Page 65 - Petrophysics

SEDIMENTARY GEOLOGY

of the sediments is determined by the geology of the peripheral areas
of weathering and erosion, and by the physiography and climate of the
entire interacting area.

CONVERGENT CONTINENTAL MARGINS

Convergent continental margins develop when two crustal plates
collide. When an ocean plate collides with a less dense continental plate,
a marginal basin forms between the island arc and the continent. This
basin fdls with carbonate deposits from marine animals and clastics from
the land mass, forming large areas for accumulation of hydrocarbons such
as the oilfields of Southeast Asia.
Continual movement of the plates against each other will result in
the formation of a long, narrow trough (several hundreds of miles long)
called a geosyncline. The resulting trough is filled with great thicknesses
of sediments that may become uplifted and folded as mountain building
(orogeny) begins, accompanied by volcanic activity. The Appalachian
Mountains in the eastern United States and the Ural Mountains in
Russia are the result of convergent continental margins where sediments
accumulated. Subsequently, they were uplifted during the orogenic
period to form the stable mountains that are eroding today and furnishing
sediments to the lowland areas on both sides of the mountains.
Some of the petroleum that may have accumulated in the sediments
is lost during the orogenic period, because the seals (caprocks) holding
the oil in geologic traps are destroyed, allowing the hydrocarbons to
migrate to the surface. Folding and faulting of the sediments, however,
also produce structural traps in other areas of the region.

TRANSFORM CONTINENTAL MARGINS

When two crustal plates slide past each other, they create a long
transform fault with branches at 30" to the main fault, creating fault
blocks at the edge of the transform fault. Numerous sealed reservoirs
occur along such faults where clastic sediments have accumulated.
An example is the San Andreas Fault in California and its associated
oilfields. Transform faults on the ocean floor are sites of sea mounts,
some of which project above the ocean floor and are accompanied by
volcanic activity [9].

TRANSGRESSIVE-REGRESSIVE CYCLES

A transgressive phase occurs when the sea level is rising or the basin
is subsiding. During this period, the volume created by subsidence