Carbonate Reservoir Rocks_ch7.doc

Magoon, L. B, and W. G. Dow, eels., 1994, The petroleum system-from source to trap: AAPG Memoir 60.

Chapter

7

Carbonate Reservoir Rocks Clifton F. Jordan, Jr.

James Lee Wilson

l11tegmted Data Services

New Bmullfels, Texas, U.S.A.

Bonne Terre, Missouri, U.S.A.

Abstract The main factors in evaluating carbonate reservoirs are lithofacies, pore types, shelf setting, sequence stratigraphy, and diagenetic overprint. Several patterns are evident based on a review of carbonate reservoirs from around the world. First, dolomites, grainstones, and boundstones are the most common carbonate reservoir rock types, but any carbonate lithofacies can be modified by diagenesis to form porous rock. Second, secondary pore types tend to dominate carbonate reservoir facies, as opposed to primary pore types. Third, inner shelf, outer shelf, and slope lithofacies belts are prime exploration fairways that are relatively predictable, with middle shelf prospects being less so. Fourth, sequence stratigraphy describes the shelf-building and basin-filling pattern of carbonate sediments and provides useful models for exploration and production. Fmally, carbonate sediments are subjected to two main types of diagenetic overprinting: steady subsidence into deep burial realms of diagenesis or subsidence interrupted by one or more periods of uplift and associ ated porosity-producing diagenetic reactions. emerge from this data: (1) the number of basins that produce hydrocarbons from carbonate rocks is lowest for South America, Africa, and Australia and highest for North America and Eurasia; and (2) Tertiary carbonate reservoir rocks are found mainly in Southeast Asia, whereas the rest of the world's production is primarily from Paleozoic and Mesozoic carbonate reservoirs, which includes the giant and supergiant Jurassic and Cretaceous fields of the Middle East.

INTRODUCTION Carbonate reservoirs with their seemingly endless variety of textures, fossil components, and diagenetic overprints present a challenge to categorize (Owen, 1964; Schmoker et al., 1985). Nonetheless, there are generalized trends and patterns that account for oil and gas produc tion from carbonate reservoir rocks on a worldwide basis. These patterns are the subject of this review and are discussed here with regard to five considerations: lithofacies, pore type, shelf setting, sequence stratig raphy, and diagenetic overprint. This review concen trates on reservoir facies development and not on source rocks, seals, or traps involving carbonates. It was necessary to avoid the diversity of rock descriptions used in the literature by standardizing lithofacies terminology for purposes of comparison. Hence, throughout this discussion, a scheme of "symbol logic" is used to describe the texture, composition, sedimentary struc tures, and diagenetic overprints of carbonate rocks (Jordan, 1985). This review is directed toward two main audiences: exploration geologists, who need to under stand carbonate reservoirs in a regional framework, and production geologists, who need to know details regarding the distribution of reservoir facies in a partic ular field. Figure 7.1 shows the worldwide distribution of basins that produce hydrocarbons from carbonate reservoirs. A list of these basins and the ages of their carbonate reser voirs are shown in Table 7.1. The following trends

LITHOFACIES Many schemes of classifying carbonate rocks have been proposed, but that of Dunham (1%2) has been used in more studies involving carbonate rocks than any other. The reasons for this are its simplicity and direct ness, as well as its effectiveness in accurately describing reservoir facies in carbonate rocks. The reader is referred to Dunham's (1%2) classic paper in which he described grain versus matrix support, the effects of particle shape on grain packing, and the spectrum of carbonate rock types observed from mudstones through grainstones and boundstones. Dunham's attention to grain- versus matrix-sup ported framework is more in accord with the principles of carbonate sedimentation than classifications that consider primarily particle size or amounts of matrix. Particulate carbonate sediment, unless winnowed completely to form a grainstone deposit, is a mixture of 141

-

MAJOR PRODUCTION

lllllli

MINOR PRODUCTION

c::J

PROSPECTIVE

Figure 7.1. Map showing the worldwide distribution of basins that produce hydrocarbons from carbonate reservoirs.(Most basins drawn after St. John,1980.)

7. Carbonate Reservoir Rocks

143

Table 7.1. Basins with Fields Producing from Carbonate Reservoirs. Map No.a

Basin

A eb

North America 1. North Slope 2. Alberta Williston 3. Rocky Mtn. Thrust Belt 4. Bighorn 5. Wind River 6. Powder River 7. 8. Paradox Denver 9. Raton 10. Salina-Forest City 11. Anadarko 12. Ardmore 13. Permian 14. Michigan 15. Illinois 16. Appalachian 17. 18. Arkoma Black Warrior 19. Sabinas 20. East Texas Salt Dome 21. Louisiana Salt Dome 22. Mississippi Salt Dome 23. 24. Gulf Coast Misanti-Tampico 25. Veracruz 26. Reforma 27. Campeche 28. NE Gulf Salt Dome 29. South Florida 30. Georges Bank 31. Scotian Shelf 32. Grand Banks 33.

mid Pal mid Pal, Tri low Pal, mid Pal mid Pal, Jur low Pal, mid Pal mid Pal mid Pal, Cret mid Pal, up Pal low Pal, mid Pal Cret low Pal low, mid, and up Pal low Pal, mid Pal low, mid, and up Pal low Pal, mid Pal low Pal, mid Pal low Pal low Pal, mid Pal low Pal Jur, Cret Jur, Cret Jur, Cret Jur, Cret Jur, Cret Jur, Cret Cret Jur, Cret Cret Jur, Cret Cret Jur Jur Jur

South A merica Maracaibo 1. Madre de Dios 2. Campos 3.

Cret, Mio up Pal Cret

Gharb Pelagian Sirte Gulf of Suez Angola South

Australia 1. Canning

Basin

A eb

Europe

Africa 1. 2. 3. 4. 5.

Map No.a

Jur, Mio Cret, Paleoc, Eoc Paleoc Mio Cret

mid Pal

1. 2. 3. 4. 5.

6. 7. 8. 9. 10. 11.

Southern North Sea Paris Aquitaine Bay of Biscay Gulf of Valentia Polish-North German Po Pannonian Carpathian Foredeep South Adriatic Caltanesetta

up Pal, Cret-Paleoc Jur Jur, Cret Cret Jur, Cret up Pal, Jur Tri, Jur Mio Cret Cret Mio

Barents Sea Pechora Volga-Urals Donetz-Dneiper North Caspian Middle Caspian North Caucasus Trough Iranian Foldbelt Arabian Yemen West Siberia Fergana Bombay Kansk Angara Trough Szechwan Nigata South China Sea Northwest Palawan Visayan South Brunei East Natuna North Sumatra South Sumatra Sibolga Benkulu Sunda West Java East Java Barito Kutei East Sengkang Salawati Bintuni Papuan

up Pal mid Pal mid Pal mid Pal, up Pal, Jur mid Pal mid Pal, up Pal Jur, Cret Cret, Mio up Pal, Jur, Cret Jur up Pal Cret, Paleoc Mio up Pal up Pal up Pal Mio Mio Mio Mio Mio Mio-Piio Mio Mio Mio Mio Mio Mio Eoc, Olig, Mio, Plio Olig Mio Mio Mio Mio Mio

Asia 1. 2. 3. 4. 5. 6. 7.

