Low Evolutionary Rate Research Articles

Norms of evidence in the classification of living fossils

Some species have held fast for millions of years as constants in a changing world. Often called “living fossils,” these species capture scientific and public interest by showing us the vestiges of an earlier world. If living fossils are defined by a holistic pattern of low evolutionary rates or stasis, however, then classifying a species as a living fossil involves the application of sophisticated norms of scientific evidence. Using examples from Crocodilia and the tuatara (Sphenodon punctatus), I show how scientists’ evidential criteria for classifying living fossils are contentious and underspecified in many cases, threatening the concept’s explanatory interest and its adequacy for sustaining a collective problem agenda as proposed by Scott Lidgard and Alan Love. While debates over the definition of the living fossil concept may appear fruitless, I suggest they can be productive insofar as the debate leads to clarified and improved evidential standards for classification. To this end, I formulate a view of the living fossil concept as an investigative kind, and compare two theoretical frameworks as a basis for shared evidential norms: the Zero Force Evolutionary Law framework, introduced by Daniel McShea and Robert Brandon, and the statistical model selection framework first developed by Gene Hunt in the 2000s.

Genome-based survey of the SARS-CoV-2 BF.7 variant from Asia.

The SARS-CoV-2 BF.7 variant represents one of the most recent subvariant under monitoring. At the beginning of the 2023 if caused several concerns especially in Asia because of a resurge in COVID-19 cases. Here we perform a genome-based integrative approach on SARS-CoV-2 BF.7 in order to shed light on this emerging lineage and produce some consideration on its real dangerousness. Both genetic and structural data suggest that this new variant currently does not show evidence of an high expansion capability. It is very common in Asia, but it appears less virulent than other Omicron variants as proved by its relatively low evolutionary rate (5.62 x 10-4 subs/sites/years). The last plateau has been reached around December 14, 2022 and then the genetic variability, and thus the viral population size, no longer increased. As already seen for several previous variants, the features that may be theoretically related to advantages are due to genetic drift that allows to the virus a constant adaptability to the host, but is not strictly connected to a fitness advantage. These results have further pointed that the genome-based monitoring must continue uninterruptedly in order to be prepared and well documented on the real situation. This article is protected by copyright. All rights reserved.

Anatomy and relationships of the bizarre Early Cretaceous pliosaurid Luskhan itilensis

Abstract Pliosaurid plesiosaurians are iconic marine reptiles that regulated marine trophic chains from the Middle Jurassic to the early Late Cretaceous. However, their evolution during the Cretaceous remains poorly documented. Recent discoveries from the Hauterivian–Aptian interval suggest that the radiation of brachaucheniine pliosaurids produced a wide disparity of forms following the Pliosaurus-dominated assemblages of the Late Jurassic. Among the most bizarre of these early brachaucheniines is Luskhan itilensis, from the Hauterivian of Russia. We describe the osteology of this tusked, longirostrine pliosaurid and discuss its possible behaviour by drawing comparisons with other marine amniotes possessing forward-pointing teeth. We take this opportunity to make extensive anatomical comparisons among Cretaceous pliosaurids, including previously overlooked cranial features. Bayesian inference of phylogenetic relationships of plesiosaurians reveals that the internal branches in Late Jurassic–Late Cretaceous pliosaurids have generally low rates of morphological evolution, indicating that the recently described Early Cretaceous pliosaurids have effectively bisected the long branch leading to the ‘classical’ brachaucheniines of the middle Cretaceous (Brachauchenius, Kronosaurus and Megacephalosaurus). Pliosaurids exhibit low evolutionary rates and a dwindling disparity before their extinction, mirroring the events seen, roughly at the same time, for ichthyosaurians.

