沉积学报  2018, Vol. 36 Issue (1): 110−119

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文章信息

李维, 朱筱敏, 陈刚, 马英俊
LI Wei, ZHU XiaoMin, CHEN Gang, MA YingJun
基于等时界面识别的浅水三角洲-河流沉积体系研究——以高邮凹陷黄珏地区古近系垛一段为例
Research based on Isochronous Surface about Shallow-water Deltas and Fluvial Sedimentary System: A case from Duo1 Member of Paleogene in Huangjue Area, Gaoyou Sag
沉积学报, 2018, 36(1): 110-119
ACTA SEDIMENTOLOGICA SINCA, 2018, 36(1): 110-119
10.3969/j.issn.1000-0550.2018.013

文章历史

收稿日期:2017-03-27
收修改稿日期: 2017-05-05
基于等时界面识别的浅水三角洲-河流沉积体系研究——以高邮凹陷黄珏地区古近系垛一段为例
李维1,2, 朱筱敏1, 陈刚2, 马英俊2     
1. 中国石油大学(北京)地球科学学院, 北京 102249;
2. 中国石化江苏油田分公司勘探开发研究院, 江苏扬州 225009
摘要: 利用岩芯观察、粒度统计、古生物对比、测井分析及地震沉积学方法研究了苏北盆地高邮凹陷黄珏地区始新世三垛组一段浅水三角洲-河流沉积体系。研究结果表明:坡缓浅水沉积背景下除了发育传统认识中由构造控制的长期基准面旋回特征转换面,还可形成另一类由气候控制的、浅水三角洲与河流沉积之间稳定的浅水-陆上沉积环境变化界面。以气候主控等时界面为基础,借助地层切片直观分析了浅水三角洲与河流相的沉积演化特征及砂体展布规律,并验证了该界面的可靠性。两类等时界面的综合约束为浅水三角洲-河流沉积体系油气勘探提供了新思路。
关键词苏北盆地     黄珏地区     古近系三垛组     浅水三角洲     等时界面     主控因素     地震沉积学    
Research based on Isochronous Surface about Shallow-water Deltas and Fluvial Sedimentary System: A case from Duo1 Member of Paleogene in Huangjue Area, Gaoyou Sag
LI Wei1,2, ZHU XiaoMin1, CHEN Gang2, MA YingJun2     
1. College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China;
2. Research Institute of Exploration and Development, Sinopec Jiangsu Oilfield Company, Yangzhou, Jiangsu 225009, China
Foundation: National Science and Technology Major Project, No.2016ZX05001002-006
Abstract: Gaoyou Sag, located in the south of Subei Basin, is a faulted lacustrine basin developed during the end of Late Cretaceous and Huangjue area is in the southwest part of Gaoyou Sag. Paleogene strata consisted of Funing Formation, Dainan Formation, Sanduo Formation and Yanchen Formation is deposited from bottom to top successively in Gaoyou Sag. Sanduo Formation consists of Duo1 and Duo2 member from bottom to top and Duo1 member is target strata of this research. During the sedimentary period of Duo1 member, tectonic movement in Gaoyou Sag dropped off gradually and the altitude difference between boundary and sag nearly disappeared, which made contributions to the characteristics of paleotopography in Gaoyou Sag such as a smooth slope. Several lines evidence have supported the fact that Duo1 member mainly deposited in shallow-water deltas, such as the brown mudstone with plenty of bioturbation structure, sedimentary structures which show strong hydropower, bad preservation of mouth bar and a large number of distributary channels, grain-size collection and analysis also indicate the influence of tractive current. However the upper Duo1 member belongs to fluvial sedimentary system. The sand content of mudstone increases obviously in the upper Duo1 member, and warm holes almost disappear. In addition to this, seismic reflection configuration also developed from imbricated progradation reflection to sub-parallel seismic reflection, which could be treated as transformation from shallow-water deltas to river. Strong fluvial-dominated function in shallow-water deltas always results in frequent changes of sand bodies. Lithological reservoirs are likely to form in such sedimentary environment and accurate reservoir description should be the key to exploration and development of such oilfields. Distribution of subtle traps in shallow-water deltas and fluvial facies is controlled by base-level cycles, so isochronous stratigraphic frameworks are foundation of reservoir prediction. Based on sufficient analysis of core, well logging and seismic data, this research identified two kinds of allo-cycle, respectively, dominated by tectonic movement and climatic variation. Generally, changes of base-level cycle dominated by a certain size of tectonic movement could always be identifiable in well logging curve. Usually convert interface of long-term base level cycle or return point in resistivity curve could be treated as recognition marks. Besides that, research suggests that a certain amount of changes in lake level could be easy to bring about big changes in the scope of lake, which makes it possible to recognize the interface between underwater deposit and continental sediment. In research found there exist a return point in resistivity logging curve, and treat such point as the surface of two sub-members. Baseline of natural gamma-ray logging above the return point is relatively lower, and AC figures of strata above the point are also distinctly smaller than that of the strata below the point. Differences in GR as well as AC shows mudstone above the point has higher sand content, and that could just be the differences of mudstone deposited in shallow-water environment and fluvial environment. Surface related to the return point was suggested to be the isochronous surface controlled by climate. There also exists a strange phenomenon; a successive seismic event appears near the climate correlatively interface while there does not develop any stable sandstone-mudstone boundary. Research makes a conclusion that mudstone with disparate sand content has different density and velocity, which generates the contrast in wave impedance, and forms the successive seismic events around the position where resistivity log begin to return. Isochronous stratigraphic framework based on interfaces mentioned above has been established all over the study area, and such framework has been used in seismic sedimentology research. Amplitude slices of strata below the climate-correlatively surface show typical characteristics of shallow-water deltas, range of reservoirs expand or shrink under control of lake level. However, slices of strata above the surface could be considered as fluvial deposit result in channels migration frequently while the entire scope of sand bodies do not change obviously. Recognition of two isochronous surfaces, especially the one controlled by climatic, could effectively guide the progressive exploration in mature oilfield. The research results above could bring about new ideas to help discover oil and gas resources in shallow-water delta and river sedimentary system.
Key words: Huangjue area     Subei Basin     Sanduo Formation of Paleogene     shall-water delta     isochronous interface     main controlling factors     seismic sedimentology    
0 引言