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

•Basin numbers correspond to those mapped in Figure 7.1 by continent; basin names are taken mainly from St. John (1980) and production data are from reviews such as Halboufy (1992). blow Pai is Cambrian-Ordovician, mid Palis Silurian-Mississippian, and up Pal is PennsytvaniarH'ermian.Boldface type indicates major producbon.

w

RECOGNIZABLE DEPOSITIONAL TEXTURE

.....J

(]J

.....J

<(

PARTICULATE SEDIMENT CONTAINS MUD MUDGRAINSUPPORTED SUPPORTED <10°/o >10°/o GRAINS GRAINS w M WACKEMUDSTONE STONE

LIME MUD CARBONATE GRAINS

ORGANICALLY BOUND SEDIMENT

w z a:

0 ::::>

<(

N

z

1-

1- (!J

0 a.. w

1-

X 0 en w ()

w a:

0

B BOUNDSTONE

CARBONATE CEMENT FRAME BUILDERS

Figure 7.2. The Dunham (1962) classification of carbonate rocks, with two minor modifications: (1) mud is defined at the sedi mentologic silt-sand boundary of 62.5 J.lm and (2) up to 5% lime mud is allowed in the category of grainstones.

various amounts of lime mud matrix with various sizes of bioclasts and peloids (with local concentrations of intraclasts or ooids). The size and shape of particles in this type of sediment are dependent on the breakdown fabric of constituent parts of calcified plants and animals and the degree of burrowing, ingesting, and reworking of the sediment. Moderate water movement and mechanical processes of sediment winnowing, so signifi cant to siliciclastic deposition, are much less important in carbonates. Two minor modifications of Dunham's (1962) classifi cation scheme are recommended. First, the size of what is considered lime mud is increased from Dunham's limit of 35 m to 62.5 , thus standardizing the use of the term mud in the sedimentologic sense. Second, the amount of lime mud allowable in a grainstone is increased from 1% to 5%, thus widening the scope of the term grainstone and incorporating many porous mud lean packstones (Figure 7.2). By using symbols to represent words (or word strings) for lithic descriptors and sedimentary structures, grain composition, remarks, and/ or porosity and by adding these to the textural terms of Dunham (Jordan, 1985), the compilation and comparison of large amounts of carbonate lithofacies information becomes more manageable. This "lithofacies shorthand" is a rigorous semi-quantitative approach that normalizes facies descriptions for comparison and mapping purposes. This system is based on the lithofacies symbols in Figure

7.3, which shows the most common symbols used to describe carbonate reservoirs. (A complete listing of nearly 500 lithofacies symbols used in describing silici clastics, carbonates, and other rocks is available from the author upon request.) Carbonate lithofacies fall into several distinct litho logic associations, ranging from various types of lime mudstones to wackestones, packstones, grainstones, boundstones, and dolomites. These six main textural families are grouped in Figure 7.4 according to the most common types of constituent grains. This grouping shows that there is a limited number of combinations of carbonate rock textures and compositions to be dealt with in nature. By using the generalized bioclastic symbol, ),, which represents an assortment of normal marine fossils (usually fragmented) from any given geologic time period, the number of common carbonate lithofacies observed reduces to about 26 (Figure 7.4) and provides boundary conditions for the variability of carbonate lithofacies throughout geologic time. Wilson (1975) and Flugel (1982) used a typical shelf to basin profile as a framework for compiling a list of standard microfacies of commonly recurring carbonate rock types through geologic time (Table 7.2). Based on this, it is evident that the natural variation in carbonate rock textures and compositions occurring on shelves and platforms through time is limited and that, to some extent, carbonate reservoir facies are predictable in their distribution.

LITHIC DESCRIPTORS CALCAREOUS

ROCK TYPE or TEXTURE

GRAIN TYPES NON-SKELETAL CARBONATES

LS

LIMESTONE (undiff.)

CONG CONGLOMERATE

RFMARKS +

WITH OR PLUS

and/or

1-'UHU:::>ITY <jl

POROSITY

@

® A

® • [;>

ARENACEOUS ARGILLACEOUS SILTY GLAUCONITE VOLCANIC GLASS CHERT(Y) IRON STAINING PYRITE BRECCIATED (undiff )

I

@

AGGREGATE GRAINS COATED GRAINS 8 INTRACLASTS MICRITIZED GRAINS 0 0 OOIDS • PELLETS PELOIDS 0 @) PISOLITES

M

w

p G B

A

LIME MUDSTONE WACKESTONE PACKSTONE GRAINSTONE BOUNDSTONE BA BAFFLESTONE Bl BINDSTONE FA FRAMESTONE RUDSTONE

CLAYSTONE MUDSTONE, TERRIGENOUS COAL AHYDRITE GYP GYPSUM SALT HALITE D DOLOMITE (undrff.) DOLOMITIZED (>50% dolomite) 0 d DOLOMITIC (<50% dolomite)

F FLOATSTONE AX RECRYSTALLIZED LS (undiff.) STYLO-BRECCIA FOSSILS AND FOSSIL FRAGS. SANDSTONE CH CALICHE or CALICHIFIED TECTONIC BRECCIA 6 MEGAFOSSILS (undiff) SILICEOUS SSm MUDDY SS sil DIAGENETICALLY MOTTLED BORED ), SKELETAL or BIOCLASTIC SSp PEBBLY SS dm BURROWED MATERIAL (undiff.) ANHYDRITIC SLT SILTSTONE A ROOT STRUCTURES SH SHALE GYPSIFEROUS G COMPACTED HORSETAIL STRUCTURES Q;) CALCISPHERES STROMATOPOROIDS (J) MOLLUSCS (undrff.) (;), MASSIVE SLUMPING or SLUMPED rn CRYPTALGAL LAMINITES (undiff.) (J) MOLLUSC FRAG'S IT\ TABULAR CONVOLUTED BEDDING @> GASTROPODS E9 COCCOLITHS DISTORTED BEDDING cS> DASYCLAD ALGAE OYSTERS fAI BRANCHING STYLOLITES (-ITIC) PELECYPODS FILAMENTOUS BLUE GRN ALGAE A FORAMINIFERA (undiff.) MICROSTYLOLITES @ ONKOLITES RUDISTS (undiff.) £ PLANKTONIC$ STYLOLIITE SWARM CAPRINIDS SMALL BENTHONIC$ "' PLATY GREEN ALGAE SUTURED GRAINS tf RED ALGAE (undiff.) LARGE BENTHONIC$ MONOPLEURIDS GRADED BEDDING (NORMAL) tf BRANCHING RED ALGAE RADIOLITIDS • FUSULINIDS EVENLY LAMINATED Ji ENCRUSTING RED ALGAE Q NUMMULITIDS TOUCASIDS WAVY LAMINATED @ RHODOLITES SPICULES "' TUBULAR FORAMS NODULAR (LENTICULAR) BOD SPONGES 0 CORALS (undifferentiated) BRACHIOPODS COLLAPSE BRECCIA

"X' Ill

'<

A

<

/

FLASER BEDDING NO APPARENT BEDDING CROSS STRATIFICATION (undiff.) PLANAR. TABULAR X- STAAT. PLANAR, WEDGE X-STAAT. TROUGH or FESTOON X-STAAT. BIDIRECTIONAL X-STAAT. LOW ANGLE X-STAAT. CURRENT DIRECTION

ss

HEAD & ENCRUSTING CORALS C) LARGE MASSIVE CORALS 'I' BRANCHING FINGER CORALS 't' THICK BRANCHING CORALS HORIZONTAL CORAL PLATES ( VERTICAL CORAL PLATES G)

#

y

*

BRYOZOA. BRYOZOA. CRINOIDS ECHINOID ECHINOID SERPULID

@

Clay MD Coal ANH

ENCRUST RAMOSE TESTS SPINES WORMS

MICROFOSSILS (undiff.) OSTRACODS CONODONTS DIATOMS RADIOLARIAN SPORES+POLLEN

@ ® @

ff f fff

® ® ,_,

-v-

V {-

+ +

11iJ •

+c +o

••

+G lA

CHERT NODULES IRON-RICH NODULES ANHYDRITE NODULES GYPSUM NODULES REPLACED GYP-ANHY LATHS OPEN FRACTURES PARTIALLY OPEN FRAC'S SEALED FRACTURES FRACTURE SWARMS

f.

