Page 253 - Geology of Carbonate Reservoirs
P. 253
234 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS
dominate. Happy Field rocks underwent a complex diagenetic history beginning
with early stabilization, dissolution, cementation, and emplacement of late burial
saddle dolomite, along with anhydrite and chalcedony as pore - filling and replace-
ment minerals. The main dissolution porosity is interpreted to have been formed
during early diagenesis, where exposure to undersaturated waters created or
enhanced porosity in the Happy rocks. Later cementation is interpreted to have
relegated the once - porous and permeable grainstones to moderate and poor reser-
voir quality. Some oolite grainstones show extensive dissolution and widespread
grain - moldic porosity, along with vuggy and solution - enhanced intergranular pores,
but later burial diagenetic calcite and anhydrite cements filled much of the pores in
such irregular distribution patterns that correlation between porosity and permea-
bility in the grainstones is very difficult. Because reservoir flow units are distributed
in such complex patterns, it was not possible to identify “ straightforward ” relation-
ships between depositional or diagenetic attributes that could serve as proxies for
porosity and permeability. Instead, a different strategy was developed whereby
stratigraphic “ slices ” 10 feet thick were mapped from top to bottom of the reservoir.
The average porosity and average permeability of each 10 - foot interval were con-
toured on translucent paper. Pairs of porosity and permeability maps for each
interval were then overlain to identify which 10 - foot slices had high porosity coin-
ciding with high permeability. In essence, this is a primitive method of 3D reservoir
visualization that could be accomplished with sophisticated, albeit expensive, com-
puter software. More about the method is explained later.
Reservoir Characteristics Petrographic and petrophysical data on (1) depositional
texture, (2) pore type and geometry, (3) porosity from core analyses, and (4) perme-
ability from core analyses were studied to define and rank flow units. Spatial distri-
bution of reservoir fl ow units and their relationship to depositional facies were not
obvious from initial study; therefore an alternative strategy was developed to defi ne
flow units and test for facies selectivity. The method involved creating “ slice maps ”
of reservoir parameters and depositional facies. The method consisted of calculating
average values of porosity and permeability from core analyses for each of the 10 -
foot stratigraphic slices in the Happy Field carbonate section and then contouring
them. Facies types for each 10 - foot slice were also mapped. The datum on which the
slices were “ hung ” is a shale marker that is the most reliable and identifi able strati-
graphic signature on borehole logs across the field. Examples of porosity and cor-
responding permeability slices for the intervals 20 - to - 30 and 60 - to - 70 feet below the
shale marker are shown in Figure 8.14 a – d. The averaged values for each parameter
were mapped in successive slices until the base of the carbonate section was reached.
A reservoir quality ranking scheme was then developed to identify fi eld segments
with the greatest, intermediate, and lowest potentials for flow communication and
recovery efficiency. Porosity and permeability values were divided into three quality
ranges defined simply as high, medium, and low. For example, high porosity values
at Happy Field were defined as the highest one - third of the range in porosity values.
The range in porosity at Happy Field is 3 – 30%; therefore the highest one - third of
that range is 21 – 30%. A similar procedure was followed for permeability. The result-
ing data were put in a 3 × 3 matrix with three porosity and three permeability, or
nine “ poroperm, ” reservoir quality rankings to be identified and mapped individu-
ally across the field. Each quality zone was outlined by examining porosity maps