Page 93 - Fundamentals of Gas Shale Reservoirs

WATER DEPTH OF MUD TRANSPORT AND DEPOSITION 73
age range of fossils is often too broad to age‐date stratigraphic Schieber and Southard, 2009) have demonstrated that
surfaces or intervals that formed during relatively short‐lived flocculation of clay‐size particles occurs in the laboratory
cycles, usually precluding the ability to definitively correlate and in nature which provides floccules large enough to be
strata within a high‐frequency, time‐stratigraphic framework. transported by currents (i.e., hydraulically equivalent to silt
Paleozoic shales can generally be resolved at third‐order and sand size particles). Floccules have been preserved in
cycles (~1–5 Myr duration) superimposed on a second‐order many Mesozoic and Paleozoic shales (O’Brien and Slatt,
cycle (~10–30 Myr duration) (Fig. 4.1c). Owing to greater 1990; Slatt and O’Brien, 2011). Hyperpycnite muds have
biostratigraphic resolution, Mesozoic and Cenozoic shales been documented for the Cretaceous Lewis Shale (Soynika
can be resolved at fourth‐order cycles (100,000–300,000 and Slatt, 2008) and other rocks in the Cretaceous western
years duration) superimposed on a third‐order cycle, as interior seaway deposits (Bhattacharya and MacEachern,
demonstrated in the following. 2009), as well as in the modern Sea of Japan, where a
transport distance of 700 km has been documented for a
hyperpycnal flows (Nakajima, 2006).
4.4 WaTEr DEPTH OF muD TraNSPOrT It would seem that microfossils such as radiolarian and
aND DEPOSITION coccoliths might offer the best chance of deposition through
the water column as individual particles (i.e., marine snow;
It has long been assumed that because of their fine‐grained Bennett et al., 1991) since they are not electrostatically
nature, precursor muds were deposited in quiet, “relatively charged, as are clay particles. However, even high concen­
deep” ocean waters. More recent studies have shown that trations of biogenic particles can move along the seabed and
siliciclastic muds can be deposited in tidal mud flats erode underlying mud (Abouelresh and Slatt, 2012a, b).
(Rine and Ginsburg, 1985) and shelf to upper slope water The presence of phosphate minerals in shales is often
depths (Loucks and Ruppel, 2007) as well as in deep attributed to upwelling currents. The upwelling model
basins. In addition to “hemipelagic settling” of mud parti­ proposes that cold, deep, oxygen‐deficient, phosphate‐rich
cles, hyperpycnal flows (Bhattacharya and MacEachern, water is drawn along the sea floor until it reaches the
2009; Mulder and Chapron, 2011) and turbidites can also continental slope, where it rises to the shelf edge. Organisms
transport sediment from continental to shelf/slope/basin thrive on the phosphate nutrients, and generate “algal
environments; storm and contour currents can rework blooms” (i.e., modern “red tides”) and further deplete
mud deposited by these processes. dissolved oxygen, creating eutrophication of the water mass
Many resource shales exhibit microsedimentary struc­ and deposition of the phosphates. A second model postulates
tures such as graded beds, cross‐laminations, and cross‐beds, that phytoplankton productivity is increased due to seasonal
indicating current transport along the sea floor (Fig. 4.2) nutrient input from continental weathering and runoff during
(Abouelresh and Slatt, 2012a, b). Such transport requires times of broad shallow seas (Lash and Blood, 2011; Rimmer
particles larger than clay size, as these would tend to be et al., 2004), particularly if the basin is silled (Molinares‐
buoyed upward by a turbulent current. Schieber et al. (2007; Blanco, 2013).

(a) (b)

(c) (d) (e)

FIGurE 4.2 Thin section photographs of sedimentary structures in the Barnett Shale. (a) Irregular (erosional) bottom surface of a siliceous
sponge spicule laminae. (b) Close‐up view of scour surface at base of light‐colored siltstone bed. (c) Ripple stratification, note the clay mate­
rials (black) delineating the ripple marks. (d) Low‐angle cross lamination. (e) Hummocky lamination. Figures from Abouelresh and Slatt
(2012a, b). Reprinted with permission of Central European Journal. Geosciences http://www.degruyter.com/