Page 171 - The Petroleum System From Source to Trap

8. Hydrocarbon Seal Rocks 163

Because all faults change their relative offset or
"throw" along their length, a near-infinity of cross
sections would be necessary to perfectly describe the
characteristics of a fault-sealed prospect. Faults that
greatly change offset along their length make lateral o.a
entrapment across the fault plane difficult. For many
types of fault, the lateral variation in offset is theoretically
predictable and systematic. Typical growth faults and 0.6
thrust faults decrease their stratigraphic offset and pass I
into bedding planes at the fault ends. In interpreting the - - - - · - - - - - �
likelihood that a fault will offset a reservoir bed laterally I I
against a sealing bed to provide a trap, the lateral I
variation in bed offset must be considered. It is easy to t
find two-dimensional "traps." Hydrocarbon accumula­
tions require three-dimensional seals. Fault seal mapping 0.2
requires analysis in three dimensions.
The quality of a fault-controlled prospect is best deter­
mined through the actual mapping of the structural
attitude of the lithologies co-joined by the fault plane 10 20 30 40 50
within the apparent closure. Such fault plane maps,
depicting the joining of reservoir and seal units at the % RESERVOIR ROCK
fault plane, are an important part of understanding
hydrocarbon migration and entrapment (Allan, 1989). Figure 8.2. Probability of a reservoir eventually being
trapped (Pt) is dependent on the proportion of reservoir to
seal rocks in the section and the number of affecting faults.
Subparallel Faults as Seals The dashed line shows the example discussed in the text.
Where a fault juxtaposes a permeable reservoir
against an updip impermeable layer, this lateral seal than the regional seals and require a detailed under­
against the reservoir rock may form a hydrocarbon trap. standing of the petrophysical properties of the sealing
In a sedimentary section composed of alternating layers surfaces. Faulting causes a redistribution of sealing
of reservoirs and seals, the probability that a trap is surfaces and needs to be carefully analyzed to determine
formed by juxtaposing a reservoir and seal is dependent the three-dimensional sealing surface of an accumulation
on the relative proportions of reservoir to seal within the of hydrocarbons.
fault-affected section. If two or more subparallel faults
affect the sedimentary section, the probability of a
reservoir eventually being laterally sealed is greatly References Cited
increased.
As a generalization, if an interbedded series of layered
reservoirs and seals contains 25% reservoirs, a single Allan, U.S., 1989, A model for the migration and entrapment
fault offset provides about a 50% chance that a reservoir of hydrocarbons within faulted structures: AAPG Bulletin,
might be completely trapped across the fault. A second v. 7 3 , p. 803-812.
paralleling fault would increase the probability of Berg, R. R., 1981, Calculation of seal capacity from porosity
and permeability data (abs.): AAPG Bulletin, v. 65, p. 900.
trapping to about 75%, a third to 88%, a fourth to 94%, Bouvier, J. D., C. H. Kaars-Sijpesteijn, D. F. Kluesner, C. C.
and a fifth to 97%. This example is graphed for one, two, Onyejekwe, and R. C. van der Pal, 1989, Three-dimen­
and five faults in Figure 8.2. Careful mapping with high­ sional seismic interpretation and fault sealing investiga­
resolution reflection seismic data may reveal the tions, Nun River field, Nigeria: AAPG Bulletin, v. 73, p.
presence of multiple subparallel faults that can strongly 1397-1414.
increase the probability of eventual updip lateral seals Downey, M. W., 1984, Evaluating seals for hydrocarbon accu­
that would form hydrocarbon traps. mulations: AAPG Bulletin, v. 68, p. 1752-1763.
Downey, M. W. 1990, Faulting and hydrocarbon entrapment:
Geophysics, Leading Edge, January, p. 20-22.
SUMMARY Foster, N. H., H. K. Veal, and C. Bortz, 1987, Fault seals in oil
fields in Nevada (abs.): AAPG Bulletin, v. 71, p. 1006.
Hydrocarbon seals are an essential element of the Grunau, H. R., 1981, Worldwide review of seals for major
petroleum system. Regional and local seal rocks act to accumulations of natural gas (abs.): AAPG Bulletin, v. 65,
confine and limit the distribution of genetically related p. 933.
hydrocarbons within the petroleum system by the source Hubbert, M. K., 1953, Entrapment of petroleum under hydro­
dynamic conditions: AAPG Bulletin, v. 37, p. 1954-2026.
rock. Understanding the structural form and the areal Mcintyre, J. F., 1988, Presence and control of evaporite top
distribution of the regional seals is a vital part of the seals on occurrence and distribution of hydrocarbon traps:
analysis of a petroleum system. The local sealing surfaces main fairway, central overthurst belt, Wyoming and Utah
for hydrocarbon accumulations are often more complex (abs.): AAPG Bulletin, v. 72, p. 221.