Maximum Burial Temperatures Research Articles

Geological Controls on Natural Gas Production in the Antrim Shale of the Michigan Basin: ABSTRACT

The Antrim Shale (Devonian-Mississippian) is a prolific natural gas reservoir with distinctive source rock and reservoir characteristics. The Antrim is composed of two distinctive lithofacies: a black shale lithofacies having total organic carbon (TOC) values as high as 12% and a gray-green shale lithofacies having TOC values generally below 0.5 %. The black shale facies is the primary source bed and reservoir for natural gas. Based on Rock-Eval data the kerogen in the Antrim Shale is only marginally mature with respect to generation of liquid hydrocarbons. This in the range of 435{degree}C, vitrinite reflectance values are 0.41, and thermal alteration index (TAI) is 2. These data indicate a maximum burial temperature of approximately 60{degree}C in Otsego County, Michigan, the main play area. Although the thermal maturation level in this area is very low, the gas produced from the Antrim is internally sourced and thermogenic; this interpretation is based on the presence of trace amounts of high-gravity oil and a thermogenic isotopic signature of produced gas samples. The black shale facies in the Antrim contains significant authigenic silica and is more susceptible to fracturing compared to the carbonate-cemented gray-green shales facies. Extremely low gas permeability values (0.1 md) indicates that fracturesmore » serve as conduits for delivery of gas to the borehole. Most of the gas must be stored in the shale matrix porosity because total gas volume produced far exceeds fracture reservoir volume.« less

Coalification in the northern Wildhorn nappe and adjacent units, western Switzerland. Implications for tectonic burial histories

Vitrinite reflectance of fourteen Eocene coal samples from the northern Wildhorn nappe in the Kander valley have been analyzed and compared to nine agewise similar coals from adjacent tectonic units. The degree of coalification decreases regularly from semi-anthracite in the Kander valley to lignite in the plateau molasse. The degree of coalification in the Tertiary coals appears to be solely dependent on their tectonic history and indicates and estimated 6 km of burial by higher (Pennine) nappes for the Kander valley. The rank at this locality (low-volatile bituminous to semi-anthracite) is significantly higher than in the Border chain at Beatenberg (high-volatile A bituminous). The latter locality therefore was never buried deeper than about 4 km. A further decrease in rank is found in the subalpine flysch and molasse (high-volatile B bituminous) which in turn is significantly more mature than lignites from the folded plateau molasse at Linden. Two samples of Dogger coal from the Pennine “Préalpes médianes” are medium- to low-volatile bituminous that is intermediate between the Kander valley and Beatenberg. Their rank, however, was achieved before their final emplacement north of the Wildhorn nappe. By comparing calculated TTI (Time-Temperature-Indices)-values for time-depth-burial curves with measured vitrinite reflectances, maximum burial temperatures were estimated for the various tectonic units. For the Wildhorn nappe, 160–180°C were obtained in the Kander valley and 100–130°C in the Border chain as a result of tectonic burial due to the thrusting by the Pennine Prealps. Sedimentary burial led to temperatures of 100–120°C in the subalpine molasse below the Border chain prior to its thrusting.

Thermal History of Alberta Deep Basin: Comparative Study of Fluid Inclusion and Vitrinite Reflectance Data

The thermal history of the Alberta Deep Basin, the gas-saturated westernmost part of the Western Canadian sedimentary basin, has been studied by analyzing aqueous and hydrocarbon fluid inclusions in diagenetic cements and comparing the results with coal maturity data. Results indicate that most of the diagenetic quartz druse precipitated from low-salinity fluids (2-3 wt. %) at a temperature of 170°-195°C. Calcite cement precipitated after quartz druse during uplift and cooling at temperatures of 108°-169°C. Inclusions rich in larger hydrocarbon molecules occur in cements that precipitated at 150°C, whereas methane-rich inclusions occur in cements that precipitated at 165°C or at lower temperatures after significant uplift and cooling of the b sin. Maximum burial temperatures (Tmb), calculated from vitrinite reflectance data using the Lopatin-Waples TTI-Ro calibration, are significantly lower (150°C) than the temperature at which quartz druse precipitated in the same area (190°C). An equation directly relating Tmb and Ro, proposed by Barker and Pawlewicz in 1986, yields maximum burial temperatures (200°-210°C) only slightly higher than the fluid temperatures. The results from this study suggest that hot fluids migrated updip along permeable conglomerates and bedding-plane fractures to produce unexpectedly high geothermal gradients (~38°C/km) in the western part of the study area. The apparent restriction of hot fluids to the western part of the study area sugge ts that convective heat transfer by fluids may have operated on a scale of only tens of kilometers in Lower Cretaceous rocks of the Alberta Deep Basin.

