Low Compositional Maturity Research Articles

Study on the Reservoir Characteristics of the Triassic Yanchang Formation in the Majiahe Area, Ordos Basin

This study thoroughly analyzes the petrological and physical properties of the Chang 4+5 and Chang 6 reservoirs of the Triassic Yanchang Formation in the Majiahe area of the Ordos Basin. The lithology in the study area is mainly composed of gray fine-grained feld-spathic sandstone, followed by medium to fine-grained and silt to fine-grained feldspathic sandstone. The overall petrological characteristics indicate low compositional maturity and high structural maturity, reflecting the complexity of the depositional environment. The reservoir properties exhibit extremely low porosity (very low to ultra-low porosity) and permeability (very low to ultra-low permeability), with significant heterogeneity in the reservoirs.

Deep Sandstone Reservoir Characteristics and Pore Structure Classification Based on Fractal Theory: A Case Study of the Bayingobi Formation in Guaizihu Sag, Yin'e Basin.

The paper focuses on deep oil and gas resources in the Bayingobi Formation of Guaizihu Sag in the Yin'e Basin. Previous studies overlooked the differences between the pores and throats, mainly focusing on pore analysis. This work aims to analyze the pore structure and petrophysical properties of the reservoir using various methods. The study utilized the constant velocity mercury intrusion method to quantify pores and throats separately. Scanning electron microscopy and casting thin section techniques characterize the pore and throat morphology. The analysis compares the pore structures in reservoirs with different petrophysical properties. Additionally, pore and throat types are classified based on fractal dimensions, and factors influencing their development are discussed. Results reveal feldspar lithic sandstone as the predominant rock type with a low compositional maturity. The sandstone reservoirs exhibit low porosity (10.23%) and ultralow permeability (0.99 mD). Primary reservoir pore spaces include intergranular pores, dissolution pores, and microfractures. Pore radius averages at 195.32 μm, while throat radius is 3.76 μm. Pore structures are categorized as micropore small-throat, small-pore small-throat, and large-pore coarse-throat types. The study area generally exhibits a high pore-to-throat ratio, impacting reservoir petrophysical properties significantly. Pore development is primarily influenced by early diagenesis, organic acid dissolution, and hydrocarbon filling. Weak compaction and cementation transformations provide a material and spatial basis for the subsequent dissolution. The presence of thick organic-rich mudstone above and below the reservoir contributes to organic acid dissolution and hydrocarbon filling.

Geochemical characteristics and detrital zircon U-Pb ages of the Yimin Formation, Kelulun Depression, Hailar Basin and constraints on uranium mineralization.

The sandstone uranium deposits in the Kelulun Depression are the first commercially viable uranium deposits discovered in the Hailar Basin and the ore-bearing strata corresponding to the Lower Cretaceous Yimin Formation. However, the source of sedimentary matter, uranium source conditions, and uranium mineralization processes in the region have not been characterized. Accordingly, we analyzed the lithology, whole-rock geochemistry, zircon U-Pb ages, and trace elements of the Yimin Formation sandstones. The Yimin Formation sandstones were primarily composed of detrital grains with low compositional maturity. A geochemical analysis indicated that the parent rocks are felsic igneous rocks formed at an active continental margin with a moderately high degree of weathering. The detrital zircon U-Pb ages of the Yimin Formation 215-287 Ma with a peak at 230-260 Ma. Based on chronological, geochemical, and lithological data, we conclude that the Yimin Formation matter is derived from the Adunchulu Uplift on the western side of the Kelulun Depression and its parent rocks are Triassic granites. The Adunchulu uplift since the late Early Cretaceous and weathering and denudation of its uranium-rich granites provided ample matter and uranium. Therefore, the Kelulun Depression is a promising area for the exploration of sandstone uranium deposits.

Tracing the sedimentary provenance of the Mesoproterozoic rocks from Taoudeni Basin (∼ 1.1 Ga) Mauritania: Evidence from Sm/Nd and elemental geochemistry

