Relinquishment Report for Licence Number P1356, Block 48/8c March 2008

Transcription

1 Relinquishment Report for Licence Number P1356, Block 48/8c March 2008

2 Table of Contents 1.0 Header Synopsis Exploration Activities Prospectivity Analysis Conclusions...12 List of Figures Figure 1 Block and prospect location map Figure 2 Key Well Ties to Seismic Figure 3 Sea level to Top Triassic Figure 4 Triassic to Bunter Isopach and Interval velocity map Figure 5 Top Bunter to Zechstein Isopach and Interval Velocity map Figure 6 Top Zechstein Depth map (meters) Figure 7 Isochron of Zechstein thickness variation Figure 8 Zechstein interval velocities from wells Figure 9 Zechstein Isopach generated at constant Int Velocities of 4360 m/s, and 6100m/s Figure 10 Top Silverpit from well velocities at regional and then detailed perspective Figure 11 Regional and detailed scale Top Silverpit Depth map constructed from a Zechstein interval velocity of 4360 m/s Figure 12 Regional and detailed Top Silverpit Depth map constructed from a Zechstein interval velocity of 6100 m/s Figure 13 Overlay of all the Pennard traps created in this project

3 1.0 Header Licence Number: P1356 Licence Round: 23 Licence Type: Promote Block Number: 48/8c Operator: Endeavour Energy (UK) Ltd. 100% Work Programme Summary: Firm Commitment Purchase released 2D over block, acquire a minimum of 50km 2 of available 3D, carry out reservoir quality study of Rotliegendes (Leman) sandstone. Drill or drop decision. 2.0 Synopsis This report marks the end of the two year term of this 23 rd Round Promote licence. The licence consists of single block 48/8c awarded 100% to Endeavour Energy (UK) Ltd. The licence was acquired to assess the potential for a four-way dip closed structure up-dip from well 48/8b-2 named the Pennard Lead (Figure 1). This lead potentially contained 288bcf GIIP but was only partly mapped on the PGS Megamerge 3D data set. Reservoir quality is an issue and the work programme was proposed to address both the structural integrity of the trap and the potential for economic flow from the tight Leman sandstone reservoir. Figure 1: Block location and structural elements map (block shown in yellow)

4 3.0 Exploration Activities The work programme has primarily focused on the integration and interpretation of 200 km 2 of WesternGeco (WG) 3D data, leading to an extensive depth conversion sensitivity study. The objectives of the geophysical project comprised 3 steps 1. Merge three 3-D volumes in time and phase (Figure 2); a. WG-SNS; the newly purchased data that covers the western portion of the Pennard prospect (red outline) b. Mega J-06; the mega merge survey on the eastern flank of the prospect (dark blue outline) c. Mega I-06; the mega merge survey on the southern flank of the prospect (light blue outline) 2. Tie the merged 3-D s to key wells within their boundaries; 48/7c-1, 48/8a-1, 48/8b-2 and 48/ Create a series of Depth maps from the TWT maps that allow for the range of interval velocities possible within the Zechstein Formation and permit a series of possible structure maps at the Leman sandstone level. Figure 2: Merged seismic surveys

5 Step 1 Merge the 3 Data sets The optimum approach was considered to be to first tie the J-6 volume to the projects key well, 48/8b-2, then to tie the remaining two volumes to J-06. To tie the well, J-06 was bulk shifted +22 ms. The I-06 volume, already tied to J-06, also needed to be bulk shifted +22 ms. To tie the WG-SNS survey to the J-06 and I-06 the following adjustments were made; Bulk time shift: -44 ms. Phase shift: +150 degrees Amplitude; x6000 Step 2- Tie the Seismic to the Wells The synthetics, were generated in SMT s SynPak program and demonstrate an excellent seismic to well tie (Figure 3). The best tie for all four wells was with a +180 degree wavelet and slight stretching and squeezing. A fifth well within the survey boundaries, 48/3-2, was not used in either well tie to seismic or depth conversion because of obvious errors in the well tops and T/D data. These errors are probably due the incorrect adjustments made to this highly deviated, shallow well. In Figure 3 (below) the picked seismic horizons are as follows; Top Triassic (blue), Top Bunter Sandstone (teal), Top Zechstein (purple) and Top Silverpit (yellow). The first three represent boundaries of formations with either significant velocity or thickness variations that could impact depth conversion. The Top Silverpit horizon was chosen to represent the Top Leman reservoir. The Silverpit lies approximately 100 m. above the Leman and creates an excellent seismic reflection whereas the Leman is unreliable. Also note the Triassic Top is slightly above the seismic pick. This is because the Triassic is picked in the wells about 15 meters above the sonic/density increases in the rock. Figure 3 Key Well Ties to Seismic

