relinquishment report · 2017-04-27 · all five wells had petrophysics and rock physics study...

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RELINQUISHMENT REPORT

LICENCE P.2175

BLOCK 21/7B UKCS

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Contents

1. Licence Information ........................................................................... 3

2. Licence Synopsis ................................................................................ 4

3. Work Programme Summary ............................................................... 5

4. Database............................................................................................ 6

4.1 Well Database ...................................................................................... 6

4.2 Seismic Database ................................................................................. 6

5. Prospectivity Update .......................................................................... 8

5.1 Tarland Prospect ................................................................................... 8

5.2 Torphins West Prospect ....................................................................... 10

6. Technical Work Undertaken ............................................................. 11

6.1 Seismic Gather Conditioning (DUG)....................................................... 11

6.2 Seismic Inversion (DUG) ..................................................................... 11

6.3 Migration Mapping .............................................................................. 13

7. Risk and Resource Summary............................................................ 15

8. Conclusions ..................................................................................... 15

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1. LICENCE INFORMATION

LICENCE INFORMATION

Licence Number: P.2175

Licence Round: 28th Round

Licence Type: Traditional

Block Number: 21/7b

Start Date: 1st December 2014

Drill or Drop: 1st December 2016

Equity Holdings:

Maersk Oil North Sea UK Limited 100% (operator)

Maersk Oil confirms that DECC / OGA is free to publish this report and that all 3rd party ownership

rights have been considered and appropriately cleared for publication purposes.

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2. LICENCE SYNOPSIS

Licence P.2175 is being relinquished having reached the end of the drill or drop term on 1st

December 2016 and subsequently the end of an extension to the 2 year drill or drop phase. For

the extension to the licence, Maersk Oil committed to both complete a farm out process and be

able to commit to drill a well on the Tarland Prospect or drop the licence so that it can be included

in the 30th Round. A relinquishment deadline of end February 2017 was set by OGA.

Block 21/7b (figure 1.0) was awarded to Maersk Oil North Sea UK Ltd (100%) on 1st December

2014.

Firm commitments on the P.2175 licence included:

Reprocess 500km2 of 3D seismic data.

Drill or Drop:

Decision required by 1st December 2015 with one well to be drilled to 1829m or 15m

below the top of the Andrew Formation, whichever is the shallower.

The licence application was submitted as part of the 28th licensing round with the purpose of

evaluating the Ypresian, Upper Forties, Tarland Prospect. The Torphins West prospect was not

evaluated further from the time of application as its volumes were assessed as sub-economic

unless developed in conjunction with a successful discovery at Tarland.

Figure 1.0: Location Map – Block 21/7b.

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3. WORK PROGRAMME SUMMARY

The firm work programme for block 21/7b was completed during the first 2 years of the initial

term and included:

Reprocessing of 500km2 of 3D seismic data.

Although the commitment was to reprocess only 500km2 of 3D seismic data, 1200km2 of 3D

seismic data was reprocessed by Down Under Geo Solutions in May-July 2015 to including gather

conditioning (linear noise reduction, residual move-out correction, radon and trim statics).

Additional products derived from this reprocessing exercise included a full elastic inversion, fluid

and lithology probability volumes (see section 6)

The uplift in quality of the gathers was adequate to more clearly discriminate the AVO response

and extent of the Tarland prospect and to reconcile the analogue discoveries and dry tests in

nearby wells.

Figure 3.0: Before and after gather conditioning showing prominent AVO response for Tarland (seismic data courtesy of CGG MCNV (GeoSpec)).

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4. DATABASE

4.1 Well Database

Maersk Oil had access to all released E&A wells and Development wells through CDA. Additional

biostratigraphic data was licensed from Ichron specifically for the Forties interval to provide a

semi-regional biostratigraphic dataset that would tie the nearby analogues including Forties Field

(figure 4.0).

Figure 4.0. Map of the study area showing all E&A wells and Development wells for which data was

available. Wells for which Biostratigraphic data was publicly available or licensed are shown in

red.

4.2 Seismic Database

Block 21/7b is covered by the CGG 2009 NW Forties 3D dataset and a CGG 2014 PSTM

reprocessing of the NW Forties data (figure 4.1), both licensed by Maersk Oil. The block is also

covered by the 2013 reprocessed PGS MegaSurvey-Plus 3D dataset, which is licensed by Maersk

Oil, but was not used for prospect evaluation. The Megasurvey Plus was used to tie distal field

analogues and to provide a wider regional perspective.

