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INTRODUCTION

A fundamental property of a reservoir rock is its porosity. However, for it to be an effective reservoir rock, THE fundamental property is permeability. Both porosity and permeability are geometric properties of a rock and both are the result of its lithologic (composition) character. The physical composition of a rock and the textural properties (geometric properties such as the sizes and shapes of the constituent grains, the manner of their packing) are what is important when discussing reservoir rocks and not so much the age of the rock.

Question: What affect do the following lithologic parameters have on how much fluid a given volume of material can contain:

  1. Grain size and pattern arrangement?

  2. Shape of the grains?

  3. Sorting [uniformity of size] of the grains?

  4. Subsequent action to the sediments (compaction)?

  5. How the grains were formed - beach (ocean) versus river sand, river sand versus clay?

Answer:
  1. Grain size and pattern arrangement: the actual size of the grain has no effect, but the arrangement pattern has a large effect.

  2. Shape of the grains -- grains of high sphericity tend to pack with minimum pore space as angularity increases, pore space. As volume increases pore space volume usually increases.

  3. Sorting of the grains -- uniformity of size has an effect; the more uniform the size, the greater volume of voids. Mixing different sizes tends to decrease total volume of void space.

  4. Subsequent action to the sediments (compaction) -- compaction decreases volume of void spaces. Compaction of sand is less dramatic than that of clay.

  5. How the grains were formed -- for example, beach sand has rounded grains while river sand is angular so the latter increases pore space. River sand is granular while clay grains form parallel plates; the latter has greater porosity, but sand has greater permeability.

Question: In the answer to Question 1 above, we said that the arrangement pattern of the grains within a rock has an effect on the volume of void space contained within that rock. Which of the following general arrangement patterns create conditions that result in the greatest volume of void space: A, B, orC? Which pattern is the most compact?
Answer: Figure A has the greatest volume of void space; it is the least compact.
Figure B has the lowest volume of void space; and is the most compact.
Figure C has intermediate volume of void space; and intermediate compaction value.

EXPERIMENT: We are going to experimentally determine what affect Sorting of Grains, i.e, uniformity of the grain size, has on the volume of voids contained in a given volume of material. Take a large, clear beaker and place it inside a larger beaker. Fill the inner beaker with water level to the top of the beaker. Gently place as many large marbles (or other suitable materials) into the beaker with water and collect the overflow in the outer beaker. Measure the amount of water and record it. Next, repeat the experiment except this time add a mixture of marbles and bb's to the full beaker. Measure and record the amount of water overflow.

OBSERVATION: The marbles of uniform [grain] size did not displace as much water as did the mixture of mables and bb's; more was left in the beaker to occupy the existing void spaces. The mixture of marbles and bb's displaced more water; the smaller bb's filled existing voids that were present in the first experiment so there was less space for the water (hydrocarbons) to occupy.

RESULT: Poorly sorted materials (more than one grain size) creates fewer pore spaces (does not hold as much fluid). Well sorted materials (uniform grain size) create more pore spaces (hold more water).

Question: Does this experiment tell you anything about permeability?
Answer: No! It tells us only about porosity.

Click for a more detailed discussion
of compaction and permeability.

Most of the prolific oil production and indeed most of the giant oilfields are in sandstones. Sandstones generally exhibit high primary permeabilities as well as secondary permeability characteristics. For example, most of the oil and gas produced in Russia is from clastic reservoir rocks. Much of the production from the USA has also been from clastic reservoir rocks. But there are some notable exceptions. For example, the Permian Basin of the southwestern U.S.A. is a carbonate (limestone) reservoir as is the huge oilfields in the Middle East.

Most geologists think of sandstones and limestones as two distinct rock types, and indeed they are. Compositionally, sandstone is formed through inorganic and clastic processes. Erosion of land surfaces containing all types of existing rocks creates sediments which are transported into a basin where compaction occurs creating sandstone rock. Looking closely at the sediments, one would see that it contains pebbles, sand grains, and other bits and pieces of rocks. All the sediment of this kind is referred to as clastic rocks meaning accumulated particles of broken rock and of skeletal remains. The clastic materials are held together in the rock by cement, generally silica.

Limestones (carbonates) are primarily made of the mineral calcite. They are the result of sediment formed by precipitation of minerals from solution in water, either the result of biochemical reactions or by inorganic chemical processes. Inorganic processes mean that calcite is precipitated directly from water; small spheroidal grains, about the size of sand grains, called oolites are found on the floor of oceans. They are composed of calcium carbonate (CaCO3). Oolites found in limestones mean that they were formed in ancient oceans. Cave deposits are also calcite, but they formed in a wet cave on land.

Biochemically formed limestones are created by the action of plant and animal life that extract calcium carbonate (CaCO3) from the water in which they live. The CaCO3 may be either incorporated into the skeleton of the organism or precipitated directly. Regardless of the mechanism, when the organism dies, it leaves behind a quantity of CaCO3 and over geologic time thick deposits of this material build up. Reefs are examples of such accumulations.

Another precipitant is the mineral dolomite CaMg(CO3)2; it occurs in large concentrations and it forms rock called Dolomite. It is the not the result of direct precipitation in sea water, but rather is formed by replacement of pre-existing deposits of calcite. For the purpose of our discussion, carbonate reservoir rocks mean limestone and dolomite.

