物源與沉積相對鄂爾多斯盆地東南部上古生界砂體展布的控制①

屈紅軍 馬 強 高勝利,3 梅志超 苗建宇

(1.西北大學大陸動力學國家重點實驗室/地質學系 西安 710069;2.中國石油吐哈油田勘探開發研究院 新疆哈密 839000; 3.陜西延長石油(集團)有限公司研究院 西安 710000)

物源與沉積相對鄂爾多斯盆地東南部上古生界砂體展布的控制①

屈紅軍1馬 強2高勝利1,3梅志超1苗建宇1

(1.西北大學大陸動力學國家重點實驗室/地質學系 西安 710069;2.中國石油吐哈油田勘探開發研究院 新疆哈密 839000; 3.陜西延長石油(集團)有限公司研究院 西安 710000)

近幾年鄂爾多斯盆地東南部上古生界陸續發現新的天然氣區帶,顯示出巨大的勘探潛力,因此在盆地東南部上古生界進行物源、沉積相、砂體展布規律的研究具有重要價值。以鉆測井、巖芯及周邊露頭資料為基礎,通過露頭剖面古流向測定、重礦物分析及相帶分析的方法,確定了研究區二疊系的物源方向;分析了上古生界沉積相類型,揭示了骨架砂體平面展布特征。研究認為:山西組與石盒子組物源可分為北北東、北北西及南部三個方向;本溪組主要發育障壁島-潟湖及潮坪沉積,海侵方向來自東部,由沉積相控制的本溪組障壁島砂體呈北北東向展布;山2、山1、盒8段主體發育不同來源方向的三角洲前緣沉積,受物源方向及沉積相控制,以延安為中心區域主體屬于匯水區,骨架砂體分別由四個向延安區域匯聚的北北西、北北東、南南東向及南部三角洲前緣水下分流河道朵體組成,在匯水區不同方向來源砂體在不同層段具有相互疊置特征;山2段南部物源砂體較發育,盒8段北部物源砂體較發育,反映研究區古構造從南高北低轉換為北高南低,指示鄂爾多斯盆地在山西組與石盒子組之間發生了南北向構造反轉。

鄂爾多斯盆地 上古生界 古流向 物源 沉積相 砂體

0 引言

鄂爾多斯盆地已發現的上古生界氣藏主要位于盆地中北部,因而盆地中北部上古生界物源、沉積、儲層、成藏規律研究程度比較高[1～11];近10年來加大了盆地東部研究力度[12～15];盆地西南部也得到了地質工作者的一定重視與研究[16～19],相對而言盆地東南部研究程度比較低,但近幾年在盆地東南部有新的天然氣區帶發現[20,21],顯示出巨大的勘探潛力,但研究僅是初步階段[22～24]。

前人研究認為鄂爾多斯上古生界受多物源控制影響,但以北部物源為主[25～29]。對于是否存在南部物源,以及南部物源的影響范圍、影響程度還不清楚,這在很大程度上影響了研究區的天然氣勘探戰略。由于鄂爾多斯盆地主體為巖性氣藏[2],因此研究物源及沉積相對上古生界儲集砂體展布的控制作用具有重要價值。

圖1 研究區區域位置圖Fig.1 Tectonic location map of the study area

研究區位于盆地東南部的陜北地區,構造單元上位于伊陜斜坡中南部(圖1)。本次研究通過35口探井詳細巖芯觀察及鉆測井資料綜合分析,結合5條野外剖面實測及古流向測定,應用重礦物分析進行物源研究,沉積相分析方法確定沉積相類型及平面展布。以物源及沉積相分析為基礎,確定研究區上古生界主力儲集段本溪組、山2段、山1段、盒8段砂體展布規律。

1 物源分析

物源研究方法已經比較成熟,古流向測定、重礦物分析等是分析物源區的有效方法[30～33],常量或微量地球化學研究方法也應用于物源研究[34,35]。本次研究應用古流向測定、重礦物分析進行物源研究。

1.1 古水流分析

古流向分析不僅可以確定骨架沉積體的走向、圈定古斜坡、推測古岸線走向,還可以按沉積物散布的樣式探索盆地的結構及幾何形態。本次研究依據各種交錯層理傾向,礫石的定向排列分析以及平行層理中的剝離線理等原生沉積構造,經過吳氏網校正后做出流向玫瑰圖,從而指示物源方向。

1 )山西組

從韓城沮水河、澄城三元橋、柳林北成家莊及柳林南三川河剖面玫瑰圖可見(圖2a),南部韓城沮水河及澄城三元橋古水流顯示,主要指向從東南向西北,反映來自為南物源;而山西柳林成家莊剖面水流方向從北向南,反映來自為北物源。總的來說,古水流方向顯示研究區中部在山西組時期整體為匯水區,說明研究區在山西期可能存在南北不同方向的物源。

2 )石盒子組

從韓城沮水河、澄城三元橋、柳林北成家莊、淳化口鎮等剖面玫瑰圖可見(圖2b),南部韓城沮水河、澄城三元橋、淳化口鎮剖面古水流方向主要指向從自南向北,反映來自為南物源;而山西柳林成家莊剖面水流方向還是自北向南,反映來自為北物源。總的來說,古水流方向顯示研究區在石盒子組時期與山西組時期略有所差別,但是研究區主體依然為匯水區說明研究區在石盒子組時期可能存在南北不同的物源。

1.2 重礦物分析

對山西組與石盒子組50個砂巖樣品的重礦物分析可以看出,各組砂巖中主體共有8種陸源重礦物,分別是鋯石、金紅石、電氣石、石榴子石、磁鐵礦、板鈦礦、白鈦礦及綠簾石,其中以鋯石、石榴子石和白鈦礦含量最高,也較穩定,三者之和平均含量可達88%,從而成為主要重礦物(只有2個樣品例外,電氣石和磁鐵礦含量大于10%)。

根據重礦物分析數據,結合前人區域研究資料,可以把研究區山西組與石盒子組物源分為北北東方向、北北西方向和南方三個物源區(圖3a,b):A區為富鋯石區,即鋯石含量較高,石榴子石與白鈦礦含量較低;B區為鋯石+白鈦礦+石榴子石區,即鋯石含量低于A區,但石榴子石與白鈦礦含量相對較高; C區為富白鈦礦區,白鈦礦含量較高,鋯石含量低于A區和B區。

圖2 研究區周邊山西組(a)及石盒子組(b)古流向玫瑰圖Fig.2 Paleo-current rose diagrams of Shanxi Formation in(a)and Shehezi Formation in(b)in surrendering areas of the study area

圖3 研究區山西組(a)及石盒子組(b)重礦物物源分區圖Fig.3 Provenance zoning diagrams of heavymineral in Shanxiand Shehezi Formation in the study area

2 沉積基地底形及沉積相帶控制本溪組砂體展布

通過巖芯詳細觀察,確定本溪組沉積相主要為海岸沉積的障壁島-潟湖及潮坪沉積;其中本2段以發育障壁島-潟湖沉積為主,本1段以發育潮坪沉積為主。由沉積相控制的本溪組主要儲層砂體為本2段障壁島砂體。通常障壁島砂體基本平行于海岸線或沉積相帶方向,因而確定海岸線或沉積相帶方向是海岸沉積研究的重點。

2.1 地層厚度與灰巖厚度反映海岸線及沉積基地底形

通過本溪組地層厚度等值線圖(圖4a)及灰巖厚度(圖4b)等值線圖可以看出,地層厚度等值線與灰巖厚度等值線都具有北北東向展布的特征,指示本溪組沉積時海岸線、沉積基地底形及沉積相帶方向為北北東向,研究區此時可能為伸向南南西方向的一個海灣,地層厚度及灰巖厚度受沉積基地底形影響是顯而易見的(圖5)。

從本溪組地層厚度與灰巖厚度從東向西逐漸變薄(圖5),而南北向變化不大(圖6),說明本溪組沉積時沉積基地底形為東低西高,鄂爾多斯本部地區本溪組時期海侵來自東部,這一認識與前人認識一致[36～38]。

2.2 沉積基地底形及沉積相帶方向指示障壁島及潮道砂體展布

本溪組骨架砂體主要沉積微相為障壁島及潮道,從理論上講二者在空間展布方向是有所區別的。但由于研究區此時為伸向南南西方向的一個海灣,二者都是平行于海灣延伸分布,因此二者在方向上難以區分。由相分析及砂地比等值線圖確定的沉積相平面圖反映的沉積相帶方向為北北東向(圖7a),通過砂巖厚度等值線圖也反映了同樣的方向(圖7b),因此由沉積相控制的本溪組障壁島及潮道砂體呈北北東向展布。

3 物源決定了不同方向三角洲前緣朵狀砂體展布

3.1 物源方向決定了山西組與石盒子組沉積相帶

以35口井詳細的巖芯觀察為基礎,通過巖石相、測井相的綜合分析,確定研究區二疊系山西組及石盒子組沉積相類型主體為三角洲相三角洲前緣亞相,其骨架砂體微相為水下分流河道。

從山西組-盒8段南北向沉積相對比剖面圖(圖6)可以看出,不同層段南北區域之間存在砂巖不發育區域,分割南北物源區,說明研究區存在北部及南部物源,且在不同層段南北物源影響范圍不同,不同來源方向水下分流河道朵狀砂體在不同層段具有相互疊置特征(圖6)。

圖4 盆地東南部本溪組地層厚度(a)及灰巖厚度(b)等值線圖Fig.4 Contourmaps of stratum thickness and limestone rock thickness in Benxi Formation in southeast Orodos basin

圖5 延111井-延125井本溪組-山西組東西向沉積相對比剖面圖Fig.5 WE direction depositional facies cross section from Well Yan-111 toWell Yan-125 well of Benxi Formation to He 8 member

通過單井巖石相、測井相、聯井沉積相對比剖面、砂地比等值線分析,繪制了研究區二疊系山2段、山1段、盒8段沉積相平面圖(圖8a,9a,10a)。從山2段、山1段、盒8段沉積相平面圖可以看出二疊系山西組與石盒子組物源可分為北北東、北北西及南部三個方向,以延安為中心區域屬于匯水區;山2段南部物源范圍較大,影響到了延安以北區域;盒8段北部物源范圍較大,越過了延安一帶,相應的南部物源范圍后撤。

圖6 延氣1井-延128井本溪組-盒8段南北向沉積相對比剖面圖Fig.6 NS direction depositional facies cross-section from Well Yanqi-1 to Well Yan-128 of Benxi Formation to He 8 member

圖7 鄂爾多斯盆地東南部本溪組沉積相及砂體展布平面圖Fig.7 Depositional facies and sandbody distribution of Benxi Formation in southeast Orodos basin

圖8 鄂爾多斯盆地東南部山2段沉積相及砂體展布平面圖Fig.8 Depositional facies and sandbody distribution plans of Shan-2 member in southeast Ordos basin

圖9 鄂爾多斯盆地東南部山1段沉積相及砂體展布平面圖Fig.9 Depositional facies and sandbody distribution of Shan-1 member in southeast Orodos basin

圖10 鄂爾多斯盆地東南部盒8段沉積相及砂體展布平面圖Fig.1 0 Depositional facies and sandbody distribution of He-8 member in southeast Orodos basin

3.2 沉積相帶決定了水下分流河道朵狀砂體展布

骨架砂體展布受控于物源方向及沉積相。在研究區山西組與石盒子組沉積期為不同方向的三角洲前緣沉積特征的認識基礎上,以物源方向及沉積相帶展布為框架,以山2段、山1段、盒8段砂巖厚度數據為依據,分別繪制不同主力儲集段砂巖厚度等值線圖(圖8b,9b,10b)。從圖可以看出,山2段、山1段、盒8段骨架砂體分別由四個向延安區域匯聚的北北西、北北東、南部、南南東向三角洲前緣水下分流河道朵體組成;山2段南部物源砂體較發育(圖6,圖8b),盒8段北部物源砂體較發育(圖6,圖10b)。

4 結論

(1)研究區本溪組主要發育障壁島-潟湖及潮坪沉積,海侵方向來自東部,由沉積相控制的本溪組障壁島砂體呈北北東向展布;山西組與石盒子組物源可分為北北東、北北西及南部三個方向;

(2)山2段、山1段、盒8段主體發育不同來源方向的三角洲前緣沉積,受物源及沉積相控制,以延安為中心區域屬于匯水區,骨架砂體分別由四個向延安區域匯聚的北北西、北北東、南部、南南東向三角洲前緣水下分流河道朵體組成,在匯水區不同來源方向砂體在不同層段具有相互疊置特征;

(3)山2段南部物源砂體較發育,盒8段北部物源砂體較發育,反映研究區古構造從南高北低轉換為北高南低,指示鄂爾多斯盆地在山西組與石盒子組之間發生了南北向構造反轉。

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Controls of Provenance and Depositional Facies on Sandbody Distributions of the Upper Paleozoic in Southeast Ordos Basin

QU Hong-jun1MA Qiang2GAO Sheng-li1,3MEIZhi-chao1MIAO Jian-yu1

(1.State Key Laboratory of Continental Dynam ics,Northwest University,Xi'an 710069;
2.Institute of Petroleum Exploration and Development,Tuha Oil Corporation,CNPC,Ham i,Xinjiang 839000; 3.Research Institute of Shaanxi Yanchang Petroleum Corporation,Xi'an 710000)

In recent years,lots of new gas plays are found continuously in the Upper Paleozoic in southeastOrdos basin,which displays great exploration prospects.The Upper Paleozoic was influenced by multisource in the former study,while themain provenance was from north.It is not clear that whether there was sourthern provenance,how much the influenced areawas,and what degree the influence was,which greatly influenced gas exploration strategy in the study area.Since lithologic play is themain reservoir in Ordos basin,the study on source and the rule of depositional facies on Upper Paleozoic sandbody distribution is of great importance in southeast Ordos basin.Based on data ofwell drilling,logging and core as long with outcrop sections of surroundings,by means of paleocurrent direction measuring in outcrop sections,heavymineral analyzing aswell as facies belt diagnosing,the provenance directions of the Permian are confined,the depositional facies styles of the Upper Paleozoic are analyzed,the rule of source,geography of depositional basement and depositional facies on sandbody distributions in the study area are studied,and also the distribution feature of skeleton sandbody are disclosured.Based on the above studies the following conclusion can be obtained.Paleocurrent direction revealed thewholemiddle partof the study areawas catchmentarea in periods of Shanxi and Shehezi Formation,which suggests the possible existence of north and south of two different sources in the very time.Heavymineral analyzing indicated that the NNE region of the studied area is rich in zircon,the NNW region of the studied area ismixed area of zircon,leucoxene and garnet,while the south region is rich in leucoxene, which disclosures that the provenance of Shanxi and Shehezi Formation can be divided into three directionswhich are NNE,NNW,and south.The coast line and geography of deposit basement reflected by thickness of strata and limestones indicated transgression direction being from east.Benxi Formation mainly developed barrier island&lagoon and tidal flat facies,the distribution of barrier island and tidal channel sandbodieswere controlled by geography of deposit basement and depositional facies beltswith the barrier sandbody being distributed along NNE direction.Depositional facies belts of Shanxi and Shehezi Formation were controlled by the orientation of provenance,which controlled the distribution of under water lobate distributary channels.Shan 2,Shan 1 and He 8 Membermainly developed delta front depositional facies sourced from different directions which were controlled by provenance directions and depositional facies belts,while Yan'an region being the center ofwater catchment.The skeleton sandbodies are composed of four underwater lobate distributary channels converging to Yan'an region coming from NNE,NNE,and SSE aswell as south direction,different sourced sandbodies in different layers display the style of overriding in water catchmentarea.The skeleton sandbodies sourced from south developed well in Shan-2 member,which influenced areas of north of Yanan,while the skeleton sandbodies sourced from north developed well in He 8 member,which overreached areas of south of Yanan while the southern provenance receded simultaneously,reflecting that palaeo-terrain in the studied area conversed from south being higher than north to north being higher than south,and also indicating that constitution reversed along NS direction in Ordos basin between periods of Shanxi and Shehezi Formation.

Ordos basin;the Upper Paleozoic;paleo-current directions;Provenance;depositional facies;sandbody

屈紅軍 男 1967年出生 博士 教授 沉積學與盆地分析 E-mail:hongjun@nwu.edu.cn

P534

A

1000-0550(2011)05-0825-10

①國家科技重大專項(編號:2008ZX05025-006)及國家重點基礎研究發展規劃“973”項目(編號:2009CB219400)聯合資助。

2010-07-28;收修改稿日期:2010-12-16