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Figure 4.1. Map of the seismic data coverage around 21/7b, showing the outline of the underlying

CGG NW Forties dataset (red outline), CGG 2014 PSTM reprocessed dataset (green outline) and

Maersk reprocessing area (black outline). Map courtesy of CGG MCNV (GeoSpec).

N

Tarland Prospect

AOI 1200km sq.

reconditioned

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5. PROSPECTIVITY UPDATE

Maersk Oil identifies prospectivity in a Ypresian Upper Forties Fm stratigraphic pinch-out exhibiting

a strong class II AVO response, Tarland Prospect. This has an estimated mean recoverable

volume of c.35mmboe. Further prospectivity is identified in the Eocene Cromarty Member,

Torphins West prospect, which has a mean recoverable volume of c.13mmboe.

5.1 Tarland Prospect

Tarland is a fully matured prominent AVO anomaly identified in the Forties sandstone Mbr of the

Sele Fm (refer to Figure 5.0). It is a 3-way stratigraphic pinch out of a Forties Formation slope

turbidite with dip closure to the east. Top seal is provided by the T80 MFS in the Sele Fm

claystones and the seat seal is defined by a mapped T70 MFS and is provided by the Lista

claystones. The AVO response is consistent with oil-filled high quality sands.

Figure 5.0. Stratigraphic column (from Millennium Atlas, 2002).

The seismic interpretation over Tarland was conducted on the 2009 reprocessed Fugro nwforties

3D and on the reprocessed data. Individual sand lobes were interpreted from the full elastic

inversion volume and sand probability volumes. Regional maps also required interpretation on the

2013 reprocessed PGS MegaSurvey-Plus 3D. A top Forties reservoir depth map isopach and

seismic section/ geo-seismic section through the Tarland prospect are shown in figures 5.1, 5.2

and 5.3.

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Figure 5.1. Top Forties depth map (ft) with Tarland P1 outline contour interval 20ft). The outline

of the Tarland lobe extent is defined using a combination of ExChroma, lithology volume and AVO.

Figure 5.2. Tarland reservoir isopach (ft) with AVO overposted (contour interval 10ft; data

courtesy of CGG MCNV (GeoSpec).

21/7b

21/8b

21/7b

21/8b

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Figure 5.3 Seismic section and geo-seismic section through the Tarland prospect (seismic data

courtesy of CGG MCNV (GeoSpec)).

Figure 5.4 Geological model for the Tarland depositional system and lateral trapping components.

5.2 Torphins West Prospect

Torphins West prospect (refer to figure 1.0) has not been further matured since the 28th Round application. The prospect is a class III AVO in the Cromarty sands and an up dip extension of the

Torphins oil and gas discovery in 21/8-4 & 4z. The development of this dual phase accumulation including a success at Torphins prospect is currently sub economic.

20km N

21/7a-3

21/3b-

5

Tarland system

Lower Forties Channel

U Forties Bypass – no

sand deposition

21/7b-4

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6. TECHNICAL WORK UNDERTAKEN

At the start of the work programme, the main geological uncertainty for the Tarland prospect

included the fluid fill and extent of the trap, which was poorly resolved on the existing data; critical

risks include reservoir development and lateral seal.

A petrophysical evaluation was conducted to determine porosity and fluid content for the

reservoirs in key wells and rock physics study was conducted to derive elastic rock property trends

for the characterisation of reservoir sands and shales. Results were fed into a QI study over 490

km2 to produce lithology and fluid prediction volumes. Stochastic modelling was used to

understand the population behaviour of key lithology and fluid combinations as a function of end-

member rock types, fluid content, reservoir quality and depth.

A further review of the biostratigraphy in offset wells was undertaken and facies maps revisited to

confirm the conclusions made for the license application. Migration modelling was conducted on

new interpretation surfaces and demonstrated that the Tarland prospect can be filled by

hydrocarbons migrated from the westerly spill of the Forties Field and exclusive of the Torphins

discovery and its separate migration route.

6.1 Seismic Gather Conditioning (DUG)

Post migration seismic gather conditioning of 1200 km2 3D marine seismic survey was undertaken

(refer to figure 4.1).

The processing sequence included linear noise reduction, residual moveout correction, radon

(linear & parabolic) and trim statics. The results show good improvements in both removal of

noise and residual multiple elimination, and improvement in event flatness and coherency (refer to

figure 3.1).

6.2 Seismic Inversion (DUG)

A comprehensive Quantitative Interpretation (QI) workflow was carried out in order to map the

lithology distribution and fluid prediction around the Tarland prospect area. Simultaneous multi-

stack absolute inversion of the seismic data was performed using low-frequency models guided by

rock physics end-member trends. The rock physics trends were also used to aid lithology and fluid

prediction. The study used DUG-conditioned seismic angle gathers as input. Five wells (15/29b-

13, 21/01b-17, 21/05a-6, 21/07b-4, 21/08-4) were available to build the rock physics trends; two

of the wells were located within the seismic area for calibration. All five wells had petrophysics

and rock physics study results from DUG's petrophysics team.

