Quartz Arenite Research Articles

PETROLOGY AND GEOCHEMISTRY OF THE LATE CRETACEOUS PAB FORMATION, WESTERN SULAIMAN FOLD- THRUST- BELT, PAKISTAN: IMPLICATIONS FOR PROVENANCE AND PALEO-WEATHERING

Late Cretaceous sandstone succession of the Pab Formation in western Sulaiman Fold Thrust belt Pakistan was investigated for petrology and bulk rock chemistry to determine its source terrain, paleo-weathering and tectonic setting. The formation is mainly comprised of sandstone with reddish to maroon color shale and arenaceous limestone. Texturally, the sandstone is fine to coarse grained, sub-angular to well-rounded and moderately to well sorted. The sandstone is petrologically and geochemically classified as quartz arenite to sub lithic arenite. The detritus was mainly derived from plutonic acidic source. QtFL and QmFLt suggests that recycled orogeny and Craton Interior setting were major sources of sediments. Geochemical models support that the detritus was derived from quartzose sedimentary source terrain, suggest deposition in a passive continental margin setting. Average values of chemical indices are CIA 59% CIW 67% and CIV 12.70%, which suggest moderate to high degree of chemical weathering in source area, that may reflect humid climate condition in the source area. The petrographic study and geochemical models demonstrate that the Pab Formation is mostly composed of mature sandstone from acidic plutonic and low-grade metamorphic rocks terrain in recycled and Craton Interior setting deposited on western passive margin of Indian plate in Tethys Ocean.

Petrography and reservoir quality controls in shallow transitional marine Cretaceous-Paleogene deposits in the Dahomey Basin, Nigeria

Upper Mesozoic and Early Cenozoic sedimentary rocks are of great significance to the petroleum geology of the Eastern Dahomey Basin as they are valuable prospects for hydrocarbon reservoirs. Diagenesis and reservoir quality (porosity and permeability) controls are deduced from core samples recovered from six shallow exploration wells in the onshore area of the Eastern Dahomey Basin using petrographic, petrophysical and mineralogical tools. The studied lithostratigraphic intervals consist of limestone, calcareous siliciclastic sandstone (Ewekoro and Araromi Formations), quartz arenite and clay-matrix supported sandstone (Afowo Formation) which were deposited within a shallow marine depositional environment.Siliciclastic samples of the Ewekoro Formation consist predominantly of calcareous sandstone and limestone, which are classified as packstone and wackestone. The porosity values in the sandstones range from 5 to 36% whereas permeability ranges from 0.006 to 109 mD. Ewekoro Limestone porosity ranges from 7 to 32% and permeability ranges from 0.003 to 13 mD. Samples of the Araromi Formation exhibit a porosity of 4 to 45% with permeability between <0.001 and 1764 mD in sandstones whereas a single limestone sample has 8% porosity and permeability of 0.03 mD. Porosity in quartz arenitic sandstones of the Afowo Formation ranges from 15 to 47% with permeability of 0.05 to >10,000 mD, while in the clay-matrix supported sandstones, the values range from 18 to 42% and 0.05 to 5 mD for porosity and permeability, respectively. The main pore-occluding authigenic minerals are carbonate cements (Fe-dolomite, Fe-calcite, siderite and dolomite), iron oxide, kaolinite, and illite-smectite mixed layer clays. The best reservoir quality is observed in the quartz arenitic sandstones of the Araromi and Afowo Formations, except for the low permeability, clay-matrix supported sandstone facies.The most significant controlling factors in sandstones are clay mineral matrix contents and the presence of carbonate cements. Fe-calcite, Fe-dolomite and kaolinite formation occurs during early diagenesis in the studied lithologies and has helped to reduce the effect of compaction. The results of this study indicate fair to good reservoir quality in the shallow marine sedimentary rocks of the Eastern Dahomey Basin, which could be targeted in further exploration campaigns in the region.

