川中须家河组致密气运移地球化学示踪

秦胜飞1,李金珊2,袁 苗1,杨 春1,杨 雨3,赵长毅1

(1.中国石油勘探开发研究院,北京 100083; 2.北京科技大学 土木与资源工程学院,北京 100083; 3.中国石油西南油气田分公司,四川 成都 610051)

:致密气成藏研究近年来备受关注,天然气在致密储集层中能否大规模运移是所关注的焦点之一。从天然气地球化学角度,以川中须家河组典型致密气藏为研究对象,用天然气组分参数对川中须家河组致密气进行地球化学示踪研究,来回答天然气能否在致密储集层中发生大规模运移的问题。川中须家河组气藏平面上大面积断续分布,纵向多个独立气藏互相叠加。研究表明,在横向上,川中须家河组相邻气田之间,相同层系的天然气组分参数有明显差异,天然气没有发生横向运移现象。在纵向上,须家河组内上、下气藏之间天然气地球化学特征具有明显差异,这种差异不符合天然气运移分馏规律,说明天然气也未发生明显的垂向运移。换言之,须家河组致密气是原地或近距离聚集成藏,天然气没有经过大规模运移。

关键词:川中;须家河组;致密气;运移;地球化学示踪

中图分类号:P618.3

文献标志码:A

文章编号:0253-9993(2018)11-3178-09

Geochemical tracing of tight gas migration in Xujiahe Formation in central Sichuan Basin

QIN Shengfei1,LI Jinshan2,YUAN Miao1,YANG Chun1,YANG Yu3,ZHAO Changyi1

(1.Research Institute of Petroleum Exploration and Development,PetroChina,Beijing 100083,China; 2.Civil and Resource Engineering School,University of Science and Technology Beijing,Beijing 100083,China; 3.Southwest Oil and Gas Company,PetroChina,Chengdu 610051,China)

Abstract:The research on the accumulation of tight gas has attracted much attention in recent years,and whether natural gas can move in large volumes in a tight reservoir is one of research focuses.In this paper,from the perspective of natural gas geochemistry,the typical tight gas reservoirs in the Xujiahe Formation in central Sichuan are studied to answer the question of whether natural gas can be transported in a large scale in tight reservoirs,using the natural gas component parameters.Horizontally,the Xujiahe Formation gas reservoirs are widely distributed intermittently,and vertically,multiple gas reservoirs are superimposed.Studies have shown that in the horizontal direction,there are significant differences in natural gas component parameters,and no horizontal migration of natural gas in the same layer between adjacent gas fields.Vertically,there are significant differences in the geochemical characteristics of natural gas between the upper and lower gas reservoirs in the Xujiahe Formation.This difference does not meet the law of natural gas migration and fractionation,indicating that there has been no significant vertical migration of natural gas.In other words,the tight gas in the Xujiahe Formation is formed in situ or at close range,and has not undergone large-scale migration.

Key words:central Sichuan Basin;Xujiahe Formation;migration;geochemical tracing

移动阅读

秦胜飞,李金珊,袁苗,等.川中须家河组致密气运移地球化学示踪[J].煤炭学报,2018,43(11):3178-3186.doi:10.13225/j.cnki.jccs.2018.0652

QIN Shengfei,LI Jinshan,YUAN Miao,et al.Geochemical tracing of tight gas migration in Xujiahe Formation in central Sichuan Basin[J].Journal of China Coal Society,2018,43(11):3178-3186.doi:10.13225/j.cnki.jccs.2018.0652

收稿日期:2018-05-15

修回日期:2018-10-25

责任编辑:韩晋平

基金项目:国家自然科学基金资助项目(41372150);中国石油天然气股份有限公司科技资助项目(2014B-0608)