Diagenesis of the Upper Proterozoic Siliciclastic Sediments of the Taoudeni Basin (West Africa) and Relation to Diabase Emplacement

ABSTRACT The predominantly siliciclastic Upper Proterozoic sediments of the Taoudeni basin have been buried to depths of 2.5 to 3 km. In a normal geothermal gradient 25°C/km), maximum burial temperatures of the section studied would be no higher than 100°C. However, detailed petrographic and fluid inclusion data of sandstones and shales from three areas in Mauritania and Mall along the western side of the basin indicate two major diagenetic events: an earlier low temperature (< 70°C) quartz cementation of the sandstones and a later high temperature (135-170°C diagenesis that affected both shales and sandstones. These latter temperatures are too high to have occurred simply in response to burial. The sediments are cut by numerous Jurassic diabase intrusions, which crop out es ecially commonly in the south and the northeast. The emplacement of these diabases was related to the opening of the South Atlantic ocean. Contact metamorphism and hydrothermal alteration are visible near intrusive contacts. In western Mall (southern part of basin) where the largest exposed sill in the basin occurs, at 50 to 60 km from any outcropping massive diabase, the late diagenetic paragenesis consists of micas, illite, chlorite, kaolinite, siderite and ankerite. High-temperature diagenesis is also observed in Adrar of Mauritania (northwestern part of basin) where outcropping diabases are extremely rare. There, calcite cement is associated with illite and chlorite. Fluid inclusions indicate formation temperatures of 135°C for the ankerite and 170°C for the calcite. Thus, in spite of a thin sedimentary pile, the Taoudeni basin experienced a high-temperature diagenesis. We propose that this resulted from circulation of hot fluids, generated during the diabase emplacement, over extensive areas within the basin.

Subnormal geothermal gradients in the Great Valley Forearc Basin, California, during Franciscan Subduction: A fission track study

Outboard parts of forearcs commonly have subnormal geothermal gradients because relatively cold lithosphere subducts beneath them. The strata of the 10‐ to 15‐km‐thick Late Jurassic‐Tertiary Great Valley forearc basin accumulated in the forearc of the Franciscan subduction zone of coastal California. In this study, paleogeothermal gradients in this basin were reconstructed to constrain the tectonic history of the convergent plate margin.Backstripping was used to reconstruct the burial histories of outcropping basin strata, and apatite fission track analysis was used to determine the maximum burial temperatures experienced by 64 rock samples. Plots of maximum burial temperatures versus maximum burial depths indicate earliest Tertiary (time of deepest burial) forearc basin geothermal gradients of about 9°C/km. Backstripping shows that strata from the lower parts of two sections were deeply buried (&gt;6.5 km) for the period 90–65 Ma without experiencing high temperatures (≥105°C), indicating that gradients were at least moderately subnormal throughout that period. The data permit gradients no greater than about 15°C/km at 90 Ma, decreasing to no greater than about 9°C/km at 65 Ma. Lower strata in another section were probably deeply buried without experiencing high temperatures from 130 or 120 Ma until the latest Cretaceous or early Tertiary, indicating that subnormal gradients probably existed throughout that period. K‐Ar ages of Franciscan Complex blueschists indicate subnormal gradients in the California forearc in the Late Jurassic and Early Cretaceous, fission track data from the Sierra Nevada indicate subnormal gradients in the early and middle Cenozoic, and modern heat flow patterns indicate subnormal gradients in the late Cenozoic. Together, these indicators suggest that thermal gradients in the forearc were continuously subnormal from the initiation of Franciscan subduction in the Late Jurassic until the termination of subduction in the late Cenozoic.Gradients in the range that would permit blueschist facies metamorphism in the Franciscan Complex were thus long‐lived. The broad range of K‐Ar ages noted for Franciscan blueschists need not reflect multiple, discrete metamorphic events, but are also consistent with a continuous, extended period of high P/T conditions. Blueschists need not have been uplifted soon after metamorphism to avoid overprinting by normal P/T conditions, but rather may have remained at depth for extended periods.Preliminary modeling indicates that an interruption in subduction of longer than 5–20 m.y. should cause detectable heating of forearc basin strata, while a much shorter interruption should heat accretionary wedge rocks. Since there is no evidence of such heating, it appears that convergence at the Franciscan trench was essentially continuous. If allochthonous terranes traveled northward parallel to the California margin during the Late Jurassic to late Cenozoic period of Franciscan subduction, they apparently did so during oblique subduction, not during extended interludes of pure transform motion.

