2nd Week Lecture:

ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS

INDUSTRY

Upstream of oil and gas include all those processes (geological, geophysical,

drilling and production engineering etc.) which are responsible to bring oil and gas

to the earth surface, from deeper parts of the earth. In such business petroleum

geologist plays a vital role and considered to be a group leader.

Petroleum geologists have yet no direct methods of locating a pool/ trap of

petroleum. They know no physical property of underground petroleum, which can be

measured at the earth surface. Their approach is indirect. Each pool / trap is unique.

For example the following Fig. 7 shows oil seepages along certain direction,

indicates that the traps are some where in the surrounding area. Petroleum

geologist who understands basic concepts and processes of different geological

disciplines (stratigraphy, paleontology, geologic history, sedimentalogy, structural

geology, fluid mechanics, petrology and geochemistry etc.) which are essential for

oil / gas discovery and production in the petroleum industry. Imagination is an

indispensible quality of the petroleum geologist. The world future supply of

petroleum is dependent upon:

- The imaginative powers of the petroleum geologist and; - The presence of favorable (reservoir and source) rocks.

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Figure 7 Typical structural (anticline - left and normal fault – right) related Traps

Petroleum geologists determine a prospect’s (deposit’s) feasibility (capability

/possibility) looking at the following main aspects /side:

Source- The source rock is where the oil was formed (if you accept the

organic theory). It is little porous and non-permeable, so it can not hold oil in

appreciable amounts. Reservoir- Oil migrates in to the porous rock unit like

sandstone or limestone that collects the hydrocarbons expelled from the

source rock and holds them inside a trap. It is possible for oil to move through

the reservoir rock all the way to the surface of the earth, if it is not sealed.

Seal- The rock unit that inhibits the oil or gas from escaping vertically out of

a hydrocarbon bearing reservoir rock.

Trap- Structural or stratigraphic feature that captures migrating

hydrocarbons into an economically producible accumulation.

Timing- Geologic events must occur in a certain order e.g. that the trap

formed before migration rather than after.

Maturation- The process of thermal maturing of a source rock, leading to

the cracking of its organic matter into oil and gas.

2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 2

Migration- The movement of the (less dense) oil or gas from the source rock

into a reservoir rock and then into a trap.

Exploration geologist, well site / drilling geologist and development geologist are

synonym names used for Petroleum geologist at different stages of exploration and

development of oil/ gas field, in petroleum industry. Petroleum geologist job never

ends, who work together with seismologist, surface / subsurface geophysicist.

Petroleum geologist correlates fragmented data derived from geology, petrophysics

and engineering disciplines. Information from such different fields is assembled

together (Figures 8 to 11 and tables 3 to 5) on various maps (structural,

stratigraphic, cross sections, facies, thickness, productivity and geothermal etc.)

and are fitted together in the mind of petroleum geologist, where it is interpreted

and translated into the best place to drill an exploratory well that will penetrate a

trap below the surface of the ground and thereby enable the well to test the

reservoir trap’s content.

All these maps and data do not tell themselves the whole story. In order to

use them in

petroleum discovery, they must be interpreted, correlated and integrated. Both

geologic and economic evaluation will justify the drilling of a wildcat well. After

exploring economic oil and

Figure 8 Layered rock sequence illustrating relative age and deposition of strata in horizontal layers

2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 3

Figure 9 Conceptual diagram for correlation of strata, in Pakistan

gas trap, petroleum geologist work

together with drilling, production

engineers and loggers (engineer or

geologist) in order to appraise, develop

and economically exploit the oil and gas

field. Petroleum geologist by his art of

imagination, based on his mental

concepts from fragmentary data of

different sources (e.g. structural and

sedimentary aspects), produce

conceptual subsurface picture/ map (Fig.

7) to identify undiscovered possible oil

and gas trap/ field. By careful mapping

Geologic Age

Age

(m.y.

ago)

% of

Field

s

Neogene

(Miocene –

Pliocene)

2 ?

22

22

63

63

145

145

210

18

Palaeogene

(Paleocene,

Eocene –

Oligocene)

21

Cretaceous 27

Jurassic 21

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petroleum geologist judges that the

rocks and structures are favorable to the

trapping of petroleum under a certain

area. He does not know whether that

area is oil or gas pool, until a discovery

well has been drilled. Drilling well

location is entirely on Petroleum

geologists decision. This is done by

locating prospects within a sedimentary

basin.

210

280

280

360

360

415

415

580

Permo-Triassic 6

Carboniferous 5

Devonian 1

Cambrian-Silurian 1

Total

100

Table 3 Distribution of world’s discovered oil and gas fields based on geologic age

Geothermal gradients and source rock maturity

Temperature Gradient

Temperature is generally a function of depth because of the earth’s natural

geothermal gradient. Normal heat flow within the earth’s crust produces a gradient

of approximately 1.5°F for each 100 feet of depth below the surface. The

temperatures required to produce crude oil occur between 5,000 (means 75o F) and

20,000 feet (=300 o F) of depth. Temperatures below 20,000 feet are generally too

high and only generate gas. Temperatures above 5,000 feet are not usually

sufficient enough to transform the material into crude oil. There are, of course,

exceptions to the rules. Geologic conditions such as volcanism and tectonics

(folding and faulting) can change or affect the temperature gradient.

Temperature plays the most important role in the transformation of organic

matter into hydrocarbons. Organic matter-time - temperature triangle is used as

one of the most significant

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Figure 10. Temperature Gradient i.e. Temperature vs. Depth

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exploration tools to determine the

hydrocarbon potential of any area.

