SEDIMENTOLOGY AND DIAGENESIS OF SOME NEOCOMIAN – BARREMIAN ROCKS (CHOUF FORMATION), SOUTHERN LEBANON

SEDIMENTOLOGY AND DIAGENESIS OF SOME NEOCOMIAN – BARREMIAN ROCKS (CHOUF FORMATION), SOUTHERN LEBANON

Mr. Georges Bellos: Educational Leadership and Management Student (LIU)

Masters Degree Holder in Geology, in Sedimentology: Option Oil and Gas (AUB)

Masters Degree Holder in Phoenician Archeology (Lebanese University)

MBA Graduate in Management (Lebanese International University)

Saloumi Road, Lebanon,

Foreword: The Neocomian – Barremian rocks are well exposed in southern Lebanon (Jezzine), yet they were not properly studied to date, except in a Masters Thesis presented and defended at the American University of Beirut (AUB) in June 2008, by Mr. Georges Bellos. Yet, the present study involves modern detailed and state of the art petrographic and mineralogical analyses of these rocks. The Chouf Formation was deposited both in fluvio-deltaic (aquatic based systems) and aeolian environments (terrigenous systems). Evidence of this is obtained in the field, looking at preserved primary structures (in the Lower parts of the Formation – for the aquatic dominated systems – and in the mid parts of the Formation – for the continentally-dominated systems). Thus, both types of sandstone strata are found involving different environments of deposition with cyclical control. (Reference)

Lithostratigraphic and petrographic analyses revealed that the Homsiyeh outcrops include five distinct facies. Each of them was studied in detail with modern petrographic and x-ray diffraction methods. This petrographic study sheds more light on the characteristics of the organic rich layers in the aquatic dominated facies mainly occurring in the lower part of the Chouf Formation. These bitumen resulted in corrosion and dissolution of the quartz grains (in the sandstones) increasing the porosity/permeability of the bulk rocks upon initial hydrocarbon migration and later on the telogenic flushing with meteoric waters. (Reference)

The discussion summarizes what the AUB Masters research discusses regarding the Chouf Formation hydrocarbon potential. Several studies in the Middle East were undertaken on sandstones of Neocomian-Barremian age. The Heletz/ Gevar Am strata in Palestine, the Nubian sandstones in the Gulf of Suez, and Cheriffe Formation (i.e. Ruthbah) in Syria were studied to assess source and reservoir rock potentials. These Levantine clastic formations were proven to contain hydrocarbon, also show similarities with the Chouf Formation (Neocomian-Barremian) in Lebanon (Reference).

Development: In general, the permeability of sandstones permits fluids to transit through them (regardless whether the fluid is water or petroleum) and often these rocks act as aquifers/ reservoirs. In the case of sandstone aquifers, waters will be fresh and filtered, as contaminants will be trapped by the network of pores, whereas for petroleum, uncemented clean arenites tend to be usually reasonably good reservoir rocks. In terms of volume, the largest sandstone reservoirs are the Mesozoic Formations of East Texas, USA (140,000 acres), the Cretaceous oilfields of Burgan, Kuwait (where productive thicknesses exceed 1300ft), and the Cretaceous fields of Pembina, Canada (755,000 acres), as well as the deltaic Athabaska tar sands of Alberta, Canada. So, 59% of the world oil production is generated by sandstone rocks (i.e. arenites, graywacke, arkose, grit, and conglomerate); whereas carbonate rocks (marly or reefal limestones and/ or dolomites) comprise 40% of the world oil production. 

Only 1 % of oil produced is generated from fractured igneous and metamorphic rocks; as well as siltstones and claystones. Porosity and permeability are important factors in reservoir rocks. Porosity types can be depositional (e.g. cubic, 48% or rhombic, 26%), and/ or diagenetic (i.e. postdepositional) and size-dependant or secondary (i.e. developing through diagenetic processes which either cause void enhancement by dissolution, or destruction by cementation). Cements may reduce bulk rock porosity by 10 to 40%. For geometrically similar grains, porosity is the same but permeability will be proportional to the square of the grain size. Permeability measurements normal to bedding will give a lower value than the original permeability value of the bed (as measured along bedding planes). Deep burial increases overall pressure, and as a result, the bulk rock porosity of sandstones may drop from 35% to 25% at depths of about 5km. Therefore, increases in temperature and pressure will result in a general determination of both porosity and permeability in sandstones. Sandstones depositing in deltaic or in regressive sea environments take form as small lenticular sand bodies which rapidly grade into clays and shales. Other siliceous rocks such as grits and conglomerates can produce excellent reservoir rocks (grits are formed from trapped feldspars in deltas). Arkosic rocks, with high feldspar content derived from the disintegration of highly acidic felsic rocks, have some hydrocarbon potentials. Calcarenites have grain sizes similar to sandstones, and are common source rocks in Saudi Arabia.

Sand bars and barrier sand beaches (grading outwards into marine silts and clays in which some organic matter is entrapped) can sometimes produce hydrocarbon Prior to the 1960s, the sandstones in Lebanon were only investigated with a rather stratigraphic approach, where field mapping of the Chouf Formation surface exposures comprised the main achievement. Later on they were analyzed petrographically as sedimentological and diagenetic techniques were made available which contributed to petroleum exploration studies in Lebanon. However, the boundaries of the Chouf Formation with the Salima and the Abeih Formations are still not well-defined. The Neocomian-Barremian rocks in southern Lebanon are characterized by large quantities of quartz-rich sandstones, which have been studied macroscopically up to the 1960s. At the time, modern techniques of sedimentary research (as petrographic and X-ray diffraction methods) were barely available. Therefore, the results of this research are used to elaborate a diagenetic history for these sandstones that may provide answers for their origin. The Chouf Formation is also known for its lignite beds, traces of amber, presence of iron, and, rarely, a few fossilized materials (Reference).

Figure 1 shows the surface exposures of the Chouf Formation all throughout Lebanon. They are widely recognized in the field, as being covered with pine trees (Pinus Pinea) and by showing strata mainly prevailing in clastic regimes, as opposed to the Jurassic and Cretaceous dominantly carbonate sequences. Table 1 shows the identified facies. So, the Basal Cretaceous strata mainly consist of the Neocomian Sandstones of the 'Grès de Base’ were dated to 130 Ma (i.e. of Hauterivian age). They occur near Mount Lebanon, and are typically seen to rest unconformably over the oolitic Salima Formation, recently dated to extend until the Valangianian. However, they can also be found to rest unconformably on the eroded top of the massive Jurassic limestones. The Chouf Formation is mainly cross bedded and orange brown in colour (Figure 2). It is hematitic at its base and in other horizons contains fossilized wood, coal, amber and/or pyrite locally. Most of the formation is very permeable and may contain numerous springs. Figure 3 shows the common types of sandstones identified in the Chouf Fm (Reference).

 

Figure 1: Generalized geologic map showing the Chouf Formation outcrops in Lebanon (Reference).

 

Figure 2: Yellowish cross-bedded sandstone, Bsalim area (pen for scale).

Table 1: General table representing the mineralogical data of the key facies from the Chouf Formation, along with their respective reservoir properties (Reference).

Figure 4 shows the phenomenon of clay draping in channel sandstone facies as the ones identified in the Jezzine area (Personal communication, Dr. Rudy Swennen, 2005). The cross strata show episodes of flooding events of different current speeds. These drapes were said to cover the ripples in times of slowing current speeds (Personal communication, Dr. Rudy Swennen, 2005). Table 2 shows depositional environments of the Chouf Fm (Reference).

 

Figure 3: Quartz rich Sandstone (A), arkose (B), Organoc matter rich sandstones (C), and calc)arenites are the most known types of sandstones (Reference).

Figure 4: Illustrative sketch representing clay draping in channel sandstone facies, associated with tidal/ flooding events (Personal communication, Dr. Rudy Swennen, 2005). A. Sketch of typical cross-beds found in cross-stratified sandstone strata. B. Flooding/ tidal curve showing differing current flow speeds. At its lowest points (e.g. A & B), clay drapes are said to be formed (the clays cover the lee side of ripples). C. Sketch of ripple laminations (A, B) showing clay draping (Personal communication, Dr. Rudy Swennen, 2005).

