Micrite Content Research Articles

Diagenetic development of rhizoliths in the Ironshore Formation (Pleistocene) of the Cayman Islands

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ13C and δ18O, which vary in accord with the internal zones are more variable than the δ13C and δ18O of the host sediments. For the rhizoliths there is a high correlation between the δ13C and δ18O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ13C values to determine if the rhizoliths developed in association with C3 or C4 vegetation failed because the δ13C values correlate with the calcite content that developed largely as diagenetic products.

A scale-consistent method for imaging porosity and micrite in dual-porosity carbonate rocks

Unlike many other clastic rocks, relating velocity and permeability to porosity for micrite-bearing carbonate rocks has been largely unsuccessful. Recent studies have shown that additional parameters, most notably the distribution and/or proportion of micrite, can be used to parameterize the velocity and permeability behavior. However, there is currently no scale-consistent, 3D methodology for differentiating macroporosity and microporosity from the total porosity measured on bench-top laboratory equipment. Previous studies estimated microporosity and micrite content by combining total porosity measurements conducted on whole 50 mm cores with measurements of phase volumes on 1 mm digital rocks (i.e., scale-inconsistent). As a step forward from those, we imaged dual-porosity carbonate rocks using X-ray microcomputed tomography and then leveraged a recently developed, optimization-based technique, called data-constrained modeling, to map the macroporosity and microporosity distribution of our samples. We evaluate the volumetric proportions of macropores, micropores, and coarse-grained calcite as a function of micrite content — with their respective uncertainties — all measured on the same digital rock and with the same method. Finally, we determine how measurements of the volumetric phase proportions could be extended using standard effective medium models to predict reservoir physical properties. The sensitivity of these models to the proportion of micrite and microporosity within the micrite is evidence that the nonuniqueness among permeability, velocity, and porosity that is commonly observed of micrite-bearing carbonate rocks can be explained by a variation of micrite content and microporosity at a similar porosity.

EFFECT OF MICRITE CONTENT ON CALCITE CEMENTATION IN AN UPPER JURASSIC CARBONATE RESERVOIR, EASTERN SAUDI ARABIA

Cementation is a primary factor reducing the porosity of carbonate rocks. It is a challenge to accurately model cementation for reservoir quality prediction because cementation is often a syndepositional process. In addition, cementation requires fluid flow to transport chemical species for precipitation within the pore spaces in a sediment. The development of fully‐coupled depositional‐hydrogeochemical models for cementation prediction is desirable, but the parameters which control the extent of cementation need to be identified and evaluated. This study uses petrographic data from 583 carbonate samples from 15 wells in an Upper Jurassic (Kimmeridgian) reservoir at a giant oilfield in eastern Saudi Arabia to investigate the controlling effects of micrite content on cementation in carbonate rocks. The results indicate that the amount of cement decreases with increasing micrite content in the carbonate rocks analysed. In addition, a modified Houseknecht method has been developed to assess the relative fractions of porosity reduction in carbonate sediments due to compaction and cementation. The method highlights variations in depositional porosity for different rock textures and distinguishes microporosity from interparticle porosity. In the studied samples, the total porosity loss due to compaction and cementation is generally less than 45%, and samples lose more porosity due to compaction than cementation. The relative importance of compaction and cementation in reducing porosity is different for different rock textures: wackestones and mudstones lose porosity mostly as a result of compaction, while grainstones, mud‐lean packstones and packstones lose porosity due to both compaction and cementation.

Porosity-permeability relationship in dual-porosity carbonate analogs

We have investigated the effect of micrite content and macroporosity on the porosity-permeability relationship of dual-porosity carbonates using analog samples created in the laboratory. Specifically, we control the micrite-to-coarse-grains ratio to produce samples in which the micrite content is the only parameter changing. We also introduce into the microstructures controlled volumes of an acetone-soluble solid material (camphor) at the expense of the micrite aggregates, which functions as macropores after being dissolved. With regard to the effect of micrite on the porosity-permeability relationship, our results indicated that adding micrite to samples exhibiting a grain-supported microstructure reduces porosity and permeability drastically. By increasing the micrite content up to approximately 30%, the sample becomes micrite supported, at which point adding more micrite increases the porosity but no longer affects the permeability significantly. Samples characterized by a high micrite content were found to have lower permeability at any given porosity. When macropores are introduced at the expense of micrite aggregates, permeability increases drastically with porosity. The rate of increase in permeability decreases, however, as the micrite content of the original microstructure increases. Additionally, at any given micrite content, the permeability increases as the percentage of macropores increases because such pores do contribute more significantly to fluid flow as compared with the micropores characterizing micrite aggregates. We used the varying micrite-to-coarse-grains ratio and its effect on the porosity-permeability relationship to inform the Kozeny-Carman relation for a pack of spheres. Our analysis determined that micrite affects the porosity-permeability relationship of carbonates by reducing the effective particle size and increasing the percolation porosity. Additionally, incorporating the content of micrite and macropores into the analysis of the porosity-permeability relationship increased the coefficient of determination ([Formula: see text]) from 0.24 to 0.78. We concluded that knowledge of micrite content and macroporosity is of paramount importance to interpret and model porosity-permeability relationships in carbonates.

