Page 47 - Fundamentals of Gas Shale Reservoirs

GEOGRAPHIC DISTRIBUTION OF ORGANIC MATTER‐RICH SHALES 27
intervals have provided more than 90% of the world’s known climates will therefore contain lower initial oxygen content
conventional hydrocarbon reserves (Klemme and Ulmishek, than present‐day bottom water (Berger, 1974).
5
1991; Tissot, 1979). Overmature oil‐prone source rocks It has long been recognized that tectonic processes have
deposited during these six stratigraphic intervals also function the potential to affect global climate (Chamberlin, 1897),
as reservoirs in the increasingly explored and exploited uncon- over both extremely short (e.g., Storey et al., 2007) and long
ventional shale gas plays. (e.g., Raymo and Ruddiman, 1992) timescales. Climate,
Correlations between geologic anomalies and black shale which is also forced by subtle cyclic variations in the earth’s
deposition (Condie, 2004; Kerr, 1998; Larson, 1991; axis and orbit (de Boer and Smith, 1994), plays a fundamental
Sheridan, 1987; Sinton and Duncan, 1997) may explain the role in the evolution of sedimentary environments on earth.
stratigraphic clustering of black shales in specific Seafloor spreading and mountain building, both driven by
stratigraphic intervals of the Phanerozoic (Klemme and tectonic processes, control earth’s climate over long time-
Ulmishek, 1991; Tissot, 1979; Trabucho‐Alexandre et al., scales. These two processes lead to changes in CO input by
2
2012b). Phanerozoic intervals characterized by enhanced volcanism and dissociation of subducted limestones, and to
tectonic activity, namely, supercontinent breakup, ocean changes in CO removal by weathering of silicates and
2
basin formation, and large igneous province emplacement, organic matter burial (Berner, 1991). Volcanism related to
are associated with greenhouse climates, eustatic high- plate tectonic processes can also drive rapid climate change
stands, vigorous ocean circulation, and abundant nutrients in both directly due to faster seafloor spreading rates (Berner
seawater (Fig. 2.2). Warm and humid greenhouse climates et al., 1983), increasing length of oceanic ridges, and
support abundant life, such as highly productive tropical rain extrusion of large igneous provinces (e.g., Kerr, 1998; Sinton
forests on land, reef communities on the shelf, whose area is and Duncan, 1997), and indirectly due to greenhouse gas
greatly expanded during eustatic highstands, and abundant generation as a consequence of increased seawater tempera-
plankton in the ocean. Abundant nutrients in seawater are a tures (e.g., Dickens et al., 1995; Hesselbo et al., 2000) and
product of intense seafloor spreading activity and large contact metamorphism (e.g., McElwain et al., 2005; Storey
igneous province emplacement, increased circulation of et al., 2007).
deep, nutrient‐rich water, and an enhanced hydrologic cycle Relative sea level, which depends both on global and local
(e.g., Larson, 1991; Sinton and Duncan, 1997; Trabucho‐ tectonics and on climate, controls the size and distribution of
Alexandre et al., 2010). paralic and shallow marine environments, where most ancient
Tectonic processes lead to changes in the geography of black shales were deposited (Arthur and Sageman, 2005;
the earth and to the evolution of depositional environments Hedges and Keil, 1995; Laws et al., 2000; Walsh, 1991;
through time (Chamberlin, 1909; Scotese, 2004; Wilson, Wignall, 1991). It is therefore unsurprising that intervals of
1968). The geographic distribution of Phanerozoic black relatively widespread black shale deposition should coincide
shales is largely independent of latitude but instead with eustatic highstands. High sea levels favor the deposition
related to the distribution of continental masses (Irving of black shales (Duval et al., 1998) by expanding sunlit shallow
et al., 1974; Klemme and Ulmishek, 1991). The distribu- marine environments where primary productivity is high and
tion of continents (and ocean basins) controls the position export paths short. Moreover, during transgressions and early
of landmasses relative to climate belts, the opening and highstands, coarser grained siliciclastic material is trapped in
closure of gateways (i.e., basin connectivity), and hence nearshore environments, such as, estuaries, reducing excessive
ocean circulation. Ocean circulation affects seawater dilution of organic matter on the shelf. The composition,
temperature, oxygenation, and nutrient content. The pres- including organic matter type and content, and texture of
ervation of organic matter on the seafloor of marine basins marine sediments are thus a function of depositional environ-
appears to be aided by a latitudinal position of continents ment and of allogenic forcing mechanisms acting on them.
that obstructs meridional ocean circulation. This position,
typical of the Mesozoic, inhibits the formation and spread
of cold, oxygenated, high‐latitude deep water which pro- 2.4 GEOGRAPHIC DISTRIBUTION OF ORGANIC
motes the destruction of organic matter in deeper water. MATTER‐RICH SHALES
Global climate also exerts an important control in this
regard, because bottom water cannot be colder than the 2.4.1 Background
coldest surface water; bottom water during greenhouse
In the early scientific literature, the main debate concerning
the origin of black shales focused on whether they had been
5 Oil‐prone kerogens have higher capacities for hydrocarbon generation per deposited in shallow or deep water (Cluff, 1981). Hard
unit organic carbon than gas‐prone kerogens. Although gas‐prone source (1931), for example, interpreted the Devonian black shales
rocks generate large amounts of gas at high maturity, late stage gas generation of New York as having been deposited in shallow water
and cracking of residual oil/bitumen in oil‐prone source rocks can account for
more gas generation than gas‐prone source rocks (Dembicki, 2013). under toxic and saline conditions, whereas Clarke (1904)