Source Rock Types Research Articles

Sources and exploration potential of Ordovician subsalt natural gas in Ordos Basin, China

With the continuous increase in exploration efforts in new zones and new strata, significant breakthroughs have been made in the natural gas exploration of the O1m56 to O1m4 formations in the Ordos Basin. Thus, the origin and exploration potential of subsalt natural gas have attracted much attention and urgently need to be addressed. On the basis of certain geochemical characteristics, genetic types, and sources of natural gas, a comprehensive study on the sedimentary environment, organic geochemical characteristics, and spatial distribution scale of source rocks are conducted in this paper by using geological and geochemical methods. The study shows that: (1) The Ordovician subsalt natural gas is mainly “pyrolysis dry gas,” among which the δ13C1 of Ordovician subsalt low sulfur (sulfur-free) natural gas is lighter, with an average value of −39.6 ‰; the δ13C2 ranges more largely from −35.6 ‰ to −25.8 ‰. In contrast, both δ13C1 and δ13C2 values are heavier in high-sulfur natural gas, revealing that different Thermochemical Sulfate Reduction (TSR) reaction stages have different degrees of influence on natural gas components and carbon isotope composition. (2) Subsalt natural gas is classified as “oil-type gas,” which is self-generated and self-accumulated, whose source rocks are mainly Ordovician subsalt marine deposits. (3) Three types of marine source rocks are developed in Ordovician subsalt, including black argillaceous rock, dark argillaceous dolomite (dolomitic mudstone), and dark micrite (bioclastic) limestone. In addition to micrite limestone, these rocks were mainly formed in a confined lagoon sedimentary environment with high salinity and anoxia. Sedimentary water was significantly stratified and the environment was highly reduced. The organic matter content of the source rocks is relatively high, with an average TOC value of 0.45 %. The hydrocarbon-generating parent materials are mainly composed of bacteria and algae, and the organic matter evolution reaches high-over maturity stage. The total gas generation amount of the marine source rocks in Ordovician subsalt is approximately 43.8 × 1012 m3, which can provide hydrocarbons and accumulate for the subsalt favorable reservoir facies located far from Upper Paleozoic gas sources.

Characteristics and Origin of Natural Gas in Yongfeng Sub-Sag of Bogda Mountain Front Belt

By systematically analyzing the natural gas composition, carbon isotopes, and source rock characteristics in the Yongfeng sub-sag of the Bogda Mountain front belt, natural gas characteristics were determined, and the genetic types and sources of natural gas were investigated. The research results indicate that methane is the main component of natural gas in the Yongfeng sub-sag, with low levels of heavy hydrocarbons and a high drying coefficient. These characteristics make it dry gas, which refers to natural gas with a methane content of over 95%. The ethane carbon isotope δ13C2 of natural gas is −28.5‰ and belongs to oil type gas. The methane carbon isotope δ13C1 of natural gas is −58.6‰~−59.4‰, has a relatively depleted methane carbon isotope value, shows significant differences from the surrounding natural gas methane carbon isotope, and belongs to the category of biogenic gas. The Permian Lucaogou Formation is the main source rock in the study area, with good organic matter abundance. The microscopic components of kerogen are mainly composed of sapropelic formations and the organic matter type is I–II1. The source rock has a high maturity and has reached the mature stage, mainly consisting of oil and wet gas. The ethane carbon isotope of natural gas in the Yongfeng sub-sag shows as oil type gas, which is consistent with the kerogen type of the Lucaogou Formation source rocks, indicating that the natural gas mainly comes from the Lucaogou Formation source rocks. Based on comprehensive data and information on natural gas composition, carbon isotopes, and burial history of the source rocks, it is believed that some of the crude oil generated from the Lucaogou Formation in the early stage underwent biodegradation due to tectonic uplift, resulting in biogenic methane and the formation of crude oil biodegraded gas.

