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Abstract: Recognition of Lowstand Wedges, Paleokarst Reservoirs, and Drownings--Insights from Seismic Modeling and Outcrop Studies

C. Robertson Handford

Exploration is increasingly dependent upon obtaining credible stratigraphic interpretations of seismic data. With respect to carbonate platforms, two of the most important seismic-imaging and interpretation problems are (1) distinguishing between lowstand unconformities and drowning unconformities, and (2) recognizing paleokarst reservoirs.

Lowstand unconformities vs. drowning unconformities. Many contend that onlapping wedges of strata above sequence boundaries but below the previous shelf break comprise the lowstand systems tract. An alternative view is that onlapping wedges do not record sea level falls, but instead chronicle sea level rises and platform demise. A key issue is whether the observed stratal discordance and onlap patterns seen in outcrops and in seismic lines are unique to either hypothesis, or could both hypotheses be correct for certain platforms? Outcrop studies have shown that some platforms have suffered from lowstand exposure and drowning during a sea level fall and rise. A Mississippian carbonate ramp exposed along the southern margin of North America is flanked by a siliciclastic lowstand wedge a d overlain by a drowning succession of black shales. This dual history of lowstand exposure and drowning formed two baselap surfaces, which lie so close to each other on the shelf that seismic dissemination is almost impossible. The paradox is that although the ramp was terminated by drowning, the visible seismic baselap was due to lowstand exposure.

Predicting paleokarst reservoirs. Numerous large fields around the world produce from carbonate reservoirs with a moderate to strong paleokarst overprint. Their discoveries, however, were structurally driven and rarely based upon predrill knowledge of paleokarst systems. In fact, there has been little effort to determine how to recognize paleocave systems in seismic reflection data. To narrow this gap, the sedimentary fill and stratal geometries of modern cave systems were examined and modeled seismically. The models show a passage from continuous reflections in the undisturbed country rock to discontinuous reflections inclined toward the cavern core. Velocity pull-ups and pull-downs are significant where velocity and density contrasts between the country rock and

collapsed chamber are important. Similar reflection character may be visible in high-resolution 2-D and 3-D data and could help delineate paleocave reservoirs.

AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers