Development characteristics and transformation mechanism of clay minerals in the organic-rich shale layers of saline-alkali lake: A case study of Fengcheng Formation in Mahu Sag, Junggar Basin

  • Xueyu YAO , 1, 2 ,
  • Xinping LIANG 3 ,
  • Zhijun JIN , 1, 2, 3 ,
  • Xiaojun WANG 1, 4 ,
  • Jiahong GAO 3 ,
  • Haiyan LEI 5 ,
  • Tao ZHU 5
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  • 1. National Key Laboratory of Petroleum Resources and Engineering of China University of Petroleum (Beijing),Beijing 102249,China
  • 2. School of Earth and Science,China University of Petroleum (Beijing),Beijing 102249,China
  • 3. Institute of Energy/School of Earth and Space Sciences,Peking University,Beijing 100871,China
  • 4. Daging Oilfield Limited Company,Daqing 163002,China
  • 5. Mahu Exploration and Development Project Department,Xinjiang Oilfield Company,PetroChina,Karamay 834000,China

Received date: 2024-05-31

  Revised date: 2024-08-31

  Online published: 2024-09-12

Supported by

The National Natural Science Foundation of China(42090025)

Abstract

To systematically study the development characteristics and transformation modes of clay minerals in alkaline lake shale, and explore the development of clay minerals in alkaline lake sedimentary environments, Based on scanning electron microscopy (SEM) observation and X-ray diffraction, this paper investigates the characteristics and dynamic transformation of clay minerals in different sedimentary environments of the Fengcheng Formation shale in Mahu Sag. In the sedimentary center of the alkaline lake, only a small amount of clay minerals (<5%) are developed, mainly characterized by smectite (S) and illite-smectite mixed layers (I/S). The detrital clay minerals were dissolved in a strongly alkaline diagenetic environment, transforming into authigenic silicate minerals such as K-feldspar. Moreover, the presence of CO3 2-/HCO3 can delay the transformation of clay minerals in the sedimentary center. In the edge slope zone, the content of clay minerals (average 11.57%), which is mainly the illite (I), chlorite (C), and I/S is higher than that in the sedimentary center. The diagenetic environment is transformed from strongly alkaline to weakly alkaline-weakly acidic, and the primitive detrital clay minerals have not been dissolved and can be preserved in the early diagenetic stage, and illitization and chloritization occurr with the increase of stratigraphic temperature and pressure. The transformation of clay minerals can produce many brittle minerals such as authigenic quartz and K-feldspar, which not only increases the brittleness of shale reservoirs but also generates secondary pores and fractures, and is beneficial for the exploration and development of shale oil and gas.

Cite this article

Xueyu YAO , Xinping LIANG , Zhijun JIN , Xiaojun WANG , Jiahong GAO , Haiyan LEI , Tao ZHU . Development characteristics and transformation mechanism of clay minerals in the organic-rich shale layers of saline-alkali lake: A case study of Fengcheng Formation in Mahu Sag, Junggar Basin[J]. Natural Gas Geoscience, 2025 , 36(1) : 72 -85 . DOI: 10.11764/j.issn.1672-1926.2024.09.001

