Everything about Petroleum Geology totally explained
Petroleum geology refers to the specific set of geological disciplines that are applied to the search for
hydrocarbons (
oil exploration).
Sedimentary basin analysis
Petroleum geology is principally concerned with the evaluation of seven key elements in
sedimentary basins:
- Source
- Reservoir
- Seal
- Trap
- Timing
- Maturation
- Migration
In general, all these elements must be assessed via a limited 'window' into the subsurface world, provided by one (or possibly more) exploration
wells. These wells present only a 1-dimensional segment through the Earth and the skill of inferring 3-dimensional characteristics from them is one of the most fundamental in petroleum geology. Recently, the availability of cheap and high quality 3D seismic data (from
reflection seismology) has greatly aided the accuracy of such interpretation. The following section discusses these elements in brief. For a more in-depth treatise, see the second half of this article below.
Evaluation of the
source uses the methods of
geochemistry to quantify the nature of organic-rich rocks which contain the precursors to hydrocarbons, such that the type and quality of expelled hydrocarbon can be assessed.
The
reservoir is a porous and permeable lithological unit or set of units that holds the hydrocarbon reserves. Analysis of reservoirs at the simplest level requires an assessment of their
porosity (to calculate the volume of
in situ hydrocarbons) and their
permeability (to calculate how easily hydrocarbons will flow out of them). Some of the key disciplines used in reservoir analysis are the fields of
stratigraphy,
sedimentology, and
reservoir engineering.
The
seal, or
cap rock, is a unit with low permeability that impedes the escape of hydrocarbons from the reservoir rock. Common seals include
evaporites,
chalks and
shales. Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified.
The
trap 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 (due to their natural
buoyancy) and being lost.
Analysis of
maturation involves assessing the thermal history of the source rock in order to make predictions of the amount and timing of hydrocarbon generation and expulsion.
Finally, careful studies of
migration reveal information on how hydrocarbons move from source to reservoir and help quantify the source (or
kitchen) of hydrocarbons in a particular area.
Major subdisciplines in petroleum geology
Several major subdisciplines exist in petroleum geology specifically to study the seven key elements discussed above.
Analysis of source rocks
In terms of source rock analysis, several facts need to be established. Firstly, the question of whether there actually
is any source rock in the area must be answered. Delineation and identification of potential source rocks depends on studies of the local
stratigraphy,
palaeogeography and
sedimentology to determine the likelihood of organic-rich sediments having been deposited in the past.
If the likelihood of there being a source rock is thought to be high, the next matter to address is the state of
thermal maturity of the source, and the timing of maturation. Maturation of source rocks (see
diagenesis and
fossil fuels) depends strongly on temperature, such that the 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. In order to determine the likelihood of oil/gas generation, therefore, the thermal history of the source rock must be calculated. This is performed with a combination of geochemical analysis of the source rock (to determine the type of
kerogens present and their maturation characteristics) and
basin modelling methods, such as
back-stripping, to model the
thermal gradient in the sedimentary column.
Analysis of reservoir
The existence of a reservoir rock (typically, sandstones and fractured limestones) is determined through a combination of regional studies (for example analysis of other wells in the area), stratigraphy and sedimentology (to quantify the pattern and extent of sedimentation) and seismic interpretation. Once a possible hydrocarbon reservoir is identified, the key physical characteristics of a reservoir that are of interest to a hydrocarbon explorationist are its porosity and permeability. Traditionally, these were determined through the study of hand specimens, contiguous parts of the reservoir that outcrop at the surface and by the technique of
formation evaluation using wireline tools passed down the well itself. Modern advances in
seismic data acquisition and processing have meant that
seismic attributes of subsurface rocks are readily available and can be used to infer physical/sedimentary properties of the rocks themselves.
Further Information
Get more info on 'Petroleum Geology'.
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