Comparison of Clay and Zeolite Mineral Occurrences in Neogene Age Sediments From Several Deep Wells

AbstractClay and zeolite mineral assemblages were determined for five deep wells in volcano-clastic sediments (Japan) and for one well in mudstones (California). The clay mineral suites in these wells showed a gradual change with depth of illite/smectite (I/S) composition, which increases in smectite content in the upper portion towards a fully expandable mineral (2–3-km depth) and then decreases in smectite content with depth (2–5 km). The temperature of transformation or recrystallization to a fully expandable smectite mineral is about 60°–70°C in non-zeolite bearing rocks and 70°–90°C in zeolite-bearing rocks, with no apparent dependence on time. Comparison is made between the I/S smectite content in the lower part of the wells (i.e., below the occurrence of the fully expandable mineral) and the zeolite mineral zone boundaries. The 60% smectite composition was found at 108°–118°C maximum burial temperatures. The zeolite II/III zone boundary, i.e., the onset of the analcime zone, occurs between 85° and 95°C and may be slightly time-related in the span of 1–15 Ma. The clay and zeolite minerals can be used as temperature indicators in the range of the Neogene age.

Oxygen Isotope Studies of Diagenesis and Pore-Water Evolution in the Western Canada Sedimentary Basin: Evidence From the Upper Cretaceous Basal Belly River Sandstone, Alberta

ABSTRACT The oxygen isotope compositions and paragenetic sequence of diagenetic minerals from the Upper Cretaceous basal Belly River sandstone, southwestern Alberta, have been used to identify changes in pore-water composition during diagenesis. These changes can be related to major geologic events within the western Canada sedimentary basin. Low-18O meteoric water has played an important role in the diagenesis of these Upper Cretaceous rocks on a basinwide scale during deposition and early diagenesis and following uplift of the basin in early Eocene time. Diagenetic phases include early chlorite (18O = +6.1 to +8.3) and calcite (18OSMOW = +21.0; 13CPDB = -1.6), followed by quartz overgrowths (18O = + 13. 1 to + 18.0), calcite (18OSMOW = + 10.4 to + 14.4; 13CPDB = - 10.6 to - 1.2), and then kaolinite (18O = +7.8 to + 11.9), illite/smectite (18O = +10.9 to +13.1) and smectite (18O = +12.6 to +13.7). The interpretation of these results is that shallow diagenesis, early in the burial history (chlorite, calcite) occurred in the presence of brackish or fresh water. With burial, the 18O value of the pore water increased due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift (Laramide Orogeny). Recharge of the basin by low-18O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz and calcite at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolinite, illite/smectite, and smectite as post-Eocene cooling and erosion progressed.

Oxygen-isotope studies of clastic diagenesis in the Lower Cretaceous Viking Formation, Alberta: implications for the role of meteoric water

Summary The oxygen-isotope compositions and paragenetic sequence of diagenetic minerals from the Lower Cretaceous Viking sandstone and conglomerate from south-central Alberta, Canada have been used to identify changes in porewater composition during diagenesis and to relate these changes to major geological events within the western Canada sedimentary basin. Large-scale influx of meteoric water has played an important role in diagenesis of this unit, especially following uplift of the basin in early Eocene time. Diagenetic phases include early kaolinite ( δ 18 O SMOW = + 24.9 to +28.2 ‰), siderite ( δ 18 O SMOW = + 22.0 to + 23.9 ‰; δ 13 C PDB = −6.3 to −1.3 ‰), calcite ( δ 18 O SMOW = + 26.0 ‰; δ 13 C PDB = −0.4 ‰) and chlorite ( δ 18 O SMOW = + 10.7 − + 13.4 ‰), followed in order by dolomite ( δ 18 O SMOW = + 19.3 − + 22.1 ‰; δ 13 C PDB = −5.9 to −2.6 ‰), calcite ( δ 18 O SMOW = + 13.9 − + 15.9 ‰; δ 13 C PDB = −8.4 to −6.1 ‰), ankerite ( δ 18 O SMOW = + 16.3 − + 17.9 ‰; δ 13 C PDB = −12.4 to −3.5 ‰), kaolin group minerals ( δ 18 O SMOW = + 12.0 − + 15.7 ‰), and illite ( δ 18 O SMOW = + 14.2 − + 15.7 ‰) and illite/smectite ( δ 18 O SMOW = + 13.8 − + 16.5 ‰). Quartz overgrowths ( δ 18 O SMOW = + 16.2 − + 27.4 ‰) began crystallizing relatively early during burial diagenesis and continued to form at least up to the onset of late diagenetic formation of clay minerals. The interpretation of these results is that shallow diagenesis, early in the burial history (glauconite, pyrite, calcite, chlorite), occurred largely in the presence of seawater-derived fluids, although a freshwater influence is indicated where siderite and(or) early kaolinite cements are abundant. As compaction and burial diagenesis proceeded (diagenetic chlorite, quartz overgrowths, dolomite), the porewaters became enriched in 18 O due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift related to the major Laramide Orogeny. Recharge of the basin by low 18 O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz, calcite and ankerite (and the dissolution and albitization of feldspar) at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolin group minerals, illite and illite/smectite as the post-Eocene erosion progressed.

