Sandy Mudstone Research Articles

The Jurassic–Cretaceous lithostratigraphy of Kilen, Kronprins Christian Land, eastern North Greenland

Kilen, Kronprins Christian Land, contains the thickest and stratigraphically most complete Jurassic and Cretaceous sediment succession in North Greenland. This study revises and formalises the lithostratigraphic framework of these deposits. The work is based on recent extensive stratigraphic field work supplemented by photogeological mapping and biostratigraphic studies, and builds on the earlier stratigraphic work conducted mainly in the 1980s and 1990s. According to the new stratigraphic scheme, the more than 500 m thick Jurassic succession is divided into four formations. The poorly dated Gletscherport Formation comprises lagoonal heterolithic sandstones. The Mågensfjeld and Birkelund Fjeld Formations consist of shallow marine fine-grained sandstones of Bajocian–Bathonian and Kimmeridgian age, respectively. The Kuglelejet Formation comprises mainly shallow marine sandy mudstone and sandstone of Volgian age and includes the mudstone-dominated Splitbæk Member. The Lower Cretaceous interval is estimated to be more than 1500 m thick and is divided into three formations. The Dromledome Formation comprises deep shelf to offshore transition, black mudstones of late Ryazanian to Hauterivian age. It is erosively overlain by unfossiliferous, fluvial and estuarine sandstones of the Lichenryg Formation. The overlying, late Aptian to middle Cenomanian Galadriel Fjeld Formation comprises six members, of which the Tågekyst and Kangoq Ryg Members occur in the Gåseslette area, whereas the Pil, Valmue, Stenbræk and Hondal Members occur in the Kilen Fjelde area. The Galadriel Fjeld Formation is characterised by interbedded mudstones and sandstones from offshore–shoreface environments. The 650 m thick Upper Cretaceous succession is assigned to the Sølverbæk Formation, which is undivided in the Gåseslette area and divided into the Skalbæk and Scaphitesnæse Members in the Kilen Fjelde area. The Sølverbæk Formation is dominated by marine mudstones and sandstonemudstone heteroliths of late Cenomanian to Santonian age. The new lithostratigraphic framework and significant biostratigraphic advances allow a closer correlation of the Mesozoic units between North Greenland and other Arctic basins.

Influence of freeze-thaw cycles on dynamic compressive strength and energy distribution of soft rock specimen

The effect of freeze-thaw cycles on the dynamic uniaxial compressive strength and energy distribution parameters including the specific energy absorbency rate (ES), energy reflectivity (ER) and energy transmitted rate (ET) are experimentally investigated in this research. The dynamic compression tests on two kinds of soft rock (i.e., sandy mudstone and mudstone) after different freeze-thaw cycles (i.e., 0, 3, 6, 9, 12, and 15 times) have been carried out with splitting Hopkinson pressure bar setup. In addition, the dynamic freeze-thaw damage coefficient (Kdf) and freeze-thaw damage variable (D) are defined. The experimental results show that the dynamic uniaxial compressive strength decreases logarithmically with increasing number of freeze-thaw cycles. The dynamic freeze-thaw damage coefficient (Kdf) of mudstone is larger than that of sandy mudstone at the same freeze-thaw cycles. All these energy distribution parameters are significantly influenced by the freeze-thaw cycles. There has a negative linear correlation between dynamic uniaxial compressive strength and freeze-thaw damage variable (D).

A taphofacies model for coquina sedimentation in lakes (Lower Cretaceous, Morro do Chaves Formation, NE Brazil)

