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TABLE OF CONTENTS

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DIAGENESIS OF CARBONATES RESERVOIR

1.1 Definition of Diagenesis

1.2 Diagenesis of Carbonate

1.2.1 Seafloor Alteration

1.2.2 Cementation

1.2.3 Dissolution

1.2.4 Mechanical Compaction

1.2.5 Chemical Compaction

1.2.6 Recrystallization and Replacement

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REFERENCES 14


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CHAPTER 1

DIAGENESIS OF CARBONATES RESERVOIRS

1.1 Definition of Diagenesis

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Diegenesis is defined as the entire physical, chemical and biological changes that

sediment is subjected to (excluding folding and fracturing) after the grains are deposited but

before they are metamorphosed. Some of these changes occur at the water-sediment interface.

But the bulk of diagenetic activity takes place after burial. During burial, the main diagenetic

processes are compaction, lithification and intrastratal solution [1].

1.2 Diagenesis of Carbonate

Diagenesis of carbonate materials begin almost as soon as skeletal material is precipitated

in a shallow water setting and can continue thoroughout the history of the rock. [2]. Whenever

the psychochemical state of the sediment of rock changes, a diagenetic response is possible.

Diagenetic processes can begin with boring and micritization of grains on the seafloor [3] and

continue through cementation, dissolution, mechanical compaction and chemical compaction

1.2.1 Seafloor Alteration

Grains can be altered to micrite to the seafloor, often through a process involving boring

mechanisms. A large variety of boring organisms may be involved including microbes, algae,

sponge, worms and anthropods, rasping echinoids and fish and boring and rasping mollusks. The

type of extent of this alteration is environmentally controlled, and both bathymetric and

latitudinal zonations are evident. Bathymetric patterns are related to light penetration, latitudinal

patterns to temperature variation. The boring and scraping performed by the organisms as part as

their search for food not only helps micrite sediment, although it is normally assumed that the

amounts are insignificant in comparison to the amounts produced by inorganic precipitation and

algal disintegration.

1.2.2 Cementation

Cementation of carbonate can occur very early in the history of the sediment. It can forms

beachrock in the intertidal zone, lithify reefs and faces termed hardgrounds on the shallow

seafloor as shown in Figure 1.1.In the moderns sear, hardground and beachrock are known from

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Bahama banks and the Persian Gulf and Shark Bay, Western Australia. Hardgrounds have been

described on carbonate slopes on the flanks of carbonate platforms and on the margins of

submarine channels and canyons. Lithfication is most intense at the surface and decrease in

intensity downward, generally ceasing within tens of centimeters of the sediment-water interface.

Many hardgrounds have been described from ancient limestone, where they signify interruptions

in sedimentation, hiatuses that typically have durations of several hundred thousand years, as

estimated on the basis of faunal discontinuities. In both modern sediments and ancient limestons,

cemets formed on the seafloor (beaches, reefs and hardground) tend to be very uniform in

thickness. The elongate nature reflects an original aragonite or high-magnesium calcite

mineralogy.

Figure 1.1: Burrowed and mineralized hardground in shallow-water skeletal wackestone to

mudstone

Early calcite cementation dominates limestones exposed to meteoric water by a lowering

of sea level (for example, during glacial episodes). In the soil zone, carbon dioxide is added to

the waters and dissolution results, particularly at the land surface and above the water table.

Farther down the flow path, water mix and/or degas; this results in precipitation of calcite

cements. If the sediments are composed aragonite and high magnesium calcite, those minerals

are metastable relative to calcite, thus they dissolve and calcite cements form. Rocks composed

100% digenetic calcite can form these processes in just tens of hundreds of thousands of years.

The cements are clear spars with planar crystals boundaries, they range in size from tens to many

hundreds of microns, and they tend to increase in crystal size toward the center of the pore. As

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shown in Figure 1.2. The abundance of the cement will be a function of many factors, including

the properties of the host sediments (mineralogy, grain size, grain sorting, porosity and

permeability), climate (amount of freshwater influx), vegetation (amount of carbon dioxide

added) and time. Cements formed above the water in the vadose zone may be distinguished from

those formed below the water by various morphological criteria as shown in Figure 1.3.

Diagenesis by meteoric water is not restricted to subaerially exposed carbonate sediments.

