Print this pageNew Integrated Formation Evaluation Technique to Meet the Unique Petrophysical Challenge Posed By Laminated Anisotropic Shaly Sand Reservoirs: A Case History from Mexico
Laminated shaly sands are important reservoirs in the operationally difficult deep water basins globally. Conventional formation evaluation methods used to estimate fluid volumes in such reservoirs use induction and nuclear logs. But these logs are unable to accurately estimate the fluid saturation because they can not resolve the true resistivity (Rt) or true porosity of the formations in the presence of electrical anisotropy and complex clay mineralogy of the laminated shaly sands.
A new technique of formation evaluation is presented that successfully meets the challenges posed by the electrically anisotropic laminated shaly sand reservoirs. The latter are characterized by significantly larger horizontal component of conductivity than its vertical counterpart, which results in grossly suppressed Rt and therefore an inflated water saturation. The anisotropy is due to high dip of the formations and due to the fact that the laminations are thinner than the vertical resolution of the conventional logs.
The new technique performs an integrated evaluation of resistivity-, nuclear-, NMR- and borehole image. The technique applies standard convolution equations from the literature to determine vertical and horizontal resistivity then solves for resistivity of the sand layer by using Thomas-Steiber relationship as well as a new clay typing method (Chitale 2000). Formation layering defined by borehole images is used to correctly map and quantify the reservoir facies. NMR-derived effective porosity and irreducible water volume are then integrated with other logs to estimate net to gross.
A case history documenting the success of the new technique in evaluating reservoirs from Burgos Basin México is presented here. This technique of formation evaluation offers a viable independent method to estimate net to gross in anisotropic laminated shaly sands.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil