Relinquishment Report

Transcription

1 Southern North Sea Block 44/29c - Licence P1593 Relinquishment Report December 2012 GDF SUEZ E&P UK Ltd BY PEOPLE FOR PEOPLE

2 Document last updated :59 GMT Licence P1593 Relinquishment Report

3 Licence P1593 Relinquishment Report 1 Licence number and Block details 1 2 Work programme summary Synopsis 2 3 Exploration Activity 3 4 Prospectivity Analysis Trap Key Wells 7 5 Play components Source Seal Reservoir 8 6 Risk - Chance of success Volumes Other prospectivity 10 7 Summary 11

4 List of figures /29c Location map BPU Horizon BPU Subcrop Maps Cross-Line Kirchhoff 2010 PreSDM Carboniferous Well Correlation

5 List of tables 1.1 L i c e n c e d e t a i l s C o n s o r t i u m d e t a i l s : V o l u m e s T a b l e

6 Licence number and Block details 1 Licence number and Block details Table 1.1: Licence details. Licence Number: P1593 Licence Round: 25 Licence Type: Traditional Block(s): 44/29c Table 1.2: Consortium details: Companies Equities GDF SUEZ E&P UK Ltd (Operator) 40% RWE DEA UK SNS Ltd 40% Faroe Petroleum (UK) Ltd 20% Fig /29c Location map 1

7 Work programme summary 2 Work programme summary Firm commitment The Licensee shall reprocess 70 km 2 of 3D seismic data to PreSDM. Drill-or-drop commitment The Licensee shall either: drill one well to 3700m or 100m below the Base Permian Unconformity, whichever is the shallower, or elect to allow the licence to automatically cease and determine pursuant to Clause Synopsis The 44/29c block was awarded to GDF SUEZ E&P UK Ltd in February 2009 as the part of the offered 25th Round. References in this document to the 44/29c block relate to its definition at the time of the 25th Round award. No drilling was undertaken in the licence area during the tenure of the licence. 2

8 Exploration Activity 3 Exploration Activity Block 44/29c lies at the north-east margin of the Silverpit Basin. The basin has experienced a high level of exploration activity over the past years and contains a number of significant Carboniferous discoveries. In the immediate vicinity to 44/29c are Orca (44/29a, 44/29b and D18a) to the east and Minke (44/24a) in the north-east. These two discoveries were used as direct analogues of Narwhal. Evaluation of the 44/29c block utilised GDF Britain's extensive Carboniferous well database across Quadrants 43 and 44 of over 100 wells, which included a comprehensive palynological and chemostratigraphic database in the Silverpit Basin. One of the key risks on prospectivety in 44/29c is the depth conversion and depth imaging due to lateral velocity variations in the overburden and a North-South orientated salt wall. The block is fully covered by a Fina 3D seismic survey acquired in 1994 and re-processed by GXT in as an anisotropic pre-stack depth migration (PreSDM), in conjunction with adjacent surveys in the Orca-Minke area to the east. The Fina D seismic survey (FH94/44) was acquired with four 3 km streamers and dual 2250 cu in sleeve gun source arrays with a streamer depth of 7 m and source depth of 6 m, recording at 2 msec sampling to 5.0 sec. The survey was shot along lines oriented NNW-SSE with a shotpoint interval of m. The surveys were shot N-S with a 3km cable, parallel to the salt wall. GXT's anisotropic PreSDM processing in used a hybrid technique involving tomographic inversion in velocity model building down to Top Zechstein, using a four layer model (Upper and Lower Tertiary, Chalk, Base Chalk to Top Zechstein). Below Top Zechstein, a well-based flooding velocity of 4570 m/s was used which closely approximates both the Zechstein and Rotliegendes interval velocities in this area. PreSDM processing resulted in improved imaging and a robust velocity model which was adopted for time depth conversion in preference to a purely well-based layer-cake V0k approach. Sub-salt imaging of the Narwhal and Narwhal South prospects, directly below a piercing Zechstein salt wall, remained sub-optimal, and was addressed by further processing tests carried out by GXT in A wave equation migration (WEM) trial proved unsuccessful in eliminating curved basal Zechstein events, and led to even more severe wavefronting. This led to a radical re-appraisal of the Zechstein salt wall geometry used in velocity model building, involving a slight overhang on the eastern flank of the salt wall. Targeted Kirchhoff pre-stack depth migration was then carried out with this revised initial velocity model over an output area of 54 km2 covering the Narwhal and Narwhal South prospects. This reprocessing project, referred to as the "Phase 3" seismic volume, involved three iterations, and was completed in September Imaging at the objective Carboniferous level showed a significant improvement, hence mapping within the licence area was carried out using the Phase 3 data wherever possible. Results from this mapping indicated there was still residual geological inconsistencies with the velocity model. These were evident in the Top Rotliegendes where migration smiles and geologically unusual uplift was present. A re-interpretation of Narwhal West was also required so the re-processing area polygon was increased to encompass a larger area of 70km2. An additional round of re-processing was commissioned with a RTM to better define the overburden velocities. Again using GXT's anisotropic PreSDM processing with a hybrid technique involving tomographic inversion in velocity model building down to Base Chalk, using a three layer model (Upper and Lower Tertiary and Chalk). Particular attention was given to the lower Chalk where higher levels of compaction may be present creating a faster velocity. A RTM was then ran to define velocities between Base Chalk to Top Zechstein and better image the salt body as well as correcting for any slower younger Triassic or Jurassic sediment that may be present. A salt flood was again used below the Top Zechstein (4570 m/s) as the narrow offset of the data limited the use of more sophisticated velocity determination techniques. 3

