东海西湖凹陷孔雀亭地区平湖组沉积相演变及其主控因素分析

吴峰, 任培罡, 谈明轩, 张福榕, 马皓然

吴峰,任培罡,谈明轩,等. 东海西湖凹陷孔雀亭地区平湖组沉积相演变及其主控因素分析[J]. 海洋地质与第四纪地质,2022,42(2): 119-130. DOI: 10.16562/j.cnki.0256-1492.2021052401
引用本文: 吴峰,任培罡,谈明轩,等. 东海西湖凹陷孔雀亭地区平湖组沉积相演变及其主控因素分析[J]. 海洋地质与第四纪地质,2022,42(2): 119-130. DOI: 10.16562/j.cnki.0256-1492.2021052401
WU Feng,REN Peigang,TAN Mingxuan,et al. Facies evolution and its controlling factors of the Pinghu Formation in the Kongqueting area of Xihu Depression, the East China Sea[J]. Marine Geology & Quaternary Geology,2022,42(2):119-130. DOI: 10.16562/j.cnki.0256-1492.2021052401
Citation: WU Feng,REN Peigang,TAN Mingxuan,et al. Facies evolution and its controlling factors of the Pinghu Formation in the Kongqueting area of Xihu Depression, the East China Sea[J]. Marine Geology & Quaternary Geology,2022,42(2):119-130. DOI: 10.16562/j.cnki.0256-1492.2021052401

东海西湖凹陷孔雀亭地区平湖组沉积相演变及其主控因素分析

基金项目: 中央高校基本科研业务费(2019B07814)
详细信息
    作者简介:

    吴峰(1990—),男,博士,讲师,主要从事海洋沉积学研究,E-mail:finncug@hotmail.com

  • 中图分类号: P736

Facies evolution and its controlling factors of the Pinghu Formation in the Kongqueting area of Xihu Depression, the East China Sea

  • 摘要: 东海陆架盆地西湖凹陷含有巨量的石油天然气资源,具有极佳的勘探潜力,其中平湖组沉积作为重要的含油气层,已成为近期油气勘探的聚焦点。孔雀亭地区为西湖凹陷研究程度较低的区块,对于该区域沉积相及其控制因素方面的研究仍然较少。基于钻井资料与三维地震资料对孔雀亭平湖组沉积微相进行精细刻画,并系统分析了全球海平面、区域构造、古气候、古地貌对研究区沉积微相演变的控制作用。研究表明,平湖组下段(包括平下下段、平下上段)以潮坪沉积为主,辫状河三角洲沉积只发育在源区附近,而平中段与平上段以辫状河三角洲沉积为主,潮坪沉积分布面积迅速降低。由平下段至平中段,随着相对海平面(全球海平面与区域构造综合效应)下降,辫状河三角洲向海方向发生进积。同时,平中段与平上段时期极高的CO2浓度使得大陆源区剥蚀量快速增加,大量沉积物随着河流输入至西湖凹陷,从而进一步加剧了辫状河三角洲的向海推进。平下段与平中段早期的古地貌断陷发育较多,对辫状河三角洲的扩张存在限制作用,而经历了平中段大量沉积物充填之后,早期的洼陷逐渐被填平,从而平上段的辫状河三角洲可以大范围地向海方向发生推进,形成规模庞大的三角洲砂体沉积。
    Abstract: The East China Sea Shelf Basin contains a huge amount of petroleum resources and the Pinghu Formation is one of the major exploration targets. However, the Kongqueting area has rarely been researched, and few studies have been made to sedimentary facies evolution and its controlling factors. Based on drilling and 3D seismic data, microfacies of the Pinghu Formation are carefully described in this paper and the controls of global sea level fluctuation, regional tectonics, paleoclimate, and paleogeography over the facies distribution patterns discussed. The Lower Pinghu Formation is dominated by tidal flat facies, and the braided river delta facies only appeared in some places near uplifts. By contrast, the Middle and Upper Pinghu Formations are dominated by deltaic deposits of braided rivers. Relative sea level falling, as a joint result of global sea level change and regional tectonics, resulted in the progradation of braided river delta towards offshore. Meanwhile, the high concentration of CO2 in the atmosphere during the Middle and Later Pinghu periods caused a sharp increase of erosion. Enormous sediments provided by the source areas were transported into the Xihu Depression, which accelerated the progradation of braided river delta into the sea. In addition, the paleogeographic framework during the Middle to Lower Pinghu Periods were characterized by many fault-controlled sags, which limited the expansion of braided river delta. These sags were filled by sediments in Middle Pinghu Period, and turned to tidal flat in Later Pinghu Period, which also helped the expansion of deltaic deposits of the Upper Pinghu Formation.
  • 西湖凹陷是东海陆架盆地油气资源最丰富的凹陷,目前已发现油气田主要集中于中央反转带和西部斜坡带。随着勘探程度加深,勘探目标由构造油气藏向构造-岩性复合油气藏转变[1],油气充满度是影响勘探成败的重要因素,因此,油气来源及运移方向刻画在油气成藏研究中愈加重要。宝武区某中型气田的发现证实了西部斜坡带平北区构造-岩性复合油气藏具有很大勘探潜力[1],孔雀亭区亟待突破。前人对孔雀亭区烃源岩特征、油气成因及成藏条件研究认为,圈闭类型及盖层发育程度是本区油气成藏的主控因素[2-4],但精细化油源对比及油气运移方向研究不够深入,难以满足现阶段油气勘探需求。本文通过烃源岩生标特征精细化分析和天然气成熟度计算,开展油气源对比厘定油气来源,并在油气地化参数分析基础上进行油气运移效应研究,梳理油气运移方向,以期对孔雀亭区油气勘探提供支持。

