Braided river depositional characteristics of the Yuquan-Longjing formations, East China Sea Basin: Case study of Y area, Xihu Sag

  • Jie YU , 1 ,
  • Xingzong YAO 1 ,
  • Congjun FENG , 1 ,
  • Hongjun QU 1 ,
  • Leigang ZHANG 1 ,
  • Mengsi SUN 2 ,
  • Zhiqiang CHEN 1
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  • 1. State Key Laboratory of Continental Dynamics,Northwest University,Xi'an 710069,China
  • 2. School of Petroleum Engineering and Environmental Engineering,Yan’an University,Yan’an 716000,China

Received date: 2024-11-01

  Revised date: 2024-12-19

  Online published: 2025-01-06

Supported by

The China National Science and Technology Major Project(2016ZX05026-007-007)

the Natural Science Basic Research Program of Shaanxi Province, China(2017JM4013)

the Research Fund of the Department of Geology, Northwest University

Abstract

Fluvial sandbodies, as an important reservoir type, hold a significant position in oil and gas exploration. The Yuquan-Longjing Formation in the Xihu Sag of the East China Sea Basin, a favorable interval layer for subsequent oil and gas exploration in the Xihu Sag, lacks comprehensive research on its sedimentary characteristics and sedimentary patterns in the existing literature. An integrated analysis of logging, core, and seismic data was conducted to study on the sedimentary characteristics, sedimentary sequences, and sedimentary evolution of the braided river sedimentary system of the Yuquan-Longjing formations of the Y area of the East China Sea Basin. The research results indicate that the Yuquan-Longjing formations mainly develop conglomerates, sandstones, siltstones, and mudstones, with low compositional maturity and structural maturity of rocks; a total of seven lithofacies types, four basic logging curve shapes, and five seismic facies are identified. It is determined that the Yuquan-Longjing formations in the study area are a braided river deposit, and three sedimentary microfacies, namely braided river channels, mid-channel bars, and floodplains, and their typical lithofacies combination types are developed. Finally, combined with paleogeomorphology and paleoclimate, the braided river sedimentary model of Yuquan-Longjing formations in the study area were established under humid environment and high-energy hydrodynamic conditions. The research results of this paper can provide a reference for the subsequent exploration and development of the shallow layers in the Xihu Sag.

Cite this article

Jie YU , Xingzong YAO , Congjun FENG , Hongjun QU , Leigang ZHANG , Mengsi SUN , Zhiqiang CHEN . Braided river depositional characteristics of the Yuquan-Longjing formations, East China Sea Basin: Case study of Y area, Xihu Sag[J]. Natural Gas Geoscience, 2025 , 36(5) : 883 -898 . DOI: 10.11764/j.issn.1672-1926.2024.12.010

0 引言

长期以来,河流系统都是陆相沉积盆地油气勘探的重要目标1-4,我国近50%的油气资源都在河流相储层内富集5-7。其中,辫状河作为最常见的河流类型之一,属于富砂型河流,其形成的砂体厚度大,连续性好,是良好的油气储集体8。由于前人在辫状河沉积特征、沉积演化及储层构型等方面的研究极大地促进了对辫状河形成过程、沉积物发展及演化的理解8-12,为后续辫状河相储层的勘探开发提供理论基础。因此,对辫状河沉积特征及沉积演化的研究具有重要的现实意义。
东海盆地西湖凹陷是中国近海最大的含油气凹陷13-17,前人15-23研究主要集中在该凹陷的斜坡带,而中央反转构造带的研究相对匮乏,有待进一步深入15。Y地区位于中央反转构造带中部,作为西湖凹陷寻找大中型油气田的最有利区带之一,其探明程度不足5%1623。前人24-25研究表明西湖凹陷玉泉组—龙井组为河流相—中浅湖泊相沉积地层,针对该层位的沉积特征及沉积体系缺乏详细深入的研究,阻碍了Y地区进一步油气勘探的进程。本文研究基于钻井岩心、测录井数据及三维地震数据,采用井震联合标定的相标志分析方法,明确Y地区的岩心相、测井相、地震相特征,总结Y地区玉泉组—龙井组辫状河沉积特征,并在此基础上建立了研究区玉泉组—龙井组的辫状河沉积模式。

