Oil and gas in deep and ultra-deep strata has been a hot topic in both research and exploration. The understanding of reservoirs in relatively shallow strata may be of significance for deep exploration. The Hetianhe gas field is the only large gas field discovered yet in the southwest region of the Tarim Basin, with a small quantity of condensate oil penetrated and a burial depth ranging from 1035 m to 2885 m, and the origin and source of the oil and gas has long been unclear. By using comprehensive 2D gas chromatography/time of flight mass spectrometry (GC x GC-TOFMS), abundant diamondoids and some thiadiamondoids were detected in condensate oil samples collected from 3 wells, additionally, the features, including dry gas with a trace amount of hydrogen sulfide, heavy carbon isotopes, and similar sulfur isotopes in hydrogen sulfide and Cambrian gypsum, indicate that the oil and gas has undergone the thermochemical sulfate reduction (TSR) and thermal cracking. The current reservoir temperature, from 35 degrees C to 83 degrees C, does not meet the condition for the onset of these secondary geochemical alterations, therefore, the oil and gas may originate from deep strata. According to the reconstruction of the geologic evolution, deep oil and gas may have migrated vertically along faults and accumulated in the Hetianhe structure formed after 10 Ma to form the secondary gas reservoirs; hence it is inferred that favorable petroleum plays may exist in deep strata. The thick evaporite in the Middle Cambrian and the high-quality dolomite reservoirs in the Lower Cambrian constitute a favorable reservoir-seal assemblage, with the current depth between 8500 m and 10000 m and reservoir temperature above 200 degrees C, where the TSR and thermal cracking may occur. The sulfur isotope of hydrogen sulfide also demonstrates the occurrence of TSR process in the Cambrian strata. Therefore, large-scale of oil and gas reserves may be preserved in the stable zones in the Cambrian sub-salt strata, and it is suggested to strengthen hydrocarbon exploration in the Cambrian sub-salt strata in the southwest region of the Tarim Basin.

鄂尔多斯中东部地区奥陶系马家沟组发育巨厚的碳酸盐岩与膏盐岩交互的沉积体系,但对其沉积发育的规律性及其内部天然气成藏聚集的基本地质特征尚未形成明确的认识。通过对奥陶系碳酸盐岩-膏盐岩体系形成时的构造与古地理背景、沉积特征、储层发育、烃源供给及圈闭聚集等方面的分析,形成新的观点与地质认识:(1)受盐洼周边古隆起影响,奥陶系碳酸盐岩-膏盐岩共生体系形成于与外部广海周期性隔绝的局限海蒸发环境,导致其岩性“相控”与“层控”分布特征极为显著;(2)首次提出“非层状沉积分异”的概念,并认为它是控制奥陶系内幕“溶孔型”储层发育的决定性因素;(3)受区域岩性相变及后期构造反转的控制,奥陶系盐下层系具有形成大区域分布的岩性圈闭体系的有利条件;(4)盐下层系虽自身的海相烃源岩生烃潜力有限,但由于燕山期东部构造抬升与西侧上古生界煤系烃源“供烃窗口”及膏盐封盖层区域性连续分布等要素密切配合,仍具有规模供烃、大区带成藏的潜力。

A sedimentary system consisting of interactive ultra-thick carbonate rock and gypsum-salt rock developed in the Ordovician Majiagou Formation in the central and eastern Ordos Basin, however, its development process and natural gas accumulation characteristics are not clear. By studying its tectonic and paleogeographic background, sedimentary features, reservoir development, hydrocarbon source supply and trap accumulation, we gain new geological insights into the carbonate-evaporite paragenesis system, which can be summarized as follows: 1) Affected by its surrounding ancient uplifts, the carbonate-evaporite paragenesis system is formed in a restricted sea with periodic isolation from the open sea, therefore its lithologic distribution has obvious “facies-control” and “strata-bound” characteristics. 2) The key factor controlling the development of the Ordovician “interior dissolved pore” type reservoir is “nonstratiform sedimentary differentiation”—a novel concept, proposed here for the first time. 3) Controlled by regional lithologic transition and later structural inversion, the Ordovician sub-salt stratum has favorable conditions for forming lithologic trap system on a large scale. 4) Although the sub-salt stratum has limited hydrocarbon-generation capacity from its own marine source rocks, it still has the potential of large-scale hydrocarbon supply/accumulation due jointly to the Yanshanian tectonic uplift in its eastern part, hydrocarbon supply from Upper-Paleozoic coal-bearing source-rock on its west side, and regional continuous distribution of gypsum-salt capping strata.

基于大量钻井、测井和地震资料分析,对鄂尔多斯盆地奥陶系盐下马家沟组四段天然气地质条件与成藏主控因素开展研究,提出古隆起控相、控储、控藏新认识：①马家沟组沉积期,中央古隆起分隔盆地中东部华北海与西南缘秦祁海两大沉积体系,在马四段台缘带发育巨厚丘状颗粒滩相白云岩,同时控制盆地中东部形成“两隆两凹”古地理格局,其中水下低隆带发育台内滩相白云岩,隆间低洼区发育泥灰岩。②由中央古隆起至盆地东缘,马四段白云岩逐渐减薄并相变为灰岩,灰岩致密带侧向封挡形成大面积白云岩岩性圈闭。③加里东末期中央古隆起遭受不同程度剥蚀,面积达6×10⁴ km²,上古生界石炭系—二叠系煤系优质烃源岩大面积披覆沉积,成为下伏奥陶系盐下白云岩岩性圈闭规模供烃主体。④印支期—燕山期盆地西倾掀斜,中央古隆起下拗转变为高效供烃窗,上古生界煤系烃源岩通过中央古隆起高孔渗白云岩体向上倾高部位侧向供烃,盐下海相烃源岩作为重要的气源补充,通过加里东期断裂、微裂缝输导供烃。以新认识为指导,转变勘探思路,综合评价优选盆地中东部马四段两大有利勘探区,部署实施两口风险探井均钻遇较厚马四段（含）气层,其中1口井获高产工业气流。该研究推动了奥陶系盐下马四段天然气勘探的历史性突破,开辟了鄂尔多斯盆地天然气勘探的重要新领域。

基于鄂尔多斯盆地天然气勘探新进展,利用地球化学与气源对比等理论与方法,分析该盆地奥陶系马家沟组海相碳酸盐岩烃源岩发育特征,探讨马家沟组自源型天然气聚集规律与潜力。研究认为加里东期古隆起东侧、环盐洼区马家沟组发育规模有效烃源岩,源自马家沟组泥质白云岩与白云质泥岩的天然气是奥陶系气藏的主要贡献者。奥陶系马家沟组不仅发育碳酸盐岩风化壳岩溶储集层,也发育内幕白云岩颗粒滩储集层,且二者分布广泛,均能成为有利储集体。马家沟组天然气以自源型油型气为主,表现为自生自储、近源聚集的特征,只在局部地区存在上生下储的天然气聚集区。同时,提出环盐洼周边地区奥陶系马家沟组具有大规模天然气生成与聚集的有利条件,具备大面积自生自储天然气聚集的优势,靖边气田以西、以北与以南的古隆起前缘洼陷区是未来奥陶系勘探的有利地区。图6表1参32

The characteristics of the marine carbonate source rocks and the accumulation and potential of self-sourced natural gas in the Ordovician Majiagou Formation of the Ordos Basin are investigated based on recent exploration progress by using geochemical and gas-source correlation methods. Massive source rocks are developed around the salt depression, east of the paleo-uplift in the Ordovician Majiagou Formation during the Caledonian; and the natural gases produced by argillaceous dolomite and dolomitic mudstone are the major sources of the Ordovician gas field. Besides widespread carbonate weathering crust karst, internal grainstone dolomite is also well-developed in the Majiagou Formation, and both can act as favorable reservoirs. The natural gas of the Majiagou Formation is mainly self-sourced oil-type gas generated by in-situ source rock and accumulated near the source area. There is only limited local accumulation of natural gas produced in the upper strata and stored below. The Majiagou Formation around the salt depression has favorable conditions for large scale natural gas generation and accumulation, has the advantage of collecting large area natural gas of self-generation and self-preservation and the frontier depression areas of the paleo-uplift to the west, north and south of Jingbian gas field are the potential Ordovician exploration targets in the future.

利用钻井岩心、测井、地震等资料开展烃源岩、储集层评价,并通过天然气同位素组成、天然气组分、流体包裹体分析,开展天然气成因识别等工作,研究鄂尔多斯盆地深层奥陶系下组合膏盐岩相关地层的天然气成藏条件。结果表明,①下组合天然气为来自海相烃源岩的高热演化干气,其天然气甲烷碳同位素组成偏轻、乙烷碳同位素组成偏重。天然气组分判识结果是原油裂解气。烃类流体与硫酸盐发生过硫酸盐热化学还原反应,依据是岩心中见到硫磺晶体,天然气中发现硫化氢,烃类及硫化氢流体包裹体广泛发育。②奥陶系下组合环绕盐洼带大面积发育泥质岩,累计厚度为20~80 m。有效烃源岩大多为富有机质纹层泥质岩或者藻团块、藻云岩,有机碳测试含量主体范围为0.1%~0.5%,平均值为0.31%,最高可达3.24%。有机酸盐恢复后平均有机碳含量可达0.58%,表明烃源岩具备较好供烃潜力。③沉积期古隆起控制储集层分布。中央古隆起东侧发育乌审旗—靖边继承性次级古隆起,神木—子洲附近发育厚层盐岩形成的低隆,均控制准同生颗粒滩白云岩储集层分布。膏盐岩咸化环境对储集层发育具有促进作用。下组合发育白云岩晶间孔型、溶蚀孔型和裂缝型3类储集层,其中晶间孔和溶蚀孔为主要储集空间。④下组合发育致密碳酸盐岩和膏盐岩两类盖层,分别控制形成两类生储盖组合。总体是海相源岩供烃、滩相灰云岩储集、小微断裂输导、构造-岩性圈闭聚集的成藏模式。⑤下组合马家沟组三段和四段是重要目的层,平面上乌审旗—靖边次级古隆起和神木—子洲低隆是白云岩与灰岩交互过渡带,隆起带东侧发育致密石灰岩,利于形成岩性上倾遮挡气藏,近期两口风险井钻探效果良好,表明两个隆起带是重要勘探方向。图11表1参46

