Reservoir characteristics of marine-continental transitional shale and gas-bearing mechanism:Understanding based on comparison with marine shale reservoir

  • Taotao CAO , 1 ,
  • Mo DENG 2 ,
  • Juanyi XIAO 1 ,
  • Hu LIU 3, 4 ,
  • Anyang PAN 2 ,
  • Qinggu CAO 2
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  • 1. School of Earth Sciences and Spatial Information Engineering,Hunan University of Science and Technology,Xiangtan 411201,China
  • 2. Wuxi Research Institute of Petroleum Geology,SINOPEC Petroleum Exploration&Production Research Institute,Wuxi 214126,China
  • 3. Sichuan Key Laboratory of Shale Gas Evaluation and Exploration,Chengdu 610091,China
  • 4. Sichuan Institute of Geological Survey,Chengdu 610072,China

Received date: 2022-06-24

  Revised date: 2022-08-27

  Online published: 2023-02-07

Supported by

The National Natural Science Foundation of China(41802163)

the Natural Science Foundation of Hunan Province, China(2021JJ30240)

the Excellent Youth Fund of Hunan Education Department(21B048)

the Open Fund of Hunan Key Laboratory of Shale Gas Resources Utilization(E22218)

Abstract

Transitional shale gas layers are widely distributed in China, whereas no significant exploration breakthrough has been made so far. In this paper, characteristics and gas-bearing mechanisms of transitional shale gas reservoirs were investigated and analyzed in detail, aiming to clarify shale gas accumulation mechanism of transitional facies and provide theoretical support for the selection of favorable intervals. Transitional shale gas layer is characterized by thin single layer thickness, rapid lithological change and poor kerogen type. Due to the limited numbers of OM nanometer-scale pores, shale pore space is dominated by pores and fractures related to clay minerals. The measured gas content is well consistent with theoretically calculated gas content for marine organic-rich shales. However, actual measured gas content is far lower than the theoretical calculated gas content for transitional shale gas reservoir. The main mechanism are summarized to be that (1) high hydrocarbon expulsion efficiency of “sandwich” space structure of sandstone-shale-coal association gives rise to most natural gas into nearby sandstone, and (2) high water saturation results in insufficient storage space for free gas in shale reservoir. Unlike marine shale gas, natural gas in transitional shale reservoir is primarily dominated by adsorbed gas in kerogen, and free gas is generally low. The favorable lithofacies types are organic-rich siliceous/calcareous shales. Multiple layers of siderite-bearing shale/siderites are developed vertically and are continuously distributed horizontally in transitional strata, particularly in flat-lagoon facies. It is easy to form “micro-trap” to storage gas in siderite-bearing shale, and siderite-bearing shale has strong sealing properties due to low porosity, low permeability and high breakthrough pressure. This property can form overpressure and trap shale gas inside the shale, which provides a new research perspective for the optimization of vertical favorable intervals, as well as exploration breakthrough in transitional shale gas. Further research should strengthen the systematic sedimentological study of transitional facies, reveal shale gas occurrence state and dynamic transformation, optimize favorable interval evaluation system and clarify the feasibility of coal-measure gas commingled production.

Cite this article

Taotao CAO , Mo DENG , Juanyi XIAO , Hu LIU , Anyang PAN , Qinggu CAO . Reservoir characteristics of marine-continental transitional shale and gas-bearing mechanism:Understanding based on comparison with marine shale reservoir[J]. Natural Gas Geoscience, 2023 , 34(1) : 122 -139 . DOI: 10.11764/j.issn.1672-1926.2022.08.015

0 引言

我国发育有海相、海陆过渡相和陆相3类富有机质泥页岩1。近年来,我国页岩气的勘探主要围绕南方古生界海相页岩层系,取得了重大勘探突破和实现了商业化生产,并形成了海相页岩气“三元”富集理论2。为突破当前单一海相龙马溪组页岩气层组,广泛发育的海陆过渡相页岩层组引起了国内学者的重视3。钻井显示海陆过渡相页岩气呈现“气测良好”“解吸气含量高”等特点。四川盆地明1井在龙潭组试采产气(3.02~3.85)×104 m3/d4,DYS1井龙潭组平均含气量为2.02 cm3/g5,黔西地区金沙参1井龙潭组平均含气量为2.93 cm3/g,湘中地区湘页1井龙潭组解吸气含量为0.16~1.41 cm3/g,鄂尔多斯盆地二叠系平均含气量为0.36~0.48 cm3/g6-8,南华北盆地牟页1井山西组和太原组平均含气量分别为1.82 cm3/g和2.64 cm3/g9。这些钻井均揭示海陆过渡相页岩储层普遍具有一定的含气量,但现有评价井实测含气量普遍不高,勘探前景有待深入认识。
对比海相页岩气成藏理论取得的显著进展,我国海陆过渡相页岩气的研究主要集中在地球化学特征和储层物性等方面。前人研究表明海陆过渡相泥页岩呈有机质以腐殖型为主,TOC含量变化快,黏土矿物及不稳定矿物(如碳酸盐矿物)等占比较高,基质孔渗较低,储集空间以矿物转化孔缝为主和有机质孔欠发育等特征;页岩吸附能力变化大,受黏土矿物和有机质含量控制,游离气/吸附气比例偏低。不同地区和沉积背景泥页岩中优势孔隙构成存在差异,目前存在3种观点:一是由黏土矿物主导10;二是由黏土矿物和碳酸盐矿物共同贡献11;三是由有机质和黏土矿物共同贡献12。尽管黏土矿物孔缝属于优势孔隙类型,但现场解吸气量主要受TOC含量控制,与矿物组成、含量、成熟度和孔隙度关联性差613-18。有限的有机质孔隙空间以何种方式储集页岩气仍不清晰。因此,需要进一步明确海陆过渡相页岩气的赋存状态和富集条件,探索高含气层段的评价标准。
基于海陆过渡相泥页岩组成的强非均质性、高黏土含量、高含水饱和度和压力系数小等特点10,本文将系统总结我国海陆过渡相泥页岩的储层特征和含气性主控因素,并以川东地区龙潭组为例,明确海陆过渡相页岩含气量较低的内在因素,揭示页岩气赋存状态、赋存机制及对页岩含气量的影响,寻找高含气层段及其形成机制,以期为海陆过渡相页岩气有利层段评价和优选提供理论支撑,助力我国海陆过渡相页岩气的勘探突破。

1 我国海陆过渡相泥页岩分布特征

我国海陆过渡相泥页岩主要分布在石炭系—二叠系(图1),在华北陆块主要分布在鄂尔多斯盆地、南华北盆地和沁水盆地本溪组、太原组和山西组,在华南陆块主要分布在龙潭组。华北陆块二叠系泥页岩分布面积为10.7×104 km2,厚度为10~60 m,单层最大厚度约为40 m。华南陆块龙潭组泥页岩分布面积为8.6×104 km2,厚度约为7~150 m,最大厚度为50 m,以四川盆地龙潭组页岩气前景最好19。华北陆块的太原组和华南陆块的龙潭组属于典型的海陆过渡相沉积,华北陆块的山西组逐渐过渡到陆相沉积,而华南陆块的大隆组和吴家坪组则呈现海相硅质建造的特点20
图1 我国页岩气盆地/地区富有机质页岩层位及页岩气勘探进展统计

Fig.1 Organic-rich shale gas layers and shale gas exploration progress statistics in shale gas basins/regions in China

2 海陆过渡相泥页岩储层特征

2.1 普遍具有较好的生烃条件

对我国典型钻井海陆过渡相泥页岩的有机质特征进行总结(表1),表明鄂尔多斯盆地、沁水盆地和南华北盆地太原组干酪根δ13C值分布在-24.3‰~-23.4‰之间,山西组干酪根δ13C值分布在-25.0‰~-22.3‰之间20,川东地区龙潭组干酪根δ13C值为-24.4‰~-22.9‰521,黔西地区龙潭组干酪根δ13C值为-23.9‰~-22.8‰22,湘中地区龙潭组干酪根δ13C值为-28.9‰~-25.8‰23。总体而言,海陆过渡相泥页岩的干酪根类型以III型为主,仅在湘中地区为II1—II2型。
表1 我国典型地区海陆过渡相泥页岩有机质特征

Table 1 Organic characteristics of transitional mud shales in typical areas of China

地区 层位 泥页岩厚度/m 有机质类型 TOC/% R O/% 数据来源
南华北 太原组 22~169 II2—III (0.92~5.06)/2.32 (3.46~3.67)/3.54 牟页1井(文献[25])
山西组 50~100 II2—III (0.50~5.10)/1.90 (3.46~3.58)/3.53
鄂尔多斯盆地 太原组 43 III (0.16~12.01)/3.43 (0.97~3.19)/1.39 LX1井等(文献[26])
山西组 24 III (1.73~3.09)/2.30 (1.12~1.35)/1.25 SSL31和SSL33井(文献[6])
沁水盆地 太原组 20~50 III (0.92~7.21)/2.91 (1.72~2.35)/2.00 文献[20
山西组 10~30 III (0.91~5.91)/2.48 (1.60~2.50)/2.00
黔北 龙潭组 44.65 III (1.30~9.58)/4.20 (2.01~3.26)/2.53 金沙参1井(文献[18])
川东 龙潭组 40 III (0.57~18.37)/3.23 (1.96~2.40)/2.22 东页深1井(文献[5])
湘中 龙潭组 125 II1—II2 (0.49~8.94)/2.20 (2.40~2.73)/2.56 龙2015-D3井(文献[17])
川东涪陵 龙马溪组一段 38 I—II1 (2.17~5.25)/3.76 (2.22~2.89)/2.58 JY-1井(文献[27])

注:(0.92~5.06)/2.32=(最小值—最大值)/平均值

华北陆块太原组泥页岩的TOC含量普遍较高,南华北地区、沁水盆地、鄂尔多斯盆地太原组泥页岩TOC含量平均值分别为2.32%、2.91%和3.43%;山西组泥页岩TOC含量平均值介于1.90%~2.48%之间。黔北和川东地区龙潭组泥页岩的TOC含量平均值分别为4.20%和3.23%,湘中地区略低,平均值为2.20%,皖南地区平均值为2.43%24。整体上看,黔北和川东地区龙潭组和华北地块山西组泥页岩的TOC含量较高,与涪陵龙马溪组页岩相当,其他层系或地区海陆过渡相泥页岩的TOC含量偏低。
华北陆块海陆过渡相泥页岩的热成熟度变化范围较大,R O值从0.97%变化到3.67%,南华北地区山西组和太原组泥页岩的R O平均值高达3.53%~3.54%,鄂尔多斯盆地临兴地区太原组泥页岩的R O值最低为0.97%(表1)。华南陆块龙潭组泥页岩的热成熟度变化较小,热成熟度较为集中,平均值为2.22%~2.56%。我国海陆过渡相泥页岩的R O值一般大于1.50%,整体处于高成熟—过成熟阶段。

