Shale Intercalations Research Articles

Storm activities during the sedimentation of late Paleocene-middle Eocene Subathu Formation, western Himalayan foreland basin

Late Paleocene-middle Eocene Subathu Formation represents the earliest deposits of the western Himalayan foreland basin. A large part of this formation is comprised of impure limestone and grey shale intercalations. The limestones contain sole marks, intraformational conglomerates, hummocky cross stratification and wave ripples. The occurrence of sole marks suggests that they are developed by the unidirectional currents at the initial phase of the storm that resulted erosion and subsequent deposition. The intra-formational limestone conglomerate also suggests erosion of the earlier deposited limestone hard grounds as a consequence of storm associated transgression. The hummocky cross stratification formed by the oscillatory flows during the long-shore littoral drift. The depth of formation of the hummocky cross stratified limestone facies was less than 40 m and most likely deposited between shore-face to backshore regions of the gulf similar to present day Persian Gulf. The identification of deeper facies (shelf facies) from Pakistan and coastal facies from India suggest that the gulf was open from the west and closed from the east.

Palynology and biostratigraphy of the Maastrichtian Coal Measures in the Anambra Basin, Southeastern Nigeria.

A total of sixty-eight (68) core samples retrieved from eleven wells penetrating the Mamu Formation in the Anambra Basin, Southeastern Nigeria were studied for their palynomorph content in order to determine the age and paleoenvironment of deposition of the Mamu Formation. The wells are characterized by intercalation of laminated fine grained sand and light-dark grey fissile shale with interbeds of coal seams. The lower part with dark grey shale is characterized by the maximum development of Longapertites marginatus Acme Zone, dated Middle Maastrichtian. The upper part defined by intercalation of sand and shale with coal seam layers interbedding belong to Spinizonocolpites baculatus Assemblage Zone. This zone is characterized by the occurrence of Spinizonocolpites baculatus, S.echinatus, Gemmazonocolpites sp, Constructipollenites ineffectus, Syncolporites marginatus, Periretisyncolpites gigantus, Zlivisporites blanensis, Retidiporites magdalenensis, Distaverrusporites simplex, Foveotriletes margaritae, L.vaneendenburgi, Proxapertites cursus, Proteacidites sp, Buttina andreevi, Periretisyncolpites sp and Monocolpites marginatus. All these forms characterize Upper Maastrichtian age on the basis of the recognized diagnostic forms in the wells. The palynomorph abundance and diversity peaks and fossil assemblage of Mamu Formation show similarity with South American forms characterized by colder and warmer climate. These are directly related to eustatic change in sea level resulting to transgression and regression phases. The marine incursion resulted in Mid-Maastrichtian leading to the deposition of basal shale sequence of Mamu Formation while the coal seams were deposited during the regressive phases in the Late Maastrichtian.

Characterization and classification of soils along a typical Hillslope in Afikpo area of Ebonyi state, Nigeria

Three pedons representing soils on the summit to shoulder (Pedon 1), backslope to footslope (Pedon 2) and toeslope (Pedon 3) positions along a typical hillslope in Afikpo area of Ebonyi State of Nigeria were studied regarding their morphological, physico-chemical and mineralogical properties. The soils were also classified according to the criteria of the USDA Soil Taxonomy and the FAO World Reference Base for Soil Resources. Soils of Pedons 1 & 2 occur on very strongly sloping, severely eroded uplands and are derived from ferruginous, feldspathic sandstone with shale intercalations. The soils in the toeslope position, occur on extensive, nearly level plains which are seasonally waterlogged with colluvio-alluvial sediments constituting the parent material. Irrespective of slope position, all the soils are very strongly acidic with a narrow pH range of 4.6 – 5. 1 and an average exch. Al3+ content of 3.77cmol kg-1 in all the horizons studied. Levels of organic C, total N, avail. P, exch. Ca, Mg, and K, range from being very low in Pedon 1, low to moderate in Pedon 2 but moderate to high in Pedon 3 indicating a gradual improvement in soil fertility from the summit to the toeslope of the hillslope . Kaolinite, mica, quartz, microcline, plagioclase, goethite and gibbsite were detected in the silt fraction of all the soils while kaolinite and poorly ordered illite as well as Fe-rich chlorite dominated the clay mineralogy. According to the criteria of the USDA Soil Taxonomy, the upland soils are Typic Udorthents while the valley bottom soils are Mollic Fluvaquents. In the FAO-WRB system, they qualify as Hper-Dystric Regosols and Mollic Gleysols respectively. Recommendations regarding soil management options have been included.Key words: acid soils, characterization, classification, Entisols, hillslope

