番禺低隆起-白云凹陷北坡第三系儲(chǔ)層次生孔隙形成機(jī)理分析①

郝樂(lè)偉 王 琪 廖 朋 唐 俊 張功成

(1.中國(guó)科學(xué)院油氣資源研究重點(diǎn)實(shí)驗(yàn)室 蘭州 730000;2.中國(guó)科學(xué)院研究生院 北京 100049;3.中國(guó)海洋石油研究中心 北京 100027)

番禺低隆起-白云凹陷北坡第三系儲(chǔ)層次生孔隙形成機(jī)理分析①

郝樂(lè)偉1,2王 琪1,2廖 朋1,2唐 俊1,2張功成3

(1.中國(guó)科學(xué)院油氣資源研究重點(diǎn)實(shí)驗(yàn)室 蘭州 730000;2.中國(guó)科學(xué)院研究生院 北京 100049;3.中國(guó)海洋石油研究中心 北京 100027)

根據(jù)砂巖薄片、鑄體薄片、掃描電鏡的分析,研究區(qū)內(nèi)發(fā)育大量的粒間和粒內(nèi)溶蝕孔。在大量實(shí)測(cè)物性參數(shù)統(tǒng)計(jì)分析的基礎(chǔ)上,發(fā)現(xiàn)了白云凹陷第三系儲(chǔ)層存在兩個(gè)次生孔隙發(fā)育帶,主要分布在2 750～3 500 m和4 100～ 4 600 m兩個(gè)深度段。通過(guò)同位素示蹤的方法,證實(shí)了形成次生孔隙的原因主要是有機(jī)質(zhì)成熟時(shí)釋放的有機(jī)酸和成巖過(guò)程中有機(jī)質(zhì)脫羧作用產(chǎn)生的CO2形成的酸性水對(duì)長(zhǎng)石、含長(zhǎng)石火山巖屑、早期碳酸鹽膠結(jié)物和生物化石等的溶解。影響次生孔隙的地質(zhì)因素還有烴源巖的分布和類(lèi)型,沉積相內(nèi)砂體的分布和砂巖類(lèi)型,構(gòu)造活動(dòng)產(chǎn)生的斷裂。另外次生孔隙還受控于烴源巖成熟時(shí)間與斷層、砂體和古構(gòu)造脊組成的輸導(dǎo)體系之間的匹配關(guān)系。綜合各種地質(zhì)因素分析后,將有機(jī)酸形成的次生孔隙模式概括為“生、運(yùn)、匹、溶”,該模式對(duì)研究區(qū)優(yōu)質(zhì)儲(chǔ)層的預(yù)測(cè)具有一定的指導(dǎo)意義。

白云凹陷 次生孔隙 有機(jī)酸 包裹體 輸導(dǎo)體系 優(yōu)質(zhì)儲(chǔ)層

隨著石油勘探向深部發(fā)展,深部?jī)?chǔ)層高孔隙的發(fā)育日益成為石油地質(zhì)工作者備受關(guān)注的問(wèn)題。深部異常高孔隙可由早期烴類(lèi)的注入或淺部發(fā)育流體超壓等多種因素造成[1],也可以由次生孔隙形成[2,3]。其中形成次生孔隙的因素很多,包括大汽水、地表淋濾水的淋濾作用,有機(jī)質(zhì)成熟演化中產(chǎn)生的有機(jī)酸的溶蝕作用[4～7],黏土礦物轉(zhuǎn)化產(chǎn)生酸性水的溶蝕作用[8]等。儲(chǔ)層次生孔隙的形成及保存機(jī)制是一個(gè)復(fù)雜的過(guò)程,既與微觀組分有關(guān),又與宏觀地質(zhì)作用相關(guān),其研究不僅要考慮古氣候條件、原始沉積相帶、成巖環(huán)境的地溫、壓力條件和流體介質(zhì)條件的影響,還要考慮多期構(gòu)造變動(dòng)對(duì)沉積及成巖作用的影響[9,10]。前人已對(duì)珠江口盆地白云凹陷-番禺地隆起第三紀(jì)的成藏組合,沉積體系和成巖作用做了大量的研究,但對(duì)該地區(qū)儲(chǔ)層物性的變化規(guī)律及影響因素缺乏綜合研究,對(duì)儲(chǔ)層孔隙演化規(guī)律了解甚微。次生孔隙的形成機(jī)制及控制因素得到合理解釋與否,直接影響著含油氣盆地碎屑巖儲(chǔ)層質(zhì)量預(yù)測(cè)模式的建立,嚴(yán)重影響著對(duì)儲(chǔ)層的客觀評(píng)價(jià)。本文通過(guò)同位素地球化學(xué),有機(jī)地球化學(xué)結(jié)合沉積學(xué)的方法,對(duì)制約研究區(qū)次生孔隙發(fā)育的因素做了系統(tǒng)的探討。

1 地質(zhì)概況

珠江口盆地位于中國(guó)南海北部,華南大陸的南緣,呈北東-南西向展布,是中國(guó)近海含油氣盆地中一個(gè)重要的盆地。該盆地東西長(zhǎng)約800 km,南北寬100～300 km,面積約17.8 X104km2。盆地的演化在中-新生代經(jīng)歷了裂前、裂谷、沉降和斷塊升降4個(gè)階段[11]。盆地可分珠一、珠二、珠三等3個(gè)坳陷和北部斷階、神狐暗沙隆起、東沙隆起、南部隆起及番禺低隆起8個(gè)二級(jí)構(gòu)造單。番禺低隆起和白云凹陷位于珠江口盆地中、南部地區(qū),二者相鄰,分屬于盆地的中央隆起帶和珠二坳陷(圖1)。白云凹陷是珠江口盆地面積最大、第三系沉積厚度最大的凹陷,最大沉積厚度超過(guò)10 km。番禺低隆起和白云凹陷的第三系自下而上發(fā)育有文昌組、恩平組、珠海組、珠江組、韓江組、粵海組、萬(wàn)山組,其中珠海組、珠江組和韓江組是主要的油氣儲(chǔ)層,發(fā)育的古近系文昌組、恩平組烴源巖,是該區(qū)主要的油氣來(lái)源[12]。

①?lài)?guó)土資源部全國(guó)油氣資源戰(zhàn)略調(diào)查與評(píng)價(jià)項(xiàng)目(編號(hào):XQ-2004-05)、國(guó)家科技重大專(zhuān)項(xiàng)海洋深水區(qū)油氣勘探關(guān)鍵技術(shù)(編號(hào):2008ZX05000-025)和中國(guó)科學(xué)院“西部之光”聯(lián)合學(xué)者項(xiàng)目資助。

圖1 白云凹陷構(gòu)造位置示意圖和地層結(jié)構(gòu)圖(據(jù)王福國(guó)[13],2004,修改)Fig.1 Tectonic location and sequence stratigraphic division of Baiyun sag(modified from Wang Fuguo,et al.2004)

2 次生孔隙類(lèi)型及演化特征

根據(jù)對(duì)研究區(qū)實(shí)際孔隙度數(shù)據(jù)與埋深的關(guān)系,孔隙度在2 000 m時(shí)偏離了理論孔隙度的變化曲線(xiàn),這種偏離一直延續(xù)到5 000 m。不考慮次生孔隙發(fā)育和早期碳酸鹽膠結(jié),孔隙度和承載壓力之間是存在良好的線(xiàn)性關(guān)系的,按照理論推斷孔隙縱向演化可以劃分為3段,即:①高孔銳減段;②穩(wěn)定下降段;③緩慢減縮段,理論孔隙度演化是逐漸降低的,不會(huì)出現(xiàn)回返[14,15]。從孔隙度曲線(xiàn)的變化上可以說(shuō)次生孔隙在研究區(qū)較為發(fā)育,在各個(gè)層位幾乎都存在,但孔隙明顯回升的層段主要分布在2 750～3 500 m和4 100～ 4 600 m兩個(gè)區(qū)間(圖2):第一區(qū)間,此帶是此間礦物顆粒和碳酸鹽膠結(jié)物發(fā)生了溶蝕,粒間溶孔和粒內(nèi)溶孔大量形成,次生孔隙開(kāi)始大量發(fā)育,孔隙在本期具有一個(gè)明顯的增大過(guò)程,屬混合孔隙發(fā)育帶。主要是由于烴源巖中有機(jī)酸的注入,形成酸性成巖環(huán)境產(chǎn)生大量溶蝕孔,從而形成了次生孔隙發(fā)育帶。珠海組主體位于此帶,此帶以粒間溶孔+粒內(nèi)溶孔+粒間孔組合為主。第二區(qū)間,隨著埋深和成巖作用的加深,有機(jī)酸逐漸被消耗,酸性環(huán)境降低逐漸向堿性環(huán)境過(guò)渡。由于主要烴源層已先期進(jìn)入成熟階段,珠江組下段和珠海組的砂巖表現(xiàn)出早期溶解作用的成巖特點(diǎn),早先形成的部分次生孔隙被其后的硅質(zhì)增生所縮小,使深部難以保存大規(guī)摸的孔隙回升現(xiàn)象。此帶孔隙度和滲透率的相關(guān)性比較差(圖3),主要由于此帶多為粒內(nèi)溶孔,多見(jiàn)火山巖屑溶孔。珠海組下部、恩平組主體位于此帶,以粒內(nèi)溶孔+粒間溶孔+粒間孔組合為主。

圖2 孔隙度隨深度的變化Fig.2 The variation of porosity with depth

3 次生孔隙形成的條件與主控因素

在本地區(qū)酸性流體的溶蝕作用是次生孔隙產(chǎn)生的主要原因,蒙脫石等黏土礦物的脫水形成酸性流體對(duì)原生孔隙保存和次生孔隙的演化僅起到輔助作用。前人研究早就指出本區(qū)酸性流體在孔隙演化上起了重要意義[15,16]。

圖3 第二區(qū)間孔隙度和滲透率相關(guān)性Fig.3 Relationship between porosity and permeability in the second interval

本次通過(guò)研究區(qū)內(nèi)碎屑巖的氧、碳同位素進(jìn)一步證實(shí)了前人的認(rèn)識(shí)和研究成果。研究區(qū)內(nèi)碎屑巖中δ13C值變化在-11.8‰ ～-3.4‰之間,δ18O為-17.8‰～2.4‰(表1)。由于大氣水中溶解CO2的 δ13C值為-7‰,湖相原生碳酸鹽的δ13C值為-2‰～ 6‰[17].rwin的研究表明[18]有機(jī)酸 δ13C值為-20‰,δ18O值為-3.5‰～-7‰。由上面各種成因碳酸鹽同位素特征值的對(duì)比可見(jiàn),僅僅由沉積時(shí)保存的孔隙水是不能形成如此低的δ13C值。在深埋過(guò)程中干酪根熱脫羧形成有機(jī)酸并加入碳酸鹽膠結(jié)物而致。這樣,依據(jù)同位素資料就可以確定白云凹陷碎屑巖儲(chǔ)層次生孔隙的成因主要是有機(jī)酸的溶蝕作用。

另外,通過(guò)對(duì)大量薄片觀察,發(fā)現(xiàn)長(zhǎng)石顆粒與巖屑顆粒被強(qiáng)烈溶蝕(圖4a),有的僅剩殘骸(圖4b),可以說(shuō)明是酸性比較強(qiáng)的酸性流體所致,而蒙脫石等黏土礦物產(chǎn)生的酸性水或有機(jī)質(zhì)過(guò)成熟產(chǎn)生的CO2難以產(chǎn)生這樣的溶蝕效應(yīng)。同時(shí),恩平組、珠海組、珠江組等多個(gè)層段溶蝕的普遍性也說(shuō)明了酸性流體的產(chǎn)出量大。如此酸性強(qiáng)、體積大的酸性流體,與本地區(qū)的烴源巖類(lèi)型和分布密切相關(guān)。

表1 碳、氧同位素分析數(shù)據(jù)Table1 The data of carbon and oxygen isotopic composition of carbonate cements

圖4 研究區(qū)典型的溶蝕現(xiàn)象a.LW3-1-1-3,SEM,3 136.00m,巖屑顆粒表面發(fā)生強(qiáng)烈溶蝕形成溶蝕孔分布特征;b.LW3-1-1-11,SEM,3 191.00m,長(zhǎng)石顆粒發(fā)生強(qiáng)烈溶蝕形成溶蝕殘骸;c.PY34-1-2-15(X100)(3 386.00 m N zj)巖屑石英細(xì)砂巖中長(zhǎng)石沿解理縫溶蝕;d.PY33-1-1-21(X50)(4 299.85 m E ep)強(qiáng)壓實(shí)巖屑細(xì)砂巖中火山巖屑粒間溶孔;e.PY33-1-1-14,(X25)3 438.20 m,E3 zh,粒間白云石膠結(jié)物溶蝕及生物化石內(nèi)部選擇性溶蝕現(xiàn)象;f.PY33-1-1-14(X50)(3 438.20m Eh)弱壓實(shí)鈣質(zhì)巖屑中-細(xì)砂巖中生物化石體腔孔Fig.4 Typical dissolution events in the study area

3.1 豐富的可溶物質(zhì)

已有的研究表明,次生孔隙主要由骨架顆粒溶解形成,長(zhǎng)石等鋁硅酸鹽是最為常見(jiàn)的易溶骨架顆粒,很多砂巖中次生孔隙的形成都是長(zhǎng)石等鋁硅酸鹽溶解的結(jié)果[19]。通過(guò)鑄體薄片觀察發(fā)現(xiàn)溶蝕現(xiàn)象有鋁硅酸鹽溶蝕、石英顆粒邊緣溶蝕和碳酸鹽膠結(jié)物溶蝕三種情況,其中以碳酸鹽膠結(jié)物溶蝕形成的次生孔隙對(duì)儲(chǔ)層的貢獻(xiàn)最大。鏡下可以觀察到大量的碳酸鹽膠結(jié)物的溶蝕現(xiàn)象,甚至膠結(jié)物全部溶蝕后,表現(xiàn)出顆粒間以點(diǎn)-線(xiàn)接觸的中-弱壓實(shí)假象。通過(guò)巖石薄片和掃描電鏡的觀察,發(fā)現(xiàn)在研究區(qū)碎屑巖儲(chǔ)層中不缺乏可溶組分,主要有以下3類(lèi)可溶蝕組分:

(1)長(zhǎng)石碎屑顆粒

含量一般,主要為鉀長(zhǎng)石、微斜長(zhǎng)石和少量酸性斜長(zhǎng)石,變化在2%～5%之間,主要沿顆粒解理縫發(fā)生溶蝕(圖4c)。

(2)含長(zhǎng)石的火山巖屑

含量較高,變化在5%～30%之間,是被溶蝕的主要成分,多形成粒內(nèi)溶孔(圖4d)。

(3)早期碳酸鹽膠結(jié)物和生物化石

膠結(jié)物含量較高,變化在5%～30%之間,溶蝕較強(qiáng),其中有孔蟲(chóng)化石發(fā)生選擇性溶蝕(圖4e),可形成體腔孔、鑄?？椎?圖4f)。

凡是巖屑含量較高的砂巖由于強(qiáng)烈的壓實(shí)作用,造成許多巖屑被壓實(shí)變形擠入孔隙之中形成了假雜基,堵塞原始粒間孔,造成原生孔隙保存較少,酸性水難以進(jìn)入產(chǎn)生溶蝕作用,因而次生孔隙不發(fā)育。而成分較純的石英砂巖其次生孔隙都很發(fā)育[20]。研究區(qū)第三系主要儲(chǔ)層砂巖成分和結(jié)構(gòu)成熟度都較高。珠江組為石英砂巖和巖屑質(zhì)石英砂巖,其次為長(zhǎng)石質(zhì)石英砂巖;珠海組為石英砂巖,其次為巖屑質(zhì)石英砂巖(圖5)。因此,研究區(qū)的儲(chǔ)層非常有利于有機(jī)酸的進(jìn)入形成次生孔隙度。

3.2 活躍的烴源巖

烴源巖是一種富含有機(jī)質(zhì)、水和多種無(wú)機(jī)礦物的沉積體.urdam等[21]認(rèn)為,有機(jī)酸主要生成于烴類(lèi)開(kāi)始大量生成之前。但經(jīng)過(guò)后來(lái)學(xué)者做的實(shí)驗(yàn)表明,有機(jī)酸的生成過(guò)程似乎可以持續(xù)到整個(gè)烴類(lèi)生成過(guò)程。陳傳平等[22]和曾濺輝等[23]的實(shí)驗(yàn)研究結(jié)果表明在300℃條件下,干酪根熱解可以大量生成有機(jī)酸.arth等[24]研究表明,在熱解溫度范圍內(nèi)最高溫度為350℃都有有機(jī)酸的產(chǎn)出,且產(chǎn)率隨溫度的升高而增加。袁佩芳等[25]對(duì)濟(jì)陽(yáng)坳陷古近系烴源巖的熱解實(shí)驗(yàn)結(jié)果表明,至450℃都有有機(jī)酸的生成。因此在干酪根成熟過(guò)程中,一直存在著有機(jī)酸的生成,可以伴隨整個(gè)烴類(lèi)生成過(guò)程。珠江口盆地存在始新統(tǒng)文昌組、始新統(tǒng)-漸新統(tǒng)恩平組、上漸新統(tǒng)珠海組和下中新統(tǒng)珠江組4套烴源巖。文昌組和恩平組為已證實(shí)的有效烴源巖。文昌組烴源巖在白云凹陷分布面積達(dá)1 900 km2,厚度最大達(dá)3 000 m;恩平組烴源巖在白云凹陷分布面積達(dá)2 860 km2,厚度最大達(dá)2 300 m[26]。并且文昌組已進(jìn)入成熟、過(guò)成熟階段,Ro最大達(dá)3.0%,恩平組烴源巖進(jìn)入成熟階段,Ro值為1.0%～1.5%[27]。近期研究成果表明,白云凹陷鉆遇珠海組泥巖TOC值大多介于1.10% ～1.15%,屬于中等烴源巖;珠江組泥巖絕大部分樣品TOC值小于0.6%,屬于差烴源巖。珠江組Ro值為0.30%～0.43%,處于未成熟階段;珠海組Ro值為0.43%～0.53%,處于低成熟階段[28]。由此可知,研究區(qū)主要烴源巖均進(jìn)入產(chǎn)酸、產(chǎn)油和產(chǎn)氣階段,可以為油氣儲(chǔ)層溶蝕作用提供大量酸性流體。

圖5 研究區(qū)砂巖類(lèi)型三角圖Fig.5 Ternary diagram of sandstone types in the study area

前人的研究表明文昌組有機(jī)質(zhì)類(lèi)型以Ⅱ型為主,部分為Ⅰ型;恩平組有機(jī)質(zhì)類(lèi)型為Ⅱ型,且以Ⅱ2型或偏Ⅲ型為主[29],珠江組和珠海組有機(jī)質(zhì)類(lèi)型主要為ⅡB型。由此可知,珠江口盆地4套烴源巖中有機(jī)質(zhì)類(lèi)型以Ⅰ和Ⅱ型為主(圖6)。模擬試驗(yàn)結(jié)果表明,在干酪根成熟過(guò)程中,一直存在著有機(jī)酸的生成,可以持續(xù)整個(gè)烴類(lèi)生成過(guò)程。于酪根的類(lèi)型對(duì)有機(jī)酸的產(chǎn)出具有明顯的控制作用,Ⅲ型干酪根烴源巖的生成量都明顯地低于Ⅰ型和Ⅱ型干酪根[25]。而研究區(qū)4套烴源巖中有機(jī)質(zhì)類(lèi)型以Ⅰ和Ⅱ型為主,因此研究區(qū)的烴源巖非常適合產(chǎn)酸,具有很強(qiáng)的產(chǎn)酸能力。

3.3 有利的輸導(dǎo)體系

輸導(dǎo)體系包括連通砂體、斷層、裂隙、不整合面。斷層對(duì)油氣的垂向輸導(dǎo)使油氣沿?cái)嗝鎻南孪蛏洗瓜蜻\(yùn)移,并連通不同的砂體輸導(dǎo)層,不整合面等進(jìn)行側(cè)向運(yùn)移。干酪根成熟過(guò)程中,有機(jī)酸先期產(chǎn)生量大,但可持續(xù)整個(gè)生烴過(guò)程中,因此有機(jī)酸的排出稍早于或者和油氣一同排出。一般流體的運(yùn)移總是沿著先期流體形成的路徑運(yùn)移,油氣的運(yùn)移應(yīng)是沿著先期有機(jī)酸運(yùn)移的路徑運(yùn)移,因此油氣的運(yùn)移路徑可以作為有機(jī)酸運(yùn)移路徑的證據(jù)。研究區(qū)內(nèi)研究區(qū)油氣主要輸導(dǎo)層為T(mén)5、T4～T3、T3～T2三個(gè)層位。在這些層位中廣泛發(fā)育有三角洲、濱海砂體、濁積扇和盆底扇等砂體,這些砂體為有機(jī)酸的側(cè)向運(yùn)移提供了側(cè)向通道。番禺低隆起-白云凹陷北坡廣泛發(fā)育第三系同沉積反向斷層,斷層延伸的距離相對(duì)較遠(yuǎn),垂向溝通性好,構(gòu)成了白云凹陷主要烴源巖產(chǎn)生的有機(jī)酸往番禺低隆起上運(yùn)移的縱向輸導(dǎo)體系[29,30]。在番禺低隆起-白云凹陷北坡發(fā)育斷層和砂體相互配置,架起了烴源巖與儲(chǔ)層之間的橋梁,使得有機(jī)酸源源不斷的從下部地層往淺部?jī)?chǔ)集層運(yùn)移。

圖6 珠江口盆地?zé)N源巖有機(jī)質(zhì)類(lèi)型分布(據(jù)朱俊章等,2008,修改)Fig.6 The types of organicmatter in source rock of Pearl River Mouth basin(modified from ZhuJunzhang,et al,2008)

流體包裹體是流體活動(dòng)的直接證據(jù),它直接記錄了流體活動(dòng)期次、溫度范圍和壓力條件。根據(jù)前人對(duì)流體包裹體的分析,番禺低隆起珠江和珠海組砂巖儲(chǔ)層中發(fā)育兩期油、兩期天然氣充注。第一期和第二期以原油充注為主,溫度范圍89.1～132.3℃,大致發(fā)生在12.8～5.8 Ma;第三期和第四期以氣態(tài)烴充注為主,溫度范129.8～171.3℃,大致發(fā)生在5.8 Ma至今[31,32]。在這四次油氣充注時(shí)白云凹陷北坡和番禺地隆起,均有斷層的活動(dòng)和砂體的存在(圖7),有機(jī)酸注入儲(chǔ)層時(shí)具有非常好的匹配關(guān)系,砂體輸導(dǎo)層與斷層相互切割,為油氣長(zhǎng)距離側(cè)向運(yùn)移提供了輸導(dǎo)通道。陳紅漢等對(duì)各期次有機(jī)流體包裹體檢測(cè)平面分布特征表明,第一期流體充注是以SW-NE一個(gè)方向充注;第二、第三和第四期均為SW-NE和SE-NW兩個(gè)方向充注(圖8)。通過(guò)從有機(jī)流體包裹體的分布,可知有機(jī)酸進(jìn)入番禺地隆起后,沿著古構(gòu)造脊運(yùn)動(dòng),沿途溶蝕儲(chǔ)層形成次生孔隙發(fā)育帶。

圖7 番禺低隆起-白云凹陷北坡油氣演化與輸導(dǎo)體系匹配關(guān)系(據(jù)王斌,2007,修改)Fig.7 Oil-gas evolution matching relation with migration system in Panyu low uplift to north slope of Baiyun Sag(modified from Wang Bin,2007)

