Utilizing Tuning/AVO Phenomena to Predict Oil Column Height in the Tui, Amokura and Pateke Fields,...
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ASEG 2007 – Perth, Western Australia 1 Utilizing Tuning/AVO Phenomena to Predict Oil Column Height in the Tui, Amokura and Pateke Fields, New Zealand Balakrishnan Kunjan Leigh Brooks James Shadlow Australian Worldwide Exploration Ltd Australian Worldwide Exploration Ltd Australian Worldwide Exploration Ltd Level 9, 60 Miller St, North Sydney Level 9, 60 Miller St, North Sydney Level 9, 60 Miller St, North Sydney [email protected] [email protected] [email protected] Eric Matthews Naomi Osman Australian Worldwide Exploration Ltd Australian Worldwide Exploration Ltd Level 9, 60 Miller St, North Sydney Level 9, 60 Miller St, North Sydney [email protected] [email protected] INTRODUCTION In this paper we describe a back to basics approach of simultaneously analysing the tuning and AVO phenomena that occurs in the presence of varying column heights of oil in reservoir rocks of the Tui, Amokura and Pateke fields. This analysis was used to predict the column heights within the fields and influenced how the field was developed. The post drill data presented demonstrates the validity of the modelling, within the limits of seismic ‘noise’. At the time of drilling of the first well there were significant uncertainties in the definition of the structure and extent of the oil pools. The existing depth structural model (Figure 1) of the fields was based on the top of reservoir seismic two- way time structure map that was converted to depth using a calibrated stacking velocity map. It was acknowledged that despite the small areal extent of the fields and the presence of one well in each field to control the velocities, there was a strong possibility of depths within each field varying by ± 10 m from the map. Such uncertainty could result in the mapped pools being substantially different to pre-drill expectations. Alternative methods were needed to improve the depth definition of these fields. The confirmation of the tuning model after drilling the first well Tui-2H, led to a radical change in the placement of the subsequent well Tui-3H. Additional modelling of the Amokura-1 and Pateke-2 wells gave increased confidence to the pre-drill structural models. SEISMIC MODELLING AND RESULTS There were three steps to this work. The first step was to evaluate the best way to express the weakly anomalous amplitudes over the fields, so that it could form the basis of a predictive model. Secondly, a predictive model had to be built to explain these anomalies. The third step was to demonstrate that the prediction has worked. Several attributes like far offset stack amplitudes only, near offset stack amplitudes only and far amplitudes minus near amplitudes were reviewed but the most useful attribute was the simple summation of the instantaneous amplitudes of the trough and peak, which broadly correspond to the top and base of the F10 reservoir. Figure 2 shows the weak to moderate build up in this amplitude coinciding with the fields. These amplitude ‘anomalies’ are weak compared to gas related anomalies seen in Tertiary basins around the world. As the oils are low GOR (375 scuf/bbl), 42° API and the depth of the reservoir is at 3700 mSS, it was not understood how or if the subtle anomalies might be related to the presence of the oil. Tui Field Seismic Modelling The first model built was with the Tui-1 well data using some simple assumptions and without correcting for filtrate invasion. SUMMARY A combined tuning/AVO model was used to predict the oil column height for the Tui, Amokura and Pateke fields in the offshore Taranaki Basin, New Zealand. The Palaeocene aged Kapuni F10 reservoir sands occur at a depth of approximately 3700 mSS. The closure height and areal extents of these fields were initially mapped by converting time structure maps to depth using average velocities derived from stacking velocities. However, the traps at Tui are broad low relief features, extending up to 4 km laterally but with only limited vertical relief. This geometry means that even very small velocity variations have a significant impact on the location and extent of closure. Weak amplitude anomalies had been noted on the fields and a study was undertaken to review the tuning and AVO behaviour of the rocks in an attempt to understand the anomalies. Vp, Vs and density logs from the Tui-1 and Amokura-1 exploration wells showed that it was possible to map oil column heights using amplitudes extracted from the F10 sand seismic horizon. This modelling predicted an oil column height of 20 m at the crest of the Tui structure prior to the drilling of development well Tui-2H. In the same area a column height of about 10 m was predicted using the average velocity based depth maps. Drilling results showed a maximum column height of 21 m, confirming the validity of the modelling. This result led to the change in the direction of the next well Tui-3H. Four horizontal development wells have been successfully drilled with total estimated gross 2P reserves of 32 MMBO. Key words: tuning, AVO, oil column height, prediction
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Transcript of Utilizing Tuning/AVO Phenomena to Predict Oil Column Height in the Tui, Amokura and Pateke Fields,...