The Barremian–Aptian Morro do Chaves Formation, NE Brazil, was deposited in lacustrine environments during the rifting between the African and South American plates. This formation contains shell beds (coquinas) interbedded with mudstone and sandstone deposits. Shell concentrations may be used to identify paleontological and paleoenvironmental data that allow to interpret the depositional energy, paleoenvironmental conditions and the accumulation history of the formation. The following features of the shell beds were analyzed: (i) taphonomic aspects (sorting, fragmentation, abrasion, rounding and orientation of the shells), (ii) sedimentological aspects (grain packing, abundance of shells, type of matrix and sedimentary structures) and (iii) stratigraphic aspects (bed dimension and shape, bounding surfaces). Two siliciclastic lithofacies (clay mudstones with mud cracks and laminated sandy mudstones) and four carbonate lithofacies (wackestones, packstones, grainstones and rudstones) are described based on the textural features. The carbonate facies were successively divided into six taphofacies (T1 to T6) with the help of an artificial neural Self-Organizing Map (SOM) and evaluated by non-metric multidimensional scaling (NMDS), based on taphonomic features of the carbonate lithofacies. Taphofacies and lithofacies formed at lacustrine margins. Clay mudstone layers with mud cracks and without shells fragments are interpreted as distal parts of alluvial fans or fluvial distributary systems at the edges of the lake system. Rudstones and packstones (taphofacies T1, T2, and T3) with fragmented and oriented shells, wave ripples and horizontal lamination correspond to upper shoreface deposits. Rudstones, packstones and wackestones (taphofacies T4, T5 and T6) that display shells with low signs of fragmentation and abrasion and laminated clay mudstone are interpreted as lower shoreface deposits. A depositional model of the coquinas of the Morro do Chaves Formation is proposed based on the characteristics and distribution of the taphofacies. Moreover, the use of an embedded Markov Chain enabled the relation of the different upper and lower shorefaces in a preferential sequence of deposits of shallowing- and deepening-upward cycles.

EVALUATION OF HYDROCARBON POTENTIALS OF CAMPANIAN SOURCE ROCKS OF NKPORO SHALE (MGBOM SECTION), AFIKPO BASIN, SOUTH EASTERN NIGERIA

This work evaluated the hydrocarbon potential of Campanian source rocks of Nkporo Shale (Mgbom section) using parameters like Soluble Organic Matter (SOM), Total Organic Carbon (TOC) and Organic Matter Quality (OM) on samples taken from the section. The Nkporo Shale is dark grey in color and is highly fissile with interbeds of sandy shale, siltstone and mudstone. The investigated section exposed at Mgbom, Ebonyi State consisted of the Nkporo Shale, a formation in the Lower Benue Trough. The rocks are fairly to moderately rich in organic matter and can therefore be considered as potential source rocks. TOC analysis on sample revealed from the Mgbom Section in the range of 0.51w%-1.02wt% which exceeds the minimal amount needed for a potential source rock. The SOM ranges from 67mg/kg – 165mg/kg and the OM ranges from 77.40% – 89.30% indicating that the source rock is fairly rich in hydrocarbon content.

Pediction method of rock stratum anti-diamond characteristics of carbonate rocks used in Qinghai YingXi block

At present domestic drillability prediction is mainly aimed at sandy mudstone formation, The aim is to study less carbonate strata. With the increase of carbonate exploration reserves, the development of carbonate is the main task facing oil companies. This paper has passed the indoor core test, combined with site logging data, research and analysis of mathematical statistics. The relationship between the rock drillability, compressive strength and acoustic jet lag, plasticity coefficient and poisson ratio, and the difference between the surface acoustic jet lag and the time difference between the wave jet and the wave jet were also found. A prediction model is established for predicting the parameters of drilling resistance in carbonate rocks using well logging data. In this paper, the anti-drilling characteristics of carbonate rocks in Yingxi block of Qinghai oilfield are analyzed, Provide reasonable basis for the selection of bit type in the area.

Detailed facies analysis of the Upper Cretaceous Tununk Shale Member, Henry Mountains Region, Utah: Implications for mudstone depositional models in epicontinental seas

Lower-Middle Turonian strata of the Tununk Shale Member of the greater Mancos Shale were deposited along the western margin of the Cretaceous Western Interior Seaway during the Greenhorn second-order sea level cycle. In order to examine depositional controls on facies development in this mudstone-rich succession, this study delineates temporal and spatial relationships in a process-sedimentologic-based approach. The 3-dimensional expression of mudstone facies associations and their stratal architecture is assessed through a fully integrative physical and biologic characterization as exposed in outcrops in south-central Utah. Sedimentologic characteristics from the millimeter- to kilometer-scale are documented in order to fully address the complex nature of sediment transport mechanisms observed in this shelf muddy environment.The resulting facies model developed from this characterization consists of a stack of four lithofacies packages including: 1) carbonate-bearing, silty and sandy mudstone (CSSM), 2) silt-bearing, calcareous mudstone (SCM), 3) carbonate-bearing, silty mudstone to muddy siltstone (CMS), and 4) non-calcareous, silty and sandy mudstone (SSM). Spatial and temporal variations in lithofacies type and sedimentary facies characteristics indicate that the depositional environments of the Tununk Shale shifted in response to the 2nd-order Greenhorn transgressive-regressive sea-level cycle. During this eustatic event, the Tununk shows a characteristic vertical shift from distal middle shelf to outer shelf (CSSM to SCM facies), then from outer shelf to inner shelf environment (SCM to CMS, and to SSM facies). Shifting depositional environments, as well as changes in dominant paleocurrent direction throughout this succession, indicate multiple source areas and transport mechanisms (i.e. longshore currents, offshore-directed underflows, storm reworking). This study provides a rare documentation of the Greenhorn cycle as exposed across the entire shelf setting. High-resolution mapping of genetically-related packages facilitate the development of process-based depositional models that can be utilized for lateral correlations into the equivalent foredeep strata of the Cretaceous Interior.