Middle Viséan (Mississippian) coral biostrome in central Guizhou, southwestern China and its palaeoclimatological implications

A middle Viséan (Mississippian) coral biostrome is reported for the first time from the Shangsi Formation in Yashui area, central Guizhou Province, southwestern China (palaeogeographically located in eastern Palaeotethys). The biostrome, which is about 500m across and 2.5–3.9m thick, is laterally variable and composed of rugose and tabulate corals with low taxonomic diversity comprising 4 rugose and 1 tabulate coral species belonging to 5 genera. Three growth stages of the biostrome are distinguished, based on different compositions of coral taxa. Average coral contents of the biostrome increase from 38.7% to 72.0% upward and the main builders are Siphonodendron pentalaxoidea, Syringopora sp. and Kueichouphyllum sinense. Associated fossils include abundant brachiopods, crinoids and common foraminifers together with rare calcareous algae, bryozoans, gastropods and ostracods. Relative sea-level changes are interpreted to have controlled growth and demise of the biostrome, which grew continuously during sea-level rise and decreasing water energy, as evident from the gradually increasing of micrite content and in situ coral colonies. However, the biostrome declined and died as the sea level fell and hydrodynamic energy strengthened, indicated by an increase of bioclasts and sparry calcite cement (indicating lack of micritic matrix due to higher energy) overlying the biostrome. This coral biostrome has similar biotic composition to middle to late Viséan coral biostromes in Europe and North Africa (western Palaeotethys). The approximately coeval occurrence of coral biostromes in both eastern and western Palaeotethys suggests that a relatively global warm episode existed during the Viséan Stage.

The effect of micrite content on the acoustic velocity of carbonate rocks

We have demonstrated the effect of micrite content on the acoustic properties of well-controlled microstructures that are created in the laboratory to closely mimic those of carbonate rocks. In particular, we examined the effect of micrite content on the acoustic velocity, sensitivity of velocity to pressure, and changes in velocity resulting from dissolution upon saturation with a reactive fluid. We followed Dunham’s classification and fabricated the samples by mixing coarse (sand-sized) and very fine (micrite-sized) calcite grains in different ratios and then cold compressing the mixture. The acoustic velocities were measured under benchtop conditions and as functions of confining pressure before and after the injection of a [Formula: see text] aqueous solution. The benchtop measurements indicated that the addition of micrite made the frame of the carbonate samples stiffer. Because the sensitivity of velocity to pressure decreases as the content of micrite increases, we hypothesized a stiffer pore structure in micrite-richer fabrics. Moreover, the content of micrite seems also to affect the change in elastic moduli upon dissolution. Micrite-rich samples experience a drop in elastic moduli after fluid injection, with respect to the moduli measured under dry conditions. This was interpreted as being likely due to dissolution, which weakens the rock frame. Such an effect seems to overcome the stiffening that results from dispersion mechanisms under high-frequency conditions.

Laboratory measurements of the acoustic and transport properties of carbonate rocks and their link with the amount of microcrystalline matrix

Laboratory data and computer tomography (CT) scan image analysis of carbonate rocks were combined demonstrating a quantitative link between acoustic and transport properties, the fraction of microcrystalline calcite (micrite) in the matrix, and the grain-to-micrite matrix ratio. Samples from the Monte Acuto formation, Gargano, Italy, a transgressive system tract (TST) whose microstructure ranges from basal pelagic mudstones to coarse calciturbidites, were analyzed. Results indicate that high micrite rocks with low grain-to- micrite matrix ratios, typical of low-energy depositional settings, are relatively stiffer and more impermeable. In contrast, lower amounts of micrite, due to high-energy depositional settings or its removal from intergranular macropores by progressive leaching of the matrix, leads to high grain-to-micrite matrix ratios. This process will increase porosity and reduce rock stiffness, both of which reduce velocity. Decreasing micrite content correlates well with increasing macroporosity. Permeability increases with macroporosity, as microporosity within micrite does contribute less significantly to fluid flow. Study results, the mechanisms producing porosity, and the classifications describing carbonate rocks in the literature prompted us to address the characterization of these rocks from a different perspective: linking the transport and acoustic properties directly to the content of the microcrystalline matrix rather than to the type and/or geometry of the pore space left behind. These trends of the acoustic and transport properties particularly apply to cycles deposited in transgressive system tracts.

Sedimentological and marine eogenetic control on porosity distribution in Upper Cretaceous carbonate turbidites (central Albania)

AbstractResults of a detailed petrographic and stable isotope study illustrate that sedimentological differences and eogenetic dissolution/precipitation processes controlled porosity distribution within carbonate turbidites of the Ionian basin (central Albania). Based on lithology characteristics and porosity distribution observed in outcrop, individual turbidite beds can be subdivided into four distinct intervals, i.e. from base to top: (A) a non‐porous wackestone/floatstone or packstone followed by (B) porous packstone–grainstone that grades into (C) wackestone and (D) non‐porous mudstone with pelagic foraminifera. Wackestone interval C is characterized by an alternation of porous and non‐porous laminae. Changes in turbidity current flow regime controlled the initial presence of matrix micrite giving rise to both matrix‐ and grain‐supported lithologies within turbidite sequences. These are non‐porous and porous, respectively. Four eogenetic diagenetic processes (dissolution, cementation, neomorphism and compaction) acted shortly after deposition and modified primary porosity characteristics and distribution. Alteration by meteoric water is excluded based on the continuous burial until the Oligocene of the studied deep marine carbonates. Moreover, the stable isotope data with values between −2·1‰ and +0·7‰ for δ18OV‐PDB and between +1‰ and +3‰ for δ13CV‐PDB, favour alteration by marine‐derived pore‐waters. Compaction and aggrading neomorphism occurred dominantly in intervals characterized by higher matrix micrite content, i.e. the floatstone base and the wackestone–mudstone upper turbidite part. Framework‐stabilizing cementation occurred dominantly in the packstone–grainstone middle part of the turbidite beds. In the latter porous lithologies, matrix micrite was not compacted because of the grain fabric and the framework‐stabilizing cements. Here, neomorphism of micrite into microrhombic euhedral calcite occurred and microporosity was preserved.

Controls of facies and sediment composition on the diagenetic pathway of shallow-water Heterozoan carbonates: the Oligocene of the Maltese Islands

Relatively few studies have so far addressed diagenetic processes in Heterozoan carbonates and the role that sediment composition and depositional facies exert over diagenetic pathways. This paper presents a study of Oligocene shallow-water, Heterozoan carbonates from the Maltese Islands. We investigate stratigraphic distribution, abundance and timing of diagenetic features and their relationship to sediment composition and depositional facies. The studied carbonate rocks comprise rud- to packstones of the Heterozoan association predominantly containing coralline red algae, bryozoans, echinoids and benthic foraminifers. XRD analyses show that all high-Mg calcite has been transformed to low-Mg calcite and that no aragonite is preserved. Diagenetic processes include dissolution of aragonitic biota, neomorphism of high-Mg calcitic biota to low-Mg calcite and cementation by fibrous, bladed, epitaxial and blocky cements. Stable isotopes on bulk rock integrated with petrographic data suggest that the study interval was not exposed to significant meteoric diagenesis. We interpret early cementation to have taken place in the marine and marine burial environment. The distribution and abundance of early diagenetic features, determining the diagenetic pathway, can be related to the primary sediment composition and depositional texture. Sorting and micrite content are important controls over the abundance of diagenetic features.

