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Volume 32 Issue 6
Dec.  2021
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Ibrahem Yousef, Vladimir Morozov, Vladislav Sudakov, Ilyas Idrisov. Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria. Journal of Earth Science, 2021, 32(6): 1545-1562. doi: 10.1007/s12583-020-1065-8
Citation: Ibrahem Yousef, Vladimir Morozov, Vladislav Sudakov, Ilyas Idrisov. Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria. Journal of Earth Science, 2021, 32(6): 1545-1562. doi: 10.1007/s12583-020-1065-8

Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria

doi: 10.1007/s12583-020-1065-8
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  • This article presents the results of cementation characteristics and their effect on sandstone reservoir quality of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations in selected fields in the Euphrates Graben area, Syria. This study emphasises the role of cementation in the evaluation of the diagenetic history of the sediments, developing effective porosity, as well as evaluation of reservoirs stimulation procedures and potential for formation damage of the sandstone reservoirs. Quartz cement is present as well developed tabular or pyramidal syntaxial overgrowths. Kaolinite cement is present as vermicular aggregates which are most abundant within sandstones of the Mulussa F Formation. Carbonate cements include siderite and dolomite. Four lithofacies were identified within the studied formations; lithofacies-1 and 2 correspond to fluvial depositional environments, lithofacies-3 and 4 correspond to fluvial to estuarine channel environments. The Post Judea Sandstone and the Lower Rutbah reservoir units are typically lithofacies-3 sequences in which quartz overgrowths are the dominant cement. Because the total cement is more extensive in the Post Judea Sandstone Formation than in the Lower Rutbah Formation, resulting in high porosity (up to 26%) and permeability (6 000 mD), the reservoir quality is predicted to be best in the Post Judea Sandstone Formation. The reservoir units in the Mulussa F Formation contain the highest cement volumes comprised of early siderite and kaolinite, which, with the development of compaction-resisting quartz overgrowths and resultant compactional pore volume loss, has resulted in typically lower porosity being preserved than in the Lower Rutbah and Post Judea Sandstone formations.
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Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria

doi: 10.1007/s12583-020-1065-8

Abstract: This article presents the results of cementation characteristics and their effect on sandstone reservoir quality of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations in selected fields in the Euphrates Graben area, Syria. This study emphasises the role of cementation in the evaluation of the diagenetic history of the sediments, developing effective porosity, as well as evaluation of reservoirs stimulation procedures and potential for formation damage of the sandstone reservoirs. Quartz cement is present as well developed tabular or pyramidal syntaxial overgrowths. Kaolinite cement is present as vermicular aggregates which are most abundant within sandstones of the Mulussa F Formation. Carbonate cements include siderite and dolomite. Four lithofacies were identified within the studied formations; lithofacies-1 and 2 correspond to fluvial depositional environments, lithofacies-3 and 4 correspond to fluvial to estuarine channel environments. The Post Judea Sandstone and the Lower Rutbah reservoir units are typically lithofacies-3 sequences in which quartz overgrowths are the dominant cement. Because the total cement is more extensive in the Post Judea Sandstone Formation than in the Lower Rutbah Formation, resulting in high porosity (up to 26%) and permeability (6 000 mD), the reservoir quality is predicted to be best in the Post Judea Sandstone Formation. The reservoir units in the Mulussa F Formation contain the highest cement volumes comprised of early siderite and kaolinite, which, with the development of compaction-resisting quartz overgrowths and resultant compactional pore volume loss, has resulted in typically lower porosity being preserved than in the Lower Rutbah and Post Judea Sandstone formations.

Ibrahem Yousef, Vladimir Morozov, Vladislav Sudakov, Ilyas Idrisov. Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria. Journal of Earth Science, 2021, 32(6): 1545-1562. doi: 10.1007/s12583-020-1065-8
Citation: Ibrahem Yousef, Vladimir Morozov, Vladislav Sudakov, Ilyas Idrisov. Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria. Journal of Earth Science, 2021, 32(6): 1545-1562. doi: 10.1007/s12583-020-1065-8
  • Diagenesis is understood as a broad spectrum of physical, chemical, and biological post-depositional processes and changes by which the original sedimentary assemblages will be transformed into sedimentary rocks (Burley et al., 1985). Many factors are involved in the diagenesis processes, including the humidity of the sediments, which contribute to the interaction of the various components and the formation of the new diagenetic minerals (Ali et al., 2010). The presence of the bacteria plays the main role in the conversion of the sedimentary matter. The chemical composition of the water solutions of the pore system and the organic matter within the sediment can cause oxygen deficiency. The appearance of carbon dioxide and hydrogen sulphide, that is, they create reducing conditions (Larsen and Chilingar, 1983). Diagenesis processes include many events, for example, compaction arising under the pressure of the new accumulated sediment layers, precipitation of the new types of cement due to the presence of the various chemical compounds that fill the pores and voids of the rock and cement the sediment particles, dissolution, and recrystallization which can lead to the transition between the different authigenic minerals (Machel, 2005). The Upper Triassic Mulussa F Formation, the Lower Cretaceous Lower Rutbah Formation, and the Upper Cretaceous Post Judea Sandstone Formation over the studied fields in the Euphrates Graben area are the main exploration, production, and development hydrocarbon targets in eastern Syria (Yousef and Morozov, 2017a, b; Yousef et al., 2016; Litak et al., 1998). Understanding the diagenesis of these sediments is very important for any future exploration (in the non-explored areas) and development activities. For example, understanding how the burial diagenesis affects the reservoir quality may place limits on the depth to which production from these reservoirs will prove economic. Also, understanding the different types and distribution of the diagenetic minerals within the reservoir's sediments may be useful for simulation strategy and development of the reservoir production (Whitaker et al., 2014).

    A diagenetic analysis based on the available core materials that involved conventional petrographically thin section analyses, modal analyses, and scanning electron microscopy (SEM) in the selected wells/fields was thus to provide results that increased understanding of the cementation characteristics and their effect on the quality of the sandstone reservoirs of the Upper Triassic Mulussa F Formation, Lower Cretaceous Lower Rutbah Formation, and the Upper Cretaceous Post Judea Sandstone Formation in the studied fields in the Euphrates Graben area. The present work aims to: (1) describe the lithofacies and cement characteristics within the sediments of the studied formations; (2) summarize the diagenesis sequences; (3) find out the cement distribution in the studied sandstones; (4) figure out the relationship between burial depth and cementation, as well as the effects of cementation on the sandstone reservoir properties of the studied formations. The main points are discussed according to the paragenetic sequences.

  • From the regional geology point of view, Syria is located on the northern slope of the Arabian Plate (Fig. 1a) (Brew et al., 2000; Litak et al., 1998). It is an important oil producer in the Middle East region (Syrian Petroleum Company, 1981). The middle Palmyrides fault belt zone, the eastern Euphrates Graben, and the northeastern Mesopotamian trough (Fig. 1b), (Litak et al., 1998, 1997), are all promising areas in Syria with proven industrial oil and gas potential. The Euphrates Graben is a 160-km-long northwest-southeast intracratonic rift basin formed by crustal extension during Middle to Late Cretaceous and characterized by a complex pattern of highly interlocking faults. According to different trends, differential subsidence is predominantly controlled by normal throws (Fig. 1c) (Yousef et al., 2018a, b). It has important economic importance as one of the largest oil regions in Syria and contains the main hydrocarbon reservoirs (Lovelock, 1984).

    Figure 1.  (a) Simplified map illustrating the tectonic settings of the Arabian Plate (Litak et al., 1998); (b) sketch map showing the Euphrates Graben region in Syria; (c) geological cross-section based on seismic interpretations showing the faulting system along the Euphrates Graben area; (d) sketch map showing the oil and/or gas fields in the Euphrates Graben; (e) a map showing the well locations over the selected studied fields in the Euphrates Graben area.

    In the Euphrates Graben, there are a fairly large number of oil and gas fields (Fig. 1d) operated mainly by Syrian Petroleum Company (SPC) and Al Furat Petroleum Company (AFPC), which indicates the presence of favorable geological factors that determine the prospects for oil and gas sedimentary cover in the Euphrates Basin (Alsdorf et al., 1995). The oil and gas potential of the Euphrates Graben is associated with deposits of the Paleozoic, Triassic, Cretaceous, Paleogene, and Neogene (Yousef et al., 2019). The studied area covers eight wells; X-1, X-2, X-3, X-4, X-5, X-6, X-7, and X-8 spread out over five fields in the Euphrates Graben (Fig. 1e).

    The interesting stratigraphical section of the research covers sediments of the Upper Triassic Mulussa F Formation, the Lower Cretaceous Lower Rutbah Formation, and the Upper Cretaceous Post Judea Sandstone Formation (Figs. 2a, 2b) over the selected fields in the Euphrates Graben area. Sediments of these three formations generally consist of sandstone and claystone sequences deposited in fluvial to mostly estuarine channel depositional environments, and they form the main oil and/or gas reservoirs over the fields of the Euphrates Graben area.

    Figure 2.  (a) General stratigraphic section of the Euphrates Graben (Brew et al, 2000); (b) correlation panel A-B between the studied wells in the Euphrates Graben area.

    The interpretations of the cementation characteristics and their effects on the quality of the sandstone reservoirs of these formations are comparably scarce so far over the fields of the Euphrates Graben area. This is one of the reasons that led to this study. Additionally, the abundance of the core materials from the wells that penetrated sediments of the studied formations over the Euphrates Graben fields provides us with a good opportunity to investigate these sediments and interprete the cementation characteristics and their effect on the quality of the sandstone reservoir units of these formations over the studied area.

  • The results presented in this article are based on analyses of the core materials from eight wells (Fig. 2b) (see also Fig. 1e) penetrated the sediments of the studied formations over the studied area with depths ranging between 2 800 to 3 600 m measured depth (MD). Almost 550 m of cored intervals were investigated for the sedimentological features of the sediments. More than 500 thin sections from the studied sediments were also investigated for this work. This is in addition to the main well log data of the wells. Thin sections were investigated using a polarized microscope which allowed us to determine the composition, porosity, pore size, and grain size of the sediments under the standard polarizing light. Sample mineralogy was determined using modal analyses (Dickson, 1966). The XRD (whole sandstone samples) and SEM analyses were performed to provide a wide range of information about the structure and composition, which made it possible to estimate the paragenesis of the authigenic minerals (Yousef et al., 2020; Li et al., 2019; Ning et al., 2019; Wang et al., 2019).

    Petrographically, thin section examination aids in defining the types of cement within the formationsʼ sandstones. The SEM and energy dispersive X-ray analyses (EDX) were the primary methods of determining the different types of cement.

    To calculate the amount of the different cements, first we classified the cements into three types: total cements, quartz cement, and other cements. Second, we applied the following formulas (Pommer and Milliken, 2015): 1. total cements=the sum of all the cements that occupy the intergranular volume; 2. quartz cement=quartz cement that occupies the intergranular volume; 3. other cements=total cements–quartz cement. Special emphasis is placed on the form and distribution of the quartz cement, its relationship with other authigenic phases, and the effects of compaction on porosity reduction. Depth plots illustrating the quartz overgrowths cement, total cements, other cements, and the remnant porosity values and charts illustrating the distribution of the major authigenic phases have been constructed also based on the available data from core and thin-sections modal analyses. The Houseknecht diagram (Houseknecht, 1988, 1984) was used when assessing the diagenetic modification and its influence on the intergranular porosity and to separate the effects of the compaction processes from the effects of the cementation processes on the porosity development of the sandstone reservoirs.

    For helium porosity and horizontal permeability: one-inch diameter plug samples were drilled using brine (200 gm/I NaCl) as the bit lubricant. Horizontal plug samples were drilled parallel to the apparent bedding at 30 cm spacing. The ends of each plug sample were trimmed to form a right cylinder. The plug samples for conventional core analysis were cleaned in a Soxhlet using hot refluxing solvents (chloroethene and methanol mix) and dried at 105 ℃. For helium porosity, the clean, dry plugs-samples were individually placed in the matrix cup of a porosimeter. Helium at a known pressure from a reference cell of known volume then expands into this matrix cup and the pressure is noted. The grain volume of the sample is measured on the principles of Boyleʼ s Law (Keelan, 1972). The bulk volume of the sample is measured by liquid displacement, allowing the porosity to be determined. For horizontal permeability, the clean, dry plug-samples were individually placed in the Hassler holder of a permeameter at a confining pressure of 400 psi. The flow rate of dry air through the sample was measured at a specified differential pressure and is used together with the measured sample length and diameter to determine permeability using Darcyʼ s Law (Keelan, 1972).

  • The Upper Triassic Mulussa F Formation is generally moderate to a good principal reservoir with a total thickness of about 400 m (Fig. 2b). It is oil-bearing in a large number of the Euphrates Graben fields. Sediments from this formation have been studied in wells X-2 and X-5 (Fig. 2b). Sedimentological investigations of the cored intervals showed that sediments of this formation in the studied wells consist of a sequence of fluviatile channel sandstone bodies, which are separated by thick flood plain claystone intervals (Yousef et al., 2019). The sandstones of the Upper Triassic Mulussa F Formation recorded helium porosity values up to 19% and horizontal permeability up to 5 000 mD (Fig. 3a).

    Figure 3.  (a) A representative chart showing the relationship between the porosity and permeability values of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations. Representative thin sections and SEM photomicrographs showing (b), (c) pore structure within the very fine-grained sandstone; (d), (e) pore structure within the fine to medium-grained sandstone; (f), (g) pore structure within the medium to coarse-grained sandstone; (h), (k) pore structure within the coarse-grained sandstone. Q. Quartz; P. pore; Py. pyrite; K. kaolinite.

    The Lower Cretaceous Lower Rutbah Formation also serves as a principal moderate, good to very good oil/gas-bearing reservoir in most of the Euphrates Graben area fields, with a total original thickness of about 100 m (Fig. 2b). Sediments of the Lower Rutbah Formation studied in the wells X-1, X-3, and X-4 (Fig. 2b) consist of sandstone/shale and/or claystone sequences perceived to be deposited mostly in fluvial to estuarine channel depositional environments (Yousef et al., 2019). The sandstone intervals of this formation recorded helium porosity values up to 21% and horizontal permeability up to 3 000 mD (Fig. 3a).

    The Upper Cretaceous Post Judea Sandstone Formation is good to very good reservoir quality over the Euphrates Graben fields with total original thickness of about 100 m. Sediments of this formation have been studied in the wells: X-4, X-6, X-7, and X-8 (Fig. 2b), which consist of basal mudstone strata passing upwards into partly bioturbated sandstone bodies that reflect sediments deposited in fluvial to estuarine channels depositional environments that are subjected to coastal marine influence. The sandstone of the Post Judea Formation recorded helium porosity values up to 26% and horizontal permeability up to 6 000 mD (Fig. 3a).

    Regional information indicates that sediments of the Upper Triassic Mulussa F Formation over the Euphrates Graben were affected and eroded partially or completely by the erosion caused by the regional Base Lower Cretaceous Unconformity (BKL) (Figs. 2a, 2b), which led to varying thicknesses of these sediments over the Euphrates Graben (Yousef et al., 2021a). Additionally, the Lower Cretaceous Lower Rutbah and the Upper Cretaceous Post Judea Sandstone formations were affected and eroded partially or completely by the erosion caused by the regional Base Upper Cretaceous Unconformity (BKU) (Figs. 2a, 2b), which also played a factor in controlling the thickness of these formation sediments over the fields of the Euphrates Graben area (Yousef et al., 2021a).

  • Based on sedimentological investigations of the core intervals from the studied wells, two main lithofacies were encountered within the sediments of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations intervals. These include sandstone and claystone lithofacies. The claystone lithofacies were ignored in this study due to the purposes of the research; the sandstone lithofacies have been described, simplified, and summarized into four main distinct lithofacies based on the following research purposes.

  • Sediments of this lithofacies are common within the Mulussa F and the Post Judea Sandstone formations (Table 1). They consist of stacked vertical sandstone bodies varying in thickness between 6 and 20 m, with the potential to form stacked vertical sandstone bodies reaching up to 60 m in thickness (Fig. 4a). The XRD analyses of the whole sandstone sample (Fig. 4b) show that the detrital components of the sandstone are represented by quartz (from 67% to 87%), clay materials are up to 10% as maximum, and they consist mainly of kaolinite. Optical microscopy shows that the sandstone is medium to coarse-grained quartz arenite. The grains vary in size between 250 and 500 µm, and most of the grains are rounded to sub-rounded with moderately to well-sorted degrees (Fig. 4c). Felspars are rare in sandstone. During diagenesis, most of the feldspars become unstable and dissolved and are partially replaced by pyrite (Fig. 4d) or by kaolinite (Fig. 4e). The SEM examinations show that the sandstone grains are very often cemented with siliceous material, and they are rarely cemented with clay materials. The pore network is formed mainly by the intergranular porosity. The pores are well connected by channels (Figs. 4c, 4f). Some of the pores are partially filled with clay materials (Fig. 4e), others are not (Fig. 4f). Sediments of this lithofacies form moderate to good major reservoir units within the Mulussa F and the Post Judea Sandstone formations with helium porosity values ranging from 2.0% to 22%, and horizontal permeability values varying from 0.03 to 6 270 mD (see Fig. 3a).

    Wells Formations Lithofacies Cementation types Reservoir quality
    X-1 Lower Rutbah Lithofacies-3
    Lithofacies-4
    Dominantly quartz (up to 7.7%) with subordinate kaolinite (up to 5.0%), and minor sphalerite (up to 2.7%) Moderate to good
    X-2 Post Judea Sandstone Lithofacies-1
    Lithofacies-2
    Lithofacies-3
    Lithofacies-4
    Lithofacies-1 and 2 dominantly dolomite (up to 90%) with subordinate quartz (up to 14.7%), and kaolinite (10.3%); lithofacies-3 and 4 are dominated by quartz (up to 14.7%) with generally minor amounts of kaolinite or dolomite Lithofacies-1 and 2 negligible; lithofacies-3 and 4 moderates to good
    X-3 Mulussa F Lithofacies-1 Dominantly quartz (up to 7.3%) with significant amounts of kaolinite (up to 11.7%) siderite (up to 23.7%), and generally minor amounts of dolomite (up to 37.0%), baryte (up to 1.0%), anhydrite (up to 1.7%), and pyrite (up to 3.0%) Moderate to good
    X-4 Mulussa F Lithofacies-1
    Lithofacies-2
    Dominantly siderite (up to 38.0%) with significant amounts of quartz (up to 9.7%), kaolinite (up to 9.0%), and minor amounts of dolomite (up to 1.0%) and pyrite (up to 2.7%) Moderate to good
    X-5 Post Judea Sandstone Lithofacies-3
    Lithofacies-4
    Dominantly quartz (up to 19.3%) with minor amounts of kaolinite (up to 9.0%), dolomite (up to 9.3%), siderite (up to 2.7%), pyrite (up to 22.0%), and hematite (up to 4.7%) Good to very good
    X-6 Post Judea Sandstone, Lower Rutbah Lithofacies-1
    Lithofacies-2
    Lithofacies-3
    Post-Judea Sandstone dominated by quartz (3.7%–15.7%) except for one dolomite (40.0%) cemented sample. The Lower Rutbah is dominated by quartz (up to 2.7%) with minor amounts of siderite (up to 7.3%), dolomite (up to 1.7%), kaolinite (up to 1.7%), and pyrite (up to 4.7%) Post-Judea Sandstone is moderate to good; the Lower Rutbah is very good
    X-7 Post Judea Sandstone Lithofacies-3
    Lithofacies-4
    Dominantly quartz (up to 9.0%) with minor amounts of kaolinite (up to 8.3%), dolomite (up to 36.3%), and pyrite (up to 2.7%) Good to very good
    X-8 Lower Rutbah Lithofacies-3 Dominantly quartz (up to 4.0%) with minor amounts of kaolinite (up to 6.0%), dolomite (up to 0.3%), pyrite (up to 0.7%), and trace amounts of siderite Very good

    Table 1.  Summary of cementation types and distribution in the studied wells, formations, and lithofacies and their effects on reservoir quality

    Figure 4.  Representative core photographs, thin sections, and SEM photomicrographs showing (a) cross-bedded sandstone from sediments of lithofacies-1, (b) XRD analysis of the whole sandstone sample from lithofacies-1, (c), (d) quartz arenite of medium to coarse grain size with a well-connected pore network and pyrite replacement in some pores, (e) coarse-grained quartz arenite and dissolved feldspars are partially replaced by kaolinite, (f) SEM photomicrographs showing the pore network structure of the lithofacies-1 sandstone, (g) cross-bedded sandstone from lithofacies-2 sediment, (h) XRD analysis of the whole sandstone sample from lithofacies-2, (k) quartz arenite, very fine to medium-grained, with kaolinite partially filling the pore network, (l) SEM photomicrographs of kaolinite embedded in a porous network, (m) quartz arenite with a fine to medium grain size, the pore network is partially filled by dolomite, (n) fine to medium-grained quartz arenite with siderite partially filling the pores network.

  • Sediments of this lithofacies are common within the Mulussa F and the Post Judea Sandstone formations (Table 1) and consist of sandstone bodies that vary in thickness between 1 and 3.5 m. The sandstone bodies are separated by thin clay interbeds (Fig. 4g). The common preserved sedimentary structures are cross- bedding. The XRD analysis of the whole sandstone sample (Fig. 4h) showed that the detrital components of the sandstone are represented by quartz (53% to 75%), while clay materials are up to 22% consisting mainly of kaolinite (Fig. 4h). Optical microscopy shows that the sandstone composed of very fine to medium- grained quartz varies in size from 100 to 200 µm. The grains are sub-rounded to angular with bad to moderate sorting (Fig. 4k). Cementation materials between the quartz grains are composed of clay mainly by kaolinite (Figs. 4k, 4l), dolomite (Fig. 4m), and siderite (Fig. 4n) also locally forms the main cementation materials between the detrital sandstone components. The pore network is formed mainly by the intergranular porosity. They are moderate to well connected by channels, but most of them are occupied by clay, dolomite, and/or siderite materials. Sediments of this lithofacies form moderate to good major reservoir units within the Mulussa F and the Post Judea Sandstone formations with helium porosity values varying from 2.0% to 17% and horizontal permeability values varying from 0.08 to 78 mD (see Fig. 3a).

  • Sediments of this lithofacies are dominant within the Lower Rutbah and the Post Judea Sandstone formations (Table 1). They consist of medium to coarse-grained quartz arenite (Figs. 5a, 5b), varying in size from 250 to 500 µm (Figs. 5c, 5d). The clay materials within the sandstones vary from terraces up to 5%, and rarely reach 10%, according to optical microscopic analysis. The intergranular networks are formed mainly by the intergranular porosity. They are well connected by channels. Some of them are partially filled with cementation clay materials (Fig. 5c), others are filled with residual hydrocarbons (Fig. 5d). Some of the quartz grains are dissolved during the diagenesis process, which leads to the generation of secondary dissolution porosity (Fig. 5d). Sediments of this lithofacies form major reservoir units within the Lower Rutbah and the Post Judea Sandstone formations with helium porosity values varying from 4% to 18.7%, and horizontal permeability values varying from 0.25 to 4 370 mD (see Fig. 3a).

    Figure 5.  Representative core photographs and thin section photomicrographs showing (a), (b) coarse-grained sandstone from sediments of lithofacies-3; (c) medium to coarse-grained quartz arenite with a well-connected pore network that is partially filled with kaolinite; (d) medium to coarse-grained quartz arenite, secondary dissolution pores are well connected and filled with bitumen; (e), (f) fine to medium-grained sandstone from sediments of lithofacies-4; (g) fine to medium-grained sandstone with a well-connected pore network; (h) sandstone, very fine to coarse-grained, with a pore network partially filled with kaolinite or pyrite. Q. Quartz; P. pore; Py. pyrite; K. kaolinite; Be. bitumen.

  • Sediments of this lithofacies are common within the Lower Rutbah and the Post Judea Sandstone formations (Table 1). They consist of cross-bedded sandstone bodies that are separated by thin clay interbeds (Fig. 5e). Locally, evidence of bioturbation exists (Fig. 5f). Based on optical microscopic examinations, the sandstones are composed of very fine to medium-grained quartz that varies in size from 100 to 200 µm (Figs. 5g, 5h). Cementation materials between the quartz grains are composed of siliceous materials (Fig. 5g), while clay forms cementation materials between the grains (Fig. 5h). The intergranular porosity is sometimes occupied by clay materials and/or by overgrown quartz. Some of the pores are well connected, others are not. Sediments of this lithofacies form minor reservoir units within the Lower Rutbah and the Post Judea Sandstone formations with helium porosity values varying from 0.3% to 19.7%, and horizontal permeability values varying from 0.01 to 673 mD (see Fig. 3a).

  • Based on the sedimentological observations of the different encountered lithofacies, sediments of lithofacies-1 are interpreted as fluvial channel deposits (Fig. 6a). This is indicated by the thickness scale and the vertically stacked pattern of the sandstone bodies. Furthermore, the cross-bedding stratification, grain size/sorting, and the low percentage of clay materials between the detrital grains provide evidence of the deposition from the fluvial channel.

    Figure 6.  Representative histogram showing the dispositional environments of the different lithofacies encountered within the sediments of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations.

    Sediments of lithofacies-2 are interpreted as products of minor fluvial channels, probably crevasse channels (Fig. 6b) related to breaching of the major fluvial channels (lithofacies-1). The small scale of the cross-bedding stratification and the average thickness of the sandstone bodies, the grain size, and the high percentage of the clay materials between the sandstone grains suggest that these sediments formed in crevasse channels. The thickness scale of the sandstone bodies of lithofacies-2 suggests that the crevasse channels were relatively shallow (Miall, 2007).

    The presence of the clay-rich laminae within sediments of the lithofacies-2 suggests a somewhat pulsatory discharge from the main fluvial channel (Liang et al., 2019; Horn et al., 2012). The presence of the disseminated siderite within sediments of the lithofacies-1 and lithofacies-2 probably reflects the mobilisation of the iron under organic-rich, reducing conditions (Ma et al., 2018; Grimm and Eriksson, 2013). Bacterial processes were possibly effective in the floodplain muds, forming siderite spherulites by progressively removing all oxygen and sulphate ions. The abundance of the siderite suggests that the paleosols were probably subjected to periods of saturation from a high-water table (Dalrymple and Choi, 2007). Sediments of lithofacies-3 and lithofacies-4 are interpreted as fluvial to estuarine channel deposits (i.e., fluvial channels that are subject to some coastal marine influence) (Fig. 6c). The features of the slight bioturbation within sediments of the lithofacies-4 suggest a depositional environment close to the coastal marine environment or located near the coastline or the delta region where biological activities are high (Yousef et al., 2021a, b, c; Song et al., 2019).

  • Different types of cementation patterns were recognized within the sandstones of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations. These include (Table 1) quartz cement, kaolinite cement, dolomite, and siderite cement. These cements are common and found in large quantities in sandstones. Other cements such as anhydrite, pyrite, baryte, hematite, and sphalerite are not so common and occur in minor amounts. Quartz is generally the dominant cement phase recorded in the form of overgrowth cements on detrital quartz grains, consisting of both small euhedral crystals and larger, well-developed tabular (Figs. 7a, 7b) or pyramidal crystals which can extensively interlock and merge to completely occlude intergranular pore throats (Fig. 7c). Alternatively, quartz overgrowths, well developed, can act to smooth pore walls and resist grain compaction, therefore preserving porosity and enhancing reservoir potential (Fig. 7d). The typically high quartz cement content (up to 19.3% with a mean of 3.1%, see Table 1) and the apparent absence of any significant grain dissolution would suggest an extra-formational origin for the silica-rich fluids (Molenaar et al., 2007; Worden and Morad, 2000; Bjørlykke and Egeberg, 1993). Cements that occur before the quartz cement in the diagenetic sequence, such as siderite and kaolinite, are important as their presence can locally restrict the later development of the quartz cement. Most of the siderite within the sandstones occurs in the form of cement between detrital quartz grains and shows signs of dissolution. This dissolution process indicates an imbalance with subsequent pore fluids (Morad, 1998). Siderite is also observed in the form of blocky elongated crystals that fill the primary and secondary pores (Fig. 7e). Siderite is also recorded as sphaerosiderite nodules (up to 100 μm) growing over the quartz grains and showing rhombohedral cleavage (Fig. 7f). This type of siderite may act as fabric replacive cement distributed within the intergranular porosity and reduce the pore connectivity (Pye et al., 1990). Kaolinite is the main clay mineral phase. The high kaolinite content in sandstones most likely indicates intense chemical weathering in a warm, humid climate (Aung et al., 2015). Kaolinite cement typically consists of localized vermicular aggregates within the secondary dissolution pores (replacing the unstable framework grains), or possibly redistributed into primary intergranular pores (Figs. 7g, 7h, 7k). Although the kaolinite cluster has microporosity varying between 5 and 15 µm, it significantly reduces the pore connectivity between the detrital grains.

    Figure 7.  Representative SEM photomicrographs showing (a) quartz cementation. Increments of growth/precipitation are recognized by overlapping crystal textures, the quartz crystals (Q) seal connecting pore throats, and often totally occlude primary pore space; (b) a remnant pore structure (P), which is usually open and well connected, with pore networks primarily sealed by interlocking quartz overgrowths cement (arrowed); (c) large euhedral quartz overgrowths (Q) form on the grain surfaces, displaying prismatic and pyramidal crystalline forms, with large quartz overgrowth shard fragments (arrowed) filling the primary pore; (d) authigenic quartz overgrowth on the grain surfaces, as well as slightly smoothing the pore (P) wall surfaces; (e) the relationship between the quartz (Q) and siderite (S) cements, the quartz occurs as large, interlocking tabular crystals, overgrowing rhombohedrally cleaved siderite; (f) sphaerosiderite nodules cement of siderite (S) within an intergranular pore, and coat the quartz (Q) overgrowth surfaces; (g) intergranular pore occlusion by interlocking quartz (Q) overgrowths and authigenic kaolinite (K) assemblages consist of < 15 µm sized euhedral plates and blocks, arranged in vermicular stack structures; (h) kaolinite cement, partially occluding primary porosity and coating the overgrowth quartz (Q), the small cluster of the kaolinite is composed of partially disaggregated vermicular stacks and loosely aggregated platy crystals, showing evidence of remobilization; (k) kaolinite cement, partially occluding the primary porosity and coating the quartz (Q) overgrowth surfaces. Q. Quartz; P. pore; Py. pyrite; S. siderite; K. kaolinite.

    The small aggregates of the authigenic kaolinite fill the cavities in the quartz grains and are surrounded by the outgrowths of polycrystalline quartz (Fig. 7g). Dolomite is the main carbonate mineral phase. Most of the authigenic dolomite is found in the form of a widespread poikilitic cement that fills the intergranular pore spaces, encloses, and postdates the quartz (Fig. 8a). Some of the dolomites found in the form of microcrystalline cement fill the fracture (Fig. 8b), or in the form of scattered rhombic crystals or nodules occluding pore spaces (Fig. 8c). Although dolomite postdates quartz precipitation and has not, therefore, influenced the movement of quartz-rich pore fluids, it may reduce pore volume and restrict inter-pore connectivity (Swart and Melim, 2000).

    Figure 8.  Representative thin sections and SEM photomicrographs showing (a) dolomite cement (D) filling and occluding the primary and secondary porosity between the quartz grains; (b) scattered rhombic dolomite fillings cement within the porosity of the secondary fracture; (c) dolomite cement occluding scattered rhombic crystals, nodules, and pervasive pores; (d) anhydrite cement (An) patches scattered throughout the intergranular filling and occluding some pore spaces; (e) anhydrite, siderite, and kaolinite, the anhydrite predating the kaolinite and overgrowth quartz; (f) intergranular porosity with early anhedral pyrite crystals (Py); (g) 5 to 15 m late nodular pyrite over quartz and within pore spaces; (h) scattered rhombic crystals of anatase (Ana); (k) hematite staining (Hm) of detrital clay. Q. Quartz; P. pore; S. siderite; K. kaolinite; D. dolomite; Py. pyrite; Hm. hematite; . An. anhydrite; Ana. anatase.

    Sulfate cements only occur in the Post-Judea Sandstone and the Mulussa F formations and consist of late poikilotopic anhydrite, gypsum, and baryte cements. Anhydrite is a later mineral among the discovered authigenic minerals, considered a late diagenetic mineral. Anhydrite usually forms spots of poikilitic cement filling the pores (Fig. 8d) and mainly replaces feldspars, clay, and carbonate minerals and/or encloses the overgrowth quartz (Fig. 8e). Sulphides are represented by ubiquitous early anhedral pyrite moderately to well-crystallized crystals in the pore spaces, replacing the clay material that fills the pores (Fig. 8f). Pyrite is also found in the form of individual poikilitic framboids of early diagenesis with a diameter of 5 to 10 μm growing over the overgrowth quartz and partially closing the pore spaces (Fig. 8g). Minor amounts of anhedral authigenic anatase (Fig. 8h) and occasional hematite staining of detrital clays are observed within the intergranular pores of the sandstones (Fig. 8k). Hematite precipitates under oxidising conditions on some quartz and feldspar grain surfaces, visible as reddish-brown rims on grain surfaces. This reveals that the formation of these grain coatings occurred during early diagenesis through iron-rich pore-water.

  • Diagenetic sequences are essentially very similar for the sandstones of the Post Judea Sandstone, the Lower Rutbah, and the Mulussa F formations, although some minor differences are noted between the studies wells. For example, sphaerosiderite is only recorded in the Mulussa F Formation, sulphates are absent from the Lower Rutbah Formation, early dolomite occurs in the Post Judea Sandstone fluvial sequence in well X-2. A generalized diagenetic sequence for sandstones of the Post Judea Sandstone, the Lower Rutbah, and the Mulussa F formations consists of the following steps illustrated in Fig. 9.

    Figure 9.  A generalized diagenetic sequence of the sandstones for the Post Judea Sandstone, Lower Rutbah, and Mulussa F formations. Evidence besides thin section; (a) infiltration of detrital clays; (b) the early stage of authigenic pyrite formation; precipitation of (c) the siderite, (d) vermicular kaolinite, (e) syntaxial quartz overgrowth; (f) early-stage of oil emplacemen; precipitation of (g) dolomite, (h) sulphates, (k) sulphides; (j) late-stage of oil emplacement; (m) compaction and fracture; (n) dissolution. Q. Quartz; B. bitumen; S. siderite; K. kaolinite; D. dolomite; Py. pyrite; An. anhydrite.

    1. Infiltration of detrital matrix clays at deposition time of the sediment in the original depositional environments, and later were either removed by etching or replaced by the authigenic minerals. 2. The formation of authigenic pyrite in sandstones is those that favor biodegradation under reducing conditions during very early diagenesis, due to the interaction of sulfate-reducing bacteria, organic matter and iron-bearing clays in the sediments that may have produced hydrogen sulphide (H2S) and generated local high alkalinity (Berner et al., 2013). In sedimentary rocks, framboidal pyrite is generally considered to be produced in the synsedimentary stage (Yue et al., 2020; Jiang et al., 2016) or early diagenetic stage (Taylor and Macquaker, 2000). Due to its earlier formation and unique structures, the pyrite framboids are likely sites for continued pyrite precipitation during burial as long as the availability of iron and sulphur is sufficient. 3. Precipitation and development of the siderite following pyrite precipitation, this was under anoxic conditions when the rate of iron reduction exceeded the rate of sulphate reduction, additionally, oxidising conditions resulting in the alteration of clay materials to hematite and the associated reddening of sediment. 4. Intrastriatal hydrolysis of clay minerals and development of secondary porosity associated with the dissolution. 5. Precipitation of vermicular kaolinite requires more acid pore waters with sufficient ions K+, Si+4, and Al+3. These ions are largely derived from the alteration of detrital minerals, in particular clay minerals and feldspars. 6. Precipitation of syntaxial quartz overgrowth cements on detrital quartz grains. 7. The first stage of oil emplacement, associated with quartz cementation. 8. Precipitation of dolomite cement and occluding partially or completely the intergranular porosity. 9. Precipitation of sulphates (anhydrite, gypsum, and baryte). 10. Precipitation of sulphides (pyrite, and sphalerite). 11. The late or the second stage of the oil emplacement.

  • In the process of studying cement distribution within sandstones of the Mulussa F, the Lower Rutbah, and the Post Judea Sandstone formations, cements were classified into three types: total cements, quartz cement, and other cements. The terms used in this section (Pommer and Milliken, 2015) are: 1. total cements= the sum of all cements that occupy the intergranular volume; 2. quartz cement=quartz cement that occupies the intergranular volume; 3. other cements=total cements–quartz cement; 4. intergranular volume=intergranular porosity+total cements; 5. intergranular porosity=intergranular volume–total cements.

  • The Post Judea Sandstone Formation in the studied wells consists of sequences lithofacies-3, and 4, although, fluvial deposits, alluvial fan lithofacies-1, and 2 are recorded in the well X-6 (Table 1). Figure 10a illustrates the distribution of the total cements within sandstones of the Post Judea Sandstone Formation, showing a large range in cement volumes (0.61% to 40.0%) with a mean of 8.5%. Figure 10d illustrates quartz cement distribution within sandstones of the Post Judea Sandstone Formation, it shows a long positively skewed tail and several of these lithologies are classified as siliceous quartz arenites, the mean value for the quartz cement (4.1%) is significantly higher than for both Mulussa F Formation sandstones (Fig. 10e) and Lower Rutbah Formation sandstones (Fig. 10f). Although quartz cementation is significantly more common in the sandstones of the Post Judea Sandstone Formation, the total cement volume is not as high as in the Mulussa F Formation sandstones (Fig. 10b). Figure 10g illustrates the distribution of other cements within the sandstones of the Post Judea Sandstone Formation, these generally consist of kaolinite, siderite, dolomite, and baryte (< 5%) with occasional dolomite-rich (> 15%), the mean value for the other cements is 4.4%, comparatively lower than in the Mulussa F Formation sandstones (Fig. 10h) but higher than in the Lower Rutbah Formation sandstones (Fig. 10i). The distribution of the intergranular volume within sandstones of the Post Judea Sandstone Formation (Fig. 10j) represents the amount of available pore space before cementation commenced. The intergranular volume of the sandstones of the Post Judea Sandstone Formation (mean 18.8%) is comparatively lower than that in the Mulussa F Formation sandstones (mean 19.8%) (Fig. 10k) which has the highest total cements value but is higher than in the Lower Rutbah Formation sandstones (mean 12.9%) (Fig. 10l). This suggests that cementation within sandstones of the Mulussa F Formation has been more effective at occluding pore volume than in sandstones of the Post Judea Sandstone Formation. Consequently, the intergranular porosity of the sandstones of the Post Judea Sandstone Formation (mean 10.3%) (Fig. 10m) is higher than in the Mulussa F Formation sandstones (mean 9.4%) (Fig. 10n) and in the Lower Rutbah Formation sandstones (mean 9.8%) (Fig. 10o).

    Figure 10.  A plate showing the distribution of the total cements within sandstones of (a) Post Judea Sandstone, (b) Mulussa F, and (c) Lower Rutbah formations; quartz cement within sandstones of (d) Post Judea Sandstone, (e) Mulussa F, and (f) Lower Rutbah formations; other cements within sandstones of (g) Post Judea Sandstone, (h) Mulussa F, and (i) Lower Rutbah formations; the intergranular volume within sandstones of (j) Post Judea Sandstone, (k) Mulussa F, and (l) Lower Rutbah formations; the intergranular porosity within sandstones of (m) Post Judea Sandstone, (n) Mulussa F, and (o) Lower Rutbah formations.

  • Sandstone cored intervals from the Lower Rutbah Formation in the studied wells consist typically of lithofacies-3, and 4. However, in well X-4, two samples represent fluvial lithofacies-1 (good reservoir). These fluvial lithologies contain little authigenic cement, but include minor amounts of pyrite and anatase, and quartz cement is absent. Overall, the total cement volume for the sandstones of the Lower Rutbah Formation is considerably lower (0.04% to 6.7%, mean 3.1%) (Fig. 10c) than in the Mulussa F Formation sandstones (Fig. 10b) and the Post Judea Sandstone Formation sandstones (Fig. 10a). Where diagenesis is important, cementation in the Lower Rutbah Formation sandstones is dominated by overgrowths quartz cement (up to 15.7%, mean 1.8%) (Fig. 10f), but in comparison with sandstones of the Mulussa F Formation (Fig. 10e), and the Post Judea Sandstone Formation (Fig. 10d), quartz cement is significantly lower.

    Other cements, i.e., minor amounts of kaolinite and dolomite (generally < 5%, mean 1.4%), are also recorded within the Lower Rutbah Formation sandstones (Fig. 10i). The intergranular volume is also generally lower in the Lower Rutbah Formation sandstones (mean 12.9%) (Fig. 10l), compared to the sandstones of the Mulussa F Formation (mean 19.8%) (Fig. 7k), and the Post Judea Sandstone Formation (mean 18.8%) (Fig. 10j), which may reflect finer grain size and higher compaction. As a result, the intergranular porosity levels in the sandstones of the Lower Rutbah Formation (mean 9.8%, Fig. 10o) are comparable with the Mulussa F Formation sandstones (mean 9.4%) (Fig. 10n), and the Post Judea Sandstone Formation (mean 10.3%) (Fig. 10m), which show a much greater diagenetic overprint and higher cement volumes. However, the cements have been less effective at reducing pore volumes and the intergranular porosity is generally good, with a mean value of 9.8% (Fig. 7o), this is higher than in Mulussa F Formation sandstones (Fig. 10n) and close to the Post Judea Sandstone Formation (Fig. 10m). Although dominantly fluvial to estuarine channels and only fluvial lithologies are recorded in the Lower Rutbah Formation, the diagenesis data suggest that quartz authigenesis has not occurred in these fluvial lithofacies assemblages. The kaolinite and dolomite cementation that is recorded probably reflects meteoric fluid processes in the early burial diagenesis (Warren, 2000).

  • Sandstone cored intervals from the Mulussa F Formation in the studied wells are characterized by both fluvial lithofacies-1 (good reservoir) and lithofacies-2 (poor reservoir). Diagenesis is predominantly facies-controlled, with cementation dominated by a combination of early siderite (up to 38.0%) in either sphaeronodular or acicular forms, and kaolinite (up to 11.7%), syn-depositional fluid reactions and meteoric fluid migration in early burial. Subsequent quartz cementation during burial has generally been less significant than these early cements.

    Figure 10b illustrates the distribution of the total cements within sandstones of the Mulussa F Formation, showing a large range of values (0.6% to 43.3%) with a mean of 10.1%. Quartz cement within the Mulussa F Formation sandstones (Fig. 10e) is generally low with a small range (0 to 9.7%) and a mean of 3.0% and is generally less significant in porosity destruction than the earlier siderite and kaolinite cements. Figure 10h illustrates the distribution of other cements and shows a large range in values (0.06% to 42.6%) with a mean of 7.4%, but a concentration of samples occurs at low cement volume (0 to 4%). The intergranular volume of the Mulussa F Formation sandstones (Fig. 10k) is generally high, up to 46.3% (mean 19.8%). However, in many sequences in the Mulussa F Formation, initial pore volumes of the sandstones were severely occluded at an early burial stage by siderite and kaolinite, which restricted or prevented extensive quartz overgrowth development. Intergranular porosity, which reflects the remnant pore volume after the effects of both cementation and compaction, is shown in Fig. 10n, recording a range of 0 to 23.7% and a mean of 9.4%.

    Comparing the cementation characteristics of the Mulussa F Formation fluvial sequences lithofacies with similar lithofacies in the Post-Judea Sandstone Formation in well X-06, quartz cementation is equally restricted in the latter. In this case, the low quartz cement volume (< 6.0%) reflects early porosity destruction by dolomitization, and therefore restricts the effective migration of SiO2 rich fluids during the deep burial diagenesis (Burley, 1993).

    Quartz cementation appears to be less well developed in the fluvial sequences lithofacies in the Euphrates Graben area, and more evident within the fluvial to estuarine channels lithofacies. Notably, early kaolinite is not extensive (dominantly < 1.0%) in the fluvial sequence's sandstones of the Post Judea Sandstone Formation in the well X-06, compared to the Mulussa F Formation. This must reflect a difference in meteoric fluid processes and clay-diagenesis reactions in early burial diagenesis stages between these two locations (Jeans, 1989). Early kaolinite is expected to be better developed in floodplain environments (well X-02) with a constant recharge of meteoric water. In alluvial fan situations (well X-06), water run-off may be more sporadic and/or high energy, thus preventing early clay reactions (Bjørlykke et al., 1989).

  • Figure 11a illustrates the relationship between the quartz cement volume and the present burial depth, cross-referenced with the different formations. As a result, very little difference in the volume of the total cement is evident with depth in any of the studied formations. In the Lower Rutbah Formation, slightly higher quartz cementation (4% to 10%) is evident below the depth 3 700 m measure depth (MD). In the Mulussa F Formation and the Post Judea Sandstone Formation, there is no evident depth controls on cement distribution and volume.

    Figure 11.  Diagrams showing the relationships between (a) the burial depth and the quartz cement distribution cross-referenced with the different formations; (b) the helium porosity and the horizontal permeability for the kaolinitic, dolomitic, and sideritic sandstone samples; (c) the helium porosity and the horizontal permeability for the clean sandstone (wackes, dolomitic, kaolinitic, and sideritic samples removed).

  • Reservoir quality is controlled by a combination of primary textural characteristics (grain size, sorting, and detrital clay content) and secondary modifications, compaction, and cementational features (Zhou et al., 2019; Bjørlykke et al., 1989). The main cementation factors are the abundance and distribution of the quartz, carbonate, and kaolinite cements which are discussed in this section. The main reservoir units are associated with lithofacies-1 (typically Mulussa F Formation) and lithofacies-3 (typically Post Judea Sandstone, and Lower Rutbah formations). These sequences generally exhibit high horizontal permeability and helium porosity values (see Fig. 3a). The poor reservoirs are associated with the fluvial lithofacies-2 (typically Mulussa F Formation) and the lithofacies-4 (typically Post Judea Sandstone and Lower Rutbah formations). These sequences are typically finer-grained and detrital clay-rich, therefore generally exhibiting low horizontal permeability and helium porosity values (see Fig. 3a). The Mulussa F Formation lithofacies-1 are meandering channel sequences where authigenic cements are significantly more extensive than in sequences of the Lower Rutbah, and the Post Judea Sandstone formations. The cements comprise a combination of locally significant quartz overgrowths, acicular or sphaeronodular siderite, and vermicular kaolinite (mean total cements 10.1%) (see Fig. 10b), they locally exert a moderate to major detrimental control on reservoir permeability. Consequently, reservoir quality for the Mulussa F lithofacies-1 sequences is variable but generally considered to be moderate to good (see Table 1). The Mulussa F Formation lithofacies-2 sequences are only present in the well X-5, these samples are typically cemented by extensive siderite and/or kaolinite resulting in low helium porosity and horizontal permeability values (Fig. 11b), consequently, reservoir quality is predicted to be poor. The Post Judea Sandstone Formation is dominated by lithofacies-3 and 4, but a small amount of fluvial lithofacies-1 and 2 samples are recorded in the well X-6. These samples have been taken from an alluvial fan sequence where the dominant cement phase is poikilotopic dolomite. This pervasive dolomite cement typically severely occludes the pore networks and exerts a major detrimental control on the reservoir permeability and therefore reservoir quality for these samples is predicted to be low. The Post Judea Sandstone Formation sequences are dominated by lithofacies-3 in which cement volumes are relatively high (mean 8.5%). The cements are dominated by quartz overgrowths, which typically act to enhance the reservoir quality by resisting compaction and smoothing pore walls. However, when quartz content exceeds about 10%, the overgrowths commonly merge and act to reduce pore volume and connectivity (Fig. 11c). The Post Judea Sandstone Formation lithofacies-3 samples typically possess the highest helium porosity and horizontal permeability values and reservoir quality is predicted to be good. The Post Judea Sandstone lithofacies-4 sequences are typically very similar to lithofacies-3 except for some silty samples in the wells; X-6, X-7, and X-8 in which reservoir quality is low. Comparing the reservoir characteristics of the similar sequences of the Post-Judea Sandstone and the Lower Rutbah formations, the Lower Rutbah lithofacies-3 typically contain significantly lower cement volumes (mean 3.1%) than the Post Judea Sandstone Formation. The cements are dominated by locally well-developed quartz overgrowths which commonly enhance the pore connectivity by reducing compaction to some extent and smoothing pore walls. Therefore, reservoir quality is predicted to be good to very good. However, the present intergranular porosity, helium porosity, and horizontal permeability values are lower than the values recorded for the Post Judea Sandstone Formation lithofacies-3. This difference is probably a result of smaller grain size and greater compaction (Figs. 12a, 12b) within the Lower Rutbah Formation sequences. The Lower Rutbah lithofacies-4 is typically clay-rich, as a result of low energy levels within the depositional environment, consequently reservoir quality is predicted to be poor. Figure 12c illustrates the relationship between the intergranular porosity and quartz cement volume, again referenced against individual formations. Figure 12d illustrates the relationship between the intergranular porosity and total cement volume, again referenced against individual formations. In both plots a lot of data scatter is evident, but it appears that overall porosity is reduced with increasing total cement volume, additionally, porosity is sometimes better preserved where quartz cement increases (over the 0 to 10% range). The increased porosity in quartz-cemented lithologies, probably reflects early overgrowth development, arresting grain compaction during burial. Provided subsequent cementation (siderite, dolomite, kaolinite) is restricted, porosity and reservoir permeability will remain reasonably high.

    Figure 12.  Houseknecht diagrams illustrating (a) the effects of compaction and cementation for all the studied formations; (b) the effects of compaction and cementation for all the studied wells; (c) the relationship between the quartz cement volume and the intergranular porosity for the studied formations; (d) the relationship between the total cement volumes and the intergranular porosity for the studied formations.

  • This article summarized cementation characteristics and their effect on sandstone reservoirs quality of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations in selected fields at Euphrates Graben, Syria. The quartz cement within the sandstones of the studied formations comprises well to developed syntaxial overgrowth within the pore spaces or over the quartz grains surfaces. The siderite cement is most abundant within the Mulussa F Formation sandstones with sphaerosiderite being exclusively recorded therein. The dolomite cement is recorded in all formations but is most extensive within the sandstones of the Post-Judea Sandstone Formation. Kaolinite comprises vermicular aggregates which are most abundant within the sandstones of the Mulussa F Formation. Reservoir units of the Post-Judea Sandstone and the Lower Rutbah formations are typically lithofacies-3 sequences in which quartz overgrowths are the dominant cement phase. Total cements are more extensive within the Post-Judea Sandstone Formation than in the Lower Rutbah Formation. The porosity and permeability are highest in the sandstones of the Post-Judea Sandstone Formation. Consequently, the sandstone reservoir quality of the Post-Judea Sandstone Formation is predicted to be best. The reservoir units of the sandstones of the Mulussa F Formation generally contain the highest cement volumes of all three formations. Cements comprise of early siderite and kaolinite.

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