0 引言

黏土矿物是组成页岩的主要矿物之一,利用黏土矿物的含量、成因及组合特征既可对古环境进行恢复,也可作为地质温度计用于岩石成岩演化研究1-2。黏土矿物的成岩过程不仅对有机质具有催化作用,且其在相互转化的过程中会对页岩储层进行改造,在有机—无机相互作用中起到不可替代的作用。但在不同沉积环境下,黏土矿物的组合特征、赋存状态、转化特征也存在明显差别,其对有机—无机作用过程的影响也不尽相同3。中国陆相富有机质页岩主要形成于中新生代湖泊沉积环境,受古气候等因素影响可分为淡水湖盆(鄂尔多斯盆地长7段页岩等)4-6、微咸水湖盆(松辽盆地青山口组页岩等)7-8、咸化湖盆(准噶尔盆地风城组页岩、芦草沟组页岩等)9-11,其中准噶尔盆地风城组作为特殊的盐碱湖沉积盆地,发育特殊的碱性矿物。不同含油气盆地的页岩矿物组成差别也较大12-14,碱湖沉积环境中主要以碳酸盐矿物及碱类矿物为主,盐湖中则更发育石膏、石盐等矿物,而淡水湖盆则不发育这些特殊矿物。在碱湖沉积环境中,其烃源岩的演化特征、矿物的发育情况、储层的发育特征及可压性等是否与黏土矿物在有机—无机相互作用中起到的作用有关仍存有疑问,因此有必要对不同沉积环境下黏土矿物的发育特征及转化规律进行研究,进而从黏土矿物发育特征及成岩过程的角度出发对碱湖页岩发育的特殊性进行解释。
准噶尔盆地玛湖凹陷风城组保存完整,是我国目前发现最古老的碱湖沉积地层,同时也记录了复杂的碱性成岩特征,其发育特殊的碱性矿物。风城组页岩含有极少量的黏土矿物且存在黏土矿物相互转化迟缓的现象,明显不同于我国其他咸化湖盆的黏土质、长英质页岩15-16。以往对黏土矿物的研究多集中于致密砂岩中,随着我国页岩油气的勘探开发,学者们也意识到页岩中黏土矿物研究的重要性17-19。国内众多学者通过对松辽盆地、鄂尔多斯盆地等含油气盆地中页岩黏土矿物含量与孔隙结构的相关关系研究,明确了黏土矿物在成岩转化过程中会产生大量次生孔隙以及黏土矿物收缩缝,对页岩孔隙发育具有一定的积极作用20-22;LI等23-24、李长志等25对玛湖凹陷风城组不同沉积区页岩的沉积—成岩环境和主要成岩作用进行研究,提出碱湖沉积区、过渡区、边缘斜坡区等不同沉积区域的差异转化特征,并认为在早成岩B期,过渡区和边缘缓坡区页岩中的蒙脱石等黏土矿物发生伊利石化,而碱湖中心区页岩黏土矿物在同生—准同生期早已发生溶解;周雪蕾等26研究了风城组页岩黏土矿物的发育特征、来源,认为风城组页岩中蒙皂石异常转化主要受物源区火山岩、气候和成岩流体的共同影响;郭佩等27明确了风城组页岩自生长英质矿物的成因,并认为玛湖凹陷“贫黏土、富自生长英质”矿物的原因是由于沸石、含镁蒙皂石等矿物的存在,在经历漫长埋藏成岩后进一步转化为更为稳定的石英、钾长石和钠长石。尽管有较多学者对风城组页岩中黏土矿物开展了相关研究,但只是作为成岩作用研究的部分内容进行简要提及,因此,有必要对碱湖页岩中黏土矿物的物质来源、发育特征以及在不同成岩环境下动态演化特征进行系统研究。
本文在前人对风城组页岩沉积、成岩研究的基础上,通过对不同沉积区(碱湖沉积中心区及边缘斜坡区)风城组页岩进行薄片、扫描电镜、XRD全岩分析,明确不同沉积区碱湖页岩黏土矿物动态演化特征并建立差异演化模型,以期对碱湖页岩中黏土矿物在不同成岩环境中的差异转化理论进行丰富与补充,深化玛湖凹陷风城组页岩的地质认识。

1 地质背景

玛湖凹陷位于准噶尔盆地西北缘,整体呈北东—南西向展布[图1(a)]。由于早期火山活动和气候频繁变化的控制,下二叠统风城组形成了独特的碱湖沉积地层28,发育天然碱、碳氢钠石、苏打石等特殊碱性矿物。风城组自下而上可划分为风一段、风二段、风三段,且各段岩相组合差异较大[图1(b)]29。风一段刚沉积时气候干旱,在强烈的火山活动作用下火山碎屑岩发育;后期在火山活动变弱、气候条件转变的条件下以凝灰质泥岩夹白云质砂岩为主。风二段沉积时期气候逐渐干热,湖盆水体萎缩、碱性增强,以碱性矿物及泥质白云岩为主并发育大量嗜碱菌藻类30。风三段在湿润气候、湖平面升高导致水体淡化等作用下,其陆源碎屑、白云质岩发育31
图1 玛湖凹陷构造位置(a)及地层柱状图(b)(改自文献[1633])

