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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 (oil exploration).
Petroleum geology is principally concerned with the evaluation of seven key elements in sedimentary basins:
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 one-dimensional segment through the Earth, and the skill of inferring three-dimensional characteristics from them is one of the most fundamental in petroleum geology. Recently, the availability of inexpensive, high-quality 3D seismic data (from reflection seismology) and data from various electromagnetic geophysical techniques (such as magnetotellurics) 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 [1] 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). [2] 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 geological 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.
Several major subdisciplines exist in petroleum geology specifically to study the seven key elements discussed above.
The critical moment is the time of the generation, migration, and accumulation of most hydrocarbons in their primary traps. The migration and accumulation of hydrocarbons occur over a short period in relation to geologic time. These processes (generation, migration, and accumulation) occur near the end of a duration of a petroleum system. The duration being the time crucial elements of the petroleum system are being accumulated.
The critical moment is crucial since it is based on the burial history of the source rock when it is at maximum burial depth. This is when most of the hydrocarbons are generated. Approximately 50%-90% petroleum is made and expelled at this point. The next step is the hydrocarbons entering the oil window. The oil window has to do with the source rock being the appropriate maturity, and also being at the right depth for oil exploration. Geoscientists will be need this to gather stratigraphic data of the petroleum system for 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. [1]
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 (140 to 248 °F) range. Gas generation starts at similar temperatures, but may continue up beyond this range, perhaps as high as 200 °C (392 °F). [1] 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.
The mid-twentieth century was when scientists began to seriously study petroleum geochemistry. Geochemistry was originally utilized for surface prospecting for subsurface hydrocarbons. Today geochemistry serves the petroleum industry by helping seek out effective petroleum systems. The use of geochemistry is relatively cost-effective that allows geologists to assess reservoir-related issues. Once oil to source rock correlation is found, petroleum geologists will use this information to render a 3D model of the basin. Now they can assess the timing of generation, migration, and accumulation relative to the trap formation. This aids in the decision-making process on whether further exploration is necessary. Additionally, this can increase recoveries of the petroleum remaining in reservoirs that were initially deemed unrecoverable.
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 (presence and quality); burial history; maturation (timing and volumes); migration and focus; and potential regional seals and major reservoir units (that define carrier beds). 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.
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 (or 3D seismic) studies are shot, and old exploration data (seismic lines, well logs, reports) 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.
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 (viscosity, chemistry, API, carbon content, etc.) of the hydrocarbon and the nature of the reservoir (porosity, permeability, etc.). [2]
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 (without damaging the formation, within commercial favorable volumes, etc.). 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 (steam injection, pumps, etc.) is used to extract more hydrocarbons or to redevelop abandoned fields.
The existence of a reservoir rock (typically, sandstones and fractured limestones) is determined through a combination of regional studies (i.e. 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 bulk rock volume, net-to-gross ratio, porosity and permeability. [2]
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 (importantly, hydrocarbons and water) can occupy. The summation of these volumes (see STOIIP and GIIP) 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 (see e.g. Guerriero et al., 2009, 2011, in references below) 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.
Kerogen is solid, insoluble organic matter in sedimentary rocks. It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes. All the kerogen on earth is estimated to contain 1016 tons of carbon. This makes it the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold.
Hydrocarbon exploration is the search by petroleum geologists and geophysicists for deposits of hydrocarbons, particularly petroleum and natural gas, in the Earth's crust using petroleum geology.
Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety of scales. The essence of the method is mapping of strata based on identification of surfaces which are assumed to represent time lines, thereby placing stratigraphy in chronostratigraphic framework allowing understanding of the evolution of the earth's surface in a particular region through time. Sequence stratigraphy is a useful alternative to a purely lithostratigraphic approach, which emphasizes solely based on the compositional similarity of the lithology of rock units rather than time significance. Unconformities are particularly important in understanding geologic history because they represent erosional surfaces where there is a clear gap in the record. Conversely within a sequence the geologic record should be relatively continuous and complete record that is genetically related.
Petroleum geochemistry is the branch of geochemistry which deals with the application of chemical principles in the study of the origin, generation, migration, accumulation, and alteration of petroleum...(John M. Hunt, 1979). Petroleum is generally considered oil and natural gases having various compounds composed of primarily hydrogen and carbon. They are usually generated from the decomposition and/or thermal maturation of organic matter. The organic matter originated from plants and algae. The organic matter is deposited after the death of the plant in sediments, where after considerable time, heat, and pressure the compounds in the plants and algae are altered to oil, gas, and kerogen. Kerogen can be thought of as the remaining solid material of the plant. The sediment - usually clay and/or calcareous (lime) ooze, hardens during this alteration process into rock i.e. shale and/or limestone. The shale or limestone rock containing the organic matter is called the source rock because it is the source, having generated the petroleum.
The geology of the Falkland Islands is described in several publications. The Falkland Islands are located on a projection of the Patagonian continental shelf. In ancient geological time this shelf was part of Gondwana, which around 400 million years ago broke from what is now Africa and drifted westwards relative to Africa. Studies of the seabed surrounding the islands indicated the possibility of oil. Intensive exploration began in 1996, although there had been some earlier seismic surveys in the region.
A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen is created in surrounding rock by the presence of high heat and pressure in the Earth's crust.
Petrophysics is the study of physical and chemical rock properties and their interactions with fluids.
