lucia 1995 rock-fabricpetrophysical classification of carbonate pore space

7/27/2019 Lucia 1995 Rock-FabricPetrophysical Classification of Carbonate Pore Space

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ABSTRACT

This paper defines the important geologic param-eters that can be described and mapped to allowaccurate petrophysical quantification of carbonategeologic models. All pore space is divided into inter-particle (intergrain and intercrystal) and vuggypores. In nonvuggy carbonate rocks, permeabilityand capillary properties can be described in termsof particle size, sorting, and interparticle porosity

(total porosity minus vuggy porosity). Particle sizeand sorting in limestones can be described using amodified Dunham approach, classifying packstoneas grain dominated or mud dominated, dependingon the presence or absence of intergrain porespace. To describe particle size and sorting in dolo-stones, dolomite crystal size must be added to themodified Dunham terminology. Larger dolomitecrystal size improves petrophysical properties inmud-dominated fabrics, whereas variations indolomite crystal size have little effect on the petro-physical properties of grain-dominated fabrics.

A description of vuggy pore space that relates topetrophysical properties must be added to the

description of interparticle pore space to completethe petrophysical characterization. Vuggy porespace is divided into separate vugs and touchingvugs on the basis of vug interconnection. Separatevugs are fabric selective and are connected onlythrough the interparticle pore network. Separate-vug porosity contributes little to permeability and

should be subtracted from total porosity to obtinterparticle porosity for permeability estimatiSeparate-vug pore space is generally consideredbe hydrocarbon filled in reservoirs; however, ingranular microporosity is composed of small psizes and may contain capillary-held connate wawithin the reservoir. Touching vugs are nonfabselective and form an interconnected pore systindependent of the interparticle system.

INTRODUCTION

The goal of reservoir characterization isdescribe the spatial distribution of petrophysiparameters, such as porosity, permeability, and uration. Wireline logs, core analyses, productdata, pressure-buildup data, and tracer tests pvide quantitative measurements of petrophysparameters in the vicinity of the well bore. Thwell bore data must be integrated with a geolomodel to display the petrophysical propertiethree-dimensional space. Studies that relate rofabric to pore-size distribution, and thus to pephysical properties, are key to quantifying geolomodels in numerical terms for input into compusimulators (Figure 1).

Geologic models are generally based on obsetions interpreted in terms of depositional envirments and sequences. In the subsurface, cores wireline logs are the main source of data for thinterpretations. Engineering models are basedwireline log calculations and average rock propties from core analyses. Numerical engineering dand interpretive geologic data are joined at rock-fabric level because the pore structure is fdamental to petrophysical properties and the pstructure is the result of spatially distributed desitional and diagenetic processes.

The purpose of this paper is to define the imptant geologic parameters that can be described amapped to allow accurate petrophysical quantiftion of carbonate geologic models by (1) descing the relationship between carbonate rock frics and petrophysical properties, (2) presentingeneric petrophysical classification of carbon

1AAPG Bulletin, V. 79, No. 9 (September 1995), P. 12751300.

Copyright 1995. The American Association of Petroleum Geologists. Allrights reserved.

1Manuscript received June 27, 1994; revised manuscript received March21, 1995; final acceptance May 1, 1995.

2Bureau of Economic Geology, University of Texas at Austin, Austin,Texas 78713-7508.

This classification of carbonate pore space is the result of researchinitiated while I was with Shell Oil Company and completed at the Bureau ofEconomic Geologys Reservoir Characterization Research Laboratory, whichis funded by industry sponsors Agip, Amoco, ARCO, British Petroleum,Chevron, Conoco, Exxon, Fina, JNOC, Marathon, Mobil, Phillips, Shell,Texaco, Total, and Unocal. I am grateful for intense discussions withcolleagues Charles Kerans, Gary Vander Stoep, Fred Wang, Susan Hovorka,Mark Holtz, Steve Ruppel, and Don Bebout. An in-depth review of themanuscript by Philip W. Choquette was especially useful. Published withpermission of the director, Bureau of Economic Geology, University of Texasat Austin.

Rock-Fabric/Petrophysical Classification of CarbonatePore Space for Reservoir Characterization1

F. Jerry Lucia2


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pore space, and (3) suggesting that rock-fabricunits are systematically stacked within high-fre-quency cycles.

PORE-SPACE TERMINOLOGY ANDCLASSIFICATION

Pore space must be defined and classified interms of rock fabrics and petrophysical propertiesto integrate geological and engineering information.Archie (1952) made the first attempt at relating rock

fabrics to petrophysical rock properties in carbon-ate rocks. The Archie classification focuses on esti-mating porosity, but is also useful for approximatingpermeability and capillary properties. Archie (1952)recognized that not all pore space can be observedusing a 10-power microscope and that the surfacetexture of the broken rock reflects the amount ofmatrix porosity. Therefore, pore space is divided

into matrix and visible porosity (Figure 2). Chalkytexture indicates a matrix porosity of about 15%,sucrosic texture indicates a matrix porosity of about7%, and compact texture indicates matrix porosityof about 2%. Visible pore space is described accord-ing to pore size, and ranges from no visible porespace to larger than cutting size. Porosity and per-meability trends and capillary pressure characteris-tics are also related to these textures.

Although the Archie method is still useful for esti-mating petrophysical properties, relating thesedescriptions to geologic models is difficult becausethe descriptions cannot be defined in depositionalor diagenetic terms. A principal difficulty is that noprovision is made for distinguishing between visibleinterparticle pore space and other types of visiblepore space, such as moldic pores. Research on car-bonate pore space (Murray, 1960; Choquette andPray, 1970; Lucia, 1983) has shown the importanceof relating pore space to depositional and diageneticfabrics and to distinguishing between interparticle(intergrain and intercrystal) and other types of pore

1276 Classification of Carbonate Pore Space

Wireline Logs

Porosity

PermeabilitySaturation

Sedimentation

Diagenesis

Tectonics

Core Analysis

Production

Pressure

Tracer Tests

ROCK FABRIC

Wireline LogsSedimentation

Diagenesis

Tectonics

Core Analysis

Production

Pressure

Tracer Tests

Figure 1Integration of spatial geologic data withnumerical engineering data through rock-fabric studies.

INTERGRAININTERCRYSTAL MOLDICINTRAFOSSILSHELTER

CAVERNOUS

FRACTURE

SOLUTION-ENLARGEDFRACTURE

PORE TYPES

VISIBLE (A,B,C, and D)

MATRIX

ARCHIE (1952)

INTERPARTICLE VUGGY

SEPARATE TOUCHING

LUCIA (1983)

FABRIC SELECTIVE NONFABRICSELECTIVE

CHOQUETTE and PRAY (1970)

Figure 2Petrophysical classification of carbonate poretypes used in this paper (Lucia, 1983) compared with

Archies original classification (1952) and the fabric-selectivity concept of Choquette and Pray (1970). A = no

visible pore space, and B, C, and D = increasing poresizes from pinpoint to larger than cutting size.

