Middle Permian Research Articles

Timing of S-saturation in the formation of the Oubulage porphyry Cu-Au deposit, Inner Mongolia, Northern China

The Oubulage porphyry Cu-Au deposit, Inner Mongolia, North China, is located in the Langshan Tectonic Belt on the southern margin of the Central Asian Orogenic Belt (CAOB). The ore-forming quartz porphyry intrusion was formed during the Middle Permian (268.4 ± 1.6 Ma). Randomly and directionally distributed sulfide blebs are observed in some porphyry rocks. These blebs are spherical or ellipsoidal in shape. There are two groups of such blebs: polymetallic sulfide blebs and silicate-carbonate-sulfide blebs. Sulfide minerals in these blebs include pyrite, chalcopyrite, and pyrrhotite. Silicate-carbonate-sulfide blebs also contain calcite, epidote, plagioclase, chlorite, fluorite, and bastnaesite. Pyrite of sulfide blebs has δ56Fe ranging from −0.38 ‰ to 1.19 ‰. Calcite in sulfide blebs has C-O-Sr isotopic compositions (δ13CVPDB = −7.35 – −5.35 ‰, δ18OVSMOW = 8.35 – 9.92 ‰, 87Sr/86Sr = 0.7032 – 0.7064) indicative of a magmatic origin. We propose a new possible genetic model in which sulfide saturation occurred in a magma chamber, leading to the formation of sulfide droplets. Assimilation added sulphur (S) into the magma chamber and caused S-saturation. The sulfide melts are transported to the shallow crust attached by fluids.

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields, Zambezi Karoo Basin of Mozambique: Implications for provenance, sediment dispersal and basin evolution

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields in the Zambezi Karoo Basin of Mozambique provides key insights into the regional provenance, sediment dispersal pathways and basin evolution. Borehole cores from the two coalfields reveal a stratigraphy spanning the early Roadian (middle Permian) to the Carnian (Upper Triassic). The Permian sandstones of the Moatize Coalfield (MC) yield three detrital zircon populations, with ages ranging from 1150 to 950 Ma, 900–780 Ma and 650–490 Ma. In contrast, the Permian sandstones of the N'Condédzi Coalfield (NC) have only one population, which ranges from 1150 to 950 Ma. During the Permian, the provenance area for the NC was the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma) to the north-northeast. In the MC, the detrital zircon populations of the early lake delta depositional setting (Roadian to Wordian) indicate a main provenance in the Zambezi Belt (900–780 Ma) located to the south of the MC, with minor sourcing from the Nampula Block (1150–950 Ma and 650–490 Ma) to the east. The transition from a lake delta to an alluvial depositional setting is attributed to a major tectonic event in the MC, which involved the formation of a braided channel belt. Sandstones from this braided channel belt yield three detrital zircons populations (650–490 Ma, 900–780 Ma and 1150–950 Ma), indicating provenance from both the Zambezi Belt and the Nampula Block. The overlying sandstones in the MC show only a minor population from the Zambezi Belt (900–780 Ma), implying a shift in provenance to the Nampula Block that was likely induced by tectonics. The absence of detrital zircon populations of 900–780 Ma and 650–490 in the Permian sandstones of NC implies that the two coalfields were not connected during the mid to late Permian. It is likely that an intra-rift horst (the Mesoproterozoic Gabbro-Anorthosite Tete Suite) separated the two coalfields. The Lower Triassic sandstones of the NC yield a main detrital zircon population indicating provenance from the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma). A minor population at 650–490 Ma is linked to increased aridity at the Permian – Triassic boundary, which caused expansion of the watershed across the Tete-Chipata Terrane and the Malawi Complex. The Upper Triassic sandstones in the NC yield a 1150–950 Ma detrital zircon population, indicating provenance from the Tete-Chipata Terrane and Malawi Complex and a return to the source-to-sink conditions seen in the mid to late Permian.

