樊 馥 張永生 高福紅 蔡進功 崔海娜 于 鵬

(1.中國地質(zhì)科學(xué)院礦產(chǎn)資源研究所 北京 100037;2.同濟大學(xué)海洋地質(zhì)國家重點實驗室 上海 200092; 3.吉林大學(xué)地球科學(xué)學(xué)院 長春 130061;4.吉林油田勘探開發(fā)研究院 吉林松原 138000;5.北京科技大學(xué)土木環(huán)境工程學(xué)院 北京 005201)

雞西盆地城子河組泥巖有機質(zhì)保存條件研究①

樊 馥1,2張永生1高福紅3蔡進功2崔海娜4于 鵬5

(1.中國地質(zhì)科學(xué)院礦產(chǎn)資源研究所 北京 100037;2.同濟大學(xué)海洋地質(zhì)國家重點實驗室 上海 200092; 3.吉林大學(xué)地球科學(xué)學(xué)院 長春 130061;4.吉林油田勘探開發(fā)研究院 吉林松原 138000;5.北京科技大學(xué)土木環(huán)境工程學(xué)院 北京 005201)

雞西盆地作為我國東北地區(qū)重要的煤炭基地,已在多口鉆井中發(fā)現(xiàn)油氣顯示。早白堊世城子河組為一套夾數(shù)個海相泥巖層的含煤巖系,形成于濱淺湖-沼澤的沉積環(huán)境,為該盆地主力烴源巖層。本次對該層位泥巖樣品的常、微量元素全分析結(jié)果表明,水體鹽度指標Sr/Ba在0.11～0.38之間,堿度指標(Ca+Mg)*20/(Si+Al)為0.22～ 1.33,氧化還原性指標V/Cr在1.17～3.23之間;生物標志物分析中反映沉積水體環(huán)境特征的指標Pr/Ph在2.4～2.8之間,平均值為2.2;伽馬蠟烷/C30-藿烷(G/C30H)平均值為0.11,均未顯示出遭受海侵影響的典型咸化水體及強還原性沉積環(huán)境特點,反映對有機質(zhì)的保存不利。結(jié)合城子河組煤巖與泥巖相比具有高的氫指數(shù)和產(chǎn)烴潛量特點,認為該區(qū)大量的油氣顯示應(yīng)該與廣泛發(fā)育的煤巖關(guān)系更為密切。

雞西盆地 泥巖 城子河組 有機質(zhì)保存

0 引言

雞西盆地為中-新生代殘余盆地,是我國東北地區(qū)重要的煤炭基地。近年來,煤田鉆孔87-20發(fā)生天然氣井噴,雞D2井鉆遇六層含油粉砂巖,在其它的多口鉆井中也見到了油氣顯示[1],說明其具有油氣勘探前景。該區(qū)穆棱組剝蝕比較嚴重[2],城子河組作為該區(qū)早白堊世主力烴源巖層,形成于濱淺湖-沼澤的沉積環(huán)境,由粉砂巖、泥巖、煤層、沉凝灰?guī)r組成。前人在該層位部分層段泥巖層中發(fā)現(xiàn)了半咸水的雙殼類化石及海相溝鞭藻,確定它為一套夾數(shù)個海相層的含煤巖系[3]。國內(nèi)外許多陸相含油氣盆地中重要生烴層位的形成與海侵有密切的關(guān)系。例如中歐盆地三疊系,它是一套典型的海陸過渡相沉積;松遼盆地青山口一段、嫩江組一、二段;蘇北-南黃海盆地的泰州組二段、阜寧組;另外,渤海灣盆地純化鎮(zhèn)組、沙河街組和東營組存在水體咸化特征,是否遭受過海侵問題一直以來也被廣泛探討[4～6]。研究認為大范圍的海侵提供豐富的成烴母質(zhì),并且形成較深的水體有利于缺氧環(huán)境的形成,這對有機質(zhì)的形成和保存是十分有益的[7]。本區(qū)城子河組的海侵是否改善了泥質(zhì)烴源巖的沉積保存環(huán)境,這是需要進一步探討的問題。目前,對于城子河組暗色泥巖的討論,除以地層對比為目的的一些溝鞭藻研究外,缺乏反映沉積環(huán)境的地球化學(xué)證據(jù)。本文主要通過對樣品常、微量元素全分析,對城子河組泥巖沉積時水體的鹽度,堿度及保存的氧化還原環(huán)境進行討論,希望為該區(qū)的油氣勘探起一定的指導(dǎo)作用。

1 采樣及樣品測試

1.1 樣品采集

圖1 雞西盆地城子河組泥巖分布及采樣位置圖Fig.1 The distribution and the sampling location ofmud stone in Chengzihe Formation

雞西盆地可劃分為雞東坳陷、恒山隆起、梨樹鎮(zhèn)凹陷和平陽斷陷四個構(gòu)造單元。城子河組泥巖在該區(qū)分布廣泛,在雞東坳陷和梨樹鎮(zhèn)凹陷內(nèi)厚度最大,最大泥巖厚度超過400 m(圖1)。地層巖性發(fā)育情況見圖2。由于受城子河組地表出露情況的限制,本次采樣點位于盆地西緣(圖1),主要分布于雞東坳陷,恒山隆起周緣及梨樹鎮(zhèn)坳陷邊部。樣品采自于城子河組砂泥巖互層的野外剖面,巖石類型為表面新鮮的灰黑色泥巖和頁巖,出露的最大單層厚度為50 cm,部分夾菱鐵礦結(jié)核及煤線。暗色泥巖最大厚度為3 m。共采集13塊樣品用于各項分析測試。

圖2 雞西盆地城子河組地層柱狀圖Fig.2 The stratigraphic column of Chengzihe Formation in Jixi basin

1.2 分析測試

常量元素分析:用去離子水對暗色泥巖樣品進行表面清洗,50℃烘干后用瓷質(zhì)研缽研磨至200目,其后采用標準壓片法,制成X射線熒光光譜分析樣品片,進行熒光光譜分析。

微量元素分析:將樣品在100℃烘箱中烘干,在瑪瑙研缽中磨至200目,稱取30～45 mg樣品,置入Teflon溶樣器中,加入1m l1∶1 HNO3,5 min后加3 ml純HF,然后放到加熱板上保溫7 d,在此期間每天超聲1次,每次約40 min,將樣品蒸干,然后加入1∶1 HNO34 m l,再超聲波振蕩30min;用2%HNO3將樣品稀釋至樣品重量的1 000倍,在稀釋1 000倍后的溶液中取出4 g左右,稀釋10倍,作為微量元素待測溶液進行ICP-MS測試。

相關(guān)常、微量數(shù)據(jù)見表1。

2 分析結(jié)果

2.1 常、微量元素

選取常、微量元素中Sr/Ba、(Ca+Mg)*20/(Si +Al)及V/Cr,Ni/Co等參數(shù)分別作為本次研究中反映水體鹽度、堿度及還原性的指標。各樣品相關(guān)參數(shù)的具體數(shù)值見表1。

城子河組泥巖樣品Sr/Ba的變化范圍在0.11～ 0.38之間,平均值為0.26。研究表明:Sr和Ba兩者在海水中的富集程度有著巨大的差別,卡欽科夫曾提出Sr/Ba>1為海相,Sr/Ba<1為陸相。王璞王君認為,Sr/Ba作為鹽度指標是直接和有效的,而作為劃相標志是間接的,且其相對值較其絕對值更有意義[8]。本區(qū)城子河組泥巖Sr/Ba特點反映沉積水體鹽度較低。

測試樣品的(Ca+Mg)*20/(Si+Al)范圍在1.26～2.00之間,平均值為1.68。因為Ca和Mg在海水中明顯富集,而Si和Al卻明顯貧化。并且現(xiàn)代沉積物的研究結(jié)果表明,泥質(zhì)巖石中的元素含量與水介質(zhì)中該元素的濃度呈顯著的正相關(guān)[8]。因此泥巖(Ca+Mg)*20/(Si+Al)的數(shù)值即反映了沉積水體的堿度情況。城子河組泥巖(Ca+Mg)*20/(Si+Al)數(shù)值特點反映該區(qū)泥巖沉積時水體的堿度偏低。

表1 樣品常、微量元素分析數(shù)據(jù)Table1 Themajor element and trace element data of samp les

表2 泥巖樣品地球化學(xué)參數(shù)特征Table2 The character of geochem ical indexes from mudstone samp les

城子河組泥巖V/Cr在1.17～3.23之間,平均值為1.73.i/Co的范圍為0.89～3.71,平均值為2.27.ones等(1994)通過對西北歐晚侏羅世沉積古氧相地球化學(xué)的研究,認為V/Cr,Ni/Co是比較可靠的指標,并首次提出相關(guān)元素比值與氧化-還原條件的對應(yīng)關(guān)系(表2)[9]。與Jones提出的標準對比,城子河組泥巖V/Cr及Ni/Co數(shù)值反映了富氧的水體環(huán)境特征。

表3 古氧相地球化學(xué)指標對比Table3 The geochem ical index of the paleo-oxidation phase

3 討論

從相關(guān)元素比值所反映的泥巖沉積保存時的水體鹽度和堿度特點來看,松遼盆地泉頭至嫩江組典型海侵層序的共生泥巖序列中Sr/Ba在0.28～1.73之間,平均值為0.69[8]。本區(qū)城子河組泥巖樣品Sr/Ba的變化范圍在0.11～0.38之間,平均值為0.26,與嫩江組相比鹽度明顯偏低。在碳酸鹽存在的情況下,該比值受碳酸鹽含量的控制,隨碳酸鹽含量升高而增加[10]。嫩江組發(fā)育大套碳酸鹽結(jié)核,而本區(qū)剖面只發(fā)現(xiàn)少數(shù)泥巖層夾菱鐵礦結(jié)核。二者Sr/Ba比值也可能與碳酸鹽含量差異有關(guān)。嫩江組(Ca+Mg)*20/(Si+Al)在0.29～9.42之間,平均值為1.70[8]。本次雞西盆地研究層位(Ca+Mg)*20/(Si+Al)的范圍在1.26～2.00之間,平均值1.68,也反映堿度明顯低于嫩江組的特點。上述對比表明,與松遼盆地湖侵層序相比,城子河組的鹽度和堿度偏低,可能是由于松遼盆地早白堊世三次海侵的規(guī)模較大,持續(xù)時間長,致使該套地層整體的鹽度、堿度背景值都明顯偏高。而雞西盆地早白堊世城子河組盡管也存在海侵,但由于規(guī)模較小,并且只是以海相夾層出現(xiàn),因此對該區(qū)泥巖整體鹽度和堿度的提高影響不大。從泥巖沉積時的氧化還原環(huán)境來看,在松遼盆地白堊系層序的共生泥巖中,發(fā)育指示沉積水體還原環(huán)境的閃鋅礦和鎳黃鐵礦[8]。而本區(qū)泥巖中僅發(fā)育少量菱鐵礦。相關(guān)研究表明,這種礦物可以形成于輕度還原環(huán)境(亞氧化帶)到強還原環(huán)境(甲烷形成帶)[11],所代表指示環(huán)境范圍十分寬泛,綜合城子河組V/Cr(數(shù)值范圍1.17～3.23)之間和Ni/Co(數(shù)值范圍0.89～ 3.71)較低的數(shù)值特點(范圍為0.89～3.71),反映泥巖沉積保存于偏氧化的環(huán)境。綜上所述,城子河組泥巖總體表現(xiàn)為低鹽度、低堿度且偏氧化的沉積保存環(huán)境特點。

高紅梅(2007)對同一批城子河組泥巖樣品進行與沉積環(huán)境相關(guān)的生物標志化合物分析,結(jié)果表明, Pr/Ph在2.4～2.8之間,平均值為2.2,反映了偏氧化的沉積保存環(huán)境特征;伽瑪蠟烷/C30-藿烷(G/C30H)平均值為0.11,反映了水體化學(xué)分層不十分明顯;孕甾烷+升孕甾烷/C27甾烷的均值為0.88,C27重排甾烷/C27甾烷均值為0.34,反映水體鹽度不高[12]。由此可以看出,泥巖中反映沉積環(huán)境特征的有機地化參數(shù)與無機地球化學(xué)指標Sr/Ba、(Ca+Mg)*20/(Si +Al)及V/Cr及Ni/Co比值反映的環(huán)境特征基本一致,并未顯示出城子河組遭受海侵后高鹽度、強還原的沉積水體特點。同時,高福紅(2007)對該盆地發(fā)現(xiàn)城子河組海相泥巖的魚亮子溝剖面取樣研究,也未找到典型海侵層位應(yīng)該具有的極高鹽度和反映強還原特征的地球化學(xué)證據(jù)[13]。盡管一些沉積相的研究證據(jù)提出,城子河組沉積期,盆地的水域面積達到最大,可能與北部的勃利盆地和東北部的虎林盆地為統(tǒng)一盆地,并且遭受了東部的海侵[14],但從雞西盆地城子河組大量煤巖發(fā)育的巖性特征以及本次泥巖樣品及前人研究得出的地球化學(xué)證據(jù)來看,海侵對于該盆地泥巖沉積保存環(huán)境的改善并不明顯,城子河組泥巖并未處于一個有利于有機質(zhì)保存的沉積環(huán)境。

先前對這批泥巖樣品生烴潛力研究反映,暗色泥巖TOC在0.11%～3.87%之間;同時,對煤巖和碳質(zhì)泥巖分析反映TOC為18.85%～80.27%,不同巖石類型有機質(zhì)豐度差別明顯,這主要與暗色泥巖通常為分散型的有機質(zhì)保存形式,而煤巖為富集有機質(zhì)保存形式有關(guān)。但同時,暗色泥巖IH為0.02～144.00 mg/g,S1+S2為0.04～4.16 mg/g;而煤和碳質(zhì)泥巖IH為53.8～207.00 mg/g,S1+S2為16.93～179.99 mg/g,不論是氫指數(shù)IH還是產(chǎn)烴潛量S1+S2均反映了煤和碳質(zhì)泥巖生烴能力要好于暗色泥巖樣品。并且熱解相關(guān)參數(shù)反映煤和碳質(zhì)泥巖的有機質(zhì)類型(Ⅱ2型)也好于暗色泥巖(Ⅲ型)[15]。同時,發(fā)生氣噴的87-20井氣樣進行CH4、N2、O2、He、CO2、δ13C分析,結(jié)果顯示氣體成分主要是甲烷,二氧化碳含量低,屬煤型氣[16]。城子河組暗色泥巖較差的有機質(zhì)保存環(huán)境特征、與煤巖對比顯示較差生烴能力特點,氣態(tài)烴類煤型氣的特征,說明盆地內(nèi)油氣顯示可能與煤巖關(guān)系更為密切。

圖3 城子河組泥巖和煤的有機質(zhì)類型[15]Fig.3 The organicmatter type of themuddy rock and the coal in Chengzihe Formation

4 結(jié)論

對雞西盆地早白堊世城子河組泥巖常、微量元素分析表明:泥巖沉積時水體的鹽度,堿度偏低,有機質(zhì)保存環(huán)境的還原性差,與有機地球化學(xué)指標的相關(guān)分析結(jié)果反映的環(huán)境特征基本一致,說明海侵對于該盆地泥巖沉積保存環(huán)境的改善并不明顯。同時,泥巖與煤巖相比,在有機質(zhì)類型及生烴能力方面均未呈現(xiàn)優(yōu)勢特征,加之,部分鉆井氣態(tài)烴類煤型氣特點,說明該區(qū)油氣顯示可能與煤巖關(guān)系更為密切。

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Organic M atter Preservation of M udstone from Chengzihe Formation in Jixi Basin

FAN Fu1,2ZHANG Yong-sheng1GAO Fu-hong3CAIJin-gong2CUIHai-na4YU peng5

(1.Institute of M ineral Resources,Chinese Academ y of Geological Sciences,Beijing 100037; 2.The State Key Laboratory of Marine Geology in Tongji University,Shanghai 200092; 3.School of Earth Science,Jilin University,Changchun 130061; 4.Institute of Oil Exp loration and Development in Jilin Field,Songyuan,Jilin 138000; 5.Beijing University of Science and Technology Beijing,Civil and Environment Engineering School,Beijing 005201)

Jixi basin is the Mesozoic-Cenozoic residue basin and the important coal base in the northeast China.At present,several drilling wells in the coal fields discovered the oil and gas shows in this area.Chengzihe Formation is the coal-bearing formation in the lower Cretaceous of the Jixi basin,which deposited in the circumstance of coastalshallow lake.The previous studies found themussels fossil of brackish water and a lotof dinoflagellates ofmarine facies,which reflected themarine interlayer existed in the Chengzihe Formation.It infers that the transgression occurred in this period.At home and abroad,many studies indicate that themain source rock layers in themajority of oil and gas bearing basin are related closely with the transgression.We choose somemud rocks from the Chengzihe Formation and perform analysis on themajor elementand trace element,combiningwith the previous organic geochemistry data, to reveal the character ofwater circumstancewhen themud rock of Chengzihe Formation deposits,and discuss how the transgression influences on the water circumstance and organic matter preservation.The analysis result indicates that the values of Sr/Ba which is salinity index ranging from 0.11 to 0.38,0.26 average;the(Ca+Mg)*20/(Si+Al),as the alkalinity index,is from 0.22 to 1.33,1.68 average;as the oxidation-reduction indexes,V/Cr is 1.17 ～3.23,and Ni/Co is 0.89～3.71.ompared with the typical transgression sequence from Quantou Formation to Nenjiang Formation in Songliao basin,the values are low obviously in the Chengzihe Formation,which reflects the sediment and preservation circumstances of low salinity,low alkalinity and weak oxidation.In recent years,the analysis results on the biomarkers from the Chengzihe Formation indicate that as the oxidation-reduction index,the value of Pr/Ph is2.4～2.8,2.2 average;the G/C30H which indicates the salinity stratification ofwater column is0.11 average;the pregnane+homopregnane/C27sterane is0.88 average,C27diasterane/C27sterane is0.34 average,reflecting the salinity of water is low,which is coincide with the analytical result of the major element and trace element.All kinds of indexes do not show the character of the typical transgression sequence,such as the high salinity water and the strong reducive sediment circumstance.It indicates that the sediment environment is not fit for the organic matter preservation.It is coincidentwith the Gaofuhong(2007)'study on the typical transgression sequence of Yuliangzigou section in the Chengzihe Formation.

In addition,the previous study on the potential of generating hydrocarbon reveals the TOC value of the blackmud rock is between 0.11%and 3.87,which is lower than the value of the coal and the carbonaceousmud rock,the TOC ofwhich ranges from 18.58%to 80.27%.esides,the hydrogen index(HI)of the black mud stone is 0.02～ 144.00 mg/g,and the generating hydrocarbon potential(S1+S2)is between 0.04～4.16 mg/g,all ofwhich is lower than the value of the coal and the carbonaceousmud rock that the IH is53.8 mg/g～207 mg/g,and the S1+S2is 16.93～179.99 mg/g.The organicmatter type(ⅡB)of the coal and carbonaceousmuddy rock is better than the organicmatter type(Ⅲ)of the black mud rock.It indicates that the coal and carbonaceousmud rock have the advantageous potential of generating hydrocarbon。

Some evidence of sedimenary facies study indicate that the water area reached themaximum during the whole periods in this basin,and it is probably that the Jixi basin,Boli basin in the north and the Hulin basin in the northeast form the uniform basin,suffering the transgression from the east.However,a lot of coal in Chengzihe Formation of Jixi basin and the geochemical evidence from the previous and present studies indicate that the transgression do not improve sediment environment evidently.Contrasting with Songliao basin,the scale of transgression is smaller in the Jixi basin,which do not improve the salinity and alkalinity evidently,so the water environment of the Chengzihe Formation is not good for the organicmatter preservation.Due to the advantageous generating hydrocarbon potential of the coal and carbonaceousmud rock,it infers the oil and gas show in this area is probably related to the coal and carbonatemud rock。

Jixi Basin;mud rock;Chengzihe Formation;organic matter preservation

樊馥 女 1982年出生 博士后 油氣地球化學(xué) E-mail:fanfu2005612033@sina.com

P593

A

1000-0550(2011)05-0980-06

①國家自然科學(xué)基金項目(批準號:40872089和41072089)和國家油氣重大專項(2008ZX05006-003)、中國石油化工股份有限公司科技局基金(P08039)資助。

2010-08-18;收修改稿日期:2010-12-16