作者简介:秦胜飞(1969—),男,安徽五河人,高级工程师,博士。E-mail:qsf@petrochina.com.cn

通讯作者:李金珊(1972—),男,山东日照人,高级工程师,博士后。E-mail:lijinshan2009@126.com

致密油和致密气是我国目前最为现实的待开发非常规油气资源[1]。四川盆地、鄂尔多斯盆地和塔里木盆地已发现众多的致密气田,致密气探明储量和年产量都在逐年提高。随着非常规油气勘探不断深入,新的问题也随之而来。其中,致密气的运移成藏问题是大家最关注的交点之一。目前致密气的研究主要侧重于致密气藏形成地质条件[2-7]、储层特征[8]和致密气地球化学等方面[9-12],关于致密气运移的研究却很少开展。致密气藏中的天然气是原地成藏还是天然气远距离运移成藏,还是近距离成藏?天然气在致密储层中能否发生大规模运移?针对这些问题,笔者选取四川盆地川中须家河组致密气藏为研究对象,以天然气地球化学为研究手段,利用天然气地球化学各种指标,开展致密气运移方面的研究,期望为致密气成藏研究带来一些启示。

1 地质背景

四川盆地须家河组为一套河流相、湖泊相和沼泽相含煤沉积[13-14],是四川盆地从海相演化到陆相之后的第1套陆相层,在整个四川盆地普遍发育。该组经历过多个沉积旋回,发育多套煤系和多套致密砂岩相互叠置的沉积组合。须家河组沉积中心在川西地区,煤系烃源岩厚度自川西到川中地区逐渐减薄[15-16],导致川中地区须家河组煤系烃源岩生气强度比较低,大部分地区都小于20×108 m3/km2(图1)。但是,在川中地区须家河组也发现一批探明储量超过1 000×108 m3的大型气田(藏),例如安岳气田须家河组气藏,广安气田,合川气田,以及一系列中小型气田。

图1 川中地区须家河组气田分布及油气系统示意
Fig.1 Map of gas field distribution in Xujiahe Formation and petroleum systems in the central Sichuan Basin

1.1 地 层

川中地区出露地层自上而下有侏罗系(J)、三叠系(T)、二叠系(P)、奥陶系(O)、寒武系(∈)和震旦系(Z),缺失石炭系(C)和志留系(S)。侏罗系上部以棕红色泥岩为主,中部以紫红、灰绿色泥岩、粉砂质泥岩、砂岩为主,下部以黑色湖湘页岩为主,是下伏气藏的优质盖层。三叠系自上而下包括须家河组(T3x)、雷口坡组(T2l)、嘉陵江组(T1j)和飞仙关组(T1f)。

须家河组是本文的研究目地层,沉积厚度自川西向川中逐渐减薄,发育多套烃源岩和砂岩相互叠置的生、储组合。自下而上发育须一段(T3x1)至须六段(T3x6),其中须一(T3x1)、须三(T3x3)和须五段(T3x5)以煤系为主,发育暗色泥岩和薄煤层,为以生气为主的烃源岩。须一段烃源岩自川西坳陷往东延续到川中东部,尖灭于合川与广安连线以东地区,有效烃源岩在川中厚度0~50 m;须三段烃源岩在川中地区都有分布,分布厚度为5~50 m,在南充、遂宁、资阳一带厚度较大,往东至广安、合川一带逐渐减薄;须五段在川中广安、潼南和威远一带厚度为50~100 m,其他地区普遍小于50 m。须二(T3x2)、须四段(T3x4)和须六段(T3x6)以灰白色、灰色细-中粒砂岩为主,是较好的储集层(图1)。

雷口坡组以白云岩为主,夹石膏和薄层灰黑色页岩,内部的白云岩也是较好的储集层;嘉陵江组以灰岩与白云岩互层,夹石膏层,兼发育较好的储集层和盖层。飞仙关组主要为溶孔鲕粒云、灰岩,是区域性的优质储集岩类。