Calcareous nannofossil biostratigraphy of the Thomel Level (OAE2) in the Lambruisse section, Vocontian Basin, southeast France

The Thomel Level of the Lambruisse section in the Vocontian Basin (southeast France), which is marked by intercalations of black shales and organic-rich marls, accumulated during the oceanic anoxic event 2 (OAE2) occurring in the Cenomanian-Turonian (C-T) boundary interval. Calcareous nannofossil biostratigraphic investigation of this interval revealed a total of five nannofossil zones, corresponding to the UC3-UC8 zones (Middle Cenomanian-Middle Turonian) as defined by Burnett. Biostratigraphically important taxa observed in the section include Cretarhabdus striatus, Axopodorhabdus albianus, Lithraphidites acutus, Corollithion kennedyi, Helenea chiastia, Quadrum gartneri, Q. intermedium, Eiffellithus eximius, Eprolithus octopetalus and E. eptapetalus. The two nannofossil events commonly used in the delineation of the C-T boundary, namely the LO of H. chiastia and the FO of Q. gartneri, occur less than 2 m apart in the studied section. These two bioevents define the limits of the UC6 nannofossil Zone and occur within the Whiteinella archaeocretacea foraminifer Zone. Previous litho- and chemostratigraphic analyses indicate that the δ 13C profile of the section corresponds well with changes in lithofacies and fluctuations in the total organic carbon (TOC) and calcium carbonate content of the section. Initial increase in the δ 13C values occurs within the UC3-UC4a undifferentiated zone, coinciding with the onset of the deposition of the organic-rich sediments of the Thomel Level and a drastic decline in the CaCO 3 values. The plateau of high δ 13C values, on the other hand, occurs within the UC5 zone, between the LO of C. kennedyi and the LO of H. chiastia (and FO of Q. gartneri). This interval of high δ 13C values also corresponds to the interval of high TOC and low CaCO 3 values. The integrated nannofossil, planktonic foraminifer and δ 13C data provide a precise biostratigraphic and chemostratigraphic framework of the C-T boundary in the Lambruisse section that can be used in future studies in the Vocontian Basin and allow correlations with other well-studied C-T boundary sections.

Observations and comments on the classification schemes proposed for the Voltaian Basin sediments on the West African craton by various Geologists

Subsurface data indicate over 6000 metres thick of sedimentary infill in the Voltaian Basin on the West African craton made up of three major lithofacies (groups) separated by distinct unconformities. These are: Lower Voltaian Group composed of over 2000 metres thick of indurated quartzitic sandstones with subordinate shales at the base, overlain by dense, greenish, poorly sorted and graded bedded sandstones and siltstones with subordinate shales of about 2000 metres thick; Middle Voltaian Group consisting of the triad: conglomerate – dolomitic limestone – bedded chert with intercalations of sandstone, siltstone, mudstone and shales of over 700 metres thick, and overlain by about 550 metres thick of greenish, flysch-type sediments. The Upper Voltaian Group is composed of distinctly continental facies (molasses) of reddish brown, chocolatebrown, brown coloured, poorly sorted sandstones, conglomerates, siltstone and mudstones of about 400 metres thick, and a sequence of thinly bedded, micaceous sandstones with clay galls of about 120 metres thick capped by massive cross-bedded arkosic sandstone of about 300 metres thick. Reviews of the classification schemes proposed by various geologists are basically similar. However, general discrepancies, inconsistencies and contradictions in the stratigraphic positions of some of the rock units have been observed, as well as terminologies to describe rock units which are inconsistent with stratigraphic concepts and principles.

Electrical Resistivity Imaging technique to delineate coal seam barrier thickness and demarcate water filled voids

Exploration and exploitation of coal seams is one of the major resources for the energy sector in any country but at the same time water filled voids/water logged areas in the old workings of these seams are very critical problems for the coal mining industry. In such situations, disasters like inundation, landslides, collapsing of the old seams may occur. In this regard, it is necessary to find out the water saturated/water filled voids and zones in the mining areas. Since no established technique is available to find such zones, an experimental study using Electrical Resistivity Imaging (ERI) has been carried out in one of the coal mining areas near Dhanbad, to find out the feasibility of finding the barrier thickness and the water logged area in underground coal mines. The area under study forms part of Jharia coalfield in Dhanbad district, Jharkhand state. The coal bearing rocks of Barakar Formation of Lower Permian age (Gondwana period) occur in the area under a thin cover (10 m to15 m) of soil and or alluvium. Coal bearing Barakar Formations consist mainly of sandstone of varying grain size, intercalation of shale and sandstone, grey and carbonaceous-shale and coal seams.

