ABSTRACT: Diagenesis in Halite-Cemented Source Rocks, Middle Devonian, Saskatchewan

Alan C. Kendall, G. D. Abbott, V. A. A. D'Elia

Porosity in Dawson Bay carbonates is halite plugged and the formation is sandwiched between thick units of bedded halite. The presence of displacive halite crystals within fine-grained carbonates (implying sediment plasticity during halite emplacement) and uncompacted organic-rich, carbonate-poor stromatolites indicate halite cementation occurred at an early stage. Also, halite cementation must have been completed prior to porosity loss in overlying bedded halites. By comparison with Holocene/Pleistocene bedded halites, this cementation occurred with only tens of meters of overburden. Early

complete halite cementation should have converted Dawson Bay carbonates into virtually a closed system and greatly curtailed or inhibited organic-matter maturation within them.

Organic-rich carbonates occur immediately below Dawson Bay evaporites as rocks containing an anomalously abundant benthos (stromatoporoids, brachiopods) or as a more restricted facies, lacking megafossils or containing gastropods. Some restricted carbonates contain more than 2% extractable organic carbon. The n-alkane, pentacyclic triterpane, nonrearranged sterane and disterane distributions suggest two distinct populations of samples are present. Biomarker distributions are difficult to interpret in terms of estimating organic maturity because of source rock environmental factors (hypersalinity), but appear to be inconsistent with the geological prognosis that these source rocks would have been isolated early in their diagenesis. The problem of how kerogens can be altered in an appar ntly closed system has yet to be resolved.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990