Shale Facies Variability in Deep Marine Depositional Settings: Seal Quantification and Prediction
William R. Almon and William C. Dawson
ChevronTexaco, Bellaire, TX
Fine-grained lithofacies are dominant components of deep-marine (middle
to upper slope) depositional systems. Failure to study the sedimentology and
architecture of fine-grained siliciclastic lithofacies results in an incomplete
characterization of marine petroleum systems. Most significantly, these
fine-grained strata are baffles and barriers to fluid flow. Petrologic analyses
of Tertiary-aged samples from wells in South Atlantic basins reveal the common
occurrence of 6 shale types
in marine stratal packages: 1)
well
-laminated
organically-enriched shales; 2) slightly silty, weakly laminated shales; 3)
silty shales weakly laminated shales; 4) distinctly mottled silty shales; 5)
very silty shales and argillaceous siltstones; and 6) calcareous shales and
claystones. Each shale type represents a limited range of depositional and
geochemical conditions. Shale
types
1, 2 and 6 have significantly greater 10%
MICP pressures relative to shale
types
3 and 4. Shale type 5 has the lowest
sealing potential. Shale facies vary systematically in terms of sequence
stratigraphy and exhibit a strong correlation with seal capacity suggesting that
at least some textural parameters have a direct effect on seal capacity.
Silt-poor transgressive shales typically have excellent to exceptional sealing
behavior. Increased percentages of silt-sized detrital grains (> 25%) can
inhibit mechanical compaction and allow preservation of relatively
large-diameter pore throats, which typify highstand and lowstand shales.
Well
-developed laminar fabrics and early marine carbonate cementation appear to
enhance seal character. Bioturbation generally degrades seal behavior. Stacking
patterns inherent to deep-marine depositional systems can result in considerable
(several hundred feet) vertical separation of lowstand reservoirs and overlying
transgressive seals.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005