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Petrophysical Characteristics of Carbonate Drift Deposits in the Maldives


In the Maldives, there are two types of drift deposits: (1) flat lying aggradational sheeted drifts and (2) mounded progradational drift deltas. Sheeted drifts lay on top of drift deltas. These deposits accumulate by the action of bottom currents, which pick up large volumes of sediment and flow through inter atoll gateways from the Indian Ocean into the Inner Sea. Drift deposits are hundreds of meters thick, skeletal rich, heavily bioturbated and contain alternating consolidated to unconsolidated units. The two drift deltas are estimated to cover approx. 350 km2 each. The coarsest grainstone-rudstone units are found at the apex of drift deposits and consists mostly of large benthic foraminifera. Moving down and off the axis into the more distal portions, the amount of mud increases and only fine-grained wackestone to packstones are found. Drift deposits have high porosity (15-80%) and do not show the typical trend of decreasing porosity with depth, but in certain intervals porosity increases with depth. Downhole fluctuations in porosity, density, velocity, and resistivity in these pure carbonate successions are controlled by changing depositional textures, grain size, facies, degree of lithification, and pore type. High percentages of microporosity decrease velocity in the deeper, finer-grained drift packages. Some spikes in resistivity are due to the occurrence of chert nodules and the amount of calcite cement. Unconsolidated units have high porosity, low density and low velocity. Gamma ray values are mostly driven by the amount of organic matter and is generally low. These drift deposits have high macro and micro porosity, relatively high permeability, and their thickness and spatial extent can potentially make them excellent reservoirs.