1 区域地质概况

1.1 构造位置及构造单元划分

东海盆地位于欧亚大陆板块东南缘大陆架之上,东邻钓鱼岛隆褶带,西邻浙闽隆褶带,总面积约为26×104 km2,是我国大型含油气盆地之一26-27。西湖凹陷位于东海盆地北部,整体呈NE向展布,总面积约为5.9×104 km2。西湖凹陷西临海礁凸起和渔山凸起,东至钓鱼岛隆褶带,北接虎皮礁凸起和福江凹陷,南抵钓北凹陷,自西往东可划分为西部斜坡带、西次凹、中央反转构造带、东次凹和东部断阶带5个构造单元,是中国近海海域最大的新生代沉积凹陷(图113-33。Y构造位于中央反转构造带中部,是东海盆地已发现的最大背斜构造,面积超过500 km2,是西湖凹陷寻找大中型油气田的最有利区带之一1623
图1 西湖凹陷区域位置、构造分区及研究区井位分布32

(a)西湖凹陷区域位置;(b)西湖凹陷构造带位置(据文献[32]改绘);(c)研究区井位

Fig.1 Regional location, structural division of Xihu Sag and well location distribution in the study area32

1.2 地层层序及构造演化

西湖凹陷沉积地层自下而上依次发育古新统、始新统、渐新统、中新统、上新统和第四系(图2)。本文研究目的层位为中新统玉泉组和中新统龙井组,为辫状河沉积充填。其中玉泉组可分为玉泉组上段(玉上段)、玉泉组中段(玉中段)和玉泉组下段(玉下段),龙井组则分为龙井组上段(龙上段)和龙井组下段(龙下段)。玉上段沉积时期岩性以灰色泥质粉砂岩与粉砂岩互层为主;玉中段和玉下段沉积时期岩性主要为灰白色厚层砂岩;龙井组沉积时期砂体十分发育,主要为灰白色巨厚层砂岩(图2)。
图2 西湖凹陷构造演化阶段及地层综合柱状图(据文献[33]修改)

Fig.2 Tectonic evolution stage and stratigraphic integrated histogram of Xihu Sag (modified from Ref.[33])

2 相标志

2.1 岩心相标志

2.1.1 岩石学特征

研究区玉泉组—龙井组岩性复杂,砾岩、砂岩、粉砂岩及泥岩均有发育。岩性以中砂岩及细砂岩为主(图3),反映玉泉组—龙井组沉积时期整体水动力条件较强。
图3 研究区玉泉组—龙井组储层岩石类型

Fig.3 Reservoir rock types of the Yuquan-Longjing formations in the study area

砂岩主要为长石岩屑质石英砂岩、长石质岩屑砂岩和岩屑砂岩。砂岩中石英含量较高,平均体积分数为59.3%,除少数岩屑砂岩外,长石和岩屑等不稳定组分含量较低(图4)。砂质碎屑颗粒磨圆较差,以次棱角状与次圆状为主,分选差—中等,稳定组分含量略高,成分成熟度和结构成熟度中等,反映研究区内玉泉组—龙井组具有稳定沉积的特征。
图4 研究区玉泉组—龙井组碎屑岩三角图

Fig.4 Triangulation of clastic rocks in the Yuquan-Longjing formations in the study area

2.1.2 沉积构造特征

研究区取心井仅有A井一口,累计长度20.6 m。基于岩心观察描述,主要识别出板状交错层理、块状层理、沙纹交错层理及透镜状层理4种沉积构造以及植物化石,详述如下:
(1)板状交错层理:是由一系列斜交于层系界面的纹层组成,纹层与层系交错,岩石粒度为细粒及以上,是沉积物顺流加积和侧向加积形成的层理,表现出强水动力条件。这种层理指示了研究区发育辫状河道或心滩沉积[图5(a),图5(b)]。
图5 研究区玉泉组—龙井组典型沉积构造

(a)中砂岩,板状交错层理(A井,2 004 m);(b)细砂岩,板状交错层理(A井,1 994.8 m),(c)中砂岩,块状层理(A井,1 993.4 m);(d)细砂岩,块状层理(A井,1 998.5 m);(e)细砂岩,沙纹交错层理(A井,1 619.2 m);(f)细砂岩,沙纹交错层理(A井,1 618.6 m);(g)粉砂岩夹灰绿色泥岩,透镜状层理(A井,1 623 m);(h)灰黑色泥岩,透镜状层理(A井,1 620.9 m);(i)黑色泥岩,炭化植物茎干碎片与炭屑(A井,1 620.3 m)

Fig.5 Typical sedimentary structure of the Yuquan-Longjing formations in the study area

(2)块状层理:是沉积物垂向加积形成的层理,在砾岩、砂岩及粉砂岩内均有发育,反映了发育位置具有充足的沉积物供应以及较快的沉积速率,指示了牵引流处于快速移动拖曳沉积物的阶段。一般出现在辫状河道的沉积环境[图5(c),图5(d)]。
(3)沙纹交错层理:是指在砂质沉积物中,由于水流向前迁移并向上生长所形成的相互叠置的波痕纹理。常见于粉砂岩或泥质粉砂岩中,一般出现于水动力条件较弱的时期,特别是呈悬浮状态的沉积物不断供给的阶段。指示该时期沉积物以悬浮搬运为主,往往出现在废弃河道或溢岸沉积中[图5(e),图5(f)]。
(4)透镜状层理:是一种多层系层理,主要由泥质构成,砂质透镜体包裹在泥质层之中。主要发育在泥岩或粉砂质泥岩中,其反映了较弱的水动力条件,指示了溢岸,废弃河道或泛滥平原沉积环境[图5(g),图5(h)]。
(5)植物化石:在研究区取心井中发现了少量植物化石,表明了该时期发育溢岸或泛滥平原亚相沉积[图5(i)]。
岩心观察描述结果表明,玉泉组取心段岩性以泥质粉砂岩与粉砂岩互层为主,发育沙纹交错层理,分析认为玉泉组取心段沉积环境应为辫状河沉积相中泥质含量较高的泛滥平原亚相;龙井组取心段岩性则为砂岩,发育块状层理,局部可见板状交错层理,分析认为该段处于辫状河沉积时期。

2.1.3 粒度特征

沉积环境决定了沉积物粒度的大小及分布状况,其直接受水体类型、水体能量高低及能量变化状况等沉积条件控制,因此粒度特征能指示沉积环境34-35。研究区砂岩样品粒度分析结果表明,玉泉组砂岩粒径Φ值集中在4~5之间[图6(a)],龙井组粒径Φ值集中在2~3之间[图6(b)],为粉砂岩至细砂岩的粒度特征,揭示该时期为稳定沉积阶段。研究区内C井和C3井的粒度概率曲线特征均为“两段式”,主要为跳跃与少数悬浮主体,其中跳跃直线的斜率远大于悬浮直线的斜率,反映了牵引流沉积特征。
图6 研究区玉泉组(a)与龙井组(b)粒度曲线

Fig.6 The grain size curves of the Yuquan Formation(a) and Longjing Formation(b) in the study area

2.1.4 岩相类型及特征

研究区内取心井岩心观察描述分析结果表明,玉泉组—龙井组发育以下7种典型岩相(表1)。块状层理砾岩相(Gm)由碎屑支撑的砾岩组成,其分选差,具有丰富的杂基和碎屑颗粒;板状交错层理砾岩相(Gp)主要由交错的细砾岩组成,分选中等,可能由位于侵蚀面之上的Gm相分化而成;槽状交错层理砂岩相(St)具有不规则的外部几何形状,由粗粒和中粒的槽状交错砂岩组成,可见泥砾;板状交错层理砂岩相(Sp)由板状交错的中细粒砂岩组成,分选中等;块状层理砂岩相(Sm)由没有明显特征和沉积结构的不等粒块状砂岩组成;块状层理粉砂岩相(Fm)以粉砂岩为主,无明显沉积构造;泥岩相(M)由深色泥岩组成,偶见植物炭屑。
表1 研究区玉泉组—龙井组典型岩相类型

Table 1 Typical lithofacies type of the Yuquan-Longjing formations in the study area

2.1.5 古气候指示

泥岩颜色可作为判断岩层形成时气候状况、水介质氧化—还原等沉积环境的重要依据34。玉泉组—龙井组泥岩以灰色、灰黑色、灰绿色为主[图5(g),图5(h),图5(i)],指示还原沉积环境,玉泉组—龙井组沉积时期发育的孢粉组合属于暖温带、亚热带被子植物35,反映该时期气候温暖潮湿。

2.2 测井相标志

测井相是测井曲线对不同沉积环境的综合响应,通过识别测井曲线的幅度、光滑度、旋回幅度、形态及顶底接触关系等特征,可以有效反映地层岩性及其组合特点、沉积环境特征34。本文研究测井相分析主要采用对岩性变化响应敏感的自然伽马曲线,并参考自然电位、声波时差曲线等变化趋势,共识别出4种基本测井曲线形态,分别为箱形、钟形、漏斗形及指形(表2)。
表2 研究区玉泉组—龙井组典型测井相特征

Table 2 Typical logging faces characteristics of the Yuquan-Longjing formations in the study area

序号 曲线形态 曲线特征 曲线实例 沉积微相
(a) 中、低幅交互指形 低幅指形与泥岩基线交互 泛滥平原
(b) 中、高幅漏斗形 下细上粗反旋回,顶部突变接触、底部渐变接触 心滩
(c) 中、高幅箱形 箱形曲线顶、底突变接触 心滩、辫状河道
(d) 中、高幅钟形 钟形曲线顶部渐变接触、底部突变接触 辫状河道
中—低幅交互的指形齿化严重,顶、底呈突变接触,且厚度较小,指示薄层砂岩与厚层泥岩互层沉积,代表辫状河泛滥平原微相[表2(a)];中—高幅的漏斗形轻微齿化,顶部突变接触、底部渐变接触,厚度较大,指示厚层砂岩沉积,代表辫状河心滩微相[表2(b)];中—高幅的箱形轻微齿化,顶、底呈突变接触,且厚度较大,指示厚层或巨厚层砂岩沉积,代表辫状河心滩或辫状河河道沉积微相[表2(c)];中—高幅的钟形轻微齿化,顶部渐变接触、底部突变接触,厚度较大,指示沉积物从下到上粒度逐渐变细,代表辫状河河道沉积微相[表2(d)]。

2.3 地震相标志

地震相是由特定地震反射参数所限定三维空间的地震反射单元,是特定的沉积相或地质体的地震响应36-39。在研究区玉泉组—龙井组共识别出5种地震相:
(1)强振幅透镜状反射为高频、强振幅、弱连续性反射地震相。在地震剖面上发育1~2对同相轴,同相轴相互平行,较为短小,连续性差。该类地震相反映了辫状河心滩沉积,发育在玉泉组[表3(a)]。
表3 研究区玉泉组—龙井组主要地震相类型

Table 3 Main seismic facies types of the Yuquan-Longjing formations in the study area

