In the Jinshan aulacogen on the southern margin of the Ordos Basin, volcanic rocks of the Mesoproterozoic Changchengian System Xiong'er Group were first encountered by Well PT1, and the drilling operations have revealed the complete strata of Changchengian System within the basin. A systematic analysis of Well PT1 data, including lithological assemblage, zircon U–Pb dating, and geochemical testing, provided the following key findings. (1) The Mesoproterozoic Changchengian System shows a typical sequence consisting of three parts: the Xiong’er Group (volcanic and volcaniclastic rocks) at the bottom, the Ruyang Group (clastic rock) in the middle, and the Luoyu Group (fine clastic–carbonate rocks) at the top. (2) The Xiong’er Group experienced multiple stages of basaltic magma eruptions, dated at 1731 ± 27 Ma and 1756 ± 19 Ma. This suggests that the Ordos Basin was rifted along with the breakup of the Columbia supercontinent, indicating that the Jinshan aulacogen functioned as a rift system. (3) A 28-m-thick dark mudstone, interval with a total organic carbon (TOC) content of up to 1.58%, was encountered in the Changchengian System Cuizhuang Formation. This mudstone has been identified as the oldest source rock discovered in the Ordos Basin. The tensional tectonic environment associated with the breakup of the Columbia supercontinent created favorable conditions for the deposition of Mesoproterozoic source rocks, making them promising targets for Proterozoic exploration in the Ordos Basin.

The Brachiopoda were one of the most successful groups of marine invertebrates during the Palaeozoic Era, exhibiting exceptional diversity and abundance. Linguloid brachiopods (superfamily Linguloidea), initially characterized by an epibenthic mode of life in the Cambrian, underwent substantial diversification during the Ordovician, including the emergence of infaunal forms. This interval marks the peak of their morphospace occupation and ecological diversification. To gain a deeper understanding of the ecological evolution of linguloid brachiopods, a comprehensive investigation of their morphological and anatomical evolution is essential. Here, we report a linguloid brachiopod species from the Upper Ordovician (Sandbian) Pingliang Formation at the Xilinggou section, southern Ordos Basin, North China Platform. Using geometric morphometrics, morphospace occupation was visualized for this species and Anomaloglossa porca, the latter of which was the most recently documented linguloid brachiopod in this section. Results indicate that the two species occupy different areas of morphospace, and the new linguloid brachiopod is assigned to Pseudolingula quadrata, which represents the first report of Pseudolingulidae from the North China Platform. Pseudolingula quadrata exhibits a clearly defined visceral area and preserves complete impressions of the musculature and mantle canal system. The absence of dorsal vascula media within the mantle canals indicates a close similarity to extant infaunal taxa. Based on geometric morphometric analysis and anatomical examination of linguloid brachiopods from the Cambrian to the present, our results not only reinforce the establishment of Pseudolingulidae at the family level, but also demonstrate significant modifications in shell shape and mantle canals that indicate their transition from an epifaunal to an infaunal lifestyle, providing critical insights into their evolutionary trajectory and ecological evolution.

The carbonate-evaporite system in the sub-salt sequence has been a key area for increasing oil and gas reserves and production.Its source rock-reservoir combination styles and distribution have become the key constraints on oil and gas exploration.Analyzing the development and distribution pattern of hydrocarbon source rock-reservoir in the sub-salt carbonate-evaporite system in the Ordos Basin,we explore the controlling factors.The results show that:the hierarchical"uplift-depression"structural pattern determines the sedimentary substrate variability,and the dry and wet paleoclimate changes alter the oceanic conditions,leading to sedimentary differentiation,which results in the formation of different carbonate-evaporite depositional sequences,and the development of various hydrocarbon source rocks and reservoirs.Paleo-uplifts and subordinate uplifts control the distribution of hydrocarbon source rocks for arid-climate argillaceous dolomites,and the paleo-depressions are favorable for the growth of hydrocarbon source rocks for wet-climate micrites;grainstone deposits are prone to developing in paleo-uplift and subordinate uplift zones,which are also a favourable habitat for benthic organisms.These zones are the dominant zones for the reflux of high-salinity brine in the process of the climate shifting from wet to arid,where the occurrence of dolomitization is widely seen,controlling the distribution of crystalline dolostone,grain dolostone,microbial dolostones and porphyritic dolostone reservoirs.Tectonic-sedimentary differentiation governs the distribution pattern of hydrocarbon source rocks and reservoirs in the carbonate-evaporite system.The slope zone around the Central paleo-uplift and the Wushenqi uplift and Shenmu-Mizhi low uplift zone adjacent to the paleo-depression have effective hydrocarbon source rock-reservoir combinations developed,favourable zones for oil and gas exploration in the sub-salt sequence.