Prograding muddy shelves in the Paleogene Wilcox deltas, south Texas Gulf Coast

Core facies analysis integrated with subsurface well-log correlations and seismic horizon mapping document two thick mudstone-dominated deltaic successions in the Wilcox Group along the south Texas Gulf Coast. Meters to tens of meter-thick upper slope to outer shelf mudstones form units that lie at the base of upward-coarsening successions which are interpreted as fluvial or storm/wave dominated shelf deltas. The muddy character of the deposits is observed in cores and inferred from well logs, whereas the larger depositional architecture is interpreted from well log correlations and seismic data.Physical sedimentary structures described from a lower Wilcox core reflect deposition on a mud dominated prodelta controlled by hyperpycnal processes. Normally and inversely graded beds are present indicating sustained flows that waxed and waned. Upward-coarsening facies successions contain current ripples, organic matter, and siderite cemented bands, low trace fossil abundance and low diversity, which suggest deposition in a fluvial prodelta to shelf environment. Centimeter-thick interbeds of normal and inversely graded mudstones, siltstones and very fine-grained sandstones with low intensities of bioturbation, reflect the high physical and chemical stresses of hyperpycnal environments. Heterolithic stratification with lenticular, wavy and flaser bedding indicates some tidal influence.On the other hand cores of the middle Wilcox reflect deposition on a muddy wave-storm dominated shelf. Bioturbation intensities tend to be high, with unstressed, fully marine ichnological suites. Pervasively bioturbated sandy mudstones and muddy sandstones with Cruziana ichnofacies, wavy parallel and low angle lamination, and structureless sandstones with Ophiomorpha record deposition in a wave-influenced prodelta to shelf environment. Hyperpycnal flows associated with river floods might have transported mud across the shelf to the outer shelf where prevailing wave and storm conditions facilitated suspension and transport of mud beyond the shelf edge.Previous studies have shown that Wilcox delta sandstones are largely trapped on the shelf by contemporaneous growth faults. However, this paper illustrates that at times the Wilcox shelf margin prograded through mud accretion.

A new charophyte flora from the Upper Triassic of Skåne (Sweden) and implications on biostratigraphy, taphonomy and the palaeoenvironment

The record of fossil charophytes from Sweden was previously restricted to the Ladinian-earliest Carnian Falsterbo Formation. Here, we present a charophyte assemblage from the Upper Triassic Kågeröd Formation exposed at the Bälteberga Gorge (Skåne, southern Sweden) from the perspectives of taxonomy, taphonomy, palaeoecology and biostratigraphy. The microfossils originate from an interval of reddish sandy mudstone and are represented by rare fossil thalli, calcium carbonate encrustations of thalli and numerous calcified fructifications, called gyrogonites. The assemblage is relatively diverse, comprising six species belonging to four genera of the family Porocharaceae (Auerbachichara cf. rhaetica, Porochara sp., Stellatochara germanica, Stellatochara aff. subsphaerica, Stenochara aff. donetziana, Stenochara aff. kisielevskyi). Both the sedimentological context and the preservation of the charophyte remains point to an autochthonous origin for the charophyte-bearing strata. An autochthonous origin together with the habitat of modern charophytes, infer that the charophyte interval was deposited in shallow ephemeral pond or lake in a terrestrial setting. Their occurrence is also indicative of low amount of nutrients and the numerous thalli encrustations suggest a rather alkaline water composition. Some of the described species (Auerbachichara cf. rhaetica and Stellatochara aff. subsphaerica) are useful for biostratigraphical correlations which attributes the assemblage to the Auerbachichara rhaetica Range zone (sensu Bilan, 1991) in the proposed Germanic Triassic charophyte zonation. This range zone is assigned to a latest Carnian to late Norian age, strengthened by an interlayering with rocks containing a characteristic assemblage of palynomorphs (Corollina meyeriana subzone a and b) in the Upper Triassic of the Polish part of the Germanic Basin. The findings of the first Triassic thalli further strengthen the suggestion that the early Mesozoic fossil record of charophytes is not solely composed of oospores and gyrogonites. A better understanding of vegetative fossil remains (silicified thalli and encrustations) of charophytes may provide important future palaeoecological implications and links between recent and extinct forms. Our findings also provide evidence that charophyte occurrences in the Kågeröd Formation are strictly controlled by palaeoenvironmental factors.