ASEG 2007 – Perth, Western Australia 1
Utilizing Tuning/AVO Phenomena to Predict Oil Column Height in the Tui, Amokura and Pateke Fields, New Zealand Balakrishnan Kunjan Leigh Brooks James Shadlow Australian Worldwide Exploration Ltd Australian Worldwide Exploration Ltd Australian Worldwide Exploration Ltd Level 9, 60 Miller St, North Sydney Level 9, 60 Miller St, North Sydney Level 9, 60 Miller St, North Sydney [email protected] [email protected] [email protected] Eric Matthews Naomi Osman Australian Worldwide Exploration Ltd Australian Worldwide Exploration Ltd Level 9, 60 Miller St, North Sydney Level 9, 60 Miller St, North Sydney [email protected] [email protected]
INTRODUCTION
In this paper we describe a back to basics approach of simultaneously analysing the tuning and AVO phenomena that occurs in the presence of varying column heights of oil in reservoir rocks of the Tui, Amokura and Pateke fields. This analysis was used to predict the column heights within the fields and influenced how the field was developed. The post drill data presented demonstrates the validity of the modelling, within the limits of seismic ‘noise’.
At the time of drilling of the first well there were significant uncertainties in the definition of the structure and extent of the oil pools. The existing depth structural model (Figure 1) of the fields was based on the top of reservoir seismic two-way time structure map that was converted to depth using a calibrated stacking velocity map. It was acknowledged that despite the small areal extent of the fields and the presence of one well in each field to control the velocities, there was a strong possibility of depths within each field varying by ± 10 m from the map. Such uncertainty could result in the mapped pools being substantially different to pre-drill expectations. Alternative methods were needed to improve the depth definition of these fields. The confirmation of the tuning model after drilling the first well Tui-2H, led to a radical change in the placement of the subsequent well Tui-3H. Additional modelling of the Amokura-1 and Pateke-2 wells gave increased confidence to the pre-drill structural models.
SEISMIC MODELLING AND RESULTS There were three steps to this work. The first step was to evaluate the best way to express the weakly anomalous amplitudes over the fields, so that it could form the basis of a predictive model. Secondly, a predictive model had to be built to explain these anomalies. The third step was to demonstrate that the prediction has worked. Several attributes like far offset stack amplitudes only, near offset stack amplitudes only and far amplitudes minus near amplitudes were reviewed but the most useful attribute was the simple summation of the instantaneous amplitudes of the trough and peak, which broadly correspond to the top and base of the F10 reservoir. Figure 2 shows the weak to moderate build up in this amplitude coinciding with the fields. These amplitude ‘anomalies’ are weak compared to gas related anomalies seen in Tertiary basins around the world. As the oils are low GOR (375 scuf/bbl), 42° API and the depth of the reservoir is at 3700 mSS, it was not understood how or if the subtle anomalies might be related to the presence of the oil. Tui Field Seismic Modelling The first model built was with the Tui-1 well data using some simple assumptions and without correcting for filtrate invasion.
