Page 242 - Geology of Carbonate Reservoirs

DEPOSITIONAL RESERVOIRS 223

SC5 SM1

8000
SC7

8000 gamma sonic
sonic
gamma
CAS1
N 8000
8000
MICROFACIES
Crinoid-Bryozoan gamma sonic
Grainstone-Packstone
RESERVOIR
Crinoid-Bryozoan
Graistone-Packstone 100 FEET
Crinoidal
Wackestone-Packstone
Crinoidal, Spiculiferous, Silty 0 NO HORIZONTAL SCALE gamma sonic
Wackstone-mudstone

Figure 8.9 A fence diagram showing facies correlation across Conley Field. Reservoir poros-

ity is limited to the crinoid – bryozoan grainstone and packstone facies shown as a darker
shade in the panels. Although grainstone facies occur across much of the antecedent high,
production is from zones with the greatest abundance of bryozoan fragments in proportion
to crinoidal fragments.
were those with highest ratios of fenestrate bryozoan fragments to crinoid frag-
ments. Rocks with high percentages of crinoid fragments were extensively cemented
by syntaxial overgrowths that nucleated on crinoids and subsequently bridged pores
to plug pores and pore throats. Rocks composed mainly of fenestrate bryozoan
fragments retained porosity as intraskeletal pores. The anatomy of bryozoan - rich
parts of the sand wave complex was determined by creating a fence diagram of the
Chappel section in the field (Figure 8.9 ). This diagram outlines the productive and

non productive zones in the field.

The Geological Concept Conley Field was found because it is easily identifi ed by
seismic surveys as a present structural anomaly. In the Hardeman Basin of North
Texas, many structural anomalies are present in the Chappel Limestone interval,
but only a few of them have reservoir porosity. In many cases the reservoirs produce
from fractures and to a lesser extent from matrix porosity. At Conley Field, however,

reservoir porosity is confined to bryozoan – crinoid grainstones that accumulated
much like the oolites and rhodolites at North Haynesville Field in the previous
example. The porosity is intra - and interparticle porosity in bioclastic grainstones
and packstones rather than nonskeletal grains that depend largely on in situ chemi-
cal precipitation for their origin. Interestingly, the Conley reservoir proves that
skeletal grainstone buildups can occur on antecedent highs just as nonskeletal,
coated grain buildups do. Finding structural anomalies is relatively easy. Finding
anomalies with porous and permeable reservoir rocks is not. The main element in
the geological concept that explains the Conley reservoir is that it became a topo-
graphic positive, or buildup, during Mississippian time. The thickened Chappel
Limestone in the field (indicated on geological cross sections and on the interval