Page 12 - The Petroleum System From Source to Trap

4 Magoon and Dow

Historical aspects have differing importance for each
[SEDIMENTARY BASIN I level of investigation. Investigations of sedimentary
� l Economics basins and petroleum systems are relative to the geologic
-
� NOT time when processes are occurring, that is, when
I PETROLEUM SYSTEM I Important sediments are being deposited and when hydrocarbons
are migrating to their traps. In contrast, the present-day
� existence of a play or prospect is the important factor.
Economics There is little interest in a play or prospect that existed at
the end of the Paleozoic but has since been eroded or
VERY 1 destroyed. A prospect is conceptual because a successful
Important PROSPECT prospect turns into an oil or gas field when drilled or
disappears when the prospect is unsuccessful.
In addition, as the focus of investigation on hydro­
carbon occurrence moves from the sedimentary basin to
Figure 1.1. Four levels of petroleum investigation. the prospect level, the cost of the investigation per unit
surface area generally increases. Investigation of sedi­
mentary basins requires a low-density information grid
that covers a large area, such as widely spaced seismic
LEVELS OF PETROLEUM lines, a few strategically placed exploratory wells, and
INVESTIGA T IONS small-scale geologic maps. In contrast, prospect evalua­
Investigations of sedimentary basins, petroleum tion requires a relatively high-density information grid
systems, plays, and prospects can be viewed as separate that covers a small area, such as closely spaced seismic
levels of hydrocarbon investigation, all of which are lines on a large-scale map. Eventually, the cost to acquire
needed to better understand the genesis and habitat of drilling rights and to drill wells must also be included in
hydrocarbons. Investigations of sedimentary basins the economics of the prospect.
describe the stratigraphic sequence and structural style of For the purpose of this volume, each level of
sedimentary rocks. Petroleum system studies describe petroleum investigation has its own descriptive term,
the genetic relation between a particular pod of gener­ such as basin analysis for the investigation of a sedimen­
ating source rock and the resulting petroleum. Investiga­ tary basin. Also, modeling has a simil set of terms, such
tions of plays describe a series of present-day traps, and as basin modeling. The difference between analysis and
of prospects, an individual trap, and determine whether modeling is that in analysis, an existing item is dissected
they have economic value and are exploitable with to determine how it functions, whereas in modeling, a
available technology and tools (Figure . 1 and Table 1.1). hypothetical item is dissected to determine how it should
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Economic considerations are unimportant in sedimen­ function. For example, prospect modeling is used on a
tary basin and petroleum system investigations, but are prospect to justify drilling, whereas a prospect analysis is
essential in play or prospect evaluation. Whenever plays carried out after drilling to understand why it lacked
or prospects are discussed, economically producible commercial hydrocarbons.
hydrocarbons are implied or anticipated. Stated in A historical summary of geologic models relevant to
another way, without favorable economics, a commercial the different levels of petroleum investigations is
petroleum play or prospect does not exist. In contrast, a outlined in Table 1.2. A descriptive comment for each
sedimentary basin and a petroleum system exist regard­ reference is followed by eight items divided into two
less of economic considerations because these broad categories: geologic factors and evaluation criteria.
phenomena are based on natural processes. Proof of a These references discuss the eight items in the two cate­
sedimentary basin is sedimentary rock; proof of a gories at various levels of detail. Although this table is
petroleum system is the presence of hydrocarbons, even incomplete, it is shown to contrast and demonstrate the
a puff of gas or a drop of oil (low volume but high relationship of a petroleum system study with other
concentration). levels of investigation.

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Table . 1 . Factor Comparison in the Four Levels of Petroleum Investigation
Sedimentary Petroleum
Factor Basin System Play Prospect
Investigation Sedimentary rocks Petroleum Traps Trap
Economics None None Essential Essential
Geologic time Time of deposition Critical moment Present-day Present-day
Existence Absolute Absolute Conditional Conditional
Cost Very low Low High Very high
Analysis and modeling Basin System Play Prospect