Shale Diagenesis Research Articles

Dating of Igneous Intrusives and Burial/Thermal History by Illite Geochronology of Clay Diagenesis in Shales: ABSTRACT

Dating of Igneous Intrusives and Burial/Thermal History by Illite Geochronology of Clay Diagenesis in Shales: ABSTRACT

The Tale of Three Basins: Illite Age Analysis, Shale Diagenesis, and Interpretation of Diagenetic History: ABSTRACT

The Tale of Three Basins: Illite Age Analysis, Shale Diagenesis, and Interpretation of Diagenetic History: ABSTRACT

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.

Estimation of the standard molal heat capacities, entropies and volumes of 2:1 clay minerals

The dearth of accurate values of the thermodynamic properties of 2:1 clay minerals severely hampers interpretation of their phase relations, the design of critical laboratory experiments and geologically realistic computer calculations of mass transfer in weathering, diagenetic and hydrothermal systems. Algorithms and strategies are described below for estimating to within 2% the standard molal heat capacities, entropies, and volumes of illites, smectites and other 2:1 clay minerals. These techniques can also be used to estimate standard molal thermodynamic properties of fictive endmembers of clay mineral solid solutions. Because 2:1 clay minerals like smectite and vermiculite are always hydrated to some extent in nature, contribution of interlayer H 2O to their thermodynamic properties is considered explicitly in the estimation of the standard molal heat capacities, entropies, and volumes of these minerals. Owing to the lack of accurate calorimetric data from which reliable values of the standard molal heat capacity and entropy of interlayer H 2O can be retrieved, these properties were taken in a first approximation to be equal to those of zeolitic H 2O in analcite. The resulting thermodynamic contributions per mole of interlayer H 2O to the standard molal heat capacity, entropy, and volume of hydrous clay minerals at 1 bar and 25°C are 11.46 cal mol −1, 13.15 cal mol −1 K −1 and 17.22 cm 3 mol, respectively. Estimated standard molal heat capacities, entropies and volumes are given for a suite of smectites and illites commonly used in models of clay mineral and shale diagenesis.

Experimental studies of silicate-carbonate reactions—I. Applications to diagenesis

The formation of clays and CO 2 from silicate-carbonate reactions and the effect of these products on reservoir quality and fluid chemistry have been well recognized in the study of sediment diagenesis. The phase relationships for the reaction of kaolinite + dolomite + quartz to calcite + chlorite-smectite + CO 2 reactions were chemographically examined and experimentally evaluated. The phase boundary of this reaction on a T− X CO 2 diagram was interpolated between high temperature experimental data and low temperature field data. The results can be applied to predict the amounts, type and timing of clay and CO 2 formation during sediment diagenesis. The variation of pore fluid composition (e.g. Ca 2+ and Mg 2+) caused by these reactions was also evaluated using a geochemical modeling program. The major conclusions and implications are the following: (1) the concentration of CO 2 in the reaction fluid controls the reaction temperature as well as the clay type (chlorite vs smectite) formed. The equilibrium boundary provides a guideline for estimating the CO 2 contents, the sealing integrity and temperatures of a diagenetic system based on the observed mineral assemblages; (2) the silicate-carbonate reactions can occur in diagenetic environments that are open to CO 2, and can therefore, be an important inorganic source of CO 2 which can lead to the formation of CO 2-rich gas fields and secondary porosity. The diagenetic reactions, however, are significantly suppressed in a closed system (e.g. overpressure zone) and therefore cannot significantly contribute to overpressure formation nore generate enough pressure to create fractures; and (3) the formation of smectite of chlorite which can serve as a sink for Mg 2+ and to a lesser extent, for Ca 2+. The pore fluid released from these reactions during shale diagenesis, in general, contains much less Mg 2+ than Ca 2+. The reactions in shales are likely sources of Ca 2+ for carbonate cements but not of Mg 2+ for dolomitization.