A petroleum geologist is an earth scientist who works in the field of petroleum geology, which involves all aspects of oil discovery and production. Petroleum geologists are usually linked to the actual discovery of oil and the identification of possible oil deposits, gas caps, or leads. It can be a very labor-intensive task involving several different fields of science and elaborate equipment. Petroleum geologists look at the structural and sedimentary aspects of the stratum/strata to identify possible oil traps or tight shale plays.
The Queen Charlotte Basin is a structural basin mostly beneath the continental shelf offshore, between Haida Gwaii, Vancouver Island, and the British Columbia mainland, roughly coincident with the physiographic region named the Hecate Depression.
Fred F. Meissner was an American geologist and engineer who contributed to the fields of geology, geophysics, engineering, petroleum engineering, geochemistry, mineralogy, physics, mining, economic geology, and fishing.
In petroleum geology, a trap is a geological structure affecting the reservoir rock and caprock of a petroleum system allowing the accumulation of hydrocarbons in a reservoir. Traps can be of two types: stratigraphic or structural. Structural traps are the most important type of trap as they represent the majority of the world's discovered petroleum resources.
In petroleum geology, source rock is rock which has generated hydrocarbons or which could generate hydrocarbons. Source rocks are one of the necessary elements of a working petroleum system. They are organic-rich sediments that may have been deposited in a variety of environments including deep water marine, lacustrine and deltaic. Oil shale can be regarded as an organic-rich but immature source rock from which little or no oil has been generated and expelled. Subsurface source rock mapping methodologies make it possible to identify likely zones of petroleum occurrence in sedimentary basins as well as shale gas plays.
Heavy oil production is a developing technology for extracting heavy oil in industrial quantities. Estimated reserves of heavy oil are over 6 trillion barrels, three times that of conventional oil and gas.
The Jeanne d'Arc Basin is an offshore sedimentary basin located about 340 kilometres to the basin centre, east-southeast of St. John's, Newfoundland and Labrador. This basin formed in response to the large scale plate tectonic forces that ripped apart the super-continent Pangea and also led to sea-floor spreading in the North Atlantic Ocean. This basin is one of a series of rift basins that are located on the broad, shallow promontory of continental crust known as the Grand Banks of Newfoundland off Canada's east coast. The basin was named after a purported 20 metres shoal labelled as "Ste. Jeanne d'Arc" on out-dated bathymetric charts and which was once thought to represent a local exposure of basement rocks similar to the Virgin Rocks.
The Brown Dense limestone is an informal name used by petroleum geologists for a layer of rock that lies beneath large parts of southern Arkansas and northwest Louisiana. The Brown Dense is a 300- to 500-foot thick interval within organic-rich, fine-grained carbonate rock that comprise the Lower Member of the Smackover Formation. Within this area, the Lower Smackover Member lies at depths of 8,000 to 10,000 feet beneath the land surface.
The Halibut Field is an oil field, within the Gippsland Basin offshore of the Australian state of Victoria. The oil field is located approximately 64 km offshore of southeastern Australia. The total area of this field is 26.9 km2 and is composed of 10 mappable units.
The Bolivar Coastal Fields (BCF), also known as the Bolivar Coastal Complex, is located on the eastern margin of Lake Maracaibo, Venezuela. Bolivar Coastal Field is the largest oil field in South America with its 6,000-7,000 wells and forest of related derricks, stretches thirty-five miles along the north-east coast of Lake Maracaibo. They form the largest oil field outside of the Middle East and contain mostly heavy oil with a gravity less than 22 degrees API. Also known as the Eastern Coast Fields, Bolivar Coastal Oil Field consists of Tía Juana, Lagunillas, Bachaquero, Ceuta, Motatán, Barua and Ambrosio. The Bolivar Coast field lies in the Maracaibo dry forests ecoregion, which has been severely damaged by farming and ranching as well as oil exploitation. The oil field still plays an important role in production from the nation with approximately 2.6 million barrels of oil a day. It is important to note that the oil and gas industry refers to the Bolivar Coastal Complex as a single oilfield, in spite of the fact that the oilfield consists of many sub-fields as stated above.
Oil and gas reserves denote discovered quantities of crude oil and natural gas that can be profitably produced/recovered from an approved development. Oil and gas reserves tied to approved operational plans filed on the day of reserves reporting are also sensitive to fluctuating global market pricing. The remaining resource estimates are likely sub-commercial and may still be under appraisal with the potential to be technically recoverable once commercially established. Natural gas is frequently associated with oil directly and gas reserves are commonly quoted in barrels of oil equivalent (BoE). Consequently, both oil and gas reserves, as well as resource estimates, follow the same reporting guidelines, and are referred to collectively hereinafter as oil & gas.
Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as clastic, igneous and carbonate rocks. Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to the existence of a fault zone. This is because different mechanisms that deform rocks can alter porosity and permeability within a fault zone. Fluids involved in a fault system generally are groundwater and hydrocarbons.
The Officer Basin is an intracratonic sedimentary basin that covers roughly 320,000 km2 along the border between southern and western Australia. Exploration for hydrocarbons in this basin has been sparse, but the geology has been examined for its potential as a hydrocarbon reservoir. This basin's extensive depositional history, with sedimentary thicknesses exceeding 6 km and spanning roughly 350 Ma during the Neoproterozoic, make it an ideal candidate for hydrocarbon production.