Page 178 - The Petroleum System From Source to Trap

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STRUCTURAL AND THERMAL Table 9.1. Factors Determining Temperature in
EVOLUTION OF SEDIMENTARY BASINS Sedimentary Basin Fill
Importance
Are the structural and thermal evolution of sedimen­
tary basins linked? In some cases the answer is yes, but Factor (Order) Qualifications
on the scale of a petroleum system, this fact may have Overburden thickness 1 s t Always important
little utility in attempts to understand the temperature Heat flow 1 s t Always important
history of the basin fill. For example, the formation of rift Thermal conductivity 1 s t Always important
basins is well understood through relatively simple theo­ Surface temperature 2nd Always important
retical models that invoke an initial extensional event. In Sedimentation 1 s t > 1 0 0 m/m.y.
these cases, it is possible to demonstrate with a fair 2nd 1 0 0 m/m.y.
amount of confidence a link between the thermal and 3rd < 1 0 m/m.y.
structural evolution of these basins. However, this may Groundwater flow
be of limited relevance in estimating temperature of the Gravity driven 1 s t-2nd Foreland basins
basin fill at a time when hydrocarbons are being Compaction driven 3rd Unless focused
thermally generated. The magnitude of the initial Free convection Unknown
thermal event associated with the creation of a rift basin Initial thermal event 1 s t (0-20 Ma) Rift basins only
decays with passing time (Figure 9.3). Thus, by the time 2nd (20-60 Ma)
sufficient overburden accumulates for hydrocarbon 3rd (>60 Ma)
maturation to begin, the influence of the initial basin­
forming thermal event may be relatively small in
comparison to other factors.
An example of a rift basin in which the initial thermal Nevertheless, steady-state conductive heat transport is a
event had little influence on the maturation of hydrocar­ useful first order approximation that provides a starting
bons is the Gulf Coast basin of the southeastern United point from which one may later consider departures.
States. This basin formed by riftg in Late Triassic-Early Fourier's law of heat conduction is
Jurassic time (-180 Ma), but it was relatively sediment
starved up to about 40 Ma. Rapid accumulation of q = k g (2)
sediments since that time has increased the burial depth
and temperature of the source rocks. Cretaceous and where q is heat flow, k is thermal conductivity, and g is
early Tertiary age source rocks are estimated to presently the thermal gradient. Applying this to the analysis of
be in the oil generation window (N unn and Sassen, temperature within sedimentary basins, we obtain
1986). Because the thermal anomaly associated with the
rifting of the Gulf Coast basin has been decaying for the T = T0 + (q/k) dz: (3)
last 180 m.y., the degree to which the lithosphere was
extended or rifted has a negligible influence on the where T is subsurface temperature, T 0 is the mean
present-day thermal state (Figure 9.3). Factors such as annual surface temperature, and ru: is thickness of the
lateral variations in overburden thickness and the overburden. Thus, heat flow, thermal conductivity, and
depression of heat flow by sedimentation have a greater overburden thickness are of equal importance in deter­
influence on source rock temperature. For example, mining subsurface temperature. However, heat flow is
Nunn and Sassen (1986) estimate that present-day heat generally a more useful measure of the thermal state of
flow in the Gulf Coast basin is depressed -30% below its sedimentary basins than temperature gradient alone
equilibrium value by high rates of sedimentation. because the geothermal gradient, g = (q/k), varies
On the scale of the petroleum system, the influence of according to thermal conductivity, which can change by
initial basin-forming thermal events is thus of indirect or as much as factor of three or four among common rock
limited importance in determining temperature of the types.
basin fill at the time hydrocarbons are generated. A more generalized description of heat transport can
Temperature of the sedimentary basin fill is more likely be obtained by considering departures from steady-state
to be sensitive to intrabasin factors such as thermal conditions and including advection of heat by moving
conductivity, groundwater flow, sedimentation, and fluids. The change of temperature with respect to time
surface temperature (Table 9.1). The following sections (()T /ot) is then described by
discuss the importance of these four factors in more
detail. pC(oT /ot) = o/oz[kz (oT /oz)] - Vz P wCw (()T /oz) A *
+
(4)
MATHEMATICAL DESCRIPTION OF
HEAT TRANSPORT where z is depth, p and C are the bulk density and heat
capacity, respectively, of a porous rock, P w is fluid
Sedimentary basins are never in complete thermal density, Cw is fluid heat capacity, Vz is the Darcy velocity
equilibrium, and groundwater flow may drastically of a fluid moving through a porous medium, k2 is
change the distribution of thermal energy within a basin. thermal conductivity, and A* is radioactive heat genera-