Micritic Grains Research Articles

Microporosity evolution and destruction in the Jurassic Arab D reservoir, Qatar

Core samples were collected from three wells, one onshore and two offshore, from Qatar’s Upper Jurassic Arab D reservoir. The samples were subjected to multiproxy petrographic and chemical analyses to identify their micro- and nanoporosity types and understand their evolution and destruction. Based on the petrographic and petrophysical properties of studied rocks, the Arab D succession was divided into seven rock types. Primary microporosity includes intergranular and interplanar, while secondary types include vuggy, intercrystalline, moldic, dissolution, pyrite displacement, microfracture, and microbial boring. Primary micropores were found mainly between the micrite grains in the lime mudstone facies, between the grains or the plates of clay minerals. Secondary micropores result from open and closed diagenetic systems. The open diagenetic system led to the development of dissolution and moldic micropores, while the closed system created pyrite displacement and boring porosity. Mechanical stress due to crystal growth or displacement generated microfractures. Micropores were destroyed either by cementation, clay minerals growth, dolomitization, or microbial pustular overgrowth. Microporosity was important in quantity and varied in nature in the mud-supported rocks. They are similar to macropores in grain-supported sediments but of less importance. The complex lithology of the studied rocks has significantly influenced the development and destruction of the porosity system of the Arab Formation.

The microbial carbonate factory of Hamelin Pool, Shark Bay, Western Australia

Microbialites and peloids are commonly associated throughout the geologic record. Proterozoic carbonate megafacies are composed predominantly of micritic and peloidal limestones often interbedded with stromatolitic textures. The association is also common throughout carbonate ramps and platforms during the Phanerozoic. Recent investigations reveal that Hamelin Pool, located in Shark Bay, Western Australia, is a microbial carbonate factory that provides a modern analog for the microbialite-micritic sediment facies associations that are so prevalent in the geologic record. Hamelin Pool contains the largest known living marine stromatolite system in the world. Although best known for the constructive microbial processes that lead to formation of these stromatolites, our comprehensive mapping has revealed that erosion and degradation of weakly lithified microbial mats in Hamelin Pool leads to the extensive production and accumulation of sand-sized micritic grains. Over 40 km2 of upper intertidal shoreline in the pool contain unlithified to weakly lithified microbial pustular sheet mats, which erode to release irregular peloidal grains. In addition, over 20 km2 of gelatinous microbial mats, with thin brittle layers of micrite, colonize subtidal pavements. When these gelatinous mats erode, the micritic layers break down to form platey, micritic intraclasts with irregular boundaries. Together, the irregular micritic grains from pustular sheet mats and gelatinous pavement mats make up nearly 26% of the total sediment in the pool, plausibly producing ~ 24,000 metric tons of microbial sediment per year. As such, Hamelin Pool can be seen as a microbial carbonate factory, with construction by lithifying microbial mats forming microbialites, and erosion and degradation of weakly lithified microbial mats resulting in extensive production of sand-sized micritic sediments. Insight from these modern examples may have direct applicability for recognition of sedimentary deposits of microbial origin in the geologic record.

Depositional Environment of the Ibrahim Formation and Determining the Oligocene-Early Miocene Boundary in Eastern Iraq

The aim of this study is to investigate the sedimentation environments and diagenetic processes of the Ibrahim Formation (Oligocene-early Miocene) in Zurbatiya, eastern Iraq. The Ibrahim Formation is comprised mostly of clayey micrite and skeletal grains composed of planktonic foraminifera, calcispheres, radiolaria, and benthic foraminifera. Glauconite and pyrite were documented in some restricted zones of this formation; they reflect quiet and reducing conditions. Radiolaria were identified in Late-Oligocene which was not known previously at this age regionally in carbonate formations of the Arabian Plate (AP). Mudstone, wackestone, and planktonic foraminiferal wackepackstone are the main microfacies that are affected by dissolution, cementation (granular), and replacement covering the age of the latest Eocene-Oligocene to the early-middle Miocene. Microfacies analysis indicates a basinal environment with a hemipelagic character of the deep shelf, toe-of-slope, and lower slope. Oligocene-Miocene outcrops have not been known previously in eastern Iraq although they have 160 m thick was first recorded in this study, whereas it was estimated as 56 m in the type section (well-1). Hence, the studied section is therefore suggested to be considered as a type section of the Ibrahim Formation. The Oligo-Miocene boundary is marked by restricted shallow-marine facies at the middle part of the formation indicating a short-term sea-level regression.