The manner in which the type of rock creating the reservoir rocks was formed is important to our discussion of reservoir rocks. The manner in which they were derived creates essential differences as it relates to an earlier discussion of the fundamental properties of porosity and permeability. Primary permeability is dependent upon the manner of formation. Sandstones generally have greater primary permeability than limestone unless the latter is a reef, or similar, structure. Clastic sedimentary rocks tend to be more porous because of the uniqueness of their origin. They are the result of erosion and transport; these two processes tend to cleanse the material of everything but silica sand grains and they are often of similar size and shape. Carbonates on the other hand, tend to have greater secondary permeability because they are more soluble and reactive. Fractures and faults within carbonates (which are highly soluble) tend to enlarge in size and lateral extent when water flows through those fractures and faults.

SANDSTONE RESERVOIR ROCKS

The term sand refers to a particular grain size (62 Ám - 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 materials, the depositional process, and the environment in which the deposition took place. Sandstone reservoirs are generally 25 meters thick, are lenticular and linear spatially, and less than 250 km2 in area. They range in age from the oldest being Cambrian (in Algeria) to the youngest being Pliocene (Caspian region in Ukraine). In the USA, two-thirds of the sandstone reservoirs are Cenozoic in age.

Question: A sandstone reservoir 1 sq km in size with an average porosity of 15%, 1 meter thick and saturated with oil contains 15 x 104 m3 of oil in place. How many barrels of oil is this?
Answer: Nearly 1 million barrels of oil (42 gal per barrel).

Question: If a well produces 4,000 m3 per day, how many barrels per day does that well produce? How many barrels per day is this for a field of 36 wells in operation? What would be the total cumulative output per year?
Answer: Do the math!!!

Sandstone reservoirs form extensive stratigraphic traps. Many of the sandstones change composition within the sandstone unit; at this point, they are said to pinchout. Others represent ancient upland river channels that have left sand in the channel that has been converted into sandstone. Often these sand channels are stacked one on top of another which means that hydrocarbons are free to migrate between 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. And successive sandstone layers exist in the subsurface that were created in alternating marine and terrestrial environments. So sandstone reservoir rocks are the result of a number of varied 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 range 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. But 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. But Oolites are not "sorted" at all, but were formed with the sizes and shapes that they have in the carbonate rock and were cemented in place.

Most carbonate rocks begin as skeletal assemblages. They are the accumulation of the remains of carbonate-secreting animals and plants. Carbonates may form on gently sloping platforms such as continental shelves in shallow, warm saline water. The resulting rock form layers of limestone in the subsurface that extend spatially over great distances as in the case of the Michigan basin. Carbonates may also form as linear or continuous reef trends, as in the case of the Cretaceous reef structure in south Louisiana.

Porosity and permeability is greatly reduced in ancient carbonate reservoir rocks due to compaction and cementation. But porosity tends to increase as a result of secondary processes of leaching and dolomitization of the limestone that occurs after the carbonate rock was formed. Recent carbonate sediments have higher porosity values, but porosity is always reduced during burial and compaction which may reduce the thickness of a limestone bed by 25% under no more than a few hundred meters of overburden.

Question: Dolomitization enables a carbonate reservoir rock to resist compaction. What does this mean in terms of thier suitability as a hydrocarbon trap?
Answer: Dolomites are normally less porous than limestones at shallow depths, but they retain their porosity better during burial and are less affected by compaction. Therefore they tend to be better reservoir rocks because they contain more voids that hydrocarbons can fill. But getting the hydrocarbons into the voids is a matter of timing. The porosity must be in place in order to receive the hydrocarbons moving from the source beds into the reservoir beds.

Carbonate reservoirs represent a broader range of producibility than do the more common sandstone reservoirs. The most prolific and sustained production rates came from carbonate reservoirs.

But carbonate reservoirs can also be at the other extreme in terms of hydrocarbon production. Many carbonate reservoirs will not yield their oil and gas at all unless they are artificially fractured. We will have more to discuss on the subject of artificial fracturing in a latter Chapter. On the average however, and despite some outstanding carbonate reservoirs, sandstone reservoirs produce more hydrocarbons per unit volume of reservoir.


REFERENCES

DOTT, R. H., and M. J. REYNOLDS, 1969, Source book for Petroleum Geology: Am. Assoc. Petrol. Geol., Mem. No. 5, 471 pp.

JUDSON, SIDNEY A. and R. A. STAMEY, 1933, Overhanging Salt On Domes of Texas and Louisiana: Bull. Am. Assoc. Petrol. Geol., Vol. 17, pp. 1492-1520.

Other Materials Utilized in Preparation of this Section

BATES, R. L. AND J. A. JACKSON, 1980, Glossary of Geology: American Geological Institute, Falls Church, VA., 2nd Ed., 749 pp.

BERGER, B. D. and K. E. Anderson, 1992, Modern Petroleum - A Basic Primer of the Industry: PennWell Books, 3rd Ed., Tulsa, OK., 517 pp.

LEVORSEN, A. I., 1967, Geology of Petroleum: W. H. Freeman and Company, San Franciso, CA., 2nd Ed., 724 pp. SELLEY, R. C., 1985, Element of Petroleum Geology: W. H. Freeman & Co., New York, 449 pp.

NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester, MA., 607 pp.

SALVADOR, A., 1991, The Gulf of Mexico Basin: The Geological Society of America, Inc., in The Decade of N. Am. Geol. Project series, The Geol. of N. Am., vol. J., 568 pp.