Genomic diversity of SARS-CoV-2 can be accelerated by mutations in the nsp14 gene

Coronaviruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), encode a proofreading exonuclease, nonstructural protein 14 (nsp14), that helps ensure replication competence at a low evolutionary rate compared with other RNA viruses. In the current pandemic, SARS-CoV-2 has accumulated diverse genomic mutations including in nsp14. Here, to clarify whether amino acid substitutions in nsp14 affect the genomic diversity and evolution of SARS-CoV-2, we searched for amino acid substitutions in nature that may interfere with nsp14 function. We found that viruses carrying a proline-to-leucine change at position 203 (P203L) have a high evolutionary rate and that a recombinant SARS-CoV-2 virus with the P203L mutation acquired more diverse genomic mutations than wild-type virus during its replication in hamsters. Our findings suggest that substitutions, such as P203L, in nsp14 may accelerate the genomic diversity of SARS-CoV-2, contributing to virus evolution during the pandemic.

The dynamics of the nickel foam formation and its effect on the catalytic properties toward hydrogen evolution reaction

Porous nickel deposits were obtained by electrodeposition via dynamic hydrogen bubble template in a galvanostatic mode at a current density of 0.3, 0.6, 0.9 and 1.2 A·cm−2. Change of nickel foam morphology (dendrite particles, pore number and their sizes) with the applied current density was analyzed. It was found that at low hydrogen evolution rate, a gradual formation of a porous structure occurs, while at high ones, the formation of the template structure ends in the first minutes of electrolysis. It is shown that the log-normal distribution can be used to describe the formation of a hydrogen template as a system of nickel foam macropores. The catalytic activity of nickel foams toward hydrogen evolution was analyzed in an alkali solution. The Tafel slope for the obtained foams is in the range of 126-107 mV·dec−1. Nickel foams obtained at 1.2 A·cm−2 are the best candidates for hydrogen evolution electrodes due to their stable structure, providing maximum access of reacting particles to the inner surface of the electrode.

Genetic Drift Shapes the Evolution of a Highly Dynamic Metapopulation.

The dynamics of extinction and (re)colonization in habitat patches are characterizing features of dynamic metapopulations, causing them to evolve differently than large, stable populations. The propagule model, which assumes genetic bottlenecks during colonization, posits that newly founded subpopulations have low genetic diversity and are genetically highly differentiated from each other. Immigration may then increase diversity and decrease differentiation between subpopulations. Thus, older and/or less isolated subpopulations are expected to have higher genetic diversity and less genetic differentiation. We tested this theory using whole-genome pool-sequencing to characterize nucleotide diversity and differentiation in 60 subpopulations of a natural metapopulation of the cyclical parthenogen Daphnia magna. For comparison, we characterized diversity in a single, large, and stable D. magna population. We found reduced (synonymous) genomic diversity, a proxy for effective population size, weak purifying selection, and low rates of adaptive evolution in the metapopulation compared with the large, stable population. These differences suggest that genetic bottlenecks during colonization reduce effective population sizes, which leads to strong genetic drift and reduced selection efficacy in the metapopulation. Consistent with the propagule model, we found lower diversity and increased differentiation in younger and also in more isolated subpopulations. Our study sheds light on the genomic consequences of extinction-(re)colonization dynamics to an unprecedented degree, giving strong support for the propagule model. We demonstrate that the metapopulation evolves differently from a large, stable population and that evolution is largely driven by genetic drift.

An atlas of small RNAs from potato.

Small RNAs, including microRNAs (miRNAs), phased secondary small interfering RNAs (phasiRNA), and heterochromatic small interfering RNAs (hc-siRNA) are an essential component of gene regulation. To establish a broad potato small RNA atlas, we constructed an expression atlas of leaves, flowers, roots, and tubers of Desiree and Eva, which are commercially important potato (Solanum tuberosum) cultivars. All small RNAs identified were observed to be conserved between both cultivars, supporting the hypothesis that small RNAs have a low evolutionary rate and are mostly conserved between lineages. However, we also found that a few miRNAs showed differential accumulation between the two potato cultivars, and that hc-siRNAs have a tissue specific expression. We further identified dozens of reproductive and non-reproductive phasiRNAs originating from coding and noncoding regions that appeared to exhibit tissue-specific expression. Together, this study provides an extensive small RNA profiling of different potato tissues that might be used as a resource for future investigations.