PERMEABILITY BETWEEN PARTICLE WP WITHIN-PARTICLE BC INTERCRYSTALLINE MO MOLDIC VUGVUGGY MV MOLDIC-VUGGY CH CHANNEL CV CAVERN BP

OPEN MICROFRACTURES

SH SHELTER

SEALED MICROFRAC'S CUT-AND-FILL STRUCTURES RIPPLES (SYMMETRIC) RIPPLES (ASSYMMETRIC) BORED SURFACE (HDGD) MUDCRACKED SURFACE LOAD STRUCTURES FLUTE CASTS GROOVE CASTS CEMENT MICROSPAR GEOPETALS CEMENT-FILLED VUG CALCITE CEMENT DOLOMITE CEMENT ANHYDRITE CEMENT GYPSUM CEMENT ISOPACHOUS RIM

BO BORING SK SHRINKAGE FR FRACTURE GF GROWTH FRAMEWORK FEN FENESTRATE BR BRECCIA KV KEYSTONE VUGS 0 OPEN VUGS SE SOLUTION-ENLARGED MOD MODIFIED

SO SYNTAXIAL OVERGROWTH MEN MENISCUS ES EQUANT SPAR CS COARSE SPAR BS BLOCKY SPAR

Figure 7.3. A chart of common lithofacies symbols used in describing carbonate reservoirs and associated rock types. (Based on Dunham, 1962; Choquette and Pray, 1970; Embry and Klovan, 1971; Swanson, 1981; Wilson and Jordan, 1983.)

7. Carbonate Reservoir Rocks

M

),w

w

•M OM

•W

),p

0G

08

D

),.p

•G

'1'8

Mo

•P

),G

't'8

),Wo

178

0G0

G

EBM

()G

fs\8

*G

1TT8

Figure 7.4. Common carbonate lithofacies, shown as six families of carbonate textures.

PORE TYPES Porosity is best described by the system of Choquette and Pray (1970), which is reproduced in Figure 7.5. The only modification from the original is the addition of "keystone vugs." Certain types of porosity in carbonate rocks occur as a function of a rock's fabric (i.e., its Dunham texture); these are the eight porosity types listed as fabric selective. The terms interparticle and intraparticle are abbreviated as BP (for between particle) and WP (for within particle), respectively, to avoid confusion with the similar sounds of "inter-" and "intra-." The terms used are descriptive, although it helps to realize that fenestra (Latin for "window") refers to rectangular voids arranged in a rectilinear pattern and that shelter refers to an umbrella effect provided by shell fragments or other large platy bioclasts. Growth-framework porosity refers to

1

large voids between branching or platy elements of a colonial organism. Claims that this is a common and significant type of porosity, especially in reefal facies, are refuted by the nearly universal filling of such voids by reef-derived sediments, either coarse-grained skeletal rainstones (),G) or skeletal wackestones-mudstones (11W /M). Four additional pore types (fractures plus three types of dissolution pores) are listed as nonfabric selective, indicating that they can be produced in any carbonate rock type. Finally, there are four unusual and less important pore types listed at the bottom of Figure 7.5 that may or may not be associated with rock fabric. From this complete listing of every possible pore type, six are recognized as being of major importance to reservoir facies development in carbonate rocks: inter particle (BP), intraparticle (WP), intercrystalline (BC), moldic (MO), fracture (FRt and vuggy (VUG). The fact that only two of these (BP and WP) are primary in origin emphasizes the role of diagenesis in porosity develop ment in carbonate reservoirs. A direct relationship exists between the most common pore types observed in carbonate rocks (Choquette and Pray, 1970) and carbonate textures (Figure 7.6). The strong association of BC porosity with dolomites, growth-framework (GF) porosity with boundstones, and BP and keystone vug (KV) porosity with grainstones further demonstrates that a rock's composition and texture control (or at least limit) the types of porosity that may be developed. Figure 7.7 shows the average porosity of carbonate reservoirs plotted against the envi ronment of deposition, using as a data set all fields described in Carbonate Petroleum Reserooirs by Roehl and Choquette (1985). The relationship between permeability

Table 7.2 Compilation of 24 Standard Microfaciesa Depositional Environment

Carbonate

LOWER SLOPE

1. SPICUUTE 2. MICROBIOCLASTIC CALCISILTITE 3. PELAGIC LIME MUOSTONE

MIDDLE and UPPER SLOPE

4. MICROBRECCIA of BIOCLASTIC LITHOCLASTIC PACKSTONE 5. BIOCLASTIC GRAINSTONE/PACKSTONE 6. REEF RUDSTONE

JISA IN and

REEF or OUTER SHELF 7. BOUNDSTONE MIDDLE SHELF

SHOALS

RESTRICTED MARINE SHOALS Inner Shelf) RESTRICTED MARINE SHELF LAGOONS (Inner Shelf)

•After Wilson (1980a,b).

8. WHOLE-FOSSIL WACKESTONE 9. BIOCLASTIC WACKESTONE 10. COATED WORN BIOCLASTS IN MICRITE (P&W'S)

Facies

gv:,

&

WIM+

Remarks silt-sized; cross-bedded Halobia common· m av have graptolites

P J\BP

A GIP

0 R PR 't'.k'R B m BA, Bl, or FR e.a. OSB UF PBI

W+O @P/W

OG

11. COATED BIOCLASTS IN SPARITE (GRAINSTONE$) 12. COQUINA, SHELL HASH, BIOCLASTIC GRAINSTONE OR RUDSTONE 13. ONKOID BIOSPARITE; GRAINSTONE 14.LAG 15. OOLITE· OOID GRAINSTONE

@

o.>.G+8

iron-staining common

16. PELSPARITE, PELOIDAL GRAINSTONE, LOFERITE 17. GRAPESTONE PELSPARITE or GRAINSTONE 18. FORAMINIFERA or DASYCLAD GRAINSTONE

OG lOG OBG OG

may have ostracods and/ or forams

OM/W o.>.w

may have ostracods and gastropods

19. PELLETED LIME MUDSTONE/WACKESTONE or a LOFERITE (which is a PELSPARITE WITH FENESTRAL POROSITY) 20. ALGAL STROMATOLITE MUDSTONE 21. SPONGIOSTROME MUOSTONE 22. MICRITE WITH LARGE ONKOIOS 23. UNLAMINATEO HOMOGENEOUS UNFOSSILIFEROUS PURE MICRITE 24. COARSE LITHOCLASTIC BIOCLASTIC RUDSTONE or FLOATSTONE

(\) G 1-GIR G

may have dasyclads

X 0G

mB mOB

WIF M

8 .>.RJF

may have selenite crystals B:M cross-bedded

FABRIC SELECTIVE

xDINTERPARTICLE BP

X

X-

NOT FABRIC SELECTIVE

FRACTURE FR

CHANNEL

INTRAPARTICLE

WP

X f: .:· : f:·::,. :X.; INTERCRYSTAL BC ,

.

CH

x

1

e

u1 VUG

VUG

CAVERN MO

cv

FEN

SH GF

ESKEYSTONE

KV

VUG

m BRECCIA BR

FABRIC SELECTIVE OR NOT

BORING

80

EE!J BURROW BU

SHRINKAGE SK

Figure 7.5. Porosity classification of carbonate rocks. Large X's indicate the most significant porosity types in carbonate reservoirs; black areas are porosity. (After Choquette and Pray, 1970).

and depositional environment is shown in Figure 7.8. These two plots together show that the highest average porosity and permeability values are in lithofacies associ ated with shoal environments, the second highest with

reef environments, and the next highest with tidal flat environments. Granted, with only one field contributing to slope facies (i.e., Poza Rica), adequate statistical treatment is not possible.

High porosity and low permeability associated with basinal and, to a lesser extent, slope deposits can be accounted for in a number of ways: (1) ineffective but abundant WP porosity within tests of planktonic

foraminifera that make up nearly 90% of certain deep water ooze deposits; (2) microcrystalline BC porosity between clay-sized coccolith fragments in true chalk deposits; or (3) microcrystalline BC porosity (also

w

M

p

G

---------

WP

18

D

B

p

)

R

16

M

14

A

@ill

R

@ill

c ---- M o l== (--) s

(

VUG

\

---

·

----

0 a: 0 10 a.. w

E

(!)

