dc.description.abstract | Quantitative AVO and Structure Analysis of Gas Hydrate and Sedimentation Features Cross China Continental Slope, SW Taiwan
Abstract
A seismic survey line, MGL0905-8RR1, which is part of TAIGER project that joint with Lamont-Doherty Earth Observatory (USA) and MOST (Taiwan) for seismic surveys associate with gas hydrate (GH) exploration project supported under NEP-MOST fund. The seismic survey line which collected the data across several targets from Formosa Ridge to Yuan-An Ridge and reach Good Weather Ridges in offshore southwest of Taiwan. Seismic attributes, AVO/AVA analysis, time-to-depth conversion and imaging interface structure features through Normal Incidence Ray Tracing (NIRT) were performed for gas hydrate expeditions. BSR (Bottom Simulating Reflector) is a seismic response to indicate the GH presence. Ten potential gas hydrates locations were proposed along the studied seismic line. Seismic attribute and AVO analysis serve as the step to identify the major GH targets that could be important for further analysis. Seismic attributes were studied first followed by AVO analysis to identify BSRs along the seismic line. These attributes include Intercept (A), Gradient (B), Product (A*B), Poisson’s ratio, Fluid Factor, Curvature, (A+B)/2 and S-wave reflectivity. Intercept and Gradient are the main attributes to confirm the GH existence. From AVO/AVA attributes studies, all the target zones have AVO classification of class IV. Seven out of ten locations are identifying as high potential locations for gas hydrates existence. Detailed studies on both Formosa Ridge and Yuan-An ridge areas are discussed on AVO effects and rock physics properties based on cross plots. Although AVO class III could be potentially and commonly found in southwest Taiwan, however, the results show that all the GH targets along the seismic line correspond to the AVO class IV. Class IV usually has properties of negative intercept and positive gradient. Such AVO feature could be caused by depositional processes and environment changes between identified target regions that produce lithology changes across active and passive margins. The sediments properties changes may produce different AVO behavior in the cross plot. The sediments contents and environment changes can affect the response of intercept and gradient.
NIRT approach serves the purpose for imaging the main interfaces and to define average velocity for each interface in order to perform time-to-depth conversion. The layer with same or similar average velocity can be interpreted and assumed as the major sedimentation process occurred at the same geologic time interval. Seismic line is mainly divided into two areas. Based on the defined model interfaces and synthetic seismograms compared with stacked section, interpretation can be performed through quantitative analysis. Through iterative time-to-depth conversion by adjusting velocity and therefore travel-time and follow-up with depth improvement, the convergence is evaluated from time and depth residuals. Both RMS travel-time residual and STD values decreases to a constant value after four iterations. NIRT technique provides the results which able to digitize structure interfaces and determine formation velocity. At northwest of Formosa Ridge, three main interfaces with the corresponding velocities values of 1484 m/s, 1528 m/s and 1644 m/s respectively were quantitatively defined. At southeast of Formosa Ridge area including Yuan-An Ridge has five interfaces with the corresponding velocities values of 1484 m/s, 1528 m/s, 1614 m/s, 1644 m/s and 1684 m/s respectively. The determined main interfaces clearly show high contrast with strong reflection amplitude in the time migration section. The distribution of continuous layering feature indicates stable sedimentation process cover both deep and shallow marine depositional environment. Meanwhile weathering, submarine slump creating turbidity current and mud diapirism (?) beneath the study area also play important roles for lateral variation of sedimentary features. In this study, the proposed geological features observed in the seismic section along MGL0905-8RR1 include Mass Transport Deposit (MTD), BSRs, faults, fluid conduct, channel fill or trough fill sedimentation features.
Keywords: gas hydrates, AVO analysis, AVO attributes, Normal Incidence Ray Tracing, geological features.
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