二叠系上部以生物碎屑灰岩、礁灰岩、白云岩为主,中部发育海陆过渡相煤系和灰岩,下部以灰岩和白云质灰岩为主,底部为薄层页岩,夹粉砂岩和灰岩。奥陶系以生物灰岩和鲕粒灰岩为主,在本区上部遭受了剥蚀。寒武系主要为灰岩和白云岩,下部发育厚层灰黑色页岩,为优质烃源岩。震旦系以白云岩为主,夹薄灰黑色页岩。

1.2 构 造

按四川盆地构造分区划分,川中地区又名“川中古隆平缓构造区”。川中地区沉积盖层薄,受力较弱,滑脱层不发育,形成低缓的构造[17]。由于四川盆地受多方面的水平挤压作用,应力场从盆缘向盆地内递进挤压,应力强度从盆缘向盆地内减弱,川中地区受构造影响相对较小。对须家河组有影响的构造运动主要有印支、燕山和喜马拉雅运动。

中三叠世末的印支早幕运动使川中地区上升为陆,结束了海相沉积。自晚三叠开始接受陆相沉积,发育煤系烃源岩和砂岩互层的沉积组合,即文中的须家河组。早侏罗世,川中地区为安定环境下的湖盆沉积,形成了湖相烃源岩。中侏罗世为快速沉积的平原河流和浅水湖相,是陆盆主要沉积期。晚侏罗世渐变为一动荡的湖泊与河流相沉积,一直继承到晚侏罗世末。古近纪末开始喜马拉雅运动,喜山运动在川中地区表现为整体抬升。在喜马拉雅期的强烈隆升和地层剥蚀作用下,大部分地区缺失了上侏罗统及以上地层,地层剥蚀厚度大约2 500 m[16]。地层抬升和剥蚀作用导致地层温度和压力降低,阻碍了烃源岩进一步生烃。现今构造受华蓥山背斜带影响,东部抬升较高,向西逐渐倾伏,形成龙女寺、南充、广安、营山、八角场等穹隆型构造,以及一些鼻状和小型穹隆构造,中间间隔向斜构造[18]。川中地区局部构造幅度较小,构造宽缓,断层少,未出现大的断裂系统,对后期的天然气保存比较有利,但对致密气大规模横向或纵向运移不利。

1.3 气藏类型

川中须家河组储层广泛分布、大面积含气。现今构造为一南高、北低的区域大单斜,其中合川、广安、磨溪-龙女寺、南充、八角场等构造受力相对较强,存在低缓短轴背斜,发育了一些潜伏构造,绝大部分地区构造比较平缓,气藏主要以构造-岩性复合气藏为主[14,17]。根据对数万个物性数据的统计,须家河组储集层储层物性较差,孔隙度主要分布在4%~8%的范围内,渗透率分布主要在0.01×10-15~1×10-15 m2,属低孔、低渗和特低孔、特低渗储层[19-20]。气藏含水饱和度较高,储量丰度(1~3)×108 m3/km2,属于中-低丰度高含水气藏[21]

2 样品和分析方法

2.1 样品采集

天然气样品取自川中地区上三叠统须家河组气藏,用钢瓶在井口进行取样,取样位置是井口压力表出口处。为了排除其外界因素干扰,确保第1手资料的准确,所选的取样井都是较长时间的正常开采且近期没有加入泡排剂等化学助采剂的钻井。

2.2 分析方法

天然气组分分析所用仪器是Agilent公司的6890A型天然气色谱分析仪,分别以He和N2为载气,用双TCD检测器。色谱柱采用30 m × 0.25 mm ×0.25 μm 石英毛细管柱。 初始炉温在40 ℃下等温保持7.5 min,然后以15 ℃/min从40 ℃升至90 ℃,最后以6 ℃/min从90 ℃升至180 ℃。

3 结果与讨论

3.1 致密气组分及其分布

关于川中须家河组天然气成因类型,前人做过很多天然气组分和烷烃气碳同位素研究工作,并得出了一致的结论,即天然气成因类型主要是煤型气,天然气来自须家河组煤系烃源岩[9-12]。前人得出的结论本文不再重复,笔者就前人容易忽略的一些天然气组分参数进行探讨。