Genetic types and provenance indication of clastic amphibole in the South Yellow Sea

The main focus of studying sediment sources in marine areas is to determine the properties of clastic minerals originating from different rivers. Variations in the characteristics of clastic minerals occur due to differences in the types of source rocks within the watershed. Determining the source rock of clastic minerals in the marine area can improve the accuracy of provenance analysis. Electron probe microanalysis of 258 amphibole grains from six surface stations in the South Yellow Sea (SYS) was conducted to calculate the numerical and characteristic values of cations in the crystal structure. The results showed that amphibole in the SYS is mainly magnesiohornblende (52.2% ~ 81.4%) in the calcic amphibole subgroup, followed by tschermakite. The source rock types of amphibole are mainly different types of magmatic (ultrabasic, basic, and intermediate acidic) and metamorphic rocks. Amphibole derived from intermediate-acid intrusive rocks accounts for a large proportion (>40%). The genetic analysis of clastic amphibole in the SYS showed that most of the medium-acid invading amphibole belong to crust-mantle type, followed by mantle type. Most of the amphibole from metamorphic origins are of medium-low pressure type (± 80%). Combined with electron probe analysis of amphibole from the Huanghe River (the Yellow River, HH) and Changjiang River (the Yangtze River, CJ), the amphibole in the SYS exhibits characteristics inherited from these two rivers. The northern sea area has a strong resemblance to the sediments from the HH, while the southern area is more influenced by sediments supply from the CJ. The central area represents a mixed zone with a higher sediment supply from the HH. The clastic amphibole deduces the type of source rock in different watersheds, serving as a crucial link between the source rock, watershed, and marine area, providing a basis for provenance analysis.

Implementation of a novel EOR screening methodology to the oilfields in Türkiye

In response to growing energy demand of Türkiye, an enhanced oil recovery (EOR) applicability assessment was conducted through an improved screening methodology, calibrated with successful applications. The methodology incorporated some improvements in applicability ranges and a novel pointing system. 159 oilfields in Türkiye were characterized based on reservoir properties and categorized as heavy, weak and strong aquifer support. The characterization revealed how source rock type and regional paleo-tectonic events dictate primary recovery and need for EOR. The EOR assessment implemented promised 58 applicable cases out of 315. Carbon dioxide (CO2) injection is applicable for many fields, which could stimulate feasibility of carbon capture projects in the country. Immiscible nitrogen injection might be an alternative for many of the medium and light oil reservoirs in the absence of CO2. Polymer flooding is also promising mainly for fields having weak-aquifer support. However, our hands-on experience suggests that conformance control is required to control flow of EOR agent in densely fractured carbonates. Thermal methods do not seem applicable in heavy oil reservoirs due to depth limitations except in-situ combustion, which requires pilot test confirmation. Provided methodology in this study may be used as a guideline for other fields to assess EOR potential.

Provenance and Petrographic Analysis of Paleogene Sandstones in the Bukit Tigapuluh Area, Jambi Subbasin, Indonesia

The study investigates the provenance of Paleogene sandstone deposits from the Lemat Formation in the Lubuk Lawas and Lubuk Bernai sections of the Bukit Tigapuluh area, Jambi Subbasin, South Sumatra Basin, Indonesia. The research aims to elucidate the formation history and stratigraphic evolution of these sedimentary rocks through detailed petrographic analysis. Twenty-two rock samples were analyzed to classify the sandstones and determine their source rock types and tectonic settings. The results indicate that the sandstones are primarily classified as Lithic Arkose, Arkosic Subarkose, Feldspathic Litharenite, Silty Claystone, Sublitharenite, and Subarkosic Wacke. The provenance analysis suggests that these sandstones originated from a tectonic setting of a recycled orogenic zone, comprising quartzose recycled, transitional recycled, and mixed sources, indicating a combination of primary and recycled orogenic materials. Paleocurrent analysis from sedimentary structures in the Lubuk Bernai area suggests deposition directions from the southwest and southeast. This study provides significant insights into the paleogeography and tectonic evolution of the Lemat Formation within the Jambi Subbasin. Further research should focus on integrating geochemical data to refine the understanding of sedimentary provenance and tectonic implications.

Experimental simulation of thermal evolution of nitrogen content and isotopes in source rocks: Implications for nitrogen cycling characterization and oil-source correlation

Sedimentary nitrogen isotopes have been widely used to reconstruct the nitrogen cycling and redox characteristics of ancient oceans, but they also have important value in the study of oil genesis, such as oil classification. However, the thermal evolution of nitrogen isotopes in the source rock and associated oil is currently unclear. In this study, the thermal evolution (from the oil generation to the dry gas stage) of three types of low-maturity source rock was simulated. And the content and isotope composition of total organic carbon (TOC) and total nitrogen (TN) of the pyrolyzed source rock, expelled oil and retained oil were measured. In combination with weight-quantitative analyses, the following conclusions can be drawn: (1) During the main oil and gas generation stage, the pyrolyzed source rocks lost 15.1–28.3% of TN, which is much lower than the loss of TOC (26.2–46.1%). This difference may be related to the fact that the TOC content of the generated oil and gas is two orders of magnitude higher than the TN. In addition, TOC loss occurred mainly within the oil generation-expulsion stage, whereas TN loss occurred mainly in the dry gas stage and higher maturity stage. (2) The nitrogen isotope compositions of three types of source rocks and associated kerogens became heavier by 0.5–1.5‰ as thermal maturity increased, which is similar to the change of organic carbon isotope compositions. The δ15N values of pyrolyzed source rocks generally have poor relationships with either the amount of expelled oil and gas or the TN loss, which is different from the δ13Corg values. The δ15N fluctuations may be attributed to three processes: oil generation-expulsion, N2 gas generation-expulsion, and inorganic N (like NH4+) expulsion. The bulk nitrogen isotope composition of source rocks is only slightly affected by thermal maturity and can be used to indicate the nitrogen biogeochemical cycle and water redox conditions during the deposition. (3) Similar to δ13Corg values, δ15N values of both expelled oil and retained oil increased gradually with enhanced thermal maturity, implying a control of kinetic isotope effect. Among the three types of source rock, two of them having relatively light nitrogen isotope compositions (δ15N value of ∼2.5‰) generated and expelled oils with similar nitrogen isotope compositions, while the third type, which has heavy nitrogen isotope composition (δ15N value of ∼13.0‰), generated and expelled oils with much lighter nitrogen isotope compositions (by 3–6‰). This finding shows that there may be significant nitrogen isotope fractionation during the generation and primary migration of oil from source rocks. The nitrogen isotopes should be used in oil-source correlation with caution, even though related mechanisms need to be further studied. Nevertheless, using a combination of organic carbon and nitrogen isotopes, the oil generated and expelled by the three types of source rocks could be distinguished. Therefore, the use of nitrogen isotopes for oil-source correlation under geological conditions should be combined with other isotope and molecular indicators.

Prediction and quantification of effective gas source rocks in a lacustrine basin: Western Depression in the Liaohe Subbasin, China

Due to limited data on the geochemical properties of natural gas, estimations are needed for the effective gas source rock in evaluating gas potential. However, the pronounced heterogeneity of mudstones in lacustrine successions complicates the prediction of the presence and geochemical characteristics of gas source rocks. In this paper, the Liaohe Subbasin of Northeast China is used as an example to construct a practical methodology for locating effective gas source rocks in typical lacustrine basins. Three types of gas source rocks, microbial, oil-type, and coal-type, were distinguished according to the different genetic types of their natural gas. A practical three-dimensional geological model was developed, refined, and applied to determine the spatial distribution of the mudstones in the Western Depression of the Liaohe Subbasin and to describe the geochemical characteristics (the abundance, type, and maturation levels of the organic matter). Application of the model in the subbasin indicates that the sedimentary facies have led to heterogeneity in the mudstones, particularly with respect to organic matter types. The effective gas source rock model constructed for the Western Depression shows that the upper sequence (SQ2) of the Fourth member (Mbr 4) of the Eocene Shahejie Formation (Fm) and the lower and middle sequences (SQ3 and SQ4) of the Third member (Mbr 3) form the principal gas-generating interval. The total volume of effective gas source rocks is estimated to be 586 km3. The effective microbial, oil-type, and coal-type gas source rocks are primarily found in the shallow western slope, the central sags, and the eastern slope of the Western Depression, respectively. This study provides a practical approach for more accurately identifying the occurrence and geochemical characteristics of effective natural gas source rocks, enabling a precise quantitative estimation of natural gas reserves.

Early-middle permian terrigenous deposits of South-Western Primorye: material composition, source areas and formation settings

The study presents the results of studying the material composition of terrigenous rocks from the Early – Middle Permian deposits of the Reshetnikovka formation of the Laoeling-Grodekovo terrane of South-Western Primorye. Studies were carried out in order to reconstruct paleogeodynamic environments for the accumulation of studied deposits, as well as to determine the tectonic type and composition of source rocks of sources areas. It has been established that in terms of mineral and geochemical parameters, the sandstones of the formation are petrogenic or “first cycle” rocks, correspond to arcoses and, only partially, subarcoses and lithite arenites, are characterized by a fairly high degree of maturity of clastic material, and their formation was due to the destruction of largely weathered parent rocks of sources areas. Paleogeodynamic interpretation of the obtained data indicates that in the Early and Middle Permian sedimentation occurred in the basins of the passive continental margin, which are intra-and intercontinental rifts and aulacogens. The main influence on sedimentation processes was exerted by continental sources areas: cratons and uplifted basement blocks, which were projections of the crystalline basement framed by rift zones. Mainly acidic igneous and metamorphic rocks were eroded with the participation of ancient sedimentary formations. U–Pb isotopic dating of detrital zircons made it possible to establish the age and possible location of magmatic complexes, due to the destruction of which formation deposits were formed.

Elemental and Sr isotopic compositions of plagioclase as an indicator of lunar source-rock type: Insights from Chang'e 5 plagioclase fragments

Plagioclase is the most abundant mineral in lunar crustal rocks, and its elemental and Sr isotopic compositions vary among different lunar surface rock types, implying that plagioclase fragments in lunar regolith can be used to trace source-rock types. In this study, we measured major- and trace-elemental and RbSr isotopic compositions of plagioclase fragments from lunar soil returned by the Chang'e 5 (CE5) mission. Correlation between Sr and An contents allows the CE5 plagioclase fragments to be divided into three groups: normal-Sr (group A), high-Sr (group B), and low-Sr (group C). The similarity of elemental and RbSr isotopic compositions between plagioclase in groups A and B and plagioclase from CE5 basalt clasts indicates that plagioclase from groups A and B originated from the comminution of local CE5 basalt. Only two of the eighty-two analyzed plagioclases (∼2.4%) are classified as group C and have Sr isotopes that are distinct from those of groups A and B plagioclases, indicating an exotic origin. One group C plagioclase has a high 87Sr/86Sr ratio (0.70242) and content of rare earth elements and might have been derived from the Sharp B or Aristarchus craters, which are enriched in the KREEP component. The other group C plagioclase has much lower 87Sr/86Sr (0.69908) and a moderately high content of An (92.3) indicating an Mg-suite source rock and possible derivation from the Pythagoras crater. This study highlights the applicability of using elemental and Sr isotopic compositions of plagioclase fragments to trace the origin of lunar regolith.

Effect of Ancient Salinity on the Distribution and Composition of Tricyclic Terpane in Hydrocarbon Source Rocks in the Mahu Depression

Ma2 and Ma3 hydrocarbon source rock samples from the Fengcheng Formation in well Maye 1, Mahu Depression, Junggar Basin, were studied using conventional geochemical analysis methods and saturated hydrocarbon gas chromatography–mass spectrometry. The distribution patterns, abundance, relative content, and ratios of different carbon compounds of tricyclic terpane in hydrocarbon source rocks from fresh-to-mildly-saline (type I), moderately saline (type II), and saline (type III) water environments significantly differed. The C28–C29TT/C30H and C19–C29TT/C30H ratios were the lowest in the type I hydrocarbon source rock. The relative ratios of C23TT/C21TT, C25TT/C24TT, C28TT/C26TT, (C23–C26TT)/(C19–C22TT), and (C28–C29TT)/(C19–C22TT) gradually increased with the increase in the salinity of the hydrocarbon source rock. The percentage of low-carbon tricyclic terpanes gradually decreased to 28%, whereas those of the medium- and high-carbon tricyclic terpanes increased to 52% and 20%, respectively. The differences in triterpane types of different hydrocarbon source rocks were mainly controlled by the depositional environment. The primary factor that controlled the distribution pattern; relative abundance, especially the high carbon tricyclic terpane content; and differences in the relative ratio of different carbon compounds in different hydrocarbon source rocks was the salinity of the ancient waterbody during deposition.

