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1.
准噶尔盆地YJ油田是一个埋藏很深,成藏机理复杂的岩性、地层圈闭油气藏。通过侏罗系西山窑组和三工河组原油、油砂抽提物地球化学特征分析后认为,西山窑组原油具有两期成藏混合的特征,三工河组成熟轻质原油、油砂抽提物又表现出煤系烃源岩的特征。这表明,YJ油田具有多源多期充注成藏的特点。对地表土壤中甲烷碳同位素、烃类比值、三维荧光图谱的分析后认为,甲烷同位素、甲烃同位素、烃类比值与凝析油伴生气、石油伴生气的经验值相符;三维荧光既有凝析油-轻油又有轻油-重油的图谱特征。这与原油和油砂抽提物的研究结果相近,也表明该油田具有多种油源。因而认为,在未知地区对土壤油气地球化学特征进行研究,对于初步判断油气来源同样具有良好的效果。 相似文献
2.
准噶尔盆地白家海凸起北临东道海子凹陷,南靠阜康凹陷,东接五彩湾凹陷,是油气聚集的有利指向区。通过对比研究区周缘生烃凹陷的烃源岩和原油地球化学特征,并结合流体包裹体分析和盆地模拟技术,对白家海凸起二叠系、三叠系油气的来源、成藏期次以及成藏模式进行了探讨。结果表明白家海凸起二叠系、三叠系油气主要来自于北部的东道海子凹陷二叠系平地泉组烃源岩。油气发生过两期充注:第一期从白垩纪早期持续到白垩纪末期,第二期从白垩纪末期持续到第四纪,为油气充注高峰期。综合研究区构造特征和油气成藏过程,建立了二叠系和三叠系近源充注、断裂高效输导、先油后气充注、晚期调整成藏的油气成藏模式。 相似文献
3.
奥连特盆地斜坡带原油油品分布复杂,重质原油开发难度大、不经济,中轻质原油分布规律不清,严重制约斜坡带油气勘探。本文从原油地化分析入手,通过分析原油分布特征和地化特征,明确重油成因机理,建立原油充注模式,指出中轻质油有利发育区,以有效指导油气勘探。研究结果如下:(1)斜坡带原油正构烷烃具有前峰型碳分布形式,表明原油主要为海相母源;(2)原油具有“双低+双高”族组成,表明原油为烃源岩低熟-成熟阶段产物;(3)重油主要为生物降解成因,证据1为:饱和烃总离子流图发育UCM现象,证据2为:检测发现25-降藿烷标志性化合物;(4)地化分析表明同一样品中同时存在降解原油和未降解原油特征,表明原油为两期充注成因;(5)据此建立油气两期充注模式:早期常规原油充注,中期原油普遍遭受生物降解,原油变重,晚期常规原油充注与降解原油混合控制平面原油油品分布特征,并指出斜坡带砂岩尖灭条带下倾方向是常规油有利分布区,部署探井、评价井获得了成功。 相似文献
4.
通过对油砂山地区露头油藏圈闭特征、流体性质和地层压力分析,结合油气成藏过程,查明该区地质特征、形成机制和主控因素。油砂山地油层(组)分布受潜水面控制,原油物性由浅至深逐渐变好,产层呈明显低压;油气藏形成经历圈闭发育期、油气充注期和油藏调整-聚集期,具"持续充注、晚期成藏"特征。此过程中深部充足油源是成藏的物质基础,优质储层是油气储集的必要条件,断裂-超压联控是油气运移的关键,后期构造改造为油气分布的主导因素。 相似文献
5.
柴达木盆地西部英东地区多期构造运动导致油气分布复杂,勘探难度大.采用油气地球化学、流体包裹体、定量荧光以及傅里叶红外光谱等方法系统分析英东地区油气成藏演化过程.研究表明:(1)英东地区新近系储层发育黄色和蓝色荧光两类油包裹体和天然气包裹体,黄色荧光和蓝色荧光油气包裹体对应两期油气充注时间分别为12 Ma和5 Ma.(2)现今油藏原油具有同源-低熟的特点,黄色荧光油包裹体成熟度较低与现今油藏原油特征相似, 是现今油藏的主要贡献者.(3)蓝色荧光油包裹体的成熟度较高,蓝色荧光油包裹体和气烃包裹体是早期低熟原油沿后期构造产生的断层疏导体系向上调整运移至浅层分馏和脱气的产物.通过以上分析明确了英东油藏经历了下油砂组(N21)沉积末期(12 Ma)的低熟原油充注和上油砂组(N22)沉积末期(5 Ma)至今油气调整与改造的成藏过程, 与喜山期主要构造活动期有很好的对应关系,反映构造活动对油气成藏具有重要控制作用. 相似文献
6.
《地球化学》2017,(3)
在准噶尔盆地玛湖凹陷斜坡区下三叠统百口泉组地层中发现大面积连片岩性油气藏。对西斜坡百口泉组一段和二段8个含油储集岩样分别提取自由态组分、束缚态组分和油气包裹体组分,并进行GC、GC-MS和GC-IRMS分析。结果表明,3类油气组分三环萜烷主峰含量远高于C_(30)藿烷,反映充注到百口泉组(T1b)储层的原油均具有很高的成熟度,相当于Ro1.20%。生物标志物指标β-胡萝卜烷/(β-胡萝卜烷+nC20)、伽马蜡烷/(C_(30)藿烷+伽马蜡烷)、三环萜烷/(三环萜烷+藿烷)、Ts/(Tm+Ts)、C_(29)Ts/(C_(29)藿烷+C_(29)Ts)、C_(30)重排藿烷/(C_(30)重排藿烷+C_(30)藿烷)和甾烷C_(21)/(C_(21)+∑C_(29))比值均显示从自由态组分、束缚态组分至油气包裹体组分成熟度依次降低的特征,表明油气组分在油藏充注过程中不断升高。与此相反,Pr/nC_(17)和Ph/nC_(18)比值以及正构烷烃单体碳同位素组成显示从自由态组分、束缚态组分至油气包裹体组分成熟度依次升高的特征,表明油气组分从成藏早期至晚期成熟度降低。由于充注的原油为不同成熟度油气组分混合产物,生物标志物在原油中属于微量及痕量组分,而Pr、Ph和正构烷烃属于常量组分,Pr/nC_(17)和Ph/nC_(18)比值以及正构烷烃单体δ~(13)C值更能代表主体原油的成熟度。因此,对于玛湖凹陷西斜坡油气藏,主体原油成熟度从成藏早期至晚期是降低的。根据8个油砂样3类油气组分的成熟度可以推断油气充注期介于晚侏罗世-白垩纪之间。盖层密封性是控制这类油气藏成藏的重要因素。在晚侏罗世之前,下伏二叠系盖层一直阻止了油气沿断层运移。至晚侏罗世-白垩纪,下伏盖层的密封性遭受破坏,油气才从下伏二叠系沿断层运聚至百口泉组一段和二段储层(T_1b~(1–2))。由于上覆百三段(T_1b~3)和克拉玛依组(T_2k)盖层自始至终保持密封状态,百口泉组一段和二段储层(T_1b~(1–2))成为玛湖凹陷油气的最终归宿。 相似文献
7.
应用储层流体包裹技术研究了恩平凹陷油气的成藏规律.结果表明,该区油气主要有两期充注,第一期充注时间较早(20 Ma),以油为主,原油主要来自深部文昌组中深湖相烃源岩,大量油气见于浅层储层;第二期充注时间较晚(5 Ma以后),以天然气为主,天然气主要源自文昌组烃源岩,油气主要分布在深层储集层中.研究认为第一期油气充注与盖层形成时间匹配不好,可能是浅层勘探失利的主要原因之一,因此勘探方向应以深层储层(文昌组)为主,寻找深层早期构造更为重要,是下一步勘探的重要领域. 相似文献
8.
