扒楼沟剖面二叠系辫状河砂体构型与非均质性特征

王 越， 陈世悦， 李天宝， 梁绘媛， 王 剑

(1.中国石油大学地球科学与技术学院,山东青岛 266580; 2.中国石化胜利油田分公司,山东东营 257015;3.新疆油田公司实验检测研究院,新疆克拉玛依 834000)

扒楼沟剖面二叠系辫状河砂体构型与非均质性特征

王 越1， 陈世悦1， 李天宝1， 梁绘媛2， 王 剑3

(1.中国石油大学地球科学与技术学院,山东青岛 266580; 2.中国石化胜利油田分公司,山东东营 257015;3.新疆油田公司实验检测研究院,新疆克拉玛依 834000)

通过露头实测、精细解剖与岩相分析等方法,结合岩石薄片与物性资料,对山西保德扒楼沟剖面二叠系山西组辫状河砂体的构型与非均质性特征进行系统研究。结果表明:辫状河主要发育河道和心滩2种成因砂体,河道砂体的垂向岩相组合为Ge-St,心滩砂体的垂向岩相组合为Ge-Sm-M-Sm-Sp-M-Sp或Ge-Sm-St-Sm-St-Sp-M-Sp;辫状河砂体可划分为5级构型单元,由小到大分别是层系、层系组、心滩/河道增生单元、心滩/辫状河道、单期河道,河道砂体由多个增生单元在垂向上加积构成,心滩砂体由多个增生单元与串沟砂体在垂向上加积构成,内部发育泥质夹层;心滩砂体物性优于河道砂体,非均质性相对较弱,整体可以作为优质储层,而河道砂体仅在靠近河心的部位物性较好,可作为优质储层;心滩砂体内规模较大的泥质夹层附近和河道砂体的边部及中上部可聚集剩余油,且心滩砂体内剩余油储量相对河道砂体较大。

扒楼沟剖面; 辫状河; 河道; 心滩; 砂体构型; 非均质性； 剩余油分布

辫状河砂体规模大、物性好,是优质的油气储集砂体,其内部复杂的构型与非均质性特征直接控制着剩余油的分布。开展砂体构型及非均质性研究对剩余油的预测和挖潜具有重要的意义,是提高油田采收率、最大限度开发油气资源的关键所在[1-3]。近年来,众多学者通过野外露头[4-6]、现代沉积[7-11]及密井网资料[12-16]对辫状河砂体构型及内部泥质夹层分布规律进行了详细研究,为砂体中剩余油的开采提供了可靠依据,但是仍然存在以下两方面问题:①目前关于辫状河砂体构型特征的研究多侧重于心滩砂体,而河道砂体内部构型单元具有怎样的几何属性及叠置关系,需要进一步研究;②前人针对辫状河砂体的纵向非均质性特征较多,而对于砂体在横向上具有怎样的非均质性特征,有待进一步探索。笔者以扒楼沟剖面二叠系山西组辫状河砂体实测和精细解剖为基础,通过岩相分析、薄片鉴定及物性测试,定量表征辫状河砂体内部构型单元的几何属性及横向非均质性特征,确定优质储层发育位置,为辫状河砂体内剩余油预测和挖潜提供依据。

1 研究区概况

扒楼沟剖面位于山西保德县城东南约25 km,该地区上古生界出露齐全且剖面发育良好,地层接触关系清楚,为露头研究创造了良好的条件(图1)。二叠系山西组为目的层段,主要发育灰色—浅黄色含砾粗砂岩、中粗砂岩,夹有薄层灰黑色炭质泥岩、泥岩及煤层,厚度一般为50～60 m,按岩性组合、沉积旋回及含煤性自下而上分为4个小层,即山22、山21和山12、山11小层。山12小层发育厚层辫状河砂体,宽度约150 m,厚度约13 m,整体表现为下粗上细的正粒序结构,底部发育大型冲刷面,下部发育大型槽状交错层理与块状层理,上部发育小型槽状交错层理与板状交错层理。

图1 研究区位置及山西组综合柱状图Fig.1 Location of study area and composite histogram of Shanxi Formation

