Spatiotemporal evolution of interseismic coupling and stress accumulation near an asperity on a vertical strike-slip fault: Insights from three-dimensional viscoelastic numerical simulation
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摘要: 了解断层凹凸体附近的运动学状态和应力积累对于评估其地震危险性至关重要。断层的闭锁程度被广泛用来刻画断层的震间运动学特征，但断层闭锁程度空间分布与凹凸体位置之间的关系很少被人关注。除此之外，地球介质的流变作用如何调节凹凸体附近滑动亏损和剪应力积累速率的时空演化目前仍不清楚。为揭示凹凸体对震间变形以及应力积累的调节作用，构建了包含直立走滑断层的三维弹性/黏弹性有限元模型，采用接触算法考虑凹凸体的闭锁作用，讨论单一凹凸体附近闭锁程度和剪应力积累速率的时空变化，并通过鲜水河断裂的实例对基于断层闭锁的震间变形方法进行验证。研究结果显示：由于凹凸体的锁定作用，距离凹凸体一定范围的断层面无法完全自由滑动，导致以凹凸体为中心的一定区域内存在滑动亏损，因此断层闭锁程度呈现出以凹凸体为中心的环状衰减样式；在完全弹性条件下，凹凸体附近区域的闭锁程度和剪应力积累速率不随时间变化；在黏弹性条件下，凹凸体附近区域的闭锁程度和剪应力积累速率等值线随时间的加载不断变大，且闭锁程度等值线随时间变化效应更加明显；对于断层两侧黏度存在差异的场景，由于断层两侧松弛时间不一样，断层的震间变形和应力积累速率主要受到松弛时间较低一侧介质流变性质控制。最终得出以下结论：由于介质的连续性，凹凸体邻近区域虽然不闭锁，但其滑动速率仍低于块体运动速度，从而导致闭锁程度的空间样式表现为由凹凸体向外逐渐衰减；黏弹性效应调节凹凸体附近的变形，导致凹凸体附近闭锁程度等值线空间范围随时间增大；断层闭锁程度可以近似作为衡量剪应力积累速率的指标，当不考虑黏弹性效应可以近似取0.5作为中强闭锁的阈值，低于该值剪应力积累不明显；考虑鲜水河断裂带炉霍至康定段的空间非均匀闭锁分布，模拟得到的地表速度与GPS观测吻合较好，验证了方法的可靠性。该项研究建立了断层闭锁程度和剪应力积累速率之间的桥梁，为潜在震源区识别提供有益思路。Abstract:
Objective Understanding the kinematic state and stress accumulation near fault protuberances is crucial for accurate assessment of earthquake hazards. Interseismic coupling (ISC) is a widely used method for characterizing the kinematic behavior of faults. Despite its importance, the correlation between the spatial distribution of ISC and the positioning of fault asperities, areas of increased frictional resistance, has not been extensively studied. Furthermore, the influence of the rheological properties of Earth materials on the temporal and spatial evolutions of slip deficits and shear stress accumulation in close proximity to these asperities remains poorly understood. Methods We developed a set of three-dimensional (3D) elastic and viscoelastic finite element models to investigate the effects of fault asperities on interseismic deformation and stress accumulation. These models incorporate vertical strike-slip faults and use sophisticated contact algorithms to simulate the mechanical locking associated with asperities. Our innovative approach, referred to as the “binary fault-locking approach”, simplifies fault behavior into a binary system, categorizing states as either “locked” or “unlocked”. The present study analyzes the spatial and temporal variations in the ISC and shear stress accumulation rates around a single asperity, providing novel insights into the mechanics of fault systems. In addition, we validate the efficacy of the “binary fault-locking approach” by applying it to the Xianshuihe fault, thereby reinforcing the relevance of our findings to real-world fault behavior. Through this study, we aim to enhance our understanding of fault mechanics and improve earthquake hazard assessments, which ultimately contributes to more effective risk-mitigation strategies. Results Because of the mechanical locking of the asperity, a fault-sliding surface within a certain distance from the asperity cannot slide freely, resulting in a slip deficit in an area centered around the asperity. Consequently, the degree of fault-locking displays a ring-shaped attenuation pattern centered on this asperity. Under purely elastic conditions, the ISC and shear stress accumulation rates near the vicinity of the asperity remained constant over time. Conversely, under viscoelastic conditions, the contours of the ISC and shear stress accumulation in the areas surrounding the asperity expanded with time under loading, and the effects of temporal changes in the locking degree became more pronounced. In scenarios where the viscosity differs on either side of the fault, the interseismic deformation and stress accumulation rate of the fault are primarily controlled by the rheological properties of the material on the side with a lower relaxation time, owing to the different relaxation times on either side of the fault. Conclusion (1) Because of the continuity of the medium, although the region adjacent to an asperity is not fully locked, its slip velocity is still lower than the movement velocity of block, resulting in a spatial pattern of decreasing ISC outward from the fault asperity. (2) Viscoelastic effects regulated the deformation near a fault asperity, leading to an increase in the spatial extent of the ISC over time. (3) The ISC can serve as an approximate indicator of the shear stress accumulation rate. Irrespective of viscoelastic effects, a value of approximately 0.5 can be used as the threshold for moderate to strong locking, and shear stress accumulation is insignificant below this value. (4) Considering the spatially nonuniform fault coupling along the Luhuo-Kangding segment of the Xianshuihe fault, the simulated surface velocities closely matched the GPS observations, thus confirming the reliability of the method. Significance This study establishes an important connection between ISC and shear stress accumulation rate, providing valuable insights for identifying potential seismic hazards. Overall, this study emphasizes the intricate interactions between fault dynamics and geological structures, and highlights the significance of detailed modeling for understanding earthquake mechanisms. By addressing the gaps in knowledge regarding the influence of protuberances on fault behavior, this research contributes valuable information to the field of seismic hazard estimation, thereby enhancing our ability to effectively mitigate earthquake risks. -
Key words:
- numerical simulation /
- asperity /
- interseismic coupling /
- stress accumulation /
- viscoelasticity /
- crustal stress /
- earthquake dynamics
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0. 引言
水溶天然气(简称水溶气)是指溶解在地层水中的气体, 气体成分以甲烷为主, 还含有少量乙烷等重烃类气体和氢气、氮气、CO2等非烃气体以及氦气等稀有气体, 是一种非常规能源[1]。这种尚未被充分开发的天然气资源在全球范围内分布很广, 潜在资源量很大。全世界含油气盆地、含煤盆地以及其他水文盆地地层水中溶解的甲烷气资源预计总量约为n×1016~n×1018 m3, 超过全球已探明的油气及煤炭资源总量的若干倍。盆地中已查明的水溶性天然气资源量约为33837×1012 m3, 是常规天然气资源量的100多倍。日本、美国、俄罗斯、乌克兰、哈萨克、乌兹别克、阿塞拜疆、土库曼、匈牙利、意大利、菲律宾、尼泊尔、伊朗等国都发现了水溶天然气, 并开展了勘探、开发及地质综合研究, 尤其在日本, 大约四分之一国土都发现了水溶气, 并累计了70多年的勘探开发工作经验[2]。
从理论上讲, 水溶气资源领域较常规天然气的分布领域更加广泛[2]。但由于水溶气在中国还属于一个较新的、研究较少的新型非常规能源, 勘探开发还处于初步阶段, 很多研究工作属于空白。
原地质矿产部第三普查勘探大队(以下简称三普)在20世纪60年代对渭河盆地进行了油气资源普查, 曾先后两上渭河盆地, 打出油气普查井32口, 其中发现气测异常井15口, 并完成《汾渭盆地石油普查阶段地质成果报告》。1974年在西安地区油气普查中, 三普在渭深13井发现8个组的气测异常, 并测得产气量208 L/d[1]。其后因工作重点转移和工业价值效果不明显而未受到重视, 使得盆地的资源勘探程度非常低。近几年随着对渭河盆地的重新认识和勘探, 在固市凹陷钻探的地热井和天然气探井均发现水溶甲烷气存在, 且含量较高, 预示了非常好的勘探前景, 成为极具工业价值的水溶甲烷气资源开发潜力盆地。
1. 水溶甲烷气类型分析
渭河盆地位于陕西省中部(关中平原), 是秦岭造山带与鄂尔多斯盆地2个大地构造单元接合部位的新生代断陷盆地, 其地层属华北地层区南缘分区。研究区固市凹陷位于渭河盆地东部, 属于渭河盆地的次级凹陷[1](见图 1)。固市凹陷富含多种资源, 主要包括水溶天然气、富氦水溶气、地热水资源等。
1.1 甲烷气成因分类
研究区内发现的水溶气以甲烷为主, 是一种多成因气体, 可分为生物化学有机成因和宇宙无机成因两大类, 水溶气中碳同位素和重烃含量的组合是判断其成因的主要参数。
① 生物化学成因气:有机盐和无机盐被微生物分解后形成, 如烃类气体、二氧化碳、氮气、氨气、硫化氢等。
② 宇宙无机成因气体:地球形成时保留下来的气体, 如惰性气体及部分无机成因的二氧化碳、甲烷气等[2]。
