Episodic hydrothermal alteration on Middle Permian carbonate reservoirs and its geological significance in southwestern Sichuan Basin,SW China

To analyze the episodic alteration of Middle Permian carbonate reservoirs by complex hydrothermal fluid in southwestern Sichuan Basin,petrology,geochemistry,fluid inclusion and U-Pb dating researches are conducted.The fractures and vugs of Middle Permian Qixia–Maokou formations are filled with multi-stage medium-coarse saddle dolomites and associated hydrothermal minerals,which indicates that the early limestone/dolomite episodic alteration was caused by the large-scale,high-temperature,deep magnesium-rich brine along flowing channels such as basement faults or associated fractures under the tectonic compression and napping during the Indosinian.The time of magnesium-rich hydrothermal activity was from the Middle Triassic to the Late Triassic.The siliceous and calcite fillings were triggered by hydrothermal alteration in the Middle and Late Yanshanian Movement and Himalayan Movement.Hydrothermal dolomitization is controlled by fault,hydrothermal property,flowing channel and surrounding rock lithology,which occur as equilibrium effect of porosity and permeability.The thick massive grainstone/dolomites were mainly altered by modification such as hydrothermal dolomitization/recrystallization,brecciation and fracture-vugs filling.Early thin-medium packstones were mainly altered by dissolution and infilling of fracturing,bedding dolomitization,dissolution and associated mineral fillings.The dissolved vugs and fractures are the main reservoir space under hydrothermal conditions,and the connection of dissolved vugs and network fractures is favorable for forming high-quality dolomite reservoir.Hydrothermal dolomite reservoirs are developed within a range of 1 km near faults,with a thickness of 30–60 m.Hydrothermal dolomite reservoirs with local connected pore/vugs and fractures have exploration potential.

Natural gas exploration potential and favorable targets of Permian Fengcheng Formation in the western Central Depression of Junggar Basin,NW China

Based on the organic geochemical data and the molecular and stable carbon isotopic compositions of natural gas of the Lower Permian Fengcheng Formation in the western Central Depression of Junggar Basin,combined with sedimentary environment analysis and hydrocarbon-generating simulation,the gas-generating potential of the Fengcheng source rock is evaluated,the distribution of large-scale effective source kitchen is described,the genetic types of natural gas are clarified,and four types of favorable exploration targets are selected.The results show that:(1)The Fengcheng Formation is a set of oil-prone source rocks,and the retained liquid hydrocarbon is conducive to late cracking into gas,with characteristics of high gas-generating potential and late accumulation;(2)The maximum thickness of Fengcheng source rock reaches 900 m.The source rock has entered the main gas-generating stage in Penyijingxi and Shawan sags,and the area with gas-generating intensity greater than 20×10^(8) m^(3)/km^(2) is approximately 6500 km^(2).(3)Around the western Central Depression,highly mature oil-type gas with light carbon isotope composition was identified to be derived from the Fengcheng source rocks mainly,while the rest was coal-derived gas from the Carboniferous source rock;(4)Four types of favorable exploration targets with exploration potential were developed in the western Central Depression which are structural traps neighboring to the source,stratigraphic traps neighboring to the source,shale-gas type within the source,and structural traps within the source.Great attention should be paid to these targets.

A review of methane leakage from abandoned oil and gas wells:A case study in Lubbock,Texas,within the Permian Basin

In the pursuit of global net zero carbon emissions and climate change mitigation,ongoing research into sustainable energy sources and emission control is paramount.This review examines methane leakage from abandoned oil and gas(AOG)wells,focusing particularly on Lubbock,a geographic area situated within the larger region known as the Permian Basin in West Texas,United States.The objective is to assess the extent and environmental implications of methane leakage from these wells.The analysis integrates pertinent literature,governmental and industry data,and prior Lubbock reports.Factors affecting methane leakage,including well integrity,geological characteristics,and human activities,are explored.Our research estimates 1781 drilled wells in Lubbock,forming a foundation for targeted assessments and monitoring due to historical drilling trends.The hierarchy of well statuses in Lubbock highlights the prevalence of“active oil wells,”trailed by“plugged and abandoned oil wells”and“inactive oil wells.”Methane leakage potential aligns with these well types,underscoring the importance of strategic monitoring and mitigation.The analysis notes a zenith in“drilled and completed”wells during 1980-1990.While our study's case analysis and literature review reiterate the critical significance of assessing and mitigating methane emissions from AOG wells,it's important to clarify that the research does not directly provide methane leakage data.Instead,it contextualizes the issue's magnitude and emphasizes the well type and status analysis's role in targeted mitigation efforts.In summary,our research deepens our understanding of methane leakage,aiding informed decision-making and policy formulation for environmental preservation.By clarifying well type implications and historical drilling patterns,we aim to contribute to effective strategies in mitigating methane emissions from AOG wells.