The Mesoproterozoic era is a crucial period in Earth history characterized by muted oxygen levels in the atmosphere–ocean system. This period is well represented in the non-metamorphosed sedimentary rocks from the Taoudeni Basin, Mauritania. Few studies have been carried out to trace the sources of these sedimentary archives. Here, we examined the sedimentary rocks from the Atar (S4 drill core) and El Mreiti groups (S2 and S3 drill cores). This study focuses on whole-rock elemental geochemistry and Sm/Nd isotope compositions to provide insights into the provenance of the fine-grained siliciclastic sediments from the basal part of the Mesoproterozoic Atar and El Mreiti groups. Paleoweathering indices reveal a moderate degree of chemical weathering in the source areas, consistent with the low compositional maturity of the studied samples. The concentrations of the relatively immobile trace elements suggest mixing sources of sediment provenance, which is dominated by felsic source rocks for the Atar sediments. However, distinctively positive ƐNd(t) values coupled with high 143Nd/144Nd and Sm/Nd ratios and high ƒSm/Nd values of the sediments from the El Mreiti Group indicate derivation from juvenile and less evolved sources rocks. Further, the depleted mantle model ages (TDM) of the provenance protolith (1.95–2.31 Ga) support the argument that the El Mreiti sediments are dominantly sourced from the Paleoproterozoic terrane exposed in the eastern Reguibat Shield while the Atar Group were predominantly from the Archean basement rocks in the western Reguibat Shield. Moreover, Sm/Nd isotope and mixing model results support the major contributions of the felsic Archean rocks of the Reguibat Shield for the Atar Group samples. Although these strata, separated by ∼1000 km, have been described as stratigraphic equivalent, our study demonstrates distinct sediment sources between the Atar Group strata and those of El Mreiti, therefore, reinforcing the importance of sediment provenance in understanding the evolution and reconstruction of modern and ancient sedimentary systems.

Mid-cretaceous rapid denudation of Eastern Tibetan plateau: Insights from detrital records at the Southwestern corner of Sichuan basin

Reconstruction of the Cretaceous tectonic evolution of the eastern margin of the Tibetan Plateau is of great significance to understanding the formation and early evolution of the Tibetan Plateau. The thick late Mesozoic sedimentary sequence in the Western Sichuan Basin may record the evolution of the basin itself and the tectonic uplift history of the eastern margin of the Tibetan Plateau during the Cretaceous period. Here we provide new multi-proxy provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages, U-Pb dating of detrital zircon, paleocurrent data, and detrital garnet geochemistry from the Cretaceous clastic units in the southwest corner of Sichuan Basin. Our analysis reveals two distinct changes in sediment provenance recorded in the Cretaceous strata at the southwest corner of the Sichuan Basin. The first significant change of detrital provenance was identified in the mid-Cretaceous Jiaguan Formation. Metamorphic rocks and volcanic rocks clast, as well as feldspar and mica, increased significantly, a heavy mineral assemblage dominated by hematite–limonite and ilmenite with high ATi and RuZi and low ZTR values, predominantly type Bi garnet and mostly 164 Ma, 207 Ma, 440 Ma, 780 Ma–824 Ma, and 1840 Ma detrital zircon ages, interpreted to be sourced from the Songpan-Ganzi fold belt and Longmenshan orogenic belt. The second shift, in the Guankou Formation, is marked by a low compositional maturity, a distinct increase in fossiliferous carbonate clasts and high GZi index with type Bii garnet, and Triassic zircon ages, indicating the exhumation of Longmenshan orogenic belt during the late Cretaceous. These data collectively indicate that the significant surface uplift and rapid denudation of the eastern margin of the Tibetan Plateau were probably initiated in the mid-Cretaceous (∼120 Ma). In conclusion, a detailed hands-on provenance analysis of the clastic sedimentary sequences in the southwest corner of the Sichuan Basin enabled us to determine catchment areas and shifts hitherto unstudied, thus contributing to the exhumation history of the eastern margin of the Tibetan Plateau.

Diagenesis and evolution of deep tight reservoirs: A case study of the fourth member of Shahejie Formation (cg: 50.4-42 Ma) in Bozhong Sag

Abstract This study focused on the deep tight sandstone reservoir (DTSR) of the fourth member of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin, a special type of reservoir. To reveal the diagenesis and evolution of the reservoir in the study area, cores observation, thin section identification, scanning electron microscopy, grain size analysis, and petrophysical properties measurements are available to analyze the mechanics of diagenesis and densification processes. The recognition is agreed on that (1) the lithology of the fourth member is mainly composed of lithic arkose and feldspathic lithic sandstone with low compositional maturity, and grain sizes vary from middle to coarse; (2) the porosity of reservoir ranges from 4 to 11.5% (av. 6.8%), which belongs to the medium low porosity sandstone reservoir; the pore structure is complex and the type of pore spaces is mainly secondary pore, while original pores are less developed due to the deep distribution of strata; (3) based on the quantitative calculation of porosity of the DTSR, it was identified that compaction is the main reason for sandstone reservoir densification, with an average porosity reduction of 62%, followed by cementation filling intergranular pores with an average pore reduction rate of 25.1%. Dissolution plays a constructive role in improving porosity, with an average increase rate of 18.5%.

Middle Eocene terrestrial paleoweathering and climate evolution in the midlatitude Bohai Bay Basin of eastern China

The middle Eocene climatic optimum (MECO, ca. ∼42 Ma) is a key time period for understanding Cenozoic cooling of the global climate. Still, midlatitude terrestrial records of climate evolution during MECO epoch are rare. In this study, continuous high-resolution record of shale sediments in mid-Eocene Shahejie Formation (MES shales) in the Bohai Bay Basin were performed with major-element and wavelet analysis. The midlatitude paleoweathering and paleoclimatic evolution during MECO epoch were analyzed in this study. The MES shales experienced weak-moderate paleoweathering under a subtropical monsoon paleoclimate with mean annual temperature of 8.3–12.9 °C and mean annual precipitation of 685–1100 mm/yr. The MES shales record a mixed provenance involving intermediate igneous rocks, and low compositional maturity. The nutrient-rich environment led to enrichment in organic matter in the MES shales. Wavelet analysis revealed good periodicity about the paleoclimate and weathering during MECO epoch. In the stage I of MES shales depositional process, the paleolake was high in nutrients, and the MES shales experienced high chemical weathering due to a relatively warmer and more humid climate. In contrast, the climate in stage II was relatively cold and dry, and the maturity of the MES shales was relatively high during this stage, suggesting a relatively stable tectonic background. This work provides more terrestrial records of MECO epoch for midlatitude region, and is benefit for better understanding of the palaeoenvironment when MES shales formed. The implication of organic matters enrichment in this study is meaningful for the shale oil/gas exploration in Nanpu Sag.

The Study on Diagenetic Characteristics of Coal Measures Sandstone Reservoir in Xishanyao Formation, Southern Margin of the Junggar Basin

The reservoir physical properties, pore types, diagenetic characteristics and reservoir quality controlling effect of the Xishanyao formation coal measure sandstone in the southern margin of the Junggar basin were discussed in this study based on thin section observation, high pressure mercury injection, low-temperature nitrogen adsorption and scanning electron microscope observation. The result shows that the porosity and permeability of the sandstone are generally low with a medium-high texture maturity and low compositional maturity. The sandstone storage space is mainly composed of residual intergranular pores, secondary dissolution pores, inter-crystalline pores and micro-fractures. The diagenetic stage of coal measure sandstone is in the mesodiagenesis A1-A2 stage, and their diagenetic interaction types mainly include compaction, cementation and dissolution. The reservoir quality of the coal measure sandstone deteriorates by compaction due to high matrix content and plastic debris content. Because of the large amounts of organic acids generated during the thermal evolution of the coal measure source rock, the coal measure sandstone suffers from strong dissolution. The secondary dissolution pores formed by the massive dissolution of feldspar, lithic fragments and early carbonate cementation in the sandstone significantly improved the reservoir quality. In the coal measure sandstone, clay mineral cementation is the most developed cementation form, followed by quartz cementation and carbonate cementation. Although kaolinite cementation and dolomite cementation can generate a small number of inter-crystalline pores, cementation deteriorates the reservoir quality. The Xishanyao formation coal measure sandstone formed in a lacustrine-delta environment, and its composition and texture make it susceptible to the influence of compaction and dissolution during diagenesis.

Contrasting styles of peraluminous S-type and I-type granitic magmatism: Identification and implications for the accretionary history of the Chinese South Tianshan