Molecular and carbon isotopic geochemistry of oils with different fluorescence color from the Upper Jurassic Qigu formation and oil accumulation process in Yongjin area, Junggar Basin in China

The Yongjin area, located in the middle of the Junggar Basin, is an important oil-producing region. Its major oil-producing formation is the Upper Jurassic Qigu Formation, which has experienced multiple oil-charging events that have resulted in oils with various fluorescent colors. However, the source of oils from the Qigu Formation is currently debated. As exploration and development progressed, the productivity of the Qigu Formation was observed to vary owing to oil viscosity differences. Conventional extraction methods can only be used to obtain a mixture of fluorescing oils of different colors, making it difficult to analyze the properties of oils that charge during different periods. Using microscopy to observe fluorescence the characteristics of oil in various samples were analyzed. Then, accelerated sequential extraction was used to separate the oils with different fluorescent colors (Groups I–IV). Through GC-MS (gas chromatography), GC-IRMS (gas chromatography-mass spectrometry), FT-IR (fourier transform infrared spectroscopy), and fluid inclusion observations and measurements, in conjunction with the geological context, the sources of oils from different charge periods and genesis of heavy oil were determined and the process of oil accumulation was reconstructed. Three stages of oil charging were identified: (1) During the Middle to Late Jurassic, low-mature and mature oil from the Lower Permian Fengcheng Formation and a low quantity of low-mature oil from the Middle Permian Lower Wuerhe Formation charged the Qigu Formation. These oils underwent notable biodegradation and oxidation, with some captured by inclusions (Group III) for preservation. (2) During the Late Cretaceous to Early Paleogene, mature oil from the Wuerhe Formation charged the Qigu Formation. This oil mixed with early oil that was not captured by the inclusions, resulting in the formation of black-brown fluorescing oil (Group IV) and yellow fluorescing inclusion oil (Group II). (3) From the Late Paleogene to the present, highly mature condensates from the Lower Wuerhe Formation charged the Qigu Formation. The oil mixed with early oil to generate yellow-brown (Group II) and blue-white (Group I) fluorescing oils. Low-mature and mature oil from the Fengcheng Formation charged the Jurassic reservoirs and underwent secondary alteration, whereas high-mature oil and gas did not. These oils likely predominantly accumulated in reservoirs between the Fengcheng and Lower Wuerhe formations. Although the Jurassic source rocks began to generate oil, this oil did not migrate upward to the Qigu Formation, indicating that the conventional and unconventional reservoirs under the Jurassic Qigu Formation have exploration potential.

Microfacies and Evolution of the Carbonate Factory During the Middle Permian in Northwest Sichuan Basin, China

Microfacies and Evolution of the Carbonate Factory During the Middle Permian in Northwest Sichuan Basin, China

Carbon isotopic record of a platform-to-basin transect through the Permian Reef Complex (Guadalupian) in the Delaware Basin of Texas and New Mexico

The Guadalupian Permian Reef Complex of the Delaware Basin is one of the most studied carbonate reef systems of the Paleozoic. Despite extensive work on the carbonate sedimentology, sequence stratigraphy, and diagenetic history of the Delaware Basin, a high-resolution carbonate carbon isotope record along a platform to basin transect for the Capitanian (264.3–259.5 Ma, the youngest age of the Guadalupian Epoch) does not yet exist. The carbon isotopic record of the Delaware Basin is important because 1) it allows us to test hypotheses about controls on the carbon isotope proxy, 2) it provides constraints on how well modern carbonate platforms like the Great Bahama Banks serve as analogues for ancient carbonate settings, and 3) these types of restricted basins likely played an important role in Permian carbon cycling and the Capitanian extinctions.In this study we present 493 new Capitanian carbonate carbon isotopic values paired with a detailed sedimentological and sequence stratigraphic framework from the platform, slope, toe of slope, and deep basin of the Delaware Basin. The bulk of the new δ13C values fall within the range of previously reported unaltered carbonates from the basin, suggesting that these results record primary environmental processes and were not significantly altered by diagenetic overprinting. With this dataset, we test hypotheses about sources of carbon isotopic variability in shallow carbonate platforms. Our results indicate that in the Delaware Basin there are no systematic and resolvable depth or lateral gradients in carbon isotopic values, that δ13C values do not vary as a function of grain type, and that there is no resolvable relationship between carbon isotopic composition and sea level change. However, we do document statistically significant differences in δ13C distributions among facies associations which we attribute to the isotopic evolution of an upwelling water mass due to direct precipitation of mud along the slope. Our results support the idea that increasing carbon isotopic values through the Capitanian were driven by increased organic carbon burial in restricted basins.

