| Fushan sag is located in south of Hainan island and covers Haikou, Qiongshan, Chengmai and Lingao and some other areas. It is a Cenozoic fault depression belonging to the east of the southern slope in Beibu gulf basin. Eocene series Liushagang Formation is a set of thick lakeshore facies-lake delta deposition, it is mainly composed of dark grey to black mudstone, shale, light grey to gray sandstone, and gravel sandstone, and it is also the main source bed and reservoir bed in the structure of Fushan sag is rather complex, it is composed by many small faults, and the igneous rock distributes widely. So the exploration of Fushan Sag is much more difficult.The conventional post-stack seismic data loss the implicit information about the original amplitude, frequency and phase in every CDP or CMP point, and the pre-stack seismic data without stacking has preserved this information. Theory and practice has proved that the "hydrocarbons anomalies" can be identified from the abnormal amplitude. Analyzing the amplitude variation with increasing offset (AVO), we can identify the amplitude anomalies caused by lithology and oil-gas bearing layers. So, bringing AVO technology into Fushan Sag has significance to recognize the lithology and to reservoir prediction, which also can enhance the exploration efficiency.AVO technology advances gradually obviously, its studying object is amplitude variation with increasing offset (or incident angle) and analytic methods include forward modeling and the analysis of inversion attribution. AVO forward modeling is based on the theory of Zoeppritz complete expression. Based on the geological background and logging data, designing the corresponding elastic parameters (mainly the P-wave velocity, S-wave velocity, density and Poisson’s ratio) with the change of the fluid character, simulating the trend of the amplitude accurately (the angle is within30°), and then analyzing the corresponding rules, we can establish AVO detection criterion fitting the research region and provide guidance to the data processing and inversion; AVO inversion is based on Shuey’s simplification. Extracting the basic attributions——P,G from CMP gather via NMO, quantifying the other attributions’relative magnitude through mathematical computation of P and G (so far, the absolute value is impossible) and then analyzing the relationship between attributions and oil-gas bearing, we can depict the plane distribution of oil and gas layers. In real application, forward modeling and inversion can test each other, we cannot ignore either, Forward modeling is the orientation, inversion is the aim. If the conclusion of forward modeling and inversion is agreed with production, we can say our objective deduction is useable.Petrophysical analysis is a bridge of the seismic data (property parameters) and reservoir characteristics (reservoir parameters). Before AVO analysis, we must have a statistical analysis on petrophysics and reservoir quality. Fluid replacement is a extension of AVO forward modeling based on Gassmann equation. Through the change of fluid properties, porosity and gas saturation, we can not only restore the original rock containing elastic characteristics of hydrocarbon, but also refine porosity and gas saturation, And then we can establish a suitable plate of AVO response and realize the semi-quantitative interpretation of gas sand by using P-G crossplot method. This is a try on quantitative seismology. After examination, the effect is really good.On the base of large number of Chinese and foreign literatures, I reviewed the development of the technology and the basic theory of AVO and the petrophysics foundation, and then discussed the effect of fluid property and reservoir parameters on AVO. We looked into the geological and seismic characteristic of Liushagang Formation, statistically analyzed the relation of elastic data and reservoir property from29given wells. Then I simulated the AVO response on the reflecting surface of typical wells based on Zoeppritz complete equation, and also summed up the trend of amplitude varying offset in every situation. At the same time, I adopted fluid replacement to simulate the different models under changing porosity (2%,6%,10%,14%,18%,22%) and changing gas saturation (0,20%,40%,60%,80%100%), and built up the corresponding AVO quantitative forecast plate. Using Shuey’s simplification, I extracted several AVO attributions, such as delta density, scaled Poison’s ratio and fluid factor, and then established the direct hydrocarbon examining standard about gas-oil bearing layers in this area. At last, I predicted the plane distribution of reservoir bed by combining the results of forward modeling and inversion and analysed coincidence rate by using production results, and also discussed the reasons which caused errors from the technique and the quality of data two aspects.Combined with the research contents mentioned above, we summarizes the AVO abnormal responses characteristics of the Liushagang formation in northern Huadong-Bailian area of Fushan Sag, as follows:1In this research area, oil-gas sand of Liushagang formation which have2-8m in thickness of single layer, belong to low-middle porosity and low-middle permeability sand and its hydrocarbon saturation mostly distributes in45%-70%, have reached industrial exploitation degree. P-wave velocity and density decrease obviously with the increase of porosity, and decrease less with the increase of hydrocarbon saturation. There are little difference of P-wave velocity and density among the gas,oil-gas and oil bearing layers.2We can draw a conclusion from AVO forward modeling of the known wells: the AVO abnormal response of oil and gas bearing layers of Liushagang Formation in this area mostly belongs to III class, sandstone type is good, seismic modeling results are consistent with logging interpretation results.3Study on the saturation changing model with the invariant porosity shows that: with the increase of gas saturation, P-value decreases, G-value increases slightly, and AVO abnormal responses change from the II class to the Ⅲclass; the AVO abnormal responses change obviously from none-gas to gas bearing, which indicates that AVO effect is very sensitive to the gas in sandstone; it is more difficult to distinguish different gas saturation using AVO response when gas saturation reaches to a certain scale (40%).Study on the porosity changing model with the invariant gas saturation shows that:the AVO response is very sensitive to the porosity. with the increase of porosity, P decreases, and G increase, and the changing is more obvious than the saturation changing model. AVO abnormal responses change from I class to II class, and then to Ⅲ class and IV class.4The analysis of AVO attribution anomaly sections shows that:the sensitive attributions including relative density, relative Poisson’s ratio and fluid factor show obviously negative abnormal response at the known oil and gas layers, that is not only consistent with logging interpretation results, but also the forward modeling results. Therefore, we know that the application of AVO technology is feasible in Fushan sag. Finally, we finish the oil and gas plane distribution prediction for upper Liushagang Formation and lower Liushagang Formation, which have high accordance rate with the production practice results. |
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