6 Step 3 Depth Conversion An SMT based Isopach summation method was used for the depth conversion. This process allows the use of wells outside and inside the 3-D surveys to calculate interval velocities. This is especially useful in this project to offer velocity control to the north, northwest and east of the prospect. Wells used to constrain velocities are as follows; The method allows for a range of interval velocities to be applied to the Zechstein Formation to account for the presence or absence of very high velocity dolomites or low velocity claystones as seen in nearby wells. The following is a summary of the 3 overburden sections summed to get down to the Top Zechstein. Figure 4: Sea level to Top Triassic Isopach (left) and interval velocity (right): Note the extreme thickness variations in this low velocity material that can have significant effects on depth conversion. Also note the Interval Velocity map covers a larger area to encompass all the relevant well control used in the velocity calculations. The yellow polygon is the approximate location of the Pennard prospect.

7 Figure 5 Triassic to Bunter Isopach (left) and Interval velocity (right). The thin, long Isopach thins (blue and green areas) are due to large salt withdrawal faults in the Triassic section. Figure 6 Top Bunter to Zechstein Isopach (left) and Interval Velocity map (right). The slower interval velocity from west to east appears to be caused by a decrease in sand to shale ratio. The sum of these 3 overburden isopachs gives the following Top Zechstein map: Figure 7 Top Zechstein Depth map (m) computed from addition of overburden isopachs. The final phase of this depth conversion exercise was believed at the beginning of this project to contain the most uncertainty. The depth to Top Silverpit /Lemon Sand calculation requires the addition of the Zechstein Isopach to the Top Zechstein Map. The Zechstein in this area has two widely varying factors; thickness and interval velocity. The isochron map in Figure 8

8 demonstrates the Zechstein thickness variation in the area in seismic two way time. The thins (blues) are areas of salt withdrawal and the thicks (reds to yellows) are areas of salt pillowing. Note, however, the Zechstein thickness over Pennard in the direction critical for structural trapping (northwest) is relatively constant. Figure 8 Isochron of Zechstein thickness variation The variability of the interval velocity is the next issue to address. Local wells that penetrate this sequence of rocks cut two dominate lithologies with different internal velocities. They are as follows; Halite and anhydrite 70 microseconds per foot = 14,300 ft. /sec. = 4360 m/s Limestone / dolomite 50 microseconds per foot = 20,000 ft. /sec. = 6100 m/s Most of the nearby wells were drilled through Zechstein thins making the ratio of the two rock groups in the thicks very uncertain. The dolomite occurs erratically as a rafted section within the flowing halite and is difficult to constrain on seismic. In the absence of hard data, three models were used to generate the maximum range of Zechstein interval velocity maps and subsequent isopachs; 1. Use interval velocities from all nearby wells as done with overburden section. 2. Use a constant interval velocity of 4360 m/s. 3. Use a constant interval velocity of 6100 m/s. The three interval velocity maps and resulting Zechstein isopach maps are shown in Figures 9 and 10 below Fig. 9 Zechstein interval velocities from wells.

9 Figure 10 Zechstein Isopach generated from constant interval velocity of a) 4360 m/s, and b) 6100m/s Although the shapes of these three isopach maps are quite similar, the relative Zechstein thickness in each is very different. The thickness of the salt pillow in the northeast portion of the maps, for example, varies between 1200 m. in the interval velocity map, 1000m in the slowest velocity map, and 1900 m in the fastest velocity map. Adding the three overburden isopachs to these Zechstein isopachs will produce the full range of Top Silverpit / Top Leman depth map (Figures 11-14). The results are as follows; Figure 11 Top Silverpit from well velocities at regional and then detailed perspective. Pennard (yellow polygon) appears to be downdip of key dry hole 48/8b-2. A more detailed look with smaller contour intervals is needed to see if Pennard trap exists in this model. The detailed Silverpit Depth Map from well velocities shows a smaller set of traps (blue polygons) than originally drawn (red). Closing contours are 3270 m. (southern trap), 3315 m. (eastern trap) and 3280 m. (northern trap). All closing contours are downdip of Silverpit in well 48/8b-2 (3262m.). Note additional closure to the northeast (white) that is 900 acres and on trend with Argo Field.

