Global Insight into Sealing Properties
Seals are the least comprehended physical elements of the petroleum system and of hydrocarbon traps, and they are often the most neglected in pre-drill play and prospect risking, even though they are commonly listed as the main reason for prospect failure. Seals are defined as rocks that have capillary entry pressures that are large enough to match or exceed the buoyancy forces of migrating or trapped hydrocarbons. They can occur as single surfaces capping a reservoir or as complex combinations of surfaces on the top, laterally, and on the bottom of reservoirs. A trapped hydrocarbon column will exert buoyancy pressure on all sealing surfaces unequally. Therefore, it is necessary to understand first how this pressure operates on the simplest sealing surface before attempting to evaluate combined mechanisms. We have evaluated this relationship by correlating the attributes of successful seals with the height of hydrocarbon columns and with the degree to which traps are filled to their maximum capacity. We suggest that to correlate seal parameters with hydrocarbon columns, seals must be described and classified by the number, type, location and combination of sealing surfaces. For the first part of the study, the focus was on simple top seals. Our analysis of ~560 reservoirs (~400 fields in ~170 basins) with simple top seals in the DAKS IQ database yields the following statistics: (1) They contain ~57% of the ultimately recoverable oil equivalent reserves found in reservoirs for which this information is documented within DAKS IQ. (2) Of the 522 giant oil (>100 MMBO) and 180 giant gas fields (>1 TCFG), 302 (69%) oil fields and 96 (53%) gas fields are sealed by simple top seals. (3) The average hydrocarbon column retained for all traps with simple top seals is ~1018 ft with a maximum of ~7516 ft. Capillary pressure measurements confirm that the displacement or the threshold pressure of shales is significantly higher than in other lithologies, with the exception of anhydrites. Over 80% of the reservoirs studied are capped by simple top seals consisting of shales, and an additional ~22% involve shales in combination with other lithologies. Although evaporites seal only 15% of the reservoirs, they trap ~26% of the oil-equivalent ultimately recoverable reserves in these reservoirs. Other sealing lithologies are argillaceous limestone, chalk, marl, undifferentiated tight carbonates, and tight sandstones/conglomerates. 96% of hydrocarbon columns >3000 ft are retained by shales or evaporites, but all other sealing lithologies are capable in most cases of retaining the maximum allowable hydrocarbon column. 88% of the reservoirs sealed by shales and interlayered rocks retain columns of <2000 ft. There is moderate correlation between seal thickness and hydrocarbon column height, but the scatter in the data suggests that thickness is not a reliable seal predictor. Thicker, intact top seals consisting of shales, anhydrites, or of interlayered combinations of both, are capable of retaining large hydrocarbon columns, but many exceptions occur in which thick seals are incapable of holding smaller columns because they are faulted. Thinner than average seals are capable of retaining larger columns.
AAPG Datapages/Search and Discovery Article #90334©2018 AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop, Abu Dhabi, UAE, April 16-19, 2018