Peraluminous granitoids have aluminum saturation indices (A/CNK) higher than 1.0, which overlap to some extent between S- and I-type granitoids. However, their source and petrogenesis are still disputed. For example, whole-rock compositions alone are not always a valid way to discriminate the sources of peraluminous granitoids. To identify the geochemical affinities, source and petrogenesis of the peraluminous granitoids, we present new geochemical data, <i>in situ</i> zircon U-Pb ages and Hf-O isotopic data, and whole-rock Sr-Nd isotopic data for the peraluminous granitoids in the South Tianshan Orogen Belt (STOB), Northwesten China. Zircon U-Pb ages suggest that these peraluminous granitoids were emplaced in the latest Carboniferous (<i>ca.</i> 299 Ma). They contain the diagnostic mineral muscovite and have high δ¹⁸O_Zrn values (&gt;8.0 ‰) demonstrating a close affinity with S-type granitoids. Their low εNd(t) values (−5.3 to −7.6), combined with variable zircon εHf(t) values (−0.35 to −10.18), indicate that these S-type granitoids were likely derived from partial melting of metasedimentary rocks. In addition, inherited zircon cores from the S-type granitoids have variable δ¹⁸O values (6.34–10.5 ‰) and zircon εHf(t) values (−4.3 to +6.3), with age populations (<i>ca</i>. 400 to 500 Ma) similar to those of detrital zircons from late Carboniferous metasedimentary rocks in the region. These data show that the S-type granitoids were dominantly derived from late Carboniferous metasedimentary rocks rather than Precambrian crustal materials. The studied granitoids have a transitional composition between I- and S-type granitoids, which could be related to low compositional maturity of the late Carboniferous metasedimentary source. According to the spatial and temporal distribution and petrogenesis of the Carboniferous intrusive rocks in the STOB, we propose that a slab roll-back model can account for the generation of late Carboniferous S-type granitoids in the STOB.

Geochemistry and mineralogy of Qaroun Lake and relevant drain sediments, El-Fayoum, Egypt

Qaroun Lake (El-Fayoum, Egypt) is a saline endorheic basin within the northern arid zone of El-Fayoum, Egypt. Mineralogical and geochemical investigations were conducted on 32 samples from the Holocene sediments of the lake and relevant drains to get information on the sediment type and maturity, source rocks, weathering, environments of deposition, and tectonic settings. The lithology of sediment samples consists of pelites and sandstones with low compositional maturity degree characterizing first cycle deposits. The sediments are mainly composed of quartz and calcite, with minor phases including feldspar, dolomite, saponite, kaolinite, hematite, goethite, gypsum, and halite minerals. The lake's sediments are enriched in Cr, Mo, Hf, Pr, and Nd and depleted in Ni, Zn, As, Rb, Y, Sn, Sb, Ba, and La, while the drain's ones are enriched in V, Sr, Zr, Cr, Mo, Hf, Pr and Nd and depleted in Co, Zn, As, Rb, Y, Sn, Sb, Ba, Pb, and La, compared to the earth's crust shale. These sediments are mainly sourced from intermediate and felsic igneous rocks, as well as, calcareous and sandstone materials incorporated in the nearby geologic formations and soils. The high SiO2 and the low Fe2O3 and MgO concentrations reflects the acidic nature of rocks in the source area. The SiO2 and (Al2O3+K2O+Na2O) binary plot indicates the prevailing semiarid to arid climatic conditions during the sedimentation. High values of CIA and PIA and low values of ICV as well as the relationship between K2O/Na2O and K2O+Na2O contents reveal relatively moderate to a high degree of chemical weathering in the source area. The elemental ratios and geochemical discrimination diagrams show that the sedimentary environments are fluviatile-deltaic environments that experienced oxic to dysoxic conditions. The geochemical characteristics of the lake sediment samples point to a transitional tectonic setting from oceanic island arc to active continental and passive continental margins, while those of the drains samples indicate an active continental tectonic setting. The obtained data can be meaningful for the development of Qaroun lake environment.

Tectonic evolution of the Dadongcha Formation, Ji'an group, Qinghe area, Tonghua, NE China: Implications from geochronological and geochemical evidence of metapelites