Hidden species diversity in an iconic living fossil vertebrate.

Ancient, species-poor lineages persistently occur across the Tree of life. These lineages are likely to contain unrecognized species diversity masked by the low rates of morphological evolution that characterize living fossils. Halecomorphi is a lineage of ray-finned fishes that diverged from its closest relatives before 200 Ma and is represented by only one living species in eastern North America, the bowfin, Amia calva Linnaeus. Here, we use double digest restriction-site-associated DNA sequencing and morphology to illuminate recent speciation in bowfins. Our results support the delimitation of a second living species of Amia, with the timing of diversification dating to the Plio-Pleistocene. This delimitation expands the species diversity of an ancient lineage that is integral to studies of vertebrate genomics and development, yet is facing growing conservation threats driven by the caviar fishery.

A Novel Nano-Composite CSNPs/PVP/CoONPs Coating for Improving Corrosion Resistance of Ti-6Al-4V Alloy as a Dental Implant

A new nano-coating of chitosan nanoparticles/polyvinylpyrrolidone/cobalt oxide nanoparticles (CSNPs/PVP/CoONPs) was performed in this work. The newly designed nano-coating comprises a copolymer and inorganic matrices. This nano-coating was used to cover the Ti-6Al-4V alloy surface as a newly designed dental alloy, and then its corrosion properties were studied through different electrochemical techniques. The results reveal that this novel coating improved the corrosion resistance of the Ti-6Al-4V alloy in artificial saliva solution by reaching 17.7 MΩ cm2. The new fabricated biocompatible coating (CSNPs/PVP/CoONPs) greatly enhanced the electrochemical corrosion resistance by giving a high protection efficiency of 90.87% and a low hydrogen evolution rate in artificial saliva solution at 37 °C. The observed results were confirmed by scanning electron microscopy (SEM), Vickers microhardness testing, coating thickness tests, high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray analysis (EDX).

Effects of oxygen supply rate on organic matter decomposition and microbial communities during composting in a controlled lab-scale composting system

The study aimed to elucidate the effect of oxygen supply rate (OSR) on the composting of model organic waste independently from other factors by using a controlled laboratory-scale reactor system. Four OSRs, 96.2, 24.2, 13.7, and 3.45 mL-O2/min/kg-WS (wet solid), were tested. The delay of organic matter degradation was observed temporarily in the early stage of composting with 13.7 mL-O2/min/kg-WS and severe oxygen deficiency was observed in lower OSR, indicating that the critical OSR existed around this value. Composting with 3.45 mL-O2/min/kg-WS resulted in constantly low CO2 evolution rate and remarkably low degree of organic matter degradation. Under deficient oxygen, all enzymes measured, such as phosphatases, esterases, lipases, proteases, and sugar degrading enzymes, showed lower activities. High-throughput sequencing revealed Caldibacillus and Ureibacillus became dominant in the later stages of the oxygen deficiency composting, while Geobacillus was the most dominant microorganism throughout composting with OSR higher than 13.7 mL-O2/min/kg-WS.

Genomic Surveillance of SARS-CoV-2: Study of Cases Intradomiciliary Infection in Brazilian Amazonia

Studies have shown that the family environment is very favorable for SARSCOV- 2 transmission, making family groups susceptible to intra-household infection because they maintain direct contact with an infected person. We screened a cohort of 416 individuals who tested positive for SARS-CoV-2 from June to October 2021 for intradomiciliary infection. Twenty-two families with an average of 2 to 4 members were located in 12 municipalities in the state of Rondônia. Nasopharyngeal samples were collected, extracted using the QIAamp viral RNA mini kit and genomic sequencing was performed using samples with ct<25 by Illumina platform, the sequences were analyzed for phylogeny, mutations, and lineages. Among the samples analyzed 72.72% matched for VOC Delta and 27.27% for Gamma variant. The median viral load for both variants was 6.95. The main symptoms presented were cough, headache, and fever. No hospitalizations or deaths were reported. We observed that among the positive individuals, 24% were immunized with the 1st and 2nd dose, 40% were partially immunized, and 36% were not immunized. For each strain, subvariants were identified, being P.1.4 and P.1.14 for the Gamma variant and AY.43, AY.99.2 and AY.122 for Delta. The mutations mostly behaved as lineage-defining, with only eight families showing point mutations. In conclusion, it was observed that despite the low adherence to immunization, household cases were equally expressed in viral characteristics and in the low rate of hospital evolution for COVID-19, and this may be a factor associated with the permanence of infection in an isolated family environment.

Ancestral morphology and taxonomic utility of squid statoliths (Cephalopoda: Loliginidae)

ABSTRACT Loliginid squids from the northeastern Pacific are partially sympatric, and their identification is difficult as their diagnostic characters often overlap. Statoliths are hard structures that may vary with respect to phylogeny and thus have potential use in taxonomy. This, however, has to be investigated using robust methods. Here, we employ geometric morphometrics to evaluate the taxonomic utility of statolith shape in loliginids caught in Mexican waters and to acquire knowledge on statolith evolution through comparative phylogenetic methods. The loliginid species we examined showed specific statolith morphology that facilitates species identification. Our results also show that statolith shape has a strong phylogenetic signal and is a powerful character for species recognition. The ancestral statolith morphologies are not very different from extant forms, probably reflecting the relatively recent origin of the two genera evaluated and their low evolutionary rate. In addition, the evolution of statolith shape in species of the genus Lolliguncula seems to have been faster than that in the genus Doryteuthis, especially in L. panamensis, which could be related to habitat differentiation in the neritic coastal environment inhabited by loliginid squids. Our analysis of statolith shape demonstrates the potential for combining neontology and paleontology in understanding the diversification of loliginid squids.

Comparative Mitochondrial Genomics of 104 Darwin Wasps (Hymenoptera: Ichneumonidae) and Its Implication for Phylogeny.

Simple SummaryNearly a hundred mitochondrial genomes of ichneumonid wasps are newly reported. Comparative mitogenomics of 104 mitochondrial genomes representing 33 subfamilies of Ichneumonidae, as well as its implications for phylogeny, were studied. We found that the mitochondrial genomes of ichneumonid wasps were highly conserved in their base composition and had low evolutionary rates, but were diverse in gene order. There are 38 types of gene rearrangement events in 104 ichneumonid mitochondrial genomes, of which 30 novel rearrangement types (R3-6, R8-R10, R12-R15, R17-R18, R20-R35 and R38) and a hot spot rearrangement around R1, with a shuffled tRNA cluster trnW-trnY-trnC and trnI-trnQ-trnM, were detected. The relationships among these subfamilies are firstly discussed based on mitochondrial genomes at a large scale. We suggest five subfamily groupings of Ichneumonidae: Brachycyrtiformes, Ichneumoniformes, Ophioniformes, Pimpliformes and Xoridiformes. Two formerly unplaced subfamilies, Eucerotinae and Microleptinae, were placed in Brachycyrtiformes and Ichneumoniformes, respectively.Ichneumonidae is one of the largest families of insects with a mega-diversity of specialized morphological and biological characteristics. We newly sequenced 92 mitochondrial genomes of ichneumonid wasps and found that they have a conserved base composition and a lower evolutionary rate than that of other families of parasitic Hymenoptera. There are 38 types of gene order in the ichneumonid mitochondrial genome, with 30 novel types identified in 104 ichneumonids. We also found that the rearrangement events occur more frequently in Ophioniformes than in Ichneumoniformes and Pimpliformes. Furthermore, the higher Ophioniformes and their relative lineages shared the transposition of trnL2 to trnI-trnQ-trnM tRNA cluster. We confirmed five higher-level groupings of Ichneumonidae: Brachycyrtiformes, Ichneumoniformes, Ophioniformes, Pimpliformes and Xoridiformes. Two formerly unplaced subfamilies, Eucerotinae and Microleptinae, were placed in Brachycyrtiformes and Ichneumoniformes, respectively. The results will improve our understanding of the diversity and evolution of Ichneumonidae.

