Unraveling Depositional and Early Diagenetic Controls in the Exshaw Formation: Coupled Lithofacies and Sulfide Geochemistry Investigation

The shale lithofacies present in the Exshaw Formation have been analyzed to gain insights into the environment of deposition. This formation, along with its lateral equivalent the Bakken Shale, is significant because it is a regional tight gas reservoir target. Previous studies have interpreted these units as having been deposited in primarily low energy, anoxic basins. In this study the link between microlithofacies variability (determined by optical and SEM techniques) and varying sulfide geochemistry (determined by SIMS, XRD and XRF) has been investigated to determine the depositional and early diagenetic processes that control rock character. Varying iron-disulfide mineralogy (proportion of marcasite to pyrite) is particularly significant because it provides insight into the geochemical conditions (oxygen concentrations, pH and redox) at the time of deposition.

The Exshaw is organized into thin (<7 mm thick), wavy sharp based, normally graded beds that are variably bioturbated. Mineralogically the mudstones are predominantly composed of quartz (62 to 79 %), feldspar (10 to 20 %), clay minerals (10 to 18 %) and pyrite. Agglutinated benthic foraminifera with tests composed of silt-sized feldspar grains are present throughout the succession. Quartz is present in a number of forms including; as an early diagenetic cement, in detrital silt grains and within tasmanites and radiolaria. A phosphate-rich lag deposit at the base of the Exshaw contains sand-sized (100 to 250 μm) detrital quartz, and a significant early diagenetic component which includes the sulfide minerals pyrite, marcasite, sphalerite, and millerite. SIMS analysis of individual grains of pyrite and marcasite identified considerable variability in δ³⁴S with a range of 14.5 ‰ to -38.6‰ within the formation. Measured δ³⁴S of marcasite was found to be consistent and the most enriched in ³⁴S of all samples with a range of 6.3 ‰ to 14.5 ‰. Measured δ³⁴S of pyrite clustered in two regions, -8.1 ‰ to -14.9 ‰ and -30.7 ‰ to -38.6 ‰.

Sulfur dynamics within the Exshaw Formation suggest that at the time of deposition the basin was a hydrodynamically active and geochemically complex environment with sulfur being derived from a number of different reservoirs. In addition, microtextural evidence (bioturbation, hyrodynamically sorted silt fractions and agglutinated benthic foraminfera) also suggest the basin was hydrodynamically active and not as persistently anoxic as reported previously.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California