Clay Volume is not Shale Volume-A Case Study in the Miocene Lower FarsFormation in Kuwait

摘要

For the Miocene Lower Fars reservoir in South Ratqa field of North Kuwait,a large investment has beenmade acquiring cores to accurately quantify rock properties.This paper will review the derivation of thefive 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 Vclindicators were used to define three different cases for effective porosity (PHIE) in probabilistic volumeestimation.Simplistic approaches for estimating Vsh and Vcl from gamma ray (GR) logs and confusion betweenwhat is Vcl and what is Vsh can lead to significant underestimation of PHIE and net sand.During a reservoirstudy 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 Vclestimates to rock-based ground truth data.As background to the study,the original documents proposing five commonly used shale volumerelationships were reviewed.The linear gamma ray (GR) method was found to be based on the averagechemical 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 syntheticshale and zero point was assumed.Steiber (1970) and Clavier (1971) were numerical solutions tied to pulseneutron log interpretations.The two Larionov equations were based on granulometry data,described in ascan of the original Russian textbook (Larionov,1969) found online.Core descriptions were available for 15 wells,XRD data were available for 52 wells,petrographic reportswere available for 57 wells,and granulometric reports were available for 18 wells.Previous work onthe field,which used a percentile-based GR Clavier(1971) solution for Vsh,was found to significantlyoverestimate Vsh.After core visual Vsh was described in six wells,a GR-based Vsh was estimated byrescaling the GR minimum and median to match core description Vsh.Clean intervals were matched thisway,but this estimate was too optimistic in the shaley intervals.Functions of the density (RHOB) andneutron (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 wasestimated from the thorium (THOR) log using the Larionov Young Rocks method and Vsh was estimatedusing 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 logVcl matched XRD total clay using the Larionov Young Rocks method in 26 out of 38 wells.The Clavierequation was too high for Vcl to match XRD data.Petrography and core description estimates of visibleshale rarely saw any shale laminations or matrix beyond trace or minor amounts at depths where XRDsometimes showed 3 to 8 percent Vcl.SEM data showed ubiquitous but volumetrically insignificant claygrain coatings on quartz grains.The challenge was how to treat the small amounts of clay that were not part of the shale visible duringthe 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 thewrong choice for correcting PHIT to PHIE.Clay grain coats on quartz and a trace to minor proportion oflithic grains have much less capillary-bound water than laminated or structural shales.The PHIT to PHIEadjustment was varied by probability class.For P90,the standard PHIT to PHIE correction was performedover the full range of Vsh.For P50,at depths where Vsh was less than 0.1,no correction was made,PHITwas used as PHIE.For P10,PHIT was used as PHIE.A probabilistic approach to the PHIT to PHIE correction avoided a signific