7. Grayburg porosity is largely confined to inner platform, cyclebase siltstones and sandstones. Because of their intercrystallinepore space, these rocks display a good porosity/permeabilityrelationship and, locally, very high permeabilities.
8. Upper San Andres (USA) porosity is most common in tidal-flatfacies. But because the pore space in these rocks is mostlycomposed of separate vugs, they generally have extremely lowpermeability and high water saturation.
9. Lower San Andres (LSA) porosity comprises moldic andintercrystalline pores and is restricted to the uppermost 70 ft ofthe sequence (HFS 4). Generally there is a good relationshipbetween porosity and permeability.
12. Altered gypsum-filling interparticle pore space in ooid/peloid grain-dominated packstone and fusulinid wackestone of the lower SanAndres Formation at Fuhrman-Mascho field. Note that the blue color is the result of blue-dyed epoxy injected into intracrystalline poreswithin the altered gypsum. The abundant gypsum in the San Andres at Fuhrman-Mascho means special precautions must be taken foraccurate petrophysical measurements and volumetric calculations.
An important complication to the interpretation of all San Andres and Grayburg carbonate reservoirsuccessions in the Permian basin is the presence of sulfate. Sulfate is ubiquitous and locally takes theform of gypsum as well as anhydrite.
GRAYBURG PETROPHYSICSSILTSTONE-SANDSTONE FACIES
Fuhrman-Mascho Field
16. The development of burrowed zones, recrystallized dolomite, and higher porosity and permeability is a direct function of proximity tothe unconformity surface. This suggests that all of these features owe their origin to diagenetic fluids entering the lower San Andresalong the unconformity surface.
15. Both porosity and permeability are markedly higher in recrystallized zones than in surrounding unrecrystallized matrix.Low permeability is normally characteristic of fusulinid-bearing facies because of the dominance of separate vug (fossil-moldic) pores. The recrystallization of these rocks along burrows has altered the rock fabric to produce significantintercrystalline porosity and, accordingly, higher permeabilities. However, although permeability can be quite high inthese zones their tubular nature and probable poor lateral interconnections suggest that effective permeability may bemuch lower.
13. Fusulinid facies in the upper part of the Lower San Andres contain abundant vertical, tubular areas of recrystallized dolomite. In many cases, there is strong evidencethat vertical tubules are a result of burrowing . Recrystallized zones diminish in abundance downsection although evidence of burrowing does not. Commonly theseburrowed vertical zones contain abundant sulfate nodules.
Microscopically, these fabrics commonly display enlarged crystals and, in many cases, leached dolomite rhomb cores.
14. Comparison of isotopes from recrystallized and adjacent unrecrystallized areas in the lower San Andres shows that recrystallized dolomite is markedly depleted in δ18O (2 to 3 ‰PDB). Theabsence of similar depletions in the upper San Andres suggests that fluids predated upper San Andres diagenesis or entered the top of the lower San Andres along the unconformity. Similarpatterns of isotopic depletion have been documented in several other San Andres and Grayburg successions, such as this example from the South Cowden Grayburg field.
11. The analysis-induced alteration of gypsum isapparent from these adjacent core pieces, onewhich was cleaned and analyzed and the otherswhich were not. The white rinds around theanhydrite nodules are dehydrated gypsum.
GRAYBURG PETROPHYSICSSILTSTONE-SANDSTONE FACIES
Fuhrman-Mascho Field
0 5 10 15 20
Porosity (%)
R=0.72
0 5 10 15 20
Porosity (%)
0.001
0.01
0.1
1
10
100
R=0.98
UPPER SAN ANDRES PETROPHYSICSTIDAL-FLAT FACIES
Fuhrman-Mascho Field
0 5 10 15 20
Porosity (%)
0.001
0.01
0.1
1
10
100
LOWER SAN ANDRES PETROPHYSICSRECRYSTALLIZED FUSULINID FACIES
Fuhrman-Mascho Field
Per
mea
bil
ity
(md
)P
erm
eab
ilit
y (m
d)
Per
mea
bil
ity
(md
)
STABLE ISOTOPE TRENDS, GRAYBURG FORMATION South Cowden Field
0
1
2
3
4
5
6
7
0 1 2 3 4 5
Recrystallized
Unrecrystallized
QAc1881c QAc1882c QAc1883c
QAc1885c
QAc1878c
QAc1879c
QAc1880c
QAc1894c QAc1897c
LOWER SAN ANDRES PETROPHYSICSRECRYSTALLIZED FUSULINID FACIES
Fuhrman-Mascho Field
UPPER SAN ANDRES PETROPHYSICSTIDAL-FLAT FACIES
Fuhrman-Mascho Field
QAc ???c
STABLE ISOTOPE TRENDS, SAN ANDRES FORMATIONFuhrman-Mascho Field
STABLE ISOTOPE TRENDS, SAN ANDRES FORMATIONFuhrman-Mascho Field
STABLE ISOTOPE TRENDS, GRAYBURG FORMATION South Cowden Field
QAc1896c
0.001
0.01
0.1
1
10
100
CAVEATS TO RESERVOIR EXPLORATIONCAVEATS TO RESERVOIR EXPLORATIONPOROSITY AND PERMEABILITYPOROSITY AND PERMEABILITY
CHEMISTRY
FABRICFABRIC
CHEMISTRY
0 1 in
0 1 in0 1 in 0 1 mm
0 1 mm 0 1 mm
QAc1884c
Recrystallized δ18O
Subtidal peloidal facies
Tidal-flat facies
Subtidal fusulinid facies
Unrecrystallized δ13O
Unrecrystallized δ18C
Recrystallized δ13C
Cycle boundary
0 1 2 3 4 5 6 7
δ13C, δ18O (‰PDB)
10. Standard core analysis techniques resultin dehydration of gypsum and produceerroneous porosity and permeabilityreading. Comparison of petrophysicalmeasurements made from low temperaturetreatment of cores versus the conventionalhigh temperature techniques shows themagnitude or analysis errors.
SGR
CGR
Porosity(%)
Perm(K)
20 10 0 100
0.01
GR Depth(ft)
3500
3600
3700
3800
3900
QAb8780c
EVIDENCE OF UNCONFORMITY-RELATED POROSITY DEVELOPMENT IN THE MID-SAN ANDRESEVIDENCE OF UNCONFORMITY-RELATED POROSITY DEVELOPMENT IN THE MID-SAN ANDRES
Porosity increase dueto core analysis
δ18O (‰PDB)
δ13C
(‰
PD
B)
Dep
th (
ft)
BureauofEconomic
Geology
up
per
San
An
dre
slo
wer
San
An
dre
s
4400
4500
4600
4700
0
4550
1200 ft.1200 ft.1000 ft. 1300 ft. 2500 ft.
Gra
ybu
rg F
orm
atio
nL
ower
San
An
dre
s F
orm
atio
nG
uad
alu
pia
n H
FS
1-4
Up
per
San
An
dre
s F
orm
atio
nG
uad
alu
pia
n H
FS
12-
13
Recrystallizeddolostone
Peritidal tidal flat
Siltstone–sandstone
Inner ramp–tidal flat mud-dominated facies
Middle ramp peloidal–skeletal wackestone–packstones
Outer ramp fusulinidwackestone–packstone
QAc1887c
Depth
100
0 0ft m
30
4600
4700
4700
4700
4700
4650
4150
4200
4200
4200
4200
EastWest
GR NGR NGR N GR N GR N
GR NWFMU 121WFMU 139 WFMU 132WFMU 39 WFMU 147
WFMU 104
4150
Average porosity
Po
rosi
ty (
%)
10.0
21.2
Unaltered Altered
25
0
5
10
15
20
QAb3942c
Average permeability
Per
mea
bil
ity
(md
)
22.55
211.37
Unaltered Altered
1000
1
10
100
QAb3942c
GEOMETRYGEOMETRY
RESERVOIR PLUMBINGFuhrman-Mascho Field
RESERVOIR PLUMBINGFuhrman-Mascho Field
PETROPHYSICSPETROPHYSICS
POROSITY AND PERMEABILITY OF ALTERED DOLOMITE ZONESSouth Cowden Grayburg Reservoir
POROSITY AND PERMEABILITY OF ALTERED DOLOMITE ZONESSouth Cowden Grayburg Reservoir
PETROPHYSICSPETROPHYSICS
