BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Figure 6 - 8
Marine Sedimentary Depositional Environments
Sandstone Reservoirs
The term sand refers to a particular grain size (62 μm – 2 mm), not to a particular composition. The
performance of the sandstone as a reservoir rock, its combination of porosity and permeability,
depends upon the degree to which it is truly a sand. Texture should reflect similar sized grains, not
a combination of coarse and fine grained material. The best sandstone reservoirs are those that are
composed primarily of quartz grains of sand size, silica cement, with minimal fragmented particles.
The quality of the initial sandstone reservoir is a function of the source area for the material, the
depositional process, and the environment in which the deposition took place. Sandstone reservoirs
2
are commonly 25 meters (80 ft.) thick, are lenticular and linear spatially, and less than 250 km
(100 mi. 2 ) in area. They range in age from the oldest being Cambrian (in Algeria) to the youngest
being Pliocene (Caspian region in Ukraine). In the U.S.A., two-thirds of the sandstone reservoirs
are Cenozoic in age.
Sandstone reservoirs form extensive stratigraphic traps. Many of the sandstones change
composition, e.g. to a shale, within the sandstone unit; at this point they are said to pinch out.
Others represent ancient upland river channels that have left sand in the channel that has converted
to sandstone. Often these sand channels are stacked one on top of another which means that
hydrocarbons are free to migrate between the reservoirs. Other sandstone reservoirs are deltaic in
origin, meaning they represent ancient river deltas similar to the Mississippi River delta that
extends into the Gulf of Mexico. Successive sandstone layers exist in the subsurface that were
created in alternating marine and terrestrial environments. Sandstone reservoir rocks are the result
of a number of processes that can occur on dry land as well as beneath the sea.
Carbonate Reservoir Rocks
The most interesting and perhaps impressive aspects of carbonate reservoir rocks are their fossil
content. Fossils rage from the very small single cell to the larger shelled animals. Prior to the
1920’s, carbonate reservoir rocks were relatively rare and prior to 1950 they were all regarded as
essentially organic rocks. This changed when textural studies of carbonates in Iraq and the
Bahamas showed that carbonates are also the result of inorganic processes. Most carbonate rocks
are deposited at or in very close proximity to the site of creation. Transportation of material is less
common and sorting is essentially non-existent. The “best-sorted” carbonate rocks are oolites in
which the “grains” are the same size and shape. Oolites are not “sorted” at all, but they are formed
with the sizes and shapes that they have in the carbonate rock and were cemented in place.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-12