Quartz Wacke Research Articles

Petrography and Geochemistry of the Lower Sylhet Sandstone Member (Therria Sandstone) of Jaintia Group Exposed Along Jowai-Badarpur Road Section, Jaintia Hills District, Meghalaya: Implication for Provenance and Palaeoclimate

ABSTRACT The Paleogene sediments of the Shillong Plateau are exposed in the southern part of Meghalaya and are designated as the Jaintia Group. The Lower Sylhet Sandstone Member (LSSM) (Therria Sandstone) is the lowermost member of the Shella Formation of Jaintia Group. To understand the depositional history of the studied sandstones, petrographic and geochemical approaches have been made. Petrographically, these sandstones are classified as quartz arenite and quartz wacke. They were derived from intermediate to upper rank metamorphic and plutonic source which were deposited in a craton interior and quartzose recycled tectonic setting. The paleoweathering indices of CIA, CIW, PIA and ICV indicate that the weathering at the provenance was intense. The tectonic discrimination diagrams suggest passive margin setting for their sedimentation and climatic condition during the deposition was humid in nature. The studied sediments were deposited in non-marine conditions as evidenced by the geochemical proxies and primary sedimentary structures existing in the LSSM.

Petrophysical rock typing based on the digenetic effect of the different microfacies types of Abu Madi clastic reservoir in Faraskur Gas Field, onshore Nile Delta, Egypt

The sandstone of Abu Madi clastic reservoir constitutes the prime gas-producing reservoir in the Nile Delta region. So, the present research aims to delineate its microfacies and dominant diagenetic features and their effect on the petrophysical characteristics of the Abu Madi reservoir in the Faraskur Gas Field. This could be achieved by integrating the petrographical studies and the conventional data to divide the reservoir into some reservoir rock types (RRTs) to delineate the impact of the mineral composition and diagenetic features on their reservoir quality and hydraulic flow zones. Petrographically, most samples are immature and described as angular to subrounded, poorly sorted, and slightly cemented. Five clastic microfacies types are recorded in the Abu Madi Formation: (1) quartz arenite, (2) feldspathic quartz arenite, (3) sub-feldspathic quartz arenite, (4) quartz wacke, and (5) sub-feldspathic quartz wacke. The most important post-depositional processes affecting the reservoir quality are the precipitation of pyrite and microsparite, as well as the feldspars alteration into authigenic dispersed clays represented by kaolinite and smectite/illite mixed layers. The dominant pore types are the primary intergranular, with some subsidiary fractures and dissolution pores. To check the reservoir quality of the Abu Madi clastics, the NPI (normalized porosity index), FZI (flow zone indicator), RQI (reservoir quality index), and λk (permeability anisotropy) were estimated. Petrophysically, the studied samples are summed up into four RRTs, with the RRT4 group has the lowest ∅ and k values (av. ∅ = 16.2%, av. kH = 6.87 md) and the lowest reservoir quality parameters (av. FZI = 0.89 μm, av. DRT = 10.4, av. RQI = 0.18 μm, av. R35 = 1.26 μm). On the contrary, the RRT1 group has the best petrophysical and reservoir parameters (av. ∅ = 25.9%, av. kH = 2695 md, av. DRT = 14.9, av. RQI = 3.12 μm, av. FZI = 8.85 μm, av. R35 = 32.7 μm). The permeability anisotropy of the RRT2 and RRT3 is the highest in Faraskur Field (av. λk = 2.4 and 2.62, respectively). This study is applicable to other extensions of the Abu Madi reservoirs in the Nile Delta basin to help in predicting their reservoir quality, petrophysical properties, and their hydrocarbon potentiality.

Implication of the micro- and lithofacies types on the quality of a gas-bearing deltaic reservoir in the Nile Delta, Egypt

The Messinian Qawasim sequence, as one of the most important reservoirs in the Egyptian Nile Delta, represents a typical gas-bearing deltaic reservoir sequence. It aims at delineating the implication of the litho- and microfacies associations on the reservoir characteristics of the different stages of the fluvial deltas including the prodelta, proximal/distal delta front, and the delta plain depositional sequences. The studied reservoir sequence was divided into two units; upper and lower clastic units. The petrophysical properties of these two units were studied on the borehole scale using the gamma-ray, caliper, sonic, density, neutron, and resistivity logs to estimate the reservoir parameters including the total and effective porosities, water saturation, shale volume, and the net-pay thickness. For more details, they are also examined on the plug-scale using the core dataset including helium porosity, density, permeability, and fluids saturations, where the flow zone indicator, the reservoir potentiality index, the reservoir quality index, and the average reservoir pore radius were then estimated. The entire sequence is divided into five reservoir rock types (RRT1-RRT5) where, the best quality is assigned to the first RRT (the upper reservoir unit), and the lowest quality is assigned to the RRT5 of the lower reservoir unit. Based on core description and the petrographical studies five lithofacies and four microfacies have been identified. The lithofacies are (1) laminated mudstones/siltstones, (2) ripple laminated sandstones, (3) cross-laminated sandstones, (4) cross-bedded sandstones, and (5) pebbly massive sandstones. These lithofacies are primarily composed of four microfacies; sublithic arenites, subarkose arenite, glauconitic quartz wacke, and sandy mudstone/siltstone. Based on this study, the pebbly massive and the cross-bedded sandstones of the upper unit which is composed of the sublithic and subarkose arenites (RRT1-RRT2) have the best reservoir quality. On the other hand, the lowest quality is assigned to the RRT5 (sandy mudstone/siltstone microfacies) which is represented on the macro scale by the laminated mudstones/siltstones lithofacies. The integration between multi-scale datasets (core-well, petrography, well logs, and seismic) gives a precise picture of the deltaic Qawasim reservoir rock units in the Nile Delta. This workflow has never been applied to the deltaic system in the onshore Nile delta and North Africa. Thereby, this study is considered a standard case study for the deltaic sequences and its proposed workflow is applicable to the Nile Delta elsewhere and worldwide for similar reservoir sequences.

Pedaro formation, equivalent of Plover sandstone at Savu Island, Outer Banda Arc

Plover Sandstone have been widely known as a good quality of oil and gas reservoir in NW Australia. As the continuity of NW Australia margin, outer Banda Arc become the distribution area of the equivalent of Plover Sandstone units. Therefore, a clear distinction and characterization of equivalent of Plover Sandstones distributed in this area is needed. Thick unit of quartz rich sandstone is scrap out in south Savu Island. Refers to the location where the lithology is found widely distributed, the unit is suggested to be called as Pedaro Formation. The characteristic of the lithology is determined through detailed measured section from two trajectories and laboratory analysis, including fourteen samples of petrography analysis, eight samples of Scanning Electron Microscope (SEM) analysis, and three samples of X-Ray Difraction (XRD) analysis. The lower part of the unit is initiated by braided fluvial conglomerates which gradually become tidal sand flat association of shale layers, coal seams and sandstone insertion. While the upper part of the unit is consisting of thick bedded quartz sandstone with thin siltstone insertion, deposited in the shoreface environment. Those facies association developed at transgressive conditions in the estuarine environment. The characteristic of the sandstone unit of Pedaro Formation is thickly bedded quartz wacke to quartz arenite, white to light grey in color, moderately to very well sorted, mostly mature sand. Pedaro Formation is identified to be deposited at Early Jurassic in interior craton tectonic setting. The characterization of sandstone unit of Pedaro Formation is showing that the unit can be correlate to the equivalent of Plover Sandstone found in Timor. Sandstone unit of Pedaro Formation can play a role as reservoir candidate in petroleum system of Savu and surrounding area.

Integrated sedimentological and petrophysical characterization of the Lower Cenomanian clastic Bahariya reservoir in Abu Gharadig Basin, Western Desert, Egypt

The Lower Cenomanian siliciclastic Bahariya Formation represents one of the main producing reservoirs in the north Western Egyptian Desert. A few previous subsurface studies exist that integrated core, petrographic, scanning electron microscope (SEM), and X-ray diffraction (XRD) analyses to study the reservoir quality of this formation. Consequently, the present study is carried out to fill this gap and outline the sedimentological, petrophysical, and digenetic characteristics of this siliciclastic reservoir in addition to the impact of these characteristics on reservoir quality. Detailed microscopic examination, XRD, SEM, and conventional core analyses were applied on the available cores. Five distinct microfacies types were petrographically identified, belonging to the arenite and wacke facies. These microfacies are represented by quartz arenite, subarkose, argillaceous quartz wacke, calcareous quartz wacke, and calcareous feldspathic greywacke. The formation was subjected to compaction and was affected by four types of cement; silica, calcite, kaolinite, and pyrite cementation. It has three types of porosities: primary intergranular and secondary porosity (microporosity and dissolution). The studied core samples of the Bahariya Formation are discriminated into two reservoir rock types (RRTs) and exhibit extreme heterogeneity. RRT1 comprises mainly quartz arenite, whereas RRT2 comprises quartz wacke. RRT1 has a reservoir quality of tight to poor due to porosity/permeability occluding by the different cementations.In contrast, RRT2 has a reservoir quality of tight to fair, despite the presence of the clay matrix. The tight to fair quality is possibly attributed to the dissolution of feldspars and authigenic clays. The Lower Cenomanian reservoir is considered a second cycle deposition and represents a tight to a fair quality reservoir in the studied wells.