Controls of facies distribution on reservoir quality in the Messinian incised-valley fill Abu Madi Formation in Salma delta gas field, northeastern onshore Nile Delta, Egypt

The Messinian incised-valley fill sediments “IVFs” of the Abu Madi Formation comprise the most prospective hydrocarbon-bearing reservoirs in the Nile Delta onshore and offshore. An integrated seismic-stratigraphic, sedimentological and petrophysical approach is applied to identify the factors controlling reservoir heterogeneity within the Abu Madi IVFs. Two separate channelized systems are interpreted and reflect the variation in flow dynamics and depositional energy. Morphologically, the channel system at the base constitutes the deeply-incised (average relief ∼ 80 ms “ms”) flat-filled channels corresponding to the braided fluvial pattern, whereas the channels at the top consists of shallower (average relief ∼ 20 ms) concave-up and shingled-filled channel facies of the meandering fluvial pattern. Coarse-grained quartzose sandstones are predominant in the channel sand bars, channel lags and channel-base drapes of the braided channel system. Higher in the section, the meandering fluvial facies hosts higher proportion of fine-grained sediments accumulated in the abandoned channels and their floodplains.The relative proportion of clays and pseudomatrix materials exercise the first-order control on the pore-scale heterogeneity and reservoir quality. Four reservoir rock types (RRT) were interpreted having different lithological and compositional characteristics and thereby petrophysical flow properties. Capillary pressure, irreducible water saturation, permeability anisotropy and pore-system heterogeneity increase from RRTI to RRTIV. RRTI intervals consist the quartzose sandstones preferentially accumulated at the base of the fining-upward cycles in the braided fluvial sediments. This study highlights the impact of sedimentological and compositional controls on the pore-scale heterogeneity, and therefore will enhance the predictive models for exploration in the incised-valley fill reservoirs.

Petrofacies investigations of the Cretaceous Pab Formation Rakhi Gorge Eastern Sulaiman Range Pakistan - Implication for reservoir potential

Petrologic investigations of the sixty representative thin sections from the Pab Formation within Rakhi Gorge section Eastern Sulaiman Range Pakistan have been used to characterize different diagenetic patterns, identification of primary composition and reservoir characters. Sublithic, lithic and quartzose sandstones have been the principal constituents of the formation. The processes responsible for the late-stage alteration and diagenetic processes identified during the petrographic study include compaction of lithic fragments and mud clasts, formation of quartz overgrowth structures, feldspar replacement and alteration, cementation, and replacement of grains by clay minerals other ferrous elements and diagenetic minerals. The study shows that the initial porosity has been primary intergranular, but the secondary porosity has been originated in the rocks as a result of the alteration of the primary and secondary constituents, as well as fracturing. These facies characters replicate the reservoir quality including, good, medium, low-quality, and non-reservoir. Samples displaying average total porosity greater than 10 % have been declared as good quality. While rocks samples having 7 % or greater porosity are grouped as a medium reservoir. Those samples consisting of porosity values between 1 and 4% are considered as a low quality reservoir while those samples having porosity ranges low than 1% have been called non-reservoir. Based on the investigated parameters the studied samples from the Pab Formation displayed the characters of a medium reservoir that may hold significant hydrocarbon. This different quality including good and low quality zonation can be attributed to the facies and diagenetic change in the formation.

Chemical Classification of Sediments in JVX -Well, Greater Ughelli Depobelt Niger Delta Basin, Nigeria

This study evaluate the Chemical classification of sediments in JVX well, Greater Ughelli Depo belt Niger Delta Basin. Samples collected from different intervals were analyzed using geochemical proxies. Agbada Formation was suggested for the sampled intervals due to the presence of shaleand sand intercalations. lithofacies units gotten from sampled intervals are Sand, Shale and Shaly sand facies . The sands are milky in colour, translucent to opaque, medium to coarse grain, subangular to subrounded and are moderately sorted while the shales are Grey in colour,fissile with the presence of lignite streak and calcareous materials.The geochemical studies of the sediments revealed that SiO2 is the dominant oxide followed by Al2O3 and Fe2O3 which constitutes over 90% while others like CaO, K2O, TiO2, Na2O and MgO constitute 10%. The sediments were classified as Fe-shale, Fe-sand and Quartz arenite. Samples that plotted in the quartz arenites region suggests an intense degree of weathering and reworking. The SiO2/Al2O3 ratios for the sediments in the well are appreciably high indicating that the samples have been heavily weathered, evidenced from the enrichment of quartz and depletion of feldspars. Also, the relatively high concentrations of Fe2O3 and TiO2 is an indication of iron-titanium minerals such as haematite and anatase retiles.

