1. Basic Carbonate

What are the most important controls on reservoir quality in carbonate sequences?

The main controls on reservoir quality (porosity and permeability) in carbonate sequences are : • depositional fabric (primary lithofacies, texture) • mineral dissolution (creation of secondary porosity) • mineral precipitation (cementation and replacement) • karstification (an important from of mineral dissolution/precipitation) • compaction • fracturing

Effects of early diagenesis on reservoir quality, Burial diagenesis will modify reservoir Quality (RQ) further information required in purple

INITIALLY POROUS AND PERMEABLE UNIT (Sedimentology)

MARINE DIAGENESIS following deposition Cementation, little effect on poroperm. no dissolution

EXPOSED

(Sedimentology)

Close to unconformity

METEORIC PHREATIC limited dissolution, cementation by low Mg calcite around grains

METEORIC VADOSE Extensive dissolution limited but patchy cementation (especially at pore throats)

Poroperm decrease, framework resistant to mechanical compaction during burial

Porosity increase, permeability lowered, framework resistant to mechanical compaction

COMPACTION LIMITED?

RQ poor RQ good RQ moderate RQ moderate RQ moderate - good

no yes

no yes

no yes

yes no

Effects of Burial diagenesis on reservoir quality

COMPACTION (Effective Stress)

CEMENTATION (Petrography)

DISSOLUTION (Petrography)

BURIAL DOLOMITISATION (Petrography)

FRACTURING (Well Data)

UPLIFT AND EXPOSURE (Seismic)

Quantification of effects?

Source of cement Extent of cement

Cementation

Yes (compartmentalise)

No source of cement? Hydrocarbon filling? (Geochemistry)

local

(Gheochemistry)

regional

Reprecipitation as cement Elsewhere in basin

No (inc. permeability)

Import of Ca MgCO3

Yes

No

mechanical

chemical

further information required in purple

RQ

REDUCED

RQ

REDUCED

RQ IMPROVED

RQ REDUCED

RQ IMPROVED

RQ MAY IMPROVED

RQ MAINTAINED

RQ REDUCED

RQ REDUCED

RQ IMPROVED

RQ MAINTAINED

Process 1 2A 2B 3A 3B 4A 4B 5A 5B Por. -ve -ve 0 +ve -ve -ve 0 0 0 Perm. -ve -ve 0 +ve -ve -ve +ve -ve +ve

1

yes no

2A 2B

3A 3B

4A

4B

5A

5B

Effects of meteoric diagenesis on reservoir quality

Effects on RQ if yes

Meteoric Diagenesis • Was there an aragonite precursor? (I.e. of Pre- Cambrian, Carboniferous, Permian, Triassic or Tertiary age) • Was it a humid climate at time of exposure? (Palaeogeographic position) • Was there a high rate of water drainage? (elevation, climate, size of hinterland) • Was reprecipitation of dissolved CaCO3 as cement limited (due to high drainage)?

+ve

+ve

+ve

+ve

Effects of karstification on reservoir quality

Effects on RQ if yes

Sequence Boundary Karst • Is there a joint/fracture system which may have had high water throuhput? (may be so if in faulted/folded terrain, if uplifted or recognizable on seismic) • Was there an aragonite precursor? (I.e. of Pre- Cambrian, Carboniferous, Permian, Triassic or Tertiary age) • Is the reservoir close to the unconformity/above the water table? • Is the pore system matrix dominated? (vuggy porosity may have poor permeability, caverns may be detrimental to drilling) • Can overlying clays/shales be ruled out? (May infiltrate porous zone beneath)

+ve

+ve

+ve

+ve

+ve

MODIFYING TERMS

GENETIC MODIFIERS SIZE* MODIFIERS

PROCESS DIRECTION OR STAGE

SOLUTION s CEMENTATION c INTERNAL SEDIMENT i

ENLARGED x REDUCED r FILLED f

CLASSES mm**

TIME OF FORMATION

PRIMARY P pre-depositional Pp depositional Pd SECONDARY S eogenetic Se mesogenetic Sm telogenetic St

MEGAPORE mg

MESOPORE ms

MICROPORE mc

large lmg

small smg

large lms

small sms

256

32

4

1/2

1/16

Use size prefixes with basic porosity types: mesovug msVUG small mesomold smsMO microinterparticle mcBP *For regular-shaped pores smaller than cavern size **Measures refer to average pore diameter of a single pore or the range in size of a pore assemblage. for tubular pores use average cross-section. For platy pores use width and note shape

