4

Evalua to discard the notion

that wa . Various tools and

techniq equently bypassed

zones, orrect petrophysi-

cal ans

The Lowdown on Low-Resistivity Pay

■■Clays are the pri-mary cause of low-resistivity pay andcan form duringand after deposi-tion. They are dis-tributed in the for-mation as laminarshales, dispersedclays and struc-tural clays. Othercauses of low-resis-tivity pay includesmall grain size, asin intervals ofigneous and meta-morphic rock frag-ments, and con-ductive mineralslike pyrite.

Austin BoydHarold DarlingJacques TabanouSugar Land, Texas, USA

Bob Davis Bruce LyonNew Orleans, Louisiana, USA

Charles FlaumRidgefield, Connecticut, USA

James KleinARCO Exploration and Production TechnologyPlano, Texas

Robert M. Sneider Robert M. Sneider Exploration, Inc. Houston, Texas

Alan SibbitKuala Lumpur, Malaysia

Julian SingerNew Delhi, India

For help in preparation of this article, thanks to JayTittman, consultant, Danbury, Connecticut, USA; Bar-bara Anderson, Ian Bryant, Darwin Ellis, Mike Herron,Bob Kleinberg, Raghu Ramamoorthy, Pabitra Sen, Chris Straley, Schlumberger-Doll Research, Ridgefield, Connecticut; David Allen, Kees Castelijns, Andrew Kirk-wood and Andre Orban, Schlumberger Wireline & Test-ing, Sugar Land, Texas, USA; Steve Bonner and TrevorBurgess, Anadrill, Sugar Land, Texas; Dale Logan,Schlumberger Wireline & Testing, Roswell, New Mexico,USA; and Pierre Berger, GeoQuest, Bangkok, Thailand.

When Cinventedresistivitycontradiresearch oil-filled water-fillever, lownized asring in Indonesi

iving the reexploration ofthods of interpreting low-e proliferated. mines the causes of low-sands, then explores theues that have been devel-

Laminat

Pore

Burrow

ting low-resistivity pay requires interpreters

ter saturations above 50% are not economic

ues have been developed to assess these fr

but there are no shortcuts to arriving at the c

wer.

onrad and Marcel Schlumberger the technique of well logging, low- pay was, practically speaking, action in terms. Their pioneeringhinged on the principle that gas- orrocks have a higher resistivity than

low oil prices drmature fields, meresistivity pay hav

This article exaresistivity pay in tools and techniq

ion of beds Shale clasts Clay-lined burrows

fillings Pore linings Clay grains

ed sand Ash shards Conductive pyrite

0.25 mm

0.5 in

Oilfield Review

ed rocks. Through the years, how--resistivity pay has become recog-

a worldwide phenomenon, occur-basins from the North Sea anda to West Africa and Alaska. With

oped to evaluate such zones. A case studyshows how log/core integration helps pin-point the causes of low-resistivity pay in theGandhar field in India.

Generally, deep-resistivity logs in low-resistivity pay read 0.5 to 5 ohm-m. “Low

In this article, AIT (Array Induction Imager Tool), ARC5(Array Resistivity Compensated), CBT (Cement BondTool), CDR (Compensated Dual Resistivity tool), CMR(Combinable Magnetic Resonance tool), CNL (Compen-sated Neutron Log), DLL (Dual Laterolog Resistivity),ELAN (Elemental Log Analysis), EPT (ElectromagneticPropagation Tool), FMI (Fullbore Formation MicroIm-ager), Formation MicroScanner, GeoFrame, GLT (Geo-chemical Logging Tool), Litho-Density, IPL (IntegratedPorosity Lithology), MicroSFL, NGS (Natural Gamma RaySpectrometry tool), Phasor, RAB (Resistivity-at-the-Bittool), SFL (Spherically Focused Resistivity), SHARP (Synergetic High-Resolution Analysis and Reconstructionfor Petrophysical Parameters) and TDT (Thermal DecayTime) are marks of Schlumberger. Sun is a mark of SunMicrosystems, Inc.1. Moore D (ed): Productive Low Resistivity Well Logs

of the Offshore Gulf of Mexico. New Orleans,Louisiana, USA: Houston and New Orleans Geologi-cal Societies, 1993.

2. Scala C: “Archie III: Electrical Conduction in ShalySands,” Oilfield Review 1, no. 3 (October 1989): 43-53.

3. One milliequivalent equals 6 x 1020 atoms.

The cation exchange capacity, or CEC,expressed in units of milliequivalent3 per100 grams of dry clay, measures the abilityof a clay to release cations. Clays with ahigh CEC will have a greater impact on low-ering resistivity than those with a low CEC.For example, montmorillonite, also knownas smectite, has a CEC of 80 to 150meq/100 g whereas the CEC of kaolinite isonly 3 to 15 meq/100 g.

Clays are distributed in the formationthree ways:• laminar shales—shale layers between

sand layers• dispersed clays—clays throughout the

sand, coating the sand grains or filling thepore space between sand grains

• structural clays—clay grains or nodules inthe formation matrix. Laminar shales form during deposition,

interspersed in otherwise clean sands (left).In the Gulf Coast, USA, finely layered sand-stone-shale intervals, or thin beds, make up

5Autumn 1995

■■The most com-mon depositionalenvironments forlow-resistivity pay: A) Lowstand basinfloor fan complexes B) Deep waterlevee-channelcomplexes andoverbank deposits C) Transgressive-marine sandsD) Lower parts(toes) of delta frontdeposits and lami-nated silt-shale-sand intervals inthe upper parts ofalluvial and dis-tributary channels. (Adapted from Dar-ling HL and SneiderRM: “Productive LowResistivity Well Logs of the Offshore Gulf of Mexico: Causesand Analysis,” in reference 1.)

contrast” is often used in conjunction withlow resistivity, indicating a lack of resistivitycontrast between sands and adjacent shales.Although not the focus of this article, low-contrast pay occurs mainly when formationwaters are fresh or of low salinity. As aresult, resistivity values are not necessarilylow, but there is little resistivity contrastbetween oil and water zones.

Because of its inherent conductivity, clay,and hence shale, is the primary cause oflow-resistivity pay (previous page).1 How

clay contributes to low-resistivity readingsdepends on the type, volume and distribu-tion of clay in the formation.

Clay minerals have a substantial negativesurface charge that causes log resistivity val-ues to plummet.2 This negative surfacecharge—the result of substitution in the claylattice of atoms with lower positive valence—attracts cations such as Na+ and K+ whenthe clay is dry. When the clay is immersedin water, cations are released, increasing thewater conductivity.

Lowstand basinfloor fan complex

A

B

C

D

Leveed channelcomplex

Transgressivemarine sands

Alluvialchannel

Distributarychannel

Delta front “toes” andshingled turbidites

Overbankdeposits

Overbank

fragments—all fine grained— mimic the logsignature of clays, featuring high gamma ray,low resistivity and little or no spontaneouspotential (SP). Unlike thin beds, this type oflow-resistivity pay can vary in thicknessfrom millimeters to hundreds of meters.

Finally, sands with more than 7% by vol-ume of pyrite, which has a conductivitygreater than or equal to that of formationwater, also produce low-resistivity readings.5This type of low-resistivity pay is consideredrare.

The challenge for interpreting low-resistiv-ity sands hinges on extracting the correctmeasurement of formation resistivity, esti-mating shaliness and then accurately deriv-ing water saturation, typically obtained fromsome modification of Archie’s law.6

Improved vertical resolution of loggingtools and data processing techniques arehelping to tackle thin beds. Nuclear mag-

about half the low-resistivity zones.4 Manylogging tools lack the vertical resolution toresolve resistivity values for individual thinbeds of sand and shale. Instead, the toolsgive an average resistivity measurement overthe bedded sequence, lower in some zones,higher in others.

Intervals with dispersed clays are formedduring the deposition of individual clay par-ticles or masses of clay. Dispersed clays canresult from postdepositional processes, suchas burrowing and diagenesis. The size differ-ence between dispersed clay grains andframework grains allows the dispersed claygrains to line or fill the pore throats betweenframework grains. When clay coats the sandgrains, the irreducible water saturation ofthe formation increases, dramatically lower-ing resistivity values. If such zones are com-pleted, however, water-free hydrocarbons

6 Oilfield Review

can be produced (see “Low-Resistivity Payin the Gandhar Field,” page 8).

Structural clays occur when frameworkgrains and fragments of shale or claystone,with a grain size equal to or larger than theframework grains, are deposited simultane-ously. Alternatively, in the case of selectivereplacement, diagenesis can transformframework grains, like feldspar, into clay.Unlike dispersed clays, structural clays actas framework grains without altering reser-voir properties. None of the pore space isoccupied by clay.

Other causes of low-resistivity pay includesmall grain size and conductive mineralslike pyrite. Small grain size can result in lowresistivity values over an interval, despiteuniform mineralogy and clay content. Theincreased surface area associated with finergrains holds more irreducible water, and, aswith clay-coated grains, the increasingwater saturation reduces resistivity readings.Intervals of igneous and metamorphic rock

Spherically FocusedResistivity

Evaluated Gas Pay Potential Gas Pay

-160 40Spontaneous Potential

0.2 20

Deep Induction0 150

Total Gamma RayGAPI ohm-m

0.2 20ohm-mCompensated

Neutron Porosityp.u.60 0

60 0

Density PorosityMDEN=2.68

Dep

th, m

X100

-160

X200

40

Spontaneous Potential

0.2 206FF40 Induction

Short Normal Resistivity

ohm-m0.2 20

■■Left: Induction Electrical Survey logs run in 1960 in a thinly bedded, gas-bearing section of the Vicksburg formation in south Texas,USA. Net pay is 7 ft. Right: Conventional triple combo—neutron, density and gamma ray tools—run in 1993 in a well offset 100 ftfrom the original 1960 well. Net pay is 14 ft.

2 ft [0.6 m] and 4 ft [1.2 m]. The FMI toolimages the borehole with an array of 192button sensors mounted on four pads andfour flaps.8 It has a vertical resolution of0.2 in. [5 mm].

