Siltstone Formation Research Articles

Provenance of the Early Permian bauxitic claystone in Huayingshan region, Sichuan Basin, South China: Constraints from U–Pb ages and trace elements of detrital zircons

The Early Permian bauxite in the Sichuan Basin is an important part of karst-type bauxite deposits in South China Block and its provenance is poorly understood. Here we present results of mineralogy, whole-rock geochemistry, detrital zircon U–Pb ages and trace element compositions of the bauxitic claystone from the Lower Permian Dazhuyuan Formation and those of the siltstone from the underlying Lower Silurian Hanjiadian Formation in Huayingshan region, Sichuan Basin, South China Block to constrain the provenance of the bauxitic claystone. Detrital zircons from bauxitic claystone and siltstone have similar age spectra, and the ages of bauxitic claystone are classified into five groups of 2600–2200 Ma, 1800–1600 Ma, 1200–900 Ma, 900–650 Ma, and 650–400 Ma respectively. Those of the siltstone are classified into four groups of 2600–2200 Ma, 1200–900 Ma, 900–650 Ma, and 650–400 Ma respectively. We propose that the bauxitic claystone in the study region has mixed sources: the siltstone of the Lower Silurian Hanjiadian Formation probably constitutes the dominant source, and the Late Paleoproterozoic magmatic rocks along the western margin of the Yangtze Block are probably the secondary source. Moreover, the source materials of the Hanjiadian Formation siltstone are mainly sourced from Grenville-aged magmatic rocks in the Cathaysia Block and Neoproterozoic magmatic rocks in the Jiangnan orogen, with minor contributions from the underlying clastic sediments in the Cathaysia Block and Pan-African magmatic rocks in the orogen in eastern Gondwana.

Lithofacies and Diagenetic Controls on Tight Silty and Sandy Upper Triassic Reservoirs of the Heshui Oil Field (Ordos Basin, North China)

Summary Tight oil, present in reservoirs of low porosity and permeability, can be regarded as a kind of unconventional resource. The tightening process in this kind of reservoir is controlled by the lithology and diagenetic history of the host formation. Upper Triassic Yanchang Formation siltstones and sandstones are the main reservoirs for hydrocarbon accumulation in the Heshui Oil Field (HOF), southwestern Ordos Basin. The reservoirs exhibit low porosity, low permeability, and strong heterogeneity. In recent years, numerous drillcores have been recovered from these units, but the porosity-permeability characteristics and burial history of these silty and sandy reservoirs have not yet been reported in detail. In this study, an integrated analysis of the lithofacies, diagenesis, and reservoir characteristics of the siltstones and sandstones was achieved using a combination of core and thin section, grain size, scanning electron microscopy (SEM), X-ray diffraction (XRD), δ13C and δ18O, mercury intrusion capillary pressure (MICP), and porosity and permeability data. Our primary goals were to quantify the porosity-permeability characteristics of these silty and sandy reservoirs, restore their diagenetic histories, and examine the paragenetic relationship of reservoir tightness to hydrocarbon accumulation. The silty and sandy reservoirs represent braided river delta facies consisting of compositionally and texturally immature sediments. In the burial environment, they underwent complex diagenetic processes that reduced porosity from an initial average of ~38% to the present ~8%. Porosity-destructive processes included compaction (~ –12.5%) and cementation (~ –21%), with increases in porosity related to grain dissolution (~ +2.2%) and tectonic fractures (~ +1.1%). The reservoirs underwent four diagenetic stages: (1) Penesyngenetic and Eogenetic A Stage (Late Triassic-Early Jurassic); (2) Eogenetic B Stage (Late Jurassic); (3) Early Mesogenetic A Stage (Early Cretaceous); and (4) Late Mesogenetic A Stage (Late Cretaceous to recent). Hydrocarbon charging of these reservoirs occurred in three pulses. Existing pore space was partly filled by hydrocarbons during the Eogenetic B Stage. A second hydrocarbon charging event occurred during the Early Mesogenetic A Stage, when residual primary intergranular pores and secondary dissolution pores were filled. A third hydrocarbon charging event occurred during the Late Mesogenetic A Stage, when the reservoirs were tight. Siltstone beds deposited in delta front environments are the main future exploration targets in the Chang 6 to 8 members. The results of this study provide a useful reference framework for future exploration of hydrocarbon resources in the Upper Triassic Yanchang Formation of the HOF, as well as potential insights into the evolution of similarly tight reservoirs in other basins.

