Paleoenvironmental contribution and visual kerogen assessment of some Upper Cretaceous sediments from southern Egypt

We investigated the palynological organic matter (POM) recovered from shale horizons, intercalating the Nubian-type sandstones in middle Egypt, for paleoenvironmental reconstruction. Our palynofacies are dominated by either amorphous organic matter (AOM) or phytoclasts whereas palynomorphs are relatively rare, but not uncommon. The palynofacies categories revealed two different palynofacies types, reflecting estuarine/tidal and fluvio-lacustrine environments for the Maghrabi and the basal Quseir formations, respectively. The kerogen composition of our investigated material and the colors of the thin-walled spores suggest possible sources of hydrocarbons, which are poorly explored in the middle and southern areas of Egypt. Our findings confirmed the previously described marine origin of the Maghrabi Formation. The complete missing of any marine palynomorphs suggests that the basal sediments of the Quseir Formation are almost exclusively of terrestrial origin. This may reveal that the worldwide Late Cretaceous transgression was not effective during deposition of these basal clastics.


Introduction
Palynological investigations in the Kharga area showed that these rocks are mostly of continental origin, with few marine incursions represented by the Abu Balls and Maghrabi formations (Schrank 1987;Schrank and Mahmoud 1998;Mahmoud 2003). The organic palynofacies and their role in the evaluation of source-rock potential have not been attempted in the Kharga area and are still poorly investigated due to the lack of fossiliferous horizons in the region's sediments. Palynofacies have demonstrated their utility to indicate a specific depositional environment or sedimentary process. Distribution of the organic matter is influenced by ecological factors and sedimentary processes (Tyson 1995).
It has been extensively used in retrieving proximal-distal variations and oxidation-reduction states (Tyson and Follows 2000;Slater et al. 2017;Cirilli et al. 2018). Palynofacies are also widely used in the evaluation of petroleum potential (El Diasty et al. 2017). Said (1990) and Hermina (1990) provided thorough descriptions of the Kharga Oasis area and noted that the former "Nubia" sandstones cover the Kharga Basin. These Nubia rocks reach a maximum thickness of about 900 m in the northern part of the basin (Senosy et al. 2013). For many years, these sediments were historically regarded as nonfossiliferous. Extensive geologic work has led to their subdivision into formal lithostratigraphic units (Klitzsch et al. 1979;Klitzsch and Lejal-Nicol 1984;Klitzsch and Wycisk 1987). The Six Hills Formation overlies the pre-Cambrian basement and is overlained by other Cretaceous Sabaya, Maghrabi, and Tarf formations (see Schrank and Mahmoud 1998 and references therein). The eastern escarpment of the Kharga area is composed of claystone and sandstone of the Middle Campanian Quseir Formation (Youssef 1957), which is overlain by the phosphate-rich layers of the Upper Campanian Duwi Formation. Despite the presence of a massive dune belt on the west of the basin edge, desert pavement forms the main surface in this region. The Cenomanian floods on the Neo-Tethys Ocean's southern edge represent the first significant marine incursion over the hinterland, resulting in the deposition of shallowwater sediments over fluvio-marine siliciclastic Early Cretaceous deposits (Abdelhady et al. 2021). A marine incursion encompassed most of Sinai, the Gulf of Suez and northwest Egypt during the Cenomanian. The incursion moved southward in the late Cenomanian, forming a narrow passageway between the Arabo-Nubian massif and the high Kufra basin. This passageway appears to have created a genuine estuary with marginal marine conditions (Said 1990). On the other hand, the Upper Cretaceous Quseir, Duwi and Dakhla Formations represent mostly coastal, shallow shelf to deep sea mudstones, claystones, and sandstones (e.g., Hermina 1990).
The present work aims principally at incorporating the palynofacies analysis and contributing information regarding the paleoecology of the investigated intervals. Petroleum exploration activities are restricted where little is known. Among the few oil shows in the south of Egypt are those discovered by the Repsol oil company (Komombo-1 well, dug in Upper Egypt in 1997 (Dolson et al. 2001). Therefore, we used a visual optical microscopic investigation of the thin-walled palynomorphs, supported by measuring percentages of the total organic carbon (TOC %) in the investigated rock units, to see if probable sources of hydrocarbons occur in the area or not.

Geological setting
The study area (Kharga Oasis) is a large geographical depression with a total size of about 7200 km 2 found in the southern central region of Egypt (Fig. 1). It occupies a distinct paleogeographic position in the NE African continent. It is bordered to the north and east by escarpments and a high plateau and is located at the southeastern part of the Dakhla Basin. Escarpments can reach heights of more than 400 m. During the Late Cretaceous period, Egypt was located at the southern margin of the Tethys Ocean. In the south of Egypt, sedimentary sequences are accumulated in two main sedimentary basins, the Dakhla basin in the west and the Upper Nile basin in the east. These two basins were isolated by the Kharga uplift (see Hendriks et al. 1987). Tectonically, the depression is influenced by N-S faults, but there is no indication of recent activity. The latter extends north-westward till it approaches the Farafra Plateau in the east. Despite considerable differences in geologic structure and formations (Hermina 1990), the Kharga and Dakhla oases can be regarded as a single physiographic unit linked by a sequence of shallower, sediment-filled depressions (Rohlfs 1874). The tectonic overview provided here is primarily taken from Morgan (1990) and Meshref (1990). Historically, there has been considerable debate about how much of the Sahara is underlain by cratonic lithosphere. Some believe that the Saharan region is an Archean-Paleoproterozoic metacraton that was remobilized during the Neoproterozoic owing to collision, resulting in the heterogeneous crust west of the Nile (Abdelsalam et al. 2002). Pan-African island arc accretion in the present-day Red Sea Hills east of the Nile during the Neoproterozoic/early Cambrian resulted in the creation of an orogenic belt that was eroded and deposited in the Paleozoic. During the Paleozoic, there was no considerable tectonic activity in this area. In the Ordovician, plate movement brought the region to its greatest southerly latitude of 70° S and the continent has been moving north ever since. Laurasia rifted from Gondwana in the Mesozoic, separating modern-day Turkey and Egypt, and this event is responsible for the Kharga area initial east-west trending fractures, which were later reactivated. During this period, differential block movement caused uplift in Egypt, resulting in a drainage change from south to north. A late Mesozoic sea level rise caused floods and a shift from a continental to a marine depositional environment. Sea level decline in the Eocene resulted in a more continental deposition. The majority of the faults in the study region were active after the Lower Eocene (Hermina 1990). The original Nile was formed by rifting and uplift near the Red Sea's borders during the Oligocene-Miocene and this event is responsible for the NNW-SSE fractures visible in limestones in the Western Desert (Tewksbury et al. 2013). During the Quaternary, there was no significant tectonic activity in the studied region (Abdel Zaher et al. 2014). Faults in the study region are typical in character, with some east-west faults displaying strike-slip activity as well (Hermina 1990).

Material and palynological sample preparation
Fifteen core samples and seven cuttings are collected from six shallow water wells drilled in the Kharga Oasis area. These wells are, Bulaq-12 (Bu-12), Bulaq-15 (Bu-15), Kharga-34 (Kh-34), ,  and El-Mahariq-1 (El-Mah-1). The lithological logs and locations of the productive samples are shown in Figs. 2 and 3. All samples were digested using standard palynological preparation techniques, mainly HCl and HF acid treatments to remove carbonates and silicates, respectively. Residues are sieved through a 15-µm nylon sieve. Neither oxidizing agents nor ultrasonic treatments were used in order to avoid oxidation and/or breaking of the organic components. Three to five slides from each sample are prepared using glycerin jelly as a mounting medium. Slides are investigated using an Axiolab Zeiss microscope (Carl Zeiss Microscopy GmbH), connected to its own digital camera, for light kerogen petrography and photomicrography. Slides and residues are stored in the Geological Museum of the Geology Department, Sohag University, Egypt.

Measuring organic carbon (TOC%)
Total organic carbon (TOC) is a critical parameter to determine reservoir quality. Chosen samples from the Quseir and Maghrabi formations are analyzed by the wet chemistry technique. The Walkley-Black approach is perhaps the most well-known of the quick dichromate oxidation procedures, having served as the "reference" method to compare with other techniques. In this procedure, potassium dichromate (K 2 Cr 2 O 2 ) and concentrated H 2 SO 4 are added to the sediment. Before adding water to stop the reaction, the solution is stirred and allowed to cool. Samples are chilled due to the exothermic reaction that occurs when potassium dichromate and sulfuric acid are combined. Then H 3 PO 4 is added to the digested mix to assist reduce interferences from ferric (Fe 3+ ) iron that may be present in the sample. Quantification of samples is carried out by adding an indicator solution to the digestate to perform the manual titrimetration. Excess Cr 2 O 7 2− is titrated with ferrous ammonium sulfate [Fe(NH 4 ) 2 (SO 4 ) 2 *6H2O] or ferrous sulfate (FeSO4) until the sample changes the color of barium diphenylamine sulfonate from purple/blue to green. For ages of these units see (Klitzsch and Hermina 1989;Schrank and Mahmoud 2000;Mahmoud 2003) 375 Page 4 of 15

Methods of semi-quantitative analyses
The organic matter of the examined rock units is categorized according to the standard grouping of the POM (Cornford 1979;Dow 1982;Tyson 1993Tyson , 1995 revised by Roncaglia and Kuijpers 2006). These classifications used generalized categories such as phytoclasts, palynomorphs and amorphous organic matter (AOM) (Fig. 4a, b), or as inertinite, vitrinite and liptinite (palynomorphs + AOM) ( Fig. 5a, b). Changes in the relative abundances of the POM categories are calculated as percentage frequency of the total palynological organic matter (TPOM). The kerogen categories liptinite-vitrinite-inertinite (LVI) are also applied. In each sample, a count of, at least, 300 particulates is established for semi-quantitative purposes. For a general descriptive way, these particulates, including palynomorphs, are classified as very abundant > 50%, abundant 36-50%, frequent 16-35%, common 5-15% and rare 5%.

Methods of visual petrography
Palynomorph colors are used to infer the degree of maturation since colors vary as a function of temperature, burial depth and maturation time. From a visual qualitative perspective, colors of the thin-walled palynomorphs such as psilate trilete Cyathidites spores are transferred to values according to the spore coloration index (SCI) or the thermal alteration index (TAI) schemes (Staplin 1969;Pearson 1990;Filho et al. 2012;Zhang et al. 2015;Koch et al. 2017). TAI can be calculated by visual examination of thin-walled palynomorph colors under light microscopy. This has long been the most preferred low-cost and quick tool for evaluating the thermal maturity history of sediments and fossils (cf. Van Bergen and Kerp 1990;Pross et al. 2007). TAI is a numerical palynomorph colors scale (e.g., Gaupp and Batten 1985) on color charts (Peters and Cassa 1994). SCI is a color gradation that varies from colorless or pale yellow to black; corresponding TAI is calculated using a five-point scale and the colors of spores and pollen before oxidation treatment (Marshall 1990;Utting and Hamblin 1991). Relative numeric frequencies (RNF %) of palynofacies assemblages, which apply liptinite-vitrinite-inertinite (LVI) and vitrinite/ Fig. 3 Integration of stratigraphic, palynofacies, and kerogen data retrieved from six boreholes at Kharga (see Figs. 1 and 2 for names and locations). It shows abundances of some selected palynomorphs and particulate organic matter of the Maghrabi and Quseir forma-tions. The change in spore color from orange brown to dark brown implies a rise in thermal maturity from Maghrabi to Quseir formations huminite-liptinite-inertinite categories are presented as ternary plots and infer fields of the corresponding types of hydrocarbons produced (Fig. 5a, b).

Results of the palynofacies recovery
The Kharga samples exhibit a semi-quantitative distribution of terrestrial phytoclasts (mainly black or brown wood) and AOM as main palynofacies components, whereas palynomorphs are relatively less abundant. Marine dinoflagellate cysts and microforaminifera linings are rare (Fig. 3). AOM forms the greater proportion in the Maghrabi Formation. At the base of the Quseir Formation, AOM declines whereas phytoclasts are dominant. As seen from Fig. 3 and Table 1, this allowed the designation of two distinct palynofacies, namely Pf-1 (Maghrabi Fm) and Pf-2 base (Quseir Fm). However, these two palynofacies differ greatly and are identified and designated as "Palynofacies Assemblage 1" (PF-1) and "Palynofacies Assemblage 2" (PF-2), as can be described in the following paragraphs.

Paleoenvironment reconstruction
Occurrence of dinoflagellate cysts in PF-1 samples (Maghrabi Fm), although rare, means deposition under some marine influence. Their association with abundant miospores suggests deposition in a transitional (estuarine/ tidal flat) environment with a strong influx of land-derived detritus. Schrank and Mahmoud (2000) attributed missing of the marine palynomorphs in adjacent areas to probable fluctuations of marine influence in a paralic environment, although they did not exclude a sampling bias as another probable reason. In basinward shelfal settings of normal marine salinity, assemblages of dinoflagellate cysts may reflect these low percentage frequencies, averaging 1% of total palynofacies (e.g., Mutterlose and Harding 1987;Lister and Batten 1988;Habib et al. 1992). The associated decrease in the total concentrations of terrestrially derived phytoclasts may support this marine origin of this palynofacies (cf. Batten 1999). This interpretation validates the previously reported estuarine and tidal flat conditions of the formation  (Klitzsch and Hermina 1989). However, an estuary is commonly identified as a semi-enclosed body of water that has an open connection with the seawater, which is measurably diluted with freshwater from land drainage (Pritchard 1967). The AOM-Phytoclasts-Palynomorphs (APP) and TPOM plots refer to deposition of PF-1 in a relatively low energy, distal marginal marine environment of dysoxic-anoxic to suboxic-anoxic circumstances (Figs. 3 and 4a, b; Table 1). AOM is a typical organic component of sediments deposited in anoxic marine settings (Summerhayes 1987;Tyson 1987). Samples of this palynofacies, except one, enter fields VI and IX in Tyson (1995) model, which refer to shelf transition (proximal-distal), with normal to low salinity. Roncaglia and Kuijpers (2006) pointed out that the palynological model of Tyson (1993Tyson ( , 1995 may be potentially good to characterize marine facies deposited distally on the shelf, but is inadequate for the facies deposited proximally with high terrigenous influx (see next Pf-2); an observation clearly supported by the present Maghrabi Fm palynofacies, where dark to translucent phytoclasts are typical of this transition (e.g., Carvalho et al. 2006). The occurrence of appreciable amounts of brown wood particles again suggests such transitional high, proximal sand and silt facies environment (Habib 1983;Firth 1993;Tyson 1995).
