Lower Jurassic Sandstones Research Articles

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.

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.

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.

Geochemical characteristics of Jurassic sandstones on the southern margin of the Junggar Basin: Constraints on provenance and sandstone-type uranium mineralization

Jurassic strata in the southern margin of the Junggar Basin are located in the piedmont zone of the North Tianshan Mountains in Xinjiang, China. Due to multiple tectonic movements, the basin has a complicated history of structural deformation. Accurate reconstructions of the tectonic setting, provenance attributes and paleoclimatic characteristics of the Jurassic strata in the area are vital to facilitate mineral resources prospecting (such as coal, oil, natural gas and uranium) in the basin. Here we used scanning electron microscopy, major and trace elements analyses (including rare earth elements; REE), clay mineralogy and heavy mineral composition in outcrop samples, to analyze the paleoclimate, tectonic setting, source rock properties and provenance characteristics in the southern margin of the Junggar Basin during the Jurassic period. The results are as follows: (1) according to major and trace elements and clay mineral assemblages, the degree of drought increased slightly from the Early Jurassic to the Late Jurassic concomitant increase. (2) Major and minor element discrimination and REE patterns indicate that the material was sourced from continental island arc and active continental margin environments, primarily from upper crustal felsic source rocks (mainly greywacke and arkose). (3) Heavy mineral and whole rock analyses show that the provenances of the Lower Jurassic sandstones were mainly from the primarily Zaire Mountain, North Tianshan Mt.-Middle Tianshan Mt., while the Upper Jurassic sediments were sourced from the primarily North Tianshan Mountain, Zaire Mountain, Middle Tianshan Mountain.

Age and Geodynamic Setting of the Lower Jurassic Sandstones in the Onon Fragment of the Aga Terrane, Mongol–Okhotsk Fold Belt

Age and Geodynamic Setting of the Lower Jurassic Sandstones in the Onon Fragment of the Aga Terrane, Mongol–Okhotsk Fold Belt

U–Pb geochronology of Upper Triassic – Lower Jurassic detrital sequences from SE margin of the South China Block: implications for Palaeo-Pacific subduction and tectonic evolution

AbstractWe performed U–Pb dating of detrital zircons and conducted petrological and whole-rock geochemical analyses to assess the provenance of the Upper Triassic – Lower Jurassic clastic rocks in the southeastern margin of the South China Block. Detrital zircon U–Pb ages are mainly classified into age groups of 2000–1700, 900–700, 490–390 and 280–210 Ma, consistent with derivation from the Jiangnan orogenic belt, Nanling Belt, as well as Wuyi and Yunkai domains. Lower Jurassic samples yield a special main age population of 200–190 Ma, and these detrital zircon grains have low Th/U and Nb/Hf ratios and high Th/Nb and Hf/Th ratios, showing they are derived from a continental magmatic arc. However, the cross-correlation and likeness coefficients of kernel density estimates of Upper Triassic and Lower Jurassic sandstones are 0.8608 and 0.8403, indicating that their populations are highly similar. Since the tectonic setting is the key factor in controlling the relationship between source and sink, the stable supply of identical provenance suggests that the tectonic setting did not significantly change during Late Triassic – Early Jurassic time. Sandstone petrography, regional facies distribution and the detrital zircon age patterns all reflect a consistent tectonic setting for the South China Block during Late Triassic – Early Jurassic time. The Palaeo-Pacific subduction therefore did not control the tectonic evolution of the South China Block until after the Early Jurassic Epoch.

Sedimentological and geochemical characteristics of Lower Jurassic Sandstones from Gümüşhane, NE Turkey: implications for source to sink processes, paleoenvironmental conditions, provenance and tectonic settings

ABSTRACT The eastern Sakarya Zone (SZ), one of the Alpine Himalayan Belt’s main components, has complex Mesozoic–Cenozoic geodynamic evolution, which remains controversial. Early-Middle Jurassic volcano-sedimentary successions (Şenköy Formation) extensively outcrops in Gümüşhane region, NE Turkey. It is regarded as a critical unit to understand the Mesozoic tectonic evolution of the eastern Sakarya Zone. One of the Şenköy Formation’s predominant units is sandstone succession, preserving implications for paleoenvironmental conditions and tectono-sedimentary development of Neotethys Ocean. Thus, a combined analysis of comprehensive petrography, whole-rock major, trace and rare earth element data is carried out on the sandstones, which are well constrained as late Sinemurian- Pliensbachian. The sandstones are mainly arkosic to lithic arenites in lithology showing poor textural maturities. They are mostly represented by relatively high ICV, low CIA, high K2O/Al2O3, Fe2O3+ MgO, TiO2, Th/U, pronounced negative Eu anomaly, enriched LREEs, elevated GdN/YbN, low Ce/Ce*, Ni/Co, La/V and Zr/Ti. The petrographic and geochemical characteristics imply that the sediments may have experienced relatively low chemical weathering processes and slight reworking and recycling processes with a short transport distance before deposition in the basin. Early Jurassic sandstone successions were interpreted as a transgressive series, showing a predominantly open sea environment with oxidizing paleoenvironmental conditions. The sandstone samples’ paleoenvironment is likely to be located along the southern margin of the arc and the Neotethyan basin to the south. The extensional and trans-extensional tectonic regime might have played an essential role in controlling the basin’s isolated-half graben and/or asymmetric geometry, leading to changes in the provenance of studied sandstone samples. Thus, the geochemical signature differences are probably associated with basement-related proximal origins and minor effects of the multi-phases petrogenesis that experienced different source to sink processes. Early Jurassic sandstones were probably derived from multi-genetic source rocks, including (i) granite and (ii) metamorphic rocks. Our new data has also indicated that other source rocks can include mafic to felsic volcanic succession. The possible provenance for granitic pluton and metamorphic rocks can be provided by the Hercynian heterogenetic crystalline basement widespread throughout the SZ. However, mafic to felsic volcanic rock source is likely to be associated with the pre-Early Jurassic nearby volcanic-arc, which is rarely recorded in the eastern SZ. The source rock signatures may imply a close genetic link with ancient magmatic-arc generation due to the southward subduction of Palaeotethys.

Geochemical constraints on the Triassic–Jurassic Amir-Abad karst-type bauxite deposit, NW Iran

The Amir-Abad karst-type bauxite deposit is located in the Irano–Himalayan bauxite belt, 8 km of south of Maragheh city in northwestern Iran. The bauxite ores occur as layers and lens-shaped pockets in the contact of the Triassic dolomitic limestone of the Elika Formation and the Lower Jurassic sandstone, siltstone, shale, and coal of the Shemshak Formation. Based on the PXRD and SEM-EDS analyses, the bauxite ores mainly consist of diaspore, hematite, and kaolinite group minerals (e.g., kaolinite and montmorillonite), with lesser amounts of goethite, anatase/rutile, and zircon. Nickel, Cr, Co, and V show a pattern similar to Fe; they were leached from the uppermost parts of the profile by acidic percolating solutions, and concentrated toward the carbonate bedrock under alkaline conditions. Aluminum, Nb, Ga, Ta, Th, Hf, and Zr can be regarded as less mobile elements. Variations of the La/Y, (ΣLREE/ΣHREE)N, and (La/Yb)N ratios across the weathering profile, together with leaching of alkali and alkaline earth metals indicate an acidic environment in the uppermost parts, and a basic environment in the basal parts of the profile. Mineral control and physico-chemical conditions (e.g., Eh and pH) probably had effective roles in the behavior of elements during weathering. Based on the concentration of trace elements, such as Zr, Cr, and Ga, and the conservative elemental indices (e.g., Sm/Nd, Ti/Zr, and Nb/Ta), the basaltic rocks were a plausible source rock for the Amir-Abad karst-type bauxite deposit.