Final inverted absolute products (P-impedance and Vp/Vs) were used in conjunction with rock

physics modelling to produce probability volumes for a range of key lithologies/fluids. The

probability volumes show good shale/sand discrimination; the oil probability distribution are

consistent with observed Near-to-Far AVO anomalies. Hydrocarbon filled Sandstones are well

discriminated from brine-saturated Sandstone; discrimination between high saturation oil and low

saturation oil/high saturation gas is possible.

Figure 6.0 compares the improved AVO response in the reprocessed data with a sand probability

volume through the Tarland prospect. A strong AVO response can be mapped over Tarland (figure

6.1). Where the AVO is locally weak to absent, this largely coincides with the thinnest areas on

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Figure 6.0 NW-SE section through Tarland for far angle stack and sand probability volume (seismic

data courtesy of CGG MCNV (GeoSpec).

Figure 6.1 AVO response Map (T.Forties to T.Forties +12ms: envelope of the fars minus envelope

of the nears x envelope of the fars) showing the Tarland Prospect P10 outline. Note the Avalon

and Torphins discoveries display AVO in the Cromarty interval – they are not hydrocarbon bearing

in the Forties reservoir (seismic data courtesy of CGG MCNV (GeoSpec)).

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the isochore. The area of the strongest contiguous AVO is taken as the P10 input area for

volumetric assessment.

6.3 Migration Mapping

Following additional well correlation (e.g. figure 6.2), a detailed regional hydrocarbon shows

analysis was conducted for >60 wells. Migration experiments were conducted on T. Forties and T. Cromarty depth maps. Lithology distributions derived from the seismic inversion were incorporated into migration experiments (chiefly lateral sand distribution from sand probability cubes).

Figure 6.2 Correlation of well calibration immediate to Tarland Prospect.

Migration was assumed to be westward from the spill at Forties Field, supported by wells with shows in the Forties. Results show that the Tarland Prospect can be charged by migration from the Migration from the spill at Forties Field. The migration routes modelled avoid wells without shows in the Forties (figure 6.3). Sensitivities using multiple lithology scenarios provide similar

results. Modelling of migration at Mid Miocene time provides similar results for the two most southern migration routes shown in figure 6.3, but shows the northern route to be unlikely due to slight regional tilt. Additional mapping on the shallower Cromarty sands shows that the Torphins, Avalon and Scolty discoveries can all be charged by an independent Cromarty migration model.

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Figure 6.3 Migration map on the top reservoir (T.Forties) regional surface conditioned with sand probability map (yellow outline is area of sand probability volume).

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7. RISK AND RESOURCE SUMMARY

Tarland and Torphins West remain the only identified prospects in P.2175. Geological risks are

recoverable volumes are summarised in Table 7.0.

Prospect Reservoir P90 P50 P10 Mean Pg

Tarland Upper Forties 4.3 26 77.8 34.7 mmboe 32

Torphins West Cromarty 8.1 12.4 17.8 12.7 mmboe 40 Table 7.0 Recoverable resources and geological risk for Tarland and Torphins West prospects.

8. CONCLUSIONS

Reprocessing of the existing seismic dataset over 21/7b, gather conditioning and elastic inversion

have provided an improved seismic dataset on which to reinterpret Tertiary prospectivity. The

Tarland prospect is characterised by a robust class II/III AVO indicative of oil-filled, high quality

sandstones in a stratigraphic closure.

The Tarland prospect trapping configuration is complex and stratigraphic, requiring up dip

pinchouts and lateral facies changes. While these are supported by the available evidence, the

resolution of the data cannot fully resolve the extent of sand continuity at the updip and lateral

edges of the trap. The lateral and base seal remain the critical risk for the prospect.

While the presence of a robust AVO for oil-filled sand is present over Tarland, defining the P10

area, the development of the reservoir remains a large uncertainty. Moderate to high net to gross

reservoirs provide a similar AVO response, impacting the range of volumes modelled.

Maersk Oil UK Limited is not prepared to drill the Tarland prospect at 100% equity and is therefore

dropping the licence. The prospect is marginally economic with limited upside potential. The

Torphins West prospect is not a viable economic prospect to support drilling of an exploration well.

relinquishment report · 2017-04-27 · all five wells had petrophysics and rock physics study...

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