    西湖凹陷是晚白垩世末期构造背景上发育的新生代沉积凹陷,位于东海陆架盆地中北部,是盆地内最大的含油气凹陷,演化过程经历断陷、拗陷和区域沉降3个阶段,可划分为5个构造带:西部斜坡带、西次凹、中央反转带、东次凹及东部断阶带(图1)。凹陷内以新生代碎屑沉积为主,自下而上发育始新统八角亭组、宝石组、平湖组,渐新统花港组,中新统龙井组、玉泉组和柳浪组,上新统三潭组以及第四系东海群(图2)。孔雀亭位于平湖斜坡带中北部地区鼻状隆起带上,其西靠海礁凸起,东邻西次凹,油气藏类型以断块、断鼻和断背斜为主。

    图  1  孔雀亭区域构造位置图
    Figure  1.  Tectonic map of the Xihu Sag and the location of Kongqueting area
    图  2  西湖凹陷地层综合柱状图
    Figure  2.  The stratigraphic column of Xihu Sag

    平湖组和宝石组是西湖凹陷主要烃源层段[5-9]。孔雀亭区平湖组烃源岩为受潮汐影响三角洲及潮坪沉积的一套煤系地层[10-11],岩性包括泥岩、碳质泥岩和煤;宝石组为滨—浅海环境下形成的一套灰色泥岩。煤是西湖凹陷重要的生烃母质,有机质类型好于泥岩[812],主要分布在平湖组地层,花港组及宝石组煤层发育规模较小[13]。孔雀亭不同部位钻井横向油气富集程度差异大,同一钻井纵向流体性质也不相同,揭示了研究区油气成藏复杂[3]

    孔雀亭区已钻遇含油气层段包括凝析气层、油层和气层,其中凝析气占探明储量60%左右,原油占比约20%。油气层大多集中于平湖组中、上段,少量分布于花港组,纵向呈现“上油下气”特征,横向呈现自东向西气层厚度和探明储量逐渐降低的趋势。原油密度较低,分布于0.75~0.86 g/cm3,平均值约0.82 g/cm3;含硫量都低于0.1%;含蜡量差别较大,分布于0.07%~19.53%(表1),整体以低密度、低硫、低原油为主,个别样品高含蜡。天然气主要分布在平湖组中段,少量分布于平湖组下段,气组分主要以烃类为主,甲烷占比73%~92%,属于湿气(表2)。非烃气主要以二氧化碳和氮气为主,含量低于10%。甲烷碳同位素分布于−37.8‰~−28.2‰,乙烷碳同位素分布于−28.4‰~−22.7‰,按照乙烷碳同位素母质遗传性,以−29‰为划分标准[14],本区天然气属于煤型气。

    表  1  西湖凹陷孔雀亭区原油物性统计
    Table  1.  Oil parameters from Kongqueting area
    井名深度/m密度/ (g/cm3凝固点/℃硫含量/%蜡含量/%
    G241500.842290.102.11
    Z24107.50.8248150.063.14
    Z241860.8152140.073.08
    Z141830.816750.043.27
    Z14548.70.8606290.0710.54
    G13542.60.8345140.0417.91
    G13565.60.8239170.0619.53
    G13810.20.7487−300.060.07
    D239490.8380−3400
    D24317.40.8443120.049.50
    D243330.8390−320.070
    G326470.831550.0212.00
    G330990.8105−10.017.70
    下载: 导出CSV 
    | 显示表格
    表  2  西湖凹陷孔雀亭区天然气组分及同位素统计
    Table  2.  Natural gas parameters from Kongqueting area
    井号深度/m组分/%同位素/‰iC4H10/nC4H10N2/C2H6成熟度%
    CH4C2H6C3H8iC4H10nC4H10iC5H12nC5H12N2CO2CH4C2H6C3H8C4H10
    G1354283.527.612.420.490.370.110.042.982.451.320.39
    G1356579.038.683.540.960.580.230.074.192.721.660.48
    G1381075.9610.335.701.800.700.250.121.703.402.570.16
    G2415089.14.480.630.160.100.050.030.365.00−37.1−22.7−20.8−21.71.600.080.79
    G4409379.438.382.841.040.650.280.200.596.29−34.6−25.3−24.5−25.41.600.071.02
    Z2411181.027.353.030.740.640.220.160.456.19−37.8−27.2−25.4−24.61.160.060.73
    Z2418681.427.292.970.720.620.220.160.186.19−28.2−23.8−23.7−23.31.160.021.97
    Z1418385.516.282.300.590.510.180.130.204.11−35.4−25.7−24.4−24.81.160.030.94
    Z1454873.208.435.831.681.400.490.310.577.79−37.7−27.6−26.3−26.21.200.070.74
    D1419889.825.511.660.440.330.140.0901.99−33.4−26.3−24.3−23.11.3201.16
    D1481092.055.381.510.360.360.170.120.020−33.6−28.4−27.1−22.71.0101.14
    下载: 导出CSV 
    | 显示表格

    厘定油气来源是准确刻画油气运移方向的基础,油气层周围、平湖组下部、宝石组及毗邻的西次凹等都属于有效供烃源岩[4],本地烃源岩处于生油阶段,西次凹烃源岩处于生气阶段[12]

    孔雀亭区平湖组中下段沉积时期属亚热带气候,高等植物和浮游生物发育,有机质生产率高,且较弱水动力条件下还原-弱还原、咸水-微咸水水介质有效保存了沉积有机质,优质烃源岩广泛发育[12];平湖组上段以河湖相为主,烃源岩发育相对较差。有机岩石学分析显示平湖组煤层样品和泥岩样品显微组分都以富含镜质组、贫惰质组,含有一定比例的壳质组和腐泥组为特征[15],不同显微组分随热演化程度增加相对含量随之变化,难以区分不同层段间烃源岩差异。生物标志化合物保留了生物有机质的原始信息,且在后期演化过程碳骨架结构中相对稳定,是烃源岩精细化对比研究的重要手段,常被用于油油对比和油岩对比[16]。由于海陆过渡相地层有机质生源变化快,纵向泥岩和煤的互层分布导致源岩生标参数非均质性强,且海上钻井取心成本高,烃源岩岩心样品不能满足大量数据统计分析条件,因此,本文从单井烃源岩岩屑生标特征入手,寻找不同层段间各生标参数变化规律,采用各层段烃源岩样品生标参数平均值代表不同层段烃源岩的整体特征,以降低地层非均质性对烃源岩判识带来的误差。