序号 地震相类型 地震相特征 地震剖面 沉积微相 发育层位
(a) 强振幅透镜状反射 高频、强振幅、弱连续 心滩 玉泉组
(b) 强振幅平行—亚平行反射 高频、强振幅、强连续 心滩或辫状河道 玉泉组、龙井组
(c) 低频中振幅空白反射 低频、中振幅、强连续 辫状河道 龙井组
(d) 弱振幅平行—亚平行反射 低频、弱振幅、强连续 泛滥平原 玉泉组、龙井组
(e) 中振幅平行—亚平行反射 中低频、中振幅、强连续 泛滥平原 玉泉组、龙井组
(2)强振幅平行—亚平行反射为高频、强振幅、强连续性反射地震相。剖面上表现为2~3对同相轴彼此平行或略微起伏,连续性好。该类地震相反映了连续稳定的水体沉积环境,为辫状河心滩或河道沉积特征,在玉泉组—龙井组内均有发育[表3(b)]。
(3)低频中振幅空白反射为低频、中振幅、强连续性反射地震相。剖面上发育1条较粗的同相轴,与相邻同相轴相互平行,连续性好。该类地震相反映了辫状河河道沉积特征,发育在龙井组[表3(c)]。
(4)弱振幅平行—亚平行反射为低频、低振幅、强连续性反射地震相。剖面上表现为2~3对同相轴彼此平行或略微起伏,连续性好。该类地震相反映了辫状河泛滥平原沉积,玉泉组—龙井组内均有发育[表3(d)]。
(5)中振幅平行—亚平行反射为中低频、中振幅、强连续性反射地震相。剖面上表现为2~3对同相轴彼此平行或略微起伏,连续性好。该类地震相反映了辫状河泛滥平原沉积,玉泉组—龙井组内均有发育[表3(e)]。
通过对研究区钻遇玉泉组—龙井组的7口钻井地震相类型与砂地比关系分析,确定了地震相与砂地比的相关性:低频中振幅空白反射地震相中砂地比波动范围较大,在0.3~0.9之间,但大部分砂地比都在0.4以上;强振幅平行—亚平行反射地震相中砂地比波动范围也比较大,在0.2~0.7之间且多数砂地比在0.4以上;强振幅透镜状地震相中砂地比范围在0.5~0.7之间;弱振幅平行—亚平行反射地震相中砂地比波动范围在0~0.5之间;中振幅平行—亚平行反射地震相中砂地比波动范围在0.1~0.4之间(图7)。这一结果证实研究区目的层段内低频中振幅空白反射地震相最为富砂,强振幅平行—亚平行反射和强振幅透镜状两类地震相次之,弱振幅平行—亚平行反射和中振幅平行—亚平行反射地震相的含砂量最少。
图7 研究区玉泉组—龙井组地震相类型与砂地比交会图

注:A为低频中振幅空白反射型砂地比;B为强振幅平行—亚平行反射型砂地比;C为强振幅透镜状反射型砂地比;D为弱振幅平行—亚平行反射型砂地比;E为中振幅平行—亚平行反射型砂地比

Fig.7 Seismic facies types and the sand-stratum ratio of Yuquan-Longjing formations in the study area

3 沉积微相类型及特征

岩相组合通常是一次沉积事件或一定环境连续演变的产物,不同的岩相组合可指示不同沉积微相的沉积过程3440。基于研究区玉泉组—龙井组岩心相,测井相和地震相综合分析,共识别出3种不同的岩相组合,并代表了不同的沉积微相(图8)。
图8 研究区玉泉组—龙井组典型岩相组合

Fig.8 Typical lithofacies associations of the Yuquan-Longjing formations in the study area

3.1 辫状河道(类型Ⅰ)

辫状河道在测井相上呈现出中、高幅箱形和钟形[表2(c),表2(d)];在地震剖面上表现为连续性低频中振幅空白反射[表3(c)]和强振幅平行—亚平行反射特征[表3(b)];岩相组合的垂向序列为Gm—St—Sm[底部为块状层理砾岩相(Gm)],多沉积在河道底板或冲刷凹陷中,与下部高能水动力侵蚀面接触,随着水动力条件逐渐稳定,向上可发育大型槽状交错层理砂岩相(St)和块状层理砂岩相(Sm),由于缺乏良好的层理,不同叠置单元之间的层理接触面可能模糊不清[图8(a)]。在下一次洪泛期,水动力再次加强,可对早期河道沉积物进行冲刷。该岩相组合为正粒序沉积,岩性以砾岩和砂岩为主,基本没有细粒沉积物的出现,沉积作用主要为垂向和侧向加积,指示了沉积物快速卸载和高能水动力条件的辫状河河道沉积。

