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Posted 10/25/01
SEG/San Antonio 2001 Expanded Abstracts
Direct Hydrocarbon Migration
Indicator, A Venting Feature in Equatorial Guinea
Joseph A. Canales, CMS Oil and Gas Company
Summary
Hydrocarbon venting and seepage features are associated with
producing fields in Equatorial Guinea, including Zafiro and
Alba Fields. The most recent discovery in Equatorial Guinea
a CMS Oil and Gas, the Estrella #1, offsets a gas chimney
and seafloor venting feature. The sea-floor expression of
this vent feature is a crater 400 meters wide and 17 meters
deep. A mound 6.5 meters high is present on the north-east
side of the crater, and a broader low relief mound of ejected
material 1.5 meters high extends over 1.5 km south-west out
of site survey area. Pinger records show hydrocarbons within
the water column rising from several streams within the crater.
Introduction
It has been stated that in general, hydrocarbon seepage can
only effect the risk of encountering hydrocarbons in a basin
wide scale (Thrasher, Flee et al., 1996a). However, the active
tectonism and high sedimentation rates present offshore Equatorial
Guinea in the Rio Del Rey basin have resulted in seeps vertically
near the focused accumulation of hydrocarbons at depth.
At CMS Oil and Gas, evidence of hydrocarbon venting or seepage
over a deeper structural trap in Equatorial Guinea is considered
a risk-reducing attribute for exploration plays. An exploration
3D survey showed a gas chimney associated with a structural
culmination a depth of the Middle Miocene Isongo Formation.
The culmination is associated with a toe-thrusted anticline.
Offset well ties indicated the presence of good reservoir
rock within the targeted interval, and nearby production at
Alba indicated hydrocarbon generation in the basin.
A piston core surface geochemical program that concluded
in February 2000 by TDI-Brooks International, Inc confirmed
active hydrocarbon migration to the seafloor. Two of three
cores targeting the seismic gas chimney recovered high levels
of extractable hydrocarbons from piston cored samples.
Details of the venting crater were illustrated by the data
gathered from a site survey over the Estrella Isongo culmination.
Method and Theory
Identification of venting hydrocarbons over the Middle Miocene
Isongo formation structure is confirmed using a variety of
data with a wide range of resolution.
A suspected gas chimney was first identified on an exploration
3D survey. This data was acquired in 1995 by PGS using five
3000m cables. 60-fold coverage was recovered over the bulk
of the survey, except for an area around an installed platform
at Alba field. Shorter 1500m cables were used to get nearer
the platform, resulting in a drop of coverage to 30 fold.
The data was reprocessed in 1999 by JDK employing a pre-stack
time migration algorithm. An incremental improvement was achieved
which has aided in interpreting the highly deformed toe-thrust
belt, where the Estrella seep has been identified.
Structural interpretation of the 3D volume indicates the
gas chimney is located in a Post-Pliocene syncline over a
toe-thrusted, middle Miocene anticline. The "hole"
on the "hill" apparently results from reactivation
of the thrust faults with a normal fault, back slide movement.
This counter-regional movement may be attributed to changing
loads as the Pliocene section progrades from behind the toe-thrust
belt, to loading over the thrust anticlines. As the overlying
section slides back, a zone of dilation occurs in the middle
of the syncline, which allows vertical migration up the shale
prone lower to middle Pliocene section. Horizontal migration
is evident in the shallower sand prone section, although much
of the hydrocarbons continue vertically up to the seafloor,
as is confirmed by piston coring.
On the basis of the 3D survey, an area-wide surface geochemical
exploration program was outlined. TDI-Brooks International,
Inc acquired 36 piston cores. Three of these piston cores
were acquired over the Estrella gas chimney. Two of the three
cores (cores #26 and 28) recovered
high amounts of extractable hydrocarbon and headspace gas,
which qualify them as containing migrated thermogenic oil
and gas (Proprietary TDI-Brooks Technical Report).
Geomark provided further geochemical work incorporated with
the TDI-Brooks study. Their findings showed core #26,which
fell within the crater, with low levels of biodegradation
suggesting a rather active seep of hydrocarbons from below.
Armed with the optimism of live oil at the seafloor, two
exploration locations were finalized and a site survey was
acquired over the area. The survey utilized an echo sounder,
sidescan sonar, hull-mounted pinger, coring and high-resolution
seismic data.
The bathymetry map shows a crater, as described above, associated
with the venting of material including the hydrocarbons. The
piston core with the highest readings of extractable hydrocarbons,
core #26, fell in the middle of the crater. The two other
gas chimney targeted cores fell on the flanks of the crater.
One had only background levels of extractable organic matter
(EOM), while the other contained elevated levels of EOM.
The pinger data showed a broad area of gas charged sediments
below the crater, and hydrocarbons in the water column above
the crater rising in several streams, Figure
2.
The high-resolution seismic data shows a history of gaseous
eruptions recorded in the about 300 milliseconds of section
below the seafloor. Data further below is masked by poor returns
of the seismic signal.
Conclusions
The geology associated with his venting feature suggests
vertical migration of hydrocarbon. The Estrella culmination
focuses hydrocarbons within the Isongo sands that are feeding
the gas chimney illuminated by the seismic data. Live oil
at the seafloor documented by the piston cores highlights
the active nature of the seep. The mud prone crater/mound
feature suggests a prolific and rapid venting.
Reference
Roberts, Harry H.,1998, Evidence of Episodic Delivery of
Fluids and Gases to the Seafloor and Impacts of Delivery Rate
on Surficial Geology,1998 AAPG Bulletin, Vol 82, Iss.13.
Thrasher, J., A.J. Flee, S.J. Hay, M. Hovland, and S. Düppenbecker,
1996a, Understanding geology as the key to using seepage in
exploration: spectrum of seepage styles, in D. Schumacher
and M.A. Abrams, eds., Hydrocarbon migration and it’s near-surface
expression: AAPG Memoir 66, p. 223–241.
Acknowledgments
The author would like to thank CMS Oil and Gas Company, Globex
Global Exploration, Inc., Samedan Oil Corporation and the
Ministry of Mining of Equatorial Guinea for permission to
publish this paper. I would like to thank John Van Horn, Jeff
Bryant and Chet Rivers for help and encouragement, and Bob
Olson for suggesting this topic.
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