    G2井烃源岩岩屑样品深度包含从平湖组到宝石组所有层段,是单井生标参数纵向对比分析的最优选择(图3)。C27规则甾烷代表水生生物对沉积有机质的贡献,C29规则甾烷代表陆源高等植物贡献,常用规则甾烷C27/C29ααα20R比值反映有机质母源特征[16],G2井岩屑样品规则甾烷C27/C29ααα20R比值小于1.0占主体,表明有机质母源以陆源高等植物为主。规则甾烷C27/C29ααα20R比值随埋深增加呈先减小后增加趋势,最低值出现在3800 m深度,揭示3000~3800 m陆源高等植物对有机质贡献逐渐增加,水生生物减少;3800 m以下水生生物对有机质贡献开始增加,最大值可达到1.0以上。藿烷中Ts/Tm参数常用来表征烃源岩成熟度,前人依据该参数比值变化范围大推测原油可能来自不同层段[15],但该参数同时受有机质来源影响较大[16]。G2井烃源岩3800 m以浅Ts/Tm比值显示该参数并没有随埋深增加而增加,表明受成熟度影响较小;Ts/Tm比值与规则甾烷C27/C29ααα20R比值变化趋势一致(图3),随深度呈先减小后增加变化规律,表明有机母质来源对本区Ts/Tm比值变化影响更大。G2井不同层段间煤层厚度统计结果显示,平湖组上、中、下段煤层累计厚度分别为13、40和12.5 m,平中段煤层最发育,对应规则甾烷C27/C29ααα20R比值及Ts/Tm比值的最低点,揭示不同层段间煤层发育差异是影响该参数变化的主要原因。为进一步验证煤层对生标参数的影响,选择平北斜坡带X井同深度煤岩和泥岩岩心样品进行生标参数对比,结果显示煤岩与泥岩生标存在较大差别,与泥岩相比煤岩具有相对高C29规则甾烷、低Ts和低伽马蜡烷特征(图4),因此,规则甾烷C27/C29ααα20R比值、Ts/Tm比值及伽马蜡烷含量能够区分不同层段烃源岩,可作为孔雀亭区油源对比有效参数。同时,重排甾烷及C24四环萜等化合物相对含量也都指示平湖组中段陆源高等植物较其他层位发育。

    图  3  G2井烃源岩生标特征随深度变化图
    Figure  3.  Variation of biomarkers with depth, from well G2
    图  4  西湖凹陷泥岩与煤生标特征对比
    Figure  4.  Biomarkers of coal and mudstone from m/z217 and m/z191 of the Xihu sag

    基于以上特征生标参数,孔雀亭区烃源岩在垂向上大致可划分为两类(表3图5),第1类包括平上段和宝石组烃源岩,以相对较高Ts/Tm(>0.4)、规则甾烷C27/C29ααα20R比值>0.8和伽马蜡烷/C30藿烷比值>0.1为特征,呈现少煤、水生生物贡献相对丰富的有机母质构成和较深的沉积水体环境;第2类包括平中段和平下段烃源岩,以相对较低Ts/Tm(<0.4)、规则甾烷C27/C29ααα20R比值<0.4和伽马蜡烷/C30藿烷比值<0.1为特征,呈现多煤、陆源高等植物贡献相对丰富的有机母质构成和较浅的沉积水体环境。此外,烃源岩成熟度参数显示,平湖组上段烃源岩C29甾烷ββ/(αα+ββ)和20S/20(R+S)比值都在0.4以下(表3),表明烃源岩演化程度较低,尚未进入成熟门限,对原油贡献少;平湖组中段烃源岩样品成熟度略高于平湖组上段,仅平中段下部进入生油门限,对原油贡献有限;平下段和宝石组烃源岩已达到成熟阶段,处于生油高峰期。因此,通过成熟度参数可进一步将两类烃源岩中不同层段加以区分。需要说明的是沿斜坡延伸方向构造相对低部位烃源岩埋深大,演化程度高于高部位同层段烃源岩,平上段和平中段烃源岩部分进入生油阶段(如D2井),对原油具有一定贡献。

    表  3  孔雀亭区烃源岩与原油生标参数统计
    Table  3.  Biomarker of source rock and oil from Kongqueting area
    样品类型甾烷C27/
    C29ααα20R
    Ts/Tm伽马蜡烷/
    C30藿烷
    C29/C30
    藿烷
    C29重排甾
    烷/C29
    则甾烷
    C24四环萜/
    C23三环萜
    C29甾烷ββ/
    (αα+ββ)
    C29甾烷20S/20
    (R+S)
    C31藿烷S/
    (S+R)
    运移效应
    烃源岩平上段0.79(14)
    0.25~2.01
    0.96(13)
    0.01~2.66
    0.18(13)
    0.01~0.83
    0.7(12)
    0.26~2.17
    0.12(14)
    0.01~0.24
    0.75(6)
    0.24~1.50
    0.39(14)
    0.25~0.50
    0.38(14)
    0.17~0.50
    0.53(14)
    0.36~0.57
    平中段0.24(16)
    0.01~0.54
    0.37(16)
    0.05~1.49
    0.07(16)
    0.01~0.20
    0.78(16)
    0.46~1.22
    0.18(16)
    0.12~0.26
    0.95(13)
    0.36~3.44
    0.36(16)
    0.24~0.53
    0.42(16)
    0.28~0.49
    0.56(16)
    0.52~0.59
    平下段0.39(4)
    0.15~0.71
    0.33(4)
    0.15~0.56
    0.05(2)
    0.09~0.10
    0.65(4)
    0.61~0.69
    0.16(4)
    0.12~0.21
    1.03(2)
    0.43~1.68
    0.45(4)
    0.43~0.48
    0.46(4)
    0.45~0.47
    0.56(4)
    0.54~0.57
    宝石组0.83(2)
    0.70~0.95
    0.82(2)
    0.76~0.88
    0.14(2)
    0.12~0.15
    0.56(2)
    0.55~0.57
    0.19(2)
    0.19~0.20
    0.25(2)
    0.27~0.24
    0.43(2)
    0.42~0.43
    0.45(2)
    0.46~0.44
    0.5(2)
    0.48~0.52
    原油G20.31(1)0.08(1)0.62(1)0.53(1)
    Z20.91(2)
    0.89~0.94
    1.16(2)
    1.16~1.16
    0.08(2)
    0.08~0.08
    0.63(2)
    0.61~0.65
    0.17(2)
    0.16~0.17
    0.56(2)
    0.56~0.56
    0.45(2)
    0.44~0.46
    0.55(2)
    0.55~0.55
    26.19(2)
    23.22~29.31
    Z10.65(2)
    0.54~0.77
    0.86(2)
    0.77~0.94
    0.08(2)
    0.07~0.09
    0.61(2)
    0.55~0.67
    0.17(2)
    0.16~0.18
    0.55(2)
    0.54~0.56
    0.44(2)
    0.44~0.45
    0.55(2)
    0.54~0.56
    24.05(2)
    21.53~26.62
    D20.67(2)
    0.65~0.69
    0.51(2)
    0.49~0.53
    0.21(2)
    0.21~0.21
    0.68(2)
    0.66~0.70
    0.13(2)
    0.12~0.13
    0.21(2)
    0.21~0.21
    0.41(2)
    0.39~0.42
    0.41(2)
    0.40~0.42
    0.53(2)
    0.53~0.54
    ~0.59(2)
    ~1.89~0.64
    G30.74(3)
    0.67~0.85
    0.52(3)
    0.36~0.70
    0.08(3)
    0.05~0.11
    0.60(3)
    0.55~0.65
    0.21(3)
    0.21~0.22
    0.27(3)
    0.15~0.50
    0.53(3)
    0.52~0.54
    0.47(3)
    0.45~0.49
    0.56(3)
    0.55~0.56
    11.39(3)
    8.14~21.03
      注:运移效应=参数(C29甾烷ββ/(αα+ββ)−C29甾烷20S/20(R+S))×100/ C29甾烷20S/20(R+S);数据格式: 平均值(样品数量)/最小值-最大值。
    下载: 导出CSV 
    | 显示表格
    图  5  孔雀亭区烃源岩与原油饱和烃色谱质谱特征
    Figure  5.  GC-MS characteristics of saturated hydrocarbon of source rock and oil from Kongqueting area

    孔雀亭区原油生标特征整体相似(表3图6),属于同一组群。其中,规则甾烷C27/C29ααα20R比值主要分布于0.6~0.91,表明有机母质来源具有陆源高等植物和水生生物共同贡献,与第1类烃源岩(平上段和宝石组)有机质母源构成相近;原油Ts/Tm比值普遍高于0.5,与第1类烃源岩特征相符,揭示煤层对原油贡献不显著,但随着烃源岩进入成熟阶段,热演化对Ts/Tm比值的影响更加显著,表现为随演化程度增加Ts/Tm比值增加,因此,综合其他参数认为孔雀亭区原油以第1类烃源岩贡献为主,同时有部分第2类烃源岩混入(图5)。结合原油成熟度参数C29甾烷ββ/(αα+ββ)和20S/20(R+S)比值都在0.4以上,高于平上段烃源岩,表明宝石组是原油的主要贡献层段,其次为平下段,而平中段和平上段贡献较少。

    图  6  孔雀亭区烃源岩与原油生标参数交会图
    Figure  6.  Correlation of biomarkers from source rocks and the oil from Kongqueting area

    甲烷碳同位素是指示气源岩热演化程度的有效指标[17],孔雀亭区天然气甲烷碳同位素数值变化范围较大,乙烷碳同位素都在−28‰以下(表2),表明天然气母源一致但成熟度范围较宽。采用刘文汇提出的煤型气甲烷碳同位素二阶演化分馏公式计算天然气成熟度[17],结果显示存在两类天然气:一类为少量中等成熟度天然气(Ro<0.9%),另一类为高成熟天然气(Ro>0.9%)。现今孔雀亭区平湖组下段和宝石组烃源岩处于中等成熟度阶段,干酪根镜质体反射率Ro<1.0%,生气量较少;西次凹平湖组源岩已进入高演化阶段,Ro>1.3%,处于主生气阶段[18]。因此,基于孔雀亭区和西次凹烃源岩有机质演化程度差异,初步推测本区少量中等成熟天然气来源于本地烃源灶,而相对较高成熟度的天然气来源于西次凹平湖煤系烃源岩,且两者发生混合,前人基于含油饱和度和储层孔隙度的相关性研究也证实了以上结论[3]

    油气地化参数是表征油气运移效应的重要指标,其原理主要基于两个方面,一是有机质生烃演化规律:烃源岩演化早期生成油气成熟度较低,运移时间早、距离远,演化后期生成油气成熟度高,运移时间晚、距离近,因此,成熟度差异可指示油气运移方向;二是流体在地层运移过程中的分馏效应:不同分子量化合物、相同分子量不同构型的化合物因极性差异导致在地层中的运移速度不同,非极性化合物受围岩影响弱、运移速度快,强极性化合物受围岩影响大、运移速度慢,不同化合物间相对含量的变化可指示油气运移方向。

    孔雀亭区原油成熟度参数甾烷C2920S/20(R+S)分布于0.4~0.49(表3),其中,D2原油比值在0.42以下,成熟度最低;Z2和Z1原油成熟度相当,比值分布于0.44~0.46;G3井原油成熟度最高,其中2861.9 m深度样品成熟度与Z2和Z1井类似,C29甾烷20S/20(R+S)比值为0.45,其余两个样品成熟度参数分别为0.48、0.49。勘探证实宁波8洼是孔雀亭区坡内供油次洼,依据原油成熟度参数推测宁波8洼烃源岩低演化阶段形成原油首先在D2井聚集成藏,中期形成的相对高成熟原油向Z2、Z1井方向以及G3井方向运移,而演化中后期形成的高成熟原油主要向G3井方向运移。其他成熟度参数如藿烷C31S/(S+R)也具有相似变化规律,而Ts/Tm由于受有机质母源影响其规律性不明显(表3)。

    甾烷C29ββ/(αα+ββ)比值除受成熟度影响外,运移距离也是重要因素,而C29甾烷20S/20(R+S)主要受成熟度影响[19],因此两者差异可间接反映油气运移距离,差异大代表运移效应强、运移距离长。以不同构型C29规则甾烷比值计算原油运移效应量化参数(表3图7),结果显示,D2井原油运移效应弱,原油运移距离短,Z2及Z1井原油运移效应强,原油运移距离长,而G3井不同深度样品存在差别,其中2861.9 m样品运移效应强,其他样品则表现为弱运移效应特征,原油运移距离短。

    图  7  孔雀亭区原油成熟度与运移效应
    Figure  7.  Maturity and migration effect of the oil from Kongqueting area

    腐泥组和壳质组是孔雀亭煤系烃源岩主要生油母质,演化早期阶段生成少量低成熟度原油[10],难以大范围运移,仅在周缘圈闭就近成藏,如D2井;演化中期阶段,生油量增加,一定数量的成熟原油可持续运移至Z2、Z1及G3井成藏;演化中后期阶段,相对高成熟原油在构造相对高部位聚集,如G3井(图8)。

    图  8  孔雀亭区原油及天然气运移方向示意图
    Figure  8.  Migration direction of oil and gas of the Kongqeuting area

    烃源岩早期生气阶段具有甲烷相对含量低、碳同位素偏轻特征,随着演化程度增加,甲烷相对含量逐渐增加、同位素趋于偏重[17],因此,甲烷相对含量和甲烷碳同位素是指示天然气运移距离的有效参数。孔雀亭天然气甲烷相对含量分布于80%~90%,其中低带D1井天然气甲烷相对含量可达90%,但中高带甲烷相对含量相近,主要分布于75%~85%;甲烷碳同位素具有类似变化趋势,表现为低带偏重、中高带偏轻(表2),揭示了天然气自西次凹沿斜坡低带向高带运移特征。

