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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Top Seal Character and Sequence Stratigraphy of
Selected Marine Shales in Gulf Coast Style Basins
By
Texaco, Inc., Houston, TX
Marine shales are top seals for approximately seventy percent of
documented hydrocarbon accumulations, but they represent
one of the least studied elements of petroleum systems
. Sealing
capacity is determined from laboratory measurements of high-pressure
mercury injection capillary pressure (MICP), which are
used to calculate hydrocarbon column heights. The largest interconnected
pore
throats control seal capacity.
Pore
throat size is influenced by numerous compositional
(mineralogy, total percentage of clay, organic enrichment) and
textural (fissility, silt content, degree of compaction) aspects of shales.
Mineralogical analyses indicate an average composition of 65% clay, 19% quartz, 4% feldspars, 5% pyrite, and 7%
accessory constituents (authigenic carbonates, glauconite, bioclasts,
and carbonaceous debris).
The total organic carbon (TOC) of these shales ranges from 0.79 to 4.00 wt% In general, seal capacity increases with increasing clay and organic content and decreases with increasing detrital silt content. The 140 Tertiary shale samples we analyzed were deposited in middle neritic to outer neritic/bathyal paleoenvironments. MICP data indicate that maximum top seal capacity ranges from 735 ft to 2,305 fl (oil column) with hydrocarbon saturations in the seal of 10 percent.
Calculated seal capacity typically exceeds hydrocarbon column
heights by a factor of 5 to 20 times. Consequently, top seal capacity
is not a significant risk in structures having four-way closure.
The interpretation of seal analysis data within a sequence stratigraphic
context reveals a systematic trend in top seal capacity.
Shales occurring in the uppermost transgressive systems
tracts
and maximum horizons are excellent top seals. Shale samples
from the lower part of transgressive
systems
tracts are good to
excellent seals, whereas shales within highstand
systems
tracts
have moderate to good sealing capacity. Some condensed
intervals contain exceptional top seals.
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