1.3 PETROLEUM SYSTEM

Petroleum: Petroleum is a complex mixture of naturally occurring hydrocarbon compounds found in rock and it can exist as solid, liquid and gaseous according to the pressure-temperature-composition, with or without impurities such as sulphur, oxygen and nitrogen; and there is considerable variation in its physicochemical properties like colour, gravity, odour, sulphur content and viscosity in petroleum from different areas.

Petroleum System: The geologic components and processes necessary to generate and store hydrocarbons, including a mature source rock, migration pathway, reservoir rock, trap and seal are collectively called the petroleum system. Appropriate relative timing of formation of these elements and the processes of generation, migration and accumulation are necessary for hydrocarbons to accumulate and be preserved. Exploration plays and prospects are typically developed in basins or regions in which a complete petroleum system has some likelihood of existing.

(Fig 2:Petroleum System)

In addition to these four basic components, a petroleum system by definition includes all the geologic processes required to create these elements. Crucial factors of proven (i.e., economic) petroleum systems include:

Organic richness/type and volume of generative source rock Adequate burial history to ensure proper time-temperature conditions for source

rock maturation Timing of maturation and expulsion in relation to timing of trap formation Presence of migration pathway linking source and reservoir rocks

Preservation of trapping conditions from time of entrapment to present day Relative efficiency of sealing layers

Petroleum systems may be identified according to three levels of certainty: known, hypothetical, and speculative (Magoon, 1988). In a known system, a good geochemical match exists between the source rock and accumulations; in the hypothetical case, a geochemical match is lacking but geochemical evidence is sufficient to identify the source rock. In the case of a speculative petroleum system, the presence of economic accumulations are lacking, but the existence of source rocks and oil/gas accumulations are postulated on the basis of geologic or geophysical evidence. Elements of petroleum system: The essential elements of a petroleum system include the following:

Source rock Reservoir rock Cap rock Trap Migration

Source Rock:1. Production , accumulation and preservation of organic matter are prerequisites

for the existence of petroleum source rocks.2. Photosynthesis is the basis for mass production of organic matter.About 2 billion

years ago in the Precambrian photosynthesis emerged asa world wide phenomenon.

3. Favourable conditions for the deposition of the sediments rich in organic matter are found on the continental shelves in the area of restricted circulation.continental slopes are also favourable for accumulation of organic matter

4. There are three major phasese in the evolution of organic matter from the time of deposition to the beginning of metamorphism.a) Diagenesis: this phase occurs in the shallow subsurface at near normal temperatures and pressures. It includes both biogenic decay, aided by bacteria, and abiogenic reactions. Methane, carbon dioxide and water and given off by the organic matter leaving a complex hydrocarbon termed Kerogenb) Catagenesis: this phase occurs in the deeper subsurface. Thermal degradation of the kerogen is responsible for the generation of most hydrocarbon i.e., oil and gas

c) Metagenesis; this third phase occurs at high temperatures and pressures verging on metamorphism . The last hydrocarbons , generally only methane are expelled.

5. The types of Kerogen present in a rock largely control the type of hydrocarbons generated in that rock. Different types of Kerogen contain different amounts of

hydrogen relative to carbon and oxygen. The hydrogen content of Kerogen is the controlling factor for oil vs. gas yields from the primary hydrocarbon-generating reactions.On the basis of chemical composition in the nature of organic matter the kerogen is classified into four basic types as:

Kerogen Type Predominant Hydrocarbon Potential

Amount of Hydrogen

Typical Depositional Environment

I Oil prone Abundant Lacustrine

II Oil and gas prone Moderate Marine

III Gas prone Small Terrestrial

IV Neither (primarily composed of vitrinite) or inert material

None Terrestrial(?)

(Table 3: Types of Kerogen)

a)Type-I Kerogen or saprophilic This is essentially algal origin.it has high hydrogen carbon ratio(H:C is about 1.2-1.7) b)Type-II Kerogen or Liptinic The organic matter of this type of kerogen consisted of algal detritus,but also contain material derived from zooplankton and phytoplankton.It has H:C ratio greater than 1. c)Type-III kerogen or humic This kerogen has a much lower H:C ratio(<0.84).Humic kerogen is produced from the lignin of the higher woody plants which grow on land.Type III sorce material is good for gas source.

5. Requirements for effective oil source systems:Oil sorce bed generation and expulsion performances are controlled by

a) Quantity of organic matter(OM): Minimum >= 0.5 wt% 0.4 wt% organic carbon)

b) Convertibilty of OM to oilA)Type-Amorphous,saprophilic,bituminous,alginate,exinite,cuticular.B)Quality-Relative high hydrogen, usually>7.0 wt%,minimal degradation:usually H/C>1.0

c) Thermal history favourable: Past-peak generation stage-83 to 84 wt% elemental carbon or higher.

d) Generating capability of system : Distribution , thickness, volume,richness,convertibility and thermal history satisfactory.

e) Critical release factor for oil.f) Expulsion mechanism efficiency-5% to 10%

Over pressuring capabilityClay size mineral

Source seals.

A source rock is a rock that is capable of generating or that has generated movable quantities of hydrocarbons. Typical source rocks, usually shales or limestone, contain about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Rocks of marine origin tend to be oil-prone, whereas terrestrial source rocks (such as coal) tend to be gas-prone. Source rocks can be grouped into four basic categories, which are described in the table-1. To be a source rock, a rock must have three features:

1. Quantity of organic matter2. Quality capable of yielding moveable hydrocarbons3. Thermal maturity.

(Table 1: Types of Source Rocks)

(Table 2: The most common methods used to determine the potential of a source rock.)

Generation: The most important factor in the generation of crude oil from the organic matter from the sedimentary rocks is temperature.A minimum temperatureof 1200 F(500

C) is necessary for the generation of oil under average sedimentary basin condition.The generation endsat 3500 F (1750 C).Time is also an important factor. The older the

sediments lower the temperature of generation.Younger sediments need higher temperature to generate oil than the average.Heavy oils are generated at the lower temperature where as the light oils are generated at high temperature.It takes millions of years to generate oil from organic matter.The youngest known source rock that has generated oil is Pliocene. At temperature higher than 3500 F crude oil is irreversibly transformed into graphite and natural gas.

Because the oil generation has a ceiling (1200 F) and a floor (3500 F), the depth in the earth where oil is generated is called the Oil Window.The type of organic matter in the source rock controls the type of petroleum generated.Woody organic matter generated crude oil.

Reservoir: A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids is a called a reservoir rock. Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and form under temperature conditions at which hydrocarbons can be preserved. The most significant property of reservoir rock is its effective permeability.Obviously, since sandstones are the best in permeability with respect to other rocks, they act as good reservoir rocks.

Cap rock: It is an impermeable rock-material to prevent further migration of hydrocarbons by buoyancy, and to seal petroleum within reservoir. Cap rocks are commonly of shale or of chemically precipitated evaporite deposits such as salt or gypsum, or biochemical alteration products of petroleum like tar.

Traps: Trap is a configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons cannot migrate. Traps are described as structural traps (in deformed strata such as folds and faults) or stratigraphic traps (in areas where rock types change, such as unconformities, pinch-outs and reefs) or their combinations. A trap is an essential component of a petroleum system. Petroleum migrates upwards and laterally from source to reservoir by buoyancy.

Being lighter than water, petroleum will displace groundwater and flow upwards, as well as laterally and will seep to the surface via faults and porous overburden unless confined under special circumstances to become trapped and to form economic petroleum deposits. Migration of petroleum is aided by its low surface tension, so that molecular attraction creates a film of water around grains, whereas the petroleum occupies the central pore spaces and is separated from the water.

Structural Traps: By juxtaposition of porous reservoir and impermeable cap rock due to folding or faulting, structural traps are created. So some tectonic or deformational mechanism (either brittle or ductile) are always involved (Figure 5). Approximately, 80 - 90% of the world's proven oil reserves are located in anticlinal traps. Anticlinal traps are commonly tens of kilometres long or even greater, and may be thousands of metres in

amplitude (e.g. Bombay high), or they may be combination of several small anticlines. Traps may be stacked vertically on top of each other where alternating reservoir and cap rocks have been folded in the same anticline. Fault traps are numerous, but only small. Faults can also be detrimental by breaching the seal of the cap rock and allowing the flow of petroleum through the fault to the surface, where it may form an oil seep.

(Fig 3(a))

(Fig 3(a) & 3(b): Schematic diagrams of structural traps)

Stratigraphic traps: By juxtaposition of porous reservoir and impermeable cap rock due to depositional variations in grain-size of different kinds of sediments stratigraphic traps are formed. This may be due to the thinning of lenses of sand and gravel (wedge-end traps), the morphology of carbonate reefs in sub-circular mounds (reef traps) or by the juxtaposition of rock types at unconformity surfaces (unconformity traps). Although unconformities are numerous, unconformity traps account for only 4% of world reserves, possibly because petroleum may have already escaped at the ancient surface prior to the formation of the unconformable beds. In Indian offshore region, especially, in the East Coast, most of the deep-water traps are stratigraphic traps like pinch-outs, unconformities etc. In Rudrasagar Oil Field of Assam is an example of stratigraphic trap, where petroleum exists in shoestring fluvial sandstone.

(Fig 4: Schematic diagrams of stratigraphic traps)

Combination situations: There are several combinations of situations. Rising salt-dome has stratigraphic traps draped against the edge with normal-fault trap caused by tension stress over the top. Some oil also accumulates in porous cap of salt-dome.In Assam Oil-field, there exists Naga Thrust upon which Tipam Sandstone terminates forming thrust propagation fold. This arrangement is a typical example of combination trap in India. Unfortunately, no salt-dome trap is yet known in India.

(Fig 5:Schematic diagrams of Combination Traps)

Fig. 7. Schematic diagram of salt plug or

Migration: Migration implies movement of hydrocarbon through rocks. There are two types of migration in a petroleum system as described below.

( Fig 6: Types of petroleum migration)

1. Primary Migration: Primary migration is the process by which hydrocarbons are expelled from the source rock into an adjacent permeable carrier bed. It is a paradoxical situation, because most source rocks are black shales, which have very low permeabilities.

2. Secondary Migration: Secondary migration is the movement of hydrocarbons along a "carrier bed" from the source area to the trap. Migration mostly takes place as one or more separate hydrocarbons phases (gas or liquid depending on pressure and temperature conditions). Main Driving force for migration is buoyancy. This force acts vertically and is proportional to the density difference between water and the hydrocarbon. So, it is stronger for gas than heavier oil.

. Examples of Different Kinds of Non-sandstone Reservoir Rocks in India:

Limestone with secondary porosity: Bombay High. Fractured shale: Indrora and Wadu Oil Field of Cambay Basin. Igneous rock: Fractured syenite of Borholla Oil Field of Assam, India.

Reserve Estimation:

,Where A is the area in km2, H the thickness in m, Φ the porosity, S0 the oil saturation, RF the recovery factor (the fraction of hydrocarbons, which can be or has been produced from a well, reservoir or field; also, the fluid that has been produced) and B0 is the reservoir formation volume factor.B0 may be of two types. It can be defined as follows:

Gas FVF: It is gas volume at reservoir conditions divided by gas volume at surface conditions. This factor is used to convert surface measured volumes to reservoir conditions, just as oil formation volume factors are used to convert surface measured oil volumes to reservoir volumes.Oil FVF: It is oil and dissolved gas volume at reservoir conditions divided by oil volume at standard conditions. Since most measurements of oil and gas production are made at the surface, and since the fluid flow takes place in the formation, volume factors are needed to convert measured surface volumes to reservoir conditions. Oil formation volume factors are almost always greater than 1.0 because the oil in the formation usually contains dissolved gas that comes out of solution in the wellbore with dropping pressure.

Accumulation:Once oil and gas migrates into the trap,it separates according to density.The gas,being lightest.goes to the top of the trap to from the free gas cap.The oil goes to the middle,and the water,which is always present,is on the bottom.The oil portion of the trap is saturated with a certain percentage of oil and water.The gas-oil and oil-water contacts are buoyantand are usually leveled.In some traps ,only gas and water are found.In other traps only oil and water are found.