Page 237 - Geology of Carbonate Reservoirs

218 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Figure 8.4 SEM micrograph of the oolitic reservoir rock at North Haynesville Field showing

the relatively undeformed and uncemented oolites from the principal oolite grainstone res-
ervoir facies that occurs along the paleostructural highs in the field. This pore system is a

good example of depositional, intergranular porosity following the genetic classifi cation
system illustrated in Chapter 2 . The average diameter of these ooids is about 1 mm. (Photo

by H. Ben Hull in Ahr and Hull ( 1983 ).)
The pore system in the oolite bar is intergranular and hybrid depositional (Figure
8.4 ). Laterally equivalent, lower porosity rocks consist of oolite – pellet – intraclast
packstones and lesser wackestone. The lower porosity in this offset facies refl ects
higher mud content. About 10% of present - day porosity was created or enhanced
by late - burial dissolution that affected both grains and lime mud.

The Geological Concept North Haynesville Field is an example of fabric - and
facies - selective hybrid - depositional porosity. Depositional facies are proxies for
porosity. The anatomy of the depositional body is elongate parallel to the local slope
break along the margin of a salt ridge, and it is thickened over the paleostructural
crest. Present structure is slightly offset from the original salt structure that infl u-
enced the location of facies boundaries, which in turn correspond generally with
reservoir boundaries. The thickest part of the grainstone buildup follows the paleo-
structural crest and includes the coarsest grainstones with large pores and pore
throats — consequently, the highest porosity and permeability. The standard deposi-
tional succession is a slope - break grainstone accumulation — or standard deposi-
tional succession number 5. The slope break in this case is at the edges of the ancient
salt ridge. Much of what is now northern Louisiana was covered by a shallow sea

during Smackover deposition; therefore the geological concept for finding this type
of reservoir is to search for other paleostructural highs with similar tectonic histories
along strike with North Haynesville Field. The paleostructural highs are salt ridges
elongate parallel to depositional strike, with or without faulted margins, and that
were paleostructurally high at just the right time. By following along strike, the
chance of fi nding a salt ridge with a similar growth history is improved. One evalu-