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作者简介:

刘景东(1984-),男,副教授,博士,研究方向为油气地质与勘探。E-mail:liujingdong@upc.edu.cn。

中图分类号:TE122

文献标识码:A

文章编号:1673-5005(2021)02-0031-11

DOI:10.3969/j.issn.1673-5005.2021.02.004

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目录contents

    摘要

    针对四川盆地元坝地区上三叠统须家河组须三段致密砂岩储层,通过流体包裹体岩相学观察及包裹体均一温度、气液比、组分等参数的准确获取,采用 PVTx 法恢复包裹体古压力,并以包裹体古压力和现今压力为约束,采用盆地模拟法对储层古压力演化史进行模拟,分析古压力演化的主控因素。 结果表明:须三段储层现今地层压力系数一般为 1. 05~1. 93,最大可达 2. 27,属于常压—强超压范畴;储层古压力演化总体经历了“增加(J1—J3 中期)、降低(J3 晚期)、 再增加(K1 )、再降低(K2—现今)”等 4 个阶段,其中晚侏罗世中期和早白垩世末期的剩余压力达到最大,平面上关键充注期的剩余压力表现出由北西向南东方向降低的趋势;烃源岩生烃、构造活动、源储配置及储层致密化是研究区储层压力演化的主控因素,其中生烃增压最为重要,其次为构造挤压增压,源夹储型的配置关系和储层致密化有利于储层超压的保存,而构造抬升剥蚀及断裂则控制了储层超压的释放。

    Abstract

    Taking the tight sandstone reservoirs of the Third Member of the Upper Triassic Xujiahe Formation in Yuanba area of Sichuan Basin as an example, the paleo-pressure of the fluid inclusions was simulated with the PVTx technique by considering petrographic characteristics and obtaining parameters such as the homogenization temperature, vapor-liquid ratio and composition of the fluid inclusions. With the paleo-pressure of fluid inclusions and the present pressure as constraints, the paleo-pressure evolution history of the reservoir was simulated using a basin simulation method, and also the main controlling factors were investigated. The results show that the present formation pressure coefficient of the formation mainly ranges from 1. 05 to 1. 93, with the maximum of 2. 27, which falls into the range between normal pressure and strong overpressure. The paleo-pressure evolution of the reservoir mainly experienced four stages, including increasing (from J1 to middle of J3 ), decreasing (late of J3 ), increasing again (K1 ), decreasing again (form K2 to present). The residual pressures in the stages of Late Jurassic and Early Cretaceous are the maximum, which show a downward trend from northwest to southeast on the plane. Our analysis shows that hydrocarbon generation of source rocks, tectonic activity, the combination relationship between source rocks and reservoirs and reservoir densification are the main controlling factors for the reservoir pressure evolution. Particularly, hydrocarbon generation and tectonic compression are the first and second most important for the pressurization, respectively. The combination relationship between source rocks and reservoirs and reservoir densification are conducive to the preservation of reservoir overpressure, while the structural uplift, erosion and formation fracture control the overpressure release.

