南海北部珠二坳陷超深水區(qū)斷層系統(tǒng)分析中的高精度相干技術(shù)

趙 釗,湯良杰,趙志剛,楊海長(zhǎng),楊東升,黃 萱

[1.中海油研究總院,北京 100028; 2.中國(guó)石油大學(xué)(北京) 盆地與油藏研究中心,北京 102249]

對(duì)于南海北部深水-超深水油氣地質(zhì)領(lǐng)域,前人在生烴特征、深水扇儲(chǔ)層、成藏動(dòng)力學(xué)與油氣勘探潛力等方面展開(kāi)過(guò)深入研究[7-16],而對(duì)于珠二坳陷超深水區(qū)域的斷裂體系與關(guān)鍵構(gòu)造期缺乏系統(tǒng)的研究。超深水區(qū)地貌地質(zhì)條件復(fù)雜,具有海水深度和坡度變化大等特點(diǎn),有強(qiáng)烈的多次波和鬼波干擾。這些特點(diǎn)導(dǎo)致地震資料中深層信噪比低、分辨率低、斷面不清、陸坡崎嶇海底區(qū)構(gòu)造畸變或不成像,構(gòu)造解釋存在多解性。由于超深水區(qū)鉆井稀少,斷裂解釋、結(jié)構(gòu)分析乃至含油氣性預(yù)測(cè)主要依靠地震資料,為降低勘探風(fēng)險(xiǎn),對(duì)構(gòu)造解釋的精度提出了更高的要求。此外，珠二坳陷超深水區(qū)內(nèi)復(fù)雜斷塊、熱流體底辟與多種沉積異常體均有發(fā)育,斷層的精細(xì)識(shí)別成為研究區(qū)構(gòu)造解釋的首要難點(diǎn),斷層解釋組合是否合理將直接影響最終構(gòu)造圖的精度,對(duì)有利圈閉的落實(shí)與勘探目標(biāo)含油氣性預(yù)測(cè)具有重要作用。近年來(lái)珠二坳陷超深水區(qū)在白云南洼與荔灣凹陷內(nèi)采集并處理了精度較高的三維地震資料。本文針對(duì)信息量豐富的三維地震體開(kāi)展以高精度沿層相干技術(shù)為代表的多敏感屬性測(cè)試,并應(yīng)用于斷層與異常體的精細(xì)表征,在此基礎(chǔ)上進(jìn)行珠二坳陷超深水區(qū)斷裂系統(tǒng)特征分析,提高了構(gòu)造解釋的精度與效率,指導(dǎo)了超深水區(qū)的油氣勘探評(píng)價(jià)。

圖1 珠江口盆地珠二坳陷構(gòu)造區(qū)劃Fig.1 Location of the ultra-deepwater area of ZhuⅡDepression,Pearl River Mouth Basin

圖2 珠江口盆地深水區(qū)地層劃分Fig.2 Stratigraphic column of deepwater area in the Pearl River Mouth Basin

1 高精度相干技術(shù)應(yīng)用

1.1 地震體相干性與預(yù)處理

斷層是由于巖層在構(gòu)造應(yīng)力的作用下發(fā)生變形,當(dāng)超過(guò)了其強(qiáng)度后發(fā)生斷裂而形成的。斷層在地震剖面上的表征為同相軸的錯(cuò)斷、不連續(xù)及突變,應(yīng)力使斷層兩側(cè)的巖層沿?cái)鄬用嫖灰?。相干技術(shù)是對(duì)地震道間的波形相似性進(jìn)行分析的一項(xiàng)地震資料解釋技術(shù),它采用水平或平面圖的方式顯示具不連續(xù)性特征的斷層和地層邊界[17-18]。相干性體現(xiàn)在規(guī)則層位面上沿層振幅變化不大,因此對(duì)應(yīng)的方差也小;而在斷層附近或巖性變化帶附近,振幅變化較大,振幅方差也相應(yīng)較大[19]。

南海北部超深水區(qū)由于地貌地質(zhì)條件復(fù)雜、水深和坡度變化大,地震數(shù)據(jù)體在經(jīng)過(guò)去噪和偏移等處理后仍然有噪音殘留,這會(huì)造成提取相干切片的切片精度和清晰度較低,對(duì)地震資料斷裂解釋造成困難。因此在進(jìn)行相干提取前,需要對(duì)三維數(shù)據(jù)體進(jìn)行構(gòu)造平滑預(yù)處理,以在壓制噪音干擾的基礎(chǔ)上保護(hù)構(gòu)造信息,使處理后的地震剖面的反射軸表面更平滑、更清晰,連續(xù)性更好。經(jīng)過(guò)構(gòu)造平滑預(yù)處理后沿層相干切片和相干體時(shí)間切片上大的斷裂并沒(méi)有被模糊,邊緣更清楚,原來(lái)比較模糊的一些小尺度的構(gòu)造細(xì)節(jié)(微小斷層)也有較好的體現(xiàn),能減少解釋的多解性并提高解釋的精度與效率。

1.2 高精度沿層相干技術(shù)

圖3 珠二坳陷超深水區(qū)地震剖面AA′示“穿時(shí)”現(xiàn)象Fig.3 Seismic profile AA′ showing diachronism phenomenon in the ultra-deepwater area of ZhuⅡDepression