Freshwater can migrate laterally for more than 100 km through the shallow sedimentary pile

under carbonate platforms or the continental shelf. The driving force is the hydrostatic head of

freshwater. This freshwater can also generate calcite cements if other reactions such as the

dissolution of evaporates or oxidation of organic matter are occurring. These reactions add

calcium and carbonate ions to the water; this causes the precipitation of calcite.

Figure 1.2: Pennyslvanian skeletal grainstone. It shows an early generation of finer crystalline

sparry calcite that overlain by a younger

generation of coarser sparry calcite.

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Figure 1.3: The different types of cement precipitated in vadose (left) and phreatic (right) parts

of the meteoric diagenetic environment.

The morphology, abundance and distribution of calcite cements in many ancient

limestone is very similar in thin section to unburied Quaternary limestones lithified only by early

meteoric diagenesis. For decades this led many carbonate geologists to consider all early calcite

cements to be products of fresh water environments. In the late 1990s, however, upper-slope

limestones recovered during the Ocean Drilling Program and Neogene limestones cored on the

margins of the Bahama Bank top revealed a different story. These limestones were deposited in

water depths too great to have received fresh water fluids even during the 100 m sea level low

stands of the Pleistocene. These limestones had only been buried in, and experience alteration in,

marine pore fluids. Yet the contained calcite cements formed concurrently with aragonite

dissolution just as in fresh water settings. This tells us that the abundance of early cements in

ancient limestones cannot be priori assumed to be the product of fresh water alteration; rather

they merely indicate the early lithification of the rocks [4].

Carbonate rocks can continue to receive cements throughout their burial histories.

Coarse, often iron-rich calcites and dolomites characterize later-formed cements. The sharp

change in crystal size and iron content relative to underlying cements is typically the evidence

for a burial origin. The geochemistry and fluid inclusions present in such cements can be used to

determine when those cements might have formed. Cements formed at > 3000m of burial and

from fluids of 200C have been documented. Anhydrite, halite and quartz are other types of

cements known from limestones often forming late in the history of rock.

1.2.3 Dissolution

Most diegenetic processes are reversible. In aragonite, calcite, anhydrite or halite can be

precipitated, they can also be dissolved. Dissolution creates secondary porosity within

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limestones, and perhaps karst cavities and solution breccias. Because of the relatively high

solubility of calcite compare to other common sedimentary materials, secondary porosity is

commonly developed in carbonates ( and in calcite-cemented sandstones) and may be essential

for the creation of pores that are sufficiently numerous for oil fields to form or for carbonate

rocks to host fresh water aquifers.

Carbonate sediments initially composed of aragonite and high magnesium calcite are

typified by solution pores that are fabric-selective, that is, molds in the size and shape of

aragonite skeletal, ooid or peloids grain. This dissolution occurs simultaneously with

cementation (both in freshwater and burial in sea water); thus some moldic pores may be

immediately back filled with calcite cements in Figure 1.4. If the original sediments were all

calcitic or the carbonate is now a completely calcite rock, the dissolution may or may not be

fabric selective. If not focused on particular fabric elements like grains, then vugs, solution-

enlarged fractures and solution-enlarged intraparticle voids forms. These non-fabric-selective

pores cut across both allochems, matrix and cements as well as stylolites, fractures, or other

earlier- formed features. In the extreme case, voids large enough to crawl through may form, and

at that point they are called caves. Caves can form by dissolution of both limestone and

evaporate deposits interlayered with limestones. With continues dissolution and/or deposition of

overlying sediments, the cavernous pore systems will collapse and breccias will form from the

collapsed ceiling and wall rocks.

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Figure 1.4: Secondary porosity in limestone as seen in thin section. Phylloid algal plate in an

algal mound and pore space created by dissolution of coarse calcite crystals within plate. Micrite

envelopes outline plate.

As in sandstones, dissolution can occur far below the land surface. The chemistry of

secondary porosity formed by dissolution of calcite in limestones during deep burial is the same

as the chemistry of secondary porosity formed by dissolution of calcite cement in sandstones.

The acidity of the pore water must increase. The most common way for this can occur is through

an increase in carbon dioxide content caused by the degradation of organic compounds,

particularly petroleum. The ability of thick sequence of limestones to quickly buffer any acid

generated and the relationship among carbon dioxide pressure, temperature and calcite solubility

in Figure 1.5make the formation of deep secondary porosity in limestones a rarity. As seen in

Figure 1.5it is clear that the greatest effect of increasing carbon dioxide pressure occurs at low

pressures. The increase in calcite solubility at any diagenetic temperature is much greater

between 0 to 10 bars than between 20-30 bars. The greatest effect of temperature on calcite

solubility is also at the lower diagenetic temperatures, those below 100C.