3.2 心滩(类型Ⅱ)

心滩沉积在测井相上呈现出中、高幅箱形和漏斗形[表2(b),表2(c)];在地震剖面上表现为强振幅透镜状[表3(a)]和强振幅平行—亚平行反射特征[表3(c)];岩相组合为Gm—Gp—Sp—Sm—Fm[底部由块状砾岩相(Gm)过渡到板状交错层理砾岩相(Gp),然后依次沉积板状交错层理砂岩相(Sp)和块状层理砂岩相(Sm),顶部可见细粒沉积物,多为块状层理粉砂岩相(Fm),图8(b)]。整个沉积序列呈正粒序,向上水动力条件逐渐减弱并趋于稳定,指示了水动力相对稳定下垂向加积的心滩沉积。

3.3 泛滥平原(类型Ⅲ)

泛滥平原在测井相上呈现出低幅交互指形[表2(a)];在地震剖面上表现为弱—中振幅连续性平行—亚平行反射[表3(d),表3(e)];岩相组合为Sm—Fm—M[在垂向上沉积厚层的细粒泥岩夹薄层细砂岩或粉砂岩为特征,图8(c)]。细粒沉积物表明是在与辫状河道相邻的河岸地区反复发生洪水,水体漫溢,在河道间快速卸载沉积。砂岩和粉砂岩与泥岩的交互出现表明悬浮物普遍沉积在细粒砂质沉积物的顶部或低地势的废弃河道。泛滥平原沉积中泥岩颜色多为灰黑色,部分含植物碎片或植物根系[图5(i)],指示了研究区处于还原环境。

4 沉积相平面展布与沉积模式

根据研究区岩心相、测井相、地震相及岩相组合特征,确定研究区玉泉组—龙井组为潮湿环境下的辫状河沉积体系,发育辫状河道、心滩和泛滥平原微相。

4.1 沉积相平面展布

本文研究提取了弧长、平均瞬时频率、平均能量、平均强度、平均波峰、平均波谷、均方根振幅、最大振幅以及最小振幅属性等共计十余种常用的地震属性,与地震相进行优选对比后,认为均方根振幅属性能够较好反映砂体平面展布[图9(a),图9(c)]。因此,本文基于岩心相,测井相和地震相分析,结合目的层段均方根振幅属性分布特征,并进一步采用古地貌约束刻画,精细厘定了研究区玉泉组—龙井组辫状河沉积相平面展布。
图9 研究区玉泉组—龙井组辫状河均方根振幅(RMS)层间属性及沉积相图

Fig.9 RMS interlaminar attribute and sedimentary facies of braided river of Yuquan-Longjing formations in the study area

研究区内发育的辫状河沉积体系的物源主要来自于研究区南部41。龙井组沉积时期,研究区处于高能水动力条件下,发育一条由南向北、稳定的辫状河道[图9(d)],河道砂体厚度及宽度较大,在纵向上和横向上砂体之间的连通性较好(图10)。玉泉组沉积时期,水动力条件相对较弱,辫状河道侧向迁移相对较少,河道较窄,沉积的砂体厚度较薄,砂体之间的连通性相对较差,泛滥平原泥岩大面积发育,在地震剖面上呈条带状分布(图10)。辫状河道在研究区内广泛发育,多条河道方向均为由南向北,与物源方向保持一致[图9(b)],多期河道砂体和心滩砂体相互叠加,形成局部富砂中心。
图10 研究区玉泉组—龙井组辫状河沉积相剖面

Fig.10 Braided-river sedimentary facies profile of the Yuquan-Longjing formations in the study area

