Genesis of overpressure in deep tight clastic rock reservoirs and its effect on reservoir quality: Case study of Longfengshan sub-sag in Changling Fault Depression, Songliao Basin

  • Yangchen ZHANG , 1 ,
  • Xiyu QU , 1 ,
  • Changsheng MIAO 2 ,
  • Xiu CHEN 3 ,
  • Jianfeng ZHU 4 ,
  • Wen XU 4
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  • 1. School of Geosciences,China University of Petroleum (East China),Qingdao 266580,China
  • 2. School of Prospecting and Surveying Engineering,Changchun Institute of Technology,Changchun 130021,China
  • 3. Research Institute of Exploration and Development,Changqing Oilfield Company,PetroChina,Xi’an 710018,China
  • 4. SINOPEC Northeast Oil and Gas Branch,Changchun 130062,China

Received date: 2022-12-06

  Revised date: 2023-01-20

  Online published: 2023-09-01

Supported by

The National Natural Science Foundation of China(41372133)

the Natural Science Foundation of Shandong Province, China(ZR2020MD027)

Abstract

Deep tight sandstone gas is a hot spot in oil and gas exploration in recent years, and it is of great significance to clarify the genetic mechanism of overpressure in deep tight clastic rock reservoirs for deep oil and gas exploration. Taking the tight clastic rock reservoirs in the Yingcheng Formation-Shahezi Formation of the Longfengshan sub-sag in the Changling Fault Depression of Songliao Basin as the research object, in view of the unclear understanding of the causes of overpressure and the lack of research on its impact on reservoir quality, using the methods of casting thin section, scanning electron microscope, well logging overpressure identification and inclusion paleo-pressure recovery, the formation of overpressure in Yingcheng-Shahezi formations and its influence on tight clastic rock reservoirs are studied. The results show that the overpressure of the Yingcheng-Shahezi formations in the Longfengshan sub-sag is mainly caused by the hydrocarbon-generation pressurization. The paleo-pressure recovery results show that there was obvious overpressure in the Yingcheng Formation during the two periods of oil and gas charging, indicating that there was overpressure caused by hydrocarbon-generating pressurization during the accumulation period. The overpressure controls the episodic hydrocarbon expulsion process, which makes the micro-fractures open in stages in the reservoir, and the unclosed micro-fractures increase the connectivity of the reservoir and improve the permeability of the reservoir.

Cite this article

Yangchen ZHANG , Xiyu QU , Changsheng MIAO , Xiu CHEN , Jianfeng ZHU , Wen XU . Genesis of overpressure in deep tight clastic rock reservoirs and its effect on reservoir quality: Case study of Longfengshan sub-sag in Changling Fault Depression, Songliao Basin[J]. Natural Gas Geoscience, 2023 , 34(9) : 1552 -1564 . DOI: 10.11764/j.issn.1672-1926.2023.03.009

0 引言

随着油气勘探开发工作的不断深入,我国油气勘探已由中浅层常规油气向深层、超深层致密油气进发。深层、超深层致密油气已成为目前油气勘探开发的热点与难点1-2。异常高压作为深层致密油气的成藏原动力,广泛分布于世界各大含油气盆地中3-6。自21世纪以来,致密储层超压的研究取得了一系列重大进展,陆续在川西坳陷须家河组7-8、鄂尔多斯盆地延长组9、莺歌海盆地黄流组10-11、吉木萨尔凹陷芦草沟组12、胜北洼陷中侏罗统13及渤中凹陷东营组14等致密储层中发现超压现象存在。不同类型超压对致密储层的质量有着不同的影响,厘清超压的成因对于致密储层的勘探开发至关重要,前人15总结的超压成因主要包括:不均衡压实、流体膨胀、成岩作用、构造挤压和压力传递。目前,生烃增压作用和欠压实作用已成为含油气盆地超压现象存在的两大主要的成因机制16-18。前人19研究认为,超压能够改善致密储层的质量,也能够抑制机械压实作用和部分胶结作用,使储层中的原生孔隙能够保存下来。同时,超压可以促进溶解作用的发生,有利于形成次生孔隙19-20。另外,超压能够促使储层形成大量微裂缝,改善储层的连通性21-23
龙凤山地区是松辽盆地长岭断陷南部重要的油气富集区,近年来在致密气资源的勘探中取得重大突破,自2013年营城组发现天然气流以来,又相继在B201井等多口井中发现高产气层,之后陆续在B210井、B220井及B5井营城组、沙河子组发现具工业产能且产量稳定的致密气23。营城组致密砂岩储层中存在丰富的油气资源,但由于实测地层压力资料有限,且现今仅在部分井的部分层段存在压力异常。因此,研究区营城组—沙河子组超压没有引起充分重视,前人对研究区超压的研究工作较少,对超压的成因机制及其对储层质量的影响研究不够深入,制约了该地区下一步的油气勘探工作。本文以龙凤山地区营城组—沙河子组超压储层为研究对象,综合利用铸体薄片、物性测试、测井资料及包裹体古压力恢复等方法,研究超压的成因机制及其对储层质量的影响,以期为深层致密砂岩气的下一步勘探开发提供理论依据。

1 区域地质概况

长岭断陷位于松辽盆地中央凹陷区南部,主要发育于晚侏罗世—早白垩世,面积约为1.3×104 km2[24-25。龙凤山次凹位于长岭断陷南部,面积约为300 km2,整体受北东向基底拆离断层控制,形成“北西断、南东超”的箕状构造格局26-28。龙凤山次凹白垩系主要发育下白垩统火石岭组、沙河子组、营城组、登娄库组、泉头组和上白垩统青山口组、姚家组、嫩江组等。在下白垩统中,营城组是主要的储集层系,营Ⅲ、营Ⅳ、营Ⅴ砂组为主要含气层,岩性主要为灰色砂砾岩、中细砂岩、粉砂岩与黑色泥岩互层。营城组沉积相类型主要包括扇三角洲、辫状河三角洲及湖相,其下部沙河子组为主要的生烃层系,岩性主要为灰黑色泥岩与灰色细砂岩、砂砾岩互层(图1)。
图1 龙凤山次凹位置(a)、构造单元(b)和地层柱状图(c)

Fig.1 The location(a), structure unit(b) and stratigraphy column(c) of Longfengshan sub-sag

2 现今地层压力特征

研究区15口井实测压力数据显示,营城组压力系数主要介于0.8~1.2之间(图2),刘文龙等29认为,在国内一些学者总结的地层压力分类方案中常压至超低压部分过于平均,高压至超高压部分过于集中,对松辽盆地深层并不适用。针对松辽盆地构造运动期次多、构造活动强烈、成岩作用强、火山岩分布广和岩性复杂等特点,提出了松辽盆地新的地层压力分类方案29表1),压力系数介于1.02~1.12之间为高压,压力系数大于1.12为超高压,现今研究区部分井中仍存在高压异常或超压,超压主要分布于埋深3 000 m以深的营城组及沙河子组中。
图2 龙凤山次凹实测压力与深度的关系

Fig.2 The relationship between measured pressure and depth in Longfengshan sub-sag

表1 松辽盆地深层压力分类及与国内外分类对比[29]

Table 1 Classification of deep stratum pressure in Songliao Basin and comparison with domestic and foreign classifications[29]

压力系数 压力分类 压力系数 压力分类
松辽盆地 <0.90 超低压 1.02~1.12 高压
0.90~0.98 低压 >1.12 超高压
0.98~1.02 常压
国外 <0.96 低压异常 1.08~1.20 高压异常
0.96~1.08 常压 >1.20 异常高压
国内 <0.75 超低压 1.10~1.50 高压
0.75~0.90 低压 >1.50 超高压
0.90~1.10 常压

3 超压成因机制

3.1 测井曲线组合分析法

测井曲线组合分析法是指利用声波时差、密度、电阻率等多条测井曲线综合判别超压成因的方法。由于测井曲线一般受多种因素影响,仅利用单一的测井曲线无法准确判断超压成因,比如声波时差测井曲线一般受孔隙度、含油气性、有机质丰度等因素的影响,声波时差偏高可能与高有机质丰度和高孔隙度有关。因此,利用多条测井曲线综合分析的方法能降低超压成因判断的不准确性。前人15研究认为,声波和电阻率测井反映岩石的传导属性,而密度和中子测井反映岩石的体积属性,所以进行超压成因判别时,可以利用声波测井、密度测井及电阻率测井来综合判别超压成因。一般认为,随着埋深的增大,声波时差增大,密度显著减小,电阻率减小,则超压可能为欠压实成因;当超压为生烃增压成因时,密度和电阻率变化趋势相反。
以龙凤山次凹B210井和B212井为例,B210井在埋深3 800 m左右泥岩声波时差曲线偏离正常趋势,说明3 800 m为B210井的超压顶界面。埋深3 800 m以深,声波时差异常增大,密度基本不变(图3),说明超压不是欠压实成因,可能为生烃增压成因;同样,B212井声波测井曲线在埋深4 000 m左右深度异常增大,密度基本不变(图4),说明研究区超压可能为生烃增压成因。
图3 B210井泥岩声波时差、电阻率、井径和密度曲线

Fig.3 AC,RLLD,CAL and DEN curves of mudstone in Well B210

图4 B212井泥岩声波时差、电阻率、井径和密度曲线

Fig.4 AC,RLLD,CAL and DEN curves of mudstone in Well B212

3.2 孔隙度对比法

欠压实作用产生的超压通常是由于泥岩的沉积速率较快,孔隙中的流体没有及时排出,从而导致地层压力异常增大。通常认为由欠压实作用形成的超压段孔隙度一般异常大,而由于生烃增压产生的超压则通常会因为幕式排烃过程在泥岩段间歇性开启微裂缝,生烃增压作用产生的超压对孔隙度的贡献不明显。因此,通过将超压段的泥岩孔隙度与实际正常压实的泥岩孔隙度对比,即可判断出地层超压的成因。
受限于连续取心的高成本,实际研究过程中泥岩的实测孔隙度的资料一般较少,所以通过声波时差、电阻率、中子及密度等测井曲线可以计算出泥岩的孔隙度。由于欠压实作用产生的超压一般会导致孔隙空间明显增大,即岩石的体积属性和传导属性都会发生变化,对应的声波、电阻率、密度、中子孔隙度都会发生明显变化。而生烃增压作用产生的超压,主要形成连通孔隙的裂缝,使岩石传导属性变化较大,声波孔隙度和电阻率孔隙度变化明显,而密度孔隙度和中子孔隙度变化不显著。
密度孔隙度可以用以下公式求出:
Φ D = ρ m a - ρ ρ m a - ρ f
式(1)中:Φ D为密度孔隙度,小数;ρma为泥岩骨架密度,取2.65 g/cm3;ρf为孔隙流体密度,取1.10 g/cm3ρ为目的层密度测井值,g/cm3
声波孔隙度可以用Wyllie方程求出,具体计算公式如下所示:
Φ S = Δ t - Δ t m a C P Δ t f - Δ t m a
式(2)中: Φ S为声波孔隙度,小数;Δt ma为岩石骨架声波时差,μs/m;Δt f为孔隙流体声波时差,取610 μs/m;Δt为目的层声波时差测井值,μs/m;C p为地层压实系数,无量纲。
声波孔隙度在计算时需要考虑地层压实系数,地层压实系数可由密度孔隙度—声波时差交会图拟合出的曲线计算出来。当密度孔隙度为0时,可认为该处对应的声波时差即为岩石骨架声波时差30-31。但由于B212井缺少浅层密度测井数据,所以利用邻井B2井浅层正常压实段测井数据进行拟合,拟合结果如图5所示。地层压实系数可以通过拟合后的曲线求出30
Φ D = a Δ t - b
a = 1 C p Δ t f - Δ t m a b = Δ t m a C p Δ t f - Δ t m a
图5 B212井密度孔隙度—声波时差交会图

Fig.5 The crossplot of density porosity-interval transit time in Well B212

根据拟合后的曲线,计算出B212井地层压实系数为1.56,岩石骨架声波时差为165.6 μs/m。
通过对B212井声波孔隙度和密度孔隙度的计算,发现B212井埋深4 000 m以浅声波孔隙度和密度孔隙度基本落在正常压实趋势线上。埋深4 000 m以深声波孔隙度明显偏离正常压实趋势线,密度孔隙度基本落在正常压实趋势线上,说明B212井深层超压可能由生烃增压产生(图6)。
图6 B212井声波孔隙度和密度孔隙度随深度的变化

Fig.6 Variation of acoustic porosity and density porosity with depth in Well B212

3.3 声波速度—密度交会图法

声波速度—密度交会图法是目前广泛应用且效果良好的一种超压成因判别方法。声波速度—密度交会图法是由鲍尔斯曲线法演变而来,其原理为不同成因的超压会导致其超压段声波速度和密度的变化趋势不同。声波速度—密度交会图法判别超压成因有4个原则15图7):①不均衡压实和正常压力落在加载曲线上;②由流体膨胀产生的超压,超压段随声波速度降低,密度基本不变;③由黏土矿物转化作用形成的超压,超压段随密度的增大而增大,声波速度基本不变或略有减小;④复合成因形成的超压,其密度—声波速度的变化介于2种超压之间。
图7 声波速度—密度超压成因判别图版7

Fig.7 Discriminant plate for causes of acoustic velocity-density overpressure7

声波速度—密度交会图在选点时遵循以下原则:曲线上的点为泥岩段的声波速度和密度的平均值,泥岩段井径正常(无明显的井径扩大或扩径率<15%),泥质含量≥90%,厚度大于2 m(降低围岩的影响)。从B212井声波速度—密度交会图(图8)可以看出,B212井超压段随声波速度的降低,密度变化较小,说明超压为生烃增压成因。
图8 B212井声波速度—密度交会图

Fig.8 The crossplot of acoustic velocity-density in Well B212

3.4 包裹体古压力恢复方法

3.4.1 包裹体拉曼成分及镜下特征

包裹体捕获时的压力能够反映成藏期的古压力,通过包裹体岩相学、测温以及拉曼光谱等技术,能够计算出包裹体捕获时的压力,进而恢复油气成藏期的流体压力32。本文共对营城组10块样品进行包裹体镜下观察、均一温度、盐度及拉曼成分测定,对3个样品进行包裹体捕获压力计算,其中包裹体拉曼成分及甲烷包裹体拉曼位移通过LABHR-VIS LabRAM HR800研究级显微激光拉曼光谱仪测得,包裹体拉曼成分表明,研究区纯气相包裹体及气液两相包裹体中的气相成分均为CH4,谱峰位置分布介于2 909~2 916 cm-1之间(表2)。纯气相甲烷包裹体主要呈深灰色,气液两相包裹体主要呈淡黄色—无色,均主要沿石英颗粒成岩期后微裂隙成带分布。
表2 研究区包裹体激光拉曼光谱分析结果

Table 2 Laser Raman spectroscopic analysis results of inclusions in the study area

井号 深度/m 测点数/个

赋存

矿物

包裹体

类型

测点

位置

成分 谱峰位置/cm-1
B206 3 234.89 14 石英 气液两相 气相 CH4 2 916
纯气相 2 910
B206 3 235.57 14 石英 气液两相 气相 CH4 2 910
B206 3 347.36 14 石英 气液两相 气相 CH4 2 916
B204 2 389.4 14 石英 气液两相 气相 CH4 2 916
B202 3 112.75 14 石英 气液两相 气相 CH4 2 914
B202 3 128.6 14 石英 气液两相 气相 CH4 2 913
B210 3 944.85 14 石英 纯气相 气相 CH4 2 909
B210 3 945.33 14 石英 纯气相 气相 CH4 2 910
B210 3 951.15 14 石英 气液两相 气相 CH4 2 916
B210 4 129.65 14 石英 纯气相 气相 CH4 2 910
流体包裹体镜下观察结果表明,研究区营城组致密碎屑岩储层中含有丰富的烃类包裹体,除个别样品发育丰度极低(GOI<1%),多数样品包裹体发育丰度极高(GOI介于6%~20%之间)。烃类包裹体多为气液两相包裹体。单偏光下,多呈无色、深灰色或淡黄色;荧光下,多呈蓝色、蓝绿色、黄绿色(图9)。烃类包裹体主要成带分布于切穿石英颗粒成岩期后微裂缝,或成群分布于长石溶蚀孔洞或粒间方解石胶结物中。
图9 龙凤山次凹营城组致密储层包裹体镜下特征

(a)石英颗粒成岩期后微裂缝中的烃类包裹体,黄绿色—蓝色荧光,B206井,3 234.89 m;(b)石英颗粒成岩期后微裂缝中的烃类包裹体,蓝色—蓝绿色荧光,B206井,3 235.57 m;(c)石英颗粒成岩期后微裂缝中的烃类包裹体,淡蓝色荧光,B210井,4 129.65 m;(d)石英颗粒成岩期后微裂缝中的烃类包裹体,单偏光下无色,B206井,3 234.89 m;(e)石英颗粒成岩期后微裂缝中的烃类包裹体,单偏光下无色,B206井,3 235.57 m;(f)石英颗粒成岩期后微裂缝中的烃类包裹体,单偏光下深灰色,B210井,4 129.65 m

Fig.9 The microscopic features of inclusions in tight reservoir of Yingcheng Formation in Longfengshan sub-sag

3.4.2 包裹体均一温度特征

与烃类包裹体共生的盐水包裹体均一温度能够反映包裹体捕获时的储层温度,进而确定出大规模油气充注的时间和期次33-35。利用Linkam THMS600型冷热台对研究区营城组致密储层样品均一温度进行了测定,共测得260个数据点,从中国石化东北油气分公司收集到124个数据点,共计384个数据点,绘制出营城组致密储层包裹体均一温度分布图。结果显示,研究区营城组致密储层盐水包裹体均一温度存在100~110 ℃及120~130 ℃ 2个峰值(图10),说明营城组储层在中成岩时期可能存在2期油气充注。
图10 龙凤山次凹营城组致密储层包裹体均一温度分布

Fig.10 Homogenization temperature distribution of inclusions in tight reservoirs of Yingcheng Formation in Longfengshan sub-sag

3.4.3 成藏期古压力恢复

甲烷包裹体拉曼位移技术是通过测定甲烷气体的拉曼位移计算近纯甲烷包裹体的密度,然后利用甲烷状态方程来计算包裹体的捕获压力36-37。甲烷包裹体的密度可由拉曼散射特征峰求取,具体公式为:
ρ = - 5.173   31 × 10 - 5 D 3 + 5.530   81 ×                    10 - 4 D 2 - 3.513   87 × 10 - 2 D
式(5)中:ρ为甲烷包裹体密度,g/cm3D=V 1V 0V 1为甲烷拉曼散射特征峰;V 0为压力约等于0时的特征峰。
研究区B206井及B210井营城组致密储层甲烷拉曼散射特征峰值V 1和压力接近0时的拉曼散射特征峰V 0的结果值如表2所示,根据得到的V 1V 0值,通过式(5)可以求出样品的甲烷包裹体密度为0.24 g/cm3
甲烷包裹体的捕获压力可由超临界甲烷体系的状态方程计算,具体计算公式如下38-39
Z   =   P V R T = P r V r T r = 1 + B V r + C V r 2 + D V r 4 + E V r 5 + F V r 2 ( β + γ V r 2 ) e x p ( - γ V r 2 )
B = a 1 + a 2 T r 2 + a 3 T r 3 C = a 4 + a 5 T r 2 + a 6 T r 3
D = a 7 + a 8 T r 2 + a 9 T r 3 E = a 10 + a 11 T r 2 + a 12 T r 3
F = α T r 3 P r = P P c
T r = T T c; V r = V V c; V c = R T c P c;
式(6)中:P为压力,Pa;T为温度,℃;R为气体常数;V为摩尔体积,dm3/mol;Z为压缩因子;P rT r分别为对比压力、对比温度,量纲为1;P cT c分别为临界压力(4 830 Pa)、临界温度(-82.75 ℃);a 1=0.087 255 392 8;a 2=-0.752 599 476;a 3=0.375 419 887;a 4=0.010 729 134 2;a 5=0.005 496 263 6;a 6=-0.018 477 280 2;a 7=0.000 318 993 183;a 8=0.000 211 079 375;a 9=0.000 020 168 280 1;a 10=0.000 016 560 618 9;a 11=0.000 119 614 546;a 12=-0.000 108 087 289;α=0.044 826 229 5;β=0.753 97;γ=0.077 167。
结合甲烷包裹体密度和均一温度,通过式(6)计算出研究区营城组致密储层甲烷包裹体捕获压力,结果表明,甲烷包裹体捕获压力介于62.86~68.74 MPa之间(表3)。所测3组样品的甲烷包裹体均一温度均分布于包裹体均一温度主峰区间(图10表3),说明所测的甲烷包裹体是油气充注高峰时所捕获的。根据邻井B201井埋藏史曲线(图11)可知,2期甲烷包裹体均一温度对应的营城组古埋深均未超过3 000 m,静水压力均小于30 MPa,而计算结果显示,2期油气充注甲烷包裹体捕获压力均大于60 MPa,说明2期油气充注时期,均存在古压力异常,即存在生烃增压作用形成的超压。
表3 甲烷包裹体密度及捕获压力计算结果

Table 3 Calculation results of methane inclusions density and capture pressure

井号 深度/m 层位 V 1/cm-1 V 0 均一温度/℃ 密度/(g/cm3 捕获压力/MPa 压力系数
B206 3 234.89 营Ⅲ砂组 2 911.26 2 917.22 110 0.24 62.86 1.94
B210 3 945.33 营Ⅵ砂组 2 911.26 2 917.22 130 0.24 68.74 1.74
B210 4 129.65 营Ⅵ砂组 2 911.26 2 917.22 130 0.24 68.74 1.66
图11 龙凤山次凹B201井埋藏史曲线(据文献[40]修改)

Fig.11 Burial history curve of Well B201 in Longfengshan sub-sag(modified from Ref.[40])

4 超压对储层质量的影响

龙凤山次凹沙河子组顶部烃源岩的镜质体反射率自西南向东北方向逐渐增大,近洼陷中心镜质体反射率大于2.8%,重点井B210、B206等井镜质体反射率均大于1.3%(图12),烃源岩处于高成熟阶段,产生大量天然气。营城组暗色泥岩厚度约为100~600 m,泥地比约为30%~50%;沙河子组烃源岩厚度约为100~400 m,泥地比约为40%~80%41。营城组烃源岩有机质丰度整体偏低,TOC值主要介于0.4%~1.0%之间,以差—中等烃源岩为主,沙河子组主力烃源岩有机质丰度较高,主要为好—极好烃源岩(图13)。整体来说龙凤山次凹营城组、沙河子组烃源岩成熟度较高、有机质丰度较高,成藏期具备形成生烃增压的烃源岩条件。
图12 龙凤山次凹沙河子组顶部R O等值线图(据文献[40]修改)

Fig.12 R O contour map at the top of the Shahezi Formation in the Longfengshan sub-sag(modified from Ref.[40])

图13 龙凤山次凹烃源岩TOCS 1+S 2交会图

Fig.13 Crossplot of TOC and S 1+S 2 of source rocks in the Longfengshan sub-sag

前人42-43研究表明,当孔隙流体压力达到上覆压力的70%及以上时,烃源岩就会破裂形成微裂缝。水热增压及黏土矿物脱水所产生的异常压力一般较小44-45,欠压实作用形成的超压一般形成于成岩阶段早期且形成深度相对较浅,与烃类的生成无关。生烃增压作用形成的超压往往能产生较大的流体压力。干酪根向烃类转化时会使体积膨胀,从而产生异常高的孔隙流体压力。当生烃增压产生的超压大于泥岩的破裂压力时,泥岩就会破裂产生排烃微裂缝,烃源岩内的流体沿微裂缝排出并进入到储集层中。随着流体不断向外排出,烃源岩内的孔隙流体压力逐渐小于破裂压力,此时微裂缝闭合,当流体压力又累积到破裂压力时,微裂缝再次开启。
当烃源岩的超压传递到储层中时,储层会由于孔隙流体压力的增大而沿颗粒接触的薄弱处形成微裂缝46。部分超压缝中充填沥青和有机质,产状不规则,宽度不一47图14(a)—图14(c)];另外,储层中还发育较多的贴粒缝[图14(d)—图14(f)],此类裂缝既无明显溶蚀特征,又非构造成因,笔者认为此类裂缝是由于源岩生烃增压产生的超压使相邻储层中的流体压力增大,颗粒接触处为薄弱点,从而产生紧贴颗粒的贴粒缝。同样石英颗粒内部成岩期后微裂缝中也存在烃类包裹体[图9(a)—图9(f)],说明超压与微裂缝的形成密切相关。
图14 研究区营城组超压缝发育特征

(a)有机质充填超压缝,B203井,3 607.8 m,(-);(b)有机质充填超压缝,B206井,3 264.26 m,(-);(c)有机质充填超压缝,B203井,3 778.27 m,(-);(d)贴粒缝发育,B210井,3 945.08 m,(-);(e)贴粒缝发育,B210井,3 945.95 m,(-);(f)贴粒缝发育,B206井,3 243.61 m,(-)

Fig.14 The development characteristics of the overpressure fractures in Yingcheng Formation in the study area

超压可以使岩石破裂形成微裂缝,也可以使已经闭合的微裂缝重新开启,或使已经形成的微裂缝保持开启状态。微裂缝既能连通孔隙,又能改善储层的渗透性,是重要的储集空间及油气运移通道。研究区营城组、沙河子组孔隙度与渗透率的关系图(图15)显示,当孔隙度小于6%时,渗透率存在异常高值,可能为深层致密背景下的裂缝发育带。实测地层压力数据和测井数据显示,龙凤山次凹营城组、沙河子组储层超压主要分布于埋深3 000 m以深(图2),且超压的分布与渗透率高值区耦合关系较好(图16),说明超压的发育有利于微裂缝的产生,改善了储层的渗透性。
图15 营城组和沙河子组孔隙度与渗透率的关系

Fig.15 Relationship between porosity and permeability of Yingcheng Formation and Shahezi Formation

图16 营城组和沙河子组渗透率、压力系数与深度的关系

Fig.16 The relationship between permeability, pressure coefficient and depth of Yingcheng Formation and Shahezi Formation

5 结论

(1)现今松辽盆地长岭断陷龙凤山次凹营城组、沙河子组超压主要发育于埋深3 000 m以深的深层,以高压、弱超压为主。
(2)松辽盆地长岭断陷龙凤山次凹营城组—沙河子组超压主要由源岩生烃增压作用产生,成藏期存在明显的古超压。
(3)有机质生烃作用产生的超压为油气的充注提供了充足的动力,超压改善了深层致密碎屑岩储层的质量,有利于微裂缝的产生,微裂缝的存在为油气运移提供了通道,增加了储层的连通性,改善了储层的渗透性。
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