Prospects and plays

24.06.2010

Several geological factors must coincide if petroleum is to be formed and accumulated in an area.

These are that:

  1. a reservoir rock is present where the petroleum can accumulate
  2. a trap has been formed so that petroleum is retained in a reservoir
  3. a source rock is present containing organic material which can convert to petroleum at sufficient temperature and pressure
  4. a migration route allows the petroleum to move from source to reservoir rock.

These factors are seldom all present simultaneously. If one or more are missing, no accumulations of oil or gas will be found in the area. Uncertainty always prevails about the presence of petroleum, and wells must be drilled to establish it. The likelihood of finding petroleum is called the discovery probability, which is estimated by assessing the likelihood of the above-mentioned factors being present.

 

Identified prospects

 

Recoverable petroleum resources have been surveyed and calculated by the NPD in 50 prospects in Nordland VI/VII and Troms II (figures 9 and 10). The “prospect” term describes an identifiable possible petroleum trap. Seismic surveys combined with data from drilling and discoveries in nearby areas form the basis for the NPD’s work. The volume of resources for each prospect cannot be determined exactly, because great uncertainty attaches to all the geological parameters used in the estimate. As a result, resources in the prospects are specified with a best estimate and with an uncertainty range providing high and low estimates which represent 10 and 90 per cent probability respectively. The discovery probability ranges from three to 25 per cent.

Some of the most important factors assessed in arriving at the discovery probability include:

  • reservoir quality
  • identification of traps on the basis of data density, data quality and
    geological complexity
  • the possibility that petroleum has leaked from the reservoir through fracturing in the overlying rocks
  • migration routes for oil and gas from source rocks to traps
  • the buried depth of the source rocks
  • uplift and subsequent erosion.

Detailed knowledge of the sandstones and the alternation between sandstone, siltstone and shale allows geologists to assess conditions prevailing when the sediments were laid down. Exploration drilling provides information on the thickness of the reservoir sands, their porosity and their hydrocarbon saturation, as well as knowledge about the types and quality of the hydrocarbons. In areas with much exploration drilling, good data are available to estimate the volumes of oil and gas likely to be found in a new prospect in the area. Uncertainty in the estimates increases with the distance from known areas.

 

 

Figure 9. Best estimate and uncertainty range for recoverable resources in prospects identified by the NPD in the Nordland VI/VII and Troms II areas (ranked by area and size).

Figure 9.
Best estimate and uncertainty range for recoverable resources in prospects identified by the NPD in the Nordland VI/VII and Troms II areas (ranked by area and size).

 

Figure 10. Prospects identified by the NPD in unopened areas. Prospects in production licences are not shown.

Figure 10.
Prospects identified by the NPD in unopened areas. Prospects in production licences are not shown.


Long distances from wells with relevant information about reservoir and liquid parameters present a challenge for resource mapping and evaluation in Nordland VII and Troms II. Nordland VI lies closer to wells, discoveries and fields in the Norwegian Sea, and the Jurassic/Triassic prospects in Nordland VI are expected to display similarities with the proven reservoirs of the same age further south.

The volume of oil and gas which might be found in an identified prospect depends on many physical parameters. Geologists map the geometric shape and volume of the petroleum trap, estimate an expected sandstone thickness, calculate the column of oil and gas which might have migrated into the prospect and assess how much petroleum could have leaked out as a result of subsequent erosion and reactivation of faults. These factors provide the basis for estimating the volumes of hydrocarbons in the prospect. The NPD’s surveys and data from scientific drilling show that the reservoir in many of the largest structures in Nordland VI/VII has been eroded before being covered by younger sediments. The reservoir will then be wedge-shaped beneath the top of the structure. That kind of shape holds substantially smaller volumes than if the sand had been preserved in its original parallel layers.

A very significant factor for calculating volume is the height of the petroleum column in a prospect. That depends not only on the strength of the seal above the prospect, but also on the amount of oil and gas which has migrated into it and how much may have leaked out. The NPD has analysed 45 discoveries in the Norwegian and western Barents Seas, and produced an overview of the oil and gas columns found there. These vary from a few metres to almost 400 metres, with 200 metres as a median value. The NPD expects geological conditions to be reasonably similar in parts of the areas assessed, and it is likely that prospects there have oil and gas column heights which correspond with the Norwegian and western Barents Seas. Columns are expected to be lower in parts of the area affected by extensive uplift and erosion because sealing properties have been weakened.

All resource estimates have been calculated on the basis of a statistical (stochastic) method which takes account of uncertainties related to all the factors. Calculated resources are presented with a best estimate and an uncertainty spread. The average discovery probability for the identified prospects is 12 per cent – in other words, a discovery is expected statistically speaking in every eighth prospect. When petroleum has been found, it will often enhance the discovery probability for similar prospects in the same area.

 

Plays

 

Various methods are available for estimating how much oil and gas could have been formed and accumulated in an area. These depend on the level of knowledge about the area. Play analysis, which is the NPD’s preferred approach, is a calculation method which involves defining geological plays within a specified geographical area where prospects with shared properties are found.