Successive improvements in resolving thinbeds are strikingly visible in a series of logsmade 33 years apart in adjacent wells in thesouth Texas Vicksburg formation (previouspage and left ).9 In 1960, induction/ shortnormal logs indicated 7 ft of net gas pay andonly two beds with resistivity greater than 2ohm-m. In 1993, a new well was drilledwithin 100 ft [30 m] of the original well andlogged with conventional wireline tools.The induction/SFL Spherically FocusedResistivity logs doubled the estimated payto 14 ft [4.3 m], with seven beds above 2

One obvious method for resolving the resis-tivity of thin beds is to develop logging toolswith higher vertical resolution, deeper depthof investigation, or both. Two loggingdevices that have proved especially helpfulin evaluating thin beds are the AIT ArrayInduction Imager Tool and the FMI FullboreFormation MicroImager tool. The AIT tooluses eight induction-coil arrays operating atmultiple frequencies to generate a family offive resistivity logs.7 The logs have mediandepths of investigation of 10, 20, 30, 60 and90 in. and vertical resolutions of 1 ft [0.3 m],

netic resonance (NMR) logging showspromise for assessing irreducible water satu-ration associated with clays and reducedgrain size (see “Nuclear Magnetic Reso-nance Imaging—Technology for the 21stCentury,” page 19). And because the mostopportune time to measure resistivity occursduring drilling, when invasion effects areminimal, resistivity measurements at the drillbit also play an important role in diagnosinglow-resistivity pay. Thin Beds

7Autumn 1995

4. Thin beds have a thickness of 5 to 60 cm [2 in. to 2ft] and laminae are less than 1-cm [0.4-in.] thick,commonly 0.05 to 1 mm [0.002 to 0.004 in.].Bates RL and Jackson JA (eds): Glossary of Geology.Falls Church, Virginia, USA: American GeologicalInstitute, 1987. Dictionary of Geological Terms. New York, NewYork, USA: Doubleday & Co., 1984.

5. Clavier C, Heim A and Scala C: “Effect of Pyrite onResistivity and Other Logging Measurements,” Transactions of the SPWLA 17th Annual LoggingSymposium, Denver, Colorado, USA, June 9-12,1976, paper HH.

6. In 1942, Gus Archie proposed an empirical relation-ship linking a rock’s resistivity, Rt, with its porosity, φ , and water saturation Sw :

.

Other terms in the equation are the formation waterresistivity Rw, and the cementation and saturationexponents, m and n. For further reading:“Archie’s Law: Electrical Conduction in Clean,Water-Bearing Rock,” The Technical Review 36, no. 3 (July 1988): 4-13.“Archie II: Electrical Conduction in Hydrocarbon-Bearing Rock,” The Technical Review 36, no. 4(October 1988): 12-21.For a discussion on the numerous versions ofArchie’s law that have been developed to handle avariety of shaly sand environments:Worthington PF: “The Evolution of Shaly-Sand Con-cepts in Reservoir Evaluation,” The Log Analyst 26(January-February 1985): 23-40.

7. Barber TD and Rosthal RA: “Using a MultiarrayInduction Tool to Achieve High-Resolution Logs withMinimum Environmental Effects,” paper SPE 22725,presented at the 66th SPE Annual Technical Confer-ence and Exhibition, Dallas, Texas, USA, October 6-9, 1991.Hunka JF, Barber TD, Rosthal RA, Minerbo GN,Head EA, Howard AQ Jr and Hazen GA: “A NewResistivity Measurement System for Deep FormationImaging and High-Resolution Formation Evaluation,”paper SPE 20559, presented at the 65th SPE AnnualTechnical Conference and Exhibition, New Orleans,Louisiana, USA, September 23-26, 1990.

8. FMI* Fullbore Formation MicroImager. Houston,Texas, USA: Schlumberger Educational Services,1992.

9. Olesen J-R, Flaum C and Jacobsen S: “WellsiteDetection of Gas Reservoirs with Advanced Wire-line Logging Technology,” Transactions of theSPWLA 35th Annual Logging Symposium, Tulsa,Oklahoma, USA, June 19-22, 1994, paper Y.

■■AIT Array Induction Imager Tool and IPL Integrated Porosity Lithology logs runin the same well as conventional triple combo on previous page. The improved vertical resolution of AIT logs and the enhanced sensitivity of the IPL-derivedneutron porosity have increased net pay to 63 ft.

Potential Gas PayD

epth

, m

X100

X200

AIT Resistivities10-90 in.

0 45

HNGS Thorium Content(HTHO)

ppm

0.2 20ohm-m

-20

60 0

Neutron PorositySandstone (APSC)

0 5p.u.

HNGS Potassium Content(HFK)

10 40c.u.

APS CaptureCross-Section (SIGF)

-160 40

Spontaneous Potential

MV

20

Differential Caliper

p.u.

60 0

Density PorosityMDEN=2.68 (DPO)

p.u.

Rt = Rw

φm Swn

(continued on page 11)

The Gandhar field, on the western coast of India,

is the largest on-land field in the country (left).

Most hydrocarbon production comes from deltaic

sands of the Hazad member, three of which con-

tain low-resistivity pay.

One of these sands, called GS-11, has resistiv-

ity values of 2 to 6 ohm-m, but contains wells

that produce clean oil on the order of 50 m3/d

[315 B/D] (next page). A detailed study of GS-11,

integrating core and log data, allowed inter-

preters to unravel the low-resistivity phenomenon

and formulate a reliable mineralogical model and

water saturation estimates.

Core Studies

Sixty core samples from three GS-11 wells pro-

vided thin sections for study of texture and miner-

alogy. Polished sections helped reveal the pres-

ence of metallic minerals. Scanning electron

microscope (SEM) and X-ray diffraction (XRD)

studies of cores identified clay minerals. In addi-

tion, laser and sieving methods were used to

analyze grain size.

The core investigations showed several mech-

anisms contributing to high conductivity.

