Chapter 13: Porosity Detection and Mapping in a Basinal Carbonate Setting, Offshore Norway
The Cretaceous chalk section of offshore Norway is a prolific hydrocarbon-producing trend. The fields are typically structural traps, often with a stratigraphic trapping component. The identification and evaluation of stratigraphic traps in this chalk section require differentiation of the porous reservoir and tight nonreservoir chalk facies. Integration of geologic models, petrophysical data bases, and seismic analysis has allowed the detection and mapping of high-porosity reservoir-quality chalks along this play trend.
Sedimentologic studies of chalk deposition, burial histories, and reworking are combined with detailed petrophysical analysis of core plug samples in the lab to define rock properties that distinguish porous from nonporous chalk facies. Seismic analysis techniques, including stratigraphic processing, velocity analysis, modeling, inversion, and AVO analysis, enable the extraction of these rock properties from the seismic data, leading to prediction of the subsurface chalk porosity distribution. The results of these analyses indicate high-porosity reservoir-quality chalk. The drilling of this prospective high-porosity chalk anomaly resulted in an oil discovery.
This study demonstrates that the distribution of porosities within the Cretaceous chalk section of offshore Norway can be detected by seismic data. Furthermore, the porosity distribution in the chalk can be accurately predicted and mapped from the analysis of seismic data integrated with a detailed understanding of the petrophysical rock properties and the geologic history of the sediments.
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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