Pore-filling Cement Research Articles

Incremental oil recovery using a horizontal drainhole in the San Andres Formation, Olson field, west Texas

Oolitic carbonates and associated rocks in the San Andres Formation produce hydrocarbons at Olson field, west Texas. Mudstones, dolomitized fusulinid-peloid packstones and wackestones, ooid-peloid grainstones and packstones, and minor siliciclastic sediments occur in the field. The reservoir is stratigraphically compartmentalized by lenticular oolite deposits that are overlain by algal-laminated mudstones and siltstones. Pore-filling anhydrite cement, common in all lithologies, adds to porosity heterogeneity. Interwell communication is poor, as indicated by variable bottomhole pressures and erratic waterflood response.A medium-radius lateral hole was drilled to a total depth approximately 50 ft (15 m) from an offset vertical well. At this depth, the horizontal well intersected a previously existing artificial fracture that occurred in the nearby vertical well. The intersection of this fracture by the lateral borehole had a significant economic impact. The production rate in the vertical well jumped from a few barrels per day to an average of 70 BOPD and less than 20 BWPD. The ensuing years maintained relatively high flow rates. Projected incremental oil recovery is 153,000 bbl, roughly equivalent to production from an average well drilled during the early life of the field.Other unsuccessful horizontal wells have been drilled at Olson field, but they have not targeted preexisting hydrofractures in offset vertical wells. Therefore, the concepts presented in this paper have not been retested. This approach, intentionally drilling a drainhole to intersect a preexisting hydrofracture, could add new life to many older, compartmentalized reservoirs.

How burial diagenesis of chalk sediments controls sonic velocity and porosity

Based on P-wave velocity and density data, a new elastic model for chalk sediments is established. The model allows the construction of a series of isoframe (IF) curves, each representing a constant part of the mineral phase contributing to the solid frame.The IF curves can be related to the progress of burial diagenesis of chalk, which is revised as follows:Newly deposited carbonate ooze and mixed sediments range in porosity from 60 to 80%, depending on the prevalence of hollow microfossils. Despite the high porosity, these sediments are not in suspension, as reflected in IFs of 0.1 or higher.Upon burial, the sediments lose porosity by mechanical compaction, and concurrently, the calcite particles recrystallize into progressively more equant shapes. High compaction rates may keep the particles in relative motion, whereas low compaction rates allow the formation of contact cement, whereby IF increases and chalk forms. Rock mechanical tests show that when compaction requires more than in-situ stress, porosity reduction is arrested.During subsequent burial, crystals and pores grow in size as a consequence of the continuing recrystallization. The lack of porosity loss during this process testifies to the absence of chemical compaction by calcite-calcite pressure dissolution, as well as to the porosity-preserving effect of contact cementation.At sufficient burial stress, the presence of stylolites indicates that pressure dissolution takes place between calcite and silicates, and depending on pore-water chemistry and temperature, pore-filling cementation may occur over a relatively short depth interval. Limestone and mixed sedimentary rock form, and porosity may be reduced to less than 20%. Isoframe increases to more than 0.6.In hydrocarbon reservoirs in North Sea chalk, relatively high porosity and high IFs are found. The reason may be that recrystallization and porosity-preserving contact cementation progress, whereas pore-filling cementation is small, probably because pressure dissolution along stylolites is arrested. Pressure dissolution may be arrested for two reasons: (1) the introduction of hydrocarbons causes a fall in effective burial stress, and (2) adsorption of polar hydrocarbons on the silicates may shield calcite from the silicates.

Reconstructing Fluid Expulsion and Migration North of the Variscan Orogen, Northern England

ABSTRACT Lower Carboniferous platform carbonates in northern England host lead-zinc-fluorite-barite deposits that have previously been classified as Mississippi Valley-type (MVT). The controls on the distribution of low-temperature, particularly MVT, minerals have long been debated, but the interplay of basin evolution and fluid movement is often neglected. Early Carboniferous sedimentation on the Derbyshire Platform took place within a back-arc extensional regime, north of the northward-migrating Variscan orogenic front, which increasingly influenced fluid movement on the platform throughout the Carboniferous. It is the interplay of fluid movement, cementation, and Variscan tectonism that is the focus of this paper, which aims to reconstruct fluid movement within a basin moving from an extensional to a compressional regime. In particular, it demonstrates the variability of fluid flux during post-rift subsidence and basin inversion and the corresponding influences of this upon mineral distribution. During early burial of the Derbyshire Platform, pore-filling cements were precipitated from meteoric porewaters, driven downdip from the east under topographic drive. These cements occluded most interparticle porosity, and therefore permeability became fracture-controlled. The waning effects of extensional tectonism during post-rift subsidence permitted intermittent expulsion of small volumes of trace metal-charged, carbonate-saturated fluids from overpressured contemporaneous clastic basins adjacent to the Derbyshire Platform. Overall, however, waning tectonism meant that the volume of fluids released during this period remained minor. With the onset of the Variscan Orogeny, and resultant compressional tectonism and basin inversion, Caledonian basement fault systems were reactivated, channeling large volumes of trace metal-charged fluids from the basins onto the Derbyshire Platform, establishing an east-west component to fluid flow. The reconstruction of fluid expulsion and migration within this tectonically active regime demonstrates variations in fluid source and migration pathways during basin evolution. Consequently, fluid movement is complex, with the platform receiving fluids from a variety of sources. This highlights the importance of understanding both the temporal and the spatial variations in cement chemistry when reconstructing flow. The results of this study have relevance to other mineralized carbonate platforms in extensional and compressional settings worldwide and especially to sedimentary basins that host MVT deposits and hydrocarbons, where it is crucial to understand the timing and mechanisms of diagenetic cementation and hydrocarbon emplacement.

Obtaining diagenetic ages from metamorphosed sedimentary rocks: U-Pb dating of unusually coarse xenotime cement in phosphatic sandstone

The depositional age of nonfossiliferous, metamorphosed sedimentary rocks is commonly bracketed between the age of the youngest detrital mineral and the age of the oldest metamorphic mineral. The technique of dating diagenetic xenotime by ion microprobe can provide robust minimum ages for sediment deposition. However, in most cases, xenotime is only a few microns (μm) in size and rarely exceeds 10 μm, the minimum size for in situ ion microprobe analysis. Phosphatic sandstone in the greenschist facies Mount Barren Group, in southwestern Australia, contains unusually abundant xenotime occurring as exceptionally coarse (200 μm) pore-filling cement that nucleated on detrital zircon grains. The optimum environmental site for the formation of the cement was sand beds within a black shale condensed section. Analysis of xenotime by sensitive high-resolution ion microprobe yields two age populations, 1696 ± 7 Ma for cement adjacent to detrital zircon grains, and 1646 ± 8 Ma for outer zones. Preserved textures show that initial xenotime growth was early diagenetic, establishing the ca. 1700 Ma age as a proxy for the depositional age of the Mount Barren Group. The younger age (ca. 1650 Ma) is regarded as burial related. The xenotime data reduce considerably the previous limits on the age of the succession, i.e., between ca. 1850 Ma (youngest zircon population) and ca. 1200 Ma (peak metamorphism). The significant achievement of our results is establishing that early diagenetic xenotime retains its physical form and U-Pb isotopic age despite greenschist-facies metamorphism and penetrative deformation.

The Relative Roles of Compaction and Early Cementation in the Destruction of Permeability in Carbonate Grainstones: A Case Study from the Paleogene of West-Central Florida, U.S.A.

