Page 167 - The Petroleum System From Source to Trap

Magoon, L. B, and W. G. Dow, eds., 1 994, The petroleum
system-from source to trap: AAPG Memoir 60.

Chapter 8

� H drocarbon Seal Rocks
y

Madan W. Downey
President, ARCO lntemational Oil [-r Gas
Plano, Texas, U.S.A.

ABSTRACf
The geographic extent of a petroleum system is defined by the observed occurrences of geneti­
cally related hydrocabons that emanated from a given pod of mature source rock. These related
hydrocn occurrences and associted migration routes, in turn, are confined and limited by the
presece of sealg surfaces. Where confing seal are lacking, hydrocns escape to the surface.
Therefore, the seal rock is an essential element of the petrleum system.
In the petroleum system, there are two important classes of seals: regional seals that roof
migrating hydrocarbons and local seal that confine accumulations. Major roofing seal act to
confine migrating hydrocns to particular stratigraphic units. Any lithology can serve as a seal
for a hydrocn accumulation. The only requirement is that minium displacement pressure of
the lithologic unit comprising the sealg surface be greater than the buoyancy pressure of the
hydrocarbon column in the accumulation. In practice, however, the overwhelming majority of
effective seal rocks are evaporites, fine-grained clastics, and organic-rich rocks. These lithologies are
commonly evaluated as seal because they have high entry pressures, are laterally continuous,
maintain uniformity of lithology over large area, and are relatively ductile.
Regional evaluation of the exploration potential of an area should start with (1) determition of
stratigraphic position and area distribution of thermaly mature sour rocks, (2) identification of
the regional seal for migrating hydrocns, (3) an analysis of trapping conditions focus on the
areas under the regional seal and updip from the thermally mature source rocks, and (4) an exami­
nation of the distribution of hydrocn shows and production.

1988). Most quantitative seal data derive from studies of
INTRODUCfiON
seals roofing local accumulations, but such field data are
The geographic extent of a petroleum system is related only inferentially to the general properties of the
defined by the observed occurrences of genetically regional seals that must roof and guide the migrating
related hydrocarbons that emanated from a given pod of hydrocarbons in the petroleum system.
mature source rock. These related hydrocarbon occur­ In a petroleum system investigation, seal analysis
rences and associated migration routes, in turn, are should start with a determination of the time and place
confined and limited by the presence of sealing surfaces. where hydrocarbons are generated and expelled. Many
A seal rock can be defined as one that has pore throats too stratigraphic horizons display properties of a seal, but
small and poorly connected to allow the passage of only those few that are above the mature source rock(s),
hydrocarbons. The geographic extent of seal rocks are regionally extensive, and have a seal-transmission
defines the effective limits of the petroleum system. (reservoir rock) couplet are important to a particular
Where confining seals are lacking, hydrocarbons escape petroleum system. Making maps, cross sections, and
to the surface. Therefore, the seal rock is an essential measurements of all possible sealing rocks in a
element of the petroleum system. petroleum province would be an inefficient and time­
Many stratigraphic horizons have properties of a seal; consuming way to understand the distribution of hydro­
it is important to identify those that define the hydro­ carbons in a petroleum system.
carbon migration and accumulation system at the critical Understanding seals that are part of a petroleum
moment. All other seals are irrelevant to the petroleum system involves focusing on just those impermeable
system. surfaces that control migration and accumulation of
Two important classes of seals occur in a petroleum genetically related hydrocarbon pools. Maps of the distri­
system: regional seals that roof migrating hydrocarbons bution, character, and structural attitude of regional seals
and local seals that confine accumulations (Ulmishek, are important in understanding the petroleum system.

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