Depositional Architecture of Stacked Reef Sigmoids during High-Amplitude 100-kyr Sea Level Cycles: Results of the Dominican Republic Drilling Project
Stratal patterns and facies heterogeneity in carbonate rocks are dependent on sediment production from biological systems, dispersal of these sediments, and available accommodation space. During high-frequency sea level cycles fluctuations in accommodation space result in complex patterns of stacked reef sigmoids and iterative phases of diagenetic alteration that modify primary depositional textures. This heterogeneity, resulting from the complex three-dimensional arrangement of disparate depositional facies, can be difficult to recognize, resolve, and predict in seismic data. The Plio-Pleistocene reefs of the southern Dominican Republic (DR) provide a unique opportunity to study the complex three-dimensional architecture and resulting petrophysical attributes of stacked reef sigmoids during a period of high frequency sea level oscillation and superimposed sequential diagenesis. Three main sigmoidal geometries (4th-order megasets) are evident based on terraces at approximately 50 m, 30 m, 15 m, and a coastal terrace at 5 m. Subaerial exposure surfaces confirm these megasets. However, within each megaset, several cosets (5th order) are resolvable based on evidence for up dip subaerial exposure and coral fauna. Higher order sigmoid sets (6th order, 1ky-10ky) represent the individual reef depositional units. The correlation of the main depositional packages is constrained by strontium-isotopic ages. New ages help to constrain deposits to within a resolution of two marine isotope stages, usually to within a range of 0.2 my in the Pleistocene. The stacking of these sigmoid-shaped reefs produce lateral progradation of approximately 15 km with geometries that generally follow the highstand systems tract model of Pomar and Ward (1994). However, uplift of the DR section, combined with the high amplitude eccentricity-driven (110 ky) sea level cycles, has also produced a step-down stacking that resembles a lowstand systems tract. Petrophysical measurements (Vp, Vs) of cylindrical miniplugs (2.5 cm in diameter) from characteristic lithologies of sigmoid depositional facies and diagenetic environments are used to resolve intra-sigmoid heterogeneity due to meteroric phreatic and vadose diagenesis. Especially important is the pore structure in the control of variability in sonic velocity. Together, these results provide a model for reservoir-scale (sub-seismic) resolution within the reef sigmoid geometry.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California