A new possible giant hydrocarbon generated formation: The Upper Triassic source rock in Southwestern Junggar Basin, NW China

The Triassic formation is a possible new giant hydrocarbon generated formation in Northwest China and Mid-Asia. Taking the Upper Triassic formation in the Sikeshu Sag in Junggar Basin as an example, based on the comprehensive analysis on the geochemical characteristics of the cores and the dark mudstone of the outcrops and reservoir formation conditions, we have evaluated the Upper Triassic source rocks by comparing with those in the Ulungu Depression, and reached the following findings. Firstly, the Upper Triassic formation is mainly composed of dark mudstone and sandy mudstone deposits, and the hydrocarbon source rock is mainly distributed in the middle and upper parts with a thickness range of 100–150 m and area of 3500 km2. Secondly, the source rock, moderate in organic matter abundance (with TOC range of 1%–3%), has the material basis for hydrocarbon generation. Thirdly, the organic matter has high percentage of sapropelinite, and is dominated by type II2. Fourthly, the degree of the thermal evolution is moderate, and the source rock with Ro higher than 0.7% has a distribution area of about 1800 km2, providing the conditions of massive hydrocarbon generation. Fifthly, the source rock has great burial depth and wide distribution; the source rock with a Ro of higher than 0.7% and thickness of more than 100 m has an area of around 1400 km2, implying huge resource potential. Sixthly, the next step exploration should focus on highly mature hydrocarbon generation central area in the Upper Triassic - Lower Jurassic in the east of the sag to search for and confirm favorable traps. The research findings have important reference value for promoting the resource status of, deepening the understanding of reservoir formation, and clarifying the exploration direction in the Sikeshu Sag and other periphery Mid-Asia areas.

An ichnological-assemblage approach to reservoir heterogeneity assessment in bioturbated strata: Insights from the Lower Cretaceous Viking Formation, Alberta, Canada

