瓊東南盆地深水區(qū)漸新統(tǒng)烴源巖有機(jī)質(zhì)碳同位素分布特征及其主控因素

何春民, 甘 軍, 梁 剛, 李 興, 王 星, 李騰飛, 田 輝*

瓊東南盆地深水區(qū)漸新統(tǒng)烴源巖有機(jī)質(zhì)碳同位素分布特征及其主控因素

何春民1,2, 甘 軍3, 梁 剛3, 李 興3, 王 星1,2, 李騰飛1, 田 輝1*

(1. 中國(guó)科學(xué)院 廣州地球化學(xué)研究所 有機(jī)地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640; 2. 中國(guó)科學(xué)院大學(xué), 北京 100049; 3. 中海石油(中國(guó))有限公司湛江分公司, 廣東 湛江 524057)

漸新世時(shí)期, 瓊東南盆地海侵作用逐漸加強(qiáng), 由海陸交互相逐漸過(guò)渡到淺海相沉積。但對(duì)崖城組與陵水組樣品有機(jī)質(zhì)碳同位素分析顯示, 從崖城組到陵水組有機(jī)質(zhì)碳同位素逐漸變重, 與通常陸生植物碳同位素重于水生生物的現(xiàn)象相左。通過(guò)對(duì)樣品顯微組分分析發(fā)現(xiàn), 有機(jī)質(zhì)碳同位素較重的樣品腐泥組分含量偏高。考慮到漸新世沉積水體為微咸水或咸水, 而咸水條件下水生藻類(lèi)有機(jī)質(zhì)碳同位素偏重, 因此顯微組分中腐泥組分含量的升高會(huì)使得有機(jī)質(zhì)碳同位素變重。然而沉積相和顯微組分基本類(lèi)似的崖城組和陵水組樣品有機(jī)質(zhì)碳同位素仍然存在差異, 這說(shuō)明有機(jī)質(zhì)碳同位素變重還存在顯微組分變化之外的其他影響因素。結(jié)合全球漸新世以來(lái)的古氣候與大氣CO2濃度變化特征可知, 該時(shí)期大氣中CO2濃度發(fā)生了巨大變化, 由崖城組沉積時(shí)期的1000～1500 μL/L下降到陵水組沉積時(shí)期的500 μL/L以下, 導(dǎo)致陸生植物碳同位素逐漸變重。因此, 陸源輸入有機(jī)質(zhì)的碳同位素變重也是造成從崖城組到陵水組有機(jī)質(zhì)碳同位素變重的重要原因之一。

漸新統(tǒng)烴源巖; 有機(jī)質(zhì)碳同位素; 顯微組分; 大氣CO2濃度; 瓊東南盆地

0 引 言

沉積有機(jī)質(zhì)碳同位素組成是一種重要的地球化學(xué)參數(shù), 成巖作用和生烴作用對(duì)有機(jī)質(zhì)碳同位素組成影響不大, 仍基本保持或繼承了早期原始有機(jī)質(zhì)的碳同位素特征[1]。原始有機(jī)質(zhì)碳同位素組成受沉積環(huán)境、有機(jī)質(zhì)類(lèi)型等因素控制[1], 其生成的原油和重?zé)N氣也會(huì)繼承母源有機(jī)質(zhì)的碳同位素特征[2–3]。因此, 沉積有機(jī)質(zhì)碳同位素在判斷有機(jī)質(zhì)類(lèi)型、沉積環(huán)境和油氣源對(duì)比中具有重要作用[4]。一般認(rèn)為, 腐殖型有機(jī)質(zhì)以高等植物生源為主, 干酪根碳同位素值較重(大于?26‰); 而腐泥型有機(jī)質(zhì)生源以低等水生生物為主, 干酪根碳同位素值較輕(小于?26‰)[5]。對(duì)瓊東南盆地深水區(qū)崖城組與陵水組有機(jī)質(zhì)碳同位素的測(cè)試結(jié)果顯示, 陵水組有機(jī)質(zhì)碳同位素總體表現(xiàn)重于崖城組, 兩者大致以?27‰為界。但從崖城組到陵水組, 總體表現(xiàn)為水深增大, 由海陸交互相過(guò)渡到淺海相沉積[6]。這有悖于經(jīng)典有機(jī)質(zhì)類(lèi)型和碳同位素特征的關(guān)系。鑒于此, 本次研究擬從有機(jī)質(zhì)碳同位素的影響因素著手, 來(lái)分析崖城組與陵水組有機(jī)質(zhì)碳同位素差異產(chǎn)生的原因。

