Glauconite Formation Research Articles

Oligocene to Holocene glauconite–phosphorite grains from the Head of the Cape Canyon on the western margin of South Africa

Abstract Glauconite and carbonate–fluorapatite (CFA) are significant components of the condensed Upper Oligocene to Holocene sediment succession recovered from the western margin of southern Africa. The detailed texture and geochemistry of these glauconite–phosphorite grains are reported here within the stratigraphic framework established on the outer continental shelf in the Head of the Cape Canyon area. Three texturally and stratigraphically distinct glauconite grain types are recognised: Oligo/Miocene infilled foraminiferal tests, Mio/Pliocene green and Quaternary dark green to black grains. Most of these glauconitic grains include a minor to significant amount of CFA. The Upper Oligocene to Lower Miocene sediment succession is dominated by fine sand-sized benthic and planktic foraminifera. Glauconite grains constitute 20–50% of the sediment and represent tests infilled by variable proportions of glauconite and CFA cement with the test wall replaced by CFA only. Glauconite–CFA formation during the Late Oligocene to Early Miocene was associated with warm-water upwelling coupled with third-order marine transgressions. Rare-earth element (REE) chemistry indicates that sub-oxic conditions prevailed during diagenesis. Mio/Pliocene green and Quaternary dark green to black medium sand-sized glauconite grains constitute between 20% and 95% of these predominantly siliciclastic sediments. The green to black glauconite grains are texturally different, but have similar major and REE chemistry. Since the mid-Miocene, formation of glauconite–CFA grains is associated with cold-water upwelling and detrital, Fe-rich terrigenous mud deposition in oxic to suboxic bottom waters analogous to the Holocene coast-parallel, organic-rich mudbelt on the shelf. The texture of these grains indicates repeated cycles of glauconite and CFA authigenesis during marine transgressions and winnowing and reworking during marine regressions.

Does the sedimentary formation of glauconite in Ratingen-Lintorf (Rhenish Massif) provide an age mark for the hydrothermal mineralization in the Niederberg Area (northwestern margin of the Rhenish Massif east of the Rhine Valley)? A petrographic short not

Does the sedimentary formation of glauconite in Ratingen-Lintorf (Rhenish Massif) provide an age mark for the hydrothermal mineralization in the Niederberg Area (northwestern margin of the Rhenish Massif east of the Rhine Valley)? A petrographic short not

CONSTRAINTS ON THE GENESIS OF FERRIAN ILLITE AND ALUMINUM-RICH GLAUCONITE: POTENTIAL IMPACT ON SEDIMENTOLOGY AND ISOTOPIC STUDIES

The term glauconite covers a series of iron-rich minerals that form in the upper layer of sediments of the sea bottom in locations where sediment input is low. Because of its potassium content and the process of its formation, it is one of few minerals that can be used in both sequence stratigraphy and in the determination of sedimentation age. Although aluminum-rich glauconite has been identified in several locations, the way it forms remains relatively unknown. A study of the ferrian illite from the Cambrian Anse Maranda Formation shows that, according to the present models for the formation of glauconite and diagenesis, the Al-for- Fe substitution responsible for the genesis of Al-rich glauconite occurs during early burial. In order to maintain charge balance while replacing Fe2+ and Mg2+ by Al3+ at the octahedral site, there is an expulsion of K, as high as 31.6% of the measured K2O. This loss is important when evaluating the time needed to form glauconite and interpreting the occurrence of Fe-rich illite; it must be accounted for when using the K–Ar system, for either dating or in diagenetic studies.

Hydraulic Fracture Design Optimization for Deep Foothills Gas Wells

Abstract The Deanne Glauconitic A pool is located approximately 80 km northwest of Rocky Mountain House, in the Alberta foothills. The reservoir is a thin, tight sandstone, located at a depth of approximately 3,500 m. In situ stress conditions in this area have been strongly influenced by the orogeny of the Rocky Mountains, with significant tectonic stresses imposed on the rock matrix. This high stress regime poses a major challenge to the very costly hydraulic fracturing treatments. A review of off-setting fracture treatments in the area showed that approximately 50% of previous treatments suffered from premature screenout, with poor post-stimulation productivity. A comprehensive study was conducted to understand the cause(s) of previous failures, and to make recommendations for an optimal fracture design. Once the key fracturing design parameters were identified, a recompletion design was carried out, including a refracture treatment, which successfully placed all of the fracture sand. This paper describes recommended procedures to optimize hydraulic fracture design for a deep gas reservoir in the foothills. These procedures include:Review off-setting fracture treatments;Identify the key drivers for successful sand placement;Examine core samples and design the fracture fluid, with emphasis on minimizing rock-fluid interactions;Model the hydraulic fracture using a 3D simulator;Size the fracture to optimize economic returns;Design the recompletion program and estimate costs;Implement the fracture treatment with stringent QA/QC processes; and,Post-fracture analysis by means of net pressure and radioactive logs to validate the fracture design. Introduction Field History The Deanne Glauconitic A pool was discovered with the completion of the well NUL el al Deanne 11-03-39-11 W5M in April 1983. The reported initial pool pressure was 55,360 kPaa at an MPP depth of 3,446.5 mKB giving a reservoir pressure gradient of 16.1 kPa/m. The well was tied in and placed on production in October 1987 at an initial rate of 100 103m3/d. Subsequently, a total of 10 wells have been completed in the Glauconitic zone in the immediate area, with a second pool (Glauconitic B) identified to the west of the original pool. The cumulative production from the Glauconitic A pool to February 2002 was 713.9 106m3 from a total of five producing wells. The reservoir has been produced by depletion drive, and the most recent available static reservoir pressure measurement was 28,160 kPaa as of June 2001. The reservoir natural gas is sweet, condensate rich with high heating value. The gas specific gravity is approximately 0.68, consisting of 83% methane, 11% ethane, and 3% propane. The average condensate gas ratio is approximately 0.14 m3/103m3 (25 Bbl/MMscf). Rock Properties and Reservoir Quality The Glauconitic formation in the Deanne area has variable gross pay thickness between 4 and 8 m. The open hole log derived porosity in the pay section varies between 9 and 12% with some streaks up to 15%. The reservoir is under-saturated with average water saturation values in the 15 to 25% range. The reservoir permeability, derived by in situ methods, was in the range of 0.1 to 1 mD.

Mineralogia e físico-química dos solos de mangue do rio Iriri no canal de Bertioga (Santos, SP)

A influência de condições halomórficas, hidromórficas e sulfato redutoras na mineralogia da fração argila e na geoquímica de três solos de mangue foi estudada no manguezal do rio Iriri. A partir de uma transeção, foram escolhidos três perfis representativos dos solos de mangue da área nos quais foram descritas as condições geoquímicas e sua conseqüência sobre a mineralogia de seus respectivos solos. A análise mineralógica (DRX e EDS) e as condições físico-químicas encontradas revelam a ocorrência de processos de piritização na área de estudo. A assembléia mineralógica está composta tanto por argilominerais alóctones (caulinitas, micas e esmectitas) como por argilominerais autóctones (micas, esmectitas e pirita). A partir de evidências provenientes da DRX e EDS, especula-se a formação de glauconita nestes solos. As condições de halomorfia e hidromorfia determinam valores de Eh e pH sob os quais elementos como Fe2+ e SO encontram-se livres na solução do solo com tendência à estabilidade da pirita (FeS2), favorecendo, portanto, o processo da piritização.

COMBINING GEOSTATISTICS AND MULTI‐ATTRIBUTE TRANSFORMS: A CHANNEL SAND CASE STUDY, BLACKFOOT OILFIELD (ALBERTA)

In this paper, we combine the methods of geostatistics and multi‐attribute reservoir parameter prediction (the multi‐attribute transform) for the integration of seismic and well log data, and illustrate this new procedure with a case study involving the prediction of porosity at the Blackfoot oilfield, central Alberta. The objectives of the survey were to delineate incised, valley‐fill sediments within the Early Cretaceous Glauconitic Formation at this field and to distinguish between sand‐fill and shale‐fill. The input consisted of twelve porosity logs together with a 3D seismic volume and the inversion of this volume. Although an excellent correlation was found between porosity and the initial inverted acoustic impedance volume, the combination of traditional geostatistics and the multi‐attribute transform produced an improved final result.Our approach uses well logs to “train” the multi‐attribute transform algorithm. We first extract average porosity values over the depth zone of interest, and compare these values to average seismic attributes over the same zone. Cross‐validation is used to show which attributes are significant. We then apply the results of the training and cross‐validation to data slices derived from both the seismic data cube and the inverted cube to produce an initial porosity map. Finally, we improve the fit between the well‐log values and the porosity map using cokriging.

