ABSTRACT

Important differences are discernible between oil basins wholly of Tertiary origin (like some Californian basins) and basins of Mesozoic origin nonethless producing from Tertiary rocks (like the Maracaibo basin). Both sets of basins differ in multiple ways from productive Palaeozoic basins. It is suggested that the differences are due primarily to distant migration, which has enabled oil in older sediments to become pooled far out on the forelands, in shelf sediments and non-orogenic traps, whereas Tertiary oils are still in their basins of origin and in traps of orogenic generation.

Combinations of Mesozoic and Palaeozoic oil are much rarer than combinations of Mesozoic and Tertiary oil. It is suggested that this has two causes: the spacing of orogenic episodes, which left the first half of Mesozoic time essentially quiescent, and the introduction of a totally new basin-creating mechanism during late Mesozoic time. This involved the creation of extensional basins consequent on continental drift.

As oil geologists have been traditionally resistant to the drift hypothesis, it is very likely that there are more new oil provinces still to be found in Mesozoic rocks than in either Tertiary or Palaeozoic. It is also likely that the familiar association between Tertiary oil basins and Tertiary orogenic belts has obscured the fact that the Mesozoic extensional basins opened up by drift continued to be basinal during the early Tertiary; the Gulf Coast is the familiar exemplar. Wherever a large delta was built outwards over the Tertiary breakover between continental and oceanic crust, equally good prospects should be provided. But if the locales are far removed from the Alpine orogenic belts, they will lie well off shorelines defined by the postglacial rise of sea level.

The ideal source sediment is deduced to be one that characterizes many oil provinces and is conspicuously rare or absent in other regions -- a dark-coloured, phosphatic pelite or marl, with organic remains almost entirely planktonic or nektonic and almost wholly microscopic except for fish remains.