EAGE 68th Conference & Exhibition — Vienna, Austria, 12 - 15 June 2006

P297Dual Inversion Applied to 2DMulti-Component Seismic DataOnshore LibyaC. Hanitzsch* (Wintershall AG), L. de Vincenzi (Wintershall Libya), J.M. Michel (CGG)& D. Semond (CGG)

SUMMARYIn line with the theme of the conference 'Opportunities in Mature Areas' (Sirt basinonshore Libya), 2D3C (multi-component) surface seismic data have been acquired,processed and inverted on behalf of Wintershall Libya in concession C97-I. In thisarea, presence of non-reservoir is the main risk for new wells. Reservoir sandstone canbe partly or fully missing because of shallow basement or intrusions of basalt ordeposition of volcanoclastics. From integrated reservoir characterisation studies itwas concluded that it is the ratio of compressional to shear wave velocity (Vp/Vs) thatallows differentiating reservoir sandstone from non-reservoir. Vp/Vs can be estimatedby simultaneous ("dual") inversion of multi-component surface seismic data. In areasof good seismic data quality, the Vp/Vs inversion results correlate very well with welldata. In areas with medium to poor seismic quality, Vp/Vs sections can still beinterpreted in a relative sense. Therefore, the technology has the potential to helpreduce the risk of dry wells in this area.

EAGE 68th Conference & Exhibition — Vienna, Austria, 12 - 15 June 2006

Introduction State-of-the-art processing applied to conventional (P-wave) seismic data does not allow discriminating the nearly 4 km deep Sarir reservoir sandstone from basement, volcanoclastics and basalt dykes and sills in the study area. Only large basalt bodies can be recognised by P-wave based seismic reservoir characterisation. Well sonic log data show low Vp/Vs ratio for reservoir (1.55 – 1.7) and high Vp/Vs ratio (1.8 – 2.1) for all non-reservoirs (with one exception). Four different types of basement have been described in the area. One of them, the Gargaf Group, shows also a low Vp/Vs ratio (about 1.7). The Gargaf Group is heavily fractured and fluid-filled, that explains the low Vp/Vs value. From the well data analyses it was recognised that the Vp/Vs ratio (or the related Poisson’s ratio) is the physical parameter that allows discrimination of reservoir from non-reservoir (basement, volcanoclastics and basalt, with the exception of the Gargaf Group) in this area. The challenge is to accurately predict Vp/Vs in a deep target prior to drilling. Therefore, a 2D3C (multi-component) survey consisting of 11 lines (212 km total length) was acquired by AGESCO / CGG in 2004 and processed by CGG in 2005 on behalf of Wintershall Libya. It was the first multi-component acquisition in Libya. A vibrator array was used as P-source and three-component geophones as multi-component receivers. The large maximum offset of 5.5 km ensures capturing reflection angles up to 40 degrees and allows for most accurate estimation of P- and S- velocities. Upholes (P- & S- mode) were recorded every kilometer to minimize potential problems with S - wave static corrections. The processing and inversion work flow was specific for the purpose of this project, because it is one of the first cases that such a technology is applied for lithology detection. A true amplitude processing sequence of full Kirchhoff prestack time migration was applied in a consistent way for PP- and PS- data. Strong acquisition noise was filtered on both PP- and PS- seismic traces. The availability of P- and S-wave well velocity data from shallow to target was one key for successful processing. The polarity was controlled using the PP- and PS- reflectivity coefficients of wells and the seismic amplitude has been calibrated to well reflection coefficients. The results of seismic processing were used for a simultaneous elastic inversion (“Dual Inversion”) that finally provided Vp/Vs sections. Dual Inversion The dual inversion (Garotta et al., 2002; Dariu et al., 2003) is a method that can be used to obtain accurate elastic rock properties from joint inversion of multi-component AVO data. Unlike conventional inversion, which uses single mode waves in the inversion process, the dual inversion reconciles simultaneously amplitude and time information from both PP- and PS- data. The inversion is done in two steps: first the Vp/Vs is computed that minimizes the misfit between the D(Vp/Vs) derived from PP- and PS- transit time and the D(Vp/Vs) derived from seismic amplitude. In the second step, the obtained optimum Vp/Vs values are used for the estimation of Vp, Vs and density values. Vp, Vs and density are randomly perturbed and from this synthetic AVO attributes are computed. The real and the synthetic AVO attributes are compared in a 1D global optimization scheme using a variant simulated annealing in order to find the optimum solution. A- priori parameter constraints are used to force the solution to be close to the low frequency parameter trend. The input data of the dual inversion are the PP- AVO attributes Rpp (PP- wave intercept), Gpp (PP- wave gradient) and Gps (PS- wave gradient). The main inversion parameters are the Vp/Vs a-priori model and the limits of search area for Vp/Vs, Vp and density. Combined inversion of PP- and PS- wave AVA attributes increases the robustness and resolution of Vp/Vs ratio definition. Matching time and amplitude information from geological events allows the tying of PP- and PS- wave data sample by sample. The global optimization using a form of simulated annealing, in which the step length is automatically

