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Recognizing pitfalls of seismic structure and amplitude anomalies in Osprey Field, Brunei

Fuping Zhu, Renato Borbajo and Sebastian Darker
Recognizing pitfalls of seismic structure and amplitude anomalies in Osprey Field, Brunei (in Pitfalls, Anonymous)
Interpretation (Tulsa) (August 2015) 3 (3): A15-A24

Abstract

The Osprey gas field was discovered in 1975 in the western offshore area of Brunei Darussalam. Two discovery wells penetrate the hanging wall of two separate growth faults and are associated with bright seismic amplitudes. Multiple thin reservoirs are separated by shales of 10 to a few hundred meters in thickness. In addition to seismic tuning of thin layers, there are many shallow gas clouds above these reservoirs, which cause significant and varying amount of push-down in seismic time data. Visualizing as seismic depth data reduces the amount of push-down but doesn't remove it totally because, to date, the velocity model has been neither detailed nor accurate enough to fully constrain the seismic processing. Time-to-depth conversion includes significant uncertainties in the reservoir time interpretation, with only one well control per fault block. The shallow gas clouds also cause severe amplitude distortion at reservoir levels in the time and depth data, resulting in difficulties for discriminating gas distribution from stratigraphic variations or tuning effects. Furthermore, the shallow gas clouds cause false or enhanced rollover structures and artificial fault features adjacent to expressions of "real" features in the seismic data. Thus, estimates of reservoir spatial distribution and hydrocarbon volumes have large uncertainties. Recognizing these structure and amplitude pitfalls and handling them properly are important to improve reservoir understanding and volumetric estimations and to optimize development of the field. We have uncovered a variety of pitfalls and developed some mitigation methods. One method uses the seismic flat spot to correct the gas push-down effect, and the check-shot velocity was adjusted accordingly. This led to reduced uncertainty in the reservoir depth prognosis of the planned wells. Four wells were drilled in 2014, and the actual depth in all wells came out within only a 4-m difference, validating the correction method for shallow gas push-downs.


ISSN: 2324-8858
EISSN: 2324-8866
Serial Title: Interpretation (Tulsa)
Serial Volume: 3
Serial Issue: 3
Title: Recognizing pitfalls of seismic structure and amplitude anomalies in Osprey Field, Brunei
Title: Pitfalls
Author(s): Zhu, FupingBorbajo, RenatoDarker, Sebastian
Author(s): Anonymous
Affiliation: Brunei Shell Petroleum Company, Seria, Brunei
Pages: A15-A24
Published: 201508
Text Language: English
Publisher: Society of Exploration Geophysicists, Tulsa, OK, United States
References: 11
Accession Number: 2016-007501
Categories: Economic geology, geology of energy sourcesApplied geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: sects., sketch maps
N04°00'00" - N05°10'00", E114°00'00" - E115°30'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by Society of Exploration Geophysicists, Tulsa, OK, United States
Update Code: 201604
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