Selective dissolution of alkali feldspars and its effect on Lower Triassic sandy conglomerate reservoirs in the Junggar Basin, northwestern China

Feldspar dissolution resulting from fluid–rock reactions in clastic rocks is common in petroliferous sedimentary basins and significantly affects the quality of reservoirs. The objective of this study is a case study of this phenomenon in the sandy conglomerate reservoirs, based on the Lower Triassic Baikouquan Formation in the Mahu Sag at the northwestern margin of the Junggar Basin in northwestern China, which has obtained exploration breakthroughs recently. In these rocks, orthoclase has undergone dissolution and the dissolution intensity decreases gradually from the basal member (T1b1) to the upper member (T1b3) of the formation, whereas albite remained stable or has even overgrown crystals, displaying a selective dissolution of alkali feldspars. This is interpreted to be mainly controlled by two factors. First, illitization of mixed‐layer illite/smectite in the sandy conglomerates consumed K, thereby increasing Na/K ratios in the formation waters and leading to orthoclase dissolution and albite overgrowths on perthite and microcline. Second, the charge of acidic, hydrocarbon‐related fluids further promoted orthoclase dissolution. These relationships are confirmed by the fact that hydrocarbon charging intensity shows a gradual upward reduction from member T1b1 to T1b3, as does the scale of orthoclase dissolution. This selective dissolution resulted from the influence of different mineral components on diagenesis, the acidic nature of reservoir fluids, and the extent of water–rock interaction. The dissolution significantly improved porosity and permeability of the reservoirs by generating substantial secondary porosity. The results indicate a favorable exploration prospect located in the up‐dip area of the Ma 18 and Aihu 2 wells on the western slope of the Mahu Sag.

High‐resolution integrated calcareous plankton biostratigraphy and magnetostratigraphy at the Oligocene–Miocene transition in Southwestern Atlantic Ocean

After the formalization of the base of the Miocene in the Lemme‐Carrosio section (Italy) at the base of Subchron C6Cn.2n, the calcareous plankton biostratigraphy was refined in several open ocean Deep Sea Drilling Project/Ocean Drilling Program sites. However, high‐resolution quantitative biostratigraphic studies, integrating planktonic foraminifera and calcareous nannofossils, are still lacking for the time interval spanning the Oligocene–Miocene transition. Here, we present a reinvestigation of Deep Sea Drilling Project Hole 516F (Rio Grande Rise) and 4 oil wells drilled by Petróbras Brasileiro SA in the Campos Basin (SW Atlantic Ocean). We identified 12 planktonic foraminiferal and 18 calcareous nannofossil bioevents that have been integrated with an updated magnetostratigraphy of Hole 516F allowing the correlation with the GPTS and the identification of the Oligocene/Miocene boundary (base of Subchron C6Cn.2n) between the Top of Sphenolithus delphix and the Base of common Paragloborotalia kugleri. Furthermore, our results give new insights on the reliability of major calcareous plankton events across the Oligocene–Miocene transition: (a) the Sphenolithus ciperoensis Top, the S. delphix Base and Top, and the Sphenolithus cometa Base are reliable events at a global scale; (b) the Bases of Globoquadrina dehiscens and Sphenolithus disbelemnos > 4 μm are correlatable events only within the study sector of the SW Atlantic Ocean; and (c) the Globoturborotalita ciperoensis Top, Globoturborotalita angulisuturalis Top, and Sphenolithus procerus Base are diachronous. Finally, previously unreported biostratigraphic data, such as the distribution range of S. disbelemnos < 4 μm and Sphenolithus cf. S. pseudoheteromorphus, the Tenuitellinata praestainforthi acme interval, and the Top of common Globigerinoides primordius were identified in the Campos Basin.

Petrography and geochemistry of sandstone–mudstone from Barakar Formation (early Permian), Raniganj Basin, India: Implications for provenance, weathering and marine depositional conditions during Lower Gondwana sedimentation

