台灣新第三系有機物熱成熟指標之探討

Organic Maturity Indices for Neogene Formations in Taiwan

沈俊卿(Jun-Chin Shen)

指導教授 ： 黃武良

國立臺灣大學/理學院/地質科學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.02707

摘要

在有機物成熟度的探討過程中，發現採自不同地區的煤層其鏡煤素反射率並未隨其氫指數增加而降低，而頁岩則與Lo,(1993)的頁岩中之鏡煤素反射率隨其氫指數增加而降低相同。為了進一步探討頁岩及煤層有機物成熟度差異的意義，本研究進行煤樣化學成份分析，以探討生物指標的成熟度工具，以作為比較參照。煤的成份霍烷類含量較豐富，固醇類相對偏低，故化學成份的成熟度指標以霍烷類較為可靠。但煤的霍烷類生物指標的成熟度指標，如Ts/(Ts+Tm)及C31 22S/ (22S+22R) 最常用成熟度工具均在煤中無法使用。因此本研究尋求新的工具。首先進行煤樣水合熱裂模擬，來探討人工熱成熟作用中生物指標的含量變化，再採集煤樣進行萃取分析後，發現以C31-22S, C31-22R homohopanes 及 C29, C30 moretanes四種成份所組成的參數如 (1) (normoretane/ moretane) / [C31 22S/ (22S+22R) ]; (2) (normoretane/ norhopane) / [C31 22S/(22S+22R) ]. (3) (moretane/hopane) / [C31 22S/ (22S+22R) ] 。可有效解決煤樣本身形成過程及外來因素所導致霍烷類的使用問題。這些參數之中又以(normoretane/ moretane) / [C31 22S/ (22S+22R) ]為最佳，因其於平衡後值的變動最小。同時也藉由這四種成份所組成的參數的達成平衡值之位置點，得知這些分析過臺灣煤樣的油窗起點約為0.6 %Ro。
另外，本研究也進行高成熟度化學成熟度指標的開發。為求工具使用的廣度，除芳香烴外，也針對飽合烴探討， pristane/ n-C17 、phytane/ n-C18及pristane/ phytane 為常用工具，但pristane/ phytane在有些樣品中可應用至1.5% Ro以上。
另外在高成熟度的應用方面，發現芳香烴中的 (3-MB)/ (4-MB+DBF)，可避免MPDF在此範圍使用之不可靠的問題迎刃而解。
沉積環境所導致的差異之探討，先以同一煤層中所含碳質物在相同的熱應力下的鏡煤素反射率，以苗栗縣三義剖面分析結果顯示煤層與下覆砂層中所夾碳質物並無差異，為確認沉積環境對有機物成熟度的影響，接著再特別蒐集並分析13個與煤形成環境不同的十六份頁岩中沈積，而所受成熟作用幾乎完全相同的獨立碳質塊體，以探討其各項成熟指標的變化(包含鏡煤素反射率及化學的成熟指標)，並以明德剖面的上福基砂岩中煤層、煤頁岩及頁岩在接近油窗時鏡煤素反射率及化學成份的變化，並探討其是否已進入油窗。探討完相同的熱應力下的影響後再進行垂直變化的探討。而根據豐富三號井採樣分析顯示，較高的鏡煤素反射率往地層淺部較年輕的卓蘭層延伸，而並未隨埋藏深度變淺而降低，此與露頭岩樣分析所得，南莊層以上年輕地層頁岩均較煤層鏡煤素反射率為高的結果相同。換言之，二次沉積有機物佔多數的現象係自南莊層開始出現。所顯示的地質意義，就是不排除自南莊層沉積時期開始，即由陸地侵蝕而帶來大量的再沉積有機物充斥於頁岩中，也可能受造山運動影響所致。而年輕地層的真正鏡煤素反射率則由頭嵙山層的碳質物分析來獲得並驗證前述發現。
十六份頁岩中獨立碳質塊體，所顯示沈積對有機物成熟度的影響，以MPI-1所計算出來的成熟度值變化較小。除兩個樣品值較高外，其餘的值變化均小且樣品間的差異不大，顯示其為一可靠的成熟度工具。換算出來的Ro%值以0.6MPI-1 +0.4所得的較接近鏡煤反射率（Ro%），也證實此一比值適用範圍可延伸至0.4 %Ro，而不是僅在0.65 % Ro以上，反而0.7 MPI-1 +0.22換算式被認為適用範圍可至0.35 %Ro，但本文所得的換算值卻都明顯偏低，顯然並不適用於此一成熟度範圍。計算值與測值的高低並無明顯正相關。鏡煤素反射率的差異則在0.1%Ro以內。此項探討所獲得的結果為就岩性的差異而言，所分析的桂竹林層十六份頁岩段樣品，夾含於粉砂層中的與頁岩中的碳質塊體之各項分析值中僅甲基菲異構物9MP/1MP比值有明顯差異。頁岩中的樣品9MP/1MP比值較高。而MPI-1則顯示粉砂層的樣品有較高值。
除前述在煤樣的油窗探討外亦進行煤樣的天然氣生成探討，以不同種類樣品的水合熱裂模擬結果天然氣累積生成量，以同為194小時加溫時間顯示原始氫指數與甲烷氣的生成量呈正相關，其關係式為甲烷生成量=1.5653(HI)0.2194，C2+亦隨氫指數增加而增加，C2+ = 0.4869(HI)0.3126，但甲烷相對乙烷生成量的比值卻隨氫指數增加而降低，其關係式為C1/C2 = 8E-0.7(HI)2- 0.0017(HI)+ 4.2356。換言之，氣體之生成量不論甲烷或C2+隨氫指數值增加，異丁烷對正丁烷隨加熱時間降低後再微幅上昇。
在完成水合熱裂模擬油氣形成後，進行火山噴發岩熱成熟作用對圍岩中有機物所造成的鏡煤素反射率變化，結果顯示碳質頁岩層有機物的成熟度，由接近火成岩體的2.44 %Ro在短距離之內急劇降為1.05 %Ro。有機物的成熟度值係由產油高峯橫跨至氣窗。這些分析樣品中殘留的可裂解碳氫化合物皆為0的結果，顯示雖然火成熱成熟作用所造成的鏡煤素反射率並不高且未達正常深埋所造成完全轉化階段，但卻能令有機物完全裂解而無殘留，可提供較預期為多的油氣，此與高溫高壓水合熱裂及深層火成岩脈所導致的結果並不同。而此一分析結果顯示殘留的可裂解碳氫化合物的存在與否並未影響煤的煤化作用。

關鍵字

煤 ； 有機物成熟度 ； 生物指標 ； 天然氣生成 ； 沉積環境

並列摘要

The distribution of hopanoids and alkanes in bitumen extracts of coals, coaly shales, and shales from northwestern Taiwan has been measured to assess the maturity of potential petroleum source rocks in the studied area. The C31 homohopane isomers of the studied coals and coaly shales attain equilibrium at maturity corresponding to 0.6% Ro measured in coals or 0.65% Ro in adjacent shales. A proposed parameter using hopanoid ratios reveals a more consistent trend and approaches a steady value at maturity similar to the C31 homohopane isomer ratio. The results show that the onset of the main phase of petroleum generation in northwestern Taiwan takes place at maturity corresponding to VR of 0.6% Ro measured in coals or coaly shales and probably higher VR in shales. The distribution of pristane, phytane n-C17 and n-C18 confirms the validity of the conventional maturity indicators if the samples are from the same source, but shows depositional or organic-source controls. The source influence on the phytane/n-C18 ratio appears to be larger than that on the pristane/n-C17 ratio. The phytane/n-C18 ratio is also less sensitive to maturity than that of the pristane/n-C17 ratio.
The pristane/phytane ratios in the studied coals and coaly shales, in contrast to the pristane/n-C17 ratio, continue to decrease to maturities beyond the oil window up to 1.5% Ro and therefore may be applicable to high maturity sources.
Thirteen desmocollinite-rich organic debrises isolated from a transgressive marine shale with a similar thermal history were characterized to study the influence of organic composition and host rocks on the variations of different maturity parameters. The results show that both maceral types of organic debrises and lithology of host rocks have little influence on the variation in vitrinite maturity. The vitrinite reflectance (0.4 to 0.55 %Ro) measured from these organic debrises, however, are slightly lower (about 0.1%Ro difference) than those from the dispersed organic matter in shale. The maturity of the organic debrises indicated by MPI-1 is consistent with that indicated by vitrinite reflectance, and the applicability of the MPI-1 indicator may be extended to maturity as low as 0.4 %Ro. The MPI-1 maturity is generally lower in samples from the siltstone than those from the shale, suggesting that the MPI-1 maturity indicator may be affected by depositional or diagenetic environments. The hopanoid index of C31 22S/(22S+22R) homohopane ratio indicates a maturity corresponding to the vitrinite reflectance measured from organic debrises. The maturities of the organic debrises indicated by three parameters (%Ro, MPI, and hapanoid ratio) are in good agreement and that the source rocks in the studied formation are very close to the onset of oil generation. The ratios of cadalene/ retene and fuoranthene/ pyrene vary significantly with the lithology: higher in samples from the siltstone than those from the shale.
Coals from different formations in Northwestern Taiwan were treated with hydrous pyrolysis to get its genetic potential. Genetive volume of gas from these samples were found in this study. It can be easily calculated by hydrogen index (C1 =1.5653(HI)0.2194，C2+ = 0.4869(HI)0.3126). C1/C2 increase with hydrogen index too(C1/C2 = 8E-0.7(HI)2- 0.0017(HI)+ 4.2356 ).

並列關鍵字

Coal ； Shale ； Biomarker ； Vitrinite ； Organic Maturity ； Sedimentary environment

參考文獻

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