Facies Detection through the Use of Calibrated Classification of Seismic
Attributes - Siderite Detection at Negage Field, Block 14, Angola
Jay Byers
Chevron, Houston, TX
Classification of single or multiple seismic
attributes is a non-linear organizational process that can enable the delineation of natural groups of
data
(“classes”) with similar properties or behaviors; these groups might otherwise be undetectable when using more conventional
seismic
interpretation
methods. Calibration of the resulting
seismic
-based “classes” to well-based information can then be used to associate various “classes” with rock- or fluid-related facies.
Calibrated classification can often distinguish the presence of very subtle rock- and fluid-related facies and their respective geometries can be spatially delineated. Additionally, the classification process can result in an increase in vertical resolution; vertical resolution can be increased by a factor of two as resolution is equal to the seismic
sample rate (e.g. 2-4 msec) as opposed to the more conventional notion of minimum resolution equal to ¼ of the
seismic
wavelet wavelength (e.g. 7-12 msec) at reservoir-depth frequencies.
During classification procedures conducted at Negage field (Block 14, Angola), a very interesting facies class was clearly delineated as an anomaly. Subsequent calibration processes led to the conclusion that siderite “tight streaks” were responsible for the anomaly and that siderite and its associated seismic
character was therefore detectable within the reservoir space; prior to this procedure, siderite was undetectable in the
seismic
data
.
Siderite is characterized as a non-reservoir quality rock that is often associated with the base of hydrocarbon-bearing reservoir sands. Normally, siderite is “buried” within the conventional seismic
signature of the base of reservoir and is not clearly delineated. This condition can cause overestimation of NRV and resulting OOIP estimates; these overestimates can be significant when the siderite layer is of considerable thickness relative to the gross reservoir interval thickness.
AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece