Jurassic Sandstones Research Articles

Influence of climate and marine transgression on Mesozoic–Cenozoic diagenesis of clastic rocks in the Tabei Uplift, Tarim Basin, China

The Mesozoic–Cenozoic clastic rocks in the Tabei Uplift form one of the most hydrocarbon-rich zones in the Tarim Basin of China. Understanding how diagenesis has affected the petrophysical properties of the sandstone is crucial for developing regional exploration strategies. Although most studies have investigated the physical conditions during sedimentation and chemical processes during diagenesis, the hydrochemical environment during sedimentation and subsequent diagenesis has received little attention. This study employed petrographic observations, paleo-geothermal reconstructions, and paleoenvironmental analysis to constrain the regional climatic conditions, hydrochemical environment of deposition, and diagenetic features of clastic rocks in the Tabei Uplift. The sandstone at depths of 3500–6000 m is located mainly in the A phase of the middle diagenetic stage, and exhibits three types of diagenesis. Triassic–Jurassic sandstones are dominated by silica, kaolinite, and chlorite cements, Cretaceous sandstones by mainly carbonate cements, and Paleogene sandstones by carbonate and evaporite cements. These diagenetic stages correspond to paleoclimatic changes during the earliest Late Jurassic and the maximum marine transgressions of the Proto-Tethys during the Paleogene, thereby highlighting the influence of climatic change and transgressions on the water conditions during deposition and the type of diagenesis.

Developing evolutionary models for deformation band formation in high-porosity suprasalt sandstones: An example from Paradox Basin, Utah

Pervasive deformation adjacent to salt diapirs can drive significant porosity and permeability reduction in potential reservoir units, yet predicting the intensity and spatial distribution of these subseismic-scale features has remained as a persistent challenge. Previously, Jurassic sandstones adjacent to salt-cored anticlines in the Paradox Basin, eastern Utah, have been used to characterize deformation banding formation. However, these host rocks have undergone significant structural and diagenetic modification following band formation, which adds significant complexity to analyses of this type. Here, we integrate detailed structural transect analysis, petrologic analysis of bands and host rocks, depth-compaction relationships, critical state deformation models, and regional burial evolution models to constrain the timing and conditions of band formation. This analysis demonstrates that bands in this region formed prior to pervasive cementation and are generally expressed as two plastic strain gradients; one characterized by distributed deformation bands across the crest of salt-cored anticlines and a second wherein deformation band density increases near mesoscale normal faults. Comparison of these results with numerical structural evolution models of similar structures indicates that band gradients may have formed as a result of outer-arc extensional stress induced by the rising diapir and are linked with normal faults. These results highlight that the integration of material behaviors with kinematic evolution can constrain models for deformation band formation and may provide a useful workflow for understanding the development of these features in geologically young petroleum systems like those of the Atlantic passive margin.

Energy characteristics of saturated Jurassic sandstone in western China under different stress paths

AbstractTo study the energy evolution and failure characteristics of saturated sandstone under unloading conditions, rock unloading tests under different stress paths were conducted. The energy evolution mechanism of the unloading failure of saturated sandstone was systematically explored from the perspectives of the stress path, the initial confining pressure, and the energy conversion rate. The results show that (1) before the peak stress, the elastic energy increases with an increase in deviatoric stress, while the dissipated energy slowly increases first. After the peak stress, the elastic energy decreases with the decrease of deviatoric stress, and the dissipated energy suddenly increases. The energy release intensity during rock failure is positively correlated with the axial stress. (2) When the initial confining pressure is below a certain threshold, the stress path is the main factor influencing the total energy difference. When the axial stress remains constant and the confining pressure is unloading, the total energy is more sensitive to changes in the confining pressure. When the axial stress remains constant, the compressive deformation ability of the rock cannot be significantly improved by the increase in the initial confining pressure. The initial confining pressure is positively correlated with the rock's energy storage limit. (3) The initial confining pressure increases the energy conversion rate of the rock; the initial confining pressure is positively correlated with the energy conversion rate; and the energy conversion rate has a high confining pressure effect. The increase in the axial stress has a much greater impact on the elastic energy than the confining pressure. (4) When the deviatoric stress is small, the confining pressure mainly plays a protective role. Compared with the case of triaxial compression paths, the rock damage is more severe under unloading paths, and compared with the case of constant axial stress, the rock damage is more severe under increasing axial stress.

Provenance of Detrital Rutiles from the Triassic–Jurassic Sandstones in Franz Josef Land (Barents Sea Region, Russian High Arctic): U-Pb Ages and Trace Element Geochemistry

Provenance study plays an important role in paleogeographic and tectonic reconstructions. Detrital zircons are commonly used to identify sediment provenance; however, a wide range of detrital zircon ages in clastic rock often represent a fingerprint of reworked older terrigenous successions rather than ages of magmatism and metamorphism in the provenance area. This study focuses on the provenance of detrital rutile grains in the Triassic–Jurassic sandstones from Franz Josef Land and shows the importance of multiproxy approaches for provenance studies. Trace element data demonstrate that most rutile grains were sourced from metapelitic rocks, with a subordinate population having a metamafic origin. The Zr-in-rutile thermometer and U-Pb geochronology suggest that detrital rutile grains were predominantly derived from rocks that underwent amphibolite facies metamorphism during the Paleozoic era, with a predominance of the Carboniferous–Permian ages. Therefore, we suggest that the provenance area for the studied sandstones on Franz Josef Land has a similar geological history to the Taimyr region and Severnaya Zemlya archipelago. We propose that this crustal domain extends across the Kara Sea and forms the basement to the north and east of FJL, representing a proximal provenance for the studied Mesozoic terrigenous rocks. This domain experienced both Middle–Late Ordovician and Carboniferous–Permian metamorphism. The comparison of U-Pb dating and the geochemistry of rutile, U-Th/He, and U-Pb dating of zircons showed that detrital rutiles are the powerful toll in provenance restoration and can give additional constrains when a provenance area locates within collisional-convergent settings.

Middle Devonian, late Carboniferous, and Triassic magmatic flare-ups in eastern Armorica (Sakarya Zone, Turkey) as revealed by detrital zircon U-Pb-Hf isotopic data

Abstract The Sakarya Zone of northern Turkey contains a well-preserved Early–Middle Jurassic and Late Cretaceous submarine magmatic arc constructed over pre-Jurassic bedrocks that are considered to be the eastward extension of the Armorican Terrane Assemblage in Europe. In this study, we present U-Pb-Hf isotopic data from the detrital zircons of middle Permian and Lower Jurassic sandstones to reveal episodes of Paleozoic–early Mesozoic magmatic flare-ups. Detrital zircon ages, together with data from the literature, define three major age groups at 400–380 Ma, 326–310 Ma, and 250–230 Ma, which indicates three distinct magmatic flare-ups. In addition, there are minor age clusters at 460–430 Ma and 215–195 Ma. Initial εHf values of the detrital zircons indicate significant juvenile input during the Triassic flare-up, the involvement of significantly reworked crustal material during the late Carboniferous magmatic flare-up, and both juvenile and reworked crustal material during the Middle Devonian magmatic flare-up. Within the pre-Jurassic continental basement rocks of the Sakarya Zone, the late Carboniferous igneous rocks are well documented and most voluminous, and the Middle Devonian rocks are known locally, while the Triassic igneous rocks—apart from those in Triassic accretionary complexes—are hardly known. Because the Sakarya Zone is a Gondwana-derived continental block that was later involved in the Variscan and Alpine orogenies, these magmatic flare-ups cannot be explained by subduction-related processes along a single subduction zone. We propose that the Sakarya Zone rifted from the northern margin of Gondwana during the Late Ordovician–Silurian, the Devonian magmatic flare-up (400–380 Ma) was related to the southward subduction of the Rheic Ocean beneath the Sakarya Zone during its northward drift, the late Carboniferous magmatic flare-up (326–310 Ma) occurred following the collision of the Sakarya Zone with Laurussia, and the Triassic flare-up (250–230 Ma) resulted from northward subduction of the Tethys Ocean beneath the Sakarya Zone. Comparison with data from the literature shows that the Triassic and late Carboniferous magmatic flare-ups are also characteristic features of neighboring Armorican domains, such as the Balkans and the Caucasus; however, the Middle Devonian flare-up appears to be restricted to the Sakarya Zone. Along with the late Carboniferous flare-up, the Late Ordovician–Silurian flare-up, which is locally recorded in the Sakarya Zone, is typical of the Armorican Terrane Assemblage as a whole.

Composition and Sources of Clastic Material of Terrigenous Rocks of the Khabarovsk Accretionary Complex (Sikhote-Alin)

Abstract ––The results of comprehensive studies of terrigenous rocks of the Khabarovsk Sikhote-Alin accretionary complex are presented. It is established that the fragments of Jurassic and Permian–Triassic sandstones are dominated by poorly rounded and poorly separated material mainly from local provenance areas. The detrital part of the rocks is mainly represented by quartz, in a smaller amount by feldspar and rock fragments. Sandstones are characterized by high silica content, moderate alumina content, low concentrations of femic elements and calcium, moderate alkali content with significant variations in the K/Na ratio. Both Jurassic and Permian–Triassic rocks are typically characterized by reduced contents of LILE, REE, to a lesser extent HFSE and negative values of the ɛNd(T) parameter – compared to PAAS. The model Nd age of Jurassic sandstones varies from 1.36 to 1.71 Ga, Permian–Triassic – from 1.14 to 1.35 Ga. Most of the detrital zircon population is of late Paleozoic–early Mesozoic age, approximately 25% are older (pre-Paleoproterozoic). The studied sandstones are mainly rocks of the first cycle of weathering (petrogenic), formed during the erosion of igneous rocks of felsic composition. The synthesis of the obtained data suggests that the main source of the cluster material for the Mesozoic sedimentary rocks was the geological formations of the northern part of the Bureya–Khanka superterrane (Bureya and Malokhingan blocks), as well as, possibly, the eastern part of the Mongol–Okhotsk belt. The Khabarovsk terrane has not drifted significantly along the Tan Lu stike-slip system and is an “autochthonous” block in the present-day structure of Sikhote-Alin.

Early Jurassic initiation of the modern drainage pattern of the Dabie orogen (East China) revealed by a multi‐proxy provenance approach

AbstractThe timing of the initiation of the present‐day tectonic architecture and drainage systems in eastern China remains debated. This study presents a comprehensive provenance study of the Early Jurassic peripheral basins surrounding the Dabie orogen including framework petrography, heavy‐mineral analysis, single‐grain chronology and chemistry. Clasts of high‐grade schist, muscovite grains, rare gneissic fragments, abundant metamorphic garnet and phengite (Si > 3.3 pfu), combined with a main 216–256 Ma rutile U–Pb population found in these Early Jurassic sandstones, indicate a source from the Triassic (U)HP belt in the Dabie orogen. Sedimentary lithics and ultra‐stable heavy‐mineral assemblages indicate an additional source of recycled sedimentary rocks. Combined with the continuous shift of the youngest detrital rutile age population toward younger ages toward the north that mimics the pattern of metamorphic bedrock ages in the Dabie orogen, we infer that the present surface tectonic architecture and paleodrainage patterns of the Dabie orogen were established in the Early Jurassic. Thus, the Early Jurassic exhumation of the Dabie orogen marked the development of the watershed between Northern and Southern China, namely the Huai River and several principal tributary systems of the middle‐lower Yangtze River.

