According to Alexander et al., (2011), shale gas refers to a natural gas that stored in organic-rich, fine-grained rocks, including shale, laminated siltstone, or mudstone. It contains a mixture of hydrocarbon gases, majorly ethane, and methane. The gases are tightly locked within the pore spaces of the sedimentary rocks. The reservoirs of the shale gas have features such as low impermeability to clay content and, small grain sized contents. The term shale does not focus on a specific rock, but rather the rocks that have fine-grained particles that are smaller than the coarse-grained particles such as siltstone and carbonate rocks among other rocks. The generation of the shale occurs through various processes that include primary and secondary thermogenic degradation alongside biogenic degradation of the organic matter. The occurrence may also occur in a combination of all of the above mechanisms. The formation of shale gas occurs through a complex process that takes years (Andrews, 2013). The process of formation begins with the deposition of material consisting of a mixture of clay and minerals in deep waters such as lakes, seas, and oceans. The material also combines with algae, plankton, and plant matter during their burial. As the mud changes into shale during its shallow burial, bacteria act on the available organic matter resulting in the release of biogenic methane as a byproduct (Bakshi, 2012).
What are shale gas reservoirs?
Shale gas originates from a source rock with hydrocarbons that are generated following the burial of clay, minerals, algae, plankton, and plant matter. The hydrocarbons migrate from the original rock via carrier beds and accumulate in the porous reservoir over time. The porous reservoir consists of carbonate and/or sandstone in discrete traps that are located on the structural high on the margins of the center basins. The low permeability of the rock that acts as the source of the gas makes it trap the shale gas and prevent it from escaping towards the surface of the earth. The gas can also be held in the natural fractures below the surface of the earth alongside the pore spaces of the sedimentary rocks (Berman, 2009). In addition, Bulletin & Norton (2003) recognize that the gas can be absorbed into organic material that can be processed to release the stored gas. Moreover, Issler et al., (2002) states in their article that the shale gas can be adsorbed to the surfaces of the minerals within the natural fractures and/or pore spaces and absorbed to mineral surfaces of the matrix rocks. The existence of the shale gas in fractures is obtained using methods such as multi-stage fracturing and drilling horizontal wells.
Geologists consider specific geochemical characteristics to evaluate the ability of the shale to have the desired production potential. The core data acts as the major source of the features of evaluating the abilities of the shale rock under consideration. Methods such as downhole sensors and calibration of the log data are effective in allowing the geologists determine the potentialities of the shale rock (Ross & Bustin, 2008). In specific, they consider the ability of the shale rock to meet the characteristics of shale resource that include the gas volume and capacity, mineralogy, permeability, thermal maturity, and total organization carbon (TOC). Total organic carbon governs the potential of the shale rock to provide the desired amount of shale gas. Therefore, rocks with high TOC values are rich in shale gas, while those with low values have less shale gas content (Lee et al., 2011).
Shale rocks have features that vary significantly between the reservoirs and within the reservoir due to the variety of materials and fabric anisotropy possessed by the organic-rich shale. The elastic characteristics of the shale gas make it to have strong anisotropic properties. The degree of anisotropy correlates with the amount of organic contents and clay in the parent rock. Vertical Young's modulus and velocity also play a role in influencing the anisotropy of the shale rocks. Significant evidence shows that the shale gas has a relatively stronger anisotropy irrespective of its limited TOC content of less than six percent. Shale rock considered a reliable source of shale gas should have high gamma-ray values. High Gamma ray value translates to high organic carbon content, thereby, a considerable source of shale gas (Shebl et al., 2010).
Moreover, the content of the organic matter defines the features of the shale rock. Shale rock should be rich in total organic matter of values greater than two percent. The high TOC content shows the potentials of the rock to provide an adequate supply of shale gas. In addition, the kerogen shale rock should be Type I, II, or IIS to make it qualify as a potential shale reservoir. Kerogen of the above classes' mean the rock is of marine or the planktonic origin, thereby, has a large amount of shale gas (Schenk, 2011). According to Speight (2013), shale rock should have an original hydrogen index of more than 250 mg/g for it to supply the required amount of shale gas. Such value is desirable because the kerogen type found in the shale rock relies on the original hydrogen index of the shale rock. Similarly, Lee et al., (2011) recognizes that the mineralogy or the clay content of the shale rock should be low, i.e. less that 35%. Low content of clay improves the quality of the shale rock by facilitating fracking, thereby, extraction of the shale gas. The content of silica should be more than 30% with some presence of some carbonate and non-swelling clays.
Jacobi et al., (2008) recognizes that shale rock should meet the requirement of thermal maturity that include maturation for gas generation. The thermal maturity of the rock should range between 1.1-3.5% to become a suitable rock for extracting shale gas. The geologists should identify the oil precursor in the rock to make it an ideal source of shale gas. In addition, Loucks & Ruppel (2007) present a system of shale rock analysis that considers the gas content, depth minimum, shale porosity, and overpressure as the key defining features of the potential shale rock for producing shale gas. In specific, the authors recognize that the gas should be present in the rock as a free and/or adsorbed gas with gas content of 60-200 bcf. The depth minimum should be greater than 5000 feet with a shale porosity of between 4% and 7% and not more than 15%. The overpressure in the parent rock should be highly over-pressured and having burial and tectonics history. Shale rocks with tectonics and burial history are rich in carbon due to involvement of planktons and algae of the sea that were buried deep into the earth. Therefore, shale rock having these features qualifies as the potential source of shale gas (Schenk, 2011).
How are shales evaluated for gas potential?
It is highly recognizable that not all the shale rocks have the potentials to provide the required volume of shale gas. Zou et al., (2012) appreciates that assessment of the shales often occurs prior to the evaluation of the potentialities of the shale rocks. Assessment entails conducting preliminary reservoir and geologic characterization of the shale formation and basins alongside establishing the area extent of the shale oil and gas formation. The assessment also entails defining the area of prospecting the gas and oil alongside calculating the risked shale oil and shale gas place. Seismic mapping using the available hydrocarbon is one of the methods of evaluating the potentiality of the shale rocks. Seismic mapping provides information related to the carboniferous basin shales that provide rich sources of carbon that determine the shale content of the shale rocks (Loucks & Ruppel, 2007).
The potentiality of the shales can also be evaluated by computing the organic content of the shale rocks using the density logs. The process entails taking the density of the shales in the area of focus alongside the density of the shale minerals and the average grain density of the shale matter. The method of evaluation operates on the premise that changes in the organic content of the shales produce significant changes in their formation, density, and the organic content. As such, slight variation in the density of the content will provide valuable information for evaluating the potentiality of the shales (Loucks & Ruppel, 2007). Moreover, Zou et al., (2012) recognize that the success of using this method to evaluate the potentiality of the shales depend on analyzing the four component system of the parent rock that has rock matrix, pyrite, organic matter, and interstitial pores.
Geochemical analysis also allows for the evaluation of the potentialities of the source rocks of the shale gas. The process involves analyzing the samples of the shales in conjunction with a detailed evaluation of the logs obtained from previously drilled wells. Geochemical testing is done to the capabilities of the shale rock in generating hydrocarbons required for producing the desired volume of shale gas. As such, rocks with high amounts of concentrate organic…