99
5
Kerogen
5.1 INTRODUCTION
The precise details regarding the twin problems of origin, and migration and accumulation of petroleum have yet to be fully answered. Recent advances in analytical chemistry and geochemistry have
advanced the knowledge and understanding, but issues remain to be resolved. Therefore, in any text
dealing with the science of petroleum, there must, of necessity, be a section dealing with kerogen.
This will help the reader to understand kerogen and its place as a naturally occurring organic material. It is not the intent of this chapter to replace the excellent texts already available on the nature
of kerogen (Tissot and Welte, 1978; Durand, 1980; Pelet and Durand, 1984; Vandenbroucke, 2003).
It is hoped that this chapter will assist the reader in understanding the nature of kerogen as well as
the role that kerogen might play in petroleum science. It is anticipated that this chapter will also
serve as an introduction to the more complex constituents of petroleum and aid in an understanding of the nature of the high molecular weight polar constituents (i.e., the asphaltene and the resin
constituents) of petroleum (Chapters 9, 10, and 12).
Petroleum scientists have always searched for new trustful technologies to provide valuable evidences regarding the origin of hydrocarbons found in sedimentary basins. Present-day analysis
of petroleum systems, when performed integrated with direct geochemistry, provides support that
crude oil and natural gas accumulations have a biological origin. The biogenic precursor to oil and
gas is believed to be kerogen.
Kerogen is the naturally occurring, solid, insoluble organic matter found in source rocks that can
yield oil upon heating. Typical organic constituents of kerogen are algae and woody plant material.
Kerogen has a high molecular weight relative to bitumen or soluble organic matter. Bitumen (not to
be confused with tar sand bitumen, Chapter 1) forms from kerogen during petroleum generation.
Kerogen is described as Type I, consisting of mainly algal and amorphous (but presumably algal)
kerogen and highly likely to generate oil; Type II, mixed terrestrial and marine source material
that can generate waxy oil; and Type III, woody terrestrial source material that typically generates
gas. A fourth type, Type IV kerogen is also now recognized (please see discussion in the following
sections).
The term kerogen is used throughout this text to mean the carbonaceous material that occurs in
sedimentary rocks, carbonaceous shale, and oil shale. This carbonaceous material is, for the most
part, insoluble in common organic solvents. A soluble fraction, bitumen, coexists with the kerogen.
The bitumen is not to be confused with the material found in tar sand deposits (Chapter 1).
5.2 PROPERTIES
Kerogen is a solid, waxy, organic substance that forms when pressure and heat from the Earth act
on the remains of plants and animals. Kerogen converts to various liquid and gaseous hydrocarbons at a depth of 7 km or more and a temperature between 50°C and 100°C (122°F and 212°F)
(USGS, 1995). The name kerogen is also generally used for organic matter in sedimentary rocks
that is insoluble in common organic and inorganic solvents. Kerogen also yields oil when the shale
containing kerogen is heated to temperatures sufficient to cause destructive distillation.
The formation of kerogen is very much open to speculation. It can only be assumed that similar
types of plant debris that went into the formation of petroleum also played a role in the formation
5
Kerogen
5.1 INTRODUCTION
The precise details regarding the twin problems of origin, and migration and accumulation of petroleum have yet to be fully answered. Recent advances in analytical chemistry and geochemistry have
advanced the knowledge and understanding, but issues remain to be resolved. Therefore, in any text
dealing with the science of petroleum, there must, of necessity, be a section dealing with kerogen.
This will help the reader to understand kerogen and its place as a naturally occurring organic material. It is not the intent of this chapter to replace the excellent texts already available on the nature
of kerogen (Tissot and Welte, 1978; Durand, 1980; Pelet and Durand, 1984; Vandenbroucke, 2003).
It is hoped that this chapter will assist the reader in understanding the nature of kerogen as well as
the role that kerogen might play in petroleum science. It is anticipated that this chapter will also
serve as an introduction to the more complex constituents of petroleum and aid in an understanding of the nature of the high molecular weight polar constituents (i.e., the asphaltene and the resin
constituents) of petroleum (Chapters 9, 10, and 12).
Petroleum scientists have always searched for new trustful technologies to provide valuable evidences regarding the origin of hydrocarbons found in sedimentary basins. Present-day analysis
of petroleum systems, when performed integrated with direct geochemistry, provides support that
crude oil and natural gas accumulations have a biological origin. The biogenic precursor to oil and
gas is believed to be kerogen.
Kerogen is the naturally occurring, solid, insoluble organic matter found in source rocks that can
yield oil upon heating. Typical organic constituents of kerogen are algae and woody plant material.
Kerogen has a high molecular weight relative to bitumen or soluble organic matter. Bitumen (not to
be confused with tar sand bitumen, Chapter 1) forms from kerogen during petroleum generation.
Kerogen is described as Type I, consisting of mainly algal and amorphous (but presumably algal)
kerogen and highly likely to generate oil; Type II, mixed terrestrial and marine source material
that can generate waxy oil; and Type III, woody terrestrial source material that typically generates
gas. A fourth type, Type IV kerogen is also now recognized (please see discussion in the following
sections).
The term kerogen is used throughout this text to mean the carbonaceous material that occurs in
sedimentary rocks, carbonaceous shale, and oil shale. This carbonaceous material is, for the most
part, insoluble in common organic solvents. A soluble fraction, bitumen, coexists with the kerogen.
The bitumen is not to be confused with the material found in tar sand deposits (Chapter 1).
5.2 PROPERTIES
Kerogen is a solid, waxy, organic substance that forms when pressure and heat from the Earth act
on the remains of plants and animals. Kerogen converts to various liquid and gaseous hydrocarbons at a depth of 7 km or more and a temperature between 50°C and 100°C (122°F and 212°F)
(USGS, 1995). The name kerogen is also generally used for organic matter in sedimentary rocks
that is insoluble in common organic and inorganic solvents. Kerogen also yields oil when the shale
containing kerogen is heated to temperatures sufficient to cause destructive distillation.
The formation of kerogen is very much open to speculation. It can only be assumed that similar
types of plant debris that went into the formation of petroleum also played a role in the formation