Depositional model for peat swamp coal facies evolution using sedimentology, coal macerals, geochemistry and sequence stratigraphy

Research output: Contribution to journalArticle

Authors

  • Jing Lu
  • Longyi Shao
  • M Yang
  • Hao Wang

Colleges, School and Institutes

External organisations

  • China University of Mining and Technology, Beijing

Abstract

As one of the most important source rocks and reservoirs of unconventional natural gas, the sedimentary environment and mode of peat swamp (the predecessor of coal seam) is important to the coal seam’s spatial distribution, material composition, hydrocarbon generation potential, reservoir physical properties, etc. To reveal the depositional characteristics and history of environmental change in a terrestrial basin during a period of peat accumulation, the Middle Jurassic aged #7 coal from Gaoquan in the Qaidam Basin (NW China) was investigated using sedimentology, maceral composition, geochemistry and sequence stratigraphy. Based on identification of the sedimentary shoreline break belt, wave energy depletion point and position of wave base, the peat swamp system can be subdivided into (1) lakeside plain, (2) low energy lakeshore, (3) high energy lakeshore, and (4) shallow lake subfacies. A new method for determining coal facies is proposed based on the combination of environmental parameters including oxidation-reduction levels, energy conditions and the influence of terrigneous sediments. The evolution of the coal seam shows that peat was deposited mainly in the low energy lakeshore and lakeside plain subfacies. Five types of sequence stratigrpahic surface and two types of parasequence were identified. Forced lake regressions and normal lake regressions are attributed as the causes of sequence boundaries. The sequence stratigraphic framework comprises six sequences and corresponding system tracts, and the curve of base-level for each demonstrates a characteristic initial period of slow rising followed by fast rising and then returning to slow rising. A model indicating the relationship among base-level changes, coal facies evolution, and the environmental features in the swamp is proposed that shows the environmental features of the swamp were controlled by both base-level changes and coal facies. Accompanying depositional environment changes from a lakeside plain to lakeshore and shallow lake caused by increasing rate of base-level rise, water paleosalinity, acidity and the percentage of woody plants decrease, and the bog type alters from the low marsh to raised bog.

Bibliographic note

paper resuting from visiting fellowship to UoB by the lead author and linked to current NNSF (CHina) grant in which Hilton is co-investigator

Details

Original languageEnglish
Pages (from-to)1163-1177
JournalJournal of Earth Science
Volume28
Issue number6
Early online date19 Dec 2017
Publication statusPublished - Dec 2017