The occurrence of terrestrial palynofloras in PF-2 (Quseir Fm), associated with absence of the marine elements, indicates terrestrial paleoenvironment for this palynofacies (Fig. 6e,  f, g, h). Pteridophyte spores reflect fern-dominated vegetation in, for example, fluvio-lacustrine niches, as previously documented by Mahmoud (2003). Abundant brown wood indicates high terrestrial inflow into the fluvial environment, near to land plants sources (Pocklington and Leonard 1979). Abundant black wood may suggest proximal, high-energy silt/ sand lithologies (Van der Zwan 1990;Baird 1992) in a fluvial delta-top system (e.g., Fischer 1980). Although black wood is often thought to reflect remote depositional environments with prolonged travel time, its abundance here may be attributed to increasing seasonal oxidation; the increased runoff under monsoon climate might be responsible for the fluctuating abundance (Fig. 3) of these opaque particles (Tyson 1995). The sand-rich sediments of the Quseir Fm might explain increasing black wood that led to a drop in the brown-to-black wood ratio, seen from some horizons in the Bul-15 core. This ratio is known to drop out far offshore (Götz et al 2005(Götz et al , 2008Habib 1982;Summerhayes 1987;Tyson 1989). But the obvious missing of small and lath-shaped black wood makes the impact of the sediment type more plausible. APP and TPOM ternary plots reflect dysoxic-anoxic conditions of this palynofacies. Samples enter the palynofacies fields II (marginal dysoxicanoxic basin) and IV (shelf-to-basin transition) in the model of Tyson (1995). In some cases, AOM may dominate the organic composition of non-marine successions at dysoxic-anoxic sites (Batten 1983;Stemmerik et al. 1990). But, in darkercolored organic-rich facies, of dysoxic-anoxic conditions, the case here, AOM declines (e.g., Dow and Pearson 1975;Bujak et al. 1977). Therefore, we accepted the interpretation of redox states, as directly inferred from Tyson, and Roncaglia and Kuijpers models (Figs. 3 and 4a, b; Table 1). But, however, the exclusively terrestrial nature of this palynofacies suggests fluvio-lacustrine rather than a proximal marine environment for this palynofacies. Further support of this interpretation can be drawn on the basis of ecological preferences and botanical affinities of our palynomorphs that play the crucial role in this respect. We recommend that recognition of the environment should not be solely based on such modeling.
In the Kharga Basin area, as in southern Egypt, the Cenomanian sediments represent a developing transgressive phase. As seen from Fig. 8a, fluvio-deltaic to shallow marine clastics of the Maghrabi Formation grade upward into deltaic to shallow marine clastics (e.g., Hantar 1990;Guiraud et al. 2001). During the Coniacian to Maastrichtian, this marine transgression was exacerbated by the accumulation of thick open marine phosphates (Duwi Fm) and shales (Dakhla Fm) (Fig. 8b). The current palynology data confirm the mixed (continental/ marine) nature of the Cenomanian Maghrabi Formation. The marine influence could not be inferred in the basal clastics of the Campanian Quseir Formation, probably due to sampling bias or to the paleogeographic position of the Kharga area at the inter-tonguing sites between fluvial and marine deposits.

General overview
Numerical SCI is used to simply determine the color variation between spores and the thermal alteration index which employs the color differences between spore-pollen assemblages to determine source rock maturation (Traverse 2007). Palynomorph color variations are thus highly valuable in establishing the thermal maturation history of fossi-bearing sediments. Color change evaluation, however, can be problematic since color varies considerably due to wall thickness, composition variation, oxidation and degradation degree and the existence of reworked and curved samples (cf. Batten 1996). Total organic carbon (TOC) is a critical parameter to determine the reservoir quality of any source and is present in all organic components of rocks (i.e., kerogen). A potential source rock, which is capable of generating hydrocarbons, must contain TOC of at least 1% as a result of thermal maturation, with increasing burial depth. Marine surface sediments can average lesser TOC content of 0.5%, where 2% is a rough minimum. Organic matter in shale generally ranges from less than 1 wt % to more than 20 wt % and is responsible for in situ gas generation (Mastalerz et al. 2012). Thermal maturity determines if the physical conditions required for hydrocarbon generation have been met or not (cf. Al-Selwi and Joshi 2015).

Kerogen evaluation of the Kharga area
Based on the discussion in the previous paragraph, the Kharga sediments contain enough TOC for the production of hydrocarbons from the Maghrabi and Quseir sediments (see Table 1); TOC ranges from 0.69 to 4.7%. The composition of the POM in the investigated Kharga area revealed the recognition of the oil/gas-prone type II kerogen (PF-1, Maghrabi Fm) with dominating AOM and the gas-prone type III kerogen (Pf-2, Quseir Fm) with abundant brown phytoclasts. Opaque/black phytoclasts in type III kerogen, are exclusively responsible for dry gas production. The microscopic examination and the plotting of counting data on the ternary plots revealed the same kerogen types, which are known to produce oil and gases. Samples in general have a liptinite group richness (mainly AOM) with somewhat moderate levels of the vitrinite and inertinite categories. The type II kerogen, with a high percentage of liptinite group, in PF-1 (Maghrabi Fm), consists primarily of AOM (average ~ 63%), with lesser opaques (average 20%) and brown wood (average ~ 19%). On the other hand, the type III kerogen, palynofacies PF-2 (basal Quseir Fm), exhibits high vitrinite phytoclasts (average ~ 40%), with relatively lesser AOM (average ~ 25%) and opaques (average ~ 34%). This kerogen type is thought to have originated from a terrestrial source, with high gas and low oil potential. We tested colors of two distinct and thinwalled palynomorph species (e.g., Deltoidospora and Triplanosporites) to estimate the visual thermal maturity using SCI and TAI standard charts (Fig. 7). Spores in the palynofacies PF-1 (Maghrabi Fm) are uniformly dark brown, with SCI values of 8 and TAI values of 3 + , suggesting thermally mature conditions, probably corresponding to the closure of the oil window and the beginning of the dry gas window. Spore colors of the PF-2 (basal Quseir Fm) reflect values of 9 on SCI and 4 − on TAI ( Fig. 3; Table 1), indicating overmature palynofacies capable of producing only dry gas. The

Conclusions
Our findings in the present article clearly indicate that studying the total palynofacies has contributed significantly to the understanding of the paleoecology of the rock units under study. Based on our achievements in this work, we were able to correlate rock units across the basin of Kharga. The main conclusions of the present work can be summarized as follows: 1. Palynofacies investigation is accepted as a powerful tool in reconstructing ancient paleoenvironments. It offered relevant information regarding redox conditions and basin-land settings of the Maghrabi and basal Quseir Formations, which were fragmentarily highlighted by using palynomorphs alone. 2. The Maghrabi and Quseir Formations reflect two distinct palynofacies (Pf-1 and Pf-2). These are: Pf-1 (AOMdominated; Maghrabi Formation) that reflects deposition in transitional environment (estuarine/tidal flat), under general anoxic conditions and Pf-2 (phytoclast-dominated; basal Quseir Formation) that were accumulated in fluvio-lacustrine settings, mostly under the same anoxic circumstances. 3. The organic matter components of the investigated rock units are of the types II and III, which are known to produce oil and gases. These kerogen types are thought to have been derived principally from terrestrial sources (gas-prone), with minor or neglected contributions from a marine origin. 4. Visual assessment of thin-walled palynomorphs colors (e.g., spores of Deltoidospora and Triplanosporites) revealed that those of the PF-1 (Maghrabi Fm) are uniformly dark brown, with SCI values of 8 and TAI values of 3 + , suggesting thermally mature gas-prone conditions. The PF-2 (Quseir Fm), displays colors corresponding to values of 9 on SCI and 4 − on TAI, indicating overmature palynofacies that can produce dry gas. 5. We believe that hydrocarbons might have generated from the Maghrabi and Quseir sediments. In the Kharga area, there occur suitable reservoir rocks (i.e., siliciclastics) and enough organic carbon (TOC% ranges from 0.69 to 4.7). In spite of this, the absence of seal rocks might explain the scarcity or absence of hydrocarbon shows in the basin area.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).
Data availability Data of this work (numerical counts of the palynofacies categories and those concerning the TOC calculations) are available with the authors whenever requested.

Conflict of interest
The authors declare no competing interests.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.