Comparative Sedimentology of Modern and Ancient Oolitic Sand Shoals and Tidal Deltas

Rush, Jason

Publication

Comparative Sedimentology of Modern and Ancient Oolitic Sand Shoals and Tidal Deltas

Rush, Jason

Abstract

This comparative study investigates modern and ancient oolitic sand shoals and tidal deltas to explore and assess predictive process-based sedimentologic and stratigraphic trends as well as their diagnostic features. Predictive stratigraphic and sedimentary tools are important for constructing reservoir simulation models that are used by the oil and gas industry for field development optimization and economic forecasting. Regulations for CO2 sequestration projects also stipulate the use of reservoir models for simulation of CO2 plume dispersal patterns as part of the permitting process and then for use in monitoring, reporting, and verification. Construction of realistic reservoir models are reliant on diagnostic properties and predictive stratigraphic and sedimentological tools. This study describes linkages and trends amongst hydrodynamics, depositional processes, and barform morphometrics and explores their occurrence and diagnostic stratal configuration in the stratigraphic record. To assess prospective modeling trends for oolitic tidal sand shoals and explore potential patterns of reservoir heterogeneity, this study examines, quantifies, and models the cycle-scale architecture of the Holocene mobile oolitic tidal sand shoal complex at Schooner Cays, Bahamas. Process-based stratigraphic trends are captured in quantitative, geocellular models of the shoal from analyses of satellite imagery; two-dimensional, high-frequency seismic (chirp) data; and sediment cores. Data show that longitudinal tidal sand ridges extend up to 8 km along depositional dip, gradually transforming bankward into channel-bound, compound barforms consisting of linear, parabolic, and shoulder bars. These bars terminate into a laterally extensive (10 km), strike-elongate sand sheet. Each bar type includes distinct internal architecture, grain size, and sorting related to feedbacks among hydrodynamics, geomorphology, and sedimentology. Building on these data and concepts from the Holocene accumulations, this study demonstrates a methodology for quantifying and validating probabilistic stratigraphic trends prior to their inclusion in stochastic-based facies modeling algorithms. Inclusion of statistically robust facies probability volumes during truncated Gaussian simulation generated ordered and geologically accurate facies distributions relative to bar-crest centerlines, water depth, and geomorphic position. Petrophysical models that incorporate facies-specific porosity, permeability, and water saturation functions display pronounced cycle-scale heterogeneity that could provide insights into variable production rates and poor sweep efficiency commonly encountered during development of analogous oolitic reservoirs. A Permian (Grayburg Formation) oolitic tidal delta system and shoal exposed in the Brokeoff Mountains (New Mexico) is described and compared to an analogous Holocene tidal delta system from the Abaco Islands (Bahamas). The depositional setting and hydrodynamics of the Grayburg oolitic depositional system is best understood in relation to its position perched below a San Andres ramp margin inflection. Over 4,000 m of measured sections, 53 linear km of bed tracing, and detailed facies mapping across key outcrop windows were used to build a robust 3-D stratigraphic framework. The uppermost San Andres forms a progradational downstepping forced-regressive wedge. The Grayburg backsteps across the San Andres forming a 90-m thick peritidal-dominated lowstand wedge situated below the San Andres shelf inflection. The San Andres lowstand antecedent shelf inflection combined with 3rd-order maximum flooding promoted development of a ramp-crest barrier and an intervening lagoon (8-m deep). Data show that a 5-m deep by 220-m wide tidal inlet formed across the ramp crest and passed lagoonward into a flood-tide delta and seaward into an ebb-tide delta. The inlet is filled by trough cross-bedded oolitic grainstone and intraclastic rudstone that progressively grade into fine grain ooid-rich siliciclastic sandstone along the outer margins of the delta lobes. Beds shingle landward and outward from the tidal inlet into the lagoon. Normal marine ichnofacies consisting of Cruziana are common across the ebb-tide delta. The ebb-tide delta is interpreted to record swash bars that build from north-to-south toward the main ebb channel. The flood-tide delta margin is situated 1,100 m from the tidal inlet, which is consistent with observations from flood-tide deltas in the Abaco Islands. A coeval oolitic sand shoal eventually built seaward across the lagoon and ramp crest and records the onset of 3rd-order highstand conditions. Collectively, observations from these oolitic shoal demonstrate spatial trends in sediment size and barform morphology that reflect bankward changes in velocity outboard of the tidal inlet that result from radial flow expansion and deceleration.