Page 169 - The Petroleum System From Source to Trap

8. Hydrocarbon Seal Rocks 161

high entry pressures, are laterally continuous, maintain a Table 8.1 . Ductile Seal Lithologies Ranked Most to Least
uniform lithology over large areas, are relatively ductile,
and are a significant portion of the fill of sedimentary Ductility Lithology
basins. Statistical analyses of the top seals of apparently Most Salt
unfaulted structural traps by Nederlof and Mohler (1981) Anhydrite
indicated that lithology is the most important factor in Kerogen-rich shales
determing a good seal rock. Silty shales
Carbonate mudstones
Ductility Least Cherts

The folding and faulting that accompany the
formation of many traps put significant strain on the Where traps are created by fault offset of reservoirs,
sealing surfaces of accumulations. Brittle lithologies thickness of the top seal can be important. In such places,
develop fractures, whereas ductile lithologies tend to the top seal can be offset to become a lateral seal, and seal
flow plastically under deformation (Table 8.1). Carbonate thickness can relate directly to the column height of
mudstones may have high entry pressures, but under entrapped hydrocarbons.
conditions of deformation, they fracture much more
readily than salt, anhydrite, clay shale, and organic-rich Uniformity
rock. Ductility is a rock property that varies with
pressure and temperature (burial depth) as well as with Stratigraphic layers identified as having the capillary
lithology. Evaporite rocks makes extraordinarily good properties of seals need to be studied to see whether
ductile seals when overburden rocks exceed several those layers are lithologically uniform throughout the
thousand feet, but they can be quite brittle at shallow areal distribution of the stratigraphic unit. Identifiable
depths. stratigraphic units may vary greatly in their capillary
Lithologies having a very fine pore structure and a properties with only modest changes in lithology. A
ductile matrix can retain sealing properties even under stratigraphic cross section of the potential seal unit, using
severe deformation. Organic-rich rocks contain electric log character and lithology, is an excellent start
deformable layers of kerogen; such rocks commonly toward establishing whether the seal unit is uniform over
have a plastic behavior during folding. The flowage of the geographic extent of the petroleum system.
the soft kerogen layers causes high displacement
pressures in the relict pores. In the overthrust provinces
of the world, where deformation and fracturing are SIGNIFICANCE OF REGIONAL SEALS
expected to be most intense, ductility becomes important
when assessing sealing layers for accumulations. The Most sedimentary sequences contain major wide­
presence of evaporite top seals is described as being spread sealing layers. These regional seals are character­
essential for hydrocarbon entrapment in the central over­ ized by having a broad extent, significant thickness,
thrust belt of Wyoming and Utah (Mcintyre, 1988). An lateral uniformity, and ductile lithologies. Where these
analysis of the characteristics of the world's 25 largest gas seals are found above a mature source rock and good
fields indicates that all of those in thrust provinces quality reservoirs, they largely control the regional distri­
depend on evaporite seals. Indeed, in reviewing the bution of genetically related hydrocarbons. A study of
world's 176 giant gas fields, almost all depended on the oil fields of the Arabian Gulf (Murris, 1980) provides
shale or evaporite seals (Grunau, 1981). excellent examples of this interdependence. Two major
source rocks are present in the area: the Upper Jurassic
Thickness Hanifa and the Aptian Shuaiba. The Upper Jurassic Hith
Anhydrite and the Albian Nahm Umr Shale are the two
Several centimeters of ordinary clay shale are theoreti­ principal seals. Hydrocarbon accumulations are concen­
cally adequate to trap a large vertical column of hydro­ trated in reservoirs under these regional seals. In areas
carbons. For example, a clay shale with a particle size of where the primary seals are absent or disrupted by
10-4 mm would be expected to have a capillary entry faulting, the hydrocarbons have migrated upward to be
pressure of about 600 psi (Hubbert, 1953), theoretically trapped by secondary seals. Here, two petroleum
capable of holding back an oil column of 3000 ft (915 m). systems exist because each pod of active source rock
Unfortunately, there is a low probability that a zone only charges separate traps isolated by mostly regional, but
a few centimeters thick could be continuous, unbroken, some local, seals.
and unbreached and also maintain uniform lithic Major roofing seals act to confine migrating hydrocar­
character over a sizable accumulation. The benefits of a bons within particular stratigraphic units. Regional eval­
thick seal are that it provides many layers of contingent uation of the exploration potential of an area should start
sealing beds and a larger probability that a sealing with (1) determination of stratigraphic position and areal
surface will actually be distributed over an entire distribution of the mature source rock, (2) identification
prospect. A thick seal is important and beneficial, but it of the regional seal for migrating hydrocarbons, (3) an
does not directly influence the amount of hydrocarbon analysis of trapping conditions under the regional seal
column that can be held by a top seal. and updip from the mature source rock, and (4) an