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本文提出得克萨斯州Calhoun县Powderhorn油田下中新统小潮汐滨岸带进积体系的一套三维(3D)地震模型。这些模型是在对面积大于13km×9km某一地区的250m厚地层剖面进行详细岩性编图的基础上得到的。作图所依据的是对115口井测井曲线的详细相分析。14个厚度平均为3～30m的砂岩沉积单元赋存于15个页岩单元中。使用了最近钻探的5口井的密度—中子和自然电位(SP)曲线计算有效孔隙度和页岩性,它们与纵波速度和体积密度测井曲线对比良好。然后通过SP和电阻率测井曲线回归建立3D特性模型(包括传播时间模型和声阻抗模型),把这两种特性扩展至没有声波、密度和其他孔隙度测井的老井。声阻抗模型的作图是从x、y、z变换到x、y、t,t指垂向双程走时。声阻抗模型转换为一次反射系数系列,然后与不同频率的零相位子波褶积产生地震模型。这些结果揭示了储层规模的沉积相是怎样利用3D地震资料表示的,以及沉积相的分辨率是怎样随着地震频率、地层位置和相类型的变化而变化的。 Powderhorn储层层序被解释为一障壁坝/泻湖沉积体系,该沉积体系与对应于频繁的相对海平面变化的沿岸河流平原、湾头三角洲和小型浪控三角洲有关。我们的地震模型很好地展示了与沿岸河流平原、湾头三角洲、三角洲平原和障壁后/泻湖过渡带等环境有关的相,这些环境中主砂岩被泥质沉积所包围。缺乏泥质沉积(例如在浪控三角洲)可能导致相边界模糊,从而使岩相图型不清晰。在本项研究中,压实差的净砂岩和纯页岩具有相似的声阻抗,因而产生相似的地震响应。如果有合适的井资料,详细分析相关系可减少这种模糊性,也许还可以通过振幅随炮检距变化(AVO)法减少模糊性。地震成象的地层分辨率不仅取决于子波频率,也受储集层的地层位置和地质干扰量级的制约。
This paper presents a set of three-dimensional (3D) seismic models of the Lower Tidal Flat Progradation of the Lower Miocene in the Powderhorn field, Calhoun, Texas. These models are based on a detailed lithographic plot of a 250-m-thick section of an area greater than 13 km x 9 km. The plot is based on a detailed phase analysis of 115 well logs. Fourteen sandstone units with an average thickness of 3 ~ 30m occur in 15 shale units. The density-neutron and natural potential (SP) curves of the five most recently drilled wells were used to calculate the effective and shale ratios, which are in good contrast with the P-wave velocity and bulk density logs. 3D models (including propagation time models and acoustic impedance models) were then established by SP and resistivity log regression to extend both features to older wells without acoustic, density, and other porosity logs. The acoustic impedance model is plotted from x, y, z to x, y, t, t for vertical two-way travel time. The acoustic impedance model is transformed into a series of first order reflection coefficients, and then the seismic model is generated by the convolution of zero-phase wavelet with different frequencies. These results reveal how reservoir-scale sedimentary facies are represented by 3D seismic data and how the resolution of sedimentary facies varies with seismic frequency, formation position and facies. The Powderhorn reservoir sequence is interpreted as a barrier-bar / lagoon sedimentary system that is associated with coastal river plains, the Bayou delta and small wave-controlled delta that correspond to frequent relative sea-level changes. Our seismic model shows well the environmental facies associated with coastal plain, bayou delta, delta plain and post / barrier / lagoon transitional zone where the main sandstone is surrounded by muddy sediments. Lack of muddy sediments (eg in the Squadron Delta) can lead to ambiguous phase boundaries, rendering the facies pattern unclear. In this study, the compacted difference of net sandstone and pure shale have similar acoustic impedance, resulting in a similar seismic response. If well-specific well data is available, a detailed analysis of the correlation can reduce this ambiguity and perhaps reduce the ambiguity by varying the amplitude versus offset (AVO) method. The resolution of seismic imaging depends not only on the wavelet frequency, but also on the stratigraphic position of the reservoir and on the magnitude of the geological disturbance.