4.2 沉积演化

中新世以来,随着菲律宾海板块向北移动,太平洋板块向西俯冲的挤压作用直接传递至东海地区,导致东海盆地发生东西向挤压1321。盆地东缘隆起褶皱抬升,伴随着大规模岩浆侵入,地层强烈抬升,研究区进入陆相沉积的演化阶段21-22。玉泉组—龙井组古地貌为南高北低,研究区中北部为沉积中心[图11(a),图11(b)]。研究区南侧发育多个沟谷,是沉积物搬运的通道,从地震剖面上看发育河道充填沉积[图11(c),图11(d)],物源方向为研究区南部40,玉泉组—龙井组沉积时期发育砂质辫状河沉积体系。龙井组沉积时期河道及心滩规模较大,水动力条件强,沉积物粒度偏粗,岩性以砾岩,砂岩为主;玉泉组沉积时期,水动力条件逐渐减弱,研究区发育多条小型辫状河道,河道及心滩发育程度较低,砂体之间的连通性变差,岩性以砂岩和泥岩为主,泛滥平原亚相在整个研究区内广泛发育。
图11 研究区玉泉组—龙井组古地貌图及地震剖面

(a)玉泉组古地貌图;(b)龙井组古地貌图;(c)玉泉组古地貌剖面;(d)龙井组古地貌剖面

Fig.11 Paleogeomorphological map and seismic section of the Yuquan-Longjing formations in the study area

4.3 沉积模式的建立

辫状河沉积模式研究在油气勘探开发领域一直备受重视42-56,基于岩心相、测井相、地震相分析,结合玉泉组—龙井组沉积相平面展布刻画,本文研究建立了Y地区玉泉组—龙井组的辫状河沉积模式(图12)。
图12 研究区玉泉组—龙井组辫状河沉积模式

Fig.12 Braided-river depositional model of the Yuquan-Longjing formations in the study area

龙井组沉积时研究区地形南高北低,处于高能水动力条件下,辫状河道和心滩发育规模大,河道侧向迁移频繁。砂体沉积厚度大,砂体之间的连通性较好,沉积颗粒较粗,泛滥平原相对不发育[图12(b)]。玉泉组沉积时,水动力条件逐渐减弱,发育多条小型辫状河道,河道和心滩发育规模减小,但较为稳定。砂体沉积厚度较薄,砂体之间的连通性较差,沉积物颗粒较细,泛滥平原在研究区广泛发育[图12(a)]。早—中中新世研究区均处于潮湿环境,南部物源供给充足,长石岩屑大量保存,岩石成分成熟度和结构成熟度中等,具有快速沉积的特征。根据以上特征判断玉泉组—龙井组为发育于潮湿环境下的辫状河沉积。

5 结论

(1)东海盆地Y地区玉泉组—龙井组储层以长石岩屑质石英砂岩为主,其成分成熟度和结构成熟度都较低,以板状交错层理和块状层理发育为主,为“两段式”牵引流沉积。
(2)东海盆地Y地区玉泉组—龙井组发育Gm—St—Sm、Gm—Gp—Sp—Sm—Fm和Sm—Fm—M共3种岩相组合,分别代表辫状河沉积体系的辫状河道、心滩及泛滥平原微相。其中,辫状河道微相以中—高幅箱形及钟形测井相为主,呈强振幅平行—亚平行反射和低频中振幅空白反射地震反射特征;心滩微相以中—高幅箱形及漏斗形测井相为主,呈强振幅透镜状和强振幅平行—亚平行反射地震反射特征;泛滥平原微相以中、高幅箱形和钟形测井相为主,呈弱—中振幅平行—亚平行反射地震反射特征。
(3)东海盆地Y地区玉泉组—龙井组辫状河沉积模式为温暖潮湿环境下的辫状河沉积。其中,龙井组沉积时期,辫状河道及心滩发育规模大,砂体沉积厚度大;玉泉组沉积时期,小型辫状河道发育,河道及心滩规模均减小,泛滥平原相对发育。
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