塔里木盆地塔中礁滩体大油气田成藏条件与成藏机制研究 总被引：4，自引：6，他引：4
中国海相碳酸盐岩油气勘探近年来进展很快,发现了一批大型油气田。塔中地区是塔里木盆地的重点勘探区和富油气区,奥陶系蕴藏了丰富的油气资源。奥陶系良里塔格组受塔中I号坡折带的控制发育陆棚边缘礁滩体,储层性质为低孔-特低孔、低渗灰岩储层,埋深在4500~6500m。储层的形成和分布受早期高能沉积相带、溶蚀作用和断裂作用等因素的控制,有效储层的空间展布控制了油气的分布与大面积成藏。油源对比认为,塔中良里塔格组礁滩体油气藏的原油主要来自于中上奥陶统烃源岩,并混有寒武系烃源岩成因的原油;天然气主要来自于寒武系油裂解气,沿塔中I号坡折带断裂向内充注。成藏过程分析表明,塔中地区曾存在三期主要成藏期,第一期为加里东晚期成藏,油气来自于寒武系-下奥陶统烃源岩,但早海西期的构造运动,对该期油气破坏严重,造成大范围油藏破坏。第二期成藏期是晚海西期,也是塔中地区最重要的油气充成藏期,油气来自于中上奥陶统烃源岩。第三期成藏期是晚喜山期,受库车前陆冲断影响,台盆区快速沉降,埋深急速增大,寒武系原油裂解气形成,沿深部断裂向浅部奥陶系充注,对油藏进行气洗改造,从而形成大面积分布的凝析气藏。 相似文献
9.
塔里木盆地沙雅隆起北部于奇西地区奥陶系为重质油勘探区，但于奇西1井（YQX1）奥陶系钻遇中-轻质油.为探求其油气来源，通过沙雅隆起52件原油样品的金刚烷化合物分析，于奇西地区奥陶系131件岩心样品的岩石学观察、流体包裹体系统分析测试和同一期次油包裹体的气相色谱（GC）分析及色谱-质谱（GC-MS）分析，运用流体包裹体均一温度、单井埋藏史-热史图确定油气充注年龄，结合研究区断裂特征和盖层分布特征总结其油气成藏模式.结果表明，研究区原油普遍遭受生物降解作用并局部存在稠油与轻质油混合现象，同时奥陶系发育2~3期油气充注，分别为加里东晚期（452.5~420.5 Ma）、燕山期（150.2~100.6 Ma）和喜山期（20.1~2.1 Ma）；早期油气藏中原油因构造抬升发生生物降解而稠油化，晚期轻质油沿基底和寒武-奥陶系NW、NNE向走滑断裂垂向充注对早期油气藏进行混合改造，形成一定范围的中-轻质油藏. 相似文献
10.
为推动准噶尔盆地腹部地区盆1井西凹陷下凹勘探,利用重点风险探井烃源岩样品分析测试资料,对准噶尔盆地腹部地区烃源岩地球化学特征及石西地区深层石炭系油藏油源开展研究,取得了以下认识:(1)准噶尔盆地腹部地区二叠系风城组烃源岩为一套富含三环萜烷、伽马蜡烷、C28规则甾烷、轻干酪根C同位素,在咸化还原环境下形成的烃源岩;(2)二叠系下乌尔禾组烃源岩为一套富含五环萜烷、C24四环萜烷、Ts,在半咸水、半氧化环境形成的烃源岩;(3)石西地区石炭系原油为高熟轻质油,富含三环萜烷、伽马蜡烷、C28、C29规则甾烷, C24四环萜烷含量较低,全油C同位素轻,来自风城组烃源岩;(4)盆1井西凹陷风城组深埋烃源岩生成原油沿断裂、不整合向石炭系风化壳运移至石炭系成藏。根据油源研究结果推测,盆1井西凹陷可能发育风城组规模烃源岩,凹陷区内具有广阔的勘探前景。 相似文献
11.
