致密砂砾岩储层孔隙结构对比及差异机制研究

致密储层孔隙结构研究对于致密油气的勘探和开发具有重要意义,也是当前致密储层研究的热点。本文以准噶尔盆地三叠系百口泉组致密砾岩和鄂尔多斯盆地三叠系延长组致密砂岩为研究对象,重点开展二维大面积背散射扫描电镜成像技术(Maps)、高压压汞、微米CT和聚焦离子束扫描电镜技术(FIB-SEM)等分析测试,采用定性和定量相结合的方法对致密砾岩和致密砂岩孔隙结构特征进行表征和对比,并进一步揭示了两者的成因机制差异。研究结果表明:百口泉组致密砾岩较延长组致密砂岩储层物性好,前者孔隙度和渗透率平均为9.29%和1.65×10-3 μm2,后者孔隙度和渗透率平均为8.85%和0.39×10-3 μm2;百口泉组致密砾岩较延长组致密砂岩孔喉尺寸偏大、孔隙连通性较差及孔隙结构更为复杂,前者孔喉半径主要分布在0~200 nm和2~10 μm范围内,平均连通率60.63%,平均孔喉配位数3.0446,后者孔喉半径主要分布于0~4 μm范围内,平均连通率73.60%,平均孔喉配位数2.7832;差异化沉积作用和成岩作用是导致百口泉组致密砾岩和延长组致密砂岩孔隙结构特征差异的根本原因。

The research on pore structure of tight reservoirs is of great significance for the exploration and development of tight oil and gas, and it is also a hotspot in the research on tight reservoirs. Taking the tight conglomerates of the Triassic Baikouquan Formation in the Junggar basin and tight sandstones of the Triassic Yanchang Formation in the Ordos basin as examples, the pore structure characteristics and comparisons of tight conglomerates and tight sandstones are studied qualitatively and quantitatively, and the differences of genetic mechanism between them are further explored using two-dimensional large area backscatter scanning electron microscopy (maps), high-pressure mercury intrusion porosimetry (HMIP), micro CT and focused ion beam scanning electron microscopy (FIB-SEM). Results show that the physical properties of tight conglomerate in the Baikouquan Formation are better than those of tight sandstone in the Yanchang Formation, and the average porosity and permeability of the former is 9.29% and 1.65×10-3 μm2, while that of the latter is 8.85% and 0.39×10-3 μm2. The pore throat distribution of tight conglomerates in the Baikouquan Formation and tight sandstone in the Yanchang Formation is characterized by multi-scale continuous distribution. The pore structure of tight conglomerates in the Baikouquan Formation is characterized by pore throat radius mainly in the range of 0~200 nm and 2~10 μm, average connectivity of 60.63% and average pore-throat coordination number of 3.0446. The pore structure of tight sandstones in the Yanchang Formation is characterized by pore throat radius mainly in the range of 0~4 μm, average connectivity of 73.60% and average pore-throat coordination number of 2.7832. Compared with the tight sandstones of the Yanchang Formation, the tight conglomerates of the Baikouquan Formation have larger pore throat size, poor pore throat sorting, poor pore connectivity and more complex pore structure. Differential sedimentation and diagenesis are the main reasons for the pore structure differences between tight conglomerates of the Baikouquan Formation and tight sandstones of the Yanchang Formation.

孔隙结构 ； 成岩作用 ； 对比研究 ； 致密砂砾岩 ； 百口泉组 ； 延长组

pore structure ； diagenesis ； comparative study ； tight sandstone or conglomerate ； Baikouquan Formation ； Yanchang Formation

21世纪以来,非常规油气快速发展,显示出巨大的资源潜力,逐渐成为油气勘探开发的重要领域(Yang Zhi et al., 2019)。致密油气作为非常规油气的重要组成,资源丰富,地质资源量达74×108~80×108 t,技术可采资源量为13×108~14×108 t(Zhu Xiaomin et al., 2018),广泛分布于四川盆地、鄂尔多斯盆地、准噶尔盆地、松辽盆地和渤海湾盆地等(Qiu Zhen et al., 2020)。不同于传统常规油气资源,致密油气具有大面积连续分布的特征,主要赋存于孔隙度小于10%、渗透率小于0.1×10-3 μm2的致密储层中(Zhu Xiaomin et al., 2018)。致密储层孔喉尺寸小、孔隙结构复杂,严重制约其流体储集和渗流能力。因此,明确致密储层孔隙结构特征,对于致密油气进一步勘探和开发具有重要意义,也是当前致密储层研究的热点。

基于前人提出的致密储层孔隙尺寸划分方案(Martin et al., 1997; Liu Na et al., 2014; Zhu Rukai et al., 2016),本文将致密储层孔隙划分为大孔(＞20 μm)、中孔(10~20 μm)、小孔(2~10 μm)、微米孔(1~2 μm)、亚微米孔(0.1~1 μm)、纳米孔(＜0.1 μm)。致密储层孔隙以微纳米孔为主(Nelson, 2009),孔径一般为40~900nm(Wang He et al., 2019),孔径下限为20~50nm(Sun Longde et al., 2019)。因此,常规分析测试技术已经不适用于致密储层孔隙结构表征,而场发射扫描电镜、高压压汞、气体吸附、核磁共振和微纳米CT等高分辨率表征技术逐渐得到推广和应用(Anovitz et al.,2015; Guo Qiyang et al., 2019; Zhang Fan et al., 2019)。鉴于上述单一方法各有优缺点,难以实现孔隙结构全尺度表征,前人通常联合多种技术手段对致密储层孔隙结构进行表征,例如高压压汞与气体吸附技术结合(Tian Hua et al., 2012; Li Zhiqing et al., 2017)、高压压汞与核磁共振技术结合(Sun Xiaolong et al., 2017; Kong Xingxing et al., 2020)以及高压压汞和微米CT扫描技术结合(Wang Qianyou et al., 2019)等。国内外学者针对不同地区、不同时代或不同类型的致密储层微观孔隙结构纷纷开展研究并取得了丰富的成果(Nooruddin et al., 2014; Xiao Dianshi et al., 2016; Li Junjian et al., 2018; Zhu Peng et al., 2020)。然而,不同致密储层之间的孔隙结构对比研究较为缺乏,有待深化。本文以三叠系鄂尔多斯盆地延长组和准噶尔盆地百口泉组为研究对象,结合氩离子抛光—场发射扫描电镜(FE-SEM)、高压压汞、核磁共振和微纳米CT扫描等技术,开展致密砂砾岩储层孔隙结构特征对比研究,并进一步探讨两者孔隙结构差异机制,为下一步致密油气勘探和甜点预测提供理论依据。

准噶尔盆地位于中亚增生造山带的中南部,是晚古生代至中、新生代多旋回叠合盆地(Huang Dingjie et al., 2015)(图1)。玛湖凹陷位于准噶尔盆地西北缘,是油气勘探重点区带。下三叠统百口泉组主要发育近源粗粒扇三角洲沉积体系,沉积物岩性以灰绿色砂质细砾岩、细砾岩、中砾岩、含砾粗砂岩为主(Qu Jianhua et al., 2017)。砾岩分选中等—较差,颗粒直径在2~40mm之间变化,磨圆以次棱角状—次圆状、次圆状为主,结构成熟度和成分成熟度均较低(Cao Yingchang et al., 2019)。百口泉组总厚度为130~240m,自下而上划分为百一段(T1b1)、百二段(T2b2)和百三段(T3b3),其中百一段和百二段为主要储集层(Xiao Meng et al., 2019)。百口泉组埋藏深度普遍大于3000m(Xiong Jian et al., 2018),表现为低孔低渗特点,孔隙度主要分布在2.5%~21.2%之间,平均7.94%,渗透率主要分布在0.01×10-3~982×10-3 μm2之间,平均为5.6×10-3 μm2(Cao Yingchang et al., 2019)。

鄂尔多斯盆地位于华北地台西部,是在前寒武系结晶基底上发展起来的大型多旋回克拉通盆地(图1),经历了中新元古代坳拉谷、早古生代浅海台地、晚古生代滨海平原、中生代内陆盆地和新生代盆缘断陷等多个构造演化阶段(Liu Dengke et al., 2019)。上三叠统延长组记录了湖盆形成、发展和消亡的全过程,为一套以河流—湖泊相为主的砂泥岩沉积。延长组自下而上划分为长10段—长1段共10个油层段,其中长2段、长6段和长8段为主要含油层段。延长组主要发育浅灰色细粒岩屑长石砂岩和长石岩屑砂岩,碎屑颗粒直径主要分布在0.06~0.25mm之间(Li Pan et al., 2018),颗粒分选中等—好,次棱角状磨圆(Zhang Xi et al., 2017)。储层致密,孔隙度一般分布在6%~14%之间,平均为10.1%,渗透率一般分布在0.01×10-3~1×10-3 μm2之间(Li Chaozheng et al., 2016)。

实验样品分别选自准噶尔盆地玛湖凹陷百口泉组百二段和鄂尔多斯盆地延长组长4+5、长7段。百口泉组样品为扇三角洲相中砾岩和含细砾中砾岩,延长组样品为河流相砂岩。

二维大面积背散射扫描电镜成像技术(Maps)是将直径为5mm的柱塞岩样进行表面离子抛光和镀膜,然后利用Helios NanoLab660仪器对样品进行小视域连续扫描,获取大量高分辨率小图像,然后将小图像组合拼接为二维大面积背散射图像,分辨率可达250nm(Zhang Tongyao et al., 2020)。该技术较好解决了视域大小与分辨率高低之间的矛盾,能够实现微米—纳米多尺度孔隙结构精细表征。

高压压汞(HMIP)采用AutoPoreⅣ 9500型全自动压汞仪,该仪器能够提供的最大进汞压力为413MPa(Yang Xiao et al., 2016),本次研究选取汞表面张力为480N/m,接触角为140°。将切割好的1cm3立方体小块样品装入封闭的膨胀计中,抽真空后进行注汞。通过测量样品在不同毛细管压力下的进汞饱和度,绘制毛细管压力曲线;然后利用Washburn方程将毛管压力转换为孔径,从而得到孔喉半径分布曲线;此外,根据Young-Dupré方程提出的界面张力与比表面积之间的关系,计算不同孔径对应的比表面积(Ma Haiyang et al., 2019)。高压压汞为破坏性实验,注汞压力过高时会导致岩样孔隙结构变形或压缩,因此该实验主要适用于宏孔(微米级和亚微米级)测量(Yu Yuxi et al., 2020),有效孔径测量范围为3nm~400 μm(Li Zhiqing et al., 2017)。高压压汞的可靠测试精度为3nm(Ning Chuanxiang et al., 2017)。

