Abstract: Indigenous Precambrian Petroleum Revisted

Grover E. Murray

In 1964-65 I pointed out that indigenous hydrocarbons are present in strata, then classed as late Precambrian, in the Amadeus basin of central Australia and, because of the widespread occurrences of early forms of life already documented at that time, I inferred (1) that these strata could be potential new frontiers for exploration in places where they are sufficiently unmetamorphosed and (2) that they should be the object of planned exploration. Subsequently, these inferences have been substantiated by discoveries of significant quantities of oil and gas in the Irkutsk region of the USSR in strata classed by the Russians as late Precambrian (Riphean and Vendian). The age of the oldest of the producing zones, assigned by the Russians to the Riphean, has been dated, using glauconites, as approximately 680 m.y. Younger producing zones assigned by the Russians to the Vendian have been dated by them as approximately 670 and 608 m.y. Additionally, numerous publications in the last decade have documented an ever greater and wider existence of varied forms of life in strata historically considered Precambrian because of their sequential position below lithostratigraphic a d biostratigraphic units commonly and generally designated as Cambrian. Some forms of life of an early evolutionary stage have been identified in strata more than 2 b.y. old.

Some geologists have questioned the assignment of the oil-and-gas-containing strata of Australia and the USSR, as well as certain rocks below the Cambrian in other parts of the world, to the Precambrian. They argue that the earliest occurrence of complex, multicellular organisms should be considered as the base of the Phanerozoic even though such occurrences may not represent temporal synchroneity.

The problem of what is the base of the Cambrian--or the top of the Precambrian--is, in many respects, a moot issue. Geologic time, under the best of circumstances, is difficult to determine and the location of a synchronous surface or boundary, in widely separated areas of the world, is infinitely more so. Should the base of the zone of Olenellus be considered the base of the Cambrian and, therefore, the base of the Paleozoic? Is this an artificially conceived boundary and should it be redefined? Should the base of the Phanerozoic coincide with the base of the Cambrian or should it be placed lower stratigraphically at the first recorded occurrence of well-organized multicellular forms of life? Should those rocks below the zone of Olenellus--and their demonstrable equivalents--be consi ered Eocambrian, late Precambrian, pre-Paleozoic Phanerozoic rocks, or what?

My purpose is not to engage in semantic, philosophic, or conceptual arguments regarding the placement of stratigraphic boundaries but rather to propound a pragmatic exploratory thesis, namely, that rocks equivalent in age to other rocks which contain oil or gas should be explored by the drill. It makes little difference that various individuals would call them Eocambrian, late Precambrian, or Precambrian. They represent, as yet, generally unaccepted exploratory goals. However, our nation's energy situation, as well as the necessity to utilize imagination and all available data in exploration, leads me to conclude that, although the chances of finding oil and gas in these "pre-Cambrian" rocks may be substantially less than in Cambrian and younger stratigraphic sequences, these earlier ossiliferous rocks should be tested. As explorationists, we should deliberately plan and program wells to penetrate sedimentary rocks below the Cambrian whenever and wherever it appears possible that they may be unmetamorphosed.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas