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Geology is regarded as the scientific study, which deals with earth’s history, origin and structure. It also provides a basic understanding about the procedures and the materials that have shaped the earth’s surface. The distinct fields of geology comprise geophysics, oceanography, sedimentary and petroleum among others (Jain 1). In this particular paper, the sedimentary branch of geology is considered by levying utmost attention on core logging and its procedure. In general, core logging is regarded as the procedure of visualizing cores in the rocks that generate due to the reasons of soil strata and the complicated structures formed within these rocks (Price 84-85).
Thesis statement. This research paper aims at identifying and examining the geology of Thamama Group by elaborating its formation based on stratigraphy and depositional environment. Various significant aspects that include the depositional setting of Habshan and Kharaib along with other rock types such as rudstones and the possible dolomitization model have been discussed in this particular research paper.
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BRIEF DESCRIPTION OF THE GEOLOGICAL SETTING OF THE UAE WITH FOCUS ON THE CRETACEOUS
The geological setting of the UAE with greater focus on the Cretaceous can be directly associated with the opening of the southern Neotethys Ocean during the Triassic and its ultimate closure in Late Cretaceous to Palaeogene Times. A majority of the rock units created during the geological period of Cretaceous, which was the last phase of the Mesozoic Era, in the UAE, were formed within Neotethys. However, the present-day distribution of these rock units is based on the procedures related to the closure of the southern Neotethys Ocean. It can hence be observed that the geology of the UAE with respect to the Cretaceous is directed by the Hajar Supergroup’s ‘carbonate platform succession.’ This succession was placed on the Arabian continental passive margin of Neothethys amid Permian and Early Cretaceous period. The Supergroup is also found to be uncovered considerably within the Hajar Mountains located on the southern region. Focusing on the Cretaceous, the sedimentary rocks were placed within the deeper sections of Neotethys. These sections are traced to expose variably in a 10-15 km broad, structurally composite zone, which extends from the UAE’s north-eastern region along with the western portion of the Hajar Mountains to the northern area of Oman (Hosani, Roure, Ellison and Lokier 61-62).
The geological setting of the UAE with immense focus on the Cretaceous can also be better understood from the deposition of the Lower Jurassic to Upper Cretaceous Hamrat Duru Group. This particular group embraces a series of turbiditic clastic based conglomerates as well as limestone that intermingle with deepwater as well as basinal facies rocks. The UAE’s geology in relation to the Cretaceous is strongly dominated by the historic Oman-UAE ophiolite, which signifies a block of oceanic shell and layer obducted on top of the Arabian platforms’ ‘eastern continental margin’ during the late Cretaceous. This obduction eventually resulted into constructing a foreland basin amid an emergent peripheral swell and the ophiolite. Moreover, the obduction posits successful loading of the Arabian platform as well. The UAE’s geological evolution emphasizing on the Cretaceous can be dispensed to five major palaeogeographical situations that include ‘outer platform margin to basin’, ‘platfrom’, ‘basinal sedimentary and volcanic rocks’, ‘platform’ and ‘syn and post-obduction foreland basin’ (Hosani, Roure, Ellison and Lokier 63). In addition, other important attributes such as the thrusting as well as the folding of the sedimentary sequences in relation to the advancement of the Zagros Mountains cannot be ignored while examining the geological setting of the UAE with greater focus on the Cretaceous (Hosani, Roure, Ellison and Lokier 62).
STRATIGRAPHY AND DEPOSITIONAL ENVIRONMENT OF THE THAMAMA GROUP
The Cretaceous is segregated into three specific groups, out of which the lowermost supersequence is recognized as the Thamama Group. This group can be categorized into various formations such as the Habshan, Kharaib, Lekhwair and Shuaiba-Bab (Eberli, Masaferro & Sarg 124). The stratigraphy environment of the Thamama Group can better be understood as it unconformably covers the Musandam Group and contains calcirudites at its base. Locally channelized series of limestone that are deposited in a lower slope setting, huge blocks of coral-bearing grainstone and lime-mudstones are recognized as certain major stratigraphic conditions of the Thamama Group (Hosani, Roure, Ellison and Lokier 68). While discussing the stratigraphy environment of the Group, it can be observed that its upper portion comprises the alphabet reservoir zones, which are classified into A, B and C, wherein the prime hydrocarbon accumulation has been discussed in a comprehensive manner. In this regard, the Thamama Zone B is viewed as the fundamental oil-producing as well as well-bearing reservoirs, deciphering the extensive nature of geologic sequences along with their evolution (Eberli, Masaferro & Sarg 124). One of the most important stratigraphy settings of the Thamama Group is identified as a deepening upwards series, entailing wackestones and lime mudstones that help in creating, developing and preserving a deeper open-marine environment. Based on this notion, as compared to the UAE, the Thamama Group holds the record of progradating an evolving ramp to shelf carbonate system into Neotethys in Oman’s Jebal Akhdar area (Hosani, Roure, Ellison and Lokier 68).