A new rhynchonellid fauna (Brachiopoda) from the Wordian (middle Permian) of Coahuila, Mexico: Stratigraphy and paleoenvironments

Seven brachiopod species of the order Rhynchonellida of a new locality (Las Margaritas I section) from the Las Delicias Formation in Coahuila, Mexico are described. The fauna includes four taxa identified as Bryorhynchus bisulcatum, Leiorhynchioidea schucherti, Leiorhynchoidea amygdaloidea, and Paranorella imperialis, as well as three species assigned with open nomenclature: Leiorhynchioidea cf. scelesta, Stenoscisma sp., and Rhynchopora sp. The brachiopod fauna identified, together with the record of different middle Permian ammonoids allowed us to establish a Wordian (middle Guadalupian) age for the Las Margaritas I section. The assignment of this relative age enabled restricting the stratigraphic range of Leiorhynchioidea schucherti and Paranorella imperialis to the middle Guadalupian since previously had only been related inaccurately to the middle Permian. The record of the species Bryorhynchus bisulcatum, Leiorhynchioidea cf. scelesta, and Leiorhynchoidea amygdaloidea, as well as the genus Stenoscisma is the first in Mexico. The preservation of specimens, lithological features, and associated fauna suggest that rhynchonellid brachiopods were deposited in different platform facies: shallow lagoonal or offshore, characterized by low to medium energy, stenohaline conditions, and continuous terrigenous input.

Geochemistry of shales of middle Permian Barren Measures Formation, West Bokaro Basin, India: implications on provenance, paleodepositional and paleoclimatic conditions

Geochemical analyses of fine-grained clastic sedimentary rocks, such as shales, are significant since they portray the nature of the control factors from source to sink in a sedimentary basin, and help characterizing the heterogeneity in potential source and/or reservoir rocks. Bulk rock geochemistry (including major element oxides, trace elements and rare earth elements) of 20 shale samples from the Permian Barren Measures Formation, West Bokaro Basin, Peninsular India, is presented to decipher the provenance, paleo-weathering, paleotectonic, paleo-depositional and paleoredox conditions of the basin within the Lower Gondwana paleogeographic framework. X-ray diffraction (XRD) peaks and scanning electron microscopy (SEM) reveals an abundance of quartz, muscovite and clay minerals, viz., illite, kaolinite, etc., with less abundant feldspar and glauconite. The concentrations of the major element oxides, the trace elements and rare earth elements (REEs) of the shale samples are compared with predefined standards such as PAAS and UCC. The CIA (chemical index of alteration) and the A–CN–K values, based on the ratios of the major element oxides, signify intense chemical weathering. The ratios of Sr/Cu versus Rb/Sr and K2O/Al2O3 versus Ga/Rb, and high ΣREE values indicate a temperate paleoclimate. Ratios of the major element oxides (e.g., K2O/Na2O versus SiO2) suggest a passive tectonic setting. The Eu/Eu∗ and (Gd/Yb)n values depict predominantly post-Archean granitic and gneissic source rocks. The ratios of trace elements, viz., Th/Sc, Th/Co, Th/Cr, La/Sc and Eu/Eu∗, and the distribution pattern of the REEs (higher LREE in comparison to HREE) attest to a felsic-intermediate source from a nearby provenance. The redox sensitive trace elements, such as V and Ni, indicate a dysoxic condition that prevailed during deposition. The ratios of Fe2O3 versus MgO and Log (MgO/Al2O3) versus Log (K2O/Al2O3), and the ternary plots of Fe2O3–MgO–SiO2/Al2O3 suggest a non-marine to deltaic transitional environment. Such evidence of marine influences during the sedimentation of the Permian Barren Measures Formation confirms the fluvio-marine transgressive paleogeography inferred from sedimentological and paleontological inputs from the Gondwana Basins in Peninsular India, and opens up a scope for regional correlations of paleogeographic changes during the Middle Permian (Guadalupian) across the Gondwanaland.

Kolymopecten gen. nov.—a New Genus of Pectinoid Bivalves from the Lower–Middle Permian of the Boreal Superrealm

Kolymopecten gen. nov.—a New Genus of Pectinoid Bivalves from the Lower–Middle Permian of the Boreal Superrealm

Mercury evidence of Emeishan volcanism driving the mid-Capitanian (Middle Permian) extinction

The mid-Capitanian (Middle Permian) extinction has been widely attributed to the eruption of the Emeishan large igneous province. Here, we investigate the nature and timing of this link by determining Hg concentrations and isotopic compositions of limestones from the Xiongjiachang section of southwestern China, where Emeishan basalts directly overlie sediments recording the mid-Capitanian extinction horizon. Results show an initial Hg-enrichment interval ∼2.2 m below the mid-Capitanian extinction horizon. Positive Δ199Hg values in the Hg-enrichment interval suggest enhanced volcanic Hg influx into the ocean via atmospheric Hg(II) deposition. The late stage of the mid-Capitanian extinction interval has lower Δ199Hg values, which indicates enhanced input of terrestrial Hg due to ecosystem collapse and soil erosion. The results of this study provide evidence that the Emeishan large igneous province eruption occurred earlier than the mid-Capitanian extinction, and establish a more reasonable temporal link between Emeishan large igneous province volcanism and the mid-Capitanian extinction.