Facies-scale trends in porosity and permeability are commonly mapped for reservoir models and flow simulation; however, these trends are too broad to capture bed and bed-set heterogeneity, and there is a need to up-scale detailed, bed-scale observations, especially in low-permeability reservoir intervals. Here we utilize sedimentology and ichnology at the bed- and bedset-scale to constrain the range of porosity and permeability that can be expected within facies of the Lower Cretaceous Viking Formation of south-central, Alberta, Canada.Three main facies were recognized, representing deposition from the middle shoreface to the upper offshore. Amalgamated, hummocky cross-stratified sandstone facies (Facies SHCS) consist of alternations between intensely bioturbated beds and sparsely bioturbated/laminated beds. Trace fossil assemblages in bioturbated beds of Facies SHCS are attributable to the archetypal Skolithos Ichnofacies, and are morphologically characterized by vertical, sand-filled shafts (VSS). Bioturbated beds show poor reservoir properties (max: 10% porosity, mean: 85.1 mD) compared to laminated beds (max 20% porosity, mean: 186 mD). Bioturbated muddy sandstone facies (Facies SB) represent trace fossil assemblages primarily attributable to the proximal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur in varying proportions, namely sediment-churning assemblages (SC), horizontal sand-filled tube assemblages (HSF), VSS assemblages, and mud-filled, lined, or with spreiten (MLS) assemblages. Ichnological assemblages containing horizontal (max: 30% porosity, mean: 1.28 mD) or vertical sand-filled burrows (max: 10% porosity, mean: 2.2 mD) generally have better reservoir properties than laminated beds (max: 20% porosity, mean: 0.98 mD). Conversely, ichnological assemblages that consist of muddy trace fossils have lower porosity and permeability (max 10% porosity, mean: 0.89 mD). Highly bioturbated, sediment churned fabrics have only slightly higher porosity and permeability overall (max: 15% porosity, mean: 1.29 mD). Bioturbated sandy mudstone facies (Facies MB) contain ichnofossils representing an archetypal expression of the Cruziana Ichnofacies. Four ichnological assemblages occur throughout Facies MB that are similar to Facies SB; SC, HSF, VSS, and MLS assemblages. The SC (max: 15% porosity, mean: 21.67 mD), HSF (max: 20% porosity, mean: 3.79 mD), and VSS (max: 25% porosity, mean: 7.35 mD) ichnological assemblages have similar or slightly lower values than the laminated beds (max: 20% porosity, mean: 10.7 mD). However, MLS assemblages have substantially lower reservoir quality (max: 10% porosity, mean: 0.66 mD).Our results indicate that the most likely occurrence of good reservoir characteristics in bioturbated strata exists in sand-filled ichnological assemblages. This is especially true within the muddy upper offshore to lower shoreface, where vertically-oriented trace fossils can interconnect otherwise hydraulically isolated laminated sandstone beds; this improves vertical fluid transmission. The results of this work largely corroborate previous findings about ichnological impacts on reservoir properties. Unlike previous studies, however, we demonstrate that the characteristics of the ichnological assemblage, such as burrow form and the nature of burrow fill, also play an important role in determining reservoir characteristics. It follows that not all bioturbated intervals (attributed to the same facies) should be treated equally. When upscaling bed-scale observations to the reservoir, a range of possible permeability-porosity values can be tested for model sensitivity and to help determine an appropriate representative elementary volume.

Chronostratigraphy of the Pliocene–Pleistocene boundary in forearc basin fill on the Pacific side of central Japan: Constraints on the spatial distribution of an unconformity resulting from a widespread tectonic event

The upper Ikego Formation (Pliocene) and the Urago Formation (Pliocene to Lower Pleistocene) have been examined for their lithostratigraphy, calcareous nannofossil biostratigraphy, and magnetostratigraphy. Both formations are forearc basin fills exposed on the Miura Peninsula on the Pacific side of central Japan. The boundary between these formations has been previously treated as the western extension of the Kurotaki Unconformity, which was considered to be related to widespread tectonic change at ca. 3 Ma in the Japanese forearc region. However, the presence of the unconformity on the Miura Peninsula remains unclear due to sparse lithostratigraphic and chronostratigraphic data around the boundary between the both formations. In the study area, the Ikego Formation consists mainly of sandy mudstone and the Urago Formation of sandstone and muddy sandstone; both formations contain numerous tuff beds. Key tuff beds and the last appearance of Discoaster tamalis have been identified in the upper Ikego Formation and the lower Urago Formation, allowing these formations to be considered conformable. The following continuous stratigraphic markers were identified (in ascending order): top of the Mammoth Subchronozone (3.21 Ma); base (3.13 Ma) and top (3.05 Ma) of the Kaena Subchronozone; last appearance of Discoaster tamalis (2.76 Ma); the widespread tuff bed KGP (ca. 2.5 Ma); and the last appearance of Discoaster pentaradiatus (2.41 Ma). Sedimentation rates are almost constant among these age-controlled horizons, which indicates continuous sedimentation from 3.2 to 2.4 Ma. The results indicate that the Kazusa Group conformably overlies the Miura Group on the Miura Peninsula, which provides a well-established continuous stratigraphic section for analyses of regional paleoenvironmental and tectonic evolution in the Japanese forearc region.

Extensive authigenic quartz overgrowths in the gas-bearing Haynesville-Bossier Shale, USA