ABSTRACT How compaction can yield a progressive decrease in the permeability and pore structure of carbonate grainstones is not well documented. In this study, the destruction of permeability by compaction relative to early cementation is examined in forty-five shallow-buried (30-440 m) grainstones with a range in permeability of 12 to 5180 md. Compaction and cementation indices calculated from thin-section point-count data indicate a group of grainstones dominated by cementation and another by compaction. Log-linear cross plots of permeability versus these indices suggest that permeability loss in these two groups occurs by different processes. The nature of those processes is elucidated by capillary pressure data and the distribution of cement and compaction fabrics as a function of permeability. As cement abundance increases in the cement-dominated samples, a continuous distribution of cement develops along grain surfaces, which is followed by the development of partial and then complete pore-filling mosaics. Permeability is progressively reduced with this succession of cement fabrics and an increasingly greater percentage of pore space is positioned behind progressively smaller throats. Permeability loss results from porosity reduction, progressive constriction of pore throats, and increased tortuosity of flow paths. Once permeability is reduced to tens of millidarcies, effective pore radii are all less than 1 µm and many are less than 0.1 µm, but there is no evidence for complete blocking of all pore throats in this suite of samples. In absolute terms, the pore-lining cements reduce permeability by thousands of millidarcies whereas the pore-filling cements reduce permeability only by hundreds of millidarcies. Mechanical compaction is the dominant cause of permeability reduction between 5000 and about 500 md in the compaction-dominated samples. Linear grain contacts and embayed grains become more abundant and close to overly close packing textures become more homogeneously distributed as permeability decreases. However, there is no concurrent change in the amount of pore space accessed by large or small throats, indicating that mechanical compaction does not uniformly constrict or seal pore throats. Instead, the data suggest that mechanical compaction reduces permeability by reducing porosity, lengthening pore throats, and increasing tortuosity. At about 500 md the compaction-dominated trend steepens on the log-linear plot of permeability versus porosity, a change that corresponds to the development of grain-to-grain pressure solution. Unlike mechanical compaction, grain-to-grain pressure solution constricts and eliminates pore throats and isolates pore space, resulting in a loss of large throats and an increase in the amount of pore volume accessed by small throats. In contrast to cement-dominated samples, the most effective pore radii are still greater than 1 µm at tens of millidarcies of permeability. Such low amounts of permeability can be reached even when pressure solution is not pervasively distributed throughout a sample.

Sublacustrine precipitation of hydrothermal silica in rift lakes: evidence from Lake Baringo, central Kenya Rift Valley

Abstract Many lakes in volcanic regions are fed by hot springs that, in some basins, can contribute a large percentage of the annual recharge, especially during times of aridity. It is important to recognize any contemporary hydrothermal contribution in paleoenvironmental reconstruction of lake basins because recharge from thermal waters can potentially confuse paleoclimatic signals preserved in the lacustrine sedimentary record. Hot spring deposits (travertine, sinter) provide the most tangible evidence for thermal recharge to lakes. Although subaerial spring deposits have been widely studied, lacustrine thermal spring deposits, especially sublacustrine siliceous sinters, remain poorly known. Detailed field, petrographic and scanning electron microscope (SEM) studies have been made of fossil sublacustrine sinter exposed at Soro hot springs along the northeastern shoreline of Ol Kokwe, a volcanic island in Lake Baringo, Kenya. Modern hot springs at Soro, which discharge Na–HCO3–Cl waters from a deep reservoir (∼180 °C ), have thin (1–10 mm), friable microbial silica crusts around their subaerial vents, but thicker (>1 cm) sinter deposits are not forming. The fossil sinter, which is present as intergranular cements and crusts in littoral conglomerates and sandstones, is composed mainly of opaline silica (opal-A). Three types of fossil sinter are recognized: (1) massive structureless silica, which fills intergranular pores and forms crusts up to 5 cm thick; (2) pore-lining silica, some of which is isopachous, and (3) laminated silica crusts, which formed mainly on the upper surfaces of detrital particles. All three types contain well-preserved diatoms including lacustrine planktonic forms. Microbial remains, mainly filamentous and coccoid bacteria (including cyanobacteria) and extracellular polymeric gels, are locally abundant in the opaline silica, together with detrital clays and thin laminae composed of authigenic chlorite (?). Most of the hydrothermal silica precipitated when the thermal springs were submerged by fresh lake water. Silica precipitated upon rapid cooling of thermal (∼90 °C ) waters at and just below their interface with the overlying cooler (∼25 °C ) lake waters. Microbial mats locally acted as a filter that limited mixing and rapid dilution of the thermal fluids. Some of the silica originally may have been soft and partly gelatinous. Planktonic diatoms and detrital clay rained down, then became incorporated in the amorphous silica. Following a fall in lake level, the opal-A lithified and partly altered to cristobalite (opal-C) and chalcedony. The lowest fossil sinters were later encrusted by calcite stromatolites, with calcite and quartz forming late pore-filling cements. The age of the sublacustrine sinters is unknown, but some of the deposits could date back to the late Pleistocene. Similar conglomerates cemented by hydrothermal silica are present along fault lines at neighbouring Lake Bogoria. Such rocks may provide evidence for deep, hot fluid recharge to lakes when encountered in the geological record.

The nature and possible origin of dolomite in Ar Rub’ Al Khali, the UAE

Surface and near-surface sediments in southeastern part of the United Arab Emirates show extensive development of dolomite that is characterized by whitish, dense rocks, partially dissected by veinlets of gypsum and well-developed coarse calcite crystals. The dolomite reveals rather monotonous aphanocrystalline to very finely crystalline crystals (2–20 μm) composed of idiotopic or planar dolomite fabric. Densely packed dolomite aggregates of clotted texture are observed. Moreover, several horizons of peloidal textures are recognized. The fossil content includes freshwater molluscs, foraminifera and gastropods embedded in very fine dolomite crystals., whereas dolomite forming the skeletal grains is considerably coarser (about 40 μm). The most common carbonate porosity type is fossil moldic followed by vug and channel porosity. Interparticle and intraparticle porosity, that may suggest possible dissolution by later diagenetic fluids, are only locally significant. Vari-sized blocky sparry calcite crystals are frequently observed as pore-filling cement.

A dolomitized diatomite in an Oligocene–Miocene deep-sea fan succession, Gonfolite Lombarda Group, Northern Italy

Abstract An unique, 5–10 cm thick layer of a dolomitized diatomite, which contains traces of volcanic material, occurs intercalated between terrigenous mudstones, thin-bedded turbidites and pebbly mudstones of the Montana Member (Burdigalian) of the Oligocene–Miocene Gonfolite Lombarda Group (“South-Alpine Molasse”). Dolomite formation took place during early diagenesis under conditions of sulfate reduction as suggested by the strongly negative δ 13 C values (∼−13‰ PDB). Stereoscan electron microscope observations show that the dolomite grew as a pore-filling cement enveloping deeply corroded diatom frustules. The dolomitized diatomite can be correlated with coeval occurrences of biosiliceous (“Tripoli”) and cineritic deposits in the Northern Apennines and the Southern Alps. Its preservation as a unique and distinct layer in the deep-sea fan facies of the Gonfolite Lombarda Group appears to reflect a particular event and suggests that it was deposited rapidly rather than during a longer time interval of slow pelagic sedimentation.

Ankerite Cementation in Deeply Buried Jurassic Sandstone Reservoirs of the Central North Sea

ABSTRACT Upper Jurassic Fulmar Formation shelf sandstones of the high-temperature, high-pressure Franklin and Elgin Fields (South Central Graben, North Sea) contain abundant disseminated and concretionary ankerite. In contrast, most Jurassic North Sea reservoirs contain only minor amounts of dispersed ankerite. Disseminated ankerite cement in the Franklin and Elgin Fields has a fairly uniform isotopic composition (18O -10 to -12.5o/oo PDB, 13C -3 to -5o/oo PDB). Ankerite concretions have 18O values similar to disseminated cements but a wider range of 13C values (+1 to -5.5o/oo PDB). They also have highly variable intergranular volumes, which (together with the 13C data) are interpreted as a combination of pore-filling cementation and in situ replacement of comminuted bioclastic debris by ankerite. Fluid-inclusion, 18O, and paragenetic evidence suggests that ankerite formed during deep burial (c. 3.5 to 4.5 km, 140-170°C), after the onset of overpressuring, but before hydrocarbon emplacement in the reservoirs. The regionally consistent 18O data suggest that ankerite formed via a temperature-influenced mechanism, and the relatively uniform 13C cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion. This can be explained by calcite dissolution in response to pH decrease during thermal breakdown of organic acids. Such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay-mineral transformations, and are likely to have been transported in pore fluids with Mg2+ and Fe2+. The presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite. A relative paucity of ankerite in other Fulmar Formation reservoirs may reflect different sedimentological compositions (less bioclastic debris) and/or lower burial temperatures (less advanced decarboxylation).

