<khalid_mirani2011@yahoo.com>
EROSION OF land surfaces containing all types of existing rocks creates sediments which are transported into a basin where compaction occurs creating sandstone rock. Sandstone is formed through inorganic and clastic processes. All kinds of the sediments is referred to as clastic rocks meaning accumulated pieces of rocks and skeletal. Sediments are held together in the rock by cementing materials, mainly silica. Sediment maturity is a measured by transportation distance/time from the source area to the depositional site.
Different factors are involved to affect on sediments such as climatic condition of weathering/transport and mineralogical makeup of the source area. Different lithological parameters are used to identify the quality of reservoir rocks their parameters are grain size, arrangement of sediments pattern and shape of the grains which shows high spherecity tend to pack with minimum pore space.
Degree of maturity is consists of immature, submature, mature and super-mature. Four types of maturity are found in reservoir rocks described as textural maturity which is depend on grain sizes and grain sorting; mineralogical maturity and compositional maturity. Sorting of the grains indicates uniformity of grains size, the more uniform the size the greater volume of voids. Mixing different sizes tends to decrease total volume of void space. 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.
There are two physical properties of reservoir rocks described as Porosity: creates the spaces to hold the oil or gas and Permeability: allows the oil and gas to flow out of the rock. Permeability is mechanical properties of reservoir rock to know how easily fluids can flow through the rock) and possible drive mechanisms, it is possible to estimate the recovery factor, or what proportion of oil in place can be reasonably expected to be produced. The recovery factor is commonly 30-35 per cent, giving a value for the recoverable reserves.
The porosity, or the percentage of the total volume that contains fluids rather than solid rock, is 20-35 per cent or less. It can give information on the actual capacity. Laboratory testing can determine the characteristics of the reservoir fluids, particularly the expansion factor of the oil, or how much the oil expands when brought from high pressure, high temperature of the reservoir to “stock tank” at the surface. Sandstone contains quartz (monocrystalline ), more stable heavy minerals, less cementing material and matrix, quartz cementing material, stable rock fragments and rapidly deposition such kind of sandstone is known as best quality reservoir rocks for hydrocarbon.
For example quartz arenite and lithic arenite. Cementing materials and matrix is very fine, sediments more oxidized, unstable composition and slow deposition are created problems in reservoir rocks. Although the process is generally the same, various environmental factors lead to the creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land surface to 30,000 ft, (9,000 m) below the surface and are a variety of shapes, sizes and ages.