Reservoir quality and rock type zonation for the Sidri and Feiran members of the Belayim Formation, in Belayim Land Oil Field, Gulf of Suez, Egypt

This paper aims to evaluate the reservoir quality and determine the rock type zonation in the subsurface Sidri and Feiran members of the Belayim Formation, in Belayim Land Oil Field (well 112–136), Gulf of Suez, Egypt. To achieve this aim, an integrated petrographical analysis (thin sections, scanning electron microscope “SEM” and X-ray diffraction “XRD”) and petrophysical estimation have been carried out on units of the Sidri and Feiran members using core samples and well logs data. Reservoir evaluation of the studied members revealed an occurrence of four reservoir rock types (RRT1, RRT2, RRT3, and RRT4) and four hydraulic flow units (HFU1, HFU2, HFU3, and HFU4).Diagenetic processes are summarised into two groups depending on their effect on the reservoir potentiality. They show dissolution processes as a porosity improvement factor and porosity diminishing aspects including compaction, mechanical infiltration, authigenic mineral, and cementation. The RRT1 rock samples are quartz arenite quartz, whereas the RRT2, RRT3, and RRT4 are dolomitic quartz arenite, dolomitic quartz wacke, and argillaceous sandy dolostone, respectively.The final findings showed that RRT1 and RRT2 provided the best reservoir zones in both Sidri and Feiran members, whereas RRT3 and RRT4 which predominate in HFU4 had the lowest potential zones of reservoir. The capillary pressure by using mercury injection test (MICP) shows that, samples of RRT1 are predominantly macro-pore sizes with some occurrences of meso- and micro-pore spaces, whereas RRT2 are dominated by meso-pores throat radius with some samples characterised by both macro and micro-pore spaces. The RRT3 reservoir rock samples are characterised by micro-pore throat radius with few meso-pore samples, whereas RRT4 samples are mainly characterised by micro-pore throat radius. Therefore, there is a similarity between the zonation obtained from the MICP method, statistical differentiation of the hydraulic flow unit, and other zonation methods in terms of: porosity-permeability and reservoir quality index (RQI) – normalised porosity index (NPI) relationships.

Benchmark study using a multi-scale, multi-methodological approach for the petrophysical characterization of reservoir sandstones

Abstract. This paper presents a detailed description and evaluation of a multi-methodological petrophysical approach for the comprehensive multi-scale characterization of reservoir sandstones. The suggested methodology enables the identification of links between Darcy-scale permeability and an extensive set of geometrical, textural and topological rock descriptors quantified at the pore scale. This approach is applied to the study of samples from three consecutive sandstone layers of Lower Cretaceous age in northern Israel. These layers differ in features observed at the outcrop, hand specimen, petrographic microscope and micro-CT scales. Specifically, laboratory porosity and permeability measurements of several centimetre-sized samples show low variability in the quartz arenite (top and bottom) layers but high variability in the quartz wacke (middle) layer. The magnitudes of this variability are also confirmed by representative volume sizes and by anisotropy evaluations conducted on micro-CT-imaged 3-D pore geometries. Two scales of directional porosity variability are revealed in quartz arenite sandstone of the top layer: the pore size scale of ∼0.1 mm in all directions and ∼3.5 mm scale related to the occurrence of high- and low-porosity horizontal bands occluded by Fe oxide cementation. This millimetre-scale variability controls the laboratory-measured macroscopic rock permeability. More heterogeneous pore structures were revealed in the quartz wacke sandstone of the intermediate layer, which shows high inverse correlation between porosity and clay matrix in the vertical direction attributed to depositional processes and comprises an internal spatial irregularity. Quartz arenite sandstone of the bottom layer is homogenous and isotropic in the investigated domain, revealing porosity variability at a ∼0.1 mm scale, which is associated with the average pore size. Good agreement between the permeability upscaled from the pore-scale modelling and the estimates based on laboratory measurements is shown for the quartz arenite layers. The proposed multi-methodological approach leads to an accurate petrophysical characterization of reservoir sandstones with broad ranges of textural, topological and mineralogical characteristics and is particularly applicable for describing anisotropy and heterogeneity of sandstones on various rock scales. The results of this study also contribute to the geological interpretation of the studied stratigraphic units.

Integrated petrophysical and petrographical studies for characterization of reservoirs: a case study of Muglad Basin, North Sudan

Petrography, scanning electron microscopy, X-ray diffraction, and both conventional and special core analyses are applied to precisely reservoir characterize the Lower Cretaceous Bentiu and Abu Gabra Formations of the Muglad Basin, which is among the largest hydrocarbon reservoirs in South Kordofan, SW North Sudan. Based on this integrated study, the Abu Gabra–Bentiu sequence comprises four petrographical microfacies, which consist of two reservoir rock types (RRTs). RRT1 is composed of quartz arenites and RRT2 is composed of quartz wackes. The greatest storage, flow capacities, and reservoir quality are assigned for RRT1 due to its good-to-excellent porosity (average ∅ = 26.7%), permeability (average k = 1365 md), reservoir quality index (RQI, average = 1.43 μm), flow zone indicator (FZI, average = 3.66 μm) and discrete rock type (DRT, average = 13). The relatively low-to-fair reservoir quality of the Abu Gabra Formation (average ∅ = 13.2%, average k = 14.7 md, average RQI = 0.17 μm, average FZI = 1.05 μm, and average DRT = 10) is due to authigenic kaolinite and siderite content, compaction and cementation by silica and clay patches.

Reply to the discussion on “Influence of structures on drainage patterns in the Tushka region, SW Egypt. Journal of African Earth Sciences 136, 262–271 by C.A. Robinson, H. El-Kaliouby, E. Ghoneimˮ by B.S. Nabawy

The present study aims at conducting a detailed reservoir characterization of the Early Cretaceous Bentiu and Abu Gabra formations in Muglad basin which is one of the most prospective hydrocarbon basins in South Kordofan, SW Sudan. This can be achieved through integration between well logging (conventional), petrography (thin sections, and scanning electron microscopy SEM), and core data.Based on this integrated study, the Abu Gabra-Bentiu sequence is composed of two reservoir rock types (RRTs). RRT1 is composed of quartz arenite, whereas RRT2 is composed of quartz wacke. The best storage and flow capacities and the best reservoir quality are assigned for RRT1 due to its good to excellent porosity (20.0 ≤ ∅ ≤ 34.1%), permeability (138 ≤ k ≤ 4140 md), reservoir quality index (0.61 ≤ RQI ≤ 2.91 μm), and flow zone indicator (2.37 ≤ FZI ≤ 6.12 μm) values. This rock type is mostly assigned to two hydraulic flow units (HFU-1, and HFU-2) of Bentiu Formation which comprises 95% of the flow capacity of the sequence. The average reservoir parameters of the studied sequence, including shale volume, effective porosity and hydrocarbon saturation, equal to 31.6, 16.2 and 39.5%, respectively with 29.5 m as a total net-pay thickness.Petrographically, the relatively low contribution of the Abu Gabra Formation is due to its authigenic mineral content, compaction, and cementation.

Rock typing and hydraulic flow units as a successful tool for reservoir characterization of Bentiu-Abu Gabra sequence, Muglad basin, southwest Sudan

The present study aims at conducting a detailed reservoir characterization of the Early Cretaceous Bentiu and Abu Gabra formations in Muglad basin which is one of the most prospective hydrocarbon basins in South Kordofan, SW Sudan. This can be achieved through integration between well logging (conventional), petrography (thin sections, and scanning electron microscopy SEM), and core data.Based on this integrated study, the Abu Gabra-Bentiu sequence is composed of two reservoir rock types (RRTs). RRT1 is composed of quartz arenite, whereas RRT2 is composed of quartz wacke. The best storage and flow capacities and the best reservoir quality are assigned for RRT1 due to its good to excellent porosity (20.0 ≤ ∅ ≤ 34.1%), permeability (138 ≤ k ≤ 4140 md), reservoir quality index (0.61 ≤ RQI ≤ 2.91 μm), and flow zone indicator (2.37 ≤ FZI ≤ 6.12 μm) values. This rock type is mostly assigned to two hydraulic flow units (HFU-1, and HFU-2) of Bentiu Formation which comprises 95% of the flow capacity of the sequence. The average reservoir parameters of the studied sequence, including shale volume, effective porosity and hydrocarbon saturation, equal to 31.6, 16.2 and 39.5%, respectively with 29.5 m as a total net-pay thickness.Petrographically, the relatively low contribution of the Abu Gabra Formation is due to its authigenic mineral content, compaction, and cementation.

