Charles E. Barker

Fluid-inclusion technique for determining maximum temperature in calcite and its comparison to the vitrinite reflectance geothermometer

Theory, laboratory experiments, and empirical observation suggest that many aqueous fluid inclusions in calcite reequilibrate during overheating, and therefore some homogenization temperatures ( T h ) record a temperature close to the maximum reached by the rock. This characteristic suggests that aqueous fluid inclusions in calcite can be used to establish maximum temperature ( T peak ). To test this hypothesis, we have compiled fluid inclusion T peak , mean random vitrinite reflectance ( R m ), and present-day T peak from 46 diverse geologic systems that have been at T peak from 10 4 to 10 6 yr. Present T peak ranged from 65 to 345 °C, T h modes and means ranged from 59 to 350 °C, and R m data ranged from 0.4% to 4.6%, spanning the temperature and thermal maturity range associated with burial diagenesis, hydrothermal alteration, and low-grade metamorphism. Plots of T h and T peak data for systems thought to be currently at maximum temperature demonstrate close agreement between T h and present T peak in sedimentary basins. Although caution should be applied, the relation suggests that T h of aqueous fluid inclusions in calcite may be a useful measure of maximum temperature. This study also compares T h to mean random vitrinite reflectance ( R m ) to offer further support for the use of T h as a measure of T peak , and to provide a better understanding of R m . T h correlates well with R m and results in a curve similar to R m vs. T peak calibrations determined by other workers. The strong correlation (correlation coefficient r = 0.93) between T peak and R m in these systems suggests that maximum temperature is the major control on thermal maturation.

Fluid inclusion and vitrinite-reflectance geothermometry compared to heat-flow models of maximum paleotemperature next to dikes, western onshore Gippsland Basin, Australia

Abstract Nine basalt dikes, ranging from 6 cm to 40 m thick, intruding the Upper Jurassic–Lower Cretaceous Strzelecki Group, western onshore Gippsland Basin, were used to study maximum temperatures ( T max ) reached next to dikes. T max was estimated from fluid inclusion and vitrinite-reflectance geothermometry and compared to temperatures calculated using heat-flow models of contact metamorphism. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a temperature of 100–135°C. Fracture-bound fluid inclusions in the host rocks next to thin dikes ( T max systematically increases towards the dike margin to at least 500°C. The estimated T max next to the thickest dike (thickness ( D )=40 m) suggests an extended zone of elevated R v-r to at least a distance from the dike contact ( X ) of 60 m or at X / D >1.5, using a normalized distance ratio used for comparing measurements between dikes regardless of their thickness. In contrast, the pattern seen next to the thin dikes is a relatively narrow zone of elevated R v-r . Heat-flow modeling, along with whole rock elemental and isotopic data, suggests that the extended zone of elevated R v-r is caused by a convection cell with local recharge of the hydrothermal fluids. The narrow zone of elevated R v-r found next to thin dikes is attributed to the rise of the less dense, heated fluids at the dike contact causing a flow of cooler groundwater towards the dike and thereby limiting its heating effects. The lack of extended heating effects suggests that next to thin dikes an incipient convection system may form in which the heated fluid starts to travel upward along the dike but cooling occurs before a complete convection cell can form. Close to the dike contact at X / D R v-r often decreases even though fluid inclusion evidence indicates that T max is still increasing. Further, fluid inclusion evidence indicates that the evolution of water vapor or supercritical fluids in the rock pores corresponds to the zone where R v-r begins to decrease. The generation of the water vapor or supercritical fluids near the dike contact seems to change vitrinite evolution reactions. These metamorphic conditions, closer to the dike than X / D =0.3 make vitrinite-reflectance unreliable as a geothermometer. The form of the R v-r profile, as it indicates T max , can be interpreted using temperature profiles estimated from various heat-flow models to infer whether the dike cooled by conduction, incipient convection, or a convection cell. A contact aureole that consists of decreasing R v-r or T max extending out to X / D ≥2 and that has a T contact ≫( T magma + T host )/2 appears to be a signature of simple conductive cooling. Incipient convection is indicated by a R v-r profile that decreases to background levels at X / D R v-r profile and consistently high R v-r that may not decrease to background levels until beyond distances of X / D >1.5.