SUMMARY A combined tuning/AVO model was used to predict the oil column height for the Tui, Amokura and Pateke fields in the offshore Taranaki Basin, New Zealand. The Palaeocene aged Kapuni F10 reservoir sands occur at a depth of approximately 3700 mSS. The closure height and areal extents of these fields were initially mapped by converting time structure maps to depth using average velocities derived from stacking velocities. However, the traps at Tui are broad low relief features, extending up to 4 km laterally but with only limited vertical relief. This geometry means that even very small velocity variations have a significant impact on the location and extent of closure. Weak amplitude anomalies had been noted on the fields and a study was undertaken to review the tuning and AVO behaviour of the rocks in an attempt to understand the anomalies. Vp, Vs and density logs from the Tui-1 and Amokura-1 exploration wells showed that it was possible to map oil column heights using amplitudes extracted from the F10 sand seismic horizon. This modelling predicted an oil column height of 20 m at the crest of the Tui structure prior to the drilling of development well Tui-2H. In the same area a column height of about 10 m was predicted using the average velocity based depth maps. Drilling results showed a maximum column height of 21 m, confirming the validity of the modelling. This result led to the change in the direction of the next well Tui-3H. Four horizontal development wells have been successfully drilled with total estimated gross 2P reserves of 32 MMBO. Key words: tuning, AVO, oil column height, prediction
Utilizing Tuning/AVO Phenomena to Predict Oil Column Height Kunjan et. al.
ASEG 2007 – Perth, Western Australia 2
Figure 3(b) shows the Tui-1 wells logs used for the modelling. For purposes of model building, sufficient section (75m) above and below the oil column was included so that all sources of potential wavelet interference have been taken into account. It was assumed that the section could be reasonably modelled by breaking it into five physical units as shown. The rock properties were obtained by plotting histograms (using the Petrolog software) of the respective log intervals and reading off the mean values. The simple assumption made for the modelling was that the Vp, Vs and density values seen in the oil column between 3664 m TVD and 3674 m TVD and the water column between 3674 m TVD and 3686 m TVD represent oil bearing and water bearing reservoir properties which can be used interchangeably within the sand between 3664 m and 3686 m TVD. Within this section, Gassmann’s substitution was not used for going from oil to water and from water to oil. The first sub model is for the case where all the reservoir units are wet. The next sub model is for the presence of 1 ms (2 m) of oil. The subsequent models are created by progressively adding oil in steps of 1ms (equivalent to 2 m) from the top of the reservoir at 3664 m TVD down to 3686 m TVD. For this sandstone depth interval all oil columns assume rock properties of Vp=4006 m/s (transit time 76.1 µs/ft), Vs=2280 m/s (transit time 133.7 µs/ft), density=2.338 g/cc and all water columns assume rock properties of Vp=4033 m/s (transit time 75.6 µs/ft), Vs=2248 m/s (transit time 135.6 µs/ft), density=2.343 g/cc. For each one of these sub models, zero offset reflectivities and 33° angle reflectivities were calculated for each of the five interfaces in the models using the Bortfeld-Aki-Richards AVO equation (Hampson Russell Knowledgebase). Although the 3D data was acquired using a relatively short 3600 m cable, refraction modelling using a ten layer velocity model shows that it is possible to obtain 33° incidence at the top of F10 reservoir depth at about 3700 mSS instead of a 26° incidence angle obtained without refraction. Figure 3 (a) shows the 33° AVO response at the five interfaces for the ‘as is’ model, i.e. representing the 10 m oil column encountered in the exploration well. The fifth interface is for the water filled case A 35 Hz Ricker wavelet was then convolved with the zero offset reflectivities and the 33° offset reflectivities. The four wavelets representing the 4 interfaces are then summed to produce a composite wavelet. This was done separately for the zero offset case and the 33° case. Figure 3 (c) shows the wavelets from the four interfaces at 33 degrees offset and the composite wavelet. The AVO surfaces and the wavelets share the same colour as the interfaces on the logs (Figure 3(b). The composite wavelet is in thick red colour. Composite wavelets were created for oil columns ranging from 0 m to 22 m in steps of 2 m (equivalent to approximately 1 ms). Figures 4(a) and 4(b) show the zero offset and 33° offset composite wavelets respectively for all these cases. From these wavelets, the peak and trough amplitudes were extracted for the various column heights to