Strengths abd weaknesses of the reef guild concepts and quantitative data: Application to the upper Capitan-massive community (Permian), Guadalupe Mountains, New Mexico-Texas

Analyses of large acatate sheet tracings, close-up photos and 105 sub-horizontal quadrat surfaces at four localities near the base of the Guadalupe Mountains Escarpment indicate that the biotic framework of the upper Capitan reef was built by about 35 species: one codiacean (Eugonophyllum sp.), 17 calcisponges, 9 bryozoans, one richthofenid brachipod, some crinoid (known only from columns), 4 Problematica and microbes. This widespread fossil community included members of the Constructor, Baffler and Binder Guilds. A re-evaluation of the Guild Concept (Fagerstrom, 1987, 1991) highlights the validity of the functional roles of the Constructor and Binder Guilds for reef construction. Members of the Baffler Guild, however, need to be revised and an interpretation of microbial micrite and cryptic biota remains controversial. Open surface phylloid algal and cryptic sponge-bryozoan dominated sub-communities were of only local importance. The upper Capitanmassive differs from its Permian conterparts in the low diversity and areal cover of the frame-building biota, low micrite content and abundant micro-frameworks, i.e, intergrown small sponges, Problematica and syndepositional cements (botryoidal and isopachous, fibrous calcite). Quantitative areal cover data were assessed at various scales. Large acetate sheets generally have low coverage of macro-biota (5.4%). By contrast, analysis of small areas of local high areal cover (selected acetate sheet quadrats, subvertical photographs, and quadrat samples: 15–21%) provide detailed insights into clustered patches forming the inital reef framework. Both data sets provide useful clues for an integrated approach to framework assessment. Mean acetate sheet data are limited by their somewhat generalized pattern, while small investigation areas may overemphasize local variation. Erect and pendant sponges with solitary, sub-cylindrical and multi-branche/clonal forms, were the predominant initial frame-builders in both open surface and cryptic habitats. Selective larval recruitment of erects sponges to firm substrates produced continous upward accretion of the initial framework. On open surfaces and in pores formed by tabular sponges and fenestrate bryozoans, erect and pendant sponges were supported in their hydrodynamically unstable growth position by encrusters, chieflyArchaeolithoporelle hidensis, Shamovella obscura, an unnamed tubular organism, and microbes. Subsequent growth ofArchaeolithoporella hidensis, microbial crusts and syndepositional cements on the outer walls of live sponges would have impeded ambient water circulation and may have led to ‘creeping sponge death by suffocation’ or complete encrustation after death. Filling of pores in the initial and encrusted reef framework by internal sediment (packstone-grainstone; derived from the framework and the back-reef shelf/platform) and voluminous syndepositional marine-phreatic cements completed the framebuilding process.

Peritidal Carbonate Platform Growth and Cyclicity in an Early Proterozoic Foreland Basin, Upper Pethei Group, Northwest Canada

ABSTRACT The upper Pethei Group is a 170-235 m thick, largely limestone, succession located in the east arm of Great Slave Lake, northwest Canada. Strata accumulated in a Paleoproterozoic foreland basin during the late stages of marine deposition. Rocks include nine predominantly stromatolitic facies that are grouped into five facies assemblages: (1) marginal peritidal complex; (2) back-barrier lagoon; (3) shelf margin; (4) subtidal shelf; and (5) slope and basin. Carbonate sedimentation on the platform involved precipitation of synsedimentary cements within and on stromatolite laminae, water-column precipitation of carbonate mud, and generation and transport of nonskeletal grains (stromaclasts, ooids, oncoids, peloids, and intraclasts). The upper Pethei platform is interpreted to have evolved through three basic platform geometries, peritidal rimmed shelf, subtidal rimmed shelf, and subtidal open shelf, in response to changes in the rate of creation of accommodation space. The peritidal complex and rim behaved in a manner consistent with models of Phanerozoic reefal rims, and alternatively, kept up, gave up, or caught up to sea level in response to fluctuations in the rate of change of relative sea level and lateral variations in sediment production and accumulation. Three scales of cycle are recognized: (1) overall formation-scale upward-deepening of the platform; (2) laterally traceable, decameter-scale shallowing-upward cycles separated by marine flooding surfaces; and (3) laterally discontinuous, meter-scale shal owing-upward peritidal cycles. The first two cycle types are interpreted to have been allogenic, controlled by changes in eustasy and subsidence rates. Meter-scale cycles are interpreted as mainly autogenic, having formed in an aggradational tidal island model. The upper Petrel platform is similar to other Paleoproterozoic foreland-basin carbonate platforms. Important differences, however, include (1) laterally discontinuous peritidal cycles, (2) lack of widespread sand shoals inboard of the shelf rim, (3) a thick, predominantly carbonate, slope and basin succession, (4) moderate to low platformal micrite content, (5) extensive lagoonal ooid sands, and (6) low contents of terrigenous clastics.

