Sedimentological Controls on Organic Carbon Preservation and Source Rock Quality in the Upper Cretaceous Greenhorn Formation, USA
Identifying the sedimentological conditions associated with organic carbon preservation and mineralization in fine-grained sediments is of critical importance for predicting shale-gas reservoir quality. Late Cenomanian-aged calcareous mudstones deposited in the center of the North American Interior Seaway are commonly cited as being excellent examples of settings where enhanced organic matter preservation was associated with deposition of laminated sediment in relatively deep, oxygen-depleted water. This detailed lithofacies study was initiated to determine mechanisms responsible for sediment delivery and deposition while this interval was being deposited.
Detailed lithofacies analyses of 53 samples collected from Rebecca Bounds #1 and Portland cores reveal that these units are highly heterogeneous at millimeter to centimeter scales. Microtextural analyses reveal that this succession is organized into very thin beds (<10 mm), that are either sharp based and normally graded or bioturbated. Where preserved many of the depositional laminae (defined by concentrations of foraminifer tests and inoceramid prisms) either downlap on to underlying bedding planes or form starved ripples. Organo-minerallic aggregates and pellets are present above foraminifer lags and bed tops are commonly homogenized.
These microfabrics indicate that during deposition of the laminated intervals sediment was being transported by storm driven combined flows that were advectively dispersing sediment across a shelf in settings starved of clastic detritus. Following initial advective deposition much of the sediment was delivered as marine snow from buoyant plumes, prior to being bioturbated and buried by the succeeding event bed. Organic carbon enrichment in this unit is not easily attributable to the presence of persistent bottom water anoxia in a low energy setting; rather it was due to event bed deposition, high primary production and high recurrence frequencies of bed emplacement.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California