High-quality reservoirs of the Feixianguan Formation of the northeast Sichuan Basin contain abundant natural gas and are dominated by dolostones. Their origin is controversial, which impedes the exploration of these high-quality carbonate reservoirs. Petrographic observation and geochemical analysis were performed on various carbonates to trace the source of dolomitizing fluids and clarify the formation mechanism of the Feixianguan dolostones. Three types of dolostones were distinguished, including fine dolomite, fine-medium dolomite, and medium dolomite according to the grain size of dolomites. Fine dolomite was characterized by crystal sizes of 5-60 mu m and high total rare earth elements (sigma REEs) of 1.87-2.45 ppm. The thin layers of fine dolomite mainly occur at the top of the depositional cycles and have the highest delta 18O values and the lowest SmSN/YbSN values of 0.65-0.87 of the three types, suggesting that they were formed by evaporation pumping dolomitization from the restricted evaporated seawater. Fine-medium dolomite was characterized by the inheritance of the original particle texture with crystal sizes of 60-120 mu m. Medium dolomite was characterized by crystal sizes of 100-200 mu m and the highest porosity among the three types of dolostones. The similar REE patterns to seawater and the same 87Sr/86Sr ratios as Feixianguan seawater for fine-medium and medium dolomites indicate that the dolo-mitizing fluids originated from coeval seawater with the Feixianguan limestone. Both the high concentrations of Na and low concentrations of Fe and Mn imply the shallow burial or near surface condition of dolomitization. According to higher delta 13C and delta 18O values than coeval limestones, and the descending tendency of delta 18O values and Na concentrations from the top to bottom of the sedimentary cycles, fine-medium and medium dolomites were formed by reflux dolomitization of evaporated Feixianguan seawater restricted by beach bridges. However, slightly brighter cathodoluminescence of medium dolomite than fine-medium dolomite and the decreasing of delta 18O values as the crystal size increasing imply that medium dolomites were slightly recrystallized from fine-medium dolomites. Moreover, a small number of coarse dolomites in pores are derived from exogenous non-seawater fluids in the burial environment.

Microbialites are important reservoirs for oil and gas. The mineralization of organic matter in microbialites during early diagenesis can produce acidic fluids that dissolve carbonate grains, and can also result in an alkaline pore water that precipitates cement. The mineralization of organic matter in microbialites and its effect on microbialite reservoirs have not yet been studied in detail. In this study, quantitative statistical analysis of the two-dimensional spatial occurrence of pores and microbial fabrics, in situ geochemical analysis of specific components (microbial, transitional zone, and fine spar fabrics), and qualitative evaluation of the implications for microbialite reservoirs were undertaken on microbialites from Member IV of the Leikoupo Formation, Sichuan Basin, China. The quantitative statistical analysis shows that pores are spatially associated with microbial fabrics, but porosity has a poor correlation with microbial fabric content. In situ geochemical data indicate that microbialites with different porosities experienced different processes of organic matter mineralization. The processes of organic matter mineralization such as oxidation and nitrate reduction can provide more dissolution micropores than the process related with sulfate reduction, whereas the process of organic matter mineralization related with Fe–Mn oxide reduction results in cementation. Micropores created by organic matter mineralization can act as fluid channels for later dissolution and are important in the development of microbialite reservoirs.