1 地質(zhì)背景

瓊東南盆地位于海南島與西沙群島之間的海域, 分布范圍為108°50′～110°50′E, 16°50′～19°00′N(xiāo), 是南海北部大陸架4個(gè)含油氣盆地之一, 面積約6.5×104km2, 其中深水區(qū)(水深大于300 m)面積大約占到了盆地面積的70%[7]。受基底斷裂控制, 盆地總體構(gòu)造格局為“三坳兩隆”, 即北部坳陷帶、崖城-松濤中央凸起帶、中央坳陷帶、南部隆起帶和南部坳陷帶(圖1)。瓊東南盆地的形成與演化受到古太平洋板塊俯沖后撤引起的南海擴(kuò)張及印度洋板塊與歐亞板塊碰撞引起的印支地塊走滑作用共同控制[8]。盆地演化經(jīng)歷了由陸到海的演化過(guò)程, 歷經(jīng)斷陷期(T100～T70)、斷坳期(T70～T60)、熱沉降期(T60～T30)和加速沉降期(T30～至今) 4個(gè)階段, 整體可劃分為裂陷期(T100～T60)與裂后期(T60～至今)兩個(gè)時(shí)期[8–10](圖2)。由于新生代以來(lái)沉積速率較快, 瓊東南盆地發(fā)育了巨厚的新生代地層, 厚度高達(dá)12000 m[12]。

始新世時(shí), 南海擴(kuò)張作用使得盆地形成一系列NE向斷裂, 同時(shí)使凹陷發(fā)生了明顯的斷塊旋轉(zhuǎn), 形成眾多犁式半地塹構(gòu)造。由于此時(shí)海水尚未涌入, 瓊東南盆地為陸相湖盆, 發(fā)育中-深湖相沉積, 為盆地重要烴源巖之一[13]。早漸新世(約32 Ma)以來(lái), 南海開(kāi)始SE向擴(kuò)張, 盆地進(jìn)入第2期快速沉降階段。隨著盆地的進(jìn)一步拉張, 發(fā)生了多次海侵海退, 使得崖城組在凸起邊緣以海陸過(guò)渡相沉積為主, 凹陷中心則發(fā)育淺海相沉積。晚漸新世時(shí), 南海繼續(xù)擴(kuò)張使得海水深度進(jìn)一步加大, 普遍發(fā)育淺海至半深海相陵水組。大約23.3 Ma時(shí), 巴拉望島北部與呂宋島弧發(fā)生碰撞, 導(dǎo)致南海北部陸緣東段停止拉張[14–16],瓊東南盆地逐漸停止裂陷活動(dòng), 整體熱沉降作用逐漸加強(qiáng)。西部次海盆在印支地塊左旋走滑活動(dòng)下繼續(xù)擴(kuò)張, 一直持續(xù)到約15 Ma[17]。在此期間瓊東南盆地沉積了三亞組淺海-深海相地層, 地層厚度自西向東遞減。在15～10 Ma期間, 紅河左旋走滑斷裂處于寧?kù)o期[17], 瓊東南盆地整體處于低沉降速率階段, 沉積了梅山組淺海-深海相沉積。在10 Ma左右, 紅河斷裂開(kāi)始轉(zhuǎn)變?yōu)橛倚呋? 造成瓊東南盆地西部進(jìn)入第3期快速沉降階段[17], 沉積了巨厚的黃流組、鶯歌海組和樂(lè)東組。紅河斷裂右旋走滑活動(dòng)對(duì)盆地構(gòu)造沉降演化的影響向東迅速遞減, 至珠江口盆地則不存在10.5～5.3 Ma以來(lái)的快速沉降過(guò)程。在4～5 Ma期間, 南海北部古擴(kuò)張脊開(kāi)始向馬尼拉海溝俯沖[18], 使得瓊東南盆地東部進(jìn)入快速沉降階段, 由于東部寶島-長(zhǎng)昌凹陷缺乏物源輸入, 盆地東部全新世以來(lái)沉積物厚度較薄, 水深迅速增大到2000 m以上(圖1)。

圖1 瓊東南盆地地理位置、構(gòu)造單元?jiǎng)澐旨熬?據(jù)文獻(xiàn)[7]修改)

2 樣品與實(shí)驗(yàn)方法

本研究所取樣品來(lái)自CC26井、WN1井、YL19井和LS33井(圖1)巖屑樣品, 共計(jì)132件。其中CC26井崖城組樣品17件, 陵水組樣品11件; LS33井崖城組樣品10件, 陵水組樣品9件; YL19井崖城組樣品39件, 陵水組樣品28件; WN1井陵水組樣品18件。為了避免油基泥漿對(duì)樣品有機(jī)質(zhì)碳同位素測(cè)定的影響, 研究中首先挑選顆粒狀巖屑, 然后對(duì)其進(jìn)行洗油處理, 具體方法是采用索氏抽提劑(二氯甲烷∶甲醇=93∶7)對(duì)樣品進(jìn)行72～96 h抽提, 直至溶劑無(wú)色為止, 接著繼續(xù)用同樣的混合試劑對(duì)樣品進(jìn)行多次超聲清洗。之后稱(chēng)取一定量樣品, 在樣品中加入稀HCl, 60 ℃反應(yīng)2 h, 以除去碳酸鹽礦物。最后, 用去離子水將樣品洗至中性, 用于總有機(jī)碳(TOC)、有機(jī)質(zhì)碳同位素和顯微組分分析。本研究中采用ELTRR CS-800碳硫分析儀對(duì)樣品TOC進(jìn)行測(cè)試, 測(cè)試標(biāo)準(zhǔn)參考GB/T 19145—2003; 有機(jī)質(zhì)碳同位素分析采用Thermo Finnigan 公司Delta XL Plus EA-IRMS進(jìn)行, 分析過(guò)程中設(shè)置平行樣, 結(jié)果采用PDB標(biāo)準(zhǔn), 測(cè)定精度為±0.2‰。顯微組分分析采用Leica MVP研究級(jí)顯微鏡, 目鏡和物鏡放大倍數(shù)分別為×10和×50, 熒光觀察采用藍(lán)光激發(fā)。

3 結(jié)果與討論

圖3展示了崖城組與陵水組巖屑樣品TOC與有機(jī)質(zhì)碳同位素頻數(shù)分布特征。從圖中可以看出, 崖城組樣品TOC含量大多數(shù)介于0.25%～1%之間, 只有少數(shù)樣品TOC含量大于1%; 陵水組樣品TOC含量基本介于0.25%～1%之間, 與李文浩等[19]測(cè)得崖城組與陵水組暗色泥巖TOC含量結(jié)果基本一致。大多數(shù)崖城組樣品有機(jī)質(zhì)碳同位素值輕于?27‰, 只有少數(shù)樣品有機(jī)質(zhì)碳同位素值較重。而絕大多數(shù)陵水組樣品有機(jī)質(zhì)碳同位素值重于?27‰, 只有極少數(shù)樣品有機(jī)質(zhì)碳同位素較輕。本研究中也制取了少量干酪根樣品進(jìn)行其他實(shí)驗(yàn), 其中獲得的YL19井崖城組4300 m處海陸過(guò)渡相干酪根碳同位素值為?26.8‰, 與該深度巖屑樣品有機(jī)質(zhì)碳同位素值接近。CC26井崖城組淺海相干酪根碳同位素值為?28.0‰, YL19井陵水組淺海相干酪根碳同位素值為?26.3‰, LS33井陵水組淺海相干酪根碳同位素值為?24.3‰, 均落在巖屑樣品有機(jī)質(zhì)碳同位素分布范圍內(nèi)。淺水區(qū)YC13井崖城組海陸過(guò)渡相干酪根碳同位素值為?27.5‰, 也落在深水區(qū)崖城組巖屑樣品有機(jī)質(zhì)碳同位素分布范圍內(nèi)。孫玉梅等[20]的測(cè)試結(jié)果顯示, 淺水區(qū)只有YC8-2-1井崖城組干酪根碳同位素值較重, 為?24.1‰; YC13-1-2井崖城組干酪根碳同位素值較輕, 為?27.9‰, 與本研究制取干酪根碳同位素值接近; YC26-1-A井崖城組干酪根碳同位素值更輕, 介于?29.5‰ ～ ?30.5‰之間; YC26-1-A井、YC13-1-2井和YC21-1-1井陵水組干酪根碳同位素值分別為?26.7‰、?27.0‰和?25.6‰, 也落在本研究中深水區(qū)陵水組巖屑樣品有機(jī)質(zhì)碳同位素值分布范圍內(nèi)。孫林婷[21]測(cè)得的淺水區(qū)崖城組與陵水組干酪根碳同位素值分布范圍雖有重合, 但仍然大致以?26‰為界, 也表現(xiàn)出陵水組干酪根碳同位素值偏重的趨勢(shì)。因此, 無(wú)論是淺水區(qū)還是深水區(qū), 巖屑樣品及干酪根均表現(xiàn)出從崖城組到陵水組有機(jī)質(zhì)碳同位素變重的特征。

