Fine-grained Sandstone Research Articles

Pore throat heterogeneity of different lithofacies and diagenetic effects in gravelly braided river deposits: Implications for understanding the formation process of high-quality reservoirs

Tight gas sandstone is a unique kind of unconventional hydrocarbon resource in that its intricate pore throat structure governs reservoir physical properties and gas reservoir productivity. However, the effect of differential diagenesis of various lithofacies on the heterogeneity of pore throat structure remains unclear. In this paper, a series of experiments were integrated to evaluate pore throat structures and fractal characteristics of the different lithofacies in a tight sandstone gas reservoir, and followed by a study of the impact of diagenetic alteration on them. Results demonstrate that the mineral composition, dominant pore types, pore throat structural parameters, fractal dimension, and physical properties of various lithofacies are notably different, which is mostly attributable to diagenetic alteration variability. There are more primary pores and less clay mineral content in coarser lithofacies. Pebbly coarse-grained sandstone and sandy conglomerate reservoirs that have undergone strong dissolution have more composite pores composed of residual intergranular pores and dissolution pores, which help to form a homogeneous pore throat system with good connectivity, resulting in the best reservoir quality. Due to the strong compaction and high clay mineral content filling pores and throats in medium-grained sandstone and fine-grained sandstone lithofacies, a large number of primary pores were lost, and throats were blocked, which easily formed intercrystalline pores, leading to poor reservoir quality. Coarse-grained sandstone reservoirs with moderate clay mineral content have undergone moderate compaction and dissolution and have moderate reservoir quality. Filamentous illite is the most destructive clay mineral to pore throat connectivity. Intergranular dissolution pores and moldic pores contribute significantly to the physical properties of pebbly coarse-grained sandstone and sandy conglomerate reservoirs. Pebbly coarse-grained sandstone and sandy conglomerate lithofacies are considered high-quality reservoirs with greater development potential for tight gas. This work has important guiding significance for the prediction of high-quality reservoirs in similar tight sandstone reservoirs.

Dinosaur tracks from the Sítio Pereiros ichnosite, Triunfo Basin (Lower Cretaceous) and the dinosaur diversity in the Rio do Peixe basins, Northeastern Brazil

The dinosaur tracks in the Rio do Peixe basins (Lower Cretaceous, Rio da Serra-Aratu stages) occur in at least 39 individual tracksites through approximately 98 stratigraphic levels in the western part of the State of Paraíba, Brazil. The Triunfo basin (one of the four Rio do Peixe basins) is a 480 km2 asymmetric graben, located in the counties of São João do Rio do Peixe, Uiraúna, Poço, Brejo das Freiras, Triunfo, and Santa Helena, controlled by a NE transcurrent fault system. To date, only four isolated footprints and two incomplete trackways have been identified in the Antenor Navarro Formation. Among the isolated footprints, three probably belong to theropods. One incomplete trackway consists of just two digitigrade, rounded digits, suggesting they were made by a small ornithopod. In this study we describe a new ichnosite, located at Sítio Pereiros, São João do Rio do Peixe county, Paraíba State. The one and a half meter thick succession of fine-grained sandstones, siltstones, mudstones and shales with ripple marks, climbing ripples and mud cracks of the Sousa Formation reveals a bedding plane with three trackways, with a total of 19 tridactyl, mesaxonic footprints. These trackways are interpreted as produced by theropods, two large and one smaller. In these beds there are also ostracods, spinicaudatans (conchostracans), and fragments of microvertebrates (fish scales, teeth and bones). The Sítio Pereiros ichnosite represents a deposition in a floodplain area, with temporary aerial exposure of the superficial sediments in which tracks were impressed. The ichnofauna from this locality increases knowledge of the theropod fauna from the Triunfo basin and the distribution of the dinosaur tracks throughout the interior basins of Northeastern Brazil.Description of these new theropod tracks permits evaluation of the behavior of these three theropods, including inferences about trackmaker speed and the type of gait of the three animals, and also of their possible size. This is the 40th ichnosite in the Rio do Peixe basins, extending analysis of the types of trackmaker associations present at such ichnosites, as well as the dinosaur diversity represented at each of them. New interpretations are presented about the environments, and the relationship between the various groups represented in this region during the Early Cretaceous.

GCS site selection in saline Miocene formations in South Louisiana

One of the key requirements of a commercial scale geologic carbon sequestration (GCS) site selection process is identifying injection sites to safely inject and retain injected CO2 over extended time scales. This requires thorough evaluation of site-specific surface and subsurface features. A site screening and selection framework was developed and used to identify potential sites in South Louisiana, incorporating surface data including site proximity to large CO2 stationary sources, vicinity of large population centres and state lands, land usage, legacy oil and gas wells, and local/regional geologic trends. Regionally the Miocene sediments thicken and dip towards the south, reflecting a prograding deltaic environment during the Miocene which resulted in the deposition of thick packages of fine-grained sandstones suitable for CO2 storage. The results of three example selected sites that are spatially separated with varying subsurface and surface features are presented in this study. The multiple stacked sand zones at these sites provide ample amount of pore volume for CO2 storage with varying degree of stratigraphic/structural traps for long terms storage containment. The screening framework presented in this study and the detailed evaluation of three specific sites in South Louisiana provides a process to screen and characterize sites for GCS projects and the suitability of these Miocene sediments for commercial scale CO2 storage projects. It is envisioned that the analysis described in this paper will be beneficial for a large number of audiences involved in GCS work and aid in the site selection process.

Systematics, ecology, and biochronology of blancan horses from Sonora, northwestern Mexico

We describe a set of dental and postcranial horse material recovered from the locality Los Hornitos, northeastern Sonora, northwestern Mexico. The fossil-bearing unit consists of brown silt with intercalation of fine-grained sandstone and bioturbated incipient soils, set in an alluvial environment and considered Pliocene in age. The comparative study of the available sample indicates the presence of two species: the small-sized hipparionine Nannippus peninsulatus and the medium-sized and primitive equine Equus simplicidens. The isotope composition of δ13C and δ18O in dental enamel tooth indicates a C3–C4 mixed diet for these species and suggests the presence of woodland and open areas in Los Hornitos during the Pliocene. The less negative δ13C values observed in E. simplicidens are related to a more C4 diet, preferring to roam open habitats. The coexistence and optimal resource partitioning between these species are explained, considering their differences in dietary behavior, body size, home range, and/or vagility. The recognized horse species are typical of the Blancan North American Land Mammal Age, and their biochronological information from the southwestern United States and central Mexico establishes the probable age of the Los Hornitos at about 3.9–2.6 Ma, corresponding to the Blancan III or late early Blancan – early late Blancan.

Development of a three-dimensional grain-based combined finite-discrete element method to model the failure process of fine-grained sandstones

This paper introduces a novel three-dimensional grain-based hybrid finite-discrete element method parallelized based on general-purpose graphic processing units (GPGPUs) and applies it to investigate the failure process of sedimentary fine-grained sandstones. The grain-based method considers the actual microstructures of rocks with Voronoi or grain-growth tessellations to model their failures, including transgranular, intergranular and intragranular crack propagations. The novel semi-adaptive contact activation approach (semi-ACAA) proposed by the authors and the efficient tetrahedron-to-point (TtoP) contact interaction algorithm developed by HOSS are implemented to speed up grain-based modelling in addition to GPGPU parallelization. Semi-ACAA and TtoP are approximately 2–20 times and 1.5 times faster than the brute-force contact activation approach and tetrahedron-to-triangle contact interaction algorithm, respectively, which are prevalent in the FDEM community. The grain-based modelling elucidates that most intragranular cracks are observed in the medium- and high-strength grains of the sandstones, while most intergranular cracks occur at the grain boundaries between low- and high-strength grains. The transgranular cracks do not discriminate any grains on their propagating paths but propagate and coalesce with intragranular and intergranular cracks, which results in the final failure of the sandstones.

Characteristics of the Tectonic Background and Sedimentary Paleoenvironment of Coal-Bearing Strata from the Yan’an Formation in the Ordos Basin: Evidence from the Geochemistry of Major and Trace Elements

The coal-bearing strata of the Jurassic Yan’an Formation in the Ordos Basin, China, contain various energy sources, including coal, oil, and gas. However, due to the complex geological conditions of the coexistence of multiple energy sources, it is difficult to explore coal, oil, and gas. Geochemical features record important geological information about rock formations at that time. Therefore, it is of great significance to study the characteristics of major and trace elements and the paleoenvironment of the coal-bearing strata of the Jurassic Yan’an Formation in the Huangling area for the subsequent smooth mining of coal mines and other energy sources. An X-ray fluorescence spectrometer and inductively coupled plasma mass spectrometer were used to determine the major and trace elements of the Yan’an Formation, respectively, and the coal-bearing strata of the Yan’an Formation were comprehensively analyzed from the vertical and different lithology aspects: the paleoweathering characteristics, provenance environment, sediment source, and tectonic background. At the same time, the paleosedimentary environment characteristics of coal-bearing strata were recovered through a series of environmental discrimination indicators, including Sr/Cu, Sr/ppm, Sr/Ba, Th/U, U/Th, V/Cr, V/(V + Ni), V/Sc, and V/Ni. The results show that the sedimentary mode of the Yan’an Formation in the Ordos Basin is mainly fine-grained deposition, and tight sand-mud interbeds and mudstone layers are developed in fine-grained sandstone deposits. The sedimentary rocks are dominated by granite (obvious negative Eu anomaly), mixed with some basement volcanic rocks and ancient sedimentary rocks, and the parent rocks have many geneses (high SiO2, low MgO, and low CaO). The degree of weathering is generally stable but has not experienced complete weathering. Among them, the lower clastic particles have undergone moderate weathering from the source area to the sedimentary basin, and the sandstone is slightly higher than the mudstone. The paleoclimate of the Yan’an Formation was warm and humid in the early stage and tended to be dry and hot. The overall paleowater depth was less than 10 m, and the local area showed a shallow lake subfacies. It experienced a transition from a saline lake facies to a freshwater environment. It is typical brackish–brackish water with a partially oxidized environment (i.e., a sub-rich oxygen environment).

Invertebrate trace fossils from Paleogene fluvial strata in Western Washington, USA

Fluvial strata in the Paleogene Chuckanut Formation contain a wealth of trace fossils that include both vertebrate and invertebrate ichnofossils. Vertebrate traces have received careful scrutiny, but invertebrate traces have not previously been described. Trace fossils occur in two sediment types. Mudstones deposited in subaqueous environments include highly bioturbated layers that are dominated by horizontal meniscate burrows. Siltstone and fine-grained sandstone beds primarily contain vermiform trails on bedding surfaces, with sparse vertical traces. Some of these sandy beds also contain vertebrate tracks. Chuckanut Formation beds commonly show rapid facies changes in sediments that were deposited along a meandering river during a time of semitropical climate. Transitions from subaqueous to subaerial conditions may have been related to changing water levels caused by variations in seasonal precipitation or episodic flood events. Other possible factors include ponds created by abandonment of channels, overbank sediment s deposited by floods, and breaching of levees to form crevasse splay deposits. Trace fossil assemblages show local variations in the diverse depositional environments of the Chuckanut Formation. Many of the ichnofossil suites clearly represent Scoyenia Ichnofacies, but some occurrences may involve overprinting of Scoyenia and Mermia ichnofacies.

Mineralogical characteristics of the heterolithic formations from the Carpathian Foredeep Miocene sediments, south-eastern Poland

Heterolithic complexes from the Carpathian Foredeep are a subject of growing attention as many natural gas accumulations of industrial importance have been discovered in such formations during the last decades. The aim of the presented work was to determine lithotypes of different lithology and mineralogical composition in the heterolithic sequences. Individual lithotypes were distinguished of macroscopic observations of cores, X-ray diffraction analysis (XRD) and X-ray fluorescence method (XRF). Preliminary distinction of the lithotypes was based on macroscopic observations of the core samples. Correlation plots between the contents of particular minerals (XRD analyses) and corresponding elements (XRF analyses) were used for verification of the lithotypes distinguished on the basis of macroscopic observations. Swelling properties of the investigated rocks were determined using the cation exchange capacity (CEC) values. The following lithotypes were identified: medium- to fine-grained sandstones, fine- to very fine-grained sandstones, clayey sandstones, sand-dominated heteroliths, heteroliths with equal proportions of sandstones and mudstones, mud-dominated heteroliths, mudstones, clayey shales. Such division provides a basis for precise petrophysical characterization of the analyzed profiles, that is assignment of particular petrophysical parameters values to individual lithotypes. More reliable petrophysical parameters in the geophysical interpretation of heterolithic sequences allow for a more precise determination of zones characterized by good reservoir parameters.

Early Late Triassic retro-foreland basin in response to flat subduction of the Paleo-Tethyan oceanic plate, SE Tibet

Syn-subduction basins bear significant implications to understand tectonic evolution of any fossil subduction zone. The late Paleozoic to early Mesozoic (Paleo-Tethyan) tectonics of the eastern and southeastern Tibetan Plateau (i.e., the Sanjiang Orogenic Belt) is featured by ocean-continent subduction systems. A huge pile of volcanic-absent sedimentary succession developed in the middle segment of the Sanjiang orogenic belt, its age and tectonic nature remain unclear. Detailed geological mapping and zircon U-Pb dating results demonstrate that the early Late Triassic volcanic-absent succession comprises the nonmarine Maichuqing Formation in the lower part and the shallow marine Sanhedong Formation in the upper part. The Maichuqing Formation consists of coarse to fine-grained sandstone, siltstone and mudstone with abundant basal erosional surfaces, trough and planar cross-beddings, ripples, mudcracks, and plant fragments. The Sanhedong Formation comprises predominantly bioclastic limestones interlayered with marl, calcareous-muddy siltstone, and calcareous sandstone with abundant bivalve fossils. Syn-sedimentation deformation structures, such as slump folds and associated normal faults are common, suggesting intense tectonism during deposition. Synthesizing sedimentary data, paleocurrent and provenance results, combined with other available data, demonstrate that the volcanic-absent succession deposited within a retro-foreland basin along the rear part of the Permian-Triassic Jomda-Weixi-Yunxian arc in response to flat-subduction of the Paleo-Tethyan Ocean during the early Late Triassic time.

Formation evaluation and reservoir quality assessment of the Albian clastic sequence in the SWM Oilfield, Southwest the Mesopotamian Basin, Iraq

This work intends to delineate the petrophysical properties and the reservoir quality of the Albian sequence in the Mesopotamian Basin which has been deposited as a siliciclastic sequence. The well log data (including the caliper, gamma-ray, density, photoelectric factor 'PEF', neutron, sonic, and resistivity logs), derived from eight wells in the SWM (Southwest Mesopotamian) Oilfield, was studied and interpreted using the appropriate software (IP3.6). These borehole data indicate the necessity to divide the sequence into five reservoir rock units (U1, U2, M, L1, and L2 units). This enabled depicting the porosity, shale volume, water saturation, and net-pay thickness in the vertical and horizontal directions. It is indicated that the M and L1 units are the most promising units with good to very good porosity (∅ ≥ 17.2%), low water saturation (Sw ≤ 28.0%), and acceptable shale volume (Vsh ≤ 8.4%). The net-pay thickness varies between 32.8 and 77.4 m in the different wells of the SWM Oilfield. The well log data were supported by more detailed conventional core data including porosity (∅He) and permeability (k), from which the reservoir quality parameters were estimated precisely. In the present study, reservoir quality assessment based on conventional core data is considered for the first time for evaluating and discriminating the Albian–Aptian sequence in the Southwest Mesopotamian field into reservoir zones and hydraulic flow units. The estimated reservoir parameters include the Flow Zone Indicator (FZI), the Reservoir Quality Index (RQI), and the effective pore radius (R35). Based on the available core data and the estimated reservoir quality parameters, the studied sequence was divided into three reservoir rock types (RRT1–3). The best petrophysical properties have been assigned for the medium-grained sandstone of the RRT1 that dominates in the L1 and the M units (av. ∅He = 23.0%, av. k = 1581 mD, av. R35 = 23.7 μm, av. FZI = 7.368 μm, and av. RQI = 2.304 μm). On the other side, the fine-grained sandstone of the RRT3 is characterized by the lowest petrophysical properties and reservoir quality parameters (av. ∅He = 17.1%, av. k = 5.49 mD, av. R35 = 0.92 μm, av. FZI = 0.603 μm, and av. RQI = 0.136 μm). The integration between the well log and core data of the studied Albian sequence stated that the M and L1 units are the most promising reservoir units. They are predominated by medium-grained and fine to medium-grained sandstones of the RRT1 and the RRT2, respectively. This well log-core data integration is applicable to the different oilfields in the different fields in the Mesopotamian Basin and other basins in south Iraq and elsewhere.

An allostratigraphic framework for the Late Albian to Early Cenomanian upper Fort St. John Group across the foredeep of the Western Canada Foreland Basin, NE British Columbia

Abstract The Upper Albian to Lower Cenomanian Hasler and Cruiser shales and intervening Goodrich Sandstone are up to about 800 m thick and constitute most of the upper Fort St. John Group in the foredeep of NE British Columbia. These rocks exhibit pronounced lateral changes in lithology and thickness from west to east, making their lithostratigraphic boundaries highly diachronous. An allostratigraphic approach to correlation, based primarily on discontinuity surfaces represented in wireline well logs, allows subdivision of these broad lithostratigraphic units, and permits correlation of genetic stratal packages across facies transitions, and from subsurface to outcrop. Upper Fort St. John strata spanning the foredeep in British Columbia are shown to be correlative with Viking allomember VD, and with the overlying Westgate and Fish Scales alloformations, previously defined in Alberta on the basis of outcrop, core and wireline log data. Bounding surfaces have also been correlated northward to connect with successions described from the Sikanni and Liard river areas of British Columbia. In the British Columbia study area, pyritic, largely unbioturbated mudstone of allomember VD thickens from 20 to 200 m, indicating syn-depositional flexural subsidence, greatest in the north-western part of the study area. The Westgate alloformation is also dominated by weakly bioturbated mudstone, but includes more common, thin, fine-grained sandstone beds, many with wave- and combined-flow ripples, organized in parasequences up to approximately 20 m thick. Westgate allomembers WA through WD take on a progressively more tabular geometry upward, reflecting diminishing flexural subsidence with time. Allomember WD includes shoreface/delta front sandstones (‘Goodrich Formation’) that extend eastward from the Foothills outcrop for up to 70 km. The Fish Scales alloformation, the base of which is defined by surface FE1, marks an abrupt introduction of sand across a muddy sea floor in response to sea-level fall. The upper boundary of the Fish Scales is the condensed section/downlap surface ‘Fish Scales Upper’ (FSU) beneath the Dunvegan alloformation. The Fish Scales alloformation is a pronounced wedge, thickening westward from approximately 100 to approximately 400 m. It consists mainly of pyritic, unbioturbated mudstone to claystone with a variable content of fish debris, indicative of dysaerobic to anaerobic bottom-water conditions. Ammonites do, however, indicate oxic surface waters. The Fish Scales alloformation comprises allomembers FA and FB, separated by an erosion surface termed the Base Fish Scales Marker (BFSM) which, throughout the Peace River region, has a veneer of chert pebbles that mark a major sea-level lowstand; pebbles were subsequently reworked by marine transgression. A 15 m thick conglomerate-filled channel (or paleovalley?), exposed in Hasler Creek, directly underlies the BFSM surface and is interpreted to be the deposit of a large river that supplied sediment to the lowstand shoreline. In the far west, close to the Foothills, the Fish Scales alloformation becomes sandy and bioturbated, and includes stacked ‘Goodrich’ shoreface sandstone bodies that prograded a few tens of km eastward. Mapped to the SE, allomember FA pinches out close to the Alberta-British Columbia border, whereas allomember FB persists as a thin and highly radioactive phosphatic mudstone, the base of which comprises the merged FE1 and BFSM erosion surfaces. Despite up to about 800 m of subsidence, sedimentary structures throughout the succession show that the sea floor, which formed a homoclinal ramp, remained above storm wave base, attesting to a very high sedimentation rate and effective offshore dispersal of sediment, primarily by storm-driven combined flows. Bivalves of the genus Posidonioceramus have recently been proposed as a marker for the base of the Cenomanian Stage. These fossils are found in the study area as low as the base of Viking allomember VD, which implies that the Albian-Cenomanian boundary may lie at a level significantly lower than the long-accepted ‘Base of Fish Scales’ erosion surface.