The study on genesis of dolomitization in different parts of oolites plays an important role in revealing the forming process and diagenetic evolution. Based on the field section observation and sampling for the second member of Lower Triassic Feixianguan Formation in the Shangba section of Yunyang, Chongqing, the location, degree and genesis of oolite dolomitization were analyzed by using thin section, cathodoluminescence, inclusions, C/O stable isotopes, and trace elements. The research results are as follows:（1）The dolomitized oolites include complete and incomplete ones, and the latter can be subdivided into outershell-dolomitized oolites,core-dolomitized oolites, and outershell-core-dolomitized oolites.（2） In layers 1–7 of the Shangba section, there develops completely dolomitized oolites, core-dolomitized oolites and outershell-dolomitized oolites. The internal dolomites are characterized by low contents of Mn, Fe and Sr, low δ 13 C and δ 18 O, and weak to very weak maroon cathodoluminescence, while intergranular calcites are nonluminous, indicating that the dolomitization occurred in the penecontemporaneous period, dominated by infiltration and reflux.（3） The core-dolomitized oolites in layer 4 are characterized by abnormally low Mn and Fe contents, high Sr, low δ 13 C and δ 18 O, deformation and fragmentation, and increased content of intergranular mud, indicating that the thermal expansion in the exposed environment gave rise to the fragmented oolites.（4） The dolomites in completely dolomitized oolites in layers 8–12 show relatively high Fe and Mn, relatively low Sr, enlarged bright edges, high internal inclusion temperature（90℃–120℃ and 130℃–140℃）, negative biases of δ 13 C and δ 18 O, and filling of intergranular dolomites, indicating that the dolomites were strongly buried dolomitized.（5） Both completely and incompletely dolomitized oolites were dolomitized in the penecontemporaneous period. However, they are different in the material basis before the dolomitization, the former experienced aragonitic homogenization cementation due to the early progressive cementation,resulting in uniform dolomitization; the latter experienced aragonitic and calcite cementation under the joint effects of progressive cementation and penecontemporaneous freshwater leaching, and then was formed by dolomitization along the aragonitic cements.

The great breakthrough and centralized evaluation results of the second member of Dengying Formation（Dengying Member2） in PT1 Well Block confirmed the development of high-quality marginal platform porous-vuggy reservoir and the favorable conditions of lithologic gas reservoir in the northern slope zone of central Sichuan paleouplift.In order to further describe the characteristics of the mound-shoal lithologic gas reservoir in Penglai area and evaluate its exploration potential, this paper analyzes the microfacies characteristics and sedimentary evolution process of the Dengying Member2in PT1 Well Block by making comprehensive use of drilling, logging and seismic data. The results show that:（1）High-energy mound-shoal deposit of typical marginal platform is developed in the Dengying Member2,the lithology is dominated by clotted dolomite and micrite-very finely crystal dolomite, and mound core, mound flat and inter-mound microfacies can be identified, presenting the characteristics of multiple facies sequence combination.（2）Based on carbon isotope, resistivity curve and well-seismic calibration results, the Dengying Member2can be divided into three submember, the thickness and microfacies of which presents the differential characteristics of different degrees in vertical and lateral.（3）Based on the correlation between the microfacies and time thickness and internal amplitude energy of submember, the distribution characteristics of sedimentary microfacies in three stages（early, middle and late）are clarified and the sedimentary evolution model of the Dengying Member2in PT1 Well Block is established.（4）Under the control of structural morphology and uniform gas-water interface, the main gas reservoir of the Dengying Member2in PT1 Well Block is mainly distributed in submember3of Dengying Member2and is differential to some extent due to the constraint of sedimentary facies internally. Accordingly, this lithologic gas reservoir is evaluated comprehensively based on multiple factors and it is divided three types of exploration area, in which class I and II areas have large water avoidance and gas column height, and are the best dessert areas for the next concentrated exploration. This proposed evaluation method can further provide important support and reference for the subsequent exploration of lithologic gas reservoirs of Dengying Formation in the slope zone of central Sichuan paleouplift.

Abundant natural gas resources are stored in the Lower Permian Qixia Formation of the Sichuan Basin. In recent years, great breakthrough for oil and gas exploration has been achieved in the Qixia Formation in the NW Sichuan Basin, and high-quality reservoirs mainly locate in dolomite strata. However, the origin of dolomites in the Qixia Formation is still unclear. Special zebra-textured dolomites were observed in the Qixia Formation from the cores of well ST 18. In this paper, a comparative study of the light and dark belts in the zebra texture was systematically carried out with petrological and geochemical methods. Results show that the light belt is mainly composed of medium to coarse crystalline saddle dolomites and its cathodoluminescence is bright red, while the dark zone is composed of medium to fine crystalline dolomites and its cathodoluminescence is dark red. The δ13C values in light and dark zones are within the δ13C range of contemporary seawater, while the δ18O values are more negative than those of contemporary seawater. It is then indicated that the dolomitization fluid was derived from seawater and the oxygen isotopes were fractionated by the affection of high temperature. The order degrees of dolomites in the light and dark zones are consistently high. It was speculated that the order degrees of dolomites in the light and dark zones had been matured to the same degree during the later burial stage. In addition, the occurrence of pyrite confirmed the involvement of hydrothermal fluids in the formation of dolomites. Therefore, the formation of zebra dolomites in well ST 18 mainly went through the following three stages: (1) Fluids dissolved and dolomitized primary limestones along tectonic fractures to form medium to fine crystalline dolomites in the dark zone; (2) Hot fluids precipitated to form medium to coarse crystalline saddle dolomites in the light zone; (3) The order degrees of dolomites in the dark and light zones had adjusted to the same degree in the later deep burial stage.

近期PT101井灯影组二段测试获得231×104 m3/d的高产工业气流,刷新了四川盆地灯影组探井测试产量纪录,显示川中古隆起北部构造斜坡区灯二段具备规模资源.利用最新的钻井岩电、地震及分析测试等资料,针对川中—川北地区灯二段含油气地质条件进行新一轮研究,明确了川中—川北地区灯二段台地边缘带勘探潜力.研究表明:①川中—川北地区灯二段发育由南向北逐渐变宽、增厚的台地边缘沉积,台缘带宽40～130 km,厚650～1000 m;②川中—川北地区灯二段储层以台地边缘丘-滩复合体藻白云岩为主,发育次生溶蚀孔、洞、缝,平均孔隙度为3.6％,平均渗透率为3.6×10-3μm2,优质储层呈大面积连片叠置分布;③川中—川北地区灯二段源储配置条件极佳,中江—蓬莱区块发育多个大型断控构造-岩性圈闭,已证实PT1井区、ZJ2井区具有独立的气-水界面并且远低于高石梯—磨溪地区,发育大型单斜构造背景下的岩性、构造-岩性气藏群.川中—川北地区灯二段台地边缘带具有有利的沉积背景、良好的储层发育条件及优越的天然气成藏要素配置关系,是四川盆地灯影组发现下一个万亿方气区的重要战略新区带.

Exploration of natural gases in the Middle Permian Qixia Formation has achieved a breakthrough in the central and western Sichuan Basin since 2010. This study discusses the quality of dolomite reservoirs and source rocks, trap types and hydrocarbon charging history and proposes that good accumulation conditions are the basis for the large-scale enrichment of natural gas in the Qixia Formation. The high-quality reservoirs in the Qixia Formation are dolomites that primarily occur along platform margins and in the inner-platform shoals surrounding the Caledonian palaeo-uplifts. The Qixia gases originated mainly from the Cambrian and Permian mudstone source rocks. The overlaying Permian source rocks and the underlying Cambrian source rocks are both widely distributed and could provide sufficient gases for dolomite reservoirs. Additionally, the structural or structural-lithologic traps formed by an overlap of the Qixia dolomites and structurally high are favourable for oil and gas accumulation. The Qixia Formation had experienced four episodes of oil and gas charging from the middle-late Triassic to the Early Cretaceous, and traps located in structural-high zones are most favourable for hydrocarbon accumulation. Thus, the Qixia Formation has excellent accumulation conditions for the formation of large-scale gas fields. The concealed structural belts in the footwall of fault No. 1, the Longmenshan–Daxingchang structural belts and the periphery Gaomo–Shehong palaeo-uplift are the key zones for large-scale gas exploration in the Qixia Formation. The Qixia Formation is an important strategic replacement field for natural gas exploration in the Sichuan Basin following the Ediacaran Dengying Formation and the Cambrian Longwangmiao Formation.

Evaporites are of great significance because they provide crucial information on paleoclimate and hydrochemistry. This paper presents a depositional system comprising a mixture of siliciclastics, carbonates, and evaporites that recorded the Eocene sedimentary evolution in thewestern Qaidam Basin, China, in response to mountain building and climate change. Based on lithology and rock textures, three types of evaporites - thickbedded halite, macrocrystalline glauberite-anhydrite association and anhydrite lumps - are identified in the Xiaganchaigou Formation (43.8-35.5 Ma). High-frequency superimposed sequences are also identified, with mineral paragenesis successively changing from argillaceous clastics to carbonates to sulfates and chloride. These associations are suggested to have formed during progressive lake salinization and early diagenesis. The results of stable isotopic data and trace element abundances suggest a semi-closed saline lacustrine environment, with intermittent terrigenous sediment and freshwater inputs during humid climatic periods. The transition of evaporitic associations from glauberite-anhydrite to halite associated with the continuous salinization of the lacustrine basin indicates a decrease inmoisture transported to the QaidamBasin due to the global cooling and the initial uplift of the Altyn Mountains in the Eocene. (C) 2021 Elsevier B.V. All rights reserved.

近年来,四川盆地西部(简称川西地区)中二叠统栖霞组油气勘探取得重大突破,展现出四川盆地栖霞组巨大的勘探潜力.为理清川西地区栖霞组天然气成藏期次、成藏演化、成藏主控因素和成藏特征等关键地质问题,为栖霞组下一步的勘探提供支撑,通过分析研究大量钻井、测井及实验分析测试等资料认为:(1)川西地区中二叠统栖霞组天然气甲烷含量一般大于96％,天然气干燥系数大于0.99,呈高演化的特征,是典型干气,主要为原油二次裂解气.(2)栖霞组天然气δ13C1介于-31.33‰～-28.31‰,δ13C2介于-28.50‰～-25.20‰.气气对比认为川西地区栖霞组天然气不存在志留系龙马溪组烃源岩的贡献,是以寒武系筇竹寺组烃源输入为主,中二叠统烃源输入为辅的混合天然气.(3)通过对所识别出流体包裹体的均一温度分析,识别出4期油气成藏事件.第Ⅰ期和第Ⅱ期分别为中—晚三叠世和晚三叠世的古油藏成藏时期;第m期为早—中侏罗世古油气藏成藏时期;第Ⅳ期为晚侏罗世—早白垩世古气藏成藏时期.(4)成藏主控因素为优越的烃源岩条件、优质的储层发育及可供油气持续聚集成藏的构造—岩性圈闭.(5)川西北双鱼石—中坝地区推覆体下盘隐伏前缘带及川西南平落坝—大兴场地区具备形成大气区的优越成藏条件组合,是下一步川西地区栖霞组勘探的最有利区带.

以柴达木盆地英西地区下干柴沟组上段(E23)为例,基于钻井岩芯、测录井和配套分析等资料发现,英西地区下干柴沟组上段(E23)整体为干旱、咸化背景下的湖相混合沉积,但内部岩石组分和结构产状变化频繁,表现为周期性垂向叠置的湖平面升降沉积旋回.旋回界面以层状岩盐、含膏斑块状灰质云岩、块状砂岩等顶部的岩性转换面或局部存在的暴露侵蚀面为典型标志.单个旋回多表现为短暂湖侵—持续湖退的向上变浅和咸化沉积序列,构成薄层状/块状灰质云岩—含膏斑块状灰质云岩、薄层状/块状灰质云岩—含膏斑块状灰质云岩—层状岩盐、薄层状/块状灰质云岩—块状砂岩等旋回结构类型.不同沉积旋回在空间的叠置组合受湖盆水动力能量、湖水盐度及古气候频繁波动的共同控制.通过旋回对比,利用井间旋回厚度差异近似反映相对古地貌起伏,研究区整体具有西高东低的古地貌格局,盆内则为低起伏的凹隆相间特征,自下而上各井区间的旋回厚度和地貌差异逐渐减小,反映湖盆填平补齐、逐渐咸化萎缩的演变规律.

以柴达木盆地英西地区古近系下干柴沟组上段(E2-3xg2)为例,基于岩心、薄片、扫描电镜、测录井、X衍射及物性分析,探讨湖平面升降对混积型碳酸盐储集层形成及其分布的控制.英西地区E2-3xg2整体为咸化背景下的湖相混合沉积,内部由多种结构类型的湖平面升降沉积旋回在纵向叠置而成,并伴随岩石结构产状及矿物组分的周期性变化.区内储集层类型多样,以混积型碳酸盐岩为主,各类储集层的纵向演化及物性分布与单个湖平面升降旋回具有良好的耦合关系,自下而上构成原生晶间微孔—溶扩晶间微孔—充填残余溶孔的孔隙组合,储集层物性具有向上变好的趋势.各类储集层的形成机理与空间分布受湖平面升降旋回控制,白云石晶间微孔型储集层形成于湖退早—中期,与准同生白云石化作用相关,物性一般,分布广泛;溶扩晶间微孔型和溶孔型储集层形成于湖退晚期,与地貌高部位的短暂暴露溶蚀相关,储渗性能优异,局部发育.