Fig.1 Structural location(a) and stratigraphic histogram(b) of Mahu Sag (modified from Refs.[1633])

按照地层岩性和厚度可将玛湖凹陷分为碱湖中心区和边缘缓坡区2532。以含碱页岩为主的碱湖中心区水体盐碱度最高,以沉积粉砂岩为主的边缘缓坡区水体盐碱度低。风城组页岩R O值为0.7%~1.2%33-34,整体处于中成岩早期。玛湖凹陷风城组页岩的残余有机质丰度较低,TOC含量分布在0.07%~2.89%之间,平均为0.94%,风城组页岩的生烃潜量(残留烃含量S 1+热解烃含量S 2)分布在0.01~19.77 mg/g之间,平均为3.97 mg/g(图2)。其中,风三段页岩的TOC含量较高,烃源岩品质较好。
图2 玛湖凹陷风城组页岩TOC含量与生烃潜量相关性

Fig.2 Relationship between TOC content and hydrocarbon generation potential of the Fengcheng Formation shale in Mahu Sag

2 黏土矿物含量及特征

2.1 黏土矿物含量

在玛湖凹陷风城组页岩岩心描述的基础上,结合扫描电镜、X射线衍射等相关实验,发现其矿物主要发育石英、长石类矿物(钾长石和斜长石)、碳酸盐矿物(白云石、铁白云石、方解石等)、碱性矿物(硅硼钠石、碳钠钙石等),以及黏土矿物(表1图3)。研究区黏土矿物含量及组合特征具有分区带发育的特点,主要发育蒙脱石(S)、伊/蒙混层(I/S)、伊利石(I)等黏土矿物。
表1 玛湖凹陷风城组页岩的主要矿物组成

Table 1 Mineral composition of the Fengcheng Formation shale, Mahu Sag

地区 井号 深度/m 石英/% 钾长石/% 斜长石/% 方解石/%

白云石

/%

铁白云石

/%

菱铁矿

/%

黄铁矿

/%

黏土矿物

/%

碱湖

沉积

中心

FN4 4 403.82~4 582.69

(7.27~13.56)

/9.46

(11.48~36.66)

/29.80

(15.65~40.73)/33.39

(7.13~34.43)

/14.68

(0~10.94)

/2.188

(0~24)

/8.75

/ / (0.88~2.6)/1.74
FN14 4 030.54~4 038.34 (4.72~76.71)/42.69 (0~14.59)/2.43

(0~19.45)

/5.08

(12.96~94.48)/47.58 /

(0~1.62)

/0.27

/ / (0.71~4.38)/1.96

边缘

斜坡

M51X 4 993.13~4 998.97 (15~41)/30.75 /

(19~34)

/27.25

(0~19)/10.75 /

(5~29)

/17.25

/

(2~4)

/2.75

(9~13)

/11.25

M54X 4 659.08~5 169.87 (30~51)/40.82 (9~32)/20.09 (6~11)/8.73 (0~3)/1.91

(6~39)

/18.55

/ /

(1~4)

/2

(5~12)

/7.91

MH57

4 627.36~

4 770.1

(4~20)/9.8 (12~20)/16 (32~61)/47.6 (1~4)/2.2 /

(0~16)

/6.2

/

(3~7)

/4.2

(4~28)

/14

MY1 4 580.85~5 027.59 (0.2~77.4)/31.67 (0.4~16.6)/4.76 (1.5~41.3)/12.83 (0.1~62.2)/10.1

(2~54.4)

/20.46

/ (0.1~3.9)/1.31

(0.5~20.5)

/4.92

(2.6~48.9)

/11.38

注:(7.27~13.56)/9.46=(最小值—最大值)/平均值;“/”无数据

图3 玛湖凹陷风城组典型井单井综合柱状图

Fig.3 Generalized column of the Fengcheng Formation of the typical well in Mahu Sag

与其他陆相页岩相比,风城组页岩具有黏土矿物含量低的特点。在碱湖沉积中心,黏土矿物含量很低(0.71%~4.38%,平均为1.86%),如FN4井、FN14井(表1图4);且在碱湖沉积中心,主要以蒙脱石发育为主,并发育R0型无序的伊/蒙混层和少量的伊利石(表2图5)。在边缘缓坡区黏土矿物含量明显增加,且分布范围比较大,平均为11.57%(2.6%~48.9%)(表1图4);主要发育伊/蒙混层、伊利石、绿泥石(C)、绿/蒙混层(C/S),蒙脱石的含量低于碱湖沉积中心(表2图5)。
图4 玛湖凹陷风城组黏土矿物含量分布

Fig.4 Distribution of clay minerals content of Fengcheng Formation in Mahu Sag

表2 玛湖凹陷风城组页岩黏土矿物相对含量

Table 2 Relative contents of clay minerals of the Fengcheng Formation shale, Mahu Sag

地区 井号 深度/m 蒙脱石/% 伊利石/%

伊/蒙混层

/%

绿泥石/% 高岭石/%

绿/蒙混层

/%

伊/蒙混层比

/%

碱湖沉积

中心

FN4

4 403.82~

4 582.69

(0~100)/75.8 (0~9)/6 (0~91)/18.2 / / / (90~100)/98
FN1426 4 061.8 7 12 79 1 1 / 5
FN126

4 097.9~

4 445.33

(0~100)/16.39 (0~36)/15.06 (0~94)/66.17 (0~10)/0.94 (0~10)/1.28 / (20~80)/51.36
边缘斜坡区 M51X

4 993.13~

4 998.97

/ (5~9)/7.5 (91~95)/92.5 / / / (90~100)/95
M54X

4 659.08~

5 169.87

/ (27~62)/43.73 (38~64)/54 (0~25)/2.27 / / (15~40)/21.82
M49

4 495.55~

4 820.9

/ (2~19)/7.79 (0~92)/41.34 (0~83)/17.28 / (0~86)/33.59 (10~95)/59.44
MY1

4 580.85~

5 027.59

(0~97)/4.16 (0~100)/36.11 (0~100)/41.26 (0~36)/6.87 (0~69)/6.63 (0~100)/4.16 (10~100)/54.22

注:(0~100)/75.8=(最小值—最大值)/平均值;“/”无数据

图5 玛湖凹陷风城组碱湖沉积中心(a)及边缘斜坡区(b)、(c)各黏土矿物含量分布

Fig.5 Distribution of clay minerals content in the alkaline lake sedimentary center (a) and the edge slope area(b),(c) of Fengcheng Formation in Mahu Sag

准噶尔盆地在石炭纪—早二叠世火山活动频繁,火山岩及火山碎屑物质的输入为蒙脱石提供了物质来源。在半干旱的气候条件、火山及热液作用的影响下,玛湖凹陷在风城组沉积时期形成碱湖。在碱性沉积环境下,火山物质等在脱玻化作用下转化为蒙脱石等矿物。泌阳凹陷核桃园组页岩中的蒙脱石来源与成因也与之相似2635

2.2 微观特征

通过对边缘斜坡区风城组泥页岩样品进行扫描电镜观察发现,伊/蒙混层以蜂窝状或不规则鳞片形态赋存,颗粒大小不均[图6(a),图6(b)];在矿物颗粒表面或粒间孔内发育弯曲片状的伊利石[图6(c)];玫瑰花状、针叶状的绿泥石多发育于颗粒之间[图6(d),图6(e)];也可在颗粒表面或粒间观察到蜂窝状、玫瑰花状的绿/蒙混层[图6(f)]。且在黏土矿物发育区域可观察到较为发育的自生石英等脆性矿物[图6(d),图6(e),图6(g)]。未在碱湖沉积中心样品中观察到较为明显的黏土矿物,这与X射线衍射分析显示的碱湖沉积中心黏土矿物含量低的结果一致。
图6 玛湖凹陷边缘斜坡区风城组黏土矿物镜下特征