According to Lopatin the rate of

hydrocarbon generation is linearly related

to time but exponentially to temperature

i.e. if a rock unit generate hydrocarbons at

130o C in 70

Temprature

range, in o C

Time frame,

million years

(m.y.)

130 70

140 35

150 17.5

Table 4 - Lapotin’s time and temperature analysis

million years, then at 140o C it will generate

hydrocarbons in 35 million years and at

150o C in 17.5 million years. This method is

known as Lapotin’s time and temperature

analysis, which has been very effectively

used to predict the presence of

hydrocarbons in a given area with known

geothermal gradients and burial history.

However, for the areas which have not gone

through major tectonic phases in the post

Hydrocarbons Temprature

range, in o C

Biogenic gas

only

69 - 82

Oil and gas

((the oil

window)

82 - 132

Gas

condensate

132 -163

Dry gas 163- 260

Barren > More than

260

Table 5 - Table showing existence of hydrocarbons in different temperature ranges.

geological history the following temperature ranges (table 5) are given for

generation of different types of hydrocarbons by various authors:

Pressure Gradient

Most pressure that effect rocks is due to the weight of overlying rocks and is called

overburden pressure. Overburden pressure is a function of depth and increases

one pound per square inch (PSI) for each foot of depth. At 3000 feet, for example,

the overburden pressure would be 3,000 pounds per square inch (psi).

Hydrocarbons evolve from an immature stage to oil generation, oil cracking (wet

gas stage), and finally to dry gas generation because of overburden pressure and

the associated increase in temperature (Fig. 11).

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Figure 11 Hydrocarbon formation as a function of burial depth of the source rock

NOTE: Good paper for Petroleum exploration

WAPLES, D.W., 1980. Time and temperature in petroleum formation: application of Lopatin’s method to petroleum exploration. AAPG Bull., 64,916-929.

Bringing oil and gas to earth surface is a complex task in petroleum industry

performed by petroleum geologist, geophysicist and petroleum engineer. All of

them are working side by side, starting from surface geophysical survey, geological

exploration, drilling, subsurface geophysical survey appraisal, development and

production of oil, gas or both. Different types of wells drilled in the industry and

phases involved are summarized below:

DIFFERENT WELLS TERMINOLOGY USED IN OIL INDUSTRY

Wildcat well – It is also known as exploratory well, drilled in the hope of

discovering a new pool (petroleum bearing area). Before such well, very little is

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known about subsurface geology of the area. If it hit petroleum reservoir then it is

called exploratory well, if not then will be either dry hole or wet well.

Discovery well – If the well traps a deposit of petroleum, it is called a discovery

well (discovery oil well or discovery gas well, according to which kind of petroleum

is found).

Dry hole – If the discovery well produces neither oil nor gas, but only water, it is

called a dry hole, or a wet well.

Test well – It is the well drilled, to know actual location and size of a petroleum

deposit / reservoir in the earth.

Confirmation well – The next well after a discovery confirms, such a well is called

confirmation well.

Development wells – Well drilled into the same reservoir after a discovery has

been made are called development wells.

Producer well – After successful exploration and development of oil / gas field,

producing wells, drilled into the reservoir, produce fluids (gas, oil or water), in order

to drain the field.

Injection well – A well in which fluids are injected rather than produced, the

primary objective typically being to maintain reservoir pressure. Two main types of

injection are common: gas and water.

Separated gas from production wells or possibly imported gas may be reinjected

into the upper gas section of the reservoir.

Water-injection wells are common offshore, where filtered and treated seawater is

injected into a lower water-bearing section of the reservoir.

DIFFERENT PHASES IN PETROLEUM INDUSTRY

Exploration The initial phase in petroleum operations that includes generation of a

prospect or play or both, and drilling of an exploration well. Appraisal, development

and production phases follow successful exploration.

Generation The formation of hydrocarbons from a source rock as bitumen forms

from kerogen and accumulates as oil or gas. Generation depends on three main

factors: the presence of organic matter rich enough to yield hydrocarbons,

adequate temperature, and sufficient time to bring the source rock to maturity.

Pressure and the presence of bacteria and catalysts also affect generation.

Generation is a critical phase in the development of a petroleum system.

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Prospect An area of exploration in which hydrocarbons have been predicted to

exist in economic quantity. A prospect is commonly an anomaly, such as a geologic

structure or a seismic amplitude anomaly, which is recommended by explorationists

for drilling a well. Justification for drilling a prospect is made by assembling

evidence for:

- an active petroleum system, or reasonable probability of encountering reservoir-

quality rock,

- a trap of sufficient size,

- adequate sealing rock, and

- appropriate conditions for generation and migration of hydrocarbons to fill the

trap.

A single drilling location is also called a prospect, but the term is more properly

used in the context of exploration. A group of prospects of a similar nature

constitutes a play.

Appraisal The phase of petroleum operations that immediately follows successful

exploratory drilling. During appraisal, delineation wells might be drilled to determine

the size of the oil or gas field and how to develop it most efficiently.

Development The phase of petroleum operations that occurs after exploration has

proven successful, and before full-scale production. The newly discovered oil or gas

field is assessed during an appraisal phase, a plan to fully and efficiently exploit it is

created, and additional wells (Development wells) are usually drilled.

Production The phase that occurs after successful exploration and development

and during which hydrocarbons are drained from an oil or gas field.

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