Studying petrophysical properties of micritic limestones using machine learning methods

It is important in geophysical applications to relate the compressional and shear ultrasonic wave velocities of micritic limestone to its porosity, volume fraction and density of micrite grains as well as the effective confining pressure. In this paper, this difficulty task is successfully realized by using the most relevant machine learning methods: The artificial neural network method, the support vector machine method and the extreme gradient boosting method (XGB). A relevant dataset available in literature is considered to train and test the models. It is observed that the XGB method significantly outperform the other methods in term of accuracy and training time. It allow obtaining a very high R-squared value of 0.96 and a very small relative root mean squared error of 3% while predicting the sonic velocities from other petrophysical properties. The robustness of the models is also confirmed by studying the sensitivity of the random splittings between the training and the testing sets

Study of a modern calcrete forming in Guadalajara, Central Spain: An analogue for ancient root calcretes

The Pajares calcrete in Central Spain, a modern calcrete, consists of an accumulation of powder carbonate around the roots of living trees and bushes which penetrates Miocene deposits by more than 3 m. Calcrete development is mostly vertical and oblique, but thinner horizontal calcified root mats also occur. Carbonate accumulation, up to 15 cm thick, is made up of micrite with some etched clasts, alveolar septal structures, calcified rootlets, coated grains and micrite grains. In detail many of these features are composed of needle fibre calcite and micro-rods. Accumulation of carbonate was produced both by roots through direct calcification of their cells and also by microbial activity within the rhizosphere. This case study provides a good example of how carbonate is accumulating around roots at present and may allow better understanding of ancient calcretes, especially those which formed directly around roots in direct contact with the host rock. In addition the calcrete's setting, and its occurrence around roots belonging to various types of plants, suggest that carbonate accumulation in modern landscapes may be a relatively rapid, common process even in erosive regimes, and unrelated to specific vegetation types.

Method Addresses Difficulty of Modeling Heterogeneous Carbonate Bimodal Rocks

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18587, “Heterogeneous Carbonates: A Modeling Method Ensuring Consistency Between the Saturation-Height and Permeability Models for Bimodal Rocks,” by Iulian N. Hulea, Harm Dijk, SPE, Danila Karnaukh, and Mirano Spalburg, Shell, prepared for the 2016 International Petroleum Technology Conference, Bangkok, Thailand, 14–16 November. The paper has not been peer reviewed. Copyright 2016 International Petroleum Technology Conference. Reproduced by permission. Because of their heterogeneity, carbonate reservoirs are more difficult to model than clastic reservoirs. The main difficulty comes from the number of different pore types, compared with the typical interparticle pore type in clastics. By using saturation-height models (SHMs) in combination with conventional permeability measurements, a new approach attempts to extract the fundamental properties of individual pore systems. The key idea centers on identifying the governing pore systems from capillary pressure curves and permeability measurements. The approach results in the ability to predict permeability continuously as a function of pore-system mixing ratios. Introduction Because of the presence of multiple pore types, carbonate rocks are difficult to model. To account for their heterogeneity, carbonate rocks are often modeled using rock-typing schemes. Two particularly challenging properties are permeability and saturation. Although these properties have been recognized as being closely connected, little information is available on how to handle them consistently. Even when permeability and SHMs satisfactorily describe the core measurements, it is not trivial to ensure their consistency in 3D models. One possible situation is building SHMs that impose consistency through the governing parameters. Throughout this work, a Brooks-Corey function is used because it was found to describe unimodal mercury-injection capillary pressures (MICPs) satisfactorily. For the examples discussed in this contribution (Fig. 1), microporosity (corresponding to the pore system accessible beyond pore throats of 2 µm or less) forms between 40 and 70% of the pore volume and is mostly located within the micritic grains. A significant difference between the two examples shown in Figs. 1a and 1d is the relative contribution of microporosity on average equaling 0.53 for Rock Type 1 (RT1) and 0.75 for Rock Type 2 (RT2). For RT2, microporosity can be located in the mud between the grains. Capillary Pressure Curve Data Analysis and Modeling The MICP modeling strategy relies on treating the two pore systems as independent, fitting each one of the pore systems first then looking for the best correlation to predict the Brooks-Corey parameters. On the basis of the measured individual-core-plug properties, the standard deviation for both plug porosity and the relative ratio of microporosity to total porosity was derived. By making the plots with the same fundamental inputs (SHM, porosity, permeability transform), the properties predicted by the models can be highlighted and contrasted.

