Experimental Co-Deposition of Sand and Flocculated Mud From Moving Muddy Suspensions - Implications for Shale Sedimentology

Abstract

Mudstones contain substantial quantities of lamina-forming detrital quartz in the coarse silt size range, and some even contain lamina/lenses of fine to medium sand. Whereas the origin of silty laminae can be understood as a-partitioning phenomenon during bedload transport of flocculated muds, the implications of sandy layers are not well understood. Because of potential bed erosion downstream of sand ripples, and the heavier load imposed by sandy layers, one may wonder whether water-rich surface muds would be eroded prior to active sand accumulation or would collapse due to the added weight. For the interpretation of ancient successions this question is critical because a substantial portion of muds with sand layers/ripples may have been eroded as a consequence. In a new set of experiments fine and medium sand was added to clay-silt mixtures used in a prior set of experiments to explore the origin of silt laminae in mudstones (Sed. Geol. 2017, v. 360, p. 22-34). The experimental conditions were identical and both fine sand and medium sand formed ripples that migrated across a surface of flocculated mud (current deposited, 85 vol % water). These experiments indicate that fine to medium sand can migrate across and be deposited on top of very water-rich muds without significant basal erosion and fabric collapse. Because we see the latter in the rock record, for example in the case of load-casted ripples, one wonders what special conditions prevailed in these cases (even higher water content? Shear warping of surface?) and what additional experimentation is need to explore this issue further. There are several implications from these experiments, one of them being that the presence of sand layers in mudstone successions does no longer imply that a substantial portion of the sedimentary record may have been lost. In addition, sandy layers interspersed with muds will initially at least have much higher permeabilities than the mud layers and enhance migration of fluids from compacting strata. If preserved, the much larger pore spaces of sand layers (when compared with silt layers and the mud matrix) should lead to high permeability zones on the final deposits that could be a critical element for “sweetspot” development within larger volumes of unconventional reservoir rock. Because formation of sand layers ties in with the broader depositional system, understanding sand layer formation may help in predicting and projecting favorable reservoir properties.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019