Chapter 4: Sonic Velocity in Carbonate Sediments and Rocks
Compressional wave and shear wave velocities (VP and Vs) of 295 carbonate minicores from different areas and different ages were measured under variable confining and pore fluid pressures. The samples range from unconsolidated carbonate mud to completely lithified limestones. The results of the measurements show that, unlike siliciclastic and shaly sediments, pure carbonate rocks show little direct correlation between acoustic properties (VP and Vs) and depth of burial or age. Velocity inversions with increasing depth are thus common. Sonic velocity in carbonates is more controlled by the combined effect of depositional lithology and diagenetic processes, such as cementation and dissolution.
At 8 MPa effective pressure, VP ranges from 1700 to 6500 m/s and Vs from 700 to 3400 m/s. These ranges are caused mainly by variations in the amount of porosity and porosity type and not by variations in mineralogy. Measured velocities generally show a positive correlation with density and an inverse correlation with porosity, but departures from general trends can be as high as 2500 m/s. These deviations can be explained by the occurrence of different pore types that form during specific diagenetic phases. Commonly used correlations such as Gardner's law (VP versus density) and the time-average equation (VP versus porosity) should be modified because they often result in velocities that are too low for carbonates.
Velocity measurements in unconsolidated carbonate mud at different stages of experimental compaction show that velocity increases due to compaction are lower than observed velocity increases at decreasing porosities in natural rocks. This difference shows that diagenetic changes such as cementation and dissolution, which may predate or accompany compaction, influence velocity more than simple compaction at increasing burial depth.
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