Provenance of Conglomerate and Sandstone from Early Permian Shoushangou Formation in Xi Ujimqin,Inner Mongolia:Implications for Understanding Paleo-Asian Ocean Subduction

During the Late Carboniferous to Early Permian,a rift was formed by post-collisional extension after ocean closure or an island arc-related basin formed by Paleo-Asian Ocean(PAO)subduction in the Xi Ujimqin area.Nevertheless,the closure time of the PAO is still under debate.Thus,to identify the origin of the PAO,the geochemistry and U-Pb age of zircons were analyzed for the extra-large deep marine,polymict clastic boulders and sandstones in the Shoushangou Formation within the basin.The analyses revealed magmatic activity and tectonic evolution.The conglomerates include megaclasts of granite(298.8±9.1 Ma)and granodiorite porphyry(297.1±3.1 Ma),which were deposited by muddy debris flow.Results of this study demonstrated that the boulders of granitoids have the geochemistry of typical I-type granite,characterized by low Zr+Nb+Ce+Y and low Ga/Al values.The granitoid boulders were formed in island arc setting,indicating the presence of arc magmatism in the area that is composed of the Late Carboniferous to Early Permian subduction-related granitoid in southern Xi Ujimqin.Multiple diagrams for determining sedimentary provenance using major and trace elements indicate that Shoushangou sediments originated from continental island arc-related felsic rocks.Detrital zircon U-Pb age cluster of 330–280 Ma was obtained,indicating input from granite,ophiolite,Xilin Gol complex,and Carboniferous sources to the south.The basin was geographically developed behind the arc during the Early Permian period because the outcropped intrusive rocks in the Late Carboniferous to Early Permian form a volcanic arc.The comprehensive analyses of source areas suggest that Shoushangou sediments developed in a backarc basin in response to the northward subduction of the PAO.The backarc basin and intrusive rocks,in addition to previously published Late Carboniferous to Early Permian magmatic rocks of arc unit in Xilin Gol,confirm the presence of an Early Permian trencharc-basin system in the region,represented by the Baolidao arc and Xi Ujimqin backarc basin.This study highlights the importance and potential of combined geochemical and geochronological studies of conglomerates and sandstone for reconstructing the geodynamic setting of a basin.

Diagenetic history and porosity evolution of the Middle Permian clastic-carbonate mixed system,Indus Basin,Pakistan:Implications for reservoir development

This study deals with unraveling the diagenesis-induced porosity evolution in a mixed clastic-carbonate sequence of the Middle Permian Indus Basin,Pakistan.Multiple data sets including outcrop,petrography,cathodoluminescence,scanning electron microscopy(SEM),mineralogy,and geochemical isotopic compositions were integrated to establish a link between porosity evolution and diagenesis.The spatial thickness and facies variations of the strata at outcrop scale are inherently controlled by the underlying bathymetry of the basin with deepening westward trend.The low values ofδ^(18)O of the target strata,relative to average values of the Permian carbonate,hints to diagenetic alteration in the strata.The data sets used in this study reveal modification of the strata in four environments,that is,i)early marine diagenesis indicated by micritization,pervasive dolomitization and isopachous fibrous cements,followed by ii)meteoric dissolution,and iii)shallow burial diagenetic processes including the precipitation of blocky cement,compaction of skeletal and non-skeletal allochems,and stylolites,and iv)a deep burial environment,characterized by pressure solution,and micro-fractures.The clastic intervals host subangular to subrounded quartz grains,floating textures,and almost complete absence of deleterious clay minerals,consequently resulting in the preservation of primary porosity.The primary porosity of carbonate intervals is preserved in the form of intercrystalline and intracrystalline porosity.The secondary porosity evolved through various diagenetic phases in the form of fractures and dissolution.The diagenetic solution mediated by organic matter in carbonates may have experienced both bacterial decomposition and thermochemical sulfate reduction,precipitating sulfides within the pores.The plug porosity/permeability analyses generally suggest high porosity in the siliciclastic unit,and carbonates with wackestone fabric while lower values were observed for the inner shelf pure carbonate facies.However,both intervals show very low permeability values probably due to isolated moldic pores and intense micritization.Therefore,clastic intervals may provide an opportunity to serve as a moderate reservoir;however,the carbonate intervals possess very low permeability values and could generally be considered as low-moderate reservoir potential.