Table 1. Pore-type Terminology and Abbreviations Used inThis Paper Compared to Abbreviations Used in Lucia(1983) and Choquette and Pray (1970)

AbbreviationsChoquette and

Term Lucia (1983) Pray (1970)

Interparticle IP BP

Intergrain IG Intercrystal IX BC

Vug VUG VUGSeparate vug SV

Moldic MO MOIntraparticle WP WP

Intragrain WG Intracrystal WX Intrafossil WF

Intragrainmicroporosity G

Shelter SH SHTouching Vug TV

Fracture FR FR Solution-enlarged

fracture SF CH*

Cavernous CV CV Breccia BR BR Fenestral FE FE

*Channel.


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space. Recognition of the importance of these factorsprompted modification of Archies classification.

The petrophysical classification of carbonateporosity presented by Lucia (1983) emphasizedpetrophysical aspects of carbonate pore space, asdoes the Archie classification. However, by com-paring rock-fabric descriptions with laboratorymeasurements of porosity, permeability, capillarity,and Archie mvalues, Lucia (1983) showed that themost useful division of pore types for petrophysical

purposes was of pore space between grains or crys-tals, called interparticle porosity, and all other porespace, called vuggy porosity (Figure 2). Vuggy porespace was further subdivided by Lucia (1983) intotwo groups depending on how the vugs are inter-connected: (1) vugs interconnected only throughthe interparticle pore network are separate vugs,and (2) vugs that form an interconnected pore sys-tem are touching vugs.

Choquette and Pray (1970) discussed the geoic concepts surrounding carbonate pore space presented a classification that is widely used. Temphasized the importance of pore-space geneand used genetic, not petrophysical, divisionstheir classification. They divided all carbonate pspace into two classes: fabric selective and nonric selective (Figure 2). Moldic and intrapartipore types were classified as fabric-selective porty by Choquette and Pray (1970) and grouped w

interparticle and intercrystal porosity. HowevLucia (1983) demonstrated that moldic and inparticle pores have a different effect on petrophcal properties than do interparticle and intercrypores and thus should be grouped separately. Potype terms used in Lucias (1983) classification listed in Table 1, which compares his terms wthose suggested by Choquette and Pray (197Although most of the terms defined by Choque

Lucia 1

Figure 3Geological and petrophysical classification of carbonate interparticle pore space based on size and sing of grains and crystals. The volume of interparticle pore space is important for characterizing the petrophysproperties.


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and Pray are also used here, interparticle and vugporosity have different definitions. Lucia (1983)demonstrated that pore space located bothbetween grains (intergrain porosity) and betweencrystals (intercrystal porosity) are petrophysicallysimilar. A term identifies these petrophysically simi-lar pore types: interparticle. Interparticle wasselected because of its broad connotation. The clas-sification of Choquette and Pray (1970) does nothave a term that encompasses these two petro-physically similar pore types. In their classification,the term interparticle is used instead of inter-grain. [See Choquette and Pray (1970, page 247)for their justification for using interparticle vs.intergrain.]

Vuggy porosity, as defined by Lucia (1983), is

pore space that is within grains or crystals or thatis significantly larger than grains or crystals; that is,pore space that is not interparticle. Vugs are com-monly present as leached grains, fossil chambers,fractures, and large, irregular cavities. Fractureporosity is categorized as a type of vuggy porosityto be inclusive. This definition deviates from therestrictive definition of vugs used by Choquetteand Pray (1970) as nondescript, nonfabric-selective

pores, but it is consistent with the Archie termi-nology and with the widespread, less restrictiveuse in the oil industry of the term vuggy porosi-ty in referring to visible pore space in carbonaterocks.

ROCK-FABRIC/PETROPHYSICALCLASSIFICATION

The foundation of the Lucia (1983) and theArchie classifications is the concept that pore-sizedistribution controls permeability and saturationand that pore-size distribution is related to rock fab-ric. To relate carbonate rock fabrics to pore-size dis-tribution, one must determine if the pore spacebelongs to one of the three major pore types: inter-particle, separate vug, or touching vug (Figure 2).Each class has a different type of pore distributionand interconnection. One must also determine thevolume of po re space in these various cl assesbecause pore volume relates to reservoir volumeand, in the case of interparticle and separate-vug

porosity, to pore-size distribution.

Petrophysics of Interparticle Pore Space

In the absence of vuggy porosity, pore-size distri-bution in carbonate rocks can be described by parti-cle size, sorting, and interparticle porosity (Figure3). Lucia (1983) showed that particle size can be


140

Mercury/airextrapolateddisplacementpressure(psia)

Average particle size (m)120100806040200

20

40

60

80

100

120

140

160

180

0

21 Percent porosity

15

14

10

19

22

2727

22

16

2116 18

1818

16 21

1812

12

2622

Figure 4Relationship between mercury displacementpressure and average particle size for nonvuggy carbon-ate rocks with greater than 0.1 md permeability (Lucia,1983). The displacement pressure is determined byextrapolating the data to a mercury saturation of zero.

Figure 5Porosity-air permeability relationship for var-

ious particle-size groups in nonvuggy carbonate rocks(Lucia, 1983).


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related to mercury capillary displacement pressurein nonvuggy carbonates having more than 0.1 mdpermeability. Because displacement pressure is afunction of the larger, well-connected pores, the par-ticle size describes the size of the larger pores(Figure 4). Whereas the displacement pressure char-acterizes the larger pores sizes and is largely inde-pendent of porosity, the shape of the capillary pres-sure curve characterizes the smaller pore sizes and isdependent on interparticle porosity (Lucia, 1983).

The relationship between displacement pressureand particle size (Figure 4) is hyperbolic and sug-gests important particle-size boundaries at 100and at 20 m. Lucia (1983) demonstrated that three

porosity-permeability fields can be defined usparticle-size boundaries of 100 and 20 m, a rtionship that appears to be limited to particle siless than 500 m (Figure 5). These three fields wbe referred to as the greater than 100-m permeaity field, 10020-m permeability field, and lthan 20-m permeability field.

Recent work has shown that permeability fiecan be better described in geologic terms if sortand particle size are considered. The approachsize and sorting used herein is similar to the grsupport/mud-support principle upon whDunhams (1962) classification is built. Dunhaclassification, however, focused on depositio

Lucia 1

0 0.5 1.0mm

0 1.0 2.0mm

0 0.5 mm

0 0.2 mm

A

C D

B

Figure 6Photomicrographs showing examples of nonvuggy limestone rock fabrics. (A) Grainstone, = 25%,1500 md. (B) Grain-dominated packstone, = 16%, k= 5.2 md. Note intergranular cement and pore space. (C) Mdominated packstone, = 18%, k= 4 md. Note microporosity. (D) Wackestone.


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texture, whereas petrophysical classifications arefocused on contemporary rock fabrics that includedepositional and diagenetic textures. Thus, minormodifications must be made in Dunhams classifica-tion before it can be applied to a petrophysical clas-sification.

Instead of dividing fabrics into grain supportedand mud supported as in Dunhams classification,fabrics are divided into grain dominated and muddominated (Figure 3). The important attributes ofgrain-dominated fabrics are the presence of openor occluded intergrain porosity and a grain-support-ed texture. The important attribute of mud-domi-nated fabrics is that the areas between the grains

are filled with mud, even if the grains appear toform a supporting framework.