Medium- to fine-grained sands ranged from gray

to green-gray, with green indicating chloritic

miles

Delhi

I N D I A

Gandhar

Dabka

Khambhat

Dhadhar River

Narmada River

Mahisaga River

G U L F O FC A M B A Y

0 km 25

0 15.5

Low-Resistivity Pay in the Gandhar Field

■■SEM photographs showing coated grains and clay matrix (left) and quartz overgrowth with chlorite coatingon quartz grains (right).

■■Gandhar field on the westerncoast of India.

8 Oilfield Review

Quartz OvergrowthClay Coating

20 µm200 µm

clays. Bioturbation created thin, fine clay lamina-

tions over clean sands. Quartz was the most

prominent mineral, with minute opaque

minerals—pyrite or magnetite—occurring in

bioturbated sections. Pyrite, which increases

the formation conductivity, was limited to the

clayey part of the matrix and constituted less

than 5% by volume.

Clay, primarily chlorite, coating the grain sur-

faces was indicated by SEM pictures and XRD

studies (previous page, bottom). Smaller grains

were coated more than larger grains. Laser

analysis of samples shows the GS-11 sand to

be in the silt range, with grain sizes averaging

22 to 32 microns.

Formation Evaluation

Logs were analyzed to identify clay types and

heavy minerals. Thorium-potassium crossplots of

the NGS Natural Gamma Ray Spectrometry logs

identified predominant clays as chlorite in the

sands and kaolinite/chlorite in the shales. The

density-neutron crossplot showed a trend toward

high density (low porosity) with little increase in

the neutron. The particles associated with this

behavior, which included fine-grained quartz and

heavy minerals such as siderite, pyrite and

ilmenite, were collectively called silt.

From core- and log-derived information, a min-

eralogical model of kaolinite, chlorite, quartz and

silt was chosen for the GS-11 sands. Validation

for the model came from geochemical analysis of

21 core samples from different wells. A few sam-

ples were analyzed to determine the weight per-

cent of oxides, such as silicon dioxide [SiO2],

using X-ray fluorescence (XRF) and the results

were interpolated between samples. The percent-

ages were then converted into weight percent of

elements using standard tables and processed

9Autumn 1995

■■Log response from Well Z shows an average resistivity reading of 3 to 4 ohm-mover the GS-11 sand, which produced clean oil during conventional testing.

GS-

11 s

and

Dep

th, m

XX80

XX90

X100

X110

1.95

45 -15

2.95

20000.26 16

-25 125

0 150

Deep Resistivity

Shallow Resistivityohm-m

Gamma Ray

SP

Caliper

GAPI

MV

in.

Density

Neutron Porosityp.u.

g/cm3

with a mineralogical model to give weight per-

cent of minerals. The model based on geochemi-

cal analysis was constrained to include only

quartz, kaolinite, chlorite and ilmenite. This

constraint allowed the weight percent of minerals

to be converted to volume percent using the

total porosity from log interpretation and the

mineral densities.

Comparison of the log and XRF mineral analy-

ses shows agreement between the total clay per-

centage and the relative volume of kaolinite and

chlorite (left). The silt and ilmenite percentages

do not agree, as might be expected since the silt

was defined to include finer grained quartz.

Conclusions

The composite results from the extensive log-

core analysis show agreement between core- and

log-derived parameters (next page). Water satu-

ration values computed from the Waxman-Smits

equation compare well with those derived from

capillary pressure measurements.1 Because little

water had been produced from existing GS-11

wells, the log-derived water saturation values

were considered to represent irreducible water

saturation values.

The core studies showed that the low-resistiv-

ity measurements in the GS-11 sand have two

sources. First, individual sand grains are coated

with clay. Second, the silt-sized formation grains

lead to higher irreducible water saturations in

the formation.

10 Oilfield Review

Free Water

Quartz

Silt

Bound Water

Chlorite

Quartz

Chlorite

Bound Water Bound Water

Ilmenite Ilmenite

Kaolinite

Quartz

Chlorite

KaoliniteKaolinite

Free Water

1:100 m

XX54

XX56

XX58

XX60

XX62

Sandstone Coarsesandstone

Bioturbatedsandstone

Silty carbon-aceous shale

Laminatedsilty shale Shale

Dep

th, m

Cor

eD

escr

iptio

n

Log AnalysisWeight % of Minerals from

XRF Analysis of Oxidesfrom Cores

Volume % of Mineralsfrom XRF Weight %

and Log Porosity

■■Comparison between log and XRF mineral analyses of Well Y. A mineralogical model of kaolinite, chlorite,quartz and silt was chosen.

1. Waxman and Smits modified Archie’s law to account forthe increased conductivity of shale by introducing a shali-ness parameter based on cation exchange capacity (CEC).See: Waxman MH and Smits LJM: “Electrical Conductivi-ties in Oil-Bearing Shaly Sands,” Society of PetroleumEngineers Journal 8, no. 2 (1968): 107-122.

ohm-m. Later the same year, the secondwell was logged with a combination of AITand IPL Integrated Porosity Lithology tools.10

The high resolution of the AIT tool—1 ft ver-sus 2 ft for the induction—and theenhanced sensitivity of the IPL-derived neu-tron porosity increased net pay to 63 ft [19.2m] and showed 13 beds with resistivitygreater than 2 ohm-m.

Resistivity Measurements at the Bit Improvements in measurements-while-drilling (MWD) technology have not onlyboosted the efficiency of directional drilling,but also enhanced thin-bed evaluation.11

Two tools‚ the RAB Resistivity-At-the-Bit tooland the ARC5 Array Resistivity Compen-sated tool—are especially useful in thin-bedenvironments by providing resistivity databefore invasion has altered the formation.

The RAB tool provides five different resis-tivity readings plus gamma ray, shock andtool inclination measurements. Configuredas a stabilizer or a slick collar, the RAB toolis run behind the bit in a rotary drillingassembly and above the motor in a steerabledrilling assembly.

One resistivity measurement, called “bitresistivity,” uses the drill bit as part of thetransmitting electrode. With the RAB toolattached to the bit, alternating current is cir-culated through the collar, bit and formationbefore returning to the drillpipe and drillcollars above the transmitter. In the case ofoil-base mud, which is an insulator, the cur-rent loop is complete only when the collarsand stabilizers touch the borehole wall. Thevertical resolution of the RAB bit resistivity isonly 2 ft and it gives the earliest possiblewarning of changes in formation resistivity.

Four additional resistivity measurements,with 1-in. vertical resolution for thin-bedapplications, are made with three buttonelectrodes and a ring electrode. The shallowdepths of investigation—3, 6 and 9 in. for

11Autumn 1995

Qv from Logs

Qv fromCo vs Cw

Qv fromWet Chemistry

m fromEPT/MicroSFL

m=f(Qv from Logs)

m fromCo vs Cw

BadHoleFlag

5.0 0.0

1:200 m 1.0 3.0φ from Core

Combined Model

Kaolinite

Chlorite

Bound Water

Silt

Quartz

Moved Water

Water

Oil

Fluid Analysisp.u.

MovedHydrocarbon

p.u.

Sw fromWaxman-Smits

p.u.

Sw fromArchie

Swirr fromCap Studies

2.00 100 0 50 100 100 0

0 100

1.0 3.0

1.0 3.0

2.00

2.00

100 0

100 0

x780

x790

x800

x810

GS-11

GS-10

■■Composite log-core analysis of Well X. Core results are shown for the cementation exponent m; the CECnormalized for pore volume, Q v, irreducible water saturation Swirr and porosity φ. Q v, the CEC per volume ofpore fluid, was calculated from cores, by measuring resistivity at different water salinities, and from logs.

10. “Neutron Porosity Logging Revisited, ”OilfieldReview 6, no. 4 (October 1994): 4-8.

11. Bonner S, Burgess T, Clark B, Decker D, Lüling M,Orban J, Prevedel B and White J: “Measurements atthe Bit: A New Generation of MWD Tools,” Oil-field Review 5, no. 2/3 (April/July 1993): 44-54.Allen D, Bagersh A, Bonner S, Clark B, Dajee G,Dennison M, Hall JS, Jundt J, Lovell J and RosthalR: “A New Generation of Electrode Resistivity Mea-surements for Formation Evaluation WhileDrilling,” Transactions of the SPWLA 35th AnnualLogging Symposium, Tulsa, Oklahoma, USA, June19-22, 1994, paper OO.

the buttons and 12 in. for the ring electrode—allow interpreters to characterize early-time invasion (left).

The recently-introduced ARC5 tool pro-vides five phase and attenuation resistivitymeasurements, like the AIT tool, with a verti-cal resolution of 2 ft. With a 43/4-in. diame-ter, it is especially useful for formation evalu-ation in slim holes typical of deviated drilling(next page, top).

The measurements and spacings of theARC5 and AIT tools are comparable,although not identical, making petrophysi-cal evaluation with either tool in the samewell or between wells seamless (below left).The multiple measurements of the ARC5tool also allow interpreters to radially mapout the invasion process. The additionalphase and attenuation measurements pro-vide a better characterization of electricalanisotropy than existing MWD tools.

Improving Thin-Bed Evaluation ThroughData ProcessingDespite the emphasis on developing high-resolution resistivity logging tools, manyopenhole tools still have a vertical resolutionof 2 to 8 ft [0.6 to 2.4 m]. Several data pro-cessing techniques have been developed toenhance the vertical resolution of these tradi-tional tools (next page, bottom). All methodsuse at least one high-resolution measure-ment to sharpen a low-resolution one andrequire a strong correlation between the two.An existing technique helpful in interpretinglow-resistivity pay is Laminated Sand Analy-sis (LSA), a computer program for evaluatingthe shaliness, porosity and water saturationin beds as thin as 2 in. [4 cm].12

A newer approach for identifying andevaluating thin beds is the SHARP Syner-getic High-Resolution Analysis and Recon-struction for Petrophysical Parameters soft-ware. SHARP processing improves theresolution of log inputs to the ELAN Elemen-tal Log Analysis module, thereby improvingsaturation and reserve estimates. Currently,SHARP software exists as an interactive,stand-alone prototype application for Sunworkstations but a second generation ver-sion will be incorporated into the GeoFramereservoir characterization system by the endof 1995.

12 Oilfield Review

12. Allen DF: “Laminated Sand Analysis,” Transactionsof the SPWLA 25th Annual Logging Symposium,New Orleans, Louisiana, USA, June 10-13, 1984,paper XX.

Res

istiv

ity, o

hm-m

100

0.2

Laminated wet sands

570 580 590 600

Distance, ft

AIT

MicroSFL

RAB ringafter drilling

RAB ringwhile drilling

Res

istiv

ity, o

hm-m

Res

istiv

ity, o

hm-m

101

102

101

102

400 450 500 550Depth, ft

90 in.60 in.30 in.20 in.10 in.