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs:A case study of Middle Permian Qixia Formation in Gaoshiti-Moxi area of Sichuan Basin,SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin,SW China,the controlling factors of reservoir development were analyzed,and the formation model of“intra-platform shoal thin-layer dolomite reservoir”was established.The Qixia Formation is a regressive cycle from bottom to top,in which the first member(Qi1 Member)develops low-energy open sea microfacies,and the second member(Qi2 Member)evolves into intra-platform shoal and inter-shoal sea with decreases in sea level.The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member.The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite,followed by vuggy dolomite.The semi-filled saddle dolomite is common in fracture-vug,and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network.Based on the coupling analysis of sedimentary and diagenesis characteristics,the reservoir formation model of“pre-depositional micro-paleogeomorphology controlling shoal,sedimentary shoal controlling dolomite,penecontemporaneous dolomite benefiting preservation of pores,and late hydrothermal action effectively improving reservoir quality”was systematically established.The“first-order high zone”micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member.Shoal facies is the basic condition of early dolomitization,and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent.The grain limestone of shoal facies is transformed by two stages of dolomitization.The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores.The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event,makes it recrystallized into medium–coarse crystal dolomite.With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures,the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed.The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Types,composition and diagenetic evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata:A case study of Mao-1 Member of Middle Permian Maokou Formation,eastern Sichuan Basin,SW China

The types,occurrence and composition of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata of the first member of the Middle Permian Maokou Formation(Mao-1 Member)in eastern Sichuan Basin were investigated through outcrop section measurement,core observation,thin section identification,argon ion polishing,X-ray diffraction,scanning electron microscope,energy spectrum analysis and laser ablation-inductively coupled plasma-mass spectrometry.The diagenetic evolution sequence of clay minerals was clarified,and the sedimentary-diagenetic evolution model of clay minerals was established.The results show that authigenic sepiolite minerals were precipitated in the Si4+and Mg2+-rich cool aragonite sea and sepiolite-bearing strata were formed in the Mao-1 Member.During burial diagenesis,authigenic clay minerals undergo two possible evolution sequences.First,from the early diagenetic stage A to the middle diagenetic stage A1,the sepiolite kept stable in the shallow-buried environment lack of Al3+.It began to transform into stevensite in the middle diagenetic stage A2,and then evolved into disordered talc in the middle diagenetic stage B1and finally into talc in the period from the middle diagenetic stage B2to the late diagenetic stage.Thus,the primary diagenetic evolution sequence of authigenic clay minerals,i.e.sepiolite-stevensite-disordered talc-talc,was formed in the Mao-1 Member.Second,in the early diagenetic stage A,as Al3+carried by the storm and upwelling currents was involved in the diagenetic process,trace of sepiolite started to evolve into smectite,and a part of smectite turned into chlorite.From the early diagenetic stage B to the middle diagenesis stage A1,a part of smectite evolved to illite/smectite mixed layer(I/S).The I/S evolved initially into illite from the middle diagenesis stage A2to the middle diagenesis stage B2,and then totally into illite in the late diagenesis stage.Thus,the secondary diagenetic evolution sequence of authigenic clay minerals,i.e.sepiolite-smectite-chlorite/illite,was formed in the Mao-1 Member.The types and evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata are significant for petroleum geology in two aspects.First,sepiolite can adsorb and accumulate a large amount of organic matters,thereby effectively improving the quality and hydrocarbon generation potential of the source rocks of the Mao-1 Member.Second,the evolution from sepiolite to talc is accompanied by the formation of numerous organic matter pores and clay shrinkage pores/fractures,as well as the releasing of the Mg2+-rich diagenetic fluid,which allows for the dolomitization of limestone within or around the sag.As a result,the new assemblages of self-generation and self-accumulation,and lower/side source and upper/lateral reservoir,are created in the Middle Permian,enhancing the hydrocarbon accumulation efficiency.

Influences of lithofacies on fluid mobility in mixed sedimentary rocks:Insights from NMR analysis of the middle Permian Lucaogou Formation,Junggar Basin

The multi-source mixed sedimentation resulted in a unique series of mixed fine-grained sedimentary rocks evolved within the Permian Lucaogou Formation in the Jimusar Sag,located in the southeastern Junggar Basin,China.The variety of lithofacies within this series resulted in pronounced heterogeneity of pore structures,complicating the analysis of fluid occurrence space and state within reservoirs.As a result,the impact of lithofacies on fluid mobility remains ambiguous.In this study,we employed qualitative methods,such as field emission scanning electron microscopy(FE-SEM)and thin section observation,and quantitative analyses,including X-ray diffraction(XRD),total organic carbon(TOC),vitrinite reflectance(Ro),high-pressure mercury intrusion(HPMI)porosimetry,and nuclear magnetic resonance(NMR),along with linear and grey correlation analyses.This approach helped delineate the effective pore characteristics and principal factors influencing movable fluids in the fine-grained mixed rocks of the Lucaogou Formation in the Jimusar Sag,Junggar Basin.The findings indicate the development of three fundamental lithologies within the Lucaogou Formation:fine sandstone,siltstone,and mudstone.Siltstones exhibit the highest movable fluid saturation(MFS),followed by fine sandstones and mudstones sequentially.Fluid mobility is predominantly governed by the content of brittle minerals,the sorting coefficient(Sc),effective pore connectivity(EPC),and the fractal dimension(D_(2)).High content of brittle minerals favors the preservation of intergranular pores and the generation of microcracks,thus offering more occurrence space for movable fluids.A moderate Sc indicates the presence of larger connecting throats between pores,enhancing fluid mobility.Elevated EPC suggests more interconnected pore throat spaces,facilitating fluid movement.A higher D_(2)implies a more intricate effective pore structure,increasing the surface area of the rough pores and thereby impeding fluid mobility.Ultimately,this study developed a conceptual model that illustrates fluid distribution patterns across different reservoirs in the Lucaogou Formation,incorporating sedimentary contexts.This model also serves as a theoretical framework for assessing fluid mobility and devising engineering strategies for hydrocarbon exploitation in mixed fine-grained sedimentary rocks.

Paleoenvironment of marine-continental transitional shales in the lower Permian Shanxi formation,southeastern Ordos Basin,China

The paleoenvironment of shales can be reconstructed to some extent using the combinations or concentrations of elements that correlate strongly with environmental conditions.In this study,we analyzed rare earth elements(REEs),major elements,and trace elements in the marine-continental transitional shales(transitional shales for short)of the Shan 2^(3)submember of the Shanxi Formation in the southeastern Ordos Basin.The purpose is to deduce the paleoenvironmental conditions of the shales,encompassing paleoredox,paleoclimate,paleoproductivity,and paleo-provenance.The Shan 2^(3)submember comprises four sections,namely Shan 2^(3)-1,Shan 2^(3)-2,Shan 2^(3)-3,and Shan 2^(3)-4.The Ba/Al,P/Al,and Cu/Al ratios,along with biogenic barium(Babio),indicate that the paleoproductivity of the submember peaked during the Shan 2^(3)-1 deposition and exhibited a downtrend upward in other sections.Trends in the Uau and the Ni/Co,V/Cr,U/Th,and V/Sc ratios suggest that suboxic conditions prevailed during the Shan 2^(3)-1 deposition,with the oxidation level gradually increasing from Shan 2^(3)-1 to Shan 2^(3)-4.C-value and the Sr/Cu vs.Ga/Rb cross-plot indicate a warm and arid paleoclimate during the Shan 2^(3)-1 deposition,which transitioned to cooler,drier conditions during the deposition of other sections.Indicators sensitive to paleoclimate,such as the K/Rb and Th/U ratios,along with the ICV,PIA,and Chemical Index of Alteration(CIA),highlight elevated weathering from Shan 2^(3)-2 to Shan 2^(3)-4,with Shan 2^(3)-1 exhibiting the weakest weathering during its deposition.As suggested by the REE data,the Zr/Sc vs.Th/Sr cross-plot,provenance discriminant functions,and the cross-plots of Hf vs.La/Th,Th vs.Hf-Co,and∑REE vs.La/Yb,the sedimentary provenance for the transitional shales of the Shan 2^(3)submember is of multiple origins,with significant contributions from the Upper Continental Crust(UCC).Discriminant diagrams,including those of Th-Co-Zr/10,Th-Sc-Zr/10,La-Th-Sc,and K_(2)O/Na_(2)O vs.SiO_(2),suggest that the transitional shales of the Shan 2^(3)submember were primarily deposited under tectonic settings such as continental island arcs(CIAs)and passive continental margins(PCMs).