2.2 黏土矿物孔缝为主的储集空间

海陆过渡相泥页岩中发育了丰富的微纳米级孔—缝体系,主要由无机矿物孔、有机质孔和微裂缝3类储集空间组成28。本文以川东地区龙潭组为例,结合文献中华北陆块山西组泥页岩孔隙发育特征,阐述海陆过渡相泥页岩中孔隙类型及发育特征。
川东地区龙潭组泥页岩中存在5种与有机质相关的孔缝,分别为生物结构孔、热解气孔、固体沥青孔、内生裂隙和边缘收缩缝。生物结构孔是有机质继承其植物先质的原始细胞腔体结构而呈现的具有一定结构形态的孔隙,常赋存于结构镜质体内部[图2(a)],直径为数微米至数十微米。腐殖型有机质以生气为主、难以改变自身结构,多形成一些圆形或不规则状热解气孔[图2(b)],连通性普遍较差。页岩中还发育少量的固体沥青孔,是液态烃裂解成气后残留在固体沥青内的孔隙[图2(c)],该类孔隙整体较少,与较低的腐泥质组分有关。腐殖型有机质生烃之后体积收缩,在有机质颗粒边缘发育收缩缝[图2(d)],在有机质内部形成内生裂隙[图2(e)]。对比海相页岩中异常发育的蜂窝状有机质孔[图2(f)],海陆过渡相泥页岩中有机质孔整体发育较差。相似的情况也出现在华北陆块山西组等泥页岩中,但在不同岩相中有机质孔的形态和发育程度存在一定的差异29,钙质硅质岩相中有机质孔发育最好,气泡状有机质孔成串分布,其次是硅质黏土质页岩,黏土页岩相中发育极少的有机质孔30-32。这与不同岩相中有机质的类型变化有关,有机质由陆源高等植物向混源转变,有机质孔发育程度变好33。尽管海陆过渡相中有机质孔有一定程度的发育,但有机质孔的体积占比一般低于20%34-35
图2 川东地区龙潭组泥页岩有机质孔发育特征

(a)生物结构孔,龙潭组;(b)干酪根热解气孔,龙潭组;(c)固体沥青孔隙,龙潭组;(d)有机质边缘收缩裂缝,龙潭组;(e)有机质内生裂隙,龙潭组;(f)固体沥青有机质孔,龙马溪组;(g)粒内溶孔,龙潭组;(h)黄铁矿晶间孔,龙潭组;(i)硬性颗粒边缘缝,龙潭组;(j)沥青充填边缘缝,龙潭组;(k)黏土矿脱收缩孔,龙潭组;(l)黏土矿物收缩孔缝,龙潭组

Fig.2 Organic matter pore development characteristics in Longtan Formation mud shales in eastern Sichuan Basin

川东地区龙潭组泥页岩中发育了大量的无机矿物孔,如黏土矿物孔、粒内溶蚀孔、晶间孔和微裂缝等。溶蚀孔主要发育在易溶矿物颗粒内及边缘,直径一般为数十纳米至微米级[图2(g)]。在草莓状黄铁矿集合体颗粒之间形成晶间孔[图2(h)],由于缺乏固体沥青难以形成黄铁矿/有机质复合体。在石英和方解石等硬性颗粒与黏土矿物或有机质的接触面常发育边缘孔[图2(i),图2(j)]。黏土矿物中发育大量的层间微孔隙,宽度为纳米级,长度为微米级[图2(k)]。在成岩演化过程中,蒙脱石向伊利石转化,伴随脱水和体积减小,在伊利石层内产生收缩缝[图2(l)]。统计表明鄂尔多斯盆地山西组页岩储集空间也呈现以黏土矿物粒间孔为主,占比约为50%,其次是矿物粒内孔和微裂缝等,无机孔在不同岩相中均较为发育3134。由此可见,海陆过渡相泥页岩的储集空间以黏土矿物孔缝为主,有机质孔和脆性矿物孔欠发育。
海相和海陆过渡相泥页岩的氮气吸附实验得出的DFT孔径分布具有显著差异。LY1井海相页岩的孔隙主要为微孔,峰值在1.30~1.45 nm之间,随着TOC含量的增加,微孔数量显著增加,高黏土矿物含量的泥页岩中微孔数量很少,且在中孔范围内孔隙数量也少于富有机质页岩[图3(a)],反映了海相泥页岩的储集空间是以有机质孔为主导的孔隙系统。HSX1井龙潭组泥页岩的孔径分布既包含一定的微孔也含有相当数量的中孔,微孔的数量少于海相页岩,但中孔的数量多于海相页岩[图3(b)],反映了海陆过渡相泥页岩的孔隙系统整体以中孔为主导。海陆过渡相的微孔峰值在1.34 nm左右,可能来自有机质,但由于分辨率的原因,在扫描电镜下这部分孔隙可能不能被有效观测到,随着TOC含量的增加,微孔的数量没有明显变化;中孔主要来自黏土矿物的贡献,相近的黏土矿物含量,泥页岩中孔的变化幅度较近,但对于低TOC含量低黏土矿物含量的泥页岩,具有明显低的中微孔数量。孔径分布特征也佐证了扫描电镜观测到的海相以有机质孔为主,海陆过渡相以黏土矿物孔为主要孔隙类型的观点。
图3 川东地区龙马溪组(a)和龙潭组(b)泥页岩孔径分布特征

Fig.3 Pore size distribution characteristics of Longmaxi Formation(a) and Longtan Formation(b) mud shales in eastern Sichuan Basin

2.3 储层物性控制因素复杂多变

华北陆块南华北地区山西组和太原组泥页岩的孔隙度平均值仅为2.10%~2.30%,鄂尔多斯盆地太原组和沁水盆地山西组泥页岩孔隙度高于4%(表2)。华南陆块(包括黔北地区、川东地区、湘中地区)龙潭组泥页岩的孔隙度普遍高于4%,与涪陵地区龙马溪组页岩相当。南华北盆地较低的孔隙度可能与其很高的成熟度有关,随着成熟度增加至过成熟阶段,有机质孔不发育,但无机矿物孔经历更大埋深致使孔隙度明显减少。无论是华北陆块还是华南陆块,海陆过渡相泥页岩的渗透率均较低,其平均值为(0.002 7~0.69)×10-3 μm2,远低于龙马溪组泥页岩,可能与较高的黏土矿物和较少的层理缝/微裂缝有关。研究表明,黏土矿物更易受压实的影响,特别是蒙脱石转化为伊利石过程中新生成的伊利石会堵塞孔隙从而降低渗透率36
表2 典型地区海陆过渡相泥页岩储集物性特征

Table 2 Physical properties of transitional mud shales reservoir in typical areas

地区 层位 孔隙度/% 渗透率/(10-3 μm2 数据来源
南华北 太原组 (0.40~4.50)/2.10 (0.001 2~0.11)/0.007 5 牟页1井、郑西页1井(文献[239])
山西组 (0.30~8.80)/2.30 (0.023~0.92)/0.045
鄂尔多斯盆地 太原组 (0.50~10.00)/4.70 (0.000 5~0.62)/0.037
山西组 (0.25~4.85)/2.30 (0.01~0.10)/0.04 Y106井等(文献[40])
沁水盆地 太原组 (1.65~4.67)/3.47 (0.22~1.92)/0.69 文献[20
山西组 (2.15~695)/4.21 (0.043~4.55)/0.15
黔北 龙潭组 (0.53~9.22)/4.24 (0.000 24~0.012)/0.002 7 HV-2井等(文献[18])
川东 龙潭组 (1.13~9.00)/5.53 <0.10 平均为0.015 东页深1井(文献[5])
湘中 龙潭组 (0.67~14.74)/4.34 <0.045 大部分<0.006 5 龙2015-D3井(文献[17])
川东涪陵 龙马溪组 (1.17~7.98)/4.61 (0.002~335.21)/23.79 JY-1井(文献[41])

注:(0.40~4.50)/2.10=(最小值—最大值)/平均值

海陆过渡相泥页岩有机质特征的差异和矿物组成的变化会导致储层空间的变化和控制因素更为复杂。南华北盆地山西组—下石盒子组、鄂尔多斯盆地山西组泥页岩的孔体积主要由黏土矿物提供1028,南华北盆地下二叠统泥页岩孔隙空间则主要由黏土矿物和碳酸盐矿物所贡献11,湘中地区龙潭组泥页岩的孔隙度的主控因素为黏土矿物含量和TOC含量37,可能与该区相对较好的有机质类型有关。由于干酪根类型较差,有机质在高热演化阶段基本上很少发育有机质孔,由此可推测多数海陆过渡相泥页岩中有机质丰度和热演化程度对有机质孔隙和总孔隙度无明显的控制作用38。总的来讲,不同地区海陆过渡相泥页岩的储集空间均突出了黏土矿物的主体贡献地位,表明黏土矿物孔缝系统决定了海陆过渡相页岩的孔隙度大小。

2.4 具有较好的可改造性

泥页岩主要矿物组成为黏土矿物、石英、碳酸盐矿物、长石和黄铁矿等,不同矿物含量的差异会影响储层物性和压裂改造效果。表3列出了我国不同地区海陆过渡相泥页岩的矿物组成,四川盆地和鄂尔多斯盆地海陆过渡相泥页岩中黏土矿物含量平均值接近50%;沁水盆地、南华北盆地和黔北地区海陆过渡相泥页岩的黏土矿物含量平均值达到36%~49.01%;湘中地区海陆过渡相泥页岩的黏土矿物含量均值仅为38%左右。总的来说,海陆过渡相泥页岩中黏土矿物含量基本在50%以下,指示泥页岩具有较高的脆性指数,利于压裂改造。
表3 典型地区海陆过渡相泥页岩矿物组成

Table 3 Mineral composition of transitional mud shales in typical areas

地区 层位 石英/% 黏土矿物/% 碳酸盐矿物/% 长石/% 黄铁矿/% 数据来源
南华北 太原组 (2~52)/35 (1~64)/36 (1~95)/25 (0~7)/3 平均4 牟页1井(文献[3])
山西组 (21~59)/45 (21~70)/46 (1~18)/4 (0~19)/4 平均1

鄂尔多斯

盆地

太原组 (2~92.4)/44.4 (3.1~98)/49.1 (4.5~18)/9 (0~1.9)/0.2 (0~6.8)/0.2 SSD1井(文献[42])
山西组 (6~77)/38.7 (23~87)/57.7 少量 (0~3)/1.43 (0~3)/0.7 Y108井等(文献[14])
沁水盆地 太原组 (27.8~43.5)/35.16 (32~63.4)/49.01 (0~30.9)/4.1 (0~3.6)/2.7 (0.7~15.2)/4.62 ZK03-2井等(文献[43])
山西组 (26.2~37.5)/32.47 (38.5~42.7)/41.33 (3.8~19.5)/12.23 (1.7~2.9)/2.63 (0~20.3)/7.2
黔北 龙潭组 (7.63~48.2)/28.34 (19.52~77.78)/46.41 少量 (0~26.91)/8.55 (0~35.51)/7.25 HV-2井等(文献[44])
川东 龙潭组 (0.3~71.9)/22.1 (6.2~90.6)/48.3 (0.2~82)/13.9 (0~5.1)/1.84 (0.1~30.1)/8.37 东页深1井(文献[5])
湘中 龙潭组 (9~61)/33 (5~68)/38 (1~44)/18 (0~15)/3.25 (0~27)/3.19 文献[23
川东涪陵 龙马溪组 (18.4~70.6)/37.3 16.6~62.8/40.5 (7.5~15.0)/11.3 (3.2~15.0)/9.3 (2.8~4.8)/3.5 文献[40

注:(2~52)/35=(最小值—最大值)/平均值

高产页岩气田的压裂开采条件需要满足弹性模量大于2.0 GPa,泊松比小于0.25。前人45研究表明川南筠连地区龙潭组泥页岩杨氏模量分布于9.89~15.30 GPa之间,泊松比分布于0.16~0.17之间,鄂尔多斯盆地山西组泥页岩杨氏模量分布于20~40 GPa之间,泊松比分布于0.20~0.27之间28。川东地区LS1井、DY3井等龙潭组泥页岩的动态杨氏模量均大于10 GPa,最高达40 GPa(图4)。这些岩石力学数据表明了我国海陆过渡相泥页岩储层具有较好的可压裂性,利于页岩气储层的改造开发。
图4 川东地区龙潭组泥页岩动态弹性模量

Fig.4 Dynamic elastic modulus of Longtan Formation shale in eastern Sichuan Basin

2.5 总含气量相对较低

含气量是页岩气选区评价、资源量和储量计算、产能预测和气藏评价的重要内容,也是决定页岩气藏能否商业开采的主要因素。表4显示南华北盆地尉参1井山西组和太原组页岩气总量分别为0.17~5.48 cm3/g和0.24~2.86 cm3/g。鄂尔多斯盆地山西页1井太原组页岩总含气量均值为1.64 cm3/g;DJ51井和Y313井山西组页岩解吸气含量均值为1.58 cm3/g20,Y313井山西组页岩总含气量为0.17~4.05 cm3/g14。沁水盆地Y1井山西组和太原组页岩总含气量为0.71~4.41 cm3/g。黔北地区西页1井龙潭组页岩总含气量为1.4~19.6 cm3/g。川东地区东页深1井潭组页岩总含气量为0.5~8.78 cm3/g。湘中地区龙2015-D 3井龙潭组页岩现场解吸气量为0.5~2.35 cm3/g。整体上看,华南陆块含气潜力略优于华北陆块,与其较大的孔隙空间、较高的TOC含量和较强的吸附性能有关(表4)。但无论是华北陆块还是华南陆块,海陆过渡相页岩总含气量均显著低于涪陵地区海相龙马溪组页岩。对于含气性较差的海陆过渡相页岩储层,如何寻找其富集的甜点层段,将成为海陆过渡相页岩气勘探突破的关键。
表4 典型地区海陆过渡相泥页岩含气特征

Table 4 Gas-bearing characteristics of transitional mud shales in typical areas

地区 层位 岩性 TOC/% 解吸气/(cm3/g) 损失气/(cm3/g) 总含气量/(cm3/g) 兰式体积/(cm3/g) 数据来源
南华北 太原组 灰黑色炭质页岩 均值3.22 0.16~1.35 0.009~4.13 0.17~5.48 /

尉参1井

(文献[9])

山西组 灰黑色炭质页岩 均值2.72 0.13~1.67 0.11~1.20 0.24~2.86 /
鄂尔多斯盆地 太原组 黑色页岩 (0.36~2.48)/1.92 (0.96~1.71)/1.03 (0.12~0.26)/0.17 (1.17~1.94)/1.64 (0.28~0.76)/0.51

山西页1井

(文献[46])

山西组 暗色泥岩 (0.93~11.93)/2.93 (0.15~3.36)/1.30 (0.01~0.79)/0.27 (0.17~4.05)/1.54 (0~3)/1.43

Y313井

(文献[14])

沁水

盆地

太原组

粉砂质泥岩

黑色炭质泥岩

(1~11)/3.73 (0.56~3.98)/1.17 0.15~0.43 0.71~4.41 (0.36~0.68)/0.62

Y1井等

(文献[47])

山西组
黔北 龙潭组

粉砂岩—炭质

页岩

(0.4~17.85)/4.25 (1.24~9.42)/6.65 0.16~10.18 1.4~19.6 (1.88~8.8)/4.51

西页1井

(文献[13])

川东 龙潭组 灰黑色泥岩 (0.57~18.37)/3.23 (0.24~2.77)/0.77 (0.32~6.01)/1.25 (0.5~8.78)/2.02 (0.74~6.83)/2.94

东页深1井

(文献[5])

湘中 龙潭组

粉砂质泥岩—

暗色泥岩

0.49~18.94

0.5~2.35

/1

/ / (1.25~2.99)/1.96

龙2015-D3井

(文献[17])

川东

涪陵

龙马

溪组

含粉砂泥岩—

炭质笔石页岩

均值3.58 / / (2.30~8.85)/4.30 (2.42~3.51)/2.87

JY1井

(文献[48-49])