Spatial and temporal variations in early diagenetic organic matter oxidation pathways in Lower Jurassic mudstones of eastern England

The Lower Liassic succession of the East Midlands Platform, eastern England, comprises shallow marine mudstones and oolitic ironstones. In these sediments, temporal and spatial variations in the relative importance of early diagenetic anaerobic organic matter oxidation pathways are recorded. The ironstones are stratigraphically condensed and were deposited upon the Market Weighton Swell (MWS), a topographic high in the basin, in shallow water and high energy conditions at times of low relative sea-level. At times of higher sea-level stratigraphically condensed mudstones were deposited upon the MWS. Away from the MWS stratigraphically expanded mudstones were deposited throughout the Lower Liassic. The ironstones contain berthierine (ooids and pore-filling cement) and siderite (pore-filling cement), with rare pyrite replacing berthierine and siderite. Mudstones deposited away from the MWS contain pyrite as the sole early diagenetic iron mineral. Contrastingly, mudstones deposited upon the MWS contain both early diagenetic pyrite and Mn-rich siderite. The presence of berthierine and siderite in the ironstones indicates that sulphide was absent from early diagenetic pore-waters and that Fe(III) reduction dominated anaerobic organic matter oxidation. In contrast, in mudstones deposited away from the MWS the presence of pyrite indicates that sulphate reduction was a predominant pathway for anaerobic organic matter oxidation. In mudstones deposited upon the MWS the presence of both pyrite and Mn-rich siderite suggests that both sulphate reduction and Fe(III) reduction were important during early diagenesis. It is proposed that this spatial and temporal variation in relative importance of early diagenetic anaerobic organic matter oxidation reactions was a result of variation in physical sedimentary parameters (sediment accumulation rate and energy of environment). At times of low net sediment accumulation rates, in shallow water, high energy environments, suboxic diagenesis (Fe(III) and Mn(IV) reduction) dominated, giving rise to the characteristic berthierine-siderite ironstone assemblage. At times of higher sedimentation accumulation rates in deeper, lower energy environments, sulphate reduction dominated, leading to pyrite formation. At intermediate conditions, Fe(III), Mn(IV) and sulphate reduction were all important, leading to precipitation of both pyrite and Mn-rich siderite.

Tafoni in the El Chorro area, Andalucia, southern Spain

Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions. © 1997 by John Wiley & Sons, Ltd.

An Fe-Berthierine From A Cretaceous Laterite: Part I. Characterization

AbstractAn Fe-berthierine occurs in a buried laterite from the Late Cretaceous (Cenomanian) in southwestern Minnesota. It formed beneath a lignitic horizon in which reducing solutions percolated through a laterite comprising gibbsite, kaolinite and goethite. Morphologic differences suggest 2 separate conditions of Fe-berthierine formation. Early forms of Fe-berthierine include radial bladed or radial blocky crystallites coating pisoids, along with alteration of kaolinite at crystal boundaries. These morphologies formed in the vadose zone. Later forms precipitated under subaqueous conditions as macroscopic, pore-filling cement. The large size of the later-formed Fe-berthierines enabled microprobe characterization. This 1st reported occurrence of Mg-free berthierine has a structural formula close to an idealized Fe-berthierine: Fe2Al2SiO5(OH)4. Apart from their chemistry, the unique feature of the Minnesota Fe-berthierines is their formation in an exclusive nonmarine depositional environment. They formed in situ as part of a lateritic weathering profile developed on a broad, low relief peneplain. Physical evidence of formation under nonmarine conditions includes the presence of 1) scattered lignitic fragments; 2) concretions forming casts and molds of woody material; and 3) a nonmarine fossil (Unio sp. undet). Chemical evidence includes siderites collected from the berthierine-bearing horizon having stable isotope values indicating freshwater formation.

Influences of depositional environment and diagenesis on geophysical log response in the South Carolina Coastal Plain: effects of sedimentary fabric and mineralogy

Interpretations of depositional environments and hydrologic units are made routinely from the study of geophysical well logs. Spontaneous potential (SP) and resistivity logs can be used as indicators of textural parameters. Gamma-ray logs denote lithologic zones based on the presence of radioactive material, particularly in fine-grained sediments. On the South Carolina Coastal Plain, surficial fluvially derived cobbles, sands and clays are characterized by very low radioactivity. Weathered paleosurfaces enriched in hydroxy-interlayered vermiculite (HIV) and associated mica produce higher gamma-ray responses at discrete intervals. The underlying sequence of siliciclastic shelf sands, deposited under conditions of eustatic sea-level rise, has high concentrations of smectite and low gamma-ray signatures. Small-scale variability of gamma radiation reflects temporal changes of sub-environments; higher values are recorded in finer-grained, organic-rich nearshore sediments. Calcareous Ecocene sediments are characterized by highly variable signals on the gamma-ray, SP and resistivity logs. Phospharic material, smectite and disseminated organic matter produced very high gamma-ray values; corresponding low SP readings are a function of low macro- to microscale porosity resulting from the fine grain size and authigenic pore-filling cement. Intervals that resemble massively bedded chalks at the macroscale are characterized by low gamma-ray and resistivity values and high SP readings. High SP values result from microposity preserved in biogenic tests rather than intergranular porosity that would be expected for a permeable sand. These observations indicate the importance of incorporating sedimentologic techniques with well-log data for a comprehensive evaluation of subsurface lithologic units.

Silica-Cemented Beachrock from Lake Taupo, North Island, New Zealand

ABSTRACT Beachrocks on the north coast of Lake Taupo are composed of volcanic grains and diatoms that are bound together by isopachous, meniscus, or pore-filling amorphous silica cements. The littoral sediments were originally stabilized by epilithic and chasmolithic filamentous microbes that bound neighboring grains together. The amorphous silica cement was precipitated from silica-rich geothermal fluids that originated from warm-water plumes that form in the northern part of Lake Taupo where hot spring waters flow into the lake and move along the shoreline. Water from the warm water plumes that is washed onshore by prevailing winds and currents then percolates down through the beach sands and gravels. Precipitation of amorphous silica, triggered by cooling of the mixed spring and lake water, is preferentially focused on the epilithic and chasmolithic filamentous microbes, which act as templates for silica nucleation and precipitation.

Chemistry, Microstructure, Petrology, and Diagenetic Model of Jurassic Dinosaur Bones, Dinosaur National Monument, Utah

ABSTRACT The fossil-bone mineral in dinosaurs from the Morrison Formation is well-crystallized, stoichiometric francolite. The closely packed, subparallel francolite crystals have equidimensional cross sections mostly 10-40 nm wide. In contrast, crystallites in unaltered mammal bone tissue are poorly crystallized, nonstoichiometric carbonate hydroxyapatite, a few nanometers thick, and plate-, rod- or needle-shaped. The diagenetic francolite crystals are elongated in the c axis direction, as are bone-tissue crystallites. Along permeable cracks, francolite crystals grew to 250 nm wide. Compared to unaltered crystallites in mammal bones, the francolite has higher concentrations of Ba, Ce, Cr, F, La, Mn, Ni, Pb, Rb, Sr, Th, U, V, and Y, and perhaps Zr. Na and Mg are lower in concentration. Diagene ic francolite grew from groundwater enriched in ions from silicic ashes in the Brushy Basin Member and from dissolution of bone-tissue crystallites. After burial, francolite grew on crystallite seeds, filling the space formerly occupied by collagen, perhaps half the tissue volume, and dissolved crystallites. The micron-scale structures such as Haversian canals and laminae are preserved because the diagenetic francolite retains the orientations of the seeds. The laminae, 1-10 µm thick, commonly preserve an orthogonal plywood structure. The diagenetic growth of francolite during this type of bone fossilization implies that oxygen isotopic ratios measured in dinosaur bones, even those that appear fresh and unaltered, may dominantly reflect groundwater temperature rather than dinosaur body temperature. Trace-element concentrations in the francolite may reflect groundwater composition rather than dinosaur diet. Pore-filling cements precipitated in the sands and bones at shallow burial depths, as evidenced by the 35-52% minus-cement porosities of the sandstones and numerous uncompacted oversize pores filled by cements. Gypsum and calcite were precipitated from shallow alkaline groundwater. As the groundwater became increasingly dominated by ions from silicic ash, argentite, chalcopyrite, native gold, native silver, tiemannite, and uraninite replaced areas of bone a few microns to tens of microns in size along cracks. A thin chlorite crust commonly mantled francolite crystals along bone cracks and lined openings in bones and sandstones. Pervasive precipitation of a chalcedony crust followed, with pseudomorphs of chalcedony after gypsum. In larger cancellous openings, the remaining space was fi led by the various forms of silica. The bones are brown due to goethite-filled cracks that cut pore-filling minerals.

