Page 173 - The Petroleum System From Source to Trap

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

Chapter 9

b
Over u rden Rock, T e m erature,
p
and Heat flow

David Deming
School of Geology and Geophysics
University of Oklahoma
Norman, Oklahoma, U.S.A.

ABSTRACT

Overburden rock, an essential element of the petrleum system, is that series of mostly sedi­
tary rock that overlies the source rock seal rock and reseoir rock. Genetion of hydrocns
from thermal degradation of organic matter in the source rock is determined by thickness of the
overbuen rock in conjunction with the physical propert and proceses that determie tempera­
ture in sedimentary basins. Thickness of the overburden rock is a by-product of the fundametal
forces and proceses that control the structur development of the sedimentary basi in which the
overbuen rock is found. Sour rock temperature is largely determined by thickness and ther
conductivity of the overbuen rock heat flow, and ground surface temperature. Procses such as
groundwater flow and sedimentation may also have signifcant effects on the thermal regime.

INTRODUCTION the overburden rock is important to the generation,
migration, and accumulation of petroleum and to the
Overburden rock is one of four essential elements of formation of traps that contain petroleum.
the petroleum system, but on the basis of volume, it is Although the role of the overburden rock in the
usually the largest part of the basin fill. Overburde�z rock is petroleum system is multifaceted, this chapter discusses
that series of mostly sedimentary rock that overlies the only the key role of the overburden rock in determining
source rock, seal rock, and reservoir rock, the three other the thermal evolution of the source rock. Oil and I or gas
essential elements. In some situations, these other is generated from organic matter (kerogen) in the source
elements may also be part of the overburden rock. The rock through one or more chemical reactions that have
remainder of the basin· fill is the underburden rock, that an exponential dependence on temperature. For a
portion of mostly sedimentary rock that lies between the normal range of geologic heating rates (1°-10°C/m.y.),
basement rock and the essential elements of the the main zone of oil generation occurs between 100° and
petroleum system. The sed imentary basin is the 150°C (Quigley et al., 1987). For these temperatures to be
geometric form defined by the interface of the basin fill reached, a source rock must be buried by overburden
and basement rock (Figure 1 . 4 in Magoon and Dow, rock through the process of sedimentation. The extent,
Chapter 1, this volume). depth, and timing of hydrocarbon generation from the
The overburden rock affects a number of physical source rock thus depend on the sedimentation rate and
processes important to the petroleum system. Because of the geothermal gradient. For a typical geothermal
burial, a source rock generates petroleum, a reservoir gradient of 25°C/km, most oil generation takes place at
rock experiences a loss of porosity through compaction, a depths of about 3--6 km. However, there is a tremendous
seal rock becomes a better barrier to petroleum range of natural variability associated with both sedi­
migration, and if oil and gas are kept in a trap at an mentation rates and geothermal gradients in sedimen­
optimum temperature, biodegradation is prevented. The tary basins.
time sequence in which the overburden rock is deposited Sedimentation rates can vary by as much as three
affects the geometry of the interface of the source rock orders of magnitude, from 1 to 1000 m/m.y.(Figure 9.1).
and overburden rock, and of the seal rock and reservoir Rates below and above these values can be important
rock. In tum, the geometry of the source-overburden locally, but burial histories between these limits are most
horizon influences the timing and direction of petroleum common. The sedimentation rate for a passive margin
migration, and the seal-reservoir horizon dictates the (e.g., Cordilleran miogeosyncline, Atlantic margin, and
timing and effectiveness of trap formation. In this way, Gulf of Lion) changes as it evolves from a rift basin

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