Understanding the Hydrocarbon Accumulation Process in Deep–ultradeep Oil and Gas Reservoirs in Complex Structural Areas—The Qixia Formation in the Shuangyushi Structure in Northwestern Sichuan Basin, China

Deep-ultradeep oil and gas reservoirs in complex structural areas have typically undergone multistage tectonic processes that have a considerable influence on the hydrocarbon composition, isotopic composition, and other geochemical characteristics of petroleum. By assessing the charging period of hydrocarbon accumulation during a particular age we can determin these changes. We studied the characteristics of fluid inclusions developed in the dolomite reservoir in the Middle Permian Qixia Formation in the Shuangyushi structure of the northwestern Sichuan Basin using methods such as inclusion petrography, microbeam fluorescence fluorescence spectroscopy, and inclusion homogenization temperature. Furthermore, the main peak wavelength of the fluorescence spectrum, red–green ratio/chromatic parameter, Q650/500, as well as the chromatic parameter corresponding to emission flux at 535 nm (QF535) and their correlations with the homogenization temperature distribution, were used to identify the charging period of hydrocarbons in the Qixia Formation. The results show that different types of fluid inclusions were developed in the Qixia Formation. The fluorescence of hydrocarbon inclusions is mainly blue, with a small amount of yellow–green and orange–yellow. The max of orange–yellow, yellow–green, and blue fluorescence of the hydrocarbon inclusions range between 562 ∼ 579 nm, 497 ∼ 548 nm, and 447 ∼ 497 nm, respectively. The distribution of the homogenization temperatures during the same period ranges from 100 °C to 110 °C, from 120 °C to 140 °C, and from 160 °C to 170 °C, respectively. By combining the burial and thermal evolution histories of the Qixia Formation, we concluded that the Qixia Formation has experienced three stages of oil and gas charging, with the time of 204, 185 and 158 Ma. This method is of great significance to determine the date and duration of oil and gas accumulation, and provides technical support for the recovery of ultra-deep hydrocarbon accumulations in complex structural areas.

Astronomical timescale across the middle Permian–Early Triassic unconformities in the northwestern Junggar Basin: Implications for the origin of the unconformities

The middle Permian–Early Triassic terrestrial successions in the Junggar Basin record an important tectonic transformation stage characterized by the development of unconformities. These unconformities control billion-ton-scale hydrocarbon reservoirs in the northwestern Junggar Basin; however, their age, duration and underlying causes remain uncertain in the absence of an accurate Permian‒Triassic timescale. This study performed a comprehensive cyclostratigraphic analysis utilizing natural gamma ray data from 14 wells to determine the age and duration of the unconformities spanning from the middle Permian Lower Wuerhe (P2w) Formation to the lower Triassic Baikouquan (T1b) Formation while also revealing their potential origins. Our results suggest that astronomically forced climate changes were recorded in the western Junggar Basin during the Permian‒Triassic period. By applying astronomical tuning through sedimentation rates across each well, 14 time-domain series were generated. Using two previously reported maximum sedimentary ages from detrital zircon U–Pb dating, a 38.2-million-year long astronomical timescale was constructed, encompassing the period from the middle Permian Xiazijie Formation to the lower Triassic Baikouquan Formation. This analysis constrains the duration of unconformities between the P2w and T1b formations to ∼7–23 Myr. By integrating the duration, order, and implications of these unconformities with previously reported evidence, we conclude that the unconformity between the middle Permian and upper Permian was driven primarily by tectonic inversion resulting from intensified regional tectonic activity. In contrast, the unconformity between the upper Permian and Lower Triassic likely arose from uplift and erosion induced by compressive stresses associated with tectonic activity, which contributed to the formation of unconformities and stratigraphic discontinuities. This work provides a crucial reference for further studies aimed at integrating detailed sedimentological analyses with high-resolution geochronological data to further elucidate the timing and effects of these tectonic events on sedimentary facies and basin development.

