P1645 Fig 1: Licence P1645 Introduction

Transcription

1 Licence Number: P.1645 commenced 12 th November 2008 Licence Round: 25 th Licence Type: Traditional Block Number: 9/8b Operator: EnQuest (60%) Partners: Norwegian Energy Company (Noreco) UK Ltd 40% Work Obligation: Obtain and reprocess 200km2 of 3D Seismic data. P1645 Fig 1: Licence P1645 Introduction Block 9/8b straddles the faulted boundary between the East Shetland Platform in the northwest and the Beryl Embayment to the southeast. The Bruce Field is located just a few kms to the southeast in blocks 9/8a and 9/9. The heavy oil discoveries at Bentley, Bressay and Kraken are in Upper Palaeocene sands within 9/2&3 and are located 5-15km north to northwest of Block 9/8b (Figure 1). A technical evaluation of 9/8b, Licence P1645 involved seismic interpretation of a 2009 Pre-Stack Time Migrated dataset between November 2009 and January

2 Database Seismic Figure 2 shows the seismic datasets that were available for the interpretation. Most of the work was carried out on the Pre-Stack Time Migrated volume processed between May and November Over 200km2 were reprocessed to fulfil the seismic work obligation on this licence. Fig 2: seismic data sets over P1645 9/8b study area. The Bruce 3D dataset provided useful well ties and the two TGS-Nopec 1988 regional 2D lines provided invaluable ties into Bentley and Kraken as well as understanding of the Palaeocene stratigraphy. In the southwest corner Conoco Phillips 2D lines were preferred to the Western D data because of concerns about data quality and also because field data for the Western survey has been lost and therefore could not be reprocessed. Considerable time and effort was expended in procurement of the PSTM dataset as it resulted from the pre stack merge of BP s HEB97 3D survey with the 1992 Bruce 3D to the southeast. Some CGGVeritas spec 3D was also merged post stack to provide coverage in the northwest corner (Figure 3). 2

3 Fig 3: seismic data sets over P1645 9/8b study area. The quality of the final dataset is good in the Tertiary section, less so in the Jurassic where Pre-Stack Depth Migration is really needed to image the complex structure. Wells Figure 2 highlights the wells used in the technical evaluation. Approximately 25 wells were incorporated in the study. Geophysics - Well Ties The synthetic tie for the Palaeocene section at well 9/8-5 shows a reasonable tie to the final seismic stack data using a 25Hz UK Normal Polarity zero phase wavelet (Figure 4). Fig 4: 25Hz UK Normal Polarity zero phase wavelet this, plus the character of the seafloor response suggests the data is correctly zero phased. However, the synthetic ties are not generally of very good quality. It is believed that many of these early (1970 s to 1980 s) wells suffer from very poor log quality and this makes reliable synthetic ties a problem. This is illustrated in Figure 5 where the deep section of 9/8-5 gives a poor correlation value (r) mainly because of an erratic sonic trace producing strong synthetic amplitudes in the Upper Cretaceous which are not present in the 3D seismic data. 3

4 Fig 5: 9/8-5 erratic sonic trace giving a poor correlation Interpretation Nine Eocene and Palaeocene horizons were picked to define structural and stratigraphic traps. The correlation with regional stratigraphy is illustrated in Figure 6 which is taken from a study on the Bentley and Bressay discoveries by Underhill (2001). Fig 5: 9/8-5 regional correlation (Underhill 2001). Mapping The main prospective intervals identified in the block application were the Upper Palaeocene Dornoch (Hermod and Teal) sands and the Lower Palaeocene (Maureen) Sands. Dornoch Sands Initial mapping concentrated on extending the Upper Dornoch Sand pick south from the Bentley area into 9/8b. The east-west regional tie line (Figure 6) shows clearly the pronounced clinoform build up at Bentley between the pink and orange picks. 4

5 Fig 6: 9/8-5 East/West Regional Seismic Line. The oil in Kraken (well 9/2-1) is within slightly older sands though there are minor shows in the Upper Palaeocene Sands. The southward correlation of the Bentley sands is shown in Figure 7. South Fig 7: 9/8-5 North/South Regional Seismic Tie Line North The clinoforms are less obvious in this orientation as the line runs across the front of the prograding beds. The higher amplitude parallel bedded appearance of this interval may be indicative of a transition to a deeper offshore setting at the base of the prograding slope. The Top Dornoch TWT map (Figure 8) shows that there is little in the way of structural closure. This is unlikely to be a southward extension of Bentley as the OWC there occurs at c secs. 5

6 Fig 9 Fig 8: Top Dornoch TWT map Line West Bentley Feature East Fig 9: 9/8-5 East/West Seismic Tie Line (See Fig 8). However a line across the shape (Figure 9) shows very similar internal architecture to Bentley and it seems that this is part of the same sedimentary system. Figure 9 also illustrates a quite well developed erosive based channel as described by Underhill from Bressay, but there is no significant compactional roll-over on the top surface. Hermod Sands Shows in the Hermod Sandstone at well 9/8-3 and 9/9c-16 suggest the possibility of stratigraphic trapping in the base of slope equivalent to the Upper Dornoch Sands at Bentley. An east west line between wells 9/8-4 and 9/9c-16 reveals a low angle prograding wedge containing Hermod Sands in the 2 wells. A shaley interval between the Hermod and underlying Teal sands may act as a bottom seal to any potential stratigraphic trap as the Hermod sand pinches out updip to the north and west. A pinchout trap might exist northwest of well 9/8-4, which is dry, but inspection of a north south line connecting the Hermod interval with the Dornoch Sand build up south of Bentley shows that there is very likely to be sand to sand contact across the fault so the seal integrity is highly questionable. Teal Sands No internal architecture has been identified within the Teal sand section where the amplitudes tend to be generally lower; however it seems likely that these sands would also be in updip communication with the Bentley Dornoch system. 6

7 Maureen Sands The top of the Maureen sands has been mapped in the hanging-wall of the main fault. These Lower Palaeocene sands were not deposited on the footwall. A number of very small closures can be seen against the main fault (Figure 10) but these are extremely high risk as they depend on fault seal. Fig 10: Small closures along main fault Some strong amplitude anomalies are also seen at Top Maureen but these do not seem to conform to structure. Furthermore the very strong trough at the Top Maureen suggests a hard kick which is unlikely to indicate hydrocarbon presence (Figure 11). South North Fig 11: Small closures along main fault The amplitude seems to be greatest where an antithetic system of faults is best developed, also shown in Figure 11. The most likely cause of these high amplitudes is shown at well 9/8-4 where a series of high velocity calcite cemented sands (actually shown as limestone on the composite logs) tie to the high amplitudes in the seismic. This contrasts with the low amplitudes and poorly cemented Maureen Sand at well 9/9c-16. It is therefore concluded that the amplitude anomalies are the result of calcite cementation around the antithetic faults south of the main platform fault. This is also likely to explain the amplitudes seen in the Hermod interval which do not conform to structure and which are in a similar position with respect to the faulting. 7

8 Lower Eocene and Balder Formation Sands Sand sections at Lower Eocene (9/8b-12, -13 and 9/8-5) and Balder (9/8-5) offer the possibility of stratigraphic traps as there is no significant structural closure at either level. Unfortunately amplitude extractions do not show any depositional patterns which could define potential sand pinchouts. Jurassic Fig 12: Base Cretaceous Unconformity TWT map The Base Cretaceous Unconformity TWT map (Figure 12) shows a significant area of closure in the hanging-wall of the main fault, just updip of well 9/8-4. The wells closest to this fault (9/8-3, -4, 9/8b-12, -13 and 9/9c-16) contain Kimmeridgian age sands directly below the Base Cretaceous. For this reason the pick is variable in character; it is usually carried in a peak, but locally in a trough where the acoustic impedance of the Jurassic exceeds that of the Cretaceous section. The mapped closure represents the top of a potential trap with top seal provided by Upper Cretaceous Chalk and marl and side seal provided by the fault itself. These seals are a major element of risk on the potential accumulation as the top seal is thin and the chalk is prone to fracture near the fault; and directly above that are the Lower Palaeocene sands. The fault itself provides a likely leakage route as it has undergone movement at least as recently as the Eocene, The potential for shale smear is low as both sides contain much coarse clastic material, typically Devonian Old Red Sandstone in the footwall and Jurassic and Palaeocene sands in the hanging-wall. Mapping beneath the Base Cretaceous is very difficult. Radical differences in the deep stratigraphy between wells 9/8-3 and 9/8-4 are not imaged at all well by the seismic, which was reprocessed primarily for Tertiary targets. Even the Base Cretaceous reflection is unclear downdip from 9/8-3. However, a top Heather pick has been made just west of well 9/8-5 where a high amplitude event was observed. Unfortunately this feature has minimal (<20ms) closure to the north as shown by Figure 30. In any case the Heather section in 9/8-5 is predominantly shale even though part of it is labelled Bruce Sandstone on the cross section. No further intra Jurassic mapping was attempted. 8

9 Volumetrics and Risking This was carried out in REP using lognormal distributions for the hanging-wall closure evident at Base Cretaceous Unconformity level. A gas case was run to reflect the situation at Bruce where there is a very large quantity of gas with some oil produced as condensate. As the fault plane was not explicitly mapped geometry factors of 80% and 50% were used to subtract the GRV underneath the fault plane for the P10 and P90 cases respectively. The Net/Gross range was centred on well 9/9c-16 which has c.30% good quality sands. The porosity range encompassed Upper Jurassic sands at Magnus as well as those seen in the Middle Jurassic of Bruce. Assuming constant average velocity of 2394 m/s: Spill point 2485 m (2.075s) Area: 5.1 x 106 m2 GRV P x 106 m3 (80% geometry factor) P x 106 m3 (50% geometry factor) (Geometry factors applied to estimate GRV above main platform boundary fault plane) N/G P10 50% - P90 10% (9/9c-16 well) Porosity: P10 24% - P90 10% (Bruce, Magnus) Hydrocarbon P P (Bruce) Saturation: Bg: P P Gas in place: P90 29 bcf P50 75 bcf P10: 188 bcf Mean 96 bcf There is a very low Chance of Success (COS) mainly due to the fault seal requirement and also the very thin topseal which comprises chalk and marl. The seal is especially critical for a gas trap with a vertical column height of 250m. Other risks would be the reservoir quality (sands in 9/8-4 are very silty) and source effectiveness as the Kimmeridge clay in the immediate vicinity is immature. Overall the modest size of the potential gas in place Figure (mean 96bcf) and the very low COS do not make this an attractive structure. Reservoir Presence 90% Effectiveness 60% Trap: Presence 80% Effectiveness 10% Source: Presence 100% Effectiveness 70% COS: 3% Summary and Recommendation No viable structural or stratigraphic traps have been identified. Some areas of high amplitude within the Tertiary section are not conformable with structure and are believed to result from calcite cementation associated with the main platform 9

10 boundary fault system. A possible trap has been mapped at Base Cretaceous level but this is extremely high risk as it depends on down-faulted seal and a very thin Upper Cretaceous chalk and marl topseal. Clearance EnQuest and co-venture partner give permission to DECC to publish this report. 10