Page 48 - Fundamentals of Gas Shale Reservoirs

28 ORGANIC MATTER‐RICH SHALE DEPOSITIONAL ENVIRONMENTS

(a) (b) (c) (d)
Deep, enclosed basin; Deep borderland basin; Western continental slope; Seamount;
positive water balance O 2 minimum zone coastal upwelling O 2 minimum zone
(e.g., Black Sea) (e.g., S. California) (e.g., Peru, Namibia) (e.g., Seine Seamount)

(e) (f) (g) (h)
Shallow strati ed basin; Coastal/intertidal zone; Paralic environments; Alluvial environments;
estuaries, fjords, etc. organic matter-rich sediments organic matter-rich sediments strati cation
(e.g., Baltic Sea) (e.g., lagoons, tidal ats) (e.g., Mississippi delta) (e.g., Lake Tanganyika)

Oxic
Anoxic
(normal marine salinity)
Dysoxic Oxic
(normal marine salinity) (low marine salinity)
FIGURE 2.3 Summary of the environments of accumulation of organic matter showing an idealized basin physiography, water mass
distribution and properties, and prevalent climate (rain cloud indicates positive water balance, cloud streamers indicate offshore winds). This
figure is an adaptation and expansion of figure 1 in Arthur and Sageman (1994, p. 507).

favored a deepwater environment similar to the bottom of analog for shelf or open ocean sedimentation in the geologic
the modern Black Sea. Pettijohn (1975, p. 284) summarized record. Tyson (2005, p. 29) summed up this idea by stating
this debate as follows: “[t]he origin of black shales has been that the “Black Sea is […] a freak of paleogeography and has
much debated. Certainly they were deposited under anaer- very specific circumstances that are unlikely to be common
obic conditions. [emphasis added] How such conditions in the geological record.” Although some authors argued for
were achieved is less certain. […] Some writers contend that an open marine origin for black shales, the view that black
black shales were deep‐marine (geosynclinal) sediments; shales were deposited in restricted basins conducive to
others have postulated comparatively shallow waters, either strongly reducing conditions, and for which the Black Sea
lagoonal or marine.” may be a good analog, was largely prevalent. Twenhofel
It is obvious from Pettijohn’s remark that reducing (1939), for example, argued for an open marine origin for
environments, whether shallow or deep, were considered the ancient black shales in general, but favored the Black Sea
key control behind the deposition of black, organic matter‐ as an analog for the Paleozoic shales of northwest Europe
rich mud. The idea that black shales are the product of and the Appalachian basins of North America. Fleming and
sedimentation in reducing environments was reinforced Revelle (1939), who discussed the role of oceanographic
early on by the study of black muds in the Black Sea processes on dissolved oxygen distribution in the water
(Pompeckj, 1901; Schuchert, 1915) and in Norwegian fjords column, also gave the Black Sea and the borderland basins
(Strøm, 1939). The Black Sea (Fig. 2.3a) has been used of Southern California as examples of modern environments
extensively as a model for the deposition of ancient of black mud deposition (Fig. 2.3a and b), and thus empha-
epicontinental and open ocean black shales. However, sized the role of sill depth in controlling the rate of renewal
the presence of a halocline in the Holocene Black Sea and the of bottom water and oxygen replenishment.
fact that it is significantly deeper (ca. 2000 m) than ancient While early authors placed great emphasis on the role of
epicontinental seas (ca. 100 m), and therefore characterized anoxia, some noted the importance of primary productivity
by a different depth‐to‐width ratio, preclude its use as an in generating organic matter‐rich sediments. Goldman