Potential effects of the Emeishan large igneous province on Capitanian marine anoxia in the Upper Yangtze region

The widespread Capitanian (late Guadalupian) marine anoxia/euxinia has long been regarded as a key driver of the end-Guadalupian (middle Permian) biotic crisis. However, the cause of this marine anoxia is debated, particularly regarding the influence of the Emeishan large igneous province (ELIP). To investigate the contribution of the ELIP to marine anoxia and the possible causal mechanisms, we undertook a conodont biostratigraphic and geochemical study of the middle Permian Maokou Formation in a platform-to-trough transect in the Upper Yangtze region, South China. Our results show that the depositional facies of the Maokou Formation changed from a carbonate ramp to an intra-platform trough within the Jinogondolella (J.) altudaensis zone in the northwestern Yangtze region, which can be attributed to the initial activity of the ELIP. Mantle-derived Sr inputs in the initial and main stages of the ELIP led to two decreases in 87Sr/86Sr during the Capitanian, in the J. shannoni–J. altudaensis and J. prexuanhanensis–J. xuanhanensis zones. The elevated 87Sr/86Sr values during the late Capitanian may have been due to enhanced continental weathering caused by rapid climate warming in response to subaerial eruptions of the ELIP. The deep-water anoxia–euxinia expanded during the middle Capitanian, as indicated by increased MoEF/UEF and V/(V + Ni) values, along with the disappearance of burrows and appearance of small pyrite framboids in the J. altudaensis zone in the Cheng 20 well. However, shallow-water anoxia occurred during the late Capitanian (i.e., J. prexuanhanensis–J. xuanhanensis zone), as evidenced by positive Ce anomalies and losses of aerobic benthic species in the Erya section. Importantly, marine anoxia and negative δ13Ccarb excursions occurred synchronously, but earlier in deep water than in shallow water, potentially indicating an expansion of the oxygen minimum zone (OMZ). The deep-water anoxia corresponded to a decrease in 87Sr/86Sr ratios and the appearance of an intra-platform trough in the J. altudaensis zone, whereas the shallow-water anoxia in the late Capitanian coincided with elevated 87Sr/86Sr ratios. This suggests that the initial activity of the ELIP promoted the development of the OMZ in deep waters during the middle Capitanian, while the subaerial eruptions of the ELIP drove climate warming that led to the expansion of the OMZ into shallow-water platforms during the late Capitanian.

Spatial and Temporal Distribution of Igneous Sills in the Central Tarim Basin and Their Geological Implications

Interpretation of the seismic reflection profiles associated with borehole data from the petroleum industry offers a novel way to study sill emplacement in sedimentary basins. This study uses this approach to reveal the intrusive part of the Tarim Large Igneous Province (LIP) within the basin, which has not been systematically reported. A large number of igneous intrusions (sills) are identified in the sedimentary layers of the Central Tarim Basin. The burial depth of the sills is 6–8 km, and they are mainly located within the upper Ordovician strata. According to their seismic facies and drilling data, it is inferred that they are dolerite sills. Based on the uplift of the overlying strata above the intrusions, it is concluded that the sills were mainly formed during the depositional period of the middle Permian Kupukuziman Formation and Kaipailezike Formation (early stage), with a few formed during the depositional period of the upper Permian strata (late stage). It is likely that these two stages of sill intrusion correspond to the main basaltic eruptions within the basin and the mafic dike emplacement in the Bachu area of the Tarim LIP, respectively. The study suggests that that the dolerite sills reported in this study are also an important component of the Permian Tarim LIP.

Post-Devonian re-emergence and demise of stromatoporoids as major reef-builders on a Carboniferous Panthalassan seamount