The Jiao–Liao–Ji Belt (JLJB) is an important Paleoproterozoic orogenic belt that has been a focus of research to understand the Precambrian evolution of the North China Craton (NCC). The widespread Paleoproterozoic Ji’an group, which was exposed in the northern margin of JLJB, is the key to solving controversies in the tectonic setting of this special belt. An intensive study on metamorphic reactions, zircon U–Pb ages, mineral thermobarometry, geochemistry, and quantitative phase equilibria modeling (pressure–temperature pseudosections) of typical metapelites from the Dadongcha Formation of the Ji’an group provides important insight to understand the tectonic setting and processes of the Paleoproterozoic JLJB. The analyzed metapelites are exposed in Toudao and Qinghe towns in NE China, include garnet–biotite schist, garnet–biotite–plagioclase gneiss, garnet–sillimanite–cordierite–biotite gneiss, and hypersthene-spinel–cordierite–biotite gneiss. Garnets contain mineral inclusions such as sillimanite, biotite, feldspar, and quartz and retain the typical cordierite reactive edge structures. The protoliths of metapelites are sets of claystone or miscellaneous sandstone with an average chemical composition that is close to that of the upper continental crust (UCC). Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) values suggest a high degree of weathering of their sources and low compositional maturity of clastic sediments. The chondrite-normalized rare earth element (REE) pattern of analyzed samples displays characteristics of medium fractionation in light and heavy REEs, and strong enrichment in light REEs. The primitive mantle-normalized trace element pattern shows enrichment of large ion lithophile elements (LILEs) such as Ba, and the content of high field strength elements (HFSEs) such as Zr and Hf is relatively low; furthermore, Nb, Ta, P, and Ti are distinctly depleted. The emphasis is on the metamorphism of metapelites, which shows a clockwise P–T path. In combination with zircon U–Pb dating, a general evolution model for the metapelites was drawn. The metapelites were formed in an active continental margin environment where the evolved island arcs developed around 1.95–2.02 Ga, then were involved in the collision between the Longgang and Langrim blocks in the eastern part of the NCC, and returned to the surface after undergoing regional metamorphism with widespread anatexis around 1.8–1.95 Ga.

Characteristics and controlling factors of Chang 8 Reserviorof the Jiyuan Area in the Ordos Basin

By analyzing the thin sections and physical property,the petrology feature and pore characteristics of the Chang 8 Formation in the Jiyuan Area,the microscopic reservoir pore structure characteristics,characteristics of porosity and permeability are studied and the reservoir performance′s main influencing factors were analyzed.The results suggest that delta sediments are the main deposits in the Chang 7 Formation,and delta front underwater distributary channel sand is the main reservoir.The lithological characteristics mainly include fine-grained lithic feldspathic sandstone and feldspathic detritus sandstone,with low compositional maturity and middle textural maturity.Granular pore and secondary feldspar-dissolution pore are the main pore types.Pore throat presents fine pore fine throat with poor sorting and high micro-heterogeneity.The Chang 7 reservior has low permeability of which the porosity is below 9.448%and the permeability is less than 0.685×10-3μm 2.The reservoir characteristics are influenced by deposition and diagenesis primarily.Sedimentary facies zone controlled development and distribution of reservoir.Besides,compaction and cementation lead to porosity decreased,but the dissolution improves the reservoir propert.

Geochemistry, detrital zircon geochronology and Hf isotope of the clastic rocks in southern Tibet: Implications for the Jurassic-Cretaceous tectonic evolution of the Lhasa terrane

In order to reconstruct tectonic evolution history of the southern margin of Asia (i.e., Lhasa terrane) before the India-Asia collision, here we present a comprehensive study on the clastic rocks in the southern Lhasa terrane with new perspectives from sedimentary geochemistry, detrital zircon geochronology and Hf isotope. Clasts from the Jurassic-Early Cretaceous sedimentary sequences (i.e., Yeba and Chumulong Formations) display high compositional maturity and experienced moderate to high degree of chemical weathering, whereas those from the late Early-Late Cretaceous sequences (Ngamring and Shexing Formations) are characterized by low compositional maturity with insignificant chemical weathering. Our results lead to a coherent scenario for the evolution history of the Lhasa terrane. During the Early-Middle Jurassic (∼192-168Ma), the Lhasa terrane was speculated to be an isolated geological block. The Yeba Formation is best understood as being deposited in a back-arc basin induced by northward subduction of the Neo-Tethys ocean with sediments coming from the interiors of the Lhasa terrane. The Middle Jurassic-Early Cretaceous Lhasa-Qiangtang collision resulted in the formation of a composite foreland basin with southward-flowing rivers carrying clastic materials from the uplifted northern Lhasa and/or Qiangtang terranes. During the late Early-Late Cretaceous (∼104-72Ma), the Gangdese magmatic arc was uplifted rapidly above the sea level, forming turbidites (Ngamring Formation) in the Xigaze forearc basin and fluvial red beds (Shexing Formation) on the retro-arc side. At the end of Late Cretaceous, the Lhasa terrane was likely to have been uplifted to high elevation forming an Andean-type margin resembling the modern South America before the India-Asia collision.

