台灣西南下枋寮盆地天然氣水合物調查同中點集的AVA/AVO模擬、分析和逆推;AVO/AVA Modeling, Analysis and Inversion of CMP Gathers for Gas Hydrates Investigation in Lower Fangliao Basin, SW-Taiwan

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Title:	台灣西南下枋寮盆地天然氣水合物調查同中點集的AVA/AVO模擬、分析和逆推;AVO/AVA Modeling, Analysis and Inversion of CMP Gathers for Gas Hydrates Investigation in Lower Fangliao Basin, SW-Taiwan
Authors:	婭妮;Effrianto, Paula Ascaryani
Contributors:	地球科學學系
Keywords:	甲烷水合物;海底仿擬反射;振幅隨支距變化分析;振幅隨支距變化合成模擬;振幅的反转随分支而变化;Hydrates;BSR;AVO/AVA Modeling;AVO/AVA Analysis;AVO Inversion
Date:	2018-01-22
Issue Date:	2018-04-13 11:01:17 (UTC+8)
Publisher:	國立中央大學
Abstract:	多頻道反射震測測線MGL0908-TST於潛在富含水合物的台灣西南海域枋寮盆地進行調查、探勘，並辨識及標示異常區域。傳統相似 (semblance)速度評估顯示存在海底仿擬反射(BSR)處的下方為低速帶而其上方為高速帶之異常速度分佈。BSR在震測剖面中被廣泛地用來判斷甲烷水合物的存在。該狀態的產生其岩性的改變扮演一個大角色。以保存震幅進行資料處理流程策略，並量測沿著同中點3541至3700之間各同中點的薄層幾何與相對反射係數(RC)曲線來進行AVO/AVA效應的調查。重合後AVO模擬結果顯示BSR的厚度大約為14公尺。重合前AVO模擬與分析可推估反射係數趨勢，並提供流體變遷的資訊。針對目標異常區域的部份同中點集建立於重合前AVO/AVA模擬的一維速度(Vp、Vs)與密度模型，該模型為逆推初始程序的關鍵。透過AVA模擬，針對特定界面的相對與絕對AVA曲線時，需要進一步細緻化其一維速度與密度模型。相對AVA反應經由量測隨著入射角變化的RC值於海床為0.27至0.1、BSR上方為-0.01至-0.12及BSR下方為0.02至0.18之間取得。而絕對AVA曲線則以所有可能的Vp、Vs與岩石物理參數組合之Zoeppritz近似來計算量測值，亦即其理論的相對AVA。根據擬合和搜索資料點，我們可以估計聲性阻抗(Ip)、彈性阻抗(Ip和Is)，並獲得於海床與BSR的上方與下方之間層面更好約束的速度(Vp、Vs)與密度分佈來做為參考。岩石物理參數可被用來當作重合後聲性阻抗(PoSAInv)與重合前彈性阻抗(PreSEInv) 逆推的初始猜測值。透過疊代修正聲性阻抗(AI)、彈性阻抗(EI)與彈性係數模型後，逆推結果展示出更細緻化的Vp、Vs、密度和Vp/Vs量測值。將細緻化物理參數的空間分佈疊合至震測剖面可進一步用於解釋。另外，逆推解可視為擬井測(pseudo-log)，並用於表現含天然氣水合物沉積帶(GHBSZ)、BSR及游離氣帶(FGZ)的地層厚度。此外，逆推後的模型可進一步被重合前AVA校驗。本論文提出量化三個主要地層GHBSZ、BSR及FGZ的厚度值與岩石參數的方法，以及該區域的含天然氣水合物砂岩系統。基於Biot-Gassmann近似，預估含天然氣水合物沉積物的孔隙率和飽和度分別約為40%和26%。故透過本論文提出的方法可以來量測天然氣水合物的潛在賦存量。 ;Seismic exploration was acquired along Fangliao Basin. The anomalous target zones had been identified along MGL0908-TST line as high potential hydrates concentration. The conventional semblance velocity estimation showed that the anomalous velocity distribution occurred with the presence of low velocity existing below Bottom Simulating Reflectors (BSR) and high velocity above BSR. The presence of methane hydrate is broadly inferred by BSR on the seismic profile. It means that lithology changes play a big role to generate this condition. The AVO/AVA effect is investigated based on amplitude preservation data processing workflow strategy to measure the effected thin layer geometry along CMP 3541 to 3700 and relative reflection coefficient (RC) curves at different CMP locations. The post-stack AVO modeling suggests that the BSR has a thickness of approximately 14 m. The pre-stack AVO/AVA modeling and analysis determined reflection coefficient trends that provide information about fluid changes. The pre-stack AVO/AVA modeling establishes the 1-D velocities (Vp, Vs) and density (ρ) for some typical CMP gathers along with the anomalous zone. The model is crucial to initiate the inversion procedure. The further AVA modeling through relative and absolute AVA curves at several specified interfaces is necessary to refine the predefined 1-D velocities and density model. The relative AVA responses are measured with RC 0.27 to 0.1 for seafloor, -0.01 to -0.12 for the top of BSR and 0.02 to 0.18 for bottom BSR as a function of incidence angles. The absolute AVA curves calculation measures by computing all possible combination of Vp, Vs, and rock physics parameters based on Zoeppritz approximation, thus representing the relative AVA responses. According to the fitting and searching cluster of data points, we could estimate the acoustic (Ip) and elastic (Ip and Is) impedance and obtain the better-constrained velocities (Vp, Vs) and density distribution of the layer above and below the seafloor, BSR-Top, and BSR-bottom which were used for references. The rock physics parameters are used as an initial guess values for post-stack acoustic impedance (PoSAInv) and pre-stack elastic impedance (PreSEInv) inversions. The inversion revealed the detail estimations of Vp, Vs, ρ, and Vp/Vs by iteratively refining the acoustic impedance (AI), elastic impedance (EI) and elastic modulus models. The refined spatial distribution of physical properties superimposed on the seismic data are further used for interpretation. In addition, the inverted solutions can be treated as pseudo-log to reveal the thickness of gas hydrate bearing sediment zone (GHBSZ), BSR, and free gas zone (FGZ). Furthermore, the inverted model can be further verified by pre-stack AVA analysis. The proposed approach quantified the thickness and rock properties of three major layers and its potential gas hydrate-bearing sand system within the study area. Those three major layers are the GHBSZ zone, BSR, and FGZ. Based on Biot-Gassmann’s approximation, the predicted porosity and saturation of gas hydrate-bearing sediments are approximately 40% and 26%, respectively. From here thereafter, we can estimate the potential gas-hydrate reserved through proposed approach.
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