) c

BR''-----,

12

y

0 (

CJ)

8

<(

a: w

6

> <(

4

N

---------------

D

(

B_C_
,)

2

A

0

R -----------

y

Figure 7.6.The correspondence between carbonate rock textures of Dunham (1962) and porosity types of Choquette and Pray (1970); bar heights indicate relative significance. M, mudstone;W, wackestone;P, packstone;G, grainstone; B, boundstone;D, dolomite.

referred to as matrix porosity) between particles of lime mud matrix material of indeterminate origin. One of the best types of secondary porosity and permeability is developed in thoroughly dolomitized packstones or grainstones in which early BP and WP pores are connected by a medium to coarsely crystalline fabric of dolomite with high intercrystalline (BC) porosity. In general, carbonate reservoir rocks in North America and Europe, especially those of Paleozoic age, more commonly exhibit secondary types of porosity, rather than primary. This includes intercrystalline porosity (and the commonly associated moldic-vuggy porosity) observed in dolomites and some recrystallized limestones of Mesozoic and Tertiary age. In contrast, reservoir facies of many of the giant carbonate fields of the Middle East occur in Cretaceous :eeloidal grainstones (OG), coated-grain grainstones (@G), and peloidal bioclastic grainstones (O),G) of Jurassic age, and in rudist boundstones (.l7B) and rudist grainstones (.l7G).

TIDAL SHOALS FLATS

SLOPE

R E

BASINAL DEPOSITS

E F

s

Figure 7.7. A plot of average porosity versus the deposi tionalenvironment of reservoir lithofacies, based on an equal weight averaging of all fields presented in Table 7.4.

100


.

80

.. I

70

<(

60

co

w 50 a: w a.. 40 : :! : w


<(

30

a: w 20 > <( 10 0

SHELF SETTING It has long been recognized (Wilson, 1975) that carbonate facies patterns show regular and somewhat predictable trends when lithofacies are mapped in a dip direction from the shallowest to the deepest part of a shelf or basin. These patterns depend on the shape of this profile, which varies in a spectrum between two end

DEPOSITS

Figure 7.8.A plot of average permeability versus the depo sitionalenvironment of reservoir lithofacies based on an equal weight averaging of all fields presented in Table 7.4.

Figure 7.9. Block diagram of carbonate lithofacies patterns on a drop-off profile across an idealized carbonate shelf during dominantly progradational stages of sedimentation during the formation of highstand systems tracts (HSTs), shelf margin wedges (SMWs), and lowstand wedges (L.SWs).

-

I SLOPE I OB 'tB

REEFS

li\B or VB

•G CG or 0G SHOALS or NEAR-REEF 1:<=>1 l>G )..p >,.p

l::;::::::::::::::q OFF-REEF, INTER-REEF, BACK REEF, and SLOPE DEPOSITS

)..G CSS mB0 or M INNER SHELF r----1 l>WIP )..eP/G l,\W or SH L J MIDDLE and MIDDLE-OUTER SHELF a.W l),W SH or lM BASIN

t.:.:.:.:.J

II&

LAND

members: the ramp and the drop-off profiles (Read, 1982; Wilson and Jordan, 1983). The relative position of sea level on either of these profiles is important in that the width of subtidal shelf facies can vary significantly (Irwin, 1965; Shaw, 1964). The relative position of sea level is a major point of emphasis in the development of sequence stratigraphy (van Wagoner et al., 1990; Schlager, 1992). Narrow belts of subtidal shelf facies (at most, a few tens of kilometers wide) correspond to lowstand wedges of carbonate sedi mentation deposited in front of older stranded shelves, whereas wide belts (up to several hundred kilometers wide) correspond to highstand systems tracts, deposited on broad, flooded shelves (Shaw, 1964; van Wagoner et al., 1990). At any given stage of sea level, a shelf can be divided into inner, middle, and outer zones. (See Figures 7.9, 7.10, and 7.12 for block diagrams of carbonate lithofa cies patterns across an idealized shelf and atolL) In general, the inner shelf setting is characterized by lithofacies containing euryhaline faunas (lacking organisms associated with normal marine salinity), by sedimentary structures, or by lithic sequences indicating the proximity of a shoreline. Examples include ostracod wackestones ('C7W) and algal stromatolite boundstones (TtJB) deposited in nearshore lagoonal and intertidal envi ronments, respectively. Low-angle accretion cross strati fication and evaporite beds are also associated with inner shelf environments.

The inner shelf zone extends from shallow subtidal to high (storm) tide levels. It includes nearshore subtidal environments, coastal lagoons, tidal flats or sabkhas, and beach environments. Consequently, lithofacies variation may be considerable along strike. For example, a 10-km length of coastline may be rocky (e.g., an eroding shore that hosts fringing reefs), sandy (a typical beach), or muddy (an algal flat where stromatolites bind and trap lime mud). The term facies mosaic has been used to describe this high degree of variability associated with inner shelf facies patterns. At the time of deposition, this facies belt ranges from a few kilometers wide to a maximum of about 15 km wide where the gradient of the sea floor is low. The shifting of inner shelf environments through time produces a wide fairway for exploration, best exemplified by three Permian San Andres zones of production from tidal flat deposits in a belt about 30 km wide on the northern shelf of the Delaware basin of West Texas (Meissner, 1974). Generally, if arid climates prevail, inner shelf lithofacies in the profile of Figure 7.9 would likely be modified to include dolomites (mainly dolomitized mudstones, M0) and possibly evaporites. In addition, the zone of nearshore faunal restriction (the inner shelf zone) would extend farther out into subtidal environments. Inner shelf deposits can also potentially form around the shoreline of any island. However, most islands, espe cially those in moderate- to high-energy middle shelf

Figure 7.10. Block diagram of carbonate lithofacies patterns across an idealized oceanic atoll during dominantly aggra dational stages of sedimenta tion associated with trans gressive systems tracts (TSTs). (See Figure 7.9 for lithofacies legend.)

I

BASIN SLOPE

I

OUTER SHELF

I

MIDDLE SHELF

ouTER SHELF

settings or those associated with open ocean atolls, show little evidence in the rock record that they were emergent. Unless tidal flat deposits formed, island deposits are difficult to distinguish from subtidal shoal deposits (Ebanks, 1975). The middle shelf setting (Wilson and Jordan, 1983) consists of a broad band of lithofacies that separates inner and outer shelf settings. Middle shelf environments generally include vast areas of subtidal sediments dominated by lime mud (e.g., skeletal wackestones, \W) and account for the bulk of carbonate deposits on a typical shelf. Burrowing in these sediments is common; by definition, they host normal marine faunas and floras. Water depths in middle shelf environments vary from about 2-3m to 60 m). Shelves commonly lie within normal wave base (about 10m deep), unless subsidence exceeds the tendency to build up to sea level. Within the regional backdrop of muddy sedimenta tion across the middle shelf, sand shoals (grainstone deposits), patch reefs (boundstones and related grain stones), and patch reef complexes can occur, all of which have high potential as reservoir facies. Patch reefs and shoals can reflect considerable environmental variation: some are emergent forming islands of storm-tossed detritus; some are capped with boundstone; others are sand shoals with small heads of reef framework; and still others are sand shoal deposits devoid of any frame builders (Figures 7.9, 7.10, and 7.11). Generally, a halo of skeletal grainstone-packstone C),G-P) occurs around individual patch reefs, demon strating the radial transport of reef-derived material away from the reef core and out into the middle shelf lagoon. Dominant paleowind directions can be inferred from asymmetric reef halos or from shoals whose spits