Provenance and depositional history of Mesozoic sediments from the Mamfe basin and Douala sub-basin (SW Cameroon) unraveled by geochemical analysis

Mineralogical and geochemical analytical methods were employed to characterize sediment samples from the Mamfe basin (MB) and Douala sub-basin (DSB) in order to constrain the provenance, weathering, paleoclimate, and depositional history of rocks from both basins. Sediments under study are classified as shales, wackes, and arkoses and are essentially derived from felsic rocks which likely correspond to nearby extensive Neoproterozoic migmatitic and gneissic basement rocks. The A-CN-K plot indicates that the K-metasomatism may have affected the bulk composition of most of the studied rocks. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and (A-K)-C-N plots suggest that the intensity of weathering was higher for DSB relative to MB samples. This differentiation can be related to active tectonism during the deposition of the MB sediments. The SiO2/Al2O3, Al2O3/Na2O, and K2O/Na2O ratios, and the Index of Chemical Variability (ICV), indicate a low compositional maturity for MB compared with DSB samples. The values of U/Th, V/Cr, Ni/Co, and authigenic U suggest an oxic depositional condition for all studied samples. The average Sr/Ba ratios for the MB (0.31 ± 0.12 and 0.59 ± 0.25, for the Okoyong and Nfaitok-Yawo areas, respectively) and DSB sediments (0.31 ± 0.08 and 0.20 ± 0.09, for southern Kumba and western Mbanga areas, respectively), suggest a depositional paleoenvironment with low salinity. The average Rb/Sr ratios for the MB (0.62 ± 0.36 and 0.17 ± 0.06, for the Okoyong and Nfaitok-Yawo areas, respectively) and DSB sediments (0.53 ± 0.11 and 0.73 ± 0.29, for southern Kumba and western Mbanga areas, respectively) are low, suggesting warm and humid condition during the deposition of MB and DSB sediments. Multi-element tectonic diagrams indicate passive and rift settings, which are consistent with the local geology, but also a collisional setting, which is probably linked to the tectonic evolution of the Neoproterozoic metamorphic source rocks.

Petrography and geochemistry of Oligocene to Lower Miocene sandstones in the Baiyun Sag, Pearl River Mouth Basin, South China Sea: Provenance, source area weathering, and tectonic setting

There is a controversy regarding the source area and weathering intensity of Oligocene to Lower Miocene sediments in the Baiyun Sag of the Pearl River Mouth Basin. Detrital modes, geochemistry, heavy mineral data, and detrital zircon U–Pb ages were analysed to understand the provenance, source area weathering, and tectonic setting of Oligocene to Lower Miocene sandstones in the Baiyun Sag. Petrographic data reveal that the sandstones were derived from a recycled orogen. With increasing transport distance, sediment maturity increased correspondingly. The Oligocene to Lower Miocene sandstones in the Baiyun Sag have low compositional maturity and have experienced low to moderate weathering. The sediments of the Zhuhai Formation have experienced relatively strong weathering intensity compared with those of the Enping and lower Zhujiang formations. Due to low degree of illitization and differences in the geochemical compositions of the sediments, the continental island arc data from different boreholes in the Baiyun Sag are more irregularly distributed in the A‐CN‐K plot. The modal composition (e.g., quartz, feldspar, and lithic fragments) and geochemical indices (Al2O3/TiO2 ratio; Al2O3‐TiO2 diagram; discriminant F1‐F2 diagram; Eu/Eu*, La/Sc, La/Co, Th/Sc, and Th/Co ratios; Hf‐La/Th and La/Sc‐Co/Th diagrams; and rare earth element patterns) indicate that the sediments in the Baiyun Sag are mainly derived from felsic source rocks. Although the uplift around the Baiyun Sag includes multistage magmatism and basic rocks, it has little influence on the sediment in the depression, and no sediments were transported by the Han River to the Baiyun Sag. Geochemical data and detrital zircon U–Pb ages suggest that the Pearl River was formed in the Early Oligocene and became the main source for the sediments in this area. The river carried felsic material from the South China Coast forming a large delta on the northern slope of the Baiyun Sag. The ratios of Eu/Eu*, La/Sc, La/Co, Th/Sc and Th/Co, and heavy mineral data indicate that the sediments transported by the Pearl River to the Baiyun Sag were changed from the arc‐dominated coastal zone to the interior of the South China Block during the Late Oligocene to Early Miocene and even reached the south‐eastern margin of the Tibetan Plateau (West Yunnan Plateau). The La‐Th‐Sc and Th‐Sc‐Zr/10 diagrams suggest that the provenance of these sandstones is evolved continental island arc or an active continental margin setting.

