November 2014 Geophysical Society of Houston 11

Summary

Hydrocarbon identification within class 1 AVO reservoirs has tended to be problematic using conventional petroacoustic analyses due to the dominance of the hard rock matrix, leaving poor differentiation of fluids from elastic parameters. Additionally, good quality long offset seismic data is a pre-requisite for any attempt at fluid and lithology discrimination.

The challenge required in this study is to de-risk a previously overlooked stratigraphic play, dominated by class 1 AVO behavior, constrained by limited offset seismic, but embellished by a recent well with high quality comprehensive wireline logs.

Integration of petroacoustic modeling with conventional interpretation, frequency and amplitude seismic attributes, and seismic co-rendering has revealed an extensive new stratigraphic play offshore Cameroon. Rock physics results from the recent well IM-5 have been compared with other drilled reservoirs in the region to calibrate different seismic attribute responses across the newly defined Intra Isongo reservoir fairway. This has led to development of a qualitative predictive model with respect to thickness, porosity, and net to gross.

Recognition and delineation of the play is presented here with a set of geophysical guidelines to aid exploration in similar petroacoustic provenances.

Introduction

The Etinde permit, located in the Cameroon sector of the Gulf of Guinea, is situated at the northern end of a string of West African coastal basins extending some 2000 kms to the south (Figure 1).

A significant geological feature which splits the Etinde permit into two basins, the Rio del Rey Basin and the Douala (and southern) Basins is the Cameroon Volcanic Line (CVL). Several hydrocarbon discoveries have been made since the 1960’s in the Rio del Rey Basin. All were drilled on structural highs and targeted the Miocene Upper and Middle Isongo reservoirs.

Early in 2013, Etinde operator Bowleven opened up a new West African stratigraphically-controlled hydrocarbon play in the Rio del Rey Basin. This was the result of

drilling exploration/appraisal well IM-5. The primary objective was appraisal of the four-way dip-closed Middle Isongo reservoir, proven to be hydrocarbon bearing by Mobil well IM-3 in 1975. Analysis of rock physics from the limited modern well stock in the area had illustrated that reservoir sands were expected to be relatively hard and behave in a similar petroacoustic manner to the volcaniclastic sediments in the area. They exhibit class 1 AVO response in all fluid cases, as confirmed by the modeling of the IM-5 well (Figure 2).

Interrogation of log-derived elastic parameter models suggest that fluid impedance generated from relative

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Integration of Rock Physics and Seismic Interpretation – An Overlooked West African

Stratigraphic Hydrocarbon Play By Ken MacAllister, Tim Daley, Bowleven PLC; Mike Bacon, Ikon Science;

Simon Tamfu, Ponce Nguema, Societe Nationale des Hydrocarbures (SNH)

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Figure 1: Location of Etinde permit (orange polygon). (Bowleven)

Figure 2: Intra Isongo fluid substituted synthetics (0 - 40°; fluid substituted blue logs brine, red curves gas) for zero-phase statistical wavelet, demonstrating Class 1 AVO at top sand. (Bowleven/SNH).

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seismic inversion may discriminate hydrocarbon from brine. However, the constraints associated with relatively short acquisition cable lengths (4 km) and poorly conditioned gathers limit the reliability of the inverted seismic data. Good near-offset data does, however, enable generation of pseudo lithology seismic for this rigid rock matrix. Conventional interpretation and integration of various seismic attributes revealed depositional geometries associated with a very hard unit positioned in a structural low, which fringed the edge of the primary structural target (Figure 3). Therefore, the amplitude driven Intra Isongo secondary objective was targeted. Pre-drill subsurface risk was high due to the proximity of the CVL and the potential of encountering nonreservoir facies.

The well encountered a 77 m clean Intra Isongo hydrocarbon bearing sandstone reservoir. No hydrocarbon contact was encountered, and the well flowed on test at a peak rate of > 40 mmscf/d from the Intra Isongo.

A comprehensive wireline logging suite was acquired in the IM-5 well which enabled a detailed petroacoustic evaluation (Figure 2), complemented by integration of frequency- and amplitude-dependent seismic attributes. The results led to the creation of a hierarchical workflow which has enabled delineation of this extensive and prolific reservoir.

The following reservoir evaluation guidelines were established.

1. Structural interpretation to establish reservoir datum.

2. 2D fluid substitution modeling.

3. Establishment of elastic parameter influence.

4. 2D tuning wedge modeling.

