lab 10

Lucrari practice - Petrlologie sedimentară

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Lucrări practice 10

Diageneza sedimentelor şi rocilor carbonatice

Diferenta intre sedimentele siliciclastice si cele carbonatice consta in faptul ca ultimele sunt produse intrabazinale, rezultate prin precipitarea chimica, biotica si biochimica a particulelor componente. Astfel, avand in vedere rata mare de precipitare carbonatica, procesele diagenetice se desfasoara cu rapiditate, uneori fiind sincrone cu procesele depozitionale.

In functie de locul in care opereaza procesele diagenetice se pot separa trei medii distincte (vezi si LP3) :

Mediul diagenezei Caracteristici Procese specifice Etape diagenetice DULCICOL (METEORIC)

solutii interstitiale dulcicole, cu pH acid si circulatie descendenta

-dizolvare selectiva -cimentare -metasomatism (dolomitizare)

-SINDIAGENEZA -EPIDIAGENEZA

MARIN DE APA PUTIN ADANCA

solutii interstitiale cu salinitate normala si pH neutru si/sau alcalin

-cimentare -micritizare microbiana -metasomatism (dolomitizare) -dizolvare selectiva -compactare -neomorfism

-SINDIAGENEZA -ANADIAGENEZA

DE INGROPARE PROFUNDA

solutii interstitiale cu salinitate foarte ridicata si pH puternic alcalin

-dizolvare sub presiune -compactare -cimentare -metasomatism (dolomitizare)

-ANADIAGENEZA

Particulele carbonatice sunt consolidate prin liant care poate fi : -ciment = cristale de calcit cu dimensiuni >4 microni = sparit -matrice = un agregat de particule carbonatice submicroscopice (<4 microni) = micrit Procese diagenetice si efectele lor:

Proces Efecte Observatii COMPACTARE reducerea

porozitatii -modificarea contactelor dintre alocheme -deformarea bioclastelor

CIMENTARE reducerea porozitatii

tipuri de cimenturi

mineralogic structural -carbonatic=calcit, aragonit, dolomit

-de pori, poikilotopic de menisc,etc

DIZOLVARE SELECTIVA

creste porozitatea -are loc in cazul circulatiei unor solutii cu agresivitate chimica pentru o parte din claste; ex:dizolvarea bioclastelor.

DIZOVARE SUB PRESIUNE

reduce porozitatea -duce la formarea stilolitelor si a contactelor concav-convexe intre particule diferite mineralogic sau la contacte suturale intre


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particule similare mineralogic. NEOMORFISM reduce porozitatea -recristalizare = aragonit-calcit; micrit-sparit

-supracretere = dezvoltari sintaxiale in jurul placilor de echinoderme

METASOMATISM reduce sau creste poroziatatea

-substituirea particula cu particula a unui mineral cu altul; afecteaza atat alochemele cat si cimentul; ex:dolomitizarea, silicifierea bioclastelor.

MICRITIZARE MICROBIANA

distruge microstructura

-formarea unor anvelope micritice in jurul bioclastelor si ooidelor ; poate duce la o micritizare completa

Procese specifice : A.Cimentare -tipurile de ciment pot fi urmărite in LP3. B. Dizolvare selectiva; bioclastele (fragmente de bivalve) dizolvate - in masa micritica rezulta o porozitate secundara in care porii au forme specifice bioclastelor. C. Dizolvare sub presiune; stilolitele (S) evidentiate prin reziduri insolubile depuse pe traiectul fluidului ; daca calcarul are un continut mare in minerale argiloase acestea se vor dispune intr-o retea anastomozata (RA; "dissolution seams").

D. Neomorfism; recristalizare agradanta micrit-sparit (RAg) si recristalizare aragonit - calcit (Re) in interiorul bioclastelor (bivalve); in acest caz se observa cum cristalele de calcit cu un aspect mozaic echigranular au inlocuit structura primara fibroasa, aragonitica. E. Neomorfism; supracresteri sintaxiale (Ss) in jurul fragmentelor monocristaline de echinoderme; in nicoli incrucisati fragmentul de echinoderm si coroana de supracrestere se comporta ca un cristal unic.

F. Metasomatism ; dolomitizarea calcarelor - cristale rombice de dolomit insotite de o porozitate secundara datorata contractiei de volum care insoteste transformarea metasomatica calcit-dolomit. G. Compactare mecanica (1); Micritizare microbiana (2); deformarea bioclastelor (1)- este deseori insotita si de procesul de dizolvare sub presiune; micritizarea microbiana (2) a bioclastelor sau ooidelor - poate fi partiala, concentrata doar in zonele marginale, sau totala generand un aspect peloidal.


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Lucrare practica

Ora I

Se vor identifica, descrie si interpreta mai multe procese si produse diagenetice, schimband mai multe sectiuni subtiri si completand fisa nr. I Ora II

Se va alege o sectiune subtire mai complexa si se vor identifica, descrie si interpreta toate procesele si produsele diagenetice urmand algoritmul fisei nr. II. Se va elabora un text la rubrica concluzii privind evolutia diagenetica a rocii respective.


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FIŞA NR. I (ora I) Proces:

Efect sau produs Descriere, observaţii

Interpretare

Desen: scara ____ mm Mediul diagenetic: Etapa diagenetică: Chimismul soluţiilor interstiţiale:

Proces:


Interpretare


Proces:


Interpretare


Proces:


Interpretare



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FIŞA NR II (ora II)

1. Cadul scheletic mineral/porozitate: ............/............% 2. Spaţiul interstiţial (porozitate) :....................%

(vezi LP3 pt. tipuri de pori)

Tipuri de pori determinaţi de fabric

.......% Tipuri de pori nedeterminaţi de fabric

.........%

Pori primari

.......% Pori secundari

.........%

3. Evoluţia spaţiului interstiţial şi a particulelor sedimentare

SD- Sindiageneza; AD- Anadiageneza; TG-Telogeneza M-Meteoric; MF-Marin freatic; ÎP- Îngropare profundă

Nr. crt.

Procese şi produse diagenetice

Etapa diagenetică Mediul diagenetic

SD AD TG M MF ÎP


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Desene reprezentative fişa II scara

scara

scara

scara

Sisteme de referinta - diageneza rocilor carbonatice Porosity Fabric-selective pores

Interparticle (intergranular) porosity: Porosity between individual particles or grains of a sedimentary rock. Corresponds generally to depositional primary porosity, but also includes secondary porosity (e.g. resulting from partial dissolution of aragonitic wid cortices). Interparticle porosity is high in modem mud-free carbonate sands (up to about 50%) as well as in modem mud-bearing sediments (about 40 to about 75%). The preservation of open interparticle pores in ancient carbonates is promoted by the absence of water in the pores in dry climates, by a protective seal of clay or evaporites, or by early oil emplacement. Common pore sizes are 0.05 to 1 mm.

Intraparticle (intragranular) porosity: Primary pore space corresponding to defined parts of skeletons (intraskeletal porosity, e.g. chambers of foraminifera) or to open spaces created by the removal of less calcified internal elements (e.g. central part of Halimeda). Depends on the morphology and microstructure of tests and skeletons of organisms as well as on the ultrastructure of the grains (e.g. ooids or aggregate grains). Common pore sizes are <0.01 to 1 mm.

Growth framework porosity: Primary porosity associated with the growth of reef-building organisms. Framework porosity may be high (in modem coral reefs) or low (in reefs dominated by encrusting organisms). It tends to become quickly reduced by infilling of sediment and by carbonate cements. Fenestral porosity: Primary porosity bound to synsedimentary open-space structures and commonly associated with supratidal and intertidal, algal- and microbial-related, mud dominated sediments.

Shelter porosity: A type of primary porosity created by the shelter and umbrella effect of relatively large grains which prevent the sediment infill'ing of pore space underneath lying. Shelter porosity is favored by the existence of large platelike fossils (e.g. larger foraminifera, platy algae).

Intercrystalfine porosity: Porosity between more or less equal-sized crystals, often related to early and late diagenetic recrystallization and dolomitization processes. Common pore sizes are c1 to 10 pm.

Moldic porosity: Results from the selective removal, commonly by solution, of grains, e.g. fossils or ooids. Requires a distinctive mineralogical or microstructural difference between the solubility of grains and matrix or cements. Molds fom preferentially in rocks of mixed mineralogies in meteoric-phreatic, but also in burial settings. Common pore sizes are 0.1 to 10 mm. Biomoldicporosity refers to porosity caused by removal of fossils. Pore sizes <0.5 to several millimeters. Dissolution of (mainly aragonitic) ooids results in the formation of oomoldic porosity, particularly in meteoric vadose and meteoric phreatic environments. Common pore sizes <0.20 to >0.5 mm. Non-fabric selective pores

Fracture porosity: Results from the presence of openings produced by the syndepositional, depositional or post-deposiI tional burial breaking of rocks . Often caused by brittle fracture of shells as a result of increasing overburden before cementation, folding, faulting, salt solution, or fluid overpressing. Common in brittle homogenous carbonates, e.g. chalks. Fractures may be healed by late diagenetic calcite. Fracture porosity is the main porosity type in many reservoir rocks.

Channel porosity: A system of secondary pores in which the openings are markedly elongate and have developed independently of texture or fabric.