A MÖSSBAUER STUDY OF EVOLUTION OF GLAUCONITE FROM CHINESE SEAS

The formation of glauconite from the Chinese Yellow Sea and the Chinese Eastern Sea was studied by the use of Mössbauer spectroscopy. The process of glauconite formation appears to differ between the respective seas, due to different initial substances of the formation. The results show the valence states and the crystallographic sites of the iron cations, and suggest that there may be a correlation between the formation process of glauconite and Mössbauer parameters.

Glauconite from lower cretaceous marine terrigenous rocks of England: A concept of biochemogenic origin

Detailed mineralogical characteristics of various forms of glauconite occurrence in Lower Cretaceous marine terrigenous rocks of the White Island (Binnel Bay, southern England) are discussed. It has been shown that glauconite was formedin situ due to the transformation of fine-dispersed and sandy-silty terrigenous materials. The influence of bacterial activity on glauconite formation is supported by the study of dissolution zones on quartz and feldspar grains, which revealed biomorphic structures akin to fossilized bacteria.

Sequence-stratigraphic significance of Miocene to Pliocene glauconite-rich layers, on- and offshore of the US Mid-Atlantic margin

Glauconite is generally agreed to be a reliable indicator of low sedimentation rate, but little systematic work has been done to specify the role of glauconite in a sequence-stratigraphic framework. Ocean Drilling Program Leg 174A recovered a good record of late Tertiary sediments along the shelf edge of the New Jersey US Atlantic margin, and glauconite was present in many intervals of the cores, sometimes in vertical proximity to sequence boundaries. Leg 174A glauconite was analyzed with binocular microscope, XRD and SEM to determine the percent of potassium and degree of maturity in order to relate occurrence to depositional environment. Seismic data were used to locate sequence boundaries, and percent glauconite was visually estimated. Glauconite samples from Site 1073 were found to have formed within a lowstand systems tract (LST), and as part of a distal condensed section (CS) within a transgressive systems tract (TST). These results are comparable to those from nearby Site 903 of Leg 150, which indicate a similar depositional setting for glauconite. Glauconites at shelf Sites 1071 and 1072 likely formed in the TST as well. Onshore, glauconite occurs mainly in transgressive systems tracts. The Miocene appears to be the upper limit of glauconite formation onshore. As the magnitude of sea-level change decreased, present onshore locations became too nearshore to maintain sediment-free environments, and the zone of glauconite deposition moved seaward. The same process did not occur offshore until the Plio–Pleistocene. Low subsidence-rate margins such as the US Atlantic are subject more to the variations of sea-level than to changes in sediment supply, tectonics, or other factors influencing their depositional patterns. Although glauconite occurrence is widespread in the stratigraphic record, this study demonstrates that for low subsidence-rate margins, primary deposition of glauconite is largely restricted to the TST.

Sulfur diagenesis and partitioning in Holocene Peru shelf and upper slope sediments

