Significance of core analysis and gamma-ray trends in depositional facies interpretation and reservoir evaluation of Cenomanian sequence, Alam El-Shawish East Oil Field, North Western Desert, Egypt

4.1 Sedimentary cycles

The detailed examination of the wireline logs detection of the marker log signatures and careful log correlation particularly those of the total gammy-ray log mirror images (Figs. 2a and 2b), enable differentiation of the Bahariya Formation and Abu Roash “G” Member into ten distinctive sedimentary cycles that are assigned from base to top, by numbers 1–10 (Figs. 2a and 2b). These cycles are easily differentiated, not only on the basis of their characterized petrophysical signatures but also on the sedimentological background of the facies studies of each subunits.

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Fig. 2a

Stratigraphic correlation panel of the sedimentary cycles recognized in the Bahariya Formation and Abu Roash “G” Member in Alam El-Shawish area (Central Block).

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Fig. 2b

Stratigraphic correlation panel of the sedimentary cycles recognized in the Bahariya Formation and Abu Roash “G” Member in Alam El-Shawish area (Block II).

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The high gamma-ray reading combined with the density log that indicates mudstone or shale thin beds, represent log markers at the tops of cycles (1), (2), (3) and (4) which compose the Lower Bahariya Member, while cycle (5) that represents the Middle Bahariya Member, is bounded by two distinctive litho-log markers. The Upper Baharyia Member represented by cycles 6 and 7, is unconformably underlain by the Middle Bahariya Member.

The log correlation (Figs. 2a and 2b) enables differentiation of the Abu Roash “G” Member into three sedimentary cycles; the lower two cycles (8 and 9) compose the lower unit, while the third one (cycle 10) composes its upper unit. The top of the upper unit (cycle 10) is determined by the high gamma-ray marker which underlies the laterally persistent Abu Roash “F” carbonate beds.

4.2 Electrofacies pattern and facies types

The interpretation of the depositional environment in the study Cenomanian sequence was carried out based on the characteristics of the identified facies types (Table 1) as well as on the well logs, particularly the gamma-ray log responses. The study facies types can be categorized into coarse siliciclastics, fine siliciclastics and carbonates lithofacies types (Table 1). On the other hand, from the gamma-ray log trend patterns, an electrofacies approach will be developed and applied for discriminating depositional environment. Therefore, well logs can be used to support the depositional environment interpretation where they provide information on physical rock properties, but not a pure straight explanation of lithology.

Major modification has been carried out trying to avoid the limitations of Cant (1992) and Siddiqui et al. (2013) electrofacies applications. According to the applications and modifications of the previously mentioned models as well as the facies types interpretation and the log response calibration that based on the available core information on the study wells, seven major log patterns were suggested to include the interpreted depositional environments (Numbers 1–20 inclusive), as given in Table 2. The following is brief description of the recognized gamma ray patterns.

Table 2 Summary of gamma-ray trends and depositional environment (modified after Cant, 1992; Siddiqui et al., 2013).

Blocky boxcar (left) pattern normally suggests a constant energy level (high in clastic systems and low in carbonate environments) and constant sediment supply and sedimentation rates. This pattern with low gamma-ray value forms in upper-middle shoreface (1a) and lower middle shoreface (1b) of wave dominated shallow marine environment. It also points to carbonate facies representing by carbonate outer ramp of transition storm shelf, where it shows low gamma-ray with high resistivity, high density and low porosity.

Boxcar right pattern normally suggests low energy showing high gamma-ray values (Table 2). The boundaries with the overlying and underlying reading are abrupt in tidal dominated mud tidal flat environment (3) and in fluvial overbank mud environment (2). This pattern also points to offshore mud of shallow shelf (4) where it forms sharp boundaries with very small internal irregularities or changes in the high gamma-ray.

