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