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作者简介:

熊万林(1987—),男,湖北仙桃人,工程师,硕士,从事油气成藏地球化学及含烃流体地质学研究工作。E-mail:xiongwl@cnooc.com.cn。

中图分类号:TE122

文献标识码:A

文章编号:1009-9603(2021)05-0050-07

DOI:10.13673/j.cnki.cn37-1359/te.2021.05.005

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目录contents

    摘要

    阳江凹陷为珠江口盆地近两年来获得油气勘探重大突破的新区。为明确阳江凹陷恩平21洼烃源岩的生烃潜力及成藏贡献,采用无机元素分析与有机地球化学分析相结合的研究方法,对阳江凹陷文昌组烃源岩的沉积环境、有机质来源及生烃潜力进行了系统分析,明确了已发现原油的来源并估算了不同沉积环境烃源岩的成藏贡献。结果表明:文昌组沉积时期恩平21洼气候湿热,湖盆水体较深、盐度较低,是整体处于亚还原环境的淡水湖盆,发育半深湖-深湖相和浅湖-半深湖相两套优质烃源岩;油-源对比及油藏中不同来源原油的占比分析发现,源于半深湖深湖相烃源岩的占比为15%,源于浅湖-半深湖相烃源岩的占比为85%。浅湖-半深湖相烃源岩同样具有较强的供烃能力,发育浅湖-半深湖相烃源岩的浅-小洼陷,也具有较好的油气资源潜力。

    Abstract

    Recently,Yangjiang Sag has become a new area that has made major breakthroughs in petroleum exploration in Pearl River Mouth Basin. To clarify the hydrocarbon generation potential and accumulation contribution of source rocks of Enping 21 Subsag in Yangjiang Sag,the sedimentary environment,source of organic matter,and hydrocarbon generation po- tential of source rocks of Wenchang Formation drilled in Yangjiang Sag were systematically analyzed,using methods com- bining inorganic element analysis with organic geochemical analysis in this paper. The sources of the discovered crude oil were identified and the accumulation contribution of source rocks in different sedimentary environments was estimated. The results show that Enping 21 Subsag is characterized by a humid-hot climate,and a deep freshwater lake basin with low sa- linity in a sub-reductive environment on the whole,and two sets of high-quality source rocks,including semi-deep to deep and shallow to semi-deep lacustrine source rocks,are developed during the deposition of Wenchang Formation. The oilsource correlation and the calculation of the proportion of crude oil from different sources in the reservoirs indicate that the crude oil from shallow to semi-deep lacustrine source rocks accounts for 85%,and that from semi-deep to deep source rocks only accounts for 15%. The shallow to semi-deep lacustrine source rocks also have strong hydrocarbon supply ability, and the shallow and small subsags developed with shallow to semi-deep lacustrine source rocks also have great petroleum resource potential.

  • 恩平 21 洼位于珠江口盆地珠三坳陷北部阳江凹陷东部,是以古生界沉积岩和浅变质岩为基底发育的新生代含油气小洼陷[1-2]。阳江凹陷油气勘探工作始于 1979 年,2011 年之前的多轮中外合作勘探,仅发现 3 个含油气构造,未获得商业发现;2011 年转入自营勘探之后,结合前人的勘探成果以及新采集的三维地震资料,对该凹陷的基础石油地质条件进行了重新评价。2018 年底钻探位于恩平 20 洼边部的阳 4 井,获得工业油流,开启了恩平 20 洼油气勘探的新篇章。至 2019 年底,恩平 20 洼周边共获得 2 个中型商业油田和 2 个潜在商业油田,实现了阳江凹陷勘探的整体突破[3-5]。然而,其后在紧邻恩平 20洼的恩平 21洼开展的油气勘探没有达到预期效果,2020年共钻探9口井,仅发现了5个含油构造。

  • 前人对阳江凹陷构造、沉积、烃源岩发育、油气来源以及成藏模式的研究主要集中在恩平20洼,研究认为阳江凹陷发育优质的中深-深湖相烃源岩,油气成藏具有“源-断-圈”耦合近源成藏的特征[3-6],对于商业发现较少的恩平21洼,研究程度较低。为此,笔者在烃源岩无机元素分析与有机地球化学分析相结合的基础上,研究了恩平21洼烃源岩的发育环境、生烃潜力及生物标志化合物特征,分析了已发现油气的成因来源并统计评估了不同沉积环境烃源岩的成藏贡献,为该洼陷后续的勘探部署风险评价以及珠江口盆地类似小洼陷的油气勘探研究提供参考。

