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作者简介:

王秀红(1981—),女,辽宁沈阳人,高级工程师,硕士,从事油藏地球化学综合研究方面工作。E-mail:wangxiuhong595.slyt@sinopec.com。

中图分类号:TE122.1

文献标识码:A

文章编号:1009-9603(2021)03-0035-07

DOI:10.13673/j.cnki.cn37-1359/te.2021.03.004

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目录contents

    摘要

    埕岛东斜坡埕北斜394井产出无色凝析油,对于其来源及成因机制尚不明确。基于轻烃组成、生物标志物、碳同位素和金刚烷等特征分析,明确埕北斜 394井凝析油气来源,探讨其形成机制。结果表明,埕北斜 394井无色凝析油与淡黄色凝析油具有相似性与关联性,无色凝析油属于渤中坳陷沙三段烃源岩在高成熟演化阶段的产物,淡黄色凝析油为无色凝析油在产出过程中混有少量本层东营组烃源岩产出的成熟原油所致。天然气为与凝析油同源的伴生气,成熟度比凝析油略高,总体上具有“早油晚气”的充注特征。轻烃指标证实凝析油的蒸发分馏作用不明显,成熟热演化作用与运移分馏作用是凝析油气藏形成的主要机制。

    Abstract

    The source and formation mechanism of the colorless condensate oil produced in Well CBX394 on the east slope of Chengdao area are not clear. According to the feature analysis of light hydrocarbon composition,biomarkers,carbon iso- topes,and adamantanes,the source of condensate oil and gas in Well CBX394 was identified and the formation mechanism was discussed. The results revealed that such colorless condensate oil is similar and relevant to the light-yellow condensate oil. The former is the product of the hydrocarbon source rocks at the high-maturity stage of Es3 in Bozhong Sag,and the light-yellow condensate oil is produced because the colorless condensate oil is mixed with a small amount of crude oil from the hydrocarbon source rock of Ed at the mature stage during production. Natural gas is the associated gas of the same source as condensate oil,and its maturity is slightly higher than that of condensate oil. On the whole,natural gas charging is featured by“early oil and late gas”. The light hydrocarbon index confirms that the evaporative fractionation of condensate oil is not obvious,and the mature thermal evolution and migration fractionation are the main mechanisms for the formation of condensate reservoirs.

  • 济阳坳陷滩海地区毗邻济阳与渤中两大富烃坳陷,具有得天独厚的聚油背景[1-2]。前期勘探开发实践证实:埕岛东斜坡东营组油藏属于常规油藏,已探明石油地质储量超千万吨[3-4]。近期部署完钻的埕北斜 394 井,于东营组获得日产油量为 42 t/d,日产天然气量高达 22.5×104 m3 /d,进一步证实了埕岛东斜坡东营组油藏具有较大的勘探开发潜力。值得强调的是,该井产出无色透明、清澈如水的高纯度“无色石油”,这类凝析油在中国实属少见,也是迄今在济阳地区唯一发现具有此特征的原油。在美国加利福尼亚、原苏联巴库、罗马尼亚和印尼的苏门答腊以及中国四川黄瓜山和华北大港油田曾产出过无色石油,但是关于其来源及成因机制则鲜见报端。为明确此类原油的来源及成因机制,笔者对埕北斜394井原油和天然气的地球化学特征及成因进行系统研究,并与渤中19-6构造带凝析油特征对比分析,厘清该井凝析油气的形成过程,并深入探讨其形成机制,以期为进一步扩大埕岛东斜坡勘探成果提供可靠依据。

  • 1 区域地质概况

  • 埕北斜 394 井位于埕岛东斜坡埕北 32 断坡带西南部(图1),该块古近系披覆或超覆于潜山之上,其构造形态受沉积基底及埕北大断层的影响和控制,由 2 条近东西向断层与 1 条近北东向小断层夹持,处于断鼻圈闭腰部,构造条件有利。埕北斜394 井同时产出天然气和凝析油,特别是凝析油颜色发生间歇性变化,无色与淡黄色交替产出,气油比为 5 257 m3 /t,主要产层为东营组4段与5—6段。

  • 图1 埕北斜394井构造位置

  • Fig.1 Structural location of Well CBX394

  • 前期研究证实,埕岛东斜坡油气主要来自渤中坳陷西南次洼与黄河口凹陷西次洼[5]。渤中坳陷西南次洼发育沙三段、沙一段与东三段 3 套主力烃源岩,有机质丰度均达到优质烃源岩标准,但是有机质类型与有机质成熟度存在一定差异,沙三段烃源岩有机质类型属于Ⅱ1-Ⅱ2型,以Ⅱ2型为主,目前处于高成熟-过成熟阶段,沙一段与东营组烃源岩的有机质类型主要为Ⅱ1型,处于成熟-高成熟阶段。黄河口凹陷西次洼主力烃源岩为沙三段与东三段,有机质类型以偏腐泥型为主,热演化程度存在明显差异,其中沙三段整体处于成熟-高成熟阶段,东营组底部现今达到成熟阶段。

  • 2 凝析油气地球化学特征

  • 2.1 凝析油的地球化学特征及成因

  • 2.1.1 物性特征

  • 埕北斜394井凝析油的密度为0.765 1 g/cm3,黏度为 1.14 mPa•s,胶质与沥青质含量低,凝固点为-3℃,含硫量为 0.04%,属于低硫原油,含蜡量未检测,但是冷藏后瓶壁可见蜡状结晶沉淀,推测可能具有一定的含蜡量,总体表现为陆相原油的特征。目前胜利滩海地区以轻质油为主,尚未发现类似的凝析油,与周缘海域凝析油进行对比来看(表1):渤中坳陷凝析油密度为 0.769 8~0.803 0 g/cm3,黏度大于 1.0 mPa•s,含蜡量多数大于 10%,黄河口凹陷及周缘地区凝析油密度相对更低,小于 0.77 g/cm3,黏度小于 1 mPa•s,含蜡量小于 5%,从物性来看,埕北斜394井与渤中坳陷凝析油物性特征[6] 更接近。

  • 表1 埕北斜394井凝析油与周缘海域凝析油物性对比

  • Table1 Comparison between physical properties of condensate oil in Well CBX394 and that in surrounding sea area

  • 2.1.2 全烃组分特征

  • 全烃组分分析测试结果显示(图2a,2b):2类凝析油峰型及碳数分布特征具有相似性。链烷烃系列是原油中的优势组分,其中正构烷烃以中-低碳数优势显著,主峰碳均为 nC6,无色凝析油最高碳数为 C30,淡黄色凝析油最高碳数为 C35。C8前的较低碳数烷烃含量丰富,C8之后的中-高碳数的烷烃含量较低,指示该井原油成熟度较高的特点。轻烃组成中(图2c,2d),正构烷烃含量为32%~34%,异构烷烃含量为 15%,环烷烃含量为 16%~19%,芳烃含量为 3%~4%。谱图直观表现为以正己烷、甲基环已烷、正庚烷为主的特征,一般来说,源于腐泥型母质的轻烃组成中富含正构烷烃,源于腐殖型的轻烃组成中富含异构烷烃和芳烃,而富含环烷烃也是陆源母质的重要特征,由此判断成烃母质类型为偏腐泥型,属于油型气。THOMPSON发现随着油气成熟度的增加,原油烷基化程度会增加,从而提出利用正庚烷值和异庚烷值来区分原油成熟度[7]。通过计算得埕北斜 394井原油正庚烷值为 29%~30%,异庚烷值为0.93~0.94,落在成熟与高成熟过渡区(图3)。

  • 图2 无色凝析油与淡黄色凝析油全烃与轻烃谱图

  • Fig.2 Total hydrocarbon and light hydrocarbon spectra of colorless condensate oil and light-yellow condensate oil

  • 利用 MANGO 建立的 2,4-二甲基戊烷与 2,3-二甲基戊烷比值的对数值与原油生成温度之间的线性关系换算[8],获得凝析油的生成温度为 142~143℃,折算出镜质体反射率为1.1%,处于有机质成熟阶段的温度范围,其结果与该井产出典型的凝析油与高气油比的特征不符。根据王培荣等研究发现,很多地区的原油因受到次生蚀变和混源作用的影响,其正庚烷值和异庚烷值都会呈现下降的趋势,并且其变化程度会随着次生蚀变和混源程度强弱的不同而有所差异,从而导致原油出现轻烃组分表征的成熟度不高的假象[9-10],排除次生作用的影响,判断埕北斜394井原油可能为混源成因。

  • 图3 埕北斜394井凝析油正庚烷值与异庚烷值关系

  • Fig.3 Relationship between n-heptane and isoheptane values of condensate oil in Well CBX394

  • 2.1.3 生物标志物特征

  • 2类凝析油的生物标志物定量分析结果表明:无色凝析油不含生物标志物,淡黄色凝析油甾烷含量为 537 μg/g,萜烷含量为 2 870 μg/g,指示二者均具有相对较高的原油成熟度。淡黄色凝析油饱和烃生物标志物谱图表现为(图4):饱和烃色谱整体呈前峰型,Pr/Ph 值大于 1,规则甾烷呈“L”型分布,重排甾烷与 4-甲基甾烷含量较高,C27重排甾烷/C27 规则甾烷值为0.30,4-甲基甾烷指数为0.33,三环萜烷与伽马蜡烷含量较低,伽马蜡烷指数为 0.08,Ts/ Tm>1,与渤中坳陷东营组烃源岩生物标志物特征具有相似性[11]。甾烷异构化参数表明,C29S/(S+R) 值为 0.43,C29ββ/∑C29 值为 0.47,处于生油高峰阶段,与甾萜含量低的整体特征不相符,推测淡黄色凝析油为无色凝析油采出过程中混有少量本层相对较低成熟度的近源原油所致,由于无色凝析油生物标志物含量几乎为 0,因此生物标志物特征呈现出混入原油的主体特征。