Post‐glacial Permian coal‐bearing sedimentary successions are traditionally interpreted as fluvially deposited within the Gondwanaland continental set‐up throughout the globe. Recent attempts to reinterpret such successions in terms of marine flooding events, mainly based on sedimentological and ichnological attributes, raised doubt on the existing palaeogeographic model of the Late Palaeozoic Gondwanaland. Reassessing these sedimentary successions using other proxies, like petrographic and geochemical properties, may provide more reliable clues to improve the present understanding.
In this paper, petrographic and geochemical analysis of sandstones, sandstone–mudstone heteroliths, and mudstones of the early Permian Barakar Formation of Lower Gondwana Supergroup, Raniganj coal Basin, India, is presented to understand the provenance, palaeoweathering pattern and palaeodepositional conditions. Petrographically, the sandstones are arkosic to sub‐arkosic in nature, with abundant unstable components and heavy minerals. Clay minerals in the matrix and the mudstones are dominated by kaolinite, illite‐smectite with authigenic glauconite, indicating a marine diagenetic realm. Mineralogical assemblage attests to sedimentation in craton interior passive margin conditions. Geochemically, the lithounits show large variations in major element and trace element (including rare earth element) concentrations. Chemical index of alteration and A–CN–K ternary plot indicate moderate to strong chemical weathering, leading to deposition of compositionally immature sediments close to their source without much recycling. Ratios of major elements (e.g., MgO, K2O, Fe2O3, Al2O3, and SiO2) signify a continental–marine transitional depositional environment developed in a stable passive margin setting under gradually warming up climatic conditions. Trace element ratios manifest an estuarine depositional setting during Barakar sedimentation, comparable with modern major river mouth estuaries.
Thus, this paper provides unequivocal evidence of significant marine influence during sedimentation in the Lower Gondwana basins in India, particularly during the Permian time, and signifies the importance of re‐examining the so called “continental fluvial” coal‐bearing deposits as considerably marine‐influenced. The interpretations provide important clues in understanding and reconstructing the Permian Lower Gondwana palaeogeography in eastern peninsular India.

Paleoenvironmental, diagenetic, and eustatic controls on the Permo–Triassic carbonate–evaporite reservoir quality, Upper Dalan and Kangan formations, Lavan Gas Field, Zagros Basin

Permo–Triassic (P–T) successions in Iran are known as one of the most extensive carbonate–evaporite sequences in the world, holding prolific hydrocarbon accumulations, such as the gas fields in the Zagros Basin. This study addresses the impact of paleoenvironmental changes across the P–T extinction boundary, variations in eustatic level and diagenesis on the carbonate reservoir quality of the Permo–Triassic Upper Dalan and Kangan formations in the Lavan Gas Field (offshore Zagros). A total of eight lithofacies have been defined and interpreted, representing the shallower parts (inner ramp to shoal) of a carbonate homoclinal ramp. The diagenetic features show the mixing influence of marine, meteoric and burial processes, with prevalence of multistage dolomitization and late meteoric‐related imprints. The main dolomitization processes are interpreted to be penecontemporaneous and associated with continuous seawater reflux and high evaporation within the intertidal environment. Combined analysis of facies and petrophysical properties has been utilized to define six rock types, including RT4 (grainstones from oolitic shoals) as the best reservoir, whereas RT1 (supratidal anhydrite) is acting as the main cap rock within the studied carbonate–evaporite sequence. Multiscale characterization of lithological and petrophysical properties suggest that the Upper Dalan Formation has a single reservoir unit (D1), whereas the Kangan Formation may be divided into two reservoir units (K1 and K2). The best reservoir quality is promoted by dissolution and dolomitization associated with late Transgressive Sequence Tracts (TST) to early Highstand Sequence Tracts (HST) sequences, while low reservoir quality corresponds to late HST sequences accompanied by pore‐occluded cements. Petrophysical analysis suggest better porosity‐permability for the Upper Dalan than for the Kangan Formation. This observation is closely associated with the different nature of lithofacies, where the Kangan Formation contains more fine‐grained carbonate rocks, and the higher intensity of porosity‐grain dolomitization in the Upper Dalan Formation. This study provides a better understanding on reservoir quality prediction across the P–T boundary and the effect of eustatic fluctuations and diagenesis in controlling porosity‐permeability evolution with similar settings elsewhere.

The syn‐depositional fault control of the evolution of axial meandering river delta‐sublacustrine fan deposits during the Ed2x deposition in the northern Liaodong Bay, Bohai Bay Basin, NE China

This research study presents the results of an integrated study employing core samples, wireline logs, and seismic dataset collected in the study area for analysing the syn‐depositional fault’s control of the evolution of the axial meandering river delta‐sublacustrine fan deposits during the Ed2x depositional period. The results indicate that the delta deposits that had developed during a retrogressive period, with the characteristics of retrogressive stacking patterns, had resulted from a high subsidence rate caused by the syn‐depositional faults with large fault throw. These appeared to be characterized by strong amplitude, low frequency, and continuous seismic reflections on the seismic profiles, and high‐amplitude serrated cylinder‐shaped stacking patterns in the wireline logs, with no obvious sedimentary structures. In contrast, the delta deposits developed during a progressive period and characterized by progressive stacking patterns were determined to have resulted from a low subsidence rate caused by the syn‐depositional faults with small fault throw. They were found to be characterized by weak‐to‐medium amplitude vermicule‐shaped seismic reflections, as well as low‐amplitude bell‐shaped or funnel‐shaped stacking patterns, with soft‐sediment deformation structures.

