75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 London, UK, 10-13 June 2013

Tu P11 08The Application of Elastic Impedance Inversionand Pre-stack Attribute Analysis for ProspectIdentification - A Case StudyH. Karbalaali (Petroleum University of Technology), S.R. Shadizadeh(Petroleum University of Technology), M.A. Riahi (University of Tehran) &S.H. Karimi* (University of Newcastle)

SUMMARYIn this study, with limited well control and pre-Stack Time Migrated (PSTM) seismic data, the advantageof performing Elastic Impedance Inversion is investigated for an offshore sandstone reservoir, south ofIran. Pre-stack simultaneous inversion is also applied in order to extract several pre-stack seismicattributes including: Lame parameters, Poisson Impedance and Poisson Dampening Factor. The attributeanalysis at the well location is studied to find the best fluid-sensitive factors for delineation of hydrocarbonbearing zone of the reservoir. The result of the analysis at the well location is generalized to the volume ofstudy since the reservoir under study is considered as a homogenous layer without any complicatingfactors of sealing faults, secondary porosity and lateral change of lithology.

75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 London, UK, 10-13 June 2013

Introduction

Reservoir characterization plays an important role in different parts of an industrial project. The results from a reservoir characterization study gives insight into rock and fluid properties which can optimize the choice of drilling locations and reduce risk and uncertainty. Delineating hydrocarbon bearing zones within the reservoir is the main objective of any seismic reservoir characterization study. In the current study with PSTM seismic data and limited well control, an attempt was made to predict the productive zone of the reservoir using Elastic Impedance Inversion, pre-stack simultaneous inversion and attribute analysis. The focus of this paper is on the application of elastic impedance inversion, Poisson Dampening Factor (PDF) and lame parameter of incompressibility to discriminate productive zone of the reservoir. However, like all seismic derived approaches, this technique is not one of a kind and needs to be validated with other types of data.

The current study is on a southern Iranian offshore oilfield. The field is a NW-SE trending anticline dipping by 1-1.5 degrees. Hydrocarbon is produced from the Oligocene-Miocene formation having a vertical closure of 62 meters. The reservoir consists of three units: Upper carbonate, target sandstone and Lower carbonate. The target reservoir in this study consists dominantly of sandstone with interbedded dolomite and shale.

Elastic Impedance Inversion

Connolly (1999) claimed that it is possible to process 3D data sets as partial offset/angle stacks. He asserted that the amplitudes of far-offset stack data can be tied to well logs using synthetics based on a function called elastic impedance (EI) which is similar in nature to acoustic impedance. EI is defined as follows:

EI(θ)= VP(1+tan2θ). Vs

(-8ksin2θ).ρ(1-4ksin2θ)

Figure 1, which is the crossplot of elastic impedance at 35 versus P-Impedance, confirms the advantage of elastic impedance inversion respect to conventional post-stack inversion in the current study for delineating hydrocarbon bearing zone of the reservoir. The analysis for obtaining proper angle range for generating partial angle stacks of seismic data revealed two appropriate ranges one for angles between 0-28 and the other for angle range of 28-42 degrees. Model-based post-stack seismic inversion was performed on these partial stacks to obtain elastic impedance at near- and far-angles of incidence.

Pre-stack simultaneous Inversion

Pre-stack simultaneous inversion introduced by Hampson et al. (2005) can be thought of as a generalization of post-stack seismic inversion into pre-stack domain. P-Impedance, S-Impedance and density can be simultaneously outputted from the inversion result. However, in the current study, the angle range at the horizon of interest does not exceed 45 degrees, so density cannot be estimated reliably. Lame parameter of incompressibility (λ) which is obtained from pre-stack seismic data is an efficient attribute for discriminating fluid. Poisson Impedance (PI) is a new attribute which was introduced by Quakenbush et al., (2006) as follows: Mazumdar (2007) extended the work of Quakenbush et al. and introduced a new attribute called

Poisson Dampening Factor (PDF) which is related to Poisson Impedance and Poisson ratio ( ) as follows:

PDF* PI.

