Minus-cement Porosity Research Articles

A late Paleoproterozoic microfossil community from siliceous granules, Dahongyu Formation, North China

After the first great oxygenation event (GOE), microorganism communities flourished in various habitats in shallow marine settings as a response to elevated oxygen contents in atmosphere and seawater. Abundant microorganisms have been reported from microbialites and siliceous dolomites and siliciclastic rocks. Here we report a diverse microfossil assemblage from siliceous granules within chert grainstones of the ca. 1.6 Ga late Paleoproterozoic Dahongyu Formation in the Jixian section, North China. These microfossils are confined to granules and absent in inter-granular chalcedony interpreted as cementing matrix. The microfossil assemblage is made up of the five morphotypes: two types of large spheroidal forms, much smaller unicells, unbranched filaments of cyanobacterial affinity, and chain-like filaments (CLFs) composed of hematitic pseudomorphs after pyritization. The CLF possesses a carbonaceous sheath structure which is mostly obscured by previous pyrite crystals, suggesting a potential bacterial origin. The unique morphology and distinctive chemical composition both distinguish these filamentous objects from filaments known in the Archean and Proterozoic worldwide. However, similar CLFs have been found in a variety of lithofacies worldwide, and coexist with other microfossils or microbial mats, implying that they all may represent a once-widespread group of microbes. The hosting granules are interpreted as depositional products that were transported from shallow marine into a deeper ramp setting, and the microfossils and mat debris therefore likely represent a phototrophic community. The inferred high minus-cement porosity and the predominantly grain-supporting texture of these granules signal the early silicification and cementation. Thus, chert grainstones formed in this way may effectively preserve traces of early life and provide snapshots of early microorganisms after the GOE.

Diagenesis and Porosity Evolution of the Pachmarhi Sandstones (Early Triassic), Satpura Gondwana Basin, Central India

This study deals with the diagenetic history of the Pachmarhi Sandstones of Satpura Gondwana Basin, Central India. The sandstones are classified as quartz-arenite and sub-arkose type. Monocrystalline quartz grains dominate the detrital mineralogy followed by polycrystalline quartz grains, feldspars (more alkali-feldspar than plagioclase), rock fragments, detrital mica and heavy minerals. The diagenetic signatures observed in the Pachmarhi Sandstones include mechanical compaction, cementation (Fe-oxide, quartz overgrowths, calcite, matrix and clay minerals), replacement and dissolution of feldspar and calcite cement. The most commonly observed type of secondary porosity is dissolution of feldspars. The porosity loss is mainly due to cementation by pore occlusion and by early stage of mechanical compaction. In addition, several clay minerals occurred as pore-filling and pore-lining cements. The nature of various types of grain- to grain contacts suggests early cementation and consequent minor compaction. Among the various cements, calcite is the earliest followed by iron oxide while silica cementation occurred probably at a late stage. The carbonate cement formed during burial by dissolution and re-precipitation represents redistributed calcite which was buried with the sandstone. The iron cement was perhaps derived from weathering and leaching of ferromagnesian minerals of overlying Deccan traps. Silica cement was derived from the corrosion of quartz and feldspar grains. Types of grain contacts, minus-cement porosity and porosity reduction indicate a shallow depth of burial conditions for these sandstones.

Sedimentary and diagenetic facies of tidal flat in epeiric sea and its related descriptive method-a case of Ma51-4 submember in Jingbian gas field of Ordos basin, China

The Ma51-4 submembers of Jingbian gas field in Ordos basin of China belongs to epeiric sea evaporative carbonate tidal flat facies. Supratidal zone of gypsum nodule dolostone and upper part of intertidal zone of gypsum crystal dolostone are most favorable reservoir. The carbonate rocks of Ma51-4 had undergone complex diagenesis. Penecontem-poraneous dolomitization and gypsification provided the material foundation of the reservoir. Epidiagenesis selective dissolution of gypseous dolostone constructs the origin shape of pore structure. The dissolution and filling of various minerals in the burial period determine whether the early dissolution pores can be preserved, and the final state of the reservoir. Burial dissolution and filling of various minerals determined the preservation of early dissolved pores and the final condition of the reservoir. The concept of “minus cement porosity” directly reflects that sedimentary facies have great influence on the growth of reservoir in Ma51-4. Combined with sedimentary microfacies and diagenetic facies, a concise numerical symbol is used to express semiquantitatively sed-imentary microfacies and diagenesis facies. This descriptive method has great benefit in reservoir evaluation and prediction.