Reservoir Quality of Upper Jurassic Corallian Sandstones, Weald Basin, UK

The Upper Jurassic, shallow marine Corallian sandstones of the Weald Basin, UK, are significant onshore reservoirs due to their future potential for carbon capture and storage (CCS) and hydrogen storage. These reservoir rocks, buried to no deeper than 1700 m before uplift to 850 to 900 m at the present time, also provide an opportunity to study the pivotal role of shallow marine sandstone eodiagenesis. With little evidence of compaction, these rocks show low to moderate porosity for their relatively shallow burial depths. Their porosity ranges from 0.8 to 30% with an average of 12.6% and permeability range from 0.01 to 887 mD with an average of 31 mD. The Corallian sandstones of the Weald Basin are relatively poorly studied; consequently, there is a paucity of data on their reservoir quality which limits any ability to predict porosity and permeability away from wells. This study presents a potential first in the examination of diagenetic controls of reservoir quality of the Corallian sandstones, of the Weald Basin’s Palmers Wood and Bletchingley oil fields, using a combination of core analysis, sedimentary core logs, petrography, wireline analysis, SEM-EDS analysis and geochemical analysis to understand the extent of diagenetic evolution of the sandstones and its effects on reservoir quality. The analyses show a dominant quartz arenite lithology with minor feldspars, bioclasts, Fe-ooids and extra-basinal lithic grains. We conclude that little compactional porosity-loss occurred with cementation being the main process that caused porosity-loss. Early calcite cement, from neomorphism of contemporaneously deposited bioclasts, represents the majority of the early cement, which subsequently prevented mechanical compaction. Calcite cement is also interpreted to have formed during burial from decarboxylation-derived CO2 during source rock maturation. Other cements include the Fe-clay berthierine, apatite, pyrite, dolomite, siderite, quartz, illite and kaolinite. Reservoir quality in the Corallian sandstones show no significant depositional textural controls; it was reduced by dominant calcite cementation, locally preserved by berthierine grain coats that inhibited quartz cement and enhanced by detrital grain dissolution as well as cement dissolution. Reservoir quality in the Corallian sandstones can therefore be predicted by considering abundance of calcite cement from bioclasts, organically derived CO2 and Fe-clay coats.

Petrophysical properties of bypassed Cenozoic clastic reservoirs in the Cesar sub-basin, Colombia

The Cesar-Ranchería basin has all the necessary elements for the generation, expulsion, and migration of hydrocarbons and considerable potential for coal bed methane (CBM) in Colombia. Previous studies in the Cesar basin focused on understanding the tectonic evolution, stratigraphy, hydrocarbon generation potential, and evaluation of reservoir potential in Cretaceous calcareous units and quartzose sandstones from the Paleocene Barco Formation. These studies had confirmed the existence of an effective petroleum system, with several episodes of oil expulsion and re-emigration in the Miocene period, turning the Cenozoic clastic succession (Barco, Los Cuervos, La Loma, and Cuesta formations) into an element of significant exploratory interest to clarify the potentiality of the basin in terms of hydrocarbon accumulation. The petrophysical parameters of Cenozoic units (shale volume, porosity, water, and oil saturation) were determined by integrating wells log and core samples analyses from three stratigraphic wells. The integration of these results synthesizes the petrophysical behavior of the units. It defines intervals with clay volumes of less than 30%, effective porosity around 20%, which means favorable characteristics as reservoir rocks that need to be considered in future exploratory projects.

Carbonate cementation in the Tithonian Jeanne d’Arc sandstone, Terra Nova Field, Newfoundland: Implications for reservoir quality evolution