75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 London, UK, 10-13 June 2013

Sensitive attribute analysis

Crossplot analysis has been made in the well location to find the sensitive factors for delineating the productive zone within the reservoir. Figure 2 illustrates the crossplot of PDF versus Poisson Impedance at the well location. PI, inherently, has the advantage of discriminating hydrocarbon bearing zone much stronger than AI and SI. As it is clear from the Figure 2, PDF is more robust in distinguishing the productive zone of the reservoir respect to Poisson Impedance in this study. A section of PDF derived from pre-stack seismic inversion is shown in Figure 3. Note the possible productive zone is highlighted as a zone of high PDF. Figure 4 displays the section of incompressibility which is consistent with the PDF result in delineating the hydrocarbon bearing zone of the reservoir. Figure 5 illustrates the crossplot of elastic impedance at near angle of incidence, i.e., 14 degrees versus elastic impedance at far angle of incidence, i.e., 35 degrees at the well location. The colour key is water saturation. Figure 6 demonstrates a crossplot of Lambda-Rho versus Mu-Rho in which the third dimension is water saturation. Note the Hydrocarbon bearing sandstones are delineated as a zone of low incompressibility and rigidity. Figure 7 displays the crossplot of elastic impedance of near-angle stack seismic volume versus elastic impedance of far-angle stack seismic volume. The hydrocarbon bearing sandstones are defined as a square in the figure according to the result at the well location. Figure 8 shows the cross section of seismic volume at inline 250 at which the predicted hydrocarbon bearing sandstones are depicted. Figure 9 is a crossplot of Lambda-Rho volume versus Mu-Rho volume in which hydrocarbon bearing sandstones are highlighted as a blue square due to the analysis result at the well location. Figure 10 displays the predicted hydrocarbon bearing sandstones at inline 250. Note the consistency of the attribute analysis outputs of elastic impedance and pre-stack inversions.

Figure 1 Crossplot of EI_35 ͦ vs.P-Impedance Figure 2 Crossplot of PDF vs.PI at at the well at the well location. location.

Figure 3 PDF attribute section. Figure 4 λρ attribute section.

75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 London, UK, 10-13 June 2013

Figure 5 Crossplot of EI_14 ͦ vs. EI_35 ͦ at the Figure 6 Crossplot of λρ vs. µρ at the well well location. location.

Figure 7 Crossplot of inverted EI_near vs. Figure 8The hydrocarbon bearing zone shown on inverted EI_far. seismic section.

Figure 9 Crossplot of λρ volume vs. µρ Figure 10The hydrocarbon bearing zone shown on volume. seismic section.

75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013 London, UK, 10-13 June 2013

Conclusion

1. Crossplot analysis of inverted elastic impedance volumes at near and far incidence angle provided a means of discriminating productive zone of the reservoir.

2. Examining several pre-stack seismic attributes including LMR parameters, Poisson Impedance and Poisson Dampening Factor revealed that PDF and incompressibility (λρ) are the best fluid-sensitive attributes for delineating producing zone within the reservoir.

3. The result of LMR crossplot analysis for delineating hydrocarbon bearing zone of the reservoir was consistent with the elastic impedance inversion result.

4. The attribute analysis at the well location was generalized to the whole seismic volume to obtain a reconnaissance view to the whole volume of study for exploration and developing plans. Although this procedure may accompany some level of uncertainty in heterogeneous reservoirs, the reservoir under study is considered as a homogeneous reservoir without any complicating factors of sealing faults, secondary porosity and lateral change of lithology. Therefore, it is possible to apply the result at the well location to the whole volume of study without any other geological control available.

Acknowledgment

The authors like to express gratitude to NIOC, Exploration Directorate, particularly Mr. Sokooti and Mr. Khoshdel for providing the technical support for this study.

References

Connolly, P. [1999] Elastic Impedance. The Leading Edge, 438-452. Goodway, B., Chen, T. and Downton, J. [1997] Improved AVO fluid detection and lithology discrimination using Lame petrophysical parameters. 67th SEG International Annual Meeting, Denver. Hampson, D.P., Russell, B.H. and Bankhead, B. [2005] Simultaneous inversion of pre-stack seismic data. Ann. Mtg. Abstracts, SEG, 1633-1637. Mazumdar, P. [2007] Poisson Dampening Factor. TLE, July 2007, 850-852. Quakenbush, M., Shang, B. and Tuttle, C. [2006] Poisson Impedance. TLE, spring 2006, 128-138.