Page 51 - Fundamentals of Gas Shale Reservoirs

GEOGRAPHIC DISTRIBUTION OF ORGANIC MATTER‐RICH SHALES 31
0º 30ºE 60ºE 90ºE 120ºE 150ºE 180º 150ºW 120ºW 90ºW 60ºW 30ºW
Greenland

Arctic Circle
Arctic Circle
Asia
60ºN
Europe
North
America
30ºN
Tropic of Cancer
Tropic of Cancer
Africa
0º Equator South
Equator
America
Tropic of Capricorn
Tropic of Capricorn Australia
30ºS

60ºS

Antarctic Circle
Antarctic Circle
Antarctica

Calcareous ooze Pelagic clay or no deposit Glacial debris
Siliceous ooze Terrigenous sediments Shelf deposits

FIGURE 2.5 The general pattern of sediment cover of the seafloor. This pattern has been known more or less in its present state since the
work of Murray and Renard (1891) following the voyage of H.M.S. Challenger. The main sediment facies are pelagic clay and calcareous
ooze. Pelagic clay, also known as red clay, is typical for the deep seafloor, whereas the calcareous facies outlines oceanic rises and platforms.
Biosiliceous deposits, characteristic for the seafloor beneath areas of high fertility, are superimposed on the previously mentioned topograph-
ically controlled dichotomy. The sediment that results in pelagic clay deposits, which is mostly eolian and cosmogenous dust associated with
hydrogenous minerals, is present everywhere in the ocean, but it is masked whenever another component is present, because its mass
accumulation rates are so low. Terrigenous sediment is present along the continental margins. In front of large rivers and submarine canyons,
terrigenous sediment can penetrate the ocean basin to a considerable distance from the shelf. Even though this map shows sharp boundaries
between well‐defined facies, the seafloor is actually covered by a complex pattern of overlapping mixtures of the various components of
fine‐grained marine sediments (Fig. 2.1).

higher than today (Fig. 2.2). Terrigenous sediment is mud beds that were deposited relatively quickly (e.g., Baird,
efficiently trapped in nearshore environments during high- 1976; Macquaker and Howell, 1999; Schieber, 1994, 2003;
stands, and thus large areas of those shelves must have been Trabucho‐Alexandre, 2014). The gaps reflect periods during
starved of new terrigenous sediment input; their terrigenous which no sediment accumulated, either because there was no
mud was probably largely derived from longshore and relict sediment to accumulate, or because sediment was being
offshore sources. Nevertheless, unlike black shales recovered removed from one area of the shelf to be deposited elsewhere
in the deep sea by ocean drilling, which are never more than on the shelf or in deeper water.
tens of centimeters in thickness, epicontinental black shales In addition to terrigenous material, neritic sediments almost
are usually meters to tens of meters thick, for example, the always contain biogenous material (Fig. 2.1), which is a
Devonian Chattanooga Shale (ca. 9 m in central Tennessee) product of high fertility of surface water along continental
and the Pliensbachian–Toarcian shales of northwest Europe margins. This is due to a high position or breakdown of the
(>100 m in northeast England). The thickness of these thermocline, which normally functions as a barrier to nutrient
successions does not necessarily equate to high terrigenous transport between deep and shallow, sunlit water. Upward
input, because the successions often represent very long mixing of nutrient‐rich water containing dissolved silica leads
periods of time represented in the rock record by gaps and by to high productivity in the photic zone and to a high proportion