Chapter 7: Carbonate Sequence Stratigraphy and its Application to Hydrocarbon Exploration and Reservoir Development
Carbonate strata differ fundamentally from siliciclastic strata and require their own set of sequence stratigraphic and facies models to aid in hydrocarbon exploration and reservoir development. Significant differences that impact sequence stratigraphic models include the following: (1) the largely autochthonous marine deposition of carbonates in the shallow-water photic zone; (2) secular changes in carbonate biota and mineralogy; (3) higher impedance of carbonates; (4)greater range of platform slope angles in carbonates due to early cementation, organic binding, and coarser grain size; (5) the virtual termination of platform growth and associated rapid freshwater cementation of carbonates during sea level lowstands; and (6)vigorous off-platform shedding of excess carbonate sediment during sea level highstands. Because carbonate deposition is linked to the photic zone, carbonates are regarded as better recorders of accommodation (sea level + subsidence) changes than siliciclastics.
Carbonate platforms are composed of a spatial and temporal hierarchy of stratal units, including parasequences, sequences, composite sequences, and supersequences, which records repetitive and cyclic changes in accommodation and sediment supply. Stratal units are recognized by their position in the stratal hierarchy and by bounding surface type and stacking patterns. Stacking patterns (landward-, vertical-, and seaward-stepping) describe changes in the geographic position and character of stratal units with respect to thickness, stratal geometry, facies types and distributions, and bounding surface type. The character of stratal units and hierarchies is a function of platform geometry, subsidence rate, amplitude and frequency of eustatic sea level fluctuations, siliciclastic supply, and environmental factors such as marine energy and climate. Furthermore, the character of smaller scale stratal units is a function of position within the larger scale stratal units. The recognition and analysis of stratal hierarchies is limited by the type, quality, location, and resolution of available data. Where possible, the analysis of stacking patterns within a stratal hierarchy is the key to improved correlation, better prediction of reservoir, source and seal facies distribution, geometry and continuity, and prediction of unconformities and related diagenesis that impact reservoir quality.
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