Isotopic time-series (δ13C and δ18O) obtained from the columnar layer of Permian brachiopod shells are a reliable archive of seasonal variations

In this paper, we develop a robust palaeo-proxy technique for inferring seasonal fluctuations in Phanerozoic seawater temperature. Specifically, we report δ13C and δ18O values across the columnar layer (inner shell) of Permian brachiopods, including five genera of the Order Spiriferida (Ingellarella, Martinia, Permophricodothyris, Cartorhium, and Trigonotreta) and two genera of the Order Athyridida (Araxathyris and Comelicania). These brachiopod specimens are inferred to have inhabited varied palaeo-depths, based on facies analysis, and were collected from low, middle and high palaeolatitudes. To obtain high-resolution ontogenetic isotopic time-series, a handheld microdrill was used to collect low-magnesium calcite samples along the longitudinal axis of the mid-valve for each specimen. Intrageneric and intergeneric comparison of statistics (mean values, variance, and range) reveals that δ18O values are consistent within and between genera although δ13C values are more variable. In specimens obtained from low palaeolatitudes (Julfa Formation, Iran; Bellerophon Formation, Italy; Late Permian), the range of δ18O values is small and consistent with weak seasonal temperature variations, irrespective of water depth. In specimens obtained from middle palaeolatitudes (Qarari Formation, Oman; Early-Middle Permian), we found a range of δ18O values dependent on inferred water depth, possibly reflecting a change in thermocline structure between warm and cold seasons. In specimens obtained from high palaeolatitudes (Wasp Head and Wandrawandian Siltstone formations, Australia; Early-Middle Permian), brachiopods from glacial and interglacial climate phases show significant variations of δ18O values, probably caused by seasonal changes in the seawater surface temperature. Based on these findings, the technique developed in this paper appears to provide reliable paleoclimate and paleoenvironmental data for Permian sites, and should be also applicable for Phanerozoic studies in general. However, we caution that δ18O values vary dependent on the growth direction, resulting in differences of up to 5 °C in seawater paleotemperature calculations, and emphasise that a consistent sampling strategy accounting for shell structure and growth direction is required.

GEOCHEMICAL ANALYSES OF SEEP OILS IN THE SOUTHERN GAFSA BASIN, SW TUNISIA: REASSESSMENT OF REGIONAL HYDROCARBON POTENTIAL

The recent discovery of surface oil seeps in the Tamerza area in the west‐central Gafsa Basin (southern Tunisia) has prompted a re‐evaluation of the hydrocarbon potential of the region. In this paper, we report the results of analyses of seep oils by Rock‐Eval pyrolysis (n = 6) and gas chromatography – mass spectrometry (n = 4). The goals of the study were to assess the composition of the seep oils, to investigate the relationship between the seep oils and potential source rocks, and to highlight the significance of the seep oils for oil exploration in the region.In the Tamerza area, surface oil seeps have been recorded in numerous formations ranging between the Upper Cretaceous Abiod Limestone Formation and the lower Miocene Sehib Siltstone Formation. The results of this study showed that all the seep oil samples analysed in general had a similar geochemical fingerprint: for example, Pr/Ph values are lower than 1; a plot of Pr/n‐C17 (0.27‐ 0.36) versus Ph/n‐C18 (0.3‐0.8) indicates a marine source rock deposited under reducing anoxic conditions; and Ts/(Ts+Tm) ratios indicate that the source rock was thermally mature. Correlation studies suggest that the oils originated from Cenomanian‐Turonian shales corresponding to the informally‐named Bahloul equivalent formation. Oil expulsion from this source rock at the seep locations is inferred to have ended by middle Miocene time. However the main phase of folding occurred here in the Pliocene – early Quaternary, and the resulting anticlinal folds are not therefore prospective structural traps for hydrocarbons because they developed after migration had already ceased. Stratigraphic traps and salt structures in the region may be of greater exploration interest.A surface oil seep sample was also recovered from the Quaternary upper Segui Formation at Jebel Orbata in the east of the Gafsa Basin. Analysis of this oil sample showed that it has similar geochemical characteristics to the seep oils from the Tamerza area, but that it appears to have had a much more recent migration history. In the eastern Gafsa Basin, Pliocene – early Quaternary anticlinal structures could therefore constitute effective structural traps charged by the same Bahloul equivalent formation source rock.

