Page 14 - The Petroleum System From Source to Trap

6 Magoon and Dow

Sedimentary Basin Investigations tectonic process, such as rifting, as a basin. On inter­
preting a seismic profile, a geophysicist refers to a thick
Over the last several decades, investigations of sedi­ package (measured in two-way time) of sedimentary
mentary basins have emphasized plate tectonics or struc­ rocks as a basin. Geologists frequently use the term
tural evolution. Basin classification schemes evolved geographically, that is, to name and locate a province,
from descriptive geology (Weeks, 1952; Knebel and such as the Williston basin, which is separate from the
Rodriguez-Eraso, 1956) to genetic interpretations genetic use of basin to mean any sedimentary basin. In
(Halbouty et al., 1970a,b; Klemme, 197la,b, 1975, 1986; some cases, the water column is implied as the basin, in
Bally, 1975; Huff, 1978, 1980; Bally and Snelson, 1980; others the sedimentary rock contents are the basin, and
Bois et al., 1982; Kingston et al., 1983a) with the advent of in yet others, the container is the basin. None of these
plate tectonics theory. With increased understanding of meanings is incorrect, and specialists from different disci­
organic geochemistry, work on the occurrence of oil and plines are usually aware that basin has more than one
gas also has gone from the descriptive (Weeks, 1952; meaning.
Knebel and Rodriguez-Eraso, 1956) to the deterministic For this volume, the sedimentary basin is a depression
(Tissot, 1969; Tissot and Pelet, 1971; Zieglar and Spotts, filled with sedimentary rocks. The depression, formed by
1978; Welte and Yukler, 1981; Demaison, 1984; Ungerer any tectonic process, is lined by basement rock, which
et al., 1984; Tissot et al., 1987). can be igneous, metamorphic, and/ or sedimentary rock.
Each new approach to the analysis of petroliferous The basin includes the rock matter, organic matter, and
sedimentary basins becomes more focused on the water deposited in this depression. In certain cases, such
genesis of petroleum. Bally (1975) pointed out that sedi­ as with coal and some carbonate deposits, the sedimen­
mentary basin type does little to improve our ability to tary material is formed in situ. Basin used by itself refers
forecast the volume of petroleum from a particular type to the sedimentary basin.
of basin. However, as more petroleum geochemistry is The term basin used with a proper noun refers to a
incorporated into the analysis of a sedimentary basin, the petroleum province, such as the Williston basin.
success ratio goes up (Demaison, 1984) and the forecast Sometimes basin is capitalized, such as in the Green
of petroleum occurrence becomes more certain (Tissot et River Basin, when it is a proper geographic name that
al., 1987). usually refers to the present-day river drainage. A
When sedimentary basins with uncomp i cated petroleum province is sometimes referred to as a
l
geologic histories are studied, a basin analysis approach petroleum basin, which is different from a petroleum
that promotes organic g e ochemistry works well. system.
However, when similar studies are carried out in fold A sedimentary basin analysis investigates, in a myriad
and thrust belts (such as in Wyoming, U.S.A.), in areas of of ways, the formation and contents of this depression.
complex geology (such as the Basin and Range of Structural and stratigraphic studies are the most conven­
Nevada and Utah, U.S.A), or in areas of uncommon heat tional way to study a sedimentary basin. More recent
source (such as in the mid-Pacific Ridge) (K venvolden et techniques include seismic stratigraphy and sequence
al., 1988), basin analysis techniques are more difficult to stratigraphy. Sequence stratigraphy, for example, can be
apply because the original sedimentary basin is severely used to understand the distribution of sandstone and
deformed or incomplete. In fact, for maps that show oil shale in a particular area as a package of related sedi­
and gas fields and basin outlines together, the petroleum mentary rock. For the petroleum geologist, in certain
accumulations occur within the basin outline as often as areas the reservoir properties of this sandstone can be
they occur on the adjacent highs or arches that are mapped as well as the organic facies of the shale. Sedi­
outside the basin outline (Vissides and Quirin, 1964; mentary basin analysis includes all aspects of basin
Wilkerson and Reed, 1982). Oil and gas fields usually formation and the basin fill up to the time petroleum is
(but not always) occur in sedimentary rocks, but not generated, at which time a petroleum system investiga­
necessarily within the boundary of basins. Therefore, to tion is required. Because petroleum is mobile, fragile,
understand the occurrence of these accumulations, at and responds to different physiochemical parameters
least two items need clarification. First, a working defini­ than does basin fill, this second type of investigation, the
tion is needed for the sedimentary basin and what is petroleum system, is needed.
being investigated, and second, a different type of inves­
tigation is needed that is separate from basin analysis Petroleum System Investigations
and deals only with oil and gas.
First, the term basin has different implications to Each investigative procedure has an appropriate
different specialties. A paleontologist uses the term in starting point. For the prospect analysis, the starting
reference to where in the water column fossils live, such point is the trap, for the play, a series of traps, and for a
as benthic or planktonic. A petroleum geochemist visual­ basin analysis, a tectonic setting and sedimentary rocks.
izes the most anoxic part of a paleoocean or continental Similarly, the investigative procedure for the petroleum
basin where organic matter accumulates and refers to system starts with discovered hydrocarbon accumula­
that as the basin. Carbonate and siliciclastic stratigra­ tions, regardless of size. Because of this, shows or traces
phers refer to the sedimentary fill that was deposited of oil and gas take on new importance. Petroleum
sometime in the past as the basin. Structural geologists geochemical analysis of oil and gas traces can provide
refer to the container that is created in response to a critical information as to the nature of the responsible