1

SEALSPresentation Agenda:

• Seal Essentials

• A Simplified Classification

• Seal Mechanisms and Characteristics

• Seal Lithology and Trap Type Occurrence

surface

subsurface

SEAL

source rock

trap

2

SEALSEssentials:

• Need lithologies with extremely low permeability and high fluid entry pressure.

• Failure of reservoir seal due to inadequate caprock or late stage faulting is a common cause of exploration well failure.

• Sealing capacity of different lithologies is variable with salt and other evaporites considered to be the best (e.g. anhydrite “superseals” of the Middle East).

• The maximum possible hydrocarbon column height of an accumulation is dependant on the sealing capacity of the caprock. If exceeded, the trap will leak.

• Other factors controlling hydrocarbon column height (HtC) are depth of burial, tectonic activity, charging, capillary pressure of both reservoir and seal lithologies and reservoir quality linked to fluid type and saturation.

3

SEALSA Simplified Classification

Français CODE EnglishARGILE SS SHALESCARBONATE: Calcaire, Craie, Dolomie SC CARBONATES: Limestone, Chalk, DolomiteMARNE SM MARLHALITE sensu stricto SH HALITE sensu strictoEVAPORITE ou ANHYDRITE sense large SE EVAPORITES or ANHYDRITE sensu latoAUTRES: Basalte, Hydrate, Faille (mineralisation ou argile de faille), etc. SO OTHERS: Basalt, Hydrates, Fault (mineralisation or shale smear), etc.

HtC = Height of Hydrocarbon Column

Seal e.g.: Shales Single Reservoir

Multi Reservoir

HtC1HtC2

HtC3 HtC

6 PRINCIPAL TYPES OF SEAL

(no dynamic communication)

SS

SM

SO

4

SEALSSeal Mechanisms and Characteristics• Seal quality is dependant on the minimum pressure

required to move connate water from the pores or fractures to cause leakage.

• The HtC has an effect on the formation pressure related to the fluid type. Lighter fluids have steeper gradients.

Buri

al D

epth

Formation Pressure

HtC Gas

HtC Oil Lithostatic

gradient ~1 psi/ft

Oil gradient ~0.33psi/ft

Salt Water gradient ~0.45 psi/ft

Gas gradient ~0.2 psi/ft

HtC may become large enough for pressure to exceed confining lithostatic pressure and fracture caprock.

Gas in particular is a common cause of this leakage.

5

SEALSSeal Mechanisms and Characteristics

Diffusion of hydrocarbons through seal rocks occurs and depends on:

• Hydrocarbon type, i.e. oil or (the more mobile fluid) gas. Note that the pressure in a gas column rises faster than in oil due to the lower fluid density.

• Characteristics of the water filled pore network in the seal lithology.• The time available, as diffusion can be a slow process.

For Oil, diffusion is unlikely through a very good seal rock.

For Gas, diffusion is more likely to occur over time.

6

SEALSSeal Mechanisms and CharacteristicsSeal Longevity and Effectiveness• Long Term Seals – effective for oil over 100’s of millions of years

after charging. The “superseals” – distinguished by high ductility at high pressures (deformation without fracturing) over time. These are:

• Evaporites of Halite (NaCl) and Anhydrite with ~30% Gypsum (CaSO4.H20)• Shales, either rich in kerogen (“plastic” deep water origin) or overpressured

shales at great depths with high fluid content.

• Average Seals – effective for less than 100 million years after charging. Less ductile than “superseals” and fracture under pressure. These are:

• Carbonates, Marls, Mudstones, Tar Mats, Igneous Rocks.

• Poor Seals – Effective for less than 10 million years, during active charging and where rate of accumulation exceeds leakage. These seals show poor ductility and easily fracture at depth. These are:

• Pure Anhydrite (no Gypsum), Dolomite, Siltstone, Tight Sandstone, Chert, Limestone

7

SEALSSeal Mechanisms and CharacteristicsSeal Thickness and Burial Depth

•Greater thicknesses for average to poor seals enhances the effective sealing capacity.•Thick seals can heal fractures more easily than thin seals.•Most seals are more ductile at shallow depths and they become more brittle below 2000m burial depth.•Salt and overpressured shales remain effective seals below 4000m burial depth.•Higher geothermal gradients make shales less ductile at shallower depths. e.g. Indonesia >2000m

8

SEALSSeal Mechanisms and CharacteristicsSeal DistributionThe spatial distribution of reservoir and seal lithologies plays

a large part in the distribution of play types:• North Sea Chalk fields such as Ekofisk are only found in the SE of

the Central Graben, where the Paleogene turbidite reservoirs have shaled out.

SENW

Anticline over Salt Dome

Leakage into Paleogene Turbidite sands, trapped by structural closures updip.

• Arabian Gulf Late Jurassic and Early Cretaceous fields distribution is ~ mutually exclusive due to basinal extent of intervening Hith Anhydrite Seal…..

L Cret. Chalk

Paleogene Shales

9

SEALSSeal Mechanisms and CharacteristicsSeal DistributionArabian Gulf Late Jurassic and Early Cretaceous Stratigraphy and

hydrocarbon distribution:

10

SEALSSeal Mechanisms and CharacteristicsSeal DistributionArabian Gulf Late Jurassic and Early Cretaceous fields areal distribution

related to extent of Hith Anhydrite “Superseal”

11

SEALSSeal Mechanisms and CharacteristicsFault Seals

• Faults may be conduits or seals – even the same fault can change from seal to conduit and back to seal over geological time, due to variations in the pressure regime.

• This is particularly the case with gas. Gas Chimneys over fault zones are a common phenomenon on seismic profiles, as the confining pressure of the fault seal is broken by the pressure build up of the accumulating gas. After the pressure release, the fault can reseal and the system then recharge. There is commonly a residual bitumen coating left in the pores of the reservoir after this has occurred as evidence of the initial charge. e.g. Elgin field, Central NorthSea.

12

SEALSSeal Mechanisms and CharacteristicsFault Seals

• The same fault can act as a seal at shallow depths, but then be a conduit at greater depth due to increased fracturing

• The downthrown side of a fault is usually only filled to the spill point in the downthrown reservoir at the intersection with the fault.

13

SEALSSeal Mechanisms and CharacteristicsFault Seals• Fault seal, other than by simple offset against a sealing lithology,

is largely the result of fault gouge.• Compressive (reverse or thrust) and strike slip faults do tend to

create an impermeable zone along the fault plane due to intense abrasion.

• Extensional faults may also act as seals, if sufficient “gouge” is produced. This process is more effective in ductile regimes andinvolves the smearing of interbedded or adjacent shales across the fault intersection with the reservoir as movement along the fault occurs. Thus “Fault Gouge” or “Shale Smear” is not common in very high net to gross reservoirs.

• A “Shale Gouge Ratio” (SGR) can be estimated from the proportion of shales present in the section studied in relation to the fault throw. This can be used to predict the sealing potential for a prospect or for the transmissibility effects during field production. A SGR > 25% can be a flow baffle and an SGR > 40% can be a true seal.

14

SEALSSeal Mechanisms and CharacteristicsOther Seal Mechanisms• Hydrodynamic barriers – water influx pressure barriers.• Permeability barriers due to diagenesis or facies changes.• Tar Mats – frequently the result of polyphase fill/spill history with

accompanying water washing/biodegradation to produce immobile bitumen compounds.

• Basement rocks in thrust fault traps in mountain belts.• Gas Hydrates.

All of theses types of barrier are difficult to detect or successfully predict prior to drilling they are all high risk seals. But they certainly do exist.

Good luck in convincing management to drill a prospect with thistype of seal.

15

SEALSSeal Mechanisms and CharacteristicsSeal Depths for Giant Oil Fields

Note: values biased by drill depths in different areas

~ 50% seal depths from 1000 to 2000m

16

SEALSSeal Mechanisms and CharacteristicsSeal Depths for Giant Gas Fields

Note: values biased by drill depths in different areas

Similar depth spread to oil, but deep gas under-represented

17

SEALSSeal Mechanisms and CharacteristicsSeal Lithologies, Depths, Ages and Oil Reserves of 25 Giant Fields

~ Split 50/50 for shales : evaporites over top 25, but overall world wide split is 62% : 38%Mid East fields dominate evaporite seals

18

Seals, Source Rocks and Reservoirs

of the Middle East

19

SEALS Bu Hasa Field

Shale over limestone reservoir

20

SEALS Burgan Field

Shale over sandstone reservoir

21

SEALSSeal Mechanisms and CharacteristicsSeal Lithologies, Depths, Ages and Gas Reserves of 25 Giant Fields

~ Split 66% : 34% for shales : evaporites over top 25.All world giant gas fields with evaporite seals are found in fold belts

22

SEALSSeal Lithology and Trap TypesShales• Statistical importance in descending order: Anticlinal,

Stratigraphic, Fault Panels.• Statistics poorly represented for Unconformity and Mixed

Traps. No Diapiric Traps known.• Anticlinal Traps - HtC Gas usually > HtC Oil for same depth

range. Can have HtC Gas of 300m at 1000m burial depth and an upper range of 560m at 4000m. Upper range for HtC Oil usually < 150m regardless of burial depth.

• Stratigraphic Traps - Top, Bottom and Side Seals are essential. No major difference between HtC for Oil and Gas. Basinal facies show largest HtC values. Deltaic facies show smallest HtC values.

• Fault Panel Traps – HtC Gas usually > HtC Oil, upper range Gas 600m versus Oil 400m. Common for decreasing HtC Gas with depth due to trap leakage as shales become more brittle and liable to fracture.

23

SEALSSeal Lithology and Trap TypesEvaporites• Best lithology is Anhydrite with ~30% Gypsum for ductility and

best represented sealing continental to platform reservoirs, where evaporitic conditions easily follow on from reservoir deposition.

• Statistical importance in descending order: Anticlinal, Fault Panels, Stratigraphic.

• Statistics poorly represented for Unconformity and Mixed Traps. No Diapiric Traps known.

• Anticlinal Traps – HtC Gas usually > HtC Oil for same depth range. Gas upper range to 560m, Oil to 125m. Giant Mid East oil fields(e.g. L. Jurassic Arab Formation sealed by Hith Anhydrite) have HtC values greater than this range.

• Fault Panel Traps – HtC Gas usually greater than HtC Oil for same depth range. Gas HtC upper range to 280m.

• Stratigraphic Traps – Platform reservoirs are best represented with evaporite seals. Oil HtC upper range to 540m.

• Halite – the best “Superseal” for ductility, but too few examples documented to evaluate statistics properly for HtC range.

24

SEALSSeal Lithology and Trap TypesCarbonates• Statistical importance in descending order: Anticlinal,

Stratigraphic, Fault Panels.• Platform reservoir environments with carbonate seals are the

most common.• Statistics poorly represented for Unconformity and Mixed

Traps. Diapiric Traps do not occur.• Anticlinal Traps – HtC Gas usually greater than HtC Oil for

same depth range. Gas HtC upper range to 700m, Oil to 300m.

• Stratigraphic Traps – General decreasing trend of HtC with burial depth due to increased fracturing and leakage.

• Fault Panel Traps – Too few examples documented for valid statistical evaluation.

25

SEALSSeal Lithology and Trap TypesMarls• Statistical importance in descending order: Anticlinal,

Stratigraphic, Fault Panels.• Platform reservoir environments with marl seals are the most

common.• Statistics poorly represented for Unconformity and Mixed

Traps. Diapiric Traps do not occur.• Anticlinal Traps – HtC Gas and Oil show similar low ranges to

100m.• Stratigraphic Traps – HtC Gas slightly > HtC Oil, with upper

range to 245m.• Fault Panel Traps – Too few examples documented for valid

statistical evaluation.

26

SEALS