川中古隆起超壓分布與形成的地溫場(chǎng)因素

劉一鋒, 鄭倫舉, 邱楠生*, 賈京坤, 常青

1 中國(guó)石油大學(xué)(北京)油氣資源與探測(cè)國(guó)家重點(diǎn)實(shí)驗(yàn)室, 北京 102249 2 中國(guó)石油大學(xué)(北京)盆地與油藏研究中心, 北京 102249 3 中國(guó)石化勘探開(kāi)發(fā)研究院無(wú)錫石油地質(zhì)研究所, 江蘇無(wú)錫 214151

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川中古隆起超壓分布與形成的地溫場(chǎng)因素

劉一鋒1,2, 鄭倫舉3, 邱楠生1,2*, 賈京坤1,2, 常青1,2

1 中國(guó)石油大學(xué)(北京)油氣資源與探測(cè)國(guó)家重點(diǎn)實(shí)驗(yàn)室, 北京 102249 2 中國(guó)石油大學(xué)(北京)盆地與油藏研究中心, 北京 102249 3 中國(guó)石化勘探開(kāi)發(fā)研究院無(wú)錫石油地質(zhì)研究所, 江蘇無(wú)錫 214151

溫度和壓力是沉積盆地兩個(gè)重要的物理場(chǎng)，溫度影響著超壓的形成和分布.本文根據(jù)鉆孔實(shí)測(cè)溫度和壓力數(shù)據(jù)分析了川中古隆起現(xiàn)今壓力與溫度的關(guān)系；在實(shí)驗(yàn)室對(duì)封閉流體進(jìn)行了多組溫-壓關(guān)系實(shí)驗(yàn)；利用等效鏡質(zhì)體反射率和包裹體測(cè)溫?cái)?shù)據(jù)恢復(fù)了川中古隆起不同井區(qū)在白堊紀(jì)抬升之前的最大古地溫，并在此基礎(chǔ)上分析了溫度降低對(duì)研究區(qū)超壓的影響；最后探討了生烴增壓和欠壓實(shí)超壓形成過(guò)程中溫度的作用.研究結(jié)果表明，川中古隆起現(xiàn)今超壓層的壓力系數(shù)與溫度呈正相關(guān)關(guān)系；在絕對(duì)密封的條件下，當(dāng)壓力大于15 MPa時(shí)，溫度每變化1 ℃，壓力變化1.076 MPa.川中地區(qū)不同井區(qū)自晚白堊世以來(lái)的差異性降溫是現(xiàn)今同一超壓層系超壓強(qiáng)度不同的主要因素，此外超壓層還應(yīng)發(fā)生了流體的橫向壓力傳遞和泄漏.下古生界原油裂解形成超壓的時(shí)間是180～110 Ma；氣態(tài)烴伴生的鹽水包裹體均一溫度暗示了在90 Ma超壓發(fā)生調(diào)整.盆地模擬結(jié)果顯示溫度對(duì)上三疊統(tǒng)須家河組的欠壓實(shí)增壓影響微弱.

川中古隆起; 溫度; 超壓; 物理模擬

1 引言

四川盆地在前震旦系基底形成以后，經(jīng)歷了復(fù)雜的構(gòu)造—沉積演化歷史，大致上可分為克拉通盆地階段和前陸盆地階段.克拉通盆地階段以海相碳酸鹽巖沉積為主，前陸盆地階段則沉積了巨厚的陸相碎屑巖.多期構(gòu)造運(yùn)動(dòng)造成現(xiàn)今盆地內(nèi)發(fā)育多個(gè)不整合面和多套地層缺失，白堊紀(jì)以來(lái)的構(gòu)造抬升運(yùn)動(dòng)使侏羅系及其以下地層出露地表(鄧賓等,2009; Liu et al.,2012; 許海龍等,2012).川中古隆起位于四川盆地中部(圖1)，屬于樂(lè)山—龍女寺鼻狀構(gòu)造的一部分.在加里東期該地區(qū)處于隆起部位，印支—燕山期為向北傾的斜坡，現(xiàn)今東部抬升較高，向西逐漸傾伏(王宓君等,1989; 許海龍等,2012).作為一個(gè)長(zhǎng)期繼承性發(fā)展的盆地內(nèi)大型正向構(gòu)造單元，川中古隆起具有廣闊的油氣勘探前景(姚建軍等,2003; 魏國(guó)齊等,2010).目前已在震旦系、寒武系、三疊系和侏羅系發(fā)現(xiàn)多個(gè)商業(yè)油氣藏.

前人對(duì)川中古隆起的現(xiàn)今地層壓力狀態(tài)已經(jīng)做了大量研究，發(fā)現(xiàn)上三疊統(tǒng)至下寒武統(tǒng)多套地層都發(fā)育超壓(楊金俠等,2003; Liu et al.,2008; 謝增業(yè)等,2009; 徐國(guó)盛等,2009; 郝國(guó)麗等,2010).鉆井實(shí)測(cè)壓力顯示，古隆起上三疊統(tǒng)須家河組壓力系數(shù)在1.0～2.0之間，平面上由西北向南逐漸降低(謝增業(yè)等,2009; 郝國(guó)麗等,2010).磨溪—龍女寺構(gòu)造帶下三疊統(tǒng)嘉陵江組壓力系數(shù)大多在2.0以上(徐國(guó)盛等,2009).川中地區(qū)震旦系主要為常壓—弱超壓，而寒武系—奧陶系則表現(xiàn)為明顯的超壓，壓力系數(shù)為1.0～1.82(Liu et al.,2008).盆地大地?zé)崃靼l(fā)生多期演化(邱楠生等,2008; Zhu et al.,2010; 王瑋等,2011; 何麗娟等,2014)，加之沉積層埋藏-抬升作用，使四川盆地地層溫度經(jīng)歷了復(fù)雜且劇烈的變化過(guò)程.地溫場(chǎng)對(duì)含油氣盆地超壓的形成和保存都有重要影響，但在我國(guó)相關(guān)研究開(kāi)展較少.前人很早就發(fā)現(xiàn)溫度升高可以引起孔隙流體壓力增大(Barker,1972)，但一些學(xué)者認(rèn)為幾乎不存在絕對(duì)封閉的蓋層以及溫度升高引起的流體體積膨脹量非常小，所以水熱增壓通常不是主要的超壓機(jī)制(Luo and Vasseur,1992; Osborne and Swarbrick,1997).當(dāng)含油氣盆地發(fā)生構(gòu)造抬升時(shí)(在此過(guò)程中泥巖欠壓實(shí)、生烴作用、成巖作用等超壓機(jī)制幾乎全部停止)，若異常壓力還能長(zhǎng)時(shí)間保存，即從側(cè)面說(shuō)明了封隔層的有效性，此時(shí)關(guān)于溫度對(duì)地層壓力的影響則不可忽略.如鄂爾多斯盆地，現(xiàn)今盆地內(nèi)的異常低壓就與構(gòu)造抬升引起的溫度降低密切相關(guān)(許浩等,2012; 李士祥等,2013).四川盆地自白堊紀(jì)以來(lái)發(fā)生了全盆范圍的巨大抬升剝蝕，最大剝蝕量超過(guò)4000 m(鄧賓等,2009; 朱傳慶等,2009; Liu et al.,2012).溫度降低對(duì)四川盆地超壓的影響的相關(guān)研究目前尚屬空白.本文利用鉆孔溫壓數(shù)據(jù)厘清現(xiàn)今超壓層的溫度-壓力關(guān)系；利用物理模擬方法探究溫度對(duì)地層壓力的控制；結(jié)合地層的溫度演化史，探討溫度對(duì)川中古隆起超壓的差異性分布和形成過(guò)程的影響.

圖1 (a) 四川盆地氣田分布及剖面與研究井位置；(b) 研究區(qū)地震剖面(據(jù)許海龍等,2012)Fig.1 (a) Gas fields distribution and locations of the section and wells; (b) A seismic section in the study area(from Xu et al.,2012)

2 現(xiàn)今地層溫度與壓力

2.1 現(xiàn)今溫度場(chǎng)特征

基于鉆孔溫度資料和大量的巖石熱導(dǎo)率、生熱率測(cè)量數(shù)據(jù)，關(guān)于盆地現(xiàn)今地溫場(chǎng)的分布特征已經(jīng)有了一定認(rèn)識(shí)(謝曉黎和于匯津,1988; 韓永輝和吳春生,1993; 盧慶治等,2005; 徐明等,2011).四川盆地現(xiàn)今大地?zé)崃髌骄禐?3.2 mW·m-2，低于我國(guó)大陸地區(qū)平均大地?zé)崃?63 mW·m-2)，但比塔里木和準(zhǔn)噶爾盆地的略高.四川盆地現(xiàn)今地溫梯度為17.7～33.4 ℃/km.由于基底埋深較淺，川中和川南地區(qū)的大地?zé)崃?60～70 mW·m-2)和地溫梯度(24～30 ℃/km)都明顯高于川東北和川西北地區(qū)的(徐明等,2011).隨著川中古隆起深層鉆井?dāng)?shù)量增多，溫度資料進(jìn)一步豐富.本文收集了來(lái)自威遠(yuǎn)地區(qū)、磨溪—高石梯地區(qū)以及八角場(chǎng)地區(qū)代表鉆井的試井溫度(表1).寒武系龍王廟組和洗象池組以及上三疊統(tǒng)須家河組是川中地區(qū)主要的天然氣產(chǎn)層，溫度數(shù)據(jù)主要集中在上述層系.為了更好地反映溫度分布特征，將各層系實(shí)測(cè)溫度數(shù)據(jù)折算為該層系底界面溫度.受埋深控制，同一套地層的溫度差異較大，磨溪—高石梯地區(qū)寒武系底界面的溫度為136～146.5 ℃，在威遠(yuǎn)地區(qū)寒武系底界面溫度僅為95 ℃；磨溪—高石梯地區(qū)上三疊統(tǒng)須家河組底界面溫度約為76.5～81.4 ℃，向西北方向溫度逐漸升高，八角場(chǎng)地區(qū)須家河組的底界溫度約為100 ℃.

表1 川中古隆起不同地區(qū)代表井溫度數(shù)據(jù)Table 1 Temperature data from bore holes in different position of the Central Paleo-Uplift in the Sichuan Basin

注：*底界溫度折算時(shí)取地溫梯度為2.5 ℃/km.

2.2 超壓分布與超壓成因

四川盆地是一個(gè)典型的超壓盆地，不少學(xué)者都曾對(duì)盆地內(nèi)超壓分布和成因機(jī)制進(jìn)行過(guò)探討(王震亮等,2004; Liu et al.,2008; Tian et al.,2008; 郝國(guó)麗等,2010; 郭迎春等,2012).川中古隆起上三疊統(tǒng)須家河組、下三疊統(tǒng)嘉陵江組、下古生界寒武系和殘留的奧陶系的地層壓力都明顯高于靜水壓力，其中嘉陵江組的壓力系數(shù)接近2.0.上二疊統(tǒng)龍?zhí)督M泥巖雖無(wú)實(shí)際測(cè)壓數(shù)據(jù)，但泥巖聲波時(shí)差的明顯異常也暗示了超高壓的存在.筆者根據(jù)實(shí)測(cè)壓力數(shù)據(jù)、鉆井泥漿、聲波測(cè)井等數(shù)據(jù)，結(jié)合流體特征和蓋層分布，在磨溪—高石梯地區(qū)劃分出多個(gè)超壓系統(tǒng)(圖2)，超壓發(fā)育深度大約在2000 m至5000 m；根據(jù)巖石物理性質(zhì)，判斷上三疊統(tǒng)須家河組和上二疊統(tǒng)超壓主要為泥巖欠壓實(shí)導(dǎo)致；下三疊統(tǒng)和下古生界(寒武系和殘留的奧陶系)的超壓主要與原油裂解生氣導(dǎo)致的孔隙流體膨脹有關(guān)(圖2).橫向上，上三疊統(tǒng)須家河組在廣安—銅梁一線的東南地區(qū)為正常壓力，向西北方向超壓逐漸增大，八角場(chǎng)地區(qū)須家河組壓力系數(shù)達(dá)1.9(郝國(guó)麗等,2010)；寒武系在威遠(yuǎn)地區(qū)表現(xiàn)為靜水壓力，在磨溪—高石梯地區(qū)壓力系數(shù)達(dá)到了1.48～1.78(Liu et al.,2008).

2.3 現(xiàn)今地層壓力與溫度的關(guān)系

溫度和壓力作為盆地內(nèi)兩個(gè)重要的物理場(chǎng)，二者的耦合關(guān)系一直是學(xué)界關(guān)注的焦點(diǎn).本文選取了溫壓測(cè)試數(shù)據(jù)較多的磨溪—高石梯地區(qū)、威遠(yuǎn)地區(qū)的寒武系和磨溪—高石梯地區(qū)、八角場(chǎng)地區(qū)的上三疊統(tǒng)須家河組兩套超壓層進(jìn)行典型分析(圖3).磨溪—高石梯地區(qū)寒武系平均地層溫度為140 ℃，地層壓力平均值為78.5 MPa；威遠(yuǎn)地區(qū)(威28井)寒武系溫度為95 ℃，地層壓力30.1 MPa.寒武系溫-壓斜率為1.075 MPa/℃.磨溪—高石梯地區(qū)須家河組平均地層溫度為78.5 ℃，地層壓力平均值為32.3 MPa.八角場(chǎng)地區(qū)須家河組平均地層溫度為100 ℃，地層壓力平均值為59 MPa.須家河組溫-壓斜率為1.24 MPa/℃.綜上所述，川中地區(qū)寒武系和上三疊統(tǒng)須家河組地層壓力及壓力系數(shù)都與地層溫度有著良好的正相關(guān)性，其中須家河組地層壓力和壓力系數(shù)隨溫度增加的增大幅度更大.

圖2 (a) 磨溪—高石梯地區(qū)超壓系統(tǒng)劃分;(b) 寒武系與上三疊統(tǒng)須家河組超壓機(jī)制判別: 須家河組超壓由欠壓實(shí)所致；寒武系超壓由流體膨脹引起Fig.2 (a) Overpressure systems in the Moxi-Gaoshiti area; (b) Mechanisms for the Cambrian and Xujiahe overpressures: the Xujiahe Formation overpressure is caused by disequilibrium compaction and the Cambrian overpressure is mainly caused by fluid expansion

圖3 川中古隆起寒武系龍王廟組底界面和三疊系須家河組底界面地層溫壓特征Fig.3 Temperature and pressure characteristics of the bottom of the Cambrian Longwangmiao formation and the Upper Triassic Xujiahe formation in the Central Paleo-Uplift of the Sichuan Basin

3 溫度與壓力的直接關(guān)系

良好的封閉條件是盆地內(nèi)異常壓力形成的前提.本文借助物理模擬實(shí)驗(yàn)探索封閉條件下溫度與壓力的關(guān)系.實(shí)驗(yàn)儀器如圖4所示，“PVT釜”(容積約250 mL)能承受高溫高壓，恒溫箱內(nèi)的加熱器可對(duì)“PVT釜”內(nèi)的溫度進(jìn)行定量調(diào)節(jié).實(shí)驗(yàn)中采用的恒壓增壓泵不僅將液體注入容器，還控制著“PVT釜”內(nèi)的初始?jí)毫?攪拌器可以縮短系統(tǒng)的溫壓平衡時(shí)間.溫度和壓力感應(yīng)計(jì)實(shí)時(shí)監(jiān)測(cè)釜內(nèi)溫壓變化，并通過(guò)電腦軟件自動(dòng)記錄.為保證實(shí)驗(yàn)過(guò)程中測(cè)得的每組溫壓數(shù)據(jù)均為系統(tǒng)平衡后的真實(shí)溫壓狀態(tài)，設(shè)置的溫度變化速率緩慢(5 ℃/h).根據(jù)不同的初始溫壓條件，以勝利油田沙河街組地層水作為實(shí)驗(yàn)流體，共完成了4組物理模擬實(shí)驗(yàn)(圖5).1、2組實(shí)驗(yàn)從高溫高壓狀態(tài)逐漸降溫；3、4組實(shí)驗(yàn)從低溫低壓狀態(tài)逐漸升溫.四組實(shí)驗(yàn)在溫度改變至預(yù)計(jì)溫度后都進(jìn)行了逆向溫度變化，使溫度再次回到初始值.逆向變溫過(guò)程中壓力也沿著之前的變化路徑逆向恢復(fù)至初始?jí)毫顟B(tài).

圖4 溫-壓關(guān)系物理模擬實(shí)驗(yàn)儀器示意圖 溫度計(jì)T和T0分別監(jiān)測(cè)“PVT釜”和 恒溫箱的溫度；壓力計(jì)P測(cè)試釜內(nèi)壓力.Fig.4 Schematic diagram of the instrument for the temperature-pressure physical simulation experiment Temperatures in the “PVT Vessel” and the thermotank are monitored by thermometers T and T0; pressure in the “PVT Vessel” is monitored by the pressure meter (P).

實(shí)驗(yàn)結(jié)果表明，當(dāng)壓力大于15 MPa時(shí)，封閉系統(tǒng)內(nèi)的溫度-壓力近乎為相互平行的直線關(guān)系，斜率為1.076 MPa/℃；當(dāng)壓力小于15 MPa時(shí)，壓力隨溫度的變化幅度明顯偏小(圖5).這是由于在低溫低壓時(shí)水的熱膨脹系數(shù)較小.從圖5中還可以看出在絕對(duì)封閉的條件下，埋深-抬升過(guò)程中溫度改變?cè)斐傻膲毫ψ兓冗h(yuǎn)大于相同深度變化時(shí)靜水壓力的變化；溫度變化引起的地層壓力變化幅度略小于同等埋深變化時(shí)靜巖壓力的改變量.

圖5 溫-壓關(guān)系物理模擬實(shí)驗(yàn)結(jié)果 兩條虛線分別代表盆地內(nèi)靜巖壓力和靜水壓力與溫度的關(guān)系線(假設(shè)地表溫度為20 ℃，地溫梯度為3 ℃/100m)；圓點(diǎn)處的 溫度和壓力值代表實(shí)驗(yàn)端點(diǎn)的溫度和壓力.Fig.5 Results of the temperature-pressure relationship physic simulation experiment Two dashed lines represent temperature-lithostatic pressure and temperature-hydrostatic pressure relations respectively.

4 溫度對(duì)川中古隆起超壓的影響

4.1 晚期降溫對(duì)超壓的影響

根據(jù)盆地沉積演化歷史分析，各地層在晚白堊世抬升前達(dá)到其所經(jīng)歷的最大古地溫.等效鏡質(zhì)體反射率(Requ)作為有機(jī)質(zhì)成熟度指標(biāo)是目前國(guó)內(nèi)外盆地?zé)崾费芯恐凶畛Ｒ?jiàn)和最成熟的古溫標(biāo)參數(shù)(邱楠生等,2004).Requ用于恢復(fù)古地溫的最大優(yōu)勢(shì)是可以反映地層在地質(zhì)歷史中所經(jīng)歷的最高溫度.圖6為川中古隆起不同井區(qū)的3口典型井的Requ剖面.如圖所示八角場(chǎng)地區(qū)地層埋深最大；威遠(yuǎn)地區(qū)地層埋深較淺，寒武系埋深小于3000 m；磨溪—高石梯地區(qū)地層埋深處于八角場(chǎng)和威遠(yuǎn)地區(qū)之間.川中地區(qū)不同構(gòu)造位置的埋深差異主要與晚白堊世以來(lái)的差異性抬升剝蝕有關(guān).現(xiàn)今川中地區(qū)中、古生界的Requ值分布在0.99%～3.25%之間.

圖6 川中古隆起典型單井Ro剖面(井位見(jiàn)圖1)Fig.6 Ro vs. depth plot for three wells in the Central Paleo-Uplift of the Sichuan Basin (wells location are shown in Fig.1)

本文在利用Requ計(jì)算最大古地溫時(shí)采用平行化學(xué)反應(yīng)模型(Easy%Ro)，計(jì)算結(jié)果見(jiàn)表2.磨溪—高石梯地區(qū)三疊系和寒武系的最大古地溫分別略高于八角場(chǎng)地區(qū)三疊系的最大古地溫和威遠(yuǎn)地區(qū)寒武系的最大古地溫.對(duì)比現(xiàn)今地溫，可得到白堊紀(jì)構(gòu)造抬升所造成的溫度減小量；再結(jié)合物理模擬結(jié)果，即可計(jì)算溫度降低對(duì)超壓層壓力降低的貢獻(xiàn)(表2).降溫幅度差異使得八角場(chǎng)地區(qū)須家河組的壓力減小量小于磨溪—高石梯地區(qū)須家河組的壓力減小量，二者相差33.4 MPa；磨溪—高石梯地區(qū)寒武系的壓力減小量小于威遠(yuǎn)地區(qū)寒武系的壓力減小量，二者相差30.1 MPa.因此，溫度降低是川中古隆起現(xiàn)今各超壓層系在不同井區(qū)壓力系數(shù)存在差異的主要原因.

此外，注意到八角場(chǎng)地區(qū)與磨溪—高石梯地區(qū)須家河組由溫度降低造成的壓力差(33.4 MPa)大于現(xiàn)今實(shí)際壓力差(約27 MPa)，而威遠(yuǎn)地區(qū)與磨溪—高石梯地區(qū)寒武系由溫度降低造成的壓力差(30.1 MPa)小于現(xiàn)今實(shí)際壓力差(約48 MPa).由于抬升前埋深基本相同(同一套地層的最大古地溫差異較小，見(jiàn)表2)，可以認(rèn)為川中地區(qū)在抬升前同一套地層的壓力狀態(tài)基本一致，從而排除抬升前就存在壓力差異的原因.須家河組實(shí)際壓力差偏小，說(shuō)明在碎屑巖層系內(nèi)發(fā)生了流體橫向運(yùn)移、壓力橫向傳遞.寒武系實(shí)際壓差偏大，可能是由于威遠(yuǎn)隆起區(qū)大幅抬升后保存條件比磨溪—高石梯地區(qū)寒武系的保存條件差，發(fā)生了流體的部分泄漏，現(xiàn)今地層壓力已降低為靜水壓力.

表2 川中古隆起不同井區(qū)T3、∈溫度降低量 及其引起的壓力下降幅度Table 2 Temperature reduction and the corresponding pressure decrease for T3 and ∈ in different wells in the Central Paleo-Uplift of the Sichuan Basin

注：*溫度降低造成的壓力減小=降溫幅度×1.076 MPa/℃.

4.2 溫度對(duì)生烴增壓的控制

泥巖欠壓實(shí)在盆地超壓形成中具有重要意義.泥巖欠壓實(shí)(也稱(chēng)不均衡壓實(shí))形成異常高壓的根本原理是快速埋藏使泥巖孔隙快速減小，流體排出受滯并承擔(dān)了部分骨架壓力，從而形成超壓.欠壓實(shí)形成的超壓隨著埋深增大而增大，在埋深最大時(shí)超壓也達(dá)到最大.地下流體流動(dòng)滿足達(dá)西定律：

(1)

其中

ρ=ρsc·exp[β(P-0.1)-α(T-25)],

(2)

μ=10n;

(3)

式中v為流體流速, m/s;K為滲透率, m2;μ為黏度, MPa·s; dh/dl為壓力差, MPa;ρ、ρsc分別為地下和地表時(shí)水的密度, kg·m-3;T為溫度, ℃;α=5×10-4℃-1,β=4.3×10-4MPa-1.公式(2)據(jù)(Bethke,1985).

根據(jù)式(2)和式(3)可以看出，溫度升高會(huì)造成地層水密度和黏度的減小，但黏度的減小幅度更加顯著；進(jìn)而在滲透率和壓差都不變的情況下，造成流速的增大(式(1)).因此，溫度升高在欠壓實(shí)形成過(guò)程中起到負(fù)面的作用.但是，在欠壓實(shí)形成時(shí)滲透率快速降低至極小值，并成為影響流速和超壓形成的最關(guān)鍵因素.相比而言，溫度引起的密度和黏度變化對(duì)流體流速及超壓的影響可以忽略不計(jì).本文通過(guò)盆地模擬結(jié)果也證明，即使川中古隆起上三疊統(tǒng)的溫度改變50 ℃，欠壓實(shí)超壓的變化也非常有限(超壓改變量小于1%).

5 討論

5.1 沉積盆地現(xiàn)今溫-壓關(guān)系

沉積盆地現(xiàn)今溫度-壓力關(guān)系是漫長(zhǎng)地質(zhì)歷史過(guò)程中多種作用的結(jié)果，分析現(xiàn)今溫度-壓力關(guān)系應(yīng)深入考慮其所處的盆地環(huán)境和演化歷程.若把地下溫度和壓力作為一個(gè)整體并認(rèn)為盆地是由一個(gè)個(gè)“封閉系統(tǒng)”組成，則每一個(gè)封閉系統(tǒng)內(nèi)溫度與壓力應(yīng)保持線性關(guān)系(劉震等,2012)：

圖7 GK1井埋藏史、原油裂解時(shí)間與磨溪—高石梯地區(qū)下古生界烴類(lèi)伴生鹽水包裹體均一溫度Fig.7 Burial history and the time of oil cracking for Well GK1 and the homogenization temperature of aqueous inclusionswhich associated with hydrocarbon inclusions from the Lower Paleozoic in the Moxi-Gaoshiti area

(4)

式中P為壓力，MPa;T為溫度，℃；K為斜率，MPa/℃；L為常數(shù).進(jìn)而得到系統(tǒng)內(nèi)的溫-壓斜率為

(5)

式中ΔP為壓力梯度，MPa/100 m；ΔT為地溫梯度, ℃/100 m.

對(duì)于淺部靜水壓力系統(tǒng)，由于ΔP基本保持在1 MPa/100 m左右，系統(tǒng)內(nèi)溫-壓斜率K主要受地溫梯度的影響.不同沉積盆地的平均地溫梯度差異很大(1.5 ℃/100 m～4.5 ℃/100 m)，對(duì)K產(chǎn)生的影響明顯.我國(guó)東部盆地地溫梯度(>3.5 ℃/100 m)明顯高于中西部盆地(<2.5 ℃/100 m)，因此，東部盆地的淺層溫-壓斜率整體上應(yīng)大于中西部盆地.對(duì)于深部有異常壓力發(fā)育的系統(tǒng)，壓力梯度(0.8 MPa/100 m～2.2 MPa/100 m)和地溫梯度一起控制著現(xiàn)今溫-壓斜率.現(xiàn)今壓力梯度是地質(zhì)歷史復(fù)雜作用的結(jié)果，現(xiàn)今地溫梯度則是目前盆地?zé)岜尘暗捻憫?yīng).

5.2 構(gòu)造抬升時(shí)其他作用對(duì)壓力的影響

構(gòu)造抬升引起上覆靜巖壓力減小，地層可能會(huì)產(chǎn)生彈性或非彈性形變.姜振學(xué)等(2007)通過(guò)物理模擬實(shí)驗(yàn)顯示，天然砂巖的體積回彈量通常小于1%，且其中80%的回彈量都是集中在5～0 MPa的低壓階段.構(gòu)造抬升可能會(huì)發(fā)生褶皺和裂縫等非彈性形變.川中古隆起遠(yuǎn)離盆地邊緣，喜山期以整體抬升為主，發(fā)生褶皺變形不明顯.裂縫和斷裂對(duì)超壓體系的封閉能力會(huì)造成極大破壞，流體泄漏之后超壓也隨之消失.四川盆地發(fā)育的多套優(yōu)質(zhì)蓋層(見(jiàn)圖2左)在抬升剝蝕過(guò)程中沒(méi)有被破壞，是現(xiàn)今川中地區(qū)超壓以及氣藏得以保存的關(guān)鍵.裂縫產(chǎn)生的孔隙很小，即使在裂縫發(fā)育地層，裂縫孔隙度主要為0.01%～0.04%(昌倫杰等,2014)，對(duì)超壓的影響有限.但裂縫會(huì)顯著提高地層的滲透率，超壓流體在超壓系統(tǒng)內(nèi)部流動(dòng)速率及壓力傳遞效率提高.綜上所述，以碳酸鹽巖和致密砂巖為主的四川盆地，構(gòu)造抬升造成的地層形變對(duì)現(xiàn)今仍舊埋藏較深的超壓系統(tǒng)的影響不是最主要的.

當(dāng)孔隙流體為烴類(lèi)(特別是氣態(tài)烴)時(shí)，其熱膨脹系數(shù)與地層水明顯不同，會(huì)改變圖5所示的溫-壓變化關(guān)系，甚至形成超壓(Katahara and Corrigan,2002).但是由于地層水是地層孔隙的主要流體(即便是川中古隆起須家河組天然氣產(chǎn)層的含水飽和度也在45%～96%(曾青高等,2009))，因此當(dāng)溫度降低時(shí)，四川盆地的孔隙壓力仍舊是逐漸減小.

6 結(jié)論

依據(jù)對(duì)川中古隆起溫壓場(chǎng)的上述分析，可以得出如下結(jié)論：

(1)川中古隆起現(xiàn)今發(fā)育多個(gè)超壓系統(tǒng).同一超壓層系在不同井區(qū)的壓力不同，例如威遠(yuǎn)地區(qū)寒武系為常壓，磨溪—高石梯地區(qū)寒武系為超壓；磨溪—高石梯地區(qū)須家河組壓力系數(shù)小于八角場(chǎng)地區(qū)須家河組的壓力系數(shù).結(jié)合現(xiàn)今地溫場(chǎng)的分布特征，發(fā)現(xiàn)同一超壓層系的壓力值和超壓強(qiáng)度與溫度呈正相關(guān)性.

(2)物理模擬實(shí)驗(yàn)結(jié)果顯示，在絕對(duì)封閉條件下流體壓力隨溫度改變而改變.當(dāng)壓力大于15 MPa時(shí)，溫度-壓力關(guān)系近乎為線性關(guān)系(1.076 MPa/℃).分析川中古隆起溫度演化歷史，認(rèn)為白堊紀(jì)以來(lái)差異性降溫是現(xiàn)今各井區(qū)超壓差異的關(guān)鍵.此外，現(xiàn)今壓力分布格局還應(yīng)疊加了壓力橫向傳遞及流體散失的影響.

(3)溫度是川中古隆起生烴增壓的“外因”，根據(jù)古溫度恢復(fù)結(jié)果，結(jié)合包裹體測(cè)溫?cái)?shù)據(jù)，確定寒武系超壓形成于180～110 Ma，在構(gòu)造抬升前(～90 Ma)發(fā)生壓力調(diào)整.溫度對(duì)于上三疊統(tǒng)須家河組的欠壓實(shí)超壓形成的影響可以忽略不計(jì).

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(本文編輯 胡素芳)

The effect of temperature on the overpressure distribution and formation in the Central Paleo-Uplift of the Sichuan Basin

LIU Yi-Feng1,2, ZHENG Lun-Ju3, QIU Nan-Sheng1,2*, JIA Jing-Kun1,2, CHANG Qing1,2

1StateKeyLaboratoryofPetroleumResourceandProspecting,ChinaUniversityofPetroleum,Beijing102249,China2ResearchCenterforBasinandReservoir,ChinaUniversityofPetroleum,Beijing102249,China3WuxiResearchInstituteofPetroleumGeology,SINOPEC,WuxiJiangsu214151,China

Temperature and pore pressure are two important physical fields in sedimentary basins. Some potential mechanisms of overpressure, such as aquathermal expansion, diagenesis and hydrocarbon generation, are related to temperature. Furthermore, temperature may control evolution and distribution characteristics of overpressure. The Sichuan Basin, located in southwest China, is a typical overpressuring basin. Combining physical simulation results with practical geological condition, the impact of temperature on overpressures in the Central Paleo-Uplift of the Sichuan Basin is analyzed.The measured temperature and pore pressure data, which were obtained from boreholes in different gas fields, were collected to analyze the present relationship between temperature and pressure. Physical simulation experiments were carried out in laboratory to study the temperature-pressure relationship in absolutely sealed condition. The Easy%Romodel for vitrinite reflectance and micro-thermometry of fluid inclusions were applied to reconstruct the maximum paleo-temperatures of various formations for different regions in the Central Paleo-Uplift. Based on the thermal history, the impacts of temperature on overpressures generation through oil cracking and disequilibrium compaction were discussed.Multiple overpressure systems exist vertically in the Central Paleo-Uplift in the Sichuan Basin and the main mechanisms for each overpressure system are different. According to petrophysical properties of overpressuring formations, the Upper Triassic overpressure is mainly generated by disequilibrium compaction and the Cambrian overpressure is mainly caused by gas generation, respectively. For the same overpressuring formation, overpressure is positively correlated with temperature in the lateral direction. The pressure-temperature gradient is 1.075 MPa/℃ for the Cambrian overpressure system and 1.24 MPa/℃ for the Upper Triassic overpressure system. Physical simulation experiment results show that fluid pressure is closely related to temperature in absolutely sealed condition. The pressure-temperature gradient is relatively small in low pressure phase and such relationship is almost linear as pressure is higher than 15 MPa, with gradient value about 1.076 MPa/℃. Formations in the Sichuan Basin have experienced high temperature and the values of Rofor the Cambrian Formation in the Central Paleo-Uplift are higher than 3%. Maximum temperatures reconstructed by Roand fluid inclusions indicate that, before the Later Cretaceous uplift, the Cambrian Formation was 225 ℃ in Moxi-Gaoshiti area and 208 ℃ in Weiyuan area and the Upper Triassic Formation was 158 ℃ in Moxi-Gaoshiti area and 148 ℃ in Bajiaochang area, respectively. Seals in the Sichuan Basin have very low porosity and permeability because of lithological character and intense compaction. Therefore, the overpressure systems could be deemed absolutely sealed. Combining the physical simulation experiments with temperature decrease since the Late Cretaceous, the pressure of the Cambrian Formation decreased 121.6 MPa in Weiyuan area and 91.5 MPa in Moxi-Gaoshiti area, and the pressure of the Upper Triassic Formation decreased 48 MPa in Bajiaochang area and 79 MPa in Moxi-Gaoshiti area. Maturity evolution of organic matter and hydrocarbon generation are mainly controlled by temperature. Oil cracking in the Cambrian reservoirs was mainly occurred during 180～110 Ma, and adjusted in 90 Ma. In this period, the Cambrian overpressure formed gradually. Based on basin modeling, the effect of temperature on disequilibrium compaction overpressure can be negligible. However, the Upper Triassic overpressure must also reach the maximum in 90Ma, because of the deepest burial depth reached in this period.Through this study, we can obtain the following conclusions: (1) Multiple overpressure systems caused by different mechanisms are developed in the Central Paleo-Uplift in the Sichuan Basin and positive correlations between pressure and temperature exist in each pressure systems. (2) When pressure is greater than 15 MPa, it would change 1.076 MPa for a temperature change of 1 ℃ in an absolutely sealed condition. The difference in temperature reduction can be regarded as the primary reason for various intensity of pressure within the Central Paleo-Uplift. Besides that, some degree of lateral transfer and leakage of pressure must occur. (3) Controlled by temperature, the Lower Paleozoic overpressure caused by oil cracking formed during 180～110 Ma and redistributed in 90 Ma. However, the effect of temperature on the Upper Triassic disequilibrium compaction overpressure generation is negligible.

Central Paleo-Uplift; Temperature; Overpressure; Physical modeling

10.6038/cjg20150715.

國(guó)家杰出青年基金項(xiàng)目 (41125010)，國(guó)家“973”項(xiàng)目(2011CB201101) 和國(guó)家科技重大專(zhuān)項(xiàng) (2011ZX05007-002)資助.

劉一鋒,男,1987年出生，博士研究生，主要從事含油氣盆地溫壓場(chǎng)和油氣成藏機(jī)理方面的研究. E-mail: liuyf1103@foxmail.com

*通訊作者 邱楠生，男，教授，主要從事沉積盆地?zé)狍w制、溫壓場(chǎng)以及油氣成藏機(jī)理等方面的研究.E-mail: qiunsh@cup.edu.cn

10.6038/cjg20150715

P314

2015-02-02，2015-05-06收修定稿

劉一鋒, 鄭倫舉, 邱楠生等.川中古隆起超壓分布與形成的地溫場(chǎng)因素.地球物理學(xué)報(bào)，58(7):2380-2390,

Liu Y F, Zheng L J, Qiu N S, et al. The effect of temperature on the overpressure distribution and formation in the Central Paleo-Uplift of the Sichuan Basin.ChineseJ.Geophys. (in Chinese),58(7):2380-2390,doi:10.6038/cjg20150715.