Chapter 15: Seismic Modeling of a Carbonate Platform Margin (Montagna della Maiella, Italy): Variations in Seismic Facies and Implications for Sequence Stratigraphy
Flavio S. Anselmetti, Gregor P. Eberli, Daniel Bernoull, 1997. "Seismic Modeling of a Carbonate Platform Margin (Montagna della Maiella, Italy): Variations in Seismic Facies and Implications for Sequence Stratigraphy", Carbonate Seismology, Ibrahim Palaz, Kurt J. Marfurt
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Synthetic seismic sections across the exposed Cretaceous-Miocene carbonate platform margin of the Montagna della Maiella (central Italy) explain the seismic facies of a carbonate platform margin system and show the limitations of relating seismic sequences to depositional sequences. To define a layered impedance model, velocities and densities of 186 minicores from all major out-cropping lithologies were determined. The impedance model was converted to synthetic seismic data by applying a computer-simulated model that uses the normal incidence ray-tracing method at variable frequencies, amplitude gains, and noise levels. The resulting synthetic seismic sections show a mostly transparent platform that is onlapped along the escarpment by a succession of high-amplitude slope reflections. The different reflectivities of platform and slope can be explained by their differences in impedance contrasts. The small impedance contrasts within platform carbonates results in weak reflections nearly indistinguishable from noise, whereas the large impedance contrasts within the slope and basin carbonates yield coherent high-amplitude reflections. The seismic image with incoherent to transparent platform, high-amplitude slope reflections, and recognizable prograding units is similar to observed seismic data across other steep carbonate platform margins (e.g., Great Bahama Bank and Adriatic Sea).
In outcrop, seven unconformity-bounded supersequences were mapped. Comparison with the synthetic seismic section shows that, at a frequency of 20 Hz, only five of these depositional super-sequences can be recognized using seismic unconformities. With an increase in frequency, an increasing number of unconformities become visible, and at a frequency of 60 Hz, all seven are imaged. The synthetic seismic sections also reveal that some of the seismic unconformities are pseudo-unconformities—they do not exist in outcrop, but the seismic image shows erroneous or nonexistent geometric patterns. These are a result of the thinning of layers below seismic resolution. These observations document the problem of seismically imaging depositional sequences. Depending on the dominant frequency, an erroneous number of sequences might be interpreted. This limitation must be taken into account when making sequence stratigraphic interpretations based solely on seismic information.
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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