THE PHYSICAL PROPERTIES OF THE CAMBRIAN ROCKS OF THE BUG STRUCTURE OF THE PODLASSKO-BREST DEPRESSION

The generalized results of studies of the physical properties of rocks along the section of Cambrian deposits (Rytskaya, Stradechskaya, Spanovskaya, Bugskaya and Orlinskaya formations) of the Pribug structure within the same-name underground gas storage (UGS) are presented. More than 120 rock samples evenly distributed throughout the thickness of the Cambrian deposits from the depth range of 889–1240 m were studied. The reservoir rocks are mainly
 quartz fine-and medium-grained sandstone with varying degrees of grading and grain rounding, type of cementation, clay content. The cap rocks are represented by clayey siltstone with the presence of small fine-grained sandstone and siltstone bands and by silty mudstone-like clays.
 Laboratory research data include the results of the study of chemical, granulometric compositions of the rocks,
 determination of carbonate and clay minerals in the rocks, determination of density, open and closed porosity, gas and
 reservoir water permeability on the specially prepared samples, study of the pore space of the rocks using digital photographs.
 The established features of filtration-capacitance properties of the rocks can be used for carrying out activities and developing recommendations to improve the efficiency of using the pore space of the deposits of the structure aimed at
 upgrading operation of the Pribug gas storage.

The sedimentology and depositional environments of the Bastians Dal and Muslingebjerg formations: evidence for the earliest phases of Jurassic rifting in North-East Greenland

The aim of this study is to elucidate the character of the earliest phases of Jurassic rifting in North-East Greenland. To achieve this, detailed sedimentological analysis and geological mapping were undertaken on the outcrops of central Kuhn Ø (74°53’55’’N,20°20’56”W). In this region the basement is overlain by the fluvial Bastians Dal Formation (Middle Jurassic) which is, in turn, overlain by the coal-bearing Muslingebjerg Formation. A maximum thickness of 140 m is calculated for the Bastians Dal Formation and mapping of stratal geometries demonstrates thinning to both the north and south, confirming that these deposits infill a palaeovalley. Predominantly south-westward palaeocurrent orientations are observed and likely reflect the orientation of the palaeovalley (NE–SW). The overlying Muslingebjerg Formation displays significant lateral variations in thickness as well as facies, thickening from a 5-m-thick coal seam in the north to 50 m in the south. Southern outcrops include two intervals of fine-grained sandstones displaying low-angle and trough cross-bedding some of which contain suggestions of tidal bundling. The arrangement of facies suggests that coal formation occurred in both fluvial- and shallow-marine (tidal?) environments. Coals are similar to those described elsewhere from the Muslingebjerg Formation and display subtle differences consistent with variable degrees of marine influence. Mapping demonstrates the presence of an NE–SW-oriented bounding fault in the south of the region into which the Muslingebjerg Formation thickens. This likely also controlled the orientation of the underlying NE–SW-aligned palaeovalley and is oblique to the proposed overall N–S orientation of faulting related to rifting through the Mid to Late Jurassic. Instead, these alignments resemble those that define pre-Jurassic phases of rifting and may therefore indicate a transitional phase of tectonism. Faulting on a similar alignment can be traced SW, cutting Lindeman Fjord and following the valleys east of the A. P. Olsen Land plateau.

Controls on reservoir quality of the Nanushuk Formation (Albian–Cenomanian), North Slope, Alaska