Marine and Nonmarine Gas-Bearing Rocks in Upper Cretaceous Blackhawk and Neslen Formations, Eastern Uinta Basin, Utah: Sedimentology, Diagenesis, and Source Rock Potential

The Upper Cretaceous Blackhawk and Neslen Formations in the eastern Uinta basin contain large amounts of thermogenic gas that was generated from interbedded humic-rich source rocks. The geometry and distribution of hydrocarbon source and reservoir rocks are controlled by depositional environment. The Blackhawk, composed of laterally extensive sandstone and locally interbedded carbonaceous siltstone and minor coal, reflects deposition in nearshore marine and backshore environments. The Neslen contains organic-rich siltstone and mudstone with lesser amounts of carbonaceous shale, coal, and lenticular sandstone that formed in coastal and lower alluvial-plain depositional settings. Potential reservoir sandstones are composed dominantly of monocrystalline quartz grains and sedimentary lithic fragments. Mechanical compaction during early burial was followed by the precipitation of quartz, carbonate, and barite later in the burial history. Variations in porosity and permeability (2-10%; < 1 md) reflects the presence of authigenic clay, mineral cements, and dissolved lithic grains. Natural fractures, cemented with carbonate, barite, and kaolinite, occur locally. Active hydrocarbon generation occurred in the Neslen and Blackhawk during the Oligocene and Miocene when these units were near their maximum burial depth and temperature. The rate of hydrocarbon generation decreased from the late Miocene to the present, owing to widespread cooling that occurred in response to regional uplift and erosion associated with the development of the Colorado Plateau. Temporally equivalent rocks in other areas of the basin may have experienced similar diagenetic and hydrocarbon generation histories.

Comment and Reply on “Distribution of maximum burial temperatures across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Comment and Reply on “Distribution of maximum burial temperatures across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Comment and Reply on “Distribution of maximum burial temperature across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Comment and Reply on “Distribution of maximum burial temperature across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Comment and Reply on “Distribution of maximum burial temperature across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Comment and Reply on “Distribution of maximum burial temperature across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Experimental stretching of fluid inclusions in calcite—Implications for diagenetic studies

Homogenization temperatures of fluid inclusions trapped in calcite crystals may change with increased heating beyond the homogenization temperature. Fluid inclusions trapped in calcite during basin subsidence will tend to yield homogenization temperatures approaching maximum burial temperatures. Diagenetic studies that depend on these data will be biased. Coarse crystalline calcite containing saline fluid inclusions was used to examine the influence of increased temperatures and pressures on measured homogenization temperatures. Stepped heating experiments were conducted at 1 atm, over a temperature range of 90 to 245 °C, along with hydrothermal experiments at confining pressures of 173 and 691 bar and temperatures of 160, 250, and 350 °C. In all cases, significant increases in the homogenization temperatures of the inclusion population were recorded. Presumably, stresses generated during heating of the fluid inclusions exceeded the combined strength of the calcite crystal and the external pore pressure and thus caused fluid-inclusion stretching. Size is a secondary factor that controls fluid-inclusion reequilibration.

Comment and Reply on “Distribution of maximum burial temperatures across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Comment and Reply on “Distribution of maximum burial temperatures across northern Appalachian Basin and implications for Carboniferous sedimentation patterns”

Distribution of maximum burial temperatures across northern Appalachian Basin and implications for Carboniferous sedimentation patterns

Clay-mineral diagenesis and apatite fission-track age data indicate that the maximum burial temperatures to which the Middle Devonian Tioga metabentonite was exposed rise abruptly from low values in western New York State to higher values in the east. The highest temperatures, which approach 175/sup 0/C, were reached just west of Syracuse. Neither the pattern nor the magnitude of burial temperatures can be explained solely by burial of the metabentonite beneath Upper Devonian sediments. Although spatial variations in the geothermal gradient could have produced the observed pattern of burial temperatures, it is more likely that Carboniferous sediments, no longer preserved in the area, were responsible for the indicated burial. The inferred presence of thick Carboniferous sequences in western New York State suggests that the Allegheny orogeny had a stronger influence on sedimentation in the northern Appalachian Basin than has been previously recognized. 25 references, 2 figures, 2 tables.

Oxygen and hydrogen isotope geochemistry of Cretaceous bentonites and shales from the Disturbed Belt, Montana

The mineralogy, δO 18, and δD of the <0.1 μm fraction of 22 Cretaceous bentonites and the mineralogy and δO 18 of the < 0.1 μm fraction of 14 adjacent shales collected from outcrops in the Sweetgrass Arch and Disturbed Belt, Montana, have been determined. Mixed-layer illite/smectite (I/S) is the dominant mineral in the bulk bentonite and usually the only mineral in the < 0.1 μm fraction. I/S is also the major clay mineral in the shales. The diagenetic grade in bentonite is qualitatively given by the percentage of illite layers in I/S, which varies from 2 to 25 (Sweetgrass Arch) to as high as 95 (Disturbed Belt). δO 18 of < 0.1 μm bentonite generally decreases from about +20%. to about +13%. with increasing diagenetic grade. On a plot of δD versus δO 18, data for the < 0.1 μm bentonite define a field that generally parallels, but falls on the meteoric water line side of the smectite-water line (Savin and Epstein, 1970). δO 18 of bulk bentonite is 1 to 3%. more negative than the δO 18 of the < 0.1 μm fraction, due to the presence of volcanic quartz and feldspar. δO 18 of several size fractions of clay-sized quartz separated from the bentonite varies from +11%. to +24%., and, in a given bentonite, generally increases with decreasing grain size. Among the different bentonites, the δO 18 range of the different grain sizes decreases as the percentage of illite layers in the coexisting I/S increases. The δO 18 of 0.1–0.5 μm shale quartz is generally 1 to 4%. more positive than clay-sized quartz from an adjacent bentonite, and the δO 18 of < 0.1 μm I/S concentrate of shales is generally < 1 to 4%. more negative than the < 0.1 μm I/S from an adjacent bentonite. Isotopic temperatures, interpreted to be maximum burial temperatures, range between about 160°C (shale), to about 250°C (bentonite). The isotopic data can be interpreted using the stages: 1. 1) deposition of volcanic glassy ash containing some quartz and feldspar; 2. 2) devitrification into mostly 100% expandable smectite with a δO 18 of about 20%. and δD of −60 to −80%.; 3. 3) tectonic burial beneath thrust sheets in the Disturbed Belt region which induced elevated temperatures that allowed oxygen and hydrogen isotope reequilibration as the percentage of illite layers in I/S increased and diagenetic quartz formed; and 4. 4) exhumation of the strata with preservation of O 18 O 16 and D/H attained during burial.

Shallow Oil in Northern Appalachian Basin: Evidence for Early Cretaceous Uplift from Fission Track Analysis: ABSTRACT

Apatite fission track data from Devonian and Carboniferous sandstones of eastern New York and Pennsylvania (e.g., Catskills, Anthracite belt) imply rapid cooling, presumably associated with significant uplift, in the Early Cretaceous (about 135 Ma). Maximum burial temperatures are unknown; however, temperatures immediately prior to uplift were greater than 130/sup 0/C, and limited zircon data suggest temperatures did not exceed 200/sup 0/C. In western New York and Pennsylvania, the data are similarly consistent with abrupt Early Cretaceous cooling, but from maximum temperatures not exceeding 105/sup 0/C. New apatite data from subsurface samples from western New York allow us to calculate a minimum geothermal gradient of approximately 22/sup 0/C/km, before the Early Cretaceous uplift. This calculation is compatible with shallow (200-600 m) Upper Devonian oil fields of the area, which have temperatures between 110/sup 0/ and 120/sup 0/C in the Cretaceous, the Oriskany gas field at 140/sup 0/C, Medina gas field at 150/sup 0/C, and Trenton gas field at 170/sup 0/C. These temperatures are compatible with oil- and gas-generation temperatures in other basins, and they provide a check for analysis. This minimum thermal gradient further implies that about 4 km of section must have been eroded from the region of these oilmore » fields. Thus, the fission track data suggest that oil fields between 200 and 600 m deep in the Upper Devonian sequences of southwestern New York and northeastern Pennsylvania contain fossil oil, which was generated prior to rapid cooling associated with significant regional uplift of the northern Appalachian basin during the Early Cretaceous.« less

