104
The Chemistry and Technology of Petroleum
functions. At the beginning of the type II path, and in general for intermediate paths between type I
and type III, elemental analysis supplies little information about the chemical structure.
5.5 ISOLATION
The first step in any study of the behavior and structure of kerogen has generally been the isolation of a
kerogen concentrate (Forsman and Hunt, 1958; Forsman, 1963; Robinson, 1969; Saxby, 1976; Durand,
1980). A variety of methods can be employed to isolate fractions of organic material without altering the structure of the native kerogen. There are also those methods intended for degradation of the
organic material in a controlled manner. The terminology of the material isolated by such methods is
often based upon the method employed. Therefore, an understanding of these methods assists in understanding the terminology. For example, a particular method may result in the generation of hydrocarbon
products as well as more complex products that are heteroatomic and/or high in molecular weight.
Physical methods to produce an organic-rich kerogen concentrate are of interest because exposure of the kerogen to strong acid and/or base is avoided, thereby lessening the chance of chemical alteration. Such methods generally involve the potential for contamination of the kerogen with
materials used to effect the separation. However, in many cases the potential impact of such contaminants can be limited by using only one or a small number of known and easily identified
chemical species. Among the more important physical methods for kerogen concentration are
sink-float, oil agglomeration, and froth flotation method (Hubbard et al., 1952; Vadovic, 1983).
By far the most common technique for kerogen isolation involves acid demineralization of the
shale to produce the kerogen concentrate. To dissolve the mineral matrix, a series of successive
treatments with a hydrochloric acid–hydrofluoric acid mixture (at approximately 65°C, 150°F) is
employed. On the other hand, demineralization with a reduced chance of organic alteration has been
achieved by carrying out the treatment at a lower temperature (20°C, 70°F) and for shorter times
(Scouten et al., 1987). The use of base to dissolve silicates has also been investigated followed by an
acid treatment to dissolve carbonates.
The isolation of kerogen from mineral matrices also depends upon the extent of the interactions
between the kerogen and the various minerals. From the results of model compound–model mineral
tests, interactions between acid clay minerals and nitrogen-containing organic compounds have been
identified as much stronger than other likely candidates for kerogen–mineral interactions (Siskin
et al., 1987a,b). The importance of this finding led to the use of differential wetting, a phenomenon
typically associated with physical separation methods, as critical to the success of kerogen separation. Thus, efficient kerogen recovery can be achieved by adding an organic solvent that wets and
swells the kerogen, thereby diminishing some of the nitrogen–mineral interactions and aiding the
physical sink-float separation. Thus both chemical and physical aspects are important for the production under mild conditions of a kerogen concentrate with, presumably, minimal structural alterations.
The laboratory sink-float methods offer mild conditions to minimize chemical alteration and can
produce a kerogen concentrate with low ash content. However, there are disadvantages to this technique, which include (1) rejection of organic compounds (leading to low recovery of the kerogen)
and (2) the possibility of kerogen fractionation along with mineral rejection. The oil agglomeration
method relies on selective wetting of kerogen particles by an oily paste material such as hexadecane
(Robinson, 1969).
5.6 METHODS FOR PROBING KEROGEN STRUCTURE
5.6.1 ultImAte (elementAl) AnAlysIs
Although not strictly a method for probing the structure of kerogen, elemental analysis offers valuable
information about the atomic constituents of kerogen. The elemental analysis of kerogen is a method
for characterizing the origin and evolution of sedimentary organic matter. Elemental analysis also
establishes a framework within which other physicochemical methods can be used more effectively.
The Chemistry and Technology of Petroleum
functions. At the beginning of the type II path, and in general for intermediate paths between type I
and type III, elemental analysis supplies little information about the chemical structure.
5.5 ISOLATION
The first step in any study of the behavior and structure of kerogen has generally been the isolation of a
kerogen concentrate (Forsman and Hunt, 1958; Forsman, 1963; Robinson, 1969; Saxby, 1976; Durand,
1980). A variety of methods can be employed to isolate fractions of organic material without altering the structure of the native kerogen. There are also those methods intended for degradation of the
organic material in a controlled manner. The terminology of the material isolated by such methods is
often based upon the method employed. Therefore, an understanding of these methods assists in understanding the terminology. For example, a particular method may result in the generation of hydrocarbon
products as well as more complex products that are heteroatomic and/or high in molecular weight.
Physical methods to produce an organic-rich kerogen concentrate are of interest because exposure of the kerogen to strong acid and/or base is avoided, thereby lessening the chance of chemical alteration. Such methods generally involve the potential for contamination of the kerogen with
materials used to effect the separation. However, in many cases the potential impact of such contaminants can be limited by using only one or a small number of known and easily identified
chemical species. Among the more important physical methods for kerogen concentration are
sink-float, oil agglomeration, and froth flotation method (Hubbard et al., 1952; Vadovic, 1983).
By far the most common technique for kerogen isolation involves acid demineralization of the
shale to produce the kerogen concentrate. To dissolve the mineral matrix, a series of successive
treatments with a hydrochloric acid–hydrofluoric acid mixture (at approximately 65°C, 150°F) is
employed. On the other hand, demineralization with a reduced chance of organic alteration has been
achieved by carrying out the treatment at a lower temperature (20°C, 70°F) and for shorter times
(Scouten et al., 1987). The use of base to dissolve silicates has also been investigated followed by an
acid treatment to dissolve carbonates.
The isolation of kerogen from mineral matrices also depends upon the extent of the interactions
between the kerogen and the various minerals. From the results of model compound–model mineral
tests, interactions between acid clay minerals and nitrogen-containing organic compounds have been
identified as much stronger than other likely candidates for kerogen–mineral interactions (Siskin
et al., 1987a,b). The importance of this finding led to the use of differential wetting, a phenomenon
typically associated with physical separation methods, as critical to the success of kerogen separation. Thus, efficient kerogen recovery can be achieved by adding an organic solvent that wets and
swells the kerogen, thereby diminishing some of the nitrogen–mineral interactions and aiding the
physical sink-float separation. Thus both chemical and physical aspects are important for the production under mild conditions of a kerogen concentrate with, presumably, minimal structural alterations.
The laboratory sink-float methods offer mild conditions to minimize chemical alteration and can
produce a kerogen concentrate with low ash content. However, there are disadvantages to this technique, which include (1) rejection of organic compounds (leading to low recovery of the kerogen)
and (2) the possibility of kerogen fractionation along with mineral rejection. The oil agglomeration
method relies on selective wetting of kerogen particles by an oily paste material such as hexadecane
(Robinson, 1969).
5.6 METHODS FOR PROBING KEROGEN STRUCTURE
5.6.1 ultImAte (elementAl) AnAlysIs
Although not strictly a method for probing the structure of kerogen, elemental analysis offers valuable
information about the atomic constituents of kerogen. The elemental analysis of kerogen is a method
for characterizing the origin and evolution of sedimentary organic matter. Elemental analysis also
establishes a framework within which other physicochemical methods can be used more effectively.