Rodney Johnston

Fracture prediction from wide-azimuth land seismic data in SE Algeria

There is evidence from core, image log, and well-test data that the Tiguentourine gas condensate field in the Illizi Basin of southeast Algeria contains fractures that provide a positive permeability assist. In 2004, the first ever wide-azimuth P-wave seismic survey in Algeria was acquired over a small part of the field to test the ability of surface seismic data to accurately predict the P-wave anisotropy resulting from the preferential orientation of open fractures. There appears to be a correlation between velocity anisotropy derived from the seismic data with image log data from the wells, thus providing confidence that future wells can be located to directly target areas with high concentrations of oriented fractures that are anticipated to provide significant permeability assist and boost well productivity.

Wavelets, well ties, and the search for subtle stratigraphic traps

We examine the conventional methodology for tying wells to processed seismic data and show why this methodology fails to allow for reliable interpretation of the seismic data for stratigraphy. We demonstrate an alternative methodology that makes the tie without the use of synthetic seismograms, but at the price of measuring the seismic source signature, the cost of such measurements being about 1% of data acquisition costs. The essence of the well tie is (1) to identify geological and seismic interfaces from the logs and core, (2) to measure the one‐way traveltime to these interfaces using downhole geophones, and (3) to use the polarity information from (1) and the timing information from (2) to identify the horizons on the zero‐phase processed seismic data. Conventional processing of seismic data usually causes the wavelet to vary from trace to trace, and conventional wavelet extraction at a well using the normal‐incidence reflection coefficients relies on a convolutional relationship between these coeff...

The first 3D/4-C ocean bottom seismic surveys in the Caspian Sea: Acquisition design and processing strategy

Bubbling mud volcanoes create havoc for conventional seismic images over the crest of the Azeri-Chirag-Gunashli (ACG) structure in the Caspian Sea. Layers of trapped gas and shallow mud flows in the overburden sediments leave areas of severely degraded data quality at the crest of the reservoir structure. Fortunately, in 2002, the application of the first 3D four-component (3D/4-C) seismic in the Caspian has helped lift the veil of mud and gas attenuation which previously obscured the details of the subsurface reservoir.

Studying shallow seafloor structure based on correlations of continuous seismic records

Ambient-noise surface wave tomography has recently emerged as a very powerful tool in regional-scale seismology and is now widely used to study Earth’s crustal structure in areas where dense regional-scale seismic networks are available (e.g. Shapiro et al. (2005a)). Here, we investigate the feasibility of this approach using continuous marine noise data to extract the information about shear-wave structure of the superficial seafloor. We use data from the Valhall Field permanent seismic system (e.g., van Gestel J.-P. et al. (2008)) and compute cross-correlations of 6-hour continuous seismic records. Resulting waveforms clearly contain low-frequency dispersive Love waves. An initial analysis of arrival times of these waves indicates that the dominant source of seismic noise is located below the platform. We then select a profile of stations suitably aligned relative to the platform and compute cross-correlations of horizontal geophones between all stations of this profile to construct record sections. We construct group velocity dispersion curves of the Love waves extracted from these sections and invert them to construct a 2D shear-velocity profile down to a 20 m depth. In this paper, we present theory and application of this new approach to estimate S-wave velocity just beneath the seafloor and S-wave statics.

Reducing Structural Uncertainty On the Azeri Field Using Ocean Bottom Seismic: Offshore Azerbaijan

The interpretation of 3D Ocean Bottom Seismic survey over part of the Azeri field, offshore Azerbaijan has delivered significant reduction in structural uncertainty. The crest and South/Central flank of the field was poorly imaged on towed streamer data; this led to “no data areas” with large depth uncertainty and a poorly resolved structural model. The depth migrated PZ products from the OBS survey have facilitated a choice between alternative structural interpretations and provided improvements in depth prediction for development drilling.

Benefits of source signature measurements for multiple removal in streamer and OBC data

We present examples of the advantages gained by knowing the source signature in the goal of suppressing free surface multiples in streamer and OBC data. This is built into the concept of multiple removal as the recovery of the earth’s reflection response in the frequency-wavenumber domain, for which it is necessary to separate the upgoing and downgoing waves. The source signature is beneficial in the calibration and QCing of OBC data for wavefield separation in the water, by permitting simulation of the vertical component of particle velocity. This also aids in the wave separation of streamer data. The reflection response must be bandlimited, both spatially and temporally, which requires knowledge of the original source signature if undue noise is to be avoided.

Depth Imaging of 3D, 4C OBS Surveys In the Caspian Sea

The 3D ocean-bottom seismic (OBS) surveys acquired by BP over the Azeri-Chirag-Gunashli (ACG) structure in the southern Caspian Sea prompted the development of new techniques for velocity model building, tomography, anisotropy estimation and event matching, to generate prestack depth-migrated datasets of both pressure-wave (PZ) and mode-converted shear-wave (PS). The OBS data are better able to resolve previously poorly imaged mud volcano features, reducing structural uncertainty particularly over the crest of the anticline, and provide a greater degree of confidence in well positioning. Subsequent drilling in the area has verified the viability of the techniques developed.

Azimuthal velocity uncertainty: estimation and application

Azimuthal velocity models for HTI (Horizontal Transverse Isotropy) media are extensively and widely used for land seismic exploration in North America, North Africa and the Middle East. A surface fitting technique honouring all azimuths can invert for an HTI velocity model, which can then be used to perform azimuthally dependent NMO to flatten the CMP gathers. When performing the velocity inversion it is important to estimate the degree of confidence in the estimated velocity model. The main subject of this paper is velocity uncertainty estimation. Furthermore, we investigate the estimated errors in the model parameters with varying acquisition direction for various offset-azimuth distributions including azimuthal sectors and Common Offset Vector (COV) classes. The application of the technique to WAZ land data from Algeria illustrates the strength of the proposed technology.

Azimuthal AVO Analysis: Stabilizing the Model Parameters

AVO is an important tool in the interpretation of seismic data. Azimuthal AVO models (AVAZ) have been used to characterize fracture distributions and directions in HTI (Horizontal Transverse Isotropy) media. The main subject of this paper is a method for the stabilization of AVAZ parameters. We present an extension of the technique of Whitcombe, Dyce, McKenzie and Hoeber (2004) for gradient stabilization in the standard 2-term AVO model given by the Shuey equation to the case of Ruger and Tsvankin’s (1997) azimuthal AVO analysis. We also investigate the estimated errors in the AVAZ model parameters with varying HTI isotropy plane direction for a selection of offset-azimuth distributions, including azimuthal sectors and Common Offset Vector (COV) classes. The application of the technique to WAZ land data from Algeria illustrates the use of the techniques.

Indonesian ocean‐bottom seismic using a seabed plough: An industry first

Field development requirements and operational difficulties acquiring seismic data prompted an innovative solution to ocean-bottom seismic for the Tangguh LNG development offshore Papua, Eastern Indonesia. A technique borrowed from the telecommunications industry allowed 70km of wide-azimuth 3D4C ocean-bottom data to be acquired using a rolling spread of receivers ploughed up to 1m below the seafloor, buried and deburied for each patch. The unique approach provided a quality of seismic data not available in this area by more conventional means on surface or seafloor. Following a detailed and staged processing effort, the dataset provided significantly improved images key to efficient field development, and estimated to be worth upwards of