Diagenetic chlorite formation in some Mesozoic shales from the Sleipner area of the North Sea

AbstractDiagenetic chlorite is forming as a result of temperature-controlled burial diagenesis in shales from the Sleipner area of the North Sea. Accompanying chlorite diagenesis, kaolinite and illite-smectite decrease in abundance, and illite increases in abundance. These clay mineral transformations occur between 122–126°C at temperatures higher than normally expected for chlorite diagenesis. Kaolinite and ordered illite-smectite are largely unaffected by diagenesis below 100°C. It is proposed that chlorite diagenesis is thus delayed due to the absence of a source of ions resulting from smectite decomposition. Clay mineralogy is of no lithostratigraphic use in the Jurassic sediments of the Sleipner area. However, the zone of chlorite diagenesis is a reliable indicator of maximum burial temperature.

The Moeraki Boulders--Anatomy of Some Septarian Concretions

ABSTRACT The Moeraki boulders are large (to 2 m) calcite concretions with septarian veins of calcite and rare late-stage quartz and ferrous dolomite. The concretions are enclosed by Paleocene marine mudstones with vitrinite reflectance values of 0.29% R suggesting maximum burial temperatures of 25-30°C. Microprobe analyses of these concretions indicate a Ca enrichment-Mg depletion trend with growth. Calcite in the core contains about 88 mole % Ca and about 93 mole % Ca in the rim. Both rim and cores contain less than 1 mole % Fe and Mn. Septarian veins also show a systematic calcium enrichment with growth of the vein. The initial vein fills are similar in composition to the outer rim of concretionary bodies, suggesting septarian cracks form contemporaneously with attainment of final concretion size. The later-stage calcite vein-fills have less than 1.5 mole % Mg and up to 11 mole % Fe + Mn. These elements systematically decrease both into and outward along the vein, with increasing calcium substitution. The above compositional trends in veins suggest continued crystallization during propagation of the fractures. The carbonate composition trends reflect interaction between the growing concretion and the enclosing mudstone pore-fluid system. The observed Fe, Mn, Mg depletion trends probably reflect depletion of elements released by short-time-scale diagenetic events of finite size. The growth time of the larger concretions is estimated at about 4 million years based on published diffusion growth models. Extrapolation of compositional trends versus growth time from these concretion bodies suggests that septarian veins form on a time scale of several million years.

Oxygen Isotopic Studies of Diagenetic Clay Minerals: Implications for Geothermometry, Diagenetic Reaction Mechanisms, and Fluid Migration: ABSTRACT

Oxygen isotope ratios of diagenetically formed minerals in shales and sandstones can provide information about temperatures of diagenesis, mechanisms of clay mineral reactions, degree of openness of the rocks to the movement of water during diagenesis, and sources of water involved in reactions. The isotopic approach has proven especially effective in studying the shales of the United States Gulf Coast in which the predominant diagenetic reaction involving clays is the conversion of smectite layers to illite layers in mixed-layer illite/smectite. Clay minerals affected by this reaction appear to undergo oxygen isotope equilibration with the ambient water. A byproduct of the smectite-illite transition is quartz. When the diagenetically formed quartz can be isolated for isotopic analysis, oxygen isotope fractionations between coexisting quartz and clay are indicative of diagenetic temperatures when temperatures are higher than 70 or 80°C. There is some evidence that at temperatures above 180°C clay-size detrital quartz may exchange isotopically with pore fluids and ot er rock constituents, perhaps permitting the determination of maximum burial temperatures of shales even in the absence of the smectite-illite conversion reaction. During diagenesis the shales studied approximate closed systems, the isotopic composition of the diagenetic waters being determined largely by isotopic exchange with the rocks. Water lost form the shale sequences during diagenesis apparently moves outward or upward along cracks, passing out of the system without isotopically affecting overlying shales. Diagenetic minerals in sandstones, like those of shales, reflect temperatures and isotopic compositions of pore waters. While few isotopic studies of minerals from sandstones have been done to date, those by Land and his coworkers indicate that this is a very promising approach for unraveling and understanding complex histories of diagenetically altered sandstones. End_of_Article - Last_Page 1447------------