Type II kerogens

Fig. 8. Typical IR spectrum of microbial (type II) kerogen. The two insets show specific absorption bands of two other kerogens —720 cm-1 related to long aliphatic chains mostly occurring in type-I kerogen —930 to 700 cm-1 related to aromatic CH (bending out of plane) mostly occuring in very mature kerogen (Adapted from Robin 26>, 1975, in Tissot Welte, 1978)24)...

Figure 3.16 Schematic representation of amount of hydrocarbons generated in, and expelled from a type II kerogen-bearing source rock as a function of organic matter maturity for the initial part of the oil window (after Leythaeuser et al., 1987. Reprinted with permission from the Proceedings 12th World Petroleum Congress, Houston, Vol. 2, Fig. 2a, p. 229).

It is difficult to represent the complex structure of kerogen, but Fig. 4.17 attempts to show the various units present in type II kerogen at the start and towards the end of catagenesis.The elimination ofbulky peripheral substituents and the increasingly compact and aromatic nature of the residual kerogen with increasing maturity are apparent. The increase in aromaticity can be seen in Table 4.8. 

Fig. 4. 18 Typical changes in oxygen-containing functional group content with increasing maturity (represented by burial depth) of type II kerogen from the Kimmeridge Clay Formation in the UK sector of the North Sea (after Patience et al. 1992). Data obtained from 13C NMR analysis, so the functional groups are represented by the signal of the C atom(s) directly bonded to each O atom (n.b. two O atoms are associated with each carboxyl C, but two C atoms with each ester O).

The Spekk Formation with its type II kerogen and its overall lower maturity, compared with the Are Formation, would be expelling petroleum with much higher relative concentrations of biomarkers. This effect will also acp to reduce possible trace signals from palaeo-Are-derived petroleum (gas-condensate) in the traps. 

The Bossier shale ranges from either mixed type 11 and type 111 kerogen in the northwestern part of the basin to pure marine (type II) kerogen in the east, thus corroborating the depositional environment of the Bossier shales. Most of the HI values are less than 150-200 mg HC/g TOC,... 

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