Vuggy porosity: Caused by irregularly distributed early and late diagenetic dissolution cutting across grains andlor cement boundaries and creating millimeter- to meter-sized holes that must be studied on different scales. Dissolution may start from molds or interparticle pores. Decimeter-sized vugs of, different size (globular; vertically elongated; irregularly elongated) and corroded walls that are lined by marcasite may be caused by synsedimentary to early diagenetic biogenic methane exhalation originating from the decay of organic matter.

Cavern porosity: Non-fabric selective porosity characterized by large caverns. The term cavern applies to man-sized or larger openings of channel or vug shapes formed predominantly by karstic solution processes. Fabric-selective or not

Breccia porosity: Depositional, solution and karst breccias may yield high porosities. Boring porosity: Micro- and macroborers contribute to the formation of very small to centimeter-sized

brings. Microborers are effective in producing microporosity. Burrow porosity:Various organisms create organic burrows in relatively unconsolidated sediment. Shrinkage porosity: Shrinkage of carbonate mud in tidal flats can result in the formation of

characteristic pore systems.

Fig. 1 diagrammatic representation of the basic fabric-selective porosity types used in the Choquette and Pray

(1970) carbonate porosity classification. What is meant by fabric selectivity is that the porosity is controlled by the grains, crystals, or other physical structures in the rock and the pores themselves do not cross those primary boundaries.

Fig. 2 diagrammatic representation of the basic non-fabric-selective or variably fabric-selective porosity types used

in the Choquette and Pray (1970) carbonate porosity classification. These are all porosity patterns that actually or potentially can cross-cut primary grains and depositional fabrics. They also include porosity types that potentially can be much larger than any single primary framework element.

Major diagenetic environments

Fig. 3 -Simplified scheme. Many of the studies dealing with the diagenesis of carbonate rocks are environment-specific and concentrate on processes affecting particular hydrogeochemically defined diagenetic environments. Carbonate diagenesis operates in the meteoric environment, the marine environment and the burial environment. In the meteoric environment pore space is occupied by freshwater and air (meteoric vadose zone above water table; hatched) or by freshwater (meteoric phreatic zone). The marine-vadose zone at the land-sea boundary and the mixing zone in coastal areas and shallow near-coastal subsurface exhibits meteoric and marine criteria. In the marine phreatic environment water is supersaturated with respect to CaCO3 in shallow seas and undersaturated in cold and deep seas. The subsurface burial environment comprises the subsurface beneath the reach of surface-related processes down to the realm of low-grade metamorphism. Conventionally shallow burial and deep burial are differentiated. The term near-sulface diagenesis refers to processes at or close to the sea floor and in the meteoric environment within the reach of surface-related processes related to depositional or weathering interfaces. Here, cementation is highly facies-specific. The terms eogenic, mesogenic and telegenic, introduced by Choquette and Pray (1970), refer to early near-surface, burial and uplift/unconformity-related processes. B - Major processes occurring in different diagenetic environments. The time involved in diagenetic processes varies significantly in different diagenetic zones. Early diagenetic solution/precipitation processes in meteoric vadose and shallow marine phreatic environments need far less time than late diagenetic deeper burial diagenesis, which can last millions of years. Similarly, unconformity-related meteoric phreatic processes may continue over very long time intervals. Early cementation in intertidal and shallow subtidal environments occurs within a range of almost recent to several tens to a few thousand years. Synsedimentary botryoidal cements on marginal slopes of platforms may grow over several tens of years, resulting in synsedimentary stabilization of steep carbonate slope deposits at or above angles of repose (Grammer et al. 1993).

Types of cements a) Type criteria:

Fig.4 Cement types part I (Flugel, 2004)

Fig. 4 Cement types part II (Flugel, 2004) a) Fabric criteria:

Fig. 5 Cement fabric part I (Flugel, 2004)

Diagenetical setting and types of cements

Meteoric vadose: Types: Dripstone, meniscus, microcrystalline, bladed, fibrous, drusy, blocky. Cement crystals small and

equant. Cements irregularly distributed, concentrated at grain contacts or beneath grains. Fabrics: Gravitational, meniscus, drusy and granular mosaics, syntaxial overgrowth.

Marine vadose: Types: Dripstone, meniscus, fibrous. Fabrics: Isopachous, gravitational, meniscus, granular mosaics, syntaxial overgrowth.

Meteoric phreatic: Types: Dogtooth, blocky, dripstone, meniscus, fibrous, microcrystalline, peloidal microcrystalline.

Cement crystals are a bit larger than vadose crystals. Cements more homogeneously distributed than vadose cements.

Fabrics: Circumgranular; isopachous, gravitational, equant, granular and blocky mosaics; syntaxial overgrowth. Marine phreatic:

Types: Fibrous, acicular, bladed, botryoidal, radiaxial, dogtooth, microcrystalline. Fabrics: Circumgranular, isopachous, crusts, splays, botryoidal.

Burial: Types: Blocky, poikilotopic, dog tooth, radiaxial.

lab 10

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