Page 21 - Petroleum Geology

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because it is in sedimentary basins that the commercial accumulations of
petroleum occur. It is therefore essential to have a clear idea from the out-
set of what a sedimentary basin is.
A sedimentary basin may be defined as “an area in which sediments ac-
cumulate during a particular time span at a significantly greater rate, and so
to a significantly greater thickness, than in the surrounding areas”. This is
not entirely satisfactory because of the vagueness about thickness - yet this
vagueness exists. The essential part of any definition must be the accumula-
tion of sediment relative to neighbouring areas, and its relative rather than
absolute thickness.
The surface of the Earth can be divided into three broad categories: areas
of erosion, areas of sediment accumulation, and neutral areas in which neither
erosion nor accumulation is dominant. Active sedimentary basins are areas
of accumulation. They may be large or small, deep (thick) or shallow (thin).
They may persist for a significant span of geological time measured usually
in tens of millions of years, and they may persist in one place, or migrate to
some extent. The age of the sedimentary basin is the age of the sediments
that accumulated in it. Sedimentary basins are of infinite variety, and so
individually unique; but they share some fundamental attributes.
The concept of a sedimentary basin is distinct from that of a physiographic
basin. A physiographic basin is a depression in the surface of the land or sea
floor that may or may not fill with sediment. Part of the area of a physio-
graphic basin is an area of erosion, providing the materials that will form
sediment in other areas of the physiographic basin, In those areas in which
sediment accumulates, the upper surface of the sediment does not neces-
sarily form a depression everywhere: it may be physiographically indistin-
guishable from the neighbouring areas that are not accumulating sediment.
For example, there is a large physiographic basin occupying the mid-
continent region of the United States of America, drained largely by the
Mississippi and Missouri rivers into the Gulf of Mexico (Fig. ,1-1). The phy-
siographic basin occupies a significant area of North America and includes
the Gulf of Mexico. Within this area, sediment is derived from the peripheral
areas, mainly the Rocky Mountains and the Appalachians, and transported
towards the sea. Some of this sediment accumulates on and in the flood
plains of the rivers, some accumulates along the Gulf Coast, and some is
carried out into the Gulf where it accumulates. One of the important sedi-
mentary basins of the world lies under the general area of the coast; and here
sediments have accumulated to a far greater thickness than the contem-
porary sediments in the deeper parts of the Gulf of Mexico. The surface of
this sedimentary basin does not form a depression, and the area of obvious
depression in the Gulf of Mexico is not the main area of sediment accumula-
tion.
The nature of the sediments in the physiographic basin is determined by
the geology of the peripheral areas of weathering and erosion, and by the