Government of Western AustraliaDepartment of Mines and PetroleumMineral Titles

Government of Western AustraliaDepartment of Mines and PetroleumEnvironment

Government of Western AustraliaDepartment of Mines and PetroleumPetroleum

Government of Western AustraliaDepartment of Mines and PetroleumPetroleum Fact Sheet

Geology Basics

www.dmp.wa.gov.au/shaleandtightgas

The Department of Mines and Petroleum (DMP) is responsible for regulating extractive industries in Western Australia; ensuring safety,

environmental practice and resource management meet relevant legislation, regulations, guidelines and policies that reflect community

expectations. DMP regulates onshore petroleum and geothermal activities through the Petroleum and Geothermal Energy Resources Act 1967,

the Petroleum Pipelines Act 1969, the Petroleum (Submerged Lands) Act 1982 and associated regulations.

IntroductionOil and gas fields are entirely found within sedimentary basins. These basins develop over tens of millions of years and gradually fill with fragmented material which hardens into rock layers. Within these formations petroleum is trapped.

The critical elements of a petroleum system are summarised below.

Source Accumulation of organic matter forming organic shales, limestones, coals etc.

Trap, Seal or Cap The trap seal or cap is an impermeable rock that prevents the further migration of hydrocarbons to the surface.

Migration The pathway of hydrocarbons from the source to the trap.

Reservoir Rock body beneath a trap seal or cap with relatively high porosity and permeability that contains hydrocarbons.

SourcePetroleum is derived from the remains of living things, as is coal. Over time this organic matter becomes kerogen a common petroleum source type. Kerogen further matures with time under pressure and heat at depth, into an assortment of hydrocarbons. The lightest hydrocarbon molecules form natural gas, the heavier hydrocarbons form oils, tars and bitumens.

Over geological time hydrocarbon molecules are released out of source rocks as it heats with increasing depth. Heavy oils form first, then light oils, then finally gas. Being lighter than rocks and water, hydrocarbons rise upward through fractures and the pores of coarse sedimentary rock formations.

MigrationThe migration of oil and gas from a source rock to a reservoir rock is related to hydrology, fluid pressures and water movement. The rate of water movement may be small and measured in mere centimetres per year, but the effect of the fluid pressure can be very important to the movement of petroleum through the rocks.

A small fraction of that migration, approximately two per cent, is captured in pools trapped by layers of impermeable rock like shale or limestone. What is not trapped continues to rise until it reaches the surface as a naturally occurring oil seep.

ReservoirTraps or petroleum reservoirs are the accumulation of hydrocarbons that have migrated from sources. Reservoir porosity is the rocks capacity to contain fluids (liquid or gases). A highly porous reservoir rock can contain a large volume of oil and gas. These fluids must be able to flow, meaning that the pores must be interconnected; this characteristic is called permeability and is the measurement of the rock’s ability to flow oil and gas.

The rocks formed in the basin have varying degrees of porosity and permeability, these two properties are one of the keys to petroleum accumulation. A rock is termed porous if it contains voids and cavities between individual grains. It is permeable if the voids are connected so that fluid can flow through the rock.

The measurement of permeability is the darcy. A good quality hydrocarbon trap will have a permeability of 1 darcy or more. A tight gas reservoir, more compact than a common house clay brick, may have permeability measured in microdarcys. The permeability of shale is even lower, as little as one one-thousandth of the permeability of tight gas formations, the unit here is the nanodarcy.

Permeability is one of the parameters which conventional gas reservoirs can be distinguished from shale and tight formations. A common feature of shale and tight gas is that both are trapped in very low permeability rock.

These compact matrixes limit and prevent migration of gas.

Porosity is the percentage of pore volume or void space, or that volume within rock that can contain fluids. Porosity typically is the space between grains that are not compacted together completely. Porosity can be generated by the development of fractures, in which case it is called fracture porosity. Shale gas reservoirs tend to have relatively high porosity, but the alignment of platy grains such as clays makes their permeability very low.

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Reservoir Rock

Confining Bed (Cap Rock)

Confining Bed Carrier Rock

Sandstone

Source Rock

DMPAUG13_2677Revised August 2013

Petroleum Fact Sheet Geology Basics

www.dmp.wa.gov.au/shaleandtightgas

A rock is porous when it has many tiny spaces or pores.

A rock is permeable when the pores are connected.

More Information

Department of Mines and Petroleum Mineral House, 100 Plain Street East Perth, Western Australia 6004 Tel: +61 8 9222 3333 Email: shaleandtightgas@dmp.wa.gov.au

Website: www.dmp.wa.gov.au/shaleandtightgas

Further Readingwww.spegcs.org