Lower Cretaceous shallow-water carbonates and a new lituolid foraminifer (Bulbobaculites felixi n. sp.) from the Northern Apuseni Mountains (Romania)

Shallow-water carbonates from the Lower Cretaceous (upper Aptian) of the Subpiatră Quarry (Apuseni Mountains, west Romania) contain a new species of lituolid foraminifera of the genus Bulbobaculites. The new foraminifer is characterized by a streptospiral initial part, followed by a rectilinear adult stage and a test-wall composed of agglutinated micritic grains. Within the Subpiatră limestones, three main facies can be distinguished: (a) coastal and littoral, (b) open-shelf/lagoon, and (c) shelf margin with bioconstructions and bioclastic shoals. The lower part of the succession contains facies typical for peritidal environments, with both low- and high-energy subtidal limestones, overlain by thinly bedded supratidal limestones. The middle part of the section contains dominantly subtidal limestones assigned to the open-shelf system, whereas the upper part consists of micro-reefs with corals and rudists interlayered with bioclastic shoals. In terms of microfacies, the new species of Bulbobaculites is most common in bioclastic grainstone/rudstone and floatstone/packstone assigned to the coastal/littoral and lagoonal facies.

Diagenetic Evolution of Selected Parasequences Across A Carbonate Platform: Late Paleozoic, Tengiz Reservoir, Kazakhstan

Abstract Predicting the diagenetic modification of reservoir quality in carbonate fields is poorly understood but of paramount importance to reservoir characterization. The diagenetic processes and current pore system are reported for two parasequences from the platform center to the margin of the Tengiz reservoir (∼ 16 km); the late Visean (Lvis13 cbs 900) and Bashkirian (Bash2.5 MSF 350) parasequences are separated by 8 million years. Syndepositional fibrous cement (> 10% rock volume) and aragonite skeletal molds developed in both parasequences. On subaerial exposure, brown soil-related calcite, and meniscus and pendant cements were emplaced in both parasequences. The application of CL petrography to these rocks for the first time has revealed the depositional components and early marine components are altered developing a mottled pattern implying that their original chemistry is not preserved. The principal calcite cement shows a complex growth pattern by CL that is unreported from any other calcite cement. CL is also used to divide the main cement into stage 1, nonluminescent, and stage 2, orange to brown luminescent, calcite. Stage 1 calcite shows vadose meniscus and pendant fabrics followed by uniform phreatic euhedral terminations. Stage 1 cement is patchily distributed at a millimeter scale; 28% to 40% in the Visean and 0.5% to 22% in Bashkirian grainstones. Stage 1 cement is more abundant at the center of the platform in the late Visean than at its margin. Abundant stage 1 calcite creates a framework that prevents later compaction preserving a high porosity at the center of the platform. This sweet spot is reinforced by the deposition of late-stage bitumen cement that occupies 8% rock volume at the margin of the platform but is almost absent at its center. Stage 1 δ13C values (−2.4‰ to +1.7‰) are derived by dissolution of host rock, and their δ18O values (−6.3‰ to −3.7‰) are compatible with precipitation from meteoric or mixed marine-meteoric water. Depositional micrite and micritic grains are transformed by dissolution–precipitation processes into porous microrhombic calcite not by Ostwald ripening. Microrhombic calcite is ubiquitous in the Tengiz icehouse, dispelling the myth that this fabric is restricted to greenhouse epochs. At ∼ 300 million years before significant burial the study grainstones were patchily cemented and dissolved to reach peak porosity; their transformed texture set up flow paths that controlled the distribution of later diagenetic products. Reservoir compartmentalization did not occur through extensive hardground formation at this time as occurs in some greenhouse sequences, but ash bands at parasequence boundaries were turned into tonsteins and wackestones were compacted perhaps sufficiently to influence vertical flow. CL examination of stage 2 calcite that adds 0.5% to 16% to rock volume reveals that its precipitation was episodic and switched sites as the pore network became blocked through precipitation and compaction and was opened by new fractures. A change from acute to equant habit occurred during stage 2 precipitation, causing the change from fibrous to equant crystal fabric. The δ18OPDB values of stage 2 calcite (−3‰ to −9‰) are linked to precipitation from an 18O-enriched fluid that evolved to ∼ +6‰ SMOW at temperatures of 60°C to 120°C, using the equation of Kim and O'Neil (1997). Many open macropores are lined with small cement crystals that CL shows have all stages of growth present and are terminated by euhedral surfaces. These macropores are only partly filled because of cement inhibition, not because they were dissolved during burial, as was previously claimed. Late-stage dissolution did create some macropores and rounded micrite crystals, but its effect on overall platform porosity is small. The CL examination of Tengiz platform calcite cement has recognized many diagenetic events that cannot be recognized without CL. The new Tengiz diagenetic textures recognized here are probably present in other carbonate platforms, especially those from icehouse epochs.

Elastic properties of microporous cemented grainstones

We investigated the effect of porosity, pore geometry, and diagenetic history on the elastic properties of dry, tightly cemented grainstones whose pore space consists dominantly of intragranular microporosity within micritic grains. The integration of laboratory petrophysical measurements (porosity, P- and S-wave velocity), petrographic analysis and scanning electron microscope (SEM) imaging of micropore space of 80 Lower Cretaceous microporous carbonate samples from Provence (south-east France) allows (1) the changes in porosity and pore geometry during the diagenetic history to be related to changes in elastic properties, and (2) the impact of micritic grain diagenesis on the elastic properties of microporous grainstones to be quantified by means of fitting parameters derived from equivalent elastic medium modeling. The Urgonian microporous cemented grainstones are elastically equivalent to a homogeneous calcitic host with spherical calcitic inclusions comprising spheroidal pores. The best fit is obtained when porous spheres are modelled using the differential effective medium (DEM) approach and the whole composite using the self-consistent (SC) method (DEM-SC model). At lower porosity values ([Formula: see text]), when the micropore volume is controlled by intercrystalline cementation processes without compaction, the equivalent pore aspect ratio (EPAR) derived from DEM-SC modelling is nearly constant and averages 0.15. At higher porosities, changes in micropore space architecture related to leaching processes result in slightly increasing EPAR. The recognition of EPAR-preserving versus EPAR-non preserving elastic property evolution is proposed as a tool for diagenetic pattern detection in microporous carbonate reservoirs.

TEM study of Mg distribution in micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian)

Microporous limestones composed of micrite crystals constitute sizeable hydrocarbon reservoirs throughout the world and especially in the Middle East. However, the crystallization history of micrites is poorly understood. Scanning electronic microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS) studies give morphological and bulk composition information about micrites, but no information exists on the distribution of minor elements inside micrite grains. This study proposes Mg maps obtained with X-ray EDS combined with scanning transmission electron microscopy (STEM) of micrite crystals from the Mishrif reservoir Formation (Middle East, Cenomanian to Early Turonian). Three types of Mg distribution were observed through micrite crystals from five different samples: (1) homogenous Mg concentration, (2) small Mg-enriched areas close to the center of the crystal, and (3) geometric Mg impoverishments near crystal edges and parallel to present crystallographic faces. The homogenous Mg distribution is the most frequent and is found both in microporous and in tight micrites. The second type of distribution showing small Mg-enriched areas inside micrite crystals relatively close to their center comes from a microporous sample located below an emersive surface. These enriched areas may correspond to crystal seeds. The third type of distribution was observed in micrite crystals from another microporous sample situated just below an emersive surface. The Mg-poor zones probably represent overgrowths that precipitated in contact with less Mg-rich meteoric fluids.

Low-temperature deformation of limestone, Isola Palmaria, northern Apennine, Italy – The role of primary textures, precursory veins and intracrystalline deformation in localization

Fine-grained limestones of the Tuscan nappe from Isola Palmaria (northern Apennine, Italy) contain sporadically developed, bedding-parallel shear zones, some of which exhibit well-developed calcite mylonites. The presence of shear zones that formed at temperatures less than 250 °C allows the study of limestone deformation mechanisms active under low-temperature, upper-crustal conditions. Two materially distinct types of shear zones developed during the syn-nappe stage of deformation and have been characterized by integrated field, strain, microstructural (optical microscopy, cathodoluminescence, transmission electron microscopy) and EBSD fabric studies. Type A shear zones occur within decametre-thick packages accommodating heterogeneously distributed strain in very fine-grained limestone (micrite). Type B shear zones are localized within calcite veins that develop into recrystallized calc-mylonites. Both types form at the contact with dolomite, although this does not play an important role in strain localization. In both cases, low-temperature ductility occurs by extensive dislocation glide sustained by athermal network-accommodated recovery. Although, the end-state microstructures of both shear zone types cannot be differentiated optically, differences do exist in the crystallographic preferred orientations (CPO) and the grain boundary structure of deformed micrite and vein calcite. Deformed micrite grain boundaries are rich in voids consistent with extensive fluid flux that can enhance material transport and independent grain displacements, while suppressing CPO development. The crystallographically defined grain boundaries in recrystallized vein calcite suggest that lower grain boundary diffusivity enables more typical dislocation-mediated CPO formation.

Ductile strain partitioning in micritic limestones, Calabria, Italy: the roles and mechanisms of intracrystalline and intercrystalline deformation

The Apennine Pollino–Ciagola limestone unit in northern Calabria is characterized by subgreenschist, heterogeneous ductile strain localized along narrow deformation zones at several stratigraphic levels. Paleogene conglomerates and Jurassic calcareous breccias and ooidal packstones have been analyzed with the aim of characterizing the deformation of limestone as a function of the strain recorded by sedimentary markers. Reference sections parallel to principal finite strain planes were prepared at each locality for the study of specific parameters. Image analysis of polished sections by scanning electron microscopy (SEM) was used to obtain the finite strain of calcite grains by Rf/ϕ, harmonic mean and normalized Fry methods. For the range of grain sizes analyzed (1–10 µm), the ellipticity of calcite grains varies as a function of grain size according to a power-law relationship, from which the size of isometric grains is empirically predicted. The finite strain (ellipticity) determined from single calcite grains shows consistently lower values than the corresponding rock strain. For a fixed grain size, grain ellipticity initially increases with rock strain; however for larger strain, scattered ellipticity values are recorded, probably because of dynamic recrystallization. Comparison of bulk strain with grain strain suggests that intercrystalline deformation involving grain boundary sliding contributes 50%–80% of the total strain, for grain sizes in the range of 2–10 µm, increasing to 90% or more for smaller grain sizes. Microstructures (optical, SEM, transmission electron microscopy) are consistent with dominant grain boundary sliding accommodated by dislocation processes. The weakly deformed samples (Rs <4) exhibit straight and subsidiary curved mechanical twins in large grains (d >10 µm), with well-developed glide dislocation substructures in both coarse and micrite grains. In the moderately to highly deformed samples (Rs >4), large grains show curved, thick, and patchy twins, with the development of undulose extinction and subgrains. Subwalls are formed from dislocation networks and relate to subgrain rotation recrystallization in the coarsest grains. Both large and small grains exhibit complex dislocation substructures comprising dislocation networks indicative of concurrent intercrystalline and intracrystalline deformation, whereby grain boundary sliding is accommodated by dislocation processes. Integration of tectonic constraints, field observations, finite strain data, microstructures, and experimental data is consistent with natural deformation at 250 °C, 15–50 MPa, and bulk shear strain rates on the order of 10–13 s–1 to 10–12 s–1.