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。

The carbonate-evaporite paragenetic deposits are developed in the Ordovician Majiagou Formation in Ordos Basin. The fourth member of Majiagou Formation (Ma 4 member) is composed of a set of transgressive carbonate rocks with the largest thickness in Majiagou Formation, which has always been the target layer for natural gas exploration. Based on geophysical, drilling, core samples and organic geochemical data, the hydrocarbon accumulation conditions such as paleo structure, reservoir distribution, source rocks and trap are restudied, and the following achievements are obtained: (1) Natural gas in Ma 4 member is oil-type gas, generated by marine source rock of the subsalt Ordovician. In addition to conventional kerogen, the source rock has a large amount of parent materials for hydrocarbon generation, such as the dispersed organic matter and organic acid salt, showing great potential of hydrocarbon generation; (2) In the Ordovician, two secondary structural units, Wushenqi-Jingbian paleo uplift and the eastern subsalt low uplift, were developed in central-eastern depression in Ordos Basin, which controlled the development of dolomite reservoir in the intra-platform shoal and intra-platform mound of Ma 4 member, with the main reservoir space of intergranular pores; (3) The wide spread self-generation and self-storage type lithologic gas reservoir is formed in the central-eastern Ordos Basin by marine source rocks of subsalt Ordovician, reservoir of the intra-platform shoal and intra-platform mound dolomite of Ma 4 member, and cap rocks of the overlying thick evaporites and the lateral barrier provided by tight limestone in the updip direction. Guided by the new geological understanding, the risk exploration Well MT1 was drilled targeting at Ma 4 member, in which a gas layer of 43.4 m was penetrated, and high gas flow of 35.24×104 m3/d was tested by applying new hydraulic sand fracturing technology, achieving a major breakthrough in strategic replacement field in the basin.

探讨鄂尔多斯盆地奥陶系盐下天然气的成因类型及来源，为勘探部署提供理论依据。以天然气地球化学特征分析为基础，结合成藏组合特点剖析，通过烷烃气及轻烃组分碳同位素组成比对、δ¹³C₁—R_O相关性分析等综合判识天然气成因类型与来源。结果显示，鄂尔多斯盆地奥陶系盐下天然气组分以烃类气体为主，烃类组分含量平均为94.1%。烃类气体中甲烷占优势，甲烷化系数[C₁/∑(C₁—C_n)]随区域热演化程度的不同而变化明显，从靖边—乌审旗—神木地区，甲烷化系数依次由0.99→0.95→0.85逐渐减小。除高含硫天然气外，奥陶系盐下天然气的δ¹³C₁值、δ¹³C₂值整体偏低，δ¹³C₁值分布于-45.90‰~-37.29‰之间，平均为-39.58‰，δ¹³C₂值分布于-35.58‰~-25.77‰之间，平均为-29.9‰。奥陶系膏盐岩下高含硫天然气的甲烷、乙烷碳同位素值显著偏高，是硫酸盐热化学还原反应(TSR)的结果。综合各项地球化学指标气气、气源比对，结果显示鄂尔多斯盆地奥陶系盐下天然气属于自生自储油型气，奥陶系海相气源岩是其主力供烃源岩。

The genetic type and source of Ordovician subsalt natural gas in Ordos Basin are discussed so as to provide theoretical basis for gas exploration.Based on the analysis of geochemical characteristics of natural gas,combined with the gas accumulation combination characteristics,the genetic type and source of Ordovician subsalt natural gas are comprehensively identified through the comparison of the stable carbon isotopic compositions of alkane gases and light hydrocarbons and the correlation analysis of <span>δ</span>¹³<span>C</span>₁-<span>R</span>_O.The results show that the Ordovician subsalt natural gas in Ordos Basin are mainly composed of hydrocarbon components and the average content is 94.1%.Methane in hydrocarbon gases is dominant and the methanation parameter (<span>C</span>₁<span>/∑(C</span>₁<span>-C</span>_n)) varies obviously with the difference of regional thermal maturity.From Jingbian to Wushen and Shenmu areas,the methanation parameter gradually decreases from 0.99-0.95-0.85.Except for high sulfur natural gas,the&nbsp; <span>δ</span>¹³<span>C</span>₁&nbsp; and&nbsp; <span>δ</span>¹³<span>C</span>₂ values of Ordovician subsalt gas are generally low.The&nbsp;<span>δ</span>¹³<span>C</span>₁&nbsp; value is in ranges from -45.90‰ to -37.29‰,with an average of -39.58‰.The&nbsp; <span>δ</span>¹³<span>C</span>₂&nbsp; value ranges from -35.58‰ to -25.77‰,with an average of -29.9‰.The carbon isotopic composition of methane and ethane in the high sulfur natural gas under the Ordovician gypsum-salt rock is significantly heavier,which is the result of the thermochemical sulfate reduction (TSR).The results of gas-gas and gas-source correlation of various geochemical index show that the Ordovician subsalt natural gas in Ordos Basin belongs to oil-type gas which is characterized by self-generation and self-accumulation,and the Ordovician marine source rocks are the chief source rocks of producing hydrocarbon.

随着新区、新层系勘探力度持续加大，鄂尔多斯盆地奥陶系盐下马五₆亚段—马四段天然气勘探取得重大突破，盐下天然气的来源及勘探潜力问题备受关注。应用地质、地球化学方法，在明确天然气地球化学特征、成因及来源的基础上，对其源岩的沉积环境、有机地球化学特征、空间发育规模及资源潜力开展综合研究。结果表明：①奥陶系盐下天然气以“高温裂解干气”为主，其中，低（不含）硫天然气的δ¹³C₁值偏低，平均值为-39.6‰，δ¹³C₂值为-35.6‰~-25.8‰，变化幅度大，高硫天然气的δ¹³C₁、δ¹³C₂值均偏高，反映TSR作用阶段不同，天然气组分及碳同位素组成受影响程度不同；②盐下天然气属于自生自储“油型气”，供气源岩为奥陶系盐下海相烃源岩；③奥陶系盐下发育黑色泥质岩、暗色泥质云岩（云质泥岩）和薄层藻泥晶灰岩3种烃源岩类型，除泥晶灰岩外，主要形成于高盐缺氧的局限海潟湖沉积环境，沉积水体分层显著，属还原环境；该套源岩有机质丰度较高，TOC平均值为0.45%，生烃母质以菌、藻类为主，有机质演化达到高-过成熟阶段，总生气量约为43.8×10¹²m³，可为远离上古生界气源的盐下有利储集体供烃。

With the continuous increase of the exploration efforts in new zones and new strata， a major breakthrough has been made in the natural gas exploration of O₁ m ₅ ⁶ to O₁ m ₄ in Ordos Basin.Thus，the origin and exploration potential of subsalt natural gas have attracted much attention and need to be solved urgently. 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 the source rocks is 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”； the δ¹³C₁ of the Ordovician subsalt low sulfur （non sulfur） natural gas is lighter， with an average value of -39.6‰；the δ¹³C₂ ranges more largely from -35.6‰ to -25.8‰； the δ¹³C₁ and δ¹³C₂ of high-sulfur natural gas are both heavier， which reveals 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 belongs to “oil-type gas” of self-generation and self-accumulation， whose source rocks are mainly Ordovician subsalt marine source rock.（3）Three types of marine source rocks such as black argillaceous rock， dark argillaceous dolomite （dolomitic mudstone）， and dark micrite （bioclastic） limestone， are developed in Ordovician subsalt. In addition to micrite limestone， it was mainly formed in a confined lagoon sedimentary environment with high salinity and anoxia. Sedimentary water was stratified significantly and the environment was reduced. The organic matter abundance of the source rocks is relatively high， with an average TOC of 0.45%.The hydrocarbon generating parent materials are mainly composed of bacteria and algae， and the organic matter evolution reaches high-over maturity stage. The total gas generation amount of the marine source rocks in Ordovician subsalt is about 43.8×10¹² m³， which can provide hydrocarbon and accumulate for the subsalt favorable reservoir facies far away from the Upper Paleozoic gas sources.

鄂尔多斯盆地中东部奥陶系天然气资源丰富，随着深层天然气勘探的进一步深入，在奥陶系盐下地层也相继发现了可观的天然气资源，但盐下天然气的成因类型及来源仍有较大争议。基于天然气的组分、稳定碳同位素等资料，结合实际地质背景，对鄂尔多斯盆地中东部奥陶系马家沟组盐下天然气成因类型及来源进行了系统的分析。研究表明：鄂尔多斯盆地中东部马家沟组盐下天然气以干气为主，非烃含量较低，甲烷碳同位素（δ¹³C₁）值主频分布在-40‰~-32‰之间，乙烷碳同位素（δ¹³C₂）值主频分布在-40‰~-28‰之间，主要表现为腐泥型气特征。综合天然气组分、碳同位素与平面分布特征，将奥陶系盐下天然气细分为油型气、煤型气和混合气，其中油型气分布范围较广，来源于奥陶系马家沟组盐下碳酸盐岩烃源岩，部分天然气发生TSR反应；煤型气和混合气分布范围小，集中在靠近盐下地层尖灭线边缘部位，主要来源于上古生界烃源岩，奥陶系烃源岩也有少量贡献。

The Ordovician natural gas resources are abundant in the middle and eastern of Ordos Basin. With the further exploration of deep natural gas， a mass of natural gas resources have been found in the Ordovician subsalt strata， but the genetic types and sources of subsalt natural gas are still controversial. Based on the data of natural gas composition and stable carbon isotope， combined with the actual geological background， this paper systematically analyzes the genetic types and sources of subsalt natural gas in Majiagou Formation， central and eastern Ordos Basin. The natural gas in the middle and east of Ordos Basin is mainly dry gas， and the gaseous non-hydrocarbon component content is less. The methane carbon isotopes composition （δ¹³C₁） is chiefly between -40 ‰ and -32 ‰， and that of ethane （δ¹³C₂） is between -40 ‰ and -28 ‰， which is characterized by sapropelic origin. According to the composition， carbon isotope and plane distribution characteristics of natural gas， the Ordovician subsalt natural gas can be divided into oil type gas， coal type gas and mixed gas. Oil type gas is widely distributed and comes from the subsalt carbonate source rock of the Ordovician Majiagou Formation， and some natural gas has TSR reaction. The distribution range of coal type gas and mixed gas is small， and they are concentrated near the edge of subsalt formation pinch out line. They mainly come from the Upper Paleozoic source rocks， and the Ordovician source rocks also make a small contribution.

鄂尔多斯盆地中东部米探1井在奥陶系马家沟组四段钻获高产工业气流，实现了奥陶系盐下天然气勘探的重大突破，但目前对其成因来源存在争议。实测结果表明，米探1井天然气以烷烃气为主（95.18%），气体干燥系数（C₁/C_1-5）为0.947，非烃气体中H₂S含量为3.49%，还有少量的N₂和CO₂。天然气中甲烷、乙烷、丙烷的碳同位素值分别为-45.5‰、-26.4‰、-24.3‰。基于区域地质背景、潜在烃源岩特征和天然气地球化学特征，认为米探1井天然气为奥陶系盐下碳酸盐岩自生自储的油型气，但其存在甲烷碳同位素组成偏轻和乙烷碳同位素组成具有煤型气特征等地球化学异常。结合生烃热模拟实验和岩石残余气特征认为米探1井特殊的地球化学特征与普遍存在的膏岩关系密切：一方面，普遍存在的膏岩提供了良好的盖层使得很多早期生成的天然气得以留存；另一方面，膏岩的存在促进了H₂S和乙烷等重烃类气体的生成。此外，小于5%的H₂S含量和较高的重烃气体（C2+）含量说明即使发生了硫酸盐热化学还原作用（TSR）也应该不太强烈，此时甲烷不会受到TSR影响，乙烷碳同位素组成可能是识别TSR的敏感指标。

The Well Mitan-1 obtained high-yield industrial gas flow in the fourth member of the Majiagou Formation of Ordovician（O₁ m₄） in the mid-eastern Ordos Basin， which has achieved a major breakthrough in the exploration of Ordovician subsalt natural gas. However， there are disputes on the origin of natural gas in Well Mitan-1 at present. The measured results show that the natural gas in Well Mitan-1 is mainly composed of alkane gas （95.18%）， the gas drying coefficient （C₁/C_1-5）is 0.947， the content of H₂S is 3.49%， and there is also a small amount of N₂ and CO₂ in non-hydrocarbon gas. The carbon isotopic compositions of methane， ethane and propane in the natural gas are -45.5‰，-26.4‰ and -24.3‰， respectively. Based on the regional geological background， the characteristics of potential source rocks and the geochemical characteristics of natural gas， it is considered that the natural gas in Well Mitan-1 is self-generated and self-accumulated oil-associated gas in Ordovician subsalt carbonate rocks. However， there are some geochemical anomalies， such as methane carbon isotope value （δ¹³C₁） is lighter and ethane carbon isotope （δ¹³C₂） has the characteristics of coal-type gas. Combined with the thermal simulation experiment of hydrocarbon generation and the characteristics of residual gas in rocks， it is considered that the special geochemical characteristics of Well Mitan-1 are closely related to the gypsum rocks. On the one hand， the ubiquitous gypsum-rock provides a good caprock， which makes retainment of the early-generated natural gas. On the other hand， the existence of gypsum rock promotes the generation of heavy hydrocarbon gases （ ${\mathrm{C}}_{{\mathrm{2}}^{+}}$） and H₂S. In addition， less than 5% H₂S content and higher ${\mathrm{C}}_{{\mathrm{2}}^{+}}$ content indicate that the thermochemical sulfate reduction （TSR）， if any， should not be so strong that methane is not affected by TSR. The δ¹³C₂ may be a sensitive parameter for identifying TSR.

碳酸盐岩是鄂尔多斯盆地古生代重要的油气储集层。综合利用岩心、薄片和测井资料,在鄂尔多斯盆地大牛地气田奥陶系马家沟组中下组合识别出蒸发台地、开阔台地和局限台地沉积亚相类型,详细分析了不同沉积亚相特征,建立受海平面升降控制的大牛地气田马家沟组中下组合沉积模式。马一段至马三段沉积时期海平面较低,沉积了蒸发台地相,其中马一段和马三段的盐湖微相主要分布于东南部,膏湖微相分布于盐湖外围,而马二段海平面略有上升,随之沉积了覆盖全区的泥云坪微相和膏云坪微相;马四段形成于最大海泛期,沉积开阔台地相,自下而上依次形成覆盖全区、稳定分布的灰坪微相和云灰坪微相;马五段形成于海退&#x02014;海侵&#x02014;海退过程,主体沉积局限台地相,马五₇_-₁₀亚段的云坪微相覆盖全区,马五₆亚段自西北往东南发育大面积的泥云坪微相,马五₅亚段中部以灰坪微相为主,云坪微相围绕灰坪分散分布。

Through the sedimentological observations of a large number of outcrops and wells, this study prepared the lithofacies paleogeographic maps of the Ordovician in Ordos Basin in a 3rd-order sequence stratigraphic framework. In the Lower Ordovician, the Ordos Basin was divided into two sedimentary areas (i.e., eastern and western carbonate platforms) by the south-north Yimeng-Central Palaeouplift, where the sedimentary paleogeography was controlled by the North China Sea and the Qilian Sea, respectively. In the Lower-to-Middle Ordovician, the transgression occurred and Yimeng-Central Palaeouplift gradually reduced in the basin. In the Middle Ordovician, the Yimeng-Central Palaeouplift was largely submerged by seawater and divided into two parts, i.e., Qingyang oldland in the south and Yimeng oldland in the north. In the late Middle Ordovician, i.e., the Darriwilian Age (deposition period of the 6th Member of Majiagou Formation), the transgression occurred in the maximum scale. In the Upper Ordovician, the regression occurred, resulting in the vast denuded zones of oldland in the present Ordos Basin with narrow open rimmed platforms and slope facies in the western and southern margins of the basin. In the middle-to-late Upper Ordovician (Katian-Himantian Age), the Ordos Basin almost entirely uplifted to the continent. From Floian Age (the late Lower Ordovician) to Darriwilian Age (the middle stage of Middle Ordovician), i.e., deposition period of the 1st-5th Members of Majiagou Formation, an evaporation environment was formed in vast areas of the eastern basin, leading to the formation of localized lagoon and even gypsum salt lagoon. The western and southern basin was controlled by the development and evolution of Helan Aulacogen and Qinqi Troughs and mainly developed continental margin sediments, which evolved from the carbonate ramps of passive continental margin in the Lower and early Middle Ordovician to the rimmed platform of active continental system in the late Middle and Upper Ordovician.

MT1井的战略突破揭示鄂尔多斯盆地马家沟组四段（简称马四段）具有良好的勘探前景，然而对马四段沉积期岩相古地理及储层认识的不足制约了天然气勘探与部署。利用地质与地球物理相结合的方法，对马四段沉积期岩相古地理进行恢复并开展岩相古地理对储层发育的控制研究，结果表明：（1）马四段沉积前鄂尔多斯盆地具有三隆一坳的古地理格局，三隆即伊盟古陆、中央古隆起和吕梁隆起，一坳即台内坳陷，又可进一步分为两凸两凹，即榆林—横山凸起、神木—米脂凸起、桃利庙凹陷和米脂凹陷。隆坳相间、坳中有凸的古地理格局控制了马四段储集相带的展布；（2）马四段沉积期由3个四级旋回组成，马四3亚段沉积期主体为海侵旋回，中央古隆起演化为弱镶边台地边缘，盆地东部整体表现为半局限台地，以石灰岩和云质石灰岩沉积为特点；马四₂ 亚段沉积期为海退早期，随着台缘滩的加积和海平面下降，台缘滩障壁作用凸显，盆地东部表现为局限台地沉积特点，在凸起区开始发育丘滩并沉积薄层白云岩和硬石膏岩；马四₁ 亚段沉积期为海退中期，台缘滩障壁作用加剧，盆地东部局限台地水体持续变浅，凸起以丘滩坪沉积为主，白云岩和硬石膏岩厚度增加、范围扩大。（3）岩相古地理控制了马四段储层发育的类型与分布，主要发育了颗粒白云岩储层、微生物白云岩储层和斑状（灰质）白云岩储层，颗粒白云岩储层和微生物白云岩储层受颗粒（丘）滩微相控制，沿古隆起和凸起区发育，而斑状（灰质）白云岩储层则分布于潟湖相。研究认为，榆林—横山凸起及神木—米脂凸起地区有利于马四段储层发育，是有利勘探区带。

The strategic breakthrough in Well MT 1 indicates promising prospects of the fourth member of Majiagou Formation (Ma 4 Member) in Ordos Basin. However, the lack of understanding on lithofacies paleogeography and reservoir development of Ma 4 Member restricts the further gas exploration and deployment. By integrating geological and geophysical methods, the lithofacies paleogeography restoration of Ma 4 member and its control on reservoir development are analyzed. The study results show that: (1) The paleogeography before the deposition of Ma 4 member had a pattern of “three uplifts and one depression”, i.e., Yimeng Ancient Land, Central Paleo Uplift, Lvliang Uplift, and Intra Platform Depression. Among them, the depression was further divided into two bulges and two sags, namely Yulin-Hengshan Bulge, Shenmu-Mizhi Bulge, Taolimiao Sag and Mizhi Sag. The paleogeographic pattern of alternating uplift and depression and bulge in depression controlled the distribution of favorable reservoir development zone of Ma 4 member; (2) Three fourth-order cycles were developed in the deposition period of Ma 4 member, including the third, second, and first submembers of Ma 4 member (i.e., Ma 4₃, Ma 4₂ and Ma 4₁) from bottom to top. Ma 4₃ submember was dominated by transgression cycle, in which the Central Uplift evolved into a weakly rimmed platform margin and the eastern basin presented a semi-restricted platform, with the main deposits of limestone and dolomitic limestone. Ma 4₂ submember was developed in the early-stage regression. With the accretion of platform marginal beach and the fall of sea level, the barrier effect of platform margin was prominent. The eastern basin was characterized by the restricted platform environment, and thin layers of dolomite and gypsum rocks of mound beach facies were deposited in bulge areas. Ma 4₁ submember was mainly deposited in the middle stage of regression. The barrier effect of the platform margin enhanced, and water depth in the restricted platform in the eastern basin continued to be shallower.The mound beach was dominated in bulge areas, with a larger thickness and wider distribution range of dolomite and gypsum rock; (3) The lithofacies paleogeography controlled the type and distribution of Ma 4 member reservoir, with the grain dolomite, microbial dolomite and porphyritic (limy) dolomite reservoirs developed. Controlled by the grain (mound) beach microfacies, the grain dolomite and microbial dolomite reservoirs were developed at the paleo uplift and bulge area. While the porphyritic (limy) dolomite reservoir was mainly developed in lagoon environments. In conclusion, Yulin-Hengshan and Shenmu-Mizhi bulges are favorable areas for reservoir development and gas exploration practice.

在四川盆地东北地区(川东北地区)找到了迄今为止该盆地最大的天然气田――飞仙关组鲕滩气田。在该气田的鲕滩储层中不同程度地含有固体沥青,沥青含量在0.09%～2.50%之间;储集孔隙发育的残余鲕粒白云岩的沥青含量最高,沥青含量与天然气产量、气藏规模及储层物性之间呈正相关关系。应用全岩热模拟和环境扫描电镜实验新技术进行的沥青的产气模拟实验表明,沥青仍有产气潜力,并随成熟度增大,产气潜力降低;沥青模拟产物的碳同位素较轻,反映出了腐泥型母质的特点;沥青的甾萜类生物标志化合物分布正常:这些特征表明了储层沥青是古油藏原油裂解成气的中间产物。此外,天然气C1-C3组分进一步证实了鲕滩天然气主要为原油裂解气。在古构造背景下,根据优质储层展布、储层沥青含量的分布趋势,预测了古油藏分布范围。

The biggest Feixianguan oolitic gas field has been found in northeast area of Sichuan basin up to date. The reservoir in northeastern Sichuan basin generally contains solid bitumen and its content is 0.09% to 2.50%. Of all the types of reservoir rocks, the content of bitumen is the most abundant in the oolitic dolomite. The abundance of bitumen is related to the gas yield, gas pool scale and reservoir continuity. Novel methods such as whole rock pyrolysis and Environmental Scanning Electron Microscope.

