Print this pageSequence-Stratigraphic Controls on Subsurface Fluid Movement: An Example of Selective Charging of Stacked Hydrocarbon Reservoirs, West Fulton Beach Field, Texas
Paul R. Knox
A study of 41 stacked Frio Formation barrier/strandplain oil and gas
reservoirs within West Fulton Beach field, Aransas County, documents the effect
of 3rd-, 4th-, and 5th-order flooding surfaces on the 
vertical
migration of
hydrocarbons and the selective charging of stacked reservoir sandstones.
Flooding surfaces were identified from regional, high-resolution stratigraphic
analyses constrained by existing barrier/strandplain depositional-facies and
facies-tract models. Recognition of flooding surfaces within a field or prospect
area aids in predicting the likelihood of a reservoir's having been charged with
hydrocarbons and may identify the dominant hydrocarbon phase in a prospective
reservoir.
Vertical trends of total hydrocarbon production, gas production, and gas/oil
ratios in West Fulton Beach field define cyclic patterns related to the
stratigraphic framework. The dominant pattern is an upward increase in both
total producible hydrocarbons (oil plus gas, in BOE) and total gas production in
4th-order sequences below 3rd-order flooding surfaces. A secondary pattern is an
upward increase in the gas-oil ratios of individual reservoir sandstones
(5th-order sequences) below 4th-order flooding surfaces. These patterns suggest
that (1) 3rd-order flooding surfaces seal vertical
migration of both oil and gas
along a bounding fault, presumably because of increased shale content in fault
gouge, (2) 4th-order flooding surfaces are gas-selective baffles within a
3rd-order sequence an distribute oil and gas under the main seal, and (3)
5th-order flooding surfaces act as baffles to selectively fill stratigraphically
higher reservoirs in a 4th-order sequence with more gas than oil. Potential oil
and gas exploration and deeper-pool targets should be evaluated within a
sequence stratigraphic framework to more accurately predict the potential for
hydrocarbon charging and the gas-oil ratio within prospective reservoirs.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995