Elements & mineralogical composition (of the evaporite rocks)
Evaporite rock is a sedimentary rock which is normally composed of mineral precipitated from an evaporating saline solution. According to Nely (1994) evaporate rocks are poor in Na and rich in K, with a K/Na ratio higher than in the other rocks. Nely continue to say that when Na is present it is actually concentrated in halite. Nely (1994) established that as one of the major components of evaporate rocks “the potassium rich character of silicates which is analyzed illites rich in potassium and authigenic potassium feldspar is well explained by an equilibrium with a solution having a high potassium and sodium ratio” (p.131).
In their further studies Nely (1994) found out that the rocks which are most typical of evaporate sedimentation is due to their magnesian character and therefore they are the least well marked in regard to boron concentration. There are also the non-sulphate samples of the evaporate rocks which are contrarily very clearly distributed in the magnesium domain delimited by this lines (Nely, 1994). He also noted that for a Fe/Mg ratio of the order of 0.3, the argillites of evaporite rocks are thus distinguished from those of a reference clastic sequence Nely (1994).
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Gornitz (2009) on the other hand commented that evaporate deposits can be established through geochemistry which helps in documenting the origin of post-depositional evolution. Gornitz (2009) continues to say that the Br/CI ratios in chloride salts provide information on the evaporite concentrations and the hydrologic history record (p.324). The marine evaporite rocks may have the composition of fluid inclusions of marine halites in the major ion chemistry. Gornitz also noted that many deposits in the evaporate rock have both marine and non marine influences and alternations between them. These alterations are known to be the result of mixing of marine and non-marine waters in isolated costal lakes and intercalations of continental with costal marine deposits Gornitz (2009).
Environment of deposition (to include tectonic setting)
Schreiber, Lugli & Geological Society of London (2007) noted that “a greater thickness of the evaporite formation is present in some wells with a deeper environment of deposition where there is a greater accommodations space” (p.59). They continued to say that such environments have over the time lead to the accumulation and preservation of great thickness of evaporates and they are composed mainly of thick gypsum and anhydrite. These are rich in halite when the conditions are suitable for precipitation of such evaporates.
Anhydrite forms most of the evaporate rocks with some residual gypsum and halite. Schreiber, Lugli, Geological Society of London (2007) also indicated that “the presence of marine organisms in the limestone beds have favored the formation of evaporates with the re-establishment and continuation of restricted marine environmental conditions between evaporate phases” (p.59). They continued to indicate that the intercalated limestone beds within the evaporites are dolomitic and they are highly affected by anhydrization and dissolution hence as e result only ghosts of fossils are preserved.
Modern evaporite deposits as indicated by Gornitz (2009) accumulate in arid and semi-arid regions of the world in the global high pressure belts of the subtropical horse latitude and the poles. In his further studies Gornitz (2009) found out that in the mid-latitude intercontinental desert and steppes that are isolated from oceanic moisture (p. 322). Other arid areas according to Gornitz (2009) where evaporates can form are the rain shadows of high mountain chains which may be present at any latitude. Examples of such areas include Patagonian and Nevada-Utah orographic deserts, which are located in the Andes and the Sierra Nevada rain shadows (Gornitz, 2009).
Gornitz (2009) continues to say that the great part of present day evaporite deposition occurs in closed continental basins. He also says that on the other hand coastal settings are volumetrically less significant. Gornitz (2009) also noted that marine evaporates are confined to coastal supratyidal settings and to low lying areas where seawater seeps into pools and small basins. He said that for example small amounts of evaporite minerals are forming in lakes by brine mixing, freezing, evaporation and sublimation.
Condie (1997) states that in attempting to relate mineral and energy deposits to plate tectonics it is important to know the relationship between the deposits and their host rocks (p. 105). According to Condie (1997) tectonic settings for evaporite rocks is characterized by restricted water circulation in which organic matter is preserved. Condie (1997) continued to say that as the rift within this environment continues to open water circulation becomes unrestricted and accumulation of organic matter and evaporite deposition cease. In his research Condie (1997) mentioned that “several requirements must be met in any tectonic setting for the production and accumulation of hydrocarbons such as oil and natural gas” (p. 107).
These conditions include: firstly the preservation of organic matter requires restricted seawater circulation to inhibit oxidations and decompositions. Also high geothermal gradients are needed to convert organic matter into oil and gas (Condie, 1997). Secondly tectonic conditions must be such as to create traps for the hydrocarbons to accumulate. Deformation which accompanies continental collisions creates a variety of structural traps in which hydrocarbons are capable of accumulating. Another example as indicated by Condie (1997) is that coal is formed as a result tectonic settings. These hydrocarbons are as a result of plant remains which must be rapidly buried before they decay were such rapid decays occur in swamps with high plant productivity (Condie, 1997).
Burial and diagenesis (sediment tectonics, sedimentary rock)
The nature of Sedimentary rocks is determined by geological processes that occur in the four main Earth surface environments (Mackenzie, 2005). Burial and diagenesis is determined by the site of sediment production, where interactions among bedrock geology occur, tectonic uplift and climate control weathering and erosion process. According to Mackenzie (2005) the “conditions of later burial, where diagenetic processes may further alter the texture and composition of buried sediments is another important factor that determines the formation of sediment rocks” (p.116). Mackenzie (2005) further mentioned that sedimentary rocks preserve records of multiple proxies that illuminate the process and conditions of sediment formation, transportation, deposition and burial.
