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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: New Resource from Old Fields:
Revitalizing Recovery in Shelf-bound Pliocene and
Miocene-age Reservoirs, Gulf of Mexico
By
School of Geosciences
Bureau of Economic Geology
University of Texas at Austin
Austin, Texas 78713-8924
[email protected]
Gas resource ultimate recovery projections have varied
greatly over the past 30 years. Recent estimates by the Gas
Research Institute put recoverable domestic resources as high as
422 TCF, nearly 50% higher than estimates made by state and
federal government organizations. High uncertainty in gas
recoverable projections is in part due to the uncertainty of
recovering resources in highly complex fluvial/deltaic/deep
marine clastic reservoirs of the Gulf Coast. Heterogeneity exists
on all levels. Varying complexity of depositional systems and
varying drive
mechanisms
are the two primary factors that cause
differences in the ultimate recovery from reservoirs.
The Secondary Gas Recovery (SGR) research program, carried
out by the University of Texas at Austin, Bureau of Economic
Geology and funded by the U.S. Department of Energy (DOE),
was begun in 1988 in response to growing realization of the
amount of gas resources being left unrecovered in U.S.
reservoirs. It is the goal of this long-standing research initiative
to seek to better resolve the stratigraphic and structural
complexities and present methods to reduce uncertainty and
improve gas production. The challenge is to identify a process
design and enhanced technology for reducing uncertainty in
between-well scale reservoir
architecture characterization, to
identify previously unrecognized stratigraphic and structural
play types and to improve economic scenarios for field development.
Outcomes must be user friendly, inexpensive to
implement and non-manpower intensive. New play concepts
must be of large enough scope to
drive
revitalization of existing
fields. Prior to 1998, projects in SGR had been confined to
onshore studies; however the most recent project marks the first
in offshore federal waters.
Miocene strata account for approximately 40% of all hydrocarbons
produced and 40% of all remaining proven reserves in the
Gulf of Mexico. These units are mostly restricted to mature
fields on the present continental shelf (< 200 m water depth).
Two fields, Starfak and Tiger Shoal, located in the Central
Planning Area of the northern Gulf of Mexico Shelf, Vermilion
and South Marsh Island Blocks, are the current study area for the
DOE Offshore SGR research initiative (Fig. 1). Integration of
sequence stratigraphy, conventional interpretative and attribute
extraction geophysical methods, well-log analysis and seismic-to-
petrophysics transform and three-dimensional reservoir
flow
simulation modeling have been used to identify bypassed
resources and new nontraditional targets across the area. At
certainty, estimates now suggest the possibility of at least 300 Bcf
of additional resources available for exploitation within the study
area.
Starfak and Tiger Shoal fields are located in offshore Louisiana immediately north of the Salt Deformation Province. Although the area's large-scale structural folds are a product of deep salt movement, geologic conditions here are structurally simple as compared with the complex diapiric deformation that occurs to the south. The fields are associated with several subregional normal faults and associated antithetic faults that are responsible for additional structural partitioning within these two large fields (Fig. 2, pg. 17). Depositional environments across the study area range from subaerial incised valleys to deltaic to deep marine basin floor fans (Fig. 3, pg.17). The section is progradational, punctuated by higher-frequency flooding and transgressive events. Sediments prograded from north-to-south.
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Several stratigraphic trap types, as well as secondary structural
traps exist unexploited across the study area. Resources show systematic preferential distribution within seven play types. New
seismic methods and attributes have been developed to improve
seismic interpretation and inversion of data to petrophysical
parameters for population of three-dimensional reservoir
models.
Seismic geomorphology and sedimentology provide a
detailed look at the architecture of these complex
reservoir
/seal
systems (Fig. 4, pg. 19). Several new targets have been tested by
industry partners and proven successful. These results are being
extrapolated into a more regional area utilizing previous play
classification work by BEG and Mineral Management Survey
researchers and will provide the roadmap by which resource
growth and new opportunity can be exploited throughout the
Gulf of Mexico.
Figure 1. Map of the Vermilion and South Marsh Islands (SMI) areas showing the study's primary target fields, Starfak and Tiger Shoal, as well as surrounding fields and the outline of the two major 3-D seismic surveys being used in the OSGR project. Pre-project resource estimates are shown for both fields. Well and production data comes from the Tiger Shoal and Starfak fields. Select additional well data were available from surrounding fields. All data were provided by industry partner, Texaco.
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Figure 2. Coherency time slice generated at 2,000 ms revealing the areal distribution of faulting throughout the study area. First order faults set up the two major study fields, Strafak and Tiger Shoal (after Deangelo and Wood, 2001).
Figure 3. Amplitude stratal slice just above sequence boundary 18 illustrating the lowstand deltaic wedge and incised0valley fill of third-order sequence seven within the general area of Starfak (SF) and Tiger Shoal (TS) fields. Valley systems fed a lowstand deltaic wedge approximately 3.5 miles southeast of the fields.
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Figure 4. Seismic sedimentology and geomorphology can add a great deal of understanding to the architecture and potential for stratigraphic traps within an area. Here stratal amplitude slices reveal an upper Miocene valley incising in and around the two study fields. Several incised distributary channels can also be seen. Texaco successfully tested the Target 3 resource-addition opportunity in 2000, and found BCF of gas.
Figure 5. Regional-scale 3D model generated using Roxar modeling software shows 7 maximum flooding surfaces (about horizontal) and 73 first- and second-order normal faults (high angle surfaces) that mate a geologically complex fluid migration system typical in the fields of offshore Louisiana in the northern Gulf of Mexico.
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