  • 图4 埕北斜394井淡黄色凝析油生物标志物谱图

  • Fig.4 Biomarker spectrum of light-yellow condensate oil in Well CBX394

  • 2.1.4 金刚烷特征

  • 金刚烷类化合物是原油中具有类似金刚石结构的刚性聚合笼型烃类化合物,存在于原油的饱和烃馏分中,它一般是多环烃类在高温热力作用下聚合反应的产物[12-14]。此类化合物在地质演化过程中性质极其稳定,一旦形成不易受到热降解和生物降解的破坏,也一般不受沉积环境以及有机质输入的影响。因此金刚烷类化合物如烷基单金刚烷和双金刚烷系列,在研究高成熟-过成熟烃源岩中有机质成熟度和判断原油热裂解程度上得到了广泛应用[15-17]

  • 定量分析结果表明:淡黄色和无色凝析油中均含有一定量的不同侧链单甲基金刚烷系列化合物,未检测到双金刚烷系列化合物,DAHL 等在实验室通过热模拟实验发现甲基双金刚烷的浓度与原油遭受热裂解的程度呈正相关[18],并认为它可以作为一种天然的内标化合物来指示原油遭受热裂解的程度,由此判断埕北斜 394 井原油受热裂解的程度甚微,主要为烃源岩热解产物,甲基单金刚烷指数为 63%~67%,对应其与成熟度的关系,推算对应的镜质体反射率为 1.1%~1.3%,判断原油处于高成熟阶段。

  • 2.1.5 碳同位素特征及来源分析

  • 碳同位素与姥植比可以反映母质来源和沉积环境。由渤中坳陷烃源岩碳同位素与 Pr/Ph的相关关系(图5)可以看出,沙三段烃源岩具有相对较重的碳同位素,通常大于-27‰,姥植比为 1.1~2.3,沙一段烃源岩碳同位素略轻,约为-27‰,姥植比较低,一般小于1.5,东营组烃源岩碳同位素最轻,主要为-29.5‰~-27.6‰,姥植比较高,为1.6~3.0,显示更为氧化的沉积环境。无色凝析油碳同位素组成与姥植比接近于渤中坳陷沙三段烃源岩,而淡黄色凝析油表现为碳同位素更轻,姥植比更高的特点,推测为无色凝析油混有少量本层东营组来源的成熟度略低的原油所致,从生物标志物绝对定量结果来看,混入后甾烷、萜烷含量仍较低,表明后者混入的比例较少。李巧梅等提出可以利用碳同位素进行混源油比例计算[19],无色凝析油的碳同位素值为-26.3‰,淡黄色凝析油的碳同位素值为-26.8‰,东营组原油的碳同位素取值为-29.5‰~-27.6‰,通过计算得到东营组原油混入比例约为10%~20%,沙三段来源原油比例为80%~90%。

  • 图5 渤中坳陷烃源岩的碳同位素与Pr/Ph相关关系

  • Fig.5 Correlation between carbon isotopes and Pr/Ph of hydrocarbon source rocks in Bozhong Sag

  • 2.2 天然气的地球化学特征及成因

  • 2.2.1 组成特征

  • 埕北斜 394 井天然气组分以烃类气为主,烃类含量为 93%。烃类气中 CH4含量为 35%,重烃含量为 59%,非烃气体 CO2与 N2总含量为 5.33%,干燥系数为0.37,属于比较典型的湿气,天然气中iC4/nC4值在一定程度上能够反映天然气的成熟度,一般该比值随着成熟度的增加而下降。凝析油伴生气为 0.45~0.68,埕北斜394井天然气为0.33,接近凝析油伴生气下限。

  • 2.2.2 碳同位素组成特征

  • 天然气甲烷碳同位素受热演化程度与母质类型双重影响,可以用作区分天然气类型的有效指标[20-21]。大量资料表明,早期生物气 δ13C1 低于-59‰,石油伴生气为-58‰~-42‰,而凝析油的伴生气为-42‰~-36‰,热裂解形成的干气大于-36‰[21-22]。埕北斜 394 井 δ13C1为-40.13‰,属于凝析油伴生气的范围。乙烷碳同位素受成熟度的影响较小,能够更准确地反映成气母质的属性。依据戴金星等提出的划分标准,腐泥型有机质形成的乙烷碳同位素值小于-29‰,而腐殖型有机质生成的乙烷则相对较重,通常大于-28‰[23]。埕北斜394井天然气乙烷、丙烷、丁烷碳同位素值分别为-29.18‰,-29.01‰,-27.71‰,并且呈现出 δ13C1δ13C2δ13C3δ13C4的正序列排列,说明该井天然气属于有机成因为主的偏腐泥型凝析油伴生气。

