A significant application of borehole image data involves using textural characteristics in conjunction with lithologic information to classify image facies. Image facies serve as proxies for depositional facies, effectively filling in for core data in areas of the field where core coverage is lacking.
Facies analysis is conducted in two steps. First, the bulk lithology is determined based on the responses of open-hole curves (such as gamma, density-neutron, and photoelectric effect), along with the color and bedding features observed in the image log. Sometimes client-generated lithology models are used to augment open-hole curve determinations. Second, a visual inspection of the images leads to a textural classification of specific intervals. Commonly observed textures include well-preserved tabular and tangential cross-bedding, planar lamination, mottling caused by bioturbating infauna, concretionary horizons, authigenic pyrite mineralization, internal soft-sediment deformation, and solution-collapse brecciation. Stick-and-pull intervals are also noted.
After identifying the image facies, they are compiled into pie charts that illustrate the relative abundance of each facies type across selected intervals. Subsequently, changes in the relative proportions of image facies are compared with changes in depth. Variations in the abundance of different facies types are also observed to fluctuate within a single stratigraphic interval in wells throughout the field.
Once the facies analysis is completed, valuable comparisons can be made both vertically within individual wells and laterally across the entire field. The analysis of image facies within a single well allows for the identification of changes in depositional environments over time. By comparing the variations in image facies composition across the field, we can capture the spatial differences in these environments. Integrating both vertical and lateral image facies data provides a three-dimensional representation of the sedimentary system.
In this example, image facies were analyzed for three sandstone packages. Sand intervals I and II exhibit a similar facies suite, predominantly consisting of cross-bedded and mottled sandstones, along with a smaller proportion of muddy sandstones and mudstones. The facies observed in these units suggest that they were deposited in a fluvial depositional environment.
Sand III represents a significant change in the facies suite compared to the underlying sands. This unit is characterized by a high percentage of mottled and laminated mudstones, with very few instances of cross-bedded sands and a notable presence of coal. These facies suggest that the Sand III unit was deposited in a deltaic environment.
Image facies data can be scaled up to improve the understanding of reservoir architecture across various fields and sub-basins. This data can provide important constraints that help refine the development of geostatistical models.
Facies Field Map
The map above displays image facies data extracted from a study of 16 wells. Image facies are represented as pie charts over a structure contour map, illustrating lateral changes in sediment composition and texture within a specific stratigraphic interval. In this example, the pie charts indicate subtle variations in the relative abundance of sand, muddy sand, and mudstone, as well as differing percentages of identifiable textures.
Image facies data can be used to inform depositional, petrophysical, and geostatistical models within a hydrocarbon reservoir. This information ultimately aids in determining in-place reserves and productivity calculations. These models also support the development of a reservoir simulation model, which helps refine estimated ultimate recovery (EUR) calculations, prioritize vertical or horizontal development, and establish optimal well spacing in conjunction with a geomechanical model.