In sandstone reservoirs, despite grain rearrangement during compaction, significant pore volumes may be retained prior to the onset of late diagenetic quartz cementation. In mudstone reservoirs, grain rearrangement during compaction results in significant pore volume reduction prior to late diagenesis. Where quartz overgrowths have been previously reported in post-compaction mudstones they have been in volumetrically low concentrations and interpreted as anomalous occurrences. Quartz cementation alters rock brittleness resulting in changes to mechanical fracture properties. Quartz overgrowths reduce reservoir porosity and permeability.We present petrographic evidence of two phases of quartz cement in the Haynesville-Bossier Shale: (i) grain replacive and (ii) quartz overgrowths. Carbonate grain replacement is volumetrically low (<1%). Quartz overgrowths identified from SEM-CL imaging are volumetrically more significant (8–13%). Quartz overgrowths were most commonly observed in the sandy and coarse mudstone microfacies, but are present in both medium and fine mudstone microfacies.Petrographic evidence indicates three processes in the development of quartz overgrowths. Mica and pyrite are (i) engulfed and (ii) displaced by quartz overgrowth cement. The absence of a supportive, primary granular framework surrounding engulfed detrital and early authigenic minerals would indicate that quartz overgrowths are also (iii) replacive. Pressure dissolution of detrital quartz silt grains and smectite-to-illite transformation are likely sources of silica for quartz cement. This study is the first to document large-scale, replacive, authigenic quartz overgrowth development within a producing mudstone.

Structural evolution and basin architecture of the Traill Ø region, NE Greenland: A record of polyphase rifting of the East Greenland continental margin

Fault block basins exposed along NE Greenland provide insights into the tectonic evolution of East Greenland and the Norwegian-Greenland Sea. We present a new geological map and cross sections of the Traill O region, NE Greenland, which formed the western margin of the Voring Basin prior to Ceno­zoic seafloor spreading. Observations support a polyphase rift evolution with three rift phases during Devonian–Triassic, Jurassic–Cretaceous, and Cenozoic time. The greatest amounts of faulting and block rotation occurred during Cenozoic rifting, which we correlate with development of the continent-ocean transition after ca. 56 Ma and the Jan Mayen microcontinent after ca. 36 Ma. A newly devised macrofaunal-based stratigraphic framework for the Cretaceous sandy mudstone succession provides insights into Jurassic–Cretaceous rifting. We identify a reduction in sedimentation rates during the Late Cretaceous; this corresponds to a transition from structurally confined to unconfined sedimentation that coincides with increased clastic sedimentation to the Voring and More Basins derived from East Greenland. With each rift phase we record an increase in the number of active faults and a decrease in the spacing between them. We attribute this to fault block rotation that leads to an excess build-up of stress that can only be released by the creation of new steep faults. In addition, we observe a stepwise migration of deformation toward the rift axis that we attribute to preexisting lithospheric heterogeneity that was modified during subsequent rift and post-rift phases. Such observations are not readily conformable to classic rift evolution models and highlight the importance of post-rift lithospheric processes that occur during polyphase rift evolution.

Siliciclastic input and sedimentation rate as controls on paleoecological distribution of Lower Miocene benthic carbonate producers in a fan–delta: Zagros foreland basin, Iran

This paper aimed to study Lower Miocene (Burdigalian) mixed carbonate–siliciclastic deposits within an Upper Cenozoic synorogenic conglomerate–dominated succession in north of Shalamzar in the Zagros foreland basin, Iran. The deposits are composed of nine facies: foraminiferal mudstone, silty mudstone, sandy mudstone, fossiliferous sandy mudstone, fossiliferous argillaceous mudstone, fossiliferous calclithite, coral limestone, calcareous claystone and hybrid sandstone. The facies represent a mixed carbonate– siliciclastic shelf–type fan–delta. The subenvironments of the fan–delta include muddy pro–delta, sandy delta– front, clastic proximal mouth bar and a subordinate delta plain. Siliciclastic input and sedimentation rate controlled the paleoecological distribution of different benthic carbonate–producing fauna in the fan–delta during its progradation into a shelf marine environment. Input of siliciclastic deposits and sedimentation rate limited the diversity and development of corals and controlled their colonization and growth morphologies in the sandy delta–front. Siliciclastic input (including plant materials and coal debris) and sedimentation rate controlled the trophic habitats of many gastropods and their abundance and distribution in the sandy delta– front and clastic proximal mouth bar. Also, increased siliciclastic input favored abundance of larger benthic foraminifera in most parts of the fan–delta with the exception of the muddy pro–delta.