Classic Scottish sandstone concretions formed from hot water

Large calcite‐cemented sandstone concretions found in Middle Jurassic sandstones from the isles of Eigg and Skye are a striking feature of the Jurassic geology of the Inner Hebrides. Recent clumped isotope data unequivocally give concretionary calcite cementation temperatures above 50°C and up to 98°C. The high temperatures contrast with the mild thermal history of the enclosing less permeable Jurassic mudrocks. The clumped isotope data also enable calculation of the oxygen isotopic composition of the precipitating fluids: these included percolating rainwater that had interacted with Paleocene volcanics; then, later hot fluids of deeper, basinal origin, squeezed out of compacting mudrocks at depth as the Paleocene lava pile loaded the crust. There is thus direct connection between ‘Jurassic’ sedimentary features and the complex Paleocene volcanic history of the region.

PETROGRAPHY OF PYROBITUMENS IN MIDDLE – LATE JURASSIC SANDSTONES FROM THERMALLY DEGRADED HYDROCARBON ACCUMULATIONS, EAST GREENLAND

A number of exhumed hydrocarbon traps have been described from the Traill Ø region of East Greenland. This study focuses on the Bjørnedal area where the distribution of bitumen has been mapped out. Bitumen staining clearly has a cross‐cutting relationship to stratigraphic units and can be shown to form distinct palaeo‐accumulations. Detailed petrographic studies show that bitumen occurs as late diagenetic phases in Middle to Late Jurassic sandstones, and is present both as both grain‐coating and pore‐filling phases. Geochemical analyses confirm that the bitumen is organic in composition and is composed largely of carbon and hydrogen. Both H/C ratios and bonds identified by FTIR behave as expected with increasing maturity measured using bitumen reflectance. Together, these results provide strong evidence that the material is pyrobitumen derived from the in situ thermal degradation of a liquid hydrocarbon precursor. On the basis of textures in transmitted and reflected light and quantitative bitumen reflectance distributions, two populations of pyrobitumen may be recognised in some samples.Two phases of Paleogene magmatism occurred in the Traill Ø region. The first late Paleocene – early Eocene phase was related to the opening of the northern North Atlantic in the earliest Eocene, and was experienced throughout East Greenland and the northwest European margin. The later magmatic phase was related to the gradual separation of the Jan Mayen microcontinent from East Greenland through the late Eocene – early Oligocene. A single pyrobitumen phase is recognised in accumulations only affected by the early magmatism, and a second phase is only observed in areas affected by both the early and later magmatism. This relationship is interpreted as evidence for a direct relationship between magmatic phases and bitumen generation. The presence of bitumen formed by the thermal degradation of liquid hydrocarbons during the later magmatic event suggests that a viable petroleum system remained active following the early magmatic event.

Detrital Mineral Chemistry of Jurassic Sandstone from the Mino Terrane in Southwest Japan

Detrital Mineral Chemistry of Jurassic Sandstone from the Mino Terrane in Southwest Japan

Effects of overpressure on deep sandstone reservoir quality: A case study of the Medium and Lower Jurassic formation in the Shawan Sag, central Junggar Basin, western China