Is I

LOPE BASIN

point toward shore. Beyond the skeletal grainstone packstone halo in deeper water off the reef, a back ground sedimentation of skeletal packstone C),p) occurs, deposited in interreef lagoonal settings. These packstones surround and include all the individual reefs of a typical middle shelf reef complex (Figure 7.11). Farther beyond the reef complex, the regional background of middle shelf sedimentation is generally muddier, e.g., consisting of skeletal wackestone-packstone C),w /P), argillaceous skeletal wackestone-packstone (l),W /P), benthonic foram-rich shale ( SH), or benthonic-foram planktonic foram packstone ( clOP). The outer shelf setting consists of a moderately narrow facies belt (2-8 km wide) of grainstone shoals or boundstone facies, forming either a linear shelf edge shoal or a barrier reef. Outer shelf facies form consistent linear to curvilinear trends that provide remarkably reliable exploration fairways along depositional strike. Figure 7.9 show the basic lithofacies pattern associated with progradational sedimentation characteristic of highstand systems tracts. A similar pattern can form farther downslope as lowstand systems tracts. The sea floor slopes gently from the shoreline to perhaps several tens of meters deep somewhere in the middle shelf envi ronment, then rises toward the shoaling environments of the outer shelf, and finally drops down the steep slope (the upper part of which may be vertical) to form an apron out into the basin. There is a general trend from muddy carbonate textures nearshore, to more grain-rich textures offshore near the shelf break and slope (or near local middle shelf highs with reefs or grainstone shoal deposits), and finally to muddy textures out in the basin. The productivity of carbonate grains in the outer shelf and slope is high, rather like a well run "carbonate

INNER SHELF ),G or SS

MIDDLE

MIDDLE

SHELF 'AW/P, l),W/P. OSH, or

SHELF

0£P

),W/P,

"l),W/P, or OSH

ISLAND

B 1

Figure 7.11. Map views of carbonate lithofacies patterns across an idealized carbonate shelf during dominantly aggrada tional stages of sedimentation associated with highstand systems tracts (HSTs). (A) Inner shelf, (B) middle shelf, (C) outer sheH and slope, and (D) basin. (See Figure 7.9 for lithofacies legend.)

factory" (James, 1984). These offshore areas are away from the input of clastic materials that interfere with photosynthesis, filter feeding, and the growth of colonial frame builders and are positioned near the shelf edge where open marine wave energies winnow out most of the lime mud in the sediments. At the outer shelf, either barrier reefs or grainstone shoals develop, forming long, narrow, nearly continuous lithofacies belts that rim shelves or major structural blocks, while considerable amounts of coarse-grained shallow water sediments slump and slide down the slope. Prime examples of prolific outer shelf deposits are rudist boundstones and grainstones of Cretaceous age around the Gulf of Mexico (Bebout and Loucks, 1977; Enos, 1985). The outer shelf lithofacies belt is the focal point of Figure 7.11C. It generally forms a topographic high on the shelf to basin profile (commonly forming small islands) and is the main factor influencing facies patterns on either side of it. Facies changes occur as a broad shoal forms and dampens marine energies enough to make

relatively low energy water (the middle shelf lagoon) behind it. The outer shelf is a site of grain production, exporting sediment downslope in front of it and back onto the shelf behind it. The sediment is completely free of lime mud along the open basinward side of this facies belt, but it consists of packstones on its leeward side. Outer shelf patch reefs occur mainly in the lee of the main barrier where they may preferentially develop opposite the mouths of tidal passes. Three such reefs are shown in Figure 7.11C, portraying tidal flow onto the shelf through a pass in the barrier reef. Oceanic swells impinge on the shelf edge from the open ocean basin, piling up broken frame builders and winnowing lime mud. In front of the outer shelf reef is a belt of slope deposits which consists of a mixture of indigenous slope sediment (a rain of planktonics and settling lime mud) and allochthonous outer shelf sediments, a considerable amount of what can be coarse sand-size to boulder-size debris with good interparticle porosity. The best example of oil production from slope facies is the Cretaceous

Figure 7.12. Block diagram of carbonate lithofacies patterns across an idealized carbonate shelf during dominantly aggradational stages of sedi· mentation associated with transgressive systems tracts (TSTs). (See Figure 7.9 for lithofacies legend.)

G.!i!J I

I

INNER SHELF

MIDDLE-OUTER SHELF

Tamabra Limestone from the Poza Rica field of eastern Mexico (Enos, 1985). A variation of the inner middle outer shelf pattern occurs on oceanic atolls (Figures 7.10 and 7.110). Here, the dominant facies belt is middle shelf, with a narrow rim of outer shelf deposits outlining the atoll. Only small, localized occurrences of inner shelf settings might be represented on small islands. Reef types include the main barrier reef of the outer shelf and numerous steep sided patch reefs in the central lagoon of the atoll. A similar diagrammatic approach is used to show aggradational lithofacies patterns (Figures 7.12 and 7.13). Here, the profile is a homoclinal ramp that gently slopes basinward (with dips commonly less than 1'-2 '), with numerous reversals of water depth (or dip) occurring at localized shoals and patch reefs. In this profile, shorelines are generally not smooth but rather digitate or barred. Extensive tidal flat deposits are formed on a gently dipping coast. Grainstone shoals or reefs form above any submarine topographic expression: faults, salt or shale diapirs, paleohighs formed as erosional remnants, or older reefs. Water depth and substrate type are critical to facies development, and slight changes affect facies patterns, resulting in complex lithofacies maps. In addition, on ramp profiles, it is difficult to distinguish easily between middle and outer shelf settings. In the proximal parts of the basin and in lower slope settings, pinnacle reefs may occur in a belt subparallel to the basin margin. They are steep sided and have narrow halos of reef-derived material around them. Pinnacles are charac teristically surrounded by and encased in basinal shales (or evaporites) that may provide source and seal.

BASIN

SEQUENCE STRATIGRAPHY The theoretical basis of sequence stratigraphy takes into account constructive and destructive interference among tectonic subsidence, eustatic sea level changes, and sediment accommodation space and accounts for trends in relative sea level change. The concepts of sequence stratigraphy as applied to carbonate rocks (Shaw, 1964; Irwin, 1965; Sarg, 1988; Schlager, 1992) are based on the lateral correlation of coeval lithogenetic units that are separated by one of two kinds of unconfor mities: type 1, extending out into the basin, and type 2, restricted mainly to inner shelf settings. Correlations are made on shelf to basin profiles across carbonate shelves and atolls that exhibit one of two main types of deposi tional profiles: ramps or drop-offs (Wilson, 1975). A shelf to basin profile shows the general concepts of sequence stratigraphy in time and depth (Figure 7.14). Within this framework, time-equivalent inner , middle , and outer shelf, slope, and basinal environments of deposition can be recognized, and associated lithofacies can be predicted for various systems tracts. With this approach, it is evident that the best developed barrier reef sequences can be expected in shelf-margin wedges (SMWs), in highstand system tracts (HSTs) where reefs can "stack up," and in lowstand wedges (LSWs). Patch reefs occur across middle shelf environments in all systems tracts and in outer shelf settings in transgressive system tracts (TSTs). Empirically, it appears that ramp profiles can evolve into drop-off profiles, but not vice versa, and that they tend to be related to TSTs and to early stages of HSTs.

MIDDLE SHELF ),P/G. ),W. or ),P/W

••• •••• •••• •••• •••• •••• •••• •••• •

0 0

0

MIDDLE- OUTER SHELF

0.

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· ···· · · · · ········ · ·· · ··· • · · ····· ·· ·· i j: j: j: j j:jj j i ·· jj:j:::j

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··· ·········:···:···:··········· PINNACLE········ :: :: :: :: :: :: :: :: :: :: :: .:=: =:· ·-::·:: ·.

m> ::;: :.> >-: -:.: R

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;: : :1: : : : :: ::; : 0 o

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not common in shallow intracratonic basins, but are logically found downslope of thick, older, well developed carbonate 1km platforms. Sources for lowstand sediments are linear and not from point sources, such as deltaic systems ·:·:·:·:·:·:·:·:·:·: associated with ·:·:·:·:·:·:·:·:·:·: clastic-dominated ·:·:·:·:·:·:·:·:·:·: ·:·: shelves. In summary, sequence stratigraphy Figure 7.13. Map views of carbonate lithofacies as applied to carbonate patterns across an idealized carbonate sheH lithofacies describes the during dominantly aggrada tional stages of detailed patterns of sedimentation associated with transgressive shelf building and systems tracts (TSTs). (A) Inner shelf, (B) middle basin filling in shelf, (C) middle outer shelf, and (D) basin. (See carbonate systems Figure 7.9 for lithofacies legend.) tracts. Original depositional Highstand development of environments favorable systems tracts in carbonate reservoir clastic-dominated facies exist: an inner systems tracts are and an outer shelf notably progradational. fairway. Other sites of In contrast, wide porosity devel opment carbonate lagoons include shoals and (middle shelf deposits) patch reefs of the may fill in with a middle shelf, porous progradational pattern sand-rich slope and building out from the lowstand fan deposits, shoreline or with a and pinnacle reefs in regular layering of slope settings. cyclic sediments, as the Carbonate LSWs are outer shelf aggrades or possibly progrades. Clasticand carbonatedominated shelves respond differently to sea levellowstands. Clastic material on stranded shelves is uncemented and is easily reworked and transported across the shelf into the adjoining basin. In contrast, carbonate sediments, upon subaerial exposure, are subject to cementation and/or dissolution but not to reworking as a secondcycle sand. Throughout the development of depositional sequences, two prime fairways of opportunity for the

o

<:: ><

.......... D .......... .. . ...