Against the bottleneck issues in the Ordovician subsalt marine gas-bearing system of the Ordos Basin, including doubtful quantity of gas generated by low-abundance source rocks, and unclear gas accumulation and preservation patterns, this study investigates the reservoir-forming conditions and near-source exploration practices of the gas-bearing system. First, the argillaceous dolomite and argillaceous gypsum dolomite of the third member of the Ordovician Majiagou Formation (Ma-3 Member) are the main subsalt marine source rocks, and the Dingbian sub-depression and its periphery are the most favorable gas-generating centers, hosting source rocks of 10-80 m thick cumulatively, dominated by Type I kerogen with total organic carbon (TOC) content of 0.58%-1.39% and vitrinite reflectance of 1.62%-2.16%. Second, reservoirs are controlled by paleogeomorphology and penecontemporaneous dissolution, with anhydrite nodule dissolution mold pores, intergranular pores, and intercrystalline pores. Regional and direct caprocks of gypsum-salt rocks are widely developed. The dense NNE-trending strike-slip faults in the east and sparse X-type strike-slip faults in the central area effectively connect source rocks and reservoirs. Third, the south-north fault-uplift and east-west nose-uplift structural setting, combined with the gypsum-bearing dolomitic flat-salt sag facies transition zone, control natural gas accumulation and preservation. Based on these findings, a new accumulation model characterized by near-source gas supply, facies transition sealing, and structural convergence is established for the Ma-3 Member, and favorable exploration zones with multi-type trap groups in low-relief structures are identified. The carbonate-gypsum-salt strata in the Ordos Basin exhibit distinct characteristics of low-abundance source rocks coupled with strong gypsum-salt rock sealing. Near-source exploration offers a new pathway for the exploration in the Ordovician sub-salt marine gas-bearing system.

Based on drilling core, thin section, physical property and logging data, taking the second member of the Ordovician Majiagou Formation (Ma 2 Member) in the Ordos Basin as an example, this paper discusses the reservoir types, distribution and forming mechanisms of the carbonate-evaporite paragenetic system. The results are obtained in three aspects. First, the Ma 2 Member was deposited in an onlapping pattern toward the Central Paleouplift and is in unconformable contact with the underlying Cambrian around the paleouplift. From the paleouplift to the eastern depression, sedimentary environments such as tidal flat, grain shoal and lagoon, as well as five types of carbonate-evaporite paragenetic sequences, developed in turn. Second, dolomicrite, silt-crystalline dolomite and grain dolomite reservoirs are developed in the Ma 2 Member. According to sedimentary and diagenetic differences, they are further subdivided into four types of reservoir rocks, including mottled silt-crystalline dolomite, grain dolomite, burrow-bearing micritic (silt-crystalline) dolomite, and gypsum-mold-pore-bearing dolomicrite. Among them, grain dolomite reservoirs have superior physical properties and high development frequency, representing the high-quality reservoirs in the study area. Vertically, the reservoirs are mainly developed in the middle and upper parts of high-frequency cycles; laterally, they show a pattern of distribution along sags and around structural highs ("along sags and around highs"), characterized by multi-stage superposition and lateral migration. Third, based on the understanding of the sedimentary geomorphic pattern and onlap sedimentary filling model, combined with the lithology, lithofacies distribution and evolution of reservoir rocks, and considering the penecontemporaneous dissolution and dolomitization under high-frequency periodic sea-level cycles, a four-element reservoir-controlling differentiation model of slope geomorphology, particle shoal, dissolution and dolomitization ("slope-shoal-dissolution-dolomitization") has been established.. The research results can provide a basis for evaluating the exploration potential of hydrocarbon replacement areas in the deep Ma 2 Member of the basin.

As a tectonically stable and extensively superimposed basin situated in the North China Craton, the Ordos Basin hosts abundant reserves of oil, natural gas, and coal within its Paleozoic strata, rendering it a focal area in energy-related geological research. The basin’s evolutionary history provides a comprehensive record of key geological transitions—from an Early Paleozoic carbonate platform to Late Paleozoic marine–continental transitional deposits and ultimately to continental clastic sedimentation—largely governed by the regional tectonic dynamics associated with the North China Plate. This study presents a systematic review of the sedimentary and tectonic evolution of the Paleozoic sequence in the basin. Findings indicate that during the Early Paleozoic, the basin developed under a passive continental margin setting, characterized by widespread epicontinental marine carbonate deposition. By the Late Ordovician, subduction of the Qinqi Ocean triggered the Caledonian orogeny, resulting in regional uplift across the basin, widespread erosion, and a significant hiatus in Middle to Late Ordovician sedimentation, which facilitated the formation of paleo-weathered crust karst reservoirs. In the Late Paleozoic, the basin evolved into an intracratonic depression. From the Late Carboniferous to the Early Permian, the Hercynian tectonic event influenced the transformation from isolated rift basins to a broad epicontinental sea, leading to the deposition of critical coal-bearing strata within marine–continental transitional facies. Starting in the Middle Permian, the closure of surrounding oceanic domains induced widespread tectonic uplift, shifting the depositional environment to a terrestrial fluvial-lacustrine system and marking the termination of marine sedimentation in the region. Based on the comprehensive research findings, this study underscores that the superposition, inheritance, and interaction of multiple tectonic events are the primary controls on the paleogeographic architecture and sedimentary.

The Wulalike Formation in the southern Ordos Basin represents an emerging marine shale oil play, with recent discoveries challenging traditional views of its low-TOC and low-porosity potential. The current study integrates 3D seismic data, geochemistry (TOC, Ro), and structural analysis to unravel tectonic controls on shale oil accumulation. The western margin features three structural zones: 1) a reverse-thrust belt (Huianbu-Shajingzi fault), 2) a thrust-extension zone (Yandunshan-Majiatan blocks), and 3) a stable eastern zone. Unlike the northern detachment systems, the southern section exhibits monoclinal thrusts, evidenced by near-vertical Ordovician strata (Qiujianjia section) and high-angle thrusts. Fault analysis highlights that fault activity reached its peak during the Permian-Triassic transition (252-201 Ma), with faults inheriting structural characteristics from Caledonian and Yanshanian-Indosinian orogenies. Early uplift in the south led to lower thermal maturity (Ro = 0.83%, TOC = 0.82%) and preserved oil, while the central-north reached gas-window maturity (Ro = 1.6%, TOC = 1.13%). Meanwhile, different unconformities in southern versus northern sections reflect differential tectonic evolution. Structurally stable inner thrust-sheet zones (e.g., Shajingzi) retained intact strata and deeper burial, whereas sheet-edge zones underwent erosion and leakage. The Wulalike shale oil model emphasizes tectonic control on preservation, prioritizing inner-sheet zones for exploration in the western margin of the Ordos Basin. These insights redefine prospects for low-TOC shale oil in thrust belts.