微米CT(Micro CT)是从微米尺度研究二维及三维孔隙结构特征的X射线无损探测技术。微米CT扫描采用MicroXCT-400型CT扫描仪对直径为5mm柱体样品进行360°扫描(砂岩扫描精度为1 μm,砾岩扫描精度为5 μm),获取一系列二维图像,在此基础上利用Avizo软件对二维图像进行二值化分割处理并重构得到三维孔隙结构模型(三维数字岩芯)(Xiong Jian et al., 2018),然后运用数理统计方法定量提取孔隙度、渗透率、形状因子和连通率等孔隙结构参数(Bai Bin et al., 2013)。

图1 准噶尔盆地和鄂尔多斯盆地在中国的地理位置(a)、准噶尔盆地构造单元划分(b) 和鄂尔多斯盆地构造单元划分(c)(据Qu Yiqian et al., 2020; Xiao Meng et al., 2020修改)

Fig.1 Geographical location of Junggar basin and Ordos basin in China (a), division of structural units in Junggar basin (b) and division of structural units in Ordos basin (c) (modified after Qu Yiqian et al., 2020; Xiao Meng et al., 2020)

聚焦离子束扫描电镜(FIB-SEM)是由聚焦离子束与电子束集成的双束系统,离子束用于对切割和剥蚀样品,电子束用于扫描成像,两者结合通过对观测样品边剥蚀边成像的方式获取样品内部的真实孔隙结构信息。FIB-SEM的扫描精度可达1~3nm(Zhang Tianfu et al., 2016)。

准噶尔盆地玛湖凹陷百口泉组致密储层发育含砾粗砂岩、砂质细砾岩、细砾岩和中砾岩等多种岩性类型,其中细砾岩为主要岩性。砾石成分以凝灰岩和花岗岩为主,砂质成分以花岗岩、石英、凝灰岩和长石为主,杂基主要为黏土矿物。X衍射结果表明(表1),百口泉组致密砾岩样品主要含有石英(48.1%~63.7%,平均值54.37%)、斜长石(16.3%~35.5%,平均值24.37%)和黏土矿物(7.6%~27.9%,平均值17.78%),其次为钾长石(0.5%~6.6%,平均值2.36%)和方解石(0.3%~2.9%,平均值0.82%),白云石、菱铁矿和黄铁矿含量极少,仅在个别样品中发育。黏土矿物以绿泥石(13%~61%,平均值37.75%)和伊蒙混层(8%~54%,平均值26.75%)为主,其次为伊利石(10%~35%,平均值20.75%)和高岭石(5%~23%,平均值14.75%)。

鄂尔多斯盆地延长组致密储层主要发育岩屑长石砂岩、长石岩屑砂岩和长石砂岩等,粒度以细砂和粉砂为主。储层填隙物主要为绿泥石、伊利石和高岭石等黏土矿物以及方解石等碳酸盐。X衍射结果表明(表1),百口泉组致密砾岩样品主要含有石英(26.0%~62.1%,平均值43.31%)、斜长石(12.9%~45.0%,平均值29.92%)、钾长石(6.1%~21.2%,平均值10.10%)和黏土矿物(7.8%~16.7%,平均值10.67%),其次为方解石(0.5%~15.2%,平均值2.82%)和白云石(0.8%~9.3%,平均值2.59%),菱铁矿含量极少,未见黄铁矿。黏土矿物以绿泥石(17%~77%,平均值44.60%)和伊利石(4%~58%,平均值28.93%)为主,其次为高岭石(5%~24%,平均值15%)和伊蒙混层(2%~19%,平均值11.47%)。

表1 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层全岩及黏土矿物组成

Table1 The compositions of whole rock and clay minerals of tight reservoirs in Baikouquan Formation of Junggar basin and Yanchang Formation of Ordos basin

结合文献调研(Dan Weidong et al., 2011; Li Chaozheng et al., 2016; Sima Liqiang et al., 2016; Kang Xun et al.,2017; Zhang Hao et al.,2017; Fu Yu et al., 2020; Qu Yiqian et al., 2020; Wang Fuyong et al., 2020; Zhang Quanpei et al., 2020)和实验测试数据,准噶尔盆地玛湖凹陷百口泉组致密砂砾岩孔隙度整体分布在5.3%~15.0%,平均值为9.29%,渗透率整体分布在0.02×10-3~16.4×10-3 μm2,平均值为1.65×10-3 μm2(图2a、b);鄂尔多斯盆地延长组致密砂岩孔隙度整体分布在1.2%~18.6%,平均值为8.85%,渗透率整体分布在0.001×10-3~3.48×10-3 μm2,平均值为0.39×10-3 μm2(图2a、b)。从分布频率上看,百口泉组致密砂砾岩孔隙度主要分布在4%~12%,渗透率集中分布在0.1×10-3~10×10-3 μm2;延长组致密砂岩孔隙度也主要分布在4%~12%,而渗透率主要分布在0.01×10-3~1×10-3 μm2。相比之下,百口泉组致密砂砾岩具有较高的孔隙度和渗透率。此外,根据孔隙图-渗透率交汇图(图2c、d),百口泉组致密砂砾岩的孔渗关系较差(R2=0.09),延长组致密砂岩的孔渗关系较好(R2=0.53),表明百口泉组致密砂砾岩的孔隙结构更为复杂,非均质性更差。

图2 准噶尔盆地百口泉组和鄂尔多斯盆地延长组物性分布特征

Fig.2 Physical properties of Baikouquan Formation in Junggar basin and Yanchang Formation in Ordos basin

