Abstract

We identify four sets of factors governing oil occurrence—climate (especially temperature), mineral nutrients, tectonic factors controlling initial basin formation, and tectonic factors controlling preservation of the oil. We argue that all factors are themselves subject to the framework imposed by plate tectonics. If we are to consider all Phanerozoic oil deposits, the only factor capable of quantitative comparison for all the periods is the first one, in that it is partly a function of latitude.A paleolatitude analysis has been made for both reservoir rocks and preferred source rocks for all petroliferous basins, with results weighted according to total reserves. No statistically satisfactory relationship was found between oil and paleolatitude that would embrace all Phanerozoic deposits. Most Paleozoic oil was formed in rocks deposited in low latitudes, but this may be an accident of preservation. The much larger Mesozoic deposits were similarly related to low paleolatitudes, but this result is heavily biased by the huge reserves of the Persian Gulf. If these are excluded, Mesozoic oil occurs with equal probability in high and in low paleolatitudes. Cenozoic oil is uniformly distributed with respect to paleolatitude.The distribution of oil with time reveals that 71% of all known oil was probably formed in the late Mesozoic, most of it (60%) in the mid-Cretaceous. The first requirement in any general theory of oil occurrence, therefore, is to understand why so much oil was formed near the present Persian Gulf, and to a lesser extent in Middle America, during such a short interval of geological time. We attempt to show that all four controlling factors were optimized in these two places for this brief time-span. In the timetable of plate tectonics, two large marine embayments opened astride the equator in the late Mesozoic, and these may or may not have been connected through the western Mediterranean. One embayment contained the Persian Gulf, and the other, Middle America. The renewal of mantle convection at about −100 m.y. activated these embayments, abruptly increased the rate of sea-floor spreading, and enlarged the oceanic ridges, causing maximum development of warm, shallow seas and releasing, through igneous activity, greatly increased quantities of mineral nutrients.The geometry of subsequent plate activity was such that the Persian Gulf was tectonically protected by the rapid northward movement of the Indian plate (which absorbed most of the impact with the Eurasian plate), and the Gulf of Mexico was protected by the northeastward movement of the Antillean arc.

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