Depositional and diagenetic controls of source-rock characteristics of the Lower Jurassic "Nordegg Member", Western Canada

Abstract

Petrographic, mineralogical, and bulk-geochemical (XRD, XRF, and Rock-Eval) data indicate that depositional and early diagenetic processes were the principal controls on the organic and inorganic characteristics of the Lower Jurassic Nordegg Member source rock. This unit comprises organic-rich, calcareous mudstones to marlstones and marly limestones that were deposited in a predominantly anoxic, hypersaline, silled marine basin during two transgressive-regressive (T-R) events. Sediments deposited during marine transgression have a higher content of carbonate (calcite and dolomite) and organic matter. Fe (mainly as pyrite) content varies inversely with carbonate content. The basal Nordegg Member is phosphatic with stratiform apatite laminae and phosphatic nodules. Early diagenetic phosphatogenesis was probably mediated by bacteria under variably dysoxic conditions during the initial transgressive event. Regressive parts of T-R cycles are expressed as an increase in abundance of silicate (quartz, illite, kaolinite, and minor K-feldspar) and lower organic carbon contents. Deposition of abundant marine organic matter fueled vigorous bacterial reduction of sulfate and almost total bacterial reworking of the sedimented organic matter, resulting in significant production of reduced sulfur species and predominantly amorphous kerogen. Pyrite formation was ubiquitous but limited by the abundance of Fe. Where Fe contents fell below 3 wt %, excess sulfides were incorporated into the kerogen. This process resulted in the early diagenetic formation of the sulfur-rich kerogen (Type I/II-S) that typifies the Nordegg. Thus Fe availability in the depositional environment exerted an important control on the amount of S available 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 incorporation of sulfur into the kerogen during early diagenesis.