for primary and diagenetic (secondary) porosity can be posi tioned on a sequence stratigraphic model. In addition, disconformable surfaces can be easily recognized, indi cating areas where more intense diagenetic processes such as meteoric dissolution, surface calichification, and dolomitization can be expected. Benefits of this approach are (1) a framework for integrating all appropriate seismic and well data, which make porosity and perme ability more predictable; (2) a natural division of the sedi mentary column into lithogenetic units where correla-

SB2

• DOLOMITIZATION q. o DISSOLUTION q. INNER SHELF FAIRWAY o BEACH DEPOSITS • FRINGING REEFS o TIDAL FLAT DEPOSITS

:

mfs

SB 1

UTHOFACIES LEGEND INNER SHELF XG ss mBc M Me r.tDDLESHELF XW/P lXW OSH X•P/G Mlh localized XG •G 0G OUTER SHELF

(a) Depth

I!

OUTER SHELF FAIRWAY :..._ 7

• NEAR-REEF DEPOSITS •SHOALS •EXPOSURE SURFACES • DISSOLUTIOt# •REEFS

T

J:

POTENTIAL SITE of PINNACLE REEFS

1Cl.

w

C

ATOLLS

..,...,..,....,...,.

r-_.. ====aiiiliiii

D

s--.

1

or OB Betc.

c:>G hG •G G X•G h•P

or OBB etc.L..:....:..-...:J

SLOPE

MiXW

BASIN

coarse BR

(b) Geologic time

UNCONFORMITY St.N-1

SB2 HST

mfs

CONDENSED SECTION SUBAERIAL HIATUS

TST LST

T w

::2:

i=

0

a g

0

w

C!)

HST

1

Figure 7.14.Carbonate lithofacies patterns and generalized reef distribution (a) in depth and (b) in geologic time, overlain on the sequence stratigraphic framework of 5arg (1988).581, sequence boundary associated with a Type 1 unconformity; 582, sequence boundary associated with a type 2 unconformity; mfs, maximum flooding surface; HST, highstand systems tract, LST,lowstand systems tract, TST, transgressive systems tract, SMW, shelf margin wedge. Major unconformity surface at the top of 581 is where porosity<+> due to dissolution and/or dolomitization is most likely to occur.

tions of fossil-poor zones are treated logically; and (3) logical and somewhat predictable progradations of facies belts, useful for prospect generation involving strati graphic traps. There is, however, the possibility of "over applying" the principles of sequence stratigraphy to situ ations where lateral correlations cannot be made (e.g., a single core 10m long from a rank wildcat well), usually due to a lack of data.

DIAGENETIC OVERPRINT Porosity in carbonate rocks results from two processes: preservation from primary conditions of deposition or creation by dissolution processes, many of which occur at relatively shallow burial depths. In general, few carbonate reservoirs-the giant Jurassic fields of Saudi Arabia being notable exceptions-display unmodified primary intergranular porosity. If primary porosity remains at all, it is commonly reduced to some degree by cementation, for example, by isopachous rim cement. More commonly, porosity in carbonate rocks is

secondary, formed by various dissolution mechanisms. One of the main debates today is how much dissolution is produced at depth (Mazzullo and Harris, 1992) by reactions involving the formation of weak organic acids, the thermal maturation of kerogen, and reactants from dewatering shales. Because rock-water reactions mainly control carbon ate cementation as well as the development of dissolu tion porosity, it is important to know the distribution of various pore fluids in the subsurface. The typical distrib ution of freshwater lenses, mixing zones, marine phreatic zones, and "subsurface brines" along a typical carbonate shelf profile is summarized in Figure 7.15. A well drilled into a middle shelf high on this profile would encounter zones of cementation, dissolution, and chemical stability or inactivity (Figure 7.16), as summarized by Longman (1980), Harris et al. (1985), and Moore (1989). Since most carbonate rocks originate as marine deposits, their diage netic history can be plotted, using the theoretical consid erations of Figure 7.16, by beginning in the marine phreatic zone and following one of two diagenetic pathways: (1) steady subsidence from the marine

Figure 7.15. Profile ofthe distrib ution of subsurface fluids showing diagenetic environ ments in an idealized shelf to basin profile.The single middle shelf high through which the columnar section of Figure 7.16 is drilled is exaggerated in height to show the freshwater lens typical of most islands.

MARINE

--------------SUBSURFACE

PHREATIC

-

M-PH

-----------------------BRINES

phreatic into shallow burial and finally deep burial diagenetic realms, or (2) uplift from the marine phreatic to be exposed to meteoric diagenesis, then subsidence back into the marine realm and finally into burial diagen esis. Carbonate sediments tend to build upward to sea level, thus meteoric exposure commonly affects at least inner shelf deposits. It also strongly affects emergent shoals or reefs of the middle and outer shelf. The cause of dissolution porosity remains problematic in that several modes of origin exist: (1) subaerial exposure, (2) regional freshwater aquifers extending out below the sea floor, or (3) deep burial reactions involving weak acids produced at depth from the dewatering of shales or from the formation of weak organic acids associated with the maturation of kerogen. Of these possibilities, the first two-occurring near or at the surface-appear to be most likely because fluids there are exchanged relatively rapidly and contain relatively high concentrations of unspent reactants. Carbonate diagenesis is greatly limited by the presence of migrating hydrocarbons. As pores become filled with less reactive substances, rock-water reactions are restricted to residual water saturations that coat pore walls as thin films (Feazel and Schatzinger, 1985).

CONCLUSIONS The lithology and types of porosity that characterize carbonate reservoirs are summarized in Figure 7.17, which also shows geographic positions favoring porosity development on shelf to basin profiles. Wilson (1980a,b) summarized the occurrence of carbonate reservoirs as seven recurrent settings. Table 7.3 lists these generically, whereas Table 7.4 presents a summary based on 39 field studies of carbonate fields by Roehl and Choquette (1985).

From these data and the basic carbonate lithofacies patterns discussed and portrayed, certain trends emerge.

First, inner shelf, outer shelf, and slope lithofacies belts are prime exploration fairways that are relatively predictable. Second, middle shelf prospects are variable in their size and distribution and present more difficult exploration problems. Third, slope facies may exist as a porous downslope extension of an outer shelf fairway, formed as debris flow deposits, and may host belts of porous pinnacle reefs. Finally, basinal or oceanic settings may produce porous chalk facies or may have shallow water carbonate facies deposited as atolls on horst blocks or volcanic pedestals, producing rimmed margins of outer shelf facies that encircle a central lagoonal area with numerous middle shelf patch reefs.

Acknowledgments This paper was improved by reviews from William A. Morgan of Conoco, Inc., and Perry 0. Roehl of Trinity University. Thanks for contributions to Figure 7.1 and Table 7.1 by Mark Longman (Consultant, Denver, Colorado), Ian Russell of Mobil Exploration and Producing, Australia, and Mateu Esteban (ERICO-Petroleum Informa tion, London, England). Computer drafting was done by Ceth Jordan.

References Cited Bebout, D. B., and R. G. Loucks, 1977, Cretaceous carbonates of Texas and New Mexico, applications to subsurface exploration: The University of Texas at Austin, Bureau of Economic Geology Report of Investigations 89, 332 p. Choquette, P. W., and L. C. Pray, 1970, Geologic nomenclature and classification of porosity in sedimentary carbonates: AAPG Bulletin, v. 54, p. 207-250. Dunham, R. J., 1962, Classification of carbonate rocks according to depositional texture, in W. E. Ham, ed., Clas sification of carbonate rocks: AAPG Memoir 1, p. 108-121.

V1 MOcp VUGcp BRcp

VADOSE

V2 V3

FRESH WATER PHREATIC

MOcp VUG cp BP+

WP+

FW-PH1

MOcp VUG cp

FW-PH2

BP+ WP+ BR+

FW-PH3

SOIL ZONE- INTENSE DISSOLUTION ZONE OF MINOR DISSOLUTION

very little cementation little or no dissolution stabilization of hi-mag calcite and aragonite { neomorphism of aragonite

FW-PH4

MIXING ZONE

MZ-1

MZ-2 M-PH1

MARINE

BP+ BP+ WP+

-·-·-·----·---·--..--

dolomite stability in 5-50% seawater .

MINOR CEMENTATION-& bcc 1 +1EcT M t: no neomorphism -- no dissolution MINOR CEMENTATION by ISOPACHOUS RIM CEMENT IR+ ARAGONITE NEEDLES FIB+ ZONE OF CEMENTATION FIBROUS RIM CEMENT FIB IR+ { MICRITIC HMC CEMENT MIC+ BOTRYOIDAL "CEMENT" BOn F

zone of micritization

M-PH3

TO DEEP SUBSURFACE

BP+ WP+ MO+ VUG+ FR+ BR+ VUG

cp (?) or

BP cpSE (?)

L-z_._.

{aragonite to calcite hi-mag calcite to calcite

DISSOLUTION OF ARAGONITE (?)

PHREATIC

SHALLOW SUBSURFACE

.

mtnor neomorphism

M-PH2

..-·-z_7

MEN+ ES+

SOME ARAGONITE DISSOLUTION neomorphism of fossils and lime mud BLADED ISOPACHOUS RIM CEMENT IR+ ZONE Of { ABUNDANT EQUANT CALCITE CEMENT ES+ CEMENTATION SYNTAXIAL OVERGROWTH CEMENT SO+ diagenetically inactive zone

MOcp BP+ WP+

MENISCUS CEMENT EQUANT CALCITE CEMENT

ZONE of MINOR

R

0

FR cp and/or

ZONE OF CEMENTATION by BLOCKY CALCITE CEMENT (SPAR) FORMATION OF WEAK ORGANIC ACIDS THERMAL MATURATION OF KEROGEN { liquid hydrocarbons MIGRATION of methane OIL and GAS carbon dioxide over-pressuring and high temperatures DEWATERING OF SHALES

If

FR+

A

c T

ss+

s

T y L

o-wL

I

T

sE

u R I N G

Figure 7.16.1dealized vertical distribution of primary and dissolution porosity and calcite cementation.The verticalprofile showing porous and tight zones is taken through a middle shelf high on a typical shelf to basin profile (see Figure 7.15 for the position of this column). t, porosity; cement. The bold type in all caps indicates phenomena associated with porosity development or preservation; normal type in all caps indicates phenomena associated with cementation.

+,

Ebanks, W. J. Jr., 1975, Holocene carbonate sedimentation and diagenesis, Ambergris Cay, Belize, inK. F. Wantland and W. C. Pusey III, eds., Belize shelf-carbonate sediments, clastic sediments, and ecology: AAPG Studies in Geology 2, p. 234-296. Embry, A. F., and Klovan, J. E., 1971, A Late Devonian reef tract on Northwestern Banks, Northwest Territories: Canadian Petroleum Geology Bulletin, v. 19, p. 730-781. Enos, P., 1985, Cretaceous debris reservoirs, Poza Rica field, Veracruz, Mexico, in P. 0. Roehl and P. W. Choquette,

eds., Carbonate Petroleum Reservoirs: Heidelberg, Springer-Verlag, p. 455-470. Feazel, C. T., and R. A. Schatzinger, 1985, Prevention of carbonate cementation in petroleum reservoirs, in N. Schneidermann and P. M. Harris eds., SEPM Special Publi cation 36, p. 97-106. Flugel, E., 1982, Microfacies Analysis of Limestones: New York, Springer-Verlag, 633 p. Halbouty, M. T., ed., 1992, Giant oil and gas fields of the decade 1978-1988: AAPG Memoir 54, 526 p.

Lithology:

D

G

B

M·W·P

Porosity:

Primary Secondary

Position on shelf to basin profiles:

Inner shelf fairway Outer shelf fairway Middle shelf highs Deep water reefs and atolls

BP WP BC MO VUG BC

Figure 7.17. General properties of carbonate reservoirs.

Harris P.M., C. G. St. C. Kendall, and I. Lerche, 1985, inN. Schneidermann and P.M. Harris, eds., Carbonate cements: SEPM Special Publication 36, p. 79-96. Irwin, M. L., 1965, General theory of epeiric clear water sedi mentation: AAPG Bulletin, v. 49, p. 445--459. James, N. P., 1984, Introduction to carbonate facies models, in R. G. Walker, ed., Facies models, 2nd ed.:Geological Asso ciation of Canada, Geoscience Canada, p. 209-211. Jordan, C. F., Jr., 1985, A shorthand notation for carbonate facie&-Dunham revisited (abs.): AAPG Bulletin, v. 69, p. 146. Longman, M. W., 1980, Carbonate diagenetic textures from near surface diagenetic environments: AAPG Bulletin, v. 64, p. 461-487. Mazzullo, S. J., and P.M. Harris, 1992, Mesogenetic dissolu tion: its role in porosity development in carbonate reser voirs, AAPG Bulletin, v. 76, p. 607--620. Meissner, F. F., 1974, Hydrocarbon accumulation in San Andres Formation of Permian basin, southeast New Mexico and West Texas (abs.): AAPG Bulletin, v. 58, p. 909-910. Moore, C. H., Jr., 1989, Carbonate diagenesis and porosity: Developments in Sedimentology 46, Amsterdam, Elsevier, 338 p. Owen, E. W., 1964, Petroleum in carbonate rocks: AAPG Bulletin, v. 48, p. 1727-1730. Read, J. F., 1982, Carbonate platforms of passive (extensional) continental margins: types, characteristics, and evolution: Tectonophysics, v. 81, p. 195--212. Roehl, P. 0.,and P. W. Choquette, 1985, Carbonate Petroleum Reservoirs: New York, Springer-Verlag, 622 p. Sarg, J. F., 1988, Carbonate sequence stratigraphy, in C. K. Wilgus, B.S. Hastings, C. G. St. C. Kendall, H. W. Posa-

Table 7.3. Recurrent Carbonate Reservoir Typesa 1.

Middle shelf grainstone bars with primary (or modified primary) porosity (4>)

2.

