Seismic Interpretation of Carbonate Depositional Environments

Abstract

Most seismic stratigraphic or seismic lithologic interpretations have been applied to clastic depositional systems. Because of their unique sedimentologic and mineralogic characteristics, carbonate rocks are more difficult to study using seismic data. Increased knowledge of carbonate deposits and their petrophysical parameters, and higher quality geophysical tools now permit a better understanding of carbonate rocks using seismic images. In this paper, seismic facies of carbonate rocks are characterized, step by step, from the basin to the supratidal environment. (1) Pelagic deposits (shales and micritic limestone layers) exhibit continuous, parallel reflections with an apparently high frequency. Chalk deposits display continuous high-amplitude reflections at the top and base with an internal reflection-free zone. (2) Talus deposits are characterized by discontinuous, oblique reflections of high amplitude. Carbonate debris flow generates chaotic reflections with hummocky surfaces. Channels exhibit erosional truncations. (3) Reef barriers are mound-shaped biogenic deposits that display marginal onlapping reflections. Overlying reflections drape the reefs, and underlying reflections exhibit pull-up or pull-down effects. Hyperbolic diffractions also may occur. (4) Platform border sands are mound-shaped oblique reflections of moderately high amplitude. (5) Inner shelf strata are characterized by parallel, continuous reflections typically displaying low frequency. Patch reefs may be distinguished within the inner shelf system (mound shape, drape of overlying reflections, velocity anomalies, and spurious events). (6) Where dolomitized, the intertidal to supratidal facies exhibit a marbled zone--a practically reflection-free zone with a few discontinuous reflections. Diagenetic events, such as paleokarst zones, may be delineated using special analyses involving amplitude offset variations. The study of carbonate depositional environments and petrophysical characteristics provides a more complete geologic insight, including relative changes in sea level, within a sedimentary basin. Such studies are fundamental in analyzing carbonate reservoirs and their paleogeographic settings.