Geochemical Characteristics of the Upper Permian Longtan Formation from Northeastern Sichuan Basin:Implications for the Depositional Environment and Organic Matter Enrichment

A set of organic-rich shales of the upper Permian Longtan Formation,which is widely developed in the northeastern part of the Sichuan Basin,is a key formation for the next step of exploration and development.At present,most studies on this set of formations have focused on the reservoir characteristics and reservoir formation mechanism of the shales,and basic studies on the palaeoenvironment and organic matter(OM)enrichment mechanism have not been fully carried out.In this paper,we recovered the sedimentary palaeoenvironment by mineralogical,elemental geochemical and organic geochemical analyses,and explored the enrichment mechanism of OM under the constraints of palaeoenvironmental evolution.The shales can be divided into two stages of sedimentary evolution:compared with the shales of the Lower Longtan Formation,the shales of the Upper Longtan Formation are relatively rich in quartz,poor in clay and carbonate minerals,and the OM type changes from typeⅢto typeⅡ_(2).The depositional environment has undergone a change from sea level rise,from warm and wet climate to dry and cold climate,and from oxygen-poor condition restricted to open reduction environment;the land source input has decreased,the siliceous mineral content has increased,the biological productivity has improved,and the deposition rate has changed from high to low.A depositional model was established for the shales of the Longtan Formation,reflecting the differential reservoir formation pattern of organic matter.For the Lower Longtan Formation shales,the most important factors controlling OM content are terrestrial source input and deposition rate,followed by paleoclimate and paleooxygen conditions.For the Upper Longtan Formation shales,the most important controlling factor is paleo-productivity,followed by sedimentation rate.The depositional model constructed for the Upper and Lower Longtan Formation shales can reproduce the enrichment of organic matter and provide a basis for later exploration and development.

Controlling factors and models of shale oil enrichment in Lower Permian Fengcheng Formation,Mahu Sag,Junggar Basin,NW China

Based on the combination of core observation,experimental analysis and testingand geological analysis,the main controlling factors of shale oil enrichment in the Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin are clarified,and a shale oil enrichment model is established.The results show that the enrichment of shale oil in the Fengcheng Formation in the Mahu Sag is controlled by the organic abundance,organic type,reservoir capacity and the amount of migration hydrocarbon in shale.The abundance of organic matter provides the material basis for shale oil enrichment,and the shales containing typesⅠandⅡorganic matters have good oil content.The reservoir capacity controls shale oil enrichment.Macropores are the main space for shale oil enrichment in the Fengcheng Formation,and pore size and fracture scale directly control the degree of shale oil enrichment.The migration of hydrocarbons in shale affects shale oil enrichment.The shale that has expelled hydrocarbons has poor oil content,while the shale that has received hydrocarbons migrated from other strata has good oil content.Lithofacies reflect the hydrocarbon generation and storage capacity comprehensively.The laminated felsic shale,laminated lime-dolomitic shale and thick-layered felsic shale have good oil content,and they are favorable lithofacies for shale oil enrichment.Under the control of these factors,relative migration of hydrocarbons occurred within the Fengcheng shale,which leads to the the difference in the enrichment process of shale oil.Accordingly,the enrichment mode of shale oil in Fengcheng Formation is established as"in-situ enrichment"and"migration enrichment".By superimposing favorable lithofacies and main controlling factors of enrichment,the sweet spot of shale oil in the Fengcheng Formation can be selected which has great significance for the exploration and development of shale oil.

Petroleum geology and sub-source hydrocarbon accumulation of Permian reservoirs in Jinan Sag,eastern Junggar Basin,NW China

According to the latest drilling and the analysis of the burial history,source rock evolution history and hydrocarbon accumulation history,the sub-source hydrocarbon accumulation characteristics of the Permian reservoirs in the Jinan Sag,eastern Junggar Basin,are clarified,and the hydrocarbon accumulation model of these reservoirs is established.The results are obtained in four aspects.First,the main body of the thick salified lake basin source rocks in the Lucaogou Formation has reached the mature stage with abundant resource base.Large-scale reservoirs are developed in the Jingjingzigou,Wutonggou and Lucaogou formations.Vertically,there are multiple sets of good regional seals,the source-reservoir-caprock assemblage is good,and there are three reservoir-forming assemblages:sub-source,intra-source and above-source.Second,dissolution,hydrocarbon charging and pore-preserving effect,and presence of chlorite film effectively increase the sub-source pore space.Oil charging is earlier than the time when the reservoir becomes densified,which improves the efficiency of hydrocarbon accumulation.Third,buoyancy and source-reservoir pressure difference together constitute the driving force of oil charging,and the micro-faults within the formation give the advantage of"source-reservoir lateral docking"under the source rock.Microfractures can be critical channels for efficient seepage and continuous charging of oil in different periods.Fourth,the Jingjingzigou Formation experienced three periods of oil accumulation in the Middle-Late Permian,Middle-Late Jurassic and Late Neogene,with the characteristics of long-distance migration and accumulation in early stage,mixed charging and accumulation in middle stage and short-distance migration and high-position accumulation in late stage.The discovery and theoretical understanding of the Permian reservoirs in the Jinan Sag reveal that the thrust belt has good conditions for forming large reservoirs,and it is promising for exploration.The study results are of guidance and reference significance for oil and gas exploration in the Jinan Sag and other geologically similar areas.