The overall depositional setting of the Thamama Group is typically based on a huge carbonate ramp mechanism, which is positioned on the Arabian shelf. The reservoir zones A, B and C, located in the Kharaib and Shuaiba Formations of the upper Thamama Group, entail a recurring series of shallow-marine carbonates. While elaborating the depositional environment of the Thamama Group, an intrashelf basin was identified to be developed namely the Shuaiba shelf margin during the late Thamama. The series of shallow-marine carbonates were principally carbonated with minimum siliciclastic material. The A, B and C reservoir zones are 18-58m thick in size and shape and are separated by intense argillaceous limestone or dense layers, whose size ranges from 6-14m. This subdivision of the reservoir zones into distinct layers are based on certain significant variables such as facies, sequence correlation and happening of stylolite-bearing dense intervals. The reservoir zone B is segregated into upper as well as lower units based on their own reservoir properties and depositional facies. These units are categorized as the B upper unit and the B lower unit, wherein the former is grain supported, whereas the latter is lime mud supported in nature. The B upper unit essentially contains creative heterogeneous bioclastic wackestones as well as grainstones having a pelletoidal surface, algal lumps, crude bioclastic-bearing layers and rudistid. Correspondingly, the B lower unit entails less creative heterogeneous bioclastic grainstones along with wackestones with moderate quantities of skeletal debris. Similar to the reservoir zone B, it is apparent that the depositional facies are present in the upper portion of the Thamama layer. This, therefore, recommends the deposition type of “layer cake” prevalent in the reservoir zones A, B and C within the Thamama Group (Eberli, Masaferro & Sarg 124-125).
DESCRIPTION OF THE MAIN DEPOSITIONAL AND DIAGENETIC FEATURES IN THE CORES OF EACH FORMATION
The lower Cretaceous rocks are acknowledged as the Thamama Group, which is categorized into four major formations such as Habshan, Shuaiba, Kharaib and Lekhwair. In general, Habshan formation symbolizes the initial marine transgression, which was initiated at the lower Cretaceous portion. Considering that the conditions at the lower Cretaceous part were immensely restricted or semi-restricted with the cyclical model of carbonate sedimentation, this eventually led towards the deposition of lagoonal wackestones and lime-mudstones as well. The foremost depositional features in the cores of Habshan formation encompass bioturbation and sedimentary structures include a series of chalky-lime mudstone and uneven quantities of skeletal as well as pelletal materials. Correspondingly, the diagenetic features in the cores of Habshan formation are typically based on dolomitization and anhydrite, as oolitic grainstone intermingled with lagoonal and supratidal dolomite as well as anhydrite. Thus, the trends of both dolomitization and anhydrite can be observed in the cores of Habshan formation that help in constructing favorable reservoir units (Metwalli and Khouri 113).
Lekhwair formation uncomformably rests on the above discussed Habshan formation, which showcases the start of usually unobstructed conditions of the marine shelf. This particular formation of the lower Cretaceous rocks, called the Thamama Group, comprises numerous cyclic series, wherein each cycle begins with the types of argillaceous limestone that encompass lime-mudstone and wackestone as well. The chief depositional characteristics in the cores of Lekhwair formation that encompass bioturbation and sedimentary structures are grading argillaceous limestone into porous clean grainstone as well as packstone and developing the regressive situations of shallow water shelf. On the other hand, the diagenetic features in the cores of Lekhwair formation are based on transgressive stage of an open-shelf subtidal setting, which represents dissolution of the normal unrestricted marine shelf situations. Based on this notion, it can be inferred that neither dolomitization nor anhydrite trend is persistent in Lekhwair formation; rather there exists the trend of dissolution of marine and water shelf conditions (Metwalli and Khouri 104-115).
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Kharaib formation holds minor unconformity with the aforementioned Lekhwair formation. This formation displays the recurring prototype of carbonate sedimentation, wherein the trangressive cycles are denoted by argillaceous limestone and the regressive cycles are signified by porous clean limestone. The key depositional features in the cores of Kharaib formation that encompass bioturbation and sedimentary structures encompass two reservoir rock units, which are acknowledged as B/ lower Aptian-Barremian and C/ Barremian and bioclastic materials. Correspondingly, the major diagenetic features in the cores of Kharaib formation typically comprise foraminiferal packstone to wackestone with rudist debris and association amid the bigger skeletal fragments and the important vuggy porosity. Based on these notions, it is apparent that the trend of pore formation is existent in this specific Kharaib formation, which can be distinguished from Lekhwair formation by the dominance of packstones, grainstones and wackestones among others (Metwalli and Khouri 116).
From a theoretical perspective, Shuaiba formation denotes the prolongation of shelf deposition, which is represented in the above discussed Kharaib and Lekhwair formations. In regressive phase, this formation entails clean wackestone as well as packstone, which is followed by deeper water transgressive argillaceous limestone. It can hence be observed that the conditions of deeper-water shelf existed in significant offshore positions of Abu Dhabi till the end portion of the lower Cretaceous. The chief depositional facets in the cores of Shuaiba formation that embrace bioturbation and sedimentary structures include thick shallow type of water rudistid carbonates and rudistid build-ups that emerge from the rise of salt plugs. On the other hand, the major diagenetic characteristics in the cores of Shuaiba formation can be determined as the advancement of submarine topographic highs, which support the development of the structures of limestone build-ups. Thus, it can be stated that there is a trend of pore creation within the Shuaiba formation due to the presence of porous and shoal-water carbonates (Metwalli and Khouri 117).
DESCRIPTION TO INTERPRET AND CONSTRUCT THE DEPOSITIONAL SETTING OF HABSHAN AND KHARAIB
In relation to Habshan and Kharaib formations, the depositional setting of rudstones is identified as fully compiled rudist and coral fragments of distinct sizes. Grainstones are related to different categories of bioclasts that comprise peloids in smaller quantities, benthic foraminifera and algal debris. These rocks can also be associated with other significant aspects as well that entail shoals, shorelines and shelf breaks. Correspondingly, the depositional surrounding of mudstones can better be described as microfacies that essentially contain pyrite crystal along wth borings in planktonic foraminifera (Al-Dabbas, Al-Jassim and Al-Jumaily 67-78). Another rock type i.e. floatstones are viewed as matrix-supported carbonate rocks that generate in excess of 10% granules bigger than 2 mm in size. This specific form of rock type is mostly used for carbonate breccias and reef carbonates (Flugel 354). The depositional setting of packstones can hence be determined as the rocks, which possess extensive level of micritic matrix and significant amounts of skeletal grains. This specific rock type is deposited into an outer shelf, basin and slop, trailing lagoonal and shoal reef settings (Al-Dabbas, Al-Jassim and Al-Jumaily 67-78).
Anhydrite and dolomite formations are different from one another in terms of functionality and procedure with respect to the characterization of carbonate reservoirs. These usually have an impact on the platform carbonates and exert extensive level of control over the reservoir quality. Emphasizing the depositional setting of Habshan and Kharaib formations, anhydrite element is identified as simply poikilotopic or pore-filling in nature. However, in contrast, dolomite component is replacive nodular in nature. From the functional perspective, anhydrite takes places in close affiliation with the hypersaline depositional conditions based on calcium sulphate precipitation. Correspondingly, dolomite operates under fabric-retentive conditions, wherein fine-scale depositional arrangements such as bioturbation, lamination and fenestral fabric are preserved in an effective manner (Bonab, Dizaji and Tavakoli 1-24).
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BRIEF OUTLINE OF THE POSSIBLE DOLOMITIZATION MODEL
The best dolomitization can be observed in the Habshan Formation, which is fundamentally composed of grainstones and ooidal packstones that are deposited in various distribution places. Habshan Formation’s dolomitization procedure is significant in nature, as the depositional grains of the carbonate rocks are vertically cyclic that have a significant effect on the distribution procedure of dolomite. Fair and successful distribution of dolomite is deemed vital to maintain reservoir quality and possess deeper comprehension about its formation mode. In the Habshan Formation, along with the ooidal limestone, few thick dolomitized beds are present that certainly enhance the dolomitization procedure as per the expectation level (Vandeginste, John and Manning 489-503). In this regard, Sabkha model can be regarded as the possible representation, which explains the dolomitization procedure being followed in the Habshan Formation. In general, Sabkhas are regarded as coastal supratidal mudflats that are widespread in various arid areas such as the Arabian Gulf. Evaporative pumping is recognized as one of the significant procedures of Sabkha model, which represents upward water flow from the saturated groundwater zone through capillary evaporation. Under this specific process, water lost from the sediments or the water table of sabkha by evaporation is restored by storm-driven seawater (Boggs 399). Therefore, as observed in the dolomitization procedure of the Habshan Formation, the sabkha model may prove to be quite effective in characterizing carbonate reservoirs with forming, maintaining and developing a shallow burial setting. This certainly has an impact on the improvement of reservoir quality and the rock fabric pattern as well. Considering that the depositional grains of the carbonate rocks present in the Habshan Formation are vertically recurrent in nature, use of sabkha model can enhance reservoir quality by raising the level of permeability (Vandeginste, John and Manning 489-503).
Based on the above discussion, it is apparent that the geological setting of the UAE with focus on the Cretaceous can be examined as the creation of extensive rock units and the deposition of the Lower Jurassic to Upper Cretaceous Hamrat Duru Group. The stratigraphy as well as the depositional settings of the Thamama Group is recognized as the presence of alphabet reservoir zones and the execution of a large carbonate ramp mechanism, which is positioned on the Arabian shelf. Sabkha can thus be regarded as the possible dolomitization model, which explains the best dolomitization procedure observed in the Habshan Formation, as this is effective in characterizing carbonate reservoirs and determining the suitable rock fabric patterns.
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