Table 2 shows the identified facies of the Chouf Fm, whereas Figure 5 shows the digenetic phases of the exposed sandstones phases in the Jezzine area. Figure 6 presents the listone fqacies of the Chouf Fm, while Figure 7 shows the full diagenetic history (Reference).

Table 2: Observed facies of the Chouf Fm, and their depositional environments (Reference).

Figure 5: Sequence of diagenetic phases for the sandstone facies exposed in the Toumatt-Jezzine/ Aazibi and Homsiyeh sections (southern Lebanon). Aren. = Arenite, O.M. = organic matter Qtz. = quartz, Lit. = Lithification, Telog. = teleogenesis, and Oxid. = oxidation (Reference).

Figure 6: Sequence of diagenetic phases for the sandstone facies exposed in the Toumatt-Jezzine/ Aazibi and Homsiyeh sections (southern Lebanon). O.M. = organic matter, Prim. = primary, Dissol. = dissolution, sol. coll. = solution collapse, arag. = aragonite, Telo. = telogenic, Frac. = fracture, and Oxid. = oxidation (Reference).

 

Figure 7: Burial curve with recorded diagenetic history of the Chouf Formation. Note that the recorded burial is estimated at 1800m, thickness is 300m, and the resulting curve is uncorrected for compaction (Creta. = Cretaceous, Pli = Pliocene, R. = Recent, and O.M. = organic matter).

 

Figure 8: Extrapolated pyrolisis results of an organic matter sample from the Lower part of the Chouf Formation. The pyrolisis was conducted at the Sedimentology Laboratory of the Institut Français du Pétrole (IFP). Note that the total organic carbon (TOC) was estimated at 67.54% (Reference).

Finally, the above diagram, through Pyrolysis tells us that the organic matters from the Chouf Fm are Type III Kerogens, and conbstitute Natural (or Wet) Gas, as per diagenesis. Figure 8 shows that (Reference).

Conclusions: This project includes the results of detailed field work, as well as petrographic and mineralogical studies conducted on several representative key Neocomian-Barremian rock facies of the Chouf Formation in the Jezzine region (southern Lebanon). Based on field observations of sandstone strata, and regional correlation, as well as petrographic and XRD characterization of the investigated rocks, the following conclusions are drawn. Six microfacies are identified in the current study, within both investigated sections, at Homsiyeh. They are listed and described as follows: i) arenites, ii) muddy quartz-rich sandstones, iii) clayey-muddy quartz-rich sandstones, iv) graywacke, v) clay, and vi) limestones (Reference).

Arenites, the most abundant microfacies of the Chouf Formation, mostly representing the middle part of the formation, are composed of submature to mature sandstones almost devoid of clays showing aeolian characteristics and are quartz-rich (content is over 95%, according to Robert Folk’s sandstone classification chart). They are typically well-sorted and develop in areas of moderate to high kinetic energy. Since they are well-sorted and the average intergranular porosity is around 10%, they may act as very good reservoir rocks. XRD studies show two or more phases of quartz, clays and goethite, and, in some cases, probable traces of calcite (Reference).

The two identified phases of quartz (detrital and authigenic) were identified under CL microscopy. In some cases, feldspar was also detected by its light blue CL pattern, which is confirmed through XRD analysis. Most of the submature sandstones were classified as muddy quartz-rich sandstones, as they were moderately sorted and contained some clay (less than 5%). They are dominantly fluvial (and likely marine, as well) and of moderate energy environments. Since they are moderately sorted and the average intergranular porosity is about 10%, they may also act as good reservoir rocks. XRD tests helped in identifying the presence of two or more quartz phases, several clays (including nontronite (lacking smectite)), and calcite (Reference).

Clayey-muddy quartz-rich sandstones include all immature sandstones that have clay contents over 5% and are poorly sorted. They are found in dominantly fluvial or overbank channel environments, of low-energy. Since they are poorly sorted and the average intergranular porosity is about 5% or less, they may act as poor reservoir rocks. XRD tests show the presence of different phases of quartz, including calcite among others, pyrite and clays (Reference). Graywackes include all strata which are extremely poorly sorted and include clay contents over 15% (feldspars and micas and other components account for about 25%). 

They are typically fine grained strata found in turbidity deposits of low kinetic energy. Since they are poorly sorted and the average intergranular porosity is about 5% or less, they may act as poor reservoir rocks. XRD tests indicate that these strata show different phases of quartz, including other constituents as calcite, pyrite, glauconite, chlorite, feldspars, and clays. Clays (generally containing lignite) were identified using X-ray diffraction. Thus, various amounts of bentonite, illite, kaolinite montmorillonite, nontronite and vermiculite were found. Most beds show the presence of kaolinite, illite and montmorillonite. However, chlorite, goethite, vermiculite often recur in some beds. In a few instances, illite/ montmorillonite interstratified layers were found (these interstratified layers have a diagenetic signature, just like the illite/ smectite or montmorillonite/ illite ratios). Since they are characterized by very low average intergranular porosity, they may act cap rocks or seals. The clays also led to a dramatic decrease in the aquiferous properties of the Chouf Formation. The limestones (defined as fossiliferous micritic muddy-wackestones, based on classificqations schmes like Robert Folk’s carbonate classification schemes dominantly comprise calcite (four phases – XRD, CL microscopy) and aragonite, including other minor amounts of other carbonates, such as dolomite (based on XRD). The limestones, which were found to be se shallow marine carbonates, show plant activity, and appear to be associated with various glauconitic marly beds that were found nearby. Since they are poorly sorted and the average intergranular porosity is about 2.5% or less, they may act as very poor reservoir rocks (Reference).

In earlier studies dealing with hydrocarbon potential, the organic matter found in terrigenous clastic rocks was thought to yield bitumens and oilshales. The results of this study point out that the organic matter of the Chouf Formation was transformed to lignite, as the burial/thermal history was recorded at 65ºC towards the mid-Eocene, causing the improperly maturing organic matter to produce coal rather than oil (due to high TOC and low HI values). Due to the telogenic fracturing, as a result of the Syrian Arc Deformation, uplifting, fracturing and erosion occurred. This research demonstrated that the studied hydrocarbon, in the lower aquatic facies of the Chouf Formation (southern Lebanon), is immature. During mild cooking at low temperature aggressive fluids helped in increasing the general porosity through dissolution and yet still led to quartz cementation. Finally, the eolian arenites (of the middle part of the Chouf Formation) could act as reservoirs due to their relatively high intergranular porosity (of about 10%) caused by good sorting and roundness. These arenites contain low amounts of cements, if any (Reference).

Recommendations: More detailed field work in the area should be conducted. The outcrops in the proximity of Machghara, Jabal Shammis, and Qaitouleh should be investigated. Such studies should yield more stratigraphic data, which should include detailed paleocurrent measurement. XRD analysis provided good information on clay mineralogy. However, for a future study, it is recommended to conduct a more thorough mineralogy analysis; by running as many tests as possible. Ideally, XRD tests should be run (at least once) on every representative sample. It is also recommended to subject the studied samples to scanning electron microscopy (SEM). Their results will be beneficial in clay studies, as SEM enables to generate 3-dimensional views of the studied clays. For a better resolution, a good alternative could be to combine the XRD and SEM results in providing very detailed assessments for clay studies. As several cements were detected, it is also recommended to do a hot CL-SEM study, in order to complete a detailed cement stratigraphy in order to complement the study. As the hydrocarbon potential of the Chouf formation has been discussed, it is recommended to study these hydrocarbons in further detail, using gas chromatography, as 204 well as analyzing the different constituents of the bitumens under fluorescence microscopy. It is also suggested to carry out a detailed geochemical analysis of the Chouf Formation by relying on geochemical methods (e.g. REE, isotope studies, fluid inclusions), and heavy mineral analysis (Reference).