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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: New Interpretations of Reservoir Architecture of the
Upper Cretaceous Woodbine Group in East Texas Field:
Sequence
Stratigraphic and Depositional Perspectives
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By
Bureau of Economic Geology
The University of Texas at Austin, John A. and
Katherine G. Jackson School of Geosciences
The East Texas Field, located on the west margin of the Sabine Uplift, is the largest oil field in the lower 48 states. Oil reservoirs occur in west-dipping Woodbine Group conglomerates and sandstones that are truncated by a subregional unconformity below the Austin Chalk, which forms the seal throughout the field. From its discovery in 1930 through mid-2007, the field had produced 5.42 billion stock tank barrels (STB) of oil. The calculated ultimate recovery of approximately 5.49 billion STB and the advanced degree of water encroachment indicate that the field is in the waning stages of production. Given these figures, about 70 million STB are still likely producible under current production practices. However, recent closer evaluation of the amount of by passed pay, deeper Woodbine pay, and poorly swept oil, all indicate that the field has remaining reserves of as much as 550 million STB. Because of this large estimated remaining-reserves volume, the numerous wells that exist in the field for potential recompletion and/or deepening (more than 31,200) , and the currently favorable price of oil, producers are now aggressively targeting recompletions—especially in deeper pay zones in the Woodbine section.
Although the East Texas Field has been producing
for more than 75 years, no modern
comprehensive geologic study of the field has
been conducted. The present study is the first
to integrate core data from the field and adjacent
areas with well
log
analyses. Our main
objectives have been to understand the role of
sequence
stratigraphy in the depositional origin
of the producing intervals, to document
facies distribution, and to describe facies distribution
controls on the potential for
additional production. We have achieved these
objectives by:
- applying
sequence
-stratigraphic
analysis
to the Woodbine Group in the East Texas Basin where the succession is complete and extending the
analysis
to the truncated Woodbine section on the adjacent Sabine Uplift to identify principal chronostratigraphic boundaries in the field;
End_Page 11---------------
- using these boundaries to map coeval sandstone units, identifying reservoir-facies trends in selected pilot study areas, and interpreting depositional facies origin and distribution and Woodbine paleogeography; and
- inferring
sequence
stratigraphic and facies controls on incompletely swept reservoir zones, potential by-passed pay, and deeper pay zones by integrating our findings from the large core and
log
data set with engineering data.
The Woodbine Group represents the dominant
episode of coarse-siliciclastic deposition during the
Late Cretaceous in the East Texas Basin and
comprises mostly on-shelf fluvial-deltaic deposits.
The succession thins gradually from the axis of the
basin westward to the Mexia-Talco Fault Zone and
eastward to the Sabine Uplift. Regional sequencestratigraphic
analysis
of approximately 225
well
logs distributed across the central part of the basin
indicates that a maximum of 14 fourth-order
sequences compose the Woodbine succession, and
the number of sequences systematically decreases
in both directions away from the basin center. The oldest five
sequences, extending to the west flank of the Sabine Uplift,
are truncated by the base-of-Austin unconformity, whereas
deposition of the upper Woodbine sequences was limited to a
zone approximately 35 miles wide centered on the basin axis.
Reservoirs in the field occur in the basal three fourth-order
sequences (S1–S3).
Analysis
of more than 1,500 feet of 30 whole cores and wireline
logs from approximately 500 wells in the north pilot area (NPA)
and south pilot area (SPA) of the field indicates that the sandstone
body architecture is more complex than that inferred by previous
workers. Moreover, the depositional settings of reservoir facies
vary considerably from those described in earlier investigations,
which inferred stacked meanderbelt-sandstones in the north part
of the field grading to sandstones of equivalent wave-dominated
deltaic and coastal-barrier systems in the southern part. Our
analysis
indicates that throughout the NPA, an S3 conglomeratic
low-stand incised-valley fill overlies a sandstone-dominated
S1 high-stand systems tract, the primary target for recent
well
deepenings. The entire S2 succession has been removed by valley
incision in this area. The Woodbine section in the SPA occurs just
east of the approximate depositional limit of the S3 incisedvalley-
fill system and comprises most or all of the S1 highstand
deltaic succession. Sandstone body heterogeneity in the
high-stand section is controlled by the fluvial-dominated deltaic
depositional architecture, with dip-elongate distributary-channel
sandstones pinching out over short distances (typically less than
500 feet) into delta-plain and interdistributary bay siltstones and
mudstones.
Two main development strategies — well
deepening and optimized
waterfloods — are options for increasing recovery efficiency
in the East Texas Field. A full understanding of reservoir
compartmentalization, fluid flow, and unswept mobile oil in the
field should consider the fluvial-dominated deltaic and lowstand
valley-fill sandstone-body architecture. For example, production
of oil by deepening of existing wells is primarily from thin
sandstones in the S1 highstand deltaic succession inferred to
contain limited untapped reservoir compartments owing to
abrupt lateral and vertical changes in facies and thickness of
sandstone bodies. Permeability and porosity data, in conjunction
with net-sandstone maps, indicate that primary reservoir facies
in the S1 highstand deltaic succession in the SPA occur in
thick (greater than 25-foot) distributary channel and channel
mouth-bar sandstones. Waterfloods can be better designed to
take advantage of the discontinuous reservoir sandstone geometry.
Production costs can be reduced by shutting off water-injection
wells in shaly areas where there is no appreciable pressure
support.
East Texas Field and Interpreted Incised Valley.
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