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1 Vertical and horizontal resistivity analysis by an electrical anisotropy template Zakir Hossain, Paola Vera de Newton* ock Solid Images Inc, 2600 South Gessner oad, Houston, TX 77063, USA Summary Analysis of electrically anisotropic reservoirs has been challenging with traditional petrophysical analysis. Several techniques were proposed as a framework for using graphical cross-plots to evaluate aly-sand reservoirs. However, there has never been a clear workflow to define ale laminations and ale anisotropy. In this study, we incorporate a depth-dependent Thomas-Stieber model to describe the ale laminations. From the vertical and horizontal resistivity, an electrical anisotropy template was built in conjunction with the modified Thomas-Stieber model. The template generated assuming isotropic ale underestimated the hydrocarbon volume. However, the template generated treating the ale as anisotropic improved the estimations of hydrocarbon presence, permitting a global assessment of the hydrocarbon potential of the aly-sand reservoirs. Using the depth-dependent Thomas-Stieber model we owed that electrical anisotropy is a function of ale laminations as well as ale compaction. Our electrical anisotropy template enhanced the accuracy of hydrocarbon identification in the anisotropic reservoir and permitted identification of more pay zones from vertical and horizontal resistivity data. Figure 1: Anisotropy resistivity template in v- v/ h attributes space. This template can be used to separate well log data into pay and non-pay zones, pivoting at the ale points (B,C). Point B indicates the sand resistivity with dispersed ale. Point C indicates the ale resistivity with laminated ale. Point B indicates the clean sand resisivity. Black contours represent volume of laminated ale whereas blue contours represent sand resistivity. ed daed boundary defines the pay zone. Dispersed ale property is assumed to be isotropic whereas laminated ale property is assumed to be anisotropic. Page 2439

2 Introduction A significant amount of the world s estimated hydrocarbon reserves are contained in thinly laminated aly-sand reservoirs. The formation resistivity of these reservoirs depends on ale properties and their distribution. At the micro-scale, ale is universally recognized as being anisotropic. At the macro-scale, this anisotropy is prevalent in the parallel bedding planes of laminated sand-ale sequences. The inherent anisotropy of the complex ale structure must be taken into consideration in order to understand the resistivity profile in the aly-sand reservoirs (Boyd et al. 1995). Evaluating thin layers comprising ale and hydrocarbon-bearing sands is difficult using measured horizontal resistivity (Clavaud et al. 2005; Anderson et al. 2005). Combining vertical resistivity with horizontal resistivity improves hydrocarbon prediction in aly-sand reservoirs (Boyd et al. 1995). However, in order to better evaluate horizontal and vertical resistivity, improved knowledge of ale properties and distributions is required. The Thomas-Stieber model can be used to describe ale distributions, whereas an electrical anisotropy template can be used to describe the anisotropic ale properties. The objectives of this study is to build an electrical anisotropy template in conjuntion with the modifed Thomas-Stieber model to improve global assessment of the hydrocarbon potential of aly-sand reservoirs. Figure 2: Anisotropy resistivity template in the v- v/ h attributes space. Dispersed ale property and laminated ale are assumed to be isotropic. ed daed boundary defines the pay zone. Some hydocarbon bearing sands are outside the defined pay zone. Page 2440

3 Mehtod and esults The equation defining the horizontal resistivity, h, aly-sand sequences is: 1 h V = 1 V + s (1) The equation defining the vertical resistivity, v, is: ) v = V + ( 1 V (2) in We generated templates as a function of laminated ale in v - v / h attributes space (Figure 1). This type of template was introduced by Klein et al. (1997) and Fanini et al. (2001) in the h - v attribute space. The v / h ratio is a useful measurement for determining the level of anisotropy (Anderson et al. 2005), therefore, we modified Klein plots using v - v / h attributes space. Infact this new template can be use to describe h, v as well as v / h. The template assuming ale isotropy does not do this very effectively, because some hydrocarbon bearing aly-sand data fall outside the defined boundary of pay zones (Figure 2). However, the generated template assuming ale anisotropy is very effective, permitting a global assessment of the hydrocarbon potential of aly-sand reservoir (Figure 3). A traditional Thomas-Stieber model has several assumptions, and ale is the only factor in porosity Figure 3: Anisotropy resistivity template in the v- v/ h attributes space. Dispersed ale property is assumed to be isotropic whereas laminated ale property is assumed to be anisotropic. ed daed boundary defines the pay zone. Hydocarbon bearing sands are inside the defined pay zone. Page 2441

4 reduction; i.e., reduction of porosity by cementation and compaction are ignored (Thomas and Stieber, 1975). Any diagenetic processes including burial and cementation are not described by this model. In order to avoid these limitations, a depth-dependent Thomas-Stieber model is presented in this study to describe the effects of burial depth and cementation. The depth-dependent Thomas- Stieber model combines rock physics depth trends with the Thomas-Stieber model in order to quantitatively characterize lithology as a function of depth (Figure 3). The upper hydrocarbon interval is associated with the mostly sand as well as a few laminated aly-sand sequences. However, lower hydrocarbon intervals are associated within diagenetic and compacted sand as well as laminated ale (Figure 4). Conclusions We demonstrate a technique to analyse the vertical and horizontal well log data. Log data ows that the electrical anisotropy template presented in v - v / h attributes space can be used to define the pay zone in aly-sand reservoirs. We found that ale anisotropy needs to be considered to improve pay zone predictions. Using a modified Thomas- Stieber model which is depth dependent, we owed the presence of compacted laminated ale. The depthdependent Thomas-Stieber model enhanced the evaluation. Acknowledgments Dr. olf Ackermann, Dr. Lucy MacGregor and Dr. Andrew Elliot are acknowledged for their comments. Figure 4: A depth-dependent Thomas-Stieber model: (a) Calibrated depth trends for ale and sand based on well-log data, (b) Modified Thomas-Stieber model. Shale volume versus porosity dara obtained from two formations (back colored data from the upper formation and red colored data from the lower formation ) ow in the Thomas-Stieber model. Inputs for A,A` are obtained from the depth trend of sand, whereas inputs for B,B` are obtained from the depth trend of ale. (c) Lithology description based on modified Thomas-Stieber model Page 2442

5 EDITED EFEENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. eference lists for the 2014 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. EFEENCES Anderson, B., J. Clavaud,. Nelson, K. U. Guru, and H. Wang, 2005, Field examples of enhanced hydrocarbon estimation in thinly laminated formation with a triaxial array induction tool: A laminated sand-ale analysis with anisotropic ale : Presented at the 46 th Annual Logging Symposium, SPWLA. Boyd, A., H. Darling, J. Tabanou, B. Davis, B. Lyon, C. Flaum, J. Klein,. M. Sneider, A. Sibbit, and J. Singer, 1995, The lowdown on low-resistivity pay: Oilfield eview, 7, no. 3, Clavaud, J.,. Nelson, K. U. Guru, and H. Wang, 2005, Field examples of enhanced hydrocarbon estimation in thinly laminated formation with a triaxial array induction tool: A laminated sand-ale analysis with anisotropic ale : Presented at the 46 th Annual Logging Symposium, SPWLA. Fanini, O. N., B. F. Kriegäuser,. A. Mollison, J. H. Schöen, and L. Yu, 2001, Enhanced lowresistivity pay, reservoir exploration and delineation with the latest multicomponent induction technology integrated with NM, nuclear, and borehole image measurements: OTC. Klein, J. D., P.. Martin, and D. F. Allen, 1997, The petrophysics of electrically anisotropic reservoirs: The Log Analyst, May-June. Schoenberg, M., and F. Muir, 1989, A calculus for finely layered anisotropic media : Geophysics, 54, , Thomas, E. C., and S. J. Stieber, 1975, The distribution of ale in sandstone and its effects upon porosity: SPWLA. Page 2443