R.A. Mollison¹, O.N. Fanini¹, B.F. Kriegshauser¹, L. Yu¹, G. Ugueto², J. van Popta³

(1) Baker Atlas, Houston, TX
(2) Shell Offshore, New Orleans, LA
(3) Shell EP Technology, The Hague

ABSTRACT: Electrical Anisotropy: A New Tool to Aid in Correlation, Facies Determination and Sequence Stratigraphy, Examples from Shell Ursa Field, Deepwater Gulf Of Mexico

The macroscopic effect of a laminated sand-shale sequence with different resistivities results in electrical anisotropy. The resulting composite resistivities measured parallel and perpendicular to bedding, horizontal and vertical resistivity respectively, provide the necessary information to quantitatively determine anisotropy coefficient, laminar sand resistivity, and laminar shale volume. In order to determine electrical anisotropy, a new multi-component induction tool (3DEX„§) has been jointly developed by Baker Atlas and Shell EP Technology to derive horizontal and vertical resistivity using three mutually orthogonal transmitters and receivers.

3DEX horizontal and vertical resistivities were obtained in two development wells penetrating multiple turbidite fan sequences in the Shell operated URSA Field, Deepwater Gulf of Mexico. Anisotropy was used to identify Bouma Tb through Te facies where traditional log indicators are not always diagnostic. Differentiation of very thinly bedded silty-sands and shales of the uppermost Bouma facies associated with distal fan deposition grading to marine shales can be extremely difficult. Changes in clay/silt volumes within shales however can create significant differences in anisotropy. Thus, shale anisotropy can be an important tool to help identify lateral proximity to fan complexes. Likewise, temporary cessation of turbidite deposition, noted as abrupt changes in shale anisotropy, may indicate major sequence boundaries. Electrical anisotropy, used within a sequence stratigraphy framework, can assist in determination of lateral reservior continuity and lateral facies relationships with a higher confidence level. Additionally, loss of electrical anisotropy in laminated sequences can assist in distinguishing slumps and deformed intervals where horizontal permeability and thus productivity have been greatly reduced.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado