南海北部白云凹陷早中新世東部沉積體系研究①

王永鳳 王英民 徐 強(qiáng) 黃振凱 李 冬

(1.中國(guó)石油大學(xué)(北京)地球科學(xué)學(xué)院 北京 102249;2.油氣資源與探測(cè)國(guó)家重點(diǎn)實(shí)驗(yàn)室 北京 102249; 3.中國(guó)海洋石油總公司研究總院 北京 100027;4.中國(guó)石油勘探開(kāi)發(fā)研究 北京 100083)

南海北部白云凹陷早中新世東部沉積體系研究①

王永鳳1,2王英民1,2徐 強(qiáng)3黃振凱4李 冬1,2

(1.中國(guó)石油大學(xué)(北京)地球科學(xué)學(xué)院 北京 102249;2.油氣資源與探測(cè)國(guó)家重點(diǎn)實(shí)驗(yàn)室 北京 102249; 3.中國(guó)海洋石油總公司研究總院 北京 100027;4.中國(guó)石油勘探開(kāi)發(fā)研究 北京 100083)

基于白云凹陷東部鉆井、巖芯和地震資料,通過(guò)地震屬性分析、地震相分析及鉆井巖芯分析,證實(shí)了白云凹陷在23.8～21 Ma之間除存在北部物源外還存在來(lái)自東沙隆起的東部物源體系,從而擴(kuò)大了白云凹陷深水碎屑巖沉積研究領(lǐng)域。通過(guò)層序地層學(xué)研究和沉積相分析,建立了東部沉積體系沉積及演化模式:該體系在低位域形成海底扇沉積;水進(jìn)域形成碳酸鹽巖和硅質(zhì)碎屑巖混合沉積;高位體系域主要發(fā)育高位三角洲沉積,其主要受相對(duì)海平面變化及物源供給控制。

白云凹陷 東沙隆起 物源 沉積模式

近年來(lái),珠江口盆地白云凹陷深水油氣勘探取得了重要進(jìn)展,發(fā)現(xiàn)的大量深水扇復(fù)合體已成為油氣勘探的重要目標(biāo)。層序地層學(xué)研究揭示白云凹陷21～ 10.5 Ma共發(fā)育6期深水扇體,垂向上相互疊置,形成了深水扇復(fù)合體[1～2],其沉積物主要來(lái)自富砂的珠江水系陸架區(qū)前期高位體系域的珠江三角洲[3]。本文從地震屬性分析入手,結(jié)合地震與鉆井資料,對(duì)白云凹陷早中新世的沉積體系進(jìn)行了精細(xì)的研究。經(jīng)研究發(fā)現(xiàn)在白云凹陷東部發(fā)育來(lái)自東沙隆起的沉積體,并對(duì)其沉積特征及其演化模式進(jìn)行了研究,為白云凹陷的進(jìn)一步研究和勘探奠定了基礎(chǔ)。

1 區(qū)域地質(zhì)背景

圖1 珠江口盆地構(gòu)造區(qū)劃圖Fig.1 Tectonic unit compartmentalization of Pearl River Mouth basin

珠江口盆地位于南中國(guó)海北部、海南島和臺(tái)灣島之間海域,面積約為17 5000 km2。白云凹陷位于南中國(guó)海北部大陸邊緣的陸坡深水區(qū),隸屬珠江口盆地珠二坳陷。凹陷北靠中央隆起帶番禺低隆,南臨南部隆起帶,西南為開(kāi)平凹陷,東側(cè)為東沙隆起(圖1),凹陷區(qū)水深200～2 000 m,為一個(gè)處于陸棚邊緣斜坡帶的沉積凹陷。研究區(qū)位于白云凹陷東側(cè),東臨東沙隆起,共發(fā)育8套地層,由上到下依次為第四系、萬(wàn)山組、粵海組、韓江組、珠江組、珠海組、恩平組和文昌組[4～6]。其中珠海組發(fā)育了大型的淺海陸架沉積,珠江組和韓江組主要發(fā)育陸坡深水沉積。

研究目的層底界為SB23.8,界面年齡為23.8 Ma,ODP1148孔顯示SB23.8界面無(wú)論是沉積速率,還是各種巖石地球化學(xué)數(shù)據(jù)、孢粉含量曲線、有機(jī)質(zhì)含量等都表現(xiàn)出明顯的突變[7],這些沉積特征表明南海北部在23.8 Ma經(jīng)歷了大的沉積環(huán)境及物源成分的改變。龐雄等認(rèn)為SB23.8是南海北部陸架坡折從白云主凹南側(cè)向北側(cè)跳躍的分界面,23.8 Ma之前白云凹陷為陸架沉積環(huán)境,之后發(fā)生強(qiáng)烈熱沉降成為陸坡深水環(huán)境[8]。而從珠江口盆地相對(duì)海平面變化曲線可知早中新世到中中新世(大約23.8 Ma到10.5Ma),珠江口盆地經(jīng)歷了7次明顯的海平面升降旋回,SB23.8形成于相對(duì)海平面下降時(shí)期,且海平面下降到陸架坡折帶以下,層序界面之下廣泛出現(xiàn)削蝕[9]。由此可知,23.8 Ma發(fā)生了強(qiáng)烈海退事件導(dǎo)致白云凹陷北緣的番禺低隆起和東緣的東沙隆起都演變?yōu)楸┞秴^(qū),前人的大量研究工作已經(jīng)證實(shí)番禺低隆起為白云深水區(qū)提供物源[3]。那么,同樣為剝蝕區(qū)的東沙隆起能否為白云凹陷提供物源,本文以此為切入點(diǎn)對(duì)東沙隆起西部的沉積體系進(jìn)行了研究。通過(guò)對(duì)近年來(lái)采集的三維地震和長(zhǎng)電纜地震剖面的地震地層學(xué)解釋,在東沙隆起西南斜坡于層序SQ23.8中識(shí)別出了來(lái)自東沙隆起方向的朵體沉積,沿西南方向伸入白云凹陷:在縱向地震剖面中具有向西南方向的前積反射,而在橫向地震剖面中具有明顯的丘狀外形。這些均表明白云凹陷東部SQ23.8沉積時(shí)期,可能發(fā)育三角洲-海底扇沉積體系。

2 物源分析

在過(guò)東沙隆起的北東向地震剖面(圖2b)上,界面SB23.8與下伏地層呈現(xiàn)明顯的削蝕接觸關(guān)系,剝蝕范圍如圖2(a)所示,面積約為20 000 km2,緊靠白云凹陷東部,從如此大的剝蝕面積來(lái)看東沙隆起在早中新世應(yīng)該為本區(qū)提供物源。而通過(guò)高分辨率三維區(qū)SB23.8與SB21層位精細(xì)解釋,在SB23.8沿層提取的均方根振幅屬性圖(圖3a)上顯示出白云凹陷北東部存在明顯呈北東向展布的強(qiáng)振幅異常帶,推測(cè)白云凹陷可能存在北東方向的物源。進(jìn)一步利用深水區(qū)與淺水區(qū)鉆井資料繪制了SQ23.8含砂率圖(圖3b),在含砂率圖中也顯示了在番禺低隆起及東沙隆起存在兩個(gè)含砂率較高的區(qū)域,并且含砂率值分別向東南方向和西南方向降低,指示了SQ23.8沉積時(shí)期沉積物可能來(lái)自上述兩個(gè)方向,分別對(duì)應(yīng)番禺低隆起與東沙隆起。此外,在白云凹陷東部NE向的地震剖面上,SB23.8界面與SB21界面之間發(fā)育明顯的前積地震反射構(gòu)型(圖4a),呈現(xiàn)自北東向南西方向的前積,說(shuō)明東沙隆起在早中新世可為研究區(qū)提供物源。綜合以上分析可知,白云凹陷在早中新世不僅存在北部物源,還存在東部物源。

3 東部沉積體系的識(shí)別

3.1 地震反射特征

圖2 東沙隆起剝蝕特征Fig.2 Erosion characteristics of Dongsha Uplift

在井控程度較低的地區(qū),以地震響應(yīng)信息為線索,依據(jù)地震反射結(jié)構(gòu)特征可以初步預(yù)測(cè)各地質(zhì)體的分布。通過(guò)地震反射特征的研究,在白云凹陷東部NE向的地震剖面中SB23.8與SB21界面之間可見(jiàn)明顯的由N向S的斜交型前積反射,底積層發(fā)育,平面上呈由NE向SW的扇形展布,面積約為600 km2(圖4a、b)。而在前積反射的北東方向的東沙隆起上可見(jiàn)明顯的削截現(xiàn)象,為物源剝蝕區(qū)。結(jié)合SQ23.8沉積時(shí)期陸架坡折帶的發(fā)育位置,斷定上述沉積體為來(lái)自東沙隆起的三角洲沉積。

圖3 a.研究區(qū)SB23.8沿層RMS屬性 b.SQ23.8含砂率圖Fig.3 a.the RMS attribute of the SB23.8 b.sandstone/strata contour of SQ23.8

圖4 各沉積體地震反射特征Fig.4 Seismic reflection characteristics of each sedimentary deposit

在三角洲西南方向的陸架坡折帶下方,還發(fā)育一個(gè)較小的扇體沉積,由E向W延伸,地震反射特征以連續(xù)性較差,振幅能量變化快為特征,縱剖面上可見(jiàn)明顯的前積反射構(gòu)型(圖4c),橫剖面呈丘形反射、雙向下超充填(圖4d),具典型的海底扇地震反射構(gòu)型,結(jié)合其發(fā)育部位為當(dāng)時(shí)的深水環(huán)境,故認(rèn)為為海底扇沉積。

3.2 鉆井巖芯特征

在鉆井方面,目前在白云凹陷鉆井中僅LHX鉆遇了上述海底扇沉積。通過(guò)巖芯觀察描述發(fā)現(xiàn),LHX井層序SQ23.8內(nèi)砂巖主要為淺灰色石英質(zhì)砂巖,細(xì)粒到中粒,次圓-次棱角狀,分選較好,鈣質(zhì)膠結(jié)較發(fā)育,碳酸鹽含量較高(圖5)。巖芯多處發(fā)育鈣質(zhì)膠結(jié)碎屑巖,如圖5中巖芯照片4、5所示,分別為鈣質(zhì)細(xì)砂巖和鈣質(zhì)泥巖沉積。

圖5 LHX井SQ23.8沉積物巖電特征Fig.5 Rock-log characteristics of the SQ23.8 deposit from LHX well

前人研究認(rèn)為由于早中新世的海侵,珠江組早期(SQ23.8)的東沙隆起發(fā)育了一套碳酸鹽巖臺(tái)地沉積,發(fā)育眾多生物礁,而北部的番禺低隆起主要為硅質(zhì)碎屑巖沉積.HX井(位置見(jiàn)圖4a)緊靠東沙隆起,而SQ23.8沉積時(shí)期LHX井處于深水環(huán)境不發(fā)育碳酸鹽巖,綜合上述分析可知LHX井SQ23.8較高碳酸鹽含量主要是由于東沙隆起碳酸鹽巖臺(tái)地所致,從而再次證明了東部物源的存在。此外,巖芯中發(fā)育多種類型的沉積構(gòu)造,發(fā)育典型的重力流沉積構(gòu)造(圖5),如遞變層理、漂礫、滑塌變形和鮑馬序列等,還發(fā)育有少量反映牽引流水流機(jī)制的交錯(cuò)層理、斜波狀層理等。前人研究認(rèn)為真正的濁流只存在懸浮沉積,僅能應(yīng)用“正遞變層理”這個(gè)標(biāo)志來(lái)鑒定濁積巖[10,11],而在LHX井巖芯中正遞變層理較發(fā)育,主要為中細(xì)砂巖沉積,可作為沉積物重力流的鑒別相標(biāo)志,且泥巖中蟲(chóng)孔多為水平蟲(chóng)孔(圖5),指示了泥巖沉積環(huán)境為較深水的靜水環(huán)境,由上述分析可知,LHX井巖芯中重力流沉積構(gòu)造發(fā)育,指示了其沉積物為半深海環(huán)境下的重力流沉積,綜合分析斷定為海底扇沉積。

圖6 SQ23.8體系域?qū)Ρ扰c剖面演化特征(剖面位置見(jiàn)圖1)Fig.6 Systems tract correlation and evolution characteristics on cross section of SQ23.8(see Fig.1 for location)

圖7 東部沉積體系沉積模式Fig.7 Sedimentarymodel of the eastern depositional system

3.3 剖面沉積特征

為建立東部沉積體系從東沙隆起到白云凹陷的沉積響應(yīng)關(guān)系,分別取兩個(gè)構(gòu)造上的井及過(guò)井地震剖面(位置見(jiàn)圖1)進(jìn)行井震結(jié)合層序劃分及沉積相分析。通過(guò)地震反射終止關(guān)系分析及井震結(jié)合對(duì)比,在SQ23.8層序內(nèi)部識(shí)別出了首次海泛面和最大海泛面,將SQ23.8層序劃分為3個(gè)體系域(圖6)。低位體系域:陸架與陸坡區(qū)均發(fā)育,陸架上LHY井為砂巖夾薄層灰?guī)r沉積,多個(gè)反韻律疊置,為三角洲前緣沉積;陸坡區(qū)LHX井為大套泥巖中夾三套海底扇朵體沉積,朵體巖性為泥灰?guī)r、砂巖,地震反射剖面可見(jiàn)明顯的前積反射構(gòu)型。海侵體系域:地震反射特征上表現(xiàn)為明顯的向東沙隆起方向的上超,陸架上LHY井碳酸鹽巖較發(fā)育,砂巖厚度較小,以白云巖為主,曲線呈鐘形;坡折帶下方LHX井主要發(fā)育碳酸鹽巖和硅質(zhì)碎屑巖混合沉積,巖性相對(duì)較細(xì),自然伽馬曲線整體呈正旋回,表現(xiàn)為退積準(zhǔn)層序組特征。高位體系域:LHY井為前面所發(fā)現(xiàn)的大型三角洲沉積,地震反射剖面表現(xiàn)為明顯的由NE向SW方向前積,下超于最大海泛面之上,巖性為厚層砂巖,低自然伽馬,以反韻律為主;坡折帶下方LHX井發(fā)育海底扇沉積,為上述取芯部位,主要為中細(xì)粒砂巖沉積,扇體規(guī)模較小。

4 沉積模式及成因探討

通過(guò)以上地震相、巖芯相及連井相的分析,結(jié)合研究區(qū)高分辨率三維地震資料及鉆井資料所揭示的海底扇、混合沉積及陸架三角洲的沉積特征,總結(jié)出了研究區(qū)沉積體系特征,建立了白云凹陷東部臺(tái)緣斜坡沉積體系沉積演化模式(圖7)。

低位體系域:23.8 Ma相對(duì)海平面下降,東沙隆起露出水面,遭受大面積剝蝕,沉積物供給增大,使得三角洲迅速向西南方向推進(jìn),由于東沙陸坡較陡,東沙隆起坡折帶沉積物處于不穩(wěn)定狀態(tài),發(fā)生滑塌,較陡的斜坡不發(fā)育可供沉積物停留的可容納空間,沉積物在斜坡區(qū)主要發(fā)生侵蝕和過(guò)路作用,沉積厚度較薄,在前方深水區(qū)即LHX部位堆積下來(lái)形成低位海底扇沉積。

海侵體系域:隨著海平面的快速上升,三角洲向岸方向退縮,東沙隆起逐漸被海水淹沒(méi),處于淺水環(huán)境,有利于造礁生物的生長(zhǎng),碳酸鹽巖比較發(fā)育。當(dāng)海平面保持穩(wěn)定時(shí),礁就會(huì)沿自身的碎石斜坡向海推進(jìn)[12],隨著海平面的次級(jí)擺動(dòng),三角洲為臺(tái)緣斜坡及周圍海盆盆地幕式地提供硅質(zhì)碎屑,從而在海侵體系域陸坡深水區(qū)形成了硅質(zhì)碎屑巖和碳酸鹽巖的混合沉積[13～15]。

高位體系域:早期相對(duì)海平面較高,伴隨著海平面開(kāi)始下降以及東沙隆起物源供給強(qiáng)度增加,三角洲開(kāi)始向白云凹陷推進(jìn),并侵蝕早期形成的生物礁,為白云深水區(qū)提供碳酸鹽巖碎屑,形成鈣質(zhì)膠結(jié)沉積物;高位晚期海平面下降,東沙隆起剝蝕面積增大,沉積物供給強(qiáng)度增加,三角洲快速向西南推進(jìn),在東沙隆起邊緣斜坡發(fā)育大型前積體,即高位三角洲沉積,并在前方深水區(qū)發(fā)育小型的海底扇沉積。

5 結(jié)論

通過(guò)對(duì)物源、地震相分析以及相關(guān)巖芯的研究,可以得到如下結(jié)論

(1)在白云凹陷東部層序SQ23.8中存在來(lái)自東沙隆起的東部物源沉積體系,該體系的主體為三角洲和海底扇沉積。

(2)東部沉積體系在SQ23.8中的沉積模式為: 23.8 Ma珠江口盆地發(fā)生大規(guī)模海退事件,導(dǎo)致東沙隆起暴露遭受剝蝕,為白云深水區(qū)提供物源,形成海底扇沉積,并在陸架區(qū)形成三角洲沉積;海侵期,東沙隆起發(fā)育碳酸鹽巖臺(tái)地,在深水區(qū)形成混合沉積;高位時(shí)期海平面開(kāi)始下降,在陸架區(qū)形成三角洲進(jìn)積沉積,而在深水區(qū)發(fā)育規(guī)模較小的海底扇沉積。

(3)該體系的形成與演化受控于海平面變化以及物源供給等因素。

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Research on the Eastern Depositional System in Early M iocene, Baiyun Sag,Northern South China Sea

WANG Yong-feng1,2WANG Ying-min1,2XU Qiang3HUANG Zhen-kai4LIDong1,2
(1.School of Geoscience,China University of Petroleum,Beijing 102249; 2.State Key Laboratory of Petroleum Resource and Prospecting,Beijing 102249; 3.The Research Institute of CNOOC,Beijing 100027; 4.Institute of Petroleum Exp loration and Development,PetroChina,Beijing 100083)

Based on a large amount of drill,core and seismic data from the eastern Baiyun Sag,by the analysis of seismic attributes,seismic facices as well as drilling sedimentary facies,it is proved that in Baiyun Sag there is not only the north provenance but also the eastern depositional system which is derived from the Dongsha Uplift in early Miocene,and this discovery will lead to the expansion of the exploration scope for the deepwater clastic deposits of Baiyun Sag.

The Eastern Depositional System ismainly developed in SQ23.8,with the bottom boundary SB23.8.revious studies reported that the“Baiyun Movement”occurred in 23.8 Ma B.P.results in tremendous changes of Baiyun sag,causing the thermal subsidence of the sag,making the shelf neritic environment transform into slope deepwater environment,and SB23.8 is the interface of the shelf break beltmigrating from the south of Baiyun sag to north.In 23.8 Ma B.P.the relative sea level fell to the lowest,whichmade the Panyu Low Uplift in the north and the Dongsha Uplift in the eastbe in a state of exposure.The formers'study has confirmed that the Panyu Low Uplift provides debris for the deep water of Baiyun sag,and then,whether the Dongsha Upliftwhich was also exposed in SQ23.8 can serve as the provenance for Baiyun sag is the breakthrough point of this paper。

The erosion area of Dongsha uplift during early Miocene is about20 000 km2,which is close to Baiyun sag,and considering such a great erosion area the uplift should providematerials for Baiyun sag.Besides,on the image of3D seismic RMS amplitude slices present an abnormally high amplitude in the east of Baiyun sag,and the graph of sandstone/strata contour of SQ23.8 also presents high values in the east.In addition,on seismic,awedge-shaped progradational reflection configuration with the direction from Dongsha Uplift to Baiyun sag in SQ23.8 is developed.Therefore,according to the above analysis,it is concluded that the Dongsha Uplift is one of the provenances of Baiyun sag during the period of SQ23.8 deposition。

On seismic,the deposits characterized by wedge-shaped progradational reflection configuration are interpreted as the deposits of deltas for their developed location,and the deposits in front characterized by high-amplitude,good continuity,progradational reflection on the trend seismic profile,a hummocky bidirectional progradation configuration on the cross profile,are interpreted as the submarine fan.Through observation of the core it is found that the carbonate content is fairly high in the core of SQ23.8,indicating that the sediments come from Dongsha Uplift,because during the period of SQ23.8 deposition,only Dongsha Uplift developed carbonate,also,again confriming the viewpoint that Dongsha Uplift is the provenance of Baiyun sag.Besides,a variety of gravity flow sedimentary structures are found in the core,such as normal graded bedding,Bouma sequence and slump-deformation bedding and so on,indicating a kind of gravity flow deposits in bathyal environment.The sediments of the core are identified as submarine fan deposits。

Based on the sequence division resultby integratingwell and seismic data,SQ23.8 can be divided into three system tracts:LST,TST and HST.Combining the sequence stratigraphic analysis and the study of the sedimentary facies,the sedimentary and evolution model of the eastern sedimentary system has been established.In this depositional system,the lowstand system tracts(LST)are represented by submarine fans,and the trangressive system tracts (TST)are dominated by themixed deposits of clastic and carbonate rock,while in the highstand system tracts(HST) the deltaic deposits are well developed.The deposits and sedimentary model aremainly controlled by the change of relative sea level and material supply。

Baiyun Sag;Dongsha Uplift;provenance;sedimentarymodel

王永鳳 女 1983年出生 博士研究生 沉積學(xué)、層序地層學(xué)、地震沉積學(xué) E-mail:wangyongf4@163.om

TE121.3

A

1000-0550(2012)03-0461-08

①國(guó)家重點(diǎn)基礎(chǔ)研究發(fā)展計(jì)劃項(xiàng)目(973計(jì)劃):(編號(hào):2009CB219407)、國(guó)家自然科學(xué)基金項(xiàng)目(批準(zhǔn)號(hào):40572067)資助。

2011-05-06;收修改稿日期:2011-08-02