Middle and outer shelf reefs a. Primary 4> in boundstones and associated grainstones b. BC 4> and FR 4> if dolomitized Grainstones and breccias of slope deposits Inner shelf dolomites (BC 4> and VUG 4>) with anhydrite seals; tidal flat deposits Dissolution, paleokarst development, and dolomitiza tion below regional unconformities Fractured carbonate reservoirs Chalks with BC 4> and FR 4>

3. 4. 5. 6. 7.

aAfterWilson (19BOa,b).

mentier, C. A. Ross, and J. C. van Wagoner, eds., Sea level changes--an integrated approach: SEPM Special Publica tion 42, p. 155--181. Schlager, W., 1992, Sedimentology and sequence stratigraphy of reefs and carbonate platforms: AAPG Continuing Education Notes Series 34, 71 p. Schmoker, J. W., K. B. Krystinik, and R. B. Halley, 1985, Selected characteristics of limestone and dolomite reser voirs in the United States: AAPG Bulletin, v. 69, p. 733--741. Shaw, A. B., 1964, Time in Stratigraphy: New York, McGraw Hill, 353p. St. John, B., 1980, Sedimentary Basins of the World (map compilation): AAPG map. Swanson, R. G., 1981,Sample examination manual: AAPG Methods in Exploration Series, 66 p. van Wagoner, J. C., R. M. Mitchum,. Jr., K. M. Campion, and V. D. Rahmanian, 1990, Siliciclastic sequence stratigraphy in well logs, cores, and outcrops: concepts for high-resolu tion correlation of time and facies: AAPG Methods in Exploration Series 7, 55 p. Wilson, J. L., 1975, Carbonate Facies in Geologic History: New York, Springer-Verlag, 471 p. Wilson, J. L., 1980a, A review of carbonate reservoirs, in A. D. Miall, ed., Facts and principles of world petroleum occur rence: Canadian Society of Petroleum Geologists Memoir 6, p. 95-115. Wilson, J. L., 1980b, Limestone and dolomite reservoirs, in G. D. Hobson, ed., Developments in Petroleum Geology, v. 2: Essex, U.K., Applied Science Publishers, p. 1-52. Wilson, J. L., and C. F. Jordan, 1983, Middle shelf, in P. A. Scholle; D. G. Bebout, and C. H. Moore, eds., Carbonate depositional environments: AAPG Memoir 33, p. 298--343.

,._.

Table 7.4. Summary of Lithofacies and Porosity Types from Carbonate Reservoirs Around the Worlda Type of

Field Name Puckett Cabin Creek Killdear

Location Texas Montana N Dakota

Formation Ellenberger Red River Red River

Age Profile Ord Ramp Ord Ramp Ord Ramp

Ord Ramp Killdear N Dakota Red River Pennel Montana Red River Ord Ramp Ramp Cabin Creek Montana Interlake Sil Sil Ramp Mt Everette Oklahoma Clarita SW Reading Oklahoma Henryhouse Sil Ramp Guelph Ramp Belle River Mills Michigan Sll Rainbow A Pool Alberta Keg River Dev Drop-off NW Lisbon Utah Leadville Miss Ramp Little Knife N Dakota Mission Canyon Miss Ramp Glenburn N Dakota Mission Canyon Miss Ramp N. Bridgeport Illinois Ste Genevieve Miss Ramp N. Bridgeport Ste Genevieve Miss Ramp Illinois Penn Drop-off Seminole SE Texas Strawn Happy Kansas Lansing-K City Penn Drop-off Seberger Kansas Lansing-K City Penn Drop-off Tarchaly Perm Ramp Poland Zechstein Rybaki Perm Ramp Poland Zechstein Sulecir Perm Ramp Poland Zechstein Perm Drop-off N. Anderson Rch New Mexico Bursum Perm Drop-off Morton New Mexico Hueco Perm Drop-off Reeves Texas San Andres Blalock Lake E. Texas Wolf camp Perm Drop-off jur Ramp Qatif Saudi Arabia Arab C Qatif Ramp Saudi Arabia Arab C Jur Oatif Saudi Arabia Arab C jur Ramp jur Ramp Qatif Saudi Arabia Arab D Qatif Saudi Arabia Arab D jur Ramp Coulommes France CC + Daile Nacree jur Ramp Ramp Chatom Arkansas Smackover Jur Mt Vernon Arkansas Smackover jur Ramp Hico Knowles Louisiana Smackover ]ur Ramp La Paz Venezuela Cogollo Cret Drop-off Fateh Dubai Mishrif Cret Ramp Sunniland Florida Sunniland Cret Drop-off Tamabra Cret Drop-off PozaRica Mexico Garoupa &. Pampa Brazil Macae Cret Drop-off Fairway Texas james Cret Drop-off Ekofisk North Sea Ekofish-Tor Cret-Pal Drop-off Gachsaran and Hakimeh Iran Asmari Oligo-Mio Drop-off West Cat Can yon California Monterey Mia Drop-off Nido A&. B Philippines St. Paul Mia Drop-off Onnagawa Fukubezawa Japan Mia Drop-off

Producing

Shelf

Environment

Settinl! Inner Inner Inner Inner Inner Inner Middle Middle Slope Middle Inner Inner Inner Middle Middle Outer Middle Middle Inner Inner Inner Middle Middle Inner Outer Mid/Inner Mid/Inner Mid/Inner Mid/Inner Mid/Inner Middle Inner Inner Mid/Outer Outer Outer Middle Distal Slope Middle Middle Basinal

Type(s) of Deposition Lithofacies ff'PD BR BC FR Tidal Flat ),eW/M 0 Tidal Flat MO VUG BC BP n-rBo .W/P0 BP Shoals 0G Pinnacle Reef mBo /5\0Bo *#Wo VUG CAV /5\),G/P /S\8o BP WP BC MV FR Reef ),ew/P0 [:>),eW/P0 MO BC 8R FR Tidal Flat )\Wo •W/P0 MO BC Tidal Flat FEN VUG BP Tidal Flat ®W ®G/P 8P WP Shoal 0G ),W/M0 Subtidal BC MO ), IA\G+ED W/P Reef BC VUG CH Algal Mounds ?:W/P ),G MO VUG Algal Mounds ?:W/P )\G MO VUG Tidal Flat BP 0Go Mo Tidal Flat BP 0Go Mo Tidal Flat BP BP SH MV Patch Reef aM a),p;w ?:8A ),G/P Tidal Flat MO BP GF ),w/M 0 Tidal Flat BC MO VUG Barrier Reef BAlM #AB/G 0G MO VUG BP Shoals BP MO BC <SoP/Go Shoals <S>OP/G BP MO Shoals BP MO Shoals OG 8P Shoals OG0 BP MO BC 0G/P 0),G/P OW Shoals MOBP Shoals BC MO VUG 0Go Shoals 0G @),G BP MO Reef 0/S\FR ),R/P BP MO @),G /7W/P Shoals MO 8P WP FR Reef /7G/P /78 MV BP WP Shoals 17G D BP MO 8C ),G C>il),G Debris Flow MO VUG BP @P Shoal MO VUG BP Patch Reef /78/W ),G MO BP Pelagic Sheets E9M 8P FR

Middle Oceanic Middle Basinal

Lagoonal muds Pelagic Sheets Reef Pelagic Sheets

),W/P D D CHT ),P/G llP D

" ' 00

Porosity

FR BC MO FR FR MV 8C 8P

aBased on Roehl and Choquette (1985). bL hofacies symbols (from Figure 7.3), displayed for producing carbonate reservoirs, are given in this order: lithic descriptor, grain size, and texture.

POROSITY(%) Range

Avg

0-12 1-25 7-15

3.5 13 12-15

0-25 2-22 6-23 0-15 0-20 3-30 3-15 1-12 8.5-27 15-20 2-22 13-40 3-18 2-12 2-12

12-25 11 15 8 7 10 10.1 5.5 14 17.3 12 27 13 4 4 8.2 3.5 10.6 9.6 7 10.4 9

na na na 1.2-12.5 3-20 7.8-17.6 8-10 12-25 25-31 21-31 5-25 15-26 5-30 20-35 13-18 10-18 2-12 1-25 0-30

na 18-30

na 0-45

na na 1-9 0.6-30.9

na na na na na

15 25 13 15

na 19 18 8 20 11 32

na 12 3 9.7

PERMEABILITY Range 0-169 0-142

na na <0.1-35 0.1-X 0-560 0.1->30 5-1000 x-570 0.01-100 1.0-167

na 0.1-9500 0.7-130 0.1-80 0.1-900 0.1-900

na na na 0.1->1000

na 0.01-230

na 12-100 80-100 250-5000 4-500 50-500

na

Avg 10-50 8

na na 9 5 93

na 8 184 22 30 23 115 12 29 1 0.3

na 5.7 124 67 2.2

na na na na na na na

40-200 63 0.01-250 108 1-25 3 0-80 2 1-102 30 1-1000 65 0.01-700 0.3-0.6 50-2450 200 3-27 na 0.1-1000 1.0

na na <0.01-3.3 0.3-8.5

na 186 1 3.2

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