Grainstone is clearly a grain-dominated fabric,but Dunhams (1962) packstone class bridges aboundary between large intergrain pores in grain-stone and small interparticle pores in wackestonesand mudstones. Some packstones have intergrainpore space, and some have intergrain spaces filledwith mud; therefore, the packstone textural classmust be divided into two rock-fabric classes: grain-dominated packstones that have intergrain porespace or cement, and mud-dominated packstonesthat have intergrain spaces filled with mud (Figure3). The pore-size distribution is controlled by the


(B)

0.1

1

10

100

Permeability(m

d)

1000

10

Interparticle porosity (%)20 30 40

0.1

1

10

100

Permeability(md)

1000

10


0.1

1

10

100

Permeability(md)

1000

10


0.1

1

10

100

Permeability(m

d)

1000

10


(C) (D)

(A)

Figure 7Porosity-air permeability crossplots for nonvuggy limestone rock fabrics compared with the three per-meability fields illustrated in Figure 5 (A) 400-m ooid grainstone, Ste. Genevieve, Illinois (Choquette and Steiner,

1985). Low-permeability, high-porosity data are deleted because they are from oomoldic and wackestone rock fab-rics (P. W. Choquette, 1993, personal communication). (B) Grain-dominated packstone data, Wolfcamp, west Texas(Lucia and Conti, 1987), a poorly sorted mixture of 80300-m grains and micrite. (C) Wackestones with micropo-rosity between 5-m crystals, Shuaiba, United Arab Emirates (Moshier et al., 1988). Data are associated with stylo-lites not shown. (D) Coccolith chalk, Cretaceous (Scholle, 1977). The presence of intragranular pore space in thecoccoliths causes the data to plot below the less than 20-m field.


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grain size in grain-dominated packstones and by themud size in mud-dominated packstones.

Permeability/Rock-Fabric Relationships

Limestone Rock FabricsExamples of nonvuggy limestone petrophysical

rock fabrics are illustrated in Figure 6. In grainstonefabrics, the pore-size distribution is controlled bygrain size; in mud-dominated fabrics, the size of themicrite particles controls the pore-size distribution.In grain-dominated packstones, however, the pore-size distribution is controlled by grain size and bythe size of micrite particles between grains. Figure7A is a crossplot between permeability and inter-grain porosity for grainstones. The data are fromChoquette and Steiners (1985) work on the Ste.Genevieve oolite (Mississippian). The average grainsize of the oolite is about 400 m. The points onthis graph are concentrated within the greater than100-m permeability field.

Figure 7C is a crossplot between air permeability

and interparticle porosity from a Middle Easternmud-dominated fabric containing microporosity.The data from this graph are abstracted fromMoshier et al. (1988). The average crystal size ofthe mud matrix is about 5 m (Moshier et al.,1988). The data are concentrated in the less than20-m permeability field.

Because grain-dominated packstone is a new fab-ric class, data are difficult to find. Lucia and Conti

(1987) reported on a nonvuggy, grain-dominaWolfcampian packstone found in a core takenOldam County, Texas. The grain-dominated pastone is described as a poorly sorted mixture150300-m grains in a matrix composed of 80-pellets and 10-m calcite crystals. A porosity-pmeability crossplot of these data (Figure 7B) shothat it plots on the boundary between the 100

m and the less than 20-m permeability fielAdditional data points have been gleaned from literature, and they plot in the 10020-m perability field.

Figure 7D is a crossplot between permeabiand porosity for North Sea coccolith chalk (Scho1977). The average size of coccoliths is aboum. The data points plot below the less than 20-permeability field. The presence of intrafossil pspace within the coccolith grains probaaccounts for the lower-than-expected permeabiin the high-porosity ranges.

Figure 8 illustrates all the data for limestocompared with permeability fields. Although

data have considerable scatter, grainstone, grdominated packstone, and mud-dominated fabare reasonably well constrained to the three perability fields. Whereas grain size and sorting defthe permeability fields, the interparticle porodefines the permeability within the fieSystematic changes in intergrain porositycementation, compaction, and dissolution will pduce systematic changes in the pore-size distrition. Therefore, the permeability field is definedinterparticle porosity, grain size, and sorting.

Dolomite Rock Fabrics

Examples of nonvuggy dolomite petrophysrock fabrics are illustrated in Figure 9. Dolomiation can change the rock fabric significantlylimestones, the grain and mud fabrics usually be distinguished with little difficulty. If the rock been dolomitized, ho wever, the overprintdolomite crystals commonly obscures the grain mud fabric. Grain and mud fabrics in fine crtalline dolostones are easily recognizable; howevas the crystal size increases, the precursor fabrbecome more difficult to determine.

Dolomite crystals (defined as particles in tclassification) commonly range in size from sevemicrons to more than 200 m. Micrite particles usually less than 20 m in size. Dolomitization omud-dominated carbonate fabric therefore cresult in an increase in particle size from less th20 to more than 100 m (Figure 9). The crosspof interparticle porosity and permeability (Fig10A) illustrates the principle that in mud-domied fabrics, permeability increases as dolomite ctal size increases. Fine crystalline (average 15

Lucia 1

0.1

1

10

100

Permeability(md)

1000

10Interparticle porosity (%)

20 30 40

Ooid grainstoneGrain-dominated packstonesMud-dominated fabrics

Figure 8Composite porosity-air permeability crossplotfor nonvuggy limestone fabrics compared with thethree permeability fields illustrated in Figure 5. Chalks

are not included because of the presence of intragrainpore space.


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0 0.5 1.0mm

0 1.0 2.0mm

0 0.2

mm0 0.5 1.0

mm

0 0.5 1.0mm

A

B

C

E

D

Figure 9Examples of nonvuggy dolomite fabrics. (A)Dolograinstone, 15-m dolomite crystal size, = 16.4%,k= 343 md, Dune field (Bebout et al., 1987). (B) Dolo-grainstone, 30-m dolomite crystal size, = 7.1%, k= 7.3md, Seminole San Andres unit, west Texas. (C) Dolo-grainstone, crystal size 400 m, = 10.2%, k= 63 md,Harmatton field, Alberta, Canada. (D) Grain-dominateddolopackstone, 10-m dolomite crystal size, = 9%, k=1 md, Farmer field, west Texas. (E) Grain-dominateddolopackstone, 30-m dolomite crystal size, = 9.5%, k= 1.9 md, Seminole San Andres unit, west Texas. (F)

Crystalline dolowackestone, 10-m dolomite crystalsize, = 11%, k= 0.12 md, Devonian, North Dakota. (G)Crystalline dolowackestone, 80-m dolomite crystalsize, =16%, k= 30 md, Devonian, North Dakota. (H)Crystalline dolowackestone, 150-m dolomite crystalsize, = 20%, k= 4000 md, Andrews South Devonianfield, west Texas.


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mud-dominated dolostones from Farmer aTaylor-Link (Lucia et al., 1992b) fields in Permian basin and from Lawrence (Bridgepofield in the Illinois basin (Choquette and Stein

1985) plot within the less than 20-m permeabifield. Medium crystalline (average 50 m) mdominated dolostones from the Dune fiePermian basin (Bebout et al., 1987), plot within 10020-m permeability field. Large crystall(average 150 m), mud-dominated dolostones frAndrews Sou th Devonian field, Permian ba(Lucia, 1962), plot in the greater than 100-m pmeability field.

