--> Reservoir Properties of Lacustrine Carbonate Buildups From Pleistocene Lake Lahontan: Analogues for South Atlantic Reservoirs

AAPG ACE 2018

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Reservoir Properties of Lacustrine Carbonate Buildups From Pleistocene Lake Lahontan: Analogues for South Atlantic Reservoirs

Abstract

The lacustrine carbonate reservoirs of the South Atlantic margin contain vast quantities of hydrocarbons, but predicting reservoir properties remains a significant challenge. Seismic-scale late-Pleistocene analogues for these reservoirs are found in carbonate buildups from paleo-Lake Lahontan, a large pluvial lake in the Basin and Range, western USA. These buildups are up to 100 m high, and consist of stacked successions of domes, pillars, and branches (< 5 m high) which exhibit meso- and microscale textural trends. Reservoir properties were examined by combining field observations, porosity and permeability measurements, and outcrop modeling. Porosity data were obtained from thin section optical porosity, and permeability measurements were made on hand samples and outcrops to create a large reservoir property dataset. We present conceptual models of depositional “building blocks”, based on observed trends in morphologies and textures, using a combination of ArcGIS, Matlab, and DecisionSpace software to model simplified reservoir properties and examine heterogeneity. We present 3D morphological models based initially on simple forms, and extended into larger stratal units. Porosity and permeability properties are stochastically incorporated into the models based on observed texture. Fluid flow through the reservoirs is modeled to examine how shape and texture affect property distribution and flow patterns. Small-scale conceptual models are applied to larger-scale, digital outcrop models constructed from imagery collected from sUAS (small unmanned aerial vehicles or drones). Reservoir property and flow simulations are applied to the larger model. We observe complex vertical and horizontal heterogeneity dependent on growth direction, texture, internal structure, overall morphology, and gross stacking patterns. The models indicate that unlike siliciclastic reservoirs, the lacustrine carbonate reservoir potential is strongly dependent upon environmental factors including water chemistry, depth, clarity, and temperature. The outcrop-based models demonstrate that stacking patterns indeed have a measure of predictability, and are useful analogues for predicting the distribution of reservoir properties and fluid pathways at the basin scale.