Evolution of the early Mesozoic Cordilleran arc: The detrital zircon record of back-arc basin deposits, Triassic Buckskin Formation, western Arizona and southeastern California, USA

Abstract A shift in the depositional systems and tectonic regime along the western margin of Laurentia marked the end of the Paleozoic Era. The record of this transition and the inception and tectonic development of the Permo-Triassic Cordilleran magmatic arc is preserved in plutonic rocks in southwestern North America, in successions in the distal back-arc region on the Colorado Plateau, and in the more proximal back-arc region in the rocks of the Buckskin Formation of southeastern California and west-central Arizona (southwestern North America). The Buckskin Formation is correlated to the Lower–Middle Triassic Moenkopi and Upper Triassic Chinle Formations of the Colorado Plateau based on stratigraphic facies and position and new detrital zircon data. Calcareous, fine- to medium-grained and locally gypsiferous quartzites (quartz siltstone) of the lower and quartzite members of the Buckskin Formation were deposited in a marginal-marine environment between ca. 250 and 245 Ma, based on detrital zircon U-Pb data analysis, matching a detrital-zircon maximum depositional age of 250 Ma from the Holbrook Member of the Moenkopi Formation. An unconformity that separates the quartzite and phyllite members is inferred to be the Tr-3 unconformity that is documented across the Colorado Plateau, and marks a transition in depositional environments. Rocks of the phyllite and upper members were deposited in wholly continental depositional environments beginning at ca. 220 Ma. Lenticular bodies of pebble to cobble (meta) conglomerate and medium- to coarse-grained phyllite (subfeldspathic or quartz wacke) in the phyllite member indicate deposition in fluvial systems, whereas the fine- to medium-grained beds of quartzite (quartz arenite) in the upper member indicate deposition in fluvial and shallow-lacustrine environments. The lower and phyllite members show very strong age and Th/U overlap with grains derived from Cordilleran arc plutons. A normalized-distribution plot of Triassic ages across southwestern North America shows peak magmatism at ca. 260–250 Ma and 230–210 Ma, with relatively less activity at ca. 240 Ma, when a land bridge between the arc and the continent was established. Ages and facies of the Buckskin Formation provide insight into the tectono-magmatic evolution of early Mesozoic southwestern North America. During deposition of the lower and quartzite members, the Cordilleran arc was offshore and likely dominantly marine. Sedimentation patterns were most strongly influenced by the Sonoma orogeny in northern Nevada and Utah (USA). The Tr-3 unconformity corresponds to both a lull in magmatism and the “shoaling” of the arc. The phyllite and upper members were deposited in a sedimentary system that was still influenced by a strong contribution of detritus from headwaters far to the southeast, but more locally by a developing arc that had a far stronger effect on sedimentation than the initial phases of magmatism during deposition of the basal members.

Investigation of alteration minerals and potential of geothermal energy on the Ambon Island

An investigation of alteration minerals has been carried out using the XRD (X-ray diffraction) method in several geothermal areas on Ambon Island. There are 4 (four) regions that are mapped as potential areas for the development of geothermal energy in Ambon islands, including Hatuasa geothermal energy, TalangHaha, Suli, and Waiyari. The results of the investigation interpret that rock samples in the Hatuasa geothermal region are dominated by quartz wacke with rock fragments in this area in the form of quartz minerals, pyroxene, opaque minerals, and feldspar while the matrix is clay minerals. The interpretation of rock samples in the TalangHaha, Suli and Waiyari regions is almost the same which is dominated by feldspathic greywacke with fragments in the form of quartz minerals, opaque minerals, lytic, and feldspar, while the matrix is clay minerals. Those minerals are alteration mineralsfrom the original minerals. The appearance of mineral changes in these areas shows that the reservoir temperature in these areas is very high. In addition to changes in the appearance of minerals, the potential of geothermal energy in some areas of Ambon Island can also be interpreted from the results of measurements of flowrate. Flow discharge in the Hatuasa, TalangHaha, Suli and Waiyari regions ranged from 0.19 m3/second; 0.16 m3/second; 0.13 m3/second and 0.20 m3/second give the results of heat loss calculations which are 86.4 kW; 71.8 kW; 72.4 kW and 114.19 kW. Thus the Hatuasa, TalangHaha, Suli and Waiyari geothermal fields can be used as a basis for recommendations on the utilization of geothermal potential for local governments (indirect use), namely as information to meet the shortage of electricity in Ambon Island.

