非常规天然气

深层页岩气储层物质组成与孔隙贡献及其勘探开发意义

  • 吴建发 , 1 ,
  • 赵圣贤 , 1 ,
  • 张瑛堃 2 ,
  • 夏自强 1 ,
  • 李博 1 ,
  • 苑术生 1 ,
  • 张鉴 1 ,
  • 张成林 1 ,
  • 何沅翰 1 ,
  • 陈尚斌 2, 3
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  • 1. 中国石油西南油气田分公司页岩气研究院,四川 成都 610056
  • 2. 中国矿业大学资源与地球科学学院,江苏 徐州 221116
  • 3. 煤层气资源与成藏过程教育部重点实验室,江苏 徐州 221116
赵圣贤(1987-),男,四川成都人,高级工程师,硕士,主要从事页岩气开发地质研究.E-mail: .

吴建发(1976-),男,甘肃靖远人,高级工程师,博士,主要从事页岩气勘探开发技术管理和研发. E-mail:.

收稿日期: 2021-05-08

  修回日期: 2021-08-17

  网络出版日期: 2022-04-22

Material composition and pore contribution of deep shale gas reservoir and its significance for exploration and development

  • Jianfa WU , 1 ,
  • Shengxian ZHAO , 1 ,
  • Yingkun ZHANG 2 ,
  • Ziqiang XIA 1 ,
  • Bo LI 1 ,
  • Shusheng YUAN 1 ,
  • Jian ZHANG 1 ,
  • Chenglin ZHANG 1 ,
  • Yuanhan HE 1 ,
  • Shangbin CHEN 2, 3
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  • 1. Shale Gas Research Institute of Southwest Oil and Gasfield Company of CNPC,Chengdu 610056,China
  • 2. School of Resources and Geosciences,China University of Mining and Technology,Xuzhou 221116,China
  • 3. Key Laboratory of the Ministry of Education on Coalbed Methane Resources and Accumulation Process,Xuzhou 221116,China

Received date: 2021-05-08

  Revised date: 2021-08-17

  Online published: 2022-04-22

Supported by

The National Natural Science Foundation of China(41772141)

the Research and Test of Key Technologies for Effective Exploitation of Deep Shale Gas(2019F⁃31)

本文亮点

深层页岩气资源丰富,勘探开发潜力大,但在复杂地质演变过程中,其储层物质组成、力学性质和相态赋存等发生了变化,具有一定特殊性,对储层孔隙发育产生重要影响。以四川盆地泸州区块五峰组—龙马溪组深层页岩为例,基于储层物质组成和孔隙结构表征等实验,获取储层的物质组成和储集空间特征,研究储层物质演化规律及其孔隙贡献。结果表明:①垂向上由浅至深,五峰组—龙马溪组页岩有机质含量先增高后降低;储层矿物组成以石英和黏土矿物为主,其他矿物包括长石、碳酸盐矿物和黄铁矿。②龙马溪组有机质孔隙贡献随埋藏深度增加总体上表现出增大的趋势,至五峰组孔隙贡献随埋深增加而减小,黏土矿物孔隙贡献变化趋势与有机质相反。③石英及黄铁矿等质地坚硬的矿物具有“抗压保孔”作用,是五峰组—龙马溪组底部有机质孔隙保存的关键因素。④区内五峰组上部—龙一1亚段是研究区内开发优质层段,其中五峰组上部—龙一1 3小层为Ⅰ类储层。为实现经济开发,建议针对深层储层高温压和大抗拉强度等特点,优化压裂技术方案。

本文引用格式

吴建发 , 赵圣贤 , 张瑛堃 , 夏自强 , 李博 , 苑术生 , 张鉴 , 张成林 , 何沅翰 , 陈尚斌 . 深层页岩气储层物质组成与孔隙贡献及其勘探开发意义[J]. 天然气地球科学, 2022 , 33(4) : 642 -653 . DOI: 10.11764/j.issn.1672-1926.2021.08.017

Highlights

The deep shale gas is rich in resources and has great potential for exploration and development. However, in the process of complex geological evolution, the material composition, mechanical properties and phase occurrence of the reservoir have changed, which has certain particularity and an important influence on the development of reservoir pores. Taking the deep shale of Wufeng-Longmaxi formations in Luzhou block as an example, based on the experiments of reservoir material composition and pore structure characterization, this paper obtained the material composition and space characteristics of the reservoir, and studied the evolution law and the pore contribution of reservoir material. The results show that: (1)Vertically, from shallow to deep, the organic matter content of Wufeng-Longmaxi formations shale increases firstly and then decreases, the content of brittle minerals increases while the content of clay minerals decreases. (2)The pore contribution of organic matter in Longmaxi Formation increases with the increase of burial depth, but decreases with the increase of burial depth in Wufeng Formation. The change trend of the pore contribution of clay minerals is opposite to that of organic matter. (3)The hard minerals, such as quartz and pyrite, have the function of “anti-compaction and pore preservation”, and are the key factors for the preservation of organic matter pores in the bottom of Wufeng-Longmaxi formations. (4)The upper part of Wufeng Formation-1st submember of 1st Member of Longmaxi Formation (Long11 submember) is a high-quality reservoir in the study area, in which the upper part of Wufeng Formation-3 sublayer of Long11 submember is a type I reservoir. In order to realize economic development, it is suggested to optimize the fracturing technology scheme according to the characteristics of high temperature and pressure in deep reservoir and high tensile strength of rock.

0 引言

自2012年以来,我国南方上奥陶统五峰组—下志留统龙马溪组页岩气在中深层(埋深小于3 500 m)实现了商业开发1-2。从资源禀赋而言,深层页岩气资源更为丰富,是未来页岩气增储上产的关键。但深层页岩气储层具有地层温度和压力高,赋存富集演化历史复杂等特点,在优质储层评价、经济开发选层和技术方案改进等方面均面临巨大难题3
川南地区埋深介于3 500~4 500 m之间的页岩气有利区面积达13 000 km2,资源量为83 000×108 m3,占埋深 4 500 m以浅资源总量的80%以上4,其中泸州区块是主要目标产区之一5-6。已有勘探和开发实践表明,深层储层孔隙度普遍偏低,按照以往评价标准,优质储层厚度有限,甚至没有优质储层,但储层压力系数较高,气显示含气性较好。因此,基于中浅层的储层孔隙度认识和评价标准是否仍然适合深层储层,有待进一步探讨。储层孔隙演化受有机质、矿物组成、成岩流体、温度和压力等多种因素影响7-13,且储层经历了漫长的地质演变过程,使储层矿物组成发生了复杂演化14,影响了孔隙结构15,但其主控因素以及孔隙贡献等问题尚未有定论。因此,本文以泸州区块深层页岩储层为例,研究储层物质组成与储集空间特征,探讨储层矿物转化及其孔隙贡献,为深层页岩气储层孔隙评价和优质层段优选提供依据。

1 地质背景

泸州区块构造上属于四川盆地南部,位于川中古隆起南斜坡和川东南拗褶带之间的川南低褶带,构造呈北东向带状分布,整体表现为“堑垒”相间的构造特征16。区内褶皱构造发育,常以直线或弧型带状展布,东北部靠近构造转换带,褶皱强度较大,向斜地带平缓宽阔,背斜地带两翼陡峭角度低,总体上断层发育较少,以逆断层为主,局部伴有地层隆升17图1(a)]。
图1 研究区构造纲要图(a)17及目的层段岩性柱状图(b)

Fig.1 Structural outline map(a)17 of the study area and lithologic histogram(b) of target strata

研究区地层从寒武系至第四系均有出露。震旦纪—中三叠世为海相沉积阶段,以碳酸盐岩沉积为主,夹多套泥岩、页岩和少量砂岩、粉砂岩,其中志留系上部被大量剥蚀,与二叠系底部呈不整合接触;中二叠统茅口组顶部被剥蚀,与上二叠统吴家坪组不整合接触;中三叠统雷口坡组遭受剥蚀,与上三叠统不整合接触。晚三叠世—第四纪为陆相沉积阶段,以砂岩、泥岩为主,侏罗系与上三叠统须家河组呈不整合接触,白垩系与下伏侏罗系不整合接触18。从晚奥陶世开始,受持续上升的乐山—龙女寺古隆起和黔中古陆影响,上扬子克拉通盆地范围进一步缩小,早—中奥陶世从具有广海特征的海域转变为“两隆夹一坳”的局限海域,沉积基底为东南高西北低的特征,海域自川东南向泸州一带逐渐变深19-21。到早志留世龙马溪期,黔中隆起进一步扩大,以西与康滇古陆相连,以东的雪峰水下古隆起雏形初现,加之川中水下古隆起进一步隆升,使泸州区块沉积水体进一步加深,处于局限陆表海(陆棚)环境中心。五峰组—龙马溪组沉积厚度普遍介于500~650 m之间4,岩性主要为灰黑色炭质页岩、黑色灰质页岩、灰黑色灰质页岩和黑灰色粉砂质页岩22

2 样品采集与实验方法

2.1 样品采集

样品采自泸州区块Y101H3-8井上奥陶统五峰组和下志留统龙马溪组一亚段1~4小层(龙马溪组一段1亚段自下而上分为1~4小层,下文中记为龙一1 1~4小层)、龙马溪组一段2亚段(下文中记为龙一2亚段)[图1(b)],样品埋深为3 695.14~3 794.20 m。

2.2 实验

(1)有机质丰度测试采用C-S分析仪,将约5 g样品用去离子水多次超声波清洗、烘干,用玛瑙研体磨碎至80~100目,粉末样品浸泡在10%的盐酸中除去碳酸盐后进行有机碳含量测试。
(2)物质成分测试采用日本Rigaku Ultima IV型X射线衍射仪,按照《沉积岩中黏土矿物和常见非黏土矿物X射线衍射分析方法》23(SY/T5163—2010),将样品粉碎至200目,实验温度为25 ℃,电压为40 kV,电流为 40 mA。
(3)孔隙度测试选用高压压汞实验,采用Auto Pore 9500全自动压汞分析仪,加热烘干和抽取真空等预处理,烘干条件为110 ℃,时间2 h。注入液态汞的压力范围为0~410 MPa,所测孔径的范围为3 nm~1 mm。
上述实验均在中国石油西南油气田公司完成。

3 结果

3.1 有机质丰度

有机碳含量测试结果表明,五峰组—龙一2亚段TOC值分布在0.14%~4.72%之间,主体介于2.0%~4.0%之间(图2),根据《页岩气资源/储量计算与评价技术规范》24(DZ/T0254—2014)的特高(TOC≥4.0%)、高(2.0%≤TOC<4.0%)、中(1.0%≤TOC<2.0%)、低(0.5%≤TOC<1.0%)、特低(TOC<0.5%)有机碳含量划分指标,区内主要为中—高有机碳含量。各小层TOC含量不同,五峰组—龙一1 3小层TOC含量较高,≥4.0%的样品占总样品的62.5%;龙一1 4小层TOC含量主体介于2.0%~4.0%之间;龙一2亚段TOC含量低,均<1.0%。五峰组与龙一1亚段有机碳含量明显高于龙一2亚段,即龙马溪组底部有机碳含量高于上部;垂向上,龙一2亚段—五峰组,TOC含量呈现先增高后降低的趋势。
图2 Y101H3-8井岩心样品TOC分布直方图

Fig.2 TOC distribution histogram of core samples of Well Y101H3-8

3.2 矿物组成

全岩及黏土矿物X射线衍射测试结果(表1)表明,五峰组—龙马溪组页岩气储层矿物组成以石英和黏土矿物为主,含量分别介于22.0%~67.0%和9.0%~63.0%之间,含长石、碳酸盐矿物和少量黄铁矿等其他矿物,含量分别介于2.0%~9.0%、0%~21.0%和0%~5.0%之间。黏土矿物主要为伊利石,含量为60.0%~89.0%,其次为高岭石和绿泥石,含量分别为6.0%~27.0%和0%~20.0%,大多数样品还含有伊/蒙混层,含量为0%~2.0%。五峰组—龙马溪组页岩矿物成分复杂,矿物种类较多,各样品矿物含量差异较大,非均质性较强。
表1 Y101H3-8井岩心样品矿物成分含量测试结果

Table 1 Test results of mineral composition content of core samples of Well Y101H3-8

