ΔlgR方法在深水沉积物层序分析中的应用
——以松辽盆地古龙凹陷古57井青山口组为例
2015-09-28刘苍宇辛仁臣
刘苍宇,辛仁臣
(中国地质大学(北京)海洋学院,北京100083)
ΔlgR方法在深水沉积物层序分析中的应用
——以松辽盆地古龙凹陷古57井青山口组为例
刘苍宇,辛仁臣
(中国地质大学(北京)海洋学院,北京100083)
为了探讨深水沉积体系地层单元层序界面及体系域的识别和划分方法,以松辽盆地古龙凹陷古57井的电阻率和声波时差测井资料为基础,采用ΔlgR方法,计算了古57井青山口组的TOCR值,在此基础上,讨论了在海平面变化周期内TOCR值的变化规律,其与层序地层单元的发育具有较好的对应关系。在古57井青山口组识别出4个三级层序旋回,9个四级层序旋回,并进一步划分出水进体系域和高位体系域。结果表明,TOCR值在水进体系域沉积期逐渐增大,到最大湖泛面处达到极大值;在高位体系域沉积期逐渐减小,到三级和四级层序界面处达到极小值。
ΔlgR方法;层序地层分析;青山口组;松辽盆地
0 引言
建立全盆地高精度等时地层格架是油气储层和隐蔽圈闭预测的基础[1-4]。浅水沉积体系位于浪基面之上,常为砂泥岩互层,旋回性明显,层序地层单元的划分和对比方法已比较成熟[5-7],而位于浪基面之下的深水沉积体系,岩性以暗色泥岩为主,与前者相比旋回性不明显,采用岩性组合特征划分和对比地层单元比较困难。因此,深水沉积体系地层单元的划分和对比就成为盆地整体高精度等时地层格架研究的难点。
层序地层学能够为石油地质分析提供准确的等时地层格架[8-10]。建立全盆地可靠的层序地层格架需要综合利用地震、露头、岩心、测井、生物地层和地球化学等资料[11-13]。然而,在盆地的深水沉积区,缺少露头资料;岩心、生物和地球化学资料均较少,且数据不连续;地震资料的分辨率较低,且多以平行与亚平行反射为主,很难见到地震剖面上的上超、顶超及削截现象。测井资料分辨率高,纵向数据连续,因此其是深水沉积层序地层分析最重要的资料。利用测井资料进行深水沉积层序地层分析是沉积学研究的热点之一[11]。
1 层序地层格架中TOC的变化规律及TOC评价方法
1.1层序地层格架中TOC的变化规律
前人对海相烃源岩和开阔湖泊相烃源岩的研究发现,地层中总有机碳(TOC)与沉积环境的水体深度有关。如果沉积环境水体相对较深,陆源沉积物供应速率较低,则TOC值很高;随着相对基准面下降,水体深度减小,沉积物进积,则TOC值较小[6,14-15]。因此,可以通过TOC值的垂相变化,分析沉积环境水体深度的变化,进而划分层序地层单元。
一个沉积层序的构成和地层叠置样式常受构造沉降、全球海平面升降、沉积物供给速率和气候等4个基本因素的综合影响[16-19]。深水沉积层序中的TOC值,除了受上述因素的影响外,还受有机质来源和保存条件差异的影响[20]。
TOC值取决于有机质聚集场所的氧化-还原性质和沉积速率,而沉积速率与相对水平面有关[图1(a)][15]。纵向上,在层序边界(SB)处,相对水平面处于较低的位置,地层大面积遭受暴露并剥蚀,沉积物的供给速率较大,这个时期水体相对较浅,在有机质聚集场所不易形成还原环境。一般地,氧气只在沉积物最表层的几厘米内存在[21],虽然快速埋藏利于有机质的保存,但局部较高的沉积速率可降低有机质的含量,可能导致TOC值较小。随着相对水平面的上升(A—C),物源供给逐渐减小,可容纳空间迅速增大,常出现欠补偿沉积,沉积速率逐渐减小,而且水体较深容易形成还原环境,这有利于有机质的保存,且TOC值呈增大趋势。在最大湖泛面(mfs)处,常可见TOC为极大值。随着相对水平面继续上升(C—E),物源供应也随之增加,可容纳空间变小[图1(b)],沉积速率逐渐增大,碎屑物质增多,有机质含量降低,TOC值呈减小的趋势。横向上,图1中③→①表示有机质向盆地方向有逐渐富集的趋势。在盆地边缘附近(③处),陆源沉积供给速率较大,水体相对较浅,不利于有机质的埋藏,所以仅B—C段见有机质沉积;在盆地中心附近(①处),一般发育半深湖—深湖亚相,水体相对较深,受陆源沉积影响较小,利于有机质的保存,有机质较为富集,A—F段皆可见有机质。
图1 层序地层格架内有机质含量剖面示意图Fig.1 Cross-section of organic content in the sequence stratigraphic framework
1.2TOC评价方法
目前,评价地层中TOC含量有2种方法:①实测地层中的TOC含量,但测试费高且获得的数据不连续;②测井方法,以ΔlgR方法[声波时差(AC)与电阻率(RLLD)曲线重叠法]使用最多、最广泛[22-24],该方法评价得到的TOC值与实测结果具有较好的相关性[25-27]。
ΔlgR方法是埃克森(Exxon)公司于1979年开发的一种烃源岩有机质含量测井预测方法[28-29],其以深度为纵坐标,以声波时差(线性坐标)和电阻率(对数坐标)为横坐标。移动声波时差曲线或电阻率曲线,使电阻率曲线的最小值和声波时差曲线的最大值重合,并将该点作为泥岩基点。2条曲线移动的幅度差即为ΔlgR,其可以反映出地层中TOC值的变化[图1(c)]。为了与实测的TOC值区别,将ΔlgR方法得到的TOC值,记作TOCR。