  • 1 区域地质概况

  • 阳江凹陷为珠江口盆地珠三坳陷北东端 NE— SW 走向的长条状二级构造单元,北部毗邻阳春凸起、海南隆起,西南部以阳江低凸起与文昌 A 洼分隔,东侧为恩平凹陷,面积约为 2 300 km2。按构造特征可划分为“两凹一凸起”3 个次级构造单元,自西向东依次为阳江西凹、阳江中低凸起、阳江东凹。其中,阳江东凹自西向东依次发育阳江 24 洼、恩平 19 洼、恩平 20 洼及恩平 21 洼 4 个洼陷[3-6] (图1),洼陷面积分别为 337,88,74及 255 km2,单个洼陷规模较小。

  • 阳江凹陷与珠江口盆地东部其他凹陷具有相似的构造、沉积演化特征,表现为“下断上拗”、“下陆上海”的双层构造沉积特征[3-47]。其中下部断陷结构包括始新统文昌组和恩平组,属于陆相沉积体系,受湖盆控洼断裂“东早西晚”演化特征所控制,文昌组仅在阳江东凹发育;上部拗陷结构包括渐新统珠海组,中新统珠江组、韩江组和粤海组,上新统万山组和第四系,属于海相沉积体系。文昌组是主要的烃源岩层系,自下而上发育文三段、文二段和文一段;韩江组和珠江组的三角洲砂岩储集体是区域内主要的储层段,珠江组和韩江组海相三角洲泥岩是主要的区域盖层。

  • 图1 阳江凹陷构造单元划分

  • Fig.1 Division of structural units in Yangjiang Sag

  • 2 实验器材与方法

  • 2.1 实验器材

  • 实验仪器包括德国飞驰公司生产的 PULVERI⁃ SETTE6 行星式球状研磨仪,荷兰帕纳科公司生产的 Axios MAX X 射线荧光光谱仪(偏差小于 0.05%) 和ELEMENT XR等离子体质谱仪(偏差小于0.1%),法国万琪公司生产的ROCK-EVAL6岩石热解仪,美国力可公司生产的CS230碳硫分析仪以及安捷伦公司生产的Agilent7890 A气相色谱仪、6890 N和5973 C色谱-质谱仪。

  • 实验样品包括泥岩、砂岩及原油样品3种类型。由于研究区恩 3 井和恩 4 井分别钻遇文昌组 400 和 200 m厚灰黑色泥岩,本次实验采集20个岩屑样品。原油样品源自研究区恩 4、阳 6 和阳 7 井,共 5 件样品。砂岩储层井壁取心样品来源于恩 2、恩 3和恩 5 井,共采集20件样品。

  • 2.2 实验方法

  • 对泥岩岩屑样品进行了室内挑样及碎样,其后采用X射线荧光光谱仪和等离子体质谱仪进行无机元素分析。采用热解仪和碳硫分析仪进行烃源岩岩石热解和碳硫分析;采用索氏抽提法获取泥岩及砂岩储层井壁取心样品的氯仿沥青“A”,采用柱色层法把氯仿沥青“A”及原油样品分离为饱和烃、芳烃、非烃和沥青质;饱和烃色谱分析使用色谱柱为 HP-5石英弹性毛细柱(30 m×0.25 mm×0.25 μm),升温步骤包括:柱始温度为 50℃,升温速率为 4℃/ min,柱终温度为300℃,恒温16 min。饱和烃色谱质谱分析使用色谱柱为 HP-5MS 石英弹性毛细柱(30 m×0.25 mm×0.25 μm),升温步骤包括:50℃恒温 2 min,从 50℃至 100℃的升温速率为 15℃/min, 100℃至 300℃的升温速率为 4℃/min,300℃恒温 20 min。岩石和原油样品分析测试均在长江大学油气资源与勘探技术教育部重点实验室完成。

  • 3 烃源岩发育特征

  • 3.1 沉积环境

  • 岩石中的元素在岩石风化、搬运和沉积过程中受外部环境的影响,会发生规律性的分散和富集,使得元素的含量或不同元素间的比值出现规律性变化。利用沉积岩中元素含量或比值来表征古沉积环境变化,恢复古水深、古氧相、古气候、古盐度等环境指标已在油气勘探领域广泛应用[8-9]

  • 根据 20 个泥岩样品的常量和微量元素测试化验结果,分析恩平21洼文昌组古气候、古水深、古盐度和古氧相等环境指标的结果(表1)表明,文昌组沉积时期恩平 21 洼气候湿热,湖盆水体较深、盐度较低,为整体处于亚还原环境的淡水湖盆。这样的古气候及湖盆的水体条件有利于优质烃源岩的发育[10-13]

  • 3.2 生烃潜力

  • 岩石热解参数被广泛用于烃源岩生烃潜力评价,文昌组烃源岩样品的热解分析表明,恩4井烃源岩现今的有机碳含量及热解生烃潜量均高于恩3井 (图2a)。恩 3 井烃源岩有机碳含量为 1.85%~3.66%,平均值为 2.50%,恩 4 井烃源岩有机碳含量为2.17%~6.00%,平均值为3.09%。尽管2口井文昌组烃源岩有机质的氢指数(HI)差异明显,恩 4 井平均值为 543 mg/g,恩 3 井平均值为 224.5 mg/g,但两者的有机质类型基本一致,均为Ⅱ1和Ⅰ型(图2b)。处于缓坡带的恩 4 井样品深度为 2 870~3 180 m,对应的镜质组反射率(R o)为 0.62%~0.71%,刚进入成熟生烃期。处于陡坡带的恩3井样品深度为3 275~3 900 m,对应的 R o值为 0.73%~1.15%,正处于生烃高峰期。2口井钻遇文昌组烃源岩的有机质成熟度差异明显,除初始有机质丰度有差异外,成熟度差异也是导致两者现今有机质丰度差异的重要原因。分析认为,恩平21洼文昌组烃源岩具有较好的生烃潜力,有机质丰度为好-优质烃源岩[14-15]

  • 表1 恩平21洼文昌组古环境指标分析结果

  • Table1 Analysis of paleoenvironmental indices of Wenchang Formation in Enping21 Subsag

  • 注:0.840.940.90 最小值 最大值 平均值

  • 图2 恩平21洼文昌组泥岩生烃潜力评价

  • Fig.2 Evaluation of hydrocarbon generation potential for mudstone of Wenchang Formation in Enping 21 Subsag

  • 3.3 生物标志化合物特征

  • 烃源岩生物标志化合物的分布与组成特征是研究其有机质类型、演化并进行油源关系研究的重要参数,不同沉积环境、不同母质类型烃源岩的生物标志化合物特征往往差异明显。

  • 位于陡坡带的恩3井文昌组烃源岩生物标志化合物组合具有“五高”特征(图3a,图3b):高 C304-甲基甾烷、高Ts、高奥利烷、高重排藿烷及高树脂化合物 T,Ts/Tm 值为 1.82~2.87,T/藿烷值为 2.14~4.83。位于缓坡带的恩4井文昌组烃源岩生物标志化合物组合具有“一高四低”特征(图3c,图3d):高 C304-甲基甾烷、低Ts、低奥利烷、低重排藿烷及低树脂化合物 T,Ts/Tm 值为 0.57~1.23,T/藿烷值为 0.09~0.16,与恩3井钻遇烃源岩差异明显。

  • 分析认为,恩 3 井钻遇烃源岩沉积于亚还原环境,有机质来源具有较高的水生藻类和陆源高等植物的双重贡献,为浅湖-半深湖相烃源岩。恩4井钻遇烃源岩也沉积于亚还原环境,但有机质贡献以水生藻类为主,为半深湖-深湖相烃源岩。

  • 4 原油类型划分及成藏贡献分析

  • 4.1 原油类型划分及油源分析

  • 前人针对珠一坳陷原油成因来源做过大量的工作[16-19],认为树脂化合物是一类特征非常明显的高等植物树脂输入的标志化合物,代表陆源高等植物贡献[20-21],Ts/Tm 参数对黏土催化剂反应很灵敏,该参数受烃源岩沉积环境影响较为明显。恩平 21 洼原油及烃源岩的生物标志化合物特征对比分析结果表明,树脂化合物 T 含量以及 Ts/Tm 的相对含量是研究区油-源对比有效的地球化学指标,依据这 2 个参数,将恩平 21 洼原油划分为 2 种类型(图4)。第 1 类原油密度为 0.80 g/cm3(以阳 6 井和阳 7 井为代表),生物标志化合物组合具有“五高”特征 (图3e,图4):高C304-甲基甾烷、高Ts、高奥利烷、高重排藿烷及高树脂化合物 T,Ts/Tm 值为 1.85~3.27, T/藿烷值为2.71~7.26,与恩3井揭示的文昌组浅湖半深湖相烃源岩生物标志化合物组合特征一致,据此判断该类原油来源于恩平21洼文昌组浅湖-半深湖相烃源岩。第 2 类原油密度为 0.87 g/cm3(以恩 4 井为代表),生物标志化合物组合具有“一高四低” 特征(图3f,图4):高 C304-甲基甾烷、低 Ts、低奥利烷、低重排藿烷及低树脂化合物T,Ts/Tm值为0.59~1.38,T/藿烷值为 0.08~1.05,与恩 4 井揭示的文昌组半深湖-深湖相烃源岩生物标志化合物组合特征一致,据此判断该类原油来源于恩平21洼文昌组半深湖-深湖相烃源岩。