  • 异常压力在沉积盆地中广泛存在,全球已发现180多个存在异常压力盆地,其中绝大部分为超压盆地[1]。异常高压的存在不仅影响油气的生成[2],同时也是油气成藏的动力来源[3],并影响油气的分布。然而,关于含油气盆地的超压成因机制与地层古压力精确恢复一直是研究的难点。根据前人提出的超压形成机制分类方案,超压成因主要包括不均衡压实、流体膨胀和压力传递[4-5] 等,且不均衡压实和流体膨胀被认为是超压形成的主要原因[6-7]。不均衡压实存在垂向与侧向两种情况,二者均可形成超压,分别主要发育于沉积速率较快的新生代盆地[8] 和构造挤压型盆地[5]。对于流体膨胀引起的超压,较多聚焦于生烃作用造成的影响,如Ungerer等[9] 通过模拟发现干酪根生成天然气可造成的流体膨胀量超过50%。关于超压成因机制的具体判识方法,主要是基于不同成因超压在测井参数上的响应存在差异,如Bowers等[10]提出利用加载-卸载曲线对超压成因进行判识,并得到一定的应用与发展[11]。目前,含油气盆地古压力的恢复方法主要包括流体包裹体PVTx模拟法[12]、PIT法[13]、盐度-均一温度法[14] 和盆地模拟法等。 Aplin等[12] 首先提出利用流体包裹体均一温度、气液比等数据通过PVTx模拟恢复包裹体组分和捕获压力;Liu等[15]、米敬奎等[16] 也利用PVTx模拟方法对塔里木盆地、鄂尔多斯盆地的流体包裹体捕获压力进行了恢复;Liu等[17]则利用盆地模拟法对渤海湾盆地霸县凹陷不均衡压实成因超压进行了恢复。然而由于不同类型盆地及不同类型储层的埋藏-热演化史、流体包裹体发育等存在较大差异,造成古压力恢复存在较多的不确定性,如何准确获得古压力演化史仍是目前亟需解决的关键问题。近年来,四川盆地常规和非常规天然气资源勘探取得一系列突破,其中非常规致密气地质资源量达5.87 × 10 12 m 3[18],展示了巨大的勘探潜力。四川盆地须家河组致密砂岩气在该盆地致密气勘探中占据重要地位,近源、源内聚集是须家河组致密砂岩气藏形成的重要特征,但由于构造演化复杂、储层异常致密,储层古压力恢复及其演化史研究较为困难且研究相对薄弱,制约了对油气成藏机制的深入认识。笔者以四川盆地元坝地区须家河组须三段源内致密砂岩储层为研究对象,首先运用流体包裹体PVTx法对单井储层古压力进行恢复,然后以恢复的古压力作为过程约束、现今实测压力作为最终约束,利用盆地模拟法对须三段储层古压力演化史进行研究,并进一步分析古压力演化的主控因素,以期为盆地须家河组致密砂岩气勘探和地层压力预测提供参考。

  • 1 地质概况

  • 元坝地区位于四川盆地北部(图1( a)),研究区北部为通南巴背斜构造带、九龙山背斜构造带的构造倾末端及池溪凹陷西南段,南部为苍溪-巴中低缓构造带[19],整体上构造低缓。研究区陆相层系主要包括上三叠统至白垩系,其中上三叠统须家河组( T x 3) 自下而上可分为须一段至须五段(T3x1-T3x5)(图1( b)),以辫状河三角洲平原、前缘和湖泊相沉积为主,累积地层厚度数百米至近千米不等。须三段(T3x3)是须家河组主要烃源岩发育层段之一,岩性以暗色泥岩为主,煤岩和碳质泥岩局部发育,而且烃源岩内部夹有薄层致密砂岩,宏观上形成源夹储型的致密砂岩储层,为须三段主力含气层。

  • 图1 四川盆地元坝地区构造位置(据文献[19],修改)及须家河组综合柱状图

  • Fig.1 Structural location of Yuanba areain Sichuan Basin(After citation[19], modified) and stratum histogram of Xujiahe Formation

  • 2 实测地层压力

  • 元坝地区陆相层系实测地层压力统计结果(图2)表明,自中侏罗统上沙溪庙组( J s 2) 开始发育超压,对应起始深度约3 000m;之后随深度增加,超压具有逐渐增大的趋势。须家河组( T x 3) 地层压力变化范围较大,整体介于37.0~103.4MPa,压力系数主体范围为1.03~1.97,属于常压—强超压。其中须家河组须三段(T3x3)地层压力同样具有较大的变化范围,介于45.4~93.8MPa,压力系数多数介于1.05~1.93,个别达2.27,属于常压—强超压。

  • 图2 元坝地区陆相地层实测地层压力与压力系数纵向分布

  • Fig.2 Vertical distribution for measured formation pressure and pressure coefficient of continental strata in Yuanba area

