欒國強，董春梅,2，林承焰,2，任麗華,2，焦紅巖，趙海燕，彭先國[.中國石油大學(華東) 地球科學與技術(shù)學院，山東 青島 266580； 2.山東省油藏重點實驗室，山東 青島 266580；.中國石化 勝利油田有限公司 現(xiàn)河采油廠，山東 東營 257068]

1 異重流研究歷史

廣義的異重流指兩種密度相差不大、可以相混的流體，在條件適宜時因密度差異而產(chǎn)生的相對運動。在運動過程中不發(fā)生全局性紊動、摻混。異重流分為雙層體系和多層體系，自然界中異重流現(xiàn)象非常廣泛，既包括水體中的分層潛流，也包括大氣中的冷暖空氣所形成的鋒面。但在水利工程和地質(zhì)學領(lǐng)域，異重流是指河流攜帶大量沉積物直接潛入蓄水盆地底部并繼續(xù)流動的高密度流體[1-3]。

1885年Forel首次報道了羅恩河注入日內(nèi)瓦湖時發(fā)生的異重流現(xiàn)象[4]，20世紀30年代以后，異重流研究才真正受到重視。1935年美國科羅拉多河胡佛壩落成蓄水，當年3月份，上游發(fā)生洪水，河流攜大量泥沙在胡佛大壩前沉入水底，不久后渾濁的泥水從大壩泄水孔流出，在此過程中水庫表面始終澄清[5]。這一現(xiàn)象讓人們意識到異重流可以攜帶大量泥沙沿水庫底部長距離搬運而不與蓄水體相混，這對于降低水庫淤積，延長水庫壽命具有重要意義。1953年Bates首次提出異重流的概念，但在地質(zhì)領(lǐng)域，對于海洋環(huán)境中是否存在異重流及其對海相深水沉積的影響卻仍然存在巨大爭議。直到20世紀90年代，相關(guān)研究表明河流入海形成異重流的現(xiàn)象具有普遍性[2,6]，異重流作為一種深水沉積物輸送機制逐漸受到地質(zhì)學家的重視[7-42](表1；圖1)。

2 異重流形成條件與影響因素

2.1 形成條件

河流與蓄水體的密度差異是異重流發(fā)生的直接原因[2,6,24,43-48]。河流所攜沉積物量、水體鹽度及溫度等都是影響河水密度的重要因素[49]。河流入海形成異重流的沉積物臨界濃度為36～44 kg/m3，臨界濃度與海水溫度和含鹽度密切相關(guān)，受氣候背景和所處緯度控制[2](表2)。洪峰到達前河流對蓄水體的稀釋作用[50]會降低河口處蓄水體密度，有利于異重流的發(fā)生。嚴格來講，異重流形成的臨界濃度是蓄水體密度(河口周圍)、流量、水深和坡降的函數(shù)，河口處坡度大、水體深，形成異重流的沉積物臨界濃度也會相應(yīng)降低。在陸相淡水湖盆中，異重流形成的沉積物臨界濃度非常低(遠低于1 kg/m3)[2,51]。如果河水與湖水溫度和鹽度相差不大，河水所攜帶沉積物濃度只要略大于千分之一，入湖后便可以形成異重流[52]。通常來說，洪泛期的中小型山區(qū)河流更易形成異重流，而大型河流由于流速低，流量大，入海過程中沉積物易發(fā)生稀釋和卸載，難以形成異重流[6]。Mulder等調(diào)研了全球147條河流的異重流發(fā)育情況，發(fā)現(xiàn)71%的河流會發(fā)育不同頻次的異重流[2]，而實際上異重流的發(fā)生可能更加普遍。

不穩(wěn)定的異輕流可以向異重流轉(zhuǎn)化，這是一種特殊而普遍的異重流形成機制[25,32,53]。溫暖的富含細粒沉積物的河水在河口形成異輕流，在沉積物自身擴散和水體擾動(波浪、潮汐等作用)影響下，異輕流與蓄水體界面之間的平衡被破壞，發(fā)生指對流現(xiàn)象(finger convection)，富沉積物流體通過指對流到達盆地底部，形成異重流[53](圖2)。通過指對流機制產(chǎn)生的異重流，其主要沉積物為富含陸源有機質(zhì)的粘土[25]。如果異輕流長時間保持穩(wěn)定，所含細粒沉積物會發(fā)生絮凝作用，以海洋雪的方式沉降[54]。

