The Far East, including Australia, is largely a gas province (the USSR and China are excluded from our study). The Middle East is largely an oil province with respect to Tertiary and Mesozoic reservoirs, and a gas province with respect to lower Mesozoic and Permian reservoirs. The geologic and geochemical parameters which determine the predominance of gas over oil or oil over gas are well known. They include type of source rock, source rock position in the total sedimentary sequence, burial history, temperature gradients, timing of hydrocarbon generation, and trap formation, retention, and related geologic factors. In the Far East, the relation between coal or coaly matter as source rocks and the occurrence of natural gas is obvious. Australia is a striking example. In th Middle East, the Sargelu formation is one of the most prolific source rocks. It is of Middle to Late Jurassic age, kerogenous and fully marine, which, in combination with other factors, explains the predominance of oil in Tertiary and Mesozoic reservoirs in and around the Arabian Gulf. At deeper stratigraphic levels, huge quantities of gas are ascribed to Paleozoic sources, the nature of which has not yet been fully assessed. Much of this gas, especially in southwest Iran, can be regarded as thermally degraded oil.

In areas of intense Neogene deformation (Tertiary basins of Indonesia and Burma), a large part of the gas phase has probably escaped, whereas the oil phase was largely retained. Many examples illustrate the validity

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of the general principles without ignoring the complexity of the gas versus oil problem.

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