Evolution characteristics and exploration targets of Permian clastic rock reservoirs in Bohai Bay Basin,East China

Based on core observation, thin section examination, fluid inclusions analysis, carbon and oxygen isotopic composition analysis, and other approaches, the structural and burial evolution histories were investigated, and the diagenetic evolution process and genetic/development models were systematically discussed of the Upper Paleozoic Permian clastic rock reservoirs in the Bohai Bay Basin, East China. The Bohai Bay Basin underwent three stages of burial and two stages of uplifting in the Upper Paleozoic. Consequently, three stages of acid dissolution generated by the thermal evolution of kerogen, and two stages of meteoric freshwater leaching occurred. Dissolution in deeply buried, nearly closed diagenetic system was associated with the precipitation of authigenic clay and quartz, leading to a limited increase in storage space. Different structural uplifting–subsidence processes of tectonic zones resulted in varying diagenetic–reservoir-forming processes of the Permian clastic reservoirs. Three genetic models of reservoirs are recognized. The Model I reservoirs with pores formed in shallow strata and buried in shallow to medium strata underwent two stages of exposure to long-term open environment and two stages of meteoric freshwater leaching to enhance pores near the surface, and were shallowly buried in the late stage, exhibiting the dominance of secondary pores and the best physical properties. The Model Ⅱ reservoirs with pores formed in shallow strata and preserved due to modification after deep burial experienced an early exposure-open to late burial-closed environment, where pore types were modified due to dissolution, exhibiting the dominance of numerous secondary solution pores in feldspar and the physical properties inferior to Model I. The Model Ⅲ reservoirs with pores formed after being regulated after multiple periods of burial and dissolution experienced a dissolution of acidic fluids of organic origin under a near-closed to closed environment, exhibiting the dominance of intercrystalline micropores in kaolinite and the poorest physical properties. The target reservoirs lied in the waterflood area in the geological period of meteoric freshwater leaching, and are now the Model Ⅱ deep reservoirs in the slope zone–depression zone. They are determined as favorable options for subsequent exploration.

Sedimentary architecture and distribution of intra-platform shoal in sequence framework of Permian Changxing Formation in central Sichuan Basin,SW China

Based on the comprehensive analysis of core, thin section, logging and seismic data, this study carried out the identification and comparison of Permian Changxing Formation sequences, clarified the typical sedimentary architectures of intra-platform shoal, investigated the vertical and horizontal development and distribution of intra-platform shoal in each sequence, and thus established the sedimentary evolution model of shoal body. The study results are reflected in four aspects.First, there are two complete third-order sequences(SQ1 and SQ2) in Changxing Formation in central Sichuan Basin. SQ1 is generally thick in the north and thin in the south, and SQ2 shows a thickness differentiation trend of “two thicknesses and three thinnesses”. Second, the Changxing Formation in central Sichuan Basin mainly develops intra-platform shoal, inter-shoal sea and intra-platform depression subfacies. In the vertical direction, the intra-platform shoal mainly presents two typical sedimentary sequences: stable superposed and high-frequency interbedded. Third, the stable superimposed sedimentary sequence is developed in the shoal belt at the edge of intra-platform depression, which is composed of two shoal-forming periods and located in the highstand systems tracts(HSTs) of SQ1 and SQ2. The high-frequency interbedded sedimentary sequence is developed in the southern shoal belt of intra-platform depression, which is composed of four shoal-forming periods and mainly located in the HST of SQ2. Fourth, during the SQ1 deposition, the intra-platform shoal was mainly developed at the edge of the intra-platform depression on the north side of the study area, and the inter-shoal sea subfacies was mainly developed on the south side. During the SQ2 deposition, the intra-platform shoal was widely developed in the area, forming two nearly parallel intra-platform shoal belts. The study results provide direction and ideas for exploration of Changxing Formation intra-platform shoal reservoirs in central Sichuan Basin.

Control of micro-wettability of pore-throat on shale oil occurrence:A case study of laminated shale of Permian Lucaogou Formation in Jimusar Sag,Junggar Basin,NW China

The control of micro-wettability of pore-throat on shale oil occurrence in different types of reservoir spaces remains unclear.Take the shale oil reservoir of the Permian Lucaogou Formation in the Jimusar Sag,Junggar Basin as an example,the reservoir space in laminated shale and the control of micro-wettability of pore-throat on shale oil occurrence were studied by using scanning electron microscope(SEM),multi-stage pyrolysis,quantitative fluorescence,nuclear magnetic resonance(NMR)and other techniques.The results show that there are mainly two types of laminated shale in the Lucaogou Formation,namely laminated shale rich in volcanic materials+terrigenous felsic,and laminated shale rich in volcanic materials+carbonate.The former type contains feldspar dissolution pores and intergranular pores,mainly with felsic mineral components around the pore-throats,which are water-wet and control the free shale oil.The latter type contains carbonate intercrystalline pores and organic pores,mainly with oil-wet mineral components around the pore-throats,which control the adsorbed shale oil.The oil-wet mineral components around the pore-throats are conducive to oil accumulation,but reduce the proportion of free oil.In the Lucaogou Formation,free oil,with high maturity and light quality,mainly occurs in the laminated shale rich in volcanic materials+terrigenous felsic.

Genesis of granular calcite in lacustrine fine-grained sedimentary rocks and its indication to volcanichydrothermal events: A case study of Permian Lucaogou Formation in Jimusar Sag, Junggar Basin, NW China

Granular calcite is an authigenic mineral in fine-grained sedimentary rocks.Core observation,thin section observation,cathodoluminescence analysis,fluid inclusion analysis,scanning electron microscope(SEM),and isotopic composition analysis were combined to clarify the genesis of granular calcite in the lacustrine fine-grained sedimentary rocks of the Permian Lucaogou Formation in the Jimusar Sag,Junggar Basin.It is found that the granular calcite is distributed with laminated characteristics in fine-grained sedimentary rocks in tuffite zones(or the transitional zone between tuffite and micritic dolomite).Granular calcite has obvious cathodoluminesence band,and it can be divided into three stages.Stage-Ⅰ calcite,with non-luminesence,high content of Sr element,inclusions containing Cos,and homogenization temperature higher than 170℃,was directly formed from the volcanic-hydrothermal deposition.Stage-Ⅱ calcite,with bright yellow luminescence,high contents of Fe,Mn and Mg,enrichment of light rare earth elements(LREEs),and high homogenization temperature,was formed by recrystallization of calcareous edges from exhalative hydrothermal deposition.Stage-IlI calcite,with dark orange luminescence band,high contents of Mg,P,V and other elements,no obvious fractionation among LREEs,and low homogenization temperature,was originated from diagenetic transformation during burial.The granular calcite appears regularly in the vertical direction and its formation temperature decreases from the center to the margin of particles,providing direct evidences for volcanic-hydrothermal events during the deposition of the Lucaogou Formation.The volcanic-hydrothermal event was conducive to the enrichment of organic matters in fine-grained sedimentary rocks of the Lucaogrou Formation,and positive to the development of high-quality source rocks.The volcanic-hydrothermal sediments might generate intergranular pores/fractures during the evolution,creating conditions for the self-generation and self-storage of shale oil.

Origin of dolomites in the Permian dolomitic reservoirs of Fengcheng Formation in Mahu Sag,Junggar Basin, NW China

Origin of authigenic dolomites in the dolomitic reservoir of the Permian Fengcheng Formation in the Mahu Sag of Junggar Basin is unclear.Occurrence and genetic evolution of the authigenic dolomites in dolomitic rock reservoir of the Fengcheng Formation in the Mahu Sag were analyzed by polarized and fluorescence thin sections,scanning electron microscope(SEM),electron microprobe(EMP),C,O and Sr isotopes analysis,and other techniques.(1)Dolomites were mainly precipitated in three stages:penecontemporaneous-shallow burial stage(early stage of the Middle Permian),middle burial stage(middle stage of the Middle Permian),and middle-deep burial stage,with the former two stages in dominance.(2)Dolomitization fluid was high-salinity brine originating from alkaline lake.In the penecontemporaneous-shallow burial stage,Mg^(2+)was mainly supplied by alkaline-lake fluid and devitrification of volcanic glass.In the middle burial stage,Mg^(2+)mainly came from the transformation of clay minerals,devitrification of volcanic glass and dissolution of aluminosilicates such as feldspar.(3)Regular changes of Mg,Mn,Fe,Sr,Si and other elements during the growth of dolomite were mainly related to the alkaline-lake fluid,and to different influences of devitrification and diagenetic alteration of volcanic materials during the burial.(4)In the penecontemporaneous stage,induced by alkaline-lake microorganisms,the micritic-microcrystalline dolomites were formed by primary precipitation,replacement of aragonite and high-Mg calcite,and other processes;in the shallow burial stage,the silt-sized dolomites were formed by continuous growth of micritic-microcrystalline dolomite and replacement of calcites,tuffs and other substances;in the middle burial stage,the dolomites,mainly silt-and fine-sized,were formed by replacement of volcanic materials.The research results are referential for investigating the formation mechanism and distribution patterns of tight dolomitic reservoirs in the Mahu Sag and other similar oil and gas bearing areas.