Hydrocarbon accumulation in deep ancient carbonate–evaporite assemblages

The Ediacaran–Ordovician strata within three major marine basins (Tarim, Sichuan, and Ordos) in China are analyzed. Based on previous studies focusing on the characteristics of the Neoproterozoic–Cambrian strata within the three major basins (East Siberian, Oman, and Officer in Australia) overseas, the carbonate–evaporite assemblages in the target interval are divided into three types: intercalated carbonate and gypsum salt, interbedded carbonate and gypsum salt, and coexisted carbonate, gypsum salt and clastic rock. Moreover, the concept and definition of the carbonate–evaporite assemblage are clarified. The results indicate that the oil and gas in the carbonate–evaporite assemblage are originated from two types of source rocks: shale and argillaceous carbonate, and confirmed the capability of gypsum salt in the saline environment to drive the source rock hydrocarbon generation. The dolomite reservoirs are classified in two types: gypseous dolomite flat, and grain shoal & microbial mound. This study clarifies that the penecontemporaneous or epigenic leaching of atmospheric fresh water mainly controlled the large-scale development of reservoirs. Afterwards, burial dissolution transformed and reworked the reservoirs. The hydrocarbon accumulation in carbonate-evaporite assemblage can be categorized into eight sub-models under three models (sub-evaporite hydrocarbon accumulation, supra-evaporite hydrocarbon accumulation, and inter-evaporite hydrocarbon accumulation). As a result, the Cambrian strata in the Tazhong Uplift North Slope, Maigaiti Slope and Mazatag Front Uplift Zone of the Tarim Basin, the Cambrian strata in the eastern-southern area of the Sichuan Basin, and the inter-evaporite Ma-4 Member of Ordovician in the Ordos Basin, China, are defined as favorable targets for future exploration.

Petroleum geological characteristics and exploration targets of the oil-rich sags in the Central and West African Rift System

Based on seismic, drilling, and source rock analysis data, the petroleum geological characteristics and future exploration direction of the oil-rich sags in the Central and West African Rift System (CWARS) are discussed. The study shows that the Central African Rift System mainly develops high-quality lacustrine source rocks in the Lower Cretaceous, and the West African Rift System mainly develops high-quality terrigenous organic matter-rich marine source rocks in the Upper Cretaceous, and the two types of source rocks provide a material basis for the enrichment of oil and gas in the CWARS. Multiple sets of reservoir rocks including fractured basement and three sets of regional cap rocks in the Lower Cretaceous, the Upper Cretaceous, and the Paleogene are developed in the CWARS. Since the Late Mesozoic, due to the geodynamic factors including the dextral strike-slip movement of the Central African Shear Zone, the basins in different directions of the CWARS differ in terms of rifting stages, intervals of regional cap rocks, trap types and accumulation models. The NE–SW trending basins have mainly preserved one stage of rifting in the Early Cretaceous, with regional cap rocks developed in the Lower Cretaceous strata, forming traps of reverse anticlines, flower-shaped structures and basement buried hill, and two types of hydrocarbon accumulation models of “source and reservoir in the same formation, and accumulation inside source rocks” and “up-source and down-reservoir, and accumulation below source rocks”. The NW–SE basins are characterized by multiple rifting stages superimposition, with the development of regional cap rocks in the Upper Cretaceous and Paleogene, forming traps of draping anticlines, faulted anticlines, antithetic fault blocks and the accumulation model of “down-source and up-reservoir, and accumulation above source rocks”. The combination of reservoir and cap rocks inside source rocks of basins with multiple superimposed rifting stages, as well as the lithologic reservoirs and the shale oil inside source rocks of strong inversion basins are important fields for future exploration in basins of the CWARS.

Trace element geochemistry of sediments in the Pearl River Estuary and its implication for provenance

The Pearl River system is China’s third longest river and the Pearl River Estuary is a major estuary in China. In the paper, a sediment core collected from the coastal zone in the Pearl River Estuary was analyzed for trace element concentrations and grain size to identify provenance during the past decades. In addition, the tectonic framework of the study area has also been studied using reliable geochemical tracers, including rare earth elements (REEs) and trace elements. The results reveal a significant contribution of reworked sediment from multiple sources. Discrimination diagrams suggest that the sediments display felsic characteristics with recycled components, and therefore, the source rock types of the sediments are presumed to be granitic rocks. The REE patterns observed in the samples primarily reflect the influence of an ancient continental crust that contains felsic materials on the sediments with an LREE/HREE ratio of 7.94–9.56 (mean 8.46) and a δ Eu value of 0.61–0.70 (mean 0.65). The sediments are plotted into a zone of continental island arcs, which was formed during the late Mesozoic subduction of the ancient Pacific. The study on the source of sediment cores shows that the main source of sediments in this area excludes the possibility of the upper middle river from the Pearl River Watered, which reflects the importance of the lower Pearl River Basin, especially the Dongjiang River, for material transport in the study area. Our results demonstrate that the geochemical characteristics of trace elements in sediment could provide an efficient approach to identify sediment provenance in estuaries worldwide.

Revisiting the Geochemical Classification of Zircon Source Rocks Using a Machine Learning Approach

Trace element fingerprints preserved in zircons offer clues to their origin and crystallization conditions. Numerous geochemical indicators have been established to evaluate the source rock characteristics from a geochemical perspective; however, multivariate trace element data have not been sufficiently investigated statistically. As substantial amounts of zircon data from a wide range of rock types have become accessible over the past few decades, it is now essential to reassess the utility of trace elements in discriminating source rock types. We employed a new zircon trace element dataset and established classification models to distinguish eight types of source rocks: igneous (acidic, intermediate, basic, kimberlite, carbonatite, and nepheline syenite), metamorphic, and hydrothermal. Whereas a conventional decision tree analysis was unable to correctly classify the new dataset, the random forest and support vector machine algorithms achieved high-precision classifications (> 80% precision, recall, and F1 score). This work confirms that trace element composition is a helpful tool for province studies and mineral exploration using detrital zircons. However, the compiled dataset with many missing values leaves room for improving the models. Trace elements, such as P and Sc, which cannot be measured by quadrupole inductively coupled plasma mass spectrometry, are vital for more accurate classification.

Size Distribution And Heavy Mineral Pattern In Some E Gbe Streams, Southwestern Nigeria

The Egbe-Isanlu schist belt has received scholarly attention from several researchers but there is no documented report on types and the distribution of heavy minerals in streams of the area. This study is intended to reveal the types of heavy minerals, the distribution and possible source rocks to guide prospectors in gems. The study area lies within latitude 08o 03' and 08o 16 ' and longitude 05o 31' and 05o 43’ of the Southwestern Basement Complex terrain in Nigeria. Fifteen stream sediment samples were collected at appropriate points along river channels, air dried at room temperature, sieved and subjected to heavy mineral separation process using bromoform. Graphic mean, sorting, skewness and kurtosis were determined and heavy minerals identified under microscope. The graphic mean ranges between 0.67 (coarse sand) to 2.49 (fine sand), skewness between 0.75 and -0.10 and Kurtosis between 0.87 and 1.88. The calculated ZTR index ranges between 6.86% and 3.35.0%. The identified heavy minerals in the study area included tourmaline, epidote, rutile, zircon, kyanite, staurolite, garnet and zeolite. These minerals are modernly distributed across the sampled area and in some locations, they are relatively high in abundance. Some of the heavy minerals are angular to sub-angular in shape suggesting a short distance of travel from source. The graphic mean gives values that suggests a moderately to poorly sorted sediment and skewness indicated a platykurtic and leptokurtic nature for the sediments. Calculated ZTR index classified the sediments as immature to sub-mature indicating a short distance of travel suggesting that the sediments could have been derived within the catchment area. The presence of zircon, tourmaline and rutile suggests that the sediments might probably have its source from acid intrusive rocks and pegmatites.
 
 

Sedimentary infill of Early-Middle Jurassic in the southeastern Tarim Basin and its constraints on the evolution of the Altyn Tagh Fault in the Northeast Tibet Plateau

The Altyn Tagh Fault (ATF) is the boundary between the Tarim Basin and the Tibetan Plateau, which is crucial for the tectonic evolution and uplift of the Tibetan Plateau. However, the Mesozoic tectonic evolution of the ATF remains controversial. This study reconstructed the Early-Middle Jurassic palaeogeographic pattern of the southeast depression of the Tarim Basin (SDTB) and revealed potential source-sink relationships between the SDTB and its adjacent mountains to constrain the Mesozoic tectonic evolution of the ATF based on sedimentology and provenance analyses. The results indicated that the SDTB was infilled with alluvial fan facies during the Early Jurassic and the late Early Jurassic, giving rise to the braided-river delta to lacustrine facies. During the Middle Jurassic, the lacustrine basin expanded with the wide development of lacustrine facies, and the lacustrine basin contracted and was primarily infilled with braided delta facies in the late Middle Jurassic. The Jurassic sedimentary center, the SDTB situated in front of the Altyn Tagh Mountain and East Kunlun Mountain, played a crucial role in the development of three types of hydrocarbon source rocks: charcoal mudstone, coal beds, and dark mudstone. Combined with seismic reflection data interpretations, the Early Jurassic the STDB is a rift basin that underwent a rift-depression transition in the late Early Jurassic, solidifying the characteristics of the depression basin in the Middle Jurassic. The extensional environment in the SDTB resulted from the tectonic effects of slab break-off following the closure of the Tethys Ocean and the spreading of the distal stress field of the Bangonghu–Nujiang Ocean. The analyses of gravel component and heavy mineral indicated that the Jurassic source of the basin was traced back to the ancestral Altyn Mountains, the East Kunlun Mountains, and the internal palaeo-uplift of the basin, which demonstrated that the SDTB and Western Qaidam Basin (WQB) shared a common lacustrine basin during the Jurassic, suggesting that the ATF did not initiate strike-slip movements during this period. This study significantly enhanced our understanding of the Mesozoic tectonic evolution in Northwest China, providing valuable insights into exploring Mesozoic oil-bearing basins in the northern Tibetan Plateau. Furthermore, it served as a useful reference for constraining the timing of the initial slip of similar plate boundary strike-slip faults.