Grain-scale analysis of proppant crushing and embedment using calibrated discrete element models

Proppant crushing and embedment in hydraulically-induced fractures is a major drawback to the recovery of unconventional oil/gas and geothermal energy production. This study provides a grain-scale analysis of the fracture evolution mechanisms of proppant crushing, rock fracture damage during proppant embedment, the influence of realistic reservoir/fracture fluid on proppant embedment, and the behaviour of proppant packs subjected to in-situ stresses using a discrete element modelling (DEM) approach. The results of this study reveal that the selection of an appropriate proppant type based on the nature of the reservoir formation plays a vital part in quantifying the degree of proppant crushing and embedment within fractures. The utilisation of frac-sand proppants instead of ceramic proppants in shallow soft sedimentary-based siltstone formations reduces proppant embedment up to 88%. However, whatever the depth of the fracture, the injection of ceramic proppants into granite-based geothermal formations is preferred to that of frac-sand proppants due to their lower proppant embedment and greater crush resistance. DEM analysis detected rock-spalling during the proppant embedment process, which ultimately led to the initiation of tensile-dominant secondary fractures in rocks. Fracture initiation, propagation, and coalescence during proppant crushing are analysed using calibrated DEM proppant-rock assemblies. Importantly, this study reveals that the saturation of formation rocks with fracturing/reservoir fluids may cause a significant increase in proppant embedment. Furthermore, proppant crushing, embedment, and re-arrangement mechanisms in proppant packs with different proppant distributions are analysed in this comprehensive numerical study.

Hydrocarbon charge and petroleum system evolution of the Montney Formation: A multidisciplinary case study of the Blueberry sub-play in Northeast British Columbia, Canada

Abstract The origin, timing, and evolution of hydrocarbons in the early Triassic Montney Formation siltstone has long been debated and is poorly understood. A proper understanding of the timing and processes involved in hydrocarbon emplacement forms an important part of in-situ fluid characterization. Presented here is a case study from northeastern British Columbia. This study integrates basin modeling, carbon isotope geochemistry, fluid inclusion microthermometry, organic and inorganic petrography, sedimentology, and stratigraphy to reconstruct the burial history and relative timing of geological events and mechanisms involved in developing the present-day Montney hydrocarbon system in the Blueberry sub-play. Petrography from whole core indicates that the organic material present is largely pyrobitumen. The lack of primary kerogen suggests that the original oil charge must have migrated into place prior to increasing thermal maturity and conversion to pyrobitumen at maximum burial. In addition, patterns in stable carbon isotope curves from extracted core fluids show a stratified oil column, indicating a lack of vertical migration that would have homogenized the hydrocarbons. Thin section petrography and facies analysis show a laminated fabric with strong permeability anisotropy suggesting preferential migration along bed-parallel permeability pathways, which would have prevented significant vertical mixing. Any interpretations about the origins of the Montney Formation hydrocarbons in this study area must satisfy these observations. Given the data available, the most likely interpretation is a bed-parallel eastern migration from a downdip source. Collective understanding of these datasets helps illustrate the processes involved in the evolution of the hydrocarbon system present in the Montney Formation. Enhanced understanding of the Montney hydrocarbon migration history and reservoir evolution is important for understanding the lateral and vertical variations in the producing gas-condensate-ratios across the Montney formation fairway.

On the records of the brachiopod ‘ Lingula ’ and associated fossils in Mid-Triassic deposits in England

Brachiopods were reported, as ‘ Lingula ’, from Anisian (early Mid-Triassic) deposits of the Tarporley Siltstone Formation (Mercia Mudstone Group) in Nottinghamshire in 1955, and from the Helsby Sandstone Formation (Sherwood Sandstone Group) in a borehole in Oxfordshire in 1975. Burrows ( Lingulichnus ) attributed to this organism were reported from the Tarporley Siltstone in Cheshire in 1981. These records are evidence of marine influence in the depositional environments of those formations.

Reservoir quality and diagenesis of Triassic sandstones and siltstones from arid fluvial and playa margin environments: A study of one of the UK's earliest producing oilfields

The Triassic stratigraphy of the UK contains many important reservoir rock units, present in all major offshore and onshore basins, that are rich in oil, gas and water resources. The Ormskirk Sandstone Formation (Sherwood Sandstone Group) and Tarporley Siltstone Formation (Mercia Mudstone Group) represent the main reservoirs found in one of the UK's earliest producing oilfields, at an unusually shallow depth of 30–90 m. In this study, environments of deposition, maximum burial depth, and reservoir quality have been evaluated from well Formby-7, drilled in 2012, using modern analytical techniques. Wireline data and core analysis data were made available by Aurora Energy Resources for the purposes of this study. Formby-7 was continuously cored so that the whole section was logged for sedimentary structures and grain size. Optical analysis of thin sections was conducted, along with SEM-EDS, using thin sections from core plugs. The Ormskirk Sandstone Formation was deposited in a proximal setting by a braided, dryland river with aeolian influence; the Tarporley Siltstone Formation was deposited in a more distal setting on the margins of a playa environment. The study found that the reservoir quality is excellent in the deeper Ormskirk sediments, but relatively poor in the overlying Tarporley sediments. Good reservoir quality present in the Ormskirk Sandstone Formation is linked to early depositional textures, grain size (medium sand) and general lack of pore-filling cements. In the Tarporley, reservoir quality is poor due to the grain size (very fine sand) and the variable presence of pore-filling gypsum associated with playa deposits. Minor feldspar diagenesis (both dissolution and precipitation) occurred in both the Sherwood and Mercia sediments. The small degree of mechanical compaction and lack of any signs of mesodiagenesis suggest that the clastic sediments in the Formby-7 well have not been buried deeper than about 1000 m. The findings reported here can be used to help the understanding of UK Triassic sedimentology and reservoir quality for oil and gas, geothermal energy, CCS (carbon capture storage) and water supply.

Compositional and diagenetic evolution of a siltstone, with implications for reservoir quality; an example from the Lower Triassic Montney Formation in western Canada

Diagenetic models for silt-rich mudstone (siltstone) are less advanced than those for sandstone and clay-rich mudstone. In addition, the influences of diagenetic processes on reservoir quality in siltstone formations are poorly understood. Here we examine the roles of depositional facies, detrital composition, and diagenetic processes in the compositional evolution and reservoir quality of the Lower Triassic Montney Formation, a prominent siltstone-dominated reservoir in western Canada with very large reserves of oil, gas-liquids and natural gas. We applied advanced, high-resolution petrographic methods including SEM (SE, BSE, EDX and SEM-CL), XRD, and QEMSCAN technologies, as well as He-pycnometry on samples from 17 different locations within the basin that represent a range of depths and thermal maturities. Cross-cutting and overgrowth relationships and volumes of authigenic phases demonstrate massive precipitation of quartz, feldspar, dolomite, calcite and anhydrite cements at shallow burial depths, leading to compositional homogeneity among different lithofacies. We propose that the precipitation of large volumes of cement resulted from high fluid flux through the formation at shallow depth, possibly driven by reflux of concentrated seawater in nearshore saltpans. Siltstones preserve relatively high porosity and permeability at shallow burial depths, differing from clay-rich mudstones. In contrast to sandstones, deep burial diagenesis played a relatively minor role in the compositional evolution of the Montney Formation siltstone and is expressed in the form of pressure solution and minor amounts of fibrous illite precipitation.

A new ichnofauna from the Permian of the Zat Valley in the Marrakech High Atlas of Morocco

A new ichnofauna from the Permian of Morocco is described in details: it is the first Palaeozoic ichnofauna from the Zat Valley in Marrakech High Atlas. The new tracksite was found in the Tighdouine region, in the middle-upper Permian of the Cham-el-Houa Siltstone Formation. An abundant and diverse ichnoassemblage composed of both protostomian (probably arthropods and annelids) traces and vertebrate tracks is recorded. The presence of protostomian burrows and traceways, associated with tetrapod tracks corresponds to the Scoyenia ichnofacies. Protostomian traces are ascribed to Diplichnites gouldi, Diplopodichnus biformis, Scoyenia cf. gracilis and Spongeliomorpha carlsbergi. Tetrapod tracks include more than 70 tracks attributed to Amphisauropus, Erpetopus, Hyloidichnus, Characichnos, and indeterminate tracks. The co-occurrence of tetrapod tracks (both walking and swimming tracks), protostomian traces, mudcracks, ripple marks, as well as the lithological features of the track-bearing levels, indicate regularly inundated depositional environments during periods of high discharge under a local seasonal climate. A sedimentological analysis shows that the depositional environments evolved from braided-meandering systems to alluvial floodplains. The track-bearing surfaces are mainly preserved in crevasse splays, levees and pond deposits. This newly discovered ichnofauna helps to better reconstruct the palaeoenvironments of the Marrakech High Atlas in Morocco during the Permian, and enlarges the palaeogeographic distribution of important ichnotaxa.

Fluvial and lacustrine successions in the youngest part of the Murihiku Supergroup, New Zealand

Fossil-bearing Late Jurassic to Early Cretaceous volcaniclastic sandstones and siltstones from the Murihiku Supergroup are described from the Kaimango Syncline South Auckland, New Zealand. These constitute the youngest known sedimentary succession within the Murihiku Supergroup (Murihiku Terrane). We concentrate on the youngest Matira Siltstone Formation which has yielded a rich but relatively low-diversity macroflora of ferns and gymnosperms, diverse miospore palynofloras, with fair to good preservation, and sporadic occurrences of bivalve and gastropod faunas. Collectively, the paleontology and sedimentology of this formation is interpreted to represent paludal and lacustrine deposition, with intercalated sandstones thought to represent fluvial channelised or deltaic settings. U-Pb dating of detrital zircons (DZ) from sandstones in the youngest formations, Mangatara Measures and Matira Siltstone, have provided significant components in the age range 140 to 143 Ma, possibly straddling the Jurassic-Cretaceous boundary, although miospores suggest a Late Jurassic correlation. Lacustrine and paludal depositional environments have not previously been recognised from the Murihiku Supergroup and this raises the prospect of their enhanced petroleum source rock potential in frontier basins to the north and east of New Zealand.

Noble and major gases released from rock core materials as intrinsic tracers for detecting carbon dioxide leakage – Laboratory evaluation

