劉全有, 戴金星, 金之鈞, 李?劍,周慶華, 馮子輝, 孫紅軍

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松遼盆地慶深氣田異常氫同位素組成成因研究

劉全有1*, 戴金星2, 金之鈞1, 李?劍3,周慶華2, 馮子輝4, 孫紅軍1

(1. 中國石化 石油勘探開發(fā)研究院, 北京?100083; 2. 中國石油 勘探開發(fā)研究院, 北京?100083; 3. 中國石油 勘探開發(fā)研究院 廊坊分院, 河北 廊坊?065007; 4. 中國石油 大慶油田勘探開發(fā)研究院, 黑龍江 大慶?163712)

對(duì)松遼盆地徐家圍子斷陷慶深氣田天然氣組分、碳?xì)渫凰睾拖∮袣怏w同位素的分析表明, 天然氣以烷烴氣為主, 烷烴氣碳同位素組成隨著碳數(shù)增加呈變輕趨勢(shì), 且13C1>–30‰, R/Ra一般大于1.0,13CCO2值介于–16.5‰ ~ –5.1‰之間; 氫同位素組成D1= –205‰ ~ –197‰, 平均值為–203‰,D2= –247‰ ~ –160‰, 平均值為–195‰,D3= –237‰ ~ –126‰, 平均值為–163‰, 且存在氫同位素組成倒轉(zhuǎn)現(xiàn)象, 即D1>D2

天然氣; 氫同位素組成; 地球化學(xué); 慶深氣田

0?引?言

在天然氣地球化學(xué)研究中, 氣態(tài)烴的碳、氫同位素蘊(yùn)含著豐富的母質(zhì)來源及母質(zhì)和產(chǎn)物所經(jīng)歷的地質(zhì)、地球化學(xué)歷程的信息, 即同位素的母質(zhì)繼承效應(yīng)和地質(zhì)歷史中生物化學(xué)、物理作用所導(dǎo)致的同位素分餾效應(yīng)。天然氣中的碳同位素組成主要反映母質(zhì)類型及其演化程度[1–7]。烷烴氣氫同位素在天然氣研究中運(yùn)用不如碳同位素廣泛, 但其蘊(yùn)涵的一些信息具有特定的意義[2,8,9], 如對(duì)沉積環(huán)境的示蹤。目前對(duì)氫同位素的研究, 主要為烷烴氣中的氫同位素組成, 絕大部分是研究有機(jī)成因烷烴氣的氫同位素, 特別是對(duì)CH4中氫同位素研究得較多。無機(jī)成因天然氣氫同位素組成研究與其它類型天然氣相比較為薄弱, 目前的研究集中在火山-地?zé)釒?、俯沖帶和裂谷帶, 主要研究CH4氫同位素組成。從目前的資料來看, 無機(jī)成因CH4碳同位素組成明顯偏重,13C1在–41‰ ~ –3.2‰之間, 一般應(yīng)大于–30‰。但是對(duì)于無機(jī)成因CH4氫同位素的報(bào)道不多。Lyon.[10]對(duì)新西蘭4個(gè)地區(qū)的地?zé)釟怏w的研究結(jié)果表明, 其CH4氫同位素組成在–197‰ ~ –142‰之間。Dai[11]曾對(duì)騰沖硫磺塘無機(jī)CH4氫同位素進(jìn)行了測(cè)試, 其值為–130‰。上官志冠[12]對(duì)滇西地區(qū)斷層氣體中2個(gè)樣品的CH4氫同位素做過測(cè)試, 其氫同位素值分別為–112‰和–106‰。Botz.[13]對(duì)希臘米洛斯島附近海底噴出氣體中的同位素組成進(jìn)行了研究, 其中CO2的13C值為0‰左右, CH4的13C值在–17.8‰ ~ –9.4‰之間, CH4的氫同位素組成在–189‰ ~ –102‰之間, 大部分重于–150‰。Botz.[14]研究了新西蘭Plenty海灣海底熱液氣體中CH4的氫同位素組成, 其值在–135‰ ~ –122‰之間?？傮w來講, 無機(jī)成因CH4的D值重于熱成因CH4以及生物CH4的D值。Sherwood Lollar.[15–16]分別對(duì)加拿大和南非前寒武紀(jì)地盾區(qū)巖石(Precambrian Shield rocks)中斷層鹽水溶解氣碳?xì)渫凰亟M成進(jìn)行了分析, 認(rèn)為通過水巖作用形成的無機(jī)成因CH4與C2H6碳?xì)渫凰亟M成比有機(jī)成因偏重。至今所報(bào)道的最重的CH4氫同位素組成是美國加利福尼亞索爾頓湖(shallow thermal wells in the Salton Sea region, California)區(qū)CO2井中的CH4, 其DCH4值為–16‰[17],最輕的是加拿大魁北克省諾里塔(Canadian Shield) N256～1985樣品, 其DCH4值為–470‰[18]。我國目前發(fā)現(xiàn)最重的CH4氫同位素在四川盆地建南氣田建35井飛三段天然氣中, 其DCH4值為–83.9‰, 最輕的為鄂爾多斯盆地城壕油田城54井延6-8伴生氣,DCH4值為–312.8‰[19]。影響油氣氫同位素的主要因素是油氣形成中外來氫源的參與[20–21]、有機(jī)質(zhì)與水的同位素交換反應(yīng)[22–23]。水在成烴演化中起重要作用, 并且參與了其化學(xué)反應(yīng), 成為有機(jī)氫的一部分[20–22,24],而且水中的氫可以與干酪根中的氫發(fā)生可逆的同位素交換反應(yīng), 發(fā)生這種交換反應(yīng)的氫主要與雜原子相連, 如N―H、S―H、O―H等中的氫[25–26]。但是在烷烴氣生成以后, 其氫同位素不會(huì)或很少與水等其他氫原子發(fā)生同位素交換反應(yīng)[22–23]。由于分析精度等原因, 無機(jī)成因天然氣的氫同位素研究較少, 隨著技術(shù)的進(jìn)步, 將會(huì)大大擴(kuò)展和提高無機(jī)成因天然氣氫同位素組成的研究和利用。

