Alfonso Gonzalez

Seismic Reservoir Characterization In Marcellus Shale

The Middle Devonian Marcellus shale that extends from Ohio and West Virginia, northeast into Maryland, Pennsylvania and New York, is believed to hold in excess of a thousand trillion ft of natural gas. High-quality surface seismic data and top-of-the-line processing are essential to characterize these reservoirs and the overburden formations for safe and cost-effective drilling. A workflow comprising data acquisition and processing to prestack seismic inversion and lithofacies classification for characterizing the shale reservoirs is presented. The key elements in this workflow are dense point-receiver data acquisition and processing in the point-receiver domain. A small data set acquired with a proprietary point-receiver system was available to demonstrate the benefits of this methodology. The data were in an area in New York, where the Marcellus formation is known to exist.

Imaging with complex decomposition: Numerical applications to seismic processing in difficult areas

Measured seismic wavefields have complexity that cannot be fully modeled by seismic processing, such as wave equation migration. Traditionally, a separation of data into signal and noise components is attempted, but it is difficult to isolate the noise component that cannot be modeled to be discarded. In this paper we use an alternative approach and assume that most energy in a measured wavefield is signal, and then we use variational methods to separate this signal into a simple part, which can be modeled by conventional seismic processing, and a complex part that is not amenable to investigation by these methods. In this approach, the complex part does have interpretation value and should not be discarded as noise. The simple part may be input to numerical processing such as depth migration, residual static after depth migration, and multiple attenuation, among others.

Resolution analysis for targeted illumination using two‐way wave‐equations

Recent advances in imaging prestack seismic data, in particular, more accurate migration algorithms such as reverse-time migration (RTM), have made higher-quality images possible. However, tools to understand the factors affecting the quality of the results such as illumination are not readily available. Typically, illumination and resolution studies involve ray-based methods, and although they capture the effects of lateral variations in velocities, intensity and directional information of wavefield propagation, and irregular acquisition geometries, they suffer from severe limitations in accuracy in complex regions where the high-frequency asymptotic approximation of rays might fail. Wave-equation methods generally do not generate the directional information of the wave propagation, which prevents us from using a waveequation method directly for illumination studies.

Waveform gather inversion and attribute-guided interpolation: A two-step approach to log prediction

The prediction of log information from seismic data plays an important role in the characterization of oil and gas reservoirs because log information can be used in such applications as lithology discrimination, porosity estimation, fluid identification, and drilling hazard assessment.

Refining 3‐D velocity models for depth migration using tomography: Application to rapid permafrost variations in Alaska's North Slope transition zone province

The presence of permafrost near the surface creates special problems for accurate imaging of seismic data. Permafrost thickness can exceed 2000 ft (600 m) onshore and completely disappear offshore, and the high contrast between permafrost velocity and unfrozen sediment velocity produces severe distortions in reflections observed in seismic data. Images from time migration have false structures and positioning errors that compromise their quality and increase uncertainty. In this type of environment 3-D depth migration should be strongly considered. In this study we use tomography to construct the velocity model of the near-surface. Tomography is an ideal tool for this estimation problem where refraction information is limited or completely absent due to the decrease of velocity with depth, and where lack of short offsets, irregular offset sampling and multiple azimuths compromise velocity resolution near the surface. The image obtained with velocities from tomography attenuates false structure introduced by the permafrost. This image is comparable to an alternative image obtained with a velocity field built using well and seismic information.

Imaging Beneath Complex Layering: Processing Seismic Data With Complex Decomposition And Renormalization Group

Seismic Imaging beneath complex layering is challenging and problematic. Examples of complex layering are complex shale, carbonate, basalt and salt. Both the quality of the image and wavelet fidelity of the reservoir beneath the complex layering are usually not satisfactory below complex layering, largely because of the associated multiples and elastic waves. These effects interfere with the primary reflection. The problem is more acute for seismic reservoir analysis where agreement between observed logs data and seismic is important. Apache Corporation and WesternGeco undertook a Strategic Technology Alliance Project of a modeling study to investigate old and new seismic processing methodology. The purpose is to do an in-depth analysis of assumptions and approximations in the conventional seismic processes. The study identifies which of these programs are ineffective in the presence of interference due to strong interaction in the overburden. Then we can replace these programs with the new processing programs like complex decomposition and renormalization group.

Amplitude Preservation for PSDM Processing

As seismic exploration moves to more complex and challenging geologic environments, Anisotropic Prestack Depth Migration is becoming the standard imaging technology. At the same time, there is high interest to use seismic amplitudes in these complex environments for risk assessment and reservoir characterization, objectives that require accurate amplitude preserving prestack depth migration. In this paper we review recent advances made to ensure depth migration algorithms maintain the quality of relative amplitudes. We verify with simple synthetics the amplitude response of Reverse Time Migration, Adaptive Beam Migration and Kirchhoff Migration algorithms, and demonstrate how these can be used successfully, under controlled circumstances, for AVO/AVA and Absolute Acoustic Inversion in media with lateral velocity variations.

A multicomponent seismic data processing approach on the Cantarell Field

In recent years the advances in acquisition and processing of multicomponent seismic data have helped to successfully apply converted wave technology to oil exploration. New seismic methodologies for the study of deep oil reservoirs are of special importance. Nowadays, higher seismic resolution with depth is required for the detection of hydrocarbon traps such as Sihil Field located underneath Cantarell Field. On the other hand, generation of seismic images at shallow depths is also of importance to locate gas reservoirs and to define zones with the presence of gas chimneys that obscure seismic images generated using P-wave data alone. In this work we present results from the processing of two multicomponent seismic lines (OBC) from Cantarell. Emphasis is made to processes applied to converted wave data such as: polarity analysis of PS reflectivity, binning, rotation, static corrections, velocity analysis, NMO, DMO, and preand post-stack migration. The PS to PP time transformation was based on two methodologies: 1) Crosscorrelation analysis of interpreted PP and PS events and, 2) Simple numerical modeling of interpreted events. Finally, different methodologies to estimate Vp/Vs ratios are discussed emphasizing their use in the estimation of rock properties and processing parameters.