Identification of natural gas genesis and source for high-matured multiple natural gases is a great challenge in the exploration of deep-ultra deep and unconventional natural gases. In this paper, the genesis identification method system of multiple natural gases is enriched through new experimental techniques and comprehensive analysis of geological data. New indexes and charts of genesis identification for multiple natural gases were determined to distinguish the sapropelic kerogen degraded gas and crude oil cracking gas, accumulated and scattered liquid hydrocarbon cracking gas in different evolution stages, nitrogen, carbon dioxide of organic and inorganic origins, inert gases of crustal and mantled origins, coal-formed gas and oil-type gas by helium, nitrogen, carbon dioxide and mercury content in natural gas. These indexes and charts have been successfully applied in the Sichuan, Tarim and Songliao basins to identify the natural gas genesis and source for complicated gas reservoirs. The research results have provided effective support for the natural gas exploration in the Sinian-Cambrian ancient carbonate formations in the Sichuan Basin, deep formations in the Kuqa depression of the Tarim Basin, and deep volcanic formations in the Songliao Basin.

针对鄂尔多斯盆地中东部盐下奥陶系内幕天然气自生自储特征及其相应烃源岩发育不清的实际，利用地球化学与测井技术相结合的方法，分析了研究区奥陶系盐下烃源岩的分布特征。奥陶系马家沟组马五₆亚段膏岩最为发育，分布最为广泛，该膏岩层之下发现的自生自储天然气出气点表明了海相碳酸盐岩烃源岩具有一定的生排气能力。研究表明:研究区马家沟组盐下碳酸盐岩烃源岩有机质丰度总体偏低，测试的有机碳含量平均值约为0.2%，大部分低于0.5%；岩石热解T_ma平均值达484℃,处于高成熟—过成熟阶段。平面分布上，盐下碳酸盐岩烃源岩有机碳含量在西部相对较高，向东逐渐降低；有机碳含量在北部相对较高，向盆地中部降低，最南部略有升高。研究区存在3个相对较好的烃源岩有利分布区。

Aimed at the fact of the self-preserved and self-generated characteristics of the Ordovician inside-scene subsalt gas reservoir and ambiguous recognition of the development of its corresponding source rocks,the distribution features of the Ordovician subsalt source rocks in the study area have been analyzed by jointly employing organic geochemistry and well logging technology.The gypsum layers are most widely developed in the 6 sub-members of the member 5 of the Ordovician Majiagou Formation with the widest distribution.The gas production points of the self-stored and self-generated natural gas discovered below the gypsum layers indicate that the marine carbonate source rocks are entitle certain ability for gas generation and discharging.It is indicated in the study that generally the subsalt carbonate source rocks in the Majiagou Formation have low organic matter abundance with a measured mean <em>TOC</em> content of about 0.2%,most of them being lower than 0.5%.The average rock pyrolysis peak temperature reaches 484℃,seating in the stage of highly-mature to overmature.On plane,the subsalt carbonate source rocks show high <em>TOC</em> content in the west,decreasing gradually in the east.It is higher in north Ordos Basin and decreased to the center;however it finally increased in south Ordos Basin.There are three favorable areas for the distribution of relative source rocks in the study area.

流体包裹体分析是研究油气运移和成藏期次等问题的一种非常有效的方法。在系统地观测了鄂尔多斯盆地中部奥陶系碳酸盐岩储层中流体包裹体的类型、分布特征、均一温度、冰点、盐度及单个包裹体成分的基础上,探讨了该盆地中部气区奥陶系风化壳天然气的成藏期次和运移方向。研究表明,在鄂尔多斯盆地中部奥陶系碳酸盐岩储层中,流体包裹体的类型和特征比较复杂,有机包裹体占5%～80%,包裹体均一温度为70～180℃,冰点为-1.0～-13.5℃,盐度多大于5.0wt.%,密度为0.922～1.065g/cm3。认为鄂尔多斯盆地中部气区天然气主要成藏期至少有两个,即晚三叠世末和早白垩世末;这两期天然气的运移方向总体上是由东向西、由南向北。

The Majiagou Middle Assemblage refers to the whole gas-bearing reservoir beds from Submember-5 to Submember-10 of Lower Ordovician Majiagou Member-5 in Ordos Basin. Intercrystalline pore, intercrystalline dissolved pores and dissolved vugs are predominating in the Middle Assemblage dolostone reservoirs that are widely distributed on the east side of the Ordovician Central Paleouplift, Ordos Basin. The Ordovician Middle Assemblage dolostone reservoirs experienced a long process of hydrocarbon accumulation and reservoir evolution. The good source-reservoir-cap assemblage consists of the Middle Assemblage karst dolostone as reservoirs and the overlying thick Carboniferous-Permian coal measure strata as cap and source rock. Attributed to the upper Paleozoic source rock that had reached to the peak of hydrocarbon generation and expulsion during late Jurassic to early Cretaceous period, the "source in the upper and reservoir in the lower" styled gas reservoirs formed widely in the Middle Assemblage. Because of modulation of the earlier gas reservoirs caused by structural evolution since early Cretaceous, nowaday gas reservoirs commonly formed in the laterally-sealling locations within Middle Assemblage. It is suggested that lithologic traps are the favorable exploration targets.