Review of hydrocarbon potential in East Denmark following 30 years of exploration activities

Between 1993 and 2017, Denmark was one of the largest oil exporting countries in Europe having gained this position from its share in the highly prolific Danish Central Graben. However, outside the Central Graben few prospects have been adequately mapped, due to a lack of data in these socalled ‘white areas.’ As such, their potential for hydrocarbon accumulation remains uncertain. This paper presents an update of the prospect and play types in this area outside the Danish Central Graben, east of 6°15´E longitude (Fig. 1), based on results from the last 30 years of exploration activities. The paper is part of a resource assessment made by the Geological Survey of Denmark and Greenland (GEUS) to the Danish Energy Agency (Schovsbo & Jakobsen 2017) and is an update of a former review of the area made in 1987 (Thomsen et al. 1987). The succeeding exploration efforts have not changed the overall low expectation for the play types in the area. Here, we show that an uncertain resource is associated with both the Zechstein carbonate play in the North German Basin and the Upper Triassic – Lower Jurassic sandstone and lower Palaeozoic shale gas plays in northern Jylland. However, questions remain as to the source of hydrocarbons in the western offshore area. Specifically, we are unable to confirm (or refute) whether these structures are sourced via long-distance migration of hydrocarbons from the Danish Central Graben.

Post-250 Ma thermal evolution of the central Cathaysia Block (SE China) in response to flat-slab subduction at the proto-Western Pacific margin

The flat-slab subduction model has been proposed to explain the complex Mesozoic–Cenozoic geotectonic record of SE China as part of the Western Pacific margin. Here we investigate the post-250 Ma thermal history of the Shaoguan-Longnan region to evaluate the time-temperature evolution of local sedimentary and magmatic rocks in the context of the flat-subduction model. Our multi-chronological approach includes zircon U-Pb geochronology, 40Ar/39Ar dating of K-feldspar and biotite, (U-Th)/He dating of zircon and apatite, and fission-track dating of apatite from basement and/or sedimentary rocks, as well as vitrinite reflectance analysis of coal seams. Zircon U-Pb dating revealed Early Silurian and Late Jurassic crystallization ages for the Fuxi (ca. 438 Ma) and Liyuan (ca. 162 Ma) granites, respectively. Syn-/pre-depositional single-grain zircon (U-Th)/He ages from Lower Jurassic sandstones record a Middle Triassic exhumational cooling, interpreted to reflect the Indosinian Orogeny, possibly caused by a flat-slab subduction event. Late Jurassic biotite 40Ar/39Ar age (ca. 156 Ma) obtained from the Late Triassic Mengdong granite, together with maximum vitrinite reflectance values of 1.7%–2.3% measured from a Lower Jurassic coal sample, document a thermal maximum in the Late Jurassic, likely related to the prior regional magmatism arising from delamination of the subducting Paleo-Pacific plate. K-feldspar 40Ar/39Ar plateau age from the Fuxi granite (ca. 125 Ma), and zircon (U-Th)/He mean ages (152–115 Ma) record a protracted cooling through the K-feldspar Ar and the zircon He partial retention zones during the Cretaceous. The Cretaceous cooling is in accord with the prediction of regional uplift and erosion in response to foundering of the flat-slab and consequent lithospheric rebound. The rapid cooling during the Paleocene–Eocene, recorded by apatite fission-track central ages (72–39 Ma) and apatite (U-Th)/He mean ages (38–33 Ma), is interpreted to reflect Cenozoic rifting in the leadup to the opening of the South China Sea.

Geochemical characteristics and accumulation of hydrocarbon in Lower Jurassic tight sandstone in Wenjisang area, Turpan Basin, NW China

In order to investigate the geochemical characteristics, sources and accumulation process of Lower-Jurassic Sangonghe Formation tight sandstone oil, multiple geochemical analyses were performed on source rocks, oil samples and sandstone particles from Wenjisang area in Turpan Basin. The results show that coal-measure source rocks have relatively higher organic matter abundance and type-II organic matter for Xishanyao and Badaowan Formations, with the potential of liquid hydrocarbon generation. Oil samples of Sangonghe Formation have the characteristics of coal-measure-formed oil. Higher maturity indicate the oil may be mainly generated by Lower-Jurassic Badaowan Formation source rocks. Quantitative fluorescence parameters also reflect that hydrocarbons charged into Sangonghe Formation through continuous migration from the bottom of sand body to the top gradually. In down dip direction of Wenjisang area may develope continuous tight oil and gas reservoirs in Lower Jurassic.