    西湖凹陷天然气中N2主要以有机质热降解形成为主[20],因N2分子直径较小、岩层吸附能力较弱,运移速度较烃类快,在运移分异作用影响下,随运移距离增加天然气中N2含量增加,此参数在斜坡带平湖油气田应用效果较好[21]。孔雀亭区低带D1井N2含量低于0.1%,向斜坡中高带方向N2含量呈增加趋势,Z1、Z2井区都在0.2%以上,至高带G1井区达到4%,(表2),氮气与乙烷比值(N2/C2)揭示了天然气由低带向高带运移(图9)。iC4/nC4比值随运移距离增加而增加,在天然气运移示踪中有着广泛应用[21],孔雀亭区iC4/nC4呈现低带(D1)相对较低、高带(G1、G2)较高的特征,是天然气由低带向高带运移的结果(图9),表明斜坡低部位天然气供给丰富,勘探潜力大(图8)。

    图  9  孔雀亭区天然气运移效应
    Figure  9.  Migration effect and direction of natural gas of the Kongqueting area

    斜坡带本地烃源岩自生天然气和西次凹外源气双重贡献条件下,两者混合作用导致斜坡低带(D1、Z1井)浅层系天然气成熟度高于深层系天然气的特征(表2)。成藏中后期,西次凹高成熟天然气沿斜坡低带向高带运移过程中沿断层垂向运移至有效圈闭聚集成藏,此时斜坡低带烃源岩部分进入生气阶段,低成熟度天然气就近聚集至附近圈闭,并与西次凹高成熟天然气混合,浅部层位烃源岩演化程度低、生气量少,混合后表现为西次凹天然气的高成熟特征,而深部层位烃源岩演化程度高、生气量大,混合后天然气成熟度较西次凹天然气程度低,且低于浅部层系天然气。

    (1)西湖凹陷孔雀亭区原油呈低密度、低硫、低蜡特征,主要分布于平湖组中、上段;天然气为煤型气,以湿气为主,主要分布于平湖组中、下段。

    (2)原油主要来自斜坡带内生油次洼,以宝石组和平下段烃源岩贡献为主,平中、上段烃源岩少量贡献;天然气以西次凹烃源岩贡献为主,同时有部分斜坡带自生天然气混入。

    (3)煤系烃源岩演化早期生成少量低熟油短距离运移成藏,中后期形成成熟原油由次洼向相对高部位运移,环次洼区域是原油优势聚集方向;天然气主要呈自西次凹沿斜坡低带向高带运移特征,斜坡低部位是天然气勘探有利区带。

  • 图  1   西湖凹陷孔雀亭地区平面位置

    C图中黄线为图3地震剖面位置。

    Figure  1.   Location of Kongqueting area in the Xihu Depression

    The yellow line in Fig.1C represents the position of seismic profile in Fig.3.

    图  2   研究区顺物源方向(a)与垂直物源方向(b)沉积相连井剖面

    Figure  2.   Well-to-well profiles of sedimentary facies distribution along (a) and vertical to (b) the directions of sediment transportation

    图  3   顺物源方向具有前积结构的地震剖面

    剖面位置见图1c。

    Figure  3.   Seismic profile showing the progradation feature

    Profile location is marked on Fig. 1c

    图  4   平湖组不同亚段振幅属性分布特征

    Figure  4.   RMS attribute features of four subsections in the Pinghu Formation

    图  5   平湖组不同亚段沉积相分布特征

    Figure  5.   Distribution patterns of sedimentary facies in four subsections of Pinghu Formation

    图  6   平湖组时期全球海平面、区域构造、相对海平面与二氧化碳含量变化

    Figure  6.   Changes in global sea level, tectonic movement, relative sea level, and CO2 concentration during the deposition of Pinghu Formation

    图  7   平湖组时期研究区古地貌特征

    Figure  7.   Paleogeography of the Kongqueting area during the Pinghu Period

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出版历程
  • 收稿日期:  2021-05-23
  • 修回日期:  2021-07-02
  • 网络出版日期:  2021-09-07
  • 刊出日期:  2022-04-27

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