Influence of time on metamorphism of sedimentary organic matter in liquid-dominated geothermal systems, western North America.

Temperature ( T in °C) and mean vitrinite reflectance ( R m in %) of sedimentary organic matter samples from six liquid-dominated geothermal systems plot about a line approximated by a regression equation of the form R m = 0.435 exp 0.00683 T . The reflectance data from these systems are strongly temperature-dependent, with a coefficient of determination ( r 2 ) of 0.8. Geologic evidence indicates that reaction duration ranges from 10 3 or 10 4 to 10 6 yr in these systems that appear to have near-maximum temperatures. The uncertainty in vitrinite reflectance and temperature determinations must account for a large part of the remaining variability not explained by the regression equation; therefore, the strong temperature dependence of R m indicates that after about 10 4 yr, reaction duration has little or no influence on metamorphism of organic matter in liquid-dominated geothermal systems. These data indicate that vitrinite reflectance can be used to determine the maximum temperature reached in hot sedimentary basins of moderate longevity.

Implications for Organic Maturation Studies of Evidence for a Geologically Rapid Increase and Stabilization of Vitrinite Reflectance at Peak Temperature: Cerro Prieto Geothermal System, Mexico (1)

A short-term rapid heating and cooling of the rock in well M-94 below 1300 m was caused by a pulse of hot water passing through the edge of the Cerro Prieto, Mexico, geothermal system. Below 1300 m, the peak paleotemperatures were about 225-250 degrees C, but equilibrium well log temperatures indicate a decrease to 150-210 degrees C at present. This hot water pulse sharply increased vitrinite reflectance to levels comparable to those measured in the central part of the system, even though studies of apatite fission-track annealing indicate that the duration of heating was only 10{0}-10{1} yr in M-94, in contrast to 10{3}-10{4} yr in the central part of the system. These data indicate that sedimentary organic matter chemically reacts quickly to temperature increases of about 125 degrees C above ambient, even when the higher temperature existed for only 10{0}-10{1} yr. The quick change of the vitrinite reflectance geothermometer indicates that thermal maturation reactions can stabilize, after a geologically short period of heating, to a level consistent with peak temperature under moderate to high-temperature diagenesis in open, fluid-rich, geothermal systems. Cerro Prieto is one of the most intensively studied and well-known geothermal systems in the world. Thus, data from Cerro Prieto are a benchmark to compare with the predictions of published thermal maturation models such as those formulated by J. Karweil, N. V. Lopatin, and A. K. Burnham and J. J. Sweeney. These thermal maturation models inaccurately predict duration of heating at Cerro Prieto. The kinetic equations used in these models explicitly allow thermal maturation to continue indefinitely at peak temperature, which does not seem to be the case at Cerro Prieto.

An empirical determination of the minimum number of measurements needed to estimate the mean random vitrinite reflectance of disseminated organic matter

Abstract In coal samples, published recommendations based on statistical methods suggest 100 measurements are needed to estimate the mean random vitrinite reflectance ( R v−r ) to within ±2%. Our survey of published thermal maturation studies indicates that those using dispersed organic matter (DOM) mostly have an objective of acquiring 50 reflectance measurements. This smaller objective size in DOM versus that for coal samples poses a statistical contradiction because the standard deviations of DOM reflectance distributions are typically larger indicating a greater sample size is needed to accurately estimate R v−r in DOM. However, in studies of thermal maturation using DOM, even 50 measurements can be an unrealistic requirement given the small amount of vitrinite often found in such samples. Furthermore, there is generally a reduced need for assuring precision like that needed for coal applications. Therefore, a key question in thermal maturation studies using DOM is how many measurements of R v−r are needed to adequately estimate the mean. Our empirical approach to this problem is to compute the reflectance distribution statistics: mean, standard deviation, skewness, and kurtosis in increments of 10 measurements. This study compares these intermediate computations of R v−r statistics with a final one computed using all measurements for that sample. Vitrinite reflectance was measured on mudstone and sandstone samples taken from borehole M-25 in the Cerro Prieto, Mexico geothermal system which was selected because the rocks have a wide range of thermal maturation and a comparable humic DOM with depth. The results of this study suggest that after only 20–30 measurements the mean R v−r is generally known to within 5% and always to within 12% of the mean R v−r calculated using all of the measured particles. Thus, even in the worst case, the precision after measuring only 20–30 particles is in good agreement with the general precision of one decimal place recommended for mean R v−r measurements on DOM. The coefficient of variation ( V = standard deviation/mean) is proposed as a statistic to indicate the reliability of the mean R v−r estimates made at n ⪡ 20. This preliminary study suggests a V V > 0.2 suggests an unreliable mean in such small samples.

Upper Cretaceous Ferron Sandstone: Major Coalbed Methane Play in Central Utah

Recent drilling for coalbed gas in the Upper Cretaceous Ferron Sandstone Member of central Utah has resulted in one of the most successful plays of this kind. Exploration to date has resulted in three fields and a potential fairway 6-10 mi (10-16 km) wide and 20-60 mi (32-96 km) long, corresponding to shallow coal occurrence at depths of about 1800-3500 ft (545-1060 m) in the Ferron, a sequence of interbedded fluvial-deltaic sandstone, shale, and coal in the lower part of the Cretaceous Mancos Shale. Coalbed methane (CBM) reservoirs in this interval consist of thin to moderately thick (3-10 ft [1-3 m]) coal beds of relatively low rank (high-volatile B bituminous) and variable gas content, ranging from 100 scf/ton or less in the south to as high as 500-600 scf/ton in the north. Productive wells have averaged more than 500 mcf/day and, after several years, continue to typically show negative production declines. In the major productive area, Drunkards Wash unit, the first 33 producers averaged 974 mcf and 85 bbl of water per day after five years of continuous production. Estimated ultimate recoverable reserves for individual wells in this unit range from 1.5 to 4 bcf. Based on several criteria, including gas content, thermal maturity, and chronostratigraphy, the play is divided into northern and southern parts. The northern part is characterized by coals that have the following characteristics: (1) high gas contents; (2) moderate thermal maturity (e.g., vitrinite reflectance [Ro] values of 0.6-0.8%); (3) good permeabilities (5-20 md); (4) lack of exposure; and (5) overpressuring, due to artesian conditions. Southern coals have much lower average gas contents (<100 scf/ton) and lower thermal maturity (Ro = 0.4-0.6%), and they are exposed along an extensive, 35 mi (56 km) outcrop belt that may have allowed a degree of flushing. These coals, however, are also thicker and more extensive than those to the north and thus may retain significant (Begin page 200) potential. Northern coals appear to contain a mixture of gas from three sources: in-situ thermogenic methane, migrated thermogenic methane from more mature sources, and late-stage biogenic gas. Current development is focused on the northern part of the stated fairway, where well control and an existing infrastructure are present. Indications are that CBM exploration in the Ferron will expand considerably in the near future.

Coalbed methane, Cook Inlet, south-central Alaska: A potential giant gas resource

Cook Inlet Basin of south-central Alaska is a forearc basin containing voluminous Tertiary coal deposits with sufficient methane content to suggest a major coalbed gas resource. Coals ranging in thickness from 2 to 50 ft (0.6 to 15 m) and in gas content from 50 to 250 scf/ton (1.6 to 7.8 cm2/g) occur in Miocene-Oligocene fluvial deposits of the Kenai Group. These coals have been identified as the probable source of more than 8 tcf gas that has been produced from conventional sandstone reservoirs in the basin. Cook Inlet coals can be divided into two main groups: (1) those of bituminous rank in the Tyonek Formation that contain mainly thermogenic methane and are confined to the northeastern part of the basin (Matanuska Valley) and to deep levels elsewhere; and (2) subbituminous coals at shallow depths (<5000 ft [1524 m]) in the Tyonek and overlying Beluga formations, which contain mainly biogenic methane and cover most of the central and southern basin. Based on core and corrected cuttings-desorption analyses, gas contents average 230 scf/ton (7.2 cm2/g) for bituminous coals and 80 scf/ton (2.5 cm2/g) for subbituminous coals. Isotherms constructed for samples of both coal ranks suggest that bituminous coals are saturated with respect to methane, whereas subbituminous coals at shallow depths along the eroded west-central basin margin are locally unsaturated. A preliminary estimate of 140 tcf gas in place is derived for the basin.

A comparison of vitrinite reflectance measurements made on whole-rock and dispersed organic matter concentrate mounts

Abstract A comparison of mean-random vitrinite reflectance ( R v-r ) values calculated from measurements on polished whole-rock and on concentrates of dispersed organic matter (DOM) mounts of the same samples generally show little difference. Compared to measurements on DOM mounts, R v-r measurements by the same microscopist on corresponding whole-rock samples are up to a few tenths of a percent lower over the range of 0.6–3.2% R v-r and have a slightly higher standard deviation. Whole-rock mounts have an advantage over DOM mounts in the identification of first-cycle vitrinite and differentiation of solid bitumen from vitrinite.

The minimal response to contact metamorphism by the Devonian Buchan Caves Limestone, Buchan Rift, Victoria, Australia

Abstract A 2.2 m thick, Late Eocene (?) dike that intruded the Devonian Buchan Caves Limestone, near Murrindal, Victoria, has produced a narrow contact aureole only centimeters wide in the adjacent host rock. Mean solid bitumen reflectance ranges from about 2.4 to 2.7% and shows little change up to 6 m away from the dike contact. Carbon and oxygen isotope values also show little change except at the dike contact. The most detectable response of the rock to contact metamorphism is the proportion of solid bitumen grains with finely granular mosaic structure increase towards the dike. Also under cathodoluminescence (CL) excitation, the host rock is initially observed to be recrystallized to a uniform bright orange color. By 1 cm away from the contact, however, CL shows preservation of sharp boundaries in finely zoned burial cements. A regional study of thermal maturation and a geothermometer based on solid bitumen reflectance indicate paleotemperatures near 200°C were widespread in the Buchan Rift during peak burial near the time of the Devonian Tabberabberan Deformation. The lack of response of the Buchan Caves Limestone to contact metamorphism is attributed to: (1) prior heating to near 200°C; and (2) the fact that the dike intruded into cool, near surface, low-porosity rocks which may have been in the vadose zone.

Fluid inclusions in vadose cement with consistent vapor to liquid ratios, Pleistocene Miami Limestone, southeastern Florida

Abstract Vadose cements in the Late Pleistocene Miami Limestone contain regions with two-phase aqueous fluid inclusions that have consistent vapor to liquid (V-L) ratios. When heated, these seemingly primary inclusions homogenize to a liquid phase in a range between 75°C and 130°C (mean = 100° C ) and have final melting temperatures between −0.3° and 0.0°C. The original distribution of T h was broadened during measurements because of fluid inclusion reequilibration. The narrow range of T h in these fluid inclusions suggest unusually consistent V-L ratios. They occur with small, obscure, single phase liquid-filled inclusions, which infer a low temperature origin (less than 60°C), and contradict the higher temperature origin implied by the two phase inclusions. The diagenetic environment producing these seemingly primary fluid inclusions can be inferred from the origin of the host calcite enclosing them. The δ 18 O composition of these cements (−4 to−5.5%., PDB) and the fresh water in the fluid inclusions are consistent with precipitation from low-temperature meteoric water. The carbon-isotope composition of the vadose cements that contain only rare two-phase fluid inclusions are comparable to the host rock matrix (δ 13 C between 0 and +4%., PDB). Cements that contain common two-phase fluid-inclusions have a distinctly lighter carbon isotopic composition of −3 to −5%.. The carbon isotope composition of cements that contain common two-phase inclusions are about 6%. lighter than those of other vadose cements; models of early meteoric diagenesis indicate that this is the result of precipitation from water that has been influenced by soil gas CO 2 . Our hypothesis is that the primary fluid inclusions, those with consistent V-L ratios and the single-phase liquid inclusions, form at near-surface temperature (25°C) and pressure when consistent proportions of soil gas and meteoric water percolating through the vadose zone are trapped within elongate vacuoles. This study corroborates that T h measurements on two phase inclusions in vadose cements can be misleading evidence of thermal diagenesis, even if the measurements are well grouped.