This study focuses on the Medium and Lower Jurassic sandstone reservoir in the Shawan Sag, central Junggar Basin to demonstrate the impacts of overpressure on deep reservoir quality. Based on thin section observation and mineral identification, compaction heterogeneity and different petrofacies were identified by mathematical statistics. The compaction degree and typical cements content of different petrofacies under different overpressure systems were analysed to qualitatively evaluate the heterogeneous influence of overpressure on reservoir quality. The evolution of palaeo-pressure and porosity for typical wells were restored using basin modelling, and then the effects of overpressure on reservoir quality were quantitatively analysed. The results reveal that deep sandstones exhibit compaction heterogeneity owing to petrological properties and calcite cement, resulting in different petrofacies: ductile-rich sandstone, calcite-cement sandstone, and ductile-lean sandstone. Ductile-rich sandstone and calcite-cement sandstone, which were nearly or already tight before the on-set of overpressure, were free from the effects of overpressure. Ductile-lean sandstone maintained better reservoir quality from overpressure. The effects of overpressure were heterogeneous, which were comprehensively controlled by grain size, the content of rigid grains, overpressure strength and the on-set and duration of overpressure. The basin modelling results show differences in grain size and the content of rigid grains for the Medium and Lower Jurassic formation, resulting in varying levels of porosity preservation. The Medium Jurassic formation, whose pressure coefficient is 1.9, shows 1.69% porosity preservation, whereas the Lower Jurassic formation, whose pressure coefficient is 1.3, shows 1.59% porosity preservation. With the development of overpressure, quartz overgrowth was restrained, whereas smectite-illitization and carbonate cements were free from the late overpressure effects. The research results deepen the existing understanding of the formation mechanism of deep effective reservoirs.

Facies-related diagenesis of Jurassic sandstones, central Junggar Basin, NW China: implications for reservoir quality evolution

Abstract Hydrocarbon exploration in China has recently expanded to deep tight oil and gas from the Lower Jurassic sandstone reservoirs in the central Junggar Basin. These deeply buried sandstones (>4.5 km) have generally been overpressured due to hydrocarbon saturation, but have not been intensively investigated in terms of diagenesis and reservoir quality. The Lower Jurassic sandstones are feldspathic litharenites (average Q 28 F 29 R 43 ) and were deposited in the lacustrine–deltaic environments of the high-palaeolatitude continental basin under warm, humid climatic conditions. Progressive burial with only slight uplift of the strata led to a relatively stable burial–thermal regime, resulting in minor diagenetic changes consisting of compaction, cementation by carbonates, quartz and clay minerals, and partial dissolution of feldspars and rock fragments. Despite generally similar diagenetic histories, the studied sandstones reveal considerable variations in the development of pore systems, porosity and permeability. The best reservoirs are represented by medium- to coarse-grained depositional facies with higher porosity (average 8.43%) and permeability (average 0.59 mD). All the evidence suggests that integrated consideration of the roles of depositional settings and diagenesis, linked with burial–thermal history and overpressure evolution conditions, provides a helpful means of predicting reservoir quality.

Identification of an oil-bearing layer by formation water resistivity: A case study of the Jurassic reservoir, Southwest Ordos Basin

In the southwestern Ordos Basin, the formation water salinity and resistivity of Jurassic sandstone reservoirs are characterized by rapid changes and great variations, which lead to difficulties in the identification of reservoir fluid types and the development and exploration of the hydrocarbons in this area. To explore appropriate methods for reservoir fluid identification provided with complex formation water salinity and resistivity, we have conducted three research steps. First, using formation water testing and well-logging data, the formation water salinity, spontaneous potential, and reservoir resistivity are used to calculate the formation water resistivity. Second, the calculation results were used to separate the reservoir fluid types. Third, the calculated formation water resistivities are classified and the relationship between the formation water resistivity and the oil-producing interval is established. The results indicate that, for oil-producing intervals, the formation water resistivity calculated using the reservoir resistivity is the highest, whereas that using the formation water salinity is the lowest. Based on this understanding, two identification methods of reservoir fluid types are developed, including the qualitative curve overlap method and the quantitative ternary graph method. The curve overlap method can be applied to visually distinguish reservoir fluid types through the differentiation of formation water resistivity. Compared with some earlier quantitative methods, the ternary graph method can achieve an efficiency of 87.7% in identifying the type of reservoir fluid. Finally, the reliability of both methods is verified by case studies.