  • 4.2 不同类型原油分布特征

  • 不同沉积期发育的烃源岩随着盆地构造演化不断迁移,并在沉积深埋过程中持续生烃,因不同烃源岩段在生烃期的油气运移成藏条件存在差异,使得所生成油气有序赋存在盆地的不同构造部位,不同来源油气的空间分布特征是油气宏观运移成藏条件的综合响应结果。

  • 恩平 21 洼不同类型的原油分布受各次级洼陷烃源岩发育特征控制作用明显。中-东次洼南部陡坡带珠江组储层中的原油主要来自于半深湖-深湖相烃源岩,恩平组-文昌组储层中原油主要来源于浅湖-半深湖相烃源岩;缓坡带的油藏无论是浅层珠江组还是深层文昌组均主要来源于浅湖-半深湖相烃源岩。西次洼洼陷中心的阳7和边缘缓坡带阳 6两个油田不论是浅层的珠江组还是深层的恩平组均主要来源于浅湖-半深湖相烃源岩。

  • 图3 恩平21洼典型烃源岩及原油生物标志化合物特征

  • Fig.3 Typical biomarker characteristics of crude oil and source rock in Enping 21 Subsag

  • 洼陷周边油藏原油分布特征表明,恩平21洼的西次洼主体发育浅湖-半深湖相烃源岩,而中-东次洼靠近陡坡带一侧发育半深湖-深湖相和浅湖-半深湖相2类烃源岩,靠近缓坡带一侧发育半深湖-深湖相烃源岩。

  • 4.3 不同类型原油成藏贡献分析

  • 恩平21洼已发现油藏原油多以单一来源为主,对各油层不同来源原油储量的统计表明,半深湖-深湖相来源原油的贡献比例为 15%,浅湖-半深湖相原油贡献比例为 85%。在已获得商业发现的恩平 20 洼,原油类型与恩平 21 洼类似,其中浅湖-半深湖相来源原油占已发现储量的30%。

  • 图4 恩平21洼典型原油及烃源岩生物标志化合物参数特征

  • Fig.4 Typical biomarker parameters of crude oil and source rock in Enping 21 Subsag

  • 文昌组半深湖-深湖相优质烃源岩的成藏贡献得到了广泛认可,而对于浅湖-半深湖相烃源岩的成藏贡献并未得到重视,这制约了低勘探程度浅-小洼陷的勘探进程。恩平 21 洼钻井揭示的烃源岩特征以及周边油藏储量来源构成表明,浅湖-半深湖相烃源岩同样具有较强的供烃能力。在珠江口盆地“下断上拗、先陆后海”构造沉积演化背景下,对上部的海相储层,油气充注需要有效的油源断裂沟通油源,对近源的陆相储层,油气充注需要有利的储集条件,因此制约商业性发现的关键因素在于通源断裂的发育程度以及深部储层的物性特征,发育浅湖-半深湖相烃源岩的浅-小洼陷,也具有较好的油气资源潜力。

  • 5 结论

  • 文昌组沉积时期恩平 21 洼为亚还原环境的淡水湖盆,湖水水体较深,气候湿热,为盆内优质烃源岩发育提供了必要的物质基础和良好的保存条件,钻井所揭示的半深湖-深湖相和浅湖-半深湖相2种不同沉积环境的烃源岩,均具有较好的生烃潜力。文昌组浅湖-半深湖相烃源岩及其所生成原油的生物标志化合物具有高 C304-甲基甾烷、高 Ts、高奥利烷、高重排藿烷及高树脂化合物T的组合特征,半深湖-深湖相烃源岩则具有高C304-甲基甾烷、低Ts、低奥利烷、低重排藿烷及低树脂化合物T的组合特征。恩平21洼周边所发现的石油地质储量中,源于半深湖-深湖相烃源岩的占比为 15%,源于浅湖-半深湖相烃源岩的占比为 85%,浅湖-半深湖相烃源岩同样具有较强的供烃能力,充注条件有利的情况下,发育浅湖-半深湖相烃源岩的浅-小洼陷,也具有较好的油气资源潜力。

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