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Figure 1.5: Solubility of calcite in the system CaCo3-CO2-H2O in distilled water as function of

temperature and pressure of CO2. Increasing CO2 pressure and increasing temperature have

opposite effect on calcite solubility.

1.2.4 Mechanical Compaction

Evidence of purely mechanical compaction is not common in limestones but it is a

process that does not occur. In grainstones, mechanical compaction features include broken

grains, collapsed molds, deformation of ooids and peloids, spalling (breakage and rotation ) of

seafloor cements rims away from grain surfaces and the the re-orienatation of grains to produce

overly close packing and a large number of longitudinal contacts (see Fig 1.6).These features are

morel likely to be seen in grianstones that did not experience much early cementation, as

cements enhance rigidity and dampen the effects of mechanical compacts. Concavo-covex shells

such as pelecypods, brachiopods and ostracods which cannot support great weights and might be

expected to be crushed by the weight of overlying sediments in nonmicritic limestones are

typically not broken. This suggests that early cementation by pore-filling sparry cement is very

common in carbonates.

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Figure 1.6: Large benthic foramas in a grainstone

Distinguishing mechanical compaction phenomena in mud-supported sediments is much

harder, and the presence of mechanical compaction in such rocks has been debated for decades.

The lack of shell breakage was long argued as evidence fro the absence of compaction. The

micritic rocks, the concavo- convex shells are protected from crushing because the strain caused

by overlying sediments is preferentially partitioned into the more easily squeezed muds. It is

those muds that compat but physical evidence for the process is lacking in thin sections.

Experimental compaction is modern mud-rich sediments from the Bahamas and South Florida,

however, reveal porosity loss from initial values > 60% to the values of ~ 40%, horizontal

alignmenr of skeletal grains, compression of circular burrows to ovoid shapes, creation of

organic-rich seams and the development of sharp lithologic of ancient mud rich limestones and

may indicate that mechanical compaction was common in the past, particularly if the carbonate

muds did not alter early to lithified micrite,

1.2.5 Chemical Compaction

Chemical compaction is the process of posrosity loss in carbonate rocks due to pressure

solution [5]. Pressure solution is indicated in thin section when the surface that separates two

allochems cut across the internal fabrics of one or both of them as shown in Figure 1.7. For

example ooids may be interpenetrate of stylolites may cut across fossil debris. Where truncation

is not obvious, pressure solution can sometimes be inferred from the presence of a film of

noncarbonated material (clay or organic matter) that remain as and insoluble residue. The stress

that causes the dissolution is transmitted through a thin film of water between the two rigid

allochems; the film also serves as the transporting pathways for the dissolved Ca 2+ and CO32- (or

HCO3-) to diffuse outward into the main pore system. The diffusing ions may be precipitated

nearby as new carbonate cements or may be transported a considerable distance by moving water

before precipitating. Depending on the purity of the rock and the lateral extent of dissolution may

result in the formation of a stylolite, an irregular surface within a bed characterized by mutual

interpenetration of the tow sides, the columns, pits and tooth-like projections on one side fitting

into their counterparts on the other as shown in Figure 1.8. Quantitative analyses of stratigraphic

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sections of limestones in which stylolites are abundant typically reveal that more than 25% of the

section has been removed by stylolitization. It was estimated that as much as 90% of the original

carbonate deposit had been dissolved, the ions having migrated elsewhere. In the absence of

seams of insoluble residue, either as gently undulating dark laminae or as fitted tooth fitted tooth

and socket featured , surfaces formed by pressure solution can easily can be mistaken for normal

bedding planes Figure 1.9. These pseudo bedding surfaces may be much more common than is

now recognized and may be an important source of calcium and carbonate ions for subsurface

cementation.

Figure 1.7: Thin section showing interpenetration of pisolith, ooids and spar cement.

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Figure 1.8: Styllolite seams (arrows) are formed of material less soluble than limestone, a

mixture of clay and carbonaceous material

Figure 1.9: Porminent weatherered surface (white arrow) passing laterally (to left) into

anastomosing stylolites which mighy me mistaken for bedding surfaces

Pressure solution is caused by enhanced solubility at localized points of anomalously

high stress. Sutured contacts ant toothy stylolites occur when the solubility of the dissolving

objects is the same. When one particle is much less soluble that the other, the particle with the

lower solubility controls the shape of the developing dissolved surface and the resulting surface

tends to be flat or perhaps gently undulating.

