Page 68 - Petrophysics 2E

42 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Figure 2.2. Accumulation of quartzose-type sediments in a bash from a low-relief
continental shelf: On a lowelief land suqace, erosion is at a minimum and chemical
degradatton of rocks to quartz is at a maximum [ll].

Wilcox Sandstone in Oklahoma, and Tensleep Sandstone in Wyoming
as examples of quartzose-type reservoirs [ 1 11.
In conditions where the uplifted land areas bordering seas are steep
enough to prevent total chemical weathering of the exposed rocks to
stable minerals such as quartz, the detrital material accumulating in the
basin will be composed of mixed rock fragments, or graywacke-type
sediments. The sedimentary particles are irregular in shape and are
poorly sorted, with variable amounts of intergranular clay particles.
Changes of the climatic conditions of the physiographic area result
in variable episodes of fine clastic deposition on top of the coarse
particles forming the layers that become the caprocks of the reservoirs
(Figure 2.3). The permeabilities of these reservoirs vary considerably
over short distances, and the vertical permeability is usually much
lower than the horizontal permeability. The permeability variation is
one reason why graywacke-type reservoirs do not produce as well
during primary production as the quartzose-type reservoirs, but exhibit
excellent secondary recovery. Due to the mixed sediments containing
clay minerals, the reservoirs are generally subject to water sensitivity
problems (clay swelling and clay particle movement). The Bradford
Sandstone in Pennsylvania and the Bartlesville Sandstone in Oklahoma
are examples of graywacke sandstone formations.
A third general class of clastics, arkose-type sediments, wiU accumulate
in basins or dendritic canyons adjacent to Iand areas of steep relief.
Due to the steep relief, chemical weathering of the sediments is
incomplete, resulting in deposition of angular grains with considerable