注:(0.36~2.48)/1.92=(最小值—最大值)/平均值;/为没有

3 海陆过渡相页岩气成藏机理

3.1 不同岩性空间配置对页岩气成藏的影响

海陆过渡相地层形成了页岩气、煤层气和砂岩气等多种类型混合的天然气叠置系统。多岩性组合对泥页岩排烃过程和气体赋存状态都有明显影响。海相页岩连续厚度一般大于30 m[图5(a)],较大的厚度不利于液态烃排出而部分滞留在页岩层内[图5(b)]。随着热演化程度的增加,滞留的液态烃裂解成气态烃和多孔固体沥青,生成的气态烃赋存在多孔固体沥青中,致使海相页岩层中天然气含量较高,往往能够形成高产页岩气藏41
图5 川东地区海相[(a)、(b)]和海陆过渡相[(c)、(d)]页岩岩石组合类型及页岩气成藏模式(据文献[4151]修改)

Fig.5 Rock combination types of marine[(a),(b)] and transitional[(c),(d)] shale and shale gas accumulation models in eastern Sichuan Basin(modified according to Refs.[4151])

海陆过渡相岩性组合在垂向上呈泥岩、煤层和砂岩/灰岩交替分布,以川东地区SY1井为例,砂岩与页岩—煤组合呈互层关系[图5(c)],形成“三明治”式的岩性空间结构,普遍较薄的页岩导致生成的页岩气发生近距离、有效地运移至临近砂岩中[图5(d)]。因此,海陆过渡相地层岩性致使页岩具有较高的排烃效率,导致页岩中滞留气含量低,这是海陆过渡相页岩含气量较低的重要机制之一。川东地区SY1井龙潭组煤层和临近泥页岩含气量分别达到18.5 cm3/g和4.08 cm3/g,远离煤层的泥页岩含气量低于2 cm3/g,说明煤层可能对邻近泥页岩贡献了一定量的游离气550。因此,海陆过渡相天然气的勘探不仅要侧重于页岩气,还需关注煤层气与致密砂岩气。

3.2 气体赋存状态及对页岩气成藏的影响

海相高产页岩气主要是来自于游离气的贡献,其平均含量为66%52。海陆过渡相泥页岩中吸附气和游离气所占比例存在较大的争议。前人53研究认为海陆过渡相页岩储层以吸附态天然气为主,占比为50%~85%。张吉振等54计算川南龙潭组页岩气含量,吸附气含量为1.88~4.70 cm3/g,游离气含量为0.60~1.34 cm3/g。川东地区DYS1井和SY1井页岩解吸气含量也表明海陆过渡相页岩气以吸附气为主,较高的黏土矿物和有机碳含量能够吸附较多的甲烷气体。海陆过渡相页岩气的赋存状态与含量的高低显著不同于海相页岩气,与海陆过渡相岩性空间结构和泥页岩储集空间大小密切相关。海相龙马溪组页岩中有机质孔隙异常发育,不仅是吸附气的赋存场所,也是游离气主要储集场所[图6(a)]。游离气/吸附气比例随着TOC含量的增加而增加,这是由于大于3 nm的有机质孔主要储集游离气而非吸附气55。由于含气充足,尽管黏土矿物孔缝大部分表面被水分占据,但仍能赋存一定的游离气[图6(b)]。由此可见,海相泥页岩中无论是干酪根还是矿物孔中均富集大量的页岩气。相比较而言,海陆过渡相页岩中有机质孔发育差,主要为表面强吸附的甲烷气体,有机质孔对游离气的储存很少[图6(c)]。页岩孔隙空间主要为黏土矿物孔缝,也是游离气的主要储存空间。在实际地质条件下,“三明治”式岩性空间导致部分游离气的逸散以及黏土矿物中很高的含水饱和度致使游离气有效储集空间减少[图6(d)]。因而,海陆过渡相页岩中游离气含量普遍较低,形成以吸附气为主的欠饱和页岩气藏。
图6 海相[(a)、(b)]和海陆过渡相页岩[(c)、(d)]中有机质孔与无机孔气体赋存状态

Fig.6 Gas occurrence states in organic and inorganic pores in marine[(a),(b)]and transitional[(c),(d)] shales

为深入探讨海陆过渡相页岩气含量及气体赋存状态,对海相和海陆过渡相典型钻井解吸气总含气量和理论含气量进行比较。解吸总含气量为现场解吸法得到的损失气与现场解吸气之和,理论含气量为吸附气与游离气之和。吸附气含量是根据实际埋藏深度的温度和压力参数进行约束后得到的室内吸附气含量,游离气是根据孔隙度、含气饱和度和地层压力等参数计算得出,具体公式见文献[56]。由计算结果可知,常压区LY1井龙马溪组上段页岩气理论含气量低于解吸气总含量,这可能是由于低TOC含量情况下孔隙度测试不准或干燥条件下吸附气估量过高引起的,但下段富有机质泥页岩的实际解吸气总含气量与理论含气量基本一致[图7(a)]。对于海陆过渡相页岩而言,实际解吸气总含气量普遍远低于理论含气量[图7(b)],这是由于:①等温吸附实验获得的吸附气含量是代表干燥条件下最大的吸附气含量,会在一定程度上高估了理论含气量;②虽然游离气含量的计算是基于地质参数得出,但并未考虑其散失程度。实际上,海陆过渡相地层所具有的岩性组合模式更利于气体的运移,致使实际含气量普遍低于理论值57。海陆过渡相页岩具有较强的吸附能力,但游离气含量较低,致使总含气量普遍较低。由此可见,吸附气是海陆过渡相页岩气的主要赋存形式。刘洪林等34指出鄂尔多斯盆地东南缘山西组页岩吸附气比例高达80%~90%,有机质孔隙的缺失致使游离气的有效储集空间减少,也进一步导致吸附气占比较高。海陆过渡相中游离气的比例虽然较小,但对页岩气的生产开发意义重大,需要进一步定量分析游离气含量及其甜点层段。
图7 海相(a)和海陆过渡相(b)页岩中气体赋存状态及其含量

Fig.7 Gas occurrence state and its content in marine(a) and transitional(b) shale

3.3 含水特征及对页岩气成藏的影响

精确评估含水饱和度对评价游离气含量和页岩气资源潜力具有重要意义。海相优质页岩储层普遍具有较高的有机碳含量和较低的黏土矿物含量,含水饱和度一般在50%以下。四川盆地长宁—威远及富顺—永川地区龙马溪组页岩含水饱和度分别为40%~46%及33%~39%58-59。海陆过渡相泥页岩高黏土矿物含量决定了其往往具有较高的含水饱和度,川东地区HSX1井和DYS1井龙潭组泥页岩含水饱和度分别为30.02%~69.62%和10.43%~92.94%,平均值分别为45.7%和71.07%。
海相页岩的孔隙度与含水饱和度呈明显的负相关性,相关性系数为0.76[图8(a)]。海相页岩中疏水性有机质孔所占的比例越高,孔隙度越高,含水饱和度越低,并确定了孔隙度2.0%和含水饱和度60%作为海相页岩气地质评价标准下限。海陆过渡相泥页岩的含水饱和度与孔隙度之间的相关性不强[图8(a)],与页岩中有机质孔发育差、孔隙构成复杂有关。但无论是海相还是海陆过渡相页岩,随着含水饱和度的增加,总含气量显著降低[图8(b)]。尤其是海陆过渡相泥页岩含水饱和度普遍超过60%,显著地降低了游离气含量,致使总含气量远低于海相页岩。
图8 泥页岩含水饱和度与孔隙度(a)和总含气量(b)的关系

Fig.8 Relationships between water saturation with porosity(a) and total gas content(b) in mud shale

因此,复杂的岩性组合致使海陆过渡相页岩气散失,含水饱和度增加游离气含量降低,不利于海陆过渡相页岩气富集成藏。

3.4 海陆过渡相页岩气主控因素

前人对海陆过渡相页岩含气量开展了较多的研究工作,主要侧重于页岩气的吸附能力,对页岩气富集的主控因素讨论较少。从物性的角度来看,海陆过渡相泥页岩的比表面积与黏土矿物含量呈正相关性,常出现与TOC含量无明显相关性的情况10-1128。对含气性而言,海陆过渡相泥页岩的吸附能力与TOC含量和黏土矿物含量均呈正相关性18。但解吸气含量仅与TOC含量之间存在正相关性[图9(a)],与黏土矿物含量之间无显著的相关性[图9(b)]。这说明虽然黏土矿物控制了孔隙空间和吸附能力,但并未控制实际含气量的高低61417-18。这与蔡光银等60阐述的页岩气赋存的优势孔隙类型为黏土矿物粒间孔,主要以吸附态存在于高岭石等黏土矿物孔隙中并不一致。海陆过渡相页岩含气量主要受控于TOC含量,是由于有机质的吸附热很高,能够提高页岩的吸附能力61。因此,以有机质为载体的吸附气是海陆过渡相页岩气主要赋存形式40,海陆过渡相页岩气的勘探也应着重关注高有机质层段。需要指出的是,孔隙不发育的有机质其储存页岩气的方式以及有效的储集能力值得高度关注。
图9 海陆过渡相页岩TOC(a)和黏土矿物含量(b)与含气量相关性(据文献[51838]修改补充)

Fig.9 Relationships between TOC(a) and clay mineral contents(b) with total gas content for transitional shales(modified and supplemented according to Refs.[51838])

川东—黔北地区海陆过渡相页岩气现场解吸测试显示高丰度有机质泥页岩的解吸气含量更高,与煤层邻近的黑色泥页岩有机碳含量最高、解吸气含量最高,其次是炭质页岩、深灰色泥岩,泥质粉砂岩解吸效果较差18385062。鄂尔多斯盆地海陆过渡相页岩气勘探也证实了高TOC含量富硅质页岩储集空间类型多样,有机质含量丰富,含气量最高,其次是中TOC含量硅质页岩,低TOC含量黏土质页岩储集空间单一、含气量最低63。造成该现象的深层次原因为:①泥页岩—煤—砂岩“三明治”式空间结构导致生成的游离气高效率排出40,高TOC含量泥页岩能够储存更多吸附气致使含气量相对较高;②高黏土矿物常具有高含水饱和度致使黏土矿物的吸附能力降低和游离气的储集空间减少,不利于页岩气在黏土质泥页岩中富集。因此,海陆过渡相页岩游离气含量低、形成以有机质吸附气为主的欠饱和页岩气藏,储层压力系数小,以常压为主2864,整体上较少存在生烃膨胀产生超压的情况53

3.5 海陆过渡相页岩气局部超压机理

普遍较低的含气量不利于海陆过渡相页岩气的勘探开发,寻找页岩气富集的“甜点”层段成为海陆过渡相页岩气勘探突破的关键。海陆过渡相页岩气含量在垂向上呈有规律的剧烈变化132850,高含气段主要分布在煤层与黑色炭质页岩中50。岩相类型的规律性变化是页岩气垂向强非均质性的主要原因,黑色页岩层段是页岩气勘探的有利层段。目前,黑色炭质页岩高含气性的内在机理仍缺乏深入的认识。近年来,研究表明含菱铁质泥岩/菱铁矿层对煤系气垂向渗流具有分划性阻隔作用65-66,控制了垂向含气单元间的独立含气性66。菱铁矿是海陆过渡相地层中最常见的自生矿物,含量在0.2%~90%之间67,它的形成往往需要一定时间内持续的弱还原环境(Eh=0~0.2)、不断补充的Fe2+、稳定的弱碱性水介质环境68,且需要与细菌/有机质分解或海水中的CO3 2-形成FeCO3胶体溶液,在水体中最终沉淀而成。特别是在潟湖—潮坪沉积区域容易形成广泛分布的菱铁矿泥岩/菱铁矿层,纵向上多层发育、横向上稳定分布(图1066。以川东地区HSX1井为例,菱铁矿层对应低孔低含气量[图11(a)],反映了菱铁矿层是页岩气有效的封堵层,利于形成独立的含气单元。
图10 菱铁质页岩/菱铁矿层在海陆过渡相地层中的发育模式(据文献[70]修改)

Fig.10 Development pattern of siderite shale/siderite layers in transitional facies stratum (modified according to Ref.[70])

图11 海陆过渡相泥页岩菱铁矿含量与页岩气含量的关系

Fig.11 Relationships between siderite content with shale gas content in transitional mud shales

近年来,一些学者认为海陆过渡相页岩气含量在垂向上有规律的剧烈变化受控于菱铁矿与TOC含量的协同变化69,随着菱铁矿含量增加,TOC含量和解吸气含量均明显地增加。菱铁矿与有机质之间存在共生组合关系,有机质为菱铁矿的形成提供了碳源,与铁质沉积物作用下形成菱铁矿,所以较高的有机质含量往往利于菱铁矿的形成,颜色越深的泥页岩中菱铁矿含量越高69。以川东地区HSX1井和LC1井为例,泥页岩的含气量与菱铁矿含量和TOC含量均呈一定的协同变化(图11)。菱铁矿含量与页岩气含量之间的正相关性可能是由于菱铁质泥页岩储层更为致密,生成的天然气难以运移至砂岩层而滞留在富有机质泥页岩层中。
进一步研究表明菱铁矿常以结核状、透镜状和细分散状分布在有机质周围,这种产出状态可视为对有机质的保存,并对有机质形成“微圈闭”封闭条件,实现对有机质内部烃类气体的有效封堵,致使页岩气原位高压聚集,使页岩气在局部层段超压富集(图12)。随菱铁矿含量的增加,“微圈闭”数量增加,封闭的气体含量随之增大。从HSX1井可知,煤层含气量为相邻页岩含气量的10倍左右,反映了菱铁矿层的阻隔作用71,导致煤层和泥页岩层甚至泥页岩与砂岩层之间缺少气体运移与交换,形成独立的“微含气系统”69。从物性特征上看,含菱铁矿泥岩的孔渗显著低于普通泥岩,却又具有更高的比表面积和微孔体积,且具有更高的突破压力72,有利于页岩气的局部富集保存。这种菱铁矿/有机质的共生组合关系能否成为海陆过渡相页岩气富集“木桶效应”的最后一块短板,实现对页岩气封盖和局部超压富集效应和形成页岩气“甜点”层段,值得深入探讨。
图12 菱铁矿形态及其对有机质(a)和页岩气体(b)富集的影响

Fig.12 Siderite morphology and its effect on organic matter accumulation(a) and shale gas accumulation(b)

4 研究展望

(1)加强海陆过渡相系统性沉积学研究,进一步明确有利于页岩气富集成藏的沉积环境。
海陆过渡相富有机质泥页岩在滨岸平原、障壁—潟湖、三角洲和浅海陆棚等环境中皆有发育,并非每一种环境都有利于有机质的聚集保存和页岩气规模聚集。海陆过渡相沉积环境对泥页岩的有机质聚集、岩石组合类型、矿物组成和储层空间的控制机理缺乏系统性的沉积学研究,对比不同沉积环境的各项地质参数,明确有利于页岩气富集成藏的沉积环境,是海陆过渡相页岩气勘探的重要内容之一。
(2)进一步明确页岩气赋存状态和控制机理,建立海陆过渡相页岩气富集模式。
目前对于海陆过渡相页岩吸附气的研究较多,对于游离气的赋存机理和主控因素等研究较少。特别是对游离气含量没有行之有效的测量方法或计算模型,一直无法量化海陆过渡相泥页岩中游离气含量,吸附气—游离气之间相互转化规律和影响赋存状态的因素缺乏系统性研究。各地质因素对海陆过渡相页岩气成藏的综合作用机理缺乏系统研究,无法确定主要控制因素,海陆过渡相页岩气的富集模式也不清晰。
(3)完善海陆过渡相页岩气评价方法,优化页岩气有利层段识别技术。
海陆过渡相地层岩性变化快、泥页岩单层厚度薄,海相页岩连续层段评价标准无法适应,现有的储层优选参数不规范、不全面,不能符合我国海陆过渡相页岩气富集的特点,海陆过渡相页岩气有利储层优选标准不统一。亟需从生—储—盖等角度开展海陆过渡相页岩气富集层段的系统性攻关研究,特别是将海陆过渡相地层的特征矿物与页岩气富集理论结合起来,体现页岩气有利层段评价的特色发展方向。
(4)明确“三气”叠置成藏机理和合采的可行性。
单独开采海陆过渡相页岩气难以获得较好的经济效益,与煤层气和砂岩气进行叠置成藏综合勘探也是今后的主要方向之一。应加强页岩气藏与其他2种气藏富集机理之间的联系,为海陆过渡相非常规天然气耦合富集机理、“三气”共探共采、甜点区评价技术和高效钻井与储层改造提供理论依据。
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