Biodegradation of occluded hydrocarbons and kerogen macromolecules of the Permian Lucaogou shales,Junggar Basin,NW China

Three kerogen samples(JJZG-1,JJZG-2 and JJZG-3)isolated from the Permian Lucaogou shales of varying biodegradation levels(BLs≈0,3 and 7,respectively)were subjected to sequential stepwise pyrolysis combined with on-line detection of gas chromatography-mass spectrometry(GC-MS).Occluded fractions(bitumenⅡ)released at low-temperature steps(≥410℃)show consistent biodegradative signatures with that reported for solvent-extracted fractions(bitumenⅠ)of the original shales,e.g.,broad range of abundant n-alkanes,isoprenoids and regular hopanes for the non-biodegraded JJZG-1;trace n-alkanes and abundant hopanes for the moderately biodegraded JJZG-2;and no n-alkanes but still prominent hopanes including the microbially produced 25-nohopanes for the severely biodegraded JJZG-3.This consistency between bitumenⅡand bitumenⅠfractions indicates the biodegradability of the kerogenoccluded bitumenⅡwith limited protection from host kerogen.A minor level of protection was suggested by the trace distribution of n-alkanes in the bitumenⅡof JJZG-2,whereas the bitumenⅠhad no nalkanes.The kerogen itself was more resistant to biodegradation as reflected by the persistence of high abundances of both n-alkanes and hopanes in the high temperature(≥460℃)products of all three kerogen samples.However,the relative abundances of these product groups did show some evidence of biodegradation alteration,e.g.,ratios of n-C_(15)alkene/C_(27)hop-17(21)-ene at 510℃pyrolysis decreased by order of magnitude from the non-biodegraded(JJZG-1=27.4)to highly biodegraded(0.3 for JJZG-3)samples.The reduced biodegradation impact on the kerogen fraction(Cf.bitumen fractions)was also evident by the absence of 25-norhopanes in the high-temperature analysis of the JJZG-3 kerogen.

A New Gleicheniaceous Fern Chansitheca (Sphenopteris) wudaensis from the Lower Permian of Wuda, Nei Mongol, China

Chansitheca (Sphenopteris) wudaensis sp. nov. is described based on the materials collected from the Lower Permian Shansi Formation of Wuda, Nei Mongol, China. The frond is small and at least two times pinnate. The pinnule is sphenopteroid. The sori are borne on the abaxial surface of the pinnule, arranged in two rows parallel to the midvein, elliptic,and composed of 4-10 round sporangia. The indusium is absent. The annulus is transverse and complete, and consists of about 18 thickened cells. The features of the fertile parts, sori, sporangia and annulus and the absence of indusium indicate that the new species belongs to the Gleicheniaceae.

A New Species of Araucarioxylon Kraus from the Early Early Permian, Nei Mongol, China

A kind of silicified fossil wood with mixed pits on the radial tracheid wall is described. The fossil wood was collected from the top of Taiyuan Formation (early Early Permian) in Wuda Mining District, Nei Mongol. Compared with the Paleozoic fossil woods in the world, it is put into Araucarioxylon Kraus and named as A. laoshidanense sp. nov. Based on the character of possessing mixed pittings (alternate and opposite pittings) on the radial tracheid wall, the fossil wood is believed to be one of the unknown primitive conifers.Diagnosis of the new species: Only secondary xylem preserved and consisting of axial tracheids and rays. Growth ring boundary, resin duct and axial parenchyma absent. One to Four (commonly 2 or 3) seriates of bordered pits (mostly alternate but sometimes opposite) on the radial tracheid wall. One to Four (commonly 1, rarely 2 to 4) Cupressoid pits in each cross_field. Rays usually uniseriate, sometimes partly_ biseriate and 2 to 39 (mainly 3-5) cells high.