PETROLEUM AND GEOLOGY: A SYNTHESIS

Abstract

A marine sedimentary basin typically begins with a transgressive phase and ends with a regressive phase; but there may be several cycles, and also periods in which neither is dominant.

Petroleum occurrences fall into two broad stratigraphie classes: those of transgressive sequences, and those of regressive sequences. Transgressions tend to accumulate potential source rocks on top of potential reservoir rocks, and the petroleum tends to migrate downwards then laterally into stratigraphic traps, especially reefs and below unconformities. It also occurs in diachronous units that are anticlinal in form due to basement irregularities. Regressions tend to accumulate potential reservoir rocks on top of potential source rocks, and the petroleum tends to migrate upwards and then laterally into anticlines and fault traps that are typically initiated contemporaneously or penecontemporaneously with sediment accumulation. There is some evidence that oil of transgressive sequences is heavier than oil of regressive sequences.

Evidence derived from subsurface geology, including petroleum occurrences, suggests that young marine sedimentary basins are typically deformed by vertical, gravity processes during and just after significant regressive phases of their development; and that these processes are a direct consequence of the accumulation of sediment in a regressive sequence. Subsequent horizontal tectonic events, in general, only modify the earlier, contemporaneous, deformation.