青山口组沉积期,松辽盆地发生大规模湖侵[30-33],齐家-古龙凹陷青山口组为半深湖—深湖沉积[34-35][图2(a)]。以古57井为例,青山口组的岩性以暗色泥岩为主,夹褐色油页岩和介形虫层[图2(b)]。测井响应以中自然伽马、低声波时差和中低电阻率为特征,测井曲线为低幅值齿化曲线,依据测井和录井资料均难以直接进行层序地层单元的划分。基于此,笔者采用ΔlgR方法进行层序地层学分析。
图2 松辽盆地青山口组沉积相(a)及古57井原始的岩性柱状图(b)Fig.2 Sedimentary facies of Qingshankou Formation(a)and primary lithologic log of Gu 57 well(b)in Songliao Basin
2 古57井TOCR的计算
2.1ΔlgR方法计算TOCR公式
TOCR的计算公式[28,36]为
式中:TOCR为预测的总有机碳含量,%;R为实测电阻率,Ω·m;Δt为实测声波时差,μs/m;Rb为泥岩基点的电阻率,Ω·m;Δtb为泥岩基点的声波时差值,μs/m;Ro为镜质体反射率,%;ΔTOCbg为区域背景校正值[37],%。
在泥岩基点处,计算得到的ΔlgR值为0,若不加上ΔTOCbg值,由此计算得到的TOCR值也为0。实际上,泥岩的TOC值很难为0,因此要得到每个深度下的TOCR值,就必须加上泥岩基点处的TOC值(ΔTOCbg)。使用ΔlgR方法划分层序时,只需考虑目的层段TOCR值垂向变化趋势及其相对数值,ΔTOCbg值加与不加只相当于将曲线左右平移,不影响TOCR值的相对大小,因此,可将式(2)精简为
2.2Ro的计算
由式(3)可知,计算不同深度的TOCR值,必须考虑相应深度下的Ro值。前人研究均表明[38-39],松辽盆地中生界的Ro与埋藏深度有较好的相关性(图3),Ro与埋藏深度的回归方程为
图3 松辽盆地Ro与深度的关系(据文献[38]修改)Fig.3 Relationship between Roand depth in Songliao Basin
式中:h为深度,m。
利用式(4),笔者计算了青山口组不同埋藏深度的Ro值。
2.3TOCR值的计算结果
TOCR值是依据泥岩的电阻率和声波测井数据计算得到的,因此,在计算TOCR值的过程中,需要对原始电阻率和声波时差测井数据进行处理,剔除非泥岩层的异常电阻率和声波时差测井数据。古57井青山口组的非泥岩主要是介形虫层和泥灰岩。
古57井的RLLD值在井深为1 947.4 m处最小,将该点作为泥岩基点,叠合声波时差曲线和电阻率曲线。泥岩基点的电阻率Rb为2.731 2 Ω·m,声波时差Δtb为98.0443 μs/m,将其代入式(1),再代入每个深度下的实测电阻率值和实测声波时差值,即可计算出对应深度下的ΔlgR值。将ΔlgR值和利用式(4)计算得到的Ro值代入式(3),即可得到TOCR随埋藏深度的变化曲线(图4)。
图4 古57井青山口组基于TOCR曲线的层序划分Fig.4 Sequence division of Qingshankou Formation in Gu 57 well based on TOCRcurve
3 古57井层序地层分析
古57井青山口组的TOCR值具有向上变大又变小的明显的多级次旋回性,不同级次旋回是不同级别层序的响应。
根据TOCR曲线出现的幅度和形态的骤变,划分出4个与三级层序[40-41]级别相当的旋回,由下向上依次命名为SQqn1,SQqn2,SQqn3和SQqn4(参见图4)。SQqn1三级层序旋回(2 357.0~2 440.0 m),岩性为黑色泥岩,TOCR值为0.28~0.95,平均为0.58,曲线形态表现为双峰形不对称旋回,以向上变大的半旋回为主。SQqn2三级层序旋回(2 153.4~2 357.0 m),岩性为深灰色泥岩,TOCR值为0.02~0.91,平均为0.48,曲线形态表现为多峰形近对称旋回。SQqn3三级层序旋回(2 033.8~2 153.4m),岩性为深灰色泥岩,TOCR值为0.02~1.09,平均为0.47,曲线形态表现为多峰形不对称旋回。SQqn4三级层序旋回(1 945.3~2 033.8 m),岩性为深灰色泥岩,TOCR值为0~1.05,平均为0.34,曲线形态表现为单峰形近对称旋回。
下面以古57井SQqn4三级层序为例,剖析TOCR值在层序内部的变化特征(图5)。SQqn4三级层序旋回可划分为2个与四级层序[41]级别相当的旋回(C8和C9)。
图5 古57井SQqn4层序地层分析Fig.5 Sequence stratigraphic analysis of SQqn4 of Gu 57 well
C8四级层序厚度为62.1 m,为水进域(TSTC8,51.1 m)占绝对优势的不对称旋回。底界面(SB8)处的TOCR值为0.15,其最大湖泛面(mfsC8)处的TOCR值为1.05,水进域(TSTC8)TOCR平均值为0.3,高位域(HSTC8,11.0 m)TOCR平均值为0.47。C8四级层序由16个准层序叠加而成,正旋回准层序略占优势,反映了7次以上升略占优势的湖平面升降。
C9四级层序厚度为28.5 m,为水进域(TSTC9,13.8 m)和高位域(HSTC9,14.7m)近对称旋回。底界面(SB9)处的TOCR值为0.14,其最大湖泛面(mfsC9)处的TOCR值为0.74,水进域TOCR平均值为0.39,高位域TOCR平均值为0.3。C9四级层序由6个准层序叠加而成,早期正旋回略占优势,晚期反旋回略占优势,反映了湖平面先上升后下降,整个沉积期湖平面在某一稳定界面附近波动的变化过程。
C8四级层序的水进域构成了SQqn4三级层序的水进域(TSTqn4),C8四级层序的高位域和C9四级层序构成了SQqn4三级层序的高位域(HSTqn4)。可见SQqn4主要发育TOCR值向上增大的正旋回三级准层序,反映了此时期以湖平面相对上升稍占优势的湖平面升降。
由TOCR曲线形态可知,SQqn1三级层序为双峰形不对称型旋回(参见图4),以水进域半旋回为主,反映湖平面以相对上升为主;SQqn2三级层序为多峰形不对称旋回,以高位域半旋回为主,反映湖平面以相对下降为主;SQqn3三级层序为多峰形近对称旋回,高位域半旋回略占优势,反映湖平面继续下降;SQqn4三级层序为单峰形近对称旋回,水进域半旋回略占优势,反映此沉积期湖平面以相对上升为主。
4 结论
(1)古57井青山口组TOCR值由下向上表现为低→高→低的9个旋回。
(2)古57井青山口组共划分出4个三级层序,对应于9个TOCR值旋回,划分出9个四级层序,并进一步划分出水进体系域和高位体系域。
(3)青山口组沉积期,松辽盆地深水区经历了相对湖平面先上升再下降后上升的变化过程。
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(本文编辑:李在光)
Application of ΔlgR method in sequence stratigraphy analysis of deep-water sediments:A case study from Qingshankou Formation of Gu 57 well in Gulong Depression,Songliao Basin
Liu Cangyu,Xin Renchen
(School of Marine Sciences,China University of Geosciences,Beijing 100083,China)
Inordertodiscussthemethodofidentifyingandclassifyingsequenceboundaryandsystemtract indeep-water sedimen-tary system,this paper calculated the values of TOCRof Qingshankou Formation with ΔlgR method on the basis of resistivity and sonic logging data of Gu 57 well in Songliao Basin.With the curve of TOCR,the variation characteristics of TOCRcorrelate well with the development of sequence stratigraphy units during the sea level change cycle.4 third-order sequences and 9 fourth-order sequences were recognized in Qingshankou Formation of Gu 57 well,and they were further divided into transgression and highstand system tracts.The result shows that the values of TOCRgradually increase during the TST period and the maximum value is in the maximum flooding surface,and that they gradually decrease during the HST period and the minimal value is in the forth-order and third-order sequence boundaries. Key words:ΔlgR method;stratigraphysequence analysis;Qingshankou Formation;SongliaoBasin
P539.2
A
1673-8926(2015)05-0030-07
2015-05-20;
2015-07-09
国家重大科技专项“岩性地层油气藏成藏规律、关健技术及目标评价”(编号:2011ZX05001002)资助
刘苍宇(1990-),男,中国地质大学(北京)在读硕士研究生,研究方向为沉积学。地址:(100083)北京市海淀区学院路29号中国地质大学(北京)校区。E-mail:1026344174@qq.com
辛仁臣(1964-),男,博士,副教授,主要从事沉积学、层序地层学与石油地质学方面的教学和科研工作。E-mail:xinrenchen@163.com。