Genetic modifier are combined as follows: ABUNDANCE MODIFIERS

Percent porosity (15%) Ratio of porosity (1 : 2) Ratio and percent (1 : 2)(15%)

or

or

PROCESS DIRECTION TIME + +

EXAMPLES: solution-enlarged sx

cement-reduced primary crP sediment-filled eogenetic ifSe

ANY MODIFYING TERMS ARE COMBINED WITH THE BASIC POROSITY TYPES IN SEQUENCE GIVEN BELOW :

GENETIC MODIFIER SIZE MODIFIER BASIC POROSITY TYPE ABUNDANCE + + +

EXAMPLES : intraparticle, 10 percent WP (10%)

primary mesointraparticle porosity P-msWP solution-enlarged primary intraparticle porosity sxP-WP micromoldic porosity, 10 percent mcMO(10%) telogenetic cavern porosity St-CV

Choquette and Pray, 1970

WP POROSITY intraparticle porosity

• Refers to pores formed where soft body parts lived in body cavities (e.g., gatropods) or pores where internal partitioning in otherwise solid material (e.g., rudist wall structures)

• may add considerably to the total porosity of grainstones and packstones

• are commonly enlarged by dissolution to form moldic or vuggy porosity

• an example follows of 10 perm plugs measured from three coral heads (Holocene), yielding average porosities of 47, 63 and 53 %

BC POROSITY intercrystalline porosity

• Forms between crystals of dolomite or limestone

• provides one of the most “evenly distributed” types of porosity in carbonates (except for vugs)

• occurs as mesoporosity and macroporosity in dolomites

• occurs as microporosity in limestone (within the lime mud matrix

BP POROSITY interparticle porosity

• Intergranular pores from spherical triangles between packed spheres and irregular between platy grain shapes

• rare in lithified rock--not commonly preserved due to cementation

• commonly is modified by thin isopachous rim cements that form in the marine phreatic

• common in jurassic carbonate of the Middle East and accounts for the success of these giant reservoirs

KV POROSITY keystone vug porosity

• Forms by the natural bridging of sand grains to form a “keystone arch” with pore space below it

• forms in the swash zone of beaches

• relatively rare porosity type

• recognized in the Pleistocene of the Bahamas

FENESTRAL POROSITY

• A porosity type commonly associated with algal stromatolite lithofacies

• voids formed within algal laminations by algae forming bridges and growing over other algal layers or by air/gas pockets within algal layers

• “fenestra” means window in latin and refers to the window-like openings within algal layers

• fenestral porosity may not be effective porosity

PROBLEMS with CARBONATE RESERVOIR

Heterogeneous porosity and

permeability complex depositional enviroments diagnetic overprints

GF POROSITY growth framework porosity

• associated with boundstone fabrics and reefs

• created by branches or tubes winding and bridging together to form pore space between their framework elements (not within them)

• one of the most difficult pore types to indentify

• may be relatively unimportant, since detritus commonly fills such spaces at the time of deposition

MO and VUG POROSITY moldic and vuggy porosity

• molds retain original particle shape

• vugs are irregular in shape

• aragonite grains are subject to dissolution and the formation of molds and vugs

• molds can leach further to form vugs

• the term MV porosity is coined for moldic-vuggy porosity combinations that are often difficult to separate

• moldic porosity (especially oomoldic) may represent isolated pores with non-effective porosity

MAJOR TECTONIC SETTINGS FOR CARBONATES

• Large offshore banks or platforms

• The sides of major cratonic blocks

• Intracratonic basins

• Sub-basins on wide shelves

FR POROSITY fracture porosity

• Brittle versus ductile behavior: dolomites fracture more readily than limestones

• Orientation of most natural fractures is verticle

• Maximum amount of porosity due to fracturing (e.g., in the Monterrey Shale of California) is about 6 %

• It is commonly beneficial to induce fracturing in the area surrounding the borehole to increase daily production rates: acid-fracs with propants

• Presence of fractures is critical for reservoir facies development in tight boundstones that grade to wackestones and in chalk deposits (coccolith mudstones)

• Fractures can be categorized descriptively as follows:

open fractures

closed fractures

fractures swarm

open microfractures

closed microfractures

PROBLEM: how to distinguish natural from articially induced fractures in cores

1. Look at the orientation of fractures, most of which should be vertical; partings caused by “unloading” will be subpareallel to bedding

2. Look for the presence of mineral cements lining the fracture

Recommendation: drill exploration holes at the intersection of fracture zones if

possible; lineament analysis by LANDSAT imagery is useful for determining where

such intersections occur. If a fracture intersection can be found within an

“exploration fairway”, such as a barrier reef trend, the odds for success are

dramatically increased.

BASIC REQUIREMENTS FOR DOLOMITE FORMATION

• SOURCE OF Mg

SEAWATER

Mf-RICH CLAYS FOR CEMENT

SKELETAL Mg CALCITE

• FLUID FLOW SYSTEM

• SUITABLE Mg/Ca RATIO

RESERVOIR QUALITY

POROSITY PERM

EXCELLENT > 20 % > 100 md

GOOD > 15 % > 50 md

FAIR 5 – 15 % 10 – 50 md

POOR < 5 % < 10 md

DIAGENETIC PHENOMENA AFFECTING CARBONATES

• MINERALOGIC STABILIZATION

ARAGONITE, CALCITE

• NEOMORPHISM (REPLACEMENT)

CALCITE, CALCITE

• DOLOMITIZATION

CALCITE, DOLOMITE

• CEMENTATION

VOID-FILLING CALCITE,

DOLOMITE or EVAPORITES

• SILICIFICATION

• PRESSURE SOLUTION / COMPACTION

• DISSOLUTION / KARSTIFICATION

• BRECCIATION / FRACTURING

DIAGENESIS

DEFINITION: THOSE NATURAL CHANGES WHICH OCCUR IN SEDIMENTS BETWEEN THE TIME OF INITIAL DEPOSITION AND METAMORPHISM

COMMON CARBONATE MINERALOGIES

MINERAL FORMULA CHARACTERISTICS ARAGONITE CaCO3 TRACE IMPURITIES; ORTHORHOMBIC MG-CALCITE CaCO3 4-25% Mg IMPIRITIES; HEXAGONAL CALCITE CaCO3 TACE IMPURITIES; HEXAGONAL DOLOMITE CaMg(CO3)2 50% or so Mg; HEXAGONAL

FRESH WATER VADOSE ENVIRONMENT

• CEMENTS TEND TO BE – MENISCUS

– PENDULOUS

• OTHER CHARACTERISTICS – LEACHING OF ARAGONITE

– SLIGHT CEMENTATION

– COMMON POROSITY

FRESH WATER PHREATIC ENVIRONMENT

• CEMENTS TEND TO BE

– ISOPACHOUS BLADED

– EQUANT CALCITE

– INTERLOCKING CRYSTALS

– COARSER TO PORE CENTER

• OTHER CHARACTERISTICS – SOME LEACHING OF ARAGONITE; LEACHING MAY BE ACCOMPANIED BY

CALCITE REPLACEMENT.

– LOW POROSITY

– RAPID CEMENTATION

– SYNTAXIAL OVERGROWTHS ON ECHINODERMS

MARINE PHREATIC ENVIRONMENT

• CEMENTS TEND TO BE – ISOPACHOUS ARAGONITE NEEDLES

– MICRITIC Mg-CALCITE

– COMMONLY INTERBEDDED WITH INTERNAL SEDIMENT

– SOMETIMES BOTRYOIDAL

– SOMETIMES BORED

• OTHER CHARACTERISTICS – NO LEACHING

– SLOW CEMENTATION EXCEPT WHERE TIDES PUMP WATER THROUGH SEDIMENT

– POLYGONAL BOUNDARIES

– MANY MINOR DISCONFORMITIES

DEEP SUBSURFACE ENVIRONMENT

CHARACTERISTICS:

• DISSOLUTION or CEMENTATION POSSIBLE

• SLOW RATES OF DIAGENESIS CAUSED BY:

– NEAR-SURFACE STABILIZATION OF ARAGONITE & Mg-CALCITE TO

FORM CALCITE

– NEAR-SURFACE CEMENTATION REDUCES POROSITY & PERMEABILITY WHICH INHIBITS WATER MOVEMENT IN THE DEEP SUBSURFACE

PERMEABILITAS OF ROCKS AND SEDIMENTS

• Tightly cemented criniodal limestone ……10 md

• Uncemented carbonate mud ……0.01 – 10 md

• Sucrosic dolomite ……0.1 – 150 md

• Cemented quartz or sandstone

or carbonate grainstone ……10 – 300 md

• Poorly cemented quartz sandstone

or carbonate grainstone ……300 – 500 md

• Unconsolidated quartz sandstone

or carbonate grainstone ……>1000 md

• Fractured sandstone or carbonate ……>1000 md

COMPARISON OF MAJOR SUBSURFACE DIAGENETIC CONTROLS

NEAR-SURFACE BURIAL DIAGNESIS

STRUCTURAL CONTROL

MINIMAL IMPORTANCE

VERY IMPORTANT

MINERAL STABILIZATION

VERY IMPORTANT MINIMAL IMPORTANCE

EQUILIBRIUM CONDITIONS

WIDE VARIATION SLIGHT VARIATION

RATE OF WATER INFLUX

VERY HIGH LOW

TIME OF RESIDENCE SHORT PERIOD LONG PERIOD

PRESSURE UNIMPORTANT VERY IMPORTANT

TEMPERATURE UNIMPORTANT IMPORTANT

TRACE ELEMENTS ISOTOPES

ENVIRONMENT Fe2+ Na+ Sr2+ Mn2+ Mg2+

MARINE H H

FRESHWATER

VADOSE L L

FRESHWATER

PHREATIC L

L

(VAR.)

SUBSURFACE

(Shallow burial) H

H

(VAR.)

ENRICHED

OCCASIONALLY ENRICHED

H = HEAVY L = LIGHT

General Properties of Carbonate Reservoir Rocks

LITHOLOGY dolomite grainstones boundstones

POROSITY PRIMARY

SECONDARY

interparticle intraparticle intercrystalline

moldic vuggy intercrystalline

POSITION ON PROFILE

DIAGENESIS

INNER-SHELF FAIRWAY OUTER-SHELF FAIRWAY MIDDLE-SHELF HIGHS DEEP-WATER REEFS/ATOLLS

STEADY SUBSIDENCE SINGLE UPLIFT MULTIPLE UPLIFTS

ORIGIN of CARBONATE CEMENTS

Cements

definition: crystal growths that form in void spaces in rock (i.e., pores and fractures) and occlude porosity

All cements form as precipitates, in mineral-saturated waters

Calcite crystal growth can occur in two main diagenetic realms:

1) shallow burial

2) deep burial

Some cements textures are diagnostic of the diagenetic environment in which they formed

COMMON ASSEMBLAGES of CARBONATE GRAIN TYPES

Fresh water / lacustrine 0 – 250 ft 0 salt low-mod energy ostracods, algal stromatolites, onkolites, chara seeds, gastropods, plant remains

Tidal flat / brackish / hypersaline 0 – 15 ft hyper- to hyposaline low-mod energy algal stromatolites, peloids, gastropods, ostracods

Inner shelf (restricted marine) 0 – 30 ft hyper- to hyposaline low- mod energy algal stromatolites, peloids, gastropods, ostracods, molluscs, mollusc fragments

Middle shelf (subtidal) 0 – 500 ft normal marine mod-high energy bioclasts, crinoids, echinoids, echinoids spines, brachiopods, bryozoa, forams, fusulinids, molluscs,

mollusc fragments, pelecypods, platy green algae, dasyclads

Outer shelf 1) Shoals 0- 30 ft normal marine high energy bioclasts, crinoids, echinoids, echinoid spines, brachiopods, bryozoa, forams, fusulinids, molluscs, mollusc

fragments, pelecypods, platy green, algae, dasyclads, ooids, pisolites, coated grains, peloids, intraclasts

2) Reef environments 0 – 200 ft normal marine mod-high energy corals, encrusting red algae, oysters, tubiphytes, rudists, thick branching, corals, thin branching corals,

bryozoa, stromatoporoids

Basinal >600 ft normal marine low energy spicules, planktonic forams, cephalopods, radiolarians, dinoflagellates, nannofossils, graptolites,

diatoms, fish scales, fish remains, echiniods