Grainstones are commonly composed of gramuch larger than the dolomite crystal size (Fig9), so that dolomitization does not have a signcant effect on the pore-size distribution. This pciple is illustrated in Figure 10B, a crossplo

interparticle porosity and permeability measuments from dolomitized grainstones. The grain sof the dolograinstones is 200 m. The fine crtalline dolograinstone from Taylor-Link fiePermian basin, the medium crystalline dologrstone from Dune field, Permian basin, and large crystalline dolograinstone from an outcon the Algerita escarpment, New Mexico, all pwithin the greater than 100-m permeabifield. The large crystalline mud-dominated frics also plot in this permeability field, indicatthat they are petrophysically similar to grastones (Figure 10A).

Interparticle porosity and permeability measuments from fine to medium crystalline, grain-donated dolopackstones are crossplotted in Fig10C. The samples are from the Seminole SAndres unit and Dune (Grayburg) field (Beboual., 1987), Permian basin. The data plot in 10020-m permeability field. The medium ctalline mud-dominated dolostones also plot in tfield (Figure 10A).

Figure 11 illustrates all dolomite data compawith permeability fields. Dolograinstones and lacrystal dolostones constitute the greater than 1m permeability field. Grains are very difficulrecognize in dolostones with greater than 100-

crystal size. However, because all large crystalldolomites and all grainstones are petrophysicsimilar, whether the crystal size or the grain sizdescribed makes little dif ference petrophysicaFine and medium crystalline grain-dominadolopackstones and medium crystalline mud-donated dolostones constitute the 10020-m perability field. Fine crystalline mud-dominated dostones constitute the less than 20-m field.

Lucia 1

0 0.5 1.0mm

0 0.5 1.0

mm

0 0.5 1.0

mm

F

G

H

Figure 9Continued.


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The dolostone permeability fields are defined bydolomite crystal size, as well as grain size and sort-ing of the precursor limestone. Within the field,permeability is defined by interparticle porosity.Systematic changes in intergrain and intercrystalporosity by predolomite calcite cementation,dolomite cementation, and compaction will sys-tematically change the pore-size distribution.Therefore, interparticle porosity defines the perme-ability, and dolomite crystal size, grain size, andsorting define the permeability field.

Limestone and Dolomite ComparisonData from limestone and dolomite rock fabrics are

combined into one porosity-permeability crossplotin Figure 12. The fabrics that make up the greater

than 100-m permeability field are (1) limestone anddolomitized grainstones, and (2) large crystallinegrain-dominated dolopackstones and mud-domi-nated dolostones. These fabrics are called rock-fab-ric/petrophysical class 1. The effect of grain size inthis field can be seen by comparing Figures 7A and10B. Ooid grainstones, which have a grain size of400 m, are more permeable for a given porosity

than dolograinstones, which have a grain size of200 m.Fabrics that make up the 10020-m field are

(1) grain-dominated packstones, (2) fine to medi-um crystalline grain-dominated dolopackstones,and (3) medium crystalline mud-dominated dolo-stones. These fabrics are called rock-fabric/petro-physical class 2. A comparison of Figures 7B and10C shows that the dolomitized grain-dominated


Figure 10Porosity-air permeability crossplots of nonvuggy dolomite fabrics compared with the three permeabili-ty fields illustrated in Figure 5. (A) Mud-dominated dolostones with dolomite crystal sizes ranging from 10 to 150

m. (B) Dolograinstones (average grain size is 200 m) with dolomite crystal sizes ranging from 15 to 150 m. (C)Grain-dominated dolopackstones with fine to medium dolomite crystal sizes.

Fine to medium crystallinegrain-dominateddolopackstone

Mud-dominateddolostones

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Fine crystal sizeMedium crystal sizeLarge crystal size

Fine crystalline dolograinstone

Medium crystalline dolograinstone

Large crystalline dolograinstone


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packstones tend to be more permeable for a given-porosity than limestone grain-dominated pack-stones. Further data may show this to be an impor-tant difference attributable to dolomite crystalmorphology. The less than 20-m permeability fieldis characterized by mud-dominated limestone andfine crystalline mud-dominated dolostones. Thesefabrics are called rock-fabric/petrophysical class 3.

Reducedmajor-axis permeability transforms are

presented for each class (Figure 12). The transformfor class 2 is slightly skewed to the field bound-aries, and the following transform is more compati-ble with the field boundaries.

Class 1:

k = (45.35 108) ip8.537 r= 0.71

Class 2:

k = (1.595 105) ip5.184 r= 0.80

or this recommended transform for class 2:

k = (2.040 106) ip6.38

Class 3:

k = (2.884 103) ip4.275 r= 0.81

where k = millidarcys and ip = fractional interpar-ticle porosity.

Unusual Types of Interparticle PorosityDiagenesis can produce unique types of interp

ticle porosity. Collapse of separate-vug fabrbecause of overburden pressure can produce frments that are properly considered diagenetic pticles. Large dolomite crystals with their centdissolved can collapse to form pockets of dolomrim crystals. Leached grainstones can collapse

form intergrain fabrics composed of fragmentsdissolved grains. These unusual pore types typicare not areally extensive; however, the collapseextensive cavern systems can produce areaextensive bodies of collapse breccia. Interbrecblock pores produced by cavern collapse included in the touching-vug category becausetheir association with fractures and other vformed by karsting processes (Kerans, 1989).

Capillary Pressure/Rock-Fabric Relationshi

Several methods have been presented for rela

porosity, permeability, water saturation, and revoir height (Leverett, 1941; Aufricht and Koe1957; Heseldin, 1974; Alger et al., 1989). Thmethods attempt to average the capillary presscurves into one relationship among saturatiporosity, permeability, and reservoir height withregard to rock fabric. The data presented demstrate that the three nonvuggy rock-fabric fields hunique porosity-permeability relationships, sugging that capillary properties of nonvuggy carbonshould also be separated into rock-fabric categori

To characterize the capillary properties of three rock-fabric fields, capillary pressure curwith different interparticle porosities from the throck-fabric fields are compared in Figure 13. Fig13A shows two curves representative of clasBoth curves are from samples of fine to medicrystalline dolograinstones. The 9.2% porosity curepresents the average of two sets of capillary psure data from dolograinstones in the Taylor-Lfield (Lucia et al., 1992b), and the 17.6% porocurve represents the average of two data sets, ofrom the Taylor-Link field and one from the Dufield (Bebout et al., 1987). Three curves represetive of class 3 are presented in Figure 13C. The curare from samples of fine crystalline dolowackestoand from the Farmer field, Permian basin. Thcurves representative of class 2 are presentedFigure 13B. Class 2 represents a very diverse clasrock fabrics, and it is difficult to combine all the rics into a few simple curves. The curves presenhere are from medium crystalline dolowackestoof the Seminole San Andres unit and may not be resentative of all grain-dominated packstones.

Pore-throat-size distribution for each capillpressure curve was calculated using the follow

Lucia 1

Figure 11Composite porosity-air permeability cross-

plot for nonvuggy dolostone fabrics compared with thethree permeability fields illustrated in Figure 6.


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formula. The results (Figure 13) show decreasingpore-throat size with decreasing porosity within apetrophysical class and a general decrease in pore-throat size from class 1 to class 3. Microporosity[pore-throat size less than 1 m (Pittman, 1971)] isconcentrated in class 3 and decreases in impor-tance from class 3 to class 1.

Ri= (2 cos C)/Pc

whereRi= pore-throat radius (m); T= air-mercuryinterfacial tension = 480 dynes/cm; = air-mercurycontact angle = 140; C= unit conversion constant= 0.145; and Pc= mercury injection pressure (psia).

Each group of curves is characterized by similardisplacement pressures and a systematic change incurve shape and saturation characteristics withchanges in interparticle porosity. The relationshipamong porosity, saturation, and rock-fabric classcan be demonstrated by selecting a reservoir heightof 150 m (which equates to a mercury capillary

pressure of about 650 psia) and plotting saturationagainst porosity for each rock-fabric class. Theresults (Figure 14) show that in nonvuggy carbon-ate reservoirs, a plot of porosity vs. water satura-tion can separate rock fabrics into three classes.

Equations relating water saturation to porosityand reservoir height are developed in two steps(Lucia et al., 1992b). First, mercury capillary pres-sure is converted to reservoir height using genericvalues (Table 2). Second, wetting-phase saturationsfrom the capillary pressure curves are plottedagainst porosity for several reservoir heights. Third,lines of equal reservoir height are drawn assumingequal slopes and a relationship between interceptsand reservoir height is developed. This relationship issubstituted for the intercept term in the porosity vs.saturation equation, resulting in a relationship amongwater saturation, porosity, and reservoir height.

The resulting equatio ns follow, and three-dimensional representations for classes 1 and 3are presented in Figure 15. These equations are


0.100.05 0.15

Interparticle porosity (fractional)

0.20 0.40

1

10

100

Permeability(md)

0.1

1000

Class 3

Class 2

0.30

Class 1

500

m

100

m

20m

Class 1

Class 2

Class 3

Figure 12Composite porosity-air permeability crossplot for nonvuggy limestones and dolostones showing statisti-cal reducedmajor-axis transforms for each class (dashed lines). See text for equations.


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0

20

40

60

80

100

2.0 1.5 1.0 0.5 0 -0.5 -1.0

Mercurysaturation(%)

17.6% porosity

Permeability = 166 md

9.2% porosity

Permeability = 72 m

Log pore-throat radius (m)

0

20

40

60

80

100

2.0 1.5 1.0 0.5 0 -0.5 -1.0


Mercurysaturation(

%)

13.7% porosity

Permeability = 5.1md

12.5% porosityPermeability = 2.3 md

8.7% porosity

0

20

40

60

80

100

2.0

Mercurysaturation(%)

1.5 1.0 0.5 0 -0.5 -1.0


15.4% porosityPermeability = 2 md

11.8% porosity

Permeability = 0.4 md

10.3% porosity

Permeability = 1.3 md

Permeability = 0.05

9.0% porosityPermeability = 0.04 md

0

1200

2000

60Mercuryin

jectionpressure(psia)

100 40

Mercury saturation (%)

20 0

9.2% porosity

80

1600

800

400

(A)

0

1200

2000

60Mercuryinjectionpressure(psia)

100 40


20 080

1600

800

400

(B)

17.60% porosity

13.7% porosity

12.5% porosity

10.3% porosity

8.7% porosity

0

1200

2000

60Mercuryinjectionpressure(psia)

100 40


20 0

15.4% porosity

80

1600

800

400

(C)

11.8% porosity

9.0% porosity

(A')

(B')

(C')

Figure 13(AC) Capillary pressure curves and (AC) pore-size distribution for petrophysical classes. (A) ClasData are from dolograinstones. (B) Class 2. Data are from medium crystalline dolowackestones. (C) Class 3. Datafrom fine crystalline dolowackestones.


14/26

specific to the capillary pressure curves used inthis paper and will not necessarily apply to specificreservoirs. However, the equations will provide rea-sonable values for original water saturations whenonly porosity and rock-fabric data are available.

Class 1:

Sw = 0.02219 H0.316 1.745

Class 2:

Sw = 0.1404 H0.407 1.440

Class 3:

Sw = 0.6110 H0.505 1.210

where H= height above capillary pressure equal tozero and = fractional porosity.

Rock-Fabric/Petrophysical Classes

Because the three rock-fabric groups define per-meability and saturation fields, the three groups,together with interparticle porosity and reservoirheight, can be used to relate petrophysical proper-ties to geologic observations. These rock-fabricgroups, herein termed rock-fabric/petrophysicalclasses (Figure 16), are described, with their gener-ic transform equations as follows:

Class 1 is composed of grainstones, dolograin-stones, and large crystalline dolostones:

k = (45.35 108) ip8.537

Sw = 0.02219 H0.316 1.745

Class 2 comprises grain-dominated packstones,fine and medium crystalline grain-dominateddolopackstones, and medium crystalline mud-domi-nated dolostones:

k = (2.040 106) ip6.38

Sw = 0.1404 H0.407 1.440

Class 3 contains mud-dominated limestones andfine crystalline mud-dominated dolostones:

k = (2.884 103) ip4.275

Sw = 0.6110 H0.505 1.210

Petrophysics of Separate-Vug Pore Space

The addition of vuggy pore space to interparti-cle pore space alters the petrophysical character-istics by altering the manner in which the porespace is connected, all pore space being connect-ed in some fashion. Separate-vug pore space isdefined as pore space that is (1) either within par-ticles or is significantly larger than the particlesize (generally greater than 2), and (2) is inter-connected only through the interparticle porosity(Figure 17). Separate vugs (Figure 18) are typicallyfabric selective in origin. Intrafossil pore space,such as the living chambers of a gastropod shell;moldic pore space, such as disso lved grains(oomolds) or dolomite crystals (dolomolds); andintragranular microporosity are examples of intra-particle, fabric-selective separate vugs. Molds ofevaporite crystals and fossils found in mud-domi-nated fabrics are examples of fabric-selective sepa-rate vugs that are significantly larger than the par-ticle size (Figure 18). In mud-dominated fabrics,shelter pore space is typica lly much larger thanthe particle size and is classified as separate-vugporosity.

In grain-dominated fabrics, extensive selectiveleaching of grains may cause grain boundaries todissolve, producing composite molds. These com-posite molds may have the petrophysical character-istics of separate vugs. If, however, dissolution of


1

10

100

10

Watersaturation(%)

5 20 30 40

Class 1 Class 2

Class 3

Figure 14Crossplot of porosity and water saturationfor the three rock-fabric/petrophysical classes at a reser-

voir height of 150 m. Water saturation (1 minus mer-cury saturation) and porosity values are taken from cap-illary pressure curves illustrated in Figure 13.

Table 2. Values Used to Convert Mercury/Air CapillaryPressure to Reservoir Height

Laboratory Reservoir (mercury/air/solid) (oil/water/solid)

T(dynes/cm) 480 28 () 140 44

Water density 1.04 0.88


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the grain boundaries is extensive, the pore spacemay be classified as interparticle porosity.

