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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Seal Character and Variability Within Deep-Marine
Depositional
Systems
: Seal Quantification and
Prediction
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By
ChevronTexaco Exploration
Technology Company
Bellaire, TX
Seals are a key element of petroleum systems
, yet they have
received limited systematic study. Textural and compositional
variations permit the recognition of six shale lithofacies in
Tertiary deep-marine depositional settings. Each shale type endmember
has distinctive textures and fabrics, which record variations
in depositional conditions. Textural and compositional
variations in shales, considered within the context of sequence
stratigraphy, provide a basis for seal risk assessment. As determined
from mercury injection capillary pressure (MICP) analysis,
the pressure required to attain critical seal
pressure (10% non-wetting phase saturation)
varies over a considerable range (15 to 20,000
psia). Tertiary shales from offshore Brazil have
consistently low critical seal pressures relative
to age-equivalent shales from offshore West
Africa. Tertiary shales from wells in the Gulf of
Mexico have intermediate critical MICP values
(mean: 4,700 psia). The organization of shale
facies within a sequence stratigraphic framework reveals systematic
variations in seal character. Silt-poor shales from uppermost
transgressive
systems
tracts
, and some condensed shales, have
good to excellent seal potential. In contrast, silt-rich shales from
highstand and lowstand
systems
tracts
have moderate to low sealing
capacities. Seal quality generally increases as total clay and carbonate
content increase; other compositional variables have limited
predictive relationship with seal character. Likewise, log-derived
parameters lack significant potential to accurately predict critical
non-wetting saturation values. Additional seal variability factors
include changes in the rate of deposition, early marine cementation,
and depositional fabric. Available data provide a compelling
argument for textural control of seal character induced by highfrequency
stratigraphic cycles.
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