王 軍 周心懷 楊 波 王清斌 郭永華 王飛龍
(1.中海石油(中國(guó))有限公司天津分公司 天津 300459; 2.中海石油(中國(guó))有限公司上海分公司 上海 200335)
渤海蓬萊9-1油田強(qiáng)烈生物降解原油油源對(duì)比*
王 軍1周心懷2楊 波1王清斌1郭永華1王飛龍1
(1.中海石油(中國(guó))有限公司天津分公司 天津 300459; 2.中海石油(中國(guó))有限公司上海分公司 上海 200335)
渤海海域近年來(lái)在中生界潛山、新近系等層系發(fā)現(xiàn)了大量的稠油油藏,但原油因遭受強(qiáng)烈的生物降解,導(dǎo)致一些常用的油源對(duì)比指標(biāo)失效而使油源對(duì)比困難。以蓬萊9-1大型稠油油田為例,研究了強(qiáng)烈生物降解對(duì)原油碳同位素、飽和烴、芳烴生物標(biāo)志化合物及含氮化合物的影響,引入降解指數(shù)C2925-降藿烷/C30藿烷定量表征強(qiáng)烈生物降解原油的降解程度,結(jié)果表明:強(qiáng)烈生物降解會(huì)造成伽馬蠟烷和咔唑含量的降低;C19三環(huán)萜烷、C24四環(huán)萜烷和Ts的優(yōu)先降解會(huì)導(dǎo)致C19/C23三環(huán)萜烷、C24四環(huán)萜烷/C26三環(huán)萜烷參數(shù)明顯減小和ETR參數(shù)明顯增大;4-甲基三芳甾類為強(qiáng)烈生物降解原油中最穩(wěn)定的生物標(biāo)志物,C274-甲基三芳甾類、C294-甲基-24-乙基三芳甾類和C294,23,24-三甲基三芳甲藻甾類對(duì)烴源巖的生源構(gòu)成及沉積環(huán)境具有指示意義,結(jié)合全油碳同位素可有效區(qū)分沙三段、沙一段和東營(yíng)組等3套烴源巖及其生成的原油;咔唑及原油物性變化規(guī)律反映了油氣的運(yùn)移方向。蓬萊9-1油田潛山原油來(lái)源于渤東凹陷沙三段和沙一段烴源巖,新近系原油主要來(lái)源于沙一段烴源巖,為渤東凹陷和廟西凹陷的共同貢獻(xiàn)。本文研究結(jié)果對(duì)研究區(qū)下步勘探具有指導(dǎo)意義,為其他類似地區(qū)強(qiáng)烈生物降解原油的油源對(duì)比提供了方法借鑒。
生物降解;油源對(duì)比;稠油油藏;蓬萊9-1油田;渤海
渤海海域以往的油源對(duì)比研究主要針對(duì)中等降解程度及以下的原油,主要利用飽和烴類的伽馬蠟烷和4-甲基甾烷豐度參數(shù)[1-4],而針對(duì)強(qiáng)烈生物降解原油的油源對(duì)比方法研究較少。渤海海域埋深小于2 000 m的新近系發(fā)現(xiàn)了大量的稠油儲(chǔ)量[5],特別是近年來(lái)發(fā)現(xiàn)的蓬萊9-1大型稠油油田潛山和新近系原油,遭受了強(qiáng)烈的生物降解,導(dǎo)致常規(guī)油源對(duì)比參數(shù)失效,無(wú)法明確原油來(lái)源以及是否存在多洼供烴,這影響到該油田周邊洼陷的資源潛力評(píng)價(jià)及勘探目的層系選擇。
國(guó)內(nèi)外關(guān)于強(qiáng)烈生物降解原油的油源對(duì)比方法可以分為直接方法和間接方法兩類,其中直接方法是利用原油中抗生物降解能力強(qiáng)的生物標(biāo)志物來(lái)進(jìn)行油源對(duì)比,包括三環(huán)萜烷、四環(huán)萜烷和伽馬蠟烷等[6-9];間接方法是利用還原原油(油砂)瀝青質(zhì)組分中鍵合的生物標(biāo)志物、恢復(fù)飽和烴組分中被降解的生物標(biāo)志物以及提取儲(chǔ)層包裹體中的生物標(biāo)志物來(lái)進(jìn)行油源對(duì)比,包括釕離子瀝青質(zhì)催化-氧化、生物標(biāo)志物定量疊加參數(shù)恢復(fù)、單體或群體包裹體烴色譜-質(zhì)譜等[10-16]。本文在對(duì)蓬萊9-1油田及圍區(qū)未遭受或僅遭受較低程度生物降解的原油及油砂樣品進(jìn)行油源對(duì)比的基礎(chǔ)上,對(duì)大量原油及油砂樣品進(jìn)行了飽和烴和芳烴氣相色譜-質(zhì)譜、碳同位素、咔唑及原油物性分析;引入降解指數(shù)定量表征強(qiáng)烈生物降解原油的降解程度,研究了該油田強(qiáng)烈生物降解對(duì)飽和烴和芳烴生物標(biāo)志物以及咔唑的影響;主要應(yīng)用4-甲基三芳甾類參數(shù)進(jìn)行油源對(duì)比,并結(jié)合全油碳同位素和原油物性等規(guī)律,明確了該油田油氣來(lái)源的層位與洼陷以及潛山和新近系油源的差異。本文研究結(jié)果拓展了生物降解原油油源對(duì)比方法,對(duì)研究區(qū)下一步勘探具有一定的指導(dǎo)意義。
蓬萊9-1油田位于渤中坳陷東部廟西北凸起上,夾持于渤東凹陷和廟西凹陷之間,距蓬萊19-3油田約38 km(圖1),為渤海近年最大的油氣發(fā)現(xiàn)之一,探明石油地質(zhì)儲(chǔ)量超過(guò)2.2×108t[17-20]。廟西北凸起潛山南北兩個(gè)高點(diǎn)為以石英片巖和云母片巖為主的元古界變質(zhì)巖,巖性致密且抗風(fēng)化能力強(qiáng),中間的鞍部為以二長(zhǎng)花崗巖和花崗閃長(zhǎng)巖為主的中侏羅統(tǒng)酸性侵入巖,潛山之上披覆新近系館陶組和明化鎮(zhèn)組淺水三角洲-曲流河碎屑巖沉積。蓬萊9-1油田位于廟西北凸起的鞍部,發(fā)育中生界潛山和新近系淺水三角洲-河流相砂體兩套含油層系,其中中生界潛山為主要的含油層系。蓬萊9-1油田兩套含油層系埋深840~1 600 m,由西向東埋深逐漸減小,原油遭受強(qiáng)烈生物降解。
圖1 研究區(qū)油田和構(gòu)造位置Fig.1 Oilfields and structures of the study area
渤東凹陷雖然沒(méi)有鉆遇優(yōu)質(zhì)烴源巖,但發(fā)現(xiàn)了來(lái)自沙三段、沙一段和東營(yíng)組烴源巖生成的原油,表明渤東凹陷存在沙三段、沙一段和東營(yíng)組等3套有效烴源巖。廟西凹陷鉆遇了東營(yíng)組優(yōu)質(zhì)烴源巖并發(fā)現(xiàn)了來(lái)自東營(yíng)組和沙一段烴源巖生成的原油,但可能缺失沙三段烴源巖。研究表明,渤海海域沙三段烴源巖沉積于淡水—微咸水?dāng)嘞莺璀h(huán)境,以渤海藻、副渤海藻為代表的溝鞭藻類化石豐富,伽馬蠟烷含量低,C304-甲基甾烷含量高,并含有甲藻甾烷;沙一段烴源巖沉積于半咸水—咸水萎縮湖盆環(huán)境,溝鞭藻類以耐鹽的多刺甲藻和菱球藻屬為特色,伽馬蠟烷含量高,C304-甲基甾烷含量低,并以甲藻甾烷為主;東營(yíng)組烴源巖沉積于湖水再次加深、淡化的環(huán)境,伽馬蠟烷和C304-甲基甾烷含量低或無(wú),不含甲藻甾烷;因較高的陸源有機(jī)質(zhì)輸入,東營(yíng)組烴源巖比沙三段和沙一段具有更高含量的C19三環(huán)萜烷及C24四環(huán)萜烷[1-3,21-25]。
樣品取自蓬萊9-1及圍區(qū)油田和含油氣構(gòu)造(圖1),其中源巖樣品為富含有機(jī)質(zhì)的泥巖巖屑;原油樣品為中途測(cè)試和電纜取樣獲得的原油,油砂樣品為含油壁心及巖心。源巖及油砂樣品可溶有機(jī)質(zhì)抽提、原油及抽提物族組分分離、原油物性分析由中國(guó)海油渤海實(shí)驗(yàn)中心完成;全油(原油,油砂和源巖抽提物)碳同位素、飽和烴和芳烴氣相色譜-質(zhì)譜、咔唑分離及氣相色譜-質(zhì)譜分析由中國(guó)石油大學(xué)(北京)重質(zhì)油國(guó)家重點(diǎn)實(shí)驗(yàn)室完成。
原油生物降解實(shí)驗(yàn)表明,微生物優(yōu)先利用輕碳同位素12C,致使殘留的飽和烴組分富集重碳同位素13C;微生物降解產(chǎn)物的加入,使非烴及瀝青質(zhì)富集12C,芳烴δ13C值幾乎沒(méi)有變化,而全油δ13C值變化較小[26-27];生物降解過(guò)程中可能因有機(jī)酸和二氧化碳等微生物降解產(chǎn)物的轉(zhuǎn)移而稍有利于富集重同位素13C[28-29]??紤]到芳烴組分可能受到族組分分離等實(shí)驗(yàn)環(huán)節(jié)的影響,而全油碳同位素受生物降解的影響較小,因此選用全油碳同位素來(lái)進(jìn)行油源對(duì)比。
實(shí)際工作中觀察到,同一含油層系的原油和油砂抽提物的全油碳同位素沒(méi)有表現(xiàn)出明顯的差別,因此將兩者合并統(tǒng)計(jì)為油樣全油碳同位素。統(tǒng)計(jì)結(jié)果表明(表1):蓬萊9-1油田圍區(qū)東營(yíng)組烴源巖及油樣全油碳同位素偏輕(-30.3‰~-27.9‰,平均-28.7‰),沙三段油樣全油碳同位素偏重(-26.9‰~-25.8‰,平均-26.4‰),沙一段烴源巖及油樣全油碳同位素居于兩者之間(-27.5‰~-27.4‰,平均-27.5‰);蓬萊9-1油田潛山油樣全油碳同位素分布范圍較窄(-27.6‰~-26.9‰,平均-27.3‰),整體介于沙一段和沙三段之間;新近系油樣全油碳同位素分布范圍寬(-28.4‰~-26.1‰,平均-27.5‰),平均值與沙一段極為相近,比潛山油樣相對(duì)偏輕,表明新近系油藏油氣來(lái)源具有多樣性。由于全油碳同位素的油源對(duì)比結(jié)果具有多解性,無(wú)法識(shí)別出可能存在的多套烴源巖來(lái)源的混源油,故油源對(duì)比結(jié)論必須應(yīng)用生物標(biāo)志物參數(shù)進(jìn)行進(jìn)一步的檢驗(yàn)。
表1 蓬萊9-1油田及圍區(qū)源巖及油樣的全油碳同位素組成Table 1 Whole oil carbon isotope of source rocks and oil samples in PL9-1 oilfield and its vicinity
渤海海域遭受強(qiáng)烈生物降解的原油幾乎都存在25-降藿烷。25-降藿烷被認(rèn)為是規(guī)則藿烷在C-10位失去一個(gè)甲基形成的,它的出現(xiàn)是原油遭受強(qiáng)烈生物降解的標(biāo)志,對(duì)應(yīng)的原油生物降解級(jí)別為6級(jí)或更高[30]。在生物降解級(jí)別為6級(jí)或更高的強(qiáng)烈生物降解原油中,隨著生物降解程度的增強(qiáng),C3017α(H)-藿烷(C30H)逐漸向C2925-降-17α(H)-藿烷(C2925-n H)轉(zhuǎn)化,因此可以用C2925-n H/C30H(稱之為降解指數(shù))來(lái)表征強(qiáng)烈生物降解原油的生物降解程度,該指數(shù)越大時(shí)反映原油生物降解程度越高。蓬萊9-1油田所有樣品均含有25-降藿烷,正構(gòu)烷烴、藿烷與規(guī)則甾烷系列部分或完全降解,但重排甾烷似乎不受影響(圖2),原油生物降解級(jí)別為6~8級(jí),因此可以應(yīng)用降解指數(shù)定量反映該油田強(qiáng)烈生物降解原油的降解程度,并通過(guò)研究其他生物標(biāo)志物參數(shù)與降解指數(shù)的相關(guān)關(guān)系來(lái)討論其在生物降解過(guò)程中的變化。
圖2 蓬萊9-1油田d井原油地化特征Fig.2 Geochemistry characteristics of oils from Well d in PL9-1 oilfield
伽馬蠟烷(Ga)相比藿烷具有更強(qiáng)的抗生物降解能力,C3017α(H)-藿烷相對(duì)于伽馬蠟烷的優(yōu)先降解會(huì)導(dǎo)致異常高的伽馬蠟烷指數(shù)(Ga/C30H)。為解決這一問(wèn)題,李水福等[8]用C29降藿烷替代C30藿烷來(lái)計(jì)算伽馬蠟烷指數(shù),并應(yīng)用于泌陽(yáng)凹陷生物降解原油的油源對(duì)比。蓬萊9-1油田原油C29降藿烷在遭受強(qiáng)烈生物降解時(shí)也逐漸消失,C29Ts抗生物降解能力比C29降藿烷更強(qiáng),當(dāng)明化鎮(zhèn)組下段原油樣品中C29降藿烷因降解幾近消失時(shí),C29Ts依然有較高的豐度(圖2)。在蓬萊9-1油田圍區(qū)烴源巖以及未遭受或遭受較低生物降解程度的原油中,伽馬蠟烷指數(shù)與Ga/C29Ts比值具有良好的正相關(guān)性(圖3a),表明Ga/C29Ts比值可以有效表征伽馬蠟烷的豐度,將其定義為新伽馬蠟烷指數(shù)。蓬萊9-1油田潛山和新近系油樣伽馬蠟烷指數(shù)隨生物降解程度增強(qiáng)而增大,但新近系油樣新伽馬蠟烷指數(shù)隨生物降解程度增強(qiáng)而減?。▓D3b),表明抗生物降解能力由強(qiáng)到弱的順序?yàn)镃29Ts>伽馬蠟烷>C30藿烷??赡苁鞘苡驮椿煸吹挠绊?,潛山油樣新伽馬蠟烷指數(shù)沒(méi)有呈現(xiàn)隨生物降解程度增強(qiáng)而規(guī)律性減小的趨勢(shì)。蓬萊9-1油田潛山和新近系油樣生物降解程度變化較大,但在相似的降解程度(C2925-n H/C30H比值相近)下,潛山油樣伽馬蠟烷和新伽馬蠟烷指數(shù)相比新近系均較低,兩個(gè)指數(shù)的相關(guān)圖版也呈現(xiàn)出潛山油樣相對(duì)較低的伽馬蠟烷豐度(圖3c),表明潛山原油中來(lái)源于高伽馬蠟烷的沙一段烴源巖的原油相比新近系偏少。依據(jù)潛山油樣全油碳同位素比新近系偏重的現(xiàn)象,認(rèn)為潛山油藏相比新近系可能有更多的原油來(lái)源于沙三段烴源巖。
圖3 蓬萊9-1油田及圍區(qū)伽馬蠟烷相關(guān)參數(shù)Fig.3 Gammacerane related index in PL9-1 oilfield and its vicinity
C19三環(huán)萜烷和C24四環(huán)萜烷豐度高是陸源有機(jī)質(zhì)輸入的反映,渤海海域東營(yíng)組、沙三段和沙一段烴源巖C19三環(huán)萜烷/C23三環(huán)萜烷(C19TT/C23TT)和C24四環(huán)萜烷/C26三環(huán)萜烷(C24Te/C26TT)比值呈逐漸降低的趨勢(shì)[21-24];長(zhǎng)鏈三環(huán)萜烷指數(shù)ETR定義為(C28三環(huán)萜烷+C29三環(huán)萜烷)/(C28三環(huán)萜烷+C29三環(huán)萜烷+Ts),該指標(biāo)與伽馬蠟烷指數(shù)呈正相關(guān),反映了沉積水體的介質(zhì)條件[9,24]。三環(huán)萜烷和四環(huán)萜烷的抗生物降解能力強(qiáng),已經(jīng)在一些生物降解原油的油源對(duì)比中得到了應(yīng)用[7,31-32]。蓬萊9-1油田潛山和新近系油樣三環(huán)萜烷和四環(huán)萜烷參數(shù)已明顯受生物降解的影響,C19TT/C23TT和C24Te/C26TT隨生物降解程度增強(qiáng)而逐漸減小,當(dāng)降解指數(shù)大于10時(shí),這2個(gè)參數(shù)快速減?。▓D4a、b),且ETR隨生物降解程度增強(qiáng)而增大,并與降解指數(shù)的對(duì)數(shù)呈正相關(guān)(圖4c),這表明上述參數(shù)的C19三環(huán)萜烷、C24四環(huán)萜烷和Ts在強(qiáng)烈生物降解過(guò)程中會(huì)優(yōu)先降解,因此三環(huán)與四環(huán)萜烷參數(shù)在蓬萊9-1油田的油源對(duì)比中已經(jīng)不適用。
圖4 蓬萊9-1油田原油地化特征與降解指數(shù)相關(guān)圖Fig.4 Correlation of oil geochemistry characteristics with biodegradation index in PL9-1 oilfield
三芳甾類因高分子量并具有苯環(huán)結(jié)構(gòu)而成為抗生物降解能力極強(qiáng)的生物標(biāo)志物之一[30],4-甲基三芳甾類被認(rèn)為是由來(lái)源于溝鞭藻類的生物標(biāo)志物4-甲基甾烷通過(guò)芳構(gòu)化作用轉(zhuǎn)化而來(lái)[33-34],包含C274-甲基三芳甾類、C294-甲基-24-乙基三芳甾類和C294,23,24-三甲基三芳甲藻甾類。與4-甲基三芳甾類結(jié)構(gòu)及保留時(shí)間相似的是3-甲基三芳甾類,包含C273-甲基三芳甾類、C283-甲基-24-甲基三芳甾類和C293-甲基-24-乙基三芳甾類。4-甲基三芳甾類和3-甲基三芳甾類已經(jīng)在塔里木盆地[35-37]、南方古生界海相烴源巖[38]、柴達(dá)木盆地陸相烴源巖[39]及勝利油田[40]的油源對(duì)比研究中得到應(yīng)用,但未見(jiàn)對(duì)4-甲基三芳甾類的各種類型所反映的地質(zhì)意義進(jìn)行討論。
4-甲基三芳甾類可以有效區(qū)分渤海海域沙三段、沙一段和東營(yíng)組等3套烴源巖及其生成的原油[41]。沙三段烴源巖及其生成的原油中C274-甲基三芳甾類、C294-甲基-24-乙基三芳甾類和C294,23,24-三甲基三芳甲藻甾類豐度高,可能反映了沙三段沉積時(shí)期耐鹽度范圍較寬的渤海藻和副渤海藻等溝鞭藻類的勃發(fā);沙一段烴源巖及其生成的原油中C274-甲基三芳甾類、C294-甲基-24-乙基三芳甾類豐度低,而C294,23,24-三甲基三芳甲藻甾類豐度高,這可能與沙一段地層中耐鹽的多刺甲藻和菱球藻等溝鞭藻類化石含量高有關(guān);東營(yíng)組烴源巖及其生成的原油中4-甲基三芳甾類豐度均較低,相應(yīng)的東營(yíng)組中溝鞭藻類化石含量較低(圖5a)。
圖5 渤海海域不同成因類型原油4-/3-甲基三芳甾類分布特征及鑒定Fig.5 4-/3-methyl triaromatic steroids distribution and identification of different origin types of oils in Bohai sea
蓬萊9-1油田油樣中4-甲基三芳甾類均具有完整性,未受生物降解的影響(圖5b)。d井新近系館陶組和明化鎮(zhèn)組原油具有低豐度的C274-甲基三芳甾類和C294-甲基-24-乙基三芳甾類、高豐度的C294,23,24-三甲基三芳甲藻甾類,與渤海海域典型沙一段生成的原油對(duì)比良好。d井潛山原油C274-甲基三芳甾類和C294-甲基-24-乙基三芳甾類豐度介于沙三段和沙一段之間,表明有部分原油來(lái)自沙三段烴源巖,原油中含有一定豐度的伽馬蠟烷(圖2b),表明也有部分原油來(lái)自沙一段烴源巖;原油中含有較高豐度的4-甲基三芳甾類,表明來(lái)自東營(yíng)組烴源巖的貢獻(xiàn)較少。
為了用4-甲基三芳甾類參數(shù)更精細(xì)地表征沙三段和沙一段烴源巖的特征,將TDSI定義為(C294,23,24-三甲基三芳甲藻甾類)/(4-甲基與3-甲基三芳甾類之和),以表征4-甲基三芳甲藻甾類的豐度,該參數(shù)值高反映了沙一段的特征;將TNSI定義為(C274-甲基三芳甾類+C294-甲基-24-乙基三芳甾類)/(4-甲基與3-甲基三芳甾類之和),以表征4-甲基三芳非甲藻甾類的豐度,該參數(shù)值高反映了沙三段的特征;兩個(gè)參數(shù)值均低則反映了東營(yíng)組的特征。利用TDSI與TNSI相關(guān)圖圖版對(duì)蓬萊9-1油田潛山和新近系的油樣進(jìn)行油源對(duì)比及族群劃分(圖6),結(jié)果表明:潛山油樣4-甲基三芳甾類參數(shù)落在沙三段油源和沙一段油源之間,為沙三段和沙一段來(lái)源的混源油,這與潛山油樣的碳同位素對(duì)比結(jié)論一致。新近系原油分為4個(gè)族群,第一個(gè)族群為沙一段來(lái)源,蓬萊9-1油田新近系油藏以該類原油為主;第二個(gè)族群為沙三段與沙一段混合來(lái)源;第三個(gè)族群為沙一段與東營(yíng)組混合來(lái)源;第四個(gè)族群為東營(yíng)組來(lái)源。新近系油源的復(fù)雜性與碳同位素分布范圍較寬是相對(duì)應(yīng)的,沙一段油源為主的結(jié)論與較高的伽馬蠟烷豐度一致。
圖6 蓬萊9-1油田及圍區(qū)原油4-甲基三芳甾類參數(shù)油源對(duì)比Fig.6 Oil-source correlation by 4-methyl triaromatic steroids of oils in PL9-1 oilfield and its vicinity
蓬萊9-1油田圍區(qū)的油源對(duì)比已經(jīng)明確渤東凹陷存在沙三段、沙一段和東營(yíng)組等3套烴源巖,廟西凹陷存在沙一段和東營(yíng)組2套烴源巖。上述分析表明,蓬萊9-1油田潛山原油主要來(lái)源于沙三段和沙一段烴源巖,從烴源層系判斷潛山油源為渤東凹陷;新近系原油主要來(lái)源于沙一段烴源巖,油源洼陷可能為渤東凹陷或廟西凹陷。含氮化合物和原油物性可以進(jìn)一步驗(yàn)證潛山原油的來(lái)源方向,并幫助判斷新近系是否存在來(lái)源于廟西凹陷的原油。
咔唑是非堿性吡咯類含氮化合物的一種,存在于非烴組分中,因其具有較強(qiáng)的極性而易于吸附在輸導(dǎo)層或儲(chǔ)層中,從而在油氣運(yùn)移過(guò)程中出現(xiàn)咔唑類的地色層分餾效應(yīng),表現(xiàn)為咔唑絕對(duì)濃度隨運(yùn)移距離增大而降低,屏蔽型咔唑(1,8-二甲基咔唑,簡(jiǎn)寫為1,8-DMCa)相對(duì)于裸露型咔唑(如2,7-二甲基咔唑,簡(jiǎn)寫為2,7-DMCa)富集,從而指示了油氣的運(yùn)移方向[42-45]。
蓬萊9-1油田及圍區(qū)原油咔唑濃度隨降解指數(shù)呈規(guī)律性變化,當(dāng)降解指數(shù)小于2時(shí),因較飽和烴抗生物降解能力更強(qiáng)[46],咔唑濃度隨生物降解程度的增強(qiáng)而增大;當(dāng)降解指數(shù)大于10時(shí),咔唑濃度降至接近于零,此時(shí)咔唑也被降解。1,8-DMCa/2,7-DMCa比值與降解指數(shù)沒(méi)有明顯相關(guān)性,表明該參數(shù)沒(méi)有明顯受強(qiáng)烈生物降解影響,而主要受運(yùn)移過(guò)程中的地色層分餾效應(yīng)影響。蓬萊9-1油田東部c井與西部b井相距僅2.5 km,c井降解指數(shù)更低,但c井潛山原油咔唑濃度更低,1,8-DMCa/2,7-DMCa比值更高,具有明顯的運(yùn)移效應(yīng)(表2),表明潛山原油運(yùn)移方向?yàn)橛晌飨驏|。受生物降解程度較輕的蓬萊A油田m井原油已經(jīng)明確來(lái)自廟西凹陷沙一段烴源巖,該井與蓬萊9-1油田g井新近系原油咔唑濃度均高達(dá)80μg/g以上,比d井和c井新近系原油咔唑濃度高很多(表2),表明g井新近系油源為廟西凹陷。
表2 蓬萊9-1油田及圍區(qū)原油降解指數(shù)、咔唑參數(shù)和原油物性Table 2 Biodegradation,carbazole index and crude oil property of PL9-1 oilfield and its vicinity
原油物性受烴源巖性質(zhì)、成熟作用、運(yùn)移距離、重力分異、保存條件及成藏期次等因素的影響[47-49]。蓬萊9-1油田埋藏較淺,原油均受到了強(qiáng)烈的生物降解,保存條件并不理想,油氣運(yùn)移過(guò)程中以稠化作用而非層析作用為主,沿運(yùn)移方向原油輕質(zhì)組分散失,遭受生物降解和水洗的程度增大,原油物性變差。蓬萊9-1油田d井和c井潛山原油密度和黏度比b井高(表2),反映了d井和c井距離渤東凹陷油源更遠(yuǎn)。蓬萊9-1油田g井新近系原油密度和黏度比蓬萊A油田m井部分樣品略高,但比d井和c井低很多(表2),反映g井油源為廟西凹陷。蓬萊9-1油田潛山和新近系原油物性的變化印證了上述油源對(duì)比結(jié)論。
1)降解指數(shù)C2925-降藿烷/C30藿烷是定量評(píng)價(jià)強(qiáng)烈生物降解原油降解程度的有效指標(biāo)。強(qiáng)烈生物降解可導(dǎo)致C19三環(huán)萜烷/C23三環(huán)萜烷、C24四環(huán)萜烷/C26三環(huán)萜烷和伽馬蠟烷/C29Ts參數(shù)減小,但伽馬蠟烷/C30藿烷參數(shù)增大。4-甲基三芳甾類抗生物降解能力極強(qiáng),是強(qiáng)烈生物降解原油中穩(wěn)定的生物標(biāo)志物,該系列生物標(biāo)志物還是烴源巖生源構(gòu)成和沉積環(huán)境的反映,可有效區(qū)分渤海海域沙三段、沙一段和東營(yíng)組等3套烴源巖。應(yīng)用4-甲基三芳甾類和全油碳同位素為主的參數(shù)組合,輔以伽馬蠟烷參數(shù),結(jié)合咔唑和原油物性變化,明確了蓬萊9-1油田強(qiáng)烈生物降解原油的烴源層位和來(lái)源洼陷。
2)蓬萊9-1油田新近系油樣全油碳同位素與圍區(qū)典型沙一段油樣相近,飽和烴組分中伽馬蠟烷含量高,芳烴組分中C274-甲基三芳甾類和C294-甲基-24-乙基三芳甾類含量低而C294,23,24-三甲基三芳甲藻甾類含量高,原油主要來(lái)源于沙一段烴源巖。潛山油樣全油碳同位素介于圍區(qū)典型沙三段和沙一段油樣之間,飽和烴組分中伽馬蠟烷含量低,芳烴組分中4-甲基三芳甾類系列生物標(biāo)志物含量均較高,原油主要來(lái)源于沙三段和沙一段烴源巖。
3)咔唑類含氮化合物依然可以應(yīng)用于蓬萊9-1油田強(qiáng)烈生物降解原油運(yùn)移方向示蹤,原油物性變化反映了以稠化作用為主的淺層原油運(yùn)移方向,沿運(yùn)移方向咔唑絕對(duì)濃度降低,1,8-二甲基咔唑/2,7-二甲基咔唑比值增大,原油密度和黏度增大。蓬萊9-1油田潛山和新近系原油主要來(lái)源于渤東凹陷,新近系南部有來(lái)自廟西凹陷的原油。
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Oil-source correlation of severely biodegraded oil of PL 9-1 oilfield in Bohai sea
WANG Jun1ZHOU Xinhuai2YANG Bo1WANG Qingbin1GUO Yonghua1WANG Feilong1
(1.Tianjin Branch of CNOOC Ltd.,Tianjin300459,China;2.Shanghai Branch of CNOOC Ltd.,Shanghai200335,China)
Many heavy oil reservoirs are discovered in Mesozoic buried hill and Neogene of Bohai sea in recent years,but oil-source correlation is difficult because the common used biomarkers become invalid due to severe biodegradation.Taking PL 9-1 oilfield in Bohai sea as an example,the influence of severe biodegradation on oil carbon isotope,biomarkers in saturated and aromatic hydrocarbon,and nitrogen containing compound is studied.A biodegradation index is introduced to quantify biodegradation level for the severely biodegraded oil.Results show that carbazole and gammacerane contents decrease when biodegradation becomes severe;the preferential biodegradation of C19tricyclic terpane,C24tetracyclic terpane and Ts results in the decrease of C19/C23tricyclic terpane and C24tetracyclic terpane/C26tricyclic terpane index,and the increase of ETRindex,respectively;4-methyl triaromatic steroids are the most reliable biomarker in severely biodegraded oil,C274-methyl triaromatic steroids,C294-methyl-24-ethyl triaromatic steroids and C294,23,24-trimethyl triaromatic dinosteroids are indicators of organic input and depositional environment for source rocks,and they can be used to distinguish oil source from the E2s3,E3s1and E3d source rocks with the combination of whole oil carbon isotope;carbazole and crude oil properties reflect the hydrocarbon migration directions.Oil source of the Mesozoic buried hills and Neogene reservoirs of PL 9-1 oilfield are E2s3and E3s1source rocks in Bodong sag.Oil source of Neogene reservoirs is E3s1,with contribution of both Miaoxi and Bodong sags.The research results have significance for exploration of the study area and provide reference for oil-source correlation of severely biodegraded oil in Bohai sea and other similar areas.
biodegradation;oil-source correlation;heavy oil reservoir;PL 9-1 oil field;Bohai sea
王軍,周心懷,楊波,等.渤海蓬萊9-1油田強(qiáng)烈生物降解原油油源對(duì)比[J].中國(guó)海上油氣,2017,29(6):32-42.
WANG Jun,ZHOU Xinhuai,YANG Bo,et al.Oil-source correlation of severely biodegraded oil of PL 9-1 oilfield in Bohai sea[J].China Offshore Oil and Gas,2017,29(6):32-42.
TE122.1
A
1673-1506(2017)06-0032-11
10.11935/j.issn.1673-1506.2017.06.004
*“十二五”國(guó)家科技重大專項(xiàng)“渤海海域已證實(shí)的富生烴凹陷再評(píng)價(jià)及新領(lǐng)域勘探方向(編號(hào):2011ZX05023-001-004)”部分研究成果。
王軍,男,工程師,主要從事油氣地球化學(xué)及成藏研究工作。地址:天津市濱海新區(qū)海川路2121號(hào)渤海石油管理局(郵編:300459)。E-mail:varran@163.com。
2017-03-20改回日期:2017-06-04
(編輯:張喜林)