Page 405 - Petroleum Geology

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of its kind. For example, the onshore Canning basin of north-western Australia
(Horstman et al., 1976) appears to have had a pre-Permian rifting, or taphro-
genic, phase followed unconformably by unfaulted Permian and (thin) Meso-
zoic. And the northern Perth basin onshore (Hosemann, 1971) contains a
hint of Permian rifting with unfaulted trans.gressive Triassic on the uncon-
formity.
From a petroleum geological point of view, we are concerned with the
effect these events had on the stratigraphy and structure of the rift basins
both in the taphrogenic phase and the following epeirogenic phase. In sum-
mary, they are these:
- Taphrogenic phase: subsidence of elongated regions, with normal fault-
ing; brief transgression in places, followed by regression with, commonly,
non-marine sediments, the accumulation of which was largely controlled by
movement on the growth faults active at the time.
- Epeirogenic phase: a period of non-accumulation and erosion (at least
locally) during which fault movement ceased; followed by marine transgres-
sion with fine-grained, commonly calcareous, sediments accumulating over a
surface of generally low relief. The lithologies of the epeirogenic phase are
commonly in strong contrast with those of the taphrogenic phase.
The taphrogenic-epeirogenic transition is the important event locally be-
cause this is a time of trap formation; but there are apparent eustatic events,
both positive and negative, superimposed on the sequence as a consequence
of events in other parts of the world. For example, the mid-Cretaceous hiatus
in southeast Australia in non-marine sediments may be the consequence of
an eustatic event caused by the taphrogenicepeirogenic transition in the north-
west shelf of Australia, the North Sea, and elsewhere. These too may form
traps for petroleum, particularly when a relative fall of sea-level is followed
by transgression and the accumulation of fine-grained sediment.
The most favourable conditions for accumulation of petroleum source
rock were generally in the transgressions immediately following the end of
the taphrogenic phase. These are the seals to the various unconformity traps,
and petroleum will migrate to any permeable rock unit that underlies or sub-
crops against the source rock, and will accumulate if there is closure on the
unconformity or disconformity surface. These fine-grained, undeformed post-
unconformity sediments may also contain petroleum source rocks for palaeo-
geomorphic and reef traps formed during the transgression, and for submarine
fans near the margins of the physiographic basin forming over the rift basin.
Pre-unconformity petroleum source rocks, both marine and non-marine,
may accumulate at any time from the inception of rifting. A contributary
favourable factor is the rapid rate of subsidence at times during the taphro-
genic phase. However, the very rate of subsidence and accumulation of sedi-
ment may mean that early petroleum source rocks generate and lose their
petroleum before large traps are formed or completed. Prg-unconformity re-
gressive sands appear to accumulate without the thick njwdstones of what