34 in.28 in.22 in.16 in.10 in.

ARC5 Phase Shift Resistivities at CAT Well

AIT Resistivities at CAT Well

■■Evaluating inva-sion with the RABtool. In laminatedwet sands, the RABlogs made afterdrilling and whiledrilling anticorre-late, showing pref-erential invasion.

■■Comparison of ARC5 log with the AIT log at 2-ft vertical resolution. The logs were runin the Customer Acceptance Test (CAT) Well in Houston, Texas, USA.

X800

X900

Dep

th, f

t

0 150

0500

ROP5

Gamma RayGAPI

ft/hr

0.2

ARC5 Phase Shift ResistivityBorehole Corrected

10 in. (P10H)

200ohm-m

34 in. (P34H)

28 in. (P28H)

22 in. (P22H)

16 in. (P16H)

-19

100

0

CCL

Borehole Sigma

c.u.

Gamma Ray

GAPI

cps

Far Detector Background

1.0

0

100 60

0.6

1500

TDT Porosity

Inelastic Count RateFar Detector

cps

0

0

Formation Sigma

c.u.

0

Near DetectorCount Rate

cps3000 0

Far DetectorCount Rate

cps1200 0

13Autumn 1995

■■ARC5 log run in wash down mode in front of thin gas stringers. Rough hole conditions precluded running wireline logs in the wellexcept for a CBT Cement Bond Tool log and a TDT Thermal Decay Time log. The TDT log confirmed the presence of gas indicated bythe ARC5 log.

Data Processing Methods for Enhancing Vertical Resolution

Technique

Enhanced PhasorProcessing (1988)

Enhanced ResolutionProcessing (1986)

Laminated SandAnalysis (1984)

Measurements

Phasor Induction log

Litho-Density log

CNL CompensatedNeutron Log

Triple combo(gamma ray, neutronand density), EPTElectromagneticPropagation Tool

Improvement in Resolution

From 7 ft to about 3 ft[2 to 1 m]

From 18 in. to 4 in.[45 cm to 10 cm]

From 24 in. to 12 in.[61 cm to 30 cm]

Down to 2 in. [5 cm]

Method

Medium-inductionmeasurement used to enhancedeep induction measurement

Near-detector measurementused to compensate for fardetector measurement

Computes bound water saturation(shaliness) from EPT tool, usedwith dual-water model toredistribute the measured inductionresistivity, yielding estimates ofthe resistivity of thin beds. Effectiveporosity, water saturationand permeability are computed.

Near-detector measurementused to compensate for fardetector measurement

SHARP analysis relies on high-resolutioninputs, such as Formation MicroScanner,FMI or EPT Electromagnetic PropagationTool logs to define a layered model of theformation (right). The program looks at thezero crossings on the second derivative ofthe high-resolution log, where the slopechanges sign, to indicate bed boundaries. Inthe case of a Formation MicroScanner orFMI log, the SHARP program examines thesecond derivative of an average resistivityreading from all button sensors.

With bed boundaries established, SHARPanalysis plots a histogram of the frequencyof a particular resistivity value within thelogged interval of interest. By studying howresistivity values cluster, an interpreter cangroup the values into different populations,or modes. SHARP analysis assumes that allresistivity data in a particular mode comefrom the same kind of formation, and furtherthat the resistivity value in a particular modeis constant. In addition, SHARP evaluationassumes that petrophysical parameters such

14 Oilfield Review

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■■Establishing bedboundaries andmodes (right) withthe SHARP programand a FormationMicroScanner log(left). SHARP analy-sis determines bedboundaries frominflection points on the secondderivative of anaverage FormationMicroScannerresistivity reading.Modes are estab-lished by groupingresistivity measure-ments on a his-togram (not shown).A square represen-tation of the aver-age FormationMicroScanner resis-tivity curve showsthe bed boundariesand modes.

■■Enhancing the reso-lution of deep lat-erolog measurements(LLD) of the DLL DualLaterolog Resistivitytool. The FormationMicroScanner aver-age resistivity curveis shown with itsSHARP-generatedaverage resistivitysquare log (far left).The bed boundariesand number ofmodes establishedby SHARP is used togenerate anenhanced-resolutionLLD curve. The mod-eled LLD measure-ment of the DLL toolis refined by compar-ing the original DLLlog with the recon-structed LLD log(middle) and interac-tively adjusting thebed boundaries andmode values toachieve a bettermatch (right).

13. Ramamoorthy R, Flaum C and Coll C: “GeologicallyConsistent Resolution Enhancement of StandardPetrophysical Analysis Using Image Log Data,paper SPE 30607, to be presented at the 70th SPE Annual Technical Conference and Exhibition,Dallas, Texas, USA, October 22-25, 1995.

14. Chapman S, Colson JL, Everett B, Flaum C, HerronM, Hertzog RC, La Vigne J, Pirie G, Quirein J,Schweitzer JS, Scott H and Wendlandt R: “TheEmergence of Geochemical Well Logging,” TheTechnical Review 35, no. 2 (April 1987): 27-35.

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as density, neutron porosity and sonic veloc-ity are also constant in a given mode.

