WIRELINE LOG-BASED IDENTIFICATION OF FACIES AND THE MID-SAN ANDRES UNCONFORMITYWIRELINE LOG-BASED IDENTIFICATION OF FACIES AND THE MID-SAN ANDRES UNCONFORMITY

17. Paired core photograph and FMI log image: upper San Andres tidal-flat rocks. Tidal-flat facies arecharacterized on FMI by thin laminated character. Note tepee structure.

18. Paired core photograph and FMI log image: lower San Andres fusulinid facies. Fusulinid facies arecharacterized by vermiform FMI pattern.

20. Paired core and FMI log image: upper San Andres. FMI has very high resolution that can be used to discriminate even the finestscale cycles and facies successions. Note: Core depths are about 5 ft higher than log.

Although cores provide the best data for characterization of facies, stratigraphy, and identification of the mid-SanAndres unconformity, two wireline logs can supplement core data: borehole imaging logs and spectral gamma ray logs.Although cores provide the best data for characterization of facies, stratigraphy, and identification of the mid-SanAndres unconformity, two wireline logs can supplement core data: borehole imaging logs and spectral gamma ray logs.

QAc1890c QAc1886c

QAc1892c QAc1893c

Tepeestructure

IMPLICATIONS FOR TARGETING ADDITIONAL RECOVERY FROM THE SAN ANDRES FORMATIONIMPLICATIONS FOR TARGETING ADDITIONAL RECOVERY FROM THE SAN ANDRES FORMATION

Regional outcrop and subsurface studies show that a major unconformity, caused by a major sea-level fall with accompanying nondeposition and possible erosion, separates upper and lower SanAndres platform carbonate successions. Studies of the San Andres in the Fuhrman-Mascho areaindicate that this unconformity is the locus of significant porosity and permeability development.The continuity of this unconformity porosity suggests that it should be a major target in reevaluatingexisting San Andres reservoir completions and in subregional exploration.

CONCLUSIONSCONCLUSIONS

This study was funded by the University of Texas System as part of the University Lands AdvancedRecovery Initiative Program. Thanks go to Steve Hartman and Wallie Gravitt of the University WestTexas Operations Office, Midland, Texas, for their support. I am grateful to Stan Decker of ArrowOperating Company for freely and promptly providing reservoir data for the study. Steve Hill, alsowith Arrow, readily shared data, interpretations, and insights into reservoir geology and operations.Jerry Lucia contributed to the petrophysical analysis of the reservoir. Peter Swart performed stableisotope analyses.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

QAc1895c

BureauofEconomic

Geology

N–D crossplot porosity(%)

SGR

CGR

Arrow Fuhrman-Mascho Unit #307Andrews County, Texas

4150

4200

4250

4300

4500

4550

4600

4350

4400

0 125 30 20 10 0 -10

Tidal-flat carbonate

Siltstone–sandstone

Mixed shallow subtidal carbonate

Fusulinid wackestone–packstone

Cycle

19. With proper core calibration, borehole imaging logs can providehigh resolution information on facies type, rock fabrics, andfracture distribution. Vertical facies successions and cycleboundaries are easily definable for this well using the FMI,providing a basis for stratigraphic correlation and rock-fabricdetermination.

Depth(ft)

GR(API)

Arrow Fuhrman-Mascho Unit #307Andrews County, Texas

FMI-BASED CORE DESCRIPTION

4250

FMI-BASED CORE DESCRIPTION

4400

QAc1889c

up

per

San

An

dre

s F

m.

low

er S

an A

nd

res

Fm

.G

rayb

urg

Fm

.

44504450

QAc1894cQAc1948c

0 1 in

Cycleboundary

0 1 in

0 1 in

Flow Regime 1. Transgressive, high continuity siltstones; highly productive and floodable

Flow Regime 2. Inner ramp. low permeability carbonates; flow baffle; poor floodability

Flow Regime 3. Outer ramp, high continuity, permeable carbonates; floodable

Flow Regime 4. Middle ramp, permeable carbonate grainstones; highly productive and floodable

South

PREDICTIVE MODEL OF HYDROCARBON DISTRIBUTION AND FLOW REGIMESan Andres-Grayburg Reservoirs, Andrews County Area

QA9980c

-1300

-1200

-1100

-1000 -300

-400

-350

Depthft m Fuhrman-Mascho Field

North

Emma Field

PREDICTIVE MODEL OF HYDROCARBON DISTRIBUTION AND FLOW REGIMESan Andres-Grayburg Reservoirs, Andrews County Area

Flow Regime 1

Flow regime 2

Flow regime 3

Flow regime 4Permeable

porosity zones

Grayburg

Upper San Andres

Lower San Andres

Emma oil/water: -1165'

FMU oil/water: -1340'

Subtidal middle ramp

Tidal flat

Inner ramp mudstone

Permeable siltstone Outer ramp wackestone

Recrystallized dolostone

The potential of reservoirs developed at the lower San Andres unconformity may not have been realized throughout its extent.In Fuhrman-Mascho field, the lower San Andres is a major contributor to production as it is in the Means field to the north.Elsewhere it may have not been fully evaluated. In the nearby Emma field, for example, the reported oil/water contact isapproximately 100 ft higher than it is in Fuhrman-Mascho. This may be a result of capillary effects associated only with therecrystallization below the mid-San Andres unconformity. The oil-water in Emma is placed in fusulinid wackestones of the upperSan Andres that display low permeabilities and apparent high water saturations despite their moderate porosity. It is possiblethat the better rock fabrics in the lower San Andres may contain higher oil saturations than those in the upper San Andresdespite their being some 100 ft below the apparent oil/water contact.

21. Although the upper San Andres is productive in many reservoirs in the Permian Basin, in inner platform settings like that of theFuhrman-Mascho field area, the succession is dominated by low permeability, tidal-flat facies, and locally porous, cycle basesubtidal facies. Although basal Grayburg siltstone/sandstone cycles are locally productive in these areas, the lower San Andres,because it consists of more stratiform and continuous, unconformity-related porosity, should be considered a primary target.