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Title: Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals

Abstract

This study was initiated to investigate if coals on the Norwegian offshore continental shelf (NOCS) expel petroleum and in which form. The results revealed that equally isotopically light methane (C{sub 1}) was released from fluid inclusions in sandstones and from adjacent coal (-60.9 to -72.7 parts per thousand). The analyzed samples were collected from cored northern North Sea and mid-Norwegian shelf wells in the depth interval 3924-5095 m. The vitrinite reflectance (R{sub o}) values of the coals range between 0.53 and 1.12%, with most values between 0.8 and 1.0%. The light C{sub 1} isotope values released both from the coals and from the fluid inclusions in the adjacent sandstones suggest that the origin of the gas is the coal, and that no isotope fractionation occurs during release of the gas in nature. Traditional isotope interpretation schemes suggest the C{sub 1} to have a biogenic origin, whereas recently published data also show the possibility for an early mature thermogenic origin. The isotope values represent averages of the total gas released from all the individual disintegrated fluid inclusions in each sample. These did not form simultaneously, but during multiple events potentially covering several million years. We speculate that significant volumes of isotopicallymore » light C{sub 1} have been expelled from the analyzed coals over time. The expelled isotopically light C{sub 1}, may mix with mature thermogenically produced gas and skew the overall methane isotope values of gas accumulations toward lighter values, thus explaining the isotopically lighter-than-expected gas accumulations on the NOCS (e.g., Troll, Frigg, and Draugen fields).« less

Authors:
;  [1]
  1. University of Oslo, Oslo (Norway). Dept. of Geoscience
Publication Date:
OSTI Identifier:
20939735
Resource Type:
Journal Article
Resource Relation:
Journal Name: AAPG Bulletin; Journal Volume: 91; Journal Issue: 5
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; 03 NATURAL GAS; SANDSTONES; COAL; NORWAY; CONTINENTAL SHELF; PETROLEUM; METHANE; INCLUSIONS; COAL SEAMS; CARBON ISOTOPES; ORIGIN

