The impact of 3D tilted resistivity anisotropy on marine CSEM measurements

Abstract

O f f s h o r e a n d o n s h o r e b r o a d b a n d s e i s m o l o g y 1374 The Leading Edge November 2013 SPECIAL SECTION: Offshore and onshore broadband seismologyThe impact of 3D tilted resistivity anisotropy on marine CSEM measurements M arine CSEM modeling and data-interpretation algorithms currently used by the E&P industry are based on application of vertical transverse isotropic (VTI) resistivity models, which in many practical cases cannot adequately describe complex tilted anisotropic geologic structures typical for accumulation of hydrocarbons (e.g., syncline or anticline types). Use of VTI models is a serious limitation of CSEM technology and might be one of the core reasons for some false-positive and false-negative outcomes of marine CSEM surveys. To investigate this phenomenon in more detail and assess the impact of arbitrary tilted resistivity anisotropy on CSEM data, we performed simulations for a set of 3D tilted transverse isotropic (TTI) and VTI benchmark models. The 3D modeling results show that ignoring the TTI effect, if it is present, might lead to incorrect assessment of the feasibility of a potential CSEM survey as well as erroneous assessments and /or mispositioning of hydrocarbon targets during interpretation of field data and, in turn, might compromise the reliability of marine CSEM technology. Therefore, there is a strong practical need to start using TTI models, which provide more accurate descriptions of complex anisotropic resistivity structures than VTI models do. Application of 3D TTI modeling will improve the reliability of CSEM technology and its derisking potential.

Comparison of TTI and VTI 3D Inversion of CSEM Data

“…This VTI assumption is suitable for sedimentary rock formations with approximately horizontal bedding. For dipping formations, however, the additional effect of the tilted transversely isotropic (TTI) formation can have a significant impact on marine CSEM data, as shown by Davydycheva and Frenkel (2013). In this paper, we show how TTI anisotropy can be included in a 3D inversion of marine CSEM data and compare VTI and TTI imaging results for a synthetic model and a CSEM field data set acquired over steeply dipping geology.…”

TTI inversion of marine CSEM data

“…This assumption works well provided that the anisotropic formations do not have significant dip. However, recent modeling studies (Davydycheva and Frenkel, 2013;Li and Dai, 2011) have shown that the effect of dipping anisotropy, known as tilted transverse isotropy (TTI), can have a significant impact on marine CSEM data, and cannot be accurately modeled under the VTI assumption. A pertinent question is therefore what effect TTI has on VTI inversion, and how to properly account for TTI in inversion.…”