Topic 4 Petroleum Generation
Average shales contain approximately 1 percent organic matter (Hunt, 1972). About 90 percent of this is the high molecular weight, insoluble, polymeric material called kerogen and the rest is solvent soluble and is the bitumen fraction (Figure 4.1). The bitumens change in amount and composition in response to changing biological and physical conditions. This leads to important differences in chemical composition between the bitumens in recent and ancient sediments, particularly for the smaller molecules (Table 4.1). The classification of sediments into “recent” or “ancient” is a matter of convenience only and does not reflect any sharp boundary — rather there is a gradual transition from one to the other. Similarly with the organic matter there is a gradual change from the composition found in recent sediments to that found in the older rocks. In this topic we will be concerned with the systematic variations in the composition of the organic matter and with the factors which cause the changes.
The biochemical processes that operate in living systems generate a wide variety of organic compounds. Some of these are not stable and do not survive when the organism dies, but others are very stable and survive deep into the sedimentary sequence. Thus the organic matter which arrives in a sediment from the overlying water column contains many compounds with varying stabilities and these react at different rates in response to changing conditions.
Thermodynamic calculations show that in a reducing environment, methane arid carbon are the stable end products produced by the