徐家圍子斷陷及周邊已陸續(xù)發(fā)現(xiàn)了汪家屯、宋芳屯、昌德和農(nóng)安村等氣田, 天然氣具有重烷烴碳同位素組成(13C1> –30‰)且呈負(fù)碳序列、高3He/4He組成(R/Ra>1.0)等特征, 為無機(jī)成因氣[27]。慶深氣田位于徐家圍子斷陷中部, 2005年底探明天然氣地質(zhì)儲(chǔ)量約1018×108m3, 天然氣成因類型值得地球化學(xué)家關(guān)注, 特別是CH4氫同位素組成是否具有無機(jī)成因氣特點(diǎn), 烷烴氣氫同位素組成能否作為判識(shí)有機(jī)與無機(jī)成因天然氣指標(biāo)對(duì)于進(jìn)一步完善天然氣成因鑒別具有重要科學(xué)意義。本文擬通過對(duì)徐家圍子斷陷21個(gè)天然氣樣品化學(xué)組分、碳?xì)渫凰亟M成和稀有氣體同位素組成的分析, 探討該氣田天然氣成因類型以及烷烴氣氫同位素組成在不同類型天然氣中變化規(guī)律, 利用地球化學(xué)參數(shù)建立判識(shí)有機(jī)與無機(jī)成因天然氣模式。

1 地質(zhì)背景

慶深氣田為近期在徐家圍子斷陷首次發(fā)現(xiàn)以烴類氣體為主的工業(yè)性氣藏, 2005年底探明天然氣地質(zhì)儲(chǔ)量超過1000×108m3,位于徐家圍子斷陷中部, 面積約5350 km2。徐家圍子斷陷是由徐西和宋西兩條邊界斷裂控制的箕狀斷陷, 由宋站低隆起和豐樂低隆起分割成3個(gè)局部深斷陷。徐西斷裂總體走向NNW, 延伸長(zhǎng)度96 km, 東傾, 傾角7~35°, 其斷距在基巖頂面一般1800 m, 最大4328 m, 最小954 m, 平面延伸近S形。宋西斷裂也是徐家圍子斷陷一條重要的邊界斷裂, 總體走向NNW, 延伸長(zhǎng)度93 km, 東傾, 傾角10~20°, 垂直斷距在基巖頂面一般為1860 m, 最大2947 m, 最小366 m, 平面延伸近S形, 其與徐西斷裂共同控制了徐家圍子斷陷2個(gè)西陡東緩斜列的箕狀斷陷。

為進(jìn)行對(duì)比分析,對(duì)位于徐家圍子斷陷東南部朝陽溝地區(qū)(包括三站和五站氣田)天然氣也進(jìn)行了分析, 該區(qū)天然氣主要來源于白堊系腐泥型暗色泥巖, 局部地區(qū)為侏羅系煤巖形成的煤型氣, 有機(jī)質(zhì)類型與徐家圍子斷陷相似。

2?天然氣樣品的采集與實(shí)驗(yàn)分析

氣體樣品均直接采自油氣井口。首先對(duì)采樣管線和不銹鋼瓶進(jìn)行15~20次沖洗以便排除空氣污染。不銹鋼瓶為1個(gè)半徑為10 cm兩端帶有砝碼的開關(guān)容器(體積大約為1000cm3), 其最大壓力為22.5 MPa。采完樣品后, 要將鋼瓶放入水中檢測(cè)是否泄漏。

氣體化學(xué)組分分析在中國科學(xué)院蘭州地質(zhì)研究所氣體地球化學(xué)重點(diǎn)實(shí)驗(yàn)室的MAT-271質(zhì)譜儀上完成。離子源為EI; 電能86 eV; 質(zhì)量范圍1~350 amu; 分辨率3000; 加速電壓8 kV; 發(fā)散強(qiáng)度0.200 mA; 真空度小于1.0×10–7Pa。根據(jù)國標(biāo)GB/T 6041-2002 和GB/T10628-89, 樣品化學(xué)組分通過標(biāo)準(zhǔn)樣氣體對(duì)比法計(jì)算出來。

碳?xì)渫凰胤治鲈谥袊涂碧介_發(fā)研究院廊坊分院天然氣地質(zhì)所有機(jī)地球化學(xué)實(shí)驗(yàn)室完成。對(duì)于碳?xì)渫凰胤治? 該實(shí)驗(yàn)室與中國科學(xué)院蘭州地質(zhì)研究所氣體地球化學(xué)重點(diǎn)實(shí)驗(yàn)室進(jìn)行過多次對(duì)比測(cè)試與檢驗(yàn)。兩個(gè)實(shí)驗(yàn)室的測(cè)定結(jié)果非常吻合, 基本處于測(cè)定精度范圍內(nèi)。穩(wěn)定碳同位素由MAT-252質(zhì)譜儀分析測(cè)試。氣相色譜柱為2 m長(zhǎng)的Porapak Q 型柱子; 加熱溫度為40~160℃, 升溫速度為15 ℃/min;純凈的氦氣作為載氣。13CPDB的分析誤差小于0.3‰。

氫同位素測(cè)試在Thermo Finnigan公司生產(chǎn)的GC/TC/IRMS色質(zhì)譜聯(lián)用儀上完成, 其中同位素質(zhì)譜計(jì)為DeltaplusXP。色譜條件: 色譜柱為ATC-2000型(30 m×0.32 mm×2.5 μm), 初始流速1.5 mL/min, 30 ℃恒溫5 min, 然后以8 ℃/min程序升溫到80 ℃, 再以4 ℃/min升溫到260 ℃并恒溫10 min。質(zhì)譜條件: 電子轟擊(EI)離子源, 電子能量124 eV, 發(fā)射電流1.0 mA, 加速電壓3 kV, 質(zhì)量分辨率70。每個(gè)樣品分析3次, 其測(cè)定結(jié)果值取3次平均值,DSMOW值分析誤差為3‰。

3?天然氣地球化學(xué)

在慶深氣田, 除芳深9和芳深701井天然氣化學(xué)組分以CO2為主外, 其余天然氣均以烴類氣體為主, 其次為N2和CO2(表1); 含有少量H2和稀有氣體(氦和氬)。雖然慶深氣田天然氣主要以烴類氣體為主, 但烷烴氣碳同位素組成隨著碳數(shù)的增加呈變輕趨勢(shì), 且13C1> –30‰, 烷烴氣碳同位素具有無機(jī)成因特征;13CCO2值介于–16.5‰ ~ –5.1‰之間。R/Ra值變化范圍為0.77~5.84, 多數(shù)樣品R/Ra值大于1.0。

慶深氣田烷烴氣體氫同位素組成D1= –205‰ ~ –197‰, 平均值為–203‰,D2= –247‰ ~ –160‰, 平均值為–195‰,D3=–237‰ ~ –126‰, 平均值為–163‰。在慶深氣田, CH4氫同位素組成(D1)變化范圍很小, 而重?zé)N氣體的氫同位素組成(D2,D3)較CH4的氫同位素(D1)變化大。同時(shí), 有相當(dāng)數(shù)量樣品的D1>D2

4?討?論

4.1?慶深氣田天然氣成因類型

雖然在慶深氣田天然氣高的重?zé)N含量(C2+)表現(xiàn)為天然氣來源于有機(jī)質(zhì)熱降解作用[3,6,28], 但烷烴氣碳同位素組成隨著碳數(shù)增加逐漸呈變輕趨勢(shì)與有機(jī)質(zhì)在單一熱動(dòng)力作用下形成的烷烴氣地球化學(xué)特征不同(圖1), 因?yàn)橛袡C(jī)質(zhì)在單一熱動(dòng)力作用下形成的烷烴氣碳同位素組成隨著碳數(shù)增加逐漸變重[3,28–32], 即13C1<13C2<13C3<13C4。在本次研究中, 烷烴氣(除汪9-12外)碳同位素組成隨著碳數(shù)增加呈變輕趨勢(shì)。Dai.[33]認(rèn)為天然氣烷烴氣碳同位素組成局部倒轉(zhuǎn)可能與以下4種因素有關(guān), 包括有機(jī)與無機(jī)氣相混合、煤型氣與油型氣混合、同型不同源氣或同源不同期氣混合、烷烴氣被細(xì)菌演化導(dǎo)致殘留組分碳同位素變重并發(fā)生倒轉(zhuǎn)[29,34]。但是對(duì)于熱成熟度較為接近的有機(jī)成因氣很難形成CH4與C2H6碳同位素組成的倒轉(zhuǎn)[3,28,29,35], 因?yàn)樵诜忾]體系熱模擬實(shí)驗(yàn)中各個(gè)溫度條件下未發(fā)現(xiàn)CH4與C2H6碳同位素組成倒轉(zhuǎn)。同時(shí), 該研究區(qū)域也不存在生物氣, 因?yàn)樯餁饩哂泻茌p的CH4碳同位素組成[2,31,36]。雖然硫酸鹽熱化學(xué)還原也可導(dǎo)致CH4碳同位素組成很重[37],但在慶深氣田天然氣中并不含有H2S, 且儲(chǔ)層以火山巖和砂巖為主, 所以在該區(qū)不存在硫酸鹽對(duì)烷烴氣還原作用。這樣, 造成慶深氣田烷烴氣碳同位素組成倒轉(zhuǎn)可能與無機(jī)成因氣有關(guān)。無機(jī)成因氣主要包括深部通過深大斷裂直接運(yùn)移成藏和在一定溫度和壓力作用下CO2和H2費(fèi)托反應(yīng)合成烷烴氣[38–41], 但這些無機(jī)氣主要以CH4為主, C2H6等重?zé)N含量很少, 有時(shí)難以檢測(cè)[39,42–44]。但在慶深氣田, 烷烴氣重?zé)N含量較高, 且重?zé)N氣碳同位素組成之間具有很好的相關(guān)性, 如13CC4和13CC5(圖2), 這種相關(guān)性暗示了慶深氣田重?zé)N氣來源于有機(jī)質(zhì)熱降解, 因?yàn)殡S著烷烴氣碳數(shù)增加, 烷烴氣與母質(zhì)之間烷烴氣碳同位素逐漸接近[6]。

圖1　慶深氣田烷烴氣碳同位素組成變化

在火山活動(dòng)過程中, 會(huì)釋放大量深部氣體, 化學(xué)組分主要以CO2、H2和CH4為主, 同時(shí)含有一定量的稀有氣體, 如He和Ar等[17]。由于稀有氣體的稀少和化學(xué)性質(zhì)上的惰性, 稀有氣體在地質(zhì)作用過程中的豐度和同位素組成變化幾乎不受復(fù)雜的化學(xué)反應(yīng)的影響, 而主要取決于溶解、吸附和核反應(yīng)等物理過程[45]。稀有氣體一般沒有呈游離聚集, 它們以摻和物形式存在于天然氣中, 其含量一般不超過1%[46]。天然氣中幔源氦主要是受深大斷裂帶、火山活動(dòng)和巖漿活動(dòng)控制, 幔源揮發(fā)分的運(yùn)移以直接與地幔相連的通道為途徑[46–47],3He為原始大氣成因的氦, 主要與深部地幔有關(guān)[48]。在本次研究中, 利用R/Ra值與CO2/3He值的關(guān)系、R/Ra值與CH4/3He值的關(guān)系來識(shí)別有機(jī)與無機(jī)成因相混合模式(圖3), 因?yàn)槿绻烊粴庵蠧O2為簡(jiǎn)單的殼源與幔源二端元混合, 那么R/Ra值和CO2/3He值應(yīng)該表現(xiàn)為線性關(guān)系。如圖3a所示, 在慶深氣田CO2數(shù)據(jù)點(diǎn)落入殼源和幔源混合區(qū)[28,47]; 盡管CO2丟失途徑很多, 包括以碳酸鈣形式沉淀和石墨還原[49], 但是在特定條件下CO2可以還原生成CH4[41,43]。此外, R/Ra值與CH4/3He值的負(fù)相關(guān)性也暗含了有機(jī)與無機(jī)成因氣的混合[41,47,50,51](圖3b); 因?yàn)橛袡C(jī)成因氣CH4/3He值為109~1012, 且R/Ra值小于0.32[27], 而東太平洋洋中脊玄武巖、熱泉?dú)?、火山噴氣等典型無機(jī)氣CH4/3He值為105~107, 且R/Ra值大于1.0[17,27]。這樣, 慶深氣田高的CH4/3He和R/Ra值可能主要與深部活動(dòng)有關(guān), 因?yàn)榛鹕交顒?dòng)過程中CO2和H2可以通過費(fèi)托反應(yīng)合成CH4。在日本海的油氣田中也發(fā)現(xiàn)類似情況, CH4/3He值高達(dá)1011~1014, 其CH4主要是通過CO2還原形成的[41]。

圖3?慶深氣田CO2/3He-R/Ra (a)和CH4/3He-R/Ra (b)關(guān)系圖

4.2?慶深氣田烷烴氣氫同位素組成

慶深氣田烷烴氣D1值變化范圍很窄, 為–205‰ ~ –197‰, 且具有氫同位素組成倒轉(zhuǎn)現(xiàn)象, 即D1>D2

圖4?慶深氣田烷烴氣氫同位素組成變化

圖5?慶深氣田CH4碳?xì)渫凰亟M成變化

圖6?慶深氣田烷烴氣氫同位素組成

5?結(jié)?論

慶深氣田是在特定的地質(zhì)背景下形成的工業(yè)性天然氣氣田, 儲(chǔ)層主要為火山巖和砂巖。由于慶深氣田天然氣具有重的CH4碳同位素組成、烷烴氣碳同位素完全倒轉(zhuǎn)、高稀有氣體同位素組成(R/Ra>1.0), 暗示了該氣田存在無機(jī)成因氣。利用R/Ra值與CO2/3He值關(guān)系、R/Ra值與CH4/3He值關(guān)系對(duì)慶深氣田天然氣成因類型進(jìn)行識(shí)別, 認(rèn)為該氣田烷烴氣中CH4有部分為無機(jī)成因, 重?zé)N氣則主要為有機(jī)成因。對(duì)朝陽溝地區(qū)天然氣氫同位素對(duì)比分析認(rèn)為, 部分無機(jī)成因CH4的混合是引起烷烴氣氫同位素組成的局部倒轉(zhuǎn)的主要原因, 即D1>D2

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Abnormal hydrogen isotopes of natural gases from the Qingshen gas field, the Songliao Basin

LIU Quan-you1*, DAI Jin-xing2, JIN Zhi-jun1, LI Jian3, ZHOU Qing-hua2, FENG Zi-hui4and SUN Hong-jun1

1. Exploration and Production Research Institute, SINOPEC, Beijing?100083, China; 2. Research Institute of Petroleum Exploration and Development, PetroChina, Beijing?100083, China; 3. Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina, Langfang?065007, China; 4. Daqing Oilfield Company, PetroChina, Daqing?163712, China

By measuring chemical compositions, carbon and hydrogen isotope compositions and noble gas isotopes of natural gases from the Qingshen gas field, Songliao Basin are characterized by a dominant component of alkanes gases, enrichment of12C with increasing molecular weight, heavy13C1values (13C1> –30‰), R/Ra>1.0, and13CCO2values ranging from –16.5‰ to –5.1‰. The distributive range of theD1,D2,D3values for natural gases from the Qingshen gas field are –205‰ to –197‰ (average values = –203‰), –247‰ to –160‰ (average value = –195‰), –237‰ to –126‰ (average value = –163‰) , respectively, associated with a partial reversal of the hydrogen isotopic trend of C1–C3alkanes, i.e.D1>D2

natural gas; hydrogen isotopic composition; geochemistry; Qingshen gas field

P597

A

0379-1726(2014)05-0460-09

2013-09-22;

2013-10-22;

2014-04-15

國家自然科學(xué)基金(41322016); 國家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃項(xiàng)目(2012CB214800); 中國石油天然氣股份有限公司科學(xué)研究與技術(shù)開發(fā)項(xiàng)目(07-01C-01-07)

劉全有(1975–), 男, 博士、高級(jí)工程師, 從事油氣地質(zhì)與地球化學(xué)研究。

LIU Quan-you, E-mail: liuqy.syky@sinopec.com; Tel: +86-10-82282405