Identification of Quartz Sand From the Hills of Gunung Walat at Sukabumi Regency for Raw Materials of Nano Silica Precipitate

Gunung Walat Hills Formation, Sukabumi Regency is an area that has abundant natural quartz sand resource potential. Quartz sand from the Gunung Walat hills is partly in the form of Quartz Wacke which is composed of monochrystaline and polychristaline quartz minerals associated with clay minerals so it has high Al2O3 and Fe2O3 impurities. Until now the use of quartz sand from the hills of Gunung walat has only been used as building material so that it has a low sale value. In this paper, we will examine the potential of quartz sand from Gunung walat hills for the raw material of nano silica precipitates so that it can increase the added value of natural resources of quartz sand. In this paper the results of identification are described by taking samples from unprocessed quartz sand, the results of washing by mining companies and dissolving with sulfuric acid. From the results of the X-Ray Fluorescent analysis it is seen that quartz sand from mining has a silica composition of 85.869% wt, water leaching results by the mining company 93.702% and sulfuric acid leaching results obtained levels of 95.672% wt. The results of SEM-EDX analysis show that impurities Al2O3 and Fe2O3 are in the mineral sludge, not in one complex compound bond on the same grain so it is relatively easier to separate. From the identification results it can be seen that quartz sand from the Gunung Walat hills can be used as a raw material for making nano silica precipitates.

Compaction, rock physics and rock properties of sandstones of the Stø Formation: Case study of five wells from the south-western Barents Sea, Norway

Five wells containing Lower-Middle Jurassic sandstones of Stø Formation from the Hammerfest Basin (7120/9–1, 7121/7–1), the Ringvassøy-Loppa Fault Complex (7119/12–1, 7119/12–4) and the Troms-Finnmark Fault Complex (7019/1-1) in the Barents Sea area are considered in this study. The Stø Formation sandstones contain dominantly very fine-to medium-grained quartz arenites with occasional coarse-grained sandstone layers. Feldspathic and quartz wackes are also present. The effect of compaction and exhumation on reservoir properties (porosity and permeability) and seismic property (P-wave velocity) of these sandstones have been investigated. Source of quartz cement has also been investigated. Forty polished thin sections embedded in blue epoxy were studied using optical microscopy, scanning electron microscopy and cathodoluminiscence. Bulk mineralogy was also analysed using X-ray diffraction.The studied sandstones have experienced Cenozoic exhumation ranging between 820 and 1050 m. P-wave velocity is higher; porosities and permeabilities are lower in the western wells (7019/1-1, 7119/12–1 and 7119/12–4) compared to the eastern wells (7120/9–1 and 7121/7–1). Rock physics models and diagnostics show that the western wells are diagenetically more mature, stiffer, more compacted and more cemented than the eastern wells. These trends are attributed largely to difference in burial history from the east to the west and less to textural variations. Quartz cement is the most important authigenic mineral in these sandstones. Quartz cement in the western well (7119/12–1) is predominantly derived from clay-induced dissolution at macrostylolites whereas the eastern wells (7120/9–1 and 7121/7–1) are mostly sourced from clay-induced dissolution at grain contacts or microstylolites. While cementational porosity loss dominates in the western wells, compactional porosity loss dominates in the eastern wells. Compaction can reduce porosities down to 26% and this might be the reason for better porosity preservation and reservoir quality in the eastern wells than in the western wells.

