瓊東南盆地北礁地區梅山組丘形反射特征

李俞鋒,蒲仁海,胡寶清,樊笑微,李斌,鄧東,王飛,郭瑞川

瓊東南盆地北礁地區梅山組丘形反射特征

李俞鋒1,2,蒲仁海1*,胡寶清2,樊笑微1,李斌1,鄧東1,王飛1,郭瑞川1

(1.西北大學 大陸動力學國家重點實驗室,陜西 西安710069;2.廣西師范學院 北部灣環境演變與資源利用教育部重點實驗室,廣西 南寧530001)

目前瓊東南盆地北礁凹陷中中新統梅山組頂部丘形反射引起廣泛關注,但對其成因有不同認識。本文通過高精度二維、三維地震、鉆井資料,研究丘形反射的特征。研究表明北礁地區梅山組頂部發育近東西向展布的長條形丘體,丘間為水道,丘內為中-弱振幅的地震反射,與西南部強振幅水道砂巖形成鮮明的對比,波阻抗反演揭示丘內為低波阻抗,屬泥巖范疇。梅山組塑性丘內地層發生重力擴展,在其上覆的脆性地層(強振幅砂巖和弱振幅泥巖)發育多邊形斷層,反推出梅山組形成于深水環境,丘為泥丘,沉積環境分析也認為北礁凹陷中中新世為半深海沉積,梅山組的丘-谷分別對應上覆地層的谷-丘,認為是底流剝蝕/沉積成因。本文的研究對南海北部丘形反射的認識有重要意義,并可降低油氣探勘風險。

北礁凹陷;梅山組;丘形反射;多邊形斷層;底流;泥丘;半深海沉積環境

1 引言

南海北部發育有多種丘形反射類型:砂巖侵入丘、泥底辟泥火山、火成巖丘、生物礁、重力流與等深流共同成因丘、沉積物波、等深積丘等[1-2];因此,對丘形反射的認識尤為重要,影響著古地理環境分析和油氣鉆探成功率等,如珠江口盆地南部低凸起之上的部分丘形反射初始被認為是生物礁成因,但鉆井BY7-1-1卻發現為這種酷似生物礁的丘形反射實則為凝灰巖火山丘。

北礁地區發育大面積分布的丘形反射[3-5],其中北礁凹陷中部YL19-1-1井鉆遇的丘為泥丘,但在北礁凹陷東北部和南西部無鉆井區的丘成因一直爭議較大[6]。本文通過二維地震和約2 000 k m2的高精度三維地震資料,分析北礁地區梅山組頂部丘的振幅、波阻抗、斷層、沉積環境等特征,并初步分析丘的成因。

2 區域地質背景

南海北部是西太平洋最大的被動大陸邊緣海之一,瓊東南盆地位于南海西北部,東靠珠江口盆地、北臨海南島(圖1),是新生代斷陷含油氣盆地[7]。該盆地經歷了3個演化階段:始新世-晚漸新世裂陷期、早中新世-中中新世熱沉降期和晚中新世以來的加速熱沉降期[8](圖1)。瓊東南盆地主要分為東北-西南伸展的北部凹陷區、北部隆起區、中央坳陷區和南部隆起區4個一級構造單元[7](圖1),研究區北礁凹陷位于松南低凸起和北礁凸起之間的中央坳陷區內,其丘形(狀)反射區分布范圍介于中央水道和北礁凸起之間(圖1),且分布在中中新統梅山組上部,中新世以來盆地整體處于構造相對穩定的坳陷期,現今深水區約占整個盆地2/3,在南部隆起上局部發育生物礁[]。

圖1 瓊東南盆地構造單元及工區位置圖Fig.1 The tectonic elements and the location of the 3Dsurvey in the Qiongdongnan Basin

3 三維區地震屬性特征

南海北部瓊東南盆地中新世以來構造穩定,構造斷裂基本不發育[10],中中新統梅山組頂面構造呈現出東南高西北低的構造格局,頂面具有近東西向長條形排列彼此相鄰的丘和水道,延伸長度10～20 k m(圖2a),這種丘和水道被認為是底流成因[3-5]。梅山組上段均方根屬性顯示中-低振幅且有0.2 k m左右的強振幅水道沉積(圖2b),黃流組底部均方根屬性僅在三維區南西部顯示近東西向的長條形強振幅,其他三維區顯示弱振幅(圖2c)。

4 丘的波阻抗特征

通過測井(BD20-1-1)約束下的二維地震反演,顯示丘內部波阻抗值為5.0×106～6.5×106kg/m3·m/s(圖3),而含油氣生物礁的波阻抗為8×106～12×106kg/m3·m/s,致密灰巖波阻抗更高超過10×106kg/m3·m/s,砂巖與灰巖的波阻抗界限在6×106～8×106kg/m3·m/s之間[5,11],說明研究區的丘形波阻抗屬于砂泥巖的范疇,同時YL19-1-1等鉆井揭示該丘巖性為泥巖和鈣質泥巖[4-5],這些似乎也都說明丘為泥丘。

5 北礁凹陷中新統多邊形斷層特征

目前各沉積盆地發現的多邊形斷層主要集中發育在被動大陸邊緣盆地的細粒沉積物(如泥巖)中[12];通過多邊形斷層推測中央坳陷區的華光凹陷梅山組古地理環境為深水饑餓沉積環境[13],同時有學者認為瓊東南盆地多邊形斷層多發育于深水盆地泥巖中[14]。處于中央坳陷區的華光凹陷、北礁凸起、長昌凹陷中新統地層中均發育多邊形斷層[15]。北礁凹陷處于上述凹陷的過渡區,前人研究也預測了該處中新統發育多邊形斷層[16]。黏土礦物中的蒙脫石含量在黃流組比梅山組低[17],其中在黏土礦物中蒙脫石含水性最好,比表面最大,具有蒙脫石含量越高,塑性指數越大,塑性越強的特點[18-19],這似乎意味著梅山組泥巖塑性很可能比黃流組泥巖塑性強,即梅山組比黃流組更易發生滑動變形。

圖2 瓊東南盆地北礁三維區地震屬性圖(平面位置見圖1)Fig.2 The various seisi mic attribute in 3Dsurvey in the Beijiao Sag of Qiongdongnan Basin(location is shown in Fig.1)a.北礁三維區梅山組頂面時間域構造圖;b.梅山組上段(丘內)均方根屬性圖;c.黃流組底部均方根屬性圖a.The ti me structure map of the top of the Meishan For mation in Beijiao Sag survey;b.the RMSattribute map of the upper Meishan For mation in Beijiao Sag survey;c.the RMSattribute map of t he bottom Huangliu For mation in Beijiao Sag survey

圖3 瓊東南盆地丘和水道二維測線波阻抗反演(剖面位置見圖1)Fig.3 The 2D wavei mpedance section for the remnant mound and channel area(location is shown in Fig.1)

一般認為多邊形斷層發育在細粒沉積物中,但近年來發現在深水砂巖中也發育多邊形斷層[12],深水砂體多邊形斷層被認為是其下伏塑性沉積物(泥巖)在重力擴展作用中形成的[20]。Victor和Moretti[20]在模擬深水水道砂巖多邊形斷層的實驗中觀察到在重力擴展作用下多邊形斷層在丘形堤岸上方地層中集中發育(形成斷層密集帶)和在水道上方發育橫截斷層。吳時國等[14]對重力擴展形成的多邊形斷層進行了詳細的描述,重力擴展初始階段,先形成垂直水道的橫截斷層,隨著重力擴展的繼續進行,在丘形堤岸上方發育平行水道走向的斷層密集帶[14,20],這似乎提供了可通過水道砂巖中的多邊形斷層反推水道下伏為深水沉積環境和塑性泥巖地層的方法。

5.1 丘與多邊形斷層特征

黃流組底部地層在丘頂上方發育多邊形斷層密集帶(圖4,圖5a、b),其中三維工區東北部丘間弱振幅水道泥巖充填(圖4a、b),三維南西區丘間強振幅砂巖充填(圖4c、d)。無論是泥巖還是砂巖充填水道,都有丘規模越大,多邊形斷層密集帶越發育的特點,即圖4b1、d1分別比圖4a1、c1多邊形斷層發育,延伸長度較長。斷層密集帶的破碎地層,易形成雜亂反射,其平面的相干屬性顯示(灰黑色)低相干(圖5a、b)。

圖4 瓊東南盆地梅山組丘頂上部斷層密集帶發育特征圖(剖面位置見圖2)Fig.4 The characteristics of the fault concentrated belts above the mound top of Meishan For mation(location is shown in Fig.2)a、b和c、d分別為工區東北部(丘間弱振幅泥巖區)和西南部(丘間強振幅砂巖區)的原始地震剖面,a1、b1和c1、d1對應的多邊形斷層密集帶解釋剖面Unexplained seismic sections a,b and c,d,arein the northeast survey(weak amplitude mudstone area bet ween the mounds)and the southwestern survey(strong amplitude sandstone zone bet ween the mounds),respectively,and theinterpretation section of the polygonal fault concentrated belts corresponding to a1,b1 and c1,d1

5.2 丘-水道與多邊形斷層平面特征

多邊形斷層在受到構造應力、刺穿鹽丘、斜坡、水道、泥底辟等的影響下,不一定呈現出多邊形形態[21],多邊形斷層在伴有水道發育的地層中,先發育垂直水道的斷層形成橫截多邊形斷層,后在丘形堤岸上方發育多邊形斷層密集帶[20]。

北礁三維西南區強振幅水道砂巖連續性好(圖4c、d),其水道顯示高相干(灰白色),呈長條形近西向展布,高相干水道上方發育橫截水道斷層(圖5a),丘頂上方發育斷層密集帶(圖4c、d,圖5c),其平面顯示低相干(灰黑色)(圖5a),呈長條形近東西向展布。在北礁三維東北區也有相同的特點(圖5b、d),只有在塑性較大的地層發生重力擴展作用時,其上覆地層才能形成多邊形斷層密集帶和橫截水道斷層[20],這些特征似乎也說明下伏梅山組丘巖性為塑性較大的深水泥巖。

圖5 瓊東南盆地北礁凹陷多邊形斷層發育特征圖(平面位置見圖2a)Fig.5 The characteristics of the polygonal faults in Beijiao Sag of Qiongdongnan Basin(location is shown in Fig.2a)

6 丘形反射巖性特征及中中新世沉積環境分析

鉆井揭示位于北部隆起的YC21-1-1井中新統以濱淺海砂巖沉積為主,位于南部隆起的XC1井中新統為碳酸鹽巖沉積,發育生物礁,屬于碳酸鹽巖的淺水臺地沉積環境,而位于中央坳陷的LS33-1-1、LS22-1-1、YL19-1-1井中新統以泥巖為主,屬于半深海相(圖6)。瓊東南盆地通過淺水區和深水區20口井,結合二維地震井網密度4 k m×2 k m,分析其北礁凹陷中中新世沉積厚度較大(圖7a),約150～300 m,梅山組頂部丘形體形成環境應屬于半深海的沉積環境(圖7b)。

7 中中新統丘-谷地震反射分別對應上中新統谷-丘地震反射特征

一些學者在研究愛爾蘭西南部Porcupine盆地底流丘形沉積物時,發現底流切割成因的丘-谷底形,分別對應上覆底流沉積成因的谷-丘形態,即丘對應上覆地層的谷,谷對應上覆地層的丘(圖8),且這類地震反射通過鉆井證實為底流成因[22],同時這類地震反射在南海北部神狐附近也有發現[23],也被解釋為底流成因。

圖6 瓊東南盆地中中新統沉積相對比(位置見圖7b)Fig.6 The comparative section of sedi mentary facies for connecting wells of the Mid-Miocene For mation in the Qiongdongnan Basin(location is shown in Fig.7b)

圖7 瓊東南盆地梅山組厚度(a)及沉積相(b)Fig.7 The isopachous(a)and sedi mentary facies(b)of the Meishan For mation in the Qiongdongnan Basin

北礁凹陷中中新統頂部丘在平面上呈長條形近東西向展布(圖2a),與Stuart和Huuse[24]研究北海底流切割成因的丘形反射的平面特征相似,且南海北部中新統已經發現底流切割成因的長條形丘,即底流底蝕下伏地層形成底流水道,水道間呈丘[25],推測南海北部最早底流侵蝕切割可追溯至早中新世晚期[25]。南海西北部穩定的底流沉積或侵蝕作用可追溯至晚中新世早期(11.5 Ma BP)[26],北礁凹陷中中新統丘狀反射有明顯的削截現象(圖8a、b),晚中新世早期底流活動加劇,底蝕下伏地層,形成底流水道(谷),水道間為殘留地層——(殘)丘,之后由于底流活動減弱,在谷內沉積受底形限制的丘形漂積體(圖8a)。底流沉積常受底形的控制,底形的“凹凸不平”也常引起上覆地層底流沉積物的“凹凸不平”,在谷內形成局限丘形漂積體或充填丘形漂積體,其地震反射特征呈丘狀反射[22,27]。梅山組底流水道流速減弱,沉積速率大于剝蝕速率,形成充填型丘形漂積體(圖8a),繼承性發育至今,而梅山組丘頂之上,流速較快,相對底流水道,沉積速率小,其上最終形成底流水道(谷),梅山組上覆的底流水道有繼承性的發育,有遷移剝蝕現象(圖8b),部分水道中發育強振幅濁流沉積。這樣就形成了中中新統梅山組的谷-丘地形分別對應晚中新統黃流組的丘-谷地形,是底流前期剝蝕后期沉積的繼承性的結果,在研究南海北部底流沉積/剝蝕應引起足夠的重視。梅山組丘X(圖9a)對應黃流組谷Y(圖9b),水道的遷移現象使得梅山組底流水道(谷)對應黃流組丘有一定的偏移(圖9c)。這些特征似乎也說明了梅山組頂部丘更大的可能為底流相關成因。

圖8 北礁凹陷底流底蝕和沉積相關的谷-丘互相對稱形態特征(a)及局部解釋(b)(剖面位置見圖9a)Fig.8 The characteristics of the mound-trough morphology originated fromthe incision and deposition of the botto m current in the Beijiao Sag of Qiongdongnan Basin(a)and localized section explained(b)(location is shown in Fig.9a)

圖9 瓊東南盆地北礁凹陷三維區的(梅山組頂部)層位T40(a)和(黃流組頂部)層位T30(b)的長條形丘相關谷-丘互相對稱關系(c)平面圖Fig.9 The characteristics of the ridge-trough morphology(c)about elongated small hu mmocks on(the top of Meishan For mation)reflection T40(a)and(the top of Huangliu For mation)reflection T30(b)in the Beijiao Sag of Qiongdongnan Basin

8 丘形體特征討論

北礁地區丘內為中-弱振幅地震反射(圖2b),與西南部強振幅水道砂巖和北東部弱振幅水道泥巖形成鮮明的對比(圖4),同時波阻抗反演顯示丘內為低波阻抗,與強振幅高阻抗的生物礁、火山丘形成鮮明的對比。北礁地區長條形丘平面延伸10～20 k m,YL19-1-1井鉆遇的丘及其附近的丘平面延伸長度較長(圖2a),部分可延伸至西南區和東北區,這些丘與YL19-1-1鉆遇的丘應該屬于同一種成因,即泥質成因。梅山組發育丘地層的重力擴展形成上覆地層的多邊形斷層密集帶(縱向斷層)和橫截水道斷層(橫向斷層)(圖5),也說明了丘內為塑性較大地層,這也進一步增大了丘內為泥巖的可能性。對瓊東南盆地中中新世沉積環境分析也認為北礁凹陷為半深海沉積(圖6,圖7),這也進一步認為丘內地層形成于深水環境中。梅山組頂部的丘-谷底形對應上覆地層的谷-丘地形,并伴有谷(水道)的遷移和濁流沉積(圖8),南海北部在早中新統晚期已發現活躍的底流[26],活躍的底流能塑造海底地形,對于該區谷-丘地形,作者認為是底流前期底蝕切割,后期沉積而形成,這樣的解釋也較為合理。然而工區臨近生物礁發育的西沙地區,有學者把該區丘解釋為生物礁也有理論依據,無論該區丘被解釋為泥質丘還是生物礁都是一種推測,還需要后期鉆井進一步證實。對于本文利用多邊形斷層識別丘的(形成于深水)沉積環境和巖性(由泥巖組成),同時梅山組的丘-谷底形分別對應上覆地層的谷-丘地形這類地質現象,被解釋為底流成因,希望能引起學者的關注。

9 結論

(1)北礁凹陷梅山組頂部丘近東西向長條形展布,近平行排列,延伸10～20 k m,丘間為水道,丘內為中-弱振幅的地震反射,與東北部弱振幅水道泥巖和西南部強振幅水道砂巖形成鮮明的對比。

(2)通過波阻抗反演揭示,丘內為低波阻抗,與高波阻抗火山丘和生物礁形成鮮明的對比,該丘應屬于泥巖的范疇。

(3)梅山組塑性地層發生重力擴展,在其上覆(黃流組)相對脆性地層(北東部弱振幅水道泥巖和南西部強振幅水道砂巖)發育多邊形斷層,即堤岸上方斷層密集帶和水道上方橫截水道斷層,推測丘內為塑性較大的泥丘,形成于深水環境。

(4)瓊東南盆地的鉆井資料和中中新統沉積厚度分析認為,北礁凹陷中中新世為半深海沉積,丘為泥丘。

(5)梅山組頂部丘-谷分別對應上覆黃流組谷-丘,認為是底流剝蝕/沉積的結果,梅山組丘為底流剝蝕殘留地層—(殘)丘。

致謝:在此衷心感謝三位審稿專家和編輯提出的寶貴的修改意見!

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The characteristics of mounded seisi mic reflections on the top of the Meishan For mation in the Beijiao Sag of Qiongdongnan Basin

Li Yufeng1,2,Pu Renhai1,Hu Baoqing2,Fan Xiaowei1,Li Bin1,Deng Dong1,Wang Fei1,Guo Ruichuan1

(1.State Key Laborator y of Continental Dyna mics,Northwest University,Xi’an 710069,China;2.Key Laborator y of Environment Change and Resources Use in Beibu Gul f,Ministr y of Education,Guangxi Teachers Education University,Nanning 530001,China)

Nowadays,a lot of mounded seis mic reflections in the Mid-Miocene Meishan For mation were found and attracted the attention of geologists,while the genesises of the mound were various.Co mbined with the 3D high resolution seis mic data and the 2Dseis mic datu m and drilling,this paper ai med at analyzing the characteristics of the mounds.The results show that co mpared with the weak amplitude channel filled with mudstone in northeast 3D area and the high amplitude channel filled with sandstone in southwest 3D area,the elongated mounds with nearly EWorientaion had mediate amplitude.The channel located bet ween mounds.Wavei mpedanceinversion unravelled that the mounds with inner lower i mpedance should belong to the scope of mudstone.The inner mound bed with ductile within Meishan For mation were subjected to gravitational spreading,which contributed to developed polygonal faults on the overlying strata with brittle property.Under this circu mstance,Meishan For mation was concluded to for min deep water depositional environ ment in Mid-Miocene and mounds on the top of Meishan For mation consisted of mudstone.Mounds and channels for med the sag-crest bedfor mcorresponded with channels and mounds above the overlying strata,respectively,which was regarded as the result of botto mcurrent of incision and deposition.This study about the Mid-Miocene mound reflections has guiding significance and can i mprove the success rate of oil and gas exploration drilling and can reduce oil and gas exploration risk.

Beijiao Sag;Meishan For mation;mound reflection;polygonal faults;bottom current;mud mound;bathyal depositional environ ment

P736.22+4

A

0253-4193(2017)12-0053-10

李俞鋒,蒲仁海,胡寶清,等.瓊東南盆地北礁地區梅山組丘形反射特征[J].海洋學報,2017,39(12):53-62,

10.3969/j.issn.0253-4193.2017.12.006

Li Yufeng,Pu Renhai,Hu Baoqing,et al.The characteristics of mounded seisimic reflections on the top of the Meishan For mation in the Beijiao Sag of Qiongdongnan Basin[J].Haiyang Xuebao,2017,39(12):53-62,doi:10.3969/j.issn.0253-4193.2017.12.006

2017-02-14;

2017-06-07。

國家科技重大專項(2011ZX05025-006-02);國家自然科學基金(41390451);國家自然基金地區項目——北部灣海陸過渡帶生態環境演化機理及其情景模擬研究(41361022);廣西創新研究團隊項目——北部灣海陸交互關鍵帶與陸海統籌發展研究(2016JJF15001)。

李俞鋒(1986—),男,四川省南充市人,博士研究生,主要從事海洋地質及其油氣效應研究。E-mail:526376337@qq.co m

*通信作者:蒲仁海(1962—),男,教授,油氣地質與地球物理專業。E-mail:purenhai@126.com