A method for the determination of nitrogen in clays, with application to the burial diagenesis of shales

A new operationally defined method for the measurement of fixed inorganic nitrogen (N[sub fix]) associated with clays has been tested and applied to a sequence of mixed-layer illite/smectite (I/S) (ranging from 40 to 68% illite in I/S) in shales from a drill site in Terrebonne Parish, Louisiana. This new method has distinct advantages over other methods in that it is simple and can accommodate small sample sizes (10 mg). N[sub fix] is measured, using a carbon-nitrogen analyzer, on a sample that has been ashed for 8 hr at 450 C to remove organic and non-fixed inorganic nitrogen. Total nitrogen is measured on an separate untreated sample. Content of organic carbon and nitrogen can be assessed by the weight-percent difference between untreated and ashed samples. Fourier-transform infrared spectroscopy confirmed the presence of nitrogen as ammonium in untreated and heat-treated samples. N[sub fix] content of the < 0.2 [mu]m fraction of the shale sequence correlates positively (R[sup 2] = 0.91) with the percent of illite in the mixed-layer I/S. Corresponding with the increase in percent of illite in I/S with depth, an increase in N[sub fix] content is also seen with increasing depth of burial. Nitrogen from thermal breakdown of organic-matter duringmore » later diagenesis probably provides the N[sub fix] found in I/S. In the range of diagenesis studied here, N[sub fix] concentration appears to depend on the available fixing capacity of illite. Because nitrogen content varies with organic-matter type, however, caution should be used when assuming this relationship for other diagenetic environments.« less

A Comparison of Shale Compositions from the Western Canada Sedimentary Basin and the U.S. Gulf Coast: The Importance of Depositional Composition on Shale Diagenesis

A Comparison of Shale Compositions from the Western Canada Sedimentary Basin and the U.S. Gulf Coast: The Importance of Depositional Composition on Shale Diagenesis

An Experimentally Derived Kinetic Model for Smectite-to-Illite Conversion and Its Use as a Geothermometer

AbstractThe smectite-to-illite conversion during shale diagenesis has recently been used to constrain the estimate of a basin's thermal history. We have systematically investigated the kinetics for the conversion of a Na-saturated montmorillonite (SWy-1) to a mixed-layer smectite/illite as a function of KCl concentration (from 0.1 to 3 moles/liter) over a temperature range of 250° to 325°C at 500 bars in cold-seal pressure vessels using gold capsules. The results show that the conversion rate can be described by a simple empirical rate equation-dS/dt = A · exp(-Ea/RT) · [K+] · S2where S = fraction of smectite layers in the I/S, t = time in seconds, A = frequency factor = 8.08 × 10-4 sec-1, exp = exponential function, Ea = activation Energy = 28 kcal/mole, R = gas constant, 1.987 cal/deg-mole, T = temperature (degree Kelvin), [K+] = K+ concentration in molarity (M) in the fluid.The results also show that Ca2+ in solutions barely affects the illitization rate, whereas Mg2+ significantly retards the rate. The retardation, however, is not as severe as previously reported. Na+ ion can significantly retard the rate only if the concentration is high.We found that by assuming a range 0.0026-0.0052 moles/liter (100-200 ppm) of K+, concentrations similar to the value typically reported in oil field brines, the present kinetic model can reasonably predict the extent of the smectite-to-illite conversion for a number of basins from various depths and age. This narrow range of potassium concentrations, therefore, is used to model the smectite-to-illite conversion in shale when the actual chemical information of pore fluid is not available.The kinetic equation has been tested using field data from a large variety of geologic settings worldwide (i.e., the Gulf of Mexico, Vienna Basin, Salton Trough Geothermal Area, East Taiwan Basin, Huasna Basin, etc). The results show that the equation reasonably predicts the extent of the reaction within our knowledge of the variables involved, such as burial history, thermal gradients, and potassium concentration.

Shale Diagenesis: A Case Study from the Albian Harmon Member (Peace River Formation), Western Canada

AbstractShales have often been cited as the source of authigenic material that occurs in coarser grained sediments, but there are few comprehensive studies of diagenesis in shales that rigorously attempt to determine if they are sources for potentially mobile constituents or if they retain those constituents within the shale body. The silicate diagenesis of a Lower Cretaceous marine mudstone, the Harmon Member, was investigated by determining the bulk chemistry, clay mineral compositions and mineral modes using standard analytical techniques and linear programming. Changes in mineralogy are observed in hemipelagic laminated mudstones (LM) that are attributable to diagenesis despite relatively constant bulk compositions. These include authigenic kaolinite formation and illitization of detrital muscovite, K-feld-spar and authigenic kaolinite. No diagenetic trends in mineralogy are observed in rapid and episodically deposited transition zone (TZ) sediments because of primary (depositional) mineralogical variability. Diagenetic changes in mineral modes for shales of the Harmon Member are small, suggesting that silicate diagenesis proceeds under relatively “closed system” conditions. The occurrence of authigenic quartz suggests that much of the Si released from the dissolution of quartz and chert is precipitated within the Harmon Member. Al, Ti, and K apparently are conserved. Burial induced increase in temperature is inferred to be the primary control on silicate diagenesis within the Harmon Member.

