Petroleum geology is the study of origin, occurrence, movement,
accumulation, and exploration of hydrocarbon fuels. It refers to the
specific set of geological disciplines that are applied to the search for
hydrocarbons. 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 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 inexpensive, high
quality 3D seismic data and data from various electromagnetic geophysical
techniques 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 and their permeability. Some of the key
disciplines used in reservoir analysis are the fields of structural analysis,
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 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 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. = Source rock analysis =
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 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
and basin modelling methods, such as back-stripping, to model the thermal
gradient in the sedimentary column. = Basin analysis =
A full scale basin analysis is usually carried out prior to defining leads and
prospects for future drilling. This study tackles the petroleum system and
studies source rock; burial history; maturation; migration and focus; and
potential regional seals and major reservoir units. All these elements are
used to investigate where potential hydrocarbons might migrate towards.
Traps and potential leads and prospects are then defined in the area that is
likely to have received hydrocarbons. = Exploration stage =
Although a basin analysis is usually part of the first study a company
conducts prior to moving into an area for future exploration, it is also
sometimes conducted during the exploration phase. Exploration geology
comprises all the activities and studies necessary for finding new hydrocarbon
occurrence. Usually seismic studies are shot, and old exploration data are used
to expand upon the new studies. Sometimes gravity and magnetic studies
are conducted, and oil seeps and spills are mapped to find potential areas for
hydrocarbon occurrences. As soon as a significant hydrocarbon occurrence is
found by an exploration- or wildcat-well the appraisal stage starts.
= Appraisal stage = The Appraisal stage is used to delineate
the extent of the discovery. Hydrocarbon reservoir properties, connectivity,
hydrocarbon type and gas-oil and oil-water contacts are determined to
calculate potential recoverable volumes. This is usually done by drilling more
appraisal wells around the initial exploration well. Production tests may
also give insight in reservoir pressures and connectivity. Geochemical and
petrophysical analysis gives information on the type of the hydrocarbon and the
nature of the reservoir. = Production stage =
After a hydrocarbon occurrence has been discovered and appraisal has indicated
it is a commercial find the production stage is initiated. This stage focuses
on extracting the hydrocarbons in a controlled way. Production wells are
drilled and completed in strategic positions. 3D seismic is usually
available by this stage to target wells precisely for optimal recovery.
Sometimes enhanced recovery is used to extract more hydrocarbons or to
redevelop abandoned fields. = Reservoir analysis =
The existence of a reservoir rock is determined through a combination of
regional studies, stratigraphy and sedimentology 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 bulk rock volume, net-to-gross ratio,
porosity and permeability. Bulk rock volume, or the gross rock
volume of rock above any hydrocarbon-water contact, is determined
by mapping and correlating sedimentary packages. The net-to-gross ratio,
typically estimated from analogues and wireline logs, is used to calculate the
proportion of the sedimentary packages that contains reservoir rocks. The bulk
rock volume multiplied by the net-to-gross ratio gives the net rock
volume of the reservoir. The net rock volume multiplied by porosity gives the
total hydrocarbon pore volume i.e. the volume within the sedimentary package
that fluids can occupy. The summation of these volumes for a given exploration
prospect will allow explorers and commercial analysts to determine whether
a prospect is financially viable. Traditionally, porosity and permeability
were determined through the study of drilling samples, analysis of cores
obtained from the wellbore, examination of 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. See also
Bituminous rocks Controlled source electro-magnetic
Important publications in petroleum geology
References V. Guerriero; et al.. "Improved
statistical multi-scale analysis of fractures in carbonate reservoir
analogues". Tectonophysics 504: 14–24. Bibcode:2011Tectp.504...14G.
doi:10.1016/j.tecto.2011.01.003. V. Guerriero; et al.. "Quantifying
uncertainties in multi-scale studies of fractured reservoir analogues:
Implemented statistical analysis of scan line data from carbonate rocks". Journal
of Structural Geology 32: 1271–1278. Bibcode:2010JSG....32.1271G.
doi:10.1016/j.jsg.2009.04.016. Further reading
Brian Frehner. Finding Oil: The Nature of Petroleum Geology, 1859-1920 232
pages External links
Petroleum Geology — A forum dedicated to all aspects of Petroleum Geology from
Exploration to Production Oil On My Shoes — Web site devoted to
the science and practical application of Petroleum Geology
AAPG — American Association of Petroleum Geologists
PetroleumGeology.org — Website about the history and technology of Petroleum
Geology