Page 127 - The Petroleum System From Source to Trap

Magoon, L. B, and W. G. Dow, eds., 1994, The petroleum
system-from source to trap: AAPG Memoir 60.

Chapter 6

S i liciclastic Reservoir Rocks

David G. Morse*
Houston, Texas, U.S.A.

Abstract

Depositional envirorunents determine the basic architectu and geometry of siliciclastic hydro­
carbon reservoir rocks. In norunae settings, sandstone reservoirs are deposited in fluvial, eolian,
and lacustrine environments, whereas in marine settings these reservoir rocks are in deltaic,
shallow marie, and deep marine settings. Facies and reseoir rock properties of giant oil and gas
fields in each depositional setting are tabulated and descrbed. The most prolific sandstone reser­
voirs are deposited in marine deltaic distributary mouth bars and distributary channels in delta
lobes. Stacking of delta lobes by channel avulsion and subsidence of inactive lobes due to
compaction of the underlying prodelta silts and days greatly increases the volume of reservoir
sand. Tertia deltaic sedients, underlying major modern deltas, offer likely places to find these
reservoir rocks in rollover anticlines. Shallow marine sediments provide the next most prolific
reservoir facies, which are deposited as banier islands, beach, shoreface, and offshore bar sands.
Great potential exists for siliciclastic reseoir rock in deep marine fans, a relatively underexplored
target occurring at the base of delta slopes or in rift or wrench basi. Advanced three-diional
seismic technology and the preset knowledge of seismic and sequence stratigraphy should help
locate these sandstone reservoirs. Norunae reservoirs offer excellent targets in some basins, such
as the fluvial sands of North Africa and the underexplored lacustrine-related reservoir rock of
China. Because of their high quality, area extent, and thickness, eolian sand reseoirs must always
be considered, particularly in continental interior strata with paleolatitudes in the 15°-40° range
north and south of the paleoequator.

INTRODUCTION depositional environments described in this chapter are
all capable of containing stratigraphic traps formed by
Siliciclastic reservoir rocks owe much of their diversity facies or unconformity pinchouts, by buried paleotopo­
and stratigraphic heterogeneity to the many different graphy, erosional relief, or updip cementation. The
depositional environments in which they are deposited. successful, creative, and knowledgeable explorationist
These hydrocarbon reservoirs are formed in settings must be able to identify the style of the particular trap and
ranging from continental alluvial sands and gravels to find the way to locate it.
deep marine fans. This chapter reviews a variety of these The petroleum system is a series of genetically related
occurrences and highlights examples from around the hydrocarbon accumulations whose provenance is a pod
world. Of the 266 giant hydrocarbon accumulations of active source rock. Even though the genetic relation­
described by Halbouty (1970), 62% have siliciclastic reser­ ship is based on the similarity of hydrocarbons in the
voirs. Although most of these accumulations are struc­ accumulations, it is often true that the reservoir rock is
turally trapped, 10% of the giant fields with siliciclastic also the same throughout the system because the
reservoirs found before 1970 occur as stratigraphic traps. plumbing from the source to trap shares the same seal
This was during an age when papers on modern sedi­ and reservoir rock. This chapter discusses single accumu­
mentary envirorunents were just beginning to appear in lations, but one should always remember that they are
the literature (Reading, 1978). part of a larger petroleum system.
Stratigraphic traps occur where "a variation in the The reservoir, an essential element of the petroleum
stratigraphy is the chief confing element in the reservoir'' system, is the storage space within a trap (Biddle and
(Levorson, 1936, p. 524). Today, a stratigraphic trap is Wielchowsky, Chapter 1 3 , this volume). The reservoir
considered to be a container formed by any variation in must be able to accommodate a significant volume of
the stratigraphy that is independent of structural defor­ fluids and be capable of transfening or exchanging fluids
mation other than regional tilting (North, 1 9 85). The to obtain its hydrocarbon charge and be produced (North,

•Present address: Environmental Quality Division, Watervliet
Arsenal-PWG, Watervliet, New York, U.S.A. 121