Petrological characteristics and the impact of mineral content on reservoir quality in coal-bearing strata of Linxing area, eastern margin of Ordos Basin, China

Tight sandstone gas is on the first position of unconventional natural gas sources, which can be developed under today’s technical conditions. In recent years, tight sandstone gas reservoirs have been found in several wells in the Linxing area, eastern margin of Ordos Basin, China. In this article, a variety of methods, including cast thin sections, X-ray diffraction analysis, scanning electron microscope, and drill core data were used to study the petrological characteristics and their influences on tight sandstone reservoir in coal-bearing strata of the Linxing area. Based on the analysis of thin section, it can be concluded that the sandstone reservoir is essentially constituted of lithic sandstone as well as lithic arkose and feldspathic litharenite. Cement types are complicated, including carbonate minerals, clay minerals, and quartz overgrowth. Illite, kaolinite, chlorite, illite–smectite mixed layer, and chlorite–smectite mixed layer are found in clay minerals. Compared with other clay minerals, illite is in the dominant position. Pores can be divided into residual intergranular pore, intragranular dissolution pore, intergranular dissolution pore, cement dissolution pore, intercrystalline pore, and microcrack in sandstone reservoir of the Linxing area. Quartz has an average content of 68% with the feature of low compositional maturity and plays a major role in increasing porosity due to dissolution and protecting of quartz. Feldspar dissolution plays a role in decreasing porosity because the by-product materials of feldspar dissolution remain in original place, instead of being transported to other areas. Dissolution pores are 2–20 µm and may be filled with kaolinite, illite, or halite. It is worth mentioning that grain-coating chlorite may be of sufficient thickness to protect reservoirs along with the increasing content of chlorite, which is testified by the crossplot between the chlorite and porosity when the absolute content of chlorite is less than 1.5%.

Provenance discrimination between Atasta and Alvarado beach sands, western Gulf of Mexico, Mexico: Constraints from detrital zircon chemistry and U–Pb geochronology

The bulk sand samples and detrital zircons collected from the Atasta and Alvarado beach areas in the southwestern Gulf of Mexico have been analysed for geochemistry and U–Pb ages to constrain their provenance. The index of chemical maturity and SiO2/Al2O3 ratio of the Atasta and Alvarado beach sands indicated low compositional maturity. The chondrite‐normalized REE patterns and Eu anomalies of the Atasta and Alvarado beach sands suggested that the sands were derived from felsic and intermediate igneous rock types, respectively. A wide range in Th and U contents of the Atasta (~1.0–884 ppm and ~39–883 ppm, respectively) and Alvarado (~13–2,641 ppm and ~21–2,457 ppm, respectively) zircons likely indicates that the detrital zircon populations consist of zircons derived from multiple sources. The total rare earth element (ΣREE) content in the Atasta and Alvarado zircons ranges from ~151 to 2,088 ppm and ~154 to 3,580 ppm, respectively. The chondrite‐normalized REE patterns of zircons from the two beach areas show slightly depleted light REE (LREE) and enriched heavy REE (HREE), as well as positive Ce and negative Eu anomalies. A few zircons of Alvarado beach lack a Ce positive anomaly but exhibit a distinct Eu anomaly. The differences in REE patterns among zircons between the two beach areas revealed the variations in the type of source terranes, which supplied zircons to the beach areas. The detrital zircon age distribution for the Atasta beach is homogeneous, showing a prominent Proterozoic peak (~596–2,461 Ma; number of samples n = 62) with few Cenozoic (~10.43–65.2 Ma; n = 4) and Palaeozoic (~380–468 Ma; n = 5) zircons. The zircon U–Pb ages of Alvarado beach revealed Cenozoic (~0.25–63.9 Ma; n = 65), Mesozoic (~67.9–181.2 Ma), Palaeozoic (~252–297 Ma), and Proterozoic (~589–1,501 Ma) age clusters with predominant Cenozoic zircons. The comparison of zircon REE chemistry and geochronological data of this study with published data suggests that the source terranes, which supplied zircons to the Atasta beach, were the Chiapas Massif and Oaxacan complexes, whereas the zircons in the Alvarado beach were derived from the nearby Mexican Volcanic Belt and Eastern Alkaline Province. The provenance variation between the two beach areas was also revealed by the dissimilarities in zircon age clusters.