In addition Tucker (2001) indicated that “considerations of sedimentary rocks do not stop with environmental interpretations” (p.5). Trucker continues to say that it is during diagenesis that an indurated rock is produced from unconsolidated, loose sediment (2001). He thus says that the diagenesis process starts immediately after deposition and continues until metamorphism takes over (Tucker, 2001). There is however a clear distinction between early diagenetic events, taking place from sedimentation until shallow burial and late diagenetic events which occur during deep burial and subsequent uplifts (Tucker, 2001).
Burial and diagenesis processes are important because they can considerably modify sedimentary rocks in both their composition and texture and also in it has been observed that in rear cases the original structures are destroyed completely (Tucker, 2001). In his further studies Tucker (2001) established that diagenesis events also affect a sediments porosity and permeability which are the properties that control a sediments potential as a reservoir for oil, gas or water. Geochemical analyses of sedimentary rocks especially limestone and shales can be used to give useful information on the environment. Some of the diagenetic processes include compaction, recrystallization, dissolution, replacement, authigenesis and cementation (Tucker, 2001). He continues to say that sedimentary rocks should be described within their lithological context such as composition and grain size (Tucker, 2001).
Distribution in space (from geographic side) and time (geologic side)
Purser & Bosence (1998) indicated that important evaporate sequences are commonly associated with rift basins and especially those located in lower palaeolatitudes (p.409). Purser & Bosence (1998) further indicated that the” association between evaporate sedimentation and the geographically and chemically restricted sedimentation exist during the initial stages of a rift basin and ocean formation” (p.409). Therefore according to their studies the favorable combination of climate, drainage and morphology which are necessary for massive evaporate development is favored by narrow, elongate depressions supplied with sea water via a vertically or laterally restricted connection with sea or ocean (Purser & Bosence, 1998)
Their research established that a well known sequence of existence of evaporates has been shown to exist in the lower Cretaceous of the West Africa and Brazilian margins. Purser & Bosence (1998) continue to say that other areas include the Triassic of the North Atlantic margins notably in Morocco. On the other hand the Miocene evaporates of the Red Sea and Gulf of Suez has been well known due to the presence and exploration of petroleum in the basin. Schreiber, Lugli & Geological Society of London (2007) also noted that marine conditions continued throughout the early Miocene and the Serikagni, Euphrates, Dhian Anhydrite and Jeribe formations were deposited in marginal basins (p. 68). As a result this gave rise to the cyclic deposition of evaporates in the Middle Miocene Fat’ha formation.
Significance (of evaporites) to petroleum industry
Condie (1997) established that evaporate formation has for a long period of time been used in the mining of petroleum products such as oil and gas. Evaporite is therefore considered as one of the major energy deposits around the globe. Both oil and gas are formed in foreare and back-are basins which can trap and preserve organic matter and where geothermal heat facilitates conversion of organic matter into hydrocarbons Condie (1997). He also established that the majority of the petroleum products that is oil and gas reserves in the world have formed either in intracratonic basins or passive continental margins.
Due to high geothermal ghradients beneath the opening rift and the increasing pressure due to burial of sediments the processes facilitate the conversion of organic matter into oil and gas. Condie (1997) also noted that oil and gas may also be trapped in structural traps as they move upward in response to increasing pressures and temperatures (Condie, 1997). In addition Melvin (1991) noted that there is a direct comparison of oil volumes to evaporate volumes throughout geologic time (p.367). Melvin continues to say that a large and appropriate reservoir for evaporites is associated with the presence of petroleum deposits as shown in the diagram figure.1.0. Melvin (1991) commented that “many petroleum occurances are may be directly associated with an evaporate environment that only produced carbonate sediment and thus would not be included in the evaporite volume data” (p.368).
Tucker (2001) established that the generation of petroleum is one of the stages in the alteration of certain types of organic matter buried in sediments. Tucker (2001) outlined that “petroleum can occur in any porous rocks and that most of the world’s oil is located in sedimentary rocks” (p. 208). As shown in the figure 2.0 below petroleum is derived largely from the maturation of organic matter deposited in fine grained marine sediments. Many marine hydrocarbon source rocks are formed at times of high organic productivity of marine plankton, coinciding with transgressive events and high stands of sea level (Tucker, 2001).
In conclusion, Nely (1994) said that a good knowledge of evaporite accumulations is very useful in petroleum exploration. Several authors have established on the relations between hydrocarbon accumulations of exploitable magnitude and evaporites. Nely (1994) also continues to argue that petroleum exploration provides access to the most interesting zones and basins with evaporites. Nely (1994) also indicated that the most the “association regarding the frequency of evaporites and the central and subsiding areas of basins gives enough evidence of the associations between evaporites and petroleum exploration” (p.189). Nely in conclusion argued that “the good quality of petroleum exploration data provides good knowledge of evaporite sequences” (p. 189). He continues to argue that given the characteristics of salt rocks a general interpretation of evaporite rocks gives enough evidence.
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