Tonian rifting coeval with the last stages of amalgamation of Rodinia: Evidence from southern Borborema province, northeastern Brazil

During the last stages of assembly of the Rodinia supercontinent at 1000-900 Ma, rifting processes were occurring in places that would be part of Gondwana in the late Neoproterozoic. In the Borborema Province, northeastern Brazil, this event is known as Cariris Velhos. It is represented in the Alto-Pajeú Domain from the Central subprovince by a 700 km-long, 70 km-wide NE-SW belt comprising bimodal metavolcanic rocks and granitic orthogneisses. Orthogneisses with similar ages were found in previous works in the western Pernambuco-Alagoas Domain from the Southern subprovince immediately to the south of the ESE-trending dextral West Pernambuco shear zone. The Airi Complex comprises several elongate bodies that extend for at least 80 km. It consists of a bimodal sequence of metafelsic (granitic orthogneisses) associated with layers and enclaves of metamafic (amphibolites) rocks. Seven orthogneiss samples dated by LA-ICPMS yielded zircon U–Pb crystallization ages of 950–1011 Ma. Zircon Hf analyses from two samples yielded εHf(t) values ranging from −1.4 to 2.1 and TDM model ages of 1.7–1.9 Ga. Geochemical analyses allow to distinct different source rocks for the protoliths of the orthogneisses. Orthogneisses derived from metasedimentary sources have modal muscovite ± garnet, are ferroan, alkali-calcic and predominantly peraluminous, exhibiting strong negative Eu anomalies in chondrite-normalized rare earth elements diagrams. Orthogneisses derived from high-K mafic rocks are calc-alkalic, slightly peraluminous, with small or absent negative Eu anomalies. Orthogneisses from tonalitic sources are magnesian, calc-alkalic to alkali-calcic, and slightly peraluminous, without Eu anomalies. There is no correlation between crystallization ages and source rock types. The results indicate that the Airi Complex originated through partial melting of ancient heterogenous crustal sources. Therefore, its geochemistry reflects the nature of these sources, not the tectonic setting of intrusion. Emplacement of the complex was broadly coeval with well-documented extensional processes in the São Francisco/Congo Craton and represents a failed attempt to break-up a larger paleocontinent encompassing this craton and the basement of the Borborema Province.

Why are S-type plutonic rocks more chemically variable than S-type volcanic rocks and S-types more variable than I-types?

S-type granitic magmas form essentially through partial melting of crustal metasedimentary rocks, so this eliminates many possibilities for the origins of chemical variations in their parental magmas. Global, whole-rock, major-oxide data show that S-type volcanic series have far less chemical variability (analytical scatter) than S-type plutonic rocks. On the other hand, published data show far smaller contrasts between metaluminous ‘I-type’ silicic plutonic and volcanic rocks. There are several possible explanations for these features, but sampling bias and analytical errors can be discounted. Likewise, the differences cannot be explained by plutonic S-types being crystal cumulates or products of wall-rock assimilation, nor can they be due to volcanic suites being products of magma mixing or having compositions controlled by a different differentiation mechanism to that which might control S-type plutonic compositions. Variations in major-oxide concentrations suggest that the most probable explanation is that S-type plutonic magmas are derived from somewhat more aluminous metasedimentary source rocks, with higher ratios of metapelites to metagreywackes. If so, there are complementary implications that plutonic S-type magmas may be formed at lower temperatures, that they may be more hydrous than the S-type volcanic magmas, and that the differences in emplacement mechanism reflect this. The weaker contrasts between ‘I-type’ volcanic and plutonic rocks may be due to their crustal magma sources being more homogeneous and the melting reactions stoichiometrically similar, regardless of source-rock type.