Grain-dominated fabrics may contain grains withintragranular microporosity (Pittman, 1971; Keithand Pittman, 1983; Asquith, 1986). Intragranularmicroporosity is classified as a separate vugbecause it is within the particles of the rock and isinterconnected only through the intergrain porenetwork. Mud-dominated fabrics also may containgrains with microporosity, but they present nounique petrophysical condition because of the sim-ilar pore sizes between the microporosity in themud matrix and in the grains.

The addition of separate-vug porosity to interpticle porosity increases total porosity, but does significantly increase permeability (Lucia, 198Figure 19A illustrates this principle in that permability of a moldic grainstone is less than wouldexpected if all of the porosity were interpartiand, at constant porosity, permeability wereincrease with decreasing separate-vug poro(Lucia and Conti, 1987). This principle is also tfor intragranular microporosity. Figure 19B crossplot of data from a San Andres dolograinstfrom the Permian of west Texas. The crosspshows that the permeability of the grainstone w

Lucia 1

Figure 15Three-dimensiodisplays of (A) class 1 and (class 3 equations relating

water saturation to reservoheight and porosity.

Watersaturation(%)

Watersaturation(%)Por

osity(%

)

Porosit

y(%)


16/26

intragranular microporosity is less than would beexpected if all the porosity were interparticle.

Separate vugs found within the oil column areusually considered to be saturated with oil becauseof their relatively large size; oil migration into sepa-rate vugs is controlled by interparticle pore size.Intragranular microporosity, however, is uniquebecause of small pore size. The small pore size pro-duces high capillary forces that trap water andlead to anomalously high water saturations withina productive interval (Pittman, 1971; Dixon andMarek, 1990). Figure 20 shows the capillary char-acteristics of a Cretaceous grainstone composed ofgrains having microporosity between 2-m crystals(Keith and Pittman, 1983) and a San Andres dolo-grainstone with grains having microporositybetween 10-m dolomite crystals. This figure illus-trates that within the oil column, water saturationsin grainstones with intragranular microporosity aresignificantly higher than would be expected if nointragranular microporosity were present. The

capillary pressure curves typically have a bimodalcharacter.

Petrophysics of Touching-Vug Pore Space

Touching-vug pore systems are defined as porespace that is (1) significantly larger than the parti-cle size, and (2) forms an interconnected pore sys-tem of significant extent (Figures 17, 18). Touchingvugs are typically nonfabric selective in or igin.Cavernous, breccia, fracture, and solution-enlargedfracture pore types commonly form an intercon-nected pore system on a reservoir scale and are typ-ical touching-vug pore types (Figure 18). Fenestralpore space is commonly connected on a reservoirscale and is grouped with touching vugs becausethe pores are normally much larger than the grainsize (Major et al., 1990).

Fracture porosity is included as a touching-vugpore type because fracture porosity is an important


Figure 16Petrophysical and rock-fabric classes based on similar capillary properties and interparticle-porosity/permeability transforms.


17/26

contributor to permeability in many carbonatereservoirs and, therefore, must be included in anypetrophysical classification of pore space. Al-though fracturing is often considered to be oftectonic origin and thus not a part of carbonategeology, diagenetic processes common to car-bonate reservoirs, such as karsting (Kerans,1989), can produce extensive fracture porosity.The focus of this classification is on petrophysi-cal properties rather than genesis, and must

include fracture porosity as a pore type irresptive of its origin.

Touching vugs are thought to be typically filwith oil in the reservoirs and can increase permability well above what would be expected frthe interparticle pore system. Lucia (1983) iltrated this fact by comparing a plot of fracture pmeability vs. fracture porosity to the three porty-permeability fields for interparticle poros(Figure 21). This graph shows that permeability

Lucia 1

Figure 17Geological and petrophysical classification of vuggy pore space based on vug interconnection. The ume of separate-vug pore space is important for characterizing the petrophysical properties.


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0 0.5 1.0

mm

0 1.0 2.0

mm

0 1.0 2.0

mm

0 1.0 2.0

mm

0 0.2mm

20 microns

A

C

E

B

D

F


19/26

3 in. 1 in.

1 in. 1 in.

0 1.0 2.0

mm

G H

I

K

J

Figure 18Photomicrograand photographs showingexamples of vug pore typeSeparate-vug types: (A)oomoldic porosity, = 26%k= 3 md, Wolfcampian, we

Texas; (B) intrafossil porespace in a gastropod shell,Cretaceous, Gulf Coast; (C)fossil molds in wackestone = 5%, k= 0.05 md;(D) anhydrite molds in

grain-dominated packston = 10%, k=


20/26

touching-vug pore systems is related principally tofracture width and is sensitive to extremely smallchanges in fracture porosity. There is no effectivemethod for measuring fracture width using therock-fabric approach.

IMPLICATIONS FOR CONSTRUCTING AGEOLOGIC MODEL

A fundamental problem in constructing a reser-voir model for input into numerical fluid-flow simu-lators is converting geologic observations into petro-physical rock properties, namely porosity, permeability,and saturation. The problem has been discussed byLucia et al. (1992a), Senger et al. (1993), and Keranset al. (1994). These workers suggested that thepetrophysical properties within a rock-fabric unitare nearly randomly distributed and thus form thebasic building blocks for reservoir models. There-fore, geologic models described in terms of rock-fab-ric units can be most effectively converted intopetrophysical parameters and into reservoir models.On the basis of data presented herein, the mostimportant rock-fabric characteristics to describe andmap in nontouching-vug reservoirs are (1) grain sizeand sorting using the modified Dunham classifica-tion, (2) dolomite crystal size using 20 and 100 mas size boundaries, (3) separate-vug porosity, (4) sep-arate-vug type with special attention to intergrainmicroporosity, and (5) total porosity.

The stacking patterns of rock-fabric units aresystematic within a depositional and diagenetic

environment (Lucia, 1993). The simplest exampleis a shoaling-upward bar complex. Mud-dominatedfabrics grade upward and laterally into grain-domi-nated fabrics with a corresponding increase in per-meability, decrease in water saturation, and littlechange in porosity (Figure 22). The permeabilityand water saturation are a function of total porosi-ty and rock fabric. The introduction of moldicporosity (separate vugs) by selective dissolution ofgrains in the grain-dominated fabric, caused per-haps by shoaling and the introduction of meteoricwater at a subaerial exposure surface, can producea diagenetic overprint that results in no significantchange in porosity, little change in water satura-tion assuming the molds are filled with hydrocar-bons, and drastically reduced permeability in thegrainstone (Figure 22). The permeability is a func-tion of interparticle porosity determined by sub-tracting separate-vug porosity from total porosity.

