張　帥，夏國(guó)清，伊海生,3，蔡占虎，李啟來(lái)

(1.成都理工大學(xué) 地球科學(xué)學(xué)院，四川 成都610059；　2.成都理工大學(xué) 沉積地質(zhì)研究院,四川 成都610059；3.成都理工大學(xué) 油氣藏地質(zhì)及開(kāi)發(fā)工程國(guó)家重點(diǎn)實(shí)驗(yàn)室，四川 成都610059)

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羌塘盆地南部隆鄂尼地區(qū)布曲組鞍形白云石成因

張帥1，夏國(guó)清2，伊海生2,3，蔡占虎2，李啟來(lái)2

(1.成都理工大學(xué) 地球科學(xué)學(xué)院，四川 成都610059；2.成都理工大學(xué) 沉積地質(zhì)研究院,四川 成都610059；3.成都理工大學(xué) 油氣藏地質(zhì)及開(kāi)發(fā)工程國(guó)家重點(diǎn)實(shí)驗(yàn)室，四川 成都610059)

羌塘盆地南部隆鄂尼地區(qū)中侏羅統(tǒng)布曲組砂糖狀白云巖中發(fā)育有大量的鞍形白云石，其巖相學(xué)特征表現(xiàn)為正交偏光下的波狀消光以及彎曲的晶面和解理，陰極發(fā)光整體呈暗紅色，無(wú)明顯的環(huán)帶。鞍形白云石中流體包裹體均一溫度主要分布于152.8～174.1 ℃，鹽度均值(23.3%NaCl)遠(yuǎn)高于現(xiàn)代海水鹽度，表明其形成經(jīng)歷了高溫高鹽度的演化過(guò)程。微區(qū)同位素分析顯示鞍形白云石δ13C值介于-4.81‰～4.29‰，δ18O值為-11.2‰～-7.51‰，根據(jù)白云石—流體氧同位素分餾方程得到成巖流體δ18O(SMOW)值為5‰～11‰。綜合分析認(rèn)為,鞍形白云石形成于相對(duì)封閉的深埋藏環(huán)境，是熱液流體調(diào)整改造作用的產(chǎn)物。地層水加熱再循環(huán)過(guò)程中在孔隙或裂縫較為發(fā)育的部位沉淀形成鞍形白云石，高鹽度的地層水可能來(lái)自沉積期古海水與埋藏期地下熱鹵水的混合。鞍形白云石的成因研究表明，晶粒相對(duì)較粗大的砂糖狀白云巖是地表/近地表形成的白云巖重結(jié)晶作用的產(chǎn)物。

碳、氧同位素；埋藏白云石化；鞍形白云石；布曲組；中侏羅統(tǒng)；羌塘盆地

羌塘盆地作為中國(guó)最大的中生代海相沉積盆地，與中東特提斯構(gòu)造域富油氣盆地具有相似的沉積演化特征和石油地質(zhì)條件[1-2]。盆地內(nèi)各類(lèi)油氣顯示多達(dá)200余處，顯示出巨大的資源潛力[3-4]。尤以盆地南部中侏羅統(tǒng)布曲組油藏帶關(guān)注程度最高，研究成果最多[5-6]。對(duì)于油藏帶砂糖狀白云巖成因的研究仍存在一定的爭(zhēng)議，早期主要根據(jù)白云巖的沉積特征和顯微結(jié)構(gòu)提出了混合水模式[7-10]。近年來(lái)包裹體測(cè)溫以及碳氧同位素?cái)?shù)據(jù)均顯示其為高溫埋藏成因[11]。進(jìn)一步研究發(fā)現(xiàn)，砂糖狀白云巖中除發(fā)育霧心亮邊白云石外，還廣泛分布一類(lèi)具有特殊形態(tài)的白云石晶體——鞍形白云石。在前期工作的基礎(chǔ)上，運(yùn)用陰極發(fā)光、包裹體測(cè)溫、微區(qū)同位素等分析手段對(duì)鞍形白云石成因展開(kāi)了討論，以期為油藏帶白云巖形成機(jī)理及空間分布特征提供參考。

1　區(qū)域地質(zhì)背景

羌塘盆地位于青藏高原中北部，處于特提斯-喜馬拉雅構(gòu)造域中段。盆地夾持于可可西里-金沙江縫合帶和班公湖-怒江縫合帶之間，總體呈“兩坳一隆”的構(gòu)造格局[12-13]。南部坳陷帶隆鄂尼地區(qū)牙爾根、格魯關(guān)那以及德如日等地出露含油白云巖帶，層位為中侏羅統(tǒng)布曲組(圖1)。白云巖油浸后呈黃褐色、深褐色，晶粒結(jié)構(gòu)，中-厚層狀構(gòu)造。巖石風(fēng)化后較疏松，常呈砂糖粒狀。實(shí)測(cè)剖面中砂糖狀白云巖與介殼灰?guī)r、鮞粒灰?guī)r、生物碎屑灰?guī)r或核形石灰?guī)r相伴生，為一套典型的臺(tái)地邊緣礁灘相沉積(圖2)。受白云石化作用和重結(jié)晶作用影響，砂糖狀白云巖原始沉積組構(gòu)消失殆盡，成巖組構(gòu)主要為不等粒結(jié)構(gòu)與霧心亮邊結(jié)構(gòu)等。

2　測(cè)試方法

樣品主要為砂糖狀白云巖，首先磨制了普通薄片，經(jīng)茜素紅染色后詳細(xì)觀察巖石結(jié)構(gòu)。在此基礎(chǔ)上選取不同剖面的典型樣品磨制陰極發(fā)光片與包裹體片。流體包裹體測(cè)試由長(zhǎng)江大學(xué)地球科學(xué)學(xué)院完成，采用英國(guó)Linkam公司THMSG600冷熱臺(tái)，分析精度±0.1 ℃，加熱冷凍過(guò)程中設(shè)置的控溫速率一般為20 ℃/min，相變點(diǎn)附近速率為4 ℃/min。

圖1　隆鄂尼地區(qū)布曲組砂糖狀白云巖地表分布及 采樣位置Fig.1　Distribution and sampling location of saccharoidal dolostones in the Buqu Formation of the Longeni area

進(jìn)一步研究中對(duì)鞍形白云石含量較高的樣品進(jìn)行激光微區(qū)碳、氧同位素測(cè)試，實(shí)驗(yàn)由西南油氣田分公司勘探開(kāi)發(fā)研究院完成。具體方法為：同軸安裝Nd，YAG激光器與偏光顯微鏡，利用顯微鏡光學(xué)系統(tǒng)定位后通過(guò)激光束加熱分解取樣(束斑直徑<20 μm)；真空提純凈化后收集CO2氣樣采用MAT252型穩(wěn)定同位素質(zhì)譜儀檢測(cè)，PDB標(biāo)準(zhǔn)，測(cè)試精度δ小于0.2‰。

3　結(jié)果分析

3.1顯微結(jié)構(gòu)特征

布曲組白云巖類(lèi)型主要包括粒屑白云巖和晶粒白云巖(圖3)。粒屑白云巖由殘余粒屑結(jié)構(gòu)組成(圖4a)，粒屑類(lèi)型主要包括鮞粒、內(nèi)碎屑、以及生物碎屑等，形態(tài)多呈球形或橢球形。晶粒白云巖中白云石自形程度較高，多呈菱面體，根據(jù)粒徑大小可分為三類(lèi)。粉-細(xì)晶白云巖中白云石晶體以它形曲面為主，晶粒大小為0.03～0.25 mm，常呈鑲嵌結(jié)構(gòu)，發(fā)育彌散狀的晶間微孔(圖4b)。細(xì)-中晶白云巖由自形—半自形白云石組成粒狀鑲嵌結(jié)構(gòu)，晶粒大小為0.1～0. 5mm，可見(jiàn)霧心亮邊構(gòu)造，晶間溶孔及溶孔、溶縫中可見(jiàn)方解石、瀝青質(zhì)充填(圖4c)。中-粗晶白云巖由晶粒大小為0.25～1 mm的白云石組成，晶體間多為凹凸接觸，發(fā)育晶間孔隙和溶蝕縫洞(圖4d)。

中-粗晶白云巖中發(fā)育有鞍形白云石，呈不均勻的斑點(diǎn)狀充填孔洞及裂縫，含量5%～10%不等，晶粒一般大于0.5 mm。正交偏光下具有典型的波狀消光，可見(jiàn)彎曲的晶面和解理，總體形態(tài)為菱面體的彎曲變形；晶體邊界呈曲折的輪廓和弧形的端面，內(nèi)部可見(jiàn)階步式的斷裂，邊緣潔凈度與透明度相對(duì)較好(圖5a，b)。鞍形白云石陰極發(fā)光整體呈暗紅色，無(wú)明顯環(huán)帶，沿解理發(fā)育部位發(fā)光呈亮紅色，局部可見(jiàn)后期溶蝕作用沉淀的方解石呈暗橘黃色(圖5c，d)。

3.2包裹體測(cè)溫與鹽度

鏡下觀察自形白云石中鹽水包裹體形態(tài)較規(guī)則，大小主要分布在5～8 μm，沿晶體生長(zhǎng)面呈帶狀分布。鞍形白云石中原生鹽水包裹體大小一般為2～5 μm，形態(tài)不規(guī)則。布曲組砂糖狀白云巖19件樣品153個(gè)氣液兩相鹽水包裹體的測(cè)試結(jié)果顯示(圖6)：自形白云石鹽水包裹體均一溫度變化范圍為139.5～180.7 ℃，平均值為150.4 ℃，采用H2O-CaCl2二元體系近似換算鹽度均值為20.1%。鞍形白云石均一溫度介于152.8～174.1 ℃，平均均一溫度為163.7 ℃，鹽度均值為23.3%。鞍形白云石的包裹體均一溫度介于自形白云石分布范圍內(nèi)，其平均值及鹽度均值高于自形白云石，整體上呈高溫高鹽度的流體特征?？紫逗土芽p中充填的方解石均一溫度及鹽度均較低(表1)，可能存在大氣淡水的改造。

圖2　隆鄂尼地區(qū)布曲組砂糖狀白云巖野外露頭及剖面柱狀圖Fig.2　Saccharoidal dolostone outcrops and stratigraphic column of the Buqu Formation of the Longeni areaa.格魯關(guān)那剖面白云巖與灰?guī)r互層；b.含生物碎屑核形石灰?guī)r；c.砂糖狀白云巖中生屑?xì)堄?；d.介殼灰?guī)r;e.地層柱狀圖

圖3　隆鄂尼地區(qū)布曲組白云巖手標(biāo)本照片F(xiàn)ig.3　Photos of dolostone samples from the Buqu Formation of the Longeni areaa.砂屑白云巖；b.粉-細(xì)晶白云巖；c.細(xì)-中晶白云巖；d.中-粗晶白云巖

根據(jù)現(xiàn)有的盆地地溫梯度資料[14-15]，羌塘盆地南部地溫梯度普遍偏低，一般介于26～28 ℃/km，其中隆鄂尼地區(qū)平均地溫梯度為26.4 ℃/km。假定地表溫度20 ℃，參考前人對(duì)盆地埋藏史的研究[15]，按布曲組最大埋深4 500 m計(jì)算，布曲組經(jīng)歷的最大古地溫約140 ℃。包裹體測(cè)溫?cái)?shù)據(jù)明顯高于地層埋藏溫度，形成鞍形白云石的流體可能經(jīng)歷了深部加熱循環(huán)或存在外部高溫流體的改造。

圖4　隆鄂尼地區(qū)布曲組白云巖顯微結(jié)構(gòu)照片F(xiàn)ig.4　Photomicrographs of dolostone from the Buqu Formation of the Longeni areaa.粒屑白云巖；b.粉-細(xì)晶白云巖；c.細(xì)-中晶白云巖，霧心亮邊結(jié)構(gòu)；d.中-粗晶白云巖，孔隙充填方解石

圖5　隆鄂尼地區(qū)布曲組鞍形白云石顯微結(jié)構(gòu)及陰極發(fā)光照片F(xiàn)ig.5　Cathodoluminescence images and photomicrographs of saddle dolomites from the Buqu Formation of the Longeni areaa.鞍形白云石呈波狀消光，正交偏光；b.鞍形白云石彎曲的晶面和解理，單偏光； c,d.鞍形白云石陰極發(fā)光照片，總體發(fā)暗紅色光，沿解理發(fā)育部位發(fā)光較亮，局部可見(jiàn)方解石膠結(jié)物發(fā)暗橘黃色光

圖6　隆鄂尼地區(qū)布曲組白云石包裹體均一溫度Fig.6　Histogram of homogenization temperature of fluid inclusions in dolomites from the Buqu Formation of the Longeni area表1　隆鄂尼地區(qū)自形白云石與鞍形白云石、方解石 包裹體均一溫度及鹽度Table 1　Homogenization temperature and salinity of fluid inclusions in the automorphic dolomite, saddle dolomite and calcite from the Buqu Formation of the Longeni area

樣品均一溫度/℃分布范圍平均值鹽度均值/%自形白云石139.5～180.7150.420.1鞍形白云石152.8～174.1163.723.3方解石64.3～114.984.85.1

3.3碳、氧同位素特征

鞍形白云石微區(qū)碳氧同位素測(cè)試結(jié)果顯示δ13C(PDB)值為-4.81‰～4.29‰，δ18O(PDB)值介于-11.2‰～-7.51‰(表2)。研究區(qū)砂糖狀白云巖全巖樣品的δ13C(PDB)值一般介于3.02‰～4.23‰，δ18O(PDB)值介于-8.94‰～-7.9‰[11]。白云石δ13C值隨溫度變化的同位素分餾很輕微，δ18O值主要受流體氧同位素以及溫度的影響[16]。鞍形白云石的δ13C值顯示其碳來(lái)源主要為封閉系統(tǒng)中的宿主地層[17-18]，較大的負(fù)偏可能受封閉環(huán)境下生物成因CO2介入的影響[19]。

4　討論

鞍形白云石作為一種特殊的白云石類(lèi)型，形成過(guò)程中發(fā)生了晶格畸變，其成因?qū)τ诎自剖黧w來(lái)源的研究具有十分重要的意義。Radke和Mathis[20]首次從形態(tài)學(xué)、晶體光學(xué)、晶體化學(xué)等多個(gè)角度對(duì)具有鞍狀形態(tài)的白云石類(lèi)型進(jìn)行了詳細(xì)論述，提出以鞍形白云石作為潛在地質(zhì)溫度計(jì)的觀點(diǎn)。嗣后，許多學(xué)者都引用了這一觀點(diǎn)，普遍認(rèn)同鞍形白云石形成于深埋藏作用下高溫、高鹽度鹵水環(huán)境[21-22]，其形成溫度很大程度上與“生油窗”到“過(guò)成熟生干氣”階段的溫度相一致[23]。也有學(xué)者認(rèn)為鞍形白云石形成于埋藏過(guò)程相對(duì)封閉系統(tǒng)中的化學(xué)壓實(shí)作用[18,24]。近年來(lái)，鞍形白云石與熱液白云巖化作用的關(guān)系受到眾多學(xué)者的重視[25-29]。鞍形白云石的廣泛發(fā)育可以作為構(gòu)造熱液白云巖儲(chǔ)層的重要標(biāo)志[30-35]。

根據(jù)白云石-水氧同位素分餾系數(shù)[36]繪制關(guān)系圖解，將鞍形白云石均一溫度和δ18O值進(jìn)行投影，結(jié)果顯示沉淀鞍形白云石的流體δ18O(SMOW)值介于5‰～11‰(圖7)，遠(yuǎn)大于現(xiàn)代海水和古海水-1‰～1‰[37]，與巖漿水δ18O(SMOW)值相似[38]。隆鄂尼地區(qū)白云巖出露區(qū)周緣無(wú)深大斷裂和火山巖分布，白云巖鏡下鑒定未見(jiàn)螢石、重晶石與菱鎂礦等熱液礦物，表明鞍形白云石的形成不是巖漿水等外來(lái)熱液流體作用的結(jié)果。鄂爾多斯盆地奧陶系馬家溝組馬五段埋藏白云巖成巖流體δ18O(SMOW)平均值為8‰[39]。沉淀鞍形白云石的流體δ18O(SMOW)值的大幅正偏可能是埋藏環(huán)境地層水高溫濃縮所致。結(jié)合包裹體測(cè)溫和鹽度

表2　隆鄂尼地區(qū)布曲組鞍形白云石微區(qū)碳氧同位素分析結(jié)果Table 2　Carbon and oxygen isotope analyses result of saddle dolomites from the Buqu Formation of the Longeni area

圖7　應(yīng)用分餾方程建立的溫度與白云石 以及流體氧同位素關(guān)系Fig.7　Plot of equilibrium relationship between δ18O(PDB) of dolomite and homogenization temperature for various δ18O(SMOW) of fluid using fractionation equation (根據(jù)Land[36]分餾方程：1 000 Lnα白云石-水=3.2×106T-2-3.3繪制。)

分析，本文提出隆鄂尼地區(qū)鞍形白云石的成巖流體應(yīng)為深埋藏環(huán)境下的地層水。由于缺乏蒸發(fā)環(huán)境的標(biāo)志[40]，高鹽度的地層水可能來(lái)自沉積期古海水與埋藏期地下熱鹵水的混合[41]。地層水加熱再循環(huán)的過(guò)程中受水巖作用影響，地表/近地表形成的白云巖調(diào)整改造(重結(jié)晶)為砂糖狀白云巖，同時(shí)在孔隙或裂縫較為發(fā)育的部位沉淀鞍形白云石。

5　結(jié)論

1)研究區(qū)鞍形白云石顯微結(jié)構(gòu)及包裹體特征顯示其形成于封閉的流體系統(tǒng)，高溫高鹽度的流體環(huán)境。

2)沉淀鞍形白云石的流體δ18O(SMOW)值介于5‰～11‰，成巖流體來(lái)自深埋藏環(huán)境下地層水的加熱再循環(huán)。

3)砂糖狀白云巖與鞍形白云石的分布緊密相關(guān)，并且具有相似的陰極發(fā)光特征和包裹體均一溫度，鞍形白云石的發(fā)育指示砂糖狀白云巖的形成與埋藏環(huán)境下熱液流體的調(diào)整改造有關(guān)。

致謝：成都理工大學(xué)博士研究生季長(zhǎng)軍與尹青，碩士研究生梁定勇、蔡占虎與金峰參加了野外剖面測(cè)量和采樣工作，在此一并表示感謝！

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(編輯董立)

Origin of saddle dolomites from the Buqu Formation of Longeni Area in southern Qiangtang Basin

Zhang Shuai1,Xia Guoqing2,Yi Haisheng2,3, Cai Zhanhu2,Li Qilai2

(1.CollegeofEarthSciences,ChengduUniversityofTechnology,Chengdu,Sichuan610059,China;2.InstituteofSedimentaryGeology,ChengduUniversityofTechnology,Chengdu,Sichuan610059,China;3.StateKeyLaboratoryofOil/GasReservoirGeologyandExploitation,ChengduUniversityofTechnology,Chengdu,Sichuan610059,China)

Saccharoidal dolostones in the Middle Jurrasic Buqu Formation of Longeni Area in southern Qiangtang Basin were found to contain large number of saddle dolomites.Petrographic study shows typical sweeping extinction,curved crystal faces and cleavage traces under cross polarized light and dark red luminescence and unzoned textures.Fluid inclusions in the saddle dolomite were measured to have homogenization temperature ranging between 152.8 and 174.1 ℃ and salinity averaged at 23.3%NaCl,much higher than the salinity of modern seawater,indicating a high temperature and high salinity diagenetic environment.In-situ isotopic analysis shows δ13C value ranging from -4.81‰ to 4.29‰,and δ18O value varying from -7.51‰ to -11.2‰.The δ18O(SMOW) value of diagentic fluid was calculated to be 5‰ -11‰ by using the fractionation equation of dolomite-fluid oxygen isotope.It is believed based on a comprehensive analysis that the saddle dolomites were formed in a relatively closed deep burial setting and were the products of thermal fluid modification.The saddle dolomites were probably formed in places where pores and fractures were well developed during a recirculation of heated formation water.The high salinity formation water could be the mixture of paleo-seawater during sedimentation period and underground hot brine during burial period.Origin of the saddle dolomite reveals that the saccharoidal dolostones with coarse grains are the result of recrystalization of surface or near surface dolostones.

carbon and oxygen isotope,burial dolomitization,saddle dolomite,Buqu Formation,Middle Jurassic,Qiangtang Basin

2015-05-14;

2016-06-06。

張帥(1985—)，男，博士研究生，儲(chǔ)層礦物巖石學(xué)。E-mail:zsdolomite@mail.com。

簡(jiǎn)介：夏國(guó)清(1982—)，男，博士、講師，沉積地質(zhì)學(xué)。E-mail:xiaguoqing2012@cdut.cn。

國(guó)家自然科學(xué)基金項(xiàng)目(41572089,41402099)。

0253-9985(2016)04-0483-07

10.11743/ogg20160404

TE121.3

A