adjusted during the cooling schedule, is a robust technique allowing increasing the accuracy of inversion results. Discussion of results Twelve wells are located inside the study area. For six of them, P- and S- wave sonic log data are available over the reservoir section. Well data are only indirectly used in the inversion (for QC, amplitude scaling and polarity control). Several well datasets were used as blind tests and only made available for comparisons afterwards. These hard datasets were most useful for gaining trust and understanding the limitations of the technology. Seismic data quality was identified to be the most crucial condition for success of Vp/Vs calculation. In the areas of good seismic data quality, it is possible

• to consistently pick and relate events on PP- and PS- seismic sections that are used for the a-priori model,

• for the inversion to successfully minimize misfits derived from meaningful transit times and seismic amplitudes,

• therefore, to obtain Vp/Vs ratio sections (Figure 1) that correlate very well with well log data (Figure 2).

Even in areas of medium to poor seismic data quality a reservoir interpretation is possible. If PP- and PS- events can be consistently picked, the inversion still manages to estimate Vp/Vs ratio sections that can be interpreted in a relative sense: lower values point towards reservoir, higher values towards non-reservoir. In areas of very weak amplitudes it is not possible to obtain reliable estimates. This is for example observed below a major graben with associated scattering and attenuation of the seismic data. Several sensitivity analyses were performed in order to gain trust and to understand the limitations of the results of the technology. The conclusion is that it is not very important which horizons we pick as top and bottom for the a-priori model. It is important that we pick consistently the same event on both, PP- and PS- seismic sections. Dual Inversion was applied twice with different a-priori models as input for all lines. The similarities and differences of results are useful for interpretation of final results. For many intersections of 2D lines, the results show a good correlation except for some intersections that are located close to major faults (Figure 3). In such cases 3D effects affected the 2D multi-component processing and inversion. A 3D multi-component survey would overcome this limitation. Conclusion In areas of good seismic data quality, the Vp/Vs ratio estimates from Dual Inversion of multi-component seismic data show an excellent match with well sonic log data. In areas with medium to poor seismic quality, Vp/Vs sections can still be interpreted in a relative sense. The Gargaf Group, a particular highly fractured basement, cannot be discriminated using this technology because it shows Vp/Vs values too close to those of reservoir. Interpretation across or close to major faults should be avoided due to 3D artefacts present in this 2D dataset. Multi-component surface seismic data have the potential to help reduce the risk of dry wells in this concession. Acknowledgements The authors acknowledge the National Oil Corporation (NOC) of Libya, Wintershall-Libya and CGG for permission to publish. We thank our colleagues Z. Ajub, W. Heerde, M. Fleckenstein (Wintershall), M. King (Monarch Technical Services) and many colleagues at CGG for their technical support in this project.

EAGE 68th Conference & Exhibition — Vienna, Austria, 12 - 15 June 2006

References Dariu, H., Garotta, R. and Granger, P. [2003] Simultaneous inversion of PP and PS wave AVO/AVA data using simulated annealing. Society of Exploration Geophysicists, Expanded Abstract, 120-123. Garotta, R., Granger, P. Y. and Dariu, H. [2002] Combined interpretation of PP and PS data provides direct access to elastic rock properties. The Leading Edge, 21, no. 6, 532-535.

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Figure 1: Vp/Vs (time) section of 2D line no. 2 estimated by Dual Inversion. Blue areas (low Vp/Vs) are interpreted as reservoir, orange / red areas (high Vp/Vs) as non-reservoir. The red dashed lines denote locations of oil wells – both are in blue areas. Intersections with two other lines are also annotated.

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Oil well Vp/Vs logInverted Vp/VsLine 2 CDP 605

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Lower ReservoirVolcanicsT.DBottom horizon for Dual Inversion

Figure 2: Vp/Vs estimated from Dual Inversion of multi-component seismic data (red line) compared to up-scaled sonic log Vp/Vs (black line), together with lithology column. It is the oil well displayed on the right of Figure 1. Note the excellent correlation.

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Figure 3: 3D view of Dual Inversion results for all eleven multi-component 2D lines.