Controlling factors of reservoir quality in low-permeability reservoir within a sequence stratigraphic framework: a case study of the lower Jurassic sandstones in the Northern margin of the Qaidam Basin, China

The Qaidam Basin, one of China’s most prolific petroliferous basins, is located in the northeastern corner of the Qinghai-Tibet Plateau. Using analytical data from cores, thin-sections and seismic data, and combined with the previous sequence stratigraphy research results, research on the reservoir characteristics was undertaken. The main controlling factors of reservoir quality in the sequence stratigraphy framework in the Lenghu area are discussed. The Lower Jurassic includes lowstand system tract (LST) and transitional systems tract (TST); the highstand systems tract are missing owing to tectonic uplift. The Lower Jurassic sandstones are composed of moderately to poorly sorted, subrounded sandstone representing several representative environments, including fan-delta and lacustrine facies. The main types of the Lower Jurassic sandstones are mostly feldspathic litharenite and lithic sandstone. The pore types of the Lower Jurassic sandstones are mainly secondary dissolution pores with few fracture types. The Lower Jurassic sandstones have experienced strong compaction that resulted in the loss of primary porosity. Carbonate cementation of the LST sandstones is greater than in the TST sandstones. Dissolution, which has an important role in reservoir development, is mainly caused by acidic fluids from the matured source rocks in the LST and meteoric water ingression from the top sequence boundary in the TST. Distributary channel sandstones adjacent to coal and mudstones are favourable reservoirs with turbidite sandstones adjacent to the sequence boundary less favourable reservoirs.

Formation phases of carbonate cements and sedimentary environments in lower Jurassic sandstones of the Lenghu V tectonic belt, North Qaidam Basin, China

Based on petrography and geochemistry, formation phases of carbonate cements and their sedimentary–diagenetic environments in Jurassic clastic rocks from the Lenghu V tectonic belt are determined. The results demonstrate that studied successions are mainly dark gray or black mudstone with interbedded thin gray siltstone and a few greywacke layers. Sandstones are lithic and feldspathic lithic greywacke with high kaolinite content. Calcite and ferrocalcite are the main carbonate cements. The carbon isotope values(δ13C) of the carbonate cements range from − 15.6 to 9.2‰ (average − 3.2‰) with a bimodal distribution. One peak is 4‰, which includes samples from the upper Xiaomeigou Formation (Fm) and the other one is − 12‰, which mainly represents the lower section. The oxygen isotope values(δ18O) have a wide range from − 18.5 to − 8.3‰ (average − 13.31‰). Combined with microscopic observations, two phases of carbonate cements are distinguished according to their isotopic characteristics. The earlier carbonate cements are mainly calcite and ferrocalcite, and they were precipitated in eodiagenesis stage. The carbon source of calcite was inorganic, and ferrocalcite was mixed with carbon from methanogenesis. This mixture led to a positive bias in the δ13C curve, which artificially inflated the paleosalinity. The later carbonate cements, formed in mesodiagenesis stage, were ferrocalcite. Decarboxylation of organic matters led to lower δ13C values. The major and trace elements data show that Sr, Ba, Fe and Mn have weak correlation with Al, Ti. So, they are effective to interpret the sedimentary environment. The Sr/Ba ratio < 1 indicates fresh water environment, Sr/Ca, Sr/Cu and Fe/Mn ratios suggest humid climate, and V/(V + Ni) ratio shows suboxic–anoxic environment. The carbon and oxygen isotopes, together with elemental geochemistry, show that the climate during the Early Jurassic was warm and wet and the studied successions were deposited in fresh water. The variations in elemental ratio curves indicate that climate became wetter and warmer later in the Early Jurassic.

Thermochronological record of Middle–Late Jurassic magmatic reheating to Eocene rift-related rapid cooling in the SE South China Block

We investigate the Mesozoic–Cenozoic thermal history of the Daxi region (central SE South China Block) to evaluate the influence of the subduction of the Paleo-Pacific oceanic plate beneath the SE South China Block along the block's southeast margin on the tectonothermal evolution of the upper plate. We apply a multi-chronological approach that includes U-Pb geochronology on zircon, 40Ar/39Ar dating on muscovite and biotite from granitic rocks as well as fission-track and (U-Th-Sm)/He analyses on zircon and apatite from granitic and sedimentary rocks. The Heping granite, located in the Daxi region, has a magmatic age of ca. 441Ma. The biotite 40Ar/39Ar ages of ca. 193Ma for the Early Jurassic Shibei granite and ca. 160Ma for the Late Jurassic Fogang granite, respectively, reflect magmatic cooling. The Triassic Longyuanba granite yielded a muscovite 40Ar/39Ar age of ca. 167Ma, recording heating to ≥350°C induced by nearby intrusion of Middle Jurassic granites. Zircon fission-track and (U-Th-Sm)/He ages from Lower Carboniferous–Lower Jurassic sandstones (140–70Ma) record continuous cooling during the Cretaceous that followed extensive Middle–Late Jurassic magmatism in the Daxi region. Cretaceous cooling is related to exhumation in an extensional tectonic setting, consistent with lithospheric rebound due to foundering and rollback of the subducted Paleo-Pacific oceanic plate. Apatite fission-track (53–42Ma) and (U-Th-Sm)/He ages (43–36Ma), and thermal modelling document rapid cooling in the Paleocene–Eocene, which temporally coincides with continental rifting in the SE South China Block in the leadup to the opening of the South China Sea.

DIAGENEZA A POROWATOŚĆ PIASKOWCÓW JURY DOLNEJ NA NIŻU POLSKIM

DIAGENESIS AND POROSITY OF THE LOWER JURASSIC SANDSTONES IN THE POLISH LOWLANDS Abstract. Sandstones from the boreholes located in Szczecin–Radom area have been subjected to petrographic and petrophysical analysis. Sandstones are represented mostly by quartz arenites and wacks, from very fine- to middle-grained. The main type of cements are matrix and cements, among which quartz, carbonates, clay minerals and locally pyrite were distinguished. Quartz cement occurring as syntaxial overgrowths developed on the quartz grains displays the greatest significance in the Lower Jurassic sandstones. Authigenic clay minerals are represented mainly by vermiform kaolinite. Authigenic illite and Fe/Mg chlorites are locally observed. Carbonate cements are represented mainly by early and late generation of siderite (mineral of series of siderite–magnesite). The early diagenetic siderite represented by siderite and sideroplesite is predominant. The siderite δ 18 C values are in the range from –24.50 to –4.56‰ PDB and δ 13 O values range from –14.37 to –0.33‰ PDB . The late siderite of pistomesite and sideroplesite chemical composition also occurs, but occasionally. Furthermore, there are Fe-dolomite/ankerite and occasionally calcite. The Jurassic sandstones are characterized by very good and good filtration properties. The primary porosity is dominant, it often exceeds 20%, and permeability is in the range of 0.001 to 1930.756 mD. The most important diagenetic processes operating in the sandstones are: compaction, cementation and dissolution. Compaction reduced primary porosity on average by about 30%, and cementation by about 21%.

How to Characterize a Potential Site for CO2Storage with Sparse Data Coverage – a Danish Onshore Site Case

The paper demonstrates how a potential site for CO<sub>2<sub/> storage can be evaluated up to a sufficient level of characterization for compiling a storage permit application, even if the site is only sparsely explored. The focus of the paper is on a risk driven characterization procedure. In the initial state of a site characterization process with sparse data coverage, the regional geological and stratigraphic understanding of the area of interest can help strengthen a first model construction for predictive modeling. Static and dynamic modeling in combination with a comprehensive risk assessment can guide the different elements needed to be evaluated for fulfilling a permit application. Several essential parameters must be evaluated; the storage capacity for the site must be acceptable for the project life of the operation, the trap configuration must be efficient to secure long term containment, the injectivity must be sufficient to secure a longstanding stable operation and finally a satisfactory and operational measuring strategy must be designed. The characterization procedure is demonstrated for a deep onshore aquifer in the northern part of Denmark, the Vedsted site. The site is an anticlinal structural closure in an Upper Triassic – Lower Jurassic sandstone formation at 1 800-1 900 m depth.

