The Petroleum Industry: Reservoir Structure

 Reservoir Structure

Reservoir location and structure play an

important role in crude oil economics

some would even say the most important

role a reservoir is any porous and

permeable stratum that will store crude

oil a very common reservoir rock is a

porous or fractured limestone especially

of the reef biome type several such

reservoir structures are found

throughout the earth

most reservoir rocks are sedimentary

rocks almost always the coarser grained

of the sedimentary rocks sand sandstone

limestone and dolomite a less common

reservoir is a fractured shale or even

an igneous or metamorphic rock

it is only rarely that shale acts as a

reservoir Rock and again fractures and

other relatively wide openings are

believed to confer the required

reservoir properties on an otherwise

unsuitable Rock petroleum accumulations

are usually found in structural highs

and anticline where reservoir rocks of

suitable porosity and permeability are

covered by a dense relatively

impermeable cap rock such as an

evaporite or shale

a reservoir rock filled by a cap rock

into position of such a geological high

that is the entik line is known as a

structural petroleum trap strati-graphic

traps are also known and occur in

various fields in all cases changes in

permeability and porosity determine the

location of oil and or a gas

accumulation such accumulations may be

several miles in length structural traps

are formed by a deformation in the rock

layer that contains the hydrocarbons

anticlines domes and folds are common

structures fault related features also

may be classified as structural traps if

closure is present

structural traps are the easiest to

locate by surface and subsurface

geological and geophysical studies they

are the most numerous among traps and

have received a greater amount of

attention in the search for oil than all

other types of traps on the other hand

strati-graphic traps are formed when

other beds seal a reservoir bed or when

the phases change that is change in the

rock type within the reservoir bed

itself leading to a change in

permeability and stoppage of the flow of

fluids

the simplest form of the structural trap

is the anticline in the dome each of

which has a convex upper surface many

oil and gas accumulations are trapped in

anticlines or domes structures that are

generally more easily detected than some

other types of traps there are also

examples in which the reservoir Rock

wedges out at its upper end as an

original depositional feature due to

lateral variation in deposition or a

butts against an old land surface

strati-graphic trap

traps associated with salt intrusions

are of various kinds limestone reefs can

also serve as reservoir rocks and give

rise to overlying traps of antic lintel

form as a result of differential

compaction lastly examples are also

known in which the reservoir rock

extends to the surface of the earth but

oil and gas are sealed in by clogging of

the pores by by Tumen or by natural

cements many reservoirs display more

than one of the factors contributing to

the entrapment of hydrocarbons

the distinction between a structural

trap and a stratigraphic trap is often

difficult to define for example an

anticline 'el trap may be related to an

underlying buried limestone reef beds of

sandstone may wedge out against an

anticline because of depositional

variations or intermittent erosion

intervals salt domes formed by flow of

salt at substantial depths also have

created numerous traps that are both a

structural trap and a strati-graphic trap.

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