Clumped-Isotope Constraints On Cement Paragenesis In Septarian Concretions

Septarian concretions exhibit multiple generations of cements that include body, fringe, and spar phases. Classic paragenetic interpretations include initial precipitation of the body followed by fringe(s) and then by spar in more or less discrete events. Traditional approaches (e.g., carbon and oxygen isotope analyses) are generally unable to distinguish paragenetic trends as they relate to specific formation environments (e.g., precipitation during burial or with meteoric influx). Here we present carbonate clumped-isotope, δ13C (δ13Ccarb), and δ18O (δ18Ocarb) values for septarian concretions taken from four host units in order to assess cement paragenesis and overcome traditional shortcomings. Clumped-isotope and δ18Ofluid data exhibit a wide range of values, with carbonate precipitation temperatures between ∼ 20 and 50°C and δ18Ofluid compositions of ∼ −14 to +4‰ (VSMOW). In stable-isotope cross-plots, specific cement phases group together and confirm the paragenesis indicated by superposition. In some cases, samples analyzed from concretion bodies yield temperature and δ18Ofluid values that indicate formation at shallow depths, consistent with independent data (e.g., high minus-cement porosity, external laminae deflection). In contrast, other concretion-body analyses indicate relatively high body temperatures that conflict with shallow-formation indices. Petrographic and backscatter scanning electron microscopy (SEM) reveal that concretion bodies partially consist of a secondary, replacement phase, which could explain the higher temperatures expressed in bulk body samples. Based on data for different phases in these septarian concretions, we suggest that initial body-cement precipitation occurred at relatively shallow depths from unmodified seawater, followed by fringe formation at elevated temperatures that likely coincided with the emplacement of the secondary body phase. When considered together, late-stage spar phases exhibit temperatures and δ18Ofluid values supportive of spar precipitation from fluids with a significant meteoric component, possibly during uplift.

Facies controlled porosity evolution of the Neoproterozoic Upper Bhander Sandstone of Western India

Abstract The Upper Bhander Sandstone is dominantly composed of quartzarenites. The basal and top portions are sandstones, with the middle section comprising thinly bedded shales with interlayer silt and sandstone units. The sandstone units are composed of several varieties of quartz, feldspar, micas, rock fragments and heavy minerals. The Upper Bhander Sandstone was deposited in a transgressive phase and later modified by tidal processes and wave- and storm-dominated processes in a tide-influenced Barrier Beach Complex of the shallow marine environment. This study reveals that, during mechanical compaction, a rearrangement of grains took place and point and long contacts were formed. The early silica cementation and shallow burial resulted in high primary porosity. This phase was followed by chemical compaction and the replacement of silica cement by iron cement (Fe-cement) under the deep burial phase of these sandstones. Dissolution of Fe-cement and feldspars resulted in secondary porosity development. Quartz overgrowths are better developed on coarse- to medium-sized grains than on fine-sized grains. These observations suggest a progressive compaction, which initiated at the sediment–water interface and continued till deep burial diagenesis in a rapidly subsiding basin. The existing optical porosity of the Upper Bhander Sandstone is 4% and the minus cement porosity is 18%.

Taphonomy of LargeCanadoceras(Ammonoid) Shells in the Upper Cretaceous Series in South Sakhalin, Russia

Based on materials from the Krasnoyarka Formation in the Naiba area in south Sakhalin, Russia, taphonomic histories of a large Campanian ammonoid, Canadoceras kossmati Matsumoto, 1954, were closely investigated. Large Canadoceras shells exceeding 30 cm in diameter are usually embedded horizontally and solitarily in muddy sandstone. A thin, lenticular calcareous concretion envelopes the shell (= envelope concretion). Their body chambers are mostly lost. The inner whorls comprising the center of the umbilicus completely disappear without exception, and only two or three outer whorls are preserved. The body and air chambers are somewhat compressed by compaction and are filled with sediments. Phycosiphon burrows are common not only in open body chambers but also in inner air chambers, which were originally closed. These observations suggest that the thin-shelled inner whorls and organic-rich siphuncular tubes degraded before final burial of the shell, and sediment infilling to the inside of the chambers followed. The early loss of inner whorls and siphuncular tubes gave rise to “draft-through currents.” The continuous supply of oxygen and nutrients by the draft-through currents supported the Phycosiphon producers in the inner air chambers. Compared with other calcareous concretions containing intact fossils, values of minus-cement porosity (MCP) remain relatively low (63–74%) and vary by areas even in the same envelope concretion. This indicates that the envelope concretions were cemented under a progressive increase of compaction during the later diagenetic stage. The formation of the envelope concretion appears to be a long-term phenomenon. Various events at different stages have been overprinted in a single large ammonoid fossil.

The effect of carbonate cementation on permeability heterogeneity in fluvial aquifers: An outcrop analog study

Spatial variations in permeability have been widely recognized as a dominant control on solute transport and petroleum migration. Outcrop studies are one means of improving our understanding of the nature of permeability heterogeneity and the underlying geologic controls. To date, these studies have either neglected the importance of cementation or investigated only the modern cemented permeability without explicitly considering how cementation has affected permeability. Two outcrops of channel sand deposits in the Plio-Pleistocene Sierra Ladrones Formation, New Mexico are investigated to quantify (1) the effect of cementation on permeability at a given location, and (2) the patterns of cementation and their impact on permeability heterogeneity. We analyzed approximately 400 spatially located samples (∼ 10 cm 3 each) from the two outcrops. Cementation is typically weak to moderate, filling less than 60% of the original pore space. Permeability and cement content are bimodally distributed and negatively correlated. Permeability reduction is investigated by measuring modern permeability, minus-cement permeability, minus-cement porosity, and volumetric cement content. Permeability is reduced locally by up to three orders of magnitude. By combining models of permeability for uncemented sediments and cemented sediments, we derive a model of cemented permeability ( k c) that depends primarily on three factors: uncemented permeability ( k u), original porosity, and cement fraction. Our results suggest that accounting for porosity reduction only may result in an overestimation of k c. Conversely, accounting for porosity reduction, increased specific surface area, and increased tortuosity appears to result in an underestimation of k c. Accounting for porosity reduction and increased specific surface area provide the best estimate of k c. Cementation is highly variable spatially and is poorly correlated with lithofacies. Variogram analysis of cementation and permeability indicates that the spatial distribution of cementation is the dominant control on the permeability correlation structure. The spatially variable cementation also results in an order-of-magnitude increase in the variance of log-permeability. Applying stochastic models of effective flow and transport properties to the results of this study suggest that cementation has a relatively small effect on effective permeability but can potentially increase dispersivity by two orders of magnitude.

Outcrop characterization of reservoir quality and interwell-scale cement distribution in a tide-influenced delta, Frontier Formation, Wyoming, USA

AbstractPetrographic study of the Frewens sandstone, Upper Cretaceous Frontier Formation, documents reservoir-scale diagenetic heterogeneity. Iron-bearing calcite cement occurs as large concretions that generally follow bedding and are most common near the top of the sandstone. Median thickness of the concretions is 0.6 m, length 4.5 m, and width 5.7 m; median volume is 5.2 m3. Concretions comprise 12% of the sandstone.The minus-cement porosity of concretion samples is low, indicating that the calcite precipitated near maximum burial depth. Isotopic and burial history data suggest that the calcite precipitated at ~54°C from evolved meteoric water enriched in 18O or from a mixed meteoric±marine pore-water. Shell-bearing transgressive shales above the Frewens sandstone are interpreted to be the source of calcium carbonate. Concretions of this size and distribution would influence fluid flow in a reservoir and would reduce the amount of hydrocarbons in place.

Feldspar albitization and feldspar cementation in the Albert Formation reservoir sandstones, New Brunswick, Canada

Feldspar albitization and feldspar cementation (especially albite) are common in the Carboniferous fluvio-lacustrine Albert Formation reservoir sandstones of the Moncton subbasin, New Brunswick. Both albitization and albite cements occur in early and late diagenesis. Burial diagenetic origin of albite is supported by: (1) the chemical purity of albite; (2) the growth of euhedral albite on numerous sites of the host feldspar; (3) the continued growth of the replacement albite into adjacent pores; (4) the delicate skeletal texture in partially dissolved areas of albitized feldspar; and (5) the occurrence of ankerite cement inclusions in authigenic albite. Backscattered electron imaging suggests that feldspar albitization occurs in irregular, vein-like and perthite elbite forms. Feldspar albitization is commonly associated with dissolution pores and twins. Backscattered electron imaging suggests that up to 50% of the mean total 4% detrital K-feldspars have been albitized. The volume percentage of feldspar albitization shows no trend with increasing burial depth or temperature. Most of the sandstones have 2–3% albite cement except in a high minus-cement porosity framework where it may exceed 30% of the sandstone.

Relative Importance of Cementation vs. Compaction to Porosity Evolution in Quartzose Sandstone: Tensleep and Quadrant Formations (Pennsylvanian), Northern Rocky Mountains: ABSTRACT

Very fine- to fine-grained, very well-sorted quartzarenites in the Tensleep (average sandstone unit thickness is 4 m) and Quadrant (average unit thickness is 2 m) formations have average porosity values of 10 and 5%, respectively (range 0-23%, Tensleep; range 0-21%, Quadrant). Porosity evolution in sandstones was strongly influenced by proximity to contacts with interbedded carbonates and the timing of cement precipitation. Early anhydrite (A) and dolomite (D) cements are preferentially located near upper and lower contacts of sandstones. The average minus-cement porosity for these types of cemented zones is 37%. A later quartz (Q) cement is concentrated toward the middle of sandstone bodies, with average minus-cement porosity values near 30%. High porosity zones ({>=} 10%) are associated with D + A/D + A + Q ratios of {<=} 0.5 and preferentially developed in the middle 50% of individual sandstone units. Estimates of initial porosity and measurements of volumes associated with cements, intergranular pressure solution, and present porosity allow assessment of the relative contributions of cementation and compaction to pore space reduction. On average in Tensleep sandstone, about 74% (lower 25% of individual sandstones) and 77% (upper 25%) of porosity reduction was the result of cementation. However, in the middle 50%more » of sandstones, compaction (47%) was subequal in importance with cementation (53%) as the cause of pore space loss. In the Quadrant, porosity was primarily destroyed by cementation regardless of location within a sandstone body (75% lower, 78% middle, 82% upper). The difference in behavior of the middle of Quadrant vs. Tensleep sandstone bodies may be related to the absolute timing and abundance of quartz cementation.« less

Hydrochemistry, origin and evolution of sedimentary subsurface fluids: III. Early diagenetic mineralization reactions in high sedimentation—rate basins

The early diagenetic evolution of pore-water chemistry is closely linked to mineralization reactions which consume significant portions of the metabolites released by bacterial organic matter decomposition. These reactions are most intense in high-sedimentation rate basins and include the precipitation of iron-sulfides and various carbonates leading to concretion growth. Early diagenetic pyrite is typically framboidal attesting to its recrystallization from precursor mackinawite, greigite or amorphous FeS which are the favored phases at high supersaturation levels during the initial sulfate reduction stages. The sulfur isotopic composition of early diagnetic pyrite can be used to differentiate diffusion-controlled, open-system conditions with isotopically light sulfide (δ 34S = − 35 to − 20‰) from closed system conditions, under which Raleigh distillation produces increasingly heaver sulfide (δ 34 S = − 35 to + 18‰). Alabandite (Mn-sulfide) is a rare authigenic sulfide in Mn-rich environments such as certain restricted, semi-stagnant basins (Baltic Sea). pH-buffering by hydrogen sulfide and hydrogen ion uptake by the reduction of manganese and iron oxides and hydroxides in the nitrate and sulfate reduction zones raise the pH sufficiently to cause supersaturation of the porewaters with respect to Ca-, Mg-, Fe- and Mn-carbonates and complex solid solutions of them. Fe-carbonates cannot form in the sulfate reduction zone in the presence of dissolved sulfide which competes for the dissolved iron. Likewise, dolomite formation appears to be inhibited or slowed down in the presence of substantial dissolved sulfate. The appearance of siderite and ankerite therefore signals carbonate precipitation below the sulfate reduction zone. Supporting evidence for the early diagenetic origin of many carbonate concertions is provided by their high carbonate contents (70 to 90% reflecting the porosity existing at the time of precipitation, called “minus-cement porosity”), isotopic composition, clay fabrics, and preservation of original bedding features including the shapes of fossils and fecal pellets. In these environments increasing carbon isotope ratios (δ 13 C = − 20 to + 15‰) indicate concretion growth below the sulfate reduction zone, i.e., in the methane generation zones. Continuing concretion growth at greater burial depth explains pore water profiles with constantly low Ca and downward decreasing Mg concentrations. Dissolved ammonia and phosphate profiles reguire adsorption and ion-exchange reactions as additional removal machanisms (besides apatite precipitation) in order to explain their downward decrease after they have reached maximum concentrations below the alkalinity maximum. Classification of early diagnetic environments into oxic and anoxic and further subdivision of the latter into sulfidic and non-sulfidic (with suboxic or post-oxic and methanic as further subcategories of the non-sulfidic environment) according to Berner (1981) is preferred over the previous classification in terms of pH/Eh fields. The temperature range of the early diagenetic stage extends from O to about 75°C, at which temperature thermocatalytic organic matter decomposition replaces the earlier bacterially mediated reactions and causes a whole set of new diagenetic reactions (such as feldspar dissolution, smectite to illite transformation) to start.

Influence of Grain Size and Temperature on Intergranular Pressure Solution, Quartz Cementation, and Porosity in a Quartzose Sandstone

ABSTRACT Cathodoluminescence petrography and scanning electron microscopy (SEM) reveal that grain size and temperature significantly influenced intergranular pressure solution, quartz cementation, and porosity evolution in the quartzose Hartshorne Sandstone (Middle Pennsylvanian) of the Arkoma basin. Among samples from each locality, a negative, linear relationship exists between mean grain size and volume of quartz dissolved by intergranular pressure solution. In contrast, a generally positive relationship exists between mean grain size and volume of quartz cement, although considerable variability exists in this relationship. A positive relationship also exists between mean grain size and porosity. Intergranular pressure solution directly reduced minus-cement porosity by causing tighter pack ng of detrital grains; it thereby controlled volumes of both cement and primary porosity. For this reason, little primary porosity is preserved in pressolved, finer-grained sandstones even though they contain small absolute volumes of quartz cement. Appreciably more primary porosity may be retained by unpressolved, coarser-grained sandstones even though they contain larger absolute volumes of quartz cement. These relationships also vary with thermal maturity, which increases eastward across the basin. This thermal maturity trend is the result of elevated temperatures associated with Mesozoic rifting and intrusive events in the Mississippi embayment. Among samples of equal grain size, more intergranular pressure solution, less quartz cement, and less porosity are evident in areas of higher thermal maturity. As is the case with grain size, the influence that temperature exerted on intergranular pressure solution is, in turn, reflected in volumes of quartz cement and porosity. Thus, primary porosity tends to be preserved preferentially in areas of lower thermal maturity even though the absolute volume of quartz cement is generally higher. In areas of higher thermal maturity, more quartz has been dissolved by intergranular pressure solution than is present as cement whereas the opposite is true in areas of lower thermal maturity. This suggests that intergranular pressure solution was an important agent of mass transfer during diagenesis of these quartzose sandstones.

Intergranular Pressure Solution and Porosity Evolution in Quartzose Sandstones: ABSTRACT

Compared to cementation by quartz, carbonates, and clay minerals, intergranular pressure solution has long been viewed as a minor control of porosity evolution in quartzose sandstones. However, quantitative cathodoluminescence petrography and scanning electron microscopy of Paleozoic and Mesozoic sandstones from various geologic settings suggest that intergranular pressure solution is the diagenetic process that most fundamentally influences porosity and permeability evolution in a majority of cases. Intergranular pressure solution dictates tightness of grain packing, size and geometry of primary pores, and diameter and morphology of contacts between framework grains; it also commonly predates pervasive cementation. Consequently, intergranular pressure solution is the fundamental control of minus-cement porosity. Although there is a close relationship between intergranular pressure solution and cementation in some sandstones, they are independent in others. For this reason, the percentage of minus-cement porosity actually occluded by cement is not systematic and is therefore difficult to predict. Numerous geologic variables have been documented that influence the amount of intergranular pressure solution that occurs in quartzose sandstones. On a local scale, relatively fine-grained sandstones and sandstones containing between 3 and 9% early authigenic clay have experienced more intergranular pressure solution than other sandstones. Regionally, among sandstones of equal age, grain size, and clay percentage, those that have been exposed to greater rates of burial, greater total depth of burial, and higher temperatures have experienced more intergranular pressure solution. These results suggest that an enhanced understanding of intergranular pressure solution may lead to a capability of predicting quartzose sandstone reservoir quality. End_of_Article - Last_Page 487------------

Influence of Early Cementation on Dolomitization and Compaction in Cambro-Ordovician Carbonates of Central Appalachians: ABSTRACT

The Cambro-Ordovician platform carbonates of the Great Valley, central Appalachians, are notable for the interlayering of limestones and dolostones with well-preserved primary sedimentary structures. Despite such primary structures there are several observations that demonstrate a secondary origin for much of the dolomite, the most significant being that dolostone-limestone boundaries commonly crosscut bedding planes. The nature of such dolomitization is clearly revealed by certain mottled dolostones in which mudcracks and burrows are haloed by limestone. Stratification can be traced through the 2 to 10-mm wide limestone halos into the dolostone without break. The limestone halos have a peloidal grainstone texture with high minus-cement porosity and unbroken shells wherea the dolostone is a sucrosic mudstone with fragmented shells. We interpret these observations to mean that early cementation around permeable mudcracks and burrows has protected these zones from later compaction and dolomitization. We extend this idea of selective early cementation of permeable sediments to explain why the limestones are invariably grainstones while the dolostones are mudrocks. The idea is supported by the observation that laminae of dolostones are bent around limestone mudcrack halos, ripple lenses, etc, showing that the limestones have acted competently and the dolostones incompetently during compaction. The timing of this post-early cementation dolomitization is not unequivocal, but it is certainly pre-stylolitization, as demonstrated by dissolution of dolomite rhombs along bedding-plane stylolites. Our observations do not rule out the possibility of a syndepositional origin for at least some dolomite in these rocks (there are dolomites and dolomites!) but they do negate the pressure-solution origin of dolomite advanced by Wanless (1979). End_of_Article - Last_Page 577------------

Early Proterozoic Microfossils and Penecontemporaneous Quartz Cementation in the Sokoman Iron Formation, Canada

Early Proterozoic microfossils from the Sokoman Iron Formation, northeastern Canada, are indistinguishable from those of the Gunflint Formation in both morphology and inferred community structure. The contemporaneity of the Sokoman assemblage with the Bitter Springs-like cyanobacteria of the Belcher Supergroup indicates that differences between the two major types of early Proterozoic microbiotas are primarily ecological and not temporal (evolutionary) in nature. In arenaceous iron formations, microfossils are restricted to peloids and are absent from pore-filling silica interpreted as cement. Cemented arenaceous intraclasts indicate that some of the silica was penecontemporaneous, and the abundance of minus-cement porosity in arenaceous iron formations demonstrates that early (pre-compaction) cementation was common.

Pressure Solution and Porosity Reduction in Shallow Buried Quartz Arenite: GEOLOGIC NOTES

The Galesville Sandstone (Cambrian) of south-central Wisconsin was studied as an example of porosity reduction in a shallow buried quartz arenite. The Galesville is a clean, well-rounded, well-sorted quartz arenite and probably has had a maximum overburden of 900 m. Porosity of the samples in thin section ranges from 15 to 28%. Minus-cement porosity averages 24%. Differences in porosity according to orientation are not statistically significant, and the percentage of pressolved quartz is similar for all outcrops, averaging 0.8%. Reduction in porosity owing to mechanical repacking of grains after deposition may have reduced porosity from about 45 or 50% to a minimum of 33%. Intergranular pressure solution of approximately 0.8% of the Galesville resulted in a minimum porosity reduction of 9% below the value established for maximum mechanical compaction.