Side lobes of wavelets impact identification of thin sand bodies

Recently, we found that side lobes of wavelets have a large impact on the identification of thin sand reservoirs when studying some gas fields in a basin in Northwest China. Reflections from the top of the H Formation, in which there are gas-bearing thin sand bodies, have the main wavelet lobe between two weak peak side lobes. The lower one always mixes with another peak reflected from the top of a thin sand reservoir. That makes it difficult to identify the sand reservoir. In order to solve this, many forward models were set up using typical well logs. 2D synthetic profiles were produced using Ricker wavelets to study the relationships between the effects of wavelet side lobes and thin sand position and frequency and between amplitude and the thin sand body. We developed the following conclusions： First, it is easier to identify thin sands in a shallower position. Second, a good way to tell sand body reflections from side lobes is by comparing profiles with different frequency windows. Third, it is helpful and effective to describe sand extent using amplitude attributes.     

地震子波旁瓣对薄砂体识别的影响

我们研究中国西部某盆地一些气田时,发现地震子波旁瓣在很大程度上影响着薄砂体的识别和解释。该区目的层段H段地层中,辫状河流相薄砂岩储层与湖相泥岩成互层。在地震资料上,来自H段地层顶面的强波谷总跟随一个弱波峰(源于子波旁瓣),此弱波峰与薄砂体顶面反射波峰相互叠合,掩盖了砂体反射,造成了薄砂体识别和解释的困难。为此,我们设计了一套正演模型,利用不同频率的零相位雷克子波合成人工地震剖面,来研究砂体位置、频率与地震子波旁瓣效应的关系,以及振幅类属性与薄砂体关系。提出处于高位置的砂体反射形态清晰,易于识别解释;认为通过对比不同带通滤波处理后的地震资料,可很好辨别旁瓣效应和砂体反射;同时发现振幅类属性可作为薄砂体识别表征参数,可有效合理地描述砂体分布范围和特征。     

基于物理小波的频谱分解方法及应用研究

在地震资料频谱分解中,采用匹配地震子波的物理小波,依据地震信号的特征,用振幅、能量衰减率、能量延迟时间及地震子波的中心频率等四类参数构造基本小波,把地震信号分解在小波域,高频分量能够得到精细的刻画.本文以物理小波变换为工具, 给出了该变换中的核函数的选择方法,进而提出了基于物理小波变换的频谱成像方法.我们将此方法用于海上某油田河流相储层的描述,并与常规软件中的小波变换频谱成像结果进行了对比, 结果表明,本文提出的方法更能精细地刻画地质事件.     

Utilizing spectral decomposition to determine the distribution of injected CO2 at the Snøhvit Field

Time‐lapse 3D seismic reflection data, covering the CO₂ storage operation at the Snøhvit gas field in the Barents Sea, show clear amplitude and time‐delay differences following injection. The nature and extent of these changes suggest that increased pore fluid pressure contributes to the observed seismic response, in addition to a saturation effect. Spectral decomposition using the smoothed pseudo‐Wigner–Ville distribution has been used to derive discrete‐frequency reflection amplitudes from around the base of the CO₂ storage reservoir. These are utilized to determine the lateral variation in peak tuning frequency across the seismic anomaly as this provides a direct proxy for the thickness of the causative feature. Under the assumption that the lateral and vertical extents of the respective saturation and pressure changes following CO₂ injection will be significantly different, discrete spectral amplitudes are used to distinguish between the two effects. A clear spatial separation is observed in the distribution of low‐ and high‐frequency tuning. This is used to discriminate between direct fluid substitution of CO₂, as a thin layer, and pressure changes that are distributed across a greater thickness of the storage reservoir. The results reveal a striking correlation with findings derived from pressure and saturation discrimination algorithms based on amplitude versus offset analysis.     

复数子波匹配追踪算法识别薄层砂体

针对东营凹陷滩坝砂岩薄砂体的预测识别问题,研究了复小波匹配追踪方法.从地震信号morlet小波分解及重构分析入手,讨论了重构地震信号时关于频率、振幅、相位及时移参数的选取方法及其具体的选取变量.并就关键的振幅参数,研究了改善的计算方法.常规的匹配追踪算法扫描地震信号的所有时间点和频率.在本文中,为了改善时频分辨率,采用局部扫描主频和子波的时间延迟.在具体实现过程中,采用计算地震信号的平均瞬时频率和地震信号的瞬时包络处的时间,做为采样频率和时间采样点,得到与地震信号最佳相关的子波.对研究的方法从理论模型和实际资料进行了论证分析,证明理论上正确合理,实际应用效果显著.     

Influence of frequency-dependent anisotropy on seismic amplitude-versus-offset signatures for fractured poroelastic rocks

Frequency-dependent amplitude variation with offset offers an effective method for hydrocarbon detections and analysis of fluid flow during production of oil and natural gas within a fractured reservoir. An appropriate representation for the frequency dependency of seismic amplitude variation with offset signatures should incorporate influences of dispersive and attenuating properties of a reservoir and the layered structure for either isotropic or anisotropic dispersion analysis. In this study, we use an equivalent medium permeated with aligned fractures that simulates frequency-dependent anisotropy, which is sensitive to the filled fluid of fractures. The model, where pores and fractures are filled with two different fluids, considers velocity dispersion and attenuation due to mesoscopic wave-induced fluid flow. We have introduced an improved scheme seamlessly linking rock physics modelling and calculations for frequency-dependent reflection coefficients based on the propagator matrix technique. The modelling scheme is performed in the frequency-slowness domain and can properly incorporate effects of both bedded structure of the reservoir and velocity dispersion quantified with frequency-dependent stiffness. Therefore, for a dispersive and attenuated layered model, seismic signatures represent a combined contribution of impedance contrast, layer thickness, anisotropic dispersion of the fractured media and tuning and interference of thin layers, which has been avoided by current conventional methods. Frequency-dependent amplitude variation with offset responses was studied via considering the influences of fracture fills, layer thicknesses and fracture weaknesses for three classes amplitude variation with offset reservoirs. Modelling results show the applicability of the introduced procedure for interpretations of frequency-dependent seismic anomalies associated with both layered structure and velocity dispersion of an equivalent anisotropic medium. The implications indicate that anisotropic velocity dispersion should be incorporated accurately to obtain enhanced amplitude variation with offset interpretations. The presented frequency-dependent amplitude variation with offset modelling procedure offers a useful tool for fracture fluid detections in an anisotropic dispersive reservoir with layered structures.     

The impact of reservoir scale on amplitude variation with offset

To investigate the vertical and horizontal impact of reservoir scale on the amplitude‐versus‐offset characteristics, we conduct seismic numerical simulations on models containing spatially confined lithologic units with different scales. We find that the reservoir scale has a nonlinear effect on the amplitude‐versus‐offset intercepts and gradients. As the reservoir width increases, amplitude‐versus‐offset intercept and gradient both first increase, then decrease, and finally remain stable. The amplitude‐versus‐offset intercept is maximum when the reservoir width is 80% larger than the Fresnel zone radius, whereas the amplitude‐versus‐offset gradient peaks at 1.5 times the Fresnel zone radius. Both amplitude‐versus‐offset intercept and gradient are approximately proportional to the reservoir width prior to reaching their maxima. When the lateral extent of the reservoir is more than three times the Fresnel zone radius, the amplitude‐versus‐offset attributes are constant. Modelling the reservoir thickness shows that intercept and gradient behave in a manner similar to that of tuning of thin beds. Both the amplitude‐versus‐offset intercept and gradient first increase and then decrease with the thickness, peaking at the tuning thickness. The thickness contribution to amplitude‐versus‐offset variations is negligible when the thickness is larger than 1.6 times of the tuning thickness. Considering the magnitude of the changes in amplitude‐versus‐offset intercept and gradient caused by reservoir scale, the width causes a maximum 433% intercept increase and a 344% gradient increase, whereas the thickness causes a maximum 100% intercept increase and a 73% gradient increase. Cross‐plotting the amplitude‐versus‐offset intercept and gradient shows the reservoir scale change gives rise to an anti‐clockwise spiraling effect. In conclusion, the lateral and vertical extents of the reservoir both play an important role in amplitude variation with offset. Our analysis shows that the lateral reservoir extent has a larger impact on the amplitude variation with offset than the vertical tuning effect.     

鄂尔多斯盆地天然气有效储层识别与评价方法

针对鄂尔多斯盆地天然气勘探开发存在的问题,通过对测井资料及天然气测试资料的分析,建立适合识别天然气有效储层的测井响应特征归一化方法,以及根据测井资料重构天然气有效储层特征曲线的方法,探索了根据测井资料评价天然气产能的方法.本文还分析了目前常用的预测天然气产能方法存在的问题及实际应用中欠缺的条件,形成了以中子、声波、密度测井资料为基本特征参数,用自然伽马、自然电位或电阻率比值参数等作为约束条件的天然气有效储层识别技术及天然气产能评价方法,并对实际测井资料进行了处理.没有试图通过井径校正的方法将受井径扩大影响的曲线恢复到原始地层的响应状态,而是通过极值变化消除井径扩大对声波、密度测井资料在天然气有效储层识别及天然气产能评价方面应用的影响.     

宽频数据在致密砂岩储层预测中的应用

西湖凹陷深层致密砂岩储层具有良好的勘探开发前景,受埋深影响,目的层地震资料品质较差.A构造通过斜缆宽频采集和处理获取宽频地震数据,提升了资料品质,然而应用常规子波提取方法对宽频数据进行子波提取并反演计算纵波阻抗,结果与井上实测数值差异较大,影响储层的定量解释.针对这一问题,提出统计性子波和确定性子波相结合的长短子波合并宽频子波提取方法,提取的宽频子波比常规子波低频丰富、旁瓣小,能更真实地反映地震信息,约束稀疏脉冲反演的纵波阻抗结果与测井曲线吻合度更高.基于宽频数据和常规数据分别进行约束稀疏脉冲弹性波阻抗反演,预测A构造优质储层分布,经已钻井证实,宽频数据比常规数据储层预测精度高,预测的储层展布特征与研究区地质沉积认识一致.结果表明:这种基于宽频子波提取的宽频资料应用方法有效降低了致密砂岩储层预测的多解性.     

岩石物理模版在储层定量解释中的应用

岩石物理模板采用测井解释的各类岩性矿物骨架点值,选取适合该地区的岩石物理模型,模拟在不同储层组合、不同孔隙及不同饱和度情况下,储层岩石物理参数变化引起的储层测井参数及地球物理响应特征的变化,定量地建立起储层参数同地球物理弹性参数间的解释关系模版.本文根据新场JS42气藏储层参数分析结果,尝试性地将岩石物理解释模板应用于储层定量解释中,对储层高产气区、含水区域进行定量解释,并预测了该气藏的气水界面,该预测结果与实钻井测试情况吻合,证实了该方法的科学性.     

地震复谱分解技术及其在烃类检测中的应用

谱分解技术在地震解释领域已得到广泛应用,但常用的谱分解方法存在两方面的不足.一是时间分辨率低,难以对薄层进行刻画;二是在烃类检测中多解性强,难以区分流体类型.为了改善该问题,本文提出一种基于地震复谱分解技术的烃类检测方法.复谱分解是指用一个包含多个不同频率Ricker子波的复子波库对地震道进行分解,从而得到时变子波频率和相位信息的过程.借助稀疏反演技术复谱分解可以获得高分辨率的时频能量谱和时频相位谱.本文首先通过拟合算例验证了复谱分解方法刻画薄层的能力以及求取子波频率和相位的准确性.然后利用基于Kelvin-Voigt模型的黏弹波动方程数值模拟对衰减引起子波相位改变的原因进行了分析.最后通过实际资料应用展示了本文方法在储层预测中的高时间分辨率优势,验证了利用子波相位信息识别气藏的有效性.     

Wavelet‐based cepstrum decomposition of seismic data and its application in hydrocarbon detection

How to use cepstrum analysis for reservoir characterization and hydrocarbon detection is an initial question of great interest to exploration seismologists. In this paper, wavelet‐based cepstrum decomposition is proposed as a valid technology for enhancing geophysical responses in specific frequency bands, in the same way as traditional spectrum decomposition methods do. The calculation of wavelet‐based cepstrum decomposition, which decomposes the original seismic volume into a series of common quefrency volumes, employs a sliding window to move over each seismic trace sample by sample. The key factor in wavelet‐based cepstrum decomposition is the selection of the sliding‐window length as it limits the frequency ranges of the common quefrency section. Comparison of the wavelet‐based cepstrum decomposition with traditional spectrum decomposition methods, such as short‐time Fourier transform and wavelet transform, is conducted to demonstrate the effectiveness of the wavelet‐based cepstrum decomposition and the relation between these two technologies. In hydrocarbon detection, seismic amplitude anomalies are detected using wavelet‐based cepstrum decomposition by utilizing the first and second common quefrency sections. This reduces the burden of needing dozens of seismic volumes to represent the response to different mono‐frequency sections in the interpretation of spectrum decomposition in conventional spectrum decomposition methods. The model test and the application of real data acquired from the Sulige gas field in the Ordos Basin, China, confirm the effectiveness of the seismic amplitude anomaly section using wavelet‐based cepstrum decomposition for discerning the strong amplitude anomalies at a particular quefrency buried in the broadband seismic response. Wavelet‐based cepstrum decomposition provides a new method for measuring the instantaneous cepstrum properties of a reservoir and offers a new field of processing and interpretation of seismic reflection data.     

基于三变量柯西分布的叠前地震反演及在优质含气储层预测中的应用

优质气层与低含气饱和度的差气层、气水同层均呈现"亮点"强振幅的地震响应特征,利用振幅类属性或者常规弹性参数难以进行有效判识。考虑到密度参数随含气饱和度的高敏感性,本文在贝叶斯叠前反演框架下,以三变量柯西分布为先验正则约束进行叠前地震反演,减小了纵波速度、横波速度与密度三参数统计相关性造成的参数估算误差,进一步提高了弹性参数的可靠性,模型测试表明新方法反演的密度精度及稳定性更高。将方法应用于马达班湾盆地X区块,在流体替代及岩石物理统计分析的指导下,利用基于三变量柯西分布的叠前地震反演方法提取密度参数,预测优质气层富集区,实钻结果与预测结果吻合度高,表明该方法具有较高的实用性。      

火山岩风化壳地层型油气藏评价预测方法研究——以新疆北部石炭系为例

针对火山岩风化壳地层型油气藏强非均质性,评价预测难度大,勘探成功率低等难题,本文采用重磁电剥层处理、沿层延拓信号增强反演方法有效顶测区域火山岩分布;建立风化淋滤剥蚀后不同岩石组合的不完整火山机构和形态识别模式,利用相干体和振幅分析等方法有效识别火山岩目标;在建立单次火山喷发岩石序列及储层分布模式基础上,开发了基于次生溶...     

Integrating seismic attributes in the accurate modeling of geological structures and determining the storage of the gas reservoir in Gorgan Plain (North of Iran)

Three dimensional seismic operation of Gorgan Plain was studied around a well, which is situated in North of Iran following the hitting of a thin overpressure gas layer (thickness of 9.6 m), with the purpose of the accurate modeling of geological structures and determining the approximate gas storages. The geological structures of the reservoir were modeled using the seismic attributes (coherence, instantaneous amplitude and spectral decomposition (FFT)). The obtained results clearly demonstrated the shape and volume of the existing structural traps in the studied area. In order to estimate the thickness of gas layer in the 3D seismic volume and determining the gas storage, the thickness changes based on the seismic amplitudes were used because its thickness was less than the critical resolution thickness for this layer. However, due to its low thickness, the lack of indicator peak in seismic sections and strong faults of area, it was difficult to pursue this layer in the seismic volume and map its exact amplitude. Considering this issue, a new method with integrating of seismic attributes was recommended. First, the instantaneous amplitude attribute of the thin reservoir layer reflector in computed synthetic seismogram were fabricated and then the frequency regarding the highest amount (dominant frequency) was chosen by Fourier Transform. Finally, spectral decomposition (FFT) with the resulting frequency was gained over the cross-section of the layer's instantaneous amplitude attribute in the 3D seismic volume choosing a proper time window. In such a situation, an increase of its thickness was seen as its amplitude increase and the minimum gas storage of this reservoir was calculated using the area of the restricted part of high thickness (over 9.6 m).     

富含煤层气储层的叠前地震反演预测方法研究——以山西晋东南测区为例

叠前地震反演技术能够将地震中振幅、偏移距和入射角等多种有效信息与油气的敏感参数结合起来,在有效识别油气藏"甜点"方面虽发挥了重要的作用,但对于煤层气的有效预测仍需研究.针对煤层气与石油天然气的赋存地质条件不同,本文以山西沁水盆地煤层气地震资料为例,借鉴叠前地震反演技术的多参数预测思想,通过模型正演和实测资料处理,得到纵波、横波速度和密度等不同的数据体,利用数据体和弹性参数关系式,采用AkiRichard近似公式与叠前宽角度反演联合的方法,寻找出富含煤层气储层的敏感参数.从而实现富含煤层气储层的有效预测.经与测井曲线结果对比,吻合率较高,说明了该方法具有可行性与实用性.     

AVO signatures of actual and synthetic reflections from different petrophysical targets1

We use a marine seismic dataset to examine the reflections from two gas sands, a lignitic sand and a cineritic bed, by means of their amplitude versus offset (AVO) responses. This offset-dependent signature is related to specific petrophysical and thus elastic situations or to peculiar interference patterns and may help to distinguish the nature of the amplitude anomalies on the stack sections. The prestack analysis is carried out on seismic data which have undergone an accurate true-amplitude processing. It is found that the lignitic-sand reflections exhibit a decreasing AVO while the two-gas sands show markedly increasing AVO trends. Also the reflections from the cineritic layer show increasing amplitudes with offset that may be due either to the petrophysical nature of the cinerites or to thin-layer interference or to both. In order to verify the reliability of the actual AVO responses we develop a detailed model from well data and compute a synthetic CMP seismogram. In order to account for mode conversions and thin-layer effects, the synthetic seismograms are computed using the reflectivity method. The wavelets used in the synthetics are retrieved from actual seismic and borehole data by means of wavelet processing. When finely layered structures are present, the estimation of a reliable wavelet is extremely important to get the correct synthetic AVO response. In particular, the AVO responses of the cineritic layer differ substantially if we make use in the computation of the synthetics of a Ricker wavelet or of a wavelet estimated through wavelet processing. The good match between the observed and modelled data confirms the reliability of the processing sequence and of the final AVO signatures.     

基于时间演化的天然气水合物CT图像阈值分割

微米X射线计算机断层扫描作为一种数字岩心探测手段,被广泛应用于研究含天然气水合物沉积物赋存形态,但由于水合物与水对X射线的衰减系数相近,二者在CT图像中灰度区间存在交集,导致在从CT图像上对水合物与水进行分割时存在强非唯一性。为提高对CT图像中水合物与水阈值分割的准确性,本文通过分析天然气水合物生长过程中不同时刻CT图像直方图特征,提出归一化CT图像及其直方图的方法。首先选定甲烷气与石英砂的峰值灰度基准;然后用高斯函数分别对当前CT图像直方图中的甲烷气与石英砂曲线进行拟合,得到当前CT图像直方图中的甲烷气与石英砂峰值灰度;再将当前CT图像直方图中的甲烷气峰值灰度与石英砂峰值灰度归一化到选定的峰值灰度基准;进而用归一化的直方图对CT图像进行归一化;最后根据归一化灰度直方图的变化趋势,定量统计得到CT图像中水合物增加和气-水减少的灰度区间,完成图像中不同组分的阈值划分。实验结果表明,提出的阈值分割方法能够为天然气水合物CT图像中水合物与水边界的确定和水合物饱和度计算提供定量的依据,具有实际的工程应用价值。      

频谱成像技术在稠油热采地震监测中的应用

频谱成像技术可有效的描述地质反射层厚度的非连续性和岩性的非均质性,其在理论上主要是依据薄层反射的调谐原理,过去通常采用以离散傅里叶变换为基础的算法,但是,该方法存在着明显的局限性,因为估算的地震振幅谱的重要特征是所选时窗长度的函数.如果所选时窗过短,振幅谱会与变换窗函数褶积,失去频率的局部化特征,而且过短的时窗会使子波的旁瓣呈现为单一反射的假象.增加时窗长度,会改善频率的分辨率.但如果所选时窗过长,时窗内的多个反射会使振幅谱以槽痕为特征,很难分清单个反射的振幅谱特征.由于在实际运用中,以傅里叶变换相关的算法的时窗问题,难以选择好时窗长度,而且无法定量分析时窗长度产生的偏差,因而会使振幅谱的估算产生偏差.以小波变换为基础的时频分析技术成了非平稳性信号的重要分析工具,在很多实际应用中已取代了傅里叶变换的分析方法.以小波变换为基础的瞬时谱分析技术能得到精确的时频分析结果,同时避免了时窗问题.它反映出了储层在纵向上时间及厚度上变化情况和横向上的地质不连续性的信息,因此能使解释人员快速而有效地描述储层特征的空间变化.     

基于小波熵水淹层识别方法探索

储层的孔隙度和电阻率曲线间微差形态波形的复杂程度隐含着储层含油气性的信息,油层水淹程度不同,其时频平面上小波系数能量团存在着显著差异,随水淹程度加重,其能量团分布由复杂逐渐趋于简单单调.油层水淹信息映射在其小波系数能量熵特征上,可使油层水淹信息得到放大,更易于水淹程度识别.基于测井信息的小波熵识别水淹层的方法,对利用传统测井解释理论判别比较困难的低阻油层有较强的识别能力.