Coalification, shale diagenesis and thermal modelling in the Alpine Foreland basin: the Western Molasse basin (Switzerland/France)

The analysis of coalification and shale diagenesis in Oligocene and Lower Miocene sediments in the western part of the Molasse basin (Western Switzerland and Haute-Savoie, France) shows a close relationship between the very low-temperature metamorphism in the foreland and the tectonic history in the orogenic belt. The degree of thermal maturation generally increases towards the alpine front. Isomaturity lines, however, run slightly oblique to today's alpine front. This is probably due to the tectonic burying under the alpine nappes in Early Miocene time. Second order variations of the coalification pattern are explained as resulting from a regional discharge of warm fluids controlled by tectonic structures.

Pitfalls and Caveats in Surface Light Hydrocarbon Surveying

The credible application of light hydrocarbon surveying in petroleum exploration requires an appreciation of the genesis, mobility, and alteration of light hydrocarbons in subsurface and surface environments. In this context, case studies and experimental work are discussed in this paper illustrating some of the pitfalls that beset light hydrocarbon surveying. These examples document a variety of spurious or false' light hydrocarbon anomalies that are genetically related to soil and sediment properties (e.g., mineralogy, parent material, indigenous organic material, biological activity) rather than to seepage phenomena. Also discussed are instances in which the natural alteration of biogenically produced light hydrocarbons in surface environments has yielded hydrocarbons that mimic thermogenically produced hydrocarbons. In addition, examples are shown in which light hydrocarbons produced during shale diagenesis (and that can be expelled via shale compaction) resemble thermogenically produced light hydrocarbons. Finally, data are presented that indicate that acid extraction, a popular analytical technique, can itself produce hydrocarbons from soil and sediment samples containing particular clay and organic constituents. Thus, the challenge in light hydrocarbon surveying is to critically evaluate the presence of light hydrocarbons in surface materials in terms of a number of possible origins, not all of which imply the existence of subsurfacemore » reservoired hydrocarbons.« less

Mineralogical Controls on Rock Fluid Properties--The Delaware Mountain Sandstones, Waha Field, West Texas

Reservoir rocks of the Guadalupian Delaware Mountain Group in Waha field, Reeves Co., Texas are very fine-grained, subarkosic sandstones interbedded with organic-rich siltstones and thin limestones. Shales and detrital clays are notably rare. The Delaware Mountain Group, including in descending order the Bell Canyon, Cherry Canyon, and Brushy Canyon formations, are currently buried at depths of 4,500-8,000 ft in this area. Production is primarily from the Bell Canyon and Cherry Canyon formations. Diagenesis of the Delaware Mountain Sandstones is unique in several aspects: The lack of shale in the sequence precludes significant changes associated with shale diagenesis. Abundant organic matter within the formations may have played an important role in diagenetic processes. Pore fluids migrating from the overlying Castile evaporite sequence are a likely source of early halite cement in the Delaware Mountain Group. Large scale dissolution subsequently removed cement and detrital material, creating abundant secondary porosity. In the study area, reservoir porosities generally range from 20 to 25%. However, permeabilities are low (usually less than 50 md) and water saturations are high due to the presence of pervasive porelining chlorite. Reservoir quality of these sandstones has been controlled to a large extent by diagenetic processes.