Acidification of pore water in geological formations by carbon dioxide (CO2) results in the release of trapped gases, the detection of which can potentially be used to signal leakage of CO2 from storage reservoirs at carbon sequestration sites. In a set of laboratory tests, core materials from a stratigraphic borehole at the proposed FutureGen 2.0 carbon storage site (Jacksonville, IL, USA) were reacted with phosphoric acid to simulate contact with aqueous CO2. Gas released by the reaction was collected and analysed for the composition of the major and noble gases (including isotopes). These experiments yielded several promising tracer candidates for intrinsic monitoring of CO2 leakage at the proposed FutureGen 2.0 site. The most robust signals are the CH4 and 4He released by carbonates in the dolomite and siltstone formations that comprise the proposed primary containment zone. Xenon released from the primary-containment zone also showed promise as a tracer, but a more robust analysis requires a different experimental set up. A general enrichment of CH4, N2 and light noble-gas isotopes with respect to Ar abundance was observed and is consistent with the addition of major gases (CO2, CH4, and to a lesser extent, N2) released by decomposition of carbonate mineral phases and/or organic matter in the dolomite and siltstone formations. No useful signal was obtained from the sandstone formation (representing the proposed injection zone) due to its very low yield of gas when acidified.

3D Particle-Based DEM Investigation into the Shear Behaviour of Incipient Rock Joints with Various Geometries of Rock Bridges

A 3D particle-based DEM model was established taking into account the geometries of rock bridges. The model was used to investigate the shear behaviour of incipient rock joints. Fifty-seven direct shear tests were conducted under constant normal load (CNL) boundary conditions using the established model, in which rock bridges with 19 different geometries and incipient joints with various areal persistence (between 0.2 and 0.96) were involved. Our results show that, for the cases having a single rock bridge, cracks often initiated around the edges of the rock bridges and coalesced first in the middle of the rock bridge areas. While for other cases containing multiple rock bridges, cracks initially appeared at the connection points (located in the middle of the joint planes) of the rock bridges and then propagated to the edges. High crack initiation stresses were measured, which were often more than 60% of the shear strength of the tested incipient rock joints. Sudden failures of the rock bridges subjected to shearing were observed, accompanying dramatic increases in the number of cracks. Another important conclusion derived from this research is that both joint areal persistence and rock bridge geometry played significant roles in the shear failure of the simulated Horton Formation Siltstone joints. The present study has shown that shear strength increased gradually when joint areal persistence was decreased. Interestingly, different shear strength values were measured for rock joints with the same areal persistence (e.g. K = 0.5). Shear velocity was also found to have a significant influence on the shear characteristics of the Horton Formation Siltstone joints. A higher shear strength was measured when the shearing velocity was increased from 0.01 to 1 m/s.

The deep-water trilobite association of the Silurian Coldwell Siltstone Formation of northern England and its wider significance

The upper Wenlock Coldwell Siltstone Formation of Cumbria and North Yorkshire, which accumulated in the Lake District Basin, contains a deep-water, trilobite-dominated fauna that is documented in detail here. The trilobites of the formation are revised on the basis of new material and existing museum collections. Delops nobilis marri Rickards and Struveria howgillensis Rickards are considered conspecific with Dalmanites mobergi Hede and Dalmanites simricus Hede respectively from the Swedish Colonus Shale. Based on the type species, Delops Rickards is recognized as a senior synonym of Lygdozoon Holloway. A new genus, Rickardsia , is erected to accommodate Dalmanites mobergi Hede, Delops nobilis (Thomas), and Delops dermolac Šnajdr, which share characters that differentiate them from the type and other species of Delops . The fauna referred to previously as the Delops–Miraspis Assemblage is renamed the Rickardsia-Miraspis Association, and redefined to take account of new data. The trilobites Decoroproetus sp. nov.? and a cheirurid, and the brachiopods Leangella segmentum (Lindström) and Protozyga ? sp., are recognized in the fauna for the first time. Comparable trilobite and brachiopod assemblages occur at the same stratigraphical level in the Welsh Basin, southern Sweden, Poland and the Czech Republic, and together with that of the Coldwell Siltstone Formation, are considered to have invaded these areas during a eustatic lowstand during the late Wenlock Epoch. Comparison with other assemblages suggests that all occupied depths that were not less than the shallowest limits of benthic assemblage (BA) 5.

