Robert H. Goldstein

Fluid inclusions in sedimentary and diagenetic systems

Abstract Some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals. Important fluids in the sedimentary realm include atmospheric gases, fresh water of meteoric origin, lake water, seawater, mixed water, evaporated water, formation waters deep in basins, oil, and natural gas. Preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global-change research, is essential for improving understanding of diagenetic systems, and provides useful information in petroleum geology. Applications of fluid inclusions to sedimentary systems are not without their complexities. Some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration, which results in stretching, or leakage and refilling, of some fluid inclusions. Similarly, overheating in the laboratory can also cause reequilibration of fluid inclusions, so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating. Natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment, fluid inclusion assemblages (FIA). Consistency in homogenization temperatures within a fluid inclusion assemblage, consisting of variably sized and shaped inclusions, is the hallmark of a data set that has not been altered through thermal reequilibration. In contrast, fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration. If a fluid inclusion assemblage has not been altered by thermal reequilibration, its fluid inclusions may be useful as geothermometers for low- and high-temperature systems, or useful as geobarometers applicable throughout the sedimentary realm. If a fluid inclusion assemblage has been altered partially by thermal reequilibration, techniques for distinguishing between altered and unaltered fluid inclusions may be applied. In studies of global change, fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments. Fluid inclusions also preserve microsamples of ancient seawater and atmosphere, the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans. In petroleum geology, fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons; they can delineate the evolution of the chemistry of hydrocarbons; and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems. In studies of diagenesis, fluid inclusions can be the most definitive record. Most diagenetic systems are closely linked to temperature and salinity of the fluid. Thus, fluid inclusions are sensitive indicators of diagenetic environments.

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.

Reequilibration of fluid inclusions in low-temperature calcium-carbonate cement

Calcium-carbonate cements precipitated in low-temperature, near-surface, vadose environments contain fluid inclusions of variable vapor-to-liquid ratios that yield variable homogenization temperatures. Cements precipitated in low-temperature, phreatic environments contain one-phase, all-liquid fluid inclusions. Neomorphism of unstable calcium-carbonate phases may cause reequilibration of fluid inclusions. Stable calcium-carbonate cements of low-temperature origin, which have been deeply buried, contain fluid inclusions of variable homogenization temperature and variable salt composition. Most inclusion fluids are not representative of the fluids present during cement growth and are more indicative of burial pore fluids. Therefore, low-temperature fluid inclusions probably reequilibrate with burial fluids during progressive burial. Reequilibration is likely caused by high internal pressures in inclusions which result in hydrofracturing. The resulting fluid-inclusions population could contain a nearly complete record of burial fluids in which a particular rock has been bathed.

U-Pb dates of paleosols: Constraints on late Paleozoic cycle durations and boundary ages

U-Pb ages of paleosol samples from cyclothemic sections in the Sacramento Mountains of New Mexico and the Permian basin of West Texas bracket the Pennsylvanian-Permian and Carboniferous-Permian boundaries. Linear interpolation and extrapolation to boundaries are accomplished by using cyclothems as periodic subdivisions. The Carboniferous-Permian boundary is 301 ± 2 Ma (2 σ), the Pennsylvanian-Permian boundary is 302 ± 2.4 (2 σ), and the Missourian-Virgilian boundary is 307 ± 3 Ma (2 σ). These ages are within the uncertainties of recent estimates for the boundaries but are significantly more precise. The average cycle duration for this Upper Pennsylvanian and Lower Permian interval is 143 ± 64 ka (2 σ), within the uncertainty of the ∼100 k.y. cycle that dominated Pleistocene sea-level history. This new estimate, which is much shorter than those published previously, suggests that the Pennsylvanian and Early Permian cyclothems may have been driven in a manner similar to that which caused glacio-eustatic cycles of the Pleistocene.

Pinning points: a method providing quantitative constraints on relative sea-level history

Abstract Quantitative constraints on the history of relative sea level allow for a better understanding of the controls on depositional sequence development. The constraints are provided by “pinning point curves”, plots of ancient relative sea-level elevations (pinning points) versus time. Constructing a pinning point curve requires identification of ancient stratigraphic positions of sea level through interpretation of facies and surfaces formed at sea level. Then, their ancient relative elevations are determined through reconstructing aspects of ancient paleotopography. Upper Miocene strata from Las Negras, southeastern Spain preserve paleotopography, contain ancient surfaces of subaerial exposure, and contain facies deposited near sea level. The pinning point curve illustrates a complex relative sea-level history with large- and small-scale relative sea-level fluctuations defined by 31 pinning points.

Extremely acid Permian lakes and ground waters in North America

Evaporites hosted by red beds (red shales and sandstones), some 275–265 million years old, extend over a large area of the North American mid-continent. They were deposited in non-marine saline lakes, pans and mud-flats, settings that are typically assumed to have been alkaline. Here we use laser Raman microprobe analyses of fluid inclusions trapped in halites from these Permian deposits to argue for the existence of highly acidic (pH <1) lakes and ground waters. These extremely acidic systems may have extended over an area of 200,000 km2. Modern analogues of such systems may be natural acid lake and groundwater systems (pH ∼2–4) in southern Australia. Both the ancient and modern acid systems are characterized by closed drainage, arid climate, low acid-neutralizing capacity, and the oxidation of minerals such as pyrite to generate acidity. The discovery of widespread ancient acid lake and groundwater systems demands a re-evaluation of reconstructions of surface conditions of the past, and further investigations of the geochemistry and ecology of acid systems in general.

Sedimentology of Ancient Saline Pans: An Example from the Permian Opeche Shale, Williston Basin, North Dakota, U.S.A.

ABSTRACT The mid-Permian Opeche Shale of North Dakota consists of bedded evaporites and red-bed siliciclastics. Detailed core and petrographic study has documented sedimentary and early diagenetic features in order to develop a depositional model, and to refine paleoclimatic data and paleogeographic setting for the late Paleozoic of the U.S. midcontinent. Lithologies and sedimentary features indicate lacustrine, distal alluvial, and minor eolian deposition, subaerial exposure, and soil formation. Bedded halites consisting of chevron and cumulate crystals, dissolution surfaces and pipes, and mudcracked microcrystalline salt crusts were deposited in a saline pan dominated by flooding, evaporative concentration, and desiccation. Bedded halites containing chevron and cumulate crystals but lacking any dissolution or desiccation features formed in perennial saline lakes. Chaotic halite, composed of red mudstone and siltstone with displacive halite crystals, represents saline mudflat deposits. Red mudstone and siltstone with little or no displacive halite but with abundant cracks and root features suggest deposition in a dry mudflat. Red-bed sandstones and conglomerates, composed of poorly sorted, subrounded quartz grains cemented with halite indicate distal alluvial deposition with possible transport by ephemeral streams, sheet floods, and debris flows. Most deposition took place in halite-dominated shallow perennial and ephemeral saline lakes surrounded by saline and dry mudflats. Evaporation, desiccation, flooding, and wind played significant roles in this environment. Therefore, the Opeche evaporites and red beds are representative of an ancient saline pan system. An inland playa setting is favored as a depositional model for the Opeche Shale. The abundance of soil features and halite dominance, as well as lack of nearshore carbonates and lack of restricted marine fossils, suggest a closed-basin nonmarine setting for the mid Permian of the U.S. midcontinent.

Geostatistical three-dimensional modeling of oolite shoals, St. Louis Limestone, southwest Kansas

In the Hugoton embayment of southwestern Kansas, reservoirs composed of relatively thin (4 m; 13.1 ft) oolitic deposits within the St. Louis Limestone have produced more than 300 million bbl of oil. The geometry and distribution of oolitic deposits control the heterogeneity of the reservoirs, resulting in exploration challenges and relatively low recovery. Geostatistical three-dimensional (3-D) models were constructed to quantify the geometry and spatial distribution of oolitic reservoirs, and the continuity of flow units within Big Bow and Sand Arroyo Creek fields. Lithofacies in uncored wells were predicted from digital logs using a neural network. The tilting effect from the Laramide orogeny was removed to construct restored structural surfaces at the time of deposition. Well data and structural maps were integrated to build 3-D models of oolitic reservoirs using stochastic simulations with geometry data. Three-dimensional models provide insights into the distribution, the external and internal geometry of oolitic deposits, and the sedimentologic processes that generated reservoir intervals. The structural highs and general structural trend had a significant impact on the distribution and orientation of the oolitic complexes. The depositional pattern and connectivity analysis suggest an overall aggradation of shallow-marine deposits during pulses of relative sea level rise followed by deepening near the top of the St. Louis Limestone. Cemented oolitic deposits were modeled as barriers and baffles and tend to concentrate at the edge of oolitic complexes. Spatial distribution of porous oolitic deposits controls the internal geometry of rock properties. Integrated geostatistical modeling methods can be applicable to other complex carbonate or siliciclastic reservoirs in shallow-marine settings.

