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GC
Seismic
Attribute
Analysis of Hydrothermal Dolomite*
By
Uwe Strecker1, Matthew Carr1, Steve Knapp2, Maggie Smith1, Richard Uden1, and Gareth Taylor1
Search and Discovery Article #40180 (2005)
Posted December 16, 2005
*Adapted from the
Geophysical Corner column, prepared by the authors and entitled “Matching
to Model Can Cut Risk,” in AAPG Explorer, July, 2005. Appreciation is expressed to
Alistair Brown, editor of Geophysical Corner, and to Larry Nation, AAPG
Communications Director, for their support of this online version.
Seismic
data courtesy of Seitel Inc; also contributing--Terry Zwicker, Samson
Canada; and Denise Poley, El Paso/British Gas.
1Rock Solid Images (RSI), Houston ([email protected])
2Steve Knapp is senior geophysicist, Oxy Petroleum ([email protected])
General Comments
To
optimize subsurface geophysical interpretations, it is beneficial to place
seismic
attributes into the proper regional geological context; knowledge of
regional geology may assist exploration/exploitation efforts in advance by
high-grading
attribute
selection and
attribute
intersection for purposes of risk
analysis.
The field-tested
exploration strategy presented here seeks to encapsulate all pertinent play
characteristics into a viable geological model, where each dominant reservoir
property is expressed as a risk parameter that in turn can be resolved by a
seismic
attribute
.
The play is Givetian (Devonian) biohermal build-ups and lagoonal deposits, which comprise prospective section within the Western Canadian Sedimentary Basin. This geologic basin hosts well known gas fields such as Ladyfern (> 1Tcf) (see Figure 1).
uGeneral commentsuFigure captionsuStructure & diagenesis
u
|
What structural and diagenetic changes caused this Canadian Paleozoic carbonate platform to become a world-class hydrothermal dolomite play? The above model of hydrothermal dolomitization contends that Mg2+-rich brines ascend along wrench-faults forcing a chemical phase transition from calcite to dolomite along favorable carbonate rock fabric/textures. Porosity development deteriorates away from faults, implying that -- in contrast to conventional wisdom -- highest reservoir quality rock may not always be encountered on structural crests, but instead on anticlinal flanks with a high wet risk (Table 1).
Using
geological knowledge about the formation,
Because of
high compressional carbonate rock velocity, well-log impedances (AI)
almost entirely respond to total porosity (PHIT) change instead of fluid
type. Unfortunately, overlapping lithology fields in a petrophysical AI/PHIT
crossplot suggest that no differentiation of shales from porous
dolomites should be possible in the The spatial distribution of relative acoustic impedance using geological knowledge about the formation, however, is employed to illuminate specific reservoir properties (Figure 2). In accord with the geological model, dolomitization (lowered relative acoustic impedance) occurs preferably at the base of the formation (phase reversal in relative AI). In contrast, high relative impedance values are associated with tight limestone (no polarity reversal). Intraformational shales, too, are associated with lowered values of relative acoustic impedance, but occur within the formation rather than at its base (Figure 3). In accord with the geological model, this lowered relative acoustic impedance spot centered within the Slave Point Formation is incompatible with the geologic model, as hydrothermal dolomitization should occur at the base of the formation first. The well drilled on this anomaly encountered a shale plug.
Conclusion
Matching
Boreen, T., and G. Davies, 2004, Hydrothermal dolomite and leached limestones in a TCF gas play: the Ladyfern Slave Point reservoir, NEBC, in Dolomites—The spectrum: Mechanisms, models, reservoir development: CSPG Seminar and Core Conference, June 13-15, 2004, Calgary, Alberta, 17 p. |