Field examples of the joint petrophysical inversion of resistivity and nuclear measurements acquired in high-angle and horizontal wells

A recently introduced interpretation workflow has confirmed that inversion-based interpretation is more reliable than conventional well-log analysis in high-angle (HA) and horizontal (HZ) wells because the former accounts for well trajectory and shoulder-bed effects on well logs. Synthetic examples show that the inversion workflow could improve the estimation of hydrocarbon volumes by 15% and 10% in HA and HZ intervals, respectively. Using field examples of thinly interbedded calcite-cemented siltstone formations, we document results of the joint petrophysical inversion of logging-while-drilling multisector nuclear (neutron porosity, density, natural gamma ray, photoelectric factor) and multiarray propagation resistivity measurement for improved formation evaluation in HA/HZ wells. Under the assumption of multilayer formation petrophysical models, the inversion approach estimates formation properties by numerically reproducing the available measurements. Subsequently, inversion-derived hydrocarbon pore volume is calculated for assessment of reservoir pay. Application of the joint inversion-based interpretation in challenging field examples highlights petrophysical characteristics such as capillary trends or water saturation variations in a hydrocarbon column influenced by reservoir quality and formation electrical anisotropy which otherwise remain inconspicuous with conventional and quick-look interpretation of well-logs.

Using Resistivity Measurements to Determine Anisotropy in Soil and Weathered Rock

This study uses electrical resistivity measurements of soils and weathered rock to perform a fast and reliable evaluation of field anisotropy. Two test sites at New Concord, Ohio were used for the study. These sites are characterized by different landform and slightly east dipping limestone and siltstone formations of Pennsylvanian age. The measured resistivity ranged from 19 Ω∙m to 100 Ω∙m, and varied with depth, landform, and season. The anisotropy was determined by a comparison of resistance values along the directions of strike and the dip. Measurements showed that the orientation of electrical anisotropy in the shallow ground may vary due to fluid connection, which is determined by the pore geometry in soil and rock, as well as by the direction of fluid movement. Results from this study indicated that a portable electrical resistivity meter is sensitive and reliable enough to be used for shallow ground fluid monitoring.

Application of till geochemical and indicator mineral data to the interpretation of the thick till sequence at Muhos, northern Finland

Recent drilling at Muhos, northern Finland has revealed an exceptionally thick Quaternary sediment cover, overlying the unmetamorphosed Neoproterozoic Muhos Siltstone Formation in the Oulujoki River Valley. Here we report preliminary petrographical, geochemical and heavy mineral results from four till units from the Muhos drill-core. The drilling site is located at 27 m asl, c. 1 km south of Oulujoki River and 30 km SE from the city of Oulu and the present shore of the Bothnian Bay of the Baltic Sea. The drilling at this site was performed with a GM 200 GTT drilling platform using 1 -m long sample tubes (HDPV, diameter 5 cm) inside the stainless steel cover tubes. The coring penetrated down to the siltstone bedrock 54 m below the ground surface and the sediment core recovery was over 75%. The sedimentary sequence from the ground surface down to the local siltstone bedrock consists of four till units interbedded with sand/silt/clay beds. The upper two till beds, Mu-1 and Mu-2, are interpreted as being deposited during the Weichselian Glaciation between 115 000–10 000 years ago, the third till bed Mu-3 during the Saalian and the lowest till bed, Mu-4, during the Saalian or pre-Saalian glaciations, i.e. till beds Mu-3 and Mu-4 prior to c. 130 000 years ago. Petrography of the coarse fraction, geochemistry of the fine fraction and heavy minerals of the sand fraction were studied from all four till units and the Muhos siltstone samples. Coarse and fine fractions were separated by dry-sieving. The fine fraction was analysed by XRF and AAS. The mineralogical composition of the heavy fraction was studied by FESEM+EDS and SEM+EDS. The lowest till bed Mu-4 can be divided into three subunits. On the basis of clast content, geochemistry of till fines the ice movement direction was from the NW or NNW during the deposition of the lowermost brown and red till units. The composition of the upper grey unit of till bed Mu-4 shows that the ice flow direction was from the WNW. Petrographical composition of till beds Mu-3 and Mu-2 indicates that the ice flow again was from the NW whereas the petrographical composition of the uppermost till bed Mu-1 suggests that the ice flow during this deposition phase was from the WNW. Younger till beds studied have in part also some reworked and re-deposited material from the older till beds. Supplementary material: The percentages of the rock types in pebble and gravel fraction, XRF and AAS analysis of the samples and identified mineral species of the heavy sand fraction are available at: http://www.geolsoc.org.uk/SUP18659

Evaluation of organic carbon content and source rock maturity using petrophysical logs and geochemical data: Case study of Horn Valley Siltstone source rock, Amadeus Basin, Central Australia

In this study, 30 samples from the Horn Valley Siltstone Formation from Amadeus Basin in Central Australia have been analyzed using geochemical method Rock-Eval (RE) pyrolysis. The analysis shows Total Organic Carbon between 0.21 to 2.74%wt . It also shows that kerogen type II, and a mixture of type II and III are the dominant kerogen. This type of kerogens are prone to oil and oil/gas production. The geochemical diagrams show that all samples are in the oil window, and they have good thermal maturation. The Horn Valley siltstone formation has been divided into 4 zones; Hv1, Hv2, Hv3 and Hv4. Although all zones are mature (i.e., have Tmax over 435 °C), Hv2 has a high organic carbon content and is located at the end of the oil window (Tmax between 450-470 °C). Also, to compare the results, petrophysical logs (Resistivity and Sonic) and ΔlogR method were used to determine the organic carbon content of source rock, and investigate zones with high pontentials to produce hydrocarbons. Zones have the potential to produce hydrocarbons was determined. And there was a close adaptation between the results of petrophysical method (ΔlogR) and Rock-Eval pyrolysis technique. The results show that Petrophysical method (ΔlogR) and the Rock-Eval pyrolysis technique have produced close results

Sample dilation and fracture in response to high pore fluid pressure and strain rate in quartz‐rich sandstone and siltstone

Natural hydraulic fractures (NHFs) are inferred to form where pore pressure exceeds the least compressive stress by an amount equal to the tensile strength of the rock. We improved upon an experimental protocol that meets the NHF criterion within cylindrical samples with the most tensile effective stress parallel to the sample axis. The effective tensile stresses achieved during these experiments ranged from 17–47 MPa. The pore fluids used had higher viscosities than water and the axial strain rate was rapid (∼10−3s−1) to delay dissipation of fluid pressure by flow. Four experiments on St. Peter Sandstone samples and two on an Abo Formation siltstone sample were performed under these conditions and under drained conditions. None of the drained experiments resulted in failure, but all of the sandstone and one of the siltstone samples fractured in response to elevated pore pressures. Consistent with field and theoretical studies, mechanical heterogeneity was a first order control on fracture location. In the absence of mesoscopic heterogeneity, fracture location coincided with the maximum pore pressure. Samples responded to elevated pore pressures and differential stresses by dilating, the magnitude of which was sufficient to achieve atmospheric pore pressure. Samples failed 2 to 250 s after experiencing the greatest pore pressures, when the effective stresses were no longer tensile. Thus, the high pore pressures and effective tensile stresses experienced early in the experiments were sufficient to fracture the rocks, even though they were not sustained until failure. These results provide insight into processes of fluid‐driven fracture formation.

A new species ofSiluraster(echinodermata: Asteroidea) from the Late Ordovician of North Wales

AbstractA new species of starfish,Siluraster caemorensissp. nov., is described from a loose block of probably the Bryn Siltstone Formation or Teirw Siltstone Formation (Caradocian) from near Glyn Ceiriog, on the north side of the Berwyn Range, North Wales. The species is similar toSiluraster perfectusfrom Bohemia and indicates that there may have been closer faunal links between the areas than are generally suggested. It is separated from known Welsh species of similar age by relatively subtle structural details that are not invariably preserved. This suggests that there is a higher diversity of asterozoan species in the Late Ordovician of Wales than has previously been recognized. Copyright © 2010 John Wiley & Sons, Ltd.