Large dataset of octocoral mitochondrial genomes provides new insights into mt-mutS evolution and function

All studied octocoral mitochondrial genomes (mt-genomes) contain a homologue of the Escherichia coli mutS gene, a member of a gene family encoding proteins involved in DNA mismatch repair, other types of DNA repair, meiotic recombination, and other functions. Although mutS homologues are found in all domains of life, as well as viruses, octocoral mt-mutS is the only such gene found in an organellar genome. While the function of mtMutS is not known, its domain architecture, conserved sequence, and presence of several characteristic residues suggest its involvement in mitochondrial DNA repair. This inference is supported by exceptionally low rates of mt-sequence evolution observed in octocorals. Previous studies of mt-mutS have been limited by the small number of octocoral mt-genomes available. We utilized sequence-capture data from the recent Quattrini et al. 2020 study [Nature Ecology & Evolution 4:1531–1538] to assemble complete mt-genomes for 94 species of octocorals. Combined with sequences publicly available in GenBank, this resulted in a dataset of 184 complete mt-genomes, which we used to re-analyze the conservation and evolution of mt-mutS. In our analysis, we discovered the first case of mt-mutS loss among octocorals in one of the two Pseudoanthomastus spp. assembled from Quattrini et al. data. This species displayed accelerated rate and changed patterns of nucleotide substitutions in mt-genome, which we argue provide additional evidence for the role of mtMutS in DNA repair. In addition, we found accelerated mt-sequence evolution in the presence of mt-mutS in several octocoral lineages. This accelerated evolution did not appear to be the result of relaxed selection pressure and did not entail changes in patterns of nucleotide substitutions. Overall, our results support previously reported patterns of conservation in mt-mutS and suggest that mtMutS is involved in DNA repair in octocoral mitochondria. They also indicate that the presence of mt-mutS contributes to, but does not fully explain, the low rates of sequence evolution in octocorals

Synthesis of a novel nitrogen-doped K2Ti6O13 nanorod with visible-light-driven water splitting performance promoted by fabrication of 1D/2D heterostructure

Although developments of visible-light-responsive nitrogen-doped oxide photocatalysts for promising solar water splitting have inspired extensive interests, the poor charge separation and low H2 evolution rate are still challenges. Here this work reports a novel nitrogen-doped 1D K2Ti6O13 nanorod (denoted as KTON hereafter) that exhibits obvious visible-light-responsive water splitting performance. The H2 evolution rate was significantly promoted by fabricating 1D KTON nanorod and 2D graphitic carbon nitride heterostructure (denoted as KTON/C3N4), which was 18.65 μmol·h−1 with an apparent quantum yield of 0.04% at 420 nm. The detailed structural characterizations demonstrated formation of intimate interface to cause remarkably promoted charge separation and 10 times of promoted H2 evolution rate on the type II heterostructure compared to KTON. Moreover, the heterostructure showed excellent photochemical stability in the experimental region. The work not only reports a novel visible-light-responsive water splitting photocatalyst, but also demonstrates the feasibility of fabricating intimate heterostructure between inorganic and organic/polymeric photocatalyst for promoting charge separation and photocatalytic performance.

How many long branch orders occur in Chelicerata? Opposing effects of Palpigradi and Opilioacariformes on phylogenetic stability

