petr 571 week4notes

20
PETR 571 Week 4--n Diag Trap: A g Seal: An i notes Hydro genetic, H geologic feature impermeable ro Highest poi Lowest poin Vertical dis Zone imme Zone adjace The produc Vertical dis Cumulative Trap may c OWC is the GOC or GW Gas always Gradational Trap must b Shales are t Evaporites ocarbon T Hydrodyna e in which petr ock medium th int of the trap i nt at which HC stance from cre diately beneath ent to the trap i ctive reservoir w stance from top e vertical thickn ontain oil, gas, e oil:water cont WC is the gas:o overlays oil (g l fluid contacts be overlain by the commonest are the most ef Traps: Stru amic, Astr roleum can acc hat isolates a re Morpholo is the crest, or C can be contai est to spill poin h the HC is the is the edge zon within the trap p of reservoir to ness that can b Fluid Distrib , or both tact (deepest le oil or gas:water gas is less dens s indicate a low Seals an effective se t seal ffective 1 uctural, St robleme cumulate eservoir and ret ogy/ Nomencla culmination ined in the trap nt/plane is the c e bottom wate ne is the pay o OWC is gros be produced is t bution Within t evel of produci r contact, respe se) w permeability and Cap Rock eal that prevent tratigraph and, of c tains petroleum ature p is the spill po closure r ss pay termed net pay the Trap ible oil) ectively (lowes with high capi ks ts the vertical m hic, Diapir course, Co m, i.e., shale, ev oint y st limit of prod illary pressure migration of th ric, mbination vaporite. ducible gas) he HC n

Upload: jhon-jairo-gonzalez

Post on 27-Oct-2015

60 views

Category:

Documents


2 download

TRANSCRIPT

PETR 571 Week 4--n 

 

Diag

Trap: A g Seal: An i

• • • • •

• • •

• • • • •

• • •

notes

Hydro

genetic, H

geologic feature

impermeable ro

Highest poiLowest poinVertical disZone immeZone adjace

The producVertical disCumulative

Trap may cOWC is theGOC or GWGas alwaysGradational

Trap must bShales are tEvaporites

ocarbon T

Hydrodyna

e in which petr

ock medium th

int of the trap int at which HCstance from crediately beneathent to the trap i

ctive reservoir wstance from tope vertical thickn

ontain oil, gas,e oil:water contWC is the gas:o overlays oil (gl fluid contacts

be overlain by the commonestare the most ef

Traps: Stru

amic, Astr

roleum can acc

hat isolates a re

Morpholo

is the crest, or C can be contaiest to spill poinh the HC is theis the edge zon

within the trap p of reservoir toness that can b

Fluid Distrib

, or both tact (deepest leoil or gas:watergas is less denss indicate a low

Seals

an effective set seal ffective

uctural, St

robleme …

cumulate

eservoir and ret

ogy/ Nomencla

culmination ined in the trapnt/plane is the ce bottom watene

is the pay o OWC is gros

be produced is t

bution Within t

evel of producir contact, respese)

w permeability

and Cap Rock

eal that prevent

tratigraph

… and, of c

tains petroleum

ature

p is the spill poclosure r

ss pay termed net pay

the Trap

ible oil) ectively (lowes

with high capi

ks

ts the vertical m

hic, Diapir

course, Co

m, i.e., shale, ev

oint

y

st limit of prod

illary pressure

migration of th

ric,

mbination

vaporite.

ducible gas)

he HC

n

PETR 571 Week 4--notes  

2  

Classification of Different Traps TYPE DESCRIPTION Structural: Fault and Fold related Stratigraphic: Due to change in lithology Diapiric: Viscous flow from depth Hydrodynamic: Due to flowing water

Astrobleme: Meteorite impact traps Trap (Structural): “…one whose upper boundary has been made concave, as viewed from below, by some local deformation,

such as folding, or faulting, or both, of the reservoir rock. The edges of a pool occurring in a structural trap are determined wholly, or in part, by the intersection of the underlying water table with the roof rock overlying the deformed reservoir rock.” (Levorsen, 1967)

Levorsen, A.I., Geology of Petroleum, 2 ed. Freeman, San Francisco, 1967.

