段 毅 馬春林 余永進(jìn) 劉軍鋒 劉一倉(cāng) 何金先 吳保祥 徐 麗 張曉麗

(1.中國(guó)科學(xué)院地質(zhì)與地球物理研究所油氣資源研究重點(diǎn)實(shí)驗(yàn)室 蘭州 730000; 2.長(zhǎng)慶油田分公司第二采油廠 甘肅慶陽(yáng) 745000;3.長(zhǎng)慶油田分公司第四超低滲項(xiàng)目部 甘肅慶陽(yáng) 745000)

鄂爾多斯盆地董志-正寧地區(qū)原油地球化學(xué)特征與成因①

段 毅1馬春林2余永進(jìn)2劉軍鋒2劉一倉(cāng)3何金先1吳保祥1徐 麗1張曉麗1

(1.中國(guó)科學(xué)院地質(zhì)與地球物理研究所油氣資源研究重點(diǎn)實(shí)驗(yàn)室 蘭州 730000; 2.長(zhǎng)慶油田分公司第二采油廠 甘肅慶陽(yáng) 745000;3.長(zhǎng)慶油田分公司第四超低滲項(xiàng)目部 甘肅慶陽(yáng) 745000)

鄂爾多斯盆地董志-正寧地區(qū)是該盆地中生界油氣勘探新區(qū),對(duì)該地區(qū)原油的地球化學(xué)進(jìn)行研究,了解原油的成因,可以為該地區(qū)石油勘探和開(kāi)發(fā)提供科學(xué)依據(jù)。本研究首次對(duì)采集于董志-正寧地區(qū)原油和鄂爾多斯盆地?zé)N源巖的烴類生物標(biāo)志化合物進(jìn)行了系統(tǒng)的分析,研究了它們的地球化學(xué)特征。原油中各類烴類生物標(biāo)志化合物分布和組成特征指示了原油形成于淡水弱還原環(huán)境;油源對(duì)比研究反映了董志-正寧地區(qū)原油主要來(lái)自長(zhǎng)7烴源巖。

鄂爾多斯盆地 董志-正寧地區(qū)原油 生物標(biāo)志化合物 分布和組成特征 成因

鄂爾多斯盆地董志-正寧地區(qū)油田位于鄂爾多斯盆地伊陜斜坡次級(jí)構(gòu)造單元的南西部(圖1),該地區(qū)油氣勘探程度較低。近幾年,在董志-正寧地區(qū)的油氣勘探獲得重要突破,西89、寧21、寧51、寧52、寧102等井在中生界長(zhǎng)2油層組獲得了工業(yè)油流,這充分說(shuō)明董志-正寧地區(qū)中生界具有較大的勘探潛力,有望成為長(zhǎng)慶油田增儲(chǔ)上產(chǎn)的接替區(qū)塊和層位。但是時(shí)至今日,有關(guān)董志-正寧地區(qū)原油的成因尚不清楚。該問(wèn)題的解決將對(duì)鄂爾多斯盆地董志-正寧地區(qū)中生界石油勘探具有重要的指導(dǎo)意義。本文系統(tǒng)采集了董志-正寧地區(qū)原油及其烴源巖樣品,分析了它們的生物標(biāo)志化合物,研究了原油的成因,并且進(jìn)行了油源對(duì)比。這些研究有助于了解該地區(qū)油藏的成因,并為盆地中石油勘探提供了科學(xué)依據(jù)。

1 樣品與分析

圖1 研究地區(qū)構(gòu)造位置Fig.1 Tectonic sketch map location of the study area

樣品均為2009年6月份采集,原油來(lái)自鄂爾多斯盆地董志-正寧地區(qū)(圖1),烴源巖樣品主要采集于鄂爾多斯盆地董志-正寧地區(qū)及其西部地區(qū)。分析方法包括:先將原油沉淀瀝青質(zhì),然后沉淀瀝青質(zhì)原油在氧化鋁/硅膠柱色譜上分離出飽和烴。飽和烴的分析鑒定是在美國(guó)安捷倫科技公司(Agilent)制造的色譜(6890N)-質(zhì)譜(5973N)聯(lián)用儀上進(jìn)行的。色譜柱為HP-5(30 m X0.32 mm),固定相涂膜厚度0.25μm,載氣為He,起始溫度80℃,以4℃/min升溫至少300℃,并且恒溫30 min。質(zhì)譜離子源溫度為250℃,電離電壓為70 eV。

2 結(jié)果與討論

2.1 原油的母源來(lái)源

正構(gòu)烷烴色譜特征蘊(yùn)含了豐富的沉積環(huán)境、生源構(gòu)成、保存條件和熱演化的信息[1–3]。鄂爾多斯盆地董志-正寧地區(qū)原油中正構(gòu)烷烴碳數(shù)分布在C11～ C37之間;主峰碳為C15、C17和C19;C21–/C21+為0.92～ 1.40,OEP分布在1.02～1.11之間(表1),說(shuō)明原油由水生生物和陸生植物混合母質(zhì)構(gòu)成。從圖2可以看出,董志-正寧地區(qū)原油樣品分布在一個(gè)狹窄的區(qū)域內(nèi),說(shuō)明原油中正構(gòu)烷烴、類異戊二烯烷烴分布極為相似,反映了它們來(lái)自一個(gè)類似的生物源[4～6]。原油中Pr/n C17和Ph/n C18比值,通常用來(lái)研究母質(zhì)類型、形成環(huán)境和成熟度。董志-正寧地區(qū)原油中Pr/ n C17和Ph/n C18比值分布特征指示了原油母質(zhì)為混合型[2,7,8](圖3)。

表1 原油和烴源巖中正構(gòu)烷烴、類異戊二烯烷烴分析數(shù)據(jù)Table1 Analytical data of n-alkanes and isoprenoid alkanes of the crude oil and source rock samples

圖2 原油中Pr/n C17與Pr/Ph(A)和Pr/n C18與Pr/Ph(B)比值相關(guān)圖Fig.2 Cross plots of Pr/n C17 vs.Pr/Ph(A)and Pr/n C18 vs.Pr/Ph(B)ratios of the crude oil samples

圖3 原油中Pr/n C17與Ph/n C18比值相關(guān)圖Fig.3 Cross plot of Pr/n C17 vs.Ph/n C18 ratios of the crude oil samples

表2 原油和烴源巖中甾烷分析數(shù)據(jù)Table2 Analytical data of steranes of the crude oil and source rock samp les

鄂爾多斯盆地董志-正寧地區(qū)原油中C27、C28和C29規(guī)則甾烷的相對(duì)含量分別為22.1%～28.2%、26.1%～29.4%和42.4%～50.5%;規(guī)則甾烷/藿烷比值在0.15～0.26之間(表2)。這些值指示了董志-正寧地區(qū)原油中甾烷的分布特征是C29規(guī)則甾烷含量最高,其次為C28規(guī)則甾烷,C27規(guī)則甾烷含量較低,同時(shí)規(guī)則甾烷/藿烷比值也較高,甾烷分布的這些特征反映了它們成油母質(zhì)為水生生物和陸生高等植物[3,9,10],同時(shí)原油來(lái)自高等植物源占有重要的比例[1,11,12]。在圖4中,這些樣品分布于一個(gè)較窄的范圍內(nèi),并且研究區(qū)長(zhǎng)2和長(zhǎng)7原油的甾烷組成極為類似,說(shuō)明它們具有類似的成油母質(zhì).obinson等[13]研究認(rèn)為,4-甲基甾烷起源于甲藻類?？墒?Philp等[3]研究結(jié)果表明4-甲基甾烷也可以來(lái)自細(xì)菌。董志-正寧地區(qū)原油中檢出較豐富的4-甲基甾烷,4-甲基甾烷/規(guī)則甾烷比值分布在0.12～0.16之間。原油中含4-甲基甾烷反映了甲藻類對(duì)原油的形成也作出了一定的貢獻(xiàn)。

圖4 原油中不同碳數(shù)規(guī)則甾烷分布圖Fig.4 Ternary diagram of C27,C28 and C29 sterane compositions of the crude oil samples

原油中四環(huán)萜烷和三環(huán)萜烷的含量與母源性質(zhì)密切相關(guān)[14,15],C24四環(huán)萜烷/(C24四環(huán)萜烷+C26三環(huán)萜烷)、三環(huán)萜烷的C19+C20/C23、C25/C26和C25三環(huán)萜烷/C24四環(huán)萜烷比值可以判識(shí)原油形成的母質(zhì)[7,16]。

表3 原油和烴源巖中萜烷分析數(shù)據(jù)Table3 Analytical data of terpanes of the crude oil and source rock samp les

