Page 67 - Petrophysics

SEDIMENTARY GEOLOGY 41

will accumulate in an area at their base level of energy, but finer grades
of the material cannot accumulate in that location and are carried in
suspension to an area of lower energy equivalent to their base level.
This is the process that leads to sorting with accumulation of sand grains
in one area and silt and clay in another area. The base level of a given
area fluctuates with time; thus, during one period of accumulation sand
particles are deposited, whereas later, finer particles of silt and clay are
deposited on top of the sand. This sequence may be repeated many
times, leading to alternate deposition of sand and shale, and formation of
sand-shale sequences.
Pirson identified three types of physiographic areas that lead to
the accumulation of either quartzose, graywacke, or arkose sands in
basins [ll]. Each depends on the relief of the land mass and thus
the time available for chemical weathering of the rocks and particles
prior to accumulation in the sedimentary basin. This is a simplifi-
cation of the sedimentary process, which is a complex interplay of
the numerous depositional situations including those idealized by
Pirson. Nevertheless, the simplifications present a clear explanation of
sedimentary accumulations that lead to different lithologies.
During periods of negligible orogenic activity in flat plains bordered
by shallow seas, erosion of the land mass is at a minimum, whereas
chemical weathering is occurring at a rapid rate because the residence
time of interstitial fluids at and near the surface is relatively long. Under
these conditions, weathering processes go to completion, furnishing
stable components from igneous and metamorphic rocks, such as quartz
and zircon, for clastic sediments. These materials are carried into the
depression forming the sea and are accumulated as clean, well-sorted
sediments with uniform composition and texture. The sediments may
remain as unconsolidated sand formations, or the grains may be cemented
by carbonate, siliceous, clayey, and ferruginous compounds precipitated
from the seawater, interstitial solution, or ferruginous waters percolating
gradually through the deposits at some later stage (Figure 2.2). Changes
of the climatic conditions of the physiographic area can change the type
of sediments accumulating in the basin, from clean granular material to
mixtures of silt, clay, and organic materials. These become shale beds
that can serve as source rocks for hydrocarbons as well as impermeable
caprocks.
Well-sorted, granular, quartzose reservoirs exhibit relatively high
vertical permeability (kv) with respect to the horizontal permeability
(kh); however, kh is still higher than k,. Therefore, primary oil recovery
will be relatively high, whereas secondary recovery will be very low
due to severe fingering and early water breakthrough. Pirson lists the
Oriskany Sandstone in Pennsylvania, St. Peter Sandstone in Illinois,