334
8 ALLOCHTHONOUS SEDIMENTS
Marl
Calcilutite
/
LIME
CLAY MINERALS
Orthoclaystone
Organic shale
Oil shale
Sapropelite
Bituminous
,,,
limestone
ORGANIC
MATTER
Fig. 8.5. Triangular diagram illustrating the nomenclature and composition of the mudrocks.
those hydrocarbon compounds which are not soluble in normal petroleum solvents,
such as ether, acetone, benzene, or chloroform (Dott and Reynolds, 1969, p. 79). Kerogen is the major constituent of the organic-rich mudrocks (see also Tissot and Welte,
1984; Hunt, 1996).
Asphalt, or bitumen, is similar to kerogen, but it is soluble in normal petroleum solvents. It contains 80-85% carbon, 9-10% hydrogen, and 2-8% sulfur, with traces of
nitrogen and negligible oxygen. Asphalt occurs both in infilling sediment pores and in
fractures.
Hydrocarbons that are liquid at normal temperatures and pressure are termed "crude
oil." Oil contains 82-87% carbon, 12-15% hydrogen, and traces of sulfur, nitrogen,
and oxygen. Oils are extremely variable in molecular composition but generally consist
of varying proportions of four main groups: paraffins, aromatics, naphthenes, and asphalts. Crude oil occurs in pore spaces of many rocks in favorable circumstances.
Table 8.2 summarizes the properties and composition of these various hydrocarbons,
including natural gas for the sake of completeness. Mudrocks with substantial traces
of organic matter are simply referred to as organic claystones. They are generally dark
colored. Oil shales are mudrocks that are sufficiently rich in organic matter to yield free
oil on heating (Duncan, 1967). They form in restricted anaerobic low-energy environments, both marine and nonmarine. A notable oil shale occurs in the Green River Formation of Utah, Colorado, and Wyoming. This was deposited in a series of Eocene lakes
(Donell et al., 1967). The presence of desiccation cracks and other shallow water features together with juxtaposition to trona (sodium carbonate) suggest a continental
sabkha depositional environment (Surdam and Wolfbauer, 1973).
Another notable oil shale occurs in the Carboniferous deltaic facies of the Midland
8 ALLOCHTHONOUS SEDIMENTS
Marl
Calcilutite
/
LIME
CLAY MINERALS
Orthoclaystone
Organic shale
Oil shale
Sapropelite
Bituminous
,,,
limestone
ORGANIC
MATTER
Fig. 8.5. Triangular diagram illustrating the nomenclature and composition of the mudrocks.
those hydrocarbon compounds which are not soluble in normal petroleum solvents,
such as ether, acetone, benzene, or chloroform (Dott and Reynolds, 1969, p. 79). Kerogen is the major constituent of the organic-rich mudrocks (see also Tissot and Welte,
1984; Hunt, 1996).
Asphalt, or bitumen, is similar to kerogen, but it is soluble in normal petroleum solvents. It contains 80-85% carbon, 9-10% hydrogen, and 2-8% sulfur, with traces of
nitrogen and negligible oxygen. Asphalt occurs both in infilling sediment pores and in
fractures.
Hydrocarbons that are liquid at normal temperatures and pressure are termed "crude
oil." Oil contains 82-87% carbon, 12-15% hydrogen, and traces of sulfur, nitrogen,
and oxygen. Oils are extremely variable in molecular composition but generally consist
of varying proportions of four main groups: paraffins, aromatics, naphthenes, and asphalts. Crude oil occurs in pore spaces of many rocks in favorable circumstances.
Table 8.2 summarizes the properties and composition of these various hydrocarbons,
including natural gas for the sake of completeness. Mudrocks with substantial traces
of organic matter are simply referred to as organic claystones. They are generally dark
colored. Oil shales are mudrocks that are sufficiently rich in organic matter to yield free
oil on heating (Duncan, 1967). They form in restricted anaerobic low-energy environments, both marine and nonmarine. A notable oil shale occurs in the Green River Formation of Utah, Colorado, and Wyoming. This was deposited in a series of Eocene lakes
(Donell et al., 1967). The presence of desiccation cracks and other shallow water features together with juxtaposition to trona (sodium carbonate) suggest a continental
sabkha depositional environment (Surdam and Wolfbauer, 1973).
Another notable oil shale occurs in the Carboniferous deltaic facies of the Midland