sellos y trampas

Geologia del petróleo

Trampas (traps)Sellos (seals) y migración

(migration)

Cristian Vallejo, PhD

migration pathway

Hydrocarbon Accumulations: the ingredients

Hydrocarbon traps

• Any geometric arrangement of rock, regardless of origin, that permits significant accumulation of oil or gas, or both, in the subsurface

• Critical components– Reservoir– Seal– Geometric arrangement

Biddle and Wielchowsky, 1994

Key elements of traps

Structural trap

Stratigraphic trap

Nomenclature of a trap

Cross-section through a simple anticlinal trap

Crest or culmination

Selley, 1998

Fluid contacts

Selley, 1983

Sharp

Gradational

Abrupt fluid contact usually indicates a permeable reservoir. Gradational contacts usually indicate low permeability reservoirs with high capillary pressure

Classification of traps

• Structural traps– Fold Traps– Fault Traps

• Stratigraphic Traps• Hydrodynamic Traps• Combination Traps


Structural traps

Formed by postdepositional tectonic modification of the reservoir

Fold-dominated traps


Dip closure

A dome-shaped trap in which strata dip outwards in all directions (four-way dip closure) in which the integrity of the trap is determined by the top seal and any uncertainty in the mapped structural spill-point. The fold may be formed by compressional tectonics, by compaction and drape, or by hanging-wall deformation in extensional faulting.

Fault-dominated traps


Fault closure

A trap which requires a lateral fault seal. In fault enhanced dip closures, a part of the closure is dip-closed but a significant upside exists if the fault seals. If not, a large part of the trap may be unfilled, due to along-fault leakage of hydrocarbons.

Stratigraphic traps

Primary or depositional Stratigraphic trap

defined purely by the shape of a sedimentary body.


Stratigraphic traps

Unconformity related Biddle and Wielchowsky, 1994

Stratigraphic/structural trap

A trap defined by depositional geometries (pinch-out, truncation at unconformities) or lateral variations in diagenesis (cementation) together with structural dips. In addition to a top-seal and fault seal, lateral seals and a seat-seal may be required.

Stratigraphic traps

Secondary or diagenetic, trap defined purely by the shape of a diagenetic body.


Hydrodynamic traps

Hydrodynamic trap

Downward movement of water prevents upward movement of oil or gas


Trap limitations


Top seals

• Top seals prevent the vertical movement of hydrocarbons

• Any rock may act as seal as long as it is impermeable

• Seal may be porous• Fine-grained rocks which have much smaller

pore throat diameters compared with reservoir rocks

• Shales most common seals• Evaporites most effective seals

Water-wet seal acts as a capillary seal to hydrocarbons unless the buoyancy pressure exceeds the capillary entry pressure, at which point leakage occurs

Top seals

Pore throats in massive top seals are commonly so small that they may only leak by hydro-fracturing or by forming linked, permeable dilatant fractures during deformation.

Diffusing seals allow light hydrocarbons (eg. gas) to pass in solution through the pore fluid in the seal, due to a saturation gradient.

Permeable seals allow slow leakage to take place by Darcy flow

Top seals

Flow barriers

Top seals

Fault seals

• Faults which prohibit fluid flow– Geometric seals – Fault gouge seals

• Determine the trap volume, compartmentalisation and production behaviour in many fields

• Effectiveness of fault seals depends on the continuity and 3-D geometry of the sealing properties in the fault network (ie. whether faults are isolated or linked), and also on the fault zone width

Fault sealsGeometric seals • Depend on the

geometrical juxtaposition of sealing lithologies

• Are analysed in juxtaposition diagrams created with the fault slice technique.

Fault seals

Geometric (juxtaposition)

Sealing fault

Fault seals

• Fault gouge seals • Caused by mechanical or chemical (diagenetic) alteration of

fault rocks• fault sealing depends on the brittle deformation mechanism,

cataclasis• Clays may be squeezed by a viscous flow mechanism into

the fault gouge to form clay smears.

Migration• Primary migration

– Expulsion of the petroleum from the source rock• Secondary migration

– Movement of the hydrocarbons from the source rock to the trap

– Driving force vertical buoyancy force due to the lower density of petroleum compared to that of formation water

– Capillary pressure differences between oil and water opposes buoyancy force discouraging entry into smaller water wet pores

Primary migration

Hydrocarbons are expelled from the source rock along the kerogen network and microfractures

Droste, 1986

Secondary migration

Reservoir filling, petroleum moves generally updip following coarsest beds. Secondary migration is

relatively fast, 1 cm/1000y for siliciclastic sandstone.