Quartz dissolution during shale diagenesis implications for quartz cementation in sandstones

Quartz dissolution during shale diagenesis implications for quartz cementation in sandstones

Geology and Diagenetic History of Overpressured Sandstone Reservoirs, Venture Gas Field, Offshore Nova Scotia, Canada (1)

Deep exploratory wells in the Scotian Basin, offshore Nova Scotia, Canada, have encountered overpressured formations with pressures 1.9 X the normal hydrostatic gradient. The overpressures occur over an area of approximately 10,000 sq km. In the Venture field, the abnormal pressures are confined below a depth of 4500 m and are associated with Upper Jurassic-Lower Cretaceous gas- and condensate-bearing sandstone reservoirs. The overpressures occur within normally compacted shales containing numerous overpressured sandstone reservoir beds. The development of overpressures, seals, and secondary reservoirs are all diagenetically driven. The volumetric increase achieved by kerogen to petroleum-gas conversion and hydrocarbon expulsion are believed to be the main driving forces or Venture overpressures. Three secondary porosity depth levels, which top at 2500 m (65 degrees C), 3700 m (95 degrees C), and 4600 m (130 degrees C), correlate with major steps in the organic matter maturation in the basin. Secondary porosity is initially achieved by alumosilicate dissolution, with ferroan sparry calcite cement dissolution dominating below 4000 m. Porosity enhancement and preservation is not the result of a single diagenetic event but instead the result of a series of diagenetic events that overlapped in time. Formation of dynamic diagenetic barriers within the zone of peak gas generation helps retard the diffusive migration of hydrocarbons and other fluids expelled during shale diagenesis resulting in pressure build up. The preservation of up to 32 percent porosity under 500-1000 atm of pressure could not be achieved without simultaneous pressuring of developing voids. Thus, the expansion of secondary reservoirs in the overpressured zone also had to be late (50 Ma or younger) and about synchronous with pressurizing of the reservoirs. Significant for hydrocarbon exploration is that Venture-type diagenetic overpressures are not associated with undercompacted sediments and, hence, they cannot be predicted from compaction trends during drilling. Petrographic, diagenetic, and lithofacies studies can be instrumental in predicting potential areas of deep subsurface secondary reservoirs development.

Diagenesis in shales from a partly overpressured sequence in the Gulf Coast, Texas, USA

Argillaceous rocks from a partly overpressured Tertiary sequence in a Texas Gulf Coast offshore well have been examined using X-ray diffraction (XRD), back scattered electron microscopy (BSEM) and electron microprobe analyses. The XRD data show that there is increasing illitization of illite-smectites at depths below 2300 m. The BSEM and microprobe data indicated that the illitization occurred in two microenvironments, the matrix and cavity infills. In the matrix the illitization appears to be a transformation process and in the cavity infills by neoformation. Other diagenetic processes identified by the BSEM analyses include the formation of kaolinite and illite in cavity infills and chlorite in the matrix and cavity infills. Calcite cementation has also been observed along with K-feldspar dissolution and mica transformation. These observations indicate the complex and heterogeneous nature of the diagenetic processes in these argillaceous sediments. The presence of overpressuring reflects a combination of processes: initial undercompaction arising from rapid sediment deposition; illitization of illite-smectites; calcite cementation creating permeability barriers. The generation of hydrocarbons and faulting may also have been contributory factors to the overpressuring pattern now observed.

Shale Diagenesis in the Bergen High Area, North Sea

AbstractIllite diagenesis in Tertiary and Mesozoic shales in the Bergen High area, northern North Sea, was studied using mineralogic, isotopic, and computerized thermal modeling techniques. The Tertiary shales are dominated by smectite, with lesser amounts of illite, kaolinite, and chlorite. At present burial temperatures of &gt;70°C smectite is absent, and the shales contain abundant lath-shaped illite which yields a mixed-layer illite/smectite (I/S) X-ray powder diffraction (XRD) pattern. Transmission electron microscopy (TEM) indicates that the illite laths increase in abundance and thickness with increasing depth; XRD patterns indicate a progressive increase in the illite component of the I/S. The deepest samples were found to contain long-range ordered (R=3) I/S, which showed platy particle morphology with the TEM. K-Ar ages of most of the &lt;0.1-μm-size illite separates imply that illitization was a relatively brief event affecting a thick sequence of sediments during late Cretaceous to early Paleocene time (65–87 Ma); however, measured ages were affected by trace levels of detrital Ar contamination and do not represent the true age of diagenesis.Several methods of quantifying Ar contamination were used to correct measured ages to obtain a reasonable estimate of the true age of diagenesis. The corrected ages are imprecise due to uncertainties in quantifying the levels of sample contamination, but generally suggest a Paleogene (38–66 Ma) period of illitization. In contrast, simple kinetic models of smectite-illitization suggest much younger ages of diagenesis (0–40 Ma at the Veslefrikk field; 0–60 Ma at the Huldra field). The timing of the diagenesis and the morphologic aspects of the authigenic illite suggest that illite precipitated before late Tertiary compaction and resulted in a decrease in fluid permeability. Low trapping efficiency of early Tertiary sediments, vertical escape of warm fluid from the Brent sandstone, and high heat flow may have promoted illite diagenesis in the shales prior to deep late-Tertiary burial.

