Page 28 - The Petroleum System From Source to Trap

20 Magoon and Dow

Devonian strata or are within the overburden rock. The plumbing of the petroleum system (within the strati­
trap and petroleum forming processes occur during graphic and geographic extent), the chance of finding
deposition of the overburden rock. commercial hydrocarbons is improved.
Over the area of the North American continent, the Ideally, a petroleum system analysis begins with an
age of these petroleum systems that have Upper oil and gas (show) map. Geochemical analyses of those
Devonian source rocks varies with the location of the hydrocarbon shows are needed to understand the origin
system. Along the eastern and southern edge of the of the oil or gas (biogenic versus thermal). Comparing oil
North American craton, these late Paleozoic foreland to oil and gas to gas can indicate whether more than one
basins (including the Appalachian, Warrior, and petroleum system is involved. The line of inquiry can be
Anadarko basins) received only minor amounts of post­ expanded to include the type of organic matter respon­
Paleozoic sediments. Since the present-day petroleum sible for those shows and the overburden rock required
accumulations must have generated and migrated to thermally mature the source rock. To determine the
around the end of Permian time or earlier (when geographic, stratigraphic, and temporal extent of the
maximum burial was achieved), the age (generation­ petroleum system, the investigator will need to acquire
migration-accumulation) of these petroleum systems specific information to make the burial history chart,
having Upper Devonian source rocks ranged from map, cross section, and events chart that define the
Mississippian to Permian time. The preservation time system (Figures 1 . 2-1.5) (see also Peters and Cassa,
extended through the Mesozoic and Cenozoic. In Figures 5.12 and 5.13, Chapter 5, this volume).
contrast, the western edge of the craton includes foreland
basin sedimentary rocks as young as Cretaceous or early Fictitious Example #1
Tertia, and one of the cratonic interior basin sags may
be as young as Tertiary. The age of these systems ranged To explain the investigative technique more graphi­
from the Cretaceous to Tertiary. cally, two fictitious examples are provided (see also
Smith, Chapter 2, this volume).
From the United States and Canada, 300 oils were
Miocene of California, U.S.A.
collected and analyzed. The oils were collected from
Another organic-rich interval that is involved in many rocks that range in age from Precambrian to Holocene,
petroleum systems is the Miocene of California. In Cali­ from a depth range of 0-3000 m, and from many litholo­
fornia, numerous strike-slip basins formed in the gies, such as fractured granite and shale, sandstone, and
Miocene and continue to develop to the present day. At dolomite. Many different types of analyses were carried
first, conditions in the basins were conducive to the out on the oils. Oil-{)il correlations indicate two groups,
1
formation and preservation of organic matter along with A and B, that form clusters in seven areas (Figure . 9A).
abundant biogenic silica and relatively little siliciclastic A geochemical profile (Peters and Cassa, Chapter 5,
material. Deposition of coarser siliciclastic material this volume) of a well in each area indicates that each
became progressively more rapid during Pliocene-Pleis­ well penetrated more than one source rock and that an
tocene time. This sediment provided the necessary over­ Upper Devonian source rock was common to all seven
burden that heated the source rock to generate hydrocar­ areas. Reexamining the vertical distribution of the oils
bons that formed petroleum systems within the Los indicates that one-third of the oils are from Carbonif­
Angeles basin, Ventura basin (Santa Barbara offshore), erous reservoirs. Using kerogen studies from the litera­
Santa Maria basin, San Joaquin basin, and several other ture and other data, an organic facies map indicates two
coastal basins. Again, what started out as organic-rich kerogen types, type II and III, in the Upper Devonian
deposits over a large area eventually developed into source rock (Figure 1 . 9B). In areas where the Upper
smaller sedimentary basins that acquired sufficient over­ Devonian source rock was eroded across the transconti­
burden rock to generate hydrocarbons, thus forming nental arch, regional mapping allowed the organic facies
separate petroleum systems. to be mapped where it was absent or too deeply buried.
By use of hydrous pyrolysis (Lewan, Chapter 11, this
volume) on immature source rock samples, oil-source
INVESTIGATIVE TECHNIQUE rock correlations indicate that the two organic facies in
the Upper Devonian are responsible for the two oil
A petroleum system investigation should begin with groups. Furthermore, the two clusters of group A oil are
hydrocarbons (Smith, Chapter 2, this volume), such as a within the type II kerogen, and the four clusters of group
show of oil or gas. In the same way that sedimentary B o il are within the type III kerogen (Figure 1.9C).
rock requires a sedimentary basin, an oil or gas show Additional well and outcrop control and burial
requires a petroleum system. With this line of investiga­ history diagrams can be used to map the the thermal
tion, it is necessary to understand the smallest accumula­ maturity of the Upper Devonian source rock. A pod of
tions or shows because they are clues to whether active source rock occurs with each of the seven oil
commercial accumulations are possible. In addition, the clusters. Computerized exploratory well and field files
petroleum system investigation approach requires that are used to map the distribution of oil, which is found to
the focus of work is on the stratigraphic and structural be within the oil clusters, further confirming the
studies of the essential elements and processes. If an geographic and stratigraphic extent of these seven
exploratory well penetrates and successfully tests the petroleum systems.