Mudstone Aggregates and their Implications for Sedimentology and Reservoir Characteristics
Laycock, Dallin P.; Pedersen, Per; Spencer, Ronald
Mudstones of the Late Cretaceous Carlile Formation in eastern Alberta host a laterally extensive gas accumulation. Detailed examination of thin sections show the succession contains abundant mudstone aggregate clasts. Interlamination of mud and silt sized quartz grains within the Carlile Formation provided color contrasts between the mudstone aggregates and the quartz grains, facilitating the distinction between the aggregates and the matrix. Chemical analyses show that the mudstone aggregates have different compositions from the mud matrix, suggesting the mudstone aggregates originated significant distances away. These mudstone aggregates are well-rounded to sub-rounded and display very little compaction, indicating they were significantly dewatered prior to deposition. Firmness of the aggregates was sufficient to occasionally preserve shelter porosity between many of the aggregates, thereby increasing the porosity and permeability of the mudstone reservoir. Ripples are abundant within the Carlile Formation, commonly comprised of silt and very fine grained sand interlaminated with mudstone aggregate clasts, indicating approximate hydrodynamic equivalency of the grains. Other ripples are even composed entirely of mudstone aggregates.
Grain size measurements of quartz grains yield a near symmetrical distribution (skewness of 0.019) and are moderately well sorted (standard deviation of 0.627) with the majority of the measured grains being silt sized. In contrast, mudstone aggregates display a strongly coarse skewed distribution (skewness of -0.311) and are moderately well sorted (standard deviation of 0.630). The median aggregate size was measured to be 0.118 mm in diameter (very fine sand), with the largest aggregate measured to be 0.522 mm in diameter (coarse sand). In the measured sample, the aggregates form approximately 55% of the sample volume, suggesting that this mudstone would more accurately be described as a sandstone in regard to depositional behavior. This rare example of visible mudstone aggregates in the rock record suggests that mud-dominated sedimentary processes are more complex than previously thought. They can represent a wider hydrodynamic range and therefore more depositional environments than previously assumed. Understanding such sedimentary processes for the mudstone fraction has a large impact on how mudstone depositional environments are interpreted, which greatly impacts hydrocarbon evaluation of mudstone and shale reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013