Litostratigrafi dan sedimentasi Formasi Kebo dan Formasi Butak di Pegunungan Baturagung, Jawa Tengah Bagian Selatan

http://dx.doi.org/10.17014/ijog.vol3no4.20081Lithologically, the Nampurejo Pillow Lava, Kebo and Butak Formations, which are dominated by volcanic rocks, spread west - eastly, along the northern foot of the Baturagung Mountains. The Nampurejo Pillow Lava, which has an Early Oligocene age, is overlain by the Late Oligocene - Early Miocene Kebo and Butak Formations successively. The Nampurejo Pillow Lava consists of basaltic pillow-lavas showing pillow structures and they are intercalated by black sandstones. The Kebo Formation comprises alternating sandstone and pebbly sandstone with intercalations of siltstone, claystone, tuff, and shale. On the other hand, the Butak For- mation is composed of polymic breccia with intercalations of sandstone, pebbly sandstone, claystone, and siltstone/shale. The three units were deposited in a deep – shallow marine basin, which was filled by volcanic prod- ucts. Compared to the lower part of the Kebo Formation, volcanic activities during the deposition of the upper part of the Kebo Formation and the Butak Formation were more active.

Litho-, bio- and chemostratigraphy across the Cenomanian/Turonian boundary (OAE 2) in the Vocontian Basin of southeastern France

The Oceanic Anoxic Event 2 (OAE 2) was one of at least two large oceanic anoxic events during the Cretaceous Period, and it is characterized in the stratigraphic record by double positive peaks in the δ 13C curves of carbonate and organic carbon. Since the positive δ 13C shift is believed to have resulted from massive burial of isotopically depleted organic carbon generated globally in anoxic oceanic environments, the positive δ 13C shift tend to coincide with widespread deposition of organic-rich sediments. In the case of OAE 2, however, the organic-rich horizons do not accord exactly with the two intervals with positive δ 13C shifts at many sites such as Pueblo, Tarfaya, Eastbourne, and Gubbio (e.g., Tsikos, H., Jenkyns, H.C., Walsworth-Bell, B., Petrizzo, M.R., Forster, A., Kolonic, S., Erba, E., Premoli Silva, I., Baas, M., Wagner, T., Sinninghe Damesté, J.S., 2004. Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic anoxic event: correlation and implications based on three key localities. Journal of the Geological Society, London 161, 711–719). The Thomel Level of the Cenomanian/Turonian (C/T) boundary in the Vocontian Basin (southeastern France) accumulated during the OAE 2, and it is marked by frequent intercalations of black shale and organic-rich marl. Detailed litho-, bio- and chemostratigraphy analyses across the C/T boundary in the Vocontian Basin indicate that the δ 13C carbonate profile fluctuation across the Thomel Level corresponds well with changes in lithofacies, total organic carbon (TOC), and calcium carbonate (CaCO 3) abundance. Positive δ 13C excursions within the OAE 2 correlate with basal and middle parts of the Thomel Level that are characterized by high TOC, low CaCO 3, and predominantly contain organic-rich sediments such as black shale and dark gray marl. On the other hand, negative δ 13C intervals within the OAE 2 correspond with lower and upper parts of the Thomel Level that consist of marly limestone and limy marl with low TOC and high CaCO 3 values. Since the fluctuation of redox conditions during Thomel Level deposition in the Vocontian Basin coincides well with the global carbon burial rate indicated by its δ 13C profile, the Thomel Level could be an ideal candidate for the reference section of the OAE 2.