4 有機(jī)酸形成次生孔隙模式

豐富的可溶物質(zhì)、活躍的烴源巖、有利的輸導(dǎo)體系以及良好的匹配關(guān)系是白云凹陷-番禺地隆起形成次生孔隙發(fā)育帶有利的地質(zhì)因素。次生孔隙的形成是一個(gè)系統(tǒng)的地質(zhì)過(guò)程,盆地?zé)崃?、?gòu)造演化、沉積作用、成巖作用、流體性質(zhì)及運(yùn)移等諸多方面均與次生孔隙度的形成和保存相關(guān)。在探討白云凹陷-番禺地隆起系統(tǒng)形成次生孔隙主控因素基礎(chǔ)上,結(jié)合本區(qū)各種地質(zhì)要素,可以歸納出有機(jī)酸形成次生孔隙模式(圖9)。研究區(qū)4套烴源巖生成有機(jī)酸沿?cái)鄬?、砂體和古構(gòu)造脊形成的輸導(dǎo)體系運(yùn)移至儲(chǔ)層進(jìn)行溶蝕形成次生孔隙發(fā)育帶,且在此過(guò)程中烴源巖成熟排烴時(shí)間與斷層形成時(shí)間要匹配。只有這樣有機(jī)酸才有可能從烴源巖中排出進(jìn)入到溶蝕層。即“生、運(yùn)、匹、溶”模式。

圖8 四期有機(jī)流體包裹體平面分布特征指示的流體運(yùn)移方向(據(jù)陳紅漢等,2004,修改)Fig.8 Planar distribution of fluid inclusion in four times showing the pathway of fluid migration (modified from Chen Honghan,et al,2004)

圖9 有機(jī)酸形成次生孔隙模式Fig.9 Formingmodel of secondary porosity caused by organic acid

5 結(jié)論

(1)研究區(qū)第三系碎屑巖儲(chǔ)層內(nèi)含有兩個(gè)次生孔隙發(fā)育帶即:2 750～3 500 m和4 100～4 600 m。發(fā)育帶內(nèi)含有大量溶蝕孔,以粒間溶孔+粒內(nèi)溶孔+粒間孔組合為主。

(2)研究區(qū)碎屑巖儲(chǔ)層中砂巖成分和結(jié)構(gòu)成熟度都較高,利于有機(jī)酸的進(jìn)入。且儲(chǔ)層中含有大量可溶蝕組分:長(zhǎng)石顆粒,含長(zhǎng)石的火山巖屑,早期碳酸鹽膠結(jié)物和生物化石等具備形成次生孔隙的物質(zhì)基礎(chǔ)。白云凹陷4套烴源巖有機(jī)質(zhì)豐度、類(lèi)型和成熟度特征均有利于連續(xù)產(chǎn)酸,為形成大量次生孔隙提供了豐富的酸性流體介質(zhì)。白云凹陷內(nèi)的烴源巖產(chǎn)生的酸性流體可沿北斜坡上的由斷層和扇三角洲、三角州和濁積扇砂體構(gòu)成的輸導(dǎo)體系從西南向西北運(yùn)移,進(jìn)入番禺低隆起后,主要沿構(gòu)造脊向北東方向運(yùn)移,并沿途溶蝕儲(chǔ)層形成次生孔隙發(fā)育帶。其有機(jī)酸形成次生孔隙模式可概括為“生、運(yùn)、匹、溶”。

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Form ing Mechanism of Secondary Porosity in Tertiary Reservoirs in Panyu Low Uplift and North Slope of Baiyun Sag

HAO Le-wei1,2WANG Qi1,2LIAO Peng1,2TANG Jun1,2ZHANG Gong-cheng3
(1.Key Laboratory of Petroleum Resources Research,Chinese Academy of Sciences,Lanzhou 730000; 2.Graduate University of Chinese Academy of Sciences,Beijing 100049; 3.Research Center,CNOOC,Beijing 100027)

As theworld's deep water oil and gas exploration continue to strengthen,the deep-water basin has increasingly become the new areas to gain growth in oil and gas reserves.The deep-sea oil and gas exploration has developed the inevitable challenges.Therefore,tomake related research about the deep exploration and development in particular characteristic of the reservoir is a necessary requirement to find deep water oil and gas and also the direction of ocean exploration.Pearl River Mouth Basin is amajor basin in China's offshore oil and gas basinswith low level of exploration and the lacking comprehensive research on the variation law and influencing factors of reservoir properties.Deep reservoir with high porosity has been increasingly developed the concern problem with oil exploration performing to deep zone by petroleum geologists.The formation mechanism and controlling factors of the secondary porosity are reasonably explained or not,which directly impacts on the establishment of oil and gas basin clastic reservoir quality predictionmodel and serious impacton the objective evaluation of the reservoir.In this paper,the constraints of the factors of the secondary porosity in the study area are discussed systemically by isotope geochemistry,organic geochemistry methods combined with sedimentology.The study area contains a large number of grains and grain dissolution pore,according to sandstone sheet,cast thin sections,scanning electron microscopy analysis.Based on reservoir property data,two secondary porosity zones can be found in the Tertiary reservoirs,Baiyun Sag,mainly in the 2 750～3 800m and 3 500～4 600 m.Mineral grains and carbonate cementare dissolved forming large of intergranular dissolution pores and dissolution pores.By isotope tracingmethod,it is found that the formation of secondary porosity is caused by the organic acid.Organic maturity and acidic water in which CO2is released by decarboxylation of organicmatter in the process of diagenesiswere themain reason.The feldspar particles,containing feldspar volcanic debris,early carbonate cements and fossils in the reservoir are dissolved by organic acid.The distribution of oxygen and carbon isotope of clastic rocks in the study area is that the value ofδ13C changes in between-11.8‰and-3.4‰andδ18O value changes in between-17.8‰and 2.4‰.f13C in carbonate cements is only from the depositional pore water,δ13C value can not be so low.So,organic acids from kerogen decarboxylationmust join in the process of the formation carbonate cements in the deep burial.Affecting the secondary porosity of the geological factors include the distribution and types of hydrocarbon source rocks,sandbodies distribution in the sedimentary facies and sandstone types,fracture resulting from tectonic activity.In Pearl River Mouth Basin,there are 4 sets of hydrocarbon source rocks including the Eocene Wenchang Formation,the Eocene-oligocene Enping Formation,upperthe Oligocene Zhuhai Formation and under the Miocene Zhujiang Formation.As the types of organicmatter aremainlyⅠandⅡin these 4 sets of hydrocarbon source rocks,the research area's hydrocarbon source rocks are suitable to produce the acid and have strong ability to produce the sour fluid.Wenchang source rocks in which the value of Ro is up to 3.0%has been inmature or even overmature.Enping source rocks has dnter themature stage,Ro=1.0%to 1.5%.he Rovalue of Zhuhai source rocks is0.43%～0.53%.hemain hydrocarbon source rocks enter the stage of producing sour fluid,which can provide themassive acidic fluid for dissolving the oil gas reservoir.The reservoir clastic rocks aremain the silicarenite and the debris silicarenite which have the high constitute and structurematurity.Therefore,it is advantageous that the organic acid enters the reservoir to form the secondary pores.The development of faults and sandstone layerwhich act as the bridge between source rocks and reservoir tomake a steady stream of organic acidsmigrating from the lower strata to shallow reservoirs in the Panyu lower uplift-Baiyun Sag North Slope.In addition,secondary porosity is also controlled by thematching hydrocarbon source rock maturity time with migration pathway system which includes faults,sand body and ancient structural ridge.When the organic acidsmigrate into the study area,there are the active faults and sandswhich can provide the long-distance lateral migration of oil and gas.Formation of secondary porosity is a systematically geological processes.Its formation and preservation are relevant tomany aspects such as basin heat flow,tectonic evolution,sedimentation,diagenesis,fluid nature,and fluid migration and so on.After comprehensively analyzing various geological factors,the model which describes the organic acids forming secondary porosity can be summed up as“production,transportation,matching,and dissolution?！監(jiān)nly the geological factorsmatchingwith each other can the secondary porosity be produced.These cognitions have good guidance for predicting high-quality reservoir in the studied area.

Baiyun Sag;secondary pores;organic acid;inclusion;migration pathway system;high-quality reservoir

郝樂(lè)偉 男 1985年出生 博士研究生 儲(chǔ)層沉積學(xué) E-mail:haolewei66@163.om

王琪 E-mail:qiwang@lzb.ac.cn

TE122.2+1

A

1000-0550(2011)04-0734-10

2010-04-10;收修改稿日期:2010-06-17