Mass transfer during hydrothermal bleaching alteration of red beds in the Lanping Basin, SW China: Implications for regional Cu(–Co) and Pb-Zn mineralization

The Lanping Basin in SW China hosts many hydrothermal Cu(–Co) and Pb-Zn deposits. Widespread bleaching alterations of red beds in the basin record mass transfer during the hydrothermal fluid migration and may provide useful information about metal sources for the deposits. Based on fluid inclusion study, mineral and elemental mapping of thin sections, and whole-rock major and trace element analyses, this study investigates fluid properties and mass transfer during the hydrothermal bleaching alterations at T1 and T2 two places where Jurassic red siltstones have been bleached in association with sulfides-free hydrothermal quartz(-siderite) veins. Mineral compositions and fluid inclusion microthermometry indicate that both T1 and T2 fluids are CO2-free, weakly acidic to neutral, S-poor, hydrocarbon-bearing reduced fluids, but the former is higher than the latter in the homogenization temperatures (188 – 265 ℃ versus 136 – 184 ℃) and salinities (17 – 36 wt% NaCl eq. versus < 11 wt.%NaCl eq.). These reduced fluids caused the bleaching alterations due to the loss of Fe3+ (in terms of hematite) from the red siltstones. The T1 fluid is comparable to the vein-type Cu-Co and Pb-Zn ore fluids in the Baiyangping district. Enrichment of Cu-Co and depletion of Pb-Zn in the bleached rocks relative to the protolith at T1 suggest that the two pairs of elements are less possibly derived from the same source rocks. Red beds probably provided Pb and Zn, while other rock types, such as mafic volcanic rocks, were likely the sources of Cu and Co. As such, various degrees of contributions from different source rock types during the ore fluid migration may determine the variable (Cu-Co)/(Pb-Zn) ratios observed in the vein-type Cu-Co and Pb-Zn deposits. The T2 fluid is similar to the ore fluids of some vein-type Cu deposits along the western margin of the Lanping Basin. Considering that the T2 hydrothermal veins are free of sulfides and the fluid inclusions from the vein-type Cu deposits in this region either contain CO2 or do not contain CO2, the abundance of reduced sulfur rather than CO2 determines whether the reduced fluids in this area are capable of causing Cu mineralization. Different from T1, Mg and TFe2O3 were introduced into the bleached rocks at T2 site. This possibly resulted from interaction between the migration fluids and widespread mafic volcanic rocks in this region. Enrichment degree of Cu is apparently greater than those of Co, Zn, and Pb in the bleached rocks, which is consistent with the Cu-only mineralization for the vein-type Cu deposits in this area.

Reservoir characteristics and controlling factor of tight sandstone in Shuixigou Group in Taibei depression, Turpan-Hami basin

The positive structure belts surrounding the Taibei Sag, Turpan-Hami Basin, have been the main targets for oil and gas exploration for years and are now left with remaining resources scattering in reservoirs adjacent to source rocks in the sag, where the Shuixigou Group with substantial oil and gas potential constitutes the primary focus for near-source exploration. Consequently, characterization of development and key controlling factors of reservoir space becomes a must for future exploration in the area. This study investigates the development traits, genesis, and controlling factors of the Xishanyao and Sangonghe formations in the Shengbei and Qiudong Sub-sags of the Taibei Sag with techniques such as cast thin-section observation, porosity and permeability tests, high-pressure mercury injection, and saturation fluid NMR analysis of reservoir rocks. The findings reveal that the Shuixigou Group in the Taibei Sag consists of lithic sandstone. Reservoirs in the group are mostly poor in terms of physical properties, with undeveloped primary pores dominated by intergranular dissolved pores as a result of a strong compaction. Comparative analysis of key controlling factors of the Sangonghe Formation reveals significant distinctions in sandstone particle size, sand body thickness, genesis and distribution, provenance location, and source rock type between the Qiudong area and Shengbei area. Vertically, the coal seams of the Xishanyao Formation exhibit heightened development with shallower burial depth and lower maturity compared to those of the Sangonghe Formation. Consequently, this environment fosters the formation of organic acids, which have a stronger dissolution effect on minerals to develop secondary dissolution pores, and ultimately resulting in better reservoir physical properties. Overall, the reservoirs within the Qiudong area of the Taibei Sag demonstrate superior characteristics compared to those in the Shengbei area. Furthermore, the reservoir physical properties of the Xishanyao Formation are better than those of the Sangonghe Formation. The research findings will provide valuable guidance for the exploration and development of lithological oil and gas reservoirs within the Taibei Sag.