Bulk geochemical parameters and biomarker characteristics of organic matter in two wells (Gaibu-1 and Kasade-1) from the Bornu basin: Implications on the hydrocarbon potential

Ditch-cutting samples from two exploratory oil wells in the Bornu basin namely Gaibu-1 and Kasade-1 were analyzed using organic geochemistry, and petrology as well as biomarkers. Two potential gas source intervals (1120–1180 m and 1285–1405 m) with the possibility of an oil source at about 1300 m depth are found within the Fika–Gongila lithology in Kasade-1. Although potential source rock is present in the Kasade-1 well, these intervals are too shallow for significant gas generation. No potential gas/oil source intervals were encountered in the Gaibu-1 well. However in this well, vitrinite reflectance values range from 1.17 to 1.45% indicating maturity in the oil to gas window. This study like similar earlier ones has found the Chad basin to have hydrocarbon generation and accumulation potential. The Fika and Gongila Formations have generally TOC > 0.5 wt.%, the minimum content for hydrocarbon generation. Potential reservoir rocks are the Bima Sandstones. The shale intercalations within this formation could serve as seals or cap rocks. More wells are to be investigated together with the application of a 3D seismic technique in order to have a better knowledge of the subsurface geology of the Bornu basin. This may lead to a final decision on the petroleum prospects of the basin.

Late Cambrian acritarchs from the “Túnel Ordovícico del Fabar”, Cantabrian Zone, N Spain

A single sample from the La Matosa Member of the Barrios Formation, exposed during the excavation of the El Fabar Tunnel in Asturias, N Spain, yielded a rich and well-preserved Late Cambrian acritarch assemblage. The Middle? to Upper Cambrian La Matosa Member consists mainly of quartz sandstone and includes locally a thick fossiliferous intercalation of dark shale (“El Fabar beds”) from which the acritarch sample was collected. The latter shale levels yielded also some linguliform brachiopods, olenid trilobites and phyllocarid crustaceans. Characteristic genera of the study sample are Acanthodiacrodium, Lusatia and Cristallinium. Other significant components are Cymatiogalea, Timofeevia and Vulcanisphaera. Lusatia dendroidea Burmann 1970, recorded for the first time in Spain, is a characteristic element of this assemblage. In the study sample, the latter species is abundant, well-preserved and exhibits appreciable morphological variability. This acritarch assemblage is correlated with the lower part of microflora RA5 of Parsons and Anderson [Parsons, M.G., Anderson, M.M., 2000. Acritarch microfloral succession from the Late Cambrian and Ordovician (early Tremadoc) of Random Island, eastern Newfoundland, and its comparison to coeval microfloras, particularly those of the East European Platform. AASP Contr. Ser. 38, 129 pp.] corresponding to the Protopeltura praecursor trilobite Zone.

Water geochemistry of the Seferihisar geothermal area, İzmir, Turkey

The Na–Cl thermal waters of Seferihisar have measured temperatures of 30–153°C, and conductivities of 7400–55200 μS/cm. They issue from Upper Cretaceous to Paleocene Bornova melange rocks, which are made up of sandstone–shale intercalations, conglomerate, mafic submarine volcanics, limestone lenses, serpentinite and limestone bodies, and their complexes, through the intersection of faults. The clay-rich zones of the overlying Neogene terrestrial sediments cap the system. The heat source is the high geothermal gradient caused by graben tectonics of the area. Na and Cl ions mainly dominate the chemistry of thermal waters, thus thermal waters of the Seferihisar area appear to be mixtures of groundwaters and seawater. The seawater contribution in thermal waters varies from 10% to 80%. Assessments from empirical chemical geothermometers, mineral equilibria and silica-mixing model applied on the thermal waters of the Seferihisar geothermal area suggest that the temperatures of the reservoir systems vary between 60 and 180°C. The Seferihisar geothermal area has been physically divided into five main groups and estimated reservoir temperatures provided by the entire methods give the increase as: Doğanbey Burnu, Karakoç spa, Doğanbey, Tuzla and Cumali spa areas in ascending order of reservoir temperature. Hydrogeochemical assessments indicate that thermal waters were mixed with cold waters before and/or after heating at depth in different proportions. The results of mineral equilibrium modeling indicate that the thermal waters of Seferihisar are undersaturated with respect to gypsum and amorphous silica, oversaturated with respect to calcite, dolomite and aragonite, and undersaturated or oversaturated with respect to quartz, chalcedony and anhydrite.

