秦向輝,陳群策,郝越進(jìn),孟 文,孫東生
1)中國(guó)地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所,北京 100081;2)國(guó)土資源部新構(gòu)造運(yùn)動(dòng)與地質(zhì)災(zāi)害重點(diǎn)實(shí)驗(yàn)室,北京 100081;3)安徽省地質(zhì)礦產(chǎn)勘查局313地質(zhì)隊(duì),安徽六安 237010
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安徽金寨沙坪溝礦區(qū)地應(yīng)力測(cè)量與現(xiàn)今應(yīng)力場(chǎng)研究
秦向輝1,2),陳群策1,2),郝越進(jìn)3),孟 文1,2),孫東生1,2)
1)中國(guó)地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所,北京 100081;
2)國(guó)土資源部新構(gòu)造運(yùn)動(dòng)與地質(zhì)災(zāi)害重點(diǎn)實(shí)驗(yàn)室,北京 100081;
3)安徽省地質(zhì)礦產(chǎn)勘查局313地質(zhì)隊(duì),安徽六安 237010
摘 要:安徽金寨沙坪溝鉬礦位于秦嶺—大別山成礦帶東段,是近年來(lái)我國(guó)發(fā)現(xiàn)的世界級(jí)特大鉬礦,其開(kāi)采設(shè)計(jì)和地質(zhì)安全評(píng)價(jià)備受重視。為了查明沙坪溝礦區(qū)現(xiàn)今應(yīng)力場(chǎng)特征,指導(dǎo)礦區(qū)開(kāi)采設(shè)計(jì),在礦區(qū)3個(gè)700~1000 m深鉆孔中開(kāi)展了水壓致裂法地應(yīng)力測(cè)量工作,測(cè)量結(jié)果表明:(a)礦區(qū)最大、最小水平主應(yīng)力隨深度增加梯度系數(shù)分別為0.0226和0.0166,與區(qū)域地應(yīng)力背景值基本一致;(b)礦區(qū)實(shí)測(cè)最大水平主應(yīng)力優(yōu)勢(shì)方位為NE?NEE向,與區(qū)域構(gòu)造應(yīng)力場(chǎng)方向一致;(c)實(shí)測(cè)地應(yīng)力數(shù)據(jù)揭示的應(yīng)力結(jié)構(gòu)與區(qū)域結(jié)果存在差異,分析認(rèn)為,沙坪溝礦區(qū)所在的桐柏—大別構(gòu)造帶特殊的構(gòu)造位置和構(gòu)造活動(dòng)機(jī)制可能是造成這種差異的主要原因。進(jìn)一步,利用庫(kù)倫滑動(dòng)摩擦準(zhǔn)則評(píng)價(jià)了實(shí)測(cè)地應(yīng)力數(shù)據(jù),探討了其對(duì)礦區(qū)開(kāi)采設(shè)計(jì)的意義,結(jié)果表明,礦區(qū)現(xiàn)今地應(yīng)力強(qiáng)度不足以導(dǎo)致以斷裂失穩(wěn)活動(dòng)或地震為表現(xiàn)形式的地應(yīng)力狀態(tài)調(diào)整事件的發(fā)生,從地應(yīng)力角度講,沙坪溝礦區(qū)目前總體處于穩(wěn)定的構(gòu)造應(yīng)力環(huán)境。本文結(jié)果,對(duì)于沙坪溝礦區(qū)開(kāi)采設(shè)計(jì)、地質(zhì)安全評(píng)價(jià)及大別山造山帶東段構(gòu)造應(yīng)力場(chǎng)研究有重要意義。
關(guān)鍵詞:沙坪溝鉬礦;地應(yīng)力;水壓致裂法;現(xiàn)今地應(yīng)力狀態(tài);庫(kù)倫滑動(dòng)摩擦準(zhǔn)則
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本文由深部探測(cè)技術(shù)與實(shí)驗(yàn)研究專(zhuān)項(xiàng)(編號(hào):Sinoprobe-06-03)、中國(guó)地質(zhì)調(diào)查局地質(zhì)礦產(chǎn)調(diào)查專(zhuān)項(xiàng)項(xiàng)目(編號(hào):1212011220122)和中國(guó)地質(zhì)科學(xué)院地質(zhì)力學(xué)研究所基本科研業(yè)務(wù)費(fèi)項(xiàng)目(編號(hào):DZLXJK201404)聯(lián)合資助。
賦存于地質(zhì)體內(nèi)未受擾動(dòng)的應(yīng)力稱(chēng)之為地應(yīng)力,地殼淺表層變形和內(nèi)部構(gòu)造運(yùn)動(dòng)及其導(dǎo)致的地質(zhì)災(zāi)害都與地應(yīng)力密切相關(guān)(Zoback et al.,1993;Hardebeck and Hauksson,2001;Liao et al.,2003;郭啟良等,2009;Chang et al.,2010;Lin et al.,2013)。地質(zhì)學(xué)家一直嘗試通過(guò)揭示地殼淺表部地應(yīng)力狀態(tài)及作用規(guī)律,解決斷裂失穩(wěn)活動(dòng)、地震地質(zhì)災(zāi)害機(jī)理、工程領(lǐng)域巖體穩(wěn)定性等問(wèn)題(吳滿(mǎn)路等,2002,2005;陳群策等,2010;Lin et al.,2013;Qin et al.,2014;譚成軒等,2014;曹輝等,2015)。對(duì)于深部礦產(chǎn)開(kāi)采、地下空間開(kāi)發(fā)而言,詳細(xì)查明工程區(qū)現(xiàn)今地應(yīng)力場(chǎng)特征,對(duì)于地下工程設(shè)計(jì)和巖體穩(wěn)定性分析有重要作用,也可為可能出現(xiàn)的工程地質(zhì)問(wèn)題的合理解決提供支持(吳滿(mǎn)路等,2002,2005;譚成軒等,2006)。
安徽省金寨縣沙坪溝鉬礦位于東秦嶺—大別成礦帶東段,是我國(guó)近年來(lái)發(fā)現(xiàn)的特大斑巖型鉬礦,資料顯示,沙坪溝鉬礦基本探明鉬金屬儲(chǔ)量可達(dá)245×104t(蘇曉靜等,2013)。目前,眾多科研機(jī)構(gòu)和學(xué)者對(duì)礦區(qū)地質(zhì)構(gòu)造與成礦地質(zhì)特征、成礦年代、成礦機(jī)理和成礦巖體礦物學(xué)特征等進(jìn)行了深入研究并取得了豐碩成果(張紅等,2011;孟祥金等,2012;張懷東等,2012),但是對(duì)于礦區(qū)現(xiàn)今構(gòu)造應(yīng)力場(chǎng)研究較少,且尚無(wú)實(shí)測(cè)地應(yīng)力數(shù)據(jù)支持。此外,沙坪溝鉬礦所處的北淮陽(yáng)帶曾作為三疊紀(jì)華南與華北陸塊碰撞造山帶的縫合線,其莫霍面錯(cuò)斷至今仍保存(董樹(shù)文等,1993,1998),查明礦區(qū)現(xiàn)今構(gòu)造應(yīng)力場(chǎng)特征,對(duì)這一關(guān)鍵構(gòu)造區(qū)帶構(gòu)造應(yīng)力場(chǎng)研究、中國(guó)大陸現(xiàn)今構(gòu)造應(yīng)力場(chǎng)分區(qū)研究都有重要意義。為此,我們利用沙坪溝鉬礦部分勘探鉆孔,開(kāi)展了水壓致裂法地應(yīng)力測(cè)量工作。
本文首先簡(jiǎn)述了沙坪溝鉬礦區(qū)域地質(zhì)條件,概述了地應(yīng)力測(cè)量方法,給出了地應(yīng)力測(cè)量結(jié)果;進(jìn)而,利用實(shí)測(cè)數(shù)據(jù)計(jì)算了礦區(qū)地殼淺表層地應(yīng)力狀態(tài)特征參數(shù),分析了礦區(qū)現(xiàn)今構(gòu)造應(yīng)力場(chǎng)特征;最后,利用庫(kù)倫滑動(dòng)摩擦準(zhǔn)則評(píng)價(jià)了實(shí)測(cè)地應(yīng)力數(shù)據(jù),并探討了地應(yīng)力測(cè)量結(jié)果對(duì)于礦區(qū)開(kāi)采的意義。
沙坪溝鉬礦地處豫皖鄂交界的金寨縣境內(nèi),構(gòu)造上屬于大別山造山帶,大別造山帶是揚(yáng)子地塊向華北地塊俯沖形成的碰撞造山帶,它是以深大斷裂構(gòu)造所圍限的“東寬西窄”的楔形地質(zhì)塊體,夾于華北與揚(yáng)子地塊之間;大別造山帶總體構(gòu)造格局被北西向斷裂控制,同時(shí)被數(shù)條近于平行的北北東向斷裂切割。具體來(lái)講,沙坪溝鉬礦位于桐柏—大別山構(gòu)造帶內(nèi),桐柏—桐城斷裂帶和商城—麻城斷裂帶的交匯部位的北東側(cè),亦即桐柏—桐城斷裂與泗河—銀山斷裂(商城—城斷裂的派生斷裂)匯構(gòu)成的“入”字型構(gòu)造銳角處(張紅等,2011;孟祥金等,2012;張懷東等,2012)(圖1)。沙坪溝礦區(qū)基底構(gòu)造層為中元古界盧鎮(zhèn)關(guān)巖群,由形成于新元古代變形變質(zhì)侵入體和變質(zhì)表殼巖組成,后者為一套中基性的火山-沉積巖組合;受燕山期巖漿活動(dòng)的強(qiáng)烈影響,該套地層已被肢解、侵蝕而零星分布,多呈殘留體和捕虜體產(chǎn)出。沙坪溝礦區(qū)巖性主要為黑云母斜長(zhǎng)片麻巖、角閃斜長(zhǎng)片麻巖、硅質(zhì)大理巖以及花崗片麻巖等,局部可見(jiàn)超鎂鐵質(zhì)變質(zhì)巖塊(張懷東等,2012)。礦區(qū)內(nèi)的構(gòu)造主要表現(xiàn)為淺層次的壓性、壓扭性斷裂,主要有北東向和北西向兩組,多為成礦后斷裂。沙坪溝礦區(qū)地貌屬大別山中低山區(qū),為構(gòu)造剝蝕類(lèi)型,海拔高度200~745 m,地形切割較深,山勢(shì)陡峻,溝谷橫截面多呈“V”型,少量規(guī)模較大的溝谷呈“U”型,地形坡度25°~40°。
1.1 地應(yīng)力測(cè)量方法
在沙坪溝礦區(qū)深孔中開(kāi)展的地應(yīng)力測(cè)量,采用了水壓致裂法。作為一種二維地應(yīng)力測(cè)量方法,水壓致裂法通過(guò)水力壓裂誘發(fā)裂縫方式來(lái)測(cè)量地應(yīng)力,是目前常用的地應(yīng)力測(cè)量方法,也是國(guó)際巖石力學(xué)學(xué)會(huì)(ISRM)推薦的五種地應(yīng)力測(cè)量方法之一(Haimson and Cornet,2003)。水壓致裂法測(cè)量的可靠性和準(zhǔn)確性在理論上和實(shí)踐中得到了廣泛認(rèn)可,其測(cè)量原理、程序等參見(jiàn)Amadei和Stephansson(1997)、Haimson和Cornet(2003)、Zoback(2007)等人著述。
水壓致裂法測(cè)量時(shí)使用了單回路水壓致裂地應(yīng)力測(cè)量系統(tǒng)(Qin et al.,2014),現(xiàn)場(chǎng)試驗(yàn)程序均嚴(yán)格按照ISRM建議步驟及要求進(jìn)行(Haimson and Cornet,2003)。對(duì)獲得的水壓致裂參數(shù)進(jìn)行處理時(shí),也采用ISRM建議方法進(jìn)行,尤其對(duì)閉合壓力(Ps)這個(gè)關(guān)鍵參數(shù),綜合了單切線法、dt/dP vs P法(Lee and Haimson,1989)和dP/dt vs P法(Hayashi and Haimson,1991)進(jìn)行取值,以提高閉合壓力計(jì)算結(jié)果準(zhǔn)確性,進(jìn)而提高主應(yīng)力計(jì)算結(jié)果的準(zhǔn)確性。在計(jì)算主應(yīng)力值時(shí),孔隙壓力根據(jù)已有研究結(jié)果,取近似等于靜水壓力處理(Barton et al.,1995;Zoback and Townend,2001)。
圖1 沙坪溝鉬礦構(gòu)造位置示意圖(據(jù)孟祥金等,2012修改)Fig.1 Tectonic map of the Shapinggou molybdenum deposit(modified after MENG et al.,2012)
圖2 沙坪溝礦區(qū)水壓致裂地應(yīng)力測(cè)量曲線Fig.2 Typical test-interval pressure-time curves obtained from hydraulic fracturing tests in the Shapinggou molybdenum deposit
圖3 沙坪溝礦區(qū)水壓致裂地應(yīng)力測(cè)量中誘發(fā)裂縫形態(tài)Fig.3 Typical induced fracture trace impressed on packer in the Shapinggou molybdenum deposit
1.2 地應(yīng)力測(cè)點(diǎn)
沙坪溝礦區(qū)三個(gè)地應(yīng)力測(cè)量鉆孔均分布于金寨縣關(guān)廟鄉(xiāng)銀山村附近(見(jiàn)圖1),相距較近,其地質(zhì)概況如下:
工程勘查鉆孔(GCZK2)位于銀山村銀山溝,孔深887 m,孔徑95 mm;鉆探揭露顯示,鉆孔巖性以花崗巖及蝕變正長(zhǎng)巖為主,巖芯總體完整,靜水位58.00 m;由于260 m深度以上巖石較破碎,地應(yīng)力測(cè)量在此深度以下開(kāi)展。
主井鉆孔(ZJZK)位于銀山村蔡灣,孔深1020 m,孔徑75 m;鉆孔揭露顯示,鉆孔巖性主要以花崗巖、黑云斜長(zhǎng)片麻巖、正長(zhǎng)巖為主,局部存在變質(zhì)特征,并且在個(gè)別深度上出現(xiàn)了餅化巖芯,巖芯總體完整,靜水位16.00 m;測(cè)量在400 m深度以下裸孔進(jìn)行。
副井鉆孔(FJZK)位于銀山村沙坪溝,孔深730 m,孔徑95 mm;鉆孔揭露顯示,地層巖性以花崗巖、正長(zhǎng)巖、片麻巖為主,除455 m深度附近存在著嚴(yán)重破碎帶外,巖芯總體完整,靜水位8.00 m。
表1 沙坪溝礦區(qū)水壓致裂地應(yīng)力測(cè)量結(jié)果Table 1 Results of hydraulic fracturing in-situ stress measurements in the Shapinggou molybdenum deposit
1.3 地應(yīng)力測(cè)量結(jié)果
在沙坪溝礦區(qū)三個(gè)鉆孔中共成功進(jìn)行了29段壓裂試驗(yàn)和12段印模試驗(yàn),圖2給出了三個(gè)鉆孔中不同深度的3段典型測(cè)試曲線,圖3給出了一段典型印模結(jié)果,表1給出了三個(gè)鉆孔水壓致裂地應(yīng)力測(cè)量主要參數(shù)以及主應(yīng)力值計(jì)算結(jié)果。
基于表1數(shù)據(jù),我們?cè)u(píng)價(jià)了沙坪溝礦區(qū)地殼淺表層應(yīng)力狀態(tài),包括:地應(yīng)力大小、最大水平主應(yīng)力方向以及應(yīng)力結(jié)構(gòu),圖4a給出了實(shí)測(cè)主應(yīng)力隨深度變化特征,而以線性擬合方式對(duì)沙坪溝礦區(qū)最大、最小主應(yīng)力隨深度變化計(jì)算結(jié)果為:
圖4a、公式(1)和(2)表明,沙坪溝礦區(qū)實(shí)測(cè)地應(yīng)力值總體上表現(xiàn)為隨著深度的增加而增大,符合已有認(rèn)識(shí)和規(guī)律。為了更加準(zhǔn)確地評(píng)價(jià)沙坪溝鉬礦地應(yīng)力分布規(guī)律,我們首先對(duì)比了沙坪溝鉬礦和中國(guó)大陸淺層地應(yīng)力隨深度增加梯度系數(shù)(景鋒等,2007;王艷華等,2012;楊樹(shù)新等,2012),結(jié)果顯示兩者基本一致,但是沙坪溝礦區(qū)地應(yīng)力地表值偏高(即擬合公式截距),表明沙坪溝礦區(qū)近地表水平構(gòu)造作用較強(qiáng)。進(jìn)一步,我們縮小對(duì)比范圍,將沙坪溝礦區(qū)結(jié)果與礦區(qū)外圍地應(yīng)力測(cè)量資料對(duì)比(謝富仁等,2003),結(jié)果顯示,沙坪溝礦區(qū)地應(yīng)力水平明顯高于礦區(qū)外圍,尤其是在450 m深度附近(圖4),進(jìn)一步分析后發(fā)現(xiàn),沙坪溝礦區(qū)450 m深度附近數(shù)據(jù)主要分布于FJZK鉆孔中,而FJZK鉆探揭露顯示,450 m深度附近巖芯十分破碎,結(jié)合該測(cè)點(diǎn)附近地形地貌特征,我們推斷此深度附近的地應(yīng)力測(cè)試數(shù)據(jù)可能受到了地形、斷層等因素的影響(Cuisiant et el.,1992;陳群策等,2004;Tan et al.,2004;譚成軒等,2006),至于那種因素起主控作用,有待進(jìn)一步分析。
圖4b和4c進(jìn)一步給出了沙坪溝鉬礦現(xiàn)今應(yīng)力狀態(tài)特征參數(shù)KHv和KHh的計(jì)算結(jié)果,KHv和KHh可以反映水平構(gòu)造應(yīng)力和水平差應(yīng)力強(qiáng)度,KHv和KHh計(jì)算結(jié)果表明,沙坪溝礦區(qū)KHv分布特征與已有研究結(jié)果大致吻合,而KHh值隨深度變化較小,其穩(wěn)定值約為1.75,略高于已有研究結(jié)果(景鋒等,2007;王艷華等,2012;楊樹(shù)新等,2012)。
圖5給出了沙坪溝礦區(qū)實(shí)測(cè)最大水平主應(yīng)力方向隨深度分布特征。分析表明,除個(gè)別測(cè)點(diǎn)外,沙坪溝礦區(qū)最大水平主應(yīng)力方向總體分布在NE?NEE之間(數(shù)學(xué)平均值為N63°E),對(duì)比圖6給出的華北地區(qū)構(gòu)造應(yīng)力場(chǎng)圖發(fā)現(xiàn),通過(guò)水壓致裂法獲得的沙坪溝礦區(qū)最大水平主應(yīng)力方向與區(qū)域構(gòu)造應(yīng)力場(chǎng)方向基本一致(謝富仁等,2003)。
圖4 沙坪溝礦區(qū)地應(yīng)力值隨深度變化圖(實(shí)心代表沙坪溝礦區(qū)數(shù)據(jù),空心表示礦區(qū)外圍數(shù)據(jù))Fig.4 Variation of measured stresses with depth in the Shapinggou molybdenum deposit(the solid and hollow logos represent data of the Shapinggou molybdenum deposit and the periphery,respectively)
公式(1)和(2)及圖4a還揭示,沙坪溝礦區(qū)實(shí)測(cè)主應(yīng)力之間關(guān)系總體表現(xiàn)為SH>Sh>Sv,應(yīng)力結(jié)構(gòu)為逆沖型,但是800 m深度以下,有轉(zhuǎn)向SH>Sv>Sh的趨勢(shì)(其應(yīng)力結(jié)構(gòu)為走滑型),由于800 m深度以下數(shù)據(jù)很少,對(duì)此暫不能下定論,但是可以確定的是,上述實(shí)測(cè)數(shù)據(jù)均揭示沙坪溝礦區(qū)現(xiàn)今應(yīng)力場(chǎng)以水平應(yīng)力作用為主導(dǎo)。對(duì)比圖4和圖6給出的資料,發(fā)現(xiàn)礦區(qū)實(shí)測(cè)地應(yīng)力數(shù)據(jù)揭示的應(yīng)力結(jié)構(gòu)和其他資料揭示的測(cè)區(qū)外圍應(yīng)力結(jié)構(gòu)存在差異。至于造成這種差異的原因,從構(gòu)造地質(zhì)角度講,沙坪溝礦區(qū)構(gòu)造位置特殊,其所處的桐柏—大別構(gòu)造帶內(nèi)是華北地塊和揚(yáng)子地塊多次碰撞縫合和長(zhǎng)期構(gòu)造演化的結(jié)果,存在著多旋回碰撞縫合,構(gòu)造環(huán)境特殊,構(gòu)造活動(dòng)演化復(fù)雜;進(jìn)一步,從構(gòu)造活動(dòng)力源方面分析,華北地塊構(gòu)造活動(dòng)的力源主要來(lái)自西太平洋板塊的俯沖,以弱張剪作用為主,而揚(yáng)子地塊構(gòu)造活動(dòng)的力源主要來(lái)自菲律賓板塊的推擠作用,以中強(qiáng)壓剪作用為主,桐柏—大別構(gòu)造帶則是兩者綜合作用的結(jié)果,加之沙坪溝礦區(qū)又被數(shù)條深切斷裂圍限,其應(yīng)力結(jié)構(gòu)又在一定程度上疊加了斷裂的影響,這也恰是豫皖—蘇北應(yīng)力區(qū)的應(yīng)力環(huán)境特點(diǎn),而測(cè)區(qū)外圍的應(yīng)力結(jié)構(gòu)資料(尤其是震源機(jī)制解資料)更多的是反應(yīng)華北和華南板塊的構(gòu)造應(yīng)力環(huán)境,礦區(qū)實(shí)測(cè)數(shù)據(jù)揭示的應(yīng)力結(jié)果與外圍區(qū)存在差異也不難理解了。因此,我們認(rèn)為除測(cè)試數(shù)據(jù)量及深度差別可能存在的影響外,沙坪溝礦區(qū)和外圍應(yīng)力結(jié)構(gòu)的差異主要與其所處特殊構(gòu)造位置和構(gòu)造活動(dòng)機(jī)制有關(guān)。
圖5 沙坪溝礦區(qū)實(shí)測(cè)最大水平主應(yīng)力方向隨深度分布特征Fig.5 Variation of the orientations of the maximum horizontal stresses with depth in the Shapinggou molybdenum deposit
圖6 華北現(xiàn)今構(gòu)造應(yīng)力場(chǎng)圖(據(jù)謝富仁等,2003修改)Fig.6 Recent tectonic stress field in North China(modified after XIE et al.,2003)
本文第二部分揭示了沙坪溝礦區(qū)現(xiàn)今地應(yīng)力狀態(tài),那么礦區(qū)現(xiàn)今構(gòu)造應(yīng)力環(huán)境是否穩(wěn)定,會(huì)不會(huì)以斷層活動(dòng)或地震的形式導(dǎo)致地應(yīng)力狀態(tài)調(diào)整呢?研究表明,現(xiàn)代構(gòu)造運(yùn)動(dòng)的性質(zhì)與強(qiáng)度,取決于區(qū)域地應(yīng)力狀態(tài)和巖體力學(xué)性質(zhì)(Zoback and Healy,1992;Zoback et al.,1993;Boatwright and Cocco,1996;陳慶宣等,1998;Townend and Zoback,2000;Zoback and Townend,2000;Rivera and Kanamori,2002;Chang et al.,2010)。Townend和Zoback(2000)認(rèn)為地殼處于破裂極限狀態(tài),并且通過(guò)斷裂的失穩(wěn)活動(dòng)或地震事件的發(fā)生,地殼應(yīng)力狀態(tài)能夠保持一個(gè)均衡的狀態(tài),Zoback和Townend(2001)分析大量地應(yīng)力實(shí)測(cè)資料后認(rèn)為地殼淺部應(yīng)力狀態(tài)是受斷裂控制的,且其強(qiáng)度的極限狀態(tài)可以用庫(kù)倫摩擦滑動(dòng)準(zhǔn)則來(lái)描述,并指出在使用庫(kù)倫摩擦滑動(dòng)準(zhǔn)則時(shí),粘聚力可以忽略,而摩擦系數(shù)可以使用Byerlee(1978)基于大量室內(nèi)實(shí)驗(yàn)而提出的除少數(shù)巖石外地殼巖石的內(nèi)摩擦系數(shù)為0.6~1.0這一結(jié)論(即著名的Byerlee定律)。在引入主應(yīng)力和有效應(yīng)力概念后,上文提到的用來(lái)描述地殼淺部強(qiáng)度極限狀態(tài)的庫(kù)倫摩擦滑動(dòng)準(zhǔn)則變成:
圖7 基于地應(yīng)力實(shí)測(cè)數(shù)據(jù)的沙坪溝礦區(qū)庫(kù)倫摩擦滑動(dòng)計(jì)算結(jié)果Fig.7 Plots displaying calculation results of measured in-situ stress data obtained in the Shapinggou molybdenum deposit using Coulomb frictional failure criterion
式中,S1,S3分別為斷裂外圍最大與最小主應(yīng)力;P0為孔隙壓力,其值與靜水壓力大致相等(Barton et al.,1995;Zoback and Townend,2001)。
使用上式評(píng)價(jià)某個(gè)地區(qū)現(xiàn)今地應(yīng)力強(qiáng)度時(shí),可將實(shí)測(cè)地應(yīng)力值帶入上式,若由實(shí)測(cè)地應(yīng)力數(shù)據(jù)計(jì)算的公式(3)左側(cè)結(jié)果大于右側(cè),那表明該區(qū)域現(xiàn)今地應(yīng)力強(qiáng)度達(dá)到地殼淺表部強(qiáng)度極限,可能通過(guò)斷層失穩(wěn)活動(dòng)或地震事件的發(fā)生來(lái)調(diào)整地應(yīng)力狀態(tài);反之,則表明該區(qū)域現(xiàn)今地應(yīng)力強(qiáng)度未達(dá)到強(qiáng)度極限,地應(yīng)力狀態(tài)不會(huì)調(diào)整,暫時(shí)處于穩(wěn)定狀態(tài)。
圖7給出了利用庫(kù)倫摩擦滑動(dòng)準(zhǔn)則對(duì)沙坪溝礦區(qū)實(shí)測(cè)地應(yīng)力數(shù)據(jù)評(píng)價(jià)結(jié)果,結(jié)果表明,沙坪溝鉬礦實(shí)測(cè)地應(yīng)力數(shù)據(jù)均未達(dá)到地殼淺表部強(qiáng)度極限的上限值(即摩擦系數(shù)μ=1.0時(shí)限值),也僅有一小部分?jǐn)?shù)據(jù)(<30%)超過(guò)了強(qiáng)度極限的下限值(即摩擦系數(shù)μ=0.6時(shí)限值),而這部分?jǐn)?shù)據(jù)又主要分布在FJZK鉆孔中,如前所述,這部分?jǐn)?shù)據(jù)還可能疊加了地形和斷裂的影響,由此可以認(rèn)為沙坪溝礦區(qū)現(xiàn)今地應(yīng)力強(qiáng)度不足以導(dǎo)致斷裂失穩(wěn)活動(dòng)或地震事件的發(fā)生,從地應(yīng)力角度講,沙坪溝礦區(qū)目前處于穩(wěn)定構(gòu)造應(yīng)力環(huán)境。
上述分析從構(gòu)造應(yīng)力場(chǎng)角度進(jìn)行,實(shí)際上對(duì)于礦產(chǎn)開(kāi)采而言,更為關(guān)心的是在巷道或豎井開(kāi)挖是否會(huì)出現(xiàn)巖爆、片幫剝落等工程問(wèn)題,由于缺少其他關(guān)鍵資料,暫時(shí)不能對(duì)此類(lèi)問(wèn)題進(jìn)行具體的分析,后續(xù)研究中將重點(diǎn)關(guān)注此問(wèn)題。此外,對(duì)于面積較大且構(gòu)造與地形條件復(fù)雜的沙坪溝礦區(qū)來(lái)講,繼續(xù)補(bǔ)充地應(yīng)力實(shí)測(cè)數(shù)據(jù),對(duì)于具體工程問(wèn)題的解決也十分有益(譚成軒等,2006;王成虎等,2009)。
本文采用水壓致裂法在金寨沙坪溝礦區(qū)3個(gè)深孔中開(kāi)展了地應(yīng)力測(cè)量工作,獲得了實(shí)測(cè)地應(yīng)力數(shù)據(jù),初步揭示了金寨沙坪溝礦區(qū)地殼淺表層現(xiàn)今應(yīng)力狀態(tài),同時(shí)利用庫(kù)倫摩擦滑動(dòng)準(zhǔn)則,探討了實(shí)測(cè)地應(yīng)力數(shù)據(jù)對(duì)礦產(chǎn)開(kāi)采的意義。通過(guò)以上分析,可以得到如下結(jié)論:
(1)沙坪溝礦區(qū)最大、最小水平主應(yīng)力隨深度增加梯度系數(shù)分別為0.0226和0.0166;地應(yīng)力狀態(tài)特征參數(shù)KHv分布特征與已有認(rèn)識(shí)一致,而KHh隨深度變化很小,總體分布在1.75左右;上述結(jié)果表明,沙坪溝礦區(qū)地殼淺表層現(xiàn)今構(gòu)造應(yīng)力場(chǎng)以水平向應(yīng)力作用為主導(dǎo)。
(2)實(shí)測(cè)地應(yīng)力數(shù)據(jù)反映,沙坪溝礦區(qū)地殼淺表層最大水平主應(yīng)力優(yōu)勢(shì)方向總體為近NE?NEE向,與區(qū)構(gòu)造應(yīng)力場(chǎng)方向基本一致;但是,實(shí)測(cè)地應(yīng)力數(shù)據(jù)揭示的應(yīng)力結(jié)構(gòu)總體表現(xiàn)為逆斷型,與其他資料解釋的區(qū)域應(yīng)力結(jié)構(gòu)存在差異,初步認(rèn)為這種差異主要由礦區(qū)所在桐柏—大別構(gòu)造帶與其兩側(cè)地塊構(gòu)造位置和構(gòu)造活動(dòng)機(jī)制差異造成。
(3)基于實(shí)測(cè)應(yīng)力數(shù)據(jù),使用庫(kù)倫滑動(dòng)摩擦準(zhǔn)則,結(jié)合Byerlee定律,對(duì)沙坪溝礦區(qū)實(shí)測(cè)地應(yīng)力結(jié)果分析表明,礦區(qū)現(xiàn)今地應(yīng)力強(qiáng)度不足以導(dǎo)致斷裂失穩(wěn)活動(dòng)或地震事件的發(fā)生,從地應(yīng)力角度講,礦區(qū)處于穩(wěn)定構(gòu)造應(yīng)力環(huán)境。
Acknowledgements:
This study was supported by the Science and Technology Project(No.Sinoprobe-06-03),China Geological Survey(No.1212011220122)and the Central Public-interest Scientific Institution Basal Research Fund(No.DZLXJK201404).
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Estimation on In-situ Stress Measurements and Present-day Stress Field in the Shapinggou Molybdenum Deposit,Jinzhai County,Anhui Province
QIN Xiang-hui1,2),CHEN Qun-ce1,2),HAO Yue-jin3),MENG Wen1,2),SUN Dong-sheng1,2)
1)Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081;
2)Key Laboratory of Neotectonic Movement &Geohazard,Ministry of Land and Resources,Beijing 100081;
3)No.313 Geological Party,Anhui Bureau of Geology and Mineral Resources,Lu′an,Anhui 237010
Abstract:The Shapinggou molybdenum deposit located in the East Qinling-Dabie metallogenic belt is a superlarge porphyry molybdenum deposit discovered in recent years.Much attention has been paid to the exploitation designing and geological safety evaluation of this deposit.To thoroughly understand the present-day stress field in the Shapinggou molybdenum deposit and put forward suggestions for the exploitation designing,the authors conducted in-situ stress measurement using hydraulic fracturing method in three drill holes at the depths of 700~1000 m in the Shapinggou molybdenum ore district.The measurement results reveal that the grade coefficients of the maximum,minimum horizontal principal stresses versus depths are 0.0226 and 0.0166,respectively,which are consistent with previous results representing the regional tectonic stress field.The measured in-situ stress data also indicate that the dominant direction of the maximum horizontal stress in the Shapinggou molybdenum ore district is in NE?NEE direction,which is consistent with the regional tectonic stress field.However,the stress regimes inferred from the measured in-situ stress data are not consistent with regional results inferred from other methods.The unique tectonic location and the tectonic evolution mechanism of thebook=224,ebook=99Tongbai-Dabie tectonic belt where the Shapinggou molybdenum deposit occurs are considered to be the main factors inducing the discrepancy between the Shapinggou molybdenum deposit and the regional results.Furthermore,the measured in-situ stress data and their implications for the exploitation designing of the Shapinggou molybdenum deposit were estimated by using the Column frictional failure criterion.It is noted that the measured in-situ stress levels in the ore district are lower than those expected to induce the stress field adjustment in the form of fault slipping or earthquake,and thus the Shapinggou molybdenum deposit can be considered to be in the relatively stable tectonic stress state now.The results achieved in this study will contribute to exploitation of the Shapinggou molybdenum deposit and geological safety evaluation as well as to the study of tectonic stress field of the Dabieshan orogenic belt.
Key words:Shapinggou molybdenum deposit;in-situ stress;hydraulic fracturing;present-day stress field;Column frictional failure criterion
作者簡(jiǎn)介:第一秦向輝,男,1984生。碩士,助理研究員。主要從事地應(yīng)力測(cè)量與監(jiān)測(cè)、區(qū)域地殼穩(wěn)定性評(píng)價(jià)等工作。E-mail:qinxiangh03@126.com。
收稿日期:2015-08-18;改回日期:2015-11-17。責(zé)任編輯:魏樂(lè)軍。
中圖分類(lèi)號(hào):P618.65;P553
文獻(xiàn)標(biāo)志碼:A
doi:10.3975/cagsb.2016.02.10