董志-正寧地區(qū)原油中三環(huán)萜烷/霍烷比值除了一個(gè)樣品為0.40外,其它原油樣品分布在0.08～ 0.29之間,低的三環(huán)萜烷/霍烷比值指示了陸源高等植物對(duì)原油做出了重要貢獻(xiàn),董志-正寧地區(qū)原油中也檢出了低豐度的具有松香烷結(jié)構(gòu)的三環(huán)萜烷(圖5),松香類/αβ-C30藿烷比值為0.021～0.136,這類化合物來(lái)自陸源高等植物。三環(huán)萜烷C25/C26和C25三環(huán)萜烷/C24四環(huán)萜烷比值可以用來(lái)劃分海陸相原油.24四環(huán)萜烷一般在陸源有機(jī)質(zhì)中含量較高.anson等對(duì)塔里木盆地原油研究表明,海相原油中C25三環(huán)萜烷/C24四環(huán)萜烷比值大于1,陸相原油中幾乎都小于1;原油中三環(huán)萜烷C25/C26比值>1,一般指示了海相原油,該比值<1指示了陸相原油[17]。在董志-正寧地區(qū)原油中,這兩個(gè)比值分別為0.40～ 0.53和0.39～0.57(表3),反映了董志-正寧地區(qū)原油成油母質(zhì)具有更多的陸源有機(jī)質(zhì)。在圖6中,研究樣品分布于一個(gè)較窄的范圍內(nèi),說(shuō)明它們具有類似的成油母質(zhì)。

圖5 寧16原油飽和烴中m/z123質(zhì)量色譜圖Fig.5 Mass chromatogram for m/z123 of diterpenoid hydrocarbons in crude oil from the sample Ning 16

圖6 原油中C23三環(huán)萜烷/(C23三環(huán)萜烷+C30藿烷)與C24四環(huán)/(C24四環(huán)+C26三環(huán)萜烷)(A)和C23三環(huán)萜烷/(C23三環(huán)萜烷+C30藿烷)與C19+C20三環(huán)萜烷/C23三環(huán)萜烷(B)比值相關(guān)圖Fig.6 Cross plot of C23 tricyclic terpane/(C23 tricyclic terpane+C30 hopane)vs.C24 tatracyclic terpane/ (C24 tatracyclic terpane+C26 tricyclic terpane)ratios(A)and C23 tricyclic terpane/(C23 tricyclic terpane+C30 hopane) vs.C19+C20 tricyclic terpanes/C23 tricyclic terpane ratios(B)in the crude oil samples

2.2 原油的形成環(huán)境

Pr/Ph比值可以反映原油烴源巖的形成環(huán)境[10]。鄂爾多斯盆地董志-正寧地區(qū)原油中Pr/Ph比值為0.95～1.39(表1),與柴達(dá)木盆地西部強(qiáng)還原環(huán)境形成的原油Pr/Ph比值(0.51～0.64)相比較[1],董志-正寧地區(qū)原油中Pr/Ph比值屬于中度,說(shuō)明董志-正寧地區(qū)原油形成于淡水湖泊相弱還原環(huán)境,圖3也說(shuō)明了研究原油形成于這種環(huán)境。

許多研究都證明了伽馬蠟烷和升霍烷可以指示沉積環(huán)境的鹽度[3,10,12,18]。伽馬蠟烷/αβ-C30霍烷比值在董志-正寧地區(qū)原油中為0.031～0.052,>C30霍烷相對(duì)含量為19.1%～26.5%之間(表3和圖7)。董志-正寧地區(qū)原油中這些值很低,說(shuō)明董志-正寧地區(qū)原油形成于淡水湖泊環(huán)境。

圖7 原油中γ-蠟烷/C30藿烷比值與>C30藿烷相對(duì)含量(%)相關(guān)圖Fig.7 Cross plot of gammacerane/αβ-C30 hopane ratio vs.relative abundance of>C30 hopane of the crude oil samples