AbstractPetrographic and in situ geochemical analyses were carried out on cores of the fluvial sandstone (&lt;50 to &gt;4 mm; sublithic quartz arenite) dominated Tithonian Jeanne d’Arc Formation interval from wells C‐09 and E‐79 of the Terra Nova oilfield to understand diagenetic control on reservoir quality of the formation. Lithic clasts are dominantly carbonates and shales. Successively, early near‐surface dolomite cementation, eogenetic dolomite dissolution, burial calcite cementation and its subsequent dissolution are the major diagenetic events that controlled the reservoir quality of the sandstone. Dolomite cement grades from mostly Fe‐rich (FeCO3 ca 12 ± 2 normalized mol.%) in the upper part of the formation to Fe‐poor (&lt;0.07 normalized mol.%) in the lower part of the formation in both wells. The dolomite and calcite cements generally have low Sr and Na; enriched Y and low δ18O and δ13C values relative to typical marine carbonates. Strontium is enriched in the Fe‐poor (higher Mg) dolomite relative to the Fe‐rich phase. Mineral liberation analyses of shale samples reveal common Fe‐chlorite and minor glauconite. Furthermore, considerable amounts of solutes in the early diagenetic pore fluid were likely derived from water—rock interaction with underlying Oxfordian Rankin Formation marine carbonate that was also exposed in the watershed of the study area at the time. Together, these suggest that the dolomites were formed from an early diagenetic bicarbonate‐rich pore fluid of mixed meteoric and seawater origin. The origin and distribution of early dolomite cementation and other successive diagenetic events have a depositional cycle control. Episodic post‐deposition transgression of seawater and/or compactional fluid expulsion from shales into overlying sandstones led to formation of dolomite cement. Subsequent infiltration of organic acid‐charged meteoric‐water into underlying dolomite‐cemented sandstones resulted in early dissolution. This porosity was mostly occluded by re‐precipitation of calcite. Increase in the abundance of relic corroded dolomite crystals with depth in each cycle indicates that early dissolution was most effective at the top of the cycles. Iron depletion in the calcite cement with depth through the formation points to a local derivation of reactants from in situ dolomite species; implying a stratigraphic control. Calcite cementation was succeeded by continuous mesogenetic dissolution concomitant mechanical compaction. The responsible low‐pH pore fluid attained its composition via enhanced H+ contribution from silicate burial diagenesis. The control of depositional settings on the major diagenetic processes makes it a key contribution to region‐wide reservoir quality prediction for the formation.

Sedimentological and provenance analysis of the Cretaceous Moro formation Rakhi Gorge, Eastern Sulaiman Range, Pakistan

The Cretaceous Moro Formation from the Rakhi Nala section Dera Ghazi Khan has been studied in detail to investigate the Sedimentology and provenance. This paper describes the litho-facies changes, depositional environment, and provenance analysis of the Cretaceous Moro Formation from the Rakhi Nala section, eastern Sulaiman Range. The studied Formation is 110-140 meters thick and consists mainly of fine to coarse-grained sandstone, with minor-siltstone, mudstone (claystone, shale), and limestone. The uppermost beds of the Moro Formation are consist of sandstone with iron types of cement. Twelve lithofacies have been identified based on a petrographic investigation related to the depositional environment of the Moro Formation ranging from deltaic to marine setting (Delta Plain-Delta front). Petrographic analysis of sandstone reveals the presence of quartz both, mono-crystalline and poly-crystalline, less feldspar; heavy minerals like hematite and magnetite, and glauconite were found in negligible amounts. Detrital mineral composition shows that in Moro Formation, the sandstone shows a litharenite. Modal composition of the sandstone from the QFL diagram was Q 66% F 0.3% L 33.7% and that of the QmFLt diagram was QM, 57% F 0.23% L 43.77%. The overall average composition is Q 61.5% F 0.27% L 38.7%. A total of 37 thin-sections are studied for provenance analysis, out of which twenty-seven samples are considered as Litharenite (this shows recycled, or craton interior origin), eight Quartz arenite categories are identified and two samples are fall in the sublitharenites category (Quarts recycled source 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.

Controls on the compositional framework and petrogenesis of Early Cretaceous first cycle quartzose sandstone, North Gondwana

Recent advances in sedimentary research have shown that quartz sandstones are scarce in the global geologic record and prolonged sediment transport does not automatically produce mineralogically mature sediments. Published data are based mainly on Cambro-Ordovician arenites. This paper discusses the formational controls of early Cretaceous syn-rift quartzose from northern Gondwana based on integrated petrographic, mineralogic, and electrofacies proxies on the Upper Sarir Sandstone Formation, Sirte Basin, Libya. The study aimed to understand the compositional framework and construe the sedimentological processes that produced the quartz-rich sandstones. Acquired samples (n = 40) from three boreholes (N8-97, N7-97, and N4-97) were subjected to petrographic analysis, x-ray diffractometry, and variable pressure scanning electron microscopy with energy dispersive x-ray spectrometry. Qualitative and quantitative datasets indicate that the mean framework composition (Q97F0L3) of the sample is quartz-dominated and primarily detrital, although diagenetic imprints, mostly quartz cementation, quartz overgrowth, and other authigenic precipitates, are present. Evidenced by the predomination of monocrystalline quartz grains (99.10%) over the polycrystalline type (0.90%), the dominance of non-undulatory extinction (72.50%), and noticeable euhedral crystals, the compositional framework of the studied quartzose reflects a predominant plutonic provenance with some metamorphic inputs. The possibility of significant sediment reworking is ruled out based on short to moderate transport distance (~100 km) interpreted from the interplay of textural parameters, depositional model, and paleogeography. A humid paleoclimate alongside its associated rigorous chemical weathering is constrained as the dominant control in the formation of the Upper Sarir quartzose sandstone from a dominant craton interior provenance. These conditions probably produced other early Cretaceous arenitic sandstones in other reaches of North Gondwanaland.

Sequence Stratigraphy and Depositional Environment of the Zubair Formation in Rumaila Oilfields, Southern Iraq: Microfacies and Geochemistry

In the Rumaila oilfields in southern Iraq, the Zubair Formation was deposited in a shallow environment as three main facies, delta plain, backshore, and delta front depositional conditions indicating a transition from delta front and delta plain to a highstand level due to the finning upward mode. The facies of the Zubair clasts show well-sorted quartz arenite sandstone, poorly sorted quartz arenite sandstone, clayey sandstone that has not been properly sorted, sandy shale, and shale lithofacies. The minor lithofacies were identified using well-logging methods (gamma ray, spontaneous potential and sonic logs) and petrography. The Zubair clasts are of transition environment that appears to be transported from freshwater and deposited in a marine environment forming many fourth-order cycles reflect sea level rise fluctuations and still-stand under tectonics developed the sequence stratigraphy. A misalignment between relative sea-level and sediment supply caused asymmetry sedimentary cycles. A shallower environment of shale-dominated rocks rich in organic matter and pyrite were exposed. The basinal shale of Ratawi at the Zubair bottom and the shallow carbonate of Shuaiba emplace on the Zubair represent the beginning of the delta build up (delta front and delta plain) to a highstand stage.

Provenance of northern Gondwana Lower Ordovician sandstones (Khabour Formation, northern Iraq) revealed by detrital zircon using LA‐ICP‐MS dating

We present new zircon U/Pb provenance data using laser ablation induced coupled plasma mass spectroscopy (LA‐ICP‐MS) for eight samples of mature quartz arenites and subarkosic sandstones of the Lower Ordovician Khabour Formation, northern Iraq, to establish their depositional age and provenance. The samples were collected from thin‐ to medium‐bedded sandstones along profiles of the Chalky Nasara and Ora sections. Most analysed detrital zircon analyses are concordant within 100 ± 10% and include a prevalence of Proterozoic over Archean ages. Collectively, the data can be divided into four main age groups: (a) Neoproterozoic ages, 570–1,000 Ma (58%); (b) Mesoproterozoic ages, 1,000–1,500 Ma (10%); (c) Palaeoproterozoic ages, 1,620–2,480 Ma (21%); and (d) Archean ages, 2,500–3,300 Ma (11%). The predominance of Cryogenian–Ediacaran ages (850–550 Ma) reflects major magmatic events associated with the assembly of northern Gondwana. The major age group at ~600 Ma likely represents the unroofing of late‐stage granitic rocks from the Arabian‐Nubian Shield. The lack of zircons younger than 490 Ma suggests that Lower Ordovician sandstones in northern Iraq were probably deposited on the distal shelf of the northern Gondwana passive margin and were not influenced by rifting and magmatism in northern Gondwana caused by the opening of Palaeo‐Tethys. Diagenetic xenotime overgrowths are common on detrital zircon grains, but radiometric dating of these was not possible.

Petrophysical assessment and permeability modeling utilizing core data and machine learning approaches – A study from the Badr El Din-1 field, Egypt