An integrated biostratigraphical analysis of the Volkhov–Kunda (Lower Ordovician) succession at Fågelsång, Scania, Sweden

The Komstad Limestone is composed of dark grey to black micritic limestone deposited in an outer shelf environment on the margin of the East European Platform. It represents an interval of low sea level during the late Volkhov to early Kunda that led to the spread of limestones into the shale-dominated western lithofacies belt. The Baltoniodus norrlandicus, Lenodus antivariabilis, Lenodus variabilis, Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones were identified in the upper Volkhov to lower Kunda interval. Based on the most recent taxonomic and biostratigraphical framework, the Megistaspis limbata (with two subzones), Asaphus expansus and Asaphus raniceps trilobite zones were identified. A shale intercalation in the lower part of the Komstad Limestone contains the graptolites Phyllograptus cor in association with Glyptograptus sp. and is referred to Darriwilian 1 (Upper Arenig). In the Baltoscandian zonation this matches the upper part of the Didymograptus hirundo graptolite Zone. The upper level of the Komstad Limestone and the basal part of the overlying Almelund Shale do not contain graptolites, whereas the succeeding black shales of the Almelund Shale belong to the Holmograptus lentus Zone (Llanvirn). The Arenig-Llanvirn boundary is situated at or very near the top of the Komstad Limestone. The conodont assemblage in the lower part of the Komstad Limestone at Fågelsång is associated with forms of Gondwanan affinity, which probably reflects the cool water environment of the outer shelf setting. The higher part of the limestone contains the Whiterockian conodont species Dzikodus sp. and Histiodella tableheadensis. These important Laurentian forms occur together with abundant Gothodus sp. 1 and Cyclopyge umbonata in the base of the Asaphus expansus Zone. The arrival of the Laurentian taxa – as well as Cyclopyge with an ‘Gondwana’ affinity – is related to a transient sea level rise at the base of the A. expansus trilobite Zone.

Rock composition and origin of late Cretaceous shales in the Duwi Formation, Upper Egypt.

Duwi Formation that crops out between Qena to the north and Idfu to the south is composed of a number of phosphorite beds intercalated with shales, marls, cherts, and limestones. The shale intercalations were subjected to field and petrographic, mineralogical and geochemical investigations in order to examine their rock composition and origin. Shales are mainly fossiliferous and composed essentially of argillaceous materials, quartz, feldspar, phosphatic pelloids and skeletal fragments. The light fraction of the separated sand fraction is composed of quartz, plagioclase and microcline. The heavy fraction consists of opaque minerals, zircon, tourmaline, epidote, garnet, rutile and phosphates. X-ray diffraction analyses of bulk rocks identified quartz, francolite, gypsum, anhydrite, microcline, albite, halite, goethite and limonite as non-clay minerals. The clay minerals are smectite, kaolinite, and illite. The study have revealed that the constituents of these shales were inherited from the weathering of mafic igneous rocks as well as quartzose sedimentary rocks situated to the south and southeast of the study area, transported via a fluvial system into shallow marine environment. The abundance of smectite suggests the deposition of those shales in a period of transgression and arid to semi-arid climatic conditions.

A correlation of accretionary complexes of southern Sikhote-Alin of Russia and the Inner Zone of Southwest Japan

This paper briefly describes the geology of southern Sikhote-Alin of Russia and the Inner zone of Southwest Japan, and presents a new correlation model in which the Samarka terrane (sensu stricto), Udeka Formation, Sebuchar Formation and Kalinovka ophiolite in Sikhote-Alin are considered as northern extensions of the Mino-Tamba terrane, Hikami Formation, Kozuki Formation, and Yakuno ophiolite in Southwest Japan, respectively. This correlation is based on the similarities in lithology, age, faunal assemblage, and structural relationship between them. The Samarka and Mino-Tamba terranes consists of Jurassic accretionary complexes including Permian greenstone-limestone complex, Permian to Lower Jurassic radiolarian bedded chert, Jurassic clastic rocks, and Jurassic melanges. The Udeka and Hikami formations are composed mainly of greenish gray sandstone with minor shale intercalations yielding Permian radiolarians. The Sebuchar and Kozuki formations are characterized by basalts accompanied with shale, limestone and chert. The limestone includes Carboniferous fusulinaceans, while Permian radiolarians occur in the chert. The Kalinovka and Yakuno ophiolites consist of a series of ultramafic to mafic igneous rocks with an ophiolitic succession. Since the ages of these ophiolites have not been clearly established, we correlate them on the basis of their lithology and tectonic positions. These units form a stack of nappes from the lower to upper horizons in the following order: Mino-Tamba terrane, Hikami Formation, Kozuki Formation, and Yakuno ophiolite in Japan, and Samarka terrane (sensu stricto), Udeka Formation, Sebuchar Formation, and Kalinovka ophiolite in Russia. This correlation supports the reconstruction model of Japan before the opening of the Sea of Japan proposed by Yamakita and Otoh (1999).