Sediments of the Peru margin were sampled in two transects across the shelf and upper slope (75 to 1000 m) in October of 1992. This region is characterized by high surface-water productivity and consequent high organic carbon fluxes, low terrigenous inputs, as well as an intense oxygen-minimum zone (OMZ) impinging on the slope at depths between 75 and 550 m. Sediments analyzed in this study were collected from one ship-deployed box core and five submersible-deployed push cores. Organic carbon (TOC) concentrations in surface sediments range from 1 to 18% on a dry weight basis. Formation of pyrite begins at the sediment/water interface (SWI). Maximum concentrations of pyrite sulfur in the cores from this study range from 0.5 to 1.8 wt.%. Concentrations of pyrite are influenced by availability of reactive iron and an oxidant capable of converting AVS to pyrite. Highly reactive iron is stripped from terrigenous detritus in the core of the OMZ. Some of this iron may contribute to glauconite formation in sediments near the base of the OMZ, where it is concentrated into a glauconitic green sand as a result of reworking and winnowing. In a shallow-water site (109 m) at the upper edge of the OMZ, high concentrations of pyrite, total iron and iron/aluminum ratios indicate that easily reduced iron species are available for sulfidation at the SWI. Pyrite formation at this site is nearly complete in the upper 4 cm of the sediment. In contrast, pyrite concentrations are slightly lower at the surface of two of the deeper-water sites (at 309 m, in the center of the OMZ, and to a lesser extent at 671 m, at the bottom edge of the OMZ), and appear to be influenced, in part, by availability of reactive iron. Concentrations of acid-volatile sulfides (AVS) are low in all sediments (≤ 0.04 wt.%), but are slightly elevated in sediments from the center of the OMZ. The ratio of organic sulfur (S-org)to organic carbon (TOC)at the sediment surface of the cores examined here is remarkably constant (0.012 ± 0.002) and within the range observed in marine biota (0.01 to 0.02), suggesting that organic sulfur at the SWI is dominantly biogenic. Although S-org/TOC ratios increase downcore, there is no clear correlation between concentrations of organic sulfur and organic carbon below the SWI. There is also no clear relation between concentrations of organic sulfur and total iron. Organic sulfur concentrations vary from site to site (0.06 to 0.33 wt.%), but are relatively constant downcore at any given site. Maximum concentrations are in cores from the center of the OMZ.

GREENSAND RESERVOIRS IN SILICICLASTIC SHORELINE SYSTEMS: FACIES MODELS FOR HYDROCARBON EXPLORATION

Greensand reservoirs are particularly significant in siliciclastic shoreline systems. Formation of autochthonous glauconite is restricted almost exclusively to the rising limb of the relative sea level (RSL) curve. Depositional environments that are typically developed in response to a rise in RSL are estuaries, barrier bars and shoreface sands. In this setting, the greensands are diachronous and exploration for hydrocarbon reservoirs should ideally be based on a sequence stratigraphic framework. These deposits are characterised by a number of significant surfaces that differ greatly in their origin, geographic extent and chronostratigraphic significance, and their thickness depends on the rate of RSL rise and sediment supply. The identification of significant surfaces bounding or contained within depositional sequences is critical, especially where drillhole samples are either unreliable or not available for geochemical and palynological analysis. Abrupt physical and chemical changes often occur on or across these surfaces, e.g. porosity, permeability and the presence of Fe-rich authigenic minerals such as glauconite and siderite. Greensands deposited in estuarine, barrier bar and shoreforce environments have excellent oil and gas potentail. Early Cretaceous greensand reservoirs from the North West Shelf are best developed where they overlie ravinement surfaces. Recognition of greensands within estuarine, barrier bar and shoreface environments is dependent on the correct identification of facies and the subsequent interpretation of the bounding discontinuities.

Pitfalls in seismic definition of incised valley reservoirs: A case study from the Lower Cretaceous Glauconitic Formation in South Central Alberta

Incised valley sandstones are common hydrocarbon targets in south‐central Alberta. An example is the Countess/Alderson trend which extends over 70 miles and contains 122 pools that have produced 100 million b/o and 300 billion ft3 gas (Figure 1). However, drilling 3-D seismic anomalies over these incised valleys occasionally produces disappointing results. This article describes a post‐drill analysis of one example.

Gas Production Modelling-A Case Study

Computer models are increasingly used in production optimization of gas fields because they offer a practical way to detect all potential bottlenecks. A single model takes into account reservoir performance, sandface inflow performance, well bore pressure losses, gathering system pressure losses, and field facility performance. Furthermore, many scenarios can be quickly evaluated without incurring significant expense. This article presents a case study of an actual Alberta gas-condensate field evaluated with computer models three times over the course of more than a year. February 1996–0riginal Model A study was commissioned by the minor partner in an Alberta gas-condensate field when the major partner wanted to install additional compression in hopes of accelerating drainage in a competitive situation. The minor partner, whose share of the cost was expected to be approximately $250,000, doubted the value of installing the compressor. Field A included a plant with compression, three wells, and the associated gathering lines (see Figure 1). Each well is completed in a tight Glauconite formation and all wells were thought to be in competitive drainage with Field B to the south and west. Reservoir Characterization A review of the static bottomhole pressures (BHP) for the wells in Field A suggested that the three wells did not share a common pool. On May 29, 1995, the static BHPs of Wells A, B, and C respectively, were 11.79 MPa, 11.59 MPa, and 8.27 MPa. On September 12, 1995, the static BHPs of the three wells were 11.73 MPa, 11.56 MPa, and 8.03 MPa respectively. Static BHPs for Wells A and B were both below the expected initial reservoir pressure for the Glauconite formation. While the pool may have experienced some drainage, the pressure decline over 106 days, as noted above was very small and close to recorder accuracy. It seemed unlikely that there was any significant drainage during this period. Wells A and B were modelled as sharing a common pool (Glauc AB) with no competitive drainage. Reserves for the Glauc AB pool were calculated by both material balance and the volumetric method. The original gas in place (OGIP) calculated by material balance was 160 E6m3. The OGIP calculated by the volumetric method, based on an assumed areal extent of two sections, was 500 E6m3. As a conservative estimate an OGIP of 160 E6m3 was initially assumed for this pool. Well C static BHPs were also below the expected initial reservoir pressure of the Glauconite formation. Unlike the Glauc AB pool, this pool continued to exhibit some drainage, with a modest pressure decline of 242 kPa over 106 days during which Well C was not producing. As an example of the observed pressure variations across Field B, a well in the extreme south of Field B had a measured static BHP of 11.80 MPa in July 1971, while a well adjacent to Well C had a measured static BHP of 11.84 MPa in October 1981. Although Well C may be in communication with wells in Field B, pressure data like those noted above suggested that the communication was not strong.

Diagenesis of the Safaniya Sandstone Member (mid-Cretaceous) in Saudi Arabia

Safaniya Member of the mid-Cretaceous Wasia Formation was deposited as fluvio-marine deltaic sediments in northeastern Saudi Arabia and the northwestern Arabian Gulf. After its deposition and initial burial, the mid-Cretaceous sequence was uplifted during Late Cretaceous structural movements. Structural traps in the region also started forming at this time, but structural growth continued during the Eocene, and new structures developed during the Plio-Pleistocene. The diagenetic mineral assemblage in the Safaniya sandstones includes kaolinite, illite, glauconite, pyrite, quartz and carbonate cements. Kaolinite and illite occur both as authigenic and detrital minerals. Authigenic kaolinite exists as skeletal and euhedral plates forming vermicules and aggregates of booklets infilling the pore spaces. Authigenic illite occurs as overgrowths of projecting laths. Quartz cement is present as syntaxial overgrowths and as microcrystalline quartz. Carbonates occur as patches and show replacement textures. Early diagenetic events include the formation of calcite, siderite, glauconite and pyrite, at or close to the sediment/water interface. Vermicular kaolinite and microcrystalline quartz probably formed by meteoric-water diagenesis during the Late Cretaceous uplift. Quartz overgrowths and the bulk of the euhedral kaolinite may have formed during the Late Cretaceous-Early Tertiary burial. Illite and ankerite were precipitated during deep burial, probably from fluids expelled from the basinal Kazhdumi shales to the east and northeast. The precipitation of authigenic minerals in the clean quartz sandstones has reduced the primary interparticle porosity by an average of about 9% of the rocks. Because the clays occur mainly as pore filling cements, their precipitation has caused only a small reduction in the permeability of these rocks.

Glauconite formation as a factor in sedimentary manganese deposit genesis

Concentrations of in the crust are dominated by a single ore type described as the orthoquartzite-glauconite-clay association or as manganese giants. Current research suggests that these large accumulations of oxide and/or carbonate are associated with relatively rapid marine transgressions and regressions across older basement rocks. Some of the larger deposits of this type, such as Nikopol and Chiatura, USSR, and Groote Eylandt, Australia, occur in petrologically simple terrigenous formations, generally sandstones and shales, interpreted as paleoshoreline sediments. A major problem lies in the very high Mn/Fe ratio of these deposits (30-50) as against that of ordinary sediments (0.02). Research on the Groote Eylandt Cenomanian age deposit suggests that the high Mn/Fe of the primary ore (15-20) was associated with enrichment in coastal waters, which in part was related to glauconitization (iron-trapping) reactions. Mineralogical evidence indicates a sequence from a well-defined glauconite phase in deeper water, off-shore sediments (with whole-rock Mn/Fe of 0.3-1.2) to a glauconite-smectite in near-shore estuaries and rivers, as a minor component of pisolitic oxides of Mn/Fe ranging from 15 to 20. Data on these glauconite minerals, including X-ray diffraction, infrared spectroscopy, microanalyzes, textural occurrences, and genesis are presented. Geologically glauconite is a product of a postoxic environment, possibly related to fluctuating water depth during transgression-regression, seaward from the site of oxide precipitation.