Recent hydrocarbon discoveries in the Albian–Cenomanian Nanushuk Formation on the North Slope of Alaska have revived interest in exploration of the Colville basin. The Nanushuk forms a segment of the Brookian sequence and together with the genetically related Torok Formation comprises part of a giant clinothem filling the western two-thirds of the basin. It consists of a succession of intertonguing marine and nonmarine strata interpreted as marine shelf, deltaic, strandplain, and fluvial deposits. Deposition occurred in two deltaic complexes, one sourced from large drainage basins extending west of present-day Arctic Alaska, the other from smaller catchment areas with headwaters in the ancestral Brooks Range to the south.The Nanushuk Formation consists of medium- to very fine-grained lithic sandstone and siltstone comprised largely of monocrystalline and polycrystalline quartz, chert, and argillaceous sedimentary and metasedimentary rock fragments. With progressive burial and compaction, ductile deformation of the argillaceous detritus is the principal mechanism of porosity and permeability loss in the sandstone. Cements are a minor component and have minimal effect on diagenesis of the strata. Reservoir quality varies extensively across the North Slope and understanding the factors controlling reservoir potential is a critical aspect of recent exploration programs.Two groups of sandstone are recognized based on differences in reservoir quality: a low-porosity group with maximum porosity less than 20 percent, and a high-porosity group with higher porosity values for a given permeability and maximum porosity exceeding 30 percent. Variation in reservoir quality within each group is delimited by depositional texture which is a primary, local control. The disparity between the groups results from differences in the maximum burial depth (Dmax) the rocks experienced which is a secondary, regional control. Linear regression models for porosity–Dmax and permeability–Dmax relations enable forecasting the reservoir potential of sandstone and siltstone containing only minor cement.

Stratigraphy and paleoenvironment of the Upper Jurassic Río Damas formation in the Alto Atuel depocenter (Malargüe fold and thrust belt)

The Río Damas Formation is an Upper Jurassic volcano-sedimentary succession, whose outcrops are largely distributed between 33°S and 35.30°S in Chile and Argentina, as part of the retroarc Neuquén Basin fill. This unit was deposited during an important marine regressive phase of the basin. We studied the Río Damas Formation exposures at the Los Gateados creek valley, in the Alto Atuel depocenter (34°40′S; westernmost Malargüe fold and thrust belt), next to the Argentinean-Chilean border. This area represents a key location where the formation interdigitates with the nonmarine red beds of the time equivalent Tordillo Formation, representing a continentalization episode of the Neuquén Basin. This paper presents a lithofacial characterization of the Río Damas Formation, a new U–Pb igneous crystallization age, and a paleographic reconstruction for this period. The Río Damas Formation in the Alto Atuel depocenter consists of porphyritic basaltic and andesitic lavas, interbedded with fine-grained sandstones, conglomerates, volcanic lithic breccias, and pyroclastic deposits. Based on field and petrographic analyses, we associate the Río Damas Formation with a piedmont environment where sedimentary fluvial and alluvial setting coexisted with volcanic effusive and pyroclastic activity. A regional comparison of the Río Damas and Tordillo Formations along the Argentinian-Chilean border shows that the composition and thickness of these units present important latitudinal and longitudinal variations. The increase in thickness and the volcanic component of the Río Damas Formation towards the west could be associated with the proximity to the Jurassic magmatic arc.

High-resolution taphonomy and sequence stratigraphy of internally complex, bakevelliid-dominated coquinas from the Aptian Romualdo Formation, Araripe Basin, NE Brazil

The Aptian Romualdo Formation (Araripe Basin, NE Brazil) was deposited in a restricted epeiric sea, during the opening of the South Atlantic Ocean, with a coeval record to that in the Brazilian Pre-Salt succession. The upper part of the Romualdo Formation encompasses the Highstand Systems Tract of a third-order stratigraphic sequence, and contains shell beds interbedded with shale, siltstone, and fine-grained sandstone. Based on distinct taphonomic features (shell sorting, fragmentation, abrasion, rounding, and orientation), sedimentological criteria (shell abundance, type of matrix, sedimentary structures, and presence/absence of quartz grains), and stratigraphic attributes (bed thickness, geometry, contacts), the paleoenvironmental conditions and the bed-by-bed accumulation history of the coquinas is disentangled. Eight carbonate microfacies were recognized, including ostracod carbonate mudstone, bivalve-gastropod wackestone, gastropod-bivalve packstone, bivalve-gastropod grainstone, bivalve-gastropod floatstone, bivalve-gastropod rudstone, bakevelliid floatstone, and bakevelliid rudstone, which are organized in six, cm-thick shell concentrations. Mudstone and wackestone facies mark the settling of carbonate muds without significant bottom currents and wave action, probably below storm wave base, whereas packstone, floatstone, and rudstone, with fragmented, rounded, and oriented shells, were deposited in agitated waters, between the fair-weather and the storm wave bases. Ripple cross-laminated grainstone is interpreted as high-energy facies deposited above fair-weather wave base. The shell beds are internally complex and consist of cm-scale alternations of microfacies differing in packing, sorting, and composition. They tend to fine upwards within dm-thick shell beds, generating shallowing-upward facies sequences. The concentrations, the uppermost one with in situ bakevelliid shells in the top, record high-frequency base level oscillations influenced by eustasy and climate changes. These bioclastic accumulations originated by the superposition of sedimentologic and primary biologic processes (e.g., post-deposition meiofaunal bioturbation). The cm-thick, low-diversity shell beds are excellent examples of multiple-event carbonate deposits generated in a siliciclastic-dominated, restricted epeiric sea, revealing high-frequency cycles in proximal to distal restricted marine settings.

