Abstract

A detailed palynofacies study was carried out on 24 shale cutting samples from the Oligocene Dabaa Formation, which was penetrated in the Amana-1X well, Abu El Gharadig Basin, north Western Desert, Egypt. The investigation targeted primarily three main groups of palynological elements: phytoclasts, palynomorphs, and amorphous marine organic matter (AMOM), using transmitted light microscopy. Two major marine palynofacies were recognized: the oldest is palynofacies 1, made up mainly of AMOM (average abundance 47.5%), phytoclasts (31.6%), and palynomorphs (20.9%). A suboxic to anoxic outer shelf to upper slope paleoenvironment is suggested for this palynofacies. Palynofacies 2 is characterized by poorly preserved translucent phytoclasts (average abundance 61.2%) in addition to palynomorphs (24%), AMOM (14.8%), and opaques (<10%). This association clearly marks a paleoenvironmental shift toward a more proximal, more oxic marine facies. The overall composition of both palynofacies indicates a general progradational trend interrupted by brief transgressional episodes during the deposition of the Dabaa Formation. The kerogen composition of palynofacies 1 suggests type II–III (mostly oil prone), and palynofacies 2 constitutes type III kerogen (largely gas prone). Spore/pollen exine colors in the studied Dabaa samples point to immature organic content of no potential for hydrocarbon production.

INTRODUCTION

The Western Desert of Egypt covers an area of ∼700,000 km2 (almost two-thirds of the area of Egypt). Despite its large area, hydrocarbon exploration activities take place within only an area of ∼250,000 km2, mainly around the Qattara Depression, north of lat 28° (Zobaa et al., 2011a). The Abu El Gharadig Basin represents one of the important sedimentary basins in the north Western Desert in terms of its hydrocarbon potential. It comprises many of the most productive oil and gas fields in the northern part of the Western Desert (El-Shaarawy et al., 1994). Because of their prominent hydrocarbon potential, the Mesozoic plays have most frequently been the main target for the palynological investigations. Only a limited number of palynological studies have been done on Cenozoic strata in general or on the Oligocene Dabaa Formation in particular (e.g., Kedves, 1971, 1985; El-Sabrouty, 1984; El-Bassiouni et al., 1988; Takahashi and Jux, 1989; Bassiouni, 2011). None of these studies dealt with the kerogen portion of the palynological assemblage, nor did they integrate the latter when interpreting paleoenvironments of deposition. This paper presents an effort to bridge this gap by utilizing palynofacies analysis to (1) determine the origin, composition, and hydrocarbon potential of the organic matter in the Dabaa Formation, and (2) provide a detailed account of the prevailing paleoenvironmental conditions during the Dabaa Formation deposition.

GEOLOGIC SETTING

The sedimentary cover of the north Western Desert includes layers spanning ages from Cambrian–Ordovician to Quaternary, attaining a thickness of >10,600 m at the east-west–oriented asymmetric graben of the Abu El Gharadig Basin (Schlumberger, 1995). The stratigraphic succession in the Western Desert preserves alternating cycles of clastic and carbonate deposition reflecting successive transgression-regression episodes. Among them, major transgressive cycles with maximum southward extension took place during the Carboniferous, Late Jurassic, middle and Late Cretaceous, middle Miocene, and Pliocene (Schlumberger, 1984). The Abu El Gharadig Basin extends ∼300 km east-west and 60 km north-south with a sedimentary sequence of Paleozoic to Miocene age. Its northern edge is marked by the Ras Qattara ridge, and its southern boundary is demarcated by the Sitra platform (Egyptian General Petroleum Corporation, 1992).

The interval studied in the Amana-1X well represents the Oligocene marine shale of the Dabaa Formation. The Dabaa Formation was proposed by Norton (1967) to encompass the 442 m section of upper Eocene–Oligocene, light gray to greenish-gray shales with thin beds of limestone encountered in the Dabaa well (lat 31°01′19″N, long 28°29′42″E; Hantar, 1990; Issawi et al., 2009). This formation has also been referred to as the Qasr El Sagha, Maadi, Birqet Qarun, and Gehannam (Hantar, 1990). Although positioned conformably beneath lower Miocene rocks, the Dabaa Formation overlies the Paleocene–middle Eocene Apollonia Formation above a brief disconformity (Issawi et al., 2009). Deposition of the Dabaa Formation is thought to have taken place in an inner shelf to littoral setting, which changed to estuarine toward its top in various areas. Southward, the Dabaa grades laterally into the littoral to deltaic deposits of the Gebel Qatrani Formation (Fayum area; Hantar, 1990; Issawi et al., 2009). In the Amana-1X well, samples of the Dabaa Formation are essentially composed of greenish-gray calcareous shale changing upward to sandy shales.

MATERIAL AND METHODS

Our study is based on 24 cutting samples collected from the Oligocene Dabaa Formation encountered in the Amana-1X well, north Western Desert, at depths between 515.1 and 304.8 m. The Amana-1X well was drilled in the Abu El Gharadig Basin (lat 29°33′23.79″N, long 29°25′23.90″E; Fig. 1). All samples were palynologically processed following the standard technique in Traverse (2007), with minor modifications. The technique involves treating the rock sample with concentrated HCl and HF acids to eliminate the carbonate and silicate contents, respectively. The residue was then sieved in a 10 µm nylon sieve to remove the remaining clay portion of the sediment matrix. Permanent kerogen slides were made using Elvacite 2044 acrylic resin as a mounting medium. Transmitted light microscopy was used to scan the slides for their particulate organic matter (POM) content; 500 particles were point counted from each kerogen slide in order to calculate relative abundances, and quantitatively represent the different kerogen categories.

Miospore color determinations in all samples are based on smooth, thin-walled species such as those of Dictyophyllidites, Deltoidospora, Inaperturopollenites, and Graminidites to avoid erroneous color observations arising from heavy sculptured, complex structured exine types. Therefore, subtle color changes, especially at the immature end of the thermal scale, could be distinguished. Thermal maturation estimations were made using Pearson’s (1984) color chart correlated with corresponding thermal alteration index (TAI) values, as illustrated in Traverse (2007, p. 584, fig. 19.2). Batten’s (1980) interpretations of miospore color alterations were also considered. All microscope slides and residues used in this study are housed in the Department of Geology, Faculty of Science, Mansoura University, Egypt.

RESULTS

Palynofacies Analysis

For the purpose of this study, the following palynofacies terms are used as defined herein. Palynomorphs encompass both continental and marine-dwelling organic-walled microfossils such as spores, pollen, dinoflagellate cysts, acritarchs, freshwater algae, and foraminiferal test linings. Phytoclasts are all terrestrial plant fragments, including those translucent with clear internal structure, degraded structureless, and opaques (e.g., cuticles, tracheids, and vessel elements). The opaque phytoclasts are black kerogen particles derived from the oxidation of translucent phytoclasts, carbonization (coalification) during postdepositional alteration, or as a result of wildfires. Amorphous marine organic matter (AMOM) includes all semitransparent, nearly colorless, fuzzy, structureless kerogen particles derived essentially from the bacterial degradation of marine phytoplankton, such as dinocysts and acritarchs (1–3 in Plate 1). Palynofacies analysts in the past identified all structureless particles as amorphous organic matter (AOM) based solely on their apparent morphology under the microscope, without regard to their environmental origin. They also lumped together such AOM graphically, even though the AOM may have represented different paleoenvironmental sources. This practice is misleading when using these data for paleoenvironmental interpretations, which essentially requires categorizing genetically related palynofacies components based on their paleoenvironmental significance rather than diagenetic structural morphology or palynological classification. To mitigate this problem, structureless particles of terrestrial origin (i.e., from the degradation of terrestrial phytoclasts) are herein named degraded phytoclasts and are not referred to as AOM. Degraded phytoclasts, particularly those of vascular plants, are lignified (contain lignin in their internal cellular structures). Lignin is a complex organic compound that tends to be firm and decay resistant (Tyson, 1995). Therefore, degraded, structureless phytoclasts can be differentiated from AMOM under the light microscope by a thicker and darker appearance (10 in Plate 1). In addition, degraded phytoclasts often show remnants of their original structures, such as cell walls and pitting (10 in Plate 1).

The interval of the Dabaa Formation sampled at the Amana-1X well can be separated into two major palynofacies associations based on the relative abundance data of the recovered POM (Fig. 2).

Palynofacies 1

Palynofacies 1 occupies the lower part of the section (samples 24–19, depth 515.1–469.4 m). It is generally dominated by AMOM (average 47.5%) and phytoclasts (31.6%) with lesser palynomorph content (20.9%). The AMOM is composed of yellow to light brown granular particles commonly showing diffused edges. Medium to fine pyrite crystals associated with the AMOM particles were not uncommon. The phytoclast portion is represented by two types of constituents: dominant, poorly preserved translucent terrestrial plant fragments (average 22.1% of the total POM count), and minor opaque particles primarily of equant shape (average 9.5% of the total POM count). Although cuticles were seen, their numbers were insignificant. Palynomorphs, the third major component in this facies, constitutes two main groups that are not far different in their abundance: the dinoflagellate cysts group (average 7.6% of the total POM count), and the pollen grains group (average 6.3% of the total POM count). Among the dinoflagellate cyst group, the chorate forms were observed most often. There was a remarkable increase in the acritarchs richness in samples 21–23. Other recorded palynomorph groups include embryophytic spores, fungal remains, Botryococcus (freshwater green alga), and microforaminiferal inner test linings.

Palynofacies 2

Palynofacies 2 occurs immediately above palynofacies 1 (samples 18–1, depth 460.2–304.8 m). It is made up essentially of phytoclasts (average 61.2%), within which poorly preserved translucent particles constitute the majority (47.6% of the total POM count) against a minor opaque fraction averaging <10%. Translucent phytoclasts consisted mainly of variably degraded woody tissues (tracheids) accompanied with cuticles of <10% of the total POM count. Cuticle fragments increased markedly in samples 13, 10, 6, and 5. Opaque phytoclast particles were mostly equant and showed a dramatic abundance rise in sample 7. After phytoclasts, palynomorphs and AMOM represented the second major POM component in this facies. Palynomorphs averaged 24% and were distinctly elevated in samples 18, 17, 16, 13, 2, and 1, while AMOM had an overall average abundance of 14.8% and was particularly high in samples 15, 12, and 11. Pollen grains and dinoflagellate cysts were the most common palynomorph elements, like that in palynofacies 1. Embryophytic spores (chiefly simple, unsculptured), fungal remains, and green algae showed a notable increase when compared to palynofacies 1. Both acritarchs and foraminiferal inner test linings continued to be present.

TAI and Spore and Pollen Color

Visual spore and pollen color observations of the Dabaa Formation samples in normal transmitted light microscopy revealed an exinal color range of colorless to pale yellow for pollen grains and pale yellow to yellowish-orange for spores. On the TAI scale, these colors correspond to values of 1–2 (Traverse, 2007).

HYDROCARBON SOURCE POTENTIAL

Kerogen Type

The kerogen composition of palynofacies 1 reflects a mixture of two major source contributors, AMOM and phytoclasts (Fig. 2). AMOM is derived largely from the degradation of phytoplankton or bacteria under reduced oxygen conditions, and is the main constituent of kerogen types I and II, oil-prone source material (Tyson, 1995). Phytoclasts are those broken up terrestrial macrophytes like wood fragments, cuticles, cortex tissues, and oxidized plant particles (opaques). They make up the bulk composition of kerogen types III and IV, gas-prone to inert source material. The average kerogen composition of palynofacies 1 (47.5% AMOM and 31.6% phytoclasts) occurs in the mid-range between the two extreme kerogen end members (highly oil prone and inert). Accordingly, palynofacies 1 is herein interpreted to contain type II–III kerogen that would expel mostly oil, but also some gas, at its peak maturity.