Concretionary cementation of a Scottish Middle Jurassic sandstone by hot, Paleocene fluids: a clumped isotope study

This study focuses on new clumped isotope data from concretionary calcite cements in the Middle Jurassic Valtos Sandstone Formation (Great Estuarine Group) of the Inner Hebrides. Clumped isotopes show that concretion cementation began at 45 ± 6°C increasing to temperatures in excess of 70°C before cooling slightly to 57 ± 7°C at the concretion margin. In the early stages of cementation, calculated δ 18 O FLUID values were c. −12‰ VSMOW, identical to an estimate of Paleocene Hebridean meteoric water based on hydrothermal reactions close to Paleocene igneous centres. These δ 18 O FLUID values imply that concretion cementation started in the Paleocene probably during the earliest stages of phreato-magmatic effusive igneous activity. As the concretion grew, temperature changes were accompanied by progressively evolving δ 18 O FLUID compositions up to +2.1 ± 1.1‰ VMOW. These evolving δ 18 O FLUID compositions were caused by isotope exchange reactions between 18 O-rich lithologies and hot basinal fluids migrating upward along faults. This fluid flow was driven by progressive crustal loading from the thickening Paleocene lava pile, which also caused sandstone compaction. Published radiometric dates that constrain the emplacement time of the Skye Lava Group, and its subsequent rapid erosion, suggest that concretion formation and final compaction was completed in no more than 2.6 myr, far more rapidly than modelled previously. Initial concretion growth that predates development of volcanic topography shows that the strongly negative compositions of Hebridean Paleocene meteoric water are mainly of latitudinal rather than orographic origin. Supplementary material: Clumped isotope data correction and uncertainties, sample details and additional figures are available at https://doi.org/10.6084/m9.figshare.c.6459860

Lithofacies logging identification for strongly heterogeneous deep-buried reservoirs based on improved Bayesian inversion: The Lower Jurassic sandstone, Central Junggar Basin, China

The strong heterogeneity characteristics of deep-buried clastic low-permeability reservoirs may lead to great risks in hydrocarbon exploration and development, which makes the accurate identification of reservoir lithofacies crucial for improving the obtained exploration results. Due to the very limited core data acquired from deep drilling, lithofacies logging identification has become the most important method for comprehensively obtaining the rock information of deep-buried reservoirs and is a fundamental task for carrying out reservoir characterization and geological modeling. In this study, a machine learning method is introduced to lithofacies logging identification, to explore an accurate lithofacies identification method for deep fluvial-delta sandstone reservoirs with frequent lithofacies changes. Here Sangonghe Formation in the Central Junggar Basin of China is taken as an example. The K-means-based synthetic minority oversampling technique (K-means SMOTE) is employed to solve the problem regarding the imbalanced lithofacies data categories used to calibrate logging data, and a probabilistic calibration method is introduced to correct the likelihood function. To address the situation in which traditional machine learning methods ignore the geological deposition process, we introduce a depositional prior for controlling the vertical spreading process based on a Markov chain and propose an improved Bayesian inversion process for training on the log data to identify lithofacies. The results of a series of experiments show that, compared with the traditional machine learning method, the new method improves the recognition accuracy by 20%, and the predicted petrographic vertical distribution results are consistent with geological constraints. In addition, SMOTE and probabilistic calibration can effectively handle data imbalance problems so that different categories can be adequately learned. Also the introduction of geological prior has a positive impact on the overall distribution, which significantly improves the accuracy and recall rate of the method. According to this comprehensive analysis, the proposed method greatly enhanced the identification of the lithofacies distributions in the Sangonghe Formation. Therefore, this method can provide a tool for logging lithofacies interpretation of deep and strongly heterogeneous clastic reservoirs in fluvial-delta and other depositional environments.

Trace metals in CO 2 -rich Green River springs, Utah, USA: an analogue for engineered storage

Abstract Sedimentary rocks with high natural CO 2 concentrations provide invaluable analogues for the long-term engineered storage of CO 2 . Some previous studies have reported high trace metal concentrations in sandstone aquifers exposed to CO 2 , a cause for concern should stored CO 2 leak into underground sources of drinking water. However, the intensively studied Jurassic sandstone aquifer in the San Rafael–Green River (Utah, USA) area has trace metal concentrations that are within US Environmental Protection Agency's limits for drinking water. Exceptions are As which is plausibly introduced into the aquifer by saline brines external to the aquifer, and salinity which largely is. This shows that CO 2 in aquifers does not inevitably cause trace metal contamination. CO 2 –water–rock batch experiments elucidated the controls on the trace metal concentrations. After the addition of CO 2 , the experiments reproduce Cu, Cd, Hg, Ni and Zn well, with less good agreement for Cr and Pb although these are still low compared to drinking water standards. Major cations used as fingerprints for mobilization mechanisms suggest that the trace metals are largely derived by desorption, possibly from grain-coating Fe-oxides, rather than by the dissolution of mineral phases. Possible exceptions are Pb and Ni, plus As which is derived from saline brines.

Detrital zircon geochronology of Jurassic strata in the Jianggalesayi–Washixia Basin, northwestern Tibet Plateau: Implications for the uplift history of the Altyn Tagh Orogen

Jurassic terrestrial sediments of the Jianggalesayi–Washixia Basin in the northwestern Altyn Tagh Orogen record abundant information regarding the evolution and uplift of the orogen. Investigation of this orogen is critical to establishing whether the Tarim and Qaidam basins were connected during the Jurassic. In this study, we present 474 single‐grain laser‐ablation–inductively coupled plasma–mass spectrometry detrital zircon U–Pb ages obtained from five Jurassic sandstone samples collected from four sections in the basin. Detrital zircons from the Lower Jurassic strata yield age populations of 997–726 and 551–509 Ma, indicating that the zircons were derived mainly from the southern–central Central Altyn Tagh Massif. Zircon age populations from the lower Middle Jurassic strata are 1024–838, 778–643, and 543–357 Ma, which suggest that the sediments were derived predominantly from the northern Central Altyn Tagh Massif and the southern South Altyn Tagh subduction–collision complex belt (SATSB). Zircon age populations from the upper Middle Jurassic strata are 980–830, 556–424, and 277–245, suggesting that these sediments were derived from the central SATSB. Detrital zircons from the Upper Jurassic strata display age groups of 515–387 and 325–226 Ma, indicating that these sediments originated from the northern SATSB. Uplift of the Altyn Tagh Orogen was initiated during the Middle to Late Triassic as a synchronous far‐field response to the collision of the Qiangtang Block with the Eurasian Plate. By the Early Jurassic, the Qaidam Basin was no longer connected with the Tarim Basin. During the Late Jurassic to Early Cretaceous, the collision of the Lhasa Block with the southern margin of the Eurasia Plate led to further uplift of the Altyn Tagh Orogen.

A Multi-proxy Provenance Study of Late Carboniferous to Middle Jurassic Sandstones in the Eastern Sverdrup Basin and Its Bearing on Arctic Palaeogeographic Reconstructions

A multi-proxy provenance study of Late Carboniferous to Middle Jurassic sandstones from the eastern Sverdrup Basin was undertaken employing optical petrography and heavy mineral analysis, chemical analysis of apatite, garnet and rutile grains, as well as detrital zircon U–Pb geochronology and Hf isotope analysis. Late Carboniferous to Middle Jurassic strata on the southern basin margin are inferred as being predominantly reworked from Silurian to Devonian strata within the adjacent Franklinian Basin succession. Higher-grade metamorphic detritus appeared during Middle to Late Triassic times and indicates exhumation and erosion of lower (Neoproterozoic to Cambrian) levels within the Franklinian Basin succession and/or a direct detrital input from the Canadian-Greenland Shield. The provenance of northern-derived sediments is more enigmatic owing to the subsequent opening of the Arctic Ocean. Northern-derived Middle Permian to Early Triassic sediments were likely derived from proximal areas of the Chukotkan part of the Arctic Alaska-Chukotka microplate. Late Triassic northern-derived sediments have different detrital zircon U–Pb age spectra from Middle Permian to Early Triassic ones and were likely derived from the Uralian orogenic belt and/or the Arctic Uralides. The loss of this sand input during latest Triassic times is interpreted to reflect drainage reorganisation farther upstream on the Barents Shelf. Middle Jurassic sands in the northern and axial parts of the basin were largely reworked from local northern-derived Late Triassic strata. This may have been facilitated by rift flank uplift of the northern basin margin in response to rifting in the adjacent proto-Amerasia Basin.

