Page 64 - Petrophysics

38 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Dynamic sedimentary basins exist when sediment accumulation
occurs simultaneously with subsidence of the basin area. The forces
producing localized subsidence are not fully understood, but they have
been related to isostatic adjustment of unbalanced gravitational forces.
The theory of isostatic equilibrium is that the outer, lighter SUI. crust
of the earth is essentially floating on a plastic-type mantle in a state of
equilibrium. Therefore, part of the Earth’s crust can gradually subside
into the plastic mantle while an adjacent area is slowly uplifted.
No earthquake foci have been recorded deeper than about 1,600 km,
where the pressure and temperature are probably great enough to
transform the mantle into a plastic-type material that can develop
slow convective currents and gradually move to adjust for changing
gravitational loads on the crust. The Great Lakes area of the United
States, Canada, and the Scandinavian peninsula are still gradually rising
in response to the melting of Pleistocene glaciers.
Continental masses have stable interiors known as cratons, or shields,
which are composed of ancient metamorphosed rocks. Examples are
the Canadian, Brazilian, Fenno-Scandian, and Indian shields that form
the nuclei of their respective continents. Sedimentary deposits from the
cratons have accumulated to form much of the dry land of the
earth’s surface, filling depressions and accumulating on the shelves of
continental margins.

DIVERGENT CONTINENTAL MARGINS

Sediments accumulated on the shelves at the margins of the continents
form several types of geologic structures that are the result of the
direction and stress imposed on them by motion of the drifting crustal
plates. Divergent continental margins develop on the sides of continents
that are moving away from the spreading ocean rifts. Examples are the
east coasts of North and South America and the west coasts of Europe
and Africa, which were originally joined together at the mid-ocean rift.
The continents are extending, leaving wide, shallow, subsea continental
shelves where carbonate sediments originate from the reefs in shallow
areas and clastic sediments result from the washing down of clastics from
the land surface.
In considering sedimentation and the attributes of a sedimentary basin,
one must include the entire region that has furnished the detrital materials
that have accumulated in the basin as sediments, and the environmental
conditions of the various episodes of sedimentation. Chapman defined
this as the physiographic basin, an area undergoing erosion which will
furnish material for the sediments accumulating in a depositional basin
or depression on the surface of the land or sea floor [9]. Thus the nature