Molecular fossils and other organic markers as palaeoenvironmental indicators of the Messinian Calcare di Base Formation: normal versus stressed marine deposition (Rossano Basin, northern Calabria, Italy)

A multidisciplinary study has been carried out on the Late Miocene (Messinian) Calcare di Base Formation in northern Calabria, Italy, with the aim of understanding the depositional conditions of these enigmatic carbonate sediments that have been interpreted as evaporitic or diagenetic limestone. The research has been developed through sedimentological (microfacies), organic petrography and geochemical analyses (palynofacies, Rock-Eval pyrolysis and gas chromatography/mass spectrometry). The carbonates studied preserve their original mineralogy (aragonite) and microstructure. The prevailing microfacies consists of clotted peloidal micrite with antigravitative fabric. Larger and darker cylindrical–subcylindrical micritic grains, attributable to faecal pellets, are more or less randomly dispersed in the peloidal micrite. The faecal pellets have been ascribed to copepods because zooclasts of these arthropods were observed in the palynofacies. Bright epifluorescence characterizes the faecal pellets and peloidal micrite, indicating their high content of organic matter. Detrital and stromatolitic microfacies occur rarely (around a few percent). The detrital microfacies is characterized by silt-sized grains organized in thin, sometimes bioturbated, graded laminae. The stromatolitic microfacies shows planar to gently curved/wrinkled laminae organized in dark and light couplets. The associated sedimentary organic matter shows low maturity, and preserves the original signatures of the source organisms. Rock-Eval pyrolysis revealed a transitional composition between Type II and Type III kerogen, suggesting a mixture of marine and terrigenous organic matter input. The molecular biogeochemical data indicate three main biological signatures: algae, bacteria and higher plants. Organic markers together with sedimentary evidence indicate a marine depositional scenario influenced by freshwater input rather than evaporitic concentration. We conclude that the most probable bacterial process involved in carbonate precipitation of the Calcare di Base, considering the freshwater input and consequent probable enrichment in organic matter and Ca 2+ ions, is ammonification of amino-acids in aerobic conditions.

Petrography and biosedimentology of the Rosso Ammonitico Veronese (middle-upper Jurassic, north-eastern Italy)

A petrographic and biosedimentological study of the Rosso Ammonitico Veronese from the Trento Plateau (north-eastern Italy) shows that diagenetic (neomorphism, recrystallization) and biological processes (microbial content and pigmentation) influenced the formation and alteration of the carbonate matrix. The subject of this article is the interaction of early diagenetic processes and an attempt to explain the different colors of the matrix (red, pink, grey). Nearly 200 samples derived from 14 sections (Callovian to Tithonian) located in the Verona area have been studied by means of classical, cathodoluminescence and SEM microscopy. Calcite and ferruginous microfilaments of different shapes and sizes are present and tentatively attributed to fungi and iron bacteria. These micro-organisms precipitated iron oxy-hydroxides at poorly dysoxic-anoxic sediment–water interfaces. Further liberation of submicronic hydroxides (now hematite) was responsible for the red pigmentation of the carbonate matrices, originally composed of less than 1 μm-sized micrite. Controversial smaller nanograins (0.1–0.5 μm) attributed to nanobacteria or planktonic picoeukaryotes have been observed in the reddish samples. Recrystallization of the micrite leads to the formation of new micritic crystals, between 2 and 4 μm in size, then to microspar crystals. Micritic textures are linked to the different colours of the samples. The intensity of the red colour is correlated with the presence of hematite (former iron hydroxides) and the presence of planar subhedral micritic grains. In contrast, pink and greyish samples are linked to the increasingly coalescent structure of anhedral micritic and microsparitic crystals.