Reservoir petrophysical characterization involving permeability estimation in the laboratory using core samples is time-consuming and that too accords limited coverage of the sub-surface reservoir. Machine learning (ML) algorithms have been utilized to overcome such difficulties, which offers better and more accurate prediction models. This work integrates core data and ML approached and reports a comprehensive petrophysical evaluation of the Cenomanian Bahariya and Albian Kharita reservoirs from the Badr El Din-1 field, Western Desert of Egypt. Three distinct reservoir rock types are identified from the detailed petrographic investigation: i) glauconitic sandstones (RRT1) and ii) subfeldspathic quartz arenites (RRT2) within Bahariya Formation and iii) quartz arenites (RRT3) within Kharita Formation. Reservoir quality index (RQI), flow zone indicator (FZI), and hydraulic flow units (HFU) are interpreted from the routine core analysis, which infers that the RRT1 has poor permeability (KH < 1 mD) and impervious (RQI < 0.25 μm, FZI <1 μm). Transgressive shoreface deposits of RRT2 display a wide range of porosity (5.9–18.5%) and permeability (0.82–76 mD) distribution and offer poor to fair reservoir qualities (1 μm < FZI < 5 μm) with moderate to high permeability anisotropy. Well sorted medium grained quartz arenites (RRT3) were deposited in a braided fluvial channel environment and contributed to the excellent reservoir quality with an average KH of 468 mD, FZI > 5 μm, and lesser permeability anisotropy. Finally, two-hybrid ML-based algorithms such as particle swarm optimization (PSO) trained neural network (NN) and least squares support vector machines (LS-SVM) have been employed to estimate the permeability. Based on the statistical performance metrics, we conclude that both the ML algorithms performed exceptionally well as compared to the conventional method, which is attributed to their enhanced competence to model the connectivity within the pore microstructures of the reservoir rocks with proper recognition of the pattern in the training data. Our results indicate that amongst the PSO-NN and LS-SVM algorithms, the latter is more superior since it is comparatively more accurate and reliable in permeability prediction.

Non-destructive in-situ classification of sandstones used in the Angkor monuments of Cambodia using a portable X-ray fluorescence analyzer and magnetic susceptibility meter

The Angkor monuments were built between the 9th and 15th centuries using gray to yellowish brown sandstone, red sandstone, and greywacke, which are petrologically classified as feldspathic arenite, quartz arenite, and feldspathic to lithic wacke, respectively. Gray to yellowish brown sandstone was generally used in all of the Angkor monument temples. Red sandstone was used as the main building material in the Banteay Srei temple but was also occasionally mixed with gray to yellowish brown sandstone in temples built in and after the late Bayon period. Greywacke was the main building material in the five sanctuaries atop the Ta Keo temple and was also used for statues and Linga-Yoni pedestals. To distinguish these sandstones using a non-destructive and in-situ approach, measurements were performed using a portable magnetic susceptibility meter and portable X-ray fluorescence analyzer. The obtained Rb, Sr, and Y contents and magnetic susceptibility values were used to effectively classify the above three rock types. The results show differences in the magnetic susceptibility, Rb and Y contents between the greywacke used in the Ta Keo temple, that used in statues, and that used in Linga-Yoni pedestals, which demonstrates different source locations.

Rock typing based on hydraulic and electric flow units for reservoir characterization of Nubia Sandstone, southwest Sinai, Egypt

The present work describes and evaluates the reservoir quality of the sandstone of the Nubia Formation at the Gebel Abu Hasswa outcrop in southwest Sinai, Egypt. Hydraulic flow unit (HFU) and electrical flow unit (EFU) concepts are implied to achieve this purpose. The Paleozoic section made up of four formations has been studied. The oldest is Araba Formation followed by Naqus formations (Nubia C and D) overlay by Abu Durba, Ahemir and Qiseib formations (Nubia B), where the Lower Cretaceous (Nubia A) is represented by the Malha Formation. The studied samples have been collected from Araba, Abu Durba, Ahemir and the Malha formations. The hydraulic flow unit (HFU) discrimination was carried out based on permeability and porosity relationship, whereas the electrical flow unit (EFU) differentiation was carried out based on the relationship between formation resistivity factor and porosity. Petrographic investigation of the studied thin sections illustrates that the studied samples are mainly quartz arenite. Important roles to enhance or reduce the pore size and/or pore throats controlling the reservoir petrophysical behavior are due to the diagenetic processes. The present study used the reservoir quality index (RQI) and Winland R35 as additional parameters applied to discriminate the HFUs. The study samples have five hydraulic flow units of different rock types, where the detected electrical flow units are only three. The differences between them are may be due to the cementation process with iron oxides that might act as pore filling, lining and pore bridging, sometimes bridges helping to decrease permeability without serious reduction in porosity. The reduction between the number of EFUs and HFUs comes from the effect of diagenesis processes which is responsible for a precipitation of different cement types such as different clay minerals and iron oxides.