Study on strength and deformation characteristics of sandy mudstone based on triaxial unloading test

To achieve the strength and deformation characteristics of sandy mudstone under the unloading condition, a triaxial unloading test was performed on a group of sandy mudstone taken from the diversion tunnel of a hydropower station. The results show that the stress-strain curve slope of sandy mudstone increases suddenly at the moment of unloading confining pressure, and there is no significant decline in the whole test. There is usually only a whole shear failure surface in the samples damaged under low initial unloading confining pressure while those damaged under high one develop tensile fracture. The introduction of semilog method helps to solve the problem that it’s difficult to get the peak intensity of sandy mudstone directly based on the variation characteristics of the stress-strain curve. From the initial stage of unloading to the damage of the samples, the reduced percentage of sandy mudstone deformation modulus is positively correlated with initial unloading confining pressure. The decrease rate of deformation modulus and increase rate of axial strain of sandy mudstone are both mutate under destructive confining pressure. The research results can provide an useful reference to research on the mechanical properties of similar soft rocks.

Experimental Study on the Dilatancy and Fracturing Behavior of Soft Rock Under Unloading Conditions

To reveal the deformation mechanism during tunneling in deep soft ground, triaxial unloading confining pressure tests and triaxial unloading creep tests were carried out on sandy mudstone specimens to study the dilatancy and fracturing behavior of soft rock. In the triaxial unloading confining pressure tests, the stress path and different unloading rates were considered to reflect the unloading characteristics of the excavation methods. The unloading rate effects and the rock damage evolution law are studied. The following conclusions are obtained from the results. Firstly, when the unloading rate is smooth, the peak strengths and deviatoric stress–strain curves under the unloading condition are close to those under the conventional loading condition. Secondly, the post-peak brittle characteristics are more apparent with the increasing unloading rates. Thirdly, the soft rock undergoes five deformation and failure regimes of elasticity, pre-peak unloading damage–dilatancy, post-peak brittle drop, linear strain softening and residual perfect plasticity under quasi-static smooth unloading of mechanized excavation which is mainly focused on in this study. Fourthly, the damage evolution law at the pre-peak damage–dilatancy stage follows an exponential function. Fifthly, during the post-peak stages, multistage microfractures are initiated, propagated and finally coalesced forming a shear-fragmentation band with a certain thickness, accompanied by significant volumetric dilatancy. In the triaxial unloading creep tests, multistep unloading of the confining pressure was applied, while the axial pressure was kept constant. The results show that when the deviatoric stress is larger and the experienced creep time is longer, the unloading effect and creep characteristics become more apparent accompanied with obvious lateral dilatancy, eventually leading to significant creep–dilatancy. The progressive failure with time is caused by the damage accumulating with time-dependent crack expansion, which can be called as ‘time-dependent damage and fracturing’. The reasons for the above evolution process are presented, then the deformation mechanism of soft rock is revealed. The soft rock deformation mainly consists of two parts. One part is the pre-peak damage–dilatancy and post-peak fracture–bulking produced at the excavation unloading instant. The other part is creep–dilatancy caused by time-dependent damage and fracturing in a period of time after excavation. The above-mentioned results of damage, dilatancy and fractures evolution process are in good agreement with the in situ monitoring results and previous studies about the surrounding rock convergence, fracturing and EDZ (excavation damaged zone) development.

Permeability heterogeneity in bioturbated sediments and implications for waterflooding of tight-oil reservoirs, Cardium Formation, Pembina Field, Alberta, Canada

Bioturbated sediments recording distal expressions of paralic depositional environments are increasingly being exploited for hydrocarbons in the super-giant Pembina Field (Cardium Formation), Alberta, Canada. These strata were previously considered unproductive due to limited vertical and horizontal connectivity between permeable beds. In these “tight oil” plays (0.1–10 mD), pressure decay profile permeametry (micropermeability) data indicate that sand-filled burrows provide vertical permeable pathways between bioturbated and parallel-laminated sandstone beds in the central, northeast and northwest parts of the field. This relationship enables the economic exploitation of hydrocarbons via horizontal drilling and multi-stage hydraulic fracturing. As the exploitation of bioturbated strata progresses in the Pembina Field, additional primary targets are being sought out, and horizontal waterflooding is being considered in areas where horizontal wells exist. Proximal to historical produced conventional targets, reservoir analyses indicate that areas where the bioturbated facies average permeability lies between 0.35 mD and 0.85 mD and sandstone isopach thicknesses are between 0.25 m and 2.5 m should be targeted in east-central Pembina.Micropermeability values enable correlation of bulk permeability from plugs and full-diameter samples to the heterogeneous permeability distributions in intensely bioturbated strata. Bulk and micropermeability data are graphically compared, and permeability distributions are mapped across the field. Using isopach thickness of bioturbated facies, production data, and permeability data, “sweet spots” are identified for placement of effective waterfloods.Production information for recently drilled horizontal wells in the Pembina Field demonstrate that bioturbated muddy sandstones and sandy mudstones in paralic environments can be economically exploited when sand-filled burrows provide connectivity between sand beds. However, well performance within these poorly understood unconventional tight oil plays can better be predicted with an in-depth characterization of their facies and complex permeability heterogeneities. Based on our results, it is clear that micropermeability analysis can be effectively employed to differentiate between economic and sub-economic plays, identify areas with high effective permeability, and high-grade areas for enhanced oil recovery schemes.

