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Abstract SUMMARY AND CONCLL’SIONS The present work deals with the application of computer techniques for the quantitative estimation of lithologies in subsurface sedimentary sequences on the bas is of well-log data. The obtained results are used for stratigraphical and sedimentological interpretations. The method elaborated in this work is applied to six wells drilled in the July Oilfield, Gulf of Suez area, Egypt. The thesis includes several aspects in order to clarify the used procedure and its relation to the interpretation of sedimentary sequences. ( 1) The first aspect of the thesis deals with the geology, stratigraphy and general structural setting of the Gulf of Suez and the Red Sea regions in general and of the July Oilfield in particular. The discussions are, here, illustrated by a geological map of the Red Sea and Gulf of Suez regions, maps showing the main faults in the July Oilfield at different horizons and a correlation chart for the six wells studied in the present work. This last mentioned correlation chart is drawn by the author on the basis of data given in composite logs ( as provided by the 6eologists of GL’PCO). The sediments drilled in the July Oilfield range in age from Holocene to Late Paleozoic. (2 The Second aspect deals with the synthesis of the information available in the literature on well logging techniques and on the methods of interpretation used for the processing of well-logging data for the quantitative estimation of lithologies. This aspect of the thesis is covered by briefly describing the devices used in welllogging and by discussing the nature of the various and most probable lithology combinations that are likely to be encountered during drilling. The latter discussion deals also with the tectonic movements of the Earth’s crust inducing the development of various types of sedimentary associations. This aspect of the thesis aims at - 126 - clarifying certain concepts related to the relationship between tectonism and sedimentation. These concepts help in the prope+ interpretation of well logs. (3) The third aspect of the thesis deals with a description and discussion of the procedure followed in the quantitative estimation of lithologies. This includes a discussion of the method followed to prepare the well-log data in a form that can be fed to the computer, together with a thorough presentation of computer programmesprepared to undertake the calculations involved in the procedure. ( 4) The fourth aspect of the thesis deals with the use of the results of the processing of the welllog data for stratigraphical and sedimentological interpretation of the logged sequences in the six wells considered in the present work. The formations involved in this study are : the South Gharib, Balayim, Kareem, Rudeis and Nukhul Formations (of Miocene age) and the Nubia Sandstone (of Early Cretaceous and Late Paleozoic age:’. The discussions involved in this part of the ·•·ork are illc.strated by a quantitative lithologic log for each well. These logs present graphically the results of the quantitative estirnation of lithologies in the studied intervals. They present also the stratigraphic subdivisions proposed in the present work together with the stratigraphical subdivisions ;;iven by the geologists of GCPCO, on their composite logs, for the considered intervals. The data of these six logs are. fc.rthermore, summarized in a correlation chart for the logged intervals in the six wells involved in the present study. The stratigraphic and sedimentological results or the above mentioned inn:stigations may be summarized as follows : (1) A comparison of the lithologic logs obtained by the procedure followed in the present work and those given by the geologists of GUPCO, based on the qualitative interpretation of electric logs and the examinationof cuttings, shows that both results match fairly well. Furthermore, it can be noticed that the quantitative method used in - ~27 - this work provideBmore information on the detailed lithologic constitutions of the studied stratigraphic intervals and on the vertical changes in lithology encountered in the logged sequences. (2) It was possible to determine some important lithological and sedimentological characteristics of the logged rock units by using the results of the programmes prepared during the present work. These characteristics may be summarized as follows, for each rock unit, arranged from top to base : The South Gharib Formation consists of alternations of anhydritic salt, clayey, dolomitic anhydrite and sandy, clayey limestone. This formation shows a lithology typical for the ”evaporite association” deposited under fluctuating extremely arid to subarid climates in a restricted intracratonic basin. The Balayim Formation is represented by its four members (Hammam Faraun, Sidri, Feiran and Baba). These members are alternations of shallow marine (Hamman Faraun and S idri) and eva pori tic (Fe iran and Baba) sequences. This formation seems to have been deposited under similar climatic conditions as those that prevailed during the sedimentation of the South Gharib Formation in a generally restricted intracratonic bas in with intermittent periods of humid climates and or free communication with the open sea (probably from the north! . The Kareem Formation is only logged in one of the SIX studied wells. from the descriptions of this formation provided by the composite logs of the studied wells, it may be assumed that it generally represents open marine sedimentation. Only the lower part of the clayey Shagar ~!ember and the evaporitic >larkha ~!ember of this formation are quantitatively studied in the present work. The lithology of this Markha Member (anhydritic salt) suggests sedimentation in a restricted intracratonic basin under extremely arid climates. Thus, the Markha Member may be considered to mark an episode of restricted (evaporitic) development before the main phase of open marine sedimentation which prevailed over - 128 - the Gulf of Suez during the time of deposition of the upper member of the Kareem Formation. Tile Rudeis Formation was subdivided for the first time-rnto four members by using the output of programmes’ LITHOL<X1Y’ and ’LDIKAGE’. These members are Mreir, Asl, Hawara Sand and a sequence of alternating clayey, calcareous sands and calcareous shales, which may represent in i”L~ totalit·.· or in part, the Mheiherrat ~!ember. Thec ::ower boundary of the Rudeis Formation coulc not be traced wit.~-] accuracy, as it was very dl:ficult to differentiate the Mheiherrat }!ember ::>i the Rudeis Formation from the ~ukhul Formatio~ by purely petrographic methods (based on lithologlc determinations from well logs). Tile ~!reir >!ember of the Rudeis FormatioD consists of shales. This is probably a sequence that was laid down in a sedimentary basin under humid c: limates. lt is interesting to note that this episode of pellitic sedimentation precede in the Gulf of Suez another phase of sedimentation characterized by the deposition of evaporite ! ~larkha ~!ember of Kareem Format ion) . These two t~:pes of deposits have been probably laid down in the same closed basin but under different climatic conditions. I’l-.e Asl \:ember of the Rudeis Formation co:J.sists of ope,-, :narine marls, sometimes dolomi t i·: . The Ha·”·ara Sand :,;ember. on the other hand. is composed of coarse clastics, denoting that lt has ~)~·en d£>posited i~ a more dynamic enYironment. However, the appreciable amounts of clavs !ound ir. these sands suggest that the sea was still deep enough to prevent effective winno”’·ing of ”’’e sediments. Tr.e influ..x of sand in :he Gulf of Suez sedimentary basin may be either d:oe to a period of ’tectonic activity or to a highly humid c 1 ima tic phase, with in tense drainage of the surrot:nding lands, or to both factors. The seqt:ence of clayey, calcareous sands and calcareo: os shales underlying the Hawara Sand in the studied wells is probably marine and c’quivalent to the· :\!beiherrat ~lember of the Rudeis Formation. It may also, however. ;>clang partly to the upper part of the ~ukhul Formation. - 129 - The ::l:ukhul Formation in the drilled wells consists of calcareous or dolomitic shales and dolomitic marls or limestones. Referring this sequence, of probably marine beds, to the Nukhul Formation remains, however, questionable as long as no more evidence of the detailed lateral variations is available. In the drilled wells there is no Oligocene sediments. The marine Eocene, Paleocene and cpper Cretaceous rocks are not properly logged, as they were not suspected to contain hydrocarbons. The Nubia Sandstone, however, was carefully logged, because this formation constitutes the reservoir for hydrocarbons in the well J 25. This formation is subdivisible into three informal rock units or ”series” (the ”A”, ”B” and ”C” series of petroleum geologists). According -r:o these geologists, the upper unit is probably ~!esozoic (corresponding to the facies of the Nubia Sandstone exposed over wide areas in Egypt). The ”B” and ”C ·• series, on the other band, are probably Paleozoic. The ’\uO:Jia Sa:1dstone consists of clayey s:<:ods, calcareous clayey sandstones, calcareotc!’o s:cales and rare dolonitic clayey l::.:nestones. ::: :s probably a paralic and or deltaic i::t ori;::·. (3. :::~e pre,·iousl:; discussed s-r:ratigraphical a:1d sedine::ttological results of the in-r:erpre:ation o: :::e quantitati·;·~ lithological daw. oiJtalned bY pro2essinh well-log data ~re o:1J..:,· pro’:isional as lon~ as they ar2 ~ot supported ~~d cu~ple~ed b·; ~,:ieq·~a:e micropaleontolo,:ical ar:d pe:ro:.:,-aphic irn·es tiga tions on samples. It can be concluded that -r:h•? metl:.od el:::.’ora-r:ed in t!1e presen-r: study prm·ides a rapid meaCJs 10:!:” obtainin;; a huge amount of detailed quantitative ll::l:ologic data from well log information. The accuracy o! the method can be better evaluated by comparing -r:he a:J.ta provided from the analysis of cores with the resc:lts based on •rell logging for the same cored inte-rvals. The method can be systematically applied to the in-r:ervals for which sufficient well log data are available in order to revise with greater precision the - 130 - stratigraphy of the different subsurface sedimentary basins in Egypt. These studies must be accompanied by parallel investigations on samples from the studied intervals. In this ·way it is possible to consider that subsurface geologists are pro\”ided with an optimum me”t;,odology for subsurface stratigraphic work and syn” t:C.es is. |