浅水三角洲一词最早由Fisk(1954)提出,指在浅水环境、构造稳定和供源充足的盆地缓坡背景下,以发育水下分流河道为典型特征的三角洲沉积体系[1-6]。我国学者通过对鄂尔多斯盆地[7]、松辽盆地[8-10]、渤海湾盆地[11]和四川盆地[12]等地区古代沉积以及鄱阳湖现代沉积[13-14]的研究,系统分析了浅水三角洲形成条件与类型划分[1-6, 15]、主要沉积微相与沉积特征[2, 10, 12]以及沉积动力机理[4, 6, 10, 13-14, 16]等方面的沉积学问题,总结了稳定缓慢沉降的大型坳陷湖盆易形成浅水三角洲;分流河道骨架砂体发育而河口坝少见;砂体空间展布主要受物源和气候共同控制;缓坡背景下的湖平面大范围变化对沉积特征起重要控制作用等认识。

浅水三角洲强烈河控作用下优势发育的河道砂体与湖相泥岩良好的储盖配置关系及其优越的油气成藏条件使之成为岩性油气藏勘探的重要领域。然而另一方面,大范围稳定泥岩段的缺失、三角洲反旋回特征的弱化、河道的频繁迁移叠置增大了浅水三角洲沉积体系等时界面的识别难度,增加了储层预测难度。前人罕有针对浅水三角洲沉积体系等时界面的系统研究和总结,而准确的界面识别正是砂体展布分析和油气勘探的基础。

研究目标为苏北盆地高邮凹陷黄珏地区始新世垛一段。不同于鄂尔多斯盆地延长组或松辽盆地青山口组等大型坳陷湖盆浅水三角洲沉积的典型代表,黄珏地区垛一段沉积于断陷湖盆发育晚期,但同样具有大量浅水三角洲沉积证据。利用钻测井与地震资料识别了浅水三角洲—河流沉积体系中分别受构造和气候主控的两类关键等时界面,以此为基础建立等时格架分析了沉积演化特征。

1 研究区概况与沉积背景

苏北盆地是位于下扬子地台北部的中—新生代盆地区,自北向南包括盐阜坳陷、建湖隆起、东台坳陷等一级构造(图 1a)。位于东台凹陷中部的高邮凹陷是晚白垩世末仪征运动基础上发展起来的箕状断陷湖盆,由南至北分为南断阶、深凹带和北斜坡三个二级构造单元,其中黄珏地区位于深凹带西部(图 1b)。

图 1 苏北盆地构造单元划分和黄珏地区位置图 a.苏北盆地构造区划;b.高邮凹陷构造单元及垛一段砂岩百分含量 Figure 1 Tectonic unit division of Subei Basin and location of Huangjue area a. tectonic unit division of Subei Basin; b. tectonic unit division of Gaoyou Sag and sandstone percent content of Duo1 member

高邮凹陷自下而上连续沉积古新世阜宁组(E1f),始新世戴南组(E2d)、三垛组(E2s)和新近纪盐城组(Ny)等地层,依次经历断坳(E1f)—断陷(E2d~E2s)—坳陷(Ny)等演化阶段。三垛组沉积于断陷活动末期,自下而上分为垛一段(E2s1)和垛二段(E2s2),其中垛一段作为主要含油层系是本次研究目标,自下而上分为E2s17至E2s11共7个砂组。垛一段沉积时期南部边界断层已基本停止活动,并且戴南组的充填也使得地势相对平缓[17],得益于此,来自北部的主物源在垛一段沉积时期基本跨越了深凹带(图 1b)。但重矿物组合显示黄珏地区以较高的锆石和较低的石榴石、钛磁铁矿含量区别于北部斜坡带,表明仍有来自南部的次要物源。总而言之,研究区在垛一段沉积时期具备形成浅水三角洲的背景条件,并且受南北物源共同控制。

古生物化石研究表明,与其下广湖相戴南组相比,高邮凹陷三垛组的介形虫化石、轮藻化石和腹足类化石数量均出现明显减少[18],浅而局限的水体成为三垛组沉积时期常态。此外,戴南组沉积晚期孢粉化石组合显示麻黄属花粉占优,指示广阔湖泊水域的消失和干旱高盐环境的出现。而三垛组沉积时期以指示温暖潮湿环境的破隙杉花粉数量占优,表明该时期重回温热潮湿的亚热带气候,低洼处应当存在浅水湖泊。

2 垛一段沉积特征与沉积体系 2.1 主要沉积特征

黄珏地区5口井(井位参见图 1)的取芯段岩石颜色、沉积构造和遗迹化石等特征表明,研究区垛一段自下而上依次发育浅水三角洲和河流沉积,主要沉积特征如下:

(1) 垛一段中下部(E2s14~E2s17)沉积于浅水环境

研究区垛一段泥岩整体呈棕色,属于偏氧化沉积环境产物。但垛一段中下部岩芯偶见灰绿色泥岩夹层(图 2d),指示了局部弱还原环境的存在;并且泥岩中常发育大量直立—斜立虫孔以及生物扰动构造(图 2bef),更可能形成于三角洲水下环境;此外在黄2井垛一段中部(E2s14)取芯段见到厚达3.5 m的泥质碎屑流沉积,棕色泥岩中漂浮着大量粗砂—细砾级别的灰白色颗粒。如此规模的重力流沉积很难出现在河流相中,应当是三角洲沉积环境的产物。综上认为垛一段中下部沉积于浅水环境。

图 2 高邮凹陷黄珏地区垛一段浅水三角洲—河流沉积体系岩性特征与沉积构造 a.黄17井,E2s14,1 466.5 m,灰色细砂岩,板状交错层理;b.黄2井,E2s14,1 330.5 m,灰绿色粉砂岩,平行层理,见近直立虫孔;c.黄3-1井,E2s15,1 527.30 m,浅灰色含砾细砂岩,细砂岩具块状层理,发育多期冲刷面,砾石定向排列,大小约3~10 mm;d.黄12井,E2s14,1 227.7 m,灰绿色泥岩与棕色泥岩过渡接触;e.黄2井,E2s14,1 299 m,灰绿色含砾泥质粉砂岩,生物扰动强烈;f.黄6井,E2s14,1 153.3 m,棕色砂质泥岩,见斜立虫孔;g.黄2井,E2s13,1 220.9 m,杂色砂质泥岩,泥岩为棕色至绿色,呈斑块状杂乱分布 Figure 2 Lithology characteristics and sedimentary structures of shallow-water deltas and fluvial facies sedimentary system in Huangjue area, Gaoyou sag

(2) 水下分流河道发育,沉积水动力较强,发育间断正韵律

垛一段浅水三角洲砂体以粉—细砂岩为主,南部近物源处粒度较粗,常见含砾砂岩。受浅水沉积环境和强烈的河控作用影响,取芯段主要发育板状交错层理、平行层理及块状层理(图 2a, b, c)。砂岩底部常见冲刷面及定向排列砾石,砾石成分相对单一且顺层分布(图 2c)。河道砂体由具泥砾的冲刷面与块状砂岩细砂岩向上过渡为平行层理粉—细砂岩,与上覆棕色、灰绿色泥岩共同构成间断正韵律。

粒度分析亦表明研究区砂体的形成主要受河流作用控制。粒度分析资料(表 1)显示垛一段偏度在0.43~0.65之间,偏态为正偏态,峰度很尖锐;标准偏差在1.27~2.36之间,平均为1.77,分选较差—中等;粒度较粗,以细砂岩为主,也有中—粗砂岩和含砾砂岩。

表 1 高邮凹陷黄珏地区垛一段粒度分析 Table 1 Grain size analysis of Duo1 Member in Huangjue area, Gaoyou sag
层位 井号 样品数 粒度中值/mm 平均粒径/mm C值/mm 偏度 峰度 标准偏差
E2s13 黄2 34 0.11 0.06 0.49 0.43 1.43 2.36
黄12 6 0.21 0.13 0.72 0.5 1.57 1.79
E2s14 黄2 14 0.13 0.09 0.4 0.53 2.05 1.68
黄17 3 0.44 0.22 0.67 0.6 1.82 2.27
黄59 7 0.2 0.15 0.54 0.55 2.37 1.32
E2s15 黄6 5 0.07 0.03 0.21 0.65 1.93 1.94
E2s16 黄3-1 10 0.18 0.12 0.52 0.55 1.93 1.84
黄17 14 0.17 0.13 0.64 0.43 2.37 1.5
黄53 16 0.21 0.17 0.57 0.49 1.89 1.27

(3) 垛一段上部(E2s11~E2s13)属于河流沉积

垛一段中上部泥岩常表现为典型的杂色漫滩沉积(图 2g),或呈现反映强烈氧化作用的砖红色,几乎未见虫孔等生物遗迹;砂岩发育块状层理或模糊的交错层理。此外钟形的SP曲线形态体现了河道砂体的正韵律特征,泥岩段较低的自然伽马和声波时差亦反映砂质含量的明显增加(图 3),综合分析认为垛一段上部已转为陆上河流沉积。

图 3 高邮凹陷黄珏地区垛一段单井相与沉积序列特征(黄70井,井位参见图 1) Figure 3 Sedimentary facies of single well and sedimentary cycles of Duo1 member in Huangjue area, Gaoyou sag(Well H70)
2.2 沉积体系

垛一段自下而上发育浅水三角洲—河流沉积体系(图 3)。关于浅水三角洲的亚相划分,有学者将洪水期与枯水期之间的大面积地区称为下三角洲平原以强调其河控作用[12];也有学者将河控作用强烈区域称为“内前缘”,湖控主导区称为“外前缘”,强调湖湖岸线对外前缘砂体展布特征的控制[19]。分类方案存在差异,但本质上都是为了突出洪水—枯水面之间大片受湖水影响区域的强烈河控作用。湖泊水体的存在除了影响砂体形态,同样会对泥岩沉积性质产生重要影响。后文将详述,泥岩性质变化正是识别气候主控等时界面的基础,故本次研究倾向于采用内前缘—外前缘的划分方案,具体亚(微)相划分如下。

(1) 浅水三角洲平原:包括分流河道与泛滥平原等微相,前已述及,生物遗迹相对少见和独特的泥岩性质是该亚相重要的识别标志。理论上河道分叉与否是识别三角洲平原与河流相的依据,但二者电性特征类似,往往难以准确区分。本次研究结合区域沉积旋回特征,认为电阻率明显回返段指示沉积水体的逐渐变浅,将其划分为浅水三角洲平原亚相,而回返段上部的电阻率基线稳定层段属于河流沉积(图 3)。

(2) 浅水三角洲内、外前缘:受湖平面的影响,砂体展布特征的差异是内、外前缘的细分依据。内前缘包括水下分流河道与水下分流河道间等微相,河口坝少见,高幅度差箱型、钟形水下分流河道是该亚相典型特征;外前缘包括残留水下分流河道、河口坝、席状砂等微相,分流河道多呈钟形,见较多反韵律河口坝和薄层席状砂。内、外前缘垂向叠置出现,反映湖平面的频繁变化,也体现了湖平面对砂体展布特征的控制作用(图 3)。

(3) 前浅水三角洲:理论上应以大套厚层泥岩为特征,研究区未见该相带典型岩芯,测井资料识别的垛一段中部泥岩段可能属于该亚相,岩屑录井显示泥岩多呈棕色,偶见灰绿色。

3 关键等时界面识别

黄珏地区垛一段下部发育两套标志层,分别是厚约10~20 m的玄武岩和厚约5~8 m的暗色泥岩(图 3),其中暗色泥岩在高邮凹陷均有分布,指示了一期大范围湖侵。垛一段下部E2s16和E2s17砂组可根据这两套区域标志层准确限定,而中上部砂组的划分则需借助旋回分析。依据主控因素可将本区关键等时界面分为两类,分别是构造控制的E2s14/E2s15分界和气候主控的E2s13/E2s14分界。

3.1 浅水三角洲自旋回改造强烈

根据成因可将沉积旋回分为异旋回和自旋回两类。异旋回由基准面变化引起,包括构造、气候和海(湖)平面升降等,因其主控因素大范围分布,故而在相当大的范围内具有可对比性;而河流改道决口、三角洲朵叶体摆动等自旋回作用不受基准面控制,在其局限的分布范围内往往破坏异旋回特征。

对比储层构型中的三类基本模型,从千层饼(滨岸相)到拼合板(三角洲相)到迷宫状(河流相),体现的正是自旋回干扰的逐渐增加,从某种意义上来说,河流作用的增强意味着更多的自旋回改造。黄珏地区垛一段河控作用占据主导,河道砂体的横向迁移与纵向叠置,必然不同程度地破坏原有异旋回规律,这一点正是在浅水三角洲内开展大范围精细分层的难点。在井资料丰富的地区,可根据沉积模式指导总结出受自旋回干扰较弱、具有全区普适性的典型异旋回规律,实现等时界面划分。

3.2 构造主控的异旋回特征

中—长期基准面旋回通常受自旋回干扰较弱,显然异旋回级别与相应等时界面的分布范围成反比,长期基准面旋回由上升到下降转换处形成的稳定泥岩发育段往往具有大范围可对比性。一般而言,大于100 m级别的旋回与幕式构造活动有关,10 m级的旋回受湖平面变化的控制,小于10 m级的旋回可能反映受米兰科维奇驱动的气候旋回[20-21]

垛一段中上部地层在全区范围都具有砂泥比先减小再增大的趋势,反应了基准面旋回特征的变化。虽然浅水环境难以沉积典型凝缩层,但一定级别基准面旋回导致的泥质含量变化仍然足以被自然伽马测井所识别。如图 3所示,研究区自然伽马曲线出现先增大后减小的普遍规律(图 3),位于垛一段中部的高GR泥岩发育段即为覆盖全区的等时界面,也是E2s14和E2s15砂组的物理界面。

3.3 气候控制的异旋回特征

通常认为气候对浅水三角洲沉积起重要控制[21],与较深水三角洲相比,浅水三角洲因为沉积背景平缓,湖平面升降将在很大范围内影响沉积相及砂体展布。干旱气候条件下湖水将大范围收缩,导致大片区域沉积环境由浅水转为陆上,若该类由湿润转为干旱的气候变化长期稳定且盆地持续稳定沉降,沉积物得以良好保存,可以形成另一类等时界面。

前已述及,黄珏地区垛一段上部存在电阻率基线的回返现象,全区分布的明显回返点所对应的界面可作为E2s13和E2s14砂组的分界。虽然E2s14和E2s13都属于泥岩发育段,但二者性质存在明显差异,E2s13及其上地层泥岩段GR基值明显更低,声波时差更小(图 3),反映了泥岩中更高的砂质含量。砂质含量的变化很可能正是漫滩沉积陆上砂质泥岩与浅水三角洲前缘水下质纯泥岩性质差异的体现,指示了全区沉积环境已由水转陆。

在制作本区合成纪录时,通过将玄武岩反射与地震反射强轴T23的精确标定,发现几乎所有井的E2s13底界都对应着一套波谷反射(负极性剖面)。这样的特征在地震剖面上更加直观(图 4,为了便于观察将波谷充填为黑色),无论主测线还是联络线,除了断层附近的波形变化,E2s13底界始终对应相对连续的同相轴。河流相泥岩更高的砂质含量意味着更大的密度和速度,从而能够形成大范围分布的阻抗差界面,因此虽然研究区E2s14至E2s13并不存在全区稳定的砂泥岩界面,却能形成稳定的反射,这在垛一段中上部同相轴断续分布的浅水三角洲—河流沉积背景中十分宝贵。此外在平行物源方向,该同相轴(T2X)下方发育叠瓦状前积反射,是浅水三角洲的典型特征[22];而同相轴上方则为指示河流沉积的亚平行席状反射(图 5)。在古地貌平缓的浅水三角洲背景中,由浅水到陆地的变化在一定尺度上显然是等时的,换而言之,气候变化形成了浅水三角洲—河流沉积体系独特的等时界面。

图 4 E2s13底界对应地震同相轴特征(黄色箭头所指处) Figure 4 Characteristics of the seismic event corresponding to the bottom of E2s13 (the yellow array)
图 5 高邮凹陷黄珏地区垛一段地震反射特征 Figure 5 Seismic reflection characteristic of Duo1 member in Huangjue area, Gaoyou sag
4 地震沉积学研究与沉积演化特征 4.1 地震等时格架建立

等时界面为地震沉积学研究提供了可能。如图 6所示,T23对应玄武岩反射,T2X是沉积环境由水到陆的变化面对应的同相轴,以这两根同相轴为基础,将井上各砂组地层厚度统计结果转换为时间域数据,在地震剖面中完成了地层标定。其中E2s14底界可由T2X至T23等比例划分得到;E2s2~E2s12底界依据河流相近似等厚的沉积模式,将T2X分别上漂48 ms,84 ms和120 ms得到(E2s11和E2s12厚约45 m,E2s13厚约60 m,研究区该段地层速度约2 500 m/s)。

图 6 高邮凹陷黄珏地区垛一段地震地层格架 Figure 6 Seismic stratigraphic framework of Duo1 member in Huangjue area, Gaoyou sag
4.2 敏感属性验证与沉积演化分析

在等时框架内以砂组为研究单元开展属性分析,认为振幅类属性能较好区分研究区砂泥岩。以E2s12~E2s14砂组为例,基于原始地震数据的均方根振幅虽不能准确实现岩性区分,但可反映砂体平面展布的整体趋势(图 7a, b, c);而通过频谱分解处理,以E2s11砂组为例,发现分频叠加后的振幅属性与砂体展布特征间具有更精细的吻合度(图 7d)。上述对比充分表明振幅类属性是本区砂泥岩识别的敏感属性,这为90°相移后的振幅切片赋予了岩性意义。

