Clay Volume Is Not Shale Volume – A Case Study In The Miocene Lower Fars Formation in Kuwait
Authors: Prasse Eric , Hornbrook John , Dharanidharan Bala , Al-Bahar Mohammad , Al-Sane Amal , Bora Anup , Shrinivisan Ashique , Dhote Prashante 
Published: October 2019

For the Miocene Lower Fars reservoir in South Ratqa field of North Kuwait, a large investment has been made acquiring cores to accurately quantify rock properties. This paper will review the derivation of the five most commonly used methodologies for estimating volume of shale (Vsh) and clay volume (Vcl), discuss three iterations of Vsh and Vcl estimates for this reservoir, and show how core-derived Vsh and Vcl indicators were used to define three different cases for effective porosity (PHIE) in probabilistic volume estimation.

Simplistic approaches for estimating Vsh and Vcl from gamma ray (GR) logs and confusion between what is Vcl and what is Vsh can lead to significant underestimation of PHIE and net sand. During a reservoir study of the Lower Fars heavy oil deposit, use was made of core visible shale descriptions, X-ray diffraction (XRD), petrography, scanning electron microscopy (SEM), and granulometric data to calibrate Vsh and Vcl estimates to rock-based ground truth data.

As background to the study, the original documents proposing five commonly used shale volume relationships were reviewed. The linear gamma ray (GR) method was found to be based on the average chemical measurement of Thorium, Uranium and Potassium in 200 midcontinent United States shales. Synthetic rock was used to calibrate GR tools at an API test facility and linearity between the synthetic shale and zero point was assumed. Steiber (1970) and Clavier (1971) were numerical solutions tied to pulse neutron log interpretations. The two Larionov equations were based on granulometry data, described in a scan of the original Russian textbook (Larionov, 1969) found online.

Core descriptions were available for 15 wells, XRD data were available for 52 wells, petrographic reports were available for 57 wells, and granulometric reports were available for 18 wells. Previous work on the field, which used a percentile-based GR Clavier(1971) solution for Vsh, was found to significantly overestimate Vsh. After core visual Vsh was described in six wells, a GR-based Vsh was estimated by rescaling the GR minimum and median to match core description Vsh. Clean intervals were matched this way, but this estimate was too optimistic in the shaley intervals. Functions of the density (RHOB) and neutron (NPHI) logs were used to identify shaley intervals and overrule the GR based interpretation.

Several years passed and many more wells were drilled. After acquisition of XRD core Vcl data, Vcl was estimated from the thorium (THOR) log using the Larionov Young Rocks method and Vsh was estimated using the density/neutron crossplot method. In the cleanest sands, where core description and the density/neutron overlay indicated shale was absent, XRD indicated trace to minor levels of Vcl. The THOR log Vcl matched XRD total clay using the Larionov Young Rocks method in 26 out of 38 wells. The Clavier equation was too high for Vcl to match XRD data. Petrography and core description estimates of visible shale rarely saw any shale laminations or matrix beyond trace or minor amounts at depths where XRD sometimes showed 3 to 8 percent Vcl. SEM data showed ubiquitous but volumetrically insignificant clay grain coatings on quartz grains.

The challenge was how to treat the small amounts of clay that were not part of the shale visible during the correction of density log total porosity (PHIT) to PHIE. Shale has significant porosity with capillary-bound water associated with non-clay shale-sized particles of quartz and other minerals, making Vcl the wrong choice for correcting PHIT to PHIE. Clay grain coats on quartz and a trace to minor proportion of lithic grains have much less capillary-bound water than laminated or structural shales. The PHIT to PHIE adjustment was varied by probability class. For P90, the standard PHIT to PHIE correction was performed over the full range of Vsh. For P50, at depths where Vsh was less than 0.1, no correction was made, PHIT was used as PHIE. For P10, PHIT was used as PHIE.

A probabilistic approach to the PHIT to PHIE correction avoided a significant understatement in net pay and PHIE. The key was understanding the difference between clay and shale as applied to estimating PHIE from PHIT.