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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
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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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