生物化学有机成因甲烷气又分为生物和热演化成因2种, 可根据甲烷碳同位素含量多少区分其成因类型。一般认为, 生物成因气碳同位素值小于-60‰, 而且几乎不含重烃气体组分; 热演化成因气分为油型气和煤型气, 油型气随着演化程度的增高, 重烃含量减少, 煤型气以重烃含量低为特征。不同热演化阶段甲烷碳同位素特征值不同, 一般为-28‰~-45‰, 随着演化程度的增高, 甲烷碳同位素值增大[3~4](见表 1)。
表 1 珠江组及其相邻地层概况(据文献[9]修改)Table 1. Stratigraphic profiles about Zhujiang Formation and the adjacent strata演化阶段 天然气类型 Ro/% δ13CCH4/‰ 计算值 确定值 生物化学作用带 生物化学气 ≤0.3 ≤-58.6 ≤-60 生物化学作用带 生物—热催化过渡气 0.3~0.6 -58.6~-44.9 -65~-45 热催化作用带 煤型热解气 0.6~2.0 -44.9~-28.0 -45~-28 热裂解作用带 煤型裂解气 ≥2.0 ≥-28.0 ≥-28 1.2 研究区甲烷气成因
① 高热演化甲烷气:煤型热解气的典型特征为甲烷碳同位素值分布在-45‰~-28‰, 重烃含量低。
此次分析中6个蓝田—灞河组较深层地热水溶气样品的碳同位素值分布于-28.1‰~-39.8‰区间(见表 2), 证明蓝田—灞河组深层水溶气属于典型的煤型热解气, 而该层不发育烃源岩, 因此水溶气应该来源于下部地层。
表 2 固市凹陷天然气碳同位素分析结果Table 2. Analysis results of the natural gas in Gushi Sag井名 取样地层 δ13 CCH4/‰ δ13 CCO2/‰ δ13 CC2H6/‰ WN1 蓝田—灞河组 -34.9 -11.4 -21.6 WN2 蓝田—灞河组 -33.8 -11.6 -26.4 WN3 蓝田—灞河组 -39.8 -10.0 -25.9 WN4 蓝田—灞河组 -28.8 -14.0 -26.2 WN5 蓝田—灞河组 -28.1 -13.1 WN6 张家坡组 -65.6 -15.2 -53.2 WN7 蓝田—灞河组 -28.4 -14.0 -26.2 ② 生物甲烷气:此次分析中WN6井气样采于浅层张家坡组, 其甲烷碳同位素(δ13 C1)值为-65.6‰(见表 2), 国内外大多数生物气成因的δ13 C1值定为-85‰~-55‰, 据此得出张家坡组浅层水溶甲烷气为典型的有机成因浅层生物化学气, 结合烃源岩和有机质类型, 该层水溶甲烷气来源于本层含碳质泥灰岩, 属自生自储的生物气。
腐泥型热解气和腐殖型热解气的乙烷碳同位素具有典型的母质继承性, 分布也明显不同。一般腐泥型热解气的乙烷碳同位素值小于-29‰, 而腐殖型热解气的乙烷碳同位素值大于-29‰[5]; 从这一标准也可以看出固市凹陷水溶天然气属于有机成因, 且蓝田—灞河组较深层地热水溶气属于腐殖型热解气(见表 2)。
从固市凹陷13口井所采地热水溶气样品气体组分数据表(见表 3)可以看出, 可燃气体所占比例最高, 全烃含量平均高达72.23%, 甲烷气质量分数大多在70%~90%, 最高为99.03%, 已大大超过水溶可燃天然气工业标准(30%), 有个别井偏低, 不排除取样时的误差。CO2质量分数多数小于10%, 个别井较高(见表 3)。
表 3 固市凹陷水溶天然气组分Table 3. Water soluble gas composition in Gushi Sag井号 甲烷/% 乙烷/% 丙烷/% 异丁烷/% 正丁烷/% 异戊烷/% 正戊烷/% 全烃/% CO2/% 1 82.44 3.405 0.343 0.012 0.064 0.013 0.021 86.29 8.824 2 89.74 3.093 0.556 0.035 0.167 0.01 0.035 93.85 1.913 3 43.17 2.300 0.545 0.115 0.185 0.039 0.048 46.40 45.678 4 12.52 0.134 0.020 0.002 0.005 0.001 0.002 12.68 17.992 5 47.52 0.442 0.053 0.006 0.013 0.003 0.004 48.43 8.241 6 98.51 0.076 0.013 0.002 0.001 0 0 98.60 1.237 7 99.03 0.092 0.014 0.004 0.001 0.001 0 99.14 0.316 8 62.77 0.018 0.002 0 0.001 0 0 62.79 15.336 9 72.20 2.727 0.536 0.040 0.166 0.016 0.044 75.73 19.830 10 98.90 0.076 0.013 0.005 0.001 0.001 0 98.99 1.249 11 84.91 0.072 0.018 0.007 0.002 0.001 0.001 85.01 0.411 12 31.17 0.420 0.160 0.028 0.025 0.009 0.009 35.10 0.256 13 95.90 0.059 0.012 0.006 0.002 0.002 0 95.98 1.292 平均 70.68 0.993 0.176 0.020 0.049 0.007 0.013 72.23 9.429 固市凹陷生烃源岩层主要为张家坡组泥灰岩段, 有机碳含量虽多在0.5%以下, 但仍有较封闭的浅湖区达到0.86%的水平, 具有生气的源岩条件, 显示未成熟的生物气生烃潜力相当可观。
1.3 二氧化碳成因
CO2可以是有机成因, 也可以是无机成因。有机成因的CO2碳同位素δ13 CCO2值一般小于-10‰; 无机成因CO2的碳同位素δ13 CCO2值一般大于-10‰, 无机成因的CO2中, 幔源成因的碳同位素δ13 CCO2值一般在-8‰~-4‰之间[3]。
固市凹陷13口井所采样品中CO2的特征为:含量低, 质量分数大部分都小于10%, 不排除个别取样误差; 其δ13 CCO2和伴生烃类气的δ13 CCH4偏轻, 所有井样品的δ13 CCO2值均小于-10‰(见表 2), 为典型的壳源型有机成因[5]。
2. 生物气地质条件
2.1 有机质特征
生物成因气形成和聚集的有利地质条件主要包括5个方面, 即高沉积速率、低地温梯度、充足的有机质、缺氧的还原环境及储盖组合形成的良好圈闭[5]。
① 固市凹陷张家坡组沉积速率较高, 上覆地层三门组的沉积速率更高。以最大沉降幅度与沉积厚度相当进行测算, 盆地新生代(距今50 Ma)的沉降速率为0.14 mm/a, 其中始新世—中新世(距今38 Ma)时期的沉降速率为0.1 mm/a, 上新世(距今8.8 Ma)时期为0.11 mm/a, 第四纪(距今3.2 Ma)时期为0.74 mm/a, 几乎与我国著名的第四纪莺歌海生物气盆地的沉积速率(0.8 mm/a)和柴达木三湖凹陷生物气沉积速率(0.715 mm/a)相当(见表 4)。
表 4 固市凹陷与柴达木三湖地区生物气资源类比Table 4. Comparison of biogas resources in Gushi Sag and Qaidam Sanhu area地区 地质年代 沉积速率/(mm·a-1) 有机碳含量/% 地层温度/℃ 地温梯度/(℃/100 m) 地层压力/MPa 储层孔隙度/% 砂层厚度/m 生物气资源量/108 m3 δ13 C1/‰ 固市凹陷 N2z 0.740 0.55 65.75 3.43 15 30 200 11784 -65.6 柴达木三湖地区 Q1s 0.715 0.3~0.4 63 3.78 16.8 25~41 300 11210 -60.58~-66.38 ② 固市凹陷张家坡组具有中等的地温梯度和低于80 ℃的地层温度。固市凹陷张家坡组产气层深度均在2000 m以内, 根据WN6井1461~1550 m深度测试, 求得平均地层温度为65.75 ℃, 地温梯度3.43 ℃/100 m。在张家坡组沉积时期, 古气候是温带向趋凉方向转化的时期, 固市凹陷张家坡组最高地层温度在80 ℃以内, 是生物气生成的理想地温条件。
③ 固市凹陷张家坡组气源岩实测结果:张家坡组气源岩有机碳含量平均0.49%, 氯仿沥青"A"含量平均0.043%, 生烃潜量平均0.98 mg/g。有机质类型多为Ⅱ1型, 其次为Ⅰ型, 即腐殖腐泥型和腐泥型, 镜质体反射率(Ro)平均0.55%, 岩石热解最高解温(Tmax)平均428 ℃(见表 5-表 8), 均表明有机质热演化程度较低, 处于未成熟阶段, 以生化作用和部分热催化作用的生物甲烷气为主。
表 5 陆相烃源岩有机质丰度评价指标Table 5. Evaluation indicators of organic matter abundance in continental hydrocarbon source rocks指标 湖盆水体类型 非烃源岩 烃源岩类型 差 中等 好 最好 总有机碳/% 淡水—半咸水 ＜0.4 0.4~0.6 ＞0.6~1.0 ＞1.0~2.0 ＞2.0 咸水—超咸水 ＜0.2 0.2~0.4 ＞0.4~0.6 ＞0.6~0.8 ＞0.8 氯仿沥青"A"/% ＜0.015 0.015~0.050 ＞0.050~0.100 ＞0.100~0.200 ＞0.200 HC/10-6 ＜100 100~200 ＞200~500 ＞500~1000 ＞1000 (S1+S2)/(mg·g-1) ＜2 2~6 ＞6~20 ＞20 表 6 固市凹陷张家坡组有机碳、氯仿沥青"A"含量Table 6. Contents of organic carbon and chloroform bitumen "A" in Zhangjiapo Formation in Gushi Sag井号 有机碳/% 氯仿沥青"A"/% 烃源岩类型 沉积相带 样品数 最小—最大/平均值 样品数 最小—最大/平均值 渭1井 12 0.112~0.858/0.47 12 0.0063~0.0674/0.032 差—中 浅湖 渭3井 36 0.034~0.423/0.18 5 0.0029~0.0241/0.0113 差—非 浅湖 渭参1井 6 0.212~0.459/0.30 2 0.0139~0.0142/0.0141 差—好 浅湖 渭参2井 8 0.060~0.143/0.10 1 0.0037 