Page 20 - Petroleum Geology

PART 1. GENERAL

CHAPTER 1

CONCEPTS OF SEDIMENTARY BASINS

SUMMARY

(1) Sedimentary basins are areas in which sediment accumulated at a sig-
nificantly greater rate than sediments of the same age in neighbouring areas,
so accumulating a greater thickness. The sediments accumulate by virtue
of subsidence.
(2) The geological concept of a sedimentary basin is distinct from the
geographical concept of a physiographic basin. Sediment accumulation is
also distinct from sedimentation and deposition, because not all sediment
deposited accumulates for a significant period of time in the geological re-
cord.
(3) The nature of the sediments that accumulate in a sedimentary basin
is related to the environments of the physiographic basin from which the
sediments were derived and in which they were deposited.
(4) Sedimentary basins typically begin with a transgressive sequence and
end with a regressive sequence, but they may have a long and complicated
history. Transgressive sequences record a general deepening of the sea, with
reduction of the land area and migration of the facies towards the land.
Regressive sequences record a general shallowing of the sea, with extension
of the land and migration of the facies seaward.
(5) M.ost significant carbonate sequences are transgressive: arenaceous se-
quences may be transgressive or regressive. All important regressive sequences
are arenaceous.
(6) Eustatic changes of sea level leave a record in all active sedimentary
basins that are accumulating sediment. Changes of sea level due to changes in
the shape of the geoid may lead to transgressive sequences in some parts
of the world contemporaneously with regressive sequences in others.
(7) Sedimentary basins are deformed by faults and folds while they accumu-
late sediment and the sedimentary column is still subsiding.

INTRODUCTION

The geology of petroleum is largely the geology of sedimentary basins