• Formed by tectonic processes after the deposition of the beds involved • Folding and Faulting

Folding: Anticline: Structural bend in strata due to (usually) comressional stress in which the general shape is

concave down with flanks dipping in opposing directions. When seen in outcrop, the age of the beds will get younger with distance from the axial plane. Usually found adjacent to a syncline. If the axis of the fold dips it is referred to as a plunging anticline. The most abundant hydrocarbon trap. Usually found to be asymmetric with the crest migrating with depth. HC may move down in a stratigraphic sense.

Syncline: Conjugate fold to an anticline. Concave up with flanks dipping toward each other. When seen

in outcrop, the age of the beds get younger with distance to the axial plane.

PETR 571 Week 4--notes  

3  

• Compressional anticlines are due to compressive stress; folds are “pushed” upward

• Compactional anticlines are anticlinal-form folds that result from tensional stress; extension stress

results in fault block “sinkage” (horst/graben type topography); overlying draping sediments “sink” with down-dropped blocks whereas sediments overlaying upthrown blocks form the culminations of the anticlinal folds

• Rollover anticlines and drag folds are pseudo-anticlinal type folds that occur along a fault plane

(normal or reverse; recall anticlines are structurally defined as vertical-relief folds, so these do not occur with transform faults) due to the relative movement of the fault blocks; beds will “sink” into normal fault planes because of extension, and will “drag” along reverse fault planes due to friction

+ Productive drag folds are common in overthrust belts as in the Rocky Mountain region.

Overthrust belts are characterized by multiple adjacent reverse faults each with fault planes whose dips die out with depth (become more horizontal) and meet at a common fault (gliding) plane known as a decollement

• Salt anticline: see section on diapiric traps

PETR 571 Week 4--notes  

4  

• Domes: Circular concave down fold. Usually the result of sediments atop dome shaped basement rock or rising diapirs of clay or salt. Commonly produce multiply stacked reservoirs for obvious reason of their characteristic geometry.

PETR 571 Week 4--notes  

5  

PETR 571 Week 4--notes  

6  

PETR 571 Week 4--notes  

7  

PETR 571 Week 4--notes  

8  

PETR 571 Week 4--notes  

9  

PETR 571 Week 4--notes  

10  

Faulting:

• Rollover anticlines are directly associated with growth faults; growth faults are fault zones characterized by normal faulting (extensional stress) and numerous antithetic minor faults

• Dip-slip faulting (normal and reverse) both contribute to HC trapping provided the fault is

noncommunicative

• Noncommunicative faults do not permit the passage of fluids through the fault plane: can be due to either clay smearing along fault plane or sufficient offset on either side of fault to preclude continuous flow through common transmitting strata

• Noncommunicative faults are common in heavily faulted reservoirs and usually compartmentalizes the HCs into numerous individual traps

• Reservoir pressure differences across a fault plane will reveal whether or not the fault is communicative or not

PETR 571 Week 4--notes  

11  

PETR 571 Week 4--notes  

12  

Trap (Stratigraphic):

“…one in which the chief trap-making element is some variation in the stratigraphy, or lithology, or both, of the reservoir rock, such as a facies change, variable local porosity and permeability, or an up-structure termination of the reservoir rock, irrespective of the cause” (Levorsen, 1967)

• Formed by changes in lithology • Lithological variation may be depositional (e.g., channel, valley) or postdepositional (e.g., truncations,

diagenetic changes)

Channel traps:

• Sedimentary environment eolian, fluvial, or delta (Continental/Coastal) • Genetically related to crevasse-splay and mouth bar traps • Can also be submarine

Shoestring traps:

• Sedimentary environment is a barrier bar (Coastal) Pinchouts traps:

• Sedimentary environment typically eolian, also deep marine (as in a submarine fan pinchout) Reef traps:

• Reefs are carbonate buildups • Are arguably the most important stratigraphic trap • Geometry is usually a domal pinnacle with an elongated antiformal platform

Diagenetic traps:

• Oil or gas migrating up a permeable reservoir rock may be retarted upon reaching a cemented zone

• Oil of gas may trap in solution porosity within a cemented rock • Oil may degrade and oxidize by bacterial action as it migrates upward and comes into contact

with shallow zone meteoric waters

Unconformity traps: Three types:

• Angular unconformity traps: HC accumulation in strata beneath erosional surface topped with impermeable layer in an angular unconformity sequence

• Disconformity traps: HC accumulation in strata within a nonchronostratigraphic sequence which is immediately overlain with an impermeable layer parallel to it and marking the unconformity

• Nonconformity traps: HC accumulation in fractured basement rocks in unconformity with overlying sediments

• HC may accumulate above the unconformity or below the unconformity (supraunconformity

trap and subunconformity trap, respectively) depending on the stratigraphy

PETR 571 Week 4--notes  

13  

Trap (Diapiric):

• Traps resulting from the upward movement of mud or salt layers and doming of the overlying strata • May give rise to many types of traps (i.e., fault or fold) • Are differentiated from structural traps because due to local lithostatic movement rather than regional

tectonic forces • Clays and salts are less dense at depth than layers overlying them… • Behave viscously…rises and may intrude upper layers • Result in large plumes (diapirs): can be bulbous or mushroom shaped, which will affect the type of

traps along the associated with it • Same principle as a lava lamp

PETR 571 Week 4--notes  

14  

PETR 571 Week 4--notes  

15  

PETR 571 Week 4--notes  

16  

PETR 571 Week 4--notes  

17  

Trap (Hydrodynamic):

• Hydrodynamic movement of waters down permeable beds will trap oils flowing upward provided the hydrodynamic force of the water is greater than the force due to the buoyancy of the oil

• These traps are rare but have been recorded (q.v., Delaware Basin, West Texas, and Paris Basin, France)

• Only achieved with hydrodynamic flow; if no flow, pressures would be hydrostatic and oil would rise above the water (because it is less dense)

• Hydrodynamic flow is important in considering HC migration down structures • Usually observed in gentle flexures in monoclinally dipping beds which have closure but no vertical

relief Trap (Astrobleme):

• Meteorite impact traps • Oil found in highly deformed central uplift in large craters or in fractured and brecciated rim facies • Unfortunately Chixculub has not produced; …however, the Williston Basin (on Canadian/U.S. border)

and the Ames Field (Oklahoma) are producing fields widely agreed to be associated with meteorite impacts

• The Lyles Ranch Field in South Texas is also believed to be associated with a large meteorite impact

Further reading for the curiously inclined: LeVie, D.S., 1986. South Texas’ Lyles ranch field: Production from an astrobleme? Oil and Gas Journal,

pp135-138, Curtiss, David K. and Wavrek, David A., 1998. Hydrocarbons in meteorite impact structures: oil reserves

in the Ames feature.

Mazur, Michael J. and Stewart, Robert R., 1997. The seismic expression and hydrocarbon potential of meteorite impact craters: current research.

Conclusion:

Combination Traps: As with many classification schemes in the field of geology, the practitioner should keep in mind there will often exist combinations of clearly defined types. • Of the above traps anticlines are by far the most abundant hydrocarbon producer to date

• Examples of Combination Traps:

1) A channel rollover anticline trap, 2) Truncated tilted fault block unconformity trap, 3) Salt dome, growth fault trap

PETR 571 Week 4--notes  

18  

Glossary: Angular unconformity: An unconformity the erosional surface of which lies between nonparallel beds. Flat, impermeable younger beds in erosional contact with tilted, permeable, petroleum-bearing older beds form an angular unconformity trap. Anticline: An upwardly convex fold with limbs that dip away from the axial position. Antithetic fault: Conjugate, usually secondary faults, at approximately 60 degrees to a primary or synthetic fault. Apparent dip: Any dip measured at other than perpendicular to the strike of an inclined surface. Asymmetric fold: A fold with an inclined axial surface or nonidentical limbs. Axial plane: The planar axial surface of a fold. Axis (fold): A line which generates the form of a fold when moved parallel to itself in space. Block fault: A type of normal fault which separates the crust into fault-bounded blocks of different elevations. Closure: The vertical distance between the highest point on an anticline or dome and its lowest closed structural contour. Complex fault trap: A fault trap involving multiple faulting and or fault block tilting. Concentric fold: A parallel fold with constant orthogonal thickness of its folded layers. Deformation (strain): Changing shape by stress application: folding, shearing. Diapir: An anticlinal fold ridge or dome formed by the squeezing of shale, salt or other mobile material into the core of the feature. Dip: The departure normal to the strike in degrees of an inclined plane from the horizontal.