  • 3 古压力恢复

  • 3.1 流体包裹体PVTx法恢复单井古压力

  • 流体包裹体PVTx法恢复古压力,主要是基于迭代计算,使所设定初始组分的包裹体气液比(Fv) 与室温下测定的包裹体气液比达到匹配,从而来求取包裹体最小捕获压力[20]。其中包裹体均一温度(Th) 和气液比是该方法准确获取古压力的关键参数[12]

  • 3.1.1 包裹体岩相学特征

  • 利用德国ZEISS AXIO Imager D1m多功能研究级显微镜对研究区须三段致密砂岩储层的流体包裹体进行镜下观察。须三段储层中发育烃类、盐水等多种类型的包裹体,其中烃类包裹体以气烃包裹体为主,其次为含沥青包裹体,石油包裹体少见,这与须三段的Ⅱ2-Ⅲ型有机质类型有关。显微镜透射光下的气烃包裹体及其伴生的盐水包裹体多呈串珠状或成群分布于石英颗粒愈合裂缝中(图3),仅少量盐水包裹体分布于石英次生加大边中,而且大多数包裹体个体较小,多介于5~12 ਈm。其中气烃包裹体在透射光下呈无色透明状,内部为均一气相(图3(a)和(c));与气烃包裹体伴生的盐水包裹体主要呈无色透明状,形态以椭圆—次圆状或短条状为主, 内部多为气液两相,且液相约占包裹体视面积的80%~90%,气相呈气泡状做无规则运动,气泡边界多呈灰色;透射光下的沥青包裹体呈黑色,多分布于石英颗粒愈合裂缝中(图3(c))。紫外光下的气烃包裹体、盐水包裹体不可见或发微弱白色荧光(图3(b)和( d)),沥青包裹体可见微弱白色荧光(图3(d))。

  • 图3 须三段储层包裹体镜下特征

  • Fig.3 Characteristics of fluid inclusions of reservoir in the Third Member of Xujiahe Formation

  • 3.1.2 包裹体均一温度

  • 包裹体均一温度代表了其被捕获时的地层温度, 而与烃类包裹体伴生的盐水包裹体由于其均一温度的稳定性较高,可以较好地反映捕获温度[21-22]。选取与烃类包裹体伴生的含气烃盐水包裹体和盐水包裹体,利用Linkam THMS600型冷热台对其进行了均一温度测试。测试结果显示(图4),研究区须三段储层与烃类包裹体伴生的盐水包裹体均一温度范围介于94.2~179℃,统计柱状图显示存在两个峰值区间,分别为100~130℃和140~160℃。

  • 3.1.3 包裹体气液比

  • 激光扫描共聚焦显微镜(CLSM)通过测定烃类包裹体的体积及其气泡体积,计算出气液比,是准确获取气液比的主要方法[23]。但对于研究区的气烃包裹体或含气烃盐水包裹体,不发荧光或荧光微弱, 无法直接使用CLSM精确测定包裹体的气液比。周振柱等[24]曾提出一种原理类似于CLSM的包裹体气液比分析方法,首先利用高分辨率显微镜通过按一定步长调节焦距获取一系列包裹体切片,并利用绘图软件计算不同切片的包裹体及其气泡面积,然后对包裹体面积-步长和气泡面积-步长关系分别进行拟合,获取二者函数并积分,可以获取包裹体气液比,该方法也得到了一定应用。采用同样的方法, 对须三段储层的含气烃盐水包裹体的气液比进行分析。结果显示,须三段储层含气烃盐水包裹体气液比变化范围为12.0%~18.4%。

  • 图4 须三段储层烃类包裹体伴生的盐水包裹体均一温度分布

  • Fig.4 Homogenization temperature distribution of brine inclusions associated with hydrocarbon inclusions of reservoir in the Third Member of Xujiahe Formation

  • 3.1.4 包裹体组分

  • 利用PVTx法对包裹体捕获压力进行模拟时,需要设定包裹体初始组分。激光拉曼光谱是测定储层中包裹体气体成分的主要手段[20]。在室温(20℃) 下利用激光拉曼光谱对须三段储层含气烃盐水包裹体成分进行测试,但结果仅显示出包裹体宿主矿物信息,对于含气烃盐水包裹体中有机组分无法准确测定。故利用现今气藏中天然气组分(有机组分: CH4、C2H6、C3H8、iC4、nC4、iC5、nC5,无机组分:CO2、 N2、H2)来近似代替包裹体中的气态烃类组分,并将其设置为初始组分。通过迭代模拟,发现须三段致密砂岩储层含气烃盐水包裹体烃类组分中CH4 含量最高,随碳原子数增多,其组分含量逐渐降低(表1);无机组分含量少于有机组分,且各无机组分的含量无明显规律。

  • 表1 须三段储层含气烃盐水包裹体模拟组分组成

  • Table1 Simulation components of brine inclusions with gaseous hydrocarbon of reservoir in the Third Member of Xujiahe Formation

  • 3.1.5 古压力恢复

  • 在获取上述包裹体的基本参数后,利用PVTx法对须三段致密砂岩储层的含气烃盐水包裹体古压力进行了模拟,同时结合包裹体均一温度对包裹体捕获时间进行了分析。模拟结果显示(表2),须三段储层古压力在中侏罗世至早白垩世(J2—K1)呈逐渐增大的趋势,模拟的古压力分布范围介于46.3~84.1MPa,压力系数介于1.14~2.04。

  • 表2 元坝地区须三段储层古压力模拟结果

  • Table2 Paleo-pressure simulation results of reservoir in the Third Member of Xujiahe Formation in Yuanba area

  • 3.2 盆地模拟法模拟古压力演化史

  • 为获得相对准确、完整的古压力演化史,以前述包裹体PVTx法获得的古压力作为过程约束,以实测地层压力作为最终约束,采用盆地模拟法对须三段致密砂岩储层的古压力演化史进行了模拟。

  • 模拟结果如图5所示,须三段致密砂岩储层地层压力演化整体表现出“增加( J1—J3 中期)、降低(J3 晚期)、再增加(K1)、再降低(K2—现今)”的变化趋势,部分井在新近纪—第四纪表现出弱增加的趋势。从各地质时期的地层压力发育情况来看,研究区须三段储层在中侏罗世普遍开始出现超压,但压力系数仅约1.1;中侏罗世晚期至晚侏罗世中期, 地层压力开始迅速增加,晚侏罗世中期的最大地层压力和剩余压力分别达80和35MPa,对应的压力系数最大约1.77;受晚侏罗世晚期的构造抬升等因素影响,地层压力和超压规模出现小幅度的降低;早白垩世以来,地层压力又再次不断增加,早白垩世末期的最大地层压力和剩余压力分别达100和42MPa,地层压力系数约1.77;晚白垩世以来的持续隆升和地层剥蚀,导致地层压力开始较大幅度降低,而局部受构造挤压影响,地层压力在新近纪以来有再次升高的趋势,但增加幅度相对较低。

  • 图5 元坝地区单井须三段储层古压力演化史

  • Fig.5 Paleo-pressure evolution of reservoir in the Third Member of Xujiahe Formation in Yuanba area

  • 3.3 关键充注期古压力平面分布

  • 根据烃源岩主生排烃期法和流体包裹体法,并结合前人对元坝地区须家河组天然气成藏期的研究[25],须三段天然气的主要充注期为中侏罗世—晚侏罗世(J2—J3)和早白垩世(K1)。根据流体包裹体PVTx法和盆地模拟法,在对单井须三段储层古压力恢复的基础上,编制了须三段在关键充注期(晚侏罗世和早白垩世)的地层剩余压力平面分布图。晚侏罗世早期,须三段储层剩余压力整体较小,变化范围介于1.0~5.0MPa,表现为由北西向南东方向降低的趋势,其中YB21井和YL12井-YL8井位置为主要超压发育中心, 剩余压力约5.0MPa( 图6(a))。早白垩世晚期,须三段储层剩余压力整体较高(图6(b)),变化范围介于12.0~24.0MPa,同样表现出由北西向南东方向降低的趋势,其中YB21井-YL11井-YL30井位置为超压发育中心,剩余压力可达21.0~25.0MPa。整体上,储层剩余压力分布与须三段烃源岩生烃强度表现为西部整体高于东部的变化规律相一致,从而为天然气运移聚集提供了动力条件。

  • 图6 须三段储层在关键充注期的剩余压力平面分布

  • Fig.6 Plane distribution of reservoir excessive pressure at critical charging period in the Third Member of Xujiahe Formation

  • 4 地层压力演化主控因素

  • 根据超压地层的测井响应差异可以有效判断超压成因类型,其中常用的参数包括声波时差、电阻率、密度及垂向有效应力[4-5]。对于垂向压实不均衡型超压,主要表现为地层流体承担了一部分上覆地层产生的压力,具有声波时差、垂向有效应力增大,电阻率和密度减小的特征[7, 11];由于密闭条件下生烃等流体膨胀作用产生的超压,其可以打开灵活的“连通孔”而无法打开“存储孔” [4],测井曲线表现为声波时差、电阻率增大,垂向有效应力减小,密度不变或略有减小[11];由构造挤压机制产生的超压则表现为声波时差减小,电阻率和密度增大,而垂向有效应力不变[26]。泥岩声波时差和电阻率对地应力响应的灵敏度不同,在强构造挤压应力下泥岩电阻率灵敏度更高[26]。因此选用测井电阻率、密度及垂向有效应力交会图来判识超压成因类型。

  • 须三段超压地层在垂向有效应力-电阻率交会图(图7(a))上,表现为垂向有效应力减小、电阻率增大和垂向有效应力不变、电阻率增大两种情况,分别表示流体膨胀和构造挤压成因机制。在密度-电阻率交会图(图7(b))上,表现为密度不变、电阻率增大和密度与电阻率同时增大两种情况,也分别表示流体膨胀和构造挤压成因机制。因此须三段超压成因类型主要为流体膨胀和构造挤压作用,根据数据点的分布情况认为流体膨胀作用对超压的贡献要明显多于构造挤压作用。

  • 图7 元坝地区超压成因类型的测井判识图

  • Fig.7 Log identification for genetic type of overpressure in Yuanba area

  • 研究区须三段主要为“源夹储”型的源储组合关系,其中烃源岩有机质类型为Ⅱ2-Ⅲ型,现今TOC最大可达6.0%,且处于高成熟—过成熟阶段,具备生成大量天然气的有利条件。通过对比研究区单井地层压力与烃源岩有机碳( TOC) 的吻合关系可知(图8),研究区陆相层系特别是须家河组剩余压力随着烃源岩TOC的增大而增大,反映了生烃作用导致流体膨胀是须三段超压形成的主要成因,此类超压主要形成于大量生排烃期。

  • 除生烃作用主导的流体膨胀增压外,构造活动也是须三段超压发育的重要因素。地层在强烈构造挤压活动下发生抬升剥蚀,由于上覆地层负荷减小、地层温度降低,岩石骨架发生回弹,地层流体体积收缩而引起孔隙空间变大,地层压力降低[27]。由研究区构造发育史可知,受到燕山—喜山运动期的龙门山、大巴山和米仓山等构造带的推覆挤压作用,使部分地层产生褶皱和断裂,然而研究区现今构造相对低缓,且断裂以四级断层为主,反映了研究区经历了一定构造挤压作用,但构造挤压作用下形成的超压幅度可能偏低于生烃作用,如李军等[28] 估算了构造挤压对川东北地区超压的贡献量,其中元坝地区增压量平均占剩余压力的39.14%,而对于须三段烃源岩层系来说,该比例应该低于39.14%。

  • 需要考虑的是,生烃作用和构造挤压作用形成的超压受燕山—喜山期的多期构造活动影响,也会不断释放,超压的保存条件对于超压同样具有重要的控制作用。从源储配置关系来看,须三段致密砂岩储层上下被烃源岩层包夹,具备相对较好的保存条件。对于须三段储层,则经历了强压实、强胶结和交代等3种主要破坏性成岩作用(图9),镜下可见云母颗粒被压弯呈“拱桥状”(图9( a))、岩石颗粒呈点—短线状接触(图9( b))、方解石孔隙式胶结(图9( b) 和( c)) 以及方解石交代岩屑颗粒(图9(d))等。这些成岩作用导致须三段储层物性极差,现今孔隙度集中分布于1.0%~4.0%,渗透率集中分布于(0.001~0.1) ×10-3 μm 2,为“特低孔、低渗” 储层。须三段储层的强压实作用和强胶结作用导致储层致密化主要发生于天然气大量生排烃期之初的中—晚侏罗世,即发生在大规模超压形成之前,这使得超压能够得到较大程度的保存。

  • 图8 YL30井陆相地层剩余压力与TOC随深度变化关系

  • Fig.8 Relationship between excessive pressure and TOC variation with depth in well YL30

  • 图9 元坝地区须三段储层破坏性成岩作用照片

  • Fig.9 Photos of destructive diagenesis of reservoir in the Third Member of Xujiahe Formation in Yuanba area

  • 综上,认为烃源岩生烃作用、构造活动、源储配置关系及储层致密化对元坝地区须三段致密砂岩储层压力演化具有重要的控制作用。其中中侏罗世至晚侏罗世中期( J1—J3 中期),受早期生排烃作用影响,须三段出现超压并逐渐增加;受晚侏罗世晚期(J3 晚期)的构造抬升剥蚀等因素影响,地层压力和超压规模开始降低,但由于储层致密化使得超压得到较大程度的保存;早白垩世(K1)以来,须三段烃源岩有机质成熟度 Ro 达2.0%,天然气大量生成并快速充注进入源内致密砂岩储层,此时由于储层的强烈致密化,使孔隙流体无法及时排出,导致须三段储层剩余压力迅速增加。晚白垩世(K2)以来发生强烈的构造隆升和地层剥蚀,生烃增压停止,虽然该时期构造挤压会产生一定程度的超压且储层已经致密化,但构造作用下形成的断层、裂缝使储层超压得到较大幅度的释放,仅局部由于新近纪以来构造挤压产生的超压规模略大于同步的超压释放影响,导致剩余压力有小幅度增加的趋势。

  • 5 结论

  • (1)元坝地区陆相地层由中侏罗统上沙溪庙组(J2 s)开始发育超压,须家河组须三段地层压力变化范围较大,介于45.4~93.8MPa,压力系数多介于1.05~1.93,属于常压—强超压。

  • (2)元坝地区须三段储层压力整体表现出“增加(J1-J3 中期)、降低(J3 晚期)、再增加(K1)、再降低(K2-现今)”的演化趋势。其中,晚侏罗世早期和早白垩世晚期的剩余压力最大分别达5.0MPa和24.0MPa;平面上,关键期的剩余压力表现出由北西向南东方向降低的趋势。

  • (3)烃源岩生烃、构造活动、源储配置及储层致密化是元坝地区须三段致密砂岩储层压力演化的主要控制因素。中侏罗世至早白垩世的生烃增压作用明显,“源夹储”型的配置关系及储层致密化使超压得到有效保存,晚侏罗世晚期和早白垩世以来的构造挤压作用不仅形成超压,也导致隆升剥蚀并形成断层、裂缝,使超压得到较大程度释放。

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