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(Table 1) Geochemical and geothermometry data from fibrous calcite vein fills in the Dabashan Fold Belt, China

Authors
Publisher
PANGAEA
Publication Date
Identifiers
DOI: 10.1594/pangaea.811050
Keywords
  • Aluminium Oxide
  • Calcium Oxide
  • Dabashan_Foreland_Belt
  • Delta 13C
  • Organic Matter
  • Delta 18O
  • Organic Matter
  • Electron Microprobe
  • Freezing Point Depression
  • Homogenization Temperature
  • Iron Oxide
  • Feo
  • Linkam Thms600 Microscope Stage
  • Lithology/Composition/Facies
  • Magnesium Oxide
  • Manganese Oxide
  • Mass Spectrometer Finnigan Mat 251Em
  • Maximal Reflectance Of Bitumen
  • Microphotometer Leitz Mpv-3
  • Phosphorus Oxide
  • Potassium Oxide
  • Sample Code/Label
  • Sampling By Hand
  • Sichuan
  • Shaanxi
  • China
  • Silicon Dioxide
  • Sodium Oxide
  • Sulphite
  • Titanium Oxide
Disciplines
  • Chemistry
  • Earth Science

Abstract

Fibrous calcite veins with organic inclusions have been widely considered as indicators of oil and gas generation and migration under overpressure. Abundant fibrous calcite veins containing organic-bearing inclusions occur in faulted Lower Paleozoic through Triassic hydrocarbon source rocks in the Dabashan Foreland Belt (DBF). d13CPDB and d18OPDB values of the fibrous calcite range from - 4.8 to -1.9 to per mil and - 12.8 to - 8.4 per mil respectively, which is lighter than that of associated carbonate host rocks ranging from - 1.7 to + 3.1 per mil and - 8.7 to - 4.5 per mil. A linear relationship between d13CPDB and d18OPDB indicates that the calcite veins were precipitated from a mixture of basinal and surface fluids. The fibrous calcite contains a variety of inclusions, such as solid bitumen, methane bearing all-liquid inclusions, and vapor-liquid aqueous inclusions. Homogenization temperatures of aqueous inclusions range from 140 to 196° with an average of 179°. Salinities of aqueous inclusions average 9.7 wt% NaCl. Independent temperatures from bitumen reflectance and inclusion phase relationships of aqueous and methane inclusions were used to determine fluid pressures. Results indicate high pressures, elevated above typical lithostatic confining pressure, from 150 to 200 MPa. The elevated salinity and high temperature and pressure conditions of the fibrous calcite veins argue against an origin solely from burial overpressure resulting from clay transformation and dehydration reactions. Instead fluid inclusion P-T data and geochemistry results and regional geology indicate abnormally high pressures during fluid migration. These findings indicate that tectonic stress generated fracture and fault fluid pathways and caused migration of organic bearing fluids from the DBF during the Yanshan orogeny.

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