Oil and gas are currently being produced by conventional techniques from more than 250 fields in the Netherlands. In addition, shale and coal layers in the subsurface may contain unconventional oil and gas resources, but whether these can ever be profitably produced has not yet been established.
Presence of hydrocarbons in the Netherlands
Conventional oil and gas accumulations are found in porous and permeable rock strata (mainly in sandstones and carbonates). Reservoirs are laterally confined by faults, folds or transitions to other, less permeable, sealing rock types. The top of a reservoir is always sealed by an impermeable rock (usually salt or clay). The base of an oil or gas accumulation is defined by the oil-water or gas-water contact (the transition from a water-bearing to a gas- or oil-bearing rock). Three key conditions have to be met for a conventionally producible oil or gas accumulation to be present:
- A source rock that is rich in organic material, in which oil and gas can be generated during deep and long burial;
- A porous and permeable (reservoir) rock, situated at a shallower depth, into which the generated oil and gas can migrate upwards;
- The presence of a sealing structure or rock above the reservoir layer, trapping the oil and gas.
Unconventional oil and gas resources are located in the same poorly permeable or impermeable shale layers (shale gas or shale oil) or coal layers (coal-bed methane) in which they formed. In this case, the source rock is also the reservoir rock, and therefor no migration of fluids or presence of a sealing structure are involved.
Once a location meeting these three conditions is identified through geological and seismic studies, an exploration well can be drilled in the most favourable prospective structures (‘prospects’) to demonstrate the presence and recoverability of hydrocarbons. In conventional prospects, oil and gas accumulate in clearly delineated structures, whereas much larger, less sharply defined areas are regarded as prospects in the case of shale oil or shale gas and coal-bed methane.
The location of a prospect is confidential information owned by the company that identified it. However, the general areas in which prospects have been recognised and regional maps showing the general geological conditions necessary for the existence of oil and gas fields can be consulted here.
Determining economic viability
After an exploration well has been drilled, the volume of oil or gas present in a reservoir can generally be estimated more precisely. Further rock measurements and production tests are required to establish the oil or gas flow rates towards the well and the expected recovery factor (i.e. the percentage of the total volume of gas or oil originally in place that can be produced, given certain technical and economic constraints). This information is used to decide how many wells need to be drilled and what technical interventions will be required to produce the resource. Any adverse effects that can be expected from production (e.g. subsidence or seismic hazards) are also assessed. The necessary licences have to be obtained before any exploration and production can start.
Potential interference with other activities and re-use of depleted gas fields
Conventional oil and gas reservoirs are often located in strata that are also attractive for geothermal energy recovery or for subsurface storage facilities. Most of these applications also benefit from good porosity and permeability of the rock. In the case of oil and gas accumulations and storage facilities, the effectiveness of the reservoir seal is also important. Geothermal energy projects will generally not be planned in undepleted oil or gas fields, but may be implemented in an adjoining aquifer if the two activities do not interfere with each other. Suitable candidates for storage facilities (e.g. for CO2 storage) are depleted gas fields – when production has ceased – or even producing gas fields (for use as a gas storage facility). During or after oil or gas production, formation water can be pumped back into the reservoir (or parts of it).
Unconventional oil and gas production (shale gas, coal-bed methane) rarely interferes with other activities. The shale layers themselves are not suitable for any other activities. However, a shale layer may form the seal of an underlying gas or oil reservoirs. In that case, the question of whether fracking may breach the reservoir seal has to be properly investigated.
Maps, data and information
Prospective areas for oil and gas production can be found in the following stratigraphic units:
- Basic information sheet on production of hydrocarbons (in Dutch)
- Carboniferous: prospective for gas
- Rotliegend: prospective for gas
- Zechstein: prospective for oil and gas
- Triassic: prospective for oil and gas
- Jurassic/Cretaceous: prospective for oil and gas
- North Sea Group: prospective for shallow gas
- Regional extent of identified prospects
- Unconventional: shale layers prospective for oil or gas
- Unconventional: coal layers prospective for gas
There are three shale stratigraphic units in the subsurface of the Netherlands that may contain oil or gas. Two of these are estimated to contain recoverable volumes of shale oil or shale gas: i.e. the Posidonia Shale Formation (PSF) of Early Jurassic age and the Carboniferous Epen Formation, the base of which is formed by the organic-rich Geverik Member (GM). The prospectivity of a third shale unit, the Coevorden Formation, of Jurassic age, is unknown because no specific geological studies have yet been done.
Exploration for shale gas
Although the Minister of Economic Affairs granted two exploration licences in 2009, all shale gas activities have been on hold since 2010. In July 2015, the Minister announced that he would not permit any shale gas activities in the next five years either. To date, no wells specifically targeting shale gas prospectivity have been drilled.
Geological characterisation of units that are prospective for shale gas
The Posidonia Shale Formation, a shale of Toarcian age (182-180 million years ago), is seen as being the most prospective for shale gas. Equivalent formations are found in other parts of Western Europe, e.g. the Jet Rock Member in Yorkshire in the UK, and the Posidonienschiefer/Ölschiefer in Germany. De Posidonia Shale Formation is encountered at depths of 1800-3800 m, while its thickness varies from 30 to 60m. It is characterised as a bituminous, dark-grey to brownish-black fissile claystone unit. More than 100 wells drilled both onshore and offshore the Netherlands have intersected this shale formation, but it was never the main target of any of these wells. The depositional setting is interpreted as pelagic with alternating oxic and anoxic conditions and occasional high-energy events, shown by erosion surfaces seen at the micro scale. The Posidonia Shale Formation is a proven oil source rock in both the Netherlands and Germany, and has sourced most onshore oilfields in the Netherlands. Adjacent parts of the underlying Aalburg and overlying Sleen Formations may also have source-rock properties.
The Epen Formation, with the Geverik Member at its base, is of Namurian age (Serpukhovian to Early-Bashkirian). Only 12 wells in the Netherlands ever intersected the Geverik Member. Data availability and our knowledge of this formation are therefore limited, certainly compared to the Posidonia Shale Formation. The unit is characterised by a succession of grey and buff, angular or sub-angular, moderately sorted claystones with a number of sandstone intercalations. The Geverik Member is a partially silicified, bituminous, calcareous black shale. The depositional environment of the succession is interpreted to reflect cycles of delta progradation into a predominantly lacustrine basin. This information is mainly based on Well Geverik-01. Over 1200 metres of core have been cut in that well, including the Geverik Member. The Geverik Member is a potential oil source rock, although no oil or gas accumulations have yet been found that are exclusively linked to the Epen Formation. The formation is expected to be present throughout the subsurface of the Netherlands. The deepest sections were encountered in wells LTG-01 and UHM-02 (at a depth of 4-5km).
The Upper-Jurassic Coevorden Formation is found at a fairly shallow depth in the east of the Netherlands. No prospectivity studies into the Coevorden Formation have yet been carried out.
ArcGIS and KMZ datasets showing the areal extent of shale layers in the deep subsurface of the Netherlands can be viewed here.