Delineation of reservoir boundary using AVO analysis

Keutapang Formation is a group of sedimentary rocks consisting of sandstone with shale intercalation. It was deposited in the coastal environment during Middle Miocene age in the north Sumatra Basin. The thickness of the formation is in range of 500-1300 m.In the sandstone reservoir, hydrocarbons were trapped within the porous zone. In this rock formation the top of the porous zone could be identified as the source of AVO anomaly. By applying Shuey’s (1985) equation to the AVO anomaly parameters of velocity of P wave Vp, ΔVp (velocity contrast of two layers), ρ (density), Δρ (density difference of two layers) and σ can be estimated by the Marquardt inversion.The relationship between ρ and Vp for water-filled sand shale sequences is explained by Gardner’s et al. (1974) equation. The difference between the derivative and the equation itself is defined as the fluid factor.Application of the fluid factor to the field where Keutapang sandstone located gives a significant change in values. These changes also shown by the value of P wave velocity and density for each CDP location. The prospective reservoir is located in the area of high value compared to the surrounding area.Borehole data in the area shows the comformable result to the predicted zone, therefor the method has given a satisfactory result in estimating the zone of interest for further delineation of reservoir boundary.

Was the Late Triassic orogeny in Turkey caused by the collision of an oceanic plateau?

Abstract A belt of Late Triassic deformation and metamorphism (Cimmeride Orogeny) extends east-west for 1100 km in northern Turkey. It is proposed that this was caused by the collision and partial accretion of an Early-Middle Triassic oceanic plateau with the southern continental margin of Laurasia. The upper part of this oceanic plateau is recognized as a thick Lower-Middle Triassic metabasite-marble-phyllite complex, named the Nilüfer Unit, which covers an area of 120 000 km 2 with an estimated volume of mafic rocks of 2 × 10 5 km 3 . The mafic sequence, which has thin stratigraphic intercalations of hemipelagic limestone and shale, shows consistent within-plate geochemical signatures. The Nilüfer Unit has undergone a high-pressure greenschist facies metamorphism, but also includes tectonic slices of eclogite and blueschist with latest Triassic isotopic ages, produced during the attempted subduction of the plateau. The short period for the orogeny (< 15 Ma; Norian-Hettangian) is further evidence for the oceanic plateau origin of the Cimmeride Orogeny. The accretion of the Nilüfer Plateau produced strong uplift and compressional deformation in the hanging wall. A large and thick clastic wedge, fed from the granitic basement of the Laurasia, represented by a thick Upper Triassic arkosic sandstone sequence in northwest Turkey, engulfed the subduction zone and the Nilüfer Plateau.

The silica supersaturated waters of northern Evia and eastern central Greece

The area of north Evia and eastern central Greece is characterized by strong geomorphological contrast and is built up mainly of consolidated rocks. Unconsolidated young sediments of Pleistocene to Holocene age cover the valley and basin flats, forming the most productive aquifers in this area. However, two more types of aquifers can be distinguished within the consolidated rock area. The first one is associated with karstified limestones and the second with strongly tectonized ultramafic rocks. The schist–chert formation, with intercalations of shales and cherts, seals the ultramafic masses underneath. Surface and spring waters associated with ultramafic rocks in north Evia and eastern central Greece were studied. Two types of water can be distinguished: (1) high Mg2+ and SiO2 , bicarbonate as the dominant anion, pH 7·4–9·2, temperature 9·5–16·3 °C, low TDS (total dissolved solutes) (459–1037 mg/l), found both in peridotite and serpentinite areas, classified as Mg–HCO3 type; (2) high Ca2+, low Mg2+ and SiO2 , hydroxyl ion as the major anion, pH 11·2, temperature 28 °C, very low TDS (122 mg/l), found in peridotite areas, classified as Ca–OH type. The studied waters are highly supersaturated with respect to quartz, amorphous silica, brucite and most low temperature magnesium silicates (antigorite, sepiolite, talc, etc.). These waters show relatively narrow SiO2 concentration ranges and a trend parallel to the amorphous silica saturation surface. The silica supersaturated waters have the potential to precipitate silica and consequently could affect the people of the local communities that use it as drinking water, causing health problems (kidney stones). Copyright © 1999 John Wiley & Sons, Ltd.