After establishing the number of beds andmodes in the logged interval—the “squarelog”—the SHARP program calculates a setof mode values that minimizes the differ-ence between the original and square logs.This model, a squared resistivity log of bedboundaries and mode values, is filtered withthe response function of a logging tool toproduce a synthetic, or so-called recon-structed, log (previous page, bottom). Themodel is refined by minimizing the differ-ence between the measured log and thereconstructed log. At a workstation screen,the log interpreter can interactively adjustthe boundaries and bed values of the modesto achieve a match.

When the synthetic and measured logsmatch, the model can be used as a high-res-olution input into the ELAN interpretation.To sharpen the resolution of other logs, suchas the gamma ray, the model of bed bound-aries determined previously is utilized toreconstruct other squared, enhanced logs forhigh-resolution formation evaluation. A low-resistivity example from the Gulf of Mexicoshows how SHARP analysis improvedreserve estimates by 28%, even whenapplied to AIT measurements and a high-res-olution triple combo of density, neutron andgamma ray logs (right and next page).

Rather than reconstruct logs using SHARPanalysis, Raghu Ramamoorthy and CharlesFlaum, of Schlumberger-Doll Research,Ridgefield, Connecticut, USA have devel-oped a simpler technique to enhance pro-ducibility and hydrocarbon content esti-mates made with conventional petrophysicalanalyses in thin beds.13 Working with logsfrom the GLT Geochemical Logging Tool,they picked a high-resolution clay indicator,either the FMI or EPT log, and calibrated it tothe clay volume derived from the GLT mea-surement. In addition to clay volume, theGLT tool combines nuclear spectrometrylogging measurements to determine mineralconcentrations and cation exchange capac-ity of the formation.14

15Autumn 1995

■■Comparison of original and SHARP-enhanced logs for a low-resistivity payexample from the Gulf of Mexico. Theinterval was logged with a high-resolu-tion triple combo. The original 10-in. and60-in. depth-of-investigation curves fromthe AIT log are shown with the enhancedAIT 60-in. log. Only the 60-in. curve wasenhanced because the SHARP prototypesoftware does not yet have the modeledtool response for other AIT measurements.An ELAN interpretation (next page) usingthe enhanced logs shows a 28% increasein estimated reserves.

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respond to clays, the GLT-derived clay vol-ume—a low-resolution measurement—isenhanced by looking at local variations ofthe high-resolution FMI measurement. Thelow-resolution GLT clay volume is adjustedby the difference between the FMI-derivedclay volume and its value averaged over theresolution of the GLT tool, which is 2 ft:

With well data, an empirical relationshipis established between clay volume andporosity. This relationship is applied to theenhanced GLT clay volumes to derive high-resolution porosity values. Enhanced GLTclay volumes and porosity values are thenprocessed with calibrated FMI resistivity val-ues to boost the resolution of hydrocarbonsaturation estimates.

Applying this technique to GLT and FMIlogs from a well in Lake Maracaibo,Venezuela reveals overlooked reserves. TheFMI image shows the highly laminatednature of the formation, with beds on theorder of 1 ft. A comparison of standard-reso-lution and high-resolution ELAN interpreta-tions shows that potential pay zones havebeen completely masked in the conven-tional processing (next page, top).

Using Electrical Anisotropy to Find Thin-Bed PayJames Klein and Paul Martin of ARCOExploration and Production Technology inPlano, Texas, and David Allen of Schlum-berger Wireline & Testing in Sugar Land,Texas are modeling electrical anisotropy todetect low-resistivity, low-contrast pay suchas thin beds.15 The researchers found that awater-wet formation with large variability ingrain size is highly anisotropic in the oil legand isotropic in the water leg. They attributethe resistivity anisotropy to grain-size varia-tions, which affect irreducible water satura-tion, between the laminations.

They tested their theory by modeling thethin, interbedded sandstones, siltstones andmudstones of the Kuparuk River formationA-sands of Alaska’s North Slope, located 10miles [16 km] west of Prudhoe Bay. Themodel, based on a Formation MicroScannerinterpretation, contains layers of low-perme-ability mudstone and layers of permeablesandstone with variable clay content.

The simulated resistivity data aredescribed as either perpendicular—mea-sured with current flowing perpendicular tothe bedding—or parallel—measured withcurrent flowing parallel to the bedding.

16 Oilfield Review

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Plotting perpendicular versus parallel resis-tivity for a given interval shows how hydro-carbon saturation influences electricanisotropy (below left). Simulated resistivitydata in the oil column curve to the right,but simulated resistivity data in the waterleg are nearly linear. The position of dataalong the oil column arc indicates thelithology of the formation.

Today, this technique works only with2-MHz MWD tools such as the CDR Com-pensated Dual Resistivity tool. The CDRphase and attenuation measurements pro-vide a unique response to anisotropy thatallows the perpendicular and parallel resis-tivities to be determined. The techniquerequires that the logging tool be parallel tothe beds so that differences in the phase andattenuation of resistivity measurements canbe used to establish anisotropy. Althoughthe technique cannot yet be applied at otherangles, its originators believe some opera-tors will value it enough to tailor the devia-tion of their wells so that logging tools canrun parallel to beds of interest.