IntroductionThe North China Craton experienced multiple episodes of Mesoproterozoic anorogenic magmatism, which provides critical insights into the breakup process of the Columbia supercontinent. While numerous previous studies have focused on the southern and eastern margins of the craton, the southwestern margin remains relatively understudied, and its tectonic setting is still poorly constrained.MethodsTo investigate the tectonic context of Mesoproterozoic magmatic events along the southwestern margin and their relationship with the breakup of Columbia, this study presents a systematic analysis of the petrology, zircon U–Pb geochronology, and geochemistry of the granite porphyry in the Qi’angou area of Longxian, southwestern North China Craton.ResultsThese data are integrated with geochemical characteristics of other Mesoproterozoic igneous rocks from the western margin to comprehensively analyze the magmatic activity in this region. Zircon U–Pb dating yielded an emplacement age of 1794 ± 10 Ma for the Qi’angou granite porphyry, indicating its formation during the Mesoproterozoic. Geochemical data show that the granite porphyry is peraluminous and belongs to the A2-type granite suite. It is enriched in large-ion lithophile elements (LILE) and depleted in high-field-strength elements (HFSE), with high Rb/Sr ratios (2.95–3.55), pronounced negative Eu anomalies, and low Mg# values, suggesting a crustal derivation.DiscussionCombined with previous studies on coeval intermediate-mafic rocks (1740–1804 Ma)—including calc-alkaline basaltic andesites and high-K calc-alkaline diabases—these findings indicate that the North China Craton underwent extension during the late Paleoproterozoic to early Mesoproterozoic. Upwelling of a mantle plume beneath the southwestern margin triggered lithospheric thinning and partial melting of the upper crust, leading to a series of magmatic events. Comprehensive geochemical features and regional tectonic analysis suggest that the western margin igneous assemblage formed in a post-orogenic intracontinental extensional setting. This represents the initial geological record of the global breakup of the Columbia supercontinent within the North China Craton and provides key chronological constraints for understanding the geodynamic mechanisms of supercontinental fragmentation.

Carbonate reservoir exhibits "low-resistivity and (ultra) low-permeability" anomaly due to complex pore structure. It means that hydraulic conductivity, closely related to permeability (K), is not simply equivalent to electrical conductivity in this case. As a result, the conventionally well-established permeability-resistivity relationship does not apply to carbonate reservoirs any longer, and this poses a big challenge for the interpretation and evaluation of carbonate reservoirs. In this study, we found that the relationship between permeability and formation factor is non-monotonic and scattering through direct-current (DC) resistivity and nuclear magnetic resonance (NMR) experiments, and this means that the hydraulic conductivity is not well correlated with the electrical conductivity in carbonate rocks as one expects. There are the two factors, the volume fraction ratio of macro- and micro-pores, and the number of dolomitization-formed micropores, predominantly control hydraulic conductivity and electric conductivity, respectively. Moreover, the non-uniform distribution of inter- crystalline pore networks will significantly reduce hydraulic conductivity. To more accurately estimate the permeability of carbonate rocks, the new permeability model based on NMR T2 spectra rather than formation factor has been proposed. In this model, the characteristic relaxation time (T2c = 83.418 ms) was defined, which is associated with the macropores that contribute most significantly to permeability and can be used to distinguish between macro- and micro-pores. This model demonstrates a minimal average absolute logarithmic deviation (d = 0.3667) and exhibits generalizability across various carbonate rocks. The study addresses the mechanism analysis of "low-resistivity and (ultra) low-permeability" phenomenon in carbonate reservoir, and has potential to facilitate the interpretation and evaluation of carbonate reservoir.