5. Mapping of petroacoustic properties.

6. Generation of dominant frequency seismic volumes.

7. Frequency filtering and blending.

8. Correlation of petroacoustic characteristics

9. Reservoir fairway delineation.

Integrated interpretation procedure

Hard ‘bright’ seismic amplitudes associated with the Intra Isongo have been recognized in this locality for a considerable time. However, with the close proximity of the Cameroon Volcanic Line (CVL) combined with the very hard seismic response, this unit was considered to be most likely of volcanic origin, and had remained undrilled.

The base of the unit forms a widespread characteristic soft seismic event which was used as the datum for horizon slicing. This procedure revealed clear depositional confinements and channel geometries, as well as apparent variations in seismic-reservoir-facies dispersion across the block. The high impedance contrast at the reservoir base also provided a platform to develop a number of interpretation guidelines (Figure 4).

Having established the top and base reservoir tuning envelope, the reservoir fairway was interpreted using the guidelines shown in Figure 4. This method demonstrated the gross reservoir fairway but gave no further indication of the potential reservoir property variation and dispersion.

A frequency spectrum of the reservoir interval introduced the bandwidth limitations which constrained a set of frequency transforms and filters to enable three dominant frequency bands to be combined and colour blended.

Figure 3: Seismic line along axis of Intra Isongo depositional system with map inset showing position of fairway on the flank of a present day structural low. (Bowleven/SNH/EAGE)

Figure 4: Tuning wedge and seismic line illustrating interpetation guidelines. 42-degree wavelet is a deterministic extraction from near stack seismic. (Bowleven/SNH/EAGE)

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Generic tuning curves created with Ricker wavelets of increasing frequency combined with the known interval velocity of the Intra Isongo sand has also enabled thickness estimates to be made for the spectrally decomposed slices through the reservoir (Figure 5). These show clearly the

dominant reservoir fairway axis and also the dispersive nature of the thinner-bedded overbank deposits. This is clearly illustrated in Figure 6.

The frequency colour blend gives clear indication of the depositional axis which corresponds to the conventional interpretation. However, the frequency blend also enabled significant secondary and tertiary channel bodies to be identified and qualitative thickness estimates to be assigned (Figures 5 and 6).

Recognition that the Intra Isongo reservoir was very extensive (> 100 km2) but variable in thickness led to further interrogation of rock physics models. This demonstrated that the data clusters fringing the nearby IE field were indeed part of the same reservoir system.

The continuum of the Vp/RhoB behavior observed from the IM area across to IE has led to establishment of an associated porosity trend (Figure 7).

Having established the reservoir thickness variation through the methods described, further changes in amplitude throughout the reservoir fairway can now be calibrated to the primary variables shale volume (Vsh) and porosity as illustrated in Figures 8 and 9. It does appear, however, that the primary factor affecting amplitude variation is porosity (Figure7).

This has led to the Intra Isongo reservoir fairway being described in terms of thickness and reservoir quality. The highly integrated geophysical interpretation methodology described herein, has been adopted as an integral part of the ongoing reservoir appraisal and development strategy. Predictive models based on the

Figure 6: RGB Colour-blended frequency bands illustrating primary reservoir fairway and secondary depositional systems. Light=thick. Dark=thin. (Bowleven/SNH).

Figure 5: Reservoir thickness estimates from tuning envelopes. (after IHS Kingdom - modified).

Figure 7: Sand trend is sub-parallel with the modified Hashin-Shtrikman upper bound for quartz/brine mixture.

Figure 8: Model of variation in acoustic impedance (AI) with increasing Vsh in Intra Isongo reservoir.

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geophysical observations have been used to optimize appraisal and development well selection for this new stratigraphically controlled reservoir.

Conclusions

Integration of multiple geophysical interpretation techniques has led to:

1. A new class 1 AVO, stratigraphically controlled, hydrocarbon play being discovered offshore Cameroon.

2. Acceptance that seismically bright ‘hard’ lithologies can be highly productive reservoirs.

3. A clear set of interpretation hierarchy to provide consistent input to field development planning.

4. A seismically driven predictive model being developed.

5. A significant risk reduction process in optimising appraisal and development well selection.

6. Identification of additional analogous seismic amplitude driven targets.

7. Recognition of the requirement for suitably conditioned surface seismic to provide a stable platform for reservoir characterization focused inversion.

Acknowledgements

Bowleven PLC: Tom Newell, Andy Imrie, Neil Ementon, Graeme Moore, Steve Cannon.

Societe Nationales des Hydrocarbures (SNH)

Joint Venture Partners

IKON Science

Figure 9: Model of variation in AI with increasing porosity in Intra Isongo reservoir.

http://dx.doi.org/10.1190/segam2014-0041.1

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