BED SEAL EVALUATION FOR LOW DATA DENSITY GEOLOGICAL CO2 SEQUESTRATION SITES

Geological sequestration of CO₂ requires identification and evaluation of reservoirs and their over and underlying mudstone bed seals, often in areas of few subsurface data. Although a robust sequence-stratigraphic framework exists for identifying and evaluating reservoirs in such situations, little has been done toward identification and evaluation of bed seal. We present a sequence-stratigraphic framework that ties bed seal presence, extent, and lateral and vertical homogeneity to the capillary seal properties of bed seals.

We use sequence-stratigraphic and petrophysical methods to predict mudstone bed seal presence and quality via integration of rock property measurements (e.g., clay-mineral content, mercury injection capillary pressure (MICP) on cuttings, core, outcrop samples), wireline logs (e.g., gamma ray), and sequence-stratigraphic surfaces and associated lithofacies packages. We first evaluate lithofacies packages and associated rock properties. Then a 1D vertical stacking pattern analysis links vertical stacking patterns to lateral facies relationships. Using available well log and seismic data we build 2D and 3D models that relate lateral facies associations to sequence-scale stratal stacking patterns. These models enable us to predict such important bed seal attributes as environment of deposition, lateral and vertical extent, and capillary seal quality.

The highest quality bed seals, MICP greater than 2000 psi, have greater than 50% clay-mineral content, low biogenic content, are clay-matrix supported and bioturbated. The lowest quality bed seals, MICP less than 1000 psi, have high biogenic content, low clay-mineral content, are silt- and sand-matrix supported, and have little bioturbation. The most consistently thick, high quality bed seals occur in foreset-dominated zones with significant terrigenous muddy strata. Distal, offshore mudstones commonly have greater biogenic content leading to significant diagenesis, micro-fracturing, and poor capillary seal quality. The environment of deposition of the highest quality seals is variable and depends on the systems tract being considered. In general, we find that transgressive to lower high-stand systems tracts and proximal to mid-shelf depositional settings hold the best seals.