3.1 顯微組分對(duì)有機(jī)質(zhì)碳同位素值的影響

本研究通過(guò)對(duì)比崖城組和陵水組有機(jī)質(zhì)碳同位素值與沉積相關(guān)系, 發(fā)現(xiàn)有機(jī)質(zhì)碳同位素值較重的樣品均來(lái)自淺海相沉積(崖城組沉積后期由于海平面上升也發(fā)育淺海相沉積), 而來(lái)自潮坪相和沼澤相沉積的樣品有機(jī)質(zhì)碳同位素值基本輕于?27‰ (圖4), 說(shuō)明沉積相對(duì)有機(jī)質(zhì)碳同位素有著重要影響。沉積相的變化可以反映有機(jī)質(zhì)來(lái)源的變化, 因此顯微組分的變化可能是導(dǎo)致樣品有機(jī)質(zhì)碳同位素不同的重要原因。從圖中可以看出, 鏡質(zhì)組為其主要組分, 含量高達(dá)70%; 其次為腐殖無(wú)定形組分, 含量約為15%; 再次為惰質(zhì)組, 含量約為8%; 殼質(zhì)組和腐泥無(wú)定形組分(含藻類(lèi))含量較低, 分別為3%和4%。有機(jī)質(zhì)碳同位素值較重的LS33井崖城組4016 m處淺海相樣品(有機(jī)質(zhì)碳同位素值為?26.5‰)中, 殼質(zhì)組和腐泥組分(含藻類(lèi))含量顯著升高(圖6), 其顯微組分中鏡質(zhì)組含量35%, 腐殖無(wú)定形組含量為30%, 惰質(zhì)組含量為15%, 腐泥無(wú)定形組分(含無(wú)結(jié)構(gòu)藻)含量為12%, 殼質(zhì)組含量為8%。到有機(jī)質(zhì)碳同位素值更重的LS33井陵水組3651 m處淺海相樣品(有機(jī)質(zhì)碳同位素值為?24.7‰), 殼質(zhì)組和腐泥組分(含無(wú)結(jié)構(gòu)藻)含量進(jìn)一步升高(圖7), 其顯微組分中鏡質(zhì)組含量30%、腐殖無(wú)定形組分含量為30%、惰質(zhì)組含量為15%、腐泥無(wú)定形含量(含無(wú)結(jié)構(gòu)藻)和殼質(zhì)組含量分別為15%和10%。Huang.[7]的研究也表明, 從崖城組到陵水組, 顯微組分中殼質(zhì)組和無(wú)定形組分含量上升。Wu.[22]通過(guò)研究伽馬蠟烷與C30藿烷的相對(duì)含量, 認(rèn)為三角洲地區(qū)表層水為淡水或微咸水環(huán)境, 而淺海區(qū)為微咸水或咸水沉積環(huán)境。咸水條件下水生生物有機(jī)質(zhì)碳同位素值偏重, 如柴達(dá)木盆地第三系鹽湖相干酪根碳同位素值介于?25.2‰ ～?21.8‰之間[1], 現(xiàn)今大西洋19°N附近懸浮顆粒有機(jī)碳的碳同位素值也偏重, 介于?22‰ ～ ?20‰之間[23]。故海洋自生沉積有機(jī)質(zhì)碳同位素值偏重, 樣品顯微組分中藻類(lèi)含量的升高將導(dǎo)致樣品有機(jī)質(zhì)碳同位素變重。

圖2 瓊東南盆地地層劃分(據(jù)文獻(xiàn)[8, 11]修改)

圖3 崖城組、陵水組巖屑樣品TOC與有機(jī)質(zhì)碳同位素頻數(shù)分布

圖5為YL19井崖城組4410 m處海陸過(guò)渡相樣品(有機(jī)質(zhì)碳同位素值為?27.2‰)干酪根顯微組分照片。

圖4 YL19井與LS33井巖屑樣品有機(jī)質(zhì)碳同位素分布

圖5 YL19井崖城組4410 m海陸過(guò)渡相樣品(全巖有機(jī)質(zhì)碳同位素值為?27.2‰)干酪根顯微組分照片

為確定顯微組分是否為影響樣品有機(jī)質(zhì)碳同位素值的唯一因素, 研究中選取長(zhǎng)昌凹陷CC26井和WN1井巖屑樣品進(jìn)行分析。CC26井位于長(zhǎng)昌凹陷緩坡邊緣, 從崖城組到陵水組均為大套泥巖, 沉積相基本保持不變, 為淺海相沉積。WN1井則靠近長(zhǎng)昌凹陷中心, 由于崖城組埋深較大, 只鉆至陵水組二段。WN1井陵水組也發(fā)育大套泥巖, 但陵水組沉積時(shí)期為水深增大的過(guò)程, 因此沉積相表現(xiàn)為從陵水組二段的淺海相沉積過(guò)渡到崖城組一段的半深海相沉積。

圖6 LS33井崖城組4016 m處淺海相樣品(全巖有機(jī)質(zhì)碳同位素值為?26.5‰)干酪根顯微照片(藍(lán)光激發(fā)熒光模式)

圖7 LS33井陵水組3651 m處淺海相樣品(全巖有機(jī)質(zhì)碳同位素值為?24.7‰)干酪根顯微照片(藍(lán)光激發(fā)熒光模式)

顯微組分分析顯示, CC26井崖城組有機(jī)質(zhì)主要由腐殖無(wú)定形體組成, 含量約占60%; 其次為殼質(zhì)組, 主要為陸源高等植物碎屑類(lèi)脂體、角質(zhì)體和孢子體等, 含量約占30%; 鏡質(zhì)組和惰質(zhì)組含量較低, 分別約為4%和5%; 此外, CC26井崖城組中還含有極少量的藻類(lèi)體, 約占1%。CC26井陵水組、WN1井陵水組樣品顯微組分與CC26井崖城組樣品基本相似。Li.[24]認(rèn)為, CC26井崖城組與陵水組沉積時(shí)期, 大量陸源碎屑的輸入稀釋了海洋生產(chǎn)力的貢獻(xiàn), 同時(shí)弱氧化的水體環(huán)境使得藻類(lèi)在沉降過(guò)程中被分解, 因此崖城組、陵水組有機(jī)質(zhì)含量與顯微組分受到陸源碎屑輸入的控制。這一觀點(diǎn)與本研究中顯微組分的分析結(jié)果一致。