Antrim Shale Depositional Environments, Diagenesis, and Source Rock Potential in Michigan Basin: ABSTRACT

The Antrim Shale in the Michigan basin is a significant reservoir of in-situ natural gas. Presently it is being drilled in only a few northern counties of Michigan, but may be producible basin-wide. Several authors have suggested that the reservoir resulted from fracturing. Two processes may have caused these fractures: (1) framework collapse due to volume reduction associated with the conversion of organic matter to gas, and (2) the loss of interstitial water resulting from the conversion of smectite to illite. It has been suggested that the sediments are thermally immature and that the natural gas is biogenic in origin. The Antrim Shale is a facies equivalent of other Mid-Continent Black Shales that are documented to have been deposited in an oxygen-stratified basin, with three zones, aerobic, dysaerobic, and anaerobic. Water depth and wave energy apparently controlled the amount of oxygen in the depositional environment and thus the preservation of organic matter in the sediments. If anaerobic conditions existed then bioturbation of the sediments was low and the preservability of organic matter was high. The organic content of the shales, and the potential production of natural gas, is related to environment of deposition. By understanding the facies relationships, the environmentmore » of deposition, and the vertical and areal distribution of these facies, nature of the fracture systems, and the maturation level of the hydrocarbons, a better exploration program can be developed for the exploitation of the Antrim Shale gas reservoirs.« less

Shale Diagenesis: An Important Factor in Development of Secondary Porosity: ABSTRACT

Shale Diagenesis: An Important Factor in Development of Secondary Porosity: ABSTRACT

Element- und isotopengeochemische Untersuchungen an Kupferschieferprofilen der Sangerhäuser Mulde (DDR)

We analyzed six profiles of the copper shale from the Basin of Sangerhausen (GDR) (determination of 52 chemical elements) to understand the vertical and lateral distribution of the elements (52 determinations) and the role of “Bote Faule” in the process of mineralization was studied. Using instrumental neutron activation analysis the concentration of 27 chemical elements were determined. On basis of the calculated correlation coefficients for element pairs their binding types could be estimated and some peculiarities of the distribution of elements were established. Results of isotope geochemical investigations of the carbonate fraction support the supply of oxidic waters across the “Bote Faule” from the ingression of the Zechstein sea to late diagenesis of copper shale.

Diagenesis, Dewatering, and Source Rock Potential of Ordovician Shales from the High Atlas, Morocco: ABSTRACT

The Ordovician shales of the High Atlas are interpreted as shelf sediments. They contain uncommon thin, wave-rippled, fine sandstone layers which record rare high-energy event and are attributed to winter storm action. The shales coarsen upward and the sandstone horizons become more common toward the top of the sequence, reflecting increasing proximality up the sequence from an outer to a mid-shelf setting. In places the shales contain large ferroan carbonate concretions. These have been analyzed together with the shales to determine the diagenetic and dewatering history of the sequence. Comparison of the chemistry of the noncarbonate fraction within the concretions with that of the host shales has led to quantitative mass-transfer models for the system. These suggest the sequence was an important source of cementing components (e.g., Ca, Si) for adjacent potential reservoir rocks. Other elements (e.g., Al, Mg, K) have not moved out of the system; the shales were not therefore a source for authigenic aluminosilicates as is often supposed. The major transport/dewatering pathways were the interbedded storm-sandstone horizons. These are now extensively quartz cemented. Local synsedimentary faults were probably also important conduits. These results have important implications regarding the relationship between shale and sandstone diagenesis. The shales presentlymore » contain up to 1% TOC. In the study area, however, they are thermally postmature with respect to hydrocarbon generation. Equivalent rocks which have experienced less burial should be considered potential source rocks.« less