Pore Fabric Anisotropy of the Cambrian–Ordovician Nubia Sandstone in the Onshore Gulf of Suez, Egypt: A Surface Outcrop Analog

Integrated studies on pore fabric anisotropy have increased the general understanding of fluid flow patterns through reservoir rocks. In this study, pore anisotropy was studied based on measuring permeability and formation resistivity factors in vertical and horizontal directions for 130 plug samples from a total of 65 oriented block samples. These samples were representatively collected for the Nubia sandstones C and D in the western southern onshore of the Gulf of Suez. In addition, the porosity was measured using two techniques, namely water and helium injection. The effective pore radius r35 and the pore radius of the displacement pressure (rdp) were also measured. Petrographic studies of some representative thin sections and scanning electron microscope studies (SEM) were applied to study the mineral compositions of the studied samples to declare the most important porosity-reducing and porosity-enhancing diagenetic factors. The results show that the studied samples can be categorized into three rock types (RRTs), namely quartz arenite (RRT1), quartz wacke (RRT2) and mudstone (RRT3). The best storage and flow capacity was assigned to the RRT1 samples, whereas the least quality was assigned to the RRT3 samples. This could be attributed to wide pore throat distributions and the anisotropy of pore spaces due to the presence of a vertical subsidiary fracture system that dominated in many samples. This system was enhanced by introducing authigenic kaolinite as pore-filling clay minerals, causing the reduction in vertical permeability but supporting the vertical electric current flow. For the RRT3 samples, this vertical micro-fracture system was reduced by silica cementation which caused the reduction in both fluid and electric current flow. The pore anisotropy of most of the studied samples is in the range of slight anisotropy with some exceptions in the RRT3 samples which are characterized by moderate anisotropy.

Depositional facies analysis using core samples from greater Ughelli Depobelt, Niger Delta Basin Nigeria

Sedimentological analysis of core samples from Well XY-62 located within the greater Ughelli depobelt of the Niger Delta basin was carried out. Some physical deterioration had set in and some gaps had reduced the total core length available to 316 .90ft. The cores contain three sandstone and one mudrock lithofacies: Fine-grained mmscale laminated sandstone (Sl.f) 40.83%, medium to coarse-grained cm-scale cross-bedded sandstone (Sx.m) 35.34%, pebbly Sandstone (PS) 4.39%, bedded Mudstone (Mb) 19.44%. The sandstone lithofacies differ from one another mainly in grain size, as appears obvious, otherwise they are largely texturally, compositionally, diagenetically and structurally broadly similar. They are generally clean (zero to 20% clay content), moderately well to well sorted, pebbly, and texturally mature. Quartz appears to be the sole framework component, making the sandstones mineralogically mature i.e. quartz arenites and quartz wackes. Being generally weakly consolidated and scantily cemented, the sandstones are all in the early stages of diagenesis, an attribute that has had a desirable effect on their reservoir quality. The bedded mudstone lithofacies contains laminated claystones and fissile shales which are all thought to have compartmentalized the sand reservoirs rather than sealed them. Depositional environments have been inferred to be the foreshore setting, with lagoon subenvironments that favoured mudrock accumulation landward of a barrier bar complex. The overall motif read from both the strip log and the wireline logs indicate neither clear progradation nor unequivocal retrogradation of the lithofacies. It is therefore concluded that the XY- 62 cores were cut from an aggradational depositional setting.Keywords: Cores, Sediment analysis, Depositional environments, Niger Delta

Mineralogic and Diagenetic Controls on Reservoir Quality of Paleozoic Sandstones, Gebel El-Zeit, North Eastern Desert, Egypt

Studying the mineral composition and petrophysical properties of the Cambro–Ordovician sequence in Gebel El-Zeit indicates that it can be discriminated into four rock types, namely quartz arenite (RRT1 of the Cambrian Araba Formation, 35 m; RRT2 of the Ordovician Naqus Formation, 320 m), quartz wacke (RRT3) and mudstone (RRT4) of the Naqus Formation. Diagenesis played a major role in reducing or enhancing the studied microfacies. Cementation by silica, compaction and authigenic clay minerals are the main porosity-reducing factors, whereas dissolution and leaching out as well as fracturing are the most important porosity-enhancing factors. For the present study, reservoir quality was determined using reservoir quality index (RQI), flow zone indicator (FZI) and reservoir potentiality index (RPI) based on helium porosity and permeability values, in addition to the mercury injection capillary pressure (MICP) tests. The best reservoir quality was assigned to the quartz arenite microfacies, whereas the least quality was assigned to RRT4. Reservoir quality is mostly controlled by permeability, which in turn is controlled by the helium porosity and the pore radius of the studied samples (r35 of Winland and r10 of Schowalter). The Araba Formation at the bottom of the sequence is characterized by the best hydraulic flow units (best RQI, FZI, RPI and MICP, rank II). In addition, the middle and top parts of the Naqus Formation have good to very good reservoir quality (rank III).

