陸表海碳酸鹽巖臺地沉積期微地貌恢復方法研究①——以四川盆地磨溪氣田嘉二2亞段A層為例

譚秀成 聶 勇 劉 宏 周 彥 李 凌 趙路子 張本健 馮 鈺,5

(1.油氣藏地質及開發工程國家重點實驗室西南石油大學 成都 610500; 2.西南石油大學資源與環境學院 成都 610500;3.西南油氣田分公司勘探處 成都 610051; 4.西南油氣田分公司川西北氣礦 四川江油 621709;5.西南油氣田分公司川中油氣礦 四川遂寧 629000)

陸表海碳酸鹽巖臺地沉積期微地貌恢復方法研究①
——以四川盆地磨溪氣田嘉二2亞段A層為例

譚秀成1,2聶 勇2劉 宏2周 彥2李 凌2趙路子3張本健4馮 鈺4,5

(1.油氣藏地質及開發工程國家重點實驗室西南石油大學 成都 610500; 2.西南石油大學資源與環境學院 成都 610500;3.西南油氣田分公司勘探處 成都 610051; 4.西南油氣田分公司川西北氣礦 四川江油 621709;5.西南油氣田分公司川中油氣礦 四川遂寧 629000)

陸表海碳酸鹽巖臺地沉積期微地貌起伏差異小,難于確定。分析了沉積期微地貌控制著顆粒灘儲層的發育及分布,因此,可通過對顆粒灘儲層的研究,反演微地貌起伏。按此原理,建議具體研究步驟為:顆粒巖沉積為主體的等時地質體選取、儲層成因確定、暴露時間確定、沉積期地貌恢復,以及成果檢驗應用。以四川盆地磨溪氣田嘉陵江組嘉二2亞段A層為例,利用豐富的鉆孔取心資料進行了實例研究,取得良好效果,表明該方法值得推廣應用,尤其適用于鉆探程度高、資料老、缺乏三孔隙度測井系列的盆地老區,這對于儲層預測和新層挖潛具有重要參考意義。

陸表海 碳酸鹽巖臺地 沉積期微地貌 顆粒灘 嘉陵江組 三疊系 磨溪氣田 四川盆地

含油氣盆地沉積期古地貌的恢復是認識區域巖相古地理及其油氣地質意義的重要關鍵,對于相對高差較大的地貌單元,通常可采用回剝法[1]、沉積學分析方法[2,3]、層序地層學方法[4,5](包括高分辨率層序地層學方法[6])等實現;相比而言,在陸表海碳酸鹽巖臺地內部,由于次一級地貌起伏較小[7],導致在某一時間段內小區域中形成的地層多為米級差異[7,8],從而使得應用常用的方法難以準確刻劃這種地貌差異。實際上,在陸表海碳酸鹽巖臺地內部,微地貌高地在海退期暴露幾率較高,因此儲層質量較之相對低洼區要好,據此理論而言,通過對儲層的研究,可以反演地層沉積期的古地貌,然而,這在過去的研究中,尚未得到系統工作。

四川盆地磨溪氣田位于盆地川中古隆中斜平緩構造帶南部,是近年來盆地天然氣勘探開發的重點地區之一[9]。在早三疊世嘉陵江期,以發育陸表海碳酸鹽巖臺地為特征,臺地內部具有次一級的地貌起伏[10],但迄今少有精確的恢復工作。目前,在磨溪構造的主體280 km2內,揭穿嘉二段鉆井共42口(資料截止2005年),其中嘉二段全取芯井16口,并且在區內分布較均勻,因此,井網合理分布的大量鉆孔資料為沉積期微地貌恢復研究提供了有利條件[7]。本文以該段為例,開展陸表海碳酸鹽巖臺地沉積期微地貌的恢復研究,主要目的是探索一種具有普遍意義的恢復方法,以供同類研究類比參考。

1 原理與方法

陸表海碳酸鹽巖臺地內部浪基面淺,一般小于5 m[7],總體擾動深度小,因此在相對海退時期,微地貌高地易處于浪基面之上,從而發育顆粒灘[7]。顆粒灘的沉積速率高于臺地內其它微相區[11],因此,地貌差異得到強化。此外,顆粒灘沉積之后,由于上覆沉積物的加積,顆粒沉積物在物理壓實的影響下,形成顆粒格架支撐,壓實率要遠低于細粒沉積物,從而使不同微相區的地貌差異導致的沉積厚度差異得到進一步強化[8],這同時也表明臺地內部沉積期的微地貌近似恢復可以不考慮壓實校正。總而言之,某一時間段內形成的顆粒巖厚度可用于近似恢復其形成時的微地貌起伏。

對于非暴露淺灘區,微地貌高地的顆粒灘始終較其它區域沉積速率快,其顆粒巖累計厚度可用來近似表征微地貌起伏(圖1a)。而陸表海碳酸鹽巖臺地內多數時期形成的顆粒灘,發育深度較小,沉積物的可容空間也相應較小,并由于其海平面變化特征為快速海侵-緩慢海退,顆粒灘的快速生長使得微地貌高地的顆粒沉積物極易在同生期暴露(暴露淺灘),使微地貌高地顆粒灘終止發育,顆粒灘向微地貌低地遷移(圖1b,c)。

圖1 水下及暴露淺灘疊置遷移模式Fig.1 Superposed migration pattern of submarine and exposed shoals

如圖1b,1c,對于同生期暴露淺灘,可以分為中短期暴露和長期暴露兩種情形。其中,中短期暴露時間一般小于50 000 a,缺乏巖溶特征,并以發育選擇性的粒內溶蝕和早期淡水膠結物為特征[12],此時,地貌高地的剝蝕不明顯,連續取芯見不到明顯的不整合面,顆粒灘在海退時期向低地遷移,但其厚度仍小于地貌高地(圖1b),因此,顆粒灘中短期暴露形成的大氣成巖透鏡體儲層與顆粒灘的累計厚度呈正相關關系。相比而言,長期暴露的淺灘,其暴露時間大于130 000 a,以發育古土壤和不規則溶溝、溶縫為特征[12],此時,地貌高地的顆粒巖存在不同程度的剝蝕,并且顆粒灘向微地貌低地遷移,低洼處的顆粒巖累計厚度可能大于微地貌高地,大氣成巖透鏡體的厚度與顆粒灘厚度不具正相關關系(圖1c)。因此,若同生期暴露時間長,就會干擾顆粒灘累計厚度對沉積期地貌的判斷,尤其是等時地質體選取的時間段越長,越難消除這種因素的影響。因而,暴露時間的確定成為利用顆粒灘厚度法恢復微地貌的關鍵。盡管準確的年齡界限目前還難以實現,但仍可以根據成巖組構與宏觀地質特征區別暴露的時間,如具有以下特征的通常反映短期暴露(反之則為長期暴露):①不整合特征不明顯,缺乏古土壤;②缺乏不規則溶溝、溶縫;③粒內溶蝕極其發育;④早期淡水膠結物與膠結不整合的出現;⑤大氣成巖透鏡體成因儲層與顆粒巖累計厚度呈正相關。

綜上所述,在陸表海碳酸鹽巖臺地內部,可以通過精細的宏微觀分析尋找符合條件的等時地質體,利用顆粒巖厚度法對暴露淺灘期的微地貌進行近似恢復。具體可以分為如下幾個步驟:①選取以顆粒巖沉積為主體的等時地質體;②確定儲層成因;③暴露時間確定;④沉積期地貌恢復;⑤成果檢驗與應用。需要注意的是,根據以上分析,未暴露和中短期的暴露灘研究效果要比長期的暴露灘研究效果好。

2 實例應用

2.1 等時地質體選擇與儲層成因

磨溪氣田嘉二段主要由海相碳酸鹽巖、蒸發巖和少量陸源碎屑巖組成,鉆厚從104 m到120 m不等,總體上表現出東厚西薄的變化趨勢。本區嘉二沉積期的沉積體系為陸表海碳酸鹽巖臺地,以發育局限-蒸發海臺地沉積為特征,其中的嘉二2亞段A層以發育臺內顆粒灘亞相為特征,主要包括灘核及灘核-灘緣微相[13]。灘核微相主要由鮞粒灰巖組成,具有顆粒巖累計厚度和單灘體厚度較大的特點,是本期沉積最有利的儲集微相帶[7]。因此,以嘉二2亞段A層為例展開重點研究。

如圖2,嘉二2亞段A層下部為藍灰色泥巖頻夾薄層砂屑灰巖,中上部為淺灰色鮞粒灰巖夾薄層泥晶灰巖或泥晶云巖,局部井區夾灰黑色塊狀硬石膏巖,上部為深灰色泥晶灰巖,儲層主要發育于鮞粒灰巖中[14]。研究表明,磨溪氣田嘉二2亞段A小層為近乎等時的地質體,并以顆粒巖沉積為主體[15],儲集空間類型以粒內溶孔和鑄模孔為主,屬同生期巖溶成因[14]。因此,開展沉積期古地貌恢復,嘉二2亞段A層具有翔實的基礎資料條件。

2.2 暴露時間確定

通過嘉二段豐富的取心資料觀測發現,嘉二2亞段A小層各向上變淺序列間未見明顯的不整合現象;巖芯上針孔狀粒內溶蝕極其發育,而未見溶溝溶縫現象(圖3);鏡下可見到大氣水成巖環境形成的刃狀方解石(圖4)。據此,對比前文歸納總結的判識標準,初步表明顆粒灘的暴露時間不長。

圖2 研究區地理位置圖1.硬石膏巖;2.泥晶云巖;3.砂屑云巖;4.粉晶云巖;5.泥晶灰巖;6.瘤狀灰巖;7.鮞粒灰巖Fig.2 Geographical location of the study area

圖3 鮞粒灰巖,逆粒序,鑄模孔,磨206井, 3186.69~3187.01 m,嘉二2 AFig.3 Oolitic limestone,inverse grading,moldic pore,Well Mo206, 3186.69~3187.01 m,the layer A of Jia22 submember.

此外,具有大氣成巖透鏡體成因的儲層與顆粒巖累計厚度呈正相關,也表明了灘體的暴露時間不長。研究表明,本區儲層段隨粒屑灘生長,加上受海平面變化等因素的影響,幾十厘米厚的單灘體也可出露水面,接受大氣淡水的淋濾作用,以顆粒巖的原生粒間孔隙為通道進行溶蝕,使得剛沉積不久的顆粒由外向內發生強烈的溶蝕作用,形成粒內溶孔發育的顆粒巖,往往成為良好的孔隙型儲層[16]。通過對磨溪和鄰區18口嘉二段全取心井的精細觀測統計,發現粒屑灘的累計厚度與針孔鮞粒灰巖的累計厚度存在明顯的正相關關系(圖5)。

圖4 鮞粒灰巖,兩期方解石膠結物,磨13井, 3124.71 m,X50(-),嘉二2亞段A層Fig.4 Oolitic limestone,two-period calcite cements,Well Mo13, 3124.71 m,X50(-),the layer A of Jia22 submember

圖5 顆粒巖累計厚度與針孔巖累計厚度相關關系圖Fig.5 The correlation graph of gross thinkness between grainstones and pinhole rocks.

上述特征表明,磨溪氣田嘉二2亞段A小層顆粒巖早期暴露時間為中-短期。大氣淡水淋溶形成的針孔狀顆粒巖發育程度與古地形高低密切相關,位于古地形高處,同生期暴露幾率較大,顆粒巖接受大氣淡水淋溶改造時間較長,因此,針孔巖發育區域即為沉積時的古地貌高地,如位于現今構造高部位的磨151井、磨13井等井,巖心觀察發現嘉二2A層針孔狀顆粒巖極為發育。而位于現今構造低部位的磨16井、磨47井等巖芯觀察表明該層針孔狀顆粒巖不發育,面孔率極低,說明這些區域古地形較低,顆粒巖暴露幾率少,很難接受大氣淡水的淋溶改造。因此,可利用該套顆粒巖厚度變化來表征嘉二2A期沉積期微地貌。

2.3 沉積期地貌恢復

基于上述原理,用顆粒巖厚度法來恢復磨溪氣田嘉二2亞段A層沉積期古地貌。除磨溪16口全取芯井直接統計外,其余未取心井則通過測井相獲取顆粒巖厚度[17],根據巖芯觀察和測井相統計出了42口井嘉二2A層顆粒巖的累計厚度,繪制出顆粒巖累計厚度等值線圖,并以此為標準恢復了嘉二2A沉積期的古地貌起伏形態,形成如圖6的認識:①微地貌高地呈近東西向展布;②微地貌高地與現今構造具一定協調性,說明磨溪構造為一同沉積水下高地,只是南部由于后期喜山運動的改造而沉降;③水下高地內部隆、凹分異明顯,出現高地和洼地等次一級微地貌單元。高地發育在Mo24-Mo205東井區、Mo206井南側、Mo207井周緣、Mo151井-Mo36井區、Mo208井區及南部、Mo48井和Mo202井區北側等(7個)。洼地發育在Mo12-Mo201、Mo005-2-Mo150、Mo22井南及Mo206井東等(4個)。

2.4 成果檢驗

嘉二2初期四川盆地遭受到一次較強海侵,沉積了一套區域上穩定分布的、富含葉肢介的藍灰色泥巖,導致嘉二2A期少見蒸發巖沉積,表明盆內海水鹽度基本正常[18]。然而有意義的是,在Mo005-2井-Mo150井一帶,以及Mo207井區卻出現了較厚的水下膏巖沉積(膏巖為深灰色塊狀,未見暴露標志)[7],其周緣是反映微高地的粒屑灘顆粒巖沉積。這反映了由于周緣微高地上灘體的生長,導致相對低洼的灘間海環境封閉受限,水體蒸發濃縮,從而形成膏巖沉積。這表明,利用顆粒巖厚度恢復出的古地貌得到了巖相沉積的支持,因此是可信的。

2.5 成果應用I:指導相鄰層系的沉積微相分析

圖6 磨溪氣田嘉二2亞段A期古地貌示意圖Fig.6 The palaeogeomorphic sketch of the layer A of Jia 22 submember,Moxi gas field

對于以中短期的早期暴露為主的陸表海碳酸鹽巖臺地內部古地貌,利用此方法恢復的沉積期地貌在較長時間內具有一定的繼承性,并控制其沉積微相的展布。如以嘉二2A沉積期后的嘉二2A期為例,圖7給出了根據實際取心資料繪制的沉積相圖,如圖可見,云坪、云坪和潮上膏云坪夾坪洼微相的沉積微地貌更高[14],沉積微相分布表明海水鹽度向北東增加,導致蒸發礦物含量增高,對比圖6,可見這種地貌特征是對嘉二2A期的繼承發展,說明該方法恢復的地貌可以指導相鄰層系的沉積微相研究。

圖7 磨溪嘉二2 B期沉積微相平面分布特征(據文獻[13])Fig.7 Microfacies distributional characteristics of the layer B of Jia22 submember,Moxigas field

圖8 磨溪氣田嘉二2 B期孔隙度>6%的儲層累計厚度等值線圖Fig.8 Gross thinkness contourmap of reservoirs(porosity is higher than 6%) in the layer B of Jia22 submember,Moxigas field

2.6 成果應用II:指導儲層精細研究及勘探開發部署

巖芯實測物性統計表明,位于古地貌高處的區域,儲層孔隙度值在5%～18%之間,平均孔隙度在9%左右,儲層有效厚度大;相比而言,位于古地貌低處的區域,水動力條件弱,顆粒巖不發育,儲層質量差,孔隙度值僅在3%左右,儲層有效厚度小。

圖9 磨溪嘉二氣田產能分布圖Fig.9 Capacity distribution of Moxi gas field

考慮到微地貌具有繼承性發展特征,因此微地貌高地也會控制相鄰時期優質儲層的分布格局,如根據嘉二2B期孔隙度大于6%的儲層累計厚度,優質儲層的分布與嘉二2B期的古地貌高地具明顯的相關性,雖然由于嘉二2B晚期向北東方向鹽度增加,膏質沉淀充填導致儲層相對變差,但不改變其分布格局(圖8),該期的優質儲層相應發育于磨151-磨22及磨207等高地。

受沉積期鹽度向北東增加的影響,儲層質量和厚度與之相適應,東北部的高地相對儲層較差,而中部高地鹽度較低,儲層質量好,實際試油資料也表明嘉二氣藏產能高的井基本上都位于中部磨151-磨22、磨160這兩個古地貌高地及附近,氣井日產能一般在十幾萬立方米/日～幾十萬立方米/日(圖9)。因此,古地貌恢復結果可對生產提供指導。

3 結論

(1)陸表海碳酸鹽巖臺地沉積期古地貌的恢復可利用等時地質體內的顆粒巖累計厚度來近似恢復,特別是對于未暴露和中短期暴露灘效果尤佳,具體研究步驟為:顆粒巖沉積為主體的等時地質體選取、儲層成因確定、暴露時間確定、沉積期地貌恢復,以及成果檢驗應用。

(2)在磨溪氣田嘉二2B期的應用結果表明,陸表海臺地內沉積期微地貌在較長時間內可繼承發展,并控制相鄰層序沉積、儲層的分布格局,其生產應用效果良好,說明對于井網密度大的開發階段氣田的沉積、儲層精細描述是一種有效輔助手段。

(3)由于臺地內灘相薄儲層厚度小于地震的垂直分別率,因此采用沉積期古地貌恢復這一方法間接預測儲層發育概率,尤其適用于鉆探程度高、資料老、缺乏三孔隙度測井系列的老區,對于儲層預測和新層挖潛具有重要意義。

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Research on the M ethod of Recoverying M icrotopography of Epeiric Carbonate Platform in Depositional Stage:A case study from the layer A of Jia 22Member in Moxi Gas Field,Sichuan Basin

TAN Xiu-cheng1,2NIE Yong2LIU Hong2ZHOU Yan2LILing2ZHAO Lu-zi3ZHANG Ben-jian4FENG Yu4
(1.State Key Laboratory of Oil and Gas Reservoir Geology and Exp loitation,Southwest Petroleum University,Chengdu 610500; 2.College of Resovoir and Environment,Southwest Petroleum University,Chengdu,610500; 3.Exp loration Department Southwest Oil and Gas Field Com pany,Chengdu,610051; 4.Northwest Sichuan M ining District,Southwest Oil and Gas Filed Com pany,PetroChina Jiangyou,Sichuan 621700; 5.Central Sichuan M ining District,Southwest Oil and Gas Field Com pany,PetraChina Suining,Sichuan 629000)

Themicrotopography difference of the epeiric carbonate platform in the depositional stage is so tiny,as a result,it is too hard to identify.This paper is then,firstly,mainly based on the development dominating factors of the grain shoal within the platform,and then points out that the microtopography highland tends to be beyond the wave base in the relative-regression stage,where the grain shoal develops.The depositional rate is higher than any othermicrofacies area within the platform,hence,the topography differencewas enhanced at that stage.Besides,after the sedimentation of the grain shoal,the grain-supported framework forms under the influence of physical compaction,where the compaction rate ismuch lower than the finer sediments,so the depositional thickness resulted from the topography difference of variousmicrofacies areas is also further enhanced,which shows that the grainstone thicknesswithin a certain period of time can be used,approximately,in recovering itsmicrotopography prominence when it began to present,and that the compaction correction can be ignored when approximately recovering themicrotopography within the platform at the depositional stage.It then further analyses the effects on the grain shoal construction and reservoir development by the exposure process during the depositional stage,and it divides the shoalwithin the epeiric platform into three genetic types:unexposed shoal,short-medium-term exposed shoal and long-term exposed shoal,meanwhile, pointing out that themethod of grainstone gross-thicknesswhich is used,approximately,in recovering the depositional palaeogeomorphology of epeiric carbonate platform is preferable for the unexposed and short-medium-term exposed shoals,where themain characteristics of the short-medium term exposed shoal can be concluded as follows(contrarily be long-term exposure):①unconformity characteristic limited,palaeosol lacks;②irregular karren and corroded fissure lack;③intargranular solution extremely develops;④the freshwater cements in early stage and cementunconformity exist;⑤lens-diagenesis reservoir of atmospheric genesis positively associates with the grainstone gross-thickness.Based on this,the paper then takes the view thatmicrotopography prominence can be inverted by the reservoir study of the grain shoal.Followed by that principle,italso suggestes that the concrete study procedure can be like this:the isochronous geologic body to be chosen as themain body of grainstone deposition,the confirmation of reservoir genesis and exposured time,the topography recovering during depositional stage and the application of achievement verification.

Sichuan Basin was characterized by developing epeiric carbonate platform during the period of Jialingjiang in early Triassic.The basin was also hit by an fierce transgression in the period of early Jia 22,which made the salinity of the seawater normal on the whole.Moxi gas field locates in the south of gentle-oblique tectonic zone of the palaeohigh in the central of the basin,where the tectonic body is about 280 km2,and the exploration wells penetrating the Jia 2 member sum up to42(data by 2005),ofwhich the total cored wells of Jia 2member sum up to 16,besides,the distribution is relatively uniform.Hence,the gas field was used in the case study,where the result indicates that highland mainly develop in the well areas of eastern side of Mo 24-Mo 205,southern side of Mo 206,the rim of Mo 207,Mo 151-Mo 36,Mo 208 and its southern part,Mo 48,Northern part of Mo 202(7 in all),which distribute in the direction of southwest-northeast,swalesmainly develop in Mo 12-Mo 201,Mo 005-2-Mo 150,southern part ofMo 22 and eastern part of Mo 206.here is thick deposition of underwater-gypsum(dark-gray and massive gypsum,no exposed marks found)developing in the well areas of Mo 005-2-Mo 150,Mo 207,which has come into being by the shoal development around themicro-highland,resulting in the sealing restrict of the low-lying environment of interbank sea and the evaporation and concentation of the waterbody,the gypsum deposition therefore comes into being.It can be believed from the above that the recovery of the palaeogeomorphology by taking advantage of the grainstone thickness can be supported by the lithofacies deposition。

According to the graph of sedimentary facies compiled by actual cored data of B Formation,Jia 22Sub-member, the palaeogeomorphology during early sedimentary stage carries on developing in the stage of Jia 22B.dominating the distributing framework of its sedimentation and reservoir.The result indicates that this sort ofmethod used in recovering themicrotopography during depositional stage is applied to the characteristics of lithofacies and so on。,besides, the recovered microtopography in the sedimentary stage can not only continue to successively develop over a long period of time and can also dominate the deposition of adjacent stratigraphy and the framework of reservoir distribution, furthermore,relatively speaking,its production application effect is good,which also indicates that dedicate description of sedimentation and reservoir is an effective and adjuvantmeasure for those gas fields at developing stage with high density ofwell pattern and that this kind ofmethod should be widely popularized,especially for those maturing basin fieldswith high level ofwell exploration,data that is out of date,rare tri-porosity logging series,all of these are significant and also has referenced significance for the reservoir prediction and tapping the new potential formation。

Epeiric Sea;carbonate platform;microtopography during depositional stage;grain shoal;Jialingjiang Formation;Triassic,Moxi Gas Field;Sichuan Basin

譚秀成 男 1970年出生 博士 副教授 儲層沉積學 E-mail:tanxiucheng70@163.om

P512.2

A

1000-0550(2011)03-0486-09

①四川省重點學科建設項目(編號:SZD0414)資助。

2010-05-26;收修改稿日期:2010-08-20