The silica supersaturated waters of northern Evia and eastern central Greece

The area of north Evia and eastern central Greece is characterized by strong geomorphological contrast and is built up mainly of consolidated rocks. Unconsolidated young sediments of Pleistocene to Holocene age cover the valley and basin flats, forming the most productive aquifers in this area. However, two more types of aquifers can be distinguished within the consolidated rock area. The first one is associated with karstified limestones and the second with strongly tectonized ultramafic rocks. The schist–chert formation, with intercalations of shales and cherts, seals the ultramafic masses underneath. Surface and spring waters associated with ultramafic rocks in north Evia and eastern central Greece were studied. Two types of water can be distinguished: (1) high Mg2+ and SiO2 , bicarbonate as the dominant anion, pH 7·4–9·2, temperature 9·5–16·3 °C, low TDS (total dissolved solutes) (459–1037 mg/l), found both in peridotite and serpentinite areas, classified as Mg–HCO3 type; (2) high Ca2+, low Mg2+ and SiO2 , hydroxyl ion as the major anion, pH 11·2, temperature 28 °C, very low TDS (122 mg/l), found in peridotite areas, classified as Ca–OH type. The studied waters are highly supersaturated with respect to quartz, amorphous silica, brucite and most low temperature magnesium silicates (antigorite, sepiolite, talc, etc.). These waters show relatively narrow SiO2 concentration ranges and a trend parallel to the amorphous silica saturation surface. The silica supersaturated waters have the potential to precipitate silica and consequently could affect the people of the local communities that use it as drinking water, causing health problems (kidney stones). Copyright © 1999 John Wiley & Sons, Ltd.

Springhill Formation, Magellan Basin, Chile: formation water characteristics and mineralogy

The Early Cretaceous Springhill Formation is a transgressive sequence of quartz sandstones with intercalations of shale and represents the principal hydrocarbon producer of the Magellan Basin. Analyses of its formation waters from the Chilean sector (surface area approximately 20,000 km 2) are classified and linked to the diagenetic alteration of the formation. Local δ 18O determinations and core leaching have been carried out to evaluate homogeneity and whole rock contribution to Springhill Formation fluid phases. Formation waters are of the chlorine–calcium type with Na +/Cl − < 1, (Na +−Cl −)/SO 4 2− < 0 and (Cl −−Na +)/Mg 2 > 1. Together with an increase in organic maturity (Ro) between 2000 and 4000 m, a change is observed from authigenic kaolinite to illite and corrensite, accompanied by a loss of porosity and permeability. With respect to the distribution of the authigenic pore phyllosilicates, the shallowest kaolinite dominated area is limited by faults against deeper sectors where corrensite and illite/smectite prevail. A later influx of high salinity fluids into the kaolinite area from the basin centre or continental graben sediments has let to the present day distribution of formation waters. Low scatter δ 18O ratios of kaolinite and corrensite reflect lithological consistency on a m-scale. Residual salt analyses shows a direct relationship between the dissolution of Ca and a high CaO content in the whole rock sample.

Effect of Vertical Heterogeneities in a Petrophysical Evaluation of Low Resistivity Pay Zones, B Sands Upper Eocene, Block III, Lake Maracaibo : ABSTRACTS

Thin-bed reservoirs can exist as intercalations of thin porous beds and shales. For a proper petrophysical evaluation and geological characterization of this type of reservoir it is necessary to implement an integrated analysis, which includes knowledge of the depositional environment to avoid missing oil zones by applying standard petrophysical evaluations. The normal resistivity logs used on this area do not have enough vertical resolution for detecting thin low resistivity pay zones. Consequently, the common water saturation models indicate high water saturation when they are evaluated. The Middle Eocene Upper B sands are part of the Misoa Formation in Block III of Lake Maracaibo. They have been exploited since 1965 when the first production test was made. The cumulative oil production to date stands at 11.49 MMSTB. In order to supplement the existing geological information in this area, a continuous core was taken which showed the high degree of shales and thin sands intercalations in the pay zone. A careful core-log calibration was made and a new set of petrophysical parameters were established for this reservoir based on the core measurements. This allowed the establishment of a new petrophysical evaluation procedure that helps the calculation of permeability, hydrocarbon volume in placemore » and productivity of this reservoir.« less