Petrography and geochemistry of the Lower Miocene Moghra sandstones, Qattara Depression, north Western Desert, Egypt

The petrographic and geochemical compositions of the Lower Miocene Moghra siliciclastic sediments in the Qattara Depression, north Western Desert, were investigated to infer their provenance, tectonic setting, palaeoweathering conditions, and diagenetic history. The Moghra Formation (~200 m thick) is dominated by sandstones intercalated with mudstone and carbonates. X‐ray diffraction analysis revealed that smectite, kaolinite, and illite are the dominant clay minerals found in mudstones. The sandstones are mainly subarkose, sublitharenite, and quartzarenite types with an average modal composition of Q89F6L5. The sandstones possess various diagenetic overprints including mechanical infiltration of clay, compaction, cementation, dissolution, and replacement. Major and trace element concentrations suggested that the parent rocks are intermediate to felsic in composition derived from the Neoproterozoic Arabian Nubian Shield. The weathering indices such as the chemical index of alteration and the plagioclase index of alteration implied that the intensity of chemical weathering in the source area was moderate under semi‐arid climatic conditions. On the other hand, the index of chemical variability indicates a low compositional maturity. The tectonic discrimination diagrams demonstrated a collision setting for the Arabian Nubian Shield, which represents the main supplier of sediments to the study area. Statistical parameters, sedimentary structures, and ichnofossil contents suggested that the Moghra sediments were deposited in a tidally‐influenced fluvial environment.

Provenance of Triassic sandstones on the southwest Barents Shelf and the implication for sediment dispersal patterns in northwest Pangaea

Thick Triassic siliciclastic units form major reservoir targets for hydrocarbon exploration on the Barents Shelf; however, poor reservoir quality, possibly associated with variation in provenance, remains a key risk factor in the area. In this study, sandstone dispersal patterns on the southwest Barents Shelf are investigated through petrographic and heavy mineral analysis, garnet and rutile geochemistry and zircon U-Pb geochronology. The results show that until the Early Norian Maximum Flooding Surface, two contrasting sand types were present: (i) a Caledonian Sand Type, characterised by a high compositional maturity, a heavy mineral assemblage dominated by garnet and low chrome-spinel:zircon (CZi) values, predominantly metapelitic rutiles and mostly Proterozoic and Archaean detrital zircon ages, interpreted to be sourced from the Caledonides, and (ii) a Uralian Sand Type, characterised by a low compositional maturity, high CZi values, predominantly metamafic rutiles and Carboniferous zircon ages, sourced from the Uralian Orogeny. In addition, disparity in detrital zircon ages of the Uralian Sand Type with contiguous strata on the northern Barents Shelf reveals the presence of a Northern Uraloid Sand Type, interpreted to have been sourced from Taimyr and Severnaya Zemlya. As such, a coincidental system is inferred which delivered sand to the Northern Barents Shelf in the late Carnian/early Norian. Following the Early Norian Maximum Flooding Surface, a significant provenance change occurs. In response to Late Triassic/Early Jurassic hinterland rejuvenation, supply from the Uralian Orogen ceased and the northern Scandinavian (Caledonian) source became dominant, extending northwards out on to the southwest Barents Shelf. The data reveal a link between reservoir quality and sand type and illustrate how provenance played an important role in the development of clastic reservoirs within the Triassic of the Barents Shelf.

Mineralogical and geochemical composition of Middle Permian Lucaogou Formation in the southern Junggar Basin, China: implications for paleoenvironment, provenance, and tectonic setting

The mineralogy and geochemistry of the Middle Permian Lucaogou shale provide information regarding tectonic setting, depositional environment, sedimentary provenance, and chemical properties. The Lucaogou shale consists predominately of quartz and clay minerals with subsidiary feldspar. The mineralogical composition is enriched in SiO2, P2O5, CaO, and Na2O relative to post-Archean Australian shales (PAAS). The abundant free SiO2 and elevated CaO relative to PAAS have led to depletions of most of trace elements due to dilution effects. The chemical index of alteration (CIA) indicates that sediment-source region has undergone low to moderate chemical weathering. The index of compositional variability (ICV) suggests that the shales have low compositional maturity and are most likely dominated by first-cycle deposits. The dominance of plagioclase over K-feldspar in amounts and extrapolation of the analyzed shales back to the plagioclase-alkali feldspar line in the A-CN-K diagram indicate that the shales could have been derived from plagioclase-rich source rocks. The Al2O3/TiO2 ratio and chondrite-normalized rare earth element (REE) patterns display a derivation from felsic rocks. Tectonic setting discriminant diagrams and the chondrite-normalized REE patterns infer a continental island arc setting for sediment-source region. The redox-sensitive elemental ratios and paleosalinity indices reflect an anoxic mildly brackish-influenced to brackish-influenced paleoenvironment.