Meteoric versus burial control on porosity evolution of the Smackover Formation

Whole-rock 13C, 18O, and Sr compositions of the uppermost ooid grainstones of the Smackover Formation at Black Creek field in Mississippi reveal characteristics of a meteoric system in this area. The trend in 13C signatures indicates that meteoric water acquired its low carbon values at a soil zone and was buffered completely with the host rock before entering the meteoric lens. Identical 18O data suggest that allochems stabilized at the same temperatures in waters of similar 18O compositions during early meteoric diagenesis; that is, calcitization continued to completion before the onset of burial. Sr concentrations reveal differential transformation mechanisms. Low Sr values in the phreatic zones are attributed to slow stabilization by exchange with the bulk solution. High Sr compositions in the vadose zone resulted from rapid, incremental transformation.Meteoric cementation was sparse; therefore, grainstones entered the burial realm with all of their porosity intact. Modification of porosity was a burial process and was directly related to the intensity of chemical compaction, which itself was controlled by three factors: (1) the degree of early cementation, (2) grain type, and (3) grain size. Early-cemented intervals experienced low to moderate compaction regardless of grain size or grain type. In the absence of early cementation, grain type and grain size were the dominant factors of porosity control. Coarse-grained intervals that were composed of micritic grains experienced less compaction than fine-grained, oolitic intervals.Average volumetric percentages of cements as determined by point counting of 63 samples are as follows: (1) marine and meteoric cements = 1.5%, (2) prebitumen calcite = 6.5%, (3) saddle dolomite = 1%, (4) postbitumen calcite = 4.3%, (5) pyrobitumen = 1%. Total cement (sum of 1–5), total burial cement (sum of 2–5), and total burial carbonate cement volumes (sum of 2–4) average 14, 13, and 12%, respectively. Out of an original porosity of 40%, 13 porosity units were lost by cementation, and 27 porosity units were reduced by compaction (mechanical and chemical).Present-day intergranular porosity of these grainstones is zero, whether they belonged to the vadose zone or the phreatic interval. Burial cementation and compaction destroyed all intergranular porosity, regardless of early diagenetic overprint or textural variables. The reservoir became compartmentalized during burial. Some intervals were active producers of calcium carbonate; others were passive recipients.

Petrography and Geochemistry of Floodplain Limestones from the Clarks Fork Basin, Wyoming, U.S.A.: Carbonate Deposition and Fossil Accumulation on a Paleocene-Eocene Floodplain

ABSTRACT Floodplain limestones are increasingly important sources of fossil vertebrates from the Clarks Fork basin, the northern extension of Wyoming's Bighorn basin. Despite their importance for understanding Paleogene floodplain faunas, little work has addressed the depositional history of these carbonates or evaluated potential taphonomic biases affecting the assemblages they produce. Recent studies of Bighorn basin paleosols have increased our understanding of parameters affecting the paleo-floodplain, and large collections of limestones now exist; this has allowed detailed investigation of limestone petrography and geochemistry and the proposal of a new model for their formation. Floodplain limestones record a complex depositional and diagenetic history. Precipitation of micritic low-Mg calcite and accumulation of bone occurred in low-energy ponded water at the earth's surface. Pedogenic overprinting obscured primary textures and produced diagenetic grainstones, packstones, and wackestones. Biological and chemical pedogenic processes formed most carbonate allochems, including micrite grains and calcified plant cells. Microbial fermentation occurred at shallow burial depths, and methane production, migration, and oxidation affected the carbon isotope composition of diagenetic carbonate and primary micrite. Pore space was fully occluded by phreatic calcite cements during late-stage diagenesis. Fossil assemblages contained within the limestones likely represent fauna derived from rarely sampled floodplain microenvironments. Hydraulic transport of bones was minimal. Accumulation of fossil material occurred episodically, and bones were rapidly encased in micrite. The uncommon body size distribution characteristic of limestone faunas is attributed, in part, to buffering of chemical and physical processes normally effecting bone decomposition during surface exposure and pedogenesis.

