most of Britain formed part of this landmass with mountains in the northwest and the open marine Devonian Sea covering southwest England.

  • The age of the Orkney dykes is 252+/-10 million years, placing the time of intrusion in the late Permian, a period of renewed tectonic movement in the sedimentary basins to the west of Orkney.

  • At the opening of the Tertiary chalk sedimentation continued on the northern North Sea. Starting around 63 Ma, the East Shetland Platform was uplifted and tilted towards the south-east. ...

  • The Quaternary deposits found on the East Shetland Platform are thin (Johnson et al., 1993). The Quaternary thickens above the Viking Graben to as much as 300m and provides a long and relatively detailed record of glaciation.

  • Rifting in the Northern North Sea commenced during the early Triassic, peaked during the late Jurassic, and terminated by the late Cretaceous. The Viking Graben can be seen as a failed arm of proto-Atlantic rift. Almost throughout this period, the Shetland area formed an area of positive relief.