Palynofacies 2 is characterized by a dramatic increase in the phytoclast/AMOM ratio (average 61.2% phytoclasts and 14.8% AMOM). This clearly indicates significant dilution of oil-prone components by gas-producing source material. Therefore, palynofacies 2 is construed to contain type III kerogen expelling largely gas, in addition to little oil, at its peak maturity.

Organic Thermal Maturity

Spore and pollen exine colors in the studied Dabaa samples collectively ranged from colorless to yellowish-orange (≈1–2 TAI), indicating negligible chemical change in the path of maturity (Batten, 1980). Therefore, the Dabaa Formation in the Amana-1X well contains immature organic matter of no potential for thermally expelled hydrocarbons. Biogenic gas is the only possible hydrocarbon product at this phase.

PALEOENVIRONMENTAL RECONSTRUCTION

Our approach to interpreting the depositional paleoenvironment of the Dabaa Formation in the Amana-1X well relies primarily on the POM composition, along with the relative abundance changes among individual components. These data were articulated with the lithologic composition of the samples studied, as well as established global paleogeographic reconstructions, and sea-level changes, to produce an integrated, holistic reconstruction of the Oligocene southeast Mediterranean. We introduce a new POM ternary diagram to represent graphically the distribution of major POM components in the context of paleoenvironmental parameters (Fig. 3); in this diagram, the POM were separated into three proportions: (1) phytoclasts plus nonmarine palynomorphs (top) indicative of terrestrial and freshwater influence, (2) AMOM (base left) reflecting the oxygenation state, and (3) marine palynomorphs (base right), which together with AMOM show the approximate basinward distance from shoreline. We note that this diagram should not be used to interpret submarine fan systems and turbidite sequences, wherein voluminous terrigenous components are known to be transported to the deep-sea realm.

The POM ternary diagram is more useful in our context than the Roncaglia and Kuijpers (2006) total sedimentary organic matter (TSOM) ternary diagram, which includes decomposition products of both marine and nonmarine sources in the AOM end member, potentially giving rise to spurious proximal-distal and redox interpretations. The same problem may also arise when using the Tyson (1989) AOM-phytoclast-palynomorph plot, which was maximized by having the palynomorphs end member include both marine and nonmarine forms. We resolved these issues by counting the unstructured amorphous nonmarine macrophytes as degraded phytoclasts, preserving their actual paleoenvironmental affiliation, and by differentiating marine from nonmarine palynomorphs, which were grouped with the terrestrial phytoclasts.

The POM composition of palynofacies 1 is largely of marine origin, mainly AMOM. Such a high percentage of AMOM is usually indicative of reducing, low-energy environments that have high organic matter preservation potential (e.g., Zobaa et al., 2011a, 2013). Prevalent reducing conditions can also be supported by the frequent occurrence of pyrite crystals within this facies. The increased AMOM is coupled with a significant population of allochthonous terrestrial phytoclasts, reflecting high terrigenous influx. The combination of these two major components implies an outer shelf or upper slope setting where marine conditions dominate, and relatively short phytoclast transport distances. The fact that the majority of recorded phytoclasts in this facies are degraded translucent fragments associated with minor opaque particles suggests transport under well-oxygenated conditions. However, transport distances were not long enough to result in complete degradation and oxidation of these phytoclast fragments.

The reported low palynomorph content in palynofacies 1 can be attributed to either the prevalence of aridity and therefore low vegetation on the hinterland, or to the dilution effect caused by the abundant AMOM and phytoclasts. We note that the palynomorph fraction contains nearly equal proportions of marine and nonmarine forms; this further supports the inferred average outer shelf to upper slope environment between both the marginal and deep-marine settings. The increased ratio of short-spined acanthomorphic acritarchs relative to the total marine palynomorph fraction is another noticeable trend occurring near the bottom of palynofacies 1 (Fig. 2). Such trends are generally known to be significant in shallow-water, high-energy, marginal marine facies (Tyson, 1993). Therefore, these acritarchs were most likely transported (perhaps reworked?) to their current deeper location.

On the POM ternary diagram, palynofacies 1 samples (orange) plot midway between proximal and distal facies, and oxic and anoxic conditions (Fig. 3). This further confirms the aforementioned arguments and conclusion concerning the lower part of the Dabaa section, i.e., outer shelf to upper slope environment under suboxic to anoxic conditions.

The upper part of the Amana-1X Dabaa section, represented by palynofacies 2, clearly marks an overall change toward a shallower facies, as indicated by the remarkable phytoclast enrichment over the AMOM (Fig. 2). This regressive trend, however, was not steady and multiple marine revivals of variable magnitudes occurred (e.g., samples 15, 11, 8, 5, 1). Barring the presence of a turbidite sequence, high percentages of phytoclasts commonly suggest a proximal facies with a strong terrestrial organic matter influence. This is also reflected on the POM ternary diagram (Fig. 3), which shows that palynofacies 2 samples were deposited under better oxygenated, more proximal, high terrestrial and freshwater input conditions in comparison to those of palynofacies 1. Consequently, the upper part of the Dabaa section is herein interpreted as to have been deposited in a marginal marine to inner shelf setting subject to strong fluvial and/or estuarine control. This interpretation seems plausible considering the visible increase of stream-transported cuticle debris and freshwater-dependent palynomorphs such as embryophytic spores and green algae.

Opaque phytoclast particles derive from three main processes: (1) the oxidation of translucent woody fragments during prolonged transport, (2) postdepositional alteration (thermal maturation), and (3) natural wildfires (as charcoal) (Tyson, 1993; Zobaa et al., 2011b). The conspicuous equant opaque phytoclast peak (about double the normal content) in sample 7 at depth of 359.7 m, together with negligible AMOM content (1%), may reflect a short-term forced regression due to a brief flooding event far on the hinterland and subsequent high amounts of terrigenous sediments supply.

Our overall shallowing-upward interpretation of the Dabaa Formation paleoenvironment in the Amana-1X well conforms with the observed lithologic change in the samples we studied from the calcareous shale at the bottom of section to sandy shale at its top. It also fits well with the global Oligocene paleogeographic model, wherein the north Western Desert was part of the eastern south Mediterranean shelf (Fig. 1). The construed sea-level drop at the boundary between palynofacies 1 and 2 may have been tied to the end of Rupelian sea-level fall ca. 28.4 Ma (Fig. 4).

CONCLUSIONS

Palynofacies data generated from the Dabaa Formation provided detailed constraints on the depositional paleoenvironment and hydrocarbon source potential of the Oligocene rocks, southeast Mediterranean. Two major marine palynofacies were recognized: palynofacies 1 (older) contains type II–III kerogen (mostly oil prone), which was deposited in an outer shelf to upper slope setting under suboxic to anoxic conditions. Palynofacies 2 comprises type III kerogen (largely gas prone) that represents shallower, terrestrially more controlled conditions. This progradational trend correlates with the global sea-level drop that occurred at the end of Rupelian time. Spore and pollen exine colors in both facies point to an organic content that is not yet thermally mature. A newly developed POM ternary diagram was used to graphically represent the distribution of major POM components in the context of paleoenvironmental parameters.

We thank Raymond M. Russo, Geosphere’s science editor, and an anonymous reviewer for their helpful comments and constructive criticism. Salah El Beialy is indebted to the Arab Fund Fellowships Program, Kuwait, for financial support through a Distinguished Scholar Award that allowed a one-year research stay at Brock University, Canada, from September 2006. This study would not have been possible without such support and generosity.