2 week - role of petroleum geologist in oil and gas industry
TRANSCRIPT
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.
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 1
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
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 4
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
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 5
Figure 10. Temperature Gradient i.e. Temperature vs. Depth
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 6
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).
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 7
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
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 8
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.
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 9
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.
2nd Week Lecture: ROLE OF PETROLEUM GEOLOGIST IN OIL AND GAS INDUSTRY Page # 10