Excepting a handful of nodes, phylogenetic relationships between chelicerate orders remain poorly resolved, due to both the incidence of long branch attraction artifacts and the limited sampling of key lineages. It has recently been shown that increasing representation of basal nodes plays an outsized role in resolving the higher-level placement of long-branch chelicerate orders. Two lineages have been consistently undersampled in chelicerate phylogeny. First, sampling of the miniaturized order Palpigradi has been restricted to a fragmentary transcriptome of a single species. Second, sampling of Opilioacariformes, a rarely encountered and key group of Parasitiformes, has been restricted to a single exemplar. These two lineages exhibit dissimilar properties with respect to branch length; Opilioacariformes shows relatively low evolutionary rate compared to other Parasitiformes, whereas Palpigradi possibly acts as another long-branch order (an effect that may be conflated with the degree of missing data). To assess these properties and their effects on tree stability, we constructed a phylogenomic dataset of Chelicerata wherein both lineages were sampled with three terminals, increasing the representation of these taxa per locus. We examined the effect of subsampling phylogenomic matrices using (1) taxon occupancy, (2) evolutionary rate, and (3) a principal components-based approach. We further explored the impact of taxon deletion experiments that mitigate the effect of long branches. Here, we show that Palpigradi constitutes a fourth long-branch chelicerate order (together with Acariformes, Parasitiformes, and Pseudoscorpiones), which further destabilizes the chelicerate backbone topology. By contrast, the slow-evolving Opilioacariformes were consistently recovered within Parasitiformes, with certain subsampling practices recovering their placement as the sister group to the remaining Parasitiformes. Whereas the inclusion of Opilioacariformes always resulted in the non-monophyly of Acari with support, deletion of Opilioacariformes from datasets consistently incurred the monophyly of Acari except in matrices constructed on the basis of evolutionary rate. Our results strongly suggest that Acari is an artifact of long- branch attraction.

Solar-to-Pharmaceutical Raw Material Production: Photoelectrochemical Naphthoquinone Formation Using Stabilized BiVO4 Photoanodes in Acid Media.

In the quest for efficient use of solar energy to produce high-value-added chemicals, we first achieved the photoelectrochemical (PEC) diketonization of naphthalene, using a BiVO4/WO3 photoanode, to obtain naphthoquinone, an important pharmaceutical raw material with excellent efficiency by solar energy conversion. In the electrochemical (EC) reaction using F-doped SnO2 (FTO) substrates and a 0.5 M H2SO4 H2O-acetone (60 vol %) mixed solution containing 5 mM naphthalene, we produced a small amount of naphthoquinone evolution in the dark. However, naphthoquinone (ηNQ)'s Faradic efficiency and its evolution rate at 1.7 VAg/AgCl were only 28.5% and 0.48 μmol·cm-2·h-1, respectively. The PEC reaction using a WO3 photoanode had very low efficiency for naphthalene diketonization, with low ηNQ and evolution rate values at 1.1 VAg/AgCl of 0.3% and 0.039 μmol·cm-2·h-1, respectively. In contrast, the BiVO4/WO3 photoanode strongly enhanced the PEC reaction, and the ηNQ and evolution rates at 1.1 VAg/AgCl were boosted up to 37.5% and 4.7 μmol·cm-2·h-1, respectively. The evolution rate of the PEC reaction in the BiVO4/WO3 photoanode was 10 times higher than that of the EC reaction with the FTO substrate regardless of the very low bias voltage. This result suggests that the BiVO4-based photoanode was very efficient for the selective oxidation of naphthalene even in acid media because of the acetone-mixed electrolyte's anti-photocorrosion effect and the multilayering of WO3 and BiVO4. At a naphthalene concentration of 20 mM, the naphthoquinone evolution rate reached its maximum value of 7.1 μmol·cm-2·h-1. Although ηNQ tended to decrease with the increase in the electric charge, it reached 100% at a low bias voltage of 0.7 VAg/AgCl. An intensity-modulated photocurrent spectroscopy analysis indicated the rate constant of charge transfer at the photoanode surface to the naphthalene molecules was strongly enhanced at a low bias voltage of 0.7-1.1 VAg/AgCl, resulting in the high ηNQ value. The acid-resistant BiVO4/WO3 photoanode functioned in acetone-mixed electrolytes enabled the realization of a new PEC oxidation reaction driven by solar energy to produce high-value-added pharmaceutical raw materials.

A Wilcoxon-Mann-Whitney spatial scan statistic for functional data

A nonparametric scan method for functional data indexed in space is introduced. The associated scan statistic is derived from the Wilcoxon-Mann-Whitney test statistic defined for infinite dimensional data. It is completely nonparametric as it does not assume any distribution concerning the functional marks. Whatever the clustering scenario, this scan test seems to be efficient to detect and locate the cluster. This method is applied to a data set for extracting features in Spanish province population growth. A significant spatial cluster of low demographic evolution rates is found, exhibiting a specific phenomenon in the North-West of Spain.

Novel avian orthoavulavirus 13 in wild migratory waterfowl: biological and genetic considerations.

Avian orthoavulavirus 13 (AOAV-13), formerly known as Avian paramyxovirus 13 (APMV-13), is found scatteredly in wild birds around the world. Although four complete genome sequences of AOAV-13 had been identified since the first discovery in Japan in 2003, the information available on the genetic variation and biological characteristics of AOAV-13 is still limited. In the present study, we isolated six AOAV-13 strains from fecal samples of wild migratory waterfowls during annual (2014-2018) viral surveillance of wild bird populations from wetland and domestic poultry of live bird markets (LBMs) in China. The phylogenetic analyses based on the HN and F genes showed that they had very close relationship and the molecular clock estimations showed a low evolutionary rate of AOAV-13. However, Bean goose/Hubei/V97-1/2015 is 1953nt in size (ORF, 1, 776nt), which is a unique size and longer than other reported AOAV-13 strains. Additionally, four repeats of conserved sequences "AAAAAT" was presented in the 5'-end trailer region of Swan goose/Hubei/VI49-1/2016, which is unprecedented in the AOAV-13. These findings highlight the importance of continuous monitoring the specific species of APMVs.

Shale gas enrichment model and exploration implications in the mountainous complex structural area along the southwestern margin of the Sichuan Basin: A new shale gas area

Marine shale gas in the southern Sichuan Basin is the most successful area of shale gas exploration and development in China. In order to open up new shale gas fields and search for new shale gas reserves and production replacement blocks, it is necessary to continuously establish and complete the standards on shale gas reservoir evaluation and area selection under different structural settings. The early exploration practice shows that shale in the mountainous complex structural area along the southwestern margin of the Sichuan Basin varies greatly in the gas bearing property, so systematical analysis and study on the shale gas enrichment mechanisms in this area is of great significance to searching for new shale gas exploration areas. Based on drilling data of 5 wells in the mountainous complex structural area along the southwestern margin of the Sichuan Basin, the main factors controlling the shale gas bearing property and the shale gas enrichment model were discussed based on the systematical experiments and analysis of the outcrops, cores and cuttings of Longmaxi Formation shale from the aspects of organic geochemistry, physical property, gas content, hydrocarbon generation history and pore evolution history, so as to lay a solid geological basis for the birth of a new shale gas area. And the following research results were obtained. First, the shale of the first submember of first Member of Longmaxi Formation (S1l11) in the mountainous complex structural area along the southwestern margin of the Sichuan Basin has an average TOC of 3.02%–4.97% and Ro of 2.38%–3.37%, and the average total gas content in local enrichment zones is up to 4.62 m3/t, so it is classified as quality shale. Second, the detailed studies on hydrocarbon generation history indicate that the shale has the characteristics of “low thermal evolution rate and low maturity”. It is characterized by late hydrocarbon generation, low thermal evolution rate, low current maturity and short late diffusion time, which are favorable for shale gas enrichment. Third, the pore evolution history reveals that shale pore evolution can be divided into six stages, i.e., sharp pore reduction, pore reduction, organic pore formation, pore preservation, organic pore dissipation and karstification, and organic pore and TOC are the most direct control factors of shale gas content. Fourth, favorable shelf facies belt is conducive to the formation of large-scale reservoir space and effective pores, and diversities of preservation conditions under different structural styles and at different structural positions control different pore evolution stages. The shale gas in the mountainous complex structural area is characterized by horizontal zoning and differential enrichment. “Low thermal evolution rate and low maturity” and “slow structural uplifting” are beneficial to the long-term enrichment and preservation of shale gas. In conclusion, the mountainous complex structural area along the southwestern margin of the Sichuan Basin has a shale gas enrichment model of “sedimentation controlling source rocks, diagenesis controlling reservoirs and structure controlling preservation”. This research result provides idea and reference for searching for new shale gas areas and fields.