值得注意的是, CC26井崖城組與陵水組有機(jī)質(zhì)組成基本一致, 但有機(jī)質(zhì)碳同位素值仍然存在差異, 也大致以?27‰為界(圖8)。顯微組分相似的WN1井陵水組有機(jī)質(zhì)碳同位素主體上也重于?27‰。因此, 沉積環(huán)境不同造成的顯微組分差異并不能解釋CC26井崖城組與陵水組有機(jī)質(zhì)碳同位素的不同。這表明除了顯微組分外, 尚存在其他因素影響有機(jī)質(zhì)碳同位素值。

3.2 氣候變化對(duì)有機(jī)質(zhì)碳同位素的影響

陸生植物有機(jī)質(zhì)碳同位素值分布范圍與植物類(lèi)型相關(guān)[25]。C4植物有機(jī)質(zhì)碳同位素值介于?8‰ ～ ?16‰之間, 平均值為?13‰; C3植物有機(jī)質(zhì)碳同位素值介于?23‰ ～ ?34‰之間, 平均值為?27‰[26–31]。由于C4植物出現(xiàn)較晚, 一般認(rèn)為不超過(guò)7 Ma[32–33], 且崖城組與陵水組樣品碳同位素值均輕于?23‰, 因此上述兩套地層中陸生植物主要來(lái)自C3植物。已有研究表明, C3植物有機(jī)質(zhì)碳同位素與其生長(zhǎng)環(huán)境相關(guān), 溫度升高以及大氣中CO2分壓降低都會(huì)使得C3植物有機(jī)質(zhì)碳同位素變重[34]。陵水組有機(jī)質(zhì)碳同位素重于崖城組可能與其沉積時(shí)期氣候和大氣CO2濃度變化相關(guān)。

圖8 CC26井和WN1井巖屑樣品有機(jī)質(zhì)碳同位素分布

前人對(duì)瓊東南盆地新生代孢粉組合研究表明, 崖城組沉積時(shí)期為溫暖潮濕的熱帶亞熱帶氣候, 到陵水組沉積時(shí)期氣溫下降趨勢(shì)明顯[35–36]。氣溫下降將導(dǎo)致有機(jī)質(zhì)碳同位素變輕, 因此崖城組、陵水組樣品有機(jī)質(zhì)碳同位素分布特征與氣溫降低無(wú)關(guān)。Pagani.[37]的研究表明, 漸新世全球大氣中CO2濃度下降明顯, 從1000～1500μL/L下降到500μL/L以下(圖9)。大氣CO2濃度的變化引起了陸生植物有機(jī)質(zhì)碳同位素值波動(dòng), 使得從崖城組到陵水組, 地層接受的陸源有機(jī)質(zhì)碳同位素值變重, 這也是從崖城組到陵水組有機(jī)質(zhì)碳同位素變重的原因之一。

圖9 50 Ma以來(lái)大氣CO2含量變化圖(據(jù)文獻(xiàn)[37])

4 結(jié) 論

(1) 瓊東南盆地深水區(qū)漸新統(tǒng)從崖城組到陵水組, 沉積相由海陸過(guò)渡相逐漸轉(zhuǎn)變?yōu)闇\海相沉積, 兩者有機(jī)質(zhì)碳同位素分布存在明顯差異, 總體表現(xiàn)為陵水組有機(jī)質(zhì)碳同位素相對(duì)偏重, 兩者大致以?27‰為界, 這與通常陸生植物有機(jī)質(zhì)碳同位素偏重的現(xiàn)象相左。

(2) 通過(guò)對(duì)顯微組分進(jìn)行分析, 發(fā)現(xiàn)從崖城組到陵水組, 顯微組分中腐泥組含量上升。由于微咸水或咸水條件下水生生物有機(jī)質(zhì)碳同位素偏重, 因此樣品中腐泥組分含量的增大會(huì)使得有機(jī)質(zhì)碳同位素變重。

(3) 崖城組與陵水組顯微組分基本一致, 主要為陸源有機(jī)質(zhì)輸入的CC26井巖屑樣品有機(jī)質(zhì)碳同位素值分布也表現(xiàn)出這一特征。因此顯微組分的變化只是有機(jī)質(zhì)碳同位素變重的原因之一, 可能還與陵水組沉積時(shí)期大氣中CO2濃度顯著下降造成陸源C3植物碳同位素變重有關(guān)。

[1] 黃第潘. 陸相烴源巖有機(jī)質(zhì)中碳同位素組成的分布特征[J]. 中國(guó)海上油氣(地質(zhì)), 1993, 7(4): 1–5. Huang Di-fan. Distributive characteristics of carbon isotope composition in organic matters of continental source rocks[J]. China Offshore Oil Gas Geol, 1993, 7(4): 1–5 (in Chinese with English abstract).

[2] Clayton C. Carbon isotope fractionation during natural gas generation from kerogen[J]. Mar Pet Geol, 1991, 8(2): 232– 240.

