Drowning Events on Carbonate Platforms: A Key to Hydrocarbon Entrapment?

Abstract

ABSTRACT Drowning of carbonate platforms occurs when carbonate production cannot keep pace with relative sea level rise. Detailed biostratigraphic and petrologic analyses suggest that "drowning unconformities" as seen on seismic data are not always unconformities in a geologic sense. These "drowning events" are often expressed in the geologic record as rapid upward-deepening (but gradational) formation contacts with little or no missing record. This interpretation is supported by high frequency (80-95 Hz) seismic data, which show the existence of "drowning sequences" not readily seen on standard (60 Hz) data. Drowning of a carbonate platform produces a distinctive sedimentologic and seismic record that can include chemical sediments (glauconite, phosphate), late-growth shelf margin buildups, and abundant alga1rhodoliths and 1arge benthic foraminifera. The characteristic sedimentary patterns that compose drowning sequences suggest that drowning of carbonate platforms is often caused by excess nutrient supply, and aided by a rise in relative sea level (tectonic + eustatic). Drowned platforms commonly have a wedge-shaped appearance in vertical profile, whether they are associated with continental margins or isolated open ocean atolls. A topographic "hinge zone" that develops as a result of rapid vertical aggradation may act as a focal point during hydrocarbon migration. Regiona1 play eva1uati on can be refined and enhanced by understanding the interplay between carbonate platform growth dynamics, tectonic subsidence, and eustatic sea level change. This can result in decreased risk associated with exploration in mature or unexplored basins. INTRODUCTION As defined by earlier workers 1, drowning of a carbonate platform occurs when " ... re1ative rise of sea level (that is, tectonic plus eustatic movements) outpaces carbonate accumulation so that the platform or reef becomes submerged below the euphotic zone of prolific carbonate production." Others2 have noted that drowning could be caused by relative sea level rise, including tectonic movements; environmental deterioration, leading to reduced benthic growth rates; and oversteepening and self erosion (i.e., slumping and density current erosion driven by the dumping of the platform's own excess sediment. Not included in this list, but identified in subsequent work3 was buria1 by prograding clastics in a submarine environment (without subaerial exposure of the platform). Recent work- has shown that drowning events that had traditionally been viewed as unconformities based on seismic data were not necessarily unconformable in a geologic sense. This important distinction recognized drowning of a carbonate platform as an event in the platform's growth history that is often represented as a distinctive sequence of rocks(drowning sequence). Drowning sequences may be difficult to detect using conventionally processed (low frequency - 60 Hz) seismic data because of their variable thicknesses, though special processing techniques (zero phase, true amplitude, 80 Hz or greater) could aid in identifying them. In a previous study4 of the seismic stratigraphy of the Upper Jurassic-Lower Cretaceous carbonate platform of the Baltimore Canyon area (Figure la), and the lower Miocene carbonate platforms of offshore China (Figure Ib), a distinction was made between the drowning response of continental margin-type platforms (Baltimore Canyon) and isolated oceanic atolls (China).