Integrated petrographical and petrophysical studies for reservoir characterization of the Middle Jurassic rocks at Ras El-Abd, Gulf of Suez, Egypt

Acoustic velocity and petrophysical parameters of thirteen samples from the Middle Jurassic Ras El-Abd exposure, west Gulf of Suez, Egypt, have been measured and integrated with the petrographical analysis to evaluate the effect of diagenetic processes on the reservoir quality. The average compressional and shear wave velocities of dry (Vp dry) and saturated (Vp sat) sandstone rock samples are lower than those of carbonate samples. The diagenetic processes affected on the acoustic properties of the studied sandstone and carbonate rocks. The neomorphism and cementation processes caused an increase in the compressional and shear wave velocities and a decrease in the reservoir quality as recorded in biomicrite and argillaceous dolomitic quartz arenite microfacies, whereas the dissolution and leaching processes caused a decrease in the acoustic characteristics and an increase in the reservoir quality as represented by quartz arenite and ferruginous quartz wacke microfacies. In order to decrease the cost of acoustic analysis, the estimated empirical equations can be used for the prediction of compressional wave velocity of wet rock samples from the compressional wave velocity of dry rock samples, and the shear wave velocity of wet rock samples can be predicted from the shear wave velocity of dry rock samples. The correlation of the relative sea-level change for the studied section according to facies changes with the global fluctuation revealed that, the lower interval of Bajocian age is dominated by a fall in sea-level manifested by the deposition of quartz arenite microfacies and hence has a higher reservoir quality than the upper part of the succession.

The quality of geothermal reservoir of the Lower Jurassic aquifer in the Mogilno-Łódź Trough (Polish Lowlands)

The assessment of the geothermal potential should always take an accurate identification of the geological conditions into account. Petrographic and petrophysical investigations of the aquifers allow to estimate their productivity and infectivity. The area of the Mogilno-Łódź Trough is next to the Podhale basin, one of the most prospective regions in terms of geothermal potential in Poland. The Lower Jurassic aquifers are characterized by the largest dispositional resources among geothermal aquifers in the Polish Lowlands. The Lower Jurassic geothermal reservoir consist of fine and mixed grain-size sands and sandstones layers from 10-650 m thick, depending on the depth. The water within the reservoir exhibits mineralization ranging from 2 to over 250 g/L and its temperature ranges from 30 to 100°C. Boreholes profiles selected for study consist Lower Jurassic formations of the Polish Lowlands, which are perspective for the heat production in Poland. Sandstones are represent mainly by quartz arenites and subordinate by quartz wackes. Parameters measured include among others porosity, permeability and surface area. The results of the petrographic and mineralogical studies of core samples confirmed the presence of rock with favorable parameters for geothermal water accumulation.

The Neostratotype of Itapecuru Formation (Lower-Middle Albian) and Its Impact for Mesozoic Stratigraphy of Parnaíba Basin.

This study defines the neostratotype of the Lower-Middle Albian Itapecuru Formation in the Parnaíba Basin, Northeast Brazil. In this sedimentary succession along the right bank of the Itapecuru River near the Itapecuru-Mirim City, Maranhão State, three lithofacies associations are recognized. The first is silty claystone with some very fine sand, micaceous, reddish color, thin laminated, showing a tabular aspect, interpreted as floodplain fines in a distal position. The second is clayey siltstone, reddish color, with ripple cross-lamination, very thin cross-laminated wacke lenses, grading to thin parallel lamination, in apparently tabular layers, interpreted as crevasse splay and distributive channels deposits in successive avulsion events. The third is fine quartz wacke, reddish to grayish color, showing grouped trough cross bedding, interpreted as channel filling deposits. These facies associations correspond to part of a fluvial depositional system, with predominantly fine-grained rocks, and small lakes in the floodplain, in an oxidizing environment and marked by seasonality. These field data, reinforced by grain size and petrographic analyses showed that, contrary to previous descriptions, the Itapecuru Formation consists mainly of mudrocks. This research also shows that the medium to coarse sandstones and conglomerates, previously included in the Itapecuru Formation, belong to the underlying Grajaú Formation.