PETR 571 Week 4--notes  

19  

Dip slip: Fault movement parallel to the dip direction of the fault plane. Disconformity: An unconformity with sediments parallel to each other above and below the erosion surface which is not necessarily planar. Disconformity trap: A petroleum trap associated with a disconformity. Disharmonic fold: A nonparallel fold in which beds of varying competence demonstrate variable deformation intensity. Displacement: Relative movement of two sides of a fault. Dome: an anticlinal structure with all dips away from the apex. Downthrown block: The fault block which is displaced downward relative to the upthrown block. “Down-to-the-coast” fault: A fault type common in areas of rapid deposition where fault blocks are displaces downward in the direction of depositional transport. Drape fold: A fold in layered rocks produced by movement of underlying brittle rocks at high angles to the layers: a forced fold. Fault: A break in the Earth’s crust along which there has been movement. Faulted anticline: An anticline disrupted or altered by faulting. Fault trap: A fault-controlled petroleum accumulation. Fold: A bent or curved stratum, cleavage plane or foliation. Foot wall: The underside wall rock of a fault, inclined vein or ore body. Fracture (stress): Stress-induced breaks in rock material occurring in conjugate sets. Geosyncline: A large, regional linear or basin like depressed area in which thick volcanic and sedimentary rocks accumulate. Related to plate margins and regional tectonism. Hanging wall: The side or block above an inclined fault. Heave (fault): The horizontal displacement of a fault. High relief salt anticline: A salt-induced, salt-cored, large relief anticline with strongly draped, often faulted strata over its apex. Nonconformity: An unconformity featuring sediment resting upon an eroded crystalline surface. Nonconformity trap: A petroleum trap resulting from a nonconformity. Overthrust trap: A fault or fold trap lying above a thrust fault. Overturned fold: A fold with an axial plane depressed below horizontal. Permeability barrier: A barrier to transmission of fluid through a medium. Plunging fold: A fold with a non-horizontal hinge line.

PETR 571 Week 4--notes  

20  

Recumbent fold: A fold with a horizontal axial plane. Reef trap: A petroleum accumulation retained by reef permeability surrounded by adjacent rock impermeability. Salt dome: A circular or elliptical, positive salt-cored structure which vertically penetrates of deforms the surrounding sediments. Seal: An impermeable rock medium that isolates a reservoir and retains petroleum, i.e., shale, evaporite. Shear: A stress-induced displacement along a zone or plane of failure. Similar fold: A fold in which bed thickness is greater in the hinge than in the limbs. Slip: Displacement along a fault surface. Strain: Deformation resulting from stress. Stratigraphic trap: A depositional, non-deformed petroleum trap. Stress: Applied force or pressure resulting in strain. Strike: The horizontal line of intersection between a dipping surface and horizontal plane. Structural trap: A petroleum trap formed by deformation. Subsidence: Downward movement of the Earth’s surface. Subthrust trap: A petroleum trap beneath a thrust fault. Syncline: A downwardly convex fold with limbs that dip toward the axis. Synthetic fault: Minor faults of the same orientation and direction of displacement as associated major faults. Tar mat: Layer of heavy oil immediately above the bottom water that impedes the flow of water into the reservoir; are believed to have been produced the thermal degradation of oils and increased gas solution with associated precipitation of asphaltenes. Throw: The vertical displacement of a fault. Trap: A geologic feature in which petroleum can accumulate. Unconformity: An interruption of the geologic record manifest as an erosion surface bounded by rocks which are not immediately chronologically successive.