Exploration Petrophysics For Intra-Salt Carbonates In Ultra Saline Environment
Authors: Saxena Vimal , McDonald Thomas Dale 
Published: May 2016

Intra-salt carbonate stringers are the Early Cambrian source rocks and reservoir in the South Oman salt basin. Reservoirs are over-pressured and drilled with salt-saturated heavy barite mud. Formation water is ultra-saline (&gt;300kppm), general porosities are low (&lt; 7%) and salt is often found filling part of the pore space. Each reservoir character creates a contrasting scenario for resistivity evaluation. Due to limited SCAL data electrical parameters of adopted Archie interpretation are not uniformly established. A minor parametric variation drastically affects the high resistivity interpretation. Above all, Stringer lithology is a combination of dolomite, calcite, anhydrite and quartz that demands careful discrimination for any meaningful reservoir characterization. Evaluation of these complex formations is a multi-facet challenge for exploration.

To overcome these difficulties, the present paper demonstrates a value based logging and interpretation strategy. The new generation geochemical logging is found effective to discriminate the complex lithology. Advanced NMR with extra-high salinity rating provides a successful evaluation of porosity, with necessary accuracy. T2-Diffusion analysis at NMR Stations is used as a crucial confirmation of hydrocarbon type and supports the real time exploration decisions. Thermal neutron capture cross-section (S) is identified as a critical log to identify salt-plugging in carbonate sequence. Monte-Carlo saturations are adopted to evaluate the relative influence of parametric variability.

Field results have demonstrated the effectiveness of this combined logging and evaluation strategy under tough reservoir conditions. A comparison of evaluation with and without geochemical logs, NMR porosities and S clearly demonstrates the significant value addition by the implemented Petrophysical workflow. The interpretation, validated by testing and subsequent core studies, not only demonstrates the reliability of the reservoir evaluation but also serves the basic need of high-quality quick log analysis for real time decisions.

INTRODUCTION

Reservoir evaluation of low porosity complex carbonate stringers in South Oman is challenging. The advantage of resistivity interpretation in commonly found ultra-high saline formation is drastically negated by tight reservoir and frequently observed salt-plugged pore system. An additional uncertainty is generated by the absence of clearly identified electrical parameter the for adopted Archie interpretation in oil-to-water mix-wet reservoirs. Low porosity demands tough error bars and complex carbonate system necessitates unambiguous lithology discrimination. Exploration Petrophysics thus requires a value based logging plan and interpretation technique that effectively provides better reservoir quantification for quick completion decisions. Clear petrophysical identification of lithology, efficient estimation of low porosities, realistic evaluation of hydrocarbon content, and creation of error bars are integral part of such reservoir characterization and reliable exploratory reserve estimations. The present paper discusses how these challenges are resolved for carbonate stringers in this salt basin.