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The Research Of AVO Response Rules Of Tight Sandstone Gas Reservoir

Posted on:2014-07-31Degree:MasterType:Thesis
Country:ChinaCandidate:Q L ZhangFull Text:PDF
GTID:2250330401480672Subject:Mineral prospecting and exploration
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As the global natural energy is quickly decreasing, tight sandstone gas reservoir as a kind of unconventional energy is getting more and more attention around the world. With the improvement of China’s oil and gas resources exploration, the proved reserves of tight sandstone gas reservoir is increasing obviously, accounts for about60%of China’s new proved reserves of crude oil and natural gas. More than ten basins such as Ordos basin and Sichuan basin have favorable geological conditions of the formation of tight sandstone gas reservoir. Therefore, the study of tight sandstone gas reservoir is one of the most important directions of natural gas exploration.Sulige gas field in ordos basin is a typical tight sandstone gas reservoir, and He-8member is the main production reservoir. The depositional environment of He-8member is fluvial facies, and structural trap does not develop. The type of gas reservoir is lithologic gas reservoir, which is mainly controlled by lateral distribution and reservoir physical properties. Because of drastic lateral variation of lithology, thin sand body, low porosity and permeability, it is difficult to use conventional seismic exploration technology to predict favorable reservoir in He-8member.AVO technology is a kind of pre-stack seismic method making full use of shear wave and the incident Angle information, based on the reflection and transmission theory of plane wave, identifies lithology and detects hydrocarbon according to the characteristics of seismic reflection amplitude changes with offset. AVO forward modeling plays an important role in AVO technology which simulates the phenomenon of amplitude varying with offset, based on the standard of gas sandstone classification, to research the AVO anomaly response types of oil and gas layer and use these characteristics to test the distribution range of oil and gas sandstone. The AVO technology is important to identify lithology, petrophysics and gas distribution in He-8tight sandstone, can effectively improve the exploration precision.Based on in-depth analysis geological and petrophysical characteristics of He-8tight sandstone according to logging data, AVO forward modeling research is applied to analyze AVO response characteristics caused by porosity, gas saturation and thickness, proves the feasibility to predict favorable reservoir.This paper analyses how petrophysical parameters and reservoir-seal associations influence on AVO response characteristics, by means of setting up single well model and statistic multiple well variable geological parameter model, summarizes and verifies the changing rule of AVO response of the tight gas sandstone reservoirs. In studies of the effect of reservoir parameters on AVO response, first of all AVO forward modeling is established by complete Zoeppritz equation of275wells in study area to identify AVO anomaly response types of the gas-bearing sandstone. Then fluid replacement is conducted based on the Gassmann equation to understand how rock physical parameters influence on AVO response by changing the gas porosity (2%-12%), gas saturation (0-100%) and thickness (2m-12m) to simulate the AVO response under different gas conditions. Finally according to original AVO forward model and logging interpretation data, statistical analysis of275wells is completed, conclude that the influence on AVO attributes (AVO intercept and AVO slope) with porosity, gas saturation and thickness, and the types of gas bearing sandstone. AVO forward modeling interpretation chart is established, can widely used in the region of Sulige gas field. In studies of the effect of reservoir-seal associations on the AVO response, first of all reservoir-seal associations are divided of275Wells in the study area, then according to AVO forward modeling results of gas-bearing sandstone, how wave impedance difference between reservoir and cap rock of gas bearing sandstone affects AVO abnormal responses is discussed.Thesis main includes the following contents:1. AVO forward modeling of gas reservoir of275wells in Sulige area determine the class Ⅱ, class Ⅲ and class Ⅳ AVO abnormal responses. Class Ⅲ is known as good gas reservoir, which can proved by well logging interpretation.2. There are4types of reservoir-seal associations:sand wrapped by mud, sand wrapped by sand, sand wrapped by overlying mud and underlying sand, sand wrapped by overlying sand and underlying mud, and the former two are better to oil and gas reservoir accumulation and preservation.3. After fluid replacement of single well and statistical analysis among multiple wells are finished in He-8member of Sulige gas field, the rock physical parameters influence on AVO abnormal responses are clear:the increase of porosity mainly affects the decrease of AVO intercept, which can make the change of AVO response of the gas bearing sandstone from class Ⅲ to class Ⅳ; the increase of gas saturation mainly affects the decrease of AVO slope, which can make the change of AVO response from class Ⅱ to class Ⅲ; the increase of thickness affects the decrease of both AVO intercept and AVO slope, which can make the change of AVO response from class Ⅳ to class Ⅲ.4. AVO anomaly response characteristics are different from difference range between different reservoir-seal associations. In sand wrapped by sand, as the impedance difference increase from10%to20%, the AVO anomaly response will change from class Ⅳ or class Ⅱ to class Ⅲ. In sand wrapped by mud, when the impedance of gas sandstone is greater than5%of shale rocks, gas reservoir mainly presents the class Ⅱ AVO anomaly response characteristic; when the impedance of gas sandstone is less than5%of tight sand cap rocks, gas reservoir mainly presents the class Ⅱ and class Ⅳ AVO anomaly response characteristic; when the impedance of gas sandstone is less than15%of shale cap rocks, gas reservoir mainly presents the class Ⅲ AVO anomaly response characteristic.5. According to statistic analysis about reservoir-seal associations, as the difference range between reservoir-seal associations is getting greater, the number of class Ⅲ sand stone increases. According to the AVO forward modeling about petro-physical parameters, gas reservoir with large thickness, high porosity and high gas saturation is shown as the class Ⅲ AVO anomaly response characteristics.
Keywords/Search Tags:tight sandstone reservoir, He-8sandstone, AVO forward modeling, fluid replacement, AVO anomaly response
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