Abstract Stromatoporoids were major reef-builders during the middle Paleozoic; however, no Carboniferous stromatoporoid reefs have been reported. The Akiyoshi Limestone Group of southwest Japan formed on a seamount in the Panthalassa Ocean from the Mississippian (Visean) to the middle Permian (Capitanian). The Early Pennsylvanian (Bashkirian) reef-core setting was well developed and laterally differentiated into several reef environments. Laminated skeletons made up of cystose or flat laminae and pillar-like vertical structures were abundant and, together with Chaetetes, contributed greatly to reef construction in most reef-core environments. The morphology of the laminated skeletons clearly indicates a stromatoporoid, probably labechiid, affinity. Thus, a lineage of reef-building stromatoporoids reappeared on a Panthalassan seamount in a locally warm-water tropical setting after the Late Devonian extinctions. Ongoing global glaciation may have resulted in enhanced ocean circulation, upwelling, and nutrient supply, especially around shallow-water seamounts, culminating in elevated carbonate saturation, which should have favored hypercalcified stromatoporoids and Chaetetes. The Chaetetes–stromatoporoid reefs remained in the Moscovian, but probably died out during the Kasimovian with intensive global cooling and frequent subaerial exposure, to be replaced by Palaeoaplysina–phylloid algal reefs. The occurrence of Bashkirian reef-building stromatoporoids indicates that Paleozoic stromatoporoids continued as reef-builders long after the Late Devonian extinction, at least in Panthalassa. This occurrence emphasizes the significance of rarely preserved open, but isolated oceanic settings like Akiyoshi for global biogeography and evolution.

Effects of the Emeishan large igneous province (ELIP) and coastal upwelling on paleoenvironment and organic matter accumulation during the Middle Permian (Capitanian), South China

The late Middle Permian (Capitanian) was characterized by many global geological, biotic and environmental events, along with the widespread deposition of organic-rich strata. Elucidating the relationships between organic matter (OM) accumulation and critical geological events is of significance for Earth system science and petroleum resource geology, but the mechanism remains unclear. To fill the knowledge gap, this study investigated the Gufeng Formation in the middle Yangtze region of South China to examine the links between OM accumulation and geological events, e.g., the Emeishan large igneous province (ELIP) and coastal upwelling. Our results show that the Gufeng Formation exhibits significant OM enrichment, with an average total organic carbon (TOC) content of 8.34 wt%. The paleoceanographic conditions went through three stages during the Capitanian: the early–middle stage of high primary productivity and anoxic–sulfidic conditions; the middle stage of moderate productivity and anoxic conditions; and the late stage of low productivity and dysoxic conditions due to weakened upwelling and a global sea-level fall. OM accumulation was influenced mainly by upwelling-induced high productivity and anoxic conditions related to the oxygen minimum zone (OMZ), with a secondary control of carbonate dilution. The ELIP possibly had a limited contribution to high TOC contents. In South China, the Gufeng and uppermost Maokou formations were marked by high OM contents (average TOC > 3 wt%; maximum = 40 wt%), resulting primarily from widespread coastal upwelling along the eastern margin of the Paleo-Tethys Ocean during the Middle Permian. ELIP volcanism mainly triggered rapid climate warming during the late Capitanian and had a regional and variable effect on OM enrichment for different regions in South China.

Complementary testing and machine learning techniques for the characterization and prediction of middle Permian tight gas sandstone reservoir quality in the northeastern Ordos Basin, China

In this study, an integrated approach for diagenetic facies classification, reservoir quality analysis and quantitative wireline log prediction of tight gas sandstones (TGSs) is introduced utilizing a combination of fit-for-purpose complementary testing and machine learning techniques. The integrated approach is specialized for the middle Permian Shihezi Formation TGSs in the northeastern Ordos Basin, where operators often face significant drilling uncertainty and increased exploration risks due to low porosities and micro-Darcy range permeabilities. In this study, detrital compositions and diagenetic minerals and their pore type assemblages were analyzed using optical light microscopy, cathodoluminescence, standard scanning electron microscopy, and X-ray diffraction. Different types of diagenetic facies were delineated on this basis to capture the characteristic rock properties of the TGSs in the target formation. A combination of He porosity and permeability measurements, mercury intrusion capillary pressure and nuclear magnetic resonance data was used to analyze the mechanism of heterogeneous TGS reservoirs. We found that the type, size and proportion of pores considerably varied between diagenetic facies due to differences in the initial depositional attributes and subsequent diagenetic alterations; these differences affected the size, distribution and connectivity of the pore network and varied the reservoir quality. Five types of diagenetic facies were classified: (ⅰ) grain-coating facies, which have minimal ductile grains, chlorite coatings that inhibit quartz overgrowths, large intergranular pores that dominate the pore network, the best pore structure and the greatest reservoir quality; (ⅱ) quartz-cemented facies, which exhibit strong quartz overgrowths, intergranular porosity and a pore size decrease, resulting in the deterioration of the pore structure and reservoir quality; (ⅲ) mixed-cemented facies, in which the cementation of various authigenic minerals increases the micropores, resulting in a poor pore structure and reservoir quality; (ⅳ) carbonate-cemented facies and (ⅴ) tightly compacted facies, in which the intergranular pores are filled with carbonate cement and ductile grains; thus, the pore network mainly consists of micropores with small pore throat sizes, and the pore structure and reservoir quality are the worst. The grain-coating facies with the best reservoir properties are more likely to have high gas productivity and are the primary targets for exploration and development. The diagenetic facies were then translated into wireline log expressions (conventional and NMR logging). Finally, a wireline log quantitative prediction model of TGSs using convolutional neural network machine learning algorithms was established to successfully classify the different diagenetic facies.

Origin of facies-controlled dolomite and exploration significance of the Middle Permian Qixia Formation in central Sichuan Basin, Western China

The intraplatform shoal dolomite of the Middle Permian Qixia Formation is currently considered the key target of hydrocarbon exploration in the central Sichuan Basin. To systematically investigate the origin of the stratabound facies-controlled porous dolomites of the Qixia Formation, integrated petrography, logging and seismic analysis were carried out in this work. The results are as following: (1) the dolomite reservoir is universal in the central Sichuan Basin, and its distribution is controlled by intraplatform shoals, with multilayer superposition vertically. Thick massive dolostone may also develop along with the fault. (2) Three replaced dolomites and one dolomite cement were identified: very finely to finely crystalline, anhedral to subhedral dolomite (Rd1); finely to medium crystalline, anhedral to subhedral dolomite (Rd2); coarsely crystalline, subhedral to euhedral dolomite (Rd3) and coarsely crystalline saddle dolomite cement (Sd). Rd2 and Rd3 are partly fabric-retentive, and preserve the original bioclastic ghosts. Sd shows wavy extinction, filled in the breccia veins. (3) The U-Pb dating and homogenization temperatures results indicate that the dolomite and Sd cement are associated with hydrothermal event during the Emeishan large igneous province. The δ13C, 87Sr/86Sr, and seawater-like REEY patterns suggest that the dolomitization and Sd precipitation fluids originate from connate seawater heated by elevated heat-flow. (4) The ELIP triggered large scale thermal anomalies in the basin during the Dongwu movement period. The increased temperature and pressure drove the formation water in the intra-platform shoal facies and overcame the binding effect of Mg2+ hydrate. Moreover, the deep hydrothermal fluid preferentially penetrated into the porous strata of shoal facies along the faults and fractures, mixed with formation water to some extent, and extensive dolomitization occurred. The facies-controlled dolomite reservoir and the underlying Cambrian source rock form a good source-reservoir assemblage, which can be a key replacement option.

Depositional environments of complex lake systems – palaeogeographical evolution and main controls: A case study of the Lower to Middle Permian Roseneath–Epsilon–Murteree strata, Cooper Basin, Australia

ABSTRACTLake systems are among the largest integrated depositional complexes in the continental realm. Their interplay with tectonics and climate often complicates the interpretation of depositional processes, usually resulting in a complex facies architecture that is challenging to correlate regionally. The Lower to Middle Permian Roseneath–Epsilon–Murteree strata of the Cooper Basin in Australia represent a glacially influenced fluvial–deltaic lake system, which hosts substantial petroleum resources. Despite their potential, the Roseneath–Epsilon–Murteree strata are inadequately understood in terms of regional facies architecture and their palaeogeographical implications. Using a multidisciplinary approach with robust data, including cores, petrology, wireline logs and geochemistry, the sedimentary evolution of the Roseneath–Epsilon–Murteree strata is described within a new basin‐wide sequence stratigraphic framework defined by key surfaces identified from cores and wireline logs. Detailed facies analysis of over 1400 m of cores and wireline logs from nine wells identified twelve lithofacies and eight facies associations, revealing various depositional processes and environments. The Roseneath–Epsilon–Murteree strata are interpreted as having formed in a fluvial–deltaic–lacustrine system influenced by a cold climate. First‐order and second‐order maximum flooding surfaces define a regional sequence stratigraphic framework, subdividing the Roseneath–Epsilon–Murteree strata into four sequence units (SU1, SU2, SU3 and SU4), deposited during four phases (I, II, III and IV). Cyclic stacking patterns observed in both cores and wireline logs reveal two transgressive–regressive cycles. Palaeogeographical maps of the four depositional phases suggest that the Roseneath–Epsilon–Murteree strata were dominated by a lacustrine system with some delta and mire deposits. Rivers primarily drained towards the south/south‐west from the Gidgealpa, Merrimelia, Packsaddle and Innamincka ridges (‘GMI Trend’) as indicated by decreasing sandstone percentages and increasing gross unit isopach maps. The deposition of the Roseneath–Epsilon–Murteree strata was primarily controlled by basin subsidence. Additionally, palaeoclimate trends and extensive glaciation in the Permian played important roles in providing sediments and affecting lake level changes. The lake system recorded by the Roseneath–Epsilon–Murteree strata serves as an analogue for other fluvial–deltaic–lacustrine systems in basins worldwide. A multidisciplinary approach, similar to the one utilized in this study, can help to elucidate their evolution and aid in resource prediction.