(a)鳞片状伊/蒙混层(I/S),MH286井,4 569.9 m;(b)蜂窝状伊/蒙混层(I/S),MH57井,4 627.36 m;(c)弯曲片状伊利石(I),MY1井,4 709.1 m;(d)针叶状绿泥石及赋存在黏土矿物间的自生石英,MH286井,4 570.9 m;(e)玫瑰花状绿泥石及赋存在黏土矿物间的自生石英,MH286井,4 570.9 m;(f)玫瑰花状绿/蒙混层(C/S),MH286井,4 570.9 m;(g)粒状自生石英,M56X井,5 947.27 m;(h)自生白云石,MH57井,4 627.36 m;(i)页岩基质中的钾长石(氩离子剖光),FN3井,4 147.5 m

Fig.6 SEM of clay minerals of Fengcheng Formation in the edge slope zone in Mahu Sag

3 黏土矿物转化特征

根据前人23-25研究结果以及XRD全岩衍射、扫描电镜等实验结果发现,玛湖凹陷风城组黏土矿物含量、组合类型等具有分区带发育的特征,不同成岩环境下(强碱性、弱碱—弱酸性),黏土矿物动态演化具有明显差别,不应一概而论地看待玛湖凹陷黏土矿物发育特征,需分区带讨论其转化特征。

3.1 碱湖沉积中心黏土矿物转化

碱湖沉积中心流体碱性强(pH>9),在准同生—早成岩阶段,陆源碎屑进入沉积中心后,原始沉积蒙脱石、石英等矿物在碱性水体环境下发生溶解。蒙脱石在碱性流体环境中不稳定,容易在K+和HCO3 -存在的水体中转化为钾长石[转化反应式(1)]、含铁伊利石等硅酸盐矿物(图7)。随着埋深逐渐增大,有机质开始大量生烃,产生有机酸等酸性流体,但发育在碱湖沉积中心的碱性矿物会迅速与其发生中和反应,且当地层中酸性流体与在火山活动提供的硼等元素结合,在富钠、硅的流体环境下形成硅硼钠石等特殊碱性矿物,因此碱湖沉积中心的整体成岩环境一直呈强碱性25
图7 玛湖凹陷黏土矿物转化示意(矿物晶体结构引自文献[46])

注:钾长石是单斜结构,是由硅氧四面体和铝氧四面体连成的曲轴状双链,钾长石的基本结构是由4个四面体构成的四元环,四元环由4个不等效的位置T1(O)、T1(m)、T2(O)、T2(m)组成,Si、Al在这4个T位置上的分布决定着钾长石的结构状态,即钾长石中Si、Al的有序无序状态

Fig.7 Schematic diagram of clay minerals transformation in Mahu Sag(crystal structure of minerals is cite from Ref.[46])