Middle-Late Jurassic chronology and source-to-sink system of the Sikeshu section in the southern margin of the Junggar Basin, Xinjiang

With the improvement of isotopic testing technology, high-precision dating methods such as SHRIMP or LA-ICP-MS of detrital zircon have been used to analyze and infer the formation age of sedimentary basins, the source of detrital materials, and the tectonic background of the formation and widely used. The LA-ICP-MS zircon U-Pb dating method was used to study the detrital zircon in the Middle-Late Jurassic sandstone in the Sikeshu section of the southern margin of the Junggar Basin, Xinjiang. The results show that the detrital zircon ages are mainly 270~447Ma,240~440Ma,245~432Ma, and 286~360Ma. There are also several ancient zircon ages, mainly 945~1080Ma. Combined with zircon mineralogical characteristics, CL images, Th/U ratio, sandstone petrographic characteristics, and regional chronological data, the primary provenance of the Middle-Late Jurassic sandstone is mainly from the Yilinghabilga Mountains in the North Tianshan, and Carboniferous-Permian volcanic rocks, pyroclastic rocks and Carboniferous granites exposed in the Borokonu Mountains of the Central Tianshan Mountains. By comparing the age changes between the four sampling points, the change process of the North Tianshan-Middle Tianshan source-sink system is obtained. From west to east, the upper Devonian glutamate, the Middle Devonian carbonate, and the large granite bodies of the Carboniferous series constituted the early watershed. From the middle to late period of the Toutunhe Formation to the Xishanyao Formation, from east to west, the Cambrian and Silurian carbonate rocks and the large granites and other rocks formed the late water line.

Experimental evaluation of microscopic pore structure and fluid migration characteristics of coal-measure sandstone reservoirs

Research on the microscopic migration characteristics of fluids in coal measure sandstone has always been a hot spot in the evaluation of reservoir properties. In this study, taking the Yan’an Formation sandstone reservoirs in the Block A of the Ordos Basin as an example, the pore structures and fluid migration characteristics of coal-measure sandstones are systematically studied using a large number of thin sections, SEM (Scanning Electron Microscope), NMR (Nuclear Magnetic Resonance), relative permeability and water-flooding test results. The results show that the Jurassic sandstones in the target layer mainly develop lithic quartz sandstone, and the main pore types are intergranular and dissolution pores, followed by a small amount of intercrystalline pores. The surface porosity of the target sandstones mainly ranges from 7.90 to 10.79%, with an average value of 8.78%. The good correlation between porosity and permeability indicates that the target layer is a pore-type reservoir. The T2 relaxation time of the target layer is mainly distributed within 100 ms. Moreover, the reservoir of the Yan’an Formation has a high saturation of movable fluids, which is mainly distributed in 43.17–71.24%, with an average value of 56.90%. Meanwhile, samples with fractures have higher movable fluid saturations. In addition, the average irreducible water saturation of the Yan’an Formation sandstone reservoir is 35.14%, and the final oil displacement efficiency is 51.14% on average. There is a good positive correlation between the oil displacement efficiency and the co-permeability zone. As the co-permeability zone range increased from 15 to 55%, the oil displacement efficiency increased from 30 to 65%. When the cores develop fractures, they have characteristics of high permeability, high oil recovery rate, high oil displacement efficiency in the anhydrous period, low irreducible water saturation and low residual oil saturation.