QUALITIES OF SANDSTONES AS A GOOD RESERVOIR ROCK
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Most of the prolific oil production and indeed most of the giant oilfields are in sandstones. Geological studies and reservoir rock characteristics are required to be studied in order to know the potential of hydrocarbon reservoir. Sandstones generally exhibit high primary permeability as well as secondary permeability characteristics. The basis of this study is to identify the texture and mineralogy of sandstones and how the diagenesis process affects the porosity of sandstones. Thus, it is important to acquaint the knowledge of the texture, mineralogy, pore structure and the formation of sandstones. Thin section microscopy analysis is applied to assist this petrographic study in order to study the rock components and the relationship of grains with matrix, cement and the diagenetic process. The sandstones which were studied are Red Sandstone, Argillaceous Sandstone and Arkose Sandstone. The results from this study justified the mineral composition in each type of sandstones. Generally, common minerals found in sandstones were quartz, feldspar, clays, hematite and mica. Quartz was known as the dominant mineral in sandstone especially in red sandstone. The percentages of these minerals were different due to several factors such as weathering, transportation and depositional environment of the sediments. Grain contacts and fractures were also observed in the samples as compaction occurred throughout diagenesis process. These features resulted to development of dissolution porosities which was secondary porosity. Cements that were commonly found in sandstones were silica cement, calcite cement, hematite cement and clay cement. Basically, the amount of cements formed was inversely proportional to the porosity values
The distribution of sandstone reservoir is controlled particularly by the processes whereby the sand was deposited. Majority of hydrocarbon reserves in the world can be found in ancient sandstones which have porosity and permeability. When sandstone contains hydrocarbon that can be produced and extracted profitably by recognized technology, it can be referred as a sandstone reservoir ( Weimer in 1982)
Most of the prolific oil production and indeed most of the giant oilfields are sandstones. Sandstone 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 (Dott, 1969)
Sandstone reservoir are usually composed by stable minerals such as feldspar quartz and rock fragments, accessory minerals and also pores which saturated with fluids, in classifying the rocks as sandstone, the size of sand grains must be between 1/16 and 2 mm in diameter. The quantity of pore volume and the nature of the interconnections between pores may be related to the primary process under which the sandstone accumulated. They may also be related to secondary changes known as diagenesis process that are post depositional.
Geologists who are connected with the basis of sedimentary rocks can observe the conditions and processes by which sediments accumulate today by thin section analysis, Hence, they can make use of the observations to interpret the origin of ancient sediment (Weimer, 1982).
A reservoir rock is a subsurface volume of rock that has sufficient porosity and permeability to permit the migration and accumulation of petroleum under adequate trap conditions. Porosity is a measure in percentage of pore volume or size of holes or voids per unit volume of rock. For example, well-sorted sand in a 300 ml container will hold about 100 ml of water in its pore space, or a porosity of 33%. If petroleum is present, it can also occupy this pore space. During burial of this sand, compaction reduces this porosity substantially to where only a small percentage of porosity is left.
Permeability is a measure of the connectivity of pores in the subsurface. The sand in the container has narrow pore throats between the large pores that allow fluid to pass from one pore to another. Permeability is measured in millidarcies (md) or Darcy (1000 md) of these narrow throats. Commonly, permeability in the range of 100 to 500 md is reasonable values for a petroleum reservoir rock. Values over a Darcy are exceptional. Fractures have infinite permeability.
Any lithology can have porosity and permeability. Siliciclastic sandstone is most likely to have vertical and lateral porosity and permeability over a relatively large distance. Frequently, carbonate rock has the most complex patterns of porosity, frequently voids porosity with little or no permeability, but dolomitized zones can have lateral porosity and permeability. Volcanic rock can have voids where hot gases bubble from molten rock. Metamorphic and other sedimentary rocks can acquire secondary porosity because of weathering or groundwater movement. All these rock types can be shattered so that unconnected pores can be connected with fractures.