Late Cretaceous Dimorphic Scaphitid Ammonoid Genus Yezoites from the Circum-North Pacific Regions

Yezoites is a Late Cretaceous (Cenomanian–Campanian) small to very small dimorphic ammonoid genus belonging to the Scaphitidae, whose microconchs have a pair of long lateral lappets at the aperture. Based on examination of previously described material including type and figured specimens and newly recovered ones, five species of Yezoites are described herein from the circum-North Pacific regions (Japan, Sakhalin, Kamchatka, Alaska, Oregon, and California); i.e., Y. perrini (Anderson), Y. seabeensis (Cobban and Gryc), and Y. puerculus (Jimbo) from the Turonian, and Y. pseudoaequalis (Yabe) and Y. matsumotoi (Tanabe) from the Coniacian. Of these species, both microconchs and macroconchs are recognized in Y. puerculus and Y. pseudoaequalis, whereas only microconchs are known in the other three species. These Yezoites species characteristically occur in very fine-grained sandstone to silty mudstone facies suggesting an intermediate between nearshore and offshore environments, and can be used supplementarily for biozonation and correlation of the Turonian and Coniacian deposits in the North Pacific regions.

Failure Characteristics and Failure Process Investigation of Fine-Grained Sandstone under Impact Load by Ultrahigh Speed DIC and SEM Techniques

Failure Characteristics and Failure Process Investigation of Fine-Grained Sandstone under Impact Load by Ultrahigh Speed DIC and SEM Techniques

Oxfordian plug-shaped trace fossils from northwestern Algeria

Plug-shaped trace fossils B. perata, B. hemispherica, Bergaueria isp., Calycraterion samsonowiczi, Conostichus ornatus and Conostichus isp. are found in the middle-upper Oxfordian fine-grained sandstone of the Argiles de Saïda Formation (northwestern Algeria). Except for the trace Calycraterion which is interpreted as an annelid trace, these traces correspond to distinct behaviours of sea anemones, and their occurrence within tempestite deposits is influenced by a stress factors in a storm-dominated environment. The abundance of these plug-shaped trace fossils suggests that the upper Jurassic siliciclastic shelves with fine-grained soft clastic substrate were colonised by actinarian sessile benthos.

Experimental study on the effect of rock pressure on sandstone permeability

The results of laboratory studies to determine the effect of effective stress on the permeability of sandstone are presented. During the test, the samples were subjected to a stepwise increase or decrease of the effective stress (at a constant pore pressure) in a specified step. The values of rock permeability at different values of effective stress were determined, and the influence of the grain size of the reservoir rock matrix on the character of the change in the sandstone permeability coefficient was also established. During the test, a decrease in permeability was observed with an increase in effective stress. It was found that as a result of gradual loading/unloading of the sandstone sample, the original permeability values were not restored, which indicates the beginning of the formation of residual strains in the rock. This effect should be taken into account when modeling field development because in the process of reserves extraction the effective stress acting on the reservoir rock skeleton changes, which results in a significant chang in rock permeability. The results of laboratory studies showed that the deviation of permeability in medium-grained sandstones relative to the initial value was greater than in medium- and fine-grained sandstones. The pressure sensitivity coefficient and constant of material, which are used in empirical relationships between permeability and effective stress, were numerically estimated. At the same time, the constant of material showed no such convergence, which indicates that the values of this parameter are individual for each rock.