2.3 原油的成熟度

許多生物標(biāo)志化合物參數(shù)可以用來(lái)確定研究原油的成熟度。鄂爾多斯盆地董志-正寧地區(qū)原油C29甾烷20S/(20S+20R)和ββ/(αα+ββ)比值分別為0.49～55和0.53～0.59(表2),Huang等[19]提出成熟原油中這兩個(gè)參數(shù)大于0.4,圖8指示董志-正寧地區(qū)原油均為成熟原油。圖9為重排甾烷/規(guī)則甾烷與C31霍烷22S/(22S+22R)相關(guān)圖,圖中原油樣品分布在較小的范圍內(nèi),并且分布特征反映了原油均為成熟原油。另外,C29重排甾烷20S/(20S+20R)和C29三環(huán)萜烷/(C29三環(huán)萜烷+C30霍烷)比值都已用來(lái)研究原油的成熟度[6,12,16]。董志-正寧地區(qū)原油中C29重排甾烷20S/(20S+20R)比值為0.45～ 0.62,該比值大都達(dá)到了平衡值;董志-正寧地區(qū)原油中C29三環(huán)萜烷/(C29三環(huán)萜烷+C30霍烷)比值為0.020～0.062,類似于塔里木盆地陸相成熟原油[16];正構(gòu)烷烴不存在奇偶優(yōu)勢(shì),OEP為1.01～1.11,數(shù)值接近于1,這些參數(shù)均反映了成熟原油的特征。

圖8 原油中C29甾烷20S/(20S+20R)與ββ/(αα+ββ)比值相關(guān)圖Fig.8 Cross plot of C29 sterane 20S/(20S+20R)vs.ββ/(αα+ββ)ratios of the crude oil samples

2.4 油源對(duì)比

為了進(jìn)行油源對(duì)比,我們對(duì)鄂爾多斯盆地延長(zhǎng)組長(zhǎng)7和延安組延6潛在烴源巖進(jìn)行了生物標(biāo)志化合物分析,結(jié)果列于表1～3和圖2～8。所研究的延長(zhǎng)組長(zhǎng)7和延安組延6烴源巖均為黑色泥巖。由表1和圖2可知,延安組潛在烴源巖的正構(gòu)烷烴和類異戊二烯烷烴組成與董志-正寧地區(qū)原油的明顯不同,前者具有較高的Pr/Ph比值;可是延長(zhǎng)組的長(zhǎng)7烴源巖該比值與董志-正寧地區(qū)原油的相似。規(guī)則甾烷含量和甾烷成熟度參數(shù)(C2920S/(20S+20R)和ββ/ αα+ββ)是重要的油源對(duì)比指標(biāo)。延安組潛在烴源巖規(guī)則甾烷是C29甾烷含量很高(表2和圖4),甾烷C2920S/(20S+20R)和ββ/(αα+ββ)值低,這些值與董志-正寧地區(qū)原油沒(méi)有可對(duì)比性,可是延長(zhǎng)組烴源巖的該比值與董志-正寧地區(qū)原油的相似(表2和圖7)。延安組烴源巖中萜烷的C24四環(huán)萜烷/(C24四環(huán)萜烷 +C26三環(huán)萜烷)、C25/C26三環(huán)萜烷和C19+20/C23三環(huán)萜烷比值較高,三環(huán)萜烷/藿烷比值較高;但是延長(zhǎng)組烴源巖中這些比值與延安組烴源巖的相反,與董志-正寧地區(qū)原油的相似(表3和圖5)。從上述資料可以看出,董志-正寧地區(qū)原油主要與長(zhǎng)7烴源巖具有親緣關(guān)系,說(shuō)明董志-正寧地區(qū)原油主要來(lái)自長(zhǎng)7烴源巖。以前的研究已經(jīng)證明了延安組烴源巖形成于湖沼環(huán)境,有機(jī)質(zhì)為腐植型,烴轉(zhuǎn)化率低,有機(jī)質(zhì)成熟度較低[20](Ro=0.53%～0.65%),因此形成工業(yè)性油藏的可能性小。延長(zhǎng)組烴源巖形成于淺湖-半深湖,有機(jī)碳含量較高,主要分布在0.6% ～6.0%之間;有機(jī)質(zhì)類型為混合型;有機(jī)質(zhì)已成熟, Ro值主要分布在0.70%～0.96%之間(未發(fā)表資料),從而延長(zhǎng)組應(yīng)是董志-正寧地區(qū)原油可能的烴源巖。其中下部長(zhǎng)7生油層組,為湖盆發(fā)育全盛期產(chǎn)物,生油性能明顯好于其他層段,是董志-正寧地區(qū)原油的主力烴源巖。

圖9 原油中重排甾烷/規(guī)則甾烷比值與C31 22S/(22S+22R)相關(guān)圖Fig.9 Cross plot of diasterane/regular steranes vs.C31 hopane 22S/(22S+22R)ratio of the crude oil samples

3 結(jié)論

鄂爾多斯盆地董志-正寧地區(qū)是鄂爾多斯盆地中生界油氣勘探新區(qū),以往對(duì)該地區(qū)原油及其烴源巖地球化學(xué)研究尚未見(jiàn)報(bào)道過(guò),對(duì)該地區(qū)原油的成因知之甚少。本文對(duì)董志-正寧地區(qū)原油中生物標(biāo)志化合物研究進(jìn)行了較為系統(tǒng)地研究。該地區(qū)原油的甾烷以C29含量較高,含有較多的四環(huán)萜烷,存在具有松香烷結(jié)構(gòu)的三環(huán)萜烷,這些說(shuō)明原油的母質(zhì)除了菌藻類外,陸源高等植物作出了重要貢獻(xiàn)。原油中類異戊二烯烷烴組成,反映了原油形成于弱還原環(huán)境;原油中伽瑪蠟烷和>C30藿烷含量很低,指示了原油形成于淡水環(huán)境。原油中C29甾烷和C31藿烷異構(gòu)體參數(shù)接近或達(dá)到平衡值,正構(gòu)烷烴CPI值接近于1,說(shuō)明原油為成熟原油。油源對(duì)比表明,董志-正寧地區(qū)原油主要來(lái)自長(zhǎng)7烴源巖。

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Geochem ical Characteristics and Genesis of Crude Oil from Dongzhi-Zhengning Area of Ordos Basin

DUAN Yi1MA Chun-lin2YU Yong-jin2LIU Jun-feng2LIU Yi-cang3HE Jin-xian1WU Bao-xiang1XU Li1ZHANG Xiao-li1

(1.Key Laboratory of Gas Geochem istry;Institute of Geology and Geophysics;CAS,Lanzhou 730000; 2.The Second Oil Production Plant of Changqing Oilfield Company,Qingyang,Gansu 745000; 3.The fourth Project of Low Permeability of Changqing Oilfield Com pany,Qingyang,Gansu 745000)

Ordos basin is a large inner continental sedimentary basin in China,with an area of about37 X104km2.t contains abundant Mesozoic crude oil resources with an reserve estimation of up to 10 X 108metric tons of oil from Mesozoic reservoirs.These crude oils are derived nonmarine source rocks,so that it has also been used as a typical model of a nonmarine sedimentary basin generating oil.It contains six structural units.Stratigraphic sequences involving Mesozoic petroleum geology principally are Upper Triassic Yanchang Formation and Lower Jurassic Yanan Formation,which are both hydrocarbon source rocks and reservoirs of Mesozoic oils and gases in Ordos basin.Dongzhi-Zhengning area is a new Mesozoic petroleum exploration target in the Ordos basin.Understanding the genesis of oil from this area is very important for revealing the formation law of oil reservoir and guiding petroleum exploration and development in this area.However,the research on geochemistry of crude oils from Dongzhi-Zhengning area has been not performed and genesis of the oil is still known little.In this paper,biomarker compounds in oils from the region and Mesozoic source rock from Ordos basin were systematically analyzed.The genesis of the oilwas discussed by studying the geochemical characteristics of these biomarker compounds.Similar geochemical features of oils from Dongzhi-Zhengning area suggestgeneration from a source withmixed terrgenous and algal-bacterial organicmatter.The biomarker distributions of the oils show that they were formed under aweak reducing and freshwater environment.Molecular maturity parameters indicate that the crude oils aremature.The results of oil-source rock correlation showed that the oilwasmainly derived from Chang-7 source rock.

Ordos Basin;crude oil from Dongzhi-Zhengning area;biomarker compound;compositional and distributional characteristics;genesis

段毅 男 1956年出生 博士 研究員 有機(jī)地球化學(xué) E-mail:duany@lzb.ac.cn

P593

A

1000-0550(2011)05-1002-08

①中國(guó)科學(xué)院支甘項(xiàng)目、國(guó)家自然科學(xué)基金項(xiàng)目(批準(zhǔn)號(hào):40872092,40772069)和“973”項(xiàng)目(編號(hào):2005CB422105)資助。

2010-11-05;收修改稿日期:2011-01-15