(a)—孔隙度分布特征;(b)—渗透率分布特征;(c)—百口泉组孔隙度-渗透率交会图;(d)—延长组孔隙度-渗透率交会图

(a)—Distribution characteristics of porosity; (b)—distribution characteristics of permeability; (c)—porosity-permeability cross plot of Baikouquan Formation; (d)—porosity-permeability cross plot of Yanchang Formation

Maps成像分析实验表明准噶尔盆地玛湖凹陷百口泉组致密砾岩储层和鄂尔多斯盆地延长组致密砂岩储层的孔隙类型均以残余粒间孔隙、溶蚀孔隙和晶间孔隙为主,见少量微裂缝。残余粒间孔隙是致密砂砾岩储层中重要的孔隙类型,为原生沉积孔隙经过埋藏压实等成岩作用后保存下来的孔隙,发育于石英、长石和岩屑等骨架碎屑颗粒之间,常表现为边缘平直的三角形或多边形形态,孔隙半径分布范围较广,为10~75 μm(图3a、4a)。溶蚀孔隙主要表现为石英、长石和岩屑等碎屑颗粒内部及边缘溶蚀形成的粒内溶孔和粒间溶孔(Cao Yingchang et al., 2019)。粒间溶孔多具有锯齿状或港湾状边缘,形状不规则,孔隙半径主要分布在5~30 μm。粒内溶孔以长石粒内溶孔为主,孔隙形状不规则,孔隙呈不定向排列。百口泉组致密砾岩样品中长石粒内溶蚀孔隙半径主要分布在0.5~20 μm(图3c),延长组致密砂岩样品中长石粒内溶蚀孔隙半径主要分布在1~10 μm(图4c)。晶间孔隙是指发育在高岭石、伊利石和绿泥石等黏土矿物集合体内部的孔隙,呈良好定向排列。百口泉组致密砾岩样品中高岭石晶间孔隙和伊利石晶间孔隙发育良好,孔隙半径为0.5~10 μm(图3d)。延长组致密砂岩样品中蒙脱石晶间孔隙和伊利石晶间孔隙发育,孔隙半径主要分布在0.1~5 μm(图4b)。微裂缝不甚发育,主要表现为石英颗粒粒间缝和长石颗粒粒内缝,百口泉组致密砾岩样品中微裂缝开度为5~50 μm(图3b),延长组致密砂岩样品中微裂缝开度为0.1~10 μm(图4d)。对比分析表明,准噶尔盆地玛湖凹陷百口泉组致密砾岩储层相对发育黏土矿物晶间孔隙,而鄂尔多斯盆地延长组致密砂岩储层相对发育长石粒内溶蚀孔隙。

高压压汞实验主要反映致密砂砾岩储层孔喉连通关系和孔喉分布特征等信息。图5a、b分别为鄂尔多斯盆地延长组致密砂岩样品和准噶尔盆地百口泉组致密砾岩样品的进汞曲线。数据分析表明,准噶尔盆地百口泉组致密砾岩样品的进汞量为55.94%~83.01%;排驱压力为0.12~1.17MPa时,对应最大孔喉半径约为1.0~10.0 μm,平均为3.1 μm(表2)。鄂尔多斯盆地延长组致密砂岩样品的进汞量为85.91%~93.77%;排驱压力为0.29~1.84MPa,对应最大孔喉半径约为0.4~2.5 μm,平均为1.3 μm(表2)。准噶尔盆地百口泉组致密砾岩与鄂尔多斯盆地延长组致密砂岩相比,具有排驱压力小和进汞量大的特点,反映前者具有更大的孔喉半径和较差的孔喉连通性。此外,从毛管压力曲线形态上看,百口泉组致密砾岩的压汞曲线中间呈“斜直状”,反映孔喉分选较差;延长组致密砂岩的压汞曲线中间平台段长,孔喉分选较好。

图3 准噶尔盆地百口泉组M139-25致密砾岩样品Maps成像分析结果

Fig.3 Map imaging analysis of M139-25tight conglomerate samples from Baikouquan Formation in Junggar basin

(a)—样品大视域图像; (b)—高岭石晶间孔隙; (c)—长石粒内溶孔; (d)—微裂缝,见石英粒间缝和长石粒内缝

(a)—Large view image of sample; (b)—intergranular pores of kaolinite; (c)—dissolution pores in feldspar grains; (d)—microfractures, such as intergranular quartz and intergranular feldspar fractures

根据孔喉半径分布特征曲线(图5c、d),准噶尔盆地百口泉组致密砾岩和鄂尔多斯盆地延长组致密砂岩的孔喉分布多数表现为“双峰”特征,其中百口泉组致密砾岩孔喉分布主要在0~0.13 μm和0.6~4.0 μm之间;延长组致密砂岩孔喉分布主要在0~0.16 μm和0.4~2.0 μm范围。对比发现,百口泉组致密砾岩主要发育微米级和亚微米级孔喉,平均占比分别约为70.6%和24.8%;而延长组致密砂岩相对发育亚微米级和纳米级孔喉,微米级孔喉较不发育,平均占比分别约为56.8%、35.2%和8%。