England, 1994

Trap Types

Traps may have structural, stratigraphic or (rarely) diagenetic origins, and are classified according to geometric elements, expressed either in map or cross-sectional view. Four basic categories of traps are: dip closures; fault closures & structural truncation traps; stratigraphic/structural traps; and pure stratigraphic traps. Structural setting, or structural style, is an important component of trap classification and description because it affects:

the interplay of tectonics and sedimentation,

trap timing in relation to charge history,

structural episodes after trap formation, which may modify or breach the trap,

vertical relationships, eg. the stacking of hydrocarbon accumulations and

lateral relationships, eg. traps sharing common hydrocarbon-water contacts and cascading fill-and-spill relationships.

Definitions

Dip closure

A dome-shaped trap in which strata dip outwards in all directions (four-way dip closure) in which the integrity of the trap is determined by the top seal and any uncertainty in the mapped structural spill-point. The fold may be formed by compressional tectonics, by compaction and drape, or by hanging-wall deformation in extensional faulting.

Fault closure

A trap which requires a lateral fault seal. In fault enhanced dip closures, a part of the closure is dip-closed but a significant upside exists if the fault seals. If not, a large part of the trap may be unfilled, due to along-fault leakage of hydrocarbons.

Stratigraphic/structural trap

A trap defined by depositional geometries (pinch-out, truncation at unconformities) or lateral variations in diagenesis (cementation) together with structural dips. In addition to a top-seal and fault seal, lateral seals and a seat-seal may be required.

Stratigraphic trap

A trap defined purely by the shape of a sedimentary or diagenetic body.

Trap

A combination of structure, reservoir and seal which has the potential to retain hydrocarbons.

Truncation trap

A trap defined by structural dips and an updip lateral seal at an unconformity or against a salt flank.

Top Seals

Hydrocarbon traps may be formed by a top seal dip closure or a combination of dip- and fault-closures (see entries in Trap Types). Top seals prevent the vertical movement of hydrocarbons and are commonly formed from fine grained rocks, which have much smaller pore throat diameters compared with reservoir rocks. They may also be flow barriers, such as permeable seals which allow slow leakage to take place by Darcy flow, or diffusing seals which allow light hydrocarbons (eg. gas) to pass in solution through the pore fluid in the seal, due to a saturation gradient. A water-wet seal acts as a capillary seal to hydrocarbons unless the buoyancy pressure exceeds the capillary entry pressure, at which point leakage occurs by permeable, two-phase flow. The pore throats in massive top seals are commonly so small that they may only leak by hydro-fracturing or by forming linked, permeable dilatant fractures during deformation. Layered top seals may leak if sufficient small faults are present to form a tortuous fault-linked leak path due to juxtaposition of leaky layers.

Fault Seals

Fault seals, ie. faults which prohibit fluid flow, determine the trap volume, compartmentalisation and production behaviour in many fields. Geometric seals depend on the geometrical juxtaposition of sealing lithologies, and are analysed in juxtaposition diagrams created with the fault slice technique. Fault gouge seals are caused by mechanical or chemical (diagenetic) alteration of fault rocks. In sediments with low net to gross ratios, along steep syn-sedimentary faults, clays are squeezed by a viscous flow mechanism into the fault gouge to form clay smears. These can be predicted with the Clay Smear Potential (CSP) formula. A sealing/non-sealing CSP cut off can be established by calibrating the prediction with known hydrocarbon occurrences or pressure anomalies. In clean reservoir sandstones, fault sealing depends on the brittle deformation mechanism. The occurrence of particulate flow, cataclastic flow, or cataclasis depends on the matrix porosity and the effective confining pressure at the time of deformation. Of these, only cataclasis causes significant permeability reductions. The effectiveness of fault seals depends on the continuity and 3-D geometry of the sealing properties in the fault network (ie. whether faults are isolated or linked), and also on the fault zone width. Under certain circumstances faults can act as migration pathways, either via tortuous linked pathways, or along the fault itself.

sellos y trampas

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