create a column height versus amplitude chart as shown in Figure 4(c). These curves show that peak and trough amplitudes at both zero offset and 33° offset increase in amplitude with oil column height. From this analysis the column height in the high amplitude region close to the toe of the Tui-2H well was estimated to be about 20 m assuming that the high amplitude in this region was achieved through maximum tuning. The pre-drill depth map predicted a maximum column height of 10 m, whereas the actual maximum column height seen in the well was 21 m. The wells were drilled using Schlumberger’s PerisScope tool to attempt to track the top of the reservoir. After drilling the Tui-2H well, column heights along the horizontal well path were estimated from the well bore trajectory, the distance to the roof of the reservoir obtained from the PeriScope tool inversion (Figure 5(a)), and the oil water contact. The well path displayed on seismic is shown in Figure 5(b). The instantaneous amplitudes of the trough and peak at the top of reservoir exported from the Kingdom interpretation system were then summed and extracted along the well path in the Petrosys mapping system. Given the noise in the seismic data, amplitude values were extracted from 100 m smoothed grids along the well path. Figure 6(a) shows the extracted amplitudes along Tui-2H well path and the other 3 drilled well paths, plotted against column height. Despite the noise, there is good evidence for amplitude increase with column height for Tui-2H as well as the other wells. The successful prediction of column height in Tui-2H provided very strong encouragement to use the method for guiding the placement of the future well paths. The original Tui-3H well path to the west was abandoned on the basis of the lack of a consistent amplitude anomaly (Figure 2) and hence a significant oil column. Instead the path was redirected to the east where a stronger expression of amplitudes and hence oil column height was interpreted. Figure 7 shows how the revised Tui-3H well path sits well above the OWC in the post drill versus the pre drill map. It also shows the ‘risky’ nature of the original Tui-3H well path given its proximity to the OWC. OTHER WELL MODELS AND EFFECT OF INVASION CORRECTION A more complete model for Tui-1 was later created by proper fluid substitution, replacing oil with water and water with oil. In addition, invasion corrections were made, given that the invaded zone water saturation was 69% compared to reservoir water saturation of 32%. Both the fluid substitution and invasion corrections were done via Gassmann’s substitution (Smith et al., 2003). The next well modelled was Amokura-1 where the invaded zone had average water saturation of 79% whereas actual reservoir water saturation was 33%. The combined effect of proper Gassmann substitution for fluid replacement, and invasion correction, for Tui-1 are quite marked as shown in Figure 6(b). Figure 6(c) for Amokura-1 also shows a marked increase in modelled amplitudes even though the original model was properly Gassmann substituted for changes from oil to water and vice versa. In both figures the broken lines represent the simpler models and the
Utilizing Tuning/AVO Phenomena to Predict Oil Column Height Kunjan et. al.
ASEG 2007 – Perth, Western Australia 3
continuous lines are for the proper fluid substituted and invasion corrected cases. Although the Pateke-2 well was modelled, it is not possible to review the results here. The Pateke-2 exploration well only had LWD logs including density but no sonic logs. A zero offset model was carried out by using density to Vp conversions obtained from analysis of the two nearby exploration wells Tui-1 and Amokura-1. No attempt was made to model 33° offset as in Tui and Amokura because of the lack of Vs data. The Pateke-2 model also showed the increase in amplitude with oil column height. The modelling of both Amokura and Pateke wells gave added confidence to the tuning phenomena. However, the well paths for Amokura-2H and Pateke-3H were not significantly changed in these cases, as the well paths were optimally placed on the existing structure maps. The modelling provided additional confidence in pre-drill structural maps. Seismic Issues The modelling from well data from all three exploration wells predicts a consistent increase of amplitude with increasing oil column height. Although the quality of the seismic data is good enough for overall structural mapping, it is less than perfect for consistent mapping of hydrocarbon columns as described in this paper. A significant component of the noise in the column height versus amplitude plot is from acquisition footprint. Reprocessing of the 3D data is currently underway to improve the fidelity of the data. Modelling Issues Prior to modelling, there was very little expectation for the use of seismic amplitudes in defining the oil pools in this area. A cursory look at the well logs would not have shown the effect that tuning can play in providing sufficient change in amplitudes for the delineation of the oil pools. One of the contributors may be the fact that invasion can mask the subtle rock property changes going from oil to water. There is a significant increase in the modelled amplitudes at maximum tuning if invasion is corrected for. Between AVO and tuning effects, in the wells modelled here, it is found that tuning provides most of the amplitude related to the presence of oil.
