A significant application of borehole image data is the use of textural characteristics combined with lithologic information to assign image facies. Image facies act as a proxy for depositional facies, effectively replacing core in portions of the field that lack core coverage.
Facies analysis occurs in two steps. First, bulk lithology is determined based upon the response of open-hole curves (gamma, density-neutron, photoelectric effect, etc.), image log color and bedding character. Second, a visual inspection of the image data is used to assign a textural classification to specific intervals. Commonly observed textures include well-preserved tabular and tangential cross-bedding, planar lamination, mottling by bioturbating organisms, concretionary horizons, authigenic pyrite mineralization, internal soft-sediment deformation and solution-collapse brecciation. Stick and pull intervals are also identified in facies analysis.
Once image facies are identified, they are compiled into pie charts that display the relative abundance of each facies type across selected intervals. Then, changes in the relative amount of image facies can be compared with changes in depth. Variation in the abundance of facies types are also observed to change within a single stratigraphic interval in wells across the field.
Once facies analysis is complete, useful comparisons are available in both the vertical well and across the field. Analysis of image facies within an individual well permits determination of changes in depositional environments through time. Comparing changes in image facies composition across a field captures spatial variation in depositional environments. Combining vertical and lateral image facies flux yields a three-dimensional portrait of the sedimentary system.
In this example, image facies were analyzed for 3 sandstone packages. Sand intervals l & ll display a similar facies suite, dominated by cross-bedded and mottled sandstones with a smaller percentage of muddy sandstones and mudstones. The facies observed in these units indicate that they were deposited in a fluvial depositional environment.
Sand III constitutes a major change in facies suite from the underlying sands. This unit displays a large percentage of mottled and laminated mudstones, with very few cross-bedded sands and a significant amount of coal. These facies indicate that the Sand III unit was deposited in a deltaic depositional environment.
Image facies data can be scaled up to enhance the understanding of reservoir architecture across fields and sub-basins. These data can contribute important constraints to fine-tune geostatistical model development.
Facies Field Map
The map above shows image facies data excerpted from a 16 well study. Image facies appear as pie charts over a structure contour map to illustrate lateral changes in sediment composition and texture within a stratigraphic interval of interest. In this example, the pie charts reveal subtle changes in the relative abundance of sand vs. muddy sand vs. mudstone and changes in the percentage of identifiable textures.
Image facies data can be used as input to constrain depositional, petrophysical and geostatistical models within a hydrocarbon reservoir, ultimately contributing to in-place reserve and productivity calculations. These models, in turn, support development of a reservoir simulation model, to fine-tune EUR calculations, establish priority for vertical or horizontal development and, with a geomechanical model, optimal well spacing.