Episodic deposition of the Lias in the Medianes nappe (western Switzerland): a record from mineralized ammonite-bearing beds

Sinemurian and Pliensbachian depositional sequences from the starved distal northern continental margin of the Tethys are preserved in the Médianes Nappe (Western Swiss Prealps). They contain mineralized beds which include a variety of facies. The northern continental margin of the Tethys was broken into blocks, less than 50 km wide, during an extensional phase. The Sinemurian and Pliensbachian sediments in the study are vary in thickness from less than 50 m in the north to more than 200 m in the south. Although the upper Sinemurian to Pliensbachian mineralized beds are thin, between 0.10 and 0.50 m thick, they commonly contain several distinct faunal horizons stacked one upon another. The mineralized beds are localized in the northern and central parts of the Préalpes Médianes in crinoidal calcarenites or between crinoidal calcarenites and alternating siliceous limestone sand marls. Petrographical and geochemical analyses of these beds show multiple phases of mineralization within an early oxic to suboxic environment. Formation of hematite, phosphates, glauconite, and silica, locally interlayered with pyrite, occurred either during a bacterial oxidation zone or during a bacterial manganese and iron reduction zone. The beds may have been reworked as intraformational conglomerates. Some ammonites in the beds have corroded upper surfaces. Although these beds can be traced from the northern to the central part of this section of the northern continental margin of the Tethys, deposition was not uniform. The episodic occurrence of these mineralized beds in space and time may be explained by local changes in accomodation potential. These changes are governed by differential subsidence and eustasy of the third order cycle scale with an added effect of higher order frequency cycles, possibly climatic in origin. Thus, there is an interplay of supraregional (i.e. global effects: eustasy, regional intraplate stresses), regional (break up of Tethyan margin into moderate scale blocks) and local (locally-derived sediment flux, local structural patterns) parameters that have affected the deposition of the beds. The interplay of these factors leads to a highly complex and unique rock record.

Interlaboratory reference-samples for K-Ar dating

Glauconitization processes proposed in the literatures include the “layer lattice” theory in which glauconite formed through K uptake by a detrital smectite precursor and the “neoformation theory” that does not require a smectite-like precursor.The non pelletal glauconite bed in the uppermost part of the Campanian Qusseir Formation at the Abu Tartur area and the underlying smectite-rich beds provided a potential example for the glauconitization process. The glauconite occurs as fine (clayey), moderately hard, and homogeneous bed ranges in thickness from 2 to 3 m. Under the optical microscope, glauconite occurs in a unique morphology as very fine green clayey matrix, which differs from the typical pelletal shape of many glauconites. Under the scanning electron microscope, it appears as dense, uniform, and very fine flakes that are composed of Si, Al, Fe, and K. X-ray diffraction analysis of both bulk samples and clay fractions indicates that the glauconite bed is composed entirely of glauconite. Geochemical analysis suggested that the investigated glauconites belong to the 2nd stage of the maturity scale (i.e. moderate maturity); or they can be alternatively ranked as evolved. Chondrite-normalized REE patterns exhibit LREE enrichment relative to HREE ((La/Yb)N ratios vary from 5 to 8) and slightly negative Eu anomalies (Eu/Eu* from 0.7 to 0.8).Clay beds that underlie the glauconite bed are composed of two types of smectites; smectite of low K2O and Fe2O3 contents and smectite of high K2O and Fe2O3 contents. Smectite of low K2O and Fe2O3 contents is considered in this study as the initial precursor of glauconite, while smectite of high K2O and Fe2O3 represents the transitional phase between smectite and glauconite. This, in turns, supports the “layer lattice” theory in which glauconite formed by K diffusion to a detrital smectite precursor from the sea-water due to transgression. Absence of mixed-layer minerals suggests that the diffusion process was active and continuous until reaction completion was reached, i.e. formation of glauconite. Maturity and REE patterns suggest the possible formation of the studied glauconite probably at shelf environment at approximately 100 m water depth. The occurrence of glauconite indicates a marine invasion of the upper part of the Qusseir Formation, which was considered previously as non-marine sediments.

Sedimentation dynamics and redox iron-cycling: controlling factors for the apatite—glauconite association on the East Australian continental margin

Abstract Detailed sedimentological and geochemical studies of phosphorites and sediments from the East Australian continental margin have shown that both apatite and glauconite are forming at a transition zone between relict, iron oxyhydroxide-rich, organic-poor (TOC<0.3%) outer shelf (200–350 m) sediments and relatively rapidly accumulating, iron oxyhydroxide-deficient, organic-rich (TOC>0.8%) deep water (460–650 m) sediments. The interaction between sediment mixing and Fe-P cycling processes (between the pore waters and the solid phase) appear critical to the formation of modern phosphorites in this area. The phosphate nodules form within the anoxic zone in the sediments at depths of approximately 10–18 cm below the sediment-seawater interface. Nodules which remain in the sediment mixed layer after they form continue to accumulate both P and Fe for up to 60 ka; during this time their apatite and iron oxyhydroxide contents more than double and the nodules become denser and more lithified. Apatite and glauconite formation are favoured by periods of high sea-level and low current velocities, as these conditions allow a relatively high organic carbon input to the sediments and thereby the maintenance of anoxia at shallow depths within the sediments. During periods of low sea-level and high current velocities, the carbon flux into the sediments decreases and the sediments become oxic. Consequently the Fe-cycling processes cease and apatite and glauconite formation stops: the glauconite is progressively transformed to goethite, and phosphorite nodules are concentrated into lag deposits and ferruginized. Alternations of high and low sea-level cycles eventually result in the formation of the massive ferruginous Neogene phosphorites that mantle much of the outer shelf. The iron enrichment processes observed in the modern to Neogene phosphorites on the East Australian continental margin provide explanations for many of the features seen in ferruginous Neogene deposits in the world’s oceans.

Origin of Hemipelagic Source Rocks During Early and Middle Miocene, Monterey Formation, Salinas Basin, California

Organic-rich hemipelagic shales of the lower Monterey Formation in the Salinas basin of central California record intervals of enhanced productivity and accumulation of fine-grained biogenic and clastic sediment under dysaerobic to anoxic conditions during the late early Miocene and early middle Miocene. Generally low oxygen conditions helped preserve abundant hydrogen-rich organic matter in laminated hemipelagic shales. These deposits are good potential oil source rocks, and consist primarily of (1) pelagic components (biogenic carbonate and silica, and marine organic matter), (2) fine-grained siliciclastic material transported mainly by sediment gravity processes, and (3) an associated assemblage of diagenetic phases. The following paragenetic sequence is observed: (1) hallow burial, with formation of glauconite, carbonate fluorapatite, pyrite, and iron-dolomite, and (2) deeper burial, with transformation of opal-CT to quartz, and precipitation of mixed layer illite/smectite, kaolinite, and chlorite. Sulfur and carbon isotope ratios from apatite suggest formation just beneath the sediment-water interface under anoxic to dysaerobic conditions. Kaolinite postdates, or is cogenetic with, diagenetic quartz and may be a dissolution product associated with the onset of thermal maturation and carboxylation of organic matter. Although conventional geochemical maturity measures suggest that these source rocks have not entered the main oil window, relatively early hydrocarbon generation is well documented from Monterey shales, and these rocks are likely a signif cant source of much Salinas basin oil.

Reservoir Description From Seismic Lithologic Parameter Estimation

Modern three-dimensional (3D) seismic data assist not only in delineating reservoir geometry, but also in predicting porosity and lithology variations away from well control. This case study of an oil-producing channel sand in the Taber/Turin area, Alta., Canada illustrates the improvement in reservoir characterization achieved with an integrated approach incorporating both well and seismic information.