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The Comparison and Study Based on the Method of Jiannan3D Brestack Seismic Attributes

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Tutor: PanRenFang
School: Yangtze University
Course: Mineral prospecting and exploration
Keywords: Jiannan Structure,Xujiahe Formation,AVO forward modeling,AVO inversion,Attribute
CLC: P631.4
Type: Master's thesis
Year:  2012
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Jiannan gas field belongs to Shizhu synclinorium in the Western Hubei-eastern. The regional tectonic belongs to the upper Yangtze, east Sichuan fold belt in the eastern Sichuan Basin, and it is a low structure lying between the anticlinorium of Fangdoushan and Qiyueshan. Affected by Himalayan movement and Indo-china movement in late Triassic epoch, Sichuan basin evolved in to a NW-SE dustpan-like inland lake basin. The deposition is primarily fluvial facies, and its featured by superimposition of channel sand, floodplain and lacustrine-bog fine sedimentary. Under the effect of several geologic factors, Xujiahe Formation resulted in thick-sand thin-storage, which restricts the exploration and exploitation.The pre-stack inversion can get more information of the reservoir and improve precise of reservoir characterization than the post-seismic impedance inversion. The pre-stack inversions mainly include the elastic impedance inversion, Pre-stack simultaneous inversion for compression impedance and shear impedance, and pre-stack waveform inversion. Pre-stack data makes full use of the origin information contained in multiple coverage, including amplitude, frequency and phase and so on, which is extremely valuable for analyzing petrophysics characters and then inferring lithology and oil-gas possibility. As the practical seismic data isn¡¯t all zero offset time records, and also the geometry of field acquisition is multiple shots and multiple channels, every seismic trace records different reflection information on every offset, that is to say, every CDP or CMP has different offset, every offset has different amplitude, especially when the offsets are relatively range, the AVO problems are more obvious. Meanwhile, the frequency and phase of wavelet is varying, so the post-stack processes inevitably cause the loss of original seismic information. There is no doubt that if we use post-stack seismic data for inversion and predicting oil-gas possibility, the accuracy and success rate will be affected by horizontal stacking. Whereas, AVO inversion is based on the pre-stack seismic traces, they fully consider the incidence angle, P-wave velocity, S-wave velocity and density, so the inversion results are plentiful and reliable. Above all, pre-stack seismic inversion technology has good prospect in the future hydrocarbon exploration and exploitation.AVO (Amplitude Various with Offset) is a kind of seismic exploration technology, which is used for analyze and identification of lithology and hydrocarbon using the variation of amplitude with offset. We usually look for natural gas in clastic rocks. The AVO theory is concise and explicit:the gas hiding in the pore of clastic ricks can cause the decrease of P-wave velocity and density, while S-wave velocity remains invariant. That is to say, the gas existence can bring the change of Vp/Vs ratio. It is these changes that cause the different amplitude varying with different offset. No doubt that we can detect the possibility of gas-bearing using AVO anomaly brought by the mentioned change. After development for many years, AVO technology has evolved from conventional "bright spot" technique to multi-pattern recognition method. AVO technology is the base line for natural gas exploration, and those AVO characteristic parameters are very sensitive to the existence of gas. It is the sensibility that brings the strong sensitivity for gas bearing detection using AVO technology. Poisson¡¯s ratio is the most sensitive factor among the AVO relevant parameter. In order to improve the accuracy of reservoir prediction, we should complement the influence of the other AVO attributes. After consulting plenty of literature and analyzing the relevant data, the seismic data has been conventionally disposed and processed. Besides I also considered the sensitive degree brought by various AVO attributes. Relying on seismic synthetic records, I chose2key wells for AVO forward model study, and summarize the variation characteristics of several AVO attributes and the difference between gas-bearing reservoir and non-gas-bearing reservoir, in favor of reliable forecasting g as-bearing reservoirs with the AVO inversion results. We define those coefficient parameters which are angle independent but formation parameters dependent as AVO attributes. Parameter inversion of AVO attributes is: obtaining these attributes using fitting different angle gathers, and then parameter inversion depending on the relationship among those attributes. Conventional AVO attributes inversion includes several ideas, mainly base on Shuey¡¯ approximation or AKI formula:(1) inversion of intercept A and gradient B (Castagna,1997), AVO attributes by linear combination of A and B;(2) inversion of zero-offset amplitude and relative Poisson¡¯s ratio(Hilterman,1995);(3) inversion of elastic parameters, such as ¦¤Vp/Vp, ¦¤Vs/Vs and fluid factor ¦¤F and so on (Smith,1987), and then crossplotting of these parameters;(4) inversion of the linear combination of frequency and intercept, called FVO attributes. The optimization of sensitive parameters related to gas-bearing can not only refer to the geological meanings showed by attributes, but also compare the log and seismic and check the goodness of fit between attributes and possibility of gas-bearing. That is to say, if some parameter have high goodness of fit of logging and seismic, it proved to be sensitive. Of cause, for different gas-bearing reservoir, the goodness of fit is different, the relevant optimization is different. The relationship between sensitive and AVO attribution depends on the feasibility of AVO technology, the quality of seismic data, the effect of processing and so on. For this area mentioned in the paper, this idea is feasible.
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