Page 90 - The Petroleum System From Source to Trap

4. Genetic Classification of Petroleum Systems 81

Geologists comfortably accept lateral migration magnitude larger than those traveled through during
distances of hundreds of kilometers ("long-distance vertical migration. For this reason alone, lateral
migration"), but are often skeptical when vertical migration of petroleum is likely to result in substan­
migration distances of two orders of magnitude less (1-5 tially larger secondary migration losses than are
km) are proposed to explain geochemical observations. incurred during vertical migration through faults and
Explorationists are often surprised when oils from reser­ fractures. This is particularly the case for natural gas,
voirs of widely different ages in the same field are shown which must continuously saturate the water phase in
by biological markers to be derived from a single source. the carrier bed if it is to persist as a separate hydro­
Yet this situation is commonly observed in extensional carbon phase during secondary migration (Sluijk and
regimes and thrust belt plays (Price, 1980). The reluc­ Nederlof, 1984).
tance in accepting vertically dominated migration may
be partially psychological: stratigraphic fabrics always Because of the physical constraints previously
display more vertical than lateral variability. Thus, it is reviewed, short-range lateral migration along permeable
intuitively assumed that vertical migration must be an carrier beds and vertical migration through faults and
unusual phenomenon. This paradigm is also reinforced fractures are the most commonly observed mechanisms
by the observation that fault and fracture zones exposed of petroleum charging into traps. Evidence for a strong
at the outcrop generally appear to the naked eye to be vertical component to migration from a deeply buried
sealed and impervious. Most significantly, the occur­ active source rock is increasingly being recognized,
rence of oil accumulations in fault traps supports the than to modem biological marker correlations (Seifert
widely held perception that faults always behave as et al., 1979; Moldowan et al., 1985). Except for unusual
barriers rather than conduits to migration in the subsur­ cases of long-distance lateral migration (e.g., peri­
face. The point can be lost that faults do not seal fault Andean foreland basins, Western Canada basin, Central
traps; the lithologies and/ or pressure regimes on the Arabia, and Williston basin), the petroleum accumula­
other sides of the faults do (Downey, 1984; Chapter 8, tions in most areas are commonly found over or immedi­
this volume). Furthermore, fault zones have a dual ately adjacent to a pod of mature source rock (Demaison,
character, in some situations acting as a conduit for fluid 1984). In fact, most commercial petroleum accumulations
flow while in other cases acting as a barrier to flow result from relatively short lateral migration distances
(Smith and Forster, 1989). Whether faults and fracture Gess than 30 km), which are constrained by the dimen­
zones act as barriers or conduits is also controlled by the sions of the structural drainage cells surrounding indi­
presence and positioning of deep, abnormally pressured vidual traps (Sluijk and Nederlof, 1984).
hydraulic systems (Hunt, 1990).
The interaction of the following three physical Recognition of Migration Styles
constraints explains why vertical migration phenomena
are so commonly observed in certain tectonic settings: For this classification, it is important to identify those
secondary migration patterns that lead to concentration
1 . Faults and fractures that are persistently reactivated rather than dispersion of hydrocarbons once they have
by tectonic movements may serve as highly efficient been expelled from the mature source rock. Recognition
avenues for vertical migration of petroleum. Given a of focused secondary migration patterns, rather than
relatively limited time (e.g., 10 m.y.), a vertical or dispersive mechanisms, helps to predict the location of
subvertical open fracture 1 mm across and 1 km long plays and their potential petroleum richness (Pratsch,
is capable of filling (or depleting) a large oil field. The 1982). Evaluation of migration style is the critical step in
role of faults and fractures as loci of hydrothermal recognizg whether drainage is focusing or dispersive.
fluid movement and sites of metallic ore deposition Migration drainage style is tectonically controlled and
has been recognized by the mining profession for well thus is predictable from the broad structural and strati­
over a century (Park and MacDiarmid, 1975). Thus, graphic fabrics of the basin fill. Petroleum systems can be
the importance of faults and fractures in the vertical classified as laterally drained or vertically drained
transport of petroleum from a deeper source rock to (Figure 4.1), as outlined below.
significantly shallower reservoir rocks should be
acceptable to petroleum geologists. Laterally Drained Petroleum Systems
2. Low-angle lateral migration is physically less efficient Lateral drainage of petroleum requires a laterally
than vertical migration. The buoyant force of an oil continuous regional seal resting on a widespread,
filament of fixed length and volume is several times permeable reservoir unit (i.e., seal-reservoir "doublets"),
higher for purely vertical displacement than for lateral a weak to moderate degree of compressive structural
upward transport along a gently dipping carrier bed. deformation, and uninterrupted homoclinal ramps.
Consequently, lateral migration is more demanding Foreland basins, such as the North Slope of Alaska
on the amount of oil needed to create an adequate (Figure 4.5) and the peri-Andean basins of South
buoyant force capable of breaking through capillary America, or saucer-shaped intracratonic sags, such as the
pressure barriers in the carrier unit (filing, 1939). Williston basin (Figure 4.6) and the Triassic salt province
3. ln general, the volumes of water-wet porous rock of eastern Algerian Sahara, are favorable settings for
percolated through during long-distance lateral focused lateral drainage, provided some low-amplitude
migration of petroleum are several orders of arches plunge into the pod of mature source rock. Lack