Buda-Georgetown Lime Play
Geology
The Georgetown was deposited in a quiet shallow Cretaceous Age sea where fine micritic limestone was the dominant lithology. Within the Upper Cretaceous, it corresponds to the European Cenomanian Stage. It is separated from the superjacent (overlying) Buda Formation by the dark grey to black, fissile Del Rio Shale. In many areas, the limestone of the Buda is very similar the limestone of the Georgetown. In most areas, the Georgetown was deposited over the thin Kiamichi Shale, separating it from the subjacent Edwards Formation limestone and dolomite. See Figure 5 for a stratigraphic correlation chart of these units.
Typically, the Buda and Georgetown have very low porosity values of 3 to 4 percent. The only porosity may be in the form of fracture porosity. Unless the fracture networks are extremely well developed, oil and gas production declines too quickly for the well to be economic, as there is no real matrix porosity to support sustained production over a long period of time. Some areas are an exception to this generalization, and they have thin, 3 to 10 foot thick beds with up to 15% porosity. The higher porosity is due to matrix intercrystalline, plus fracture porosity. These anomalous areas tend to be the best producing areas.
Figure 6 is a complete type-log display of a vertical well at ICI field. It demonstrates both fracture and matrix porosity.
It is likely that the cause of the local matrix porosity is that the rock type is similar to Edwards lithology, where larger grains of carbonate fragments and the secondary alteration of these grainstones to dolomite has produced a better quality reservoir. The local “Sweet Spots” were probably once grainstone shoals located on paleo-highs in the shallow Cretaceous seaway. This type of rock dwindles away on the flanks of the bar, where porosities and perms are lower.