2 Key ingredients for petroleum accumulation
2.5: Combining the ingredients
Having examined the essential ingredients for a petroleum accumulation, this section discusses how knowledge about them is combined to create a petroleum play. This is a particularly useful concept, since it consolidates what is known (or not known) about the petroleum potential of a particular level within a basin and forms the basic strategy for oil and gas exploration. A play is defined as a perception or model of how a petroleum charge system, reservoir, seal and trap may combine to produce petroleum accumulations at a specific stratigraphic level. By examining whether each of the play ingredients is both present and effective, it is possible to define parts of a basin where petroleum accumulations can reasonably be expected to exist. This process can be conducted systematically in a given area to generate a play fairway map that depicts where the ingredients coexist, even though the precise details of trap location and size may not be known. As more data become available the play becomes better defined, but even when the play is proven by a discovery it does not imply that every trap within the same fairway will contain a petroleum accumulation. It is in the nature of exploration that more often than not geoscientists are wrong with their predictions, but this approach at least helps to reduce their uncertainty.
To illustrate the petroleum play approach an example is provided from the Upper Jurassic of the North Sea (Table 4).
Table 4: Upper Jurassic petroleum plays of the North Sea.
There are two play types: the Kimmeridgian–Volgian deep marine play and the slightly older Oxfordian–Volgian shallow marine play. Whilst the depositional settings for the two reservoir types are quite different, both plays share the same petroleum charge, seal and trap ingredients. Importantly, notice that the onset of petroleum generation comfortably post-dates trap formation. For simplicity the two plays can be combined and their distribution plotted to create an Upper Jurassic play fairway map (Figure 5). This illustrates the close correspondence between the limit of mature Upper Jurassic Kimmeridge Clay source rocks and the fairway, such that the migration pathways between the two are short (less than 15 km) and highly permeable. More than 60 Upper Jurassic fields have been discovered to date beneath the North Sea. They have combined oil reserves of about 2.5×109 toe (tonnes of oil equivalent) and account for 23% of total North Sea production (Evans et al., 2003).