Sequence
Stratigraphic Setting of Siliciclastic Source
Rocks, Reservoir Rocks and Seals
John M. Armentrout
Sequence
stratigraphy
is the study of stratigraphic relationships within a
time-stratigraphic
framework
. It combines detailed analysis of sedimentary
facies and depositional geometries, and defines a hierarchy of stratigraphic
units that stack into progressively larger scale cycles. Each depositional cycle
consists of four phases of relative accommodation change which can be related to
relative water level change, such as sea level. These four phases are rising,
highstand, falling, and lowstand, and the rocks deposited during each phase are
called systems tracts. Each systems tract has the potential to contain
organic-rich rocks, and porous and permeable reservoir rocks. However, specific
depositional environments within some systems tracts are likely to have higher
proba ility as an effective source rock, seal or reservoir. For example,
regionally extensive muds are deposited during transgression and effective seals
may extend over broad areas. If sedimentation rates and biologic productivity
rates are optimal, some outer shelf and upper slope areas of these muds may be
organic rich and could yield significant hydrocarbons upon burial into the
generative window. Reservoirs also occur in predictable patterns. High-energy
nearshore environments produce porous and permeable sandstones, both within
lowstand and highstand prograding depositional systems.
Systems tracts maps may be overlain one-on-the-other to assess the relative
distribution of potential reservoir, seal or hydrocarbon source rock versus
possible hydrocarbon traps
. For example, reservoir sands occur within the basin
floor thicks, lowstand prograding complexes, and the highstand prograding
complexes. The regional mudstones deposited by transgressive systems tract
provide the best top seal. The transgressive systems tract may overlies all of
the updip ends of the underlying lowstand prograding complexes, providing
top-seal. Because the lowstand prograding complexes are geographically
restricted to re-entrants into the underlying slope, lateral seal may exist if
the underlying slope is sand-poor. Such lateral and top-seal could provide a
stratigraphic trap for the coastal plain or near-shore sandstones of the
lowstand prograding complex. Sandstones of the highstand systems tract and the
basin floor thick of the lowstand systems tract are often widely distributed and
the potential for effective top-seal or lateral-seal is more problematic. For
such highstand and basin floor reservoir targets structural closure is needed to
form effective
traps
.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995