Page 125 - Introduction to Petroleum Engineering

SEDIMENTARY BASINS AND TRAPS 111
6.3 SEDIMENTARY BASINS AND TRAPS

A sedimentary basin is “an area of the earth’s crust that is underlain by a thick
sequence of sedimentary rocks” (Selley and Sonnenberg, 2015, page 377). The
sediments have accumulated to a greater extent in basins than in adjacent areas.
Sedimentary basins are formed in large crustal regions that are lower than sur-
rounding regions. Deposition of sediment in these relatively low‐lying regions can
result in thick accumulations of sedimentary rock.
The rock in sedimentary basins can exhibit significant variability throughout the
basin. Rock heterogeneity is due to different depositional environments and
changes in pressure, density, and composition of deposited material. The applica-
tion of tectonic forces such as folding, faulting, and fracturing can result in major
changes in the orientation and continuity of rock strata. The three‐dimensional
distribution of rock strata that have been folded and faulted is an example of a
geologic structure.
Some layers of sediment may contain organic matter. These layers are called
source rock if there is enough organic matter to generate petroleum. A source rock
can be characterized by a factor called the transformation ratio. The transformation
ratio is the ratio of the amount of petroleum formed in the basin to the amount of
material that was available for generating petroleum.
Pressure and temperature at the base of the accumulation of sediment increase as
the accumulation thickens. The increasing temperature and pressure can facilitate the
decay of organic matter and the formation of petroleum. The volume of petroleum
generated in a sedimentary basin depends on the area of the sedimentary basin, the
average total thickness of source rock, and the efficiency of transforming organic
matter to petroleum.
Hydrocarbon fluids generated in source rocks are usually less dense than water.
They will migrate upward and follow permeable pathways until they encounter an
impermeable boundary. A permeable pathway is typically a permeable or fractured
formation. The permeable pathway can be referred to as carrier rock to distinguish
it from source rock. Traps are structures where hydrocarbon fluids accumulate since
they can no longer migrate.

6.3.1 Traps

There are two primary types of traps: structural and stratigraphic. A structural trap is
present when the geometry of the reservoir prevents fluid movement. Structural traps
occur where the reservoir beds are folded and perhaps faulted into shapes which can
contain commercially valuable fluids like oil and gas. Anticlines are a common type
of structural trap. Folding and faulting can be caused by tectonic or regional activity.
Tectonic activity is a consequence of moving plates, while regional activity is exem-
plified by the growth of a salt dome. Formation of a structural trap by regional activity
is also known as a diapiric trap.
Stratigraphic traps occur where the fluid flow path is blocked by changes in the
character of the formation. The formation changes in such a manner that hydrocar-
bons can no longer move upward. Types of stratigraphic traps include the thinning