A Physical Model of Cementation and Its Effects on Single-Phase Permeability

In this paper, we present a mathematical model to quantify the effects of cements on the single-phase permeability estimate of clastic rocks. The model represents pore-filling, pore-lining, and pore-bridging cements by treating the cement particles as rectangular crystals attached radially to detrital grains. These clusters influence permeability through an increase in effective specific surface area and tortuosity. Because the model is based on physical principles, it is largely devoid of empiricism and should be applicable to a wide range of rock types. Permeability estimates by our model match experimental data from two sets of sandstone samples of five hydrocarbon-bearing formations in the North Sea. The model also confirms the well-known result that the effect of pore-bridging cement is a factor of ten greater in reducing the single-phase permeability compared to pore-lining and pore-filling cements.

Approaches to Predicting Reservoir Quality in Sandstones

Despite limited understanding of the details of many diagenetic processes, empirical techniques can be used effectively to predict reservoir quality prior to drilling. The predictive approach depends mainly on the availability of empirical data in the area of interest. In frontier basins, mean or maximum porosity of a potential sandstone reservoir can be estimated for a given composition and level of thermal exposure (or burial depth). The reliability of the estimates is constrained by input data. Approximate values of input parameters can be obtained from seismic data in combination with geological analysis of the area. In basins with sufficient information to generate calibration data sets, the predictive technique uses regression analysis. The applicability of this approach is constrained by the limits imposed by the calibration data set and is generally limited to samples containing less than 10% pore-filling cement. Quartz-rich sandstones (>85% framework quartz), cemented with quartz, are an exception to the 10% limit because quartz cementation commonly is related to burial history and rock texture. In weakly cemented sandstones, the critical variables controlling porosity are detrital composition, sorting, and burial history. Permeability can be predicted independently of porosity using the same variables plus grain size. These variables can be evaluated from seismic data and facies models. T is approach is best suited either for sandstones in which compaction is the main porosity-reducing process or for quartz-rich sandstones. An adequate predictive model for sandstone suites with a wide range of cement content can be obtained by dividing the calibration data set into two or more subsets and developing a predictive model for each. For example, one subset can be limited to samples with less than 10% cement, whereas the second subset will consist of samples with more than 10% nonquartz cement. Porosity and permeability in the first subset are then expressed by linear regression, whereas controls on reservoir quality in the more heavily cemented sandstones are determined independently based on understanding of cement distribution patterns.