Funnel Pattern shows upward decrease of gamma-ray values consistently from maximum value, usually indicating decreasing shale content to form coarsening upward trend. This shape related to a transformation from shale to shale-free

lithologies due to upwardly increasing depositional energy (Siddiqui et al., 2013). In the study well sequences, this pattern is formed in channel crevasse splay (6), tidal sand bar (7), flood tidal delta mudstone (8a), tidal lagoonal silty shale (8b) environments belonging to fluvial, tidal and wave dominated estuary system respectively (Table 2).

Bell pattern formed by the increasing upward consistently from a minimum gamma-ray values usually indicated an increasing in shale content. In the study sequence the bell shape or fining upward trend are formed in tide dominated (tidal channel, 10) which corresponds to a retreating shoreline shelf system with decreasing energy. It is also formed in fluvial point bar (9).

Left bow pattern is developed by a cleaning upward overlain by dirtying up trend of similar grain size and with no sharp breaks between the two, shows left bow shape (Siddiqui et al., 2013). In the study well sequences, this trend form different deposition facies such as mixed muddy tidal flat (11), reworked offshore carbonate bar (12), shoreface delta (13), brecciated barrier tidal inlet (15a), clastic barrier tidal inlet (15b) and sandy carbonate barrier tidal inlet (15c) that belonging to variable environmental setting (Table 2). Occasionally, the log shows blocky to fining upward overlain by coarsening upward to form “hour glass” shape of the well log. This case of gamma-ray trend is very common characterizes the barrier tidal inlet containing carbonate sand shoals package which is altered from well to well along their vertical sequences. The coarsening upward pattern indicates a gradual shift toward progressively higher energy depositional environment (Catuneanu, 2006). This variety of left bow pattern is usually complicated case showing variable changes in log trend according to lithological variation and thickness of package as well as the glauconitic and calcareous (shell fragments) material contents.

Right bow pattern is the opposite of left shape; the trend is right bow shape in which sand bodies with medium to coarse grained overlying and underlying by muddy deposits without internal structure. In the study well sequences this trend form in mixed sandy dominated tidal flat (14).

Irregular/serrated pattern represents the fluctuated gamma-ray reading with high and low values (Table 2). It could be subdivided into three groups according the gamma-ray values ranges. This trend is formed in silty mud tidal flat (16) that belonging to group (a), with thin laminated silty mud intercalation having rare planner cross-stratification. The gamma-ray fluctuating in group (b) is smaller than group (a) where it is formed in silty sand tidal flat (17). This pattern of the gamma-ray trend (group c) is formed in fluvial flood plain (18), storm dominated inner shelf (19) and lower shoreface (20) environment belonging to the depositional macroforms of fluvial, transition storm shelf and shallow marine shelf respectively (Table 2).

4.3 Depositional macroforms

The recognized facies types (Table 1) and the interpreted depositional environments (Table 2) of the study sequence associate together in different proportion and stacking pattern forming six macroforms. These are fluvial (F), tidal-dominated estuary (TE), wave-dominated estuary (WE), wave-dominated shallow marine (shorefaces) (WM), transition storm (TS) and shallow marine shelf (SH) (Table 3). Some of these macroforms are compiled together to form new mixed macroforms (as given in Table 3). Each of these macroforms contains a facies association or more (A1-15), which inturn contains a vertical stacking of the associated facies types. The succession of the interpreted depositional environments, facies associations, sedimentary cycles, subcycles associated with sets of gamma-ray trends are shown in Figs. 3a and 3b as examples of the reservoir sequences in WHG 2/1 well.

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Fig. 3a

Graphic sedimentary log with facies interpretation of upper part (Cycles 6&7) of the Bahariya Formation.

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Fig. 3b

Graphic sedimentary log with facies interpretation of AR/G Member (Cycles 8, 9 and 10).