Conversion of the shoaling-upward bar complexfrom limestone to 50-m dolostone will not changethe grain size and sorting characteristics of thegrain-dominated fabrics, but will alter the particlesize characteristics of the mud-dominated fabrics,changing the petrophysical class from class 3 toclass 2. Dolomitization may reduce the total porosi-ty of the grain-dominated fabrics, but the perme-ability and water saturation will still be a functionof grain size, sorting, and intergrain porosity. If theprecursor limestone is a moldic grain-dominatedfabric, the resulting grain-dominated dolostone willbe moldic (Figure 22). Dolomitization of the mud-dominated fabric to a 50-m dolomite may reduce


(B)

0.1

10

1000

Permeabil

ity(md)

Grainstonefield

100

1

(A)

0.1

10

1000

Permeabil

ity(md)

100

1Svug

porosityaverage

8%

105 20

Porosity (%)

30 40

Porosity (%)

105 20 30 40

Grainstonefield

Dolograinstonewith intragrainmicroporosity

Svugporosityaverage

20%

Figure 19Crossplot illustrating the effect of separate-vug porosity on air permeability. (A) Grainstones with sepa-rate-vug (Svug) porosity in the form of grain molds plot to the right of the grainstone field in proportion to the vol-

ume of separate-vug porosity. (B) Dolograinstones with separate vugs in the form of intragranular microporosityplot to the right of the grainstone field.


21/26

the porosity, but will increase the permeability anddecrease the water saturation significantly.

Another simple example is the high-energy tidal-flatcapped cycle where intertidal and supratidalsediments overlie a shoreface grain-dominatedpackstone (Figure 23). Carbonate cement and com-paction have reduced porosity in the tidal-flat sedi-ments, resulting in a low-permeability, highwater-saturation flow unit overlying a permeablegrain-dominated limestone and a low-permeabilitymud-dominated limestone. In a seaward direction,shoaling-upward bar complexes replace the tidal-flatcapped cycles.

Evaporitic tidal-flat environments are a sourcedolomitizing water, and downward f low of hypersaline water can dolomitize underlying cbonate sediments (Figure 23). In Permian bareservoirs, intertidal and supratidal dolostones dense because of compaction and occlusionpore space by anhydrite and dolomite, and grdominated dolostones underlying the tidal-flat sments commonly are dense because of pore-filanhydrite. High porosity is confined to subtimud-dominated dolostones, and permeability awater saturation are related to intercrystal poroand dolomite crystal size. In this example (Fig

Lucia 1

100 80 60 40 20 0

0


500

1000

1500

2000

Bimodal pore system18.2%, 14.0 md

Mercurycapillarypressure

(psia)

(B)

Unimodal pore system17.6%, 166 md

(A)


0

500

1000

1500

2000

Mercuryinjectio

npressure(psia)

100 80 60 40 20 0


Unimodal pore system11.2%, 44.9 md

0

20

40

60

80

100

1.5

MercurySaturation(%)

1.0 0.5 0 -0.5 -1.0


Unimodal pore system11.2%, 44.9 md

0

20

40

60

80

100

2.0

MercurySaturation(%)

1.5 1.0 0.5 0 -0.5 -1.0


Unimodal pore system

17.6%, 166 md


Bimodal pore sys16.4 %, 5.4 md

(B')

(A')

Figure 20(A, B) Capillary pressure curves and (A, B) pore-size distribution illustrating the effect of intragranmicroporosity on capillary properties. Notice the bimodal character of the samples with both intergranular intragranular microporosity pore types. (A) Ooid grainstone with intergranular porosity compared with ooid grstone with both intergranular and intragranular microporosity pore types, Rodessa limestone, Cretaceous,

Texas (after Keith and Pittman, 1983). (B) Dolograinstone with intergranular porosity compared with dologrstone with both intergranular porosity and intragranular microporosity pore types, San Andres dolomite, Perm(Farmer San Andres field), west Texas.


22/26

23), dolomite crystal size increases from fine tomedium with depth and the petrophysical proper-ties change correspondingly.

Grain-dominated bodies seaward of the tidal-flatdeposits are dolomitized by hypersaline water froman overlying cycle and are not cemented by anhy-drite. The permeability and saturation characteris-tics are related to grain size, sorting, and intergrainporosity (Figure 23). The underlying mud-dominat-ed dolostone is composed of medium dolomitecrystals and has higher permeability than a mud-dominated limestone with the same interparticleporosity. The underlying limestone has low porosi-ty and permeability in this example.

These examples illustrate the importance ofmapping rock-fabric units within the geologicframework. If the shoaling-upward cycle is mappedas a bar facies or the tidal-flatcapped cycle ismapped as a peritidal facies, the permeability andsaturation structure is compromised and the result-ing model is oversimplified. In addition, if the dia-genetic facies are not included as mapping parame-ters, the permeability and saturation values in theresulting reservoir model may be in serious error.For example, permeability can be significantlyoverestimated if the estimate is based on total

porosity rather than total porosity less separate-vugporosity, and permeability and saturation can beseriously underestimated if the changes in particlesize resulting from dolomitization are not includedin the geologic model.

SUMMARY

The goal of reservoir characterization is todescribe the spatial distribution of petrophysicalparameters, such as porosity, permeability, and sat-uration. The rock-fabric approach presented here isbased on the premise that pore-size distributioncontrols the engineering parameters of permeabili-ty and saturation, and that pore-size distribution isrelated to rock fabric, which is a product of geolog-ic processes. Thus, rock fabric integrates geologicinterpretation with engineering numerical mea-surements.

To determine the relationships between rockfabric and petrophysical parameters, one must

define and classify pore space as it exists today interms of petrophysical properties. This is bestaccomplished by dividing pore space into porespace located between grains or crystals, calledinterparticle porosity, and all other pore space,called vuggy porosity. Vuggy pore space is furthersubdivided into two groups, depending on how thevugs are interconnected: (1) vugs interconnectedonly through the interparticle pore network aretermed separate vugs, and (2) vugs in direct con-tact are termed touching vugs.

The petrophysical properties of interparticleporosity are related to particle size, sorting, andinterparticle porosity. Grain size and sorting of grainsand micrite are based on Dunhams (1962) classifica-tion and are modified to make the classification com-patible with petrophysical considerations. Instead ofdividing fabrics into grain-supported and mud-sup-ported categories, fabrics are divided into grain-dom-inated and mud-dominated categories. The impor-tant attributes of grain-dominated fabrics are thepresence of open or occluded intergrain porosityand a grain-supported texture. The importantattribute of mud-dominated fabrics is that the areasbetween the grains are filled with mud even if thegrains appear to form a supporting framework.

Grainstone is clearly a grain-dominated fabric,but Dunhams (1962) packstone class bridges animportant petrophysical boundary. Some pack-stones, as we see them now, have intergrain porespace, and some have the intergrain spaces filledwith mud. Therefore, the packstone textural classmust be divided into two rock-fabric classes: (1)grain-dominated packstones having intergrain porespace or cement, and (2) mud-dominated pack-stones having intergrain spaces filled with mud.


10010110-110-410-5 10-3 10-2

10

1

10-1

10-4

10-3

10-2

105

102

103

104

Permeability(d)

Porosity (%)

w=0.05

cm

w=0.1

cmw

=0.5

cm

w=1cm

w=0.01cm

w=0.005

cm

w

=0.001

cm

Z=

1000

cm

Z=1c

m

Z=

100

cm

Inter-particle

>100m

100 to 20m


23/26

Figure 22Three examples of how the stacking of rock-fabric units affects the distribution of porosity, permeaty, and water saturation in a shoaling-upward sequence with selective dissolution and dolomitization overprints

Lucia 1


24/26


Figure 23Examples of how the stacking of rock-fabric units affects the distribution of porosity, permeability, andwater saturation in a tidal-flatcapped sequence with hypersaline reflux dolomitization and sulfate emplacementoverprints.