基于微米CT测试结果,分析准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层的孔隙和喉道的分布特征(表3、图6)。微米CT结果表明,准噶尔盆地百口泉组致密砂砾岩和延长组致密砂岩样品的孔喉均呈现多尺度连续分布的特征。从孔隙数量分布上看,百口泉组致密砂砾岩样品在孔隙半径2~10 μm区间的孔隙数量最多,占比达74.19%;延长组致密砂岩样品中孔隙数量在0~4 μm半径范围内的比例高达90.88%。从孔隙体积分布上看,百口泉组致密砂砾岩样品孔隙体积为2.11×107~96.17×107 nm3(平均值为3.42×108 nm3),不同尺度的孔隙分布相对均匀,以半径为8~10 μm和半径＞20 μm的孔隙体积占比相对较高,分别为17.88%和17.97%;延长组致密砂岩样品孔隙体积为1.46×107~7.63×107 nm3(平均值为4.36×10 7 nm3),孔隙体积主要集中分布在0~8 μm孔隙半径范围内,占比为74.02%,其中半径为2~4 μm孔隙体积占比达30.78%。从喉道数量分布上看,百口泉组致密砂砾岩样品喉道半径在0~10 μm区间的喉道数量比例达90.49%;延长组致密砂岩样品喉道半径在0~4 μm区间的喉道数量比例达93.28%。从喉道体积分布上看,百口泉组致密砂砾岩样品喉道体积为0.37×107~41.36×107 nm3(平均值为1.45×108 nm3),主要集中分布在0~10 μm喉道半径范围内,占比达74.88%,其中半径在2~6 μm区间喉道体积比例高达38.66%;延长组致密砂岩样品喉道体积为0.64×107~4.04×107 nm3(平均值为2.11×107 nm3),其中喉道半径在0~6 μm区间的喉道体积比例为84.03%。

图4 鄂尔多斯盆地延长组W96-28致密砂岩样品Maps成像分析结果

Fig.4 Map imaging analysis of W96-28tight sandstone samples from Baikouquan Formation in Junggar basin

(a)—样品大视域图像; (b)—黏土矿物晶间孔隙,见伊利石晶间孔和蒙脱石晶间孔; (c)—长石粒内溶孔; (d)—石英粒间缝

(a)—Large view image of sample; (b)—the intergranular pores of clay minerals including illite and montmorillonite; (c)—dissolution pores in feldspar grains; (d)—intergranular quartz fractures

表2 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层高压压汞孔隙结构参数

Table2 Pore structure parameters of high pressure mercury injection in tight reservoirs of Baikouquan Formation in Junggar basin and Yanchang Formation in Ordos basin

图5 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层毛管压力曲线及孔喉半径分布曲线

Fig.5 Capillary pressure curve and pore throat radius distribution curve of tight reservoirs in Baikouquan Formation of Junggar basin and Yanchang Formation of Ordos basin

(a)—百口泉组致密砾岩毛管压力曲线;(b)—延长组致密砂岩毛管压力曲线;(c)—百口泉组致密砾岩孔喉分布曲线; (d)—延长组致密砂岩孔喉分布曲线

(a)—Capillary pressure curve of tight conglomerate in Baikouquan Formation; (b)—capillary pressure curve of tight sandstone in Yanchang Formation; (c)—pore throat distribution curve of tight conglomerate of Baikouquan Formation; (d)—pore throat distribution curve of tight sandstone of Yanchang Formation

表3 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层微米CT孔隙结构参数

Table3 Micro CT pore structure parameters of tight reservoirs in Baikouquan Formation of Junggar basin and Yanchang Formation of Ordos basin

百口泉组致密砾岩和延长组致密砂岩样品的孔喉分布特征表明,百口泉组致密砾岩与延长组致密砂岩相比孔喉更为发育。此外,在百口泉组致密砾岩和延长组致密砂岩样品中,微米孔、小孔等小尺度孔隙喉道的数量要远远多于中孔、大孔等大尺度孔隙喉道的数量,但大尺度孔隙喉道对孔隙体积的贡献不可忽视。以孔隙分布特征为例,百口泉组大尺度孔隙数量占比为16.73%,而对应的孔隙体积占比高达52.39%;延长组大尺度孔隙数量仅占总孔隙的1.19%,其孔隙体积占比达到20.80%。百口泉组致密砂砾岩与延长组致密砂岩相比,大尺度孔隙喉道更为发育,并且对孔隙空间的贡献更大。

图6 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层微米CT孔喉分布特征

Fig.6 Micro CT pore throat distribution of tight reservoirs in Baikouquan Formation of Junggar basin and Yanchang Formation of Ordos basin

(a)—百口泉组致密砾岩孔喉数量占比;(b)—延长组致密砂岩孔喉数量占比;(c)—百口泉组致密砾岩孔喉体积占比; (d)—延长组致密砂岩孔喉体积占比

(a)—Percentage of pore throats in tight conglomerate of Baikouquan Formation; (b)—percentage of pore throats in tight sansstone of Yanchang Formation; (c)—proportion of pore throat volume of tight conglomerate in Baikouquan Formation; (d)—proportion of pore throat volume of tight sandstone in Yanchang Formation

聚焦离子束扫描电镜(FIB-SEM)最小分辨率可以达到0.5nm,能够反映纳米尺度孔隙分布特征。利用Avizo软件对FIB-SEM观测结果进行定量化表征,分别得到百口泉组致密砂砾岩和延长组致密砂岩储层的纳米级孔隙数量和体积分布特征,如图7所示。百口泉组致密砂砾岩储层中半径在0~200nm的孔隙数量最多,占总孔隙数量的比例超过95%;同时这部分孔隙也是孔隙体积的主要贡献者。延长组致密砂岩储层中孔隙数量主要集中在半径0~100nm范围内,孔隙体积主要由半径0~100nm和300~400nm孔隙提供。对比聚焦离子束扫描电镜与微米CT测试结果,两者得到的孔喉半径分布特征差异较大,造成这一差异的原因是这两种测试技术的分辨率不同,故所能表征的孔喉尺度范围也不同,微米CT测试结果主要反映微米尺度的孔喉分布特征,而聚焦离子束扫描电镜测试结果主要反映纳米尺度的孔喉分布特征。

微米CT能够反映孔隙连通性和复杂程度。百口泉组致密砂砾岩储层孔隙连通性整体较差,以M604-24样品为例,微米CT总孔隙度为5.41%,有效孔隙度为3.28%,连通率为60.63%(图8a)。延长组致密砂岩储层孔隙连通性整体较好,以W234-19样品为例,微米CT总孔隙度为9.65%,有效孔隙度为7.86%,连通率为81.45%(图8b)。进一步结合微米CT孔喉配位数、迂曲度、孔隙形状因子等参数对比百口泉组和延长组致密储层的孔隙结构复杂程度。百口泉组致密砂砾岩样品的孔喉配位数平均为3.0446,迂曲度平均为13.3,孔隙形状因子平均为0.0492;而延长组致密砂岩样品的孔喉配位数平均为2.7832,迂曲度平均为4.0,孔隙形状因子平均为0.0478(表3)。对比表明,百口泉组致密砂砾岩的孔隙结构与延长组致密砂岩的孔隙结构相比更为复杂。

图7 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层聚焦离子束扫描电镜纳米孔喉分布特征

Fig.7 Distribution characteristics of nano pore throat in tight reservoirs of Baikouquan Formation in Junggar basin and Yanchang Formation in Ordos basin by focused ion beam scanning electron microscopy

(a)—致密砂砾岩储层孔喉数量占比;(b)—致密砂砾岩储层孔喉体积占比

(a)—Proportion of pore throats in tight sandy conglomerate reservoir; (b)—proportion of pore throat volume in tight sandy conglomerate reservoir

图8 准噶尔盆地百口泉组和鄂尔多斯盆地延长组致密储层微米CT三维孔隙连通模型

Fig.8 3D micro CT pore connectivity model of tight reservoirs in Baikouquan Formation of Ordos basin and Yanchang Formation of Ordos basin

(a)—百口泉组M604-24致密砾岩样品孔隙连通模型;(b)—延长组W234-19致密砂岩样品孔隙连通模型

(a)—Pore connectivity model of M604-24tight conglomerate sample of Baikouquan Formation; (b)—pore connectivity model of W234-19tight sandstone sample in Yanchang Formation

沉积作用决定储层的原始孔隙结构,表现为不同沉积相带的沉积物组分、碎屑颗粒大小及结构等因素不同从而控制储层原始孔隙结构(Morad et al., 2000)。此外,沉积作用也进一步影响了成岩作用及致密储层的孔隙结构演化。例如,沉积物组分以其不同的物理、化学稳定性影响压实作用的减孔效应。石英、长石等刚性颗粒含量高的岩石抗压实能力强,对原生粒间孔隙起到较好的保护作用;岩屑、云母等塑性颗粒含量高的岩石抗压实能力弱,在压实作用下颗粒容易变形而紧密堆积,从而造成原生粒间孔隙的大量损失;杂基在埋藏压实作用下易变形而充填原生粒间孔隙,因此杂基含量高的岩石不易保存粒间孔隙。碎屑颗粒大小影响孔隙结构,颗粒较大的沉积物通常具有较大的孔喉尺寸、孔隙体积和比表面积以及更好的孔隙连通性(Qiao Juncheng et al., 2020)。此外,碎屑颗粒的分选、磨圆等结构特征也会影响孔隙结构,通常分选好、磨圆度好的碎屑颗粒能够较好保留原生孔隙。百口泉组的沉积环境为扇三角洲,以扇三角洲前缘水下分流河道为优势沉积微相,水动力较强,沉积物粒度粗,以细砾为主,磨圆度较好,抗压实能力强,原生孔喉保存较好,孔喉连通性较好。延长组沉积时主体处于三角洲-湖泊沉积环境,主要发育三角洲前缘和半深湖-深湖重力流沉积。三角洲前缘以水下分流河道为优势沉积微相,沉积水动力较强,颗粒粒度以中细粒为主,颗粒分选、磨圆较好,泥质含量较少,岩石抗压实能力较强,有利于原生孔隙发育和保存。深湖-半深湖重力流沉积主要是三角洲前缘沉积物沿斜坡滑塌变形的结果(Ran Yixuan and Zhou Xiang, 2020),颗粒粒度较细,以细砂岩和粉砂岩为主,由于较短距离快速堆积,沉积物颗粒分选较差,杂基含量较多,抗压实能力较差,原生孔隙结构保存较差。综合对比百口泉组和延长组沉积作用特征,前者沉积物粒度较粗,杂基含量较低,石英含量较高,有利于原始孔隙结构保存;后者沉积物粒度较细,杂基含量较高,长石和岩屑含量较高,抗压实能力相对较差,不利于原始孔隙结构保存。

压实作用包括机械压实作用和化学压实作用。化学压实作用通常在储层达到一定埋藏深度和地层温度时才会广泛进行,百口泉组(埋深2400~3900m)和延长组(埋深2300~2800m)埋藏深度均较浅,因此不考虑化学压实作用的影响(Yang Shangfeng et al., 2020)。机械压实作用是导致百口泉组砂砾岩和延长组砂岩储层原生粒间孔隙损失、原生孔隙度减小的主要成岩因素。前人研究表明,百口泉组致密砂砾岩储层原始孔隙度约为36%(Zhu Ning et al., 2019),由于压实作用损失孔隙度为10.51%~24.01%,平均为16.92%(Kuang Yan et al., 2017),平均减孔率约为47%;延长组致密砂岩储层原始孔隙度为35.35%~40.44%(平均38.45%),在埋藏过程中由于压实作用损失孔隙度为5.31%~32.26%(平均23.47%)(Li Pan et al., 2018),平均减孔率达到60%。对比发现,百口泉组致密砂砾岩储层在埋藏成岩过程中由于压实作用导致的原生粒间孔隙损失相对延长组致密砂岩储层较小,可能与百口泉组致密砂砾岩石英含量较高和碎屑颗粒粒度较大等有关。

胶结作用对储层孔隙具有建设性和破坏性双重效应,总体体现为破坏孔隙的成岩作用,主要包括黏土矿物胶结、碳酸盐岩胶结和硅质胶结等类型。结合X衍射结果,百口泉组主要发育绿泥石、伊蒙混层等黏土矿物胶结物,延长组主要发育方解石、白云石等碳酸盐胶结物和绿泥石、伊利石等黏土矿物胶结物。绿泥石胶结物主要在早成岩阶段发育,围绕碎屑颗粒表面形成包膜,在一定程度上减缓压实作用以及抑制石英、方解石等胶结作用,对原生粒间孔隙保存起到正向作用。自生伊利石主要由蒙脱石或高岭石转化而成,呈毛发状或丝状充填孔隙和喉道。伊蒙混层呈网状充填于粒间孔隙并一定程度上堵塞喉道。自生高岭石多充填于长石粒内溶蚀孔隙,集合体呈书页状或蠕虫状形态。黏土矿物胶结物虽然充填破坏了部分粒间孔隙和粒内溶蚀孔隙,但是提供了大量微米—纳米级晶间孔隙。百口泉组黏土矿物含量(平均17.78%)明显高于延长组黏土矿物含量(平均10.67%),提供了更多的微纳米级晶间孔隙,这也给百口泉组微纳米孔隙数量和体积大于延长组微纳米孔隙的现象提供了解释。百口泉组碳酸盐胶结物较不发育,一方面绿泥石薄膜的发育抑制早期方解石胶结物的形成,另一方面溶蚀作用可能溶解了部分早期形成的方解石;晚期发育的粗晶方解石主要赋存在长石粒内溶蚀孔隙中。延长组碳酸盐胶结物相对发育,以铁方解石和铁白云石广泛分布,充填粒间孔隙。硅质胶结物主要以石英加大边的形式填充剩余粒间孔隙、或是以自生石英晶体的形式填充长石和岩屑的粒内溶蚀孔隙。

溶蚀作用以长石和岩屑溶蚀为主,局部可见碳酸盐溶蚀。溶蚀作用能够产生大量的粒内溶蚀孔隙,对储层孔隙具有建设性作用。长石溶蚀广泛发育,与油气充注伴生的酸性流体密切相关。百口泉组主要经历两期溶蚀作用:早期溶蚀作用较弱,以斜长石溶蚀为主,产生的粒内溶蚀孔隙多被方解石胶结物充填;晚期溶蚀作用较强,以钾长石溶蚀为主,产生大量粒内溶蚀孔隙。根据X射线衍射结果,百口泉组含有较高含量的斜长石(平均值24.37%)和较低含量的钾长石(平均值2.36%),间接反映了长石选择性溶蚀作用。延长组主要经历了一期溶蚀作用,与大规模油气充注时期相匹配。根据X射线衍射结果,延长组的长石含量较高,斜长石和钾长石平均含量分别为29.92%和10.10%,比百口泉组的长石含量高,一定程度上反映延长组相对于百口泉组经历的长石溶蚀作用较弱,长石粒内溶孔较不发育。与剩余粒间孔隙相比,粒内溶蚀孔隙相对孤立,连通性差,对储层渗透率贡献不大。

图9 准噶尔盆地百口泉组致密砾岩孔隙成岩演化(据Kang Xun et al., 2019修改)

Fig.9 Diagenetic evolution of tight conglomerate pores in Baikouquan Formation, Junggar basin (modified after Kang Xun et al., 2019)

基于储层成岩作用及孔隙结构演化特征,对致密储层的历史孔隙度进行定量恢复,具体计算过程如下:① 根据Beard et al.(1973)提出的经验公式对原始孔隙度φ0进行恢复(式1),原始孔隙度φ0等于压实损失孔隙度φp加胶结损失孔隙度φc加残余孔隙度φr之和(式2);② 选取致密储层样品的铸体薄片,利用光学显微镜和Image Pro Plus图像分析软件分别统计致密砂岩样品的总面孔率m、胶结物面孔率mc、溶蚀孔面孔率md及残余孔面孔率mr,建立总面孔率m与总孔隙度φ的关系,进而求取胶结损失孔隙度φc、溶蚀增加孔隙度φd及残余孔隙度φr(式3);③ 结合百口泉组和延长组的埋藏演化过程和成岩作用序列,分别恢复它们的孔隙度演化历史:

图10 鄂尔多斯盆地延长组致密砂岩孔隙成岩演化(据Wang Wurong et al., 2019修改)

Fig.10 Diagenetic evolution of tight sandstone pores in Yanchang Formation, Ordos basin (modified after Wang Wurong et al., 2019)

式中:φ0为原始孔隙度(%);S0为分选系数;φp为压实损失孔隙度(%);φc为胶结损失孔隙度(%);φr为残余孔隙度(%);φd为溶蚀增加孔隙度(%);m为总面孔率(%);mc为胶结物面孔率(%);md为溶蚀孔面孔率(%);mr为残余孔面孔率(%)。

综合地层埋藏史、成岩作用等判断百口泉组主要处于中成岩B期演化阶段(Wang Wei et al., 2016; Jin Jun et al., 2017; Guo Huajun et al., 2018; Kang Xun et al., 2019)(图9)。早三叠世,百口泉组沉积后快速沉降埋藏,在压实作用下原生粒间孔隙遭到破坏,孔隙体积减小;成岩环境表现为低温碱性还原条件,有利于在矿物和岩屑等颗粒表面形成绿泥石包膜。晚三叠世—早侏罗世,随着埋藏深度增加,压实作用进一步增强,原生粒间孔隙进一步损失。该时期发生第一次油气充注,油气充注强度较低,受有机酸影响成岩环境由碱性转化为弱酸性,富钙斜长石、岩屑等开始溶解,长石、岩屑粒内溶蚀孔隙开始发育;析出的Ca2+等碱性金属离子进入孔隙水中,为早期方解石胶结物提供物质来源和碱性环境。晚侏罗世—早白垩世,百口泉组发生第二次油气充注,有机酸和CO2明显降低了地层水pH值,促进酸性成岩环境的形成。该阶段钾长石加速溶解,早期沉淀的方解石也发生溶解,长石粒内溶蚀孔隙大量发育;蒙脱石向伊利石转化,伴生高岭石和自生石英,粒内溶蚀孔隙部分被充填,黏土矿物晶间孔隙发育。随着溶蚀作用析出的逐渐增多,地层流体pH值逐渐增大,晚期富Mn方解石胶结物开始形成(Zhu Ning et al., 2019)。

根据碎屑岩成岩阶段划分标志,延长组处于中成岩A期演化阶段(Lu Jiehe et al., 2017; Wang Wurong et al., 2019; Wang Junjie et al., 2020)(图10)。晚三叠世—中侏罗世(早成岩A期),在浅埋藏环境下沉积物处于弱固结或半固结状态,原始粒间孔隙发育。该阶段成岩作用以机械压实作用为主,随着埋藏深度增加,原生粒间孔隙数量和体积都下降。自生绿泥石在碎屑颗粒表面沉淀,形成包膜,抑制石英次生加大的形成。晚侏罗世—早白垩世早期(早成岩B期),先沉淀方解石,该阶段压实作用强烈,伊利石胶结物等发育导致粒间孔隙急剧减小。该阶段发生一次油气充注,有机酸进入地层流体,对长石、岩屑颗粒进行溶蚀,粒内溶蚀孔隙发育,同时溶蚀作用释放的SiO2为石英胶结物提供了物质基础。早白垩世末期(中成岩A期),机械压实作用达到最大化(Lu Jiehe et al., 2017),石英次生加大边发育,阻塞原生孔隙,长石岩屑等碎屑颗粒进一步溶蚀,发育少量的粒间和长石溶蚀孔隙。伴随古地温增加,高岭石逐渐向伊利石和伊蒙混层转化,铁方解石和铁白云石胶结物广泛发育,是造成该阶段储层孔隙体积减小的主要原因。

(1)准噶尔盆地百口泉组致密砾岩和鄂尔多斯盆地延长组致密砂岩孔隙类型以残余粒间孔隙、粒内溶蚀孔隙和黏土矿物晶间孔隙为主,孔喉呈多尺度连续分布特征,微—纳米孔喉占主导地位。百口泉组致密砾岩与延长组致密砂岩相比,前者储层物性较好、孔喉尺寸偏大、孔隙连通性较差及孔隙结构更为复杂。

(2)准噶尔盆地百口泉组沉积于扇三角洲环境,沉积物以细砾岩为主,原生粒间孔隙尺寸较大;在埋藏成岩演化过程中,经历了较强的压实作用和一定的胶结作用,损失了较多的原生粒间孔隙和部分粒内溶蚀孔隙,强烈的溶蚀作用产生大量的岩屑和长石粒内溶蚀孔隙,而较高含量的自生黏土矿物一定程度上增加了晶间孔隙,最终形成现今的致密砾岩储层。

(3)鄂尔多斯盆地延长组致密砂岩储层为三角洲—湖泊沉积体系,沉积物以细砂岩为主,原生粒间孔隙尺寸较小;在埋藏成岩演化过程中早期经历强烈压实作用,损失了大量的原生粒间孔隙,而后伴随油气充注的有机酸溶蚀作用产生大量长石粒内溶孔,后期铁方解石、铁白云石等胶结物的广泛发育对粒间孔隙造成进一步破坏,最终形成现今的致密砂岩。

(4)差异化沉积作用和成岩作用是导致百口泉组致密砾岩和延长组致密砂岩孔隙结构特征差异的根本原因。针对以上两种类型的致密储层,考虑到它们不同的孔隙结构特征,采取不同的开采技术手段,以期提高致密油气采收率。

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