The Ordovician carbonate formations in the Ordos Basin provide a crucial stratigraphic unit for prospective oil and gas exploration. Significant progress has been made in the exploration of natural gas within the Ordovician subsalt formations. Nonetheless, understanding its accumulating properties requires additional investigation. Clarifying the formation periods of the carbonate rock reservoirs in the Majiagou Formation of the basin can furnish a theoretical foundation for advanced exploration of carbonate rock oil and gas. This study uses fluid inclusion petrography, laser Raman spectroscopy, and microscopic temperature measurement methods, along with information about the basin’s history of burial and thermal evolution, to look at the oil and gas charging periods of Majiagou Formation reservoir in the central-eastern basin. The results show that there are two stages of hydrocarbon inclusions. The first stage has blue fluorescence and temperature peaks between 85 and 95 °C in the central basin and between 105 and 115 °C in the eastern basin. For the second stage, no fluorescence can be observed. Meanwhile, the temperature peaks are between 175 and 185 °C in the central basin, and between 165 and 175 °C in the eastern basin. In the central part of the basin, oil charging began in the Late Triassic (231–203 Ma) and reached the gas generation stage in the Late Early Cretaceous (121–112 Ma), peaking in natural gas charging. In contrast, the reservoirs in the eastern part of the basin experienced a primary oil charging stage in the Early Jurassic (196–164 Ma) and entered the gas generation stage in the Late Early Cretaceous (110–101 Ma). The hydrocarbon charging process in the study area is mainly controlled by the thermal evolution history of the basin. The study determines that the central basin enters the threshold of hydrocarbon generation earlier than the eastern basin, leading to earlier oil and gas charging.

To advance the understanding of Mesoproterozoic sedimentary facies and delineate their distribution patterns in the Ordos Basin, this study systematically analyzes the sedimentary characteristics and stratigraphic architecture of the Mesoproterozoic sequences using integrated datasets, including core samples, well logs, seismic profiles, and field outcrops. The results indicate that the Mesoproterozoic Changcheng System, shaped by rift-related processes, displays a southwest-to-northeast thinning trend, culminating in stratigraphic pinch-out. Three Facies associations are identified: rift-related sandstone-shale and volcanic eruption facies (dominated by sandstones with abundant basaltic eruptives in the Changcheng rift trough), shoreface sandstone facies (primarily lower shoreface subfacies with quartz sandstones and mudstones), and fluvial facies (sandstone-siltstone sequences). During the Jixian System, post-Changcheng "compensatory sedimentation" led to a significantly reduced depositional area dominated by tidal flat environments. A southwest-to-northeast facies transition progresses from shelf facies (siltstones interbedded with shales and bioclastic limestones) through mid-ramp facies to tidal flat and mixed tidal flat facies, collectively forming a carbonate tidal flat system dominated by siliceous-banded dolomites. The Mesoproterozoic succession in the Ordos Basin thus records an evolutionary trajectory from rift basin infilling ("leveling") to the development of a shallow marine clastic-carbonate platform during the Jixian System, reflecting progressive tectonic stabilization and paleoenvironmental transitions.

With the continuous increase in exploration efforts in new zones and new strata, significant breakthroughs have been made in the natural gas exploration of the O1m56 to O1m4 formations in the Ordos Basin. Thus, the origin and exploration potential of subsalt natural gas have attracted much attention and urgently need to be addressed. On the basis of certain geochemical characteristics, genetic types, and sources of natural gas, a comprehensive study on the sedimentary environment, organic geochemical characteristics, and spatial distribution scale of source rocks are conducted in this paper by using geological and geochemical methods. The study shows that: (1) The Ordovician subsalt natural gas is mainly "pyrolysis dry gas," among which the 513C1 of Ordovician subsalt low sulfur (sulfur-free) natural gas is lighter, with an average value of -39.6%; the 513C2 ranges more largely from -35.6%to -25.8%. In contrast, both 513C1 and 513C2 values are heavier in high-sulfur natural gas, revealing that different Thermochemical Sulfate Reduction (TSR) reaction stages have different degrees of influence on natural gas components and carbon isotope composition. (2) Subsalt natural gas is classified as "oil-type gas," which is self-generated and self-accumulated, whose source rocks are mainly Ordovician subsalt marine deposits. (3) Three types of marine source rocks are developed in Ordovician subsalt, including black argillaceous rock, dark argillaceous dolomite (dolomitic mudstone), and dark micrite (bioclastic) limestone. In addition to micrite limestone, these rocks were mainly formed in a confined lagoon sedimentary environment with high salinity and anoxia. Sedimentary water was significantly stratified and the environment was highly reduced. The organic matter content of the source rocks is relatively high, with an average TOC value of 0.45%. The hydrocarbon-generating parent materials are mainly composed of bacteria and algae, and the organic matter evolution reaches highover maturity stage. The total gas generation amount of the marine source rocks in Ordovician subsalt is approximately 43.8 x 1012 m3, which can provide hydrocarbons and accumulate for the subsalt favorable reservoir facies located far from Upper Paleozoic gas sources. (c) 2024 Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences AND Research Institute of Petroleum Exploration and Development, PetroChina. Publishing services by Elsevier B.V. on behalf of KEAI Communications Co. Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Based on the global typical karst characteristics reported in recent decades and the latest petroleum and gas exploration results, it is found that the characteristics of the karst reservoirs in the Ordos Basin are dramatically different from those of traditional karst reservoirs, indicating that the main contribution to the formation of the karst reservoirs in the region may not have been karst dissolution. The previous view of the formation mechanism of karst reservoirs suggests that we should think about the formation and development processes of Ordovician karst and the reservoir pores in the Ordos Basin from another perspective. In this investigation of reservoir formation, karstology, geography, sedimentology, and reservoir geology were all utilized. On the basis of the initial understanding of the control of large-scale landforms on the sediments in sedimentary basins, this study focuses more on the dynamic spatial-temporal relationships between the evolutions of the landforms, karsts, and reservoirs. By studying the petrological characteristics, pore filling, karst zonation, and planation model of the weathering crust reservoir in the study area, it has been determined that the primary reservoir space was formed in advance rather than in the supergene dissolution period. A gypsum-containing dolomite flat served as the material foundation for reservoir construction and later karst planation transformation. The penecontemporaneous dissolution dominated the spatial development of the early dolomite reservoir. The traditional karst cycle is actually caused by the erosion of the bottom of the karst floor on the double-layer leveling surface. The karst planation during the Caledonian period played a discontinuous and destructive filling role throughout the entire reservoir zone by changing the diagenetic environment, the main contribution of which was communication between the pores. Therefore, this weathered crust reservoir is actually a residual reservoir controlled by karst planation rather than a traditional karst reservoir. The “sweet spots” are primarily dispersed in the upper portion of the karst slope and the top of the karst residual hillock in the gypsum-containing dolomitic flat environment.

近年来,通过创新地质认识,突出风险勘探、强化规模勘探、精细效益勘探,鄂尔多斯盆地油气勘探取得丰硕成果;2020-2022年,年产量均超过6000×104t油当量,其中风险勘探在"四新"领域的探索过程中发挥了重要作用.在前期研究工作的基础上,通过深化奥陶系盐下、二叠系太原组石灰岩、三叠系延长组长7段页岩油等领域油气成藏条件分析,创新形成"盐下源内白云岩丘滩体含气新系统""太原组石灰岩'三明治'成藏模式""长73纹层型页岩油甜点标准和特有工艺改造技术"等成藏认识和技术成果,成为"十五五"及中长期油气勘探的接替领域.同时,突出"向源、向新、向深"的勘探理念,按重点攻关和区带准备两个层次,重新认识盆地低勘探程度区带油气成藏特征与勘探潜力,指出了今后一个时期风险勘探领域的前景与方向.其中重点攻关方向为已经发现勘探苗头且资源潜力较大的领域,优选出奥陶系海相页岩气、延长组下组合、煤系致密气及元古宇长城系等主要方向;区带准备则针对具备基本油气成藏条件,但研究程度较低且勘探难度较大的领域,提出中央古隆起周缘寒武系、西缘复杂构造区及盆地东部盐下马家沟组马三段—马二段新层系等具有较大勘探潜力目标.通过风险勘探领域的地质认识、成果总结和潜力分析,将实现盆地后备油气勘探区带的有序接替,以及起到对长庆油田二次加快发展战略需求重要的指导作用.