Diagenesis of the Rotliegend Sandstones in the V-Fields, southern North sea: a fluid inclusion study

Abstract Illite, quartz, Fe-dolomite (with some dolomite), anhydrite and barite are the principal pore-filling cements in the analysed samples of the Rotliegend sandstones from the Vulcan, South Valiant and Vanguard Fields of the southern North Sea. Apart from illite, these cements also infill fractures cutting the Rotliegend sandstones, but in the fractures there is a predominance of anhydrite and barite whereas in the pore infills, quartz and Fe-dolomite predominate. The quartz, Fe-dolomite and sulphate cements have been studied using a variety of analytical techniques but with the emphasis on fluid inclusion microthermometry. The results indicate that fluids at temperatures of 70 to 125°C, with salinities of 17 to 24 eq. wt% NaCl and an overall composition of Na-Ca-Mg-K-Cl-SO 4 , were involved in authigenic mineral formation in pore spaces and fractures. Hydrocarbons were detected in the fluid inclusions hosted in the cements and indicate that cementation occurred in the presence predominantly of methane and only rarely liquid hydrocarbons. Relating the microthermometry homogenization/formation temperatures to the burial history, quartz cementation occurred mainly in the late Permian to late Cretaceous, Fe-dolomite during the Jurassic to mid-Cretaceous, and anhydrite and barite mainly in the mid-to late Cretaceous.

Two-Stage Neomorphism of Jurassic Aragonitic Bivalves: Implications for Early Diagenesis

ABSTRACT Aragonitic bivalves in Middle Jurassic paralic limestones from Lincolnshire (Eastern England) have been neomorphically replaced by inclusion-rich calcite spar. Textural features of spar are analogous to examples from other calcitized Jurassic molluscs, but Lincolnshire bivalves show evidence for two distinct stages of replacement. Staining, cathodoluminescence, and backscatter scanning electron microscopy reveal an initial volumetrically minor and frequently fabric-selective replacement by nonferroan calcite, with most of the remaining skeletal aragonite subsequently replaced by ferroan calcite spar. Stable-isotope data from individual bivalves define a negative covariant mixing trend ranging from early, relatively 13C-depleted and 18O-enriched nonferroan calcite (d13C = -2 PDB), to ferroan calcite with composition similar to associated late pore-filling cements (d13C » +2 PDB, d1 O » -7 PDB). The early diagenetic end member precipitated from depositional pore fluids, with bicarbonate supplied both from dissolution of skeletal aragonite and bacterial oxidation of organic material. In contrast, ferroan spar precipitated during burial diagenesis and dewatering of shales surrounding the limestone. Bulk trace-element signatures of the bivalves are dominated by late ferroan calcite, but electron probe microanalysis and intrinsic blue cathodoluminescence shows that early calcite contains negligible amounts of Fe, Mn, and Mg. Together with isotopic and fabric data this indicates that early neomorphism occurred at shallow depths and may even have begun at the sediment-water interface. In the absence of through-flowing meteoric waters, early replacement did not proceed via an inward-migrating neomorphic front. Instead, calcitization was initiated at numerous individual reaction centers throughout the shells and was principally controlled by the distribution of the skeletal organic matrix. This contrasts strongly with most previous reports of aragonite neomorphism during arly diagenesis.

Assessing brittle volume-gain and pressure solution volume-loss processes in quartz arenite

Quartz arenite of the Tuscarora Sandstone has been deformed by dislocation flow, pressure solution and microfracturing of which the last two are most important. Pressure solution involved shortening with no extension. Most samples have approximately 20% shortening normal to bedding interpreted to be compaction, and a few have up to 20% layer-parallel shortening of tectonic origin. Because there is no extension, these shortenings resulted in an equivalent amount of bulk volume loss. Multiple sets of microfractures in the form of fluid inclusion planes and microveins account for up to 10% extension and bulk volume increases up to 17%. When both microfracture volume gains and pressure solution volume losses are considered together, all samples show a net bulk volume loss ranging from 14 to 35%. Changes in material volume are different than bulk volume because of the presence of porosity at the time of pressure solution. Whereas all samples show bulk volume losses, most samples show net material volume gains of up to 16% with only those having experienced both compactional and tectonic pressure solution showing material volume losses. Pressure solution during compaction can account for most but not all cement. Compactional pressure solution surfaces do not extend into pore-filling cement suggesting that much of the cement was externally derived after compaction. Therefore, the flux of material may have been considerably greater than the material volume changes would indicate.