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4.4 Reservoir facies

The facies analysis of the cored reservoir intervals of the Upper Bahariya Member indicates a deposition in wave-dominated shallow marine (WM) and to subordinate a deposition in wave-dominated estuary (WE) (Tables 2 and 3). The upper-middle shoreface (Table 2) is mainly sandstone with lenticular cross bedding and hummocky structure. It is associated with shoreface delta sediments consisting of laminated-current rippled bedded bioturbated muddy sandstone or sandy mudstone (Fig. 4b). The clastic (sandstone) barrier tidal inlet (Table 2) is dominated by flaser cross lamination and bioclastics framework. It is usually intercalated with laminated silty shales of tidal delta wave dominated estuary (facies association A6c, Table 3 and Fig. 4a). The sandy carbonate barrier tidal inlet is composed of calcareous fine sandy laminated glauconitic limestone. It is usually associated with laminated hummocky flaser bedded fine sandy mudstone of tidal flat depositional facies or with tidal delta mudstone.

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Fig. 4a

Reservoir characterization of Upper Member of Bahariya Formation and lower unit of AR/G Member (cycle 8) in well WHG 2/1 (Core 2), 3250–3277.8 m.

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Fig. 4b

Reservoir characterization of Upper Member of Bahariya Formation in well HG 34/4 (Core 2), 3255–3283 m.

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Facies analysis of the cored reservoir intervals of the Abu Roash “G” Member (mainly within the sedimentary cycle 10) indicates a deposition in wave-dominated estuary and shallow marine macroforms (Table 3, Figs. 3b, 5a and 5b). Occasionally, the estuarine environment is invaded by transition storm shelf condition particularly along the basal part of the sedimentary cycle 10 (Figs. 5a and 5b). The top of this cycle (non-cored interval) is dominated by shoreface delta clastic sequence (Fig. 3b). The main reservoir which belongs to cycle 10 is represented by several periods of repeated deposition of clastic barrier tidal inlet sediments intercalated with tidal lagoonal silty shale (Table 3 and Fig. 3b). The basal reservoir interval is represented occationally by brecciated barrier tidal inlet (Table 2). This reservoir rock is mottled flaser-cross laminated glauconitic (pyritic) bioclastic sandstone that characterized by low porosity and moderate permeability showing low oil saturation (Fig. 5b).

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Fig. 5a

Reservoir characterization of upper unit of AR/G Member in well WHG 2/1 (Core 1), 3135–3162.7 m.

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Fig. 5b

Reservoir characterization of upper unit of AR/G Member in well HG 34/4 (Core 1), 3083–3110 m.

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4.5 Petrophysical evaluation of oil shows

Based on the petrophysical data, several representative reservoir intervals were tested and cored as given in Table 4. The petophysical evaluation of the studied wells revealed an existence of hydrocarbon accumulation or oil shows in the permeable zones of the Upper Member of the Bahariya Formation as well as in the Abu Roash “G” Member in WHG 2/1, WHG 1/1 and HG 34/4 wells (representing the northern sector) and Karima-1X well (representing the eastern sector). This evaluation has revealed also, an existence of water accumulation in the dry wells in Sedfa-1X, Hend-1X and Mosaad-1X wells which are located in the southern sector of the study area as given in Table 4. The representative litho-saturation cross-plots of the study reservoir intervals in WHG 2/1 are shown in Figs. 6a and 6b.

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Fig. 6a

Litho-saturation cross-plot of Upper Member of Bahariya Formation in WHG 2/1 well, showing hydrocarbon sub-reservoir interval.

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Fig. 6b

Litho-saturation cross-plot of upper unit of Abu Roash “G” Member in WHG 2/1 well, showing hydrocarbon sub-reservoir interval.

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5.1 Significance of gamma-ray trends

The application of electrofacies patterns in the study succession is limited to recognize the micro-variations in heterogenetic lithological characteristics of both siliciclastics and carbonates. The gamma-ray logging may loss the accuracy to differentiate the lithology and its radioactive minerals (e.g. in highly glauconitic sandstone intervals). Another important limitation was concluded by Siddiqui et al. (2013); that the gamma-ray trend in each depositional environment shows different shape/trend that may be similar within two or more depositional environments. According to Siddiqui et al. (op.cit.) the tide-dominated major environment shows blocky left boxcar, funnel shaped, bell shaped, right boxcar and irregular gamma-ray trends, while the wave-dominated major environment shows left and right boxcar, funnel shaped and irregular trends. The main difference between the tide and wave-dominated depositional environments is the fluctuating GR values (Siddiqui et al., 2013). According to Cant (1992) cylindrical (blocky boxbar) gamma-ray pattern characterizes many depositional environments (e.g. eolian, braided fluvial, channel-fill, submarine canyon-fill). Funnel and bell-shaped patterns characterize crevasse splay, river mouth bar, delta front, shoreface, submarine fan, fluvial point bar, tidal point bar, deep tidal channel fill and tidal flat, respectively. The hour glass (symmetrical prograding and retrograding) gamma-ray trend indicates reworked offshore bar, regressive to transgressive storm face delta (Cant, 1992).

He also concluded that serrated (irregular) trend may characterize fluvial floodplain, storm-dominated shelf, deep marine slope. In conclusion electrofacies are characterized and grouped into sub-populations which would provide a method for estimating the lithologic characteristics of the formation and ultimately to distinguish it from surroundings. Furthermore, the environmental interpretation of the cored sequences in the study wells (WHG 2/1, HG 34/4, Hend-1X and Sedfa-1X) is used as a guide and starting point for the construction of the depositional model in this study. The core information has been used to calibrate the log response which was used to interpret the non-cored wells. However, the generalized log pattern shows micro-variations in log shape based on the location relative to depositional environment. Accordingly, major modification has been carried out trying to avoid the limitations of Cant (1992) and Siddiqui et al. (2013) electrofacies applications. The result of this modification is given in Tables 2 and 3. According to the applications and modifications of the previously mentioned models (e.g. Cant, 1992; Siddiqui et al., 2013) as well as the facies types interpretation and the log response calibration that based on the available core information on the study wells, seven major log patterns were suggested to include the interpreted depositional environments (Numbers 1–20 inclusive), as given in Table 2.

5.2 Modeling of depositional setting

The field of facies models is still a relatively young field with a history of less than forty years where observations which were collected through measurements in modern environments, outcrops, wireline log, core and borehole studies, have been incorporated into depositional facies models since the middle of the twentieth century (Boyd et al., 2006). The facies analysis and the interpreted facies associations revealed that the study Upper Albian-Cenomanian succession of the Bahariya Formation and AR/G Member is one of the best representative's ancient depositional records for fluvio-marine facies models. Much siliciclastic and carbonate sediments of these rock units were deposited along fluvial, estuarine and marine shoreline-shallow shelf mixed environments. Along these environmental setting, sediment supply, tidal range, wave action and storm frequency are generally discussed. The effects of external and internal factors (e.g. sea-level changes, tectonic setting and climatic conditions) on the sedimentation of the study succession in terms of facies sequences and evolution were discussed (El-Tehiwy, 2018). Most of the ancient environments deviate in some way from the idealized models due to such factors in addition to mixed influence of marine and fluvial processes.

Try to applying the concepts of distillation and simplification of Walker (1984) and Boyd et al. (2006) respectively, the study succession comprises depositional facies sequences belonging to three proposed sedimentary models or mixture of them; they are: Fluvial sedimentary model, Estuarine sedimentary model and Marine shoreline-shallow shelf model.

Many factors influence the nature of the spatial changes in depositional facies models, from fluviatile model of sedimentary cycle (1), (2) and lower part of (3) of Bahariya Formation to tidal estuarine to shoreface and offshore shallow shelf models of Abu Roash “G” Member, however, the most important factor is the impact of sea level sequence changes. This changes are controlled by combination of eustatic changes, subsidence/uplift of depositional basin of sub-basins and/or substantial variations in the rate sediment supply.

It is not by chance the majority of hydrocarbon accumulation with moderate to good reservoir quality, occur within the barrier tidal inlet complex. The majority of the identified deposional facies associations (e.g. A6a, A7, A8; Table 3) essentially exist within or near the unconformity sequence boundary surfaces. Exceptionally, the oil shows occur within sandy-silty tidal flat representing mixed marine estuarine facies that may be developed upon the wave ravinement surface.