Sedimentological and palynological assessment of three wells in eastern Dahomey Embayment, southwestern Nigeria, West Africa

Integrated studies were carried out on one hundred and eleven (111) ditch samples retrieved from three wells drilled in Lagos area within the Dahomey Embayment, coastal southwestern Nigeria. The purpose of this study is to determine the provenance, heavy mineral assemblage(s), establish the lowstand - highstand episodes, Quaternary palaeoenvironment and date the encountered lithofacies which constitute sandstones, siltstones with thin beds of lignite. The textural parameters show that the sandstones are medium to fine grained with the lignite consisting of plant fragment remains. Petrographic analyses showed abundant detrital minerals with average modal composition of quartz (Q = 90%), feldspar (F = 2%) and rock fragments (RF = 4%). The QFR ternary plots revealed that the sandstones are quartz arenites of continental block provenance origin deposited under humid climatic setting. This lithofacies has undergone medium distance range of transportation and have been subjected to mechanical and chemical weathering prior deposition. The heavy mineral assemblages and the ratios of Zircon-Tourmaline (ZTR index) in the wells suggest matured sediments. The Apatite: Tourmaline (ATi) ratios showed variations within the studied wells indicating differences in rate of weathering. It is evident that the periods of residence on the floodplain despite similar provenances and fluctuations in ratios, suggest sea level changes. The fluctuations in apatite: tourmaline ratios with respect to weathering are attributed to rapid changes in climatic conditions during the floodplain residence. The high ATi values in sandstone were deposited during relative sea level highstand (transgression), while sandstones with low ATi were deposited during lowstand (regression) in Nigeria and along the West African region. The abundance species of Rhizophora sp., Avicennia africana, Acrostichu mauruem, Laevigatosporite sp., Combretaceae sp., Mitrogynaciliata, Sapotaceae sp., Disopyros sp., Elaeis guineensis and Alchronea spp. indicated depositional transition from mangrove to fresh water swamp due to the episodic highstand and lowstand regimes. The abundance of Podocarpus milianjanus and Retitricolporites hians and the absence of Miocene diagnostic forms suggest late Pleistocene (Tarantian = 0.012–0.126 Ma) deposition.

A new lithostratigraphic framework for portions of the Pongola Supergroup within the Nkandla sub-basin, southern Kaapvaal Craton, South Africa; insights into Mozaan Group stratigraphy

AbstractA revised lithostratigraphic framework for Mozaan Group-equivalent strata within the Nkandla sub-basin is presented based on new field data, remote sensing and genetic sequence stratigraphic interpretations. Although previous literature has suggested that no Mozaan Group lithologies were deposited within the sub-basin, reinterpretations presented here indicate that 90% of the lithostratigraphy developed within the main basin occurs within the Nkandla and Mhlatuze inliers. Mozaan Group units previously defined as the Vutshini and Ekombe formations are correlated with stratigraphy from the lowermost Sinqeni Formation to the Gabela Formation. Although thinner than units within the type area in the main basin, thicknesses of the Sinqeni Formation are comparable to those observed within the White Mfolozi Inlier. A ~1 000 m composite reference profile is measured within the Mdlelanga Syncline of the Nkandla Inlier. Further profiles were measured for sequences in the Gem-Vuleka Syncline of the Nkandla Inlier, as well as within the Mhlatuze Inlier. These latter profiles, however, host only lower Mozaan Group strata. In all sections the basal portion of the sequence comprises two quartz arenite units, separated by a ferruginous shale, which hosts minor iron formation interbeds. This predominantly coarse-grained lower sequence is overlain by a shale-dominated succession with multiple sandstone interbeds. A prominent coarse-grained quartz arenite unit forms a distinct marker in the middle portion of the sequence. This is overlain by a sequence of shales and sandstones with two prominent igneous units present. Genetic sequence stratigraphic interpretations indicate cyclical deposition of dominantly shallow marine sediments with condensed sections, marked by iron formations or ferruginous shales, denoting periods of marine highstand along the southeastern margin of the Kaapvaal Craton. The evidence of Mozaan Group stratigraphy within the Nkandla sub-basin supports a passive margin tectonic model whereby deposition occurred in an arcuate shallow continental margin which opened to the southeast. The extension of Mozaan Group strata into the Nkandla sub-basin suggests that the Mozaan Basin likely formed a single depository rather than separate sub-basins as previously proposed.

Diagenetic Control on the Distribution of Porosity within the Depositional Facies of Proterozoic Rajgarh Formation, Alwar Sub-basin, Northeastern Rajasthan

Abstract The sandstones of the Proterozoic Rajgarh Formation are texturally immature and compositionally sub-mature quartz arenite to arkose in composition. The reservoir quality of sandstones is strongly influenced by depositional facies and various types of diagenetic modifications. A significant reduction in primary porosity in the studied sandstone is mainly due to high mechanical compaction followed by chemical compaction rather than the cementation process. Arkose from the river-dominated estuary facies show infiltrated clay coating, feldspar dissolution, iron-oxide cementation, leaching of detrital by iron-oxide cement, kaolinization of feldspar, the fracturing of detrital, partial albitization and folding of mica. The pore-lining clay has helped in retaining primary porosity, and leaching and dissolution processes were the main process generating secondary porosity in this facies. Tidal flat sub-arkose petrofacies display compaction of micas and unstable rock fragments into the pseudomatrix, expansion of muscovite by kaolinite, and pore-filling silty matrix that occupied intergranular spaces. Quartz-arenite petrofacies from shallow marine deposit show grain replacive kaolinite, pyrite precipitation, glauconite authigenesis, muscovite bending, albitization of plagioclase, feldspar overgrowths, dissolution, and grain fracturing. Pyrite plays a significant role in the reduction of both primary and secondary porosity by precipitating in intergranular spaces and in dissolution voids of feldspar. Moreover, fractures in detrital act as a second cycle porosity generation process in the shallow marine petrofacies. The linking between the diagenetic processes and depositional facies provides a geologic model for hydrocarbon exploration through reservoir quality prediction. Therefore, the petrographic observations indicate quartz-arenite (shallow marine facies) and sub-arkosic sandstones (tidal flat facies) are the least suitable latent reservoir because of their lowest primary and secondary porosity and unconnected pore spaces. Arkosic (river-dominated estuary facies) compositions are considered to be appropriate frameworks for potential reservoir rocks. This study reveals that the depositional environment largely controls the diagenetic pathways and hence, influences the reservoir quality.

Facies Architecture and Sedimentary Structures in the drill cores of Uranium Bearing Sediments of Banganapalle Formation of Palnad Sub-basin, Guntur District, Andhra Pradesh

The Banganapalle Formation, the lowest member of the Neoproterozoic Kurnool Group of rocks, resting over the basement granites, has been identified as the host rock for uranium in Koppunuru area in the western part of Palnad sub-basin. The uppermost arenite facies of the Banganapalle Formation is exposed on surface and shows only few bedform indicators like ripple marks, planer laminations etc. Down-hole lithological examinations on course of core drilling in Koppunuru and adjoining areas identified five recognizable lithofacies of Banganapalle Formation, viz. basal conglomerate unit, quartzite-shale intercalated facies, and two quartz arenite facies separated by a grey shale dominated argillaceous facies.&#x0D; The polymictic conglomerate, with unsorted grit to pebble size clasts of granite, shale, quartzite, vein quartz and dolerite indicate short distance transportation and derivation from nearby granitoids traversed by quartz reef/dolerite dykes. Cyclic repetitions of arenaceous and argillaceous sediments in Banganapalle lithocolumn above the basal conglomerates point to alternate rhythmic marine transgression and regression regimes. These units can also be discriminated based on their sedimentary texture, bedforms and several soft-sedimentary penecontemporaneous deformational structures (PCD) like load structure, convolute bedding/laminations, and slump structures such as micro-slips, gravity faults and folds. These syn-sedimentary structures clearly indicate perturbation and submergence of the basin contemporaneous to deposition. Cross-beds suggests change in current direction/intensity while bi-directional symmetrical ripples in outcrops of upper arenite facies suggests that the Banganapalle sediments are derived from the basement granitoids exposed to the north as well as upper Cuddapah sediments to its west. Overall, the sedimentary structures, textural and composition variation of the lithounits suggest deposition of these sediments in marginal marine, inter- to supra-tidal flat environment. The porous and permeable nature of the quartz arenite and the basal conglomerates and the presence of available reductants in the form of sulphides and carbonaceous matter make them the best suited loci for fluid movement and precipitation of uranium.