ABSTRACT: Value addition through stochastic evaluation of gamma rays- a geostatistical approach to geological modeling and characterization of the reservoir

Jain, A. K., and A. Carnegie , ONGC Limited, New Delhi, India

Gamma-ray logs measuring natural radioactivity of formations indicate shale content, thus enabling identification of various lithofacies and their vertical distribution. Inter- well extension of this knowledge through geostatistics adds quantitative value while interpreting the reservoir. This concept has been used for geological modeling and characterization of two reservoirs from widely separated fields of Indian peninsula. Gandhar field in western India has multiple sandstone layers separated by shale markers. First part of study addresses one of those layers. Through evaluation of GR values in 65 wells, various lithofacies have been identified. They have then been extended between different wells by convergent contouring of GR values to visualize their distribution in three dimensions. Closely spaced NS, EW and horizontal slices from 3D volume establish geological model and characterize the reservoir. Second exercise has been performed in a reservoir of Lakwa field located in north- eastern region of India. Geostatistical exercise here has been even more rigorous and supported by core data. Various litho- facies were identified and lithologs in terms of these facies were prepared for 26 wells. Through Markovian Analysis, probabilities of different types of sequences within the reservoir have been calculated in each well to establish the geological model. Inter- well extension of the facies and 3D realization of reservoir through lithology variograms, indicator kriging and truncated Gaussian simulation are consistent with Markovian Analysis, thus ratifying such attempts. The concept, therefore, provides value addition to quantitative geological analyses preceding reservoir engineering and simulation studies.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia