Developing a specific data processing flow in areas of shallow carbonates, as in other seismically difficult areas, requires a clear understanding of the problems associated with such environments. Once the initial problems are defined, a specific processing procedure is designed to achieve the objectives of seismic data processing. This design properly starts from the end, at the final required results, and proceeds backward to identify the processing tools needed to achieve these results. A number of severe data problems are associated with the shallow carbonate environment, the most obvious one being the strong multiples generated by the surface and carbonate interfaces. Water-trapped multiples propagate with water velocity, hence they are more easily discernible (due to their NMO differences) from the deeper, higher velocity primary reflections. The surface-connected and interbed multiples generated by carbonate interfaces, however, propagate with higher velocities, hence they are not as easily identified as the low-velocity multiples. As a result, a variety of multiple suppressing methods must be included in the processing stream.
Carbonate layers also cause the generation of horizontally traveling waves, such as the refracted events on marine data and large-amplitude ground roll on land data. These must be suppressed before proceeding with processes such as deconvolutions and wavelet processing. On land data, carbonates are normally covered by weathered low-velocity layers, and in some areas, carbonate layers are cut by old riverbeds. These conditions create short-wavelength disturbances in travel-times and thus adversely affect the imaging process. The processing flow must therefore include computation and removal of these time disturbances as well. Large-velocity differences between carbonates and clastic rocks cause most of the pressure wave energy to be reflected, with little transmitted to the layers below. This results in low signal-to-noise ratios in the primary reflections below carbonate layers. The velocity difference also causes the transversely isotropic layer velocity anisotropy, which in turn affects the apparent NMO velocities of longer offset data.
Figures & Tables
We first collaborated in the area carbonate seismology in 1990 while mapping Cretaceous and Tertiary carbonate reservoir facies from neighboring seismic data surveys gathered in the Pelagic Sea of Tunisia and Malta. Both areas, one shallow water (<50 m) and one deep water (>500 m), were plagued by a “penetration” problem through shallow carbonates and by a resolution problem of low-relief stratigraphic targets at depth. While the geologists on our teams had an ample supply of up-to-date sources devoted to the details of carbonate sedimentology and sequence stratigraphy, those of us working on the seismic data were left to our own devices. With considerable effort, we were able to come up with a handful of technical papers, some good notes from continuing education courses, and a thick pile of expanded abstracts from diverse sources to help us understand the seismic expression of carbonates. We augmented this sparse material with expert advice from our Amoco colleagues, our contractors, and our partners.
It was at this point when we first saw the need for an integrated reference book on carbonate seismology, and we vowed that once we were finished with our assignments, we would attempt to put such a book together. The years of 1992–1994 were tumultuous in the petroleum industry, with most of the major oil companies downsizing and the competing service companies consolidating. During this period, we saw many of our experienced colleagues who had provided us with expert advice leave the oil industry. With this additional lack of available “folk” wisdom in the area of carbonate seismology, we found it more imperative than ever to capture the current state-of-the-art before it was lost to posterity.
Our goal was to produce a book that would integrate the principles of carbonate geology with its seismic expression and would be readily understandable to the practicing geologists, geophysicists, and engineers that form the exploration and exploitation teams in the petroleum industry. The result is a single integrated volume, written in plain language by acknowledged experts in their fields, that illustrates the interrelationships of carbonate geology, petrology, sequence stratigraphy, rock properties, seismic data acquisition, seismic data processing, and integrated interpretation.
We have taken care in the editing process to ensure that every concept is explained clearly and concisely without getting lost in domain-specific terminology. Our hope is that this volume will sit dog-eared on the desk of every practicing geoscientist, to help the seismic data processor determine parameters to enhance the fidelity of carbonate images, to help the seismic interpreter better recognize the expression of sequence stratigraphy, to help the engineer understand patterns of permeability and fractures, and to help the carbonate geologist understand the expression of the rock record at the seismic scale and differentiate it from common seismic acquisition and processing artifacts.
We have provided ample examples on the application of carbonate AVO and acoustic logging. Tying acoustic logs to seismic is a common theme throughout the book. We have included two chapters by Fischer et al. and by D’Angelo et al. that show how, with the aid of careful seismic modeling, AVO can be calibrated and used to map porosity in carbonate rocks.
We wish to thank all the contributing authors for their hard work, perseverance, and patience. We also want to thank those authors who had hoped to contribute to this volume and did much of the work but, through the turmoil in the oil industry, found themselves severed from their data and ultimately unable to contribute.
Kurt J. Marfurt