ĐỊA CHẤT DẦU KHÍ ( PETROLEUM GEOLOGY ) - CHƯƠNG 4 potx

CHAPTER 04
CHAPTER 04
SEAL
SEAL
(
(
ROOF
ROOF
OR
OR
CAP
CAP
ROCK)
ROCK)
Definition
Definition
:
:
Seal is impermeable rock that forms barrier
Seal is impermeable rock that forms barrier
on top of the reservoir rock of an oil and/or gas
on top of the reservoir rock of an oil and/or gas
reservoir.
reservoir.
In the case of anticlines (
In the case of anticlines (
Figure
Figure
1
1

, (a)), only a vertical seal,
, (a)), only a vertical seal,
or
or
caprock
caprock
, is required; but faults (
, is required; but faults (
Figure
Figure
1
1
, (b)) and
, (b)) and
stratigraphic
stratigraphic
traps (
traps (
Figure
Figure
1
1
, (
, (
c,d
c,d
)) must be sealed both
)) must be sealed both
vertically and laterally.
vertically and laterally.

Figure 1
4.1 TYPES: the seal is commonly:
4.1 TYPES: the seal is commonly:
° Best SEAL: Formed by ductile sedimentary
rock: clay or shale (for most sandstone
reservoir, >60% of known giant oilfields have
shale seal).
Shale is the dominant caprock of worldwide
reserves (Figure 2) and is overwhelmingly
the seal in basins rich in terrigenous
sediments, where sandstones are the
dominant reservoir rock.
° Idea cap rock: evaporates (especially favorable
where the reservoir rock are carbonates, its density
being almost 3.0). Evaporites, however, are the
most efficient caprock. They are particularly
common in carbonate-rich basins, and they often
form seals for carbonate reservoirs. Furthermore,
evaporites commonly develop in restricted basin
settings, where accumulations of organic-rich
source rocks are also favored. (Figure 2)
° •Third common type: Dense carbonates are the
third most abundant caprock lithology and seal
about 2% of the world's reserves, cemented rocks,
argillaceous rocks, chalk… (Figure 2)
Figure 2
4.2 General properties
° Permeability in seal are mostly < 10
-4
darcies.

°
Seal are important and commonly overlooked
component in the evaluation of a potential
hydrocarbon accumulation.
°
Effective seals for hydrocarbon accumulation are
typically thickness, laterally continuous, ductile
rocks with high capillarity entry pressure.
° To calculate the seal capacity, the geologist
needs also to know the pore size and
parameters permitting the fluids to pass
through pores of that sizes, the fluid densities,
the interfacial tention between the fluids, and
the wettability
° Seal need to be evaluated at two different
(micro and macro) scales.
4.3 MICRO PROPERTIES OF SEAL
4.3 MICRO PROPERTIES OF SEAL
° Capillary pressure, Pc
Pc= 2γcosθ/ R
γ: Hydrocarbon –water interfacial tension;
θ: Wettability;
R: Radius largest pore throats.
° Hydrocarbon pressure, P
P = (ρ
w
-ρhc) ×gh
ρ
w
: density of the water; ρhc: density of the HC.;

g: the acceleration of gravity;
h: the height of HC. column.
A seal is broken when P > Pc.
DIFFUSION LOSSES THROUGH SEALS
DIFFUSION LOSSES THROUGH SEALS
Diffusion of Hydrocarbon through seals is
dependent mainly on:
° Hydrocarbon type
° The characteristics of the water filled pore,
network of the contacting seal
° Time available for diffusion
4.4 MACRO CHARACTERISTIC OF SEAL
4.4 MACRO CHARACTERISTIC OF SEAL
° LITHOLOGY.
° DUCTILITY.
° THICKNESS.
° STABILITY.
LITHOLOGY
LITHOLOGY
° Almost effective seals are evaporite, fine grained classtics,
and organic-rich rocks.
° These lithologies are seen as seals because:
 Have high entry pressure
 Are laterally continuous
 Maintain stability of lithology over large areas
 Are relative ductile
 Are a significant portion of the fill of
sedimentary basins.
DUCTILITY
DUCTILITY

° Ductility is a rock property to deform and flow without
visible fracturing that varies with pressure and
temperature (burial depth) as well as with lithology.
° Ductile lithologies
tend to flow plastically under
deformation, whereas brittle lithologies develop
fractures.
° The evaporite
rock group make good ductile seal under
overburden of several thousand feet, but can quite
brittle at shallow depths.
SEAL LITHOLOGIES ARRANGED BY
SEAL LITHOLOGIES ARRANGED BY
DUCTILITY
DUCTILITY
° SALT
° ANHYDRITE
° •KEROGEN – RICH SHALES
° •CLAY SHALES
° •SILTY SHALES
° •CARBONATE MUDSTONES
° •CHERTS
(most ductile lithologies at top of column)
THICKNESS
THICKNESS
° A few inches of ordinary clay shale are
theoretically adequate to trap very large column
heights of hydrocarbons (particle size of 10-4mm 
have 600 psi ≅ 915m of hydrocarbon column)
° •Unfortunately, there is a low probability that a

zone only a few inches thick could be continuous,
unbroken, unbreached, and maintain stable lithoic
character over a sizable accumulation.
STABILITY
STABILITY
° STABILITY IN LITHOLOGY
° STABILITY IN THICKNESS
QUESTION TO DISCUSS
QUESTION TO DISCUSS
Determine the type and characteristics (petrographic content,
thickness, colour, main minerals, rock facies, original,
tectonics) of below seal rock (Cuu Long Basin)