Petrological study of the early Mesoproterozoic Glauconitic Sandstone and Olive Shale members from the Semri Group, Vindhyan Supergroup in Central India: Implications to input from intrabasinal felsic volcanic source and glauconitization

The Semri Group of the Vindhyan Supergroup contains widespread intrabasinal felsic volcanic rocks in the Porcellanite Formation formed during the early Mesoproterozoic. This widespread felsic volcanism was followed by the deposition of siliciclastic sedimentary rocks of the Kheinjua Formation containing the Olive Shale, Fawn Limestone, Glauconitic Sandstone members, the last one dominantly arkose and litharenite. The volcanic products resulting from volcanic activity during deposition of Porcellanite Formation were predominantly fine volcanic ash or dust that settled very slowly in the water column of the basin. The ranges of 28 elemental ratios for the Olive Shale and Glauconitic Sandstone members overlapped with those of the Porcellanite Shale Formation. The contemporaneous intermingling of the detrital input from volcanic products with the terrigenous detritus received by the basin from the Bundelkhand Craton in the north is reflected in the geochemical signatures. Various plots of elemental ratios and ternary plots for the siliciclastic rocks overlap those of the Porcellanite Shale Formation and Bundelkhand Tonalite‐trondhjemite‐granodiorite (TTG) suggesting that they might be the source rocks. The mixing of detritus from volcanic products was perhaps responsible for the widespread appearance of glauconite‐bearing sediments in the subsequent deposits. Glauconitization of detrital K‐feldspars, clay peloids, and volcanic lithic fragments is evident from the petrography. The shallow marine environment, slow sedimentation rates, elevated surface temperatures and mildly reducing conditions (CO2 got released from explosive volcanism) are considered as the favourable conditions for glauconitization, which prevailed within the Vindhyan Basin. Under these conditions the detrital input received as the result of previous volcanism during deposition of the porcellanite shale (glass, pumice lithic fragments, and feldspars) were transformed into clays such as illite and glauconite during the deposition of siliciclastic rocks of the Kheinjua Formation within the basin.

A new pancryptodiran turtle from the Late Jurassic of Poland and palaeobiology of early marine turtles

Although Western Europe has yielded numerous Jurassic turtle taxa, several represented by cranial material or complete skeletons, the fossil record of the Jurassic turtles remains scarce to the north and east from Germany. Although some Late Jurassic testudinates were historically described from Poland, they were, thus far, represented by fragmentary remains that never were properly figured or described in detail. Therefore, very little is known about the mid‐Mesozoic diversity of turtles in that region of the continent. A new pancryptodiran turtle genus and species, Owadowia borsukbialynickae, is described from the uppermost Jurassic (Tithonian, ca. 148 Ma) carbonate sediments of the Kcynia Formation in Owadów‐Brzezinki Quarry, near Tomaszów Mazowiecki in central Poland. The lower jaw morphology and palaeoecological setting inhabited by the new genus and species, together with the trophic relationships of the Jurassic pancryptodiran turtles, are discussed in an attempt to determine the potential range of mode of life of O. borsukbialynickae. We propose that the new specimen belongs to a new durophagous pancryptodiran turtle taxon. O. borsukbialynickae might have spent considerable time in the marine environment and specialized on eating hard‐shelled invertebrates like bivalves and decapod crustaceans, common to that setting.

Pennsylvanian–early Permian palaeokarst development on the Yangtze Platform, South China, and implications for the regional sea‐level history

The Pennsylvanian to lower Permian succession at Madiyi, South China, represents a nearshore mixed siliciclastic‐carbonate system characterized by cyclic sedimentation patterns along a depth gradient from continental siliciclastics to marine open platform carbonates. Various palaeokarst features related to subaerial exposure are widely distributed. Additionally, pedogenesis was pervasive. The identification of twenty‐eight sedimentary cycles grouped into five sequences allows the reconstruction of a relative sea‐level curve. The cycles represent higher frequency changes superimposed on the long‐term evolution. After the initial transgression, the sea level remained relatively low in the late Bashkirian to early Moscovian. Then, it rose stepwise until a major drop occurred in the late Moscovian. Afterwards, the sea level rose again, but marine sedimentation ceased during at least the early Kasimovian. The area was flooded in the late Kasimovian, and the sea level rose until a late Gzhelian sea‐level drop. The subsequent marine sequence contains uppermost Gzhelian to upper Asselian deposits, which are capped by lower Permian continental facies associated with a major regional regression.
Shallow to deeper marine sedimentary settings recorded high‐frequency sea‐level changes throughout South China during the late Bashkirian to Asselian. The main transgression–regression cycles are clearly associated with different depositional settings during several time intervals. Furthermore, the good correlations among South China, Ukraine, and the U.S. Mid‐continent are rooted in the impact of Gondwanan glaciations on the global sedimentation patterns. However, not all changes at Madiyi are the result of glacio‐eustatic sea‐level fluctuations; some are related to the local subsidence history and the rise of the Xuefeng Uplift.

