Shale is a fine-grained clastic sedimentary rock formed by consolidation of clay and silt particles. The presence of fissibility allows shales to be distinguished from mudrocks. Although mudrocks do not exhibit fissibility, it still has similar depositional processes and composition to shales. Most shales are relatively impermeable with the potential to accumulate significant amounts of organic matter resulting in rich hydrocarbon source rocks. In addition to being long-term energy source rocks and prolific unconventional reservoirs, shales are also common seal rocks for hydrocarbon traps.
Due to the development of advanced technology to produce oil and gas from unconventional reservoirs, it is important to understand the properties of unconventional resource shales including its depositional processes. Shales are traditionally thought to be deposited from suspension settling as ‘hemipelagic rain’ in low energy environments . However, recent studies have suggested multiple processes in association with shale deposition: A. Hemipelagic rain; B. Hyperpycnal flow; C. Turbidity current flow; and D. Tempestites  (Figure 1).
Figure 1. Potential transport, deposition, and reworking processes associated with shale deposition. A: Hemipelagic rain, B: Hyperpycnal flow, C: Turbidity current flow, D: Tempestite (shelf storm deposits).
Recent studies using flume experiments suggest that clay-sized particles can only move along the seafloor bottom in the presence of hydraulic processes similar to the transport of coarser grains . The hydraulic component is attained through the formation of ‘floccules’ (Figure 2) promoting fine-grained sediment transport and creating mud ripples which are vital to the transportation of large volumes of mud (Figure 3). The phenomenon of flocculation not only enhances the deposition of fine-grained sediments, but also proves that shale deposition is not confined to low energy conditions; and instead, it occurs universally over short time intervals .
Figure 2. A: Illustration of flocculated clay particles in the form of ‘domains’. B: Flocculated clay in salt water. C: Floccules in varved clay from Great Salt Lake in Utah.
Figure 3. Mud ripple obtained through flume tank experiments. Flocculated particles move along the flume tank floor by tractive processes.
Hemipelagic sediment or hemipelagite consists of fine-grained sediments of terrigenous and biogenic material derived from close continental margins or near terrigenous sediment sources . When hemipelagic sediments from suspension settle down to the bottom of the seafloor, it creates a semi-parallel arrangement of the grains which after compaction and cementation forms shales. This process is called ‘hemipelagic rain’ and was widely assumed for many several years as the only shale depositional mechanism (Figure 1A) . In seismic data, hemipelagic sediments tend to appear with high to moderate amplitudes, undeformed, and laterally continuous with notches (Figure 4) .
Figure 4. Example of mapped seismic units and its respective facies from Offshore Angola. The changes in acoustic impedance are indicated by a downward increase (blue) and downward decrease (red). Note the upper Pliocene hemipelagites and how the semi-parallel to parallel arrangement is preserved
Hyperpycnal flow occurs when freshwater containing sediments discharges into the ocean basin. Since freshwater has a lower density than seawater, freshwater sediments float on the surface as hypopycnal flow or sediment plumes (Figure 1B). During large or extreme floods, the ocean basin then becomes saturated with sediments traveling along the seafloor bottom as hyperpycnal flow  (Figure 5). Advanced studies have been conducted to understand hyperpycnal flow motion as well as its relation to other deposits ; however, hypopycnal flow tends to appear more laterally continuous than hyperpycnal flow.
Figure 5. Proposed model for depositional processes of Barnett Shale lithofacies. Note transportation mechanisms of hyperpycnal flow, hypopycnal flow, turbidity currents, and hemipelagic plumes.
Turbidity Current Flow
Turbidity current flow is another shale depositional process partially governed by gravity; that is, transportation and sedimentation from shallow to deep-water environments . Turbidity currents (Figure 1C) occur when sediments build up on a continental shelf and fail as slides due to geological disruptions. The turbid water then moves downslope carrying sediments and gaining velocity as it flows. The turbidity current grades as the sediments are deposited, creating a bouma sequence with finer grains on top. On seismic profiles, turbidity current deposits usually present continuous strong and multiple parallel reflectors indicating stratified sandy and muddy deposits; in some cases, the reflectors can appear slightly chaotic (Figure 4) .
Tempestites or shelf storm deposits are products of storms that result in currents extending just below the wave base in shallow shelf environments (Figure 1D) . Tempestites are not great contributors to new sediments, but the reworked sediments from storms still contain evidence of depositional energy reduction in the form of normal grading or specific sedimentary structures (wavy to lenticular bedding) . Though storm deposits reflect a vast range of erosional and depositional models, shale facies are still formed from this type of depositional process.
- Schlumberger glossary (shale)
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