9 Exploration Activity Mapping of the licence area was based primarily on structural interpretation of SEG positive, zero-phase, reflectivity depth domain seismic data from the 2007 (Phase 3) and 2010 (RTM) PreSDM reprocessed volumes. Additionally, reference was made to corresponding time, relative acoustic impedance, near-, faroffset and similarity volumes. Key interpreted horizons included Top Chalk, Base Chalk, Top Zechstein, Top Rotliegendes, Base Permian Unconformity (Fig. 3.1), Intra Lower Ketch 1, Top Murdoch Sandstone and an Inter Caister horizon. The Top Chalk and Base Chalk horizons highlight the major Tertiary and Chalk depocentre which occupies the Zechstein depletion zone adjacent to the salt wall that crosses the eastern part of the 44/29c block. Although picked with confidence in relatively undisturbed areas as a high amplitude, positive peak on the 2010 RTM reflectivity data, considerable uncertainty is attached to the precise geometry of the Top Zechstein horizon along the 44/29c salt wall. The improved imaging of the 2010 Kirchhoff data below the Zechstein salt wall is attributed largely to the revised interpretation of the horizon, constrained by reflection terminations on the flanks and the Base Chalk horizon above. This resulted in steeper flanks, a greater degree of piercement and a slight overhang on the eastern flank. Fig. 3.1 BPU Horizon. BPU with outline of Narwhal and Narwhal South. The Top Rotliegendes horizon comprises a prominent regional marker, expressed as a strong negative trough on the SEG positive, zero-phase reflectivity data, originating from the interface between basal Zechstein anhydrite and Silverpit Formation shales. The final map of the Top Rotliegendes horizon was picked on the 2010 Kirchhoff data. Although the Base Permian Unconformity (BPU) is locally picked as a negative trough, confidence in its picking is generally poor in the 44/29c area, and its mapping has been based on a well-based Rotliegendes isopach below the Top Rotliegendes horizon. Within the Westphalian section subcropping the Base Permian Unconformity (Fig. 3.2), mapping was carried out at three horizons: an intra Lower Ketch 1 horizon, Top Murdoch Sandstone and an Intra Caister horizon. As in the case of the Top Rotliegendes, these horizons were picked with a higher level of 4

10 Exploration Activity Fig. 3.2 BPU Subcrop Maps. BPU subcrop using well isopachs up from Murdoch to BPU (A) 44/24-3, (B) 44/24-4 and isopach down from BPU to Murdoch (C) 44/29b-4 confidence on the 2010 Kirchhoff data and maps were likewise based primarily on the 2010 dataset. Despite generally poor reflectivity within the objective Lower Ketch Formation, an Intra Lower Ketch 1 horizon was tentatively correlated from the Orca field to the east and ultimately tied to near the base of the Lower Ketch in the Minke well 44/24a-5 to the northeast. Correlation of this horizon below the salt wall and across the Narwhal prospect to the west of Orca was not possible. The Top Murdoch Sandstone horizon, picked as a positive peak on the SEG positive data, was picked with a greater degree of confidence as a fairly continuous marker extending further westwards in block 44/29c. Additional support for Carboniferous character correlation was gained from mapping a deeper Intra Caister horizon, close to the top of the Namurian. Fault interpretation comprised fault segment picking on vertical sections, aided by inspection of depth slices through the similarity volume and horizon similarity extraction of the 2010 data and low-angle illumination of horizon data. 5

11 Licence P1593 Relinquishment Report 2012 Prospectivity Analysis 4 Prospectivity Analysis 4.1 Trap Structurally, the 44/29c block occupies a position on the southwestern limb of a major, southeasterly plunging synform which preserves Upper Ketch and possibly younger deposits at its core in the southeastern part of the Orca field to the east (Fig. 4.1). Within the block, however, preservation of Westphalian C/D deposits is confined to the Lower Ketch 1 in its northeastern part, where the Narwhal South and Narwhal West prospects are located. Towards the southwest, the Permian subcrop becomes progressively older along a northwest-southeast trending palaeohigh which represents a continuation of the Murdoch Ridge. Fig. 4.1 Cross-Line Kirchhoff 2010 PreSDM. Cross Line 1558 Kichhoff 2010 PreSDM. Orca prospect is located further to the right (east). Narwhal is indicated on top of X-axis. The Narwhal prospect is bounded to the west by the major bounding fault of the north northwest-south southeast trending graben that aided creation of the salt wall. Some degree of footwall uplift may have contributed to the relief across Narwhal. Timing of this faulting is likely to be late Permian but could be younger. The north-east of the structure is bounded by a northwest-southeast trending fault of early Permian age although reactivation during the later phase of extension is likely. The faulting follows the same structural trend as the Orca prospect. The eastern flank of the structure is bounded by a small north-south trending fault probably of similar age to the main bounding fault. The southern tip of the structure may link via a saddle to a structure further south. This gives a structural spill point of -3767m TVDSS. 6

12 Prospectivity Analysis 4.2 Key Wells Key wells in block 44/29c are 44/29-1A and 44/29-3. Both these wells are located along the northwestsoutheast trending paleohigh of the Murdoch Ridge and were 'dry'. These wells were targeting the Murdoch Sandstone member which is tight this far south but wells provide valuable stratigraphic information for the prospectivity of the block. The PreSDM was tied to the 44/29-1A well which penetrated the Tertiary, Chalk Group, Triassic, Rotliegendes and Westoe Fm. No Jurassic was apparent with minimal Zechstein encountered due to salt withdrawal. The Triassic predominantly consisted of the Bacton Group. The Upper Rotliegendes Group consisting of the Silverpit Shale was penetrated and Westoe Shale was penetrated directly below the Saalian Unconformity on both wells. 44/29-3 lay just outside the area of seismic coverage but aided as an additional tie. These well ties were primarily used for mapping the west of the block but included tentative ties of pre-salt stratigraphy to the east of the block beneath the salt wall where imaging is poor. To the east of the block lies the Orca discovery in 44/29b. The nearest well 44/29b-4 penetrated the Tertiary, Chalk Group, Triassic and Rotliegendes Group Silverpit Shale. Below the BPU, the Lower Ketch 1 & 2 Formation was encountered at TD. This well was a gas discovery and is used as an analogue for block 44/29c prospectivity. Well 44/24-3 further north was used as a tie to the deeper stratigraphy below the Ketch Formation for additional confidence in mapping the east of block 44/29c. Well 44/24-4 is a gas discovery producing from the Lower Ketch 1 sandstone. Well 44/24-3 lies on a similair structural trend as the main Narwhal prospect where Lower Ketch 1 was encountered beneath the BPU (Fig. 4.2). Fig. 4.2 Carboniferous Well Correlation. Well correlation of wells surronding Narwhal prospect. 7

13 Play components 5 Play components 5.1 Source The source rock is composed of the coal beds and of the Westphalian A Caister Coal Formation and Westphalian B Westoe Coal Formation that are known to be mature for gas generation in this area. 5.2 Seal The ultimate seal in 44/29c is provided by the shales and evaporites of the Rotliegendes Formation. Upper Ketch Formations may also provide potential seals for Ketch reservoirs as well. The lower Zechstein evaporites will also act as a seal where they juxtapose the Ketch reservoir unit. 5.3 Reservoir Lower Ketch 1 and 2 has been proved many times as the main reservoir in surrounding fields and discoveries. 8

14 Risk - Chance of success 6 Risk - Chance of success An overall Probability of Geological Success of 53% was assigned to the Narwhal prospect (segment risk for LK2 and LK1 are 32% each). The primary contribution to the risk is related to risks to effective reservoir, seal and closure, with effective reservoir risked at 50% and closure/seal at 80%. The high rating for closure was due to the RTM depth migration and V 0 k mid-point depth conversion both giving fairly robust closure, however the salt overhang on the eastern flank has the potential of distorting the structure as does the internal velocity of the salt wall. The amount of side seal capacity coming from Upper Ketch shales is difficult to determine because of the lack of wells which have penetrated this formation in the 44/29 area, so that it is based on experiences from neighboring blocks. The lower confidence in effective reservoir is due to the lack of Ketch sands west of the salt wall and the lack of Lower Ketch 2 sands to the north of the Narwhal prospect. A thick section of Zechstein suggest a reliable seal is present, a proven source and migration path of the nearby Orca field have helped lower the risk of source, seal and migration and these factors are not considered an issue in this region. 6.1 Volumes Table 6.1: Volumes Table GIIP units P90 P50 P10 Mean Gas bcf Reserves Gas bcf Condensate mmbc Oil bbls Total mmboe

15 Risk - Chance of success 6.2 Other prospectivity Separated from the Narwhal prospect to the north by a saddle at the Base Permian level, the Narwhal South prospect shows a structural spillpoint at 3760 m TVDSS and a column height of 85 m over a closed area of 2.0 km 2, entirely within the 44/29c block. Its western closure is defined by a poorly imaged, westhading fault, while a pre-permian graben may influence its southern closure. A spillpoint towards the southeast may run along strike degrading the integrity of the enclosure. Imaging of the Top Rotliegendes in this region is poor and mapping deeper Carboniferous horizons was done with a very low level of confidence. The deterministic GIIP is 11 bcf risked at 43%. Narwhal West is located to the west of Narwhal on the western flank of the main graben. It is a structural high that is likely to have a valid closure. However, the analysis of the subcrop at the BPU leads to a most likely case of Westoe with no real reservoir properties. 10

16 Summary 7 Summary Mean Gas Initially in Place figures of 47 bcf were derived for the Narwhal prospect. Approximately 55% of Narwhal lies in the 44/29c block with the remainder located in the northern block 44/24a. In the absence of production data from the Orca field (due on stream in 2014) Narwhal is seen as being a subcommercial prospect. 11