准噶尔盆地西北缘玛湖凹陷是全球著名的湖相富烃凹陷,最新勘探成果展示出大油(气)区特征。为及时总结,丰富发展当代中国特色叠合盆地油气地质理论与勘探实践,本文主要分析了这一大油(气)区的基本形成条件与勘探领域。结果表明,玛湖凹陷大油(气)区得以形成主要缘于三大有利基础条件:前陆碱湖优质高效烃源岩、立体输导体系、三类规模有效储层与储盖组合,由此造就油气在充足的烃源条件下,沿立体的输导体系,被三套区域性盖层有效封盖,向着三类规模有效储层,在有利圈闭中运聚成藏,因此油气富集规律可归纳为"源-输控烃、储-盖控藏"。相应,按目的层和圈闭类型形成了三大有利油气成藏领域,由浅至深依次为三叠系下统百口泉组岩性油(气)藏群、二叠系下统风城组致密油、石炭系—下二叠统大构造油气藏群。玛湖凹陷大油(气)区的勘探突破缘于勘探理念创新,具有巨大勘探潜力和基础科学研究意义。 相似文献
12.
LIU Zuodong WANG Hongjun Graham BLACKBOURN MA Feng HE Zhengjun WEN Zhixing WANG Zhaoming YANG Zi LUAN Tiansi WU Zhenzhen 《《地质学报》英文版》2019,(1):199-212
Global recoverable resources of heavy oil and oil sands have been assessed by CNPC using a geology-based assessment method combined with the traditional volumetric method, spatial interpolation method, parametric-probability method etc. The most favourable areas for exploration have been selected in accordance with a comprehensive scoring system. The results show:(1) For geological resources, CNPC estimate 991.18 billion tonnes of heavy oil and 501.26 billion tonnes of oil sands globally, of which technically recoverable resources of heavy oil and oil sands comprise 126.74 billion tonnes and 64.13 billion tonnes respectively. More than 80% of the resources occur within Tertiary and Cretaceous reservoirs distributed across 69 heavy oil basins and 32 oil sands basins. 99% of recoverable resources of heavy oil and oil sands occur within foreland basins, passive continental-margin basins and cratonic basins.(2) Since residual hydrocarbon resources remain following large-scale hydrocarbon migration and destruction, heavy oil and oil sands are characterized most commonly by late hydrocarbon accumulation, the same basin types and source-reservoir conditions as for conventional hydrocarbon resources, shallow burial depth and stratabound reservoirs.(3) Three accumulation models are recognised, depending on basin type: degradation along slope; destruction by uplift; and migration along faults.(4) In addition to mature exploration regions such as Canada and Venezuela, the Volga-Ural Basin and the Pre-Caspian Basin are less well-explored and have good potential for oil-sand discoveries, and it is predicted that the Middle East will be an important region for heavy oil development. 相似文献
13.
LIU Zuodong WANG Hongjun Graham BLACKBOURN MA Feng HE Zhengjun WEN Zhixing WANG Zhaoming YANG Zi LUAN Tiansi WU Zhenzhen 《《地质学报》英文版》2019,93(1):199-212
Gl obal recoverable resources of heavy oil and oil sands have been assessed by CNPC using a geology-based assessment method combined with the traditional volumetric method, spatial interpolation method, parametric-probability method etc. The most favourable areas for exploration have been selected in accordance with a comprehensive scoring system. The results show: (1) For geological resources, CNPC estimate 991.18 billion tonnes of heavy oil and 501.26 billion tonnes of oil sands globally, of which technically recoverable resources of heavy oil and oil sands comprise 126.74 billion tonnes and 64.13 billion tonnes respectively. More than 80% of the resources occur within Tertiary and Cretaceous reservoirs distributed across 69 heavy-oil basins and 32 oil-sands basins. 99% of recoverable resources of heavy oil and oil sands occur within foreland basins, passive continental-margin basins and cratonic basins. (2) Since residual hydrocarbon resources remain following large-scale hydrocarbon migration and destruction, heavy oil and oil sands are characterized most commonly by late hydrocarbon accumulation, the same basin types and source-reservoir conditions as for conventional hydrocarbon resources, shallow burial depth and stratabound reservoirs. (3) Three accumulation models are recognised, depending on basin type: degradation along slope; destruction by uplift; and migration along faults. (4) In addition to mature exploration regions such as Canada and Venezuela, the Volga-Ural Basin and the Pre-Caspian Basin are less well-explored and have good potential for oil-sand discoveries, and it is predicted that the Middle East will be an important region for heavy-oil development. 相似文献
14.
The Erlian basin is a continental rift basin located in Inner Mongolia, Northern China. It is a typical representative of Cretaceous Northeast Asian Rift System, which includes many small petroliferous basins in Mongolia Republic and Northern China. Although Lower Cretaceous source rocks are understood to be most important in the Erlian petroleum systems, the precise identification of these source rock intervals and their determination on oil families distribution and composition are poorly understood in this tectonically complicated, nonmarine basin. New bulk data have been gathered from source rock intervals, oil sands and crude oil samples in eight main oil-producing subbasins. Geochemical analyses indicate that Lower Cretaceous Aershan formation (K1ba) and Tengger 1 formation (K1bt1) are two main source intervals in the Erlian basin and their source rock facies vary from profundal lacustrine to marginal lacustrine according to biomarker and trace elements calibration, the profundal lacustrine facies is characterised by brackish water and anoxic environment, which is similar to their correlative oils (Family 1 oils). The marginal lacustrine facies is characterised by freshwater and suboxic environment, which sourced the most common Family 2 oils. Meanwhile, different maturation processes exercise the second control on oil groups and their compositions, the profundal lacustrine source rocks characterised by their sulphur-rich kerogens lead to two oil groups (group 1 and group 2 oils), whose maturity range from low to normal; while, the marginal lacustrine source rock only lead to normal-maturity oils. At last, biodegradation affected the composition of a certain oils and formed group 4 heavy oils. In addition, short migration distance in small subbasins made the contamination or fractionation less notable in the Erlian basin. 相似文献
15.
构造活动是源-汇系统中形成古地貌和砂分散体系的重要影响因素。塔木察格盆地塔南凹陷在早白垩世是一个典型的断陷湖盆,经历了多期幕式断陷活动,但同沉积构造活动及其配置所产生的古地貌对层序结构特征、沉积体系及砂体分布的控制作用仍不明确。文中以地震、测井、岩心资料为基础,以层序地层学理论和古地貌控砂理论为指导,建立了塔南凹陷下白垩统层序地层格架;结合断层活动性分析,阐明了不同类型古地貌对沉积体系和砂体分布的控制作用和规律。构造演化和沉积充填的综合研究表明: 受控盆主断裂的幕式断陷活动及同沉积断裂的差异断陷活动影响,塔南凹陷发育横向凸起和走向斜坡2种类型的构造调节带,这些构造调节带是水系入盆的通道,控制着盆地的主体物源方向、沉积体系类型与分布特征;根据断层的几何形态及发育部位,共识别出4种类型构造坡折带,分别是陡坡断崖型坡折带、陡坡断阶型坡折带、缓坡反向断阶型坡折带和盆内坡折带,它们各自控制了不同类型沉积体系的形成与分布;沉积物入盆后,沉积物分散体系和砂体的分布特征受同沉积断裂的组合样式控制,研究区共发育梳状断裂系、叉状断裂系和平行断裂系3种同沉积断裂平面组合;与构造调节带相对应的断裂坡折带低部位是大型储层砂体,特别是低位扇三角洲或盆底扇砂体发育的有利部位,是寻找砂岩油气藏的有利区带。研究成果对于深化断陷湖盆源-汇系统理论、指导断陷湖盆砂体分布预测具有重要的理论意义和实际价值。 相似文献
16.
The petroliferous central Junggar Basin in northwest China is predominantly an oil exploration region. However, its gas exploration also might have good prospects. Thus to assist in gas exploration, the geochemistry and origins of gases are discussed in this paper based on relatively comprehensive analyses of compositions, carbon isotopes and light hydrocarbons of gases. Based on the results, the gas genetic types are grouped into families and combined with the geological setting (e.g., biomarkers of retrograde condensates and source rock characteristics). We show that there are four representative genetic types of gases. The first consists of gases derived from Permian lacustrine mudstones with type I–II kerogen and type III kerogen sources in the Penyijingxi sag. Their representative geochemical feature is δ13C2 ranging from −31.4‰ to −24.7‰. The second is gas sourced from Carboniferous tufaceous mudstones of type III kerogen in the Dishuiquan sag, whose representative geochemical feature is the heaviest values of δ13C1 in the studied samples, ranging from −32.0‰ to −30.4‰. The third consists of gases sourced from Jurassic coals and mudstones in the Shawan–Fukang sag. The light hydrocarbon fingerprints of these gases are similar to those of gases and oils typically derived from Jurassic source rocks in the southern Junggar Basin. The fourth is gas most likely generated from the degradation of crude oil. It is mainly found in the Luliang area and has dryness values as much as 0.999 and δ13C1 ranging from −54.8‰ to −43.2‰. Among these four types of gases, the first (mainly sourced from the Permian lacustrine mudstones in the Penyijingxi sag) is the predominant type. 相似文献
17.
准噶尔盆地玛湖凹陷深层油气流体相态研究 总被引：1，自引：0，他引：1
准噶尔盆地玛湖凹陷深层(5000m)是我国深层油气勘探当前的一个热点风险领域。为给区域下步油气勘探提供参考,通过对区内烃源岩和油气进行系统的地质地球化学正演、反演对比综合研究,重点分析了油气流体相态。结果表明,研究区深层稳定发育石炭系—二叠系四套烃源岩,有机质丰度高,普遍达到中等—好质量,类型多种多样,既可生油亦可生气,加之所有烃源岩均已基本进入高—过成熟演化,因此深层油气流体相态总体应高熟轻质,可能包括高熟轻质油、干酪根裂解气、油裂解气等。原油地球化学研究表明,原油随埋藏深度增加、层位变老,油质有逐渐变轻的趋势,成熟度有逐渐增大的趋势,保存条件亦逐渐变好,因此预测深层原油油质更轻、成熟度更高、保存条件更好。天然气地球化学研究表明,中浅层天然气可能属于混合来源,包括干酪根裂解气和油裂解气,规模性的高过成熟天然气可能在深层。因此,研究区深层油气资源丰富,相态轻质,成藏有利,值得勘探。这些认识还可供具有相似油气地质背景地区在研究和勘探时类比参考。 相似文献
18.
惠民凹陷临邑洼陷岩性油藏控制因素分析 总被引：7，自引：3，他引：7
临邑洼陷古近系和新近系断裂发育，沉积相和沉积体系频繁变化，为岩性油藏的形成提供了良好条件。古近系岩性油藏主要包括砂岩透镜体油藏、砂体上倾尖灭油藏和砂体被断层切穿的岩性油藏三种类型。砂岩透镜体油藏主要分布于深凹陷带及斜坡上的三角洲前缘相带，砂体上倾尖灭油藏主要分布于斜坡带，砂体被断层切穿的油藏一般位于断裂相对发育的构造带附近。临邑洼陷具备丰富的油源条件、不整合与断层的沟通或侧向遮挡，有效的直接盖层等优越条件。因此，三角洲前缘砂体、滑塌浊积岩砂体、湖相砂体的分布，成为控制岩性油藏形成的主控因素。上述主要因素控制下，在不同构造部位和沉积相带，油藏类型有规律地变化。 相似文献
19.
