Page 235 - Geology of Carbonate Reservoirs

216 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Depositional Setting Porosity data were not available from enough wells to gener-

ate a porosity contour map for the field, so that step was skipped. The next analytical
step was mapping depositional facies. Smackover rocks in North Haynesville Field
consist of grainstones and packstones composed of ooids, peloids, rhodoliths, and
intraclasts. The originally published facies map by Bishop ( 1968 ) illustrates “ oolite
bar, mixed oolite, and oolite - pellet - superficial oolite facies ” (Figure 8.3 ). The term

“ pellet ” is common in older literature. Peloid is used in this book unless it is known
that grains are of fecal origin, in which case they are called pellets. Studies of bore-
hole cores and thin sections by Ahr and Hull ( 1983 ) determined that the coarser
grains in Bishop ’ s “ oolite bar ” facies are algal encrusted grains otherwise known as
rhodoliths. Rhodoliths are common in Smackover reservoirs and are usually among
the coarsest of all grains in the rocks. Some workers determine the largest grain size

in thin section, usually of coated grains, as a “ clasticity index ” as defined by Carozzi
( 1958 ) to aid in correlating individual reservoir zones or depositional cycles in
barrier island and sand wave complexes. That technique was used by Erwin et al.
( 1979 ) to correlate individual depositional cycles in carbonate dune deposits at the
Oaks Field, in the Jurassic, Smackover Formation of Louisiana. Ahr and Hull ( 1983 )
plotted maximum coated grain size against depth, against core analysis porosity, and
against trace element composition. Reservoir porosity did not correlate well with
clasticity index, or maximum coated grain size, because rocks with the largest coated
grains did not always have good sorting and uniformly large pores. Still, the North
Haynesville “ oolite bar ” facies of Bishop ( 1968 ) with its rhodoliths, ooids, peloids,
and scattered intraclasts has the highest porosity and permeability of the three
facies. Petrographic study revealed that the northeastern segment of the oolite bar
facies underwent extensive calcite cementation that reduced its reservoir quality.
The primary reservoir occurs in the remaining “ bar ” and its adjacent oolite – pellet
facies. Sedimentary structures and textures in borehole cores indicate that the North
Haynesville reservoir body is a marine sand wave, or tidal bar complex, similar to
the “ slope - break grainstone ” succession found along the edge of the Great Bahama
Bank near Eleuthra Island. Although the Smackover platform is a ramp, some parts
of it were elevated by salt tectonics to form local highs with localized slope breaks
along their margins. These localized slope breaks became sites for deposition of the
grainstone “ sand waves ” or “ bars. ”
Antecedent topography — the paleo - high that existed on the crest of the salt ridge
during Smackover deposition — influenced the size and shape of the North Haynes-

ville grainstone body. Reservoir thickness was determined by the rate and duration
of sedimentation. In this case, the reservoir is thickest near its axis, which is inter-
preted by Ahr and Hull ( 1983 ) to have been directly over the crest of the salt ridge
at Smackover time. Deposition kept pace with relative sea - level rise, producing the
grainstone – packstone accumulation on the antecedent high where hydrological
conditions were most favorable for the formation and accumulation of oolites and
rhodolites. As sedimentation and salt movement continued, the oolite sand wave
grew thicker and depositional topography was accentuated.

Reservoir Characteristics Bishop ( 1968 ) and Ahr and Hull ( 1983 ) did not identify

and rank individual flow units in the field. However, petrographic studies by Ahr

and Hull revealed that the highest porosity and permeability occur in Bishop ’ s
oolite bar facies, except where its northeastern tip has been cemented by calcite.