Clay Lithofacies and Depositional Environment, as Deduced ... - IDOSI

Middle-East Journal of Scientific Research 16 (8): 1027-1036, 2013 ISSN 1990-9233 © IDOSI Publications, 2013 DOI: 10.5829/idosi.mejsr.2013.16.08.85178

Clay Lithofacies and Depositional Environment, as Deduced from Natural Gamma-Ray Spectrscopy and Petrographical Data of the Abu Roash “E” Member, GPT Field, Western Desert, Egypt Ali Younis Ahmed Abdel-Rahman Department of Geophysical Sciences, National Research Centre, Cairo, Egypt Abstract: The present study is concerned with the identification of clay minerals and depositional environment for the Abu Roash “E” Member in the GPT Field, Abu Sennan area, Abu El-Gharadig basin, using the natural gamma-ray spectroscopy (NGS) and lithodensity log. In addition, geologic and extensive petrographic study were preformed on some selected thin sections of rock samples representing the studied member in the study area. The present study revealed, that the Abu Roash “E” Member is dominated by silty shale with subordinate limestones, siltstones and sandstones and that the shales are composed of a mixture of kaolinite and mixed-layer clay minerals. The sediments were deposited under shallow marine shelf conditions with an occasional more open water marine environment. Key words: GPT Field Abu El-Gharadig basin of clay minerals

Petrography

INTRODUCTION The study area is located in Abu Sennan concession within Abu El-Gharadig basin in the northern part of the Western Desert. It is approximately bounded by longitudes 28° 00 and 29° 00 E and by latitudes 29° 00 and 30° 00 N (Fig. 1). One of the important applications of electrical logs, especially natural gamma-ray spectroscopy (NGS), is the identification of depositional environment, using the ratio of Th/U in order to differentiate between the sedimentary facies. Based on the values of Th/U ratio, the main depositional environment of the Abu Roash “E” Member is determined. The evaluation of sedimentary facies depends mainly on the identification of clay minerals witin the formation. In order to highlight the depositional environment, the radioactive clay minerals have to be first identified. This is available from a combination of the NGS tool with the improved litho-density log. The latter allows a measurement of the photoelectric absorption index, Pe, in barns/electron, which is related to the atomic number and thus varies from one lithology to another. Such Corresponding Author:

Depositional environment

Identification

combination of NGS and Pe permits a powerful and precise evaluation of clay minerals. The Turonian Abu Roash “E” Member rocks were subjected to a comprehensive integrated study (geologic, petrographic, identification of clay minerals and depositional environment), as it represents one of the most important members of high hydrocarbon potential. In the present work, 20 wells were studied through varying subsurface geologic methods and 4 wells of them were chosen for providing information on the petrographic, identification of clay minerals and depositional environment. Geological Setting: The northern part of the Western Desert of Egypt has been studied by many geologists, such as Abdine [1], Deibis [2], Barakat et al. [3], Robertson [4], Demerdash, et al. [5], Moussa [6], Ibrahim [7], Puglies and Kamel [8], Said [9], Darwish, et al. [10], El Shaarawy and Montasser [11], Abd El-Rahman [12], Shalaby, et al. [13], Abd El-Gawad, et al. [14], Khalifa, et al. [15], Shalaby, et al. [16], Ali, et al. [17], Yousef, et al. [18] and others. The geological column penetrated by those (20) wells can be summarized as follows:

Ali Younis Ahmed Abdel-Rahman, Department of Geophysical Sciences, National Research Centre, Cairo, Egypt.