Dolomites and Early Mississippian Bioherms, Leadville Formation, Molas Lake, Colorado: ABSTRACT

Two dolomite facies that exhibit little petrographic evidence of their original textures are interpreted to be integral parts of two bioherms. A core facies of rugose corals (Vesiculophylum), pelmatozoans, cephalopods, and brachiopods in a peloid wackestone-packstone matrix forms two mounds 50 × 40 m and 110 × 75 m in diameter and 7 and 20 m high, resting on a basal unit of foraminiferal (endothyrid) ooid, coated-grain grainstone. These mounds are surrounded and onlapped by a bedded flank facies with relic cross-bedding that forms a halo 15-40 m wide around each mound. The facies consists of very coarsely to coarsely crystalline dolomite, but field evidence shows that it was originally detrital and is coeval with the core facies. It grades laterally away from th core into a distal flank-intermound facies of dolomudstone interbedded with millimeter-thick laminae of dolomite peloid packstone. This facies occurs up to 100 m from the mounds. Corals in the core facies have been replaced and cemented by nonferroan, nonluminescent sparry calcite at temperatures of at least 200°C. The matrix of micrite and skeletal grains is composed of nonferroan, red-orange luminescent calcite. Diagenetic changes have been modest. In contrast, the two flank facies show obliteration of original textures and replacement by inclusion-rich, nonferroan, red-luminescent, anhedral to subhedral dolomite at temperatures of a least 165°C. Other than appealing to differences in original porosity and susceptibility to subsurface fluids, it is difficult to explain why these closely associated facies have followed such divergent diagenetic paths. End_of_Article - Last_Page 318------------

Factors Controlling the Formation of Microspar in the Bromide Formation

ABSTRACT A common form of neomorphism in ancient carbonate rocks is the recrystallization of micrite to microspar. This occurs when a cage of magnesium ions is flushed from 2 µm micrite grains and permits them to recrystallize to microspar grains 5 to 10 µm in diameter. Some clays play an important role in this type of neomorphism. In the Bromide Formation (Middle Ordovician, Oklahoma) shale beds of chlorite and/or montmorillonite attracted magnesium ions from adjacent micrite beds and greatly facilitated the formation of microspar. In the absence of clays, porosity controlled the formation of microspar. Some neomorphism of micrite to microspar occurred during outcrop weathering, but most neomorphism seems to have occurred after burial to moderate depths and after cementation of ssociated limestones.

An SEM Study of Microfabrics from the Irish Chalk

Sediment compaction and subsequent cementation must be considered important mechanisms for the reduction of porosity in the fine grained rocks studied. Evidence for early post-depositional compaction includes micrite grain flow around large bioclasts and grain orientation. Fracture of delicate grains causing some displacement was observed with suturing or serration at the grain boundaries between broken fragments of microfossils. Styolites cutting through grains and matrix suggests continued compaction. Geopetal structures and veins with incorporated whole microfossils indicate some grain support from partial cementation. Micrite veins with crystal orientation parallel to the trend of the vein may represent anastomosis tubes for the dewatering of the sediment.

Lithologic Controls on the Development of Solution Porosity in Carbonate Aquifers

Movement of ground water in maturely karsted limestone aquifers is commonly through solution conduits of considerable size. The object of this study was to determine the lithologic controls on the distribution of conduits within a heterogeneous sequence of carbonate rocks. The volume of all accessible caves more than 100 feet in length in the Nittany Valley area of central Pennsylvania was measured. Most caves are entirely within limestones, cave development in dolomite is extremely rare. Within the limestone sequence the bulk of the cave volume is concentrated in a few members. Chemical and petrographic analyses of the carbonate rocks were associated with cave volume by bar graphs and components analyses. The more cavernous limestones seem to be those with low amounts of dolomite, clay, and other impurities, and a high micrite grain fraction. Coarse limestones and dolomites are less cavernous.