Logging assessment of tight clastic rock reservoir fractures via the extraction of effective pore aspect ratios: A case study of lower Permian strata in the southern Qinshui Basin of eastern China

Research on tight gas enrichment in coal measure strata is contentious worldwide. Gas logging anomalies are very common in the upper Paleozoic coal-bearing strata in the Southern Qinshui Basin of eastern China. The distribution characteristics of these free gases are closely related to the degree of fracture development. The rock pore aspect ratio (α) can reflect the morphology of rock pores and fractures. In this paper, we use full wave logging data to propose a new logging assessment method for tight clastic rock reservoir fractures via the extraction of effective pore aspect ratios. The basic concept is as follows: (1) we extract the rock matrix mineral modulus and dry rock modulus of the rocks using an optimized algorithm, and (2) we invert the effective pore aspect ratio of coal measure strata using the theoretical model of a differential equivalent medium. The research results show that the studied clastic rocks in coal measure strata are very tight, with porosity levels that are generally less than 5%, and sections in which fractures have formed often correspond to relatively low α values of less than 0.1. The microfracture-bearing sections identified based on α inversions correspond well to the sections with fractures at the core scale. Thus, this method can be used to identify fractured sections at the core scale. For sandy mudstone, microfractures mainly exist in sandy intervals, and α is negatively related to rock porosity and positively related to shale content. Regardless of lithology, i.e., sandstone, mudstone or coal, the communication and transport capacity of fracture systems are greater than those of the pore system, and coal-derived gas reservoirs form when there are good gas sources, transportation systems, fracture systems and preservation conditions. Similar to the formation mechanism of fractures in rocks, α is also derived from mechanical mechanisms, and this study shows that the inversion process is effective in characterizing the degree of fracture development in tight clastic rocks in coal measure strata based on elastic rock properties.

A record of the upper Olduvai geomagnetic polarity transition from a sediment core in southern Yokohama City, Pacific side of central Japan

A detailed paleomagnetic record of the upper Olduvai polarity transition was obtained from a 106.72 m-long sediment core drilled in southern Yokohama City, located on the northern Miura Peninsula, on the Pacific side of central Japan. The core spans the upper part of the Nojima Formation and the lowermost part of the Ofuna Formation, both of which correspond to the middle Kazusa Group (Lower Pleistocene forearc basin fill). The record was reconstructed using discrete specimens taken throughout mudstone and/or sandy mudstone sequences in the Nojima Formation. In this record, the virtual geomagnetic pole (VGP) fluctuation accompanying the polarity transition was determined to occur between depths of 66.99 and 63.60 m. These depths have been dated at 1784.4 and 1779.9 ka, respectively, and the duration of the polarity transition is estimated to be 4.5 kyr using an age model based on a δ18O record from that core. The VGP paths during the transition do not appear to show any preferred longitudinal bands. However, the VGP positions cluster in five areas: (A) eastern Asia near Japan, (B) the Middle East, (C) eastern North America (North Atlantic), (D) off southern Australasia, and (E) the southern South Atlantic off South Africa. The primary locations of the observed VGP clusters coincide with the areas on the Earth’s surface that possess a strong downward flux of the vertical component of the present geomagnetic non-axial dipole field. The relative paleointensity rapidly decreased approximately 1 kyr before the beginning of the polarity transition and gradually recovered to its initial level in 12 kyr.

Investigations on Mechanical Properties and Crack Propagation Characteristics of Coal and Sandy Mudstone Using Three Experimental Methods

Investigations on Mechanical Properties and Crack Propagation Characteristics of Coal and Sandy Mudstone Using Three Experimental Methods