Nuclear Magnetic Resonance LoggingAlthough thin-bed evaluation is challenging,the tools and techniques described so farprovide answers in most cases. More trou-blesome to interpreters than thin beds isanother prominent cause of low-resistivitypay, reduced grain size, which contributesto high irreducible water saturations. TheCMR Combinable Magnetic Resonance toolshows potential for measuring irreduciblewater saturation and pore size.16

The CMR tool looks at the behavior ofhydrogen nuclei—protons—in the presenceof a static magnetic field and a pulsed radio

17

■■Using a high-resolution measurement to enhance a low-resolution one. The clay vol-umes derived from a GLT log of well in Lake Maracaibo, Venezuela were enhancedwith an FMI log from the same well. The enhanced ELAN interpretation (track 3) fea-tures several pay zones that were missed on the standard ELAN interpretation (track 2).

■■Effect of saturation on electrical anisotropy in the Kuparuk River formation, Alaska,USA. Resistivity data taken in the oil column curve to the right, but resistivity datataken in the water leg are nearly linear. The position of data along the oil column arcindicates the lithology of the formation. Strong anisotropy may be present in the oil col-umn, depending on the saturation in the more resistive component. In the water leg,the same formation might display little or no anisotropy.

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15. Anisotropy is the variation of a property with direction. In this case, it is variation of resistivity in the vertical (perpendicular) versus horizontal (parallel) planes. For a review of electrical anisotropy:Anderson B, Bryant I, Helbig K, Lüling M and Spies B: “Oilfield Anisotropy: Its Origins and Electri-cal Characteristics,” Oilfield Review 6, no. 4 (January 1995): 48-56.Allen DF, Klein JD and Martin PR: “The Petrophysicsof Electrically Anisotropic Reservoirs,” Transactions ofthe SPWLA 36th Annual Logging Symposium, Paris,France, June 26-29, 1995.

16. Morriss CE, MacInnis J, Freedman R, Smaardyk J, Straley C, Kenyon WE, Vinegar HJ and Tutunjian PN:“Field Test of an Experimental Pulsed Nuclear Mag-netism Tool,” Transactions of the SPWLA 34th AnnualLogging Symposium, Calgary, Alberta, Canada, June13-16, 1993, paper GGG.Chang D, Vinegar H, Morriss C and Straley C: “Effective Porosity, Producible Fluid and Permeabilityin Carbonates from NMR Logging,” Transactions ofthe SPWLA 35th Annual Logging Symposium, Tulsa,Oklahoma, USA, June 19-22, 1994, paper A.

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Oilfield Review

frequency (RF) signal (right ). A proton’smagnetic moment tends to align with thestatic field. Over time, the magnetic fieldgives rise to a net magnetization—more pro-tons aligned in the direction of the appliedfield than in any other direction.

Applying an RF pulse of the right fre-quency, amplitude and duration can rotatethe net magnetization 90° from the static fielddirection. When the RF pulse is removed, theprotons precess in the static magnetic field,emitting a radio signal until they return totheir original state. Because the signalstrength increases with the number of mobileprotons, which increases with fluid content,the signal strength is proportional to the fluidcontent of the rock. How quickly the signaldecays—the relaxation time—gives informa-tion about pore sizes and, to some extent,the amount and type of oil.

A CMR log displays distributions of relax-ation, or T2 times, which correspond to pore

size distributions. The area under a spec-trum of T2 times is called CMR porosity.

Unlike previous NMR tools, the CMRtool is a pad-mounted device. Permanentmagnets in the tool provide a static mag-netic field focused into the formation. TheCMR tool’s depth of investigation, about 1 in.[2.5 cm], avoids most effects from mudcakeor rugosity. Its vertical resolution of 6 in.[15 cm] allows for comparison with high-resolution logs.

A low-resistivity example from theDelaware formation in West Texas showshow the NMR response allows log inter-preters to measure residual oil saturationdirectly from the CMR log (below). NMRmeasurements on core samples from theDelaware formation show that the NMRresponse will decay within the first 200 mil-liseconds (msec) if the pores are filled withwater. If the pores are filled with oil, how-ever, the signal decays after about 400 msec.

The T2 distributions in track 4 have beendivided into three parts. The area under theT2 curve to the left of the first cutoff, shownas a blue line at 33 msec, represents irre-ducible water saturation. The area under thecurve from 33 msec to 210 msec (red line)represents producible fluid. Above 210msec, the area under the curve representsoil, presented as a CMR oil show in track 3.This measurement of oil actually refers toresidual oil saturation since the CMR toollooks only at the flushed zone.

With the introduction of the CMR tool,log interpreters are gaining the upper handin the struggle to assess low-resistivity pay.Although there are no easy answers whenevaluating low-resistivity pay, the tools andinterpretation techniques are in place tomore efficiently find these frequentlybypassed zones. —TAL

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■■Principle behind the CMR tool. Permanentmagnets in the CMR tool create a staticmagnetic field B that gives rise to a netmagnetization among hydrogen nuclei (top).A pulsed radio frequency signal rotates thenet magnetization 90° away from the staticmagnetic field (middle). After the RF pulse isremoved, the protons precess back to theiroriginal state, emitting a radio signal whosestrength is proportional to the fluid contentof the rock (bottom).

■■Early field test of the CMR tool in the Delaware formation, West Texas, USA. Based onNMR measurements of core samples from the Delaware formation, the T2 distributionsin track 4 have been divided into three parts. The area under the T2 curves to the left ofthe 33-msec cutoff is irreducible water saturation. From 33 msec to 210 msec, the areaunder the curve represents producible fluid. Above 210 msec, the area under the curverepresents oil. In track 3, the mud log show curve was derived from the total gas mea-sured on the mud log. It indicates that there is an oil-water contact halfway throughthe interval, at about X320 ft.

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