Lab Experiments Recreating Typical Laminations of Black Shales, Unconventional Thoughts About the Unconventional Eagle Ford Play
The Eagle Ford Fm contains organic-rich laminated marls interbedded with limestones and bentonites. The highest TOC values (7-13 wt%) occur in the lower part, characterized by what at first glance appear to be simple parallel laminae. This mm-scale lamination consists of: 1) dark mudstone laminae with pellets, organic matter and scattered small forams and 2) light laminae of sand size forams and other bioclasts. Contacts between laminae are typically sharp, laminae may also be lenticular and show low-angle cross stratification. Whereas one initially might interpret these laminae as reflecting alternating depositional conditions, in which quiescent hemipelagic background sedimentation (mudstone laminae) is interrupted by plankton blooms (foram laminae) in a generally anoxic setting (lack of macroscopic bioturbation), new flume-tank experiments suggest an alternative interpretation: laminae are generated by currents, interpretation consistent with evidence of erosion and winnowing from outcrop and core. Flume-tank experiments show that the style of lamination recorded in the lower part of Eagle Ford Fm, where TOC is the highest, can be produced by a continuous unidirectional flow of a muddy suspension. The muddy suspension consisted of a mixture of kaolinite clay and plastic beads (with sizes and densities covering the range of the observed forams). At flow velocities of 20 cm/sec these materials migrated as bedload ripples across the flume bottom. Continuous ripple migration and stacking of ripple-deposits produced laminated sediments. With 2 g/l concentration, a 3 cm thick bed was deposited over a 3 days period, equivalent to ca. 5 mm of rock record. If deposition continued at this rate for 1000 years, it would result in 600 m of rock record. In contrast, constraints from dated bentonites put the average compacted accumulation rate for the laminated facies of the lower Eagle Ford at 1-2 cm per 1000 years. Collectively, these observations and deductions suggest that laminated black shales deposited in an intrashelf basin, such as the Eagle Ford Fm, could be the product of bedload deposition from moving suspensions, and that the bulk of the sedimentation would be destroyed by intermittent erosion events and therefore not preserved. Whereas these experiments were just concluded and in-depth analysis will take some time, our observations provide thought provoking alternatives to conventional ways of thinking about black shale accumulation.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014