2 成因砂体

2.1 岩相类型

岩相反映了沉积体形成的水动力条件、搬运方式及沉积作用机制等[17-18],是划分沉积微相、识别成因砂体的重要依据。通过对扒楼沟剖面山西组辫状河露头的岩性、粒度、沉积构造和颜色等特征分析总结,共识别出5种岩相类型(表1、图2)。

表1 岩相类型划分及成因解释Table 1 Type division and genetic explantion of lithofacies

图2 扒楼沟剖面山西组辫状河露头典型岩相类型Fig.2 Typical lithofacies types of braided river outcrop of Shanxi Formation in Palougou profile

2.2 成因砂体

山12小层以辫状河砂体沉积为主,包括河道和心滩2种成因砂体,两者在外部几何形态、内部结构与岩相特征等方面存在显著差异。

2.2.1 河 道

露头上可见辫状水道砂体呈底凸顶平的外部几何形态,长度约为60.5 m,厚度约为9.3 m,其岩相组合为Ge-St(表2、图3(a))。辫状水道底部可见明显的冲刷面,冲刷面上发育滞留砾岩相,夹有植物茎干化石,砾石最大粒径约为7 cm,平均粒径约为3 cm(图2(a));下部过渡为槽状交错层理中粗砂岩相,层理规模较大,层系宽约3 m,高约0.5 m(图2(b)),上部发育槽状交错层理中细砂岩相,层理规模变小,层系宽度约为1.5 m,高度约为0.3 m(图2(c)),反映了水动力条件较强,且向上逐渐减弱。

2.2.2 心 滩

心滩砂体呈底平顶凸的外部几何形态,长度约为90.8 m,厚度约为13.2 m,其岩相组合为Ge-Sm-M-Sm-Sp-M-Sp或Ge-Sm-St-Sm-St-Sp-M-Sp(表2、图3(b)、(c))。心滩底部发育滞留砾岩相,由次棱角状—次圆状、分选中等、平均粒径为1.5 cm的砾石组成,厚度一般为10～20 cm(图2(d));下部发育块状层理粗砂岩,厚度约为5～6 m(图2(e)),夹有灰绿色薄层泥岩(图2(f)),即落淤层;上部发育槽状交错层理中粗砂岩相,为强水流冲裂心滩形成的冲沟沉积(图2(g));顶部发育多组低角度下截型板状交错层理中细砂岩相,纹层界面倾角一般为14°～20°,层系厚度10～20 cm(图2(h)),内部夹有薄层灰绿色泥岩(图2(i))。

表2 扒楼沟剖面山西组辫状河砂体构型特征Table 2 Architectural characteristics of braided river sand body of Shanxi Formation in Palougou profile

图3 扒楼沟剖面山西组辫状河砂体岩相组合垂向序列Fig.3 Lithofacies combination characteristics of braided river sand body of Shanxi Formation in Palougou profile

3 砂体构型特征

通过对扒楼沟剖面山12小层辫状河砂体进行详细观察及实测,根据砂体规模可以划分为5级构型单元,由小到大分别是层系、层系组、心滩/河道增生单元、心滩/辫状河道、单期河道,分别被不同级别的界面所限定(表2)。该辫状河砂体长度约为150 m,厚度约为13 m,走向为E-W向,近似垂直于主物源方向[19-20],其整体为一个单期河道,由1个河道砂体与1个心滩砂体侧向拼接构成(图4)。

表3 扒楼沟剖面山西组辫状河砂体构型单元与界面级次划分Table 3 Architectural elements and bounding surfacies of braided river sand body of Shanxi Formation in Palougou profile

河道砂体由3个河道增生单元在垂向上加积构成,在增生单元的底部可见较明显的冲刷面。下部河道增生单元宽厚比较小,呈底凸顶凹状,发育滞留砾岩相与大型槽状交错层理中粗砂岩相,反映河流发展早期水动力条件较强。上部河道增生单元宽厚比较大,呈底凸顶平状,发育小型槽状交错层理中细砂岩相,反映河流发展后期水动力强度减弱。

