DEPOSITIONAL AND DIAGENETIC CONTROLS ON SOURCE-ROCK CHARACTERISTICS OF THE LOWER JURASSIC NORDEGG MEMBER, WESTERN CANADA

Petrographic, mineralogical, and bulk-geochemical (XRD, XRF, and Rock- Eval) data indicate that depositional and early diagenetic processes w ere the principal controls on the organic and inorganic characteristic s of the Lower Jurassic ''Nordegg Member'' source rock, This unit comp rises organic-rich, calcareous mudstones to marlstones and marry limes tones that were deposited in a predominantly anoxic, hypersaline, sill ed marine basin during two transgressive-regressive (T-R) events, Sedi ments deposited during marine transgression have a higher content of c arbonate (calcite and dolomite) and organic matter, Fe (mainly as pyri te) content varies inversely with carbonate content. The basal ''Norde gg Member'' is phosphatic with stratiform apatite laminae and phosphat ic nodules, Early diagenetic phosphatogenesis was probably mediated by bacteria under variably dysoxic conditions during the initial transgr essive event, Regressive parts of T-R cycles are expressed as an incre ase in abundance of silicate (quartz, illite, kaolinite, and minor K-f eldspar) and lower organic carbon contents. Deposition of abundant mar ine organic matter fueled vigorous bacterial reduction of sulfate and almost total bacterial reworking of the sedimented organic matter, res ulting in significant production of reduced sulfur species and predomi nantly amorphous kerogen, Pyrite formation was ubiquitous but limited by the abundance of Fe, Where Fe contents fell below 3 wt %, excess su lfides were incorporated into the kerogen, This process resulted in th e early diagenetic formation of the sulfur-rich kerogen (Type I/II-S) that typifies the ''Nordegg'', Thus Fe availability in the depositiona l environment exerted an important control on the amount of S availabl e for incorporation into the kerogen. Furthermore, content of organic S increases with increasing Hydrogen Index, indicating the importance of type and reactivity of organic matter on the extent of incorporatio n of sulfur into the kerogen during early diagenesis.