圖4 珠二坳陷超深水區(qū)4 000 ms相干切片示兩套斷裂系統(tǒng)(a)與沿層相干切片(b)Fig.4 Two sets of fault systems showing in the 4 000 ms coherence-slice(a)& coherent slice along horizon(b)in the ultra-deepwater area of ZhuⅡDepression

南海超深水區(qū)地質(zhì)條件復(fù)雜,泥底辟、熱流體底辟與三角洲等多種沉積異常體均有發(fā)育,反映在相干切片中,不相干數(shù)據(jù)異常不一定都是斷層,也可能存在因?yàn)閹r性變化導(dǎo)致的地震識(shí)別陷阱與假象。圖5中相干切片平面特征上疑似為多條規(guī)模較大的斷層,而經(jīng)研究區(qū)地震剖面BB’與多屬性分析后認(rèn)為是下漸新統(tǒng)恩平組多期三角洲沉積,反映典型的三角洲前積結(jié)構(gòu)特征。所以在應(yīng)用相干體識(shí)別斷層過(guò)程中應(yīng)結(jié)合地震剖面與多種地球物理屬性進(jìn)行具體分析,規(guī)避異常體干擾。

2 基于高精度沿層相干技術(shù)的珠二坳陷超深水區(qū)斷層系統(tǒng)分析

2.1 基底斷裂特征

2.2 晚漸新世斷裂特征

2.3 早中新世斷裂特征

圖6 珠二坳陷白云超深水區(qū)沿層相干切片與構(gòu)造Fig.6 Coherent slice along horizon and structural map in the Baiyun ultra-deepwater area of ZhuⅡDepressiona.沿層相干切片反射層等t0構(gòu)造圖

圖7 珠二坳陷白云超深水區(qū)地震剖面CC′示斷裂發(fā)育特征 Fig.7 Seismic section CC′ showing fault characteristics in the Baiyun ultra-deepwater area of ZhuⅡDepression

圖8 珠二坳陷白云超深水區(qū)沿層相干切片與構(gòu)造Fig.8 Coherent slice along horizons and structural map in the Baiyun ultra-deepwater area of ZhuⅡDepressiona.沿層相干切片反射層等t0構(gòu)造圖

2.4 斷裂系統(tǒng)與斷-拗轉(zhuǎn)換

白云超深水區(qū)斷裂系統(tǒng)劃分為三級(jí)(表1),劃分依據(jù)是斷層的發(fā)育規(guī)模及對(duì)構(gòu)造與沉積控制作用的大小。從白云超深水區(qū)基底等時(shí)構(gòu)造圖上可見(jiàn)(圖9)，該區(qū)的構(gòu)造格局和主要斷裂的空間展布。其中發(fā)育1條一級(jí)控凹斷裂,該斷層位于白云主洼與白云南洼的交界地帶,與白云凹陷結(jié)構(gòu)走向一致為NE向。作為控制白云凹陷南部邊界的斷層,斷層發(fā)育早,延伸長(zhǎng),水平斷距大,活動(dòng)強(qiáng)度大,但活動(dòng)時(shí)間相對(duì)短,繼承性不強(qiáng)。從珠二坳陷斷-拗轉(zhuǎn)換期(早漸新世末)古地貌圖可見(jiàn),該NE向斷層對(duì)白云-荔灣凹陷恩平組沉積時(shí)期的連通性起到了重要的控制作用(圖10)。研究區(qū)的二級(jí)斷裂是一條位于白云凹陷南端的近EW向斷層,該斷層水平斷距大,延伸較長(zhǎng),控制了白云南洼的構(gòu)造形態(tài)。三級(jí)斷裂為NE向、NW與近EW向展布的斷層。三級(jí)斷裂數(shù)量雖多,對(duì)凹陷的構(gòu)造格局影響小,但對(duì)局部構(gòu)造圈閉的形成發(fā)育具有控制作用。除主控?cái)嗔?大部分NE向斷層在漸新統(tǒng)沉積前結(jié)束活動(dòng),個(gè)別繼續(xù)活動(dòng)的斷層在漸新世結(jié)束后活動(dòng)明顯減弱。

表1 珠二坳陷白云超深水區(qū)主要斷裂要素Table 1 Fault characteristics of the Baiyun ultra-deepwater area of ZhuⅡDepression

圖9 珠二坳陷白云超深水區(qū)主要斷裂系統(tǒng)Fig.9 Main fault system in the Baiyun ultra-deepwater area of ZhuⅡDepression

圖10 珠二坳陷斷-拗轉(zhuǎn)換期(早漸新世末)古地貌Fig.10 Palaeogeomorphology of rift to sag transition period in ZhuⅡDepression(Early Oligocene)F1. NE向一級(jí)斷層；F2. EW向二級(jí)斷層

3 結(jié)論

1) 在珠二坳陷超深水區(qū)的斷層研究中應(yīng)用高精度沿層相干技術(shù),有效提高了斷裂系統(tǒng)解釋與組合的精度,高精度沿層相干技術(shù)改善了超深水區(qū)內(nèi)復(fù)雜斷塊、陡傾地層與沉積異常體的影響,對(duì)超深水三維資料中特定地質(zhì)時(shí)期斷層的空間展布具有敏感表征。

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