Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
GPR Constraints on 3-D Stratigraphic Architecture and Porosity in Microbialite-Oolite Sequences, Upper Miocene, SE Spain
(1) Department of Geology, University of Kansas, Lawrence, KS.
Microbialite-oolite systems form important reservoirs, yet optimal recovery can be hindered by complex facies geometries and porosity distribution. This study integrates high-resolution ground-penetrating radar (GPR) and detailed field data to investigate paleotopographic influences on deposition and porosity distribution in a microbialite-oolite reservoir analog (Upper Miocene Terminal Carbonate Complex, TCC; La Molata, Spain). The TCC contains four topography-draping sequences that were deposited in association with high-amplitude, high-frequency glacioeustasy. GPR data integrated with petrophysical, petrological, and lithological data allow development of comprehensive 3D Petrel models of the distribution of porosity, permeability, and stratigraphic architecture.
Twenty kilometers of 2D GPR data of differing frequencies (100, 50, and 25 MHz) were collected to create pseudo-3D geophysical models of facies and porosity patterns at several scales. Lower frequencies (25 and 50 MHz) provide resolution of 1 m and 0.5 m, respectively, and are useful in identifying large-scale features, including stratal packaging and sequence boundaries. Higher frequency (100 MHz) data provide resolution of 0.25 m and are useful for resolving finer scale facies, geometry and porosity changes within sequences. GPR profiles collected within 1-2 m of well-exposed outcrops are the ground truth for confident correlation of GPR reflections to specific geologic features. These include sequence boundaries, porous versus nonporous facies and fractures. Three additional general GPR facies are identifiable and mappable in 3D for each frequency. They are recognized on the basis of reflection signatures (strong, weak, and no internal reflections). The GPR facies are linked predominantly to porosity variations in porous and nonporous microbialite and oolite facies. Two densely-spaced 3D grids allow geometries of porous and nonporous microbialite and oolite bodies to be imaged in 3D and related to paleotopography in the subsurface.
Some of the porosity signatures cut across facies and stratigraphy and appear to be related to diagenetic overprinting. Therefore, our results demonstrate that GPR is a valuable tool for recognition of both depositional and diagenetic controls on porosity variability. The findings provide a 3D analog useful for understanding paleotopographic controls on geometries and associated heterogeneity in analogous hydrocarbon reservoirs.