心滩砂体由若干个心滩增生单元与串沟砂体在垂向上加积构成,内部夹有平水期形成的细粒悬浮沉积物。心滩底部的增生单元宽度约为90 m,厚度约为4 m,发育滞留砾岩相与块状层理中粗砂岩相,为河流形成早期强水动力条件下粗碎屑物质快速堆积的产物;上部心滩增生单元在剖面上表现为底平顶凸,宽度一般为12～52 m,厚度为1.5～3.2 m,宽厚比较下部增生单元减小,发育块状层理中粗砂岩相与板状交错层理中细砂岩相。串沟为平水期小规模流水在心滩顶部冲出的沟道,常分布在靠近心滩中部的位置,剖面上表现为底凸顶平,厚度约为2 m,宽度为19～28 m,发育槽状交错层理中粗砂岩相。

图4 扒楼沟剖面山西组辫状河砂体构型特征与发育模式Fig.4 Architectural characteristics and development mode of braided river sand body of Shanxi Formation in Palougou profile

4 砂体非均质性

在砂体构型研究的基础上,通过在河道与心滩砂体底部进行横向连续取样,对各砂体物性与岩性特征开展研究,分析整个辫状河砂体的横向非均质性,确定优质储层发育位置。

河道砂体孔隙度一般为9.66%～11.60%,平均值为10.44%,渗透率一般为(0.002 7～0.006 7)×10-3μm2,平均值为0.003 8×10-3μm2,渗透率均质系数为0.57,变异系数为0.85。心滩砂体孔隙度一般为10.76%～12.67%,平均值为11.46%,渗透率一般为(0.006 5～0.011 9)×10-3μm2,平均值为0.008 5×10-3μm2,渗透率均质系数为0.71,变异系数为0.48(表4)。通过对比可知心滩砂体孔隙度、渗透率明显大于河道砂体,而且渗透率均质系数较大、变异系数较低,表明心滩砂体物性优于河道砂体,且非均质性相对较弱。

河道以中细粒长石质岩屑砂岩、中粗粒岩屑砂岩为主,从河道边部向河心方向岩石粒度变粗,杂基含量减少,孔渗性能变好;心滩砂体岩石粒度较河道砂体有所变粗,以中粗粒岩屑砂岩、中粗粒长石质岩屑砂岩为主,横向上岩石粒度变化不大,杂基含量较少,孔渗性能整体较好(图5)。结合物性与岩性特征分析可知,在辫状河沉积中河道砂体非均质程度较强,仅在靠近河心部位物性较好,可作为优质储层;心滩砂体物性优于河道砂体,非均质程度相对较弱,整体可作为优质储层。

表4 扒楼沟剖面山西组辫状河道与心滩砂体物性特征Table 4 Physical properties of the braided river channel and channel bar of Shanxi Formation in Palougou profile

图5 扒楼沟剖面山西组辫状河道与心滩砂体物性及岩性特征对比Fig.5 Physical properties and lithology comparison of braided river channel and channel bar of Shanxi Formation in Palougou profile

5 剩余油分布预测

在河道砂体与心滩砂体之间不发育泥质夹层,因此辫状河砂体整体上为一个泛连通体,但两种砂体在构型及非均质性特征方面存在显著差异,导致在注水开发过程中具有不同的水驱波及特征和剩余油分布特征。河道砂体在靠近河心的部位物性较好,且在纵向上不发育泥质夹层,因此底水和注入水容易在河心附近向上突进,造成油层快速水淹,形成强水淹层。在河道的边部和中上部砂体物性相对较差,底水锥进和注入水速度较慢,水淹程度较低,分布有一定量的剩余油。心滩砂体物性整体较好,但内部发育水平分布的泥质落淤层,其对底水和注入水的垂向遮挡作用明显,导致心滩的砂体的水淹程度低于河道砂体,在规模较大的泥质夹层附近分布有剩余油。综合考虑心滩砂体与河道砂体的规模、构型特征及非均质性特征,认为河道砂体内剩余油储量较小,而心滩砂体内剩余油储量相对较大。