强烈的火山活动为玛湖凹陷风城组泥页岩提供大量的火山碎屑物质,且火山碎屑等物质会转化生成大量的黏土矿物36,如在火山频发环境下发育的塞尔斯湖、柏哥利亚湖等其黏土矿物含量可达20%37-38。但玛湖凹陷碱湖沉积中心黏土矿物含量低至5%以下,主要是因为在强碱性的成岩环境下,蒙脱石等转化为更为稳定的自生硅酸盐矿物。前人23对玛湖凹陷碱湖中自生硅酸盐研究结果表明,玛湖凹陷发育的自生钾长石、钠长石生成原因之一是由黏土矿物在碱性条件下发生蚀变形成,但自生长石类矿物与黏土基质矿物之间的边界较为模糊,且黏土矿物到自生长石之间的过渡带其元素成分复杂,也进一步说明了黏土矿物可在碱性条件下向自生长石进行转化。
2.5 [ X 0.5 + ( A l 1.2 F e 0.3 3 + M g 0.5 ) S i 4 O 10 ( O H ) 2 ] ( 蒙脱 ) + 3 K + + 0.85 H 2 O + 1.25 H C O 3 - 3 K A 1 S i 3 O 8 ( 钾长 ) + H 4 S i O 4 + 1.5 M g C O 3 + 0.75 F e ( O H ) 3 + 0.45 H +
黏土矿物在强碱环境下的转化特征在其他碱性沉积盆地中也具有相似的特征。在美国塞尔斯湖(pH>9)沉积区域,也大量发育由碎屑蒙脱石等黏土矿物被大量溶蚀后生成自生钾长石等矿物37。中国非海相沉积盆地中方沸石成因研究中也发现,成岩方沸石主要由盐碱湖中火山玻璃、黏土或沸石前体的蚀变或在埋藏成岩阶段沉淀形成39。泌阳凹陷安棚地区核桃园组碱矿中发育的微晶纹层状自生方沸石成因之一是在早成岩过程中,黏土矿物在碱性流体环境下转化而成40-41。但与核桃园组不同的是,碱湖沉积中心的风城组中方沸石并不发育,主要是因为风城组存在较多的自生石英,方沸石与自生石英发生反应继续蚀变成自生长石类矿物[转化反应式(2)],但核桃园组碱矿发育区缺乏自生石英的发育,因此核桃园组的方沸石得以保留。当地层流体环境中Na+/H+值越高时,Si—Al则更易于与钠离子结合形成架状硅酸盐矿物(方沸石等),而不是层状硅酸盐矿物(蒙脱石等黏土矿物)4042
N a A l S i 2.6 O 7.2 H 2 O ( 方沸 ) + 0.4 S i O 2 = N a A l S i 3 O 8
( 钠长 ) + H 2 O
玛湖凹陷风城组泥页岩处于中成岩阶段,按照正常的黏土矿物转化序列,该沉积阶段蒙脱石已转化消失,应以伊利石、伊/蒙混层为主(图8)。但是根据风城组页岩黏土矿物特征,研究区出现了黏土矿物非正常转化的现象(表2)。泌阳凹陷核桃园组页岩中黏土矿物也出现相同的非正常转化的现象26图8)。产生这一现象的原因可能是碱湖中CO3 2-/HCO3 -的存在导致蒙脱石转化迟缓。随着温压的增加,蒙脱石八面体中的Al3+替换四面体中的Si4+过程中导致电荷失衡,此时周围流体中的K+会替换层间Ca2+、Mg2+、Na+等阳离子来维持电荷平衡,进而促进蒙脱石向伊利石等转化。但若在碱性环境下,置换出的Ca2+、Mg2+会迅速与地层流体中的CO3 2-/HCO3 -结合成碳酸钙或碳酸镁沉淀,CaCO3等沉淀会吸附在黏土矿物表面,从而阻碍流体与岩石的渗流通道,导致K+供应不足,延缓黏土矿物转化43。在对碱湖非海洋环境下化学沉淀过程研究中发现在碱性环境下仅需中等浓度(0.5~1 mmol/kg)的Ca2+即可沉淀形成球状CaCO3 44,且可溶性SiO2浓度升高会缩短CaCO3成核时间。通过对盐—碱湖沉积物与盐湖沉积物中离子组成进行调研后发现,盐—碱湖沉积盆地(玛湖凹陷风城组、泌阳凹陷核桃园组)中主要以CO3 2-/HCO3 -离子组合为主,而盐湖(柴达木盆地干柴沟组)则主要发育Cl-图9)。因此在玛湖凹陷碱湖沉积中心,蒙脱石转化的延缓作用明显,主要以蒙脱石为主、伊/蒙混层为辅的黏土矿物组合特征。
图8 中国不同沉积盆地黏土矿物发育特征(部分数据引自[47-51])

Fig.8 Development characteristics of clay minerals in different sedimentary basins in China(partial data is cited from Refs.[47-51])

图9 不同盆地沉积物中主要离子组成特征(数据引自文献[4352-53])

Fig.9 Ion composition characteristics in sediments from different basins(date is cited from Refs.[4352-53])

3.2 边缘斜坡区黏土矿物转化

玛湖凹陷风城组边缘缓坡沉积区水体盐碱度明显小于碱湖沉积中心,其碱性矿物欠发育,黏土矿物发育也明显不同于碱湖沉积中心。边缘缓坡区湖水具有水体浅、碱性弱等特点,且pH值受降水等外界条件的控制明显,所以在准同生—早成岩阶段对石英、黏土矿物等溶蚀作用减弱,因此早期蒙脱石等黏土矿物得以保存,且在边缘缓坡沉积区发育大量燧石45。随着有机质成熟度的增加,烃源岩在中成岩早期开始生烃,并产生大量有机酸等酸性流体,由于酸性流体的进入,使得沉积环境由弱碱性转换至弱酸性,不稳定矿物(长石、碳酸盐矿物等)发生溶蚀作用,钾长石的溶蚀为水体环境中输入大量K+。随着地层温压的升高,原始沉积的蒙脱石发生脱水、离子取代等反应,逐渐生成中间产物伊/蒙混层,最终形成伊利石(表2图7)。碳酸盐矿物等矿物被溶蚀,释放出Mg2+、Fe2+等离子,且Mg2+、Fe2+等离子会进入到蒙脱石层间,发生绿泥石化,形成赋存于颗粒表面或粒间的绿/蒙混层、绿泥石等矿物(表2图7)。在pH值升高以及二氧化硅存在的情况下,只需要适量浓度的Mg2+就可沉淀出大量结晶度较差的镁硅酸盐,如海泡石、滑石等44。有学者在边缘缓坡区页岩中发现了海泡石,且在海泡石晶体之间发育大量孔隙,为页岩油气提供了富集空间25
在弱碱性—弱酸性的沉积环境中,碎屑黏土矿物几乎不会或者很少会转化成钾长石等自生硅酸盐,但会随着地层温度和压力的增加,发生伊利石化、绿泥石化等反应,并生成大量自生石英[图6(d),图6(e),图6(g)]。且在边缘缓坡区,其CO3 2-/HCO3 -含量低于碱湖沉积区,在黏土矿物转化过程中并不会形成大量的碳酸钙吸附在黏土矿物表面从而延缓其转化,所以边缘斜坡区黏土矿物组合符合热力学驱动下的黏土矿物转化的特征,即以伊/蒙混层和伊利石为主,并存在绿泥石及绿/蒙混层,不含或者含极少的蒙脱石(表2)。

4 碱湖各沉积区黏土矿物动态演化机制

在前人2325-27对风城组页岩成岩作用研究的基础上,结合本文对碱湖沉积中心及边缘斜坡区黏土矿物含量及转化特征研究,明确强碱性和弱碱—弱酸性的成岩环境中黏土矿物成岩转化特征,并总结了不同沉积区域黏土矿物动态演化机制。

4.1 不同成岩环境下黏土矿物动态演化机制

在准同生—早成岩阶段,玛湖凹陷碱湖沉积中心碱性强,大量蒙脱石等黏土矿物在强碱性的条件下发生蚀变,转化成自生硅酸岩矿物。与此同时,边缘斜坡区的水体呈弱碱性,黏土矿物并未发生大规模的蚀变,多数由火山碎屑物质转化而成的蒙脱石等黏土矿物在该区域得以保留(图10)。
图10 碱湖不同沉积区黏土矿物动态转化模式图

Fig.10 Dynamic transformation patterns of clay minerals in different sedimentary zones in alkaline lake

随着地层温度和压力的升高,在早成岩—中成岩阶段,有机质开始生烃,并伴随大量有机酸等酸性物质产生并进入地下流体当中。但在碱湖沉积区,有机酸等酸性流体会立刻被碱性矿物所中和,成岩环境整体呈强碱性。按照黏土矿物正常转化序列,当地层温度达到60 ℃,蒙脱石开始向伊利石转化,但在碱湖沉积区并未发现大量转化而来的自生伊利石[表2图5(a)],分析原因主要有2方面:一方面,碱湖沉积区蒙脱石多在强碱性条件下转化生成钾长石等矿物,仅有极少部分保存下来,导致其向伊利石转化困难;另一方面,在强碱水体中,CO3 2-/HCO3 -含量较高,蒙脱石溶解蚀变等反应置换出的Ca2+会迅速与地层流体中的CO3 2-/HCO3 -结合成CaCO3,并黏附在蒙脱石等黏土矿物表面,阻挡K+供应通道,导致蒙脱石伊利石化延缓2643。边缘斜坡区碱性矿物不发育,酸性流体产生后,成岩环境逐渐转化为弱酸性,并发生不稳定矿物溶蚀现象,提供了黏土矿物转化所必须的K+、Mg2+、Fe2+等。在准同生—早成岩阶段保存下来的蒙脱石等矿物在该种成岩水体环境下,当地层温度和压力达到反应门限则发生伊利石化和绿泥石化,因为边缘斜坡区碱性较弱,其CO3 2-/HCO3 -含量低,对黏土矿物转化延迟作用不明显。

4.2 黏土矿物转化对页岩油气的控制作用

黏土矿物是陆相页岩的重要组成部分之一,其转化机制也对页岩油气的勘探具有一定的控制作用,主要体现为以下2个方面:一方面,黏土矿物在转化过程中会产生大量的自生石英等硅质矿物,风城组黏土矿物转化形成的自生石英其自形形态较好,在扫描电镜下多呈微米级粒状,偶见六方柱状的形态嵌在黏土矿物中[图6(d),图6(e),图6(g)]。且在碱湖沉积中心,蒙脱石等黏土矿物在强碱的环境下转化为自生钾长石等硅酸盐矿物[图6(i)]。风城组页岩中的自生长英质矿物大多以粒状形态嵌在黏土矿物间,且经过多期成岩作用与黏土矿物紧密黏结在一起252754,所以该类自生石英、长石等脆性矿物可以提高页岩储层可压性。郭佩等27研究也表明风城组自生石英和长石类矿物的来源之一是原始黏土矿物在成岩转化过程中形成的,且盐碱度越高,自生长石含量则越高。另一方面,黏土矿物在成岩转化过程形成的黏土矿物收缩缝可以有效地增加次生孔隙,有利于后续页岩油气开发55-56。黏土矿物转化形成的孔隙、斜长石及白云石受成岩作用形成的溶蚀孔隙等主要贡献中孔,随着埋深及有机质成熟度的增加,中孔的比表面积及孔体积也随之增大16

5 结论

(1)准噶尔盆地玛湖凹陷风城组黏土矿物具有“分区带”分布的特征,在碱湖沉积中心,黏土矿物含量较低(最高为4.38%,最低为0.71%,平均为1.86%),且以蒙脱石及R0型无序的伊/蒙混层为主。边缘缓坡区黏土矿物含量明显增加(最高为48.9%,最低为2.6%,平均为11.57%),以伊/蒙混层、伊利石、绿泥石为主。
(2)在碱湖沉积中心,火山物质蚀变而成的黏土矿物及陆源碎屑黏土矿物(蒙脱石等)在强碱性的成岩环境下发生溶蚀,转化形成钾长石等自生硅酸盐矿物。蒙脱石在转化过程中释放Ca2+,在富集CO3 2-/HCO3 -碱性流体环境中生成CaCO3沉淀吸附在黏土矿物表面从而堵塞K+运移通道,延缓蒙脱石向伊利石转化。
(3)边缘斜坡区成岩环境经历了弱碱—酸性的转变,原始沉积的黏土矿物在准同生—早成岩期未发生溶蚀转化,得以保留。在热力学驱动下,蒙脱石在富K+条件下转化成伊利石,在富Fe2+、Mg2+条件下转化为绿泥石。
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