Late Paleozoic intra-oceanic arc and its accretionary complex in East Junggar (NW China): implications for multiple arc amalgamation in the southern Altaids

The East Junggar is a key to understanding the accretionary history of the southern Altaids. However, the tectonic setting and amalgamation history of the East Junggar during the Carboniferous–Permian remain controversial. Here we report new field, geochronological and geochemical data for the Carboniferous–Permian volcanic and sedimentary rocks from the Jiangjun unit and the Hongliugou ophiolitic complex of East Junggar to constrain its tectonic evolution. Carboniferous volcanic rocks, with zircon and apatite U–Pb ages from 337 ± 2 to 303 ± 32 Ma, display arc-related geochemical characteristics. Carboniferous–Permian sedimentary rocks predominantly contain Carboniferous–Permian detrital zircons and have maximum depositional ages from 318 ± 5 to 281 ± 6 Ma. Provenance analysis indicates that the sediments from the west Jiangjun unit were sourced from the Yemaquan arc and those from the east Jiangjun unit may be sourced solely from a new intra-oceanic arc. Integrated data indicate that Carboniferous–Permian strata in the Jiangjun unit were formed in the intra-oceanic arc setting that remained until the middle Permian. These analyses suggest an amalgamation process of multiple convergence and accretion for the southern Altaids during the formation of NE Pangaea in the late Paleozoic.

Comparative study on pore-connectivity and wettability characteristics of the fresh-water and saline lacustrine tuffaceous shales: triggering mechanisms and multi-scale models for differential reservoir-forming patterns

Although particular attention has been paid to responses of hydrocarbon storage and percolation capacity to the devitrification concerning lacustrine tuffaceous shale reservoirs in recent years, there is still a lack of systematical and comparative investigation on differential patterns and potential triggering mechanisms concerning development of the pore-microfracture systems and characteristics of surface wettability between the fresh-water and saline lacustrine settings, which is of considerable importance in fully understanding of genesis and spatial distribution of dessert reservoir intervals of tuffaceous shale reservoirs, and to provide further conceptual basis for deciphering shale-oil movability of saline lacustrine fine-grained mixed sedimentary sequences. In this study, tuffaceous shales from both the Upper Triassic Yanchang Formation in the Ordos Basin and Middle Permian Jingjingzigou Formation in the Junggar Basin are targeted to unravel the differential behavior of tuff devitrification and potential impacts on reservoir wettability and pore connectivity concerning fresh-water and saline lacustrine settings, and we present new results here from integrated analyses and combined interpretation of FE-SEM, Image Pro Plus (IPP) software image processing, contact angle and spontaneous imbibition experiments. In view of comparative analysis from representative samples, the tuffaceous shales from saline lacustrine environments are characterized by well-developed intergranular-intercrystalline and dissolution pores, and inorganic microfractures, generally yield a higher plane porosity of representative pore-fracture spaces and spontaneous imbibition slopes, a relatively lower average of n-decane contact angles and corresponding wettability parameters. The saline lacustrine tuffaceous shales are thus suspected to have undergone more intense devitrification resulting in a higher amount of devitrification and associated dissolution pores, and a relatively better connectivity between isolated micropore systems with adjacent microfractures. This would significantly facilitate the interface wettability reversal and occurrence of movable hydrocarbon fluid in microscopic reservoir spaces. Finally, a comprehensive and conceptual model is established illustrating the effects of differential devitrification on reservoir-forming patterns concerning tuffaceous shales developed in the fresh-water and saline lacustrine settings, respectively. These findings are of great theoretical and practical significance to enrich theory of high-quality reservoir formation and shale-oil accumulation in saline lacustrine tuffaceous shale reservoirs, and lay the foundation for guiding efficient exploration of continental fine-grained mixed sedimentary sequences.