Primary Aqueous Inclusions Research Articles

Burial history of the Penrith Sandstone (Lower Permian) deduced from the combined study of fluid inclusion and palaeomagnetic data

Abstract The Lower Permian Penrith Sandstone in the Vale of Eden represents the deposits of fossil aeolian dunes, which are well cemented by syntaxial quartz overgrowths in the northern part of the outcrop. The overgrowths possess a complex internal growth fabric as revealed by scanning electron microscopy-cathodoluminescence. Fluid inclusions are common in the overgrowths and occur in three associations. Small, subrounded, mostly liquid-rich, two-phase, primary aqueous inclusions occur in the concentrically banded growth zone. Similar primary inclusions occur concentrated in growth imperfections aligned orthogonally to the growth direction. A third association comprises irregular, elongate inclusions that occur along healed microfractures. Salinities in all three associations vary from 13.5 to 23 wt.% NaCl equivalents, whereas measured homogenization temperatures range from 69 to 280°C. This variation results from necking of the inclusion populations after vapour phase nucleation and, as a consequence, the measured homogenization temperatures are geologically meaningless. However, the smallest inclusions yield consistent T h data of around 70°C, which is equivalent to a T t of around 95°C in the absence of methane. A variety of iron oxyhydroxides carry the magnetic remanance in the Penrith Sandstone. The characteristic magnetization is carried by hematite, which occurs in several textural forms. This remanence is multicomponent and shows directional scatter along a NNW-SSE great circle. Individual components are difficult to isolate because of the similarity of their unblocking temperatures, but are best interpreted as a mixture of Tertiary normal and reverse elements. Above 530°C a weak, but stable, Early Permian magnetization can be isolated from a number of specimens. A model is proposed in which quartz authigenesis and magnetization took place during burial from alkaline-rich fluids derived from the Eden Shales. Authigenic hematites precipitated before and during quartz cementation at temperatures of around 95°C. The fluid inclusion microthermometry and thermal demagnetization data are consistent with the Penrith Sandstone attaining a maximum burial temperature of about 120°C. For necking to have caused entrapment of inclusions containing both liquid and vapour suggests that the Penrith Sandstone underwent burial into the all-liquid field, trapped inclusions and then underwent cooling, equivalent to a temperature decrease of 30–40°C. This temperature decrease may be a consequence of regional uplift or, alternatively, the Penrith Sandstone may have been heated and cooled following a transient flush of hot, migrating fluids, possibly related to the shallow intrusion of magma into the basin in Tertiary times. The origin of the Tertiary remanence in the Penrith Sandstone is compatible with fluid inclusion data and is considered to be a combination of chemical remanent magnetization and partial thermoremanent magnetization which was blocked during the Tertiary cooling.

Fluid inclusions as tectonothermobarometers: Relation between pressure-temperature history and reequilibration morphology during crustal thickening

Results of experimental reequilibration studies of natural primary aqueous inclusions in quartz under conditions of hydrostatic compressive loading at 1, 2, 3, 4, and 5 kbar and 500 °C indicate that the intensity of reequilibration features is inversely proportional to inclusion volume. The smallest inclusions show the first evidence of reequilibration (dissolution of inclusion walls) after ∼1.8 kbar of effective pressure. For effective pressures less than ∼3.8 kbar, decrepitation of inclusions is probably controlled by the corrosive action of water on the inclusion walls. At higher effective pressures, decrepitation is related to high compressive stress and is less dependent on fluid interaction with the inclusion walls. The observed correlation between inclusion size, intensity of reequilibration features, and amount of loading, combined with paragenetic and microthermometric data for the inclusions, may be used to determine the pressure-temperature history of the inclusions and their host rocks.

The Role of Active and Ancient Geothermal Systems in Evolution of Grant Canyon Oil Field, Railroad Valley, Nye County, Nevada

Since discovery in 1983, the Grant Canyon field has been among the most prolific oil producers (on a per-well basis) in the US. Production through June 1990 was 12,935,630 bbl of oil, principally from two wells which in tandem have consistently yielded more than 6,000 bbl of oil per day. The field is hosted by highly porous Devonian dolomite breccia loosely cemented with hydrothermal quartz. Results of fluid-inclusion and petrographic research in progress at Grant Canyon suggest that paleogeothermal and perhaps currently circulating geothermal systems may have played a major role in oil-reservoir evolution. For example, as previously reported, the breccia-cementing quartz hosts primary aqueous, aqueous/oil, and oil fluid inclusions which were trapped at about 120C (average homogenization temperature) and document initial oil migration and entrapment as droplets or globules dispersed in dilute (< 2.2 wt.% equivalent NaCl) aqueous solutions. Additional evidence of geothermal connection is that the horst-block trap at Grant Canyon is top and side sealed by valley-fill clastic and volcanic rocks which are locally hydrothermally altered and calcite flooded. These secondary seals are enhanced by disseminated, solid asphaltic residues locally accounting for 23% (volume) of the rock. Current reservoir temperatures at Grant Canyon (120C) and the adjacentmore » Bacon Flat field (171C) attest to vigorous contemporary geothermal activity. Based on results of the authors' Grant Canyon work to date, they suggest that active and paleohydrothermal systems could be viable petroleum exploration targets in otherwise favorable terrain elsewhere in the Basin and Range.« less

Thermo-Chronological Approach to Reservoir Diagenesis in the Offshore Angola Basin: a Fluid Inclusion, {40}Ar-{39}Ar and K-Ar Investigation (1)

Microthermometric analyses of fluid inclusions and geochronological surveys of authigenic silicates in Neocomian sandstones and Albian dolomitic reservoirs from the offshore Angola basin have allowed determination of the age and the physical-chemical conditions for fluid migration in a late diagenetic stage. The {40}Ar-{39}Ar laser probe dating of adularia overgrowths from the Albian reservoirs shows that the trapping of primary aqueous and oil inclusions occurred during the early Miocene (25.6 + or - 3.2 Ma). The K-Ar age of authigenic illites from the Neocomian sandstones is slightly younger (19 + or - 1 Ma), and the crystallization of regular illite-smectite mixed layers is related to a previous Eocene to Paleocene diagenetic stage (51-58 Ma). There appears to have bee both a regional and vertical variation in the salinity of aqueous fluids during the Miocene. The fluid inclusion trapping temperatures were estimated by correcting the homogenization temperatures of aqueous fluids, assuming that pressures during the Miocene were close to the present reservoir pressures, and by intersecting the isochores of aqueous and oil inclusions, assuming they were trapped at strictly similar pressure and temperature conditions. The fluid inclusion temperatures are consistent with those derived from the tetrahedral aluminum content (AlIV) of authigenic chlorites: 168-206 degrees C in the Albian reservoirs at depths between 2350 and 3580 m, and 175-237 degrees C in the Neocomian sandstones at depths between 2770 and 4470 m. In both reservoir formations, Miocene tempe atures were higher than the present ones and related either to deeper burial or higher geothermal gradient.

Fluid-inclusion evidence for the formation conditions of apatite from the Tororo carbonatite complex of eastern Uganda

SummaryPortions of the carbonatitic fluids responsible for the deposition of apatite from the Tororo car-bonatite complex are trapped and preserved as minute (&lt; 100 µm) inclusions within the apatites. These fluids consist predominantly of alkali-carbonate-bearing brines and demonstrate that water and alkalis played an important role in the formation of the carbonatites from this complex. Furthermore, the opinion that carbonatite ‘magmas’ are richer in alkalis and water than the chemistry of the carbonatites themselves reveal is clearly substantiated. The mean minimum formation temperature of the apatites from five separate carbonatite samples were determined from the homogenization temperatures of 200 primary aqueous inclusions. The results from Limekiln Hill samples (328 °C 321 °C 359 °C) are similar to those from samples of the separate carbonatite mass at Tororo Rock (353 °C, 365 °C).