10 Figure 12 Regional and detailed scale Top Silverpit Depth map constructed from a Zechstein interval velocity of 4360 m/s. The detailed Silverpit Depth Map from a constant Zechstein velocity of 4360 m/s shows a smaller set of traps (blue polygons) than originally drawn (red). Closing contours are 3270 m. (southern trap), 3320 m. (eastern trap) and 3282 m. (northern trap). All closing contours are downdip of Silverpit in well 48/8b-2 (3262m.). Note additional closure to the northeast (white) that is 980 acres and on trend with Argo Field. Figure 13 Regional and detailed scale of Top Silverpit Depth map constructed from a Zechstein interval velocity of 6100 m/s. The detailed Silverpit Depth Map from a constant Zechstein velocity of 6100 m/s shows a smaller set of traps (blue polygons) than originally drawn (red). Closing contours are 3265 m. (southern trap) and 3315 m. (eastern trap). All closing contours are downdip of Silverpit in well 48/8b-2 (3262m.). Note additional closure to the northeast (white) that is 1000 acres and on trend with Argo Field.

11 Figure 14 Overlay of all the Pennard traps created in this project. Obviously the variance in the Zechstein interval velocity has little effect on the size or location of the Pennard trap. The largest effect on the size and location is probably due to the consideration of the low velocity pre-triassic section in this depth conversion. The red polygon (above) used throughout this report to illustrate the approximate location of the Pennard prospect was the closure generated up-dip of the well when a slightly slower pretriassic velocity was used to generate the pre-triassic isopach. The push down effect of this low velocity section has a greater effect on the trap image in depth than the pull up effect of the Zechstein. Reservoir studies incorporating offset fields analyses conclude that fraccing would be required in order to deliver reasonable economic rates of deliverability: the expected average permeability, based on the offset well 48/8b-2 is expected to be less around 0.1mD. For fraccing to be feasible, a substantial hydrocarbon column would be required (> 250ft) and significant in-place volumes would need to be present. 4.0 Prospectivity Analysis The results of this mapping with new 3-D data and a more involved method of depth conversion adds significant risk to the presence of a structural trap up-dip of the 48/8b-2 well. The largest currently mapped area for the Pennard lead, based on using a Zechstein interval velocity of 6100m/s, gives a lowest closing contour of 3265m with closure of around 1200 acres and a maximum column height of around 70m. This is compared to the licence round estimate of over 5800acres of closure in the maximum case. In place reserves are now estimated at a maximum of 17bcf. The imaging of Pennard is more dependent on correcting for the low velocity pre-triassic than on the high velocity Zechstein section.

12 5.0 Conclusions Structural integrity was identified as one of the key risks for the Pennard Lead. Further work has indicated that there is significant risk attached to the likelihood of the presence of a trap up-dip from dry hole 48/8b-2. It is considered that a pre-stack depth conversion would be needed to answer the trap risk of Pennard in a more definitive way. Based on various depth conversions, the largest that the Pennard lead could now be is calculated at 17bcf in place (versus the 288bcf GIIP originally mapped in the application). A second small but more reliable and lower risk trap has been located to the east of the original lead, but calculations indicate only around 12bcf in place for this feature. The local reservoir quality remains a concern a reservoir study of offset fields and the 48/8b-2 well has indicated that fraccing would be required in order to deliver economic rates of flow. For fraccing to be feasible, a significant column and large in place reserves would be required to access sufficient reserves in such low permeability sandstones. None of the leads remaining have column heights that would be sufficient and all have less than 20bcf GIIP. Therefore they are not considered to be viable fraccing candidates. For these reasons, Endeavour considers that there is not a viable prospect to drill on block and therefore elects to drop the block.