Cement Stratigraphy of Pennsylvanian Holder Formation, Sacramento Mountains, New Mexico

Cyclic strata of the Holder Formation (Virgilian, New Mexico) were deposited across a Pennsylvanian shelf-to-basin transition during a time when sea level fluctuated over tens of meters. Cement-stratigraphy studies indicate abundant calcite precipitation from low-temperature fresh water within shelf, shelf-crest, and shelf-edge marine limestones. Freshwater cementation occurred during 15 periods of intraformational subaerial exposure. These early cements are least abundant in basin and lagoon limestones. The distribution of the early cement zones suggests that cementation was controlled by paleotopography, stratigraphic position below subaerial exposure surfaces, lithology, and configuration of the paleoaquifer system. Distribution of early calcite cements provides new data for interpretation of cycles, diagenetic systems, and porosity evolution in petroleum reservoirs. Trace-element analyses support a low-temperature, freshwater origin for the early cements. Cement-stratigraphy studies, fluid-inclusion analyses, and trace-element analyses indicate a later cement that occluded remaining limestone porosity precipitated from a sodium- and calcium-rich brine at a temperature of about 100°C. Stratigraphic reconstruction dates this cementation as Cretaceous or later. Fluorescent, oil-filled fluid inclusions were trapped along fractures in the late cement, indicating oil migration during or after late-stage cementation.

History of diagenetic fluids in a distant foreland area, Middle and Upper Pennsylvanian, Cherokee basin, Kansas, USA: Fluid inclusion evidence☆

Abstract Analysis of fluid inclusion data in diagenetic cements from Pennsylvanian limestones and sandstones of the Cherokee basin in southeastern Kansas reveals the succession of diagenetic fluids in a distant foreland of the Arkoma-Ouachita system. This succession includes early low-salinity (0.0–2.4 wt% NaCl eq.) fluids of meteoric affinity (Fluid I) followed by low-temperature Na-Ca-Cl brines (Fluid II with salinities between 8.4 and 24.1 wt% NaCl eq.). Fluids I and II were present in the system during precipitation of early-stage calcite cements at temperatures less than about 50°C. Another Na-Ca-Cl brine (Fluid III with salinity up to 25 wt% NaCl eq.) was present in the system later, at temperatures of maximum burial (at least 80–85°C) and higher. Fluid III is followed by a Na-Cl brine (Fluid IV, with salinities about 19–21 wt% NaCl eq.) characterized by temperatures distinctly higher than maximum burial, up to 150°C. Fluid III and Fluid IV were entrapped during precipitation of late-stage baroque dolomite and Fe-dolomite in Pennsylvanian limestones, and late-stage Fe-dolomite and ankerite in Pennsylvanian sandstones. The record of progression from Fluid III to Fluid IV may have been partially obscured by thermal re-equilibriation of some inclusions during migration of Fluid IV. Fluids with Na-Ca-Cl chemistry (Fluid II and III) were either indigenous subsurface fluids of the Cherokee and Arkoma basins, or might have originated as reflux fluids in a Permian evaporitic basin of Central Kansas. Later Na-Cl brine (Fluid IV) originated in deeper parts of the Arkoma-Ouachita system and might have acquired their salinity by dissolution of hypothetical salts buried beneath the Ouachitas. Temperatures recorded by fluid inclusions in late-diagenetic carbonates are 20–60°C higher than those calculated for the maximum burial of the studied section. This thermal anomaly suggests an advective heat transfer from the Arkoma-Ouachita system onto the shelf of the Cherokee basin related to the invasion of late-diagenetic fluids.