样品编号 主要矿物/% 黏土矿物/%
石英 长石 碳酸盐矿物 黄铁矿 黏土 伊利石 伊/蒙混层 高岭石 绿泥石
Y-1 30 7 0 0 63 65 0 21 14
Y-2 38 8 0 3 51 67 0 18 15
Y-3 35 7 0 3 55 67 3 10 20
Y-4 39 7 0 3 51 66 3 20 11
Y-5 25 7 5 2 61 70 4 12 14
Y-6 22 6 27 2 43 60 4 27 9
Y-7 26 8 19 3 44 71 5 18 6
Y-8 32 6 13 4 45 68 6 15 11
Y-9 37 7 12 3 41 72 6 14 8
Y-10 43 4 7 5 41 71 6 15 8
Y-11 38 5 11 3 43 73 6 12 9
Y-12 38 6 10 3 43 77 5 12 6
Y-13 24 9 14 2 51 70 7 15 8
Y-14 42 5 10 2 41 72 4 14 10
Y-15 48 8 9 3 32 75 7 11 7
Y-16 62 5 13 4 16 77 8 11 4
Y-17 67 3 16 3 11 68 12 10 10
Y-18 56 4 18 5 17 89 3 8 0
Y-19 64 4 21 2 9 88 6 6 0
Y-20 67 5 7 4 17 75 3 15 7
Y-21 30 2 20 4 44 69 3 21 7
Y-22 24 6 18 1 51 74 5 17 4

3.3 孔隙度

孔隙度测试结果表明,五峰组—龙马溪组样品孔隙度介于2.56%~5.41%之间(图3),平均为4.18%。其中,五峰组孔隙度介于2.58%~5.07%之间,平均为4.11%,且随埋藏深度增加,孔隙度显著降低;龙一1 1-3小层孔隙度介于3.77%~5.41%之间,平均为4.52%;龙一1 4小层孔隙度介于2.87%~5.32%之间,平均为4.36%;龙一2亚段孔隙度介于2.56%~3.78%之间,平均为3.22%。可见,页岩气储层孔隙度整体较低,大多未达到Ⅰ类储层孔隙度标准(>5.0%)。
图3 Y101H3-8井岩心样品孔隙度测试结果

Fig.3 Porosity test results of core samples of Well Y101H3-8

4 讨论

4.1 储层矿物组成特征与成因

五峰组—龙马溪组各小层矿物组成存在差异[图4(a)],五峰组下部石英含量略有下降,黏土矿物含量有所升高。奥陶系五峰组沉积时期,扬子地区火山活动频繁25-26,五峰组沉积早期海平面下降,沉积水体变浅,陆源碎屑输入增多27-29,导致黏土矿物含量增加;五峰组沉积末期处于赫南特冰期,冰川和冰融事件交替发生使海平面升降较为频繁30-31,但不足以导致沉积界面上升到浪基面附近,因此五峰组上部仍属于深水沉积,发育硅质页岩相。龙一1 4小层下部页岩中硅质矿物含量整体上呈现增长趋势,是由于龙马溪组页岩沉积早期,受加里东运动影响,扬子板块周缘产生了一些古隆起,形成了一种由隆起包围的滞留环境32,以湿热气候与高海面(海侵)为背景,因海底地形起伏(水下障壁)而导致海水循环连通受阻,自广海漂浮、浮游而来的生物,特别是生烃母质生物得到富集并繁衍、繁盛,死亡后降落至缺氧还原的海底而保存在欠补偿沉积环境中33-34。海洋自生成分构成了海洋沉积物组分,Si存在于生物自生成分中,因此区内五峰组—龙马溪组下部页岩石英含量增多,主要为生物石英注入;而黏土矿物含量随埋深增加逐渐减小,与硅质矿物含量呈此消彼长的关系[图4(a)],主要是在成岩历程中蒙脱石伊利石化过程中排出的硅质再沉淀也可形成石英35-36。蒙脱石在温度达到60~80 ℃时可以发生溶解释放Si离子,致使孔隙水中硅的浓度升高,当浓度超过石英的饱和度时就可以形成微米级的石英晶体。这一反应过程为37-38
蒙脱石+Al3++K+=伊利石+Si4++nH2O
图4 五峰组—龙马溪组页岩矿物组成变化趋势

(a)全岩矿物含量 (b)黏土矿物含量

Fig.4 Variation trend of shale mineral composition in Wufeng-Longmaxi formations

硅质矿物中长石的含量略有减少而石英含量增加[图5(a)],其可能原因是随埋深增加,温度和压力增大,有机质生烃产生的有机酸溶蚀长石,释放SiO2 39,反应式为940
KAlSi3O8(钾长石)+有机酸→K++Al2Si2O5(OH)4(高岭石)+SiO2+H2O
图5 Y101H3-8井五峰组—龙马溪组页岩储层SEM图像

(a)Y3,有机质孔,3 709 m;(b)Y7,有机质孔与微裂缝,3 728 m;(c)Y10,石英溶蚀孔及粒内孔,3 748 m;(d)Y15,石英粒间孔与黏土矿物粒间孔,3 771 m;(e)Y21,黏土矿物层间孔缝与有机质孔,3 791 m;(f)Y19,白云石粒内孔,3 785 m

Fig.5 SEM image of shale reservoir in Wufeng-Longmaxi formations of Well Y101H3-8

龙一1 4小层至五峰组碳酸盐类矿物含量明显增多,可能是加里东运动导致海退,海平面下降和前陆隆起继续抬升,虽然该时期仍处于深水陆棚环境,但沉积水体逐渐变浅,物源区输入增强,碳酸盐矿物逐渐增多41
龙一2亚段整体上矿物组成变化不大,黏土矿物含量较高,硅质矿物含量较低,含有少量黄铁矿,反映研究区在龙马溪组晚期处于浅水陆棚沉积环境,以陆源黏土矿物供给为主。

4.2 黏土矿物特征与演化规律

由黏土矿物分析(图4)可知,研究区五峰组—龙马溪组页岩储层垂向上由浅至深,黏土矿物含量整体上呈减少趋势,主要受沉积环境影响。五峰组—龙马溪组海相页岩形成于沉积水体较深的深水陆棚环境中,其距离剥蚀区较远,陆源输入量少,浅水表层生物死亡后的硅质、钙质下沉到底部,加之深水洋流影响,使来自陆源的黏土矿物在沉积埋藏过程中被稀释,造成页岩中石英矿物含量较高而黏土矿物含量较低41-42
样品中以伊利石和绿泥石等稳定矿物为主[图4(b)],反映五峰组—龙马溪组页岩已达到高成岩演化阶段43。所有样品中均无绿/蒙混层矿物,表明蒙皂石的绿泥石化不发育,绿泥石主要来源于陆源碎屑;随埋深增加,伊利石含量增加,高岭石含量略有降低,并有伊/蒙混层出现,表明受成岩演化影响,存在2个黏土矿物转化序列:蒙脱石→伊/蒙混层→伊利石[式(3)44-45和高岭石→伊利石[式(4)式(5)46-47
蒙脱石+Al3++K+=伊利石+Si4++nH2O
K++Al2Si2O5(OH)4(高岭石)+I/S(伊/蒙混层) →KAl3Si3O10(OH)2(伊利石)
K++Al2Si2O5(OH)4(高岭石)→2KAl3Si3O10(OH)2(伊利石)+2H++H2O

4.3 储层组成物质的孔隙贡献

为了定量研究五峰组—龙马溪组页岩气储层主要物质成分的孔隙贡献,结合TOC含量、XRD矿物成分与孔隙度数据,运用偏最小二乘多元线性回归分析,建立关系方程:
Φi =aTOCi +bQtzi +cFspi +dCbi +ePyi + fCMi +Ci
式中:Φ为孔隙度,%;TOC为有机质含量,%;Qtz为石英含量,%;Fsp为长石含量,%;Cb为碳酸盐矿物含量,%;Py为黄铁矿含量,%;CM为黏土矿物含量,%;i为样品编号,值为1~naf为拟合系数;C为残差项。
结合样品测试数据,得到研究区储层孔隙贡献拟合结果:
Φ=0.528 9TOC+0.012 6Qtz+0.026 5Fsp+0.004 9Cb+0.232 6Py+0.038 8CM,R=0.767 3
计算可得,有机质孔隙贡献介于2.35%~54.66%之间,平均为27.27%;脆性矿物孔隙贡献介于22.49%~42.48%之间,平均为34.53%;黏土矿物孔隙贡献介于8.07%~75.16%之间,平均为38.20%。其中,五峰组有机质、脆性矿物和黏土矿物的孔隙贡献依次为33.04%、35.77%和31.18%;龙一1 1~3小层三者孔隙贡献依次为42.85%、40.35%和16.80%;龙一1 4小层三者的孔隙贡献依次为25.56%、32.36%和42.08%;龙一2亚段三者孔隙贡献依次为5.03%、33.08%和61.88%。
由SEM图像可知,研究区内五峰组—龙马溪组页岩有机质孔隙与无机矿物孔隙均较为发育,有机质、脆性矿物和黏土矿物对孔隙均有贡献(图5);微观孔隙结构形态各异,普遍发育有机质孔[图5(a),图5(b),图5(e)]、粒内孔[图5(c),图5(f)]、粒间孔[图5(d)]、溶蚀孔[图5(c)]以及微裂缝[图5(b),图5(e)]。
有机质孔隙贡献在龙一2亚段—龙一1 1小层随埋藏深度增加总体上表现出增大的趋势,但在龙一1 4小层底部时孔隙贡献变化不大,至五峰组孔隙贡献随埋深增加而减小;黏土矿物孔隙贡献变化趋势与有机质相反,二者此消彼长;脆性矿物孔隙贡献随埋深增加基本保持稳定或略有增大(图6)。五峰组—龙马溪组页岩孔隙贡献随深度的演化特征主要受控于储层物质成分的变化。此外,上覆岩层压力对有机质与黏土矿物孔隙也有影响,石英和黄铁矿等骨架矿物具有抵御上覆岩层压力的作用,因此,有机质与骨架矿物的特殊接触关系很大程度上降低了上覆压力对于有机质孔隙的破坏。石英及黄铁矿等质地坚硬矿物的抗压保护作用是五峰组—龙马溪组底部有机质孔隙发育的关键因素 (图7)。
图6 五峰组—龙马溪组页岩组成物质孔隙贡献

Fig.6 Pore contribution of shale components in Wufeng-Longmaxi formations

图7 石英和黄铁矿对孔隙的保护作用

(a)Y8,有机质孔与黄铁矿聚集体,3 736 m;(b)Y12,黏土矿物粒间孔缝、微裂缝与石英,3 761 m;(c)Y16,黏土矿物粒间孔与黄铁矿,3 779 m;(d)Y20,有机质孔与石英,3 789 m

Fig.7 Protection of quartz and pyrite on pores

4.4 深层页岩气储层评价标准

准确识别优质储层,是深层页岩气勘探开发取得成功的关键。已有学者将孔隙连通性、微观孔隙结构、含气量、矿物含量、泊松比和杨氏模量等作为评价指标,建立了“甜点层”的评价标准48-50。本文综合前人研究结果,参考能源行业标准中页岩储层分类评价规范(NB/T 14001—2015)51,将TOC作为储层评价的基础要素,寻找孔隙度、脆性矿物含量、黏土矿物含量与TOC的匹配关系(图8),建立泸州区块深层页岩气储层评价标准(表2),依据该标准对区内页岩储层进行评价。结果表明,五峰组上部—龙一1 3小层储层参数最优,TOC>3.0%,孔隙度介于3.77%~5.01%之间,脆性矿物含量介于68.0%~91.0%之间,黏土矿物含量介于9.0%~32.0%之间,为Ⅰ类储层。
图8 泸州区块五峰组—龙马溪组页岩储层参数相关图

Fig.8 Correlation diagram of shale reservoir parameters of Wufeng-Longmaxi formations in Luzhou block

表2 泸州区块深层页岩气储层分类评价标准

Table 2 Classification and evaluation criteria for deep shale gas reservoirs in Luzhou block

储层类别 TOC/% 孔隙度/% 脆性矿物含量/% 黏土矿物含量/%
Ⅰ类 >3.0 >3.5 >50.0 <42.0
Ⅱ类 2.0~3.0 2.8~3.5 36.0~50.0 42.0~54.0
Ⅲ类 1.0~2.0 2.2~2.8 18.0~36.0 54.0~67.0
Ⅳ类 <1.0 <2.2 <18.0 >67.0

4.5 勘探开发意义

五峰组—龙一1 4小层孔隙度较高,介于2.58%~5.41%之间,均值为4.34%,其中五峰组上部—龙一1 4 小层TOC含量较高,主体介于2.0%~5.0%之间,均值为2.6%,为页岩气成藏提供了良好的物质基础;有机质和脆性矿物孔隙贡献较大,脆性矿物含量较高,黏土矿物含量较低,降低了上覆压力对于有机质孔隙的破坏,有利于有机质孔的保存。因此,根据Y101H3-8井五峰组—龙马溪组页岩储层的物质组成、物质演化特征及其孔隙贡献可初步预测,五峰组上部—龙一1亚段为该区域勘探开发的有利层段。
泸州区块深层页岩气储层有利层段孔隙度明显低于周边浅部页岩气储层有利层段(表2),大多小于5%;有机碳含量与长宁区块相当,低于威远和涪陵区块;脆性矿物含量高于其他浅层储层区块。根据《页岩气资源/储量计算与评价技术规范》24(DZ/T 0254—2014),泸州区块有利层段总有机碳含量总体处于高—特高水平(2.0%~5.0%),能为页岩气藏提供良好的物质基础;参考能源行业标准中页岩储层分类评价规范(NB/T 14001—2015)51,泸州区块有利层段达到Ⅰ类脆性层标准(脆性矿物含量大于55.0%或脆性指数大于60.0%),岩石脆度大,压裂中易于形成复杂缝网,可压裂性好。
此外,泸州区块有利层段页岩埋藏深度大,且多为低陡多褶皱构造类型,无深大断裂发育(图1),且地层压力系数比其他浅层页岩气储层大,超过2.0(表3),而异常高压对高硅质含量页岩储层的有机孔隙和无机孔隙均具有保护作用,受后期压实作用和成岩作用的影响则相对较小,从而使原生有机和无机孔隙得到了有效保留4。因此泸州区块页岩气保存条件良好。随埋深增大,地层温度升高,在特定压力下页岩储层吸附甲烷的能力降低,游离气含量增大,有利于深层页岩气井在生产初期获得较高的产量。
表3 深层与中浅层页岩气储层有利层段特征对比[52-57]

Table 3 Characteristics comparison of favorable strata between deep and middle-shallow shale gas reservoirs[52-57]

区块 目标层 埋深/m TOC/% 孔隙度/% 脆性矿物含量/% 压力系数
中浅层 长宁 龙马溪组 1 285.0~3 174.5 2.70~3.25 2.0~7.6 30.0~69.0 1.3~2.0
威远 龙马溪组 1 440.0~1 560.0 0.51~8.12 3.8~8.0 40.0~78.0 1.4~2.0
涪陵 龙马溪组 2 330.0~2 420.0 1.04~5.89 2.7~7.1 31.0~70.6 1.8~2.0
深层 泸州 五峰组—龙马溪组 3 713.0~3 792.0 0.40~4.72 2.8~5.4 46.0~89.0 >2.0
综上所述,长宁、威远等地区浅层页岩气储层的分类评价标准不完全适用于深层储层。泸州区块深层页岩气储层成藏及保存条件较好,满足勘探开发需要。但需要注意的是,随储层深度增加,深层页岩气储层温度和压力较中浅层明显增大,导致复杂缝网的形成和储层体积改造难度加大;同时,岩石抗压强度显著增大,压裂施工难度增加。因此,对深层页岩气储层而言,压裂相关参数则更为苛刻,需要对现有压裂技术进行改进以实现深层页岩气储层的经济开发。

5 结论

(1)研究区五峰组—龙马溪组页岩各小层有机质含量不同,垂向上,由浅至深呈现出先增高后降低的趋势。储层矿物组成以石英和黏土矿物为主,含有长石、碳酸盐矿物和黄铁矿等其他矿物;黏土矿物主要为伊利石,其次为高岭石和绿泥石;页岩储层矿物成分复杂,矿物含量差异较大,非均质性较强。
(2)龙马溪组有机质孔隙贡献随埋藏深度增加总体上表现出增大的趋势,至五峰组孔隙贡献随埋深增加而减小,黏土矿物孔隙贡献变化趋势与有机质相反;石英及黄铁矿等质地坚硬的矿物对孔隙具有保护作用,是五峰组—龙马溪组底部有机质孔隙发育的关键因素。
(3)将TOC作为储层评价的基础要素,结合孔隙度、脆性矿物含量和黏土矿物含量等指标,建立泸州区块深层页岩气储层评价标准,泸州区块五峰组上部—龙一1 3小层储层参数最优,为Ⅰ类储层,是开发优质层段。
(4)深层页岩气储层温度压力大,岩石抗拉强度大,需优化压裂技术方案以实现经济开发。
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