3D GPR of the Miami Oolite: Resorvoir Scale Internal Anatomy

RALF WEGER AND MARK GRASMUECK

Active oolitic sand shoals like the modern ones in the Bahamas as well as those in the ancient exhibit a complex internal architecture with a multitude of stacked sedimentary structures. As a result, the anatomy of these shoals is usually too complex to be captured with two-dimensional outcrop and one-dimensional well information. In order to avoid imprecise interpolations and speculations knowledge of three-dimensional (3-D) sedimentary structures is necessary for accurate reservoir flow modeling. This is especially important for units where production of fluids and gases is bedding controlled. In order to improve our understanding of ooid sand shoal anatomy we collected a 3D ground-penetrating radar (GPR) dataset in the Miami Limestone. The data cube consists of a 48x24 m data grid with 10 cm in-line spacing, 20 cm cross-line spacing, and 7 m average penetration depth. This study shows that: o 3D GPR provides a high-resolution volume image of ooid sand shoal architecture o 3D GPR images overcome the limitations of 2D outcrop and 1D core information on a sub-meter scale o 3D GPR resulted in a revision of previously derived paleocurrent and sandwave migration directions based on local nearby outcrops. This study combines outcrop information about the Miami Oolite, a Pleistocene Limestone formation that forms the bedrock of the greater Miami area (Fig. 1), and a 48x24 m 3-D 100 Mhz ground-penetrating radar (GPR) data cube (Fig. 2). The Miami Oolite was accumulated during the last sea-level highstand (approx. 120 ka). The oolite ridge of Miami Limestone consists of two alternating major facies, the bedded facies, and the mottled, bioturbated facies. Near the location of the 3-D GPR survey outcrops of cross-bedded oolitic grainstones and burrowed peloidal-ooid grainstones record alternating high-energy ooid sand deposition and low-energy periods of bioturbation. The bedded facies consist of regularly spaced, coarsening upward, 1-2 centimeter couplets that are differently cemented and can be observed in sets of 0.5 to 1

1 m thickness. The apparent eastward dipping couplets suggest the existence of prograding forsets created by eastward migrating decimeter scale sandwaves during high-energy times. The burrowed peloidal-ooid grainstone of the mottled facies at this location is separated from the overlaying cross-bedded oolitic grainstone by a thin layer of mud. The first-order bounding surface, an approx. 2 cm thick layer of micritic mudstone, suggests rapid burial of the area by migrating ooid sandwaves. Our GPR survey, for the first time, images this environment in three-dimensions. The dataset reveals several 1-2 m thick sets of southward dipping 5-10 m long reflectors at varying depths of 1-7 m. The main reflectors can be correlated to bounding surfaces between cross-bedded sets visible on the outcrop. Dip direction of the reflectors observed in the 3D GPR cube deviate by up to 90º from the small-scale cross-bedding observed in the outcrop. This discrepancy is likely caused by the two-dimensional nature of the outcrop observation. Gonzalez and Eberli (1997) have previously documented truncated large-scale prograding forsets superimposed by small scale sinuous and linear ripples with dip direction deviating by up to 90º on an active Bahamian ooid shoal. This highly successful first test of using 3D GPR in an oolitic limestone environment produces a new tool to help understand the spatial relationship between outcrop and well observations in a larger context of tens of meters. This tool allows a more accurate description of the 3D internal oolite reservoir architecture and detailed paleoenviromnment description such as dominant wave and current direction and location within an ooid shoal. Future 3D GPR surveys in other locations of the Miami Limestone ooid complex will further establish the variations in spatial relationships on a larger scale. Eventually, knowledge from this case study can be applied to ooid-sand reservoirs.

2 Inter-Shoal Channel Shoal Barrier Bar Back-Barrier Channel

GPR Survey Location

Figure 1. Top: Paleogeographic reconstruction N of Pleistocene ooid shoal complex after Halley & Evans. Left: Loacation of 3D GPR data aquired in Coral Gables Florida and 2D GPR profile linking outcrop to 3D data volume.

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Fig.2 48x24m 3-D ground-penetrating radar (GPR) dataset with 10cm in-lines spacing, 20cm cross-line spacing, and 7m average penetration depth was collected in the Miami Oolite.

ripples on channel floor ripples on top of sand wave sand bars base level tidal channel spillover lobe prograding foresets

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Fig.3 Typical structural features of carbonate sand bodies and how they appear in outcrops when fossilized. Note the complex hierarchies of erosional horizons. From Gonzalez (1993)