准噶尔盆地玛湖凹陷三叠系源上砂砾岩扇—断—压三控大面积成藏模式 总被引：1，自引：0，他引：1
准噶尔盆地玛湖富烃凹陷下三叠统百口泉组新近发现了我国首个源上大面积连续型砂砾岩油藏群,是对全球"连续型"油气藏研究的新补充,但其成藏机理和模式并不很清楚。为加强对其的理论认识,并为下步勘探提供参考,基于油气生、储、盖、圈、运、保等基础石油地质条件,并结合油气藏特征,进行了成藏条件与成藏模式的综合研究。结果表明,优质充足的下二叠统风城组碱湖油气来源、规模有效的扇三角洲前缘砂砾岩储层、多重组合的扇三角洲平原致密砾岩、泥岩和断裂封盖保存、沟通良好的高角度断裂输导体系、平缓连续的构造背景奠定了大面积连续成藏的地质基础。在此背景下,高成熟的风城组所生成油气,在切穿烃源灶和储层的高角度压扭性断裂沟通下,优先充注物性相对好的扇三角洲前缘水下河道砂岩和砂质细砾岩,并且在地层异常高压促进下,控制着油气富集程度,使得油气成藏表现为大型缓坡浅水扇三角洲沉积控制下的源上扇-断-压三控大面积"连续型"。百口泉组油气藏具有的油质轻且含气、微裂缝广泛发育,以及异常高压等,决定了砾岩储层虽总体低孔低渗,但依旧能够高产。在油源断裂沟通的斜坡区上倾方向,叠合地层异常高压以及扇三角洲前缘水下河道砂砾岩的区域是下步有利勘探方向。 相似文献
20.
G.T Philippi 《Geochimica et cosmochimica acta》1977,41(1):33-52
Paraffinic crude oils are designated ‘primary’ because their composition is very close or identical to that of the hydrocarbons extracted from the corresponding oil source rocks. Heavy and medium-gravity naphthenic crude oils, on the other hand, typically are quite different compositionally from hydrocarbon mixtures in either mature or immature shales.The normal paraffin carbon number odd/even ratio 2C29/(C28 + C30) of all the heavy to medium-gravity crude oils which could be analysed are in exactly the same range as is observed for the primary paraffinic crude oils, namely 0.95–1.42. The naphthene indices of the medium to heavy gravity naphthenic crude oils and of the primary paraffinic crude oils from the same area are identical or close. These facts are significant because both the n-paraffin carbon number odd/even ratio and the naphthene index of shale hydrocarbons are strongly depth and subsurface temperature dependent. The facts observed demonstrate beyond question that, in the same area, the paraffinic precursors of the heavy to medium-gravity naphthenic crude oils are generated and expelled in the identical depth range, and from the same mature relatively deep oil source beds as the primary paraffinic crude oils. Later, during and/or after a generally upward migration into oil reservoirs, the primary crude may be transformed compositionally into a naphthenic crude oil.In none of the five widely scattered oil basins studied are medium to heavy naphthenic crude oils found at temperatures greater than a limiting subsurface temperature. The abruptness of the temperature cutoff of the change in oil compositions in all five oil basins, as well as the average value of the cutoff temperature of 66°C (150°F), leaves no doubt that the mechanism of this crude oil transformation process is microbial.Optical activity, which was observed in narrow saturate hydrocarbon fractions of the 80–325°C range of all microbially transformed crude oils, but not in the primary untransformed oils, is strong additional evidence for the microbial nature of the crude oil transformation process. The observed optical activity is explained by the microbial digestion at different rates of optical antipodes present in the primary paraffinic crude oils.To gain perspective the vast scale of the microbial oil transformation process in nature is pointed out. Billions of tons of heavy to medium-gravity naphthenic crude oils, originating from the microbial transformation of primary paraffinic oils, are present in oil fields and tar sands all over the world. 相似文献