  • 2.2.3 热演化成熟度

  • 受热解引起的同位素分馏效应的影响,有机质生成的天然气碳同位素随其演化程度的增加而变重[24]。基于中国不同沉积盆地天然气地化资料,根据油型气和煤成气甲烷碳同位素与镜质体反射率 (R o)的关系式[23] 推断,埕北斜 394 井天然气属于油型气范畴,计算对应 R o值为 1.2%~1.4% 左右,表明天然气为成熟-高成熟阶段的演化产物(图6)。依据生气母质的差异,成熟-过成熟阶段生成的天然气可以进一步划分为干酪根裂解气和原油裂解气。运用 LORANT 等建立的基于乙烷、丙烷含量和碳同位素的鉴别图版[25] 可知,埕北斜 394 井天然气主要为偏腐泥型干酪根裂解气(图7)。

  • 图6 埕北斜394井天然气碳同位素与R o的关系

  • Fig.6 Relationship between carbon isotopes and R o of gas from Well CBX 394

  • 图7 埕北斜394井天然气(δ13C2-δ13C3)与C2/C3关系

  • Fig.7 Relationship between δ13C2-δ13C3 and C2/C3 of gas from Well CBX 394

  • 3 凝析油气形成机制

  • 3.1 油气充注过程

  • 根据THOMPSON提出的甲苯/正庚烷与正庚烷/ 甲基环己烷2个比值判断凝析油的形成机制图版[26] 可以看出:埕北斜 394井东营组凝析油甲苯/正庚烷值小于 0.5,芳香化程度都不高,表明未发生明显的生物降解和蒸发分馏作用,主要受成熟作用影响,且与 BZ19-6井区深层源于渤中坳陷沙三段烃源岩的凝析油参数接近,与该井区浅层来自于东营组的常规油特征明显不同(图8),代表了二者不同的来源及成因过程。

  • 图8 区分原油次生变化的Thompson模式图

  • Fig.8 Thompson pattern for distinguishing secondary changes of crude oils

  • 3.2 形成机制

  • 油气藏的形成是油气生成、运移和聚集的综合结果。目前埕北斜394井表现出原油成熟度略低于天然气成熟度的特点,原油与天然气同源但为烃源岩不同演化阶段的产物,且该油藏经历了“早油晚气”的成藏过程。凝析油中不同烃类组分表征的成熟度存在一定差异:其中轻烃组分反映成熟特征,中等生物标志物组分体现出高成熟-过成熟特征,重质金刚烷类组分表现出高成熟特征,碳同位素对比表明 2 类凝析油主要源自渤中坳陷沙三段烃源岩,造成轻烃、全烃和金刚烷类化合物的分布与组成差异是大量气体注入配以长期发育断裂引起的运移分馏作用导致,垂向运移过程中的重力分异,导致相对低分子质量烃类富集,而相对高分子质量烃类明显减少,结果显示相对偏低的热裂解程度; 同时在运移过程中,胶质和沥青质被岩石吸附形成了无色凝析油,淡黄色凝析油是混有一定量的东营组的贡献导致。不同成熟度油气混合亦是不同参数判识成熟度指标存在差异的主要原因。

  • 4 结论

  • 埕北斜 394 井区东营组油藏是一个多源供烃、多期次油气充注、经历多种后生作用的复合成因油气田。渤中坳陷沙三段烃源岩生成的高成熟原油沿埕北 30 北断层由北向南、由深至浅充注,受后期构造运动的影响,演化程度更高的烃源岩生成的天然气沿着断裂大量注入,从而引起油藏发生运移分馏作用,天然气携带原油中轻质组分溶出并向上运移到较浅部位成藏,然后再溶出再运移,最后在浅部有利圈闭聚集形成凝析油气藏,凝析油在开采过程中又混入了少量同层东营组生成的成熟原油,随着地层压力的交替变化,导致了无色与淡黄色交替产出的现象。埕北斜 394 井气油比明显比埕岛中东排山目前发现的油藏气油比高很多,证实埕北斜 394井所在的构造位置为这类凝析油气优先充注的位置,考虑温压条件是影响油气相态的重要因素的同时,加强埕北30北断层走向运移输导条件研究有望获得更大的突破。

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