川中须家河组天然气以烃类气体占绝对优势,为95.2%~99.5%,平均98.3%。含少量N2和CO2等非烃气体,2者平均分别为0.70%和0.39%。烃类气体中,乙烷等重烃气含量较高,天然气干燥系数(C1/C1+)为0.803~0.972,平均0.910。如果以干燥系数0.95为划分干气和湿气的界线,小于该值为湿气,大于该值为干气,则川中须家河组天然气主要为湿气(表1)。不同层段天然气iC4/nC4iC5/nC5也明显不同,总体上,层位越往下,比值越高。T3x6,T3x4和T3x2iC4/nC4平均分别为0.95,1.28和1.29,iC5/nC5的比值平均分别为1.87,2.20和2.25。不同的气田,该比值也有差异。例如T3x2气藏中,iC4/nC4值合川气田最高,平均为1.46;潼南气田最低,为1.07。

表1 川中须家河组天然气组分含量
Table 1 Molecular composition of natural gases in Xujiahe Formation of central Sichuan Basin

气田井号层位主要组分/%N2CO2C1C2C3iC4nC4iC5nC5C6+C1+计算参数 iC4/nC4iC5/nC5C1/C1+合川 1T3x21.420.1288.846.391.580.390.270.160.070.3398.021.452.430.906合川101T3x20.080.1993.294.540.640.120.070.030.010.1198.821.772.900.944合川102T3x200.2387.055.651.430.360.240.150.061.2196.151.542.300.905合川105T3x20.160.2291.415.711.010.210.150.070.030.2298.811.392.180.925合川 106T3x200.1690.155.951.480.380.290.180.080.5799.061.292.330.910合川 110T3x200.0392.834.430.760.180.110.070.030.2498.651.692.440.941合川 112T3x200.1990.476.051.420.360.230.130.050.2498.961.562.540.914合川合川 117T3x20.040.2390.435.881.410.400.270.140.050.1698.741.492.770.916合川 118T3x20.010.189.416.281.570.420.310.190.080.8399.101.372.260.902合川 119T3x20.100.1289.935.641.230.370.260.200.090.6198.331.462.360.915合川 124T3x20.020.4289.785.831.460.430.330.250.110.5798.751.312.330.909合川 4T3x20.810.0688.886.211.710.670.490.410.17098.541.372.410.902合川 5T3x20.150.0992.965.080.960.260.170.090.03099.551.533.000.934合川 6T3x20.980.2788.236.971.840.450.330.180.080.4798.551.342.330.895合川 7T3x20.690.3990.825.881.350.330.240.120.05098.791.382.400.919潼南001-1T3x20.020.3188.077.222.050.460.400.200.090.4198.901.142.250.891潼南001-5T3x20.050.2587.027.282.250.580.510.280.130.4198.471.142.100.884潼南102T3x21.150.5385.778.272.600.580.510.220.100.2098.251.122.130.873潼南107T3x20.060.2987.947.492.020.440.370.170.080.2998.801.192.250.890潼南潼南108T3x200.384.738.432.800.580.600.270.150.5098.050.971.820.864潼南110T3x200.4386.827.352.050.490.450.300.161.1098.711.091.890.880潼南113T3x20.100.6382.298.262.890.760.690.360.170.4195.821.112.060.859潼南114T3x200.3783.987.982.980.610.700.320.210.7697.530.881.540.861潼南3T3x22.000.3278.918.763.781.251.431.130.641.6397.520.881.760.809潼南6T3x20.050.3788.566.621.780.390.350.180.090.5698.521.142.010.899西 20T3x20.880.3589.825.722.070.400.430.120.060.0798.700.931.950.910西 35-1T3x21.480.1786.776.081.880.580.470.350.182.0198.321.252.010.883西 13-1T3x41.501.6985.006.642.740.520.650.260.170.6996.660.791.560.879西 32T3x43.990.2182.727.203.300.630.840.310.210.5495.760.761.480.864西 48T3x41.320.8788.186.072.090.420.470.180.110.2097.730.891.620.902西 56T3x40.150.6688.006.222.320.480.530.200.120.3498.200.891.660.896西 57T3x40.940.183.788.553.600.751.050.410.2397.970.721.580.855南充西 58T3x41.420.8285.627.082.840.520.600.190.130.6397.610.861.540.877西 62T3x42.140.3882.337.583.570.741.120.490.381.2597.450.661.310.845西 64T3x40.760.2888.926.032.160.440.480.200.130.5398.880.911.580.899西 65T3x41.650.2487.716.222.320.440.540.210.140.5098.080.811.540.894西 69T3x40.020.7489.625.271.890.410.460.180.120.3698.310.881.590.912西 71T3x40.160.7388.665.992.000.420.470.180.120.4598.290.901.590.902西 72T3x40.200.2687.316.682.830.570.720.300.190.9099.500.791.560.877西 73XT3x40.880.7079.016.243.491.432.261.861.292.8398.400.631.440.803

气田井号层位主要组分/%N2CO2C1C2C3iC4nC4iC5nC5C6+C1+计算参数 iC4/nC4iC5/nC5C1/C1+广安 003-2T3x400.3193.973.710.630.130.080.040.010.1398.701.612.500.952广安 106T3x40.030.3493.254.320.750.140.100.040.020.1698.781.502.320.944广安 112T3x40.920.4192.714.520.860.150.120.050.020.1998.621.262.380.940广安 113T3x40.900.3693.514.010.700.140.100.050.020.1498.651.442.650.948广安 114T3x40.350.2794.723.840.550.080.060.020.010.0799.351.332.000.953广安 116T3x40.830.2093.074.600.770.160.100.050.020.1698.931.642.720.941广安 121T3x41.170.2992.494.670.830.180.120.060.030.0898.451.452.320.939广安 122T3x41.420.2792.214.590.760.190.110.060.020.1498.071.762.770.940广安 123T3x41.050.3791.855.110.950.220.170.090.030.0798.491.262.780.933广安 125T3x41.370.2492.114.730.900.210.130.070.020.0998.261.632.960.937广安 126T3x40.750.3393.324.430.750.150.100.050.020.0998.901.492.420.944广安 127T3x41.290.3392.214.760.820.170.100.040.010.0598.161.633.330.939广安 128T3x40.140.3793.014.380.760.150.100.050.020.1698.631.512.300.943广安 130T3x41.090.2692.374.940.960.160.110.030.010.0598.621.472.700.937广安 131T3x40.150.2493.164.390.740.140.100.040.010.0898.661.482.690.944广安 133T3x40.920.9292.304.200.680.130.090.050.020.5097.981.442.080.942广安 134T3x40.190.0792.994.450.680.130.080.030.010.0798.431.682.550.945广安 136T3x40.890.2892.154.940.940.220.140.070.030.1998.681.562.390.934广安 142T3x40.140.3392.334.690.900.210.140.080.040.3098.691.562.370.936广安 143T3x40.200.2993.103.910.620.110.070.030.010.1998.041.612.580.950广安 144T3x40.030.3093.354.270.700.150.090.040.020.1398.741.682.560.945广安 145T3x40.151.6094.062.320.230.040.030.010.010.0996.791.482.000.972广安广安 16T3x40.640.4192.324.941.050.240.180.070.030.0798.891.352.670.934广安 17T3x40.610.4693.104.500.880.190.130.060.020.0598.921.422.620.941广安 20T3x40.220.2791.894.680.970.230.170.110.050.5198.591.352.190.932广安 3T3x41.990.3593.733.310.420.070.040.020.01097.601.732.830.960广安 002-21T3x60.840.6389.466.261.680.330.350.130.070.1798.450.961.830.909广安 002-X77T3x60.010.4489.375.971.720.350.360.160.090.4398.440.961.720.908广安 103T3x60.720.6487.407.592.100.400.370.140.070.1598.221.072.030.890广安 104T3x60.830.6388.856.561.770.350.350.150.070.3698.451.002.030.902广安 105T3x60.920.5489.206.101.810.390.400.160.080.2798.400.962.110.907广安 109T3x60.710.5789.166.131.720.340.340.140.070.2998.191.012.090.908广安 110T3x60.810.3989.816.081.650.330.320.130.070.3398.711.021.970.910广安 111T3x60.060.3689.296.391.790.350.360.150.090.2998.710.971.610.905广安 112T3x61.610.3389.665.921.390.240.260.110.060.1897.820.901.980.917广安 114T3x62.300.3590.614.990.980.1430.1490.0540.0220.09197.040.962.450.934广安 115T3x62.230.1988.056.581.740.360.350.130.070.2397.511.021.940.903广安 118T3x61.530.3890.335.841.230.170.190.070.030.1698.010.872.410.922广安 122T3x60.710.3190.146.141.530.340.300.130.070.2398.881.111.890.912广安 130T3x600.7686.226.001.750.370.360.170.090.2495.201.032.010.906广安 133T3x60.130.4586.217.152.700.710.760.380.200.6598.760.931.870.873广安 15T3x61.480.0586.477.292.810.550.610.200.150.3598.430.901.330.878广安 2T3x60.150.1989.036.651.930.390.400.160.090.3498.990.961.680.899广安 3T3x60.970.2090.924.201.740.260.530.180.210.7098.740.490.860.921广安 7T3x60.250.6388.307.322.100.390.400.170.080.3399.100.962.040.891

T3x6,T3x4和T3x2天然气分别来自各自气藏下伏的T3x5,T3x3和T3x1煤系烃源岩。从埋藏深度来看,T3x5,T3x3和T3x1烃源岩演化程度随埋藏深度逐渐增加。总体上,丁烷和戊烷的异/正构比值随烃源岩成演化程度增加而增大。在图2中,T3x4和T3x2的样品点几乎分布于同一区域,这是因为这些样品分别来自不同的气田,这些气田所分布的区域对应的烃源岩成熟度不同。须六段样品主要来自广安气田,丁烷和戊烷的异/正构比值落在演化序列的中段,是因为广安气田须五段烃源岩演化程度相对较高。总体上川中地区T3x1烃源岩成熟度在遂宁、南充、广安一带Ro大于1.3%,在其他部位Ro分布在1.0%~1.3%;T3x3烃源岩Ro分布在1.0%~1.3%,遂宁、南充、广安一带成熟度高于其他地区;T3x5烃源岩成熟度分布在0.7%~1.0%,广安地区成熟度相对较高。

图2 川中须家河组不同层位天然气丁烷和戊烷异/正构比值对比
Fig.2 Ratio iC4/nC4 vs.iC5/nC5 of gases in different layers of Xujiahe Formation in central Sichuan

3.2 相邻气田之间天然气没有发生横向运移

川中地区内部相邻气田之间天然气是否发生了明显横向运移?笔者分别选取了川中南部相邻的合川和潼南气田、川中北部的广安和南充气田相同层段天然气分别进行对比研究。

目前,合川和潼南气田探明的天然气储量都集中在须二段。尽管两气田相隔距离较近,但2者天然气甲和乙烷碳同位素却有明显区别,合川气田都明显重于潼南气田,合川气田甲烷和乙烷碳同位素平均分别为-39.0‰、乙烷是-27.0‰;潼南气田平均分别为-40.9‰、乙烷是-27.8‰(表1、图3)。说明合川气田天然气对应的须家河组烃源岩成熟度高于潼南气田。另外,合川气田天然气干燥系数也明显高于潼南气田(图4)。这也是由于烃源岩成熟度差异造成,并非由天然气横向运移发生组分分馏所致。如果是运移分馏造成合川气田干燥系数偏高,那么合川气田甲烷碳同位素不应该重于潼南气田。

图3 合川和潼南气田须二段天然气甲烷与乙烷碳同位素对比(数据来自文献[22])
Fig.3 δ13C1 vs.δ13C2 of natural gases from T3x2 reservoirs in Hechuan and Tongnan gas fields

图4 合川和潼南气田须二段、广安和南充气田须四段天然气地球化学参数对比
Fig.4 Geochemical parameters of natural gases from T3x2 reservoirs in Hechuan and Tongnan gas fields and T3x4 reservoirs in Guang’an and Nanchong gas fields

合川和潼南气田烷烃气的异构比也有明显差异,合川气田高于潼南(图4(a))。异构烷烃由于沸点低,饱和蒸汽压高,分子间作用力较小,因而扩散系数要大于同碳数的正构烷烃。所以,iC4iC5的运移速率分别大于nC4nC5。研究表明,在38 ℃盐水饱和的泥岩中,iC4的扩散系数大于nC4,分别为1.26×10-7和1.24×10-7 cm2/s;iC5的扩散系数大于nC5,分别为7.0×10-7和5.2×10-7 cm2/s[23]。在气相色谱进行天然气组分测定的时候,异构烷烃也比同碳数的正构烷烃先被测出,所以随天然气运移距离的增加,iC4/nC4iC5/nC5比值都分别会增大。但潼南、合川气田烷烃气异构比的差异,貌似也可以用天然气运移分馏所致,但这又与合川甲烷碳同位素重于潼南相矛盾。由图3上看,合川气田甲烷碳同位素重于潼南气田,说明合川气田烃源岩成熟度大于潼南气田,这与图4反映出的演化趋势一致,天然气干燥系数合川气田大于潼南气田。一般情况下,在横向上相同层位、厚度相似的烃源岩生成的天然气往往自烃源岩高成熟区往低成熟区运移,因烃源岩厚度相似的情况下,烃源岩成熟度高,其生气强度也会相对较高。

川中北部的南充气田和广安气田T3x4气藏,天然气地球化学特征也具有明显差异,干燥系数广安T3x4气藏天然气大于南充气田,丁烷异/正构比前者也明显高于后者(图4(b))。

由此可见,相邻的川中须家河组气田,天然气地球化学特征也有明显差异,这种差异并非由运移分馏所致,是为各自气藏供烃的烃源岩成熟度不同导致的。这说明,横向上,相同层位的川中须家河组气藏之间天然气并未发生明显的横向运移。

3.3 同一气田上下气藏之间天然气差异明显

在须家河组内部上、下叠置的气藏之间,是否发生了天然气规模性的运移?笔者以须家河组天然气开采层系较多的广安气田为例来进行研究。广安气田主采气藏为T3x4和T3x6,天然气最有可能分别来自气藏下部的T3x3和T3x5煤系烃源岩,并且T3x3烃源岩的成熟度要高于T3x5

如确有天然气自T3x4气藏运移至T3x6气藏,T3x6气藏中的天然气干燥系数和烷烃气和异构比都应该都分别高于T3x4气藏,而事实却相反。由图5可看出,T3x4气藏中的天然气干燥系数和烷烃气的异构比都明显高于T3x6。这现象可以用烃源岩成熟度的不同进行合理的解释。T3x4和T3x6气藏中天然气截然不同的地球化学参数特征也很好地说明了川中须家河组上、下气藏中天然气没有明显的混合现象。

图5 广安气田须四和须六气藏天然气地球化学参数对比
Fig.5 Geochemical parameters of natural gases from T3x4 and T3x6 reservoirs in Guang’an gas field

笔者选取了一批分别能取到了T3x4和T3x6气藏天然气样品的钻井,来进行上、下气藏天然气特征的差异研究。同一口钻井,T3x4气藏中天然气干燥系数(C1/(C1+))明显高于T3x6气藏,重烃气浓度T3x6气藏天然气高于T3x4(图6)。

图6 广安气田相同钻井T3x4和T3x6天然气干燥系数和湿度系数对比
Fig.6 Dryness and wetness coefficients of gases of T3x4 and T3x6 reservoirs from the same wells in Guang’an gas field

笔者还罗列了广安和其他气田一些钻穿整个须家河组的钻井,把不同深度气藏中的天然气某些地球化学特征进行对比。对比发现,自下而上,也就是说自T3x2到T3x4和T3x6,天然气密度逐渐增大,甲烷体积分数逐渐降低(表2),与T3x1,T3x3和T3x5烃源岩成熟度逐渐降低相吻合。也说明上、下气藏之间天然气没有混合现象,天然气为近源成藏。

3.4 构造相对稳定使得川中须家河组天然气为近源成藏

须家河组砂岩储集层分布广,但储层强烈的非均质性以及较平缓的区域地层,限制了油气进行长距离运移。川中地区硬性基底、区域构造稳定、沉积盖层受到的区域构造应力较弱,断层和裂缝相对不发育,油气运聚作用受到一定限制[24]。储层中流体区域性的超压也反映了川中地区须家河组储层流体的运移受到一定的阻碍。川中地区须家河组各层段储层普遍存在高压与超压,同一深度不同地区超压程度有所不同(表3)。反映出须家河组储层孔隙度、渗透率低,储层连通性差,非均质性强,未形成有效的孔、缝体系,未形成“泄压通道”,造成川中须家河组超压异常和压力的非均质性。

表2 川中须家河组相同钻井不同层段天然气密度和甲烷体积分数φ(CH4)
Table 2 Density and methane concentration of natural gases in Xujiahe Formation of Central Sichuan

气田井号T3x2密度φ(CH4)/%T3x4密度φ(CH4)/%T3x6密度φ(CH4)/%八角场角 480.60293.000.62091.000.63788.00遂南遂 120.63889.000.64886.000.68683.00磨溪磨 120.63888.000.64886.00广安 1120.60392.700.62189.66广安广安 1140.58794.720.61490.72广安 1330.61092.300.67086.21

表3 川中须家河组地层压力统计
Table 3 Statistics of stratigraphic pressure of Xujiahe Formation in Central Sichuan

气田井号测试深度/m层位压力/MPa压力系数潼南潼南1012 241.40T3x231.601.44潼南1022 240.85T3x229.651.35合川12 135.00T3x222.571.08合川合川32 130.80T3x225.281.21合川52 265.00T3x230.071.35南充充深12 205.60T3x431.231.44充深22 225.25T3x429.031.33八角场角13T3x455.141.81角45T3x456.191.79广安1352 475.22T3x436.191.49广安1382 526.75T3x432.411.31广安1392 370.40T3x435.721.54广安广安21 782.45T3x619.551.12广安1312 589.00T3x634.411.36广安1031 799.70T3x618.381.04

4 结 论

(1)川中须家河组天然气是以烃类气体为主的煤成湿气,干燥系数(C1/C1+)为0.803~0.972,平均0.910。不同层段天然气iC4/nC4,iC5/nC5也明显不同,总体上,层位越低,比值越高。

(2)须家河组相邻气田之间天然气干燥系数、iC4/nC4、碳同位素差异明显,未发生明显的横向运移;在纵向上,同一气田不同深度的气藏中天然气地球化学特征也具明显差异。自下而上,天然气密度逐渐增大,甲烷浓度逐渐降低,与T3x1、T3x3和T3x5烃源岩成熟度逐渐降低相吻合,上、下气藏之间天然气没有明显的混合现象。

(3)须家河组构造相对稳定,但未经历过大规模的挤压变形,气藏之间未形成有效的沟通体系,限制了川中须家河组天然气的大规模运移,天然气为近源成藏,成藏后没有经过明显的大规模运移。

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