2.2 主要影響因素

異重流的形成受構(gòu)造和氣候影響，與源匯系統(tǒng)密切相關(guān)。

2.2.1 源區(qū)特征

一定的沉積物濃度是異重流形成的重要條件[48,55]，如果源區(qū)能夠提供大量的碎屑物質(zhì)，則有利于異重流的形成。源區(qū)構(gòu)造演化與地形起伏是碎屑沉積物形成的基礎(chǔ)，母巖固結(jié)程度低，構(gòu)造高差大，易發(fā)生侵蝕搬運作用。氣候是沉積物形成的重要條件，一般認為半干旱氣候下，物源區(qū)植被稀少，容易發(fā)生水土流失[56]，同時伴隨周期性洪水的發(fā)生，有利于河流攜帶大量懸浮物質(zhì)形成異重流[48,57-58]。

2.2.2 河流流域

河流是源匯系統(tǒng)內(nèi)沉積物輸送的重要通道[59]，河流流域特征決定了河流類型、流量及沉積物輸送能力[6,60]。河流從物源區(qū)攜帶的沉積物只有少部分能直接到達沉積盆地，其余大部分會在搬運路途中發(fā)生暫時性沉積，形成心灘、邊灘、決口扇、山麓沖積扇及沖積平原等[61]，大量暫時性沉積的發(fā)生會緩沖河流向盆地搬運沉積物的效率。一般來說，流域面積越大，地形高差越低，河流輸送能力越差[6,62-65]。流域氣候條件也會對河流輸送能力產(chǎn)生重要影響，蒸發(fā)量大，導致河流流量降低，沉積物在河流區(qū)域大量攜載，不利于異重流發(fā)生。因此，相對于大型河流，中小型的山間河流因為流域面積小，高程大，蒸發(fā)量低，易于形成異重流[6]。

2.2.3 匯水盆地

蓄水體鹽度、水深、溫度以及河口坡降等都是影響河流潛入的重要因素，盆地水體鹽度低、溫暖、有一定水深和較大的河口坡降，會降低異重流形成的沉積物臨界濃度。異重流發(fā)生還與海/湖平面高低有密切關(guān)系[26,48,66-73]，在狹窄大陸架條件下，低水位體系域時，河流可直接進入大陸坡峽谷，有利于異重流的形成；而在寬闊大陸架地帶，低水位體系域時，河流流域面積增大，中小型河流發(fā)生合并，形成大河，更利于三角洲和滑塌型重力流的發(fā)生。

表1 異重流沉積研究實例Table 1 Case study of hyperpycnal flow deposition

圖1 異重流沉積研究實例全球分布示意圖Fig.1 Global distribution of case study of hyperpycnal flow deposition

表2 不同氣候環(huán)境下海水溫度、鹽度、密度及對應(yīng)的河流沉積物臨界濃度(引自文獻[2])Table 2 Temperature,salinity and density of seawater in different climates,and the correspondingcritical concentration of river sediments(cited from reference [2])

3 異重流演化過程及沉積特征

3.1 演化過程

在洪水到達之前，由于河水中所攜帶沉積物濃度低，河水密度低于蓄水體密度，在河口處粗碎屑物質(zhì)卸載，形成三角洲河口沙壩，河水緩慢向前漂浮在蓄水體表面，形成異輕流(圖3a)。隨著洪水發(fā)育，河流攜帶沉積物濃度越來越大，當河水濃度與蓄水體濃度相等時，河水與海水整體混合，形成等密度流(圖3b)。在洪水作用下，河流流速加快，所攜帶沉積物濃度增加，河水密度大于蓄水體密度，達到潛入臨界條件，河流直接潛入水底，形成向盆地深處運移的異重流(圖3c)。異重流的發(fā)生會帶動周圍水體的運動，形成局部水循環(huán)，倒吸先前漂浮在水面的異輕流，大量水體表面漂浮物在潛入點處聚集，形成分明的清渾交界面(圖4a)。

圖2 尼羅河河口異輕流-異重流轉(zhuǎn)化作用(據(jù)文獻[25]修改)Fig.2 Conversion from hypopycnal flow to hyperpycnal flow at the estuary of Nile River(modified from reference [25])

在海洋和鹽湖環(huán)境中，如果異重流頭部速度過低，其所攜帶沉積物的濃度小，局部密度低于周圍水體密度，會發(fā)生浮力反轉(zhuǎn)，形成上浮相(圖3，圖4)，上浮相開始產(chǎn)生的位置為上浮點，上浮點位置隨異重流頭部運動發(fā)生遷移，在異重流增強期，上浮點隨異重流向盆運動，在異重流衰退期，上浮點隨異重流向岸運動(圖3c—f)。上浮點的運移方式有連續(xù)運移和幕式運移，這取決于異重流能量演化特征。上浮相與異重流所攜帶的沉積物類型和流速有密切關(guān)系，以粘土為主的異重流不易產(chǎn)生上浮相[74]。異重流衰退末期完全上浮，回歸等密度流和異輕流(圖3g，h)。

3.2 主要巖相類型

異重流存在3種沉積物搬運方式——底床載荷、懸浮載荷和上浮載荷(圖5)。其中底床載荷沉積物主要以滾動和跳躍方式搬運；懸浮載荷是異重流主要沉積物運載方式，沉積物以懸浮狀態(tài)發(fā)生搬運；上浮載荷是指異重流末端沉積物大量卸載，異重流密度低于環(huán)境水體密度后，沉積物發(fā)生上浮擴散的一種搬運方式，主要發(fā)生在海相和鹽湖環(huán)境中。

異重流沉積的巖相類型與沉積物負載方式密切相關(guān)[75-77]，底床載荷主要沉積細礫巖與礫質(zhì)砂巖，常見的沉積構(gòu)造有塊狀層理、疊瓦狀構(gòu)造、低角度交錯層理、平行層理等。懸浮載荷主要沉積細砂巖、粉砂巖，典型的沉積構(gòu)造有平行層理、爬升沙紋層理、波狀層理等。沉積物重新聚集和上浮部分主要沉積水平層理泥巖和粉砂質(zhì)泥巖，層面富含有機質(zhì)和異地搬運的植物碎屑[78](圖5)。異重流由近源到遠源過程中，能量虧損，流速降低，底床載荷、懸浮載荷和上浮載荷依次卸載形成特有的沉積序列。

3.3 沉積序列

單期異重巖沉積過程與異重流能量演化過程密切相關(guān)，因此異重巖垂向沉積序列記錄了異重流能量增強和減弱兩個演化階段[24](圖3，圖6，圖7)。在洪水發(fā)育初期，異重流能量不斷增強，沉積物具有反序特征，并依次發(fā)育爬升波紋層理、平行層理和低角度交錯層理等沉積構(gòu)造；洪峰過后，隨著洪水能量衰減，異重流所攜帶沉積物由粗到細依次卸載，沉積物具有正序特征，沉積構(gòu)造與異重流能量增強階段相反。這種反映洪水能量增強的反序沉積單元(Ha)和反映洪水能量減弱的正序沉積單元(Hb)所組成的二元結(jié)構(gòu)是異重巖典型沉積序列(圖6)[9,24,33,74,79-80]。異重流除了經(jīng)歷增強和減弱兩個大的變化階段外，同一位置異重流能量會有多次波動(圖6)，這一特點體現(xiàn)為異重流流動過程的脈動性。

洪峰活動期間，如果異重流能量未達到對下伏沉積物造成侵蝕的臨界條件，Ha與Hb之間連續(xù)接觸，沉積物最大粒度代表的接觸界面響應(yīng)于洪峰(圖6Ⅲ，圖7Ⅲ)；如果洪峰期間異重流能量超過對下伏沉積物造成侵蝕的臨界條件，異重流能量增強階段形成的沉積物會被部分侵蝕，洪峰過后隨異重流能量減弱，沉積物繼續(xù)在侵蝕面之上沉積，Ha與Hb侵蝕接觸(圖6Ⅱ，圖7Ⅱ)；當洪峰能量足夠大，持續(xù)時間足夠長，異重流能量增強階段形成的反序單元(Ha)在洪峰期會被完全侵蝕，洪峰過后隨異重流能量降低，沉積物卸載，僅保存正序單元(Hb)(圖6Ⅰ，圖7Ⅰ)。實際上由于河流注入量和所攜帶沉積物濃度的變化，異重流存在多個能量波動過程[41]，導致單期異重巖沉積序列更加復(fù)雜。異重流不僅有時間上的波動性，還有空間上的遲滯性。離河口越遠，異重流速度越低，一期洪峰總是先到達近端，一段時間之后才能傳播到遠端(圖6)，導致同一能量波動在不同位置異重巖中的時間響應(yīng)不同，同一期異重流在不同位置形成的沉積序列不同(圖7)。

圖3 異重流演化過程[74] Fig.3 Evolution of hyperpycnal flow[74] a.洪水初期，異輕流發(fā)育；b.隨洪水能量增強，河流攜帶沉積物濃度達到臨界濃度，等密度流形成；c.隨洪水能量增強，異重流形成并加速向深水流動；d.洪峰到達，異重流能量達到最強；e.洪峰之后，異重流能量開始衰減；f.異重流能量進一步衰減；g.異重流衰減為等密度流；h.洪水過后，等密度流恢復(fù)為異輕流

圖4 紅海阿卡巴灣異重流[28]Fig.4 Hyperpycnal flows in the Gulf of Aqaba,Red Sea[28]a.異重潛入點處清渾交界；b,c.異重流表面上浮相

圖5 異重巖主要巖相類型[74]Fig.5 Major lithofacies of hyperpycnites[74]

異重流近端洪峰期能量強，對下伏沉積物侵蝕強烈，以正序水道充填沉積為主；中部洪峰期異重流能量中等，形成侵蝕接觸的Ha-Hb二元結(jié)構(gòu)，侵蝕面附近富含泥礫。遠端洪峰期異重流能量低，形成連續(xù)接觸的Ha-Hb二元結(jié)構(gòu)(圖6，圖7)。

圖6 異重流沉積演化過程(據(jù)文獻[9]修改)Fig.6 Depositional evolution of hyperpycnal flows (modified from reference [9])

圖7 異重流沉積序列(圖中Ⅰ,Ⅱ,Ⅲ位置與圖6中Ⅰ,Ⅱ,Ⅲ位置相對應(yīng))Fig.7 Sedimentary sequence of hyperpycnal flows (location Ⅰ，Ⅱand Ⅲ are also the location Ⅰ，Ⅱand Ⅲ in Fig.6)

3.4 沉積模式

河流攜帶沉積物直接入水所形成的異重流沉積與盆內(nèi)滑塌型濁流存在差異。本文在總結(jié)阿根廷內(nèi)烏肯盆地、美國阿巴拉契亞盆地、中國的松遼盆地和鄂爾多斯盆地等古代異重流沉積的基礎(chǔ)上，結(jié)合地中?，F(xiàn)代深水水道發(fā)育特征，建立了由溝道充填沉積-天然堤-溝道側(cè)緣沉積-前緣朵葉體組成的異重流沉積的理想模式(圖8)。異重流沉積近端，由于能量較強，侵蝕作用顯著，溝道陡深，由近端向遠端逐漸變寬緩，溝道底部充填底負載相的細礫巖和礫質(zhì)砂巖，向上依次為懸浮相細砂巖、粉砂巖和上浮相泥巖，具有顯著的正序特征。兩側(cè)天然堤主要由懸浮相的細砂巖、粉砂巖和上浮相泥質(zhì)粉砂巖、泥巖組成，由溝道向兩側(cè)粒度減小，泥質(zhì)含量增加，逐漸過渡到溝道側(cè)緣沉積。溝道側(cè)緣沉積主要由上浮相的泥質(zhì)粉砂巖、粉砂質(zhì)泥巖和泥巖組成，常見大量植物碎屑和紅色層理。在異重流沉積前緣末端，由于沒有溝道約束，沉積物大量溢出，形成以懸浮相和上浮相為主的前緣朵葉體，該部分沉積物與天然堤和側(cè)緣沉積相似，具有典型的Ha-Hb二元結(jié)構(gòu)。

4 討論

4.1 異重流與經(jīng)典濁流的區(qū)別

沉積物重力流包括盆內(nèi)滑塌型重力流和盆外洪水型異重流[81-82]，兩類重力流在成因機制、流體動力學過程及沉積特征等方面都存在差異性，進一步造成油氣儲層分布的不同[21,82]。同時，由于傳統(tǒng)的涌浪型濁流與異重流存在相似性，致使在以往研究中有大量異重流沉積被解釋為經(jīng)典濁流沉積[9]。因此認識傳統(tǒng)濁流沉積與異重流沉積的區(qū)別與聯(lián)系，對于建立和完善深水重力流沉積模式具有重要意義。

圖8 異重流沉積模式Fig.8 Depositional model of hyperpycnal flows

根據(jù)Ha-Hb二元結(jié)構(gòu)、內(nèi)部侵蝕突變界面、富含陸源植物碎屑等特征可以從巖心和露頭對異重流和經(jīng)典濁流沉積進行初步識別，但更多的識別標志仍需要進一步發(fā)掘，兩種不同沉積的地球物理差異性仍然有待研究(表3)。

在實際研究中，三角洲地帶往往存在三角洲-異重流-盆內(nèi)重力流綜合沉積作用(圖9)[83]。洪水期間異重流的發(fā)生可以作為觸發(fā)機制，造成斜坡失穩(wěn)，產(chǎn)生盆內(nèi)重力流，導致多種重力流事件同時發(fā)生[20,84]；進積型三角洲帶來的沉積物大量堆積，也可以為盆內(nèi)重力流提供物質(zhì)基礎(chǔ)，導致盆外和盆內(nèi)重力流交替發(fā)生。盆內(nèi)和盆外重力流的混合發(fā)育以及復(fù)雜的流體轉(zhuǎn)化過程，增加了重力流研究的挑戰(zhàn)性。

4.2 異重流與細粒沉積

異重流對深水細粒沉積有重要影響。細粒沉積物形成于弱水動力條件一直是沉積學界的共識，然而近年來沉積學家們通過大量的實際觀察和實驗研究逐漸認識到,在較強水動力條件下也可以沉積細粒物質(zhì)[85-89]。異重流是細粒沉積物重要的運載方式之一[48,74,81,90]，異重流中的懸浮相和上浮相都以細粒沉積物為主，泥質(zhì)異重流和通過指對流方式形成的異重流所攜沉積物以粘土為主，異重流可以攜帶大量陸源細粒沉積物直接進入深水形成細粒沉積，對正常深水環(huán)境產(chǎn)生重要影響。

洪水期間，泥質(zhì)異重流從三角洲前緣溢出，可在平緩的地形條件下將大量陸源細粒沉積物輸送上百千米，在近海大陸架形成大規(guī)模泥質(zhì)沉積。這種成因的細粒沉積巖在淺海陸架地區(qū)具有普遍性，被稱為泥質(zhì)前三角洲異重巖[29]。泥質(zhì)前三角洲異重巖通常由粉砂巖和泥巖組成，單層厚度為厘米尺度，下部發(fā)育侵蝕面。單層內(nèi)部主要由3部分組成，下部為發(fā)育脈動式牽引流特征的泥質(zhì)粉砂巖或粉砂質(zhì)泥巖，中間為無明顯宏觀特征的泥巖，最上部為發(fā)育大量生物變形構(gòu)造的泥巖[29](圖10)。單層的沉積過程大致可分為3個階段，首先是過路侵蝕階段，形成下部的侵蝕界面，之后在異重流能量虧損階段，開始沉積形成下部的泥質(zhì)粉砂巖、粉砂質(zhì)泥巖(推移質(zhì))和中間的泥巖(懸浮質(zhì))，最后形成上部的暗色泥巖并發(fā)育生物擾動(圖10)。

表3 異重流與經(jīng)典濁流沉積差異對比Table 3 Similarities and differences between typical turbidites and hyperpycnal flows

圖9 三角洲-異重流-滑塌重力流綜合沉積模式Fig.9 Composite depositional model of delta,hyperpycnal flow,and slumping gravity flow

由于異重流能量的波動性，泥質(zhì)前三角洲異重巖在單層系和層系組級別發(fā)育正粒序和反粒序，但在幾十到上百米的準層序級別具有向上變粗變厚的反序特征[32]。

異輕流、層間流、滑塌型重力流都是深水細粒沉積物的重要供給方式，但異重流特殊性在于將河水和陸源沉積物直接輸送到盆地底部，大量陸源物質(zhì)的加入，會改變深層水體含氧量、含鹽度等物理化學性質(zhì)，從而對原地有機質(zhì)的保存產(chǎn)生影響，同時形成大量陸源有機質(zhì)沉積[32,79,91]。

4.3 中國陸相湖盆異重流沉積研究

相比于海相環(huán)境，陸相湖盆由于水體鹽度低、物源近、以中小型河流為主等特點，更容易發(fā)生異重流。楊仁超等(2017)從構(gòu)造和氣候角度，對鄂爾多斯盆地異重流沉積特征開展研究，并提出了異重巖測井識別標志[21]。馮志強等(2010)發(fā)現(xiàn)松遼盆地嫩江組發(fā)育大規(guī)模湖底水道系統(tǒng)，起源于杏樹崗三角洲，在平緩的湖底延伸70 km，最后成扇體撒開(圖11)，巖心中發(fā)育植物葉片、平行層理和交錯層理[92]，綜合分析這些特征，可初步判斷為異重流沉積。我國陸相湖盆普遍發(fā)育的洪水型湖底扇，其形成機制及沉積特征都與底負載型異重流類似。渤海灣盆地濟陽坳陷東營三角洲在沙三中亞段發(fā)生大規(guī)模進積，同時形成了一系列滑塌濁積體，是巖性油氣藏勘探重點區(qū)塊[93-94]，但巖心觀察發(fā)現(xiàn)，該地區(qū)存在大規(guī)模異重流沉積過程(圖12)，有效區(qū)分滑塌濁積和異重流沉積，對于進一步推動該地區(qū)的勘探工作有重要意義。

圖10 紐約中部中泥盆世杰納西奧組細粒沉積巖異重流沉積模式及理想巖相特征[29] Fig.10 Depositional model and idealized lithofacies features of hyperpycnal flows with fine-grained sedimentary rocks in the Middle Denovian Genesee Formation,central New York[29]a. Northern Appalachian盆地泥盆世素描圖，發(fā)生異重流沉積;b.圖a中沿A—B剖面圖，異重流發(fā)育底負載、懸浮負載和上浮負載;c.理想泥質(zhì)異重流巖相序列素描圖

圖11 松遼盆地嫩江組陸相湖盆湖底水道體系(據(jù)文獻[92]修改)Fig.11 Bottom channel system of a continental lacustrine basin in the Neijiang Formation,Songliao Basin(modified from reference [92])

圖12 東營凹陷沙三中亞段異重流沉積構(gòu)造Fig.12 Sedimentary structure of hyperpycnal flows in the middle Es3,Dongying saga.上攀層理，濱424井，埋深2 496.1 m；b.逆正粒序，局部炭質(zhì)碎屑成層富集，發(fā)育火焰狀構(gòu)造，說明上部沉積時下部泥質(zhì)富含水，為同一期異重流能量波動成因，牛116井，埋深3 102.1 m；c.層面富集植物碎片，河155井，埋深2 979.49 m；d.逆正粒序泥質(zhì)粉 砂巖，發(fā)育紅色層理，河155井，埋深3 002.4 m

5 結(jié)論

1) 異重流作為一種深水沉積物輸送機制，其發(fā)育具有普遍性。對異重流的成因機制、演化過程及沉積特征開展研究有助于加深對深水重力流沉積的理解。有效區(qū)分異重流與盆內(nèi)滑塌型濁流，建立異重流沉積巖心-測井-地震識別標志，完善異重流沉積模式，是異重流研究的當務(wù)之急。

2) 異重流能夠沿相對平緩地形將陸源沉積物輸送上百公里，泥質(zhì)異重流對于盆地細粒沉積具有重要影響，異重流研究對于進一步拓展隱蔽油氣藏勘探范圍，指導非常規(guī)油氣勘探具有重要意義。

3) 中國陸相湖盆異重流沉積研究處于起步階段，將異重流沉積體系與現(xiàn)有陸相湖盆沉積體系融合，建立陸相湖盆異重流沉積模式，必將進一步推動中國湖相油氣勘探進程。

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