Effects of Texture and Composition on Mechanical Behavior of Experimentally Deformed Carbonate Rocks

Dry, low-porosity carbonate rocks with a wide range of texture and dolomite content were experimentally deformed at room temperature, strain rate of 10-4 sec-1, and at confining pressures of 0, 50, and 100 MPa (1 MPa = 10 bars = 145 psi) to evaluate the effects of texture and composition on ultimate strength and ductility. Of all the factors considered, weighted mean grain size and microcrystalline carbonate (micrite) content have the highest linear correlation coefficients with ultimate strength. Grain size is, therefore, the dominant intrinsic rock property that affects ultimate strength in low-porosity carbonate rocks. Systematic increases in total dolomite content (determined by X-ray diffraction) do not correlate significantly with ultimate stre gth when all of the variously textured rocks are compared. However, ultimate strength does increase with increasing dolomite content in rocks of similar texture. The approximate ultimate strengths of carbonate rocks with intermediate dolomite and/or micrite content can be predicted by the best-fit plane to the ultimate strengths of pure end members (Yule marble, Solenhofen limestone, Hasmark dolomite, Blair dolomite). For experiments at 100 MPa confining pressure, this plane is defined by the equation: 0.90 D + 2.07 M + 269 = ^sgru, where D is dolomite content in percent, M is microcrystalline carbonate content in percent, and ^sgru is ultimate strength in MPa. The equation of the plane defined by ultimate strengths of rocks of intermediate composition and texture is: 1.07 D + 2.29 M + 258 = ^sgru. The mean difference between the strengths predicted by these two planes is only 9 MPa. Most precursive microfractures in nonmicritic elements are intragranular. Although the density of intragranular microfractures increases with increasing confining pressure, grain boundary cracks are suppressed by confining pressure. Very few microfractures occur in the micrite itself. At a given confining pressure, highly micritic rocks exhibit much less abundant precursive microfractures than coarser grained rocks, and hence they are stronger.

Early Diagenesis and Lithification in Carbonate Sediments

ABSTRACT The dominant carbonate minerals in carbonate depositional terrains are aragonite and high-magnesian calcite. These undergo mineralogical and textural changes on subaerial exposure leading to lithification. If these minerals after deposition continue to be exposed to marine water, they retain their original mineralogical composition and textural characteristics and resist lithification. Under subaerial conditions during lithification, mineralogical and textural changes occur concurrently. In carbonate sediments devoid of micrite or with only a minor micrite content which are exposed to subaerial processes, low-magnesian calcite is precipitated as an interparticle cement to bind the grains together and at an early stage only partially occludes pore space. The calcium carbonate necessary to provide this cement is derived from the dissolution of aragonite. Magnesium is removed from high-magnesian calcite to yield low-magnesian calcite or high-magnesian calcite is removed and low-magnesian calcite formed on a micro scale by solution-deposition without a textural change of the grains affected. By contrast, aragonite grains are leached out to form moldic porosity, and the molds are infilled by a drusy low-magnesian calcite mosaic. Additional low-magnesian calcite, derived from the dissolution of aragonite, is precipitated as a drusy mosaic in the interparticle pore space. pH and salinity of the waters control the changes involved in lithification. In the presence of waters of requisite pH and salinity, lithification can probably occur in the subsurface. Paramorphic replaceme t of aragonite grains and micrite by calcite without an intervening stage of mold formation is common. Water probably takes part in this reaction with solution-deposition on a scale so small that cavities of molecular dimension are involved in this change. Thus small textures are fully preserved. Aragonite grains under subaerial conditions have a metastability sequence which in order of increasing stability is skeletal grains<ooids<pellets and cryptocrystalline grains. Pellets, cryptocrystalline grains and, to some degree, oooids tend to resist mold formation under subaerial conditions. During diagenesis they prefer to undergo paramorphic replacement by calcite. Dolomite was found to form under three depositional-diagenetic conditions: as a presumably penecontemporaneous sediment in the intertidal zone of pellet muds among algal mats in the Bahamas, under deep water marine conditions below the water-sediment interface on the Bermuda Apron, and by replacement of calciteinfilled molds of originally aragonitic grains. These three modes of formation during early diagenesis emphasize the diversity of conditions under which dolomite is apt to form.