Citation Formats

Ohm, S.E., and Karlsen, D.A. Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals. United States: N. p., 2007. Web. doi:10.1306/11280606021.
Ohm, S.E., & Karlsen, D.A. Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals. United States. doi:10.1306/11280606021.
Ohm, S.E., and Karlsen, D.A. Tue . "Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals". United States. doi:10.1306/11280606021.
@article{osti_20939735,
title = {Biogenic gas(?) in fluid inclusions from sandstones in contact with oil-mature coals},
author = {Ohm, S.E. and Karlsen, D.A.},
abstractNote = {This study was initiated to investigate if coals on the Norwegian offshore continental shelf (NOCS) expel petroleum and in which form. The results revealed that equally isotopically light methane (C{sub 1}) was released from fluid inclusions in sandstones and from adjacent coal (-60.9 to -72.7 parts per thousand). The analyzed samples were collected from cored northern North Sea and mid-Norwegian shelf wells in the depth interval 3924-5095 m. The vitrinite reflectance (R{sub o}) values of the coals range between 0.53 and 1.12%, with most values between 0.8 and 1.0%. The light C{sub 1} isotope values released both from the coals and from the fluid inclusions in the adjacent sandstones suggest that the origin of the gas is the coal, and that no isotope fractionation occurs during release of the gas in nature. Traditional isotope interpretation schemes suggest the C{sub 1} to have a biogenic origin, whereas recently published data also show the possibility for an early mature thermogenic origin. The isotope values represent averages of the total gas released from all the individual disintegrated fluid inclusions in each sample. These did not form simultaneously, but during multiple events potentially covering several million years. We speculate that significant volumes of isotopically light C{sub 1} have been expelled from the analyzed coals over time. The expelled isotopically light C{sub 1}, may mix with mature thermogenically produced gas and skew the overall methane isotope values of gas accumulations toward lighter values, thus explaining the isotopically lighter-than-expected gas accumulations on the NOCS (e.g., Troll, Frigg, and Draugen fields).},
doi = {10.1306/11280606021},
journal = {AAPG Bulletin},
number = 5,
volume = 91,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Recent studies of fluid inclusions in quartz overgrowths have shown quartz cementation to have taken place at temperatures within the range 60--145 C in several sandstones from the North Sea and offshore mid-Norway (Malley et al. 1986; Konnerup-Madsen and Dypvik 1988; Burley et al. 1989; Walderhaug 1990; Ehrenberg 1990; Saigal et al. 1992; Nedkvitne et al. 1993). This study aims at determining whether these results are typical for quartz cementation of sandstones by presenting homogenization temperatures for 274 aqueous and 366 hydrocarbon inclusions in quartz overgrowths from Jurassic reservoir sandstones on the Norwegian continental shelf, and by reviewing previously publishedmore » fluid-inclusion data. Possible explanations for different ranges of homogenization temperatures in different sandstones are also discussed, and possible sources of quartz cement and the effect of hydrocarbon emplacement on quartz cementation are considered.« less
  • The Frio fluvial/deltaic sandstone along the Vicksburg fault zone play of south Texas has produced nearly 1 billion bbl of oil from fluvial/deltaic sandstones since field development began in the 1940s. More than half of the reservoirs in this depositionally complex play have been abandoned, even though large volumes of oil remain. Current efforts integrating geological and engineering reservoir characterization are being used to identify the location of unrecovered mobile oil, estimated at more than 1 billion bbl, that remains in unproduced reservoir zones in fields within this very mature play. Engineering data from Frio reservoirs in Rincon field weremore » used to assess past production behavior, determine completion density, and prioritize zones for incremental reserve growth opportunities. Geologic data have been evaluated to identify interwell stratigraphic heterogeneity and potential for compartmentalization of significant volumes of unrecovered oil. Major oil reservoirs represent deposition in broad, dip-elongate fluvial systems. Individual zones consist of multiple thin (0-40 ft) sandstone units that stack to form gross thicknesses of 50 to 100 ft. They occur both as narrow channel fills isolated vertically and laterally by very low-permeability overbank facies and flood-plain mudstones and as large channel complexes with multiple laterally coalescing sand lobes. Large dip-elongate channel-sandstone complexes provide ideal conditions for the isolation of oil accumulations in multiple reservoir compartments, many of which are now incompletely drained or completely untapped. Reservoir architectural mapping and core analysis data from more than 100 wells are being combined with reserve volumetrics to describe heterogeneity and to identify possible locations of additional reserves within a 100-ft reservoir interval that has already produced more than 15 million bbl of oil.« less
  • Fluid inclusions were examined in 13 samples from wollastonite-zone rocks, calc-silicate skarns, and granites from the Notch Peak, Utah, contact-metamorphic aureole using microthermometric techniques. All inclusions are aqueous, with no evidence for the presence of CO{sub 2} or daughter crystals. Homogenization temperatures range from 280 to 110C, and melting temperatures range from +2 to {minus}28C. Calculated trapping temperatures, at 2-kbar total pressure, range from {approximately}250 to 475C, and calculated salinities range from 0 to 15 eq wt% NaCl. Wollastonite-zone inclusions have the highest homogenization temperatures of the rocks studied and have salinities of {approximately}5 eq wt% NaCl. Granite sample inclusionsmore » have similar salinities but have homogenization temperatures in the range of 200 to 140C. Skarn samples have inclusions that are generally more saline than the other two sample groups and that have homogenization temperatures lower than {approximately}180C. The low melting temperatures and lack of daughter crystals indicate that the solute consists of divalent salts, most likely CaCl{sub 2}, in addition to NaCl. The lack of CO{sub 2} is consistent with metamorphic fluid compositions indicated by mineral assemblages. Even though the calculated trapping temperatures and pseudosecondary or secondary nature of most inclusions indicate that the fluid was trapped after the peak of metamorphism, the compositions indicate that the trapped fluids could be the late metamorphic fluid.« less
  • A new analytical method, fluid inclusion stratigraphy (FIS), allows past and near-present-day pore fluids, trapped as fluid inclusions, to be chemically characterized in drill cuttings, cores or outcrop material. This novel mass spectrometric technique is capable of evaluating thousands of samples in a matter of days, thereby enabling whole-well stratigraphic mapping of fluid chemistries. Coupled with other complementary fluid inclusion technologies, the process provides unique solutions to a variety of exploration and production questions. Examining fluid inclusions in more than 20,000 core samples from 180 wells and analyzing the resulting data maps generated 39 oil and gas leads in themore » Gulf of Mexico within six weeks. A similar study would have taken labs with conventional instrumentation at least a year to complete.« less