25/26

The important fabric elements to recognize forpetrophysical classification of dolomites are precur-sor grain size and sorting, dolomite crystal size, andinterparticle (intergrain and intercrystal) porosity.Important dolomite crystal size boundaries are 20and 100 m. Dolomite crystal size has little effecton the petrophysical properties of grain-dominateddolostones. The petrophysical properties of mud-

dominated fabrics, however, are significantlyimproved when the dolomite crystal size is lessthan 20 m.

Permeability and saturation characteristics ofinterparticle porosity can be grouped into threerock-fabric/petrophysical classes. Class 1 is com-posed of limestone and dolomitized grainstones,and large crystalline grain-dominated dolopack-stones and mud-dominated dolostones. Class 2 iscomposed of grain-dominated packstones, fine tomedium crystalline grain-dominated dolopack-stones, and medium crystalline mud-dominateddolostones. Class 3 is composed of mud-dominatedlimestone and fine crystalline mud-dominated dolo-

stones.Generic porosity-permeability transforms and

water saturation, porosity, reservoir-height equa-tions for each rock-fabric/petrophysical class arepresented in the section Rock-Fabric/Petro-physical Classes.

The addition of separate-vug porosity to inter-particle porosity increases total porosity, but doesnot significantly increase permeability; therefore,one must determine interparticle porosity by sub-tracting separate-vug porosity from total porosityand use interparticle porosity to estimate perme-ability. Separate-vug porosity is thought to be typi-cally filled with hydrocarbons in the reservoir.Intergranular microporosity, however, may containsignificant amounts of capillar y-bound water,resulting in water-free production of hydrocarbonsfrom intervals with higher than expected watersaturation.

Touching-vug pore systems cannot be related toporosity, but are related principally to fracturewidth. Because there is no effective method formaking this observation in the reservoir, the rock-fabric approach cannot be used to characterizetouching-vug reservoirs.

The key to constructing a geologic model thatcan be quantified in petrophysical terms is to selectfacies or units that have unique petrophysical quali-ties for mapping. Petrophysical properties in rock-fabric facies or units are nearly randomly distributedand can be legitimately averaged, making these geo-logic units ideal for petrophysical quantification. Innontouching-vug reservoirs, the most importantrock-fabric characteristics to describe and map are(1) grain size and sorting using the modifiedDunham classification, (2) dolomite crystal size

using 20 and 100 m as size boundaries, (3) serate-vug type with special attention to intergrmicroporosity, (4) total porosity, and (5) separvug porosity.

In touching-vug reservoirs, characterizing pore system is difficult because the pore systemnot related to a precursor depositional fabric, butypically wholly diagenetic. Although the pore

tem may conform to bedding, as in evaporite clapse brecciation, it more commonly cuts acrstratal boundaries. Recognizing the presence otouching-vug pore system is paramount, howebecause it may dominate the flow characteristicthe reservoir.

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Archie, G. E. , 1952, Classi fication of carbonate reservoir roand petrophysical considerations: AAPG Bulletin, v. 36, no

p. 278298.Asquith, G. B., 1986, Microporosity in the OHara oolite zon

the Mississippian Ste. Genevieve Limestone, Hopkins CouKentucky, and its implications for formation evaluatCarbonates and Evaporites, v. 1, no. 1, p. 712.

Aufricht, W. R., and E. H. Koepf, 1957, The interpretation of clary pressure data from carbonate reservoirs: Transactionthe American Institute of Mining, Metallurgical, and PetrolEngineers, v. 210, p. 402405.

Bebout, D. G., F. J. Lucia, C. F. Hocott, G. E. Fogg, and GVander Stoep, 1987, Characterization of the Grayburg revo ir , U nivers ity Lands Dune fi eld, Cran e County, TeUniversity of Texas at Austin, Bureau of Economic GeoReport of Investigations No. 168, 98 p.

Choquette, P. W., and L. C. Pray, 1970, Geologic nomenclaand classification of porosity in sedimentary carbonates: ABulletin, v. 54, no. 2, p. 207250.

Choquette, P. W., and R. P. Steiner, 1985, Mississippian oolitenon-supratidal dolomite reservoirs in the Ste. GenevFormation, North Bridgeport field, Illinois basin, in P. O. Rand P. W. Choquette, eds., Carbonate petroleum reservNew York, Springer-Verlag, p. 209238.

Dixon, F. R., and B. F. Marek, 1990, The effect of bimodal psize distribution on electrical propert ies of some MiEastern limestones: Society of Petroleum Engineers TechnConference, SPE 20601, p. 743750.

Dunham, R. J., 1962, Classification of carbonate rocks accorto depositional texture, inW. E. Ham, ed., Classificationcarbonate rocksa symposium: AAPG Memoir 1, p. 1081

Heseldin, G. M., 1974, A method of averaging capillary prescurves: Society of Professional Well Log Analysts AnLogging Symposium, paper E, p. 17.

Keith, B. D., and E. D. Pittman, 1983, Bimodal porosity in ooreservoireffect on productivity and log response, Rod

limestone (Lower Cretaceous), East Texas basin: AABulletin, v. 67, no. 9, p. 13911399.

Kerans, C., 1989, Karst-controlled reservoir heterogeneity inEllenburger Group carbonates of west Texas: AAPG Bull

v. 72, no. 10, p. 11601183.Kerans, C., F. J. Lucia, and R. K. Senger, 1994, Integrated ch

terization of carbonate ramp reservoirs using PermianAndres Formation outcrop analogs: AAPG Bulletin, v. 78, np. 181216.

Leverett, M. C., 1941, Capillary beha vior in porous so

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Transactions of the American Institute of Mining, Metallurgical,and Petroleum Engineers, v. 142, p. 151169.

Lucia, F. J., 1962, Diagenesis of a crinoidal sediment: Journal ofSedimentary Petrology, v. 32, no. 4, p. 848865.

Lucia, F. J., 1983, Petrophysical parameters estimated from visualdescription of carbonate rocks: a field classification of carbon-ate pore space: Journal of Petroleum Technology, March,

v. 35, p. 626637.Lucia, F. J., 1993, Carbonate reservoir models: facies, diagenesis,

and flow characterization, in D. Morton-Thompson and A. M.

Woods, eds., Development geology reference manual: AAPGMethods in Exploration 10, p. 269274.

Lucia, F. J., and R. D. Conti, 1987, Rock fabric, permeability, andlog relationships in an upward-shoaling, vuggy carbonatesequence: University of Texas at Austin, Bureau of EconomicGeology Geological Circular 87-5, 22 p.

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F. Jerry Lucia

F. Jerry Lucia retired from ShellOil Company in 1985, where hehad worked as a consulting geolog-ical engineer, after 31 years inresearch and operations. Currently,he is a senior research fellow withthe University of Texas at Austin,Bureau of Economic Geology,developing new techniques andmethods of characterizing carbon-

ate reservoirs by applying outcropstudies to subsurface reservoirs.

ABOUT THE AUTHOR

lucia 1995 rock-fabricpetrophysical classification of carbonate pore space

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