6 结 论

(1)扒楼沟剖面二叠系山西组辫状河发育河道和心滩两种成因砂体,河道砂体在剖面上呈底凸顶平状,长度约为60.5 m,厚度约为9.3 m,其岩相组合为Ge-St;心滩砂体在剖面上呈底平顶凸状,长度约为90.8 m,厚度约为13.2 m,发育Ge-Sm-M-Sm-Sp-M-Sp和Ge-Sm-St-Sm-St-Sp-M-Sp两种岩相组合类型。

(2)辫状河砂体可划分为5级构型单元,由小到大分别是层系、层系组、心滩/河道增生单元、心滩/辫状河道、单期河道。河道砂体由多个增生单元在垂向上加积构成,增生单元自下而上宽厚比值变大。心滩砂体由多个增生单元与串沟砂体在垂向上加积构成,心滩增生单元呈底平顶凸的外部几何形态;串沟砂体表现为底凸顶平的外部几何形态,常分布在靠近心滩中部的位置。

(3)河道砂体的横向非均质性相对较强,在其边部的岩石粒度较细,杂基含量较高,物性较差;在靠近河心的位置,岩石粒度较粗,杂基含量较低,物性较好,可作为优质储层。心滩砂体岩石粒度比河道砂体粗,储层物性优于河道砂体,横向非均质性相对较弱,整体可作为优质储层。

(4)河道砂体内水淹程度较高,剩余油储量较小,主要分布在河道砂体的边部和中上部;心滩砂体水淹程度相对较低,剩余油储量相对较大,主要分布在规模较大的泥质夹层附近。

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(编辑 徐会永)

Braided river sand body architecture and heterogeneity of Permian in Palougou outcrop

WANG Yue1, CHEN Shiyue1, LI Tianbao1, LIANG Huiyuan2, WANG Jian3

(1.School of Geosciences in China University of Petroleum, Qingdao 266580, China;2.ShengliOilfield,SINOPEC,Dongying257015,China;3.ExperimentalTestingInstituteofXinjiangOilfield,Karamay834000,China)

Through field measurement, sand body anatomy, lithofacies analysis, and with the aid of thin sections and physical data, the architecture and heterogeneity characteristics of the braided river sand bodies of Shanxi Formation of Permian in the Palougou profile in Baode county, Shanxi province, were systematically studied. The results show that two kinds of genetic sand bodies, channel and channel bar are mainly developed in braided rivers. The vertical lithofacies combinations in channel sand body and channel bar sand body are Ge-St and Ge-Sm-M-Sm-Sp-M-Sp or Ge-Sm-St-Sm-St-Sp-M-Sp, respectively; Braided river sand bodies can be further divided into five levels of architectural elements, which are bed, bed set, channel bar/channel accretion, channel bar/braided channel, and braided river from small to large. Channel sand body is usually composed of several accretions superimposed vertically, while channel bar sand body is composed of several accretions and chute sand bodies superimposed vertically, with mud layers in it. Channel bar sand body has better physical properties and weaker heterogeneity than channel sand body, and therefore, the whole channel bar can be high quality reservoir. On the other hand, channel sand body only has good physical properties near the river center which may become potential reservoir. Remaining oil mainly accumulates near large scales of mud layers in the channel bar, and in the side and upper part of the channel sand body, but with large deposition potentials in the channel bar.

Palougou profile; braided river; channel; channel bar; sand body architecture; heterogeneity; remaining oil distribution

2016-06-05

“十二五”国家油气重大专项(2011ZX05013-002)

王越(1988-)，男，博士研究生，研究方向为层序地层学及沉积学。E-mail：620788364@qq.com。

1673-5005(2016)06-0001-08

10.3969/j.issn.1673-5005.2016.06.001

TE 121.3

A

王越,陈世悦,李天宝,等. 扒楼沟剖面二叠系辫状河砂体构型与非均质性特征[J].中国石油大学学报(自然科学版),2016,40(6):1-8.

WANG Yue, CHEN Shiyue, LI Tianbao, et al. Braided river sand body architecture and heterogeneity of Permian in Palougou outcrop [J]. Journal of China University of Petroleum(Edition of Natural Science),2016,40(6):1-8.