8.3 MUDROCKS
333
CLAY
80
o5
:~
80
15 ~~,~ ~~
7s,,,
Silty send Sandy silt
sAND
SILT
Fig. 8.4. An end-member triangle for classifying unconsolidated sediment on the basis of grain size. (From
Shepard, 1954. Courtesy of the Society for Sedimentary Geology.)
detrital grains, organic matter, and carbonates. The clay minerals are described in some
detail later. The detrital grains consist of fine angular particles of quartz, micas, and
heavy minerals such as zircon and apatite. Organic matter in mudrocks is chemically
very complex and the nomenclature organic mudrocks is not rigidly defined.
Essentially mudrocks can be named with reference to an end-member triangle whose
apices represent pure organic matter, pure lime, and pure clay minerals (Fig. 8.5). Mudrocks composed largely of admixtures of various clay minerals can be termed "claystones," or more pedantically, "orthoclaystones," to indicate their purity. With increasing lime content, claystones grade into marls and on into micrites, which are pure lime
mudrocks.
8.3.1 Sapropelites, Oil Shales, and Oil Source Rocks
Organic matter is present in at least small quantities in all sediments. It is, however,
most abundant in the mudrocks and has been studied intensively, both because organicrich shales are commonly thought to be the source of crude oil, and because certain mudrocks, termed "oil shales," release crude oil on heating (Yen and Chilingarian, 1976).
Sapropelite is a name given to organic-rich mudrocks.
Essentially the organic matter in sediments is of four types: kerogen, asphalt, crude
oil, and natural gas. These are organic compounds of great complexity. Their nomenclature is ill defined and these four groups span a continuous spectrum of hydrocarbons.
Kerogen is a dark gray-black amorphous solid that is present in varying quantities in
mudrocks; when pure it is termed "coal." It contains between 70 and 80% carbon, 7 and
11% hydrogen, 10 and 15% oxygen, and traces of nitrogen and sulfur. The exact molecular structure of kerogen is not well known, but, by definition, kerogen includes
333
CLAY
80
o5
:~
80
15 ~~,~ ~~
7s,,,
Silty send Sandy silt
sAND
SILT
Fig. 8.4. An end-member triangle for classifying unconsolidated sediment on the basis of grain size. (From
Shepard, 1954. Courtesy of the Society for Sedimentary Geology.)
detrital grains, organic matter, and carbonates. The clay minerals are described in some
detail later. The detrital grains consist of fine angular particles of quartz, micas, and
heavy minerals such as zircon and apatite. Organic matter in mudrocks is chemically
very complex and the nomenclature organic mudrocks is not rigidly defined.
Essentially mudrocks can be named with reference to an end-member triangle whose
apices represent pure organic matter, pure lime, and pure clay minerals (Fig. 8.5). Mudrocks composed largely of admixtures of various clay minerals can be termed "claystones," or more pedantically, "orthoclaystones," to indicate their purity. With increasing lime content, claystones grade into marls and on into micrites, which are pure lime
mudrocks.
8.3.1 Sapropelites, Oil Shales, and Oil Source Rocks
Organic matter is present in at least small quantities in all sediments. It is, however,
most abundant in the mudrocks and has been studied intensively, both because organicrich shales are commonly thought to be the source of crude oil, and because certain mudrocks, termed "oil shales," release crude oil on heating (Yen and Chilingarian, 1976).
Sapropelite is a name given to organic-rich mudrocks.
Essentially the organic matter in sediments is of four types: kerogen, asphalt, crude
oil, and natural gas. These are organic compounds of great complexity. Their nomenclature is ill defined and these four groups span a continuous spectrum of hydrocarbons.
Kerogen is a dark gray-black amorphous solid that is present in varying quantities in
mudrocks; when pure it is termed "coal." It contains between 70 and 80% carbon, 7 and
11% hydrogen, 10 and 15% oxygen, and traces of nitrogen and sulfur. The exact molecular structure of kerogen is not well known, but, by definition, kerogen includes