Permeability, compressibility and porosity of Jurassic shale from the Norwegian–Danish Basin

The Fjerritslev Formation in the Norwegian–Danish Basin forms the main seal to Upper Triassic–Lower Jurassic sandstone reservoirs. In order to estimate the sealing potential and rock properties, samples from the deep wells Vedsted-1 in Jylland, and Stenlille-2 and Stenlille-5 on Sjælland, were studied and compared to samples from Skjold Flank-1in the Central North Sea. Mineralogical analyses based on X-ray diffractometry (XRD) show that onshore shales from the Norwegian–Danish Basin are siltier than offshore shales from the Central Graben. Illite and kaolinite dominate the clay fraction. Porosity measurements obtained using helium porosimetry–mercury immersion (HPMI), mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR) techniques on the shale samples show that MICP porosity is 6–10% lower than HPMI or NMR porosity. Compressibility, from uniaxial loading, and elastic wave velocities were measured simultaneously on saturated samples under drained conditions at room temperature. Uniaxial loading tests indicate that shale is significantly stiffer in situ than is normally assumed in geotechnical modelling. Permeability can be predicted from elastic moduli, and from combined MICP and NMR data. The permeability predicted from Brunauer–Emmett–Teller (BET)-specific surface-area measurements using Kozeny’s formulation for these shales, being rich in silt and kaolinite, falls in the same order of magnitude as permeability measured from constant rate of strain (CRS) experiments but is two–three orders of magnitude higher than the permeability predicted from the 1998 model of Yang &amp; Aplin, which is based on clay fraction and average pore radius. When interpreting CRS data, Biot’s coefficient has a significant and systematic influence on the resulting permeability of deeply buried shale.

Determination of changes in the reservoir and cap rocks of the Chabowo Anticline caused by CO2–brine–rock interactions

The article describes the changes taking place in the rock matrix (reservoir and cap rock) due to the effect of carbon dioxide in the presence of brine. The experiment was carried out on samples of Lower Jurassic sandstones (reservoir rocks) and claystones (cap rocks) from Chabowo 1 and Chabowo 3 boreholes, Chabowo Anticline (NW Poland)—a potential structure for underground storage of carbon dioxide. Rock samples were placed for a period of 18months in a designed apparatus system and the experiment was performed under the conditions of T=25°C and P=6MPa, by soaking the rock samples in a brine of simplified chemical composition similar to the brine present in the rock formation analysed. Mineral composition was determine by observation in transmitted light, XRD and SEM-EDS. In order to perform the changes in the individual mineral phases, using the SEM- EDS method, the results of mineralogical and petrographic observations of the same rock samples before and after the experiment are presented. Dissolution and precipitation processes of minerals were observed as the result of the experiment. Corrosion-prone minerals were feldspars, etchings were observed on pyrite and mica grains. The dominant mineral phase formed after the experiment was halite (which caused the precipitation of this mineral after the evaporation of brine). Precipitation of poorly developed kaolinite crystals was also observed. The long-term experiment conducted at low temperature and pressure showed similar changes that are obtained in the experiments carried out for the actual reservoir conditions, but on a smaller scale.

Late Palaeozoic and early Mesozoic tectonic and palaeogeographic evolution of central China: evidence from U–Pb and Lu–Hf isotope systematics of detrital zircons from the western Qinling region

The western Qinling region of central China is situated centrally in the Kunlun, Qilian, Qinling, Longmenshan, and Songpan–Ganzi orogens. Late Palaeozoic and Early Mesozoic sediments deposited here may provide keys to understanding the tectonic evolution of the Palaeo-Tethys and collision of the North China and Yangtze Cratons. We conducted in situ U–Pb and Lu–Hf isotope analyses of 568 detrital zircons collected from Upper Palaeozoic to Mesozoic sandstones in the central Qinling block, Taohe depression, and Bailongjiang block in western Qinling to constrain the sources of these sandstones. Our results reveal that the Bailongjiang block has affinities with the Yangtze Craton, from which it may have been rifted. Therefore, the Palaeo-Tethyan Animaqen suture between the two cratons lies north of the Bailongjiang block. We identified the North China Craton as the main source for Triassic flysch in central China. It is possible that the Bailongjiang block could have blocked detritus shed from the North China Craton into the main depositional basins in the Songpan–Ganzi area. The dominance of 300–200 Ma detrital zircons of metamorphic origin in Lower Jurassic sandstones indicates that the Dabie–Qinling orogen was elevated during Early Jurassic time. In addition, our Lu–Hf isotopic results also reveal that Phanerozoic igneous rocks in central China were mostly products of crustal reworking with insignificant formation of juvenile crust.