Major and minor reservoir rocks are determined from the percentage of in-place petroleum that originated from a particular pod of active source rock. If the volume of in-place petroleum is unavailable, the volume of recoverable hydrocarbons is used. All oil and gas fields included in a petroleum system are listed and the original in-place (recoverable) hydrocarbons are determined by stratigraphic interval. The volumes of in-place hydrocarbons for each stratigraphic interval are added up, and the percentage of in-place hydrocarbons by stratigraphic interval is determined. Usually, one stratigraphic interval contains most of the in-place hydrocarbons, so this interval is the major reservoir rock. Reservoir rocks that contain minor or lesser amounts of in-place hydrocarbons are the minor reservoir rocks. The major reservoir rock is the second name used in the name of the petroleum system. The major reservoir rock indicates the optimum migration path between the pod of active source rock and the traps that include the major reservoir rock. The minor reservoir rock indicates the least effective migration path, or it indicates a migration path that should be studied for overlooked prospects. Major and minor reservoir rocks should be included on the petroleum system events chart.
A reservoir is a rock that contains hydrocarbon accumulation in large quantity (although other fluids beside Hydrocarbons may be present) by virtue of its porosity, permeability and sealing mechanism. A reservoir rock cannot exist without adequate porosity and permeability. Porosity refers to the pores in the rock that permit the rock to store fluids. Pores here refer to the interstitial spaces between rock aggregates.
There exist spaces between the particles that lead to the formation of rock. These spaces are at times interconnected. The interconnection between the pores of a rock is a huge factor of the attribute permeability. A reservoir rock with large hydrocarbon accumulation but no permeability is just as useless as a rock without oil. Crude oil travels distance through rocks before it finally gets to an impermeable rock. The impermeable rock prevents the oil from moving any further and keeps it stored there. The sealing mechanism refers to the impermeable rock (cap rock). The diagram below can be used to illustrate a reservoir rock using an anticline reservoir.
A reservoir rock as stated above contains other fluids but Hydrocarbon, water inclusive. Hydrocarbon exist in three forms namely; Gaseous, Liquid and Solid. All of the three forms are dependent on the carbon chain of the Hydrocarbon with the lighter end being the gas and the heaviest of the end being the solid. Petroleum and natural gas are often found in sandstones. They do not form there, but seek to float to the surface by percolating through water-saturated sandstones. Sandstones layers shaped into domes by folding or other processes (and overlaid by non-porous rock) act as traps for migrating oil and gas that ascend into them but then have no way out. Such traps are much sought after by oil companies; indeed, most sandstone sedimentologists work for the petroleum industry.
1.2 Types of Petroleum reservoir
Reservoir rocks are on a large scale classified into:
Carbonate reservoir rocks
Sandstone reservoirs rocks
Carbonates are made from a mineral called calcite. They are formed as a result of sediments created by precipitation of minerals. They are formed from biochemical reactions or inorganic process. Inorganic process means 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). Most of the world’s oil is found carbonate reservoirs
Sandstone rocks are made from dominant constitution of Quartz and Feldspar particles. 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 materials. The best sandstone reservoirs are those that are composed primarily of quarts grains of sand size, silica cement, with minimal fragmented particles. Sandstone are the most common reservoir rocks and can be found in most every oil field. The best sandstone reservoir are those that are composed mainly of quartz grains of sand size of nearly equal size or silica cement, with minimal fragmented particles. The maturity of the sandstone rock influences the grain fraction and in turn, influences its reservoir characteristics.
- To determine the mineralogy of the rock from different type of sandstones
- To identify the texture of the sandstones in terms of grains sizes and shapes, grain sorting and the grain contacts of the rocks
- To study the effects of the rock textures to the porosity
- To determine the quality of the different types of sandstone as being a good reservoir rock
- To determine were these sandstones are found in Cameroon
- Analysis of petrography (texture and mineralogy) using thin section microscopy and stereo microscope with LAS v3.8 software (Leica Application Suite).
- Different types of sandstone rock samples were used in this study:
- Red Sandstone
- Argillaceous Sandstone
- Arkose Sandstone
- The effects of rock texture and mineralogy on different sandstones were analyzed.