图 7 高邮凹陷黄珏地区垛一段部分砂组敏感属性分析 a. E2s14砂组RMS与砂地比;b. E2s13砂组RMS与砂地比;c. E2s12砂组RMS与砂地比;d. E2s11砂组8 Hz,16 Hz,24 Hz,30 Hz,42 Hz振幅属性叠合图与砂地比 Figure 7 RMS and sand-shale ratio of some sands group of Duo1 member in Huangjue area, Gaoyou sag a. RMS and sand-shale ratio of E2s14 sands group; b. RMS and sand-shale ratio of E2s13 sands group; c. RMS and sand-shale ratio of E2s12 sands group; d. Amplitude attribute overlay map on 8 Hz, 16 Hz, 24 Hz, 30 Hz, 42 Hz and sand-shale ratio of E2s11 sands group

以2 ms采样间隔制作了E2s11~E2s14砂组地层切片并分析沉积演化。受研究区地质情况(砂泥岩互层、单砂体厚度基本不超过10 m且断层发育)与地震资料分辨率(主频约20~25 Hz,垂向分辨率有限,并且导致分频处理时噪声相对突出)的综合影响,地层切片难以精确刻画河道或单砂体边界,但可借助切片分析沉积演化及指导砂体发育区预测。

E2s14砂组浅水三角洲沿湖盆环状分布的特征十分典型(图 8),砂体展布范围明显受湖平面升降控制,地层切片共呈现两期对称的基准面旋回和一期不对称的基准面下降半旋回,与测井旋回特征良好吻合;E2s13砂组上部至E2s11砂组地层的切片则展现出完全不同的沉积特征,砂体不再环湖分布,其分布范围也不再有规律地扩大—缩小,而是表现出全区普遍覆盖的特征和平面分布位置的不断变化(图 8),反映了河道的不断迁移。地层切片清晰展示了从E2s14到E2s13沉积环境由水到陆的转变,因此前文关于气候主控异旋回特征变化界面的推论是可靠的。

图 8 高邮凹陷深凹带西部垛一段E2s14~E2s11砂组沉积演化特征 Figure 8 Sedimentary evolution under different environments from E2s14 sand group to E2s11 sand group in west part of deep sag, Gaoyou sag

浅水三角洲是国内外沉积学研究和油气生产热点,准确的沉积模式与砂体展布规律认识是勘探开发关键。大范围等时地层格架的建立,为在储层非均质性较强的浅水三角洲—河流沉积体系中实现岩性预测提供了约束。浅水三角洲前缘砂体延伸范围与沉积规模明显受湖平面升降控制,以黄珏地区E2s14砂组为例,有利储层集中发育在该段中上部和下部,并且内前缘砂体应当以厚层条带状为特征,而外前缘砂体则具有相对更好的连续性;河流沉积储层非均质性更强,更易形成岩性圈闭。虽然本次研究中单一振幅属性未能清晰刻画河道砂体,但等时界面的识别与沉积模式的认识已经能够为砂体预测提供指导和帮助。

5 结论

(1) 岩芯观察、粒度分析、古生物资料以及井震资料分析表明,苏北盆地黄珏地区垛一段沉积早、中期发育浅水三角洲沉积,晚期转为河流沉积,共同构成浅水三角洲—河流沉积体系。

(2) 黄珏地区垛一段浅水三角洲发育两类等时界面,分别是受构造主控的长期基准面旋回变化面和受气候主控的沉积环境由水到陆的转换面。两类等时界面是构建浅水三角洲—河流沉积体系大范围等时地层格架的基础。

(3) 浅水三角洲坡缓水浅的沉积背景有利于气候主控等时界面的形成,地层切片显示该界面之下的浅水三角洲与界面之上的河流沉积具有截然不同的砂体展布规律和沉积演化特征。

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