SUMMARY AND CONCLUSIONS
Simultaneous analysis of the tuning/AVO phenomena occurring with varying column heights allowed prediction of the column heights from seismic amplitudes. The confirmation of the modelling from drilling Tui-2H, led to a radical change in the placement of the Tui-3H well. The drilling of the revised well path of Tui-3H provided further support for the model and led to modelling of the Amokura-1 and Pateke-2 wells. These wells showed the same tuning/AVO phenomenon and gave additional confidence to the pre-drill structural maps of these fields. Comparison of the pre drill depth map and the post drill depth map (Figure 7), in the Tui field shows the increased oil column heights which match with the increased amplitudes observed pre-drill. A relatively simple model was able to predict the tuning/ AVO behaviour in the Tui oil field. Later corrections enhanced the modelled basic relationship of increasing seismic amplitudes with oil column heights.
ACKNOWLEDGMENTS The authors wish to thank the Tui development Joint Venture partners of PMP 38158, Mitsui E&P New Zealand Limited, Stewart Petroleum Company Ltd (“New Zealand Oil and Gas”), WM Petroleum Ltd (“Pan Pacific Petroleum NL”) and Australian Worldwide Exploration for permission to publish this paper.
REFERENCES Hampson Russell Knowledgebase website, 2006 Mavko, G., Mukerji, T., Dvorkin, J., 1998 – The Rock Physics Handbook. Smith, T.M., Sondergeld, C. and Rai, C.S., 2003, Gassmann fluid substitutions: A tutorial: Geophysics, 68, 430-440. Wang, Z., Wang, H. and Cates, M.E., 2001, Effective elastic properties of solid clays: Geophysics, 66, 428-440
Utilizing Tuning/AVO Phenomena to Predict Oil Column Height Kunjan et. al.
ASEG 2007 – Perth, Western Australia
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Figure 1. Pre-drill depth structure map, top F10 Sand.
Figure 2. Summation of the instantaneous amplitudes (grid) from top and base F10 Sand, pre drill depth contours overlain. Note the moderate build up of amplitudes associated with the fields.
(a) (b) (c)
Figure 3. (a) Computed AVO for the four interfaces and the water case, (b) Tui-1 well logs and modelled interfaces, and (c) the wavelets from the four interfaces for 33 degrees offset, 10m oil - ‘as is’ case. Note that a trough equals a decrease in acoustic impedance.
Wavelets from the interfaces
at 33 Degrees
Top of Shale 3635mTVD
Top of F10 Sand 3664mTVD
OWC 3674mTVD
Top of Silty Sand 3686mTVD
Base of Silty Sand 3699mTVD
78.7μsec/ft
74.9μsec/ft
75.6μsec/ft
76.1μsec/ft
74.2μsec/ft
3710mTVD
133.7μsec/ft
135.6μsec/ft
135.7μsec/ft
134.8μsec/ft
141.4μsec/ft2.536 g/cc
2.338 g/cc
2.343 g/cc
2.378 g/cc
2.354 g/cc
75 m
35 Hz Ricker used
Composite wavelet
ρ Vs GR Vp
Tui-1 Well logs and properties
Zerodeg reflectivity
33deg reflectivity
-0.0300
-0.0200
-0.0100
0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0 10 20 30 40 50 60 70
Angle
Am
plitu
de
33 Degrees
Computed AVO at the interfaces
Shale to Water case
Wavelets from the interfaces
at 33 Degrees
Top of Shale 3635mTVD
Top of F10 Sand 3664mTVD
OWC 3674mTVD
Top of Silty Sand 3686mTVD