[3] Liu Q Y, Wu X Q, Wang X F, Jin Z J, Zhu D Y, Meng Q Q, Huang S P, Liu J Y, Fu Q. Carbon and hydrogen isotopes of methane, ethane, and propane: A review of genetic identification of natural gas[J]. Earth Sci Rev, 2019, 190: 247–272.

[4] 戴金星. 天然氣地質(zhì)和地球化學(xué)論文集(卷二)[M]. 北京: 工業(yè)出版社, 2000. Dai Jin-xing. Selected works of Natural Gas Geology and Geochemistry (Vol.2)[M]. Beijing: Petroleum Industry Press, 2000 (in Chinese).

[5] 孫玉梅, 李友川, 黃正吉. 部分近海湖相烴源巖有機(jī)質(zhì)異常碳同位素組成[J]. 石油勘探與開(kāi)發(fā), 2009, 36(5): 609–616.Sun Yu-mei, Li You-chuan, Huang Zheng-ji. Abnormal carbon isotopic compositions in organic matter of lacustrine source rocks close to sea[J]. Pet Explor Develop, 2009, 36(5): 609– 616 (in Chinese with English abstract).

[6] 王子嵩, 劉震, 黃保家, 孫志鵬, 姚哲, 陳宇航, 劉鵬, 王兵. 瓊東南盆地深水區(qū)中央坳陷帶東部漸新統(tǒng)烴源巖分布及評(píng)價(jià)[J]. 天然氣地球科學(xué), 2014, 25(3): 360–371.Wang Zi-song, Liu Zhen, Huang Bao-jia, Sun Zhi-peng, Yao Zhe, Chen Yu-hang, Liu Peng, Wang Bing. Distribution and evaluation of Oligocene source rocks in the east of Central Depression Belt in deep-water area, Qiongdongnan Basin[J]. Nat Gas Geosci, 2014, 25(3): 360–371 (in Chinese with English abstract).

[7] Huang B J, Tian H, Li X S, Wang Z F, Xiao X M. Geochemistry, origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin, South China Sea[J]. Mar Pet Geol, 2016, 72: 254–267.

[8] 謝文彥, 張一偉, 孫珍, 姜建群. 瓊東南盆地?cái)嗔褬?gòu)造與成因機(jī)制[J]. 海洋地質(zhì)與第四紀(jì)地質(zhì), 2007, 27(1): 71–78. Xie Wen-yan, Zhang Yi-wei, Sun Zhen, Jiang Jian-qun. Characteristics and formation mechanism of faults in Qiongdongnan Basin[J]. Mar Geol Quatern Geol, 2007, 27(1): 71–78 (in Chinese with English abstract).

[9] 雷超, 任建業(yè), 裴健翔, 林海濤, 尹新義, 佟殿君. 瓊東南盆地深水區(qū)構(gòu)造格局和幕式演化過(guò)程[J]. 地球科學(xué), 2011, 36(1): 151–162. Lei Chao, Ren Jian-ye, Pei Jian-xiang, Lin Hai-tao, Yin Xin-yi, Tong Dian-jun. Tectonic framework and multiple episode tectonic evolution in deepwater area of Qiongdonan Basin, northern continental margin of South China Sea[J]. Earth Sci, 2011, 36(1): 151–162 (in Chinese with English abstract).

[10] 趙民, 張曉寶, 吉利明, 張功成. 瓊東南盆地構(gòu)造演化特征及其對(duì)油氣藏的控制淺析[J]. 天然氣地球科學(xué), 2010, 21(3): 494–502. Zhao Min, Zhang Xiao-bao, Ji Li-ming, Zhang Gong-cheng. Characteristics of tectonic evolution in the Qiongdongnan Basin and brief discussion about its controlling on reservoirs[J].Nat Gas Geosci, 2010, 21(3): 494–502 (in Chinese with English abstract).

[11] 謝玉洪. 南海北部自營(yíng)深水天然氣勘探重大突破及其啟示[J]. 天然氣工業(yè), 2014, 34(10): 1–8. Xie Yu-hong. A major breakthrough in deepwater natural gas exploration in a self-run oil/gas field in the northern South China Sea and its enlightenment[J]. Nat Gas Ind, 2014, 34(10): 1–8 (in Chinese with English abstract).

[12] 于鵬, 王家林, 鐘慧智, 陳冰, 劉鐵樹(shù), 劉學(xué)考. 瓊東南盆地基底結(jié)構(gòu)綜合地球物理研究[J]. 中國(guó)海上油氣(地質(zhì)), 1999, 13(6): 55–62. Yu Peng, Wang Jia-lin, Zhong Hui-zhi, Chen Bing, Liu Tie-shu, Liu Xue-kao. Comprehensive geophysical research on the basement structure in Qiongdongnan Basin[J]. China Offshore Oil Gas Geol, 1999, 13(6): 55–62 (in Chinese with English abstract).

[13] Zhu W L, Huang B J, Mi L J, Wilkins R W T, Fu L, Xiao X M. Geochemistry, origin, and deep-water exploration potential of natural gases in the Pearl River Mouth and Qiongdongnan basins, South China Sea[J]. AAPG bulletin, 2009, 93(6): 741–761.

[14] Hamilton W B. Tectonics of the Indonesian Region[M]. Washington: US Government Printing Office, 1979: 345– 1078.

[15] Holloway N H. North Palawan block, Philippines: Its relation to Asian mainland and role in evolution of South China Sea[J]. AAPG Bulletin, 1982, 66(9): 1355–1383.

[16] Fuller M, Haston R, Schmidtke E. Paleomagnetism in SE Asia: sinistral shear between Philippine Sea plate and Asia [M]// Kissel C, Laj C. Paleomagnetic Rotations and Continental Deformation. Dordrecht: Springer, 1989: 411–430.

[17] 袁玉松, 楊樹(shù)春, 胡圣標(biāo), 何麗娟. 瓊東南盆地構(gòu)造沉降史及其主控因素[J]. 地球物理學(xué)報(bào), 2008, 51(2): 376–383. Yuan Yu-song, Yang Shu-chun, Hu Sheng-biao, He Li-juan. Tectonic subsidence of Qiongdongnan Basin and its main control factors[J]. Chinese J Geophys, 2008, 51(2): 376–383 (in Chinese with English abstract).

[18] Yang T F, Lee T, Chen C H, Cheng S N, Knittel U, Punongbayan R S, Rasdas A R. A double island arc between Taiwan and Luzon: Consequence of ridge subduction[J]. Tectonophysics, 1996, 258(1–4): 85–101.

[19] 李文浩, 張枝煥, 李友川, 傅寧, 黃儼然, 黎瓊, 張慧敏. 瓊東南盆地古近系漸新統(tǒng)烴源巖地球化學(xué)特征及生烴潛力分析[J]. 天然氣地球科學(xué), 2011, 22(4): 700–708. Li Wen-hao, Zhang Zhi-huan, Li You-chuan, Fu Ning, Huang Yan-ran, Li Qiong, Zhang Hui-min. Geichemical characteristics and hydrocarbon generation of Paleogene Oligocene source rocks in Qiongdongnan Basin[J]. Nat Gas Geosci, 2011, 22(4): 700–708 (in Chinese with English abstract).

[20] 孫玉梅, 郭迺嬿, 王彥. 鶯-瓊氣區(qū)天然氣主氣源及注入史分析[J]. 中國(guó)海上油氣(地質(zhì)), 2000, 14(4): 23–30. Sun Yu-mei, Guo Nai-yan, Wang Yan. Main gas sources and gas charge history in Yinggehai-Qiongdongnan region[J]. China Offshore Oil Gas (Geol), 2000, 14(4): 23–30 (in Chinese with English abstract).

[21] 孫林婷. 鶯-瓊盆地原油地球化學(xué)特征及來(lái)源[D]. 杭州: 浙江大學(xué), 2017: 77. Sun Linting. Geochemical characteristics and origins of oils in Yinggehai and Qiongdongnan Basins[D]. Hangzhou: Zhejiang University, 2017: 77 (in Chinese with English abstract).

[22] Wu P, Hou D J, Gan J, Li X, Ding W J, Liang G, Wu B B. Paleoenvironment and controlling factors of Oligocene source rock in the eastern deep-water area of the Qiongdongnan basin: Evidences from organic geochemistry and palynology[J]. Energ Fuel, 2018, 32(7): 7423–7437.

[23] Hofmann M, Wolf-Gladrow D A, Takahashi T, Sutherland S C, Six K D, Maier-Reimer E. Stable carbon isotope distribution of particulate organic matter in the ocean: A model study[J]. Mar Chem, 2000, 72(2–4): 131–150.

[24] Li W H, Zhang Z H. Paleoenvironment and its control of the formation of Oligocene marine source rocks in the deep-water area of the northern South China Sea[J]. Energ Fuel, 2017, 31(10): 10598–10611.

[25] Gr?cke D R. The carbon isotope composition of ancient CO2based on higher-plant organic matter[J]. Philos Trans A Math Phys Eng Sci, 2002, 360(1793): 633–658.

[26] Park R, Epstein S. Carbon isotope fractionation during photosynthesis[J]. Geochim Cosmochim Acta, 1960, 21(1/2): 110–126.

[27] Bender M M. Variations in the13C/12C ratios of plants in relation to the pathway of photosynthetic carbon dioxide fixation[J]. Phytochemistry, 1971, 10(6): 1239–1244.

[28] Smith B N, Epstein S. Two categories of13C/12C ratios for higher plants[J]. Plant Phys, 1971, 47(3): 380–384.

[29] Smith B N, Brown W V. The Kranz syndrome in the Gramineae as indicated by carbon isotopic ratios[J]. Am J Botany, 1973, 60(6): 505–513.

[30] Tieszen L L. Natural variations in the carbon isotope values of plants: Implications for archaeology, ecology, and paleoecology[J]. J Archaeol Sci, 1991, 18(3): 227–248.

[31] Vogel J C. 4-Variability of carbon isotope fractionation during photosynthesis[M]//Ehleringer J, Hall A E, Farquhar G D. Stable Isotopes and Plant Carbon-Water Relations. Pittsburgh: Academic Press, 1993: 29–46.

[32] Morgan M E, Kingston J D, Marino B D. Carbon isotopic evidence for the emergence of C4plants in the Neogene from Pakistan and Kenya[J]. Nature, 1994, 367(6459): 162.

[33] Hodell D A, Williams D F, Kennett J P. Late Pliocene reorganization of deep vertical water-mass structure in the western South Atlantic: Faunal and isotopic evidence[J]. Geol Soc Am Bull, 1985, 96(4): 495–503.

[34] Wolberg D L, Stump E, Rosenberg G R. Dinofest? International: Proceedings of a symposium held at Arizona State University[C]. Philadelphia: The Academy of Natural Sciences. 1997: 457–461.

[35] 張一凡, 劉東生, 張訓(xùn)華. 瓊東南盆地新生代孢粉組合及其古氣候意義[J]. 海洋地質(zhì)與第四紀(jì)地質(zhì), 2017, 37(1): 93–101. Zhang Yi-fan, Liu Dong-sheng, Zhang Xun-hua. Neogene Palynological assemblages from Qiongdongnan Basin and third paleoclimatic implications[J]. Mar Geol Quatern Geol, 2017, 37(1): 93–101 (in Chinese with English abstract).

[36] 覃軍干, 吳國(guó)瑄, 李君, 王任, 謝金有, 張強(qiáng). 瓊東南盆地漸新統(tǒng)-上新統(tǒng)孢粉、藻類(lèi)記錄[J]. 微體古生物學(xué)報(bào), 2016, 33(4): 335–349. Qin Jun-gan, Wu Guo-xuan, Li Jun, Wang Ren, Xie Jin-you, Zhang Qiang. Spores, pollen, freshwater algae and dinoflagellatecysts recorded in the Oligocene-Pliocene from southeast Hainan Basin, Sourth China Sea [J]. Acta Micropalaeontol Sinica, 2016, 33(4): 335–349 (in Chinese with English abstract).

[37] Pagani M, Zachos J C, Freeman K H, Tipple B, Bohaty S. Marked decline in atmospheric carbon dioxide concentrations during the Paleogene [J]. Science, 2005, 309(5734): 600–603.

Characteristics of the organic carbon isotope of Oligocene source rocks in deepwater area of the Qiongdongnan Basin and its main controlling factors

HE Chun-min1,2, GAN Jun3, LIANG Gang3, LI Xing3, WANG Xing1,2, LI Teng-fei1and TIAN Hui1*

1.State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; 2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Zhanjiang Branch of China National Offshore Oil Corporation Ltd., Zhanjiang 524057, China

During the Oligocene, transgression gradually increased in the Qiongdongnan Basin, and the marine- continental transitional facies went into neritic facies progressively. However, organic carbon isotope analysis of the Yacheng Formation and Lingshui Formation revealed that the organic carbon isotope values grew heavy from the Yacheng Formation to the Lingshui Formation, contrary to the general phenomenon in which terrestrial plants have heavier organic carbon isotope values than aquatic organisms. Analysis of the maceral indicated that samples with heavy organic carbon isotope values contained more sapropelic amorphous. Considering the sedimentary water body is brackish water or saltwater during the Oligocene, the algae in saltwater have heavier organic carbon isotope values than freshwater. The increase in the sapropelic amorphous content makes the organic carbon isotope heavy. Nevertheless, differences in organic carbon isotope still existed between the Yacheng Formation and the Lingshui Formation samples with similar sedimentary facies and maceral composition, indicating other factors influenced the organic carbon isotopes. Climate and atmospheric CO2concentration changes since Oligocene caused the atmospheric CO2concentration to decline significantly during this period, from 1000–1500 μL/L in the deposition period of the Yacheng Formation to less than 500 μL/L in the sedimentary period of the Lingshui Formation; this led to heavier organic carbon isotopes in terrestrial plants. Therefore, the input of terrestrial organic matter with heavy carbon isotopes is also an important cause of the heavier organic carbon isotopes in the Lingshui Formation than in the Yacheng Formation.

hydrocarbon source rocks in the Oligocene; organic carbon isotope; maceral composition; atmospheric CO2concentration; Qiongdongnan Basin

P593; TE122

A

0379-1726(2021)02-0175-10

10.19700/j.0379-1726.2021.02.004

2019-11-08;

2020-01-07;

2020-01-09

國(guó)家油氣重大專(zhuān)項(xiàng)(2016ZX05026-002-00)

何春民(1992–), 男, 博士研究生, 地球化學(xué)專(zhuān)業(yè)。E-mail: hechmin@mail2.sysu.edu.cn

TIAN Hui, E-mail: tianhui@gig.ac.cn; Tel: +86-20-85290309