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Middle-East J. Sci. Res., 16 (8): 1027-1036, 2013

Moghra Formation (Miocene - Recent). Dabaa Formation (Oligocene - Upper Eocene). Apollonia Formation (Middle - Lower Eocene). Khoman Formation (Santonian - Maastrichtian). Abu Roash Formation (Coniacian - Upper Cenomanian). Bahariya Formation (Lower Cenomanian). Kharita Formation (Albian). Abu Roash “E Member: It shows that, the Abu Roash “E Member is dominated by silty shale with subordinate limestones, siltstones and sandstones. A number of wells showing the top part of the member to be absent due to local erosion. Shallow marine shelf conditions (which was normally a low energy environment dominated by the deposition of mud and silt) with an occasional more open water marine environment are postulated.The thin limestones in the Abu Roash “E” Member were able to accumulate only, when terrigeneous input was reduced. The thickness contour map of Abu Roash “E” Member (Fig. 2) shows a marked thinning toward the southeastern part of the study area, illustrating the effect of faulting in this part. Generally, the thickness increases northwards. The variation in thickness of the Abu Roash “E” Member is due to fault movements (Mid-Turonian event, as shown by Dia El-Din, [19]), which led to a local unconformity between the Abu Roash “E & D” Members, that is thought to be one of the major tectonic events in the study area. The maximum thickness penetrated by the Abu Roash “E” Member is 312 m in GPT-15 well. The minimum thickness cannot precisely be determined, since the wells with minimum thickness are faulted out due to partial missing. The 2D and 3D representation or the depth contour map of the Abu Roash “E” Member (Fig. 3) indicates that, the Abu Roash “E” Member is an asymmetrical anticline towards the northeast. Petrographical Studies: Six samples from the Member “E” of the Abu Roash Formation have been studied from GPT - 2 and 4 wells. A thin section from one of these (Fig. 4A of GPT - 2 well) shows the limestone to be a bioturbated bioclastic lime packstone. It is very fine grained, moderately well sorted sandstone, with angular to subangular grains and tangential to concave/convex grain contacts. A wide variety of the detrital grains are present, dominated by monocrystalline quartz (47%). The second most abundant grains are the fragments of bored shell and bone (8%). The feldspar component (4%) includes leached grains of orthoclase, plagioclase and microcline. The other detrital minerals are muscovite mica (2%),

quartzitic rock fragments (trace), clay pellets (2%), glauconite (5%), zircon (trace) and pyritised plant debris (2%). The sample has a mixed matrix of detrital clay (3%) and lime mud (14%). No authigenic clay minerals have been identified, but authigenic mineral cements are present. Non-ferroan calcite is the dominant mineral cement (5%). This calcite is preferentially located adjacent to skeletal carbonate grains. Small volumes (1%) of poikilotopic ferroan calcite have also been identified. The total volume of silica cement (as the authigenic quartz overgrowths) is small (2%), but the individual overgrowths form localized tightly cemented patches. Around (1%) of framboidal pyrite is also present. The visible porosity is around 4% primarily microporosity (3%), with a small volume (1%) of secondary porosity produced by grain dissolution. Figure (4B - F) represent, the five clastic samples, that are commonly moderately sorted, fine to medium grained sandstones with subangular to rounded grains and straight to concave/convex grain contacts. However, Fig. (4B) is somewhat finer grained (silt to very fine sand) and has tangential to straight grain contacts. Significant volumes of detrital clay (7%) are present in Fig. (4B). The detrital grain mineralogy of all the sandstones is dominated by monocrystalline quartz (57% to 73%), with subordinate feldspar (traces to 4%), quartzose rock fragment (absent to traces) and heavy minerals (traces to 1%). The feldspar component includes orthoclase and microcline (commonly leached) and the heavy mineral component includes opaques and zircon. Glauconite (2% to 7%) is present in most samples and small volumes of fossil debris (shelly fragments, plant debris and fish remains) are common. Most of the sandstones contain little evidence of authigenic clay (commonly 100. Th or U (ppm) as; low = < 3, moderate = 3 - 6 and high = >6. K (%) as; low = 2. Th/K ratio as; low = < 5 and high = > 5. Th/U ratio as; low = < 2 and high = > 2. Therefore, the spectral gamma-ray log of the Abu Roash “E” Member (Figs. 6, 7 and 8) has values of SGR ranging from low to moderate, ranging between (15 and 70 GAPI). The Th is low to high (0.2 - 11.5 ppm), the U is low to moderate (-0.9 - 5.3 ppm) and the K is low (0.001 - 0.026 %). Consequently, the ratios of Th/K =10^ 4 are low (0.003 - 0.4) and Th/U are low to high (0.4 - 234). The Th/K against Th/U ratios (Fig. 8) are distinguishable into two groups, both of them has low and high values. The variation in Th/K ratios within the Abu Roash “E” Member can be adequatly accounted for by the changes in detrital clay mineral assemblages. Thus, the high Th/K ratios are associated with clay mineral suites dominated by kaolinite and/or smectite, whereas the low Th/K ratios are associated with clay mineral assemblages dominated by illite. The change from high Th/K to low Th/K ratios may be explained by a change from aeolian to marine transport for the clay and by a change in the climate and/or sediment source area. Identification of Clay Minerals: In addition to the X-ray diffraction analysis, the natural gamma-ray spectroscopy (NGS) and litho-dnsity log (Pe) are used for the identification of clay minerals. The (NGS) log has the ability of resolving the overall radiation emitted from the sedimentrey rocks and minerals into the relative contributions from the sedimentary mineral radioactive isotops of potassium K%, uranium U in ppm and thorium in ppm (Rider, [21]). The greatest amount of potassium exists in illite and mica. Regarding the thorium, it is generally distributed throughout most shales, where it is fixed by absorption. On the other hand, uranium shows a very loose attachment to the principle molecules of rocks, due to its continued solubility; therefore, it has a very heterogeneous and irregular sedimentary distribution. In order to identify the types of clay minerals in the Abu Roash “E” Member, the log values were plotted on two crossplots, that distinguish clay mineral species. The first crossplot (Fig. 9) is the thorium-potassium concentration crossplot, on which the data are plotted partly on the kaolinite zone and partly on the montmorillonite-mixed layer clay zone. The second

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crossplot (Fig. 10) is the thorium-potassium ratio versus the photoelectric absorption effect crossplot, on which the data are also plotted on the mixed-layer clay zone, with some data deviated towards the kaolinite zone. Accordingly, it can be concluded that, the Abu Roash “E” Member shale are composed of a mixture of kaolinite and mixed-layer clay minerals.

Th/U values are present, suggesting that the Abu Roash “E” Member was deposited under fluvio-marine conditions. CONCLUSIONS

NGS Identification of Depositional Environment: The distribution of the radioactive elements: thorium, uranium and potassium, reflects the process of sedimentation and depositional environments. According to Koszy [22], Adams and Weaver [23] and Hassan, et al. [24], the ratio of thorium to uranium is very important in this manipulation. As the marine shale has a considerable amount of uranium and the affinity of thorium for terrestrial sediments has been reported, therefore, the ratio of Th/U can give an implication on the influence of marine affiliation on the depositional environment. Depending on such ratio (Th/U), the sedimentary facies can be classified into low (below 2), intermediate (2 - 7) and high (over 7); lower values indicate marine conditions (Adams and Weaver, [23]). On the other hand, place deposits reflect high ratio, in which the uranium content is removed, leaving behind thorium in high concentrations. Shallow marine conditions represent an intermediate stage between marine and continental conditions. The data plotted on (Fig. 11) show that both, high and intermediate

Mineralogic composition was identified in the petrographic study through the examination of some selected thin sections of rock samples representing the Abu Roash “E” Member and by using different well logging analyses, especially the natural gamma-ray spectroscopy logs (K, Th and U) and the photoelectric absorption effect (Pe). It is found that, the Abu Roash “E” Member consists mainly of silty shale with subordinate limestones, siltstones and sandstones. A numper of wells show the top part of the member to be absent due to local erosion. Shallow marine shelf conditions (which was normally a low energy environment dominated by the deposition of mud and silt), with an occasional more open water marine environment are postulated. Illite is the dominant altered mineral and cholorite is also present. The shale of this member is composed of a mixture of kaolinite and mixed-layer clay minerals. A variety of illitic clays (illite, mixed - layer illite / chlorite and mixed - layer illite / smectite) are occurred. The Th/U- resistivity crossplots conclded that, the Abu Roash “E” Member is of intermediate to high Th/U ratios zones, which reflects that the sediments were deposited under fluvio-marine conditions.

Fig. 1: Location map of the study field, Western Desert, Egypt 1030


Fig. 2: Thickness contour map of the Abu Roash “E” Member GPT Field Abu Sennan Area, Western Desert, Egypt (C.l. = 25 m.)

Fig. 3: 2D and 3D repesentation of the depth contour map of the Abu Roash “E” Member, GPT Field, Abu Sannan Area, Western Desert, Egypt (C.l. = 50 m.) 1031


Fig. 4: Photomicrographs: A: Non-ferroan skeletal calcite debris and extensive chloritic clay, (1); skeletal calcite debris and (2); chlorite clay matrix. B: Sample containing extensive detrital clay matrix. Porosity is negligible and permeability is severely restricted, (1); detrital clay matrix. C: Detail of authigenic pore filling kaolinite and chloritic clays. Note the abundant microporosity within the authigenic kaolinite, (1); grain coating/pore-lining chlorite, (2); pore-lining kaolinite, (3); chlorite and (4); kaolinite. D: Partially leached shell fragment with the associated ferroan calcite fringe cements and spar. Note also the grain-coating to grainreplacing chlorite, (1); fringe cements, (2); leached calcite, (3); skeletal calcite, (4); chlorite and (5); ferroan calcite spar. E: Cement stratigraphy with pore partially filled first by siderite, secondly by authigenic quartz overgrowths and lastly by poikilotopic calcite, (1); siderite cement, (2); detrital quartz grain, (3); calcite and (4); authigenic quartz overgrowths. F: Authigenic quartz overgrowths, locally leached and post-dating sphaerosiderite formation, (1); authigenic quartz, (2); corroded overgrowth, (3); sphaerosiderite and (4); calcite cement. G: Partially leached phosphate nodule, (1); phosphate nodule. H: Skeletal feldspar grain within phosphate nodule, (1); phosphate cement and (2); feldspar.

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Fig. 5: Scanning electron micrographs: A: B: C: D: E:

General view of fine grained, argillaceous sample. General view of coarser grained, open packed samples with significant macroporosity. Detail of open packed sample with primary pores, (1); pores. Sphaerosiderite and authigenic chlorite rosettes, (1); sphaerosiderite. Secondary pore produced by dissolution of feldspar and compacted, altered mica grain, (1); mica and (2); leached feldspar. (After Robertson, [4]).

Fig. 6: Spectral gamma-ray logs of the Abu Roash "E" Member, GPT -16 well, Abu Sennan Area, Western Desert, Egypt

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Fig. 7: Spectral gamma-ray logs of the Abu Roash "E" Member, GPT -18 well, Abu Sennan Area, Western Desert, Egypt

Fig. 8: Th/K ratio against Th/U ratio for spestral gamma-ray of the Abu Roash “E” Member, GPT Field, Abu Sennan Area, Western Desert, Egypt

Fig. 9: Crossplot of thorium against potassium over the Abu Roash “E” Member, GPT Field, Abu Sennan Area, Western Desert, Egypt 1034


Fig. 10: Th/K ratio plotted against Pe for varrious clay types of the Abu Rosh “E” Member, GPT Field, Abu Sennan Area, Western Desert, Egypt REFERENCES 1.

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Fig. 11: Th/U ratio and resistivity crossplot of the Abu Roash “E” Member, GPT Field, Abu Sennan Area, Western Desert, Egypt 8.

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