Base of Silty Sand 3699mTVD
78.7μsec/ft
74.9μsec/ft
75.6μsec/ft
76.1μsec/ft
74.2μsec/ft
3710mTVD
133.7μsec/ft
135.6μsec/ft
135.7μsec/ft
134.8μsec/ft
141.4μsec/ft2.536 g/cc
2.338 g/cc
2.343 g/cc
2.378 g/cc
2.354 g/cc
75 m
35 Hz Ricker used
Composite wavelet
ρ Vs GR Vp
Tui-1 Well logs and properties
Zerodeg reflectivity
33deg reflectivity
-0.0300
-0.0200
-0.0100
0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0 10 20 30 40 50 60 70
Angle
Am
plitu
de
33 Degrees
Computed AVO at the interfaces
Shale to Water case
Top of Shale 3635mTVD
Top of F10 Sand 3664mTVD
OWC 3674mTVD
Top of Silty Sand 3686mTVD
Base of Silty Sand 3699mTVD
78.7μsec/ft
74.9μsec/ft
75.6μsec/ft
76.1μsec/ft
74.2μsec/ft
3710mTVD
133.7μsec/ft
135.6μsec/ft
135.7μsec/ft
134.8μsec/ft
141.4μsec/ft2.536 g/cc
2.338 g/cc
2.343 g/cc
2.378 g/cc
2.354 g/cc
75 m
35 Hz Ricker used
Composite wavelet
ρ Vs GR Vp
Top of Shale 3635mTVD
Top of F10 Sand 3664mTVD
OWC 3674mTVD
Top of Silty Sand 3686mTVD
Base of Silty Sand 3699mTVD
78.7μsec/ft
74.9μsec/ft
75.6μsec/ft
76.1μsec/ft
74.2μsec/ft
3710mTVD
133.7μsec/ft
135.6μsec/ft
135.7μsec/ft
134.8μsec/ft
141.4μsec/ft2.536 g/cc
2.338 g/cc
2.343 g/cc
2.378 g/cc
2.354 g/cc
75 m
35 Hz Ricker used
Composite wavelet
ρ Vs GR Vp
Tui-1 Well logs and properties
Zerodeg reflectivity
33deg reflectivity
-0.0300
-0.0200
-0.0100
0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0 10 20 30 40 50 60 70
Angle
Am
plitu
de
33 Degrees
Computed AVO at the interfaces
Shale to Water case
Zerodeg reflectivity
33deg reflectivity
-0.0300
-0.0200
-0.0100
0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0 10 20 30 40 50 60 70
Angle
Am
plitu
de
33 Degrees
Computed AVO at the interfaces
-0.0300
-0.0200
-0.0100
0.0000
0.0100
0.0200
0.0300
0.0400
0.0500
0 10 20 30 40 50 60 70
Angle
Am
plitu
de
33 Degrees
Computed AVO at the interfaces
Shale to Water case
Utilizing Tuning/AVO Phenomena to Predict Oil Column Height Kunjan et. al.
ASEG 2007 – Perth, Western Australia
5
(a) (b) (c) Figure 4. Composite wavelets for various oil column heights , (a) Zero degree offset , (b) 33 degrees offset, (c) tuning curves for the peaks and troughs at Zero offset and 33 degrees offset.
(a) (b) Figure 5. (a) Tui-2H PeriScope inversion showing top F10 reservoir structure with post drill depth structure map below, (b) Tui-2H well path (pink dashed line) displayed on seismic. Green horizon is the top of the reservoir. In the seismic section, a trough, represented by brown, represents an interface where acoustic impedance decreases.
Utilizing Tuning/AVO Phenomena to Predict Oil Column Height Kunjan et. al.
ASEG 2007 – Perth, Western Australia
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(a) (b) (c) Figure 6. (a) Cross plot of actual extracted seismic amplitudes along the four drilled development well paths (b) Invasion correction effects on the tuning curves for (b) Tui-1 and (c) Amokura-1 wells. Figure 7. Note that the revised Tui-3H well path is optimally placed well above the oil water contact in the post versus the pre drill map. The original well path to the west still looks ‘risky’ on the post drill map.