Ore genesis of the Xiyi Pb‐Zn deposit, western Yunnan province, China: Geologic, fluid inclusion, and isotopic (C‐H‐O‐S‐Pb) evidence

The Xiyi Pb‐Zn deposit, located in the Baoshan Block of the Sanjiang region in western Yunnan, China, contains 1.2 Mt of Pb‐Zn resources. The orebodies are hosted by Devonian‐Carboniferous limestone, structurally controlled by the NE‐trending fractures, and occur in stratiform, vein, and lenticular forms. The mineral assemblage of the ores includes sphalerite, galena, pyrite, arsenopyrite, chalcopyrite, and marcasite. Calcite, barite, and quartz occur as gangue minerals. Three stages of hydrothermal mineralization are recognized based on the ore textures, crosscutting relationship, and mineral assemblages: an early stage of sphalerite + galena + calcite (Stage 1), a middle stage of galena + arsenopyrite + calcite ± sphalerite (Stage 2), and a late stage of calcite + pyrite ± barite ± quartz (Stage 3). Two types of fluid inclusions are associated with the different ore‐forming hydrothermal stages: aqueous inclusions and hydrocarbon‐H2O inclusions. The ore‐forming fluids were characterized by low temperatures (100 ~ 200 °C), medium salinities (8 ~ 20 wt% NaCleq), and medium densities (1.0 ~ 1.1 g/cm3) and contained minor amounts of CH4. The δDH2O and calculated δ18OH2O values of the Stage 1 calcite range from −89‰ to −103‰ and from 1.5‰ to 5.5‰, respectively, indicating that the H2O in the hydrothermal fluids was derived from seawater that had reacted with organic matter. The δ13CPDB and δ18OSMOW values of the calcite associated with Stages 1 and 3 range from −5.2‰ to 3.2‰ and from 12.5‰ to 20.0‰, respectively. These data plot in the field between marine carbonate and granites and are close to the marine carbonate field, suggesting that the CO2 in the ore‐forming fluids was derived from the dissolution of marine carbonate in the sedimentary rocks. The galena, sphalerite, and pyrite in Stages 1 and 3 have a narrow range of δ34S values (0–3‰), which is lower than the δ34S values of early Carboniferous to Middle‐Late Devonian seawater sulfate (+18‰ to +23‰). Therefore, because the mineralization is unrelated to magmatic activity, the sulfur in the hydrothermal sulfides was derived from regional marine sulfates primarily via organic reduction at temperatures of 100 to 200 °C. The lead isotopic compositions in the galena, sphalerite, and pyrite in Stages 1 and 3 are very similar to those in the diagenetic pyrite in the host rocks, plotting in the field of the upper crust Pb evolution curve. Thus, the lead source was likely the Carboniferous and Devonian strata. The geologic, fluid inclusion, and isotopic (C‐H‐O‐S‐Pb) evidence in this study suggests that the Xiyi Pb‐Zn deposit is a Mississippi Valley‐type deposit.

The Late Carboniferous Khuhu Davaa ophiolite in northeastern Mongolia: Implications for the tectonic evolution of the Mongol–Okhotsk Ocean

The newly discovered Khuhu Davaa ophiolite is located in the Adaatsag suture zone, which marks the closure of the Mongol–Okhotsk Ocean in northeastern Mongolia. In this paper, we present new geochemical and SIMS zircon U–Pb data for the ophiolite to provide some constraints on the tectonic evolution of the Mongol–Okhotsk orogenic belt. Metagabbro and plagiogranite samples from the ophiolite yielded SIMS zircon U–Pb ages of 321 ± 4 Ma and 314 ± 3 Ma, respectively, which are interpreted to represent the formation age of the ophiolite. These data suggest that the Carboniferous Khuhu Davaa ophiolite is likely related to the contemporaneous Adaatsag ophiolite to the southwest, defining the suture zone of the Mongol–Okhotsk Ocean in northeastern Mongolia. Tholeiitic basalt samples from the ophiolite show N‐MORB and E‐MORB geochemical affinities but are enriched in Ba, Pb, and Sr, which may reflect an input of subduction‐related fluids. Metagabbro samples exhibit HSFE depletion and LILE enrichment, which are diagnostic geochemical features of arc magmatism. Plagiogranites with low TiO2 contents appear to have formed through partial melting rather than fractional crystallization of a mafic crust in the subduction zone environment. Thus, we suggest that the Khuhu Davaa ophiolite resulted from generation of oceanic crust during the closure of the Mongol–Okhotsk Ocean and documents the initiation of subduction during the Late Carboniferous in northeastern Mongolia.