Calcium carbonate although very soluble in water relative to silicate minerals, is quite

insoluble by most other measures. The amount of calcium carbonate in subsurface waters

typically less than 1g/L; if 10% of this were precipitated, it would require more than 27L of

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water to precipitate only 1cm3 of calcite. Thus, the amount of water that would have to flow

through carbonate sediment to fill the pores with calcite to fill the pores with calcite is

prohibitively large except very near the surface where permeability is great, and water can move

vertically and rapidly. On the basis of this reasoning we might assume that nearly all cementation

of carbonate sediments occurs within perhaps 103m of the surface. But porosity and permeability

in limestone decrease continually with increasing depth indicating that cementation must

continue progressively to great depth. Such cementation cannot be result of near-lateral flow of

water through continually less permeable rocks because the time required for essentially

complete lithification is prohibitive. This conclusion leads to the interference that a large and

perhaps major source for the calcium and carbonate ions in surface waters must be internal to the

sediment itself. Cannibalization is required and the fact that many limestone beds do not contain

stylolitic seams suggests that nonstylolitic pressure solution is an important source for calcite

cement. Because pressure solution would increase in abundance and intensity with increasing

effective stress, nonstylolitic pressure solution seams may be very common at depths of perhaps

103m or more.

1.2.6 Recrystallization and Replacement

Recrystallization refers to a diagenetic reaction in which a mineral recrystallizes to the

same mineral [6]. Examples include high-magnesium calcite skeletal grains (e.g: echinoderms,

bryozoa, foraminifera and red algae) recrystallizing to low magnesium calcite and early-formed

metastable dolomite rhombs recrystallizing to more chemically stable dolomite. In this section,

the products may be physically identical to the precursors; although on some scale (usually

angstroms to microns) there are physical changes. Recrystallization generally happens early in a

carbonate rocks history because the original phase is metastable. Once a mineralogically stable

product is formed, recrystallization is suppressed. The trace-element and isotopic composition of

different calcitic grains and cements in ancient limestone are typically quite varied, indicating a

lack of recrystallization once this stable calcite formed.

Replacement refers to diagenetic reactions in which one mineral replaces to another.

Example include the aforementioned replacement of calcite grains and limestone by chert, the

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replacement of calcite by anhydrite (common in evaporate-rich limestone formed on arid

coastlines), the replacement of aragonite by calcite. All replacement reaction involved

dissolution of the host phase and precipitation of the new mineral phase. The dissolution-

precipitation process may be almost concurrent in time and separated in space by only a thin

water film. In this case, there will be some preservation of the texture of the precursor, such as

the growth banding or microstructure of a mollusk shell or the concentric coats aragonitic ooid.

When there is partial preservation of texture, the term neomorphism is sometimes used. The

dissolution and precipitation process however may also be widely separated in time and space

and in extreme case, replacement is then merely the formation of avoid by dissolution and the

subsequent infilling of that void by cement.

MAIN REFERENCES:

[1] Harvey Blatt, Robert J.Tracey and Brent E.Owens, Petrology: Igneous, Sedimentary

and Methamorphic, 3rd Edition, W.H.Freeman and Company, New York.

[2] Melim, L.A, and P.K.Swart and R.G. Maliva.2001. Meteoric and marine burial

diagenesis in the subsurface of Great Bahama Bank. In Subsurface Geology of a

Prograding Carbonate Margin, Great Bahama Bank: Results of the Bahama Drilling

Project, SEPM Special Publication 70, ed.R.N.Ginsburg 137-162

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[3] Reid, R.P and L.G. Macintyre,2000. Microboring versus recrystallization: further insights

into the micritization process:J.Sela.Res,70: 24-28

[4] Melim,L.A, H.Westpal,P.K.Swart, G.P.Eberli and A.Munnecke.2002. Questioning

carbonate diegenetic paradigms: evidence from Neogene of the

Bahamas,Mar,Geol.185:27-53

[5] Budd,D.A.2002. The relative roles of compaction and early cementation in the

destruction of permeability in carbonate grainstones: a case study from the Paleogene of

west-central Florida,USA.J. Sed.Res.72:116-128

[6] Reid, R.P and L.G. Macintyre,1998. Carbonate recrystallization in shallow marine

environments :a wide sprad diagenetic process forming micritized

grains.J.Sed.Res.68:928-946.

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