Introduction to Petroleum Geoscience
An Overview
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons during oil exploration. Petroleum geology is principally concerned with the evaluation of seven key elements in a sedimentary basin to obtain an idea of the subsurface and overall petroleum system.
* Source
* Reservoir
* Seal
* Trap
* Timing
* Maturation and
* Migration
We shall now consider these seven sources in a step by step manner.
1. Evaluation of the Source
This involves quantification and evaluation of the nature of organic-rich rocks so that the type
analysis.
Steps in Source Rock Analysis
* First establish likelihood of presence of organic-rich sediments deposited in the past on the basis of studies of local stratigraphy, paleogeography and sedimentology of the area.
* Identification and delineation of area of potential source rock.
* Determine the type of KEROGEN and state of its maturation.
* Calculation of thermal maturity and timing of maturation.
* Finally determine the likelihood of oil / gas generation in the area and calculate the depth of oil window.
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(Majority of oil generation occurs in the 60° to 120°C range. Gas generation starts at similar temperatures, but may continue up beyond this range, perhaps as high as 200°C.)
Geochemical Log
2. The Reservoir
It is a porous and permeable lithological unit or set of units that holds the hydrocarbon reserves. The common types include sandstone and limestone.
Steps in Analysis of Reservoirs
* First determine the type of lithofacies of the reservoir.
* Assessment of their porosity (to calculate the volume of in situ hydrocarbons) and permeability (to calculate how easily hydrocarbons will flow out of them).
* Study the Post depositional Diagenetic changes in reservoir.
* Establish depositional environment and geometry of the reservoirs.
Some of the key disciplines used in reservoir analysis are stratigraphy, sedimentology, reservoir engineering and the technique of Formation evaluation using wireline tools. Seismic attributes of subsurface rocks generated through seismic data processing are used to infer physical/sedimentary properties of the rocks.
Reservoir Rock
Reservoir Rocks
3. The Seal or Cap Rock
It is a unit with low permeability that impedes the escape of hydrocarbons from the reservoir rock. Common seals include evaporites, chalks and shale.
Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified.
4. The Trap
It is the stratigraphic or structural feature that ensures the juxtaposition of reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather than escaping and being lost.
The common types are Structural, Stratigraphic and Combination Traps.
A structural trap is where a fault has juxtaposed a porous and permeable reservoir against an impermeable seal. Oil (shown in red) accumulates against the seal, to the depth of the base of the seal. Any further oil migrating in from the source will escape to the surface and seep.
The Essay on Rock Joints
A joint is defined as a fracture in a rock between the sides of which there is no observable relative movement. They are present is most consolidated rocks of igneous, metamorphic and sedimentary origin. Joints may form as a result of either diastrophism or contraction. Description: (i) A series of parallel joints is called a 'joint set'. (ii) Two or more joint sets intersecting each other produce ...
Types of Traps
* Structural Trap: Here the trap has been produced by deformation of the beds after they were deposited, either by folding or faulting.
* Stratigraphic Trap: Here the trap is formed by changes in the nature of the rocks themselves, or in their layering, the only structural effect being a tilt to allow the oil to migrate through the reservoir.
* Combination Traps: Here the trap is formed partly by structural and partly by stratigraphic effects, but not entirely due to either.
* Hydrodynamic Traps: These Traps is due to water flowing through the reservoir and holding the oil in places where it would not otherwise be trapped.
Common Trap Types
5. Analysis of Maturation
It involves assessing the thermal history of the source rock in order to make predictions of the amount and timing of hydrocarbon generation and expulsion.
6. The Time and Nature of Migration
Finally, careful studies reveal information on how hydrocarbons move from source to reservoir and help quantify the source (or kitchen) of hydrocarbons in a particular area.
Origin of Petroleum Forms
* Petroleum is derived from the remains of living things which contains a material called kerogen.
* Before dead organic matter becomes petroleum with time, the kerogen matures into an assortment of hydrocarbon molecules of all sizes and weights.
* The lightest (small) hydrocarbon molecules waft away as natural gas, and the heavier ones make up an oily liquid.
* Petroleum source rocks are of terrestrial and marine origin.
* Terrestrial source rocks are deposited in lakes, delta and river basins having woody plant matter, algae etc.
* Marine source rocks contain dead planktons, algae, organic remains etc.
* In both the settings, the mixture is buried under conditions of no oxygen. The kerogen is classified as type I, II & III as per their origin and are capable of producing oil or gas or both.
* Under the anaerobic conditions, the kerogen is transformed into a flammable substance called bitumen by the action of heat and anaerobic microbes in the sediment and natural catalysts.
The Essay on Identification of Petroleum Hydrocarbons
Abstract: Unknown hydrocarbon K was identified by using purification, determination of boiling point by using the capillary tube technique, the calculation of density and obtained refractive index value from the Abbe-3L refractometer. The density was not useful due to an error of the machine but the boiling point and refractive index were used to confirm that the unidentified hydrocarbon K was 2,2 ...
* Most of the bitumen is eventually cooked into tarry asphalt releasing hydrocarbon molecules (as well as water and carbon dioxide) out of the source rock as it heats.
* Heavy oils form first, then light oils. As temperatures rise to and above 100° C, source rocks produce more gas.
* Being lighter than rocks, petroleum tends to rise upward through fractures and the pores of coarse sandstone beds.
* A small fraction of that leakage, perhaps 2% is preserved in large pools having an impermeable cap/seal over it.
Characteristics of Petroleum Reservoirs
* A reservoir is sponge-like rock with open space between its grains -porosity.
* The porosity may be primary or it might be secondary as groundwater dissolves pores in the rock or as minerals undergo alteration or may have formed due to tectonic activities.
* One source of porosity is the transformation of calcite to dolomite by fluids rich in magnesium, which takes up less space.
* Besides porosity, there must be high permeability i.e. the connectedness of pores that allows fluid to move easily through the reservoir rock.
* Permeability, porosity in combination with geologic structure is all of interest to petroleum geologists as it provides sites of hydrocarbon accumulation under favourable conditions.