非 河湖 渭参3井 5 0.326~0.408/0.36 5 0.0107~0.0225/0.0174 差 浅湖 渭参4井 25 0.083~1.185/0.48 28 0.0021~0.257/0.0362 差—中 较深湖 渭参5井 18 0.239~0.786/0.45 21 0.0044~0.348/0.0468 差—中 较深湖 渭深12井 27 0.24~0.92/0.52 27 0.0011~0.0663/0.0275 差—好 较深湖 渭深16井 23 0.08~1.17/0.50 23 0.0025~0.0368/0.0174 差—好 较深湖 渭深17井 16 0.22~1.3/0.69 24 0.0059~0.045/0.0181 差—好 较深湖 渭南1井 1 0.59 1 0.1195 中—好 较深湖 渭南2井 1 0.56 1 0.0256 差—中 较深湖 渭南3井 2 0.08~0.30/0.19 2 0.0061~0.0072/0.00665 差—非 浅湖 渭南4井 1 0.41 1 0.0115 差—非 较深湖 渭南5井 2 0.26~0.33/0.295 2 0.0082~0.0090/0.0086 差—非 较深湖 渭南6井 2 0.65~0.82/0.735 2 0.061~0.089/0.075 中—好 较深湖 表 7 固市凹陷张家坡组烃源岩热解分析数据Table 7. Pyrolysis analysis data of hydrocarbon source rock in Zhangjiapo Formation in Gushi Sag样品号 井号 井深/m 岩性 可溶烃
S1/(mg·g-1)热解烃
S2/(mg·g-1)最高峰温
Tmax/℃产油潜率
(S1+S2)/(mg·g-1)wh-1 渭南5 1764.75 灰黑色泥岩 0.05 2.84 430 2.89 wh-2 渭南6 1687.85 灰黑色泥岩 0.02 0.97 421 0.99 wh-3 渭南11 1673.76 灰黑色泥岩 0.05 2.42 432 2.47 wh-4 1669.76 灰黑色泥岩 0.03 1.32 422 1.35 wh-5 渭南10 1479.16 灰黑色泥岩 0.01 0.17 426 0.18 wh-6 1610.15 灰黑色泥岩 0.01 0.24 428 0.25 wh-7 渭南8 1843.54 深灰色泥页岩 0.01 0.49 430 0.50 wh-8 1845.83 深灰色泥岩 0 0.03 431 0.03 wh-9 渭南9 1543.26 深灰色泥岩 0.01 0.16 431 0.17 表 8 固市凹陷张家坡组镜质体反射率分析结果Table 8. Vitrinite reflectance of hydrocarbon source rock in Zhangjiapo Formation in Gushi Sag样品号 井号 井深/m 岩性 测点数 Ro/% 最小值 最大值 平均值 wh-1 渭南5 1764.75 灰黑色泥岩 5 0.50 0.65 0.58 wh-2 渭南6 1687.85 灰黑色泥岩 5 0.49 0.62 0.54 wh-3 渭南11 1673.76 灰黑色泥岩 1 0.53 wh-4 1669.76 灰黑色泥岩 1 0.49 wh-5 渭南10 1479.16 灰黑色泥岩 1 0.59 wh-6 1610.15 灰黑色泥岩 1 0.48 wh-7 渭南8 1843.54 深灰色泥页岩 10 0.52 0.64 0.57 wh-8 1845.83 深灰色泥岩 1 0.60 wh-2 渭南9 1543.26 深灰色泥岩 8 0.50 0.68 0.59 2.2 凹陷基底及气源分析
根据前人研究, 渭河盆地基底以渭河断裂为界, 断裂以北基底为下古生界碳酸盐岩地层, 断裂南部基底为秦岭北侧的太古界花岗岩和变质岩层[7]。固市凹陷沉积于渭河断裂之上, 在断裂南北均有分布, 因此凹陷南部基底为花岗岩和变质岩, 断裂北部基底属于下古生界碳酸盐岩地层的范围。
固市凹陷西部三原凸起某探井中钻遇二叠系[8], 也佐证了固市凹陷北部基底有下古生界碳酸盐岩地层沉积。
凹陷内深层蓝田—灞河组大量分布高热演化煤型裂解甲烷气与有机成因的CO2气, 证实固市凹陷北部鄂尔多斯南缘下部可能沉积高热演化的煤系地层, 经过水动力系统运移至凹陷中心, 而固市凹陷新生界烃源岩成熟度低, 难以大量生成CO2, 因此CO2来源也说明了该凹陷周边下部存在高热演化煤系地层。
3. 油气地质分析
基础地质资料、地热井与水溶气井钻井资料显示, 固市凹陷新近系沉积厚度达数千米, 以河湖相沉积为主, 岩性组合为砂(砾)岩-泥灰岩组合, 表现为下粗上细的正旋回沉积特征, 即旋回下部河流相为良好的储集层, 上部为巨厚的湖相泥灰岩, 是良好的盖层, 纵向上构成了良好的储盖组合。自下而上发育2~3套下粗上细的储盖组合, 分别为高陵群、蓝田—灞河组、张家坡组[1]。
新近系张家坡组、蓝田—灞河组储层均为层状砂岩体, 属中、高孔隙-裂隙型储层, 砂岩孔隙性和渗透性好, 为主要的地热水赋水层段和水溶甲烷气成藏层段。钻探显示, 该区潜在油气藏的主要盖层岩性为湖相泥灰岩、粉砂质泥岩, 单层厚度一般为数米, 分布面积较广。
高陵群早期为河流相, 晚期为湖相—河流相沉积, 其中冷水沟组岩性主要为紫红色泥岩、砂岩, 寇家沟组岩性为棕红色、桔黄色泥岩和灰白色、棕黄色砂岩。由下向上岩性变细, 构成一套储盖组合, 储层物性相对较好。
蓝田—灞河组与高陵群具有相似的沉积相特征及岩性组合, 为一套红色调为主的砂泥岩交替沉积。其中蓝田组岩性主要为深红色黏土及棕红、灰白色砾岩, 属河湖相沉积; 灞河组是河流相为主的河湖相沉积, 上部以泥页岩为主, 普遍含石膏, 下部砂体发育。孔隙度平均15%左右; 渗透率一般在30×10-3 μm2以上。
张家坡组为浅湖—半深湖相沉积, 以泥灰岩为主; 张家坡组砂岩储层渗透率普遍较大, 分布在128.5×10-3~1529.0×10-3 μm2之间, 平均386.5×10-3 μm2, 储层物性较好, 孔隙度在15.20%~42.30%, 平均29.65%, 相对较高; 非储集层岩性以泥灰岩为主, 尤其是上部泥灰岩厚度大, 分布面积广, 为该凹陷水溶天然气成藏的良好盖层。
固市凹陷新生界发育多套良好储盖组合, 张家坡组又发现了储量丰富的非常规水溶甲烷生物气。该区张家坡组虽然埋藏深度不大, 深埋历史较短, 但其断裂体系发育, 具有良好的运移通道[9], 而且发育良好储盖结合, 为固市凹陷新生界生物气藏成藏提供了良好的地质基础。
4. 结论
通过对地层水溶甲烷气碳同位素δ13C1及重烃的含量研究发现, 不同层位的水溶甲烷气成因类型不同。新近系张家坡组水溶甲烷气主要为有机成因生物气, 来源于本层含碳质较高的灰黑色泥灰岩生物分解, 为自生自储式; 下部蓝田—灞河组水溶甲烷气以未成熟的煤型热解气(煤型腐殖型热解气)为主, 来源于下部地层。
对CO2碳同位素的分布范围和质量分数的含量进行分析得出δ13CCO2小于-10‰, 为典型的壳源型有机成因。
结合CO2碳同位素和乙烷碳同位素分析认为, 蓝田—灞河组水溶甲烷气和CO2来源于下部地层的混合型气, 即下部地层可能存在有机成因的煤型热解气层系。
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图 1 模型网格图
模型坐标为右手系，其中x轴正向代表东，y轴正向代表北，z轴正向代表上
Figure 1. Mesh of the finite element method (FEM) model
An east-north-up (ENU) system is used in the model.
图 2 弹性模型的计算结果
a—闭锁程度；b—剪应力积累速率
Figure 2. Elastic model results
(a) Interseismic coupling (ISC); (b) Shear stress accumulation rates
图 3 均匀黏弹性模型计算的闭锁程度和剪应力积累速率的时空演化特征
a—闭锁程度；b—剪应力积累速率
Figure 3. Spatiotemporal evolution of the ISC and shear stress accumulation rates derived from the homogeneous viscoelastic model
(a) Interseismic coupling (ISC); (b) Shear stress accumulation rates
图 4 横向非均匀黏度模型计算的闭锁程度和剪应力积累速率的时空演化特征
a—闭锁程度；b—剪应力积累速率
Figure 4. Spatiotemporal evolution of the ISC and shear stress accumulation rates derived from the heterogeneous model
(a) Interseismic coupling (ISC); (b) Shear stress accumulation rates
图 5 监测点闭锁程度及剪应力积累速率随时间的变化
a—闭锁程度；b—剪应力积累速率
Figure 5. Temporal evolution of the ISC and shear stress accumulation rates at the monitoring points
(a) Interseismic coupling (ISC); (b) Shear stress accumulation rates
图 6 北向位移速率云图
a—完全弹性模型；b—横向均匀黏弹性模型（时间为5 λ，即1.25×1010 s）；c—横向非均匀黏弹性模型（时间为5 λ，即1.25×1010 s）
Figure 6. Contour map of the northward displacement rate
(a) Elastic model; (b) Homogeneous viscoelastic model; (c) Heterogeneous viscoelastic modelThe observed depth is 8 km for all panels. The snapshots in panels (b) and (c) represent a time of 5 λ (1.25 × 1010 s).
图 7 由弹性模型得出的凹凸体附近闭锁程度和剪应力归一化因子等值线
红线表示ISC等值线；黑线表示Φτ等值线，等值线间隔为0.2，由凹凸体向外扩展的4根黑线分别为Φτ=0.8、0.6、0.4、0.2
Figure 7. Contour map of the ISC and shear stress normalization factor near an asperity derived from the elastic model
Red lines represent the ISC contour lines at an interval of 0.1. Black lines represent the Φτ contour lines at an interval of 0.2. The four black lines extending outward from the asperity were 0.8, 0.6, 0.4 and 0.2.
图 8 由黏弹性模型给出的凹凸体附近闭锁程度和剪应力归一化因子的时空演化特征
a—均匀黏弹性模型；b—非均匀黏弹性模型红线表示ISC等值线；黑线表示Φτ等值线，由凹凸体向外扩展的4根黑线分别为Φτ=0.8、0.6、0.4、0.2
Figure 8. Spatiotemporal evolution of the ISC and shear stress normalization factor near an asperity derived from the viscoelastic model
(a) Homogeneous viscoelastic model; (b) Heterogeneous viscoelastic model Red lines represent the ISC contour lines at an interval of 0.1 Black lines represent the Φτ contour lines at an interval of 0.2. The four black lines extending outward from the asperity were 0.8, 0.6, 0.4 and 0.2.
图 9 鲜水河断裂带邻区的震间变形
a—鲜水河断裂带闭锁程度分布图（Jiang et al., 2015；黑色划线区域为此次模型中设置的凹凸体的位置）；b—地表速度模拟值和GPS速度的比较（带误差椭圆的红色箭头表示模拟的地表地震间速度，带误差椭圆的蓝色箭头表示模型内部的GPS速度观测值，带误差椭圆的白色箭头表示模型外的GPS速度观测值）
Figure 9. Interseismic deformation near the Xianshuihe fault
(a) Map of the locking degree of the Xianshuihe fault (Jiang et al., 2015; The area outlined by the black line represents the asperities used in this model.) (b) Comparison of simulated surface and GPS velocities (The red arrow with an error ellipse represents the simulated interseismic velocity, blue arrow with an error ellipse represents the GPS velocity, and white arrow with an error ellipse represents the GPS velocity outside the model domain.)
表 1 数值模拟中使用的弹性参数
Table 1. Elastic parameters used in the numerical simulation
分层 弹性模量/Pa 泊松比 密度/(kg/m3) 上地壳 8×1010 0.25 2700 下地壳 1×1011 0.25 3000 岩石圈地幔 1.5×1011 0.28 3300 
下载: 导出CSV
表 2 模型中使用的黏度参数
Table 2. Viscosity parameters used in the model
上地壳块体1 上地壳块体2 下地壳块体1/Pa s 下地壳块体2/Pa s 上地幔块体1/Pa s 上地幔块体2/Pa s 模型1 − − − − − − 模型2 − − 1×1020 1×1020 1×1020 1×1020 模型3 − − 1×1019 1×1020 1×1019 1×1020
下载: 导出CSV
表 3 鲜水河断裂带数值模拟中使用的参数
Table 3. Parameters used in the numerical simulation of the Xianshuihe fault zone
分层 深度/km 弹性模量/Pa 泊松比 密度/(kg/m3) 黏度/（Pa s） 上地壳 18 8× 1010 0.25 2700 下地壳 18 1× 1011 0.25 3000 1×1020 岩石圈地幔 64 1.5× 1011 0.28 3300 1×1020
下载: 导出CSV
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