The probability of reservoir and source rocks, traps and migration routes being present is assessed for each play. This is called the play probability. It is uncertain whether the play actually functions until discoveries have been made in one of its prospects. Should producible petroleum be unproven in a play, it will be unconfirmed. When a discovery is made, the play is confirmed. It will then no longer be uncertain whether the play functions, and the play probability is set at one. This will normally have the effect of increasing estimated resources for the play. Several plays can be found within the same geographical area. They could be of different geological age, for instance, and accordingly lie one above the other at various depths in the strata.

When a discovery has been made, it remains unclear how much of the proven resources are technically and financially recoverable. In other words, uncertainty prevails about the volume of resources within the various geographical areas, whether they lie in large or small discoveries, and whether they consist principally of oil or gas. The more information available, the more certain is the evaluation of the plays and prospects for making discoveries.

The NPD has defined plays within the surveyed areas which break down into six main groups by age. Each play could extend over several of the areas.

 

Eocene and Palaeocene

 

The youngest plays are defined as lying in the westernmost part of the Barents Sea, including the Egga Edge (figure 11). and embrace reservoir sandstones of Eocene age. One exploration well has been drilled in the area, but the play is unconfirmed. It has not been possible to map prospects, but several possibilities have been identified. The NPD nevertheless believes that it could be possible to identify prospects and prove petroleum through drilling. This play accordingly contributes to the calculated resource volumes.

 

Figure 11. Plays in the Eocene (yellow).

Figure 11. Plays in the Eocene (yellow).

 

Two plays, both unconfirmed, have also been defined in the Palaeocene (figure 12). The larger and best mapped lies in the outer part of the West Fjord Basin. The structure is thought to be sandstone with very good reservoir quality, which has been proven by a number of wells to the south-west. These formations are sandstone fans deposited in the sea towards the south from a land area in Lofoten. The source rock is probably of Late Jurassic age. Two prospects have been identified in Nordland VI along with a number of possibilities. Sequences very similar to those interpreted as a Palaeocene play in Nordland VI have been mapped in Troms II. No Palaeocene prospects have been identified in Troms II.

 

Figure 12. Plays in the Palaeocene (orange).

Figure 12. Plays in the Palaeocene (orange).

 

Cretaceous

 

Plays in the Upper and Lower Cretaceous (figures 13 and 14) comprise sandstone fans deposited along the edge of the sedimentary basins. Substantial thicknesses of Cretaceous sediments are found in Troms II. Source rocks are organically rich shales of Early and Late Jurassic age. Extending into Nordland V and the West Fjord, the Lower Cretaceous play has been confirmed by discoveries on the Halten Bank but it has not been possible to define prospects in this area. Thick Cretaceous sediments are found in Nordland VI/VII, but the play is unconfirmed. The boundary between Lower and Upper Cretaceous has been mapped in Nordland VI, but cannot be determined with certainty in Nordland VII. Fourteen prospects of Cretaceous age have been identified, one in Nordland VI, seven in Nordland VII and six in Troms II.

The resource potential in Cretaceous plays is substantial.

 

Figure 13. Plays in the Upper Cretaceous (light green).

Figure 13. Plays in the Upper Cretaceous (light green).

 

Figure 14. Plays in the Lower Cretaceous (dark green).

Figure 14. Plays in the Lower Cretaceous (dark green).

  

Jurassic

 

The Jurassic play located in Nordland VI/VII is unconfirmed (figure 15). Jurassic sandstones have been identified by scientific drilling, and corresponding rocks are known from Andøya. The source rock is expected to be able to form oil in the deeper parts of the Ribbe Basin, while it probably forms gas in the deeper parts of the Træna Basin (figure 4). Uplift and erosion have probably reduced sealing properties and could have caused leaks. Combined with a thin coverage of younger sedimentary strata which may have poorer sealing properties, this is regarded as a substantial risk with the play. Extending into Nordland V and the West Fjord, the play has been confirmed by discoveries on the Halten Terrace.

In Troms II, the play has been confirmed by several discoveries in the Barents Sea (7019/1-1 and 7119/12-3).

Fifteen prospects of assumed Jurassic age have been identified in Nordland VI, 10 in Nordland VII and two in Troms II. The Jurassic plays contribute the largest resources in the whole area surveyed.

 

Figure 15. Plays in the Jurassic (blue).

Figure 15. Plays in the Jurassic (blue).

 

Pre-Jurassic

 

Two pre-Jurassic plays have been defined in the eastern Norwegian Sea (figure 16). These are thought to comprise sandstone reservoirs deposited on land as turbidite fans and river sediments, and as deltas and shallow marine sediments. Chalk deposits laid down in shallow water could also be present. The plays are unconfirmed. No prospects associated with this play have been identified. The resource potential is regarded as low.

 

Figure 16. Plays in the pre-Jurassic (indigo).

Figure 16. Plays in the pre-Jurassic (indigo).

 

Plays in basement rocks


A new play has been defined with fractured and weathered basement rock as the expected reservoir. This play extends from Nordland VI into Nordland VII (figure 17). A total of seven prospects have been identified in this play, but the resource potential is regarded as low.

 

Figure 17. Plays in basement rocks (light brown).

Figure 17. Plays in basement rocks (light brown).

 

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