碳酸盐岩?膏盐岩组合蕴含了全球46％的碳酸盐岩油气储量,是重要的油气富集层位.鄂尔多斯盆地奥陶系马家沟组发育大套厚层碳酸盐岩?膏盐岩混合沉积,目前在盐上已发现了近万亿方探明储量.盐下和盐间有无勘探潜力?对该沉积体系的古环境恢复是关键.通过对盆内T112井400 m系统取芯进行描述,结合薄片鉴定、微量元素、稀土元素、碳、氧、锶同位素、岩石组分等分析,对马家沟组碳酸盐岩?膏盐岩沉积体系的沉积环境进行了恢复.根据海平面升降和水体氧化程度的变化,该体系可以划分为4个亚环境,分别是缺氧段、次氧化段、氧化段和硫化段.随着氧化程度加剧,U、Mo等微量元素含量降低,碳氧锶同位素逐渐正偏,但由于硫化段导致含氧量急剧降低,U、Mo和碳氧锶同位素急剧升高,并呈现随海平面频繁振荡分布.基于古环境与TOC和孔隙度的对比分析,硫化段和缺氧段是最有利的烃源岩发育层段,强烈蒸发作用导致水体分层及硫化,使有机质得到有效保存.氧化段和次氧化段是储层发育的有利层段,台内局部隆起沉积的丘滩体频繁暴露遭遇准同生溶蚀作用,发育大量溶蚀孔隙,经白云石化以后得到有效保存.硫化段的烃源岩与氧化段的白云岩储层纵向上呈层状频繁交互,平面上呈指状交叉,构成良好的源储配置关系.乌审旗隆起带上发育的立体型储集层网络是鄂尔多斯盆地奥陶系盐下勘探的有利区带.

鄂尔多斯盆地奥陶系马家沟组发育碳酸盐岩—膏盐岩共生体系沉积,马家沟组四段(马四段)是马家沟组沉积厚度最大的一套海侵碳酸盐岩沉积层,长期以来是天然气勘探关注的重点层位.基于地球物理、钻井岩心及有机地球化学等资料,重新认识了鄂尔多斯盆地奥陶系盐下古构造、储层分布、烃源岩和圈闭等成藏条件,并取得以下成果认识:(1)盐下马四段天然气为油型气,主要来自奥陶系盐下海相烃源岩,该套烃源岩生烃母质除常规干酪根外,还发育分散有机质、有机酸盐,生烃物质丰厚,具备规模生烃潜力;(2)奥陶纪鄂尔多斯盆地中东部坳陷存在乌审旗—靖边古隆起和东部盐下低隆两大次级构造单元,分别控制了盐下马四段台内滩、台内丘白云岩储层的发育,储集空间主要为白云岩晶间孔;(3)盆地中东部盐下马四段台内滩、台内丘白云岩上覆厚层膏盐岩封盖,上倾方向致密石灰岩侧向遮挡,海相烃源岩供烃,形成了大面积分布的自生自储式岩性气藏.在新的地质理论认识的指导下,针对鄂尔多斯盆地奥陶系盐下马四段部署的风险探井MT1井钻遇气层43.4m,采用水力加砂压裂新工艺,试气获35.24×104m3/d的高产工业气流,实现了盆地战略接替领域的重大突破.

近期,米探1井的勘探突破表明鄂尔多斯盆地东部米脂地区奥陶系马家沟组四段(以下简称马四段)具有天然气勘探的巨大潜力,然而,对米脂地区马四段沉积相的认识还存在着争议,在一定程度上制约了该区天然气的勘探进程.为此,基于野外露头、钻井岩心、微观薄片、测井和碳/氧同位素等基础资料,系统开展了沉积相和有利储层方面的研究,确定了沉积演化模式,并预测了有利储层发育区.研究结果表明:①马四段发育灰质潟湖、台内丘和含云膏坪3种沉积微相,灰质潟湖微相中的石灰岩、灰质围岩δ13C、δ18O与同期海水相似;台内丘微相中的白云质围岩 δ13C、δ18O仍处于同期海水值范围,但较灰质潟湖沉积石灰岩偏正;含云膏坪微相中的白云质围岩 δ13C、δ18O较同期海水偏负.②马四段可识别出海侵体系域、高位体系域早期、高位体系域晚期3种沉积模式——海侵体系域期,水体最深,米脂地区为灰质潟湖沉积;高位体系域早期,水体变浅、咸化,凸起带发育台内丘,凹陷区发育灰质潟湖;高位体系域晚期,水体最浅,米脂地区整体演化为含云膏坪沉积.③台内丘内部沉积的微生物白云岩、斑状粉晶白云岩和斑状灰质白云岩发育晶间孔,物性相对较好,是有利沉积相带.结论认为,米脂地区的瑶镇—大保当、神木—米脂、绥德—碛口一带沉积期位于地势较高的凸起带上,且广泛发育台内丘,是下一步天然气勘探的重要方向.

Based on outcrop profiles, drilling cores, cast thin sections etc., the types, microfacies combinations and distribution pattern of microbial carbonates in the Ordovician middle assemblage of the mid-eastern Ordos Basin have been systematically analyzed. The middle assemblage of Ordovician in the mid-eastern Ordos Basin has microbial carbonates formed by the calcification of cyanobacteria, including microbial biostromes and microbial mounds made of stromatolites, thrombolites, and oncolites. The distribution of the carbonates shows obvious "stratum-control" and "regional" characteristics. The microbial biostromes 2–3 m thick each are controlled by sequence cycles and sedimentary facies changes, and were mainly formed in the tidal flat environment during the depositional stages of the Ma56 and Ma55 sub-members. The microbial biostrome in the Ma55 sub-member occurring near the carbonate-evaporite transition interface in the early stage of the transgression is distributed mainly in the Mizhi subsag in the eastern part of the basin; the microbial biostrome in the Ma56 sub-member turns up near the carbonate-evoporite transition zone in ring shape in the east of the central uplift. The ancient landform had noticeable control on the distribution of microbial mounds. The microbial mounds or mound-shoal complexes developing mainly during the depositional stages of Ma57_Ma510 sub-members are about 15–25 m thick in single layer and distributed largely in the Wushenqi-Jingbian paleouplift. The development model of the microbial carbonate rocks shows that the carbonate-evaporite lithologic transition zone and the Wushenqi-Jingbian paleouplift are favorable exploration zones of microbial carbonates in the Ordovician middle assemblages.

鄂尔多斯盆地中东部地区奥陶系马家沟组发育巨厚的盐岩、膏岩沉积,封盖性能好,膏盐岩之下的碳酸盐岩一直是备受关注的重要勘探领域.在对区域构造沉积背景及沉积相带分布研究的基础上,以马家沟组马五段膏盐岩盖层之下的马四段碳酸盐岩为目标,主要开展了储层及成藏特征研究.研究认为:马四段白云岩分布主要受控于古隆起控制下的台内分异作用,发育滩相晶间孔型白云岩储层,马三段等层段的溶孔型白云岩储层也具有较好的储集性能;加里东期的风化剥蚀作用造成马四段白云岩与上古生界煤系烃源岩、奥陶系盐下海相烃源岩形成了空间配置关系,天然气成藏具有双源供烃的特点,形成岩性相变控制下的白云岩岩性气藏.在对不同区域生储盖配置关系及成藏控制因素的差异性对比的基础上,综合分析认为盆地东部盐下马四段盐岩底辟隆起带和中部乌审旗古隆起控制下的岩性相变带是盐下勘探的有利目标,深层的马三段、马二段也具有较好的勘探潜力.