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ABSTRACT: Shaly sand petrophysics in the Bombay offshore basin, India - integrating old principles with new technology

Bailey, Steven W.1, Michael D. Van Horn1, Charles J. Kaiser1, and Paul A. Connolly2
(1) Enron Global Exploration & Production Inc, Houston, TX 
(2) EOG Resources

The South Tapti gas field consists of multiple Miocene and Oligocene age shaly sands trapped in a Late Miocene inversion structure. During initial development, the first well from each of the three platforms was drilled vertically through the entire reservoir section to acquire a comprehensive suite of formation evaluation data. Core was acquired from every major reservoir unit and facies. Full-suite LWD measurements were recorded. Modern wireline logs, including Array Induction, Platform Express, Dipole Sonic, Magnetic Resonance, FMI, VSP, and MDT were recorded.

Core measurements revealed unusual and complex petrophysical properties. High porosities (25-35%) and high permeabilities (1-40,000 md) are present, even in the shale-rich reservoir facies. Matrix densities ranged from 2.65 to 3.00 gm/cc. Petrology indicated abundant iron-rich minerals, and clay minerals present in laminated, dispersed and structural forms. Core NMR measurements indicated an unusually low 20 msec T2 cut-off. CEC measurements showed significant clay conductance. Capillary pressure measurements showed bound water saturations as high as 90 percent.

High-tech, modern logging suites proved effective only after detailed calibration to core data using fundamental petrophysical principles. A porosity model was developed to account for variable matrix densities. A shaly sand water saturation model was calibrated to core CEC and capillary pressure data. Pay cut-offs were selected based on capillary pressure concepts. Log evaluation results have been verified and compare exceptionally well with well tests, NMR, and MDT pressure gradient data. LWD measurements proved a reliable and accurate alternative to wireline measurements in deviated development wells.

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