PETROLEUM GEOLOGYByDr. Muhammad Mujtaba : PETROLEUM GEOLOGYByDr. Muhammad Mujtaba Definition : Definition Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration). Major subdisciplines in petroleum geology : Major subdisciplines in petroleum geology Analysis of source rocks
Analysis of reservoir Slide 4: A structural trap, 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. Principal Concern of Petroleum Geology. : Principal Concern of Petroleum Geology. Petroleum geology is principally concerned with the evaluation of seven key elements in sedimentary basins Source
Migration Source : Source 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. Reservoir : Reservoir The reservoir is a porous and permeable lithological unit or set of units that holds the hydrocarbon reserves. Reservoir : Reservoir 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). Reservoir : Reservoir Some of the key disciplines used in reservoir analysis are the fields of stratigraphy, sedimentology, and reservoir engineering. Seal : Seal The seal, or cap rock, is a unit with low permeability that impedes the escape of hydrocarbons from the reservoir rock. Seal : Seal Common seals include evaporites, chalks and shales. Analysis of seals involves assessment of their thickness and extent, such that their effectiveness can be quantified. Trap : Trap 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. Maturation : Maturation 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. Migration : Migration 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 : 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 : 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. Analysis of source rocks : Analysis of source rocks 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. Analysis of source rocks : Analysis of source rocks 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. Analysis of source rocks : Analysis of source rocks 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 : Analysis of reservoir 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 porosity and permeability. Analysis of reservoir : Analysis of reservoir 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. Sedimentary basin : Sedimentary basin The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation.
As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification. Methods of Formation : Methods of Formation Lithospheric stretching
Lithospheric compression/shortening and flexure
Strike-slip deformation Sedimentary basin : Sedimentary basin It is common to categorise sedimentary basins according to the mechanism of formation:
tectonic compression (e.g., foreland basins, caused by lithospheric flexure)
tectonic extension (e.g., back-arc basins, caused by lithospheric stretching)
tectonic strike-slip (such as pull-apart basins). Lithospheric stretching : Lithospheric stretching If the lithosphere is caused to stretch horizontally, by mechanisms such as ridge-push or trench-pull, the effect is believed to be twofold.
The lower, hotter part of the lithosphere will "flow" slowly away from the main area being stretched, whilst the upper, cooler and more brittle crust will tend to fault (crack) and fracture. Lithospheric stretching : Lithospheric stretching The combined effect of these two mechanisms is for the earth's surface in the area of extension to subside, creating a geographical depression which is then often infilled with water and/or sediments.
(An analogy might be a piece of rubber, which thins in the middle when stretched.) Lithospheric stretching : Lithospheric stretching An example of a basin caused by lithospheric stretching is the North Sea - also an important location for significant hydrocarbon reserves.
Another such feature is the Basin and Range province which covers most of the USA state of Nevada, forming a series of horst and graben structures. Lithospheric stretching : Lithospheric stretching Another expression of lithospheric stretching results in the formation of ocean basins with central ridges;
The Red Sea is in fact an incipient ocean, in a plate tectonic context.
The mouth of the Red Sea is also a tectonic triple junction where the Indian Ocean Ridge, Red Sea Rift and East African Great Rift Valley meet. Ongoing development of sedimentary basins : Ongoing development of sedimentary basins As more and more sediment is deposited into the basin, the weight of all the newer sediment may cause the basin to subside further because of isostasy.
A basin can continue having sediment deposited into it, and continue to subside, for long periods of geological time; this can result in basins many kilometres in thickness.
Geologic faults can often occur around the edge of, and within, the basin, as a result of the ongoing slippage and subsidence. Study of sedimentary basins : Study of sedimentary basins The study of sedimentary basins as a specific entity in themselves is often referred to as basin modelling or Sedimentary Basin Analysis.
The need to understand the processes of basin formation and evolution are not restricted to the purely academic.
Indeed, sedimentary basins are the location for almost all of the world's hydrocarbon reserves and as such are the focus of intense commercial interest. Stratigraphy : Stratigraphy A branch of geology, studies rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks.
Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy. Lithostratigraphy or lithologic stratigraphy : Lithostratigraphy or lithologic stratigraphy It is the most obvious.
It deals with the physical lithologic, or rock type, change both vertically in layering or bedding of varying rock type and laterally reflecting changing environments of deposition, known as facies change.
Key elements of stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries mean in terms of depositional environment. Lithostratigraphy or lithologic stratigraphy : Lithostratigraphy or lithologic stratigraphy One of stratigraphy's basic concepts is codified in the Law of Superposition, which simply states that,
In an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. Biostratigraphy or paleontologic stratigraphy : Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers.
Strata from widespread locations containing the same fossil fauna and flora are correlatable in time.
Biologic stratigraphy was based on William Smith's principle of faunal succession, which was one of the first and most powerful lines of evidence for, biological evolution.
It provides strong evidence for formation (speciation) of and the extinction of species. Biostratigraphy or paleontologic stratigraphy : Biostratigraphy or paleontologic stratigraphy The geologic time scale was developed during the 1800s based on the evidence of biologic stratigraphy and faunal succession.
This timescale remained a relative scale until the development of radiometric dating, which based on an absolute time framework, leading to the development of chronostratigraphy.
One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from world-wide stratigraphic patterns. Biostratigraphy or paleontologic stratigraphy : Biostratigraphy or paleontologic stratigraphy Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals and traps in petroleum geology. Biostratigraphy or paleontologic stratigraphy : Biostratigraphy or paleontologic stratigraphy Chronostratigraphy is based upon deriving geochronological data for rock units, both directly and by inference, so that a sequence of time relative events of rocks within a region can be derived.
In essence, chronostratigraphy seeks to understand the geologic history of rocks and regions. Biostratigraphy or paleontologic stratigraphy : Biostratigraphy or paleontologic stratigraphy The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth. Palaeogeography : Palaeogeography It is the study of what the geography was in times past.
It is most often used about the past physical landscape.
In petroleum geology the term paleogeographic analysis is used for the detailed study of sedimentary basins, since the ancient geomorphological environments of the Earth's surface are preserved in the stratigraphic record.
Paleogeographers also study the sedimentary environment associated with fossils to Palaeogeography : Palaeogeography aid in the understanding of evolutionary development of extinct species.
The reconstruction of prehistoric continents and oceans depends on paleogeographic evidence.
Thus paleogeography provides critical evidence for the development of continental drift and current plate tectonic theories. Palaeogeography : Palaeogeography For example, knowledge of the shape and latitudinal location of supercontinents such as Pangaea and ancient oceans such as Panthalassa result from paleogeographic studies.
Sedimentology encompasses the study of modern sediments such as sand, mud (silt), and clay and the processes that result in their deposition. Sedimentology : Sedimentology Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary structures.
Sedimentary rocks cover most of the Earth's surface, record much of the Earth's history, and harbor the fossil record. Sedimentology : Sedimentology Sedimentology is closely linked to stratigraphy, the study of the physical and temporal relationships between rock layers or strata.
The premise that the processes affecting the earth today are the same as in the past is the basis for determining how sedimentary features in the rock record were formed.
By comparing similar features today to features in the rock record—for example, by comparing modern sand dunes to dunes preserved in ancient aeolian sandstones—geologists reconstruct past environments. Sedimentology : Sedimentology In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism.
These changes happen at relatively low temperatures and pressures and result in changes to the rock's original mineralogy and texture. Sedimentology : Sedimentology The boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure, is gradational.
After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Sedimentology : Sedimentology Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis.
Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization The role of diagenesis in hydrocarbon generation : The role of diagenesis in hydrocarbon generation When animal or plant matter is buried during sedimentation, the constituent organic molecules (lipids, proteins, carbohydrates and lignin-humic compounds) break down due to the increase in temperature and pressure.
This transformation occurs in the first few hundred meters of burial and results in the creation of two primary products: kerogens and bitumens. The role of diagenesis in hydrocarbon generation : The role of diagenesis in hydrocarbon generation It is generally accepted that hydrocarbons are formed by the thermal alteration of these kerogens (the biogenic theory).
In this way, given certain conditions (which are largely temperature-dependent) kerogens will break down to form hydrocarbons through a chemical process known as cracking, or catagenesis. Fossil fuels or mineral fuels : Fossil fuels or mineral fuels These are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms.
The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years.
These fuels contain high percentage of carbon and hydrocarbons. Fossil fuels or mineral fuels : Fossil fuels or mineral fuels Fossil fuels range from volatile materials with low carbon:hydrogen ratios like methane, to liquid petroleum to nonvolatile materials composed of almost pure carbon, like anthracite coal.
Methane can be found in hydrocarbon fields, alone, associated with oil, or in the form of methane clathrates. Fossil fuels or mineral fuels : Fossil fuels or mineral fuels It is generally accepted that they formed from the fossilized remains of dead plants and animals by exposure to heat and pressure in the Earth's crust over hundreds of millions of years.
This biogenic theory was first introduced by Georg Agricola in 1556 and later by Mikhail Lomonosov in the 18th century. Fossil fuels or mineral fuels : Fossil fuels or mineral fuels It was estimated by the Energy Information Administration that in 2006 primary sources of energy consisted of petroleum 36.8%, coal 26.6%, natural gas 22.9%, amounting to an 86% share for fossil fuels in primary energy production in the world.
Non-fossil sources included hydroelectric 6.3%, nuclear 6.0%, and (geothermal, solar, tide, wind, wood, waste) amounting 0.9 percent. World energy consumption was growing about 2.3% per year. Fossil fuels or mineral fuels : Fossil fuels or mineral fuels Fossil fuels are non-renewable resources because they take millions of years to form, and reserves are being depleted much faster than new ones are being formed.
The production and use of fossil fuels raise environmental concerns.
A global movement toward the generation of renewable energy is therefore under way to help meet increased energy needs. Fossil fuels or mineral fuels : Fossil fuels or mineral fuels The burning of fossil fuels produces around 21.3 billion tonnes (21.3 gigatonnes) of carbon dioxide per year, but it is estimated that natural processes can only absorb about half of that amount, so there is a net increase of 10.65 billion tonnes of atmospheric carbon dioxide per year (one tonne of atmospheric carbon is equivalent to 44/12 or 3.7 tonnes of carbon). Fossil fuels or mineral fuels : Fossil fuels or mineral fuels Carbon dioxide is one of the greenhouse gases that contributes to global warming, causing the average surface temperature of the Earth to rise in response, which climate scientists agree will cause major adverse effects. Characterization of source rock samples : Characterization of source rock samples To characterize the hydrocarbon potential of rock samples, a combination of methods for sample preparation and the determination of the type of organic material, its richness, maturity and genetic potential are required.
Two fractions of organic matter can be identified in sedimentary rocks: 1) kerogen and 2) bitumen. Kerogen : Kerogen It is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks.
It is insoluble in normal organic solvents because of the huge molecular weight (upwards of 1,000 Daltons) of its component compounds.
The soluble portion is known as bitumen.
When heated to the right temperatures in the Earth's crust, (oil window ca. 60°-120°C, gas window ca.120°-150°C) some types of kerogen release crude oil or natural gas, collectively known as hydrocarbons (fossil fuels). Kerogen : Kerogen When such kerogens are present in high concentration in rocks such as shale they form possible source rocks.
Shales rich in kerogens that have not been heated to a sufficient temperature to release their hydrocarbons may form oil shale deposits. Types of Kerogen : Types of Kerogen Kerogen Type-I initially has a high H/C and a low O/C atomic ratio. It mainly consists of algal lipids (e.g. fatty acids, wax, alcohol, oils) and of products of microbial activity. Type-I kerogen is typical of fresh water oil shales but also occur in marine sediments too.
Kerogen Type-II has a small O/C and a relatively high H/C atomic ratio. It is a mixture of all marine organisms (phyto and zoo plankton) plus normaly a certain Slide 60: percentage of land plants deposited in a reduding environment. Type-II kerogen is the most common oil source rock.
Kerogen Type-III has a very low H/C and initially the highest O/C atomic ratio and consists mainly of organic matter from terrestrial plant (vitrinite). It is the main natural gas source rock even at greater depths butonly a lean to moderate oil source rock. In the initial stage of maturity it generates abundant carbon dioxide. Slide 61: Residual Kerogen (Type-IV or IIIB) has the lowest H/C atomic ratio. It consists mostly of reworked and oxidized or weathered organic matter (inertinite) and has no potential, neither for oil nor for gas.
Note: In fact, however, most kerogens in nature are mixture of these four types. Basin modelling : Basin modelling Basin modelling is the term broadly applied to a group of geological disciplines that can be used to analyse the formation and evolution of sedimentary basins, often but not exclusively to aid evaluation of potential hydrocarbon reserves. Basin modelling : Basin modelling At its most basic, a basin modelling exercise must assess:
The burial history of the basin (back-stripping).
The thermal history of the basin (thermal history modelling).
The maturity history of the source rocks.
The expulsion, migration and trapping of hydrocarbons. Basin modelling : Basin modelling By doing so, valuable inferences can be made about such matters as hydrocarbon generation and timing, maturity of potential source rocks and migration paths of expelled hydrocarbons.
In petroleum exploration and development, formation evaluation is used to determine the ability of a borehole to produce petroleum.
Essentially, it is the process of "recognizing a commercial well when you drill one". formation evaluation : formation evaluation Modern rotary drilling usually uses a heavy mud as a lubricant and as a means of producing a confining pressure against the formation face in the borehole, preventing blowouts.
Only in rare, catastrophic cases and in Hollywood movies, do oil and gas wells come in with a fountain of gushing oil.
In real life, that is a blowout—and usually also a financial and environmental disaster. formation evaluation : formation evaluation But controlling blowouts has drawbacks—mud filtrate soaks into the formation around the borehole and a mud cake plasters the sides of the hole.
These factors obscure the possible presence of oil or gas in even very porous formations. formation evaluation : formation evaluation Further complicating the problem is the widespread occurrence of small amounts of petroleum in the rocks of many sedimentary provinces.
In fact, if a sedimentary province is absolutely barren of traces of petroleum, one is probably foolish to continue drilling there. formation evaluation : formation evaluation The formation evaluation problem is a matter of answering two questions:
What are the lower limits for porosity, permeability and upper limits for water saturation that permit profitable production from a particular formation or pay zone; in a particular geographic area; in a particular economic climate.
Do any of the formations in the well under consideration exceed these lower limits. formation evaluation : formation evaluation It is complicated by the impossibility of directly examining the formation. It is, in short, the problem of looking at the formation indirectly. GENERATION AND MIGRATION OF PETROLEUM : GENERATION AND MIGRATION OF PETROLEUM The precise details of petroleum generation and migration are still debatable. Recent advances in geochemistry, especially in analytical techniques, have resulted in rapid progress on this front, but many problems remain to be solved. FORMATION OF KEROGEN : FORMATION OF KEROGEN Generation and preservation of organic matter at the earth's surface is now well established, therefore, it will be now appropriate to consider what happens to this organic matter when buried in a steadily subsiding sedimentary basin.
As time passes, burial depth increases, exposing the sediment to increased temperature and pressure. FORMATION OF KEROGEN : FORMATION OF KEROGEN Tissot (1977) defined three major phases in the evolution of organic matter in response to burial:
Diagenesis. This phase occurs in the shallow subsurface at near normal temperatures and pressures. It includes both biogenic decay, aided by bacteria. Methane, carbon dioxide, and water are given off by the organic matter, leaving a complex hydrocarbon termed kerogen. The net result of the diagenesis of organic matter is the reduction of its oxygen content, leaving the hydrogen:carbon ratio largely unaltered. Maturation of Kerogen : Maturation of Kerogen Catagenesis. This phase occurs in the deeper subsurface as burial continues and temperature and pressure increase. Petroleum is released from kerogen during catagenesis, first in the form of oil and later gas. The hydrogen: carbon ratio declines, with no significant change in the oxygen:carbon ratio. Maturation of Kerogen : Maturation of Kerogen Metagenesis. This third phase occurs at high temperatures and pressures verging on metamorphism.
The last hydrocarbons, generally only methane, are expelled. The hydrogen:carbon ratio declines until only carbon is left in the form of graphite. Maturation of Kerogen : Maturation of Kerogen Establishing the level of maturation of kerogen in the source rocks of an area subject to petroleum exploration is vital. Maturation of Kerogen : Maturation of Kerogen When kerogen is immature, no petroleum has been generated; with increasing maturity, first oil and then gas are expelled; when the kerogen is overmature, neither oil nor gas remains.
The maturation of kerogen can be measured by several techniques.
The rate of maturation may be dependent on temperature, time, and, possibly, pressure. PETROLEUM MIGRATION : PETROLEUM MIGRATION There is a number of observational evidences that show that oil and gas do not generally originate in the rock in which they are found, but that they must have migrated into it from elsewhere.
This theory is proved by the following observations: PETROLEUM MIGRATION : PETROLEUM MIGRATION As previously discussed, organic matter is easily destroyed by oxidization in porous, permeable sediments at the earth's surface. It must therefore have invaded the reservoir rock after considerable burial and raised temperature. PETROLEUM MIGRATION : PETROLEUM MIGRATION Oil and gas often occur in solution pores and fractures that must have formed after the burial and lithification of the host rock. PETROLEUM MIGRATION : PETROLEUM MIGRATION Oil and gas are trapped in the highest point (structural culmination, or stratigraphic pinchout) of a permeable rock unit, which implies upward and lateral migration. PETROLEUM MIGRATION : PETROLEUM MIGRATION Oil, gas, and water occur in porous, permeable reservoir rock stratified according to their relative densities. This stratification implies that they were, and are, free to migrate vertically and laterally within the reservoir. PETROLEUM MIGRATION : PETROLEUM MIGRATION These observations demonstrate that hydrocarbons migrate into reservoir rocks at a considerable depth below the surface and some time after burial.
An important distinction is made between primary and secondary migration. PETROLEUM MIGRATION : PETROLEUM MIGRATION Primary migration is understood as the emigration of hydrocarbons from the source rock (clay or shale) into permeable carrier beds (generally sands or limestones).
Secondary migration refers to subsequent movement of oil and gas within permeable carrier beds and reservoirs. THE PETROLEUM SYSTEM : THE PETROLEUM SYSTEM Having established that commercial quantities of petroleum are of organic origin, and having discussed the primary migration of petroleum from the source rock into the carrier bed, let us now discuss the petroleum system, that is to say the integration of petroleum migration with the thermal and tectonic evolution of a sedimentary basin. THE PETROLEUM SYSTEM : THE PETROLEUM SYSTEM This involves consideration of the distance of secondary migration of petroleum, and the mathematical modelling of the time and amounts of petroleum that may have been generated within a given sedimentary basin. Measurement of the Distance of Petroleum Migration : Measurement of the Distance of Petroleum Migration The lateral distance to which petroleum can migrate has always been debated.
It is a difficult parameter to measure.
Traditionally, it is done by physically measuring the distance between the petroleum accumulation and the nearest mature source rock. Measurement of the Distance of Petroleum Migration : Measurement of the Distance of Petroleum Migration Where oil is trapped in sand lenses surrounded by shale, the migration distance must have been short.
Where oil occurs in traps with no obvious adjacent source rock, extensive lateral migration must have occurred. Measurement of the Distance of Petroleum Migration : Measurement of the Distance of Petroleum Migration Correlation between source rock and reservoir oil can be carried out by fingerprinting using gas chromatography (Bruce and Schmidt,1994). Measurement of the Distance of Petroleum Migration : Measurement of the Distance of Petroleum Migration Table 5.4 cites some documented examples of long-distance lateral petroleum migration.
The record for the longest distance of oil migration is held by the West Canadian basin, where a migration distance of more than 1000 km has been calculated (Garven, 1989). Measurement of the Distance of Petroleum Migration : Measurement of the Distance of Petroleum Migration A new geochemical method for calculating migration distances has been developed by Larter et al. (1996).
This is based on the regional variation of traces of nonalkylated benzocarbazoles.
The method is apparently effective, irrespective of the maturity of the oils. Some Published Accounts of Long-Distance Petroleum Migration. : Some Published Accounts of Long-Distance Petroleum Migration. Measurement of the Distance of Petroleum Migration : Measurement of the Distance of Petroleum Migration 'Previously based in geometric analysis, it is now possible to use geochemistry to measure migration distance.
Accurate estimates of the distance from the "devil's kitchen" to the petroleum trap is an essential part of basin modeling. The Petroleum System and Basin Modelling : The Petroleum System and Basin Modelling It is useful to be able to assess the amount of petroleum that has been generated in a sedimentary basin.
Such an assessment is obviously very difficult in a virgin area with no data. The Petroleum System and Basin Modelling : The Petroleum System and Basin Modelling In a mature petroleum province where a large quantity of data are available, it is considerably easier.
Knowledge of the quantity of reserves yet to be discovered is important for deciding whether continuing exploration is worth the expense if only small reserves remain to be found. The Petroleum System and Basin Modelling : The Petroleum System and Basin Modelling The volume of oil generated in an area may be calculated using the geochemical material balance method (White and Gehman, 1979). The basic equation may be expressed as follows : The basic equation may be expressed as follows Volume of oil generated = Basin area X
Average total thickness of source rock x Transformation ratio The Petroleum System and Basin Modeling : The Petroleum System and Basin Modeling The volume of source rock can be calculated from isopach maps.
The average amount of organic matter must be estimated from the geochemical analysis of cores and cuttings, extrapolating from wireline logs where possible. The Petroleum System and Basin Modeling : The Petroleum System and Basin Modeling The genetic potential of a formation is the amount of petroleum that the kerogen can generate (Tissot and Welte, 1978).
The transformation ratio is the ratio of petroleum actually formed to the genetic potential, and, as described earlier, these values are determined from the pyrolysis of source rock samples. The Petroleum System and Basin Modeling : The Petroleum System and Basin Modeling The concept of the petroleum system is an old one, though it has become very fashionable now.
Because it facilitates the modeling of sedimentary basins as a means of finding out how much petroleum they may have generated and where it may be located. The Petroleum System and Basin Modeling : The Petroleum System and Basin Modeling The petroleum system integrates the sedimentary and structural history of a basin with its petroleum characteristics,in terms of the richness, volume, and maturity of source rocks. Economics of increasing significance : Economics of increasing significance Sedimentary Basin, Petroleum System, Play, Prospect.
The concept of the petroleum system can be usefully applied to the computer modelling of sedimentary basins. Slide 103: Basin modelling may take place in one, two, or three dimensions. Basin modelling is done by computer. Several software packages are available.
One-dimensional modelling involves no more than the construction of a burial history curve for a particular point in a basin, such as a well location. Slide 104: This may be used to establish the maturity of a source rock interval, either using a modern geothermal gradient data (if the well has been drilled) or developing a geothermal history based on the tectonic regime of the location. Slide 105: A two-dimensional model consists of a cross-section. This may be constructed by "backstripping" the geological history, based on a seismic section, calibrated with well data, if available.
To do this, we need to estimate the depositional depth and compaction history of the sedimentary sequence. Then a geohistory scenario can be plugged in. This can then be used to establish the pressure system of the section and the migration and entrapment history of petroleum. Slide 106: A three-dimensional model involves the same operations as just described; however, not for a cross-section, but for a volume of rock. An example of 3D modelling in the Gulf of Mexico basin has been published by Anderson et al. (1991). Hydrocarbon Generation and Migration: Summary : Hydrocarbon Generation and Migration: Summary Understanding the primary migration of hydrocarbons is one of the last problems of petroleum geology. Research in this field is currently very active. A summary of this complex topic is given below:
Commercial quantities of oil and gas form from the metamorphism of organic matter. Slide 108: Kerogen, a solid hydrocarbon disseminated in many shales, is formed from buried organic detritus and is capable of generating oil and gas.
Three types of kerogen are identifiable: type I (algal), type II (liptinitic), and type III (humic). Type I tends to generate oil; type III, gas.
The maturation of kerogen is a function of temperature and, to a lesser extent, time. Oil generation occurs between 60 and 120°C, and gas generation between 120 and 225°C. Slide 109: Source rocks generally contain more than 1500 ppm organic carbon, but yield only a small percentage of their contained hydrocarbon.
Several techniques may be used to measure the maturity of a source rock.
The exact process of primary migration, whereby oil and gas migrate from source beds, is unclear. Slide 110: An empirical relationship between oil occurrence and clay dehydration suggests that the flushing of water from compacting clays plays an important role in primary migration. THE RESERVOIR : THE RESERVOIR As described earlier, one of the five essential prerequisites for a commercial accumulation of hydrocarbons is the existence of a reservoir.
Theoretically, any rock may act as a reservoir for oil or gas. In practice, the sandstones and carbonates contain the major known reserves, although fields do occur in shales and diverse igneous and metamorphic rocks. Slide 112: For a rock to act as a reservoir it must possess two essential properties: It must have pores to contain the oil or gas, and the pores must be connected to allow the movement of fluids; in other words, the rock must have permeability. POROSITY : POROSITY Porosity is the first of the two essential attributes of a reservoir. The pore spaces, or voids, within a rock are generally filled with connate water, but contain oil or gas within a field. Porosity is either expressed as the void ratio, which is the ratio of voids to solid rock, or, more frequently, as a percentage:
Porosity (%) = volume of voids x 100
total volume of rock PERMEABILITY : PERMEABILITY The second essential requirement for a reservoir rock is permeability. Porosity alone is not enough; the pores must be connected.
Permeability is the ability of fluids to pass through a porous material.
Average permeabilities in reservoirs are commonly in the range of 5 to 500 milli darcy (md). Permeability is generally referred to by the letter K. RELATIONSHIP BETWEEN POROSITY, PERMEABILITY, AND TEXTURE : RELATIONSHIP BETWEEN POROSITY, PERMEABILITY, AND TEXTURE The texture of a sediment is closely correlated with its porosity and permeability. The texture of a reservoir rock is related to the original depositional fabric of the sediment, which is modified by subsequent diagenesis.
The effects of diagenesis on sandstone reservoirs include the destruction of porosity by compaction and cementation, and the enhancement of porosity by solution. Slide 116: Cementation reduces the porosity and permeability of a sand. In some cases, however, solution of cement or grains can reverse this trend.
It generally involves the leaching of carbonate cements and grains, including calcite, dolomite, siderite, shell debris, and unstable detrital minerals, especially feldspar.
As reservoirs, carbonates are as important as sandstones, but their development and production present geologists and engineers with a different set of problems. Slide 117: Some 90% of the world's oil and gas occur in sandstone or carbonate reservoirs. The remaining 10% occur in what may therefore be termed atypical reservoirs, which range from various types of basement to fractured shale. RESERVOIR CHARACTERIZATION : RESERVOIR CHARACTERIZATION Once an accumulation of petroleum has been discovered it is essential to characterize the reservoir as accurately as possible in order to calculate the reserves and to determine the most effective way of recovering as much of the petroleum as economically as possible.
Reservoir characterization first involves the integration of a vast amount of data from seismic surveys, from geophysical well logs, and from geological samples. Slide 119: The first aim of reservoir characterization is to produce a geological model that honours the available data and can be used to predict the distribution of porosity, permeability, and fluids throughout the field.
Reservoirs possess a wide range of degrees of geometric complexity.
Geologists apply their knowledge to produce a predictive model for the layer-cake model with ease, and the jigsaw puzzle variety with some difficulty. RESERVE CALCULATIONS : RESERVE CALCULATIONS Estimates of possible reserves in a new oil or gas field can be made before a trap is even drilled. The figures used are only approximations, but they may give some indication of the economic viability of the prospect.
As a proven field is developed and produced, its reserves are known with greater and greater accuracy until they are finally depleted. Slide 121: The calculation of reserves is more properly the task of the petroleum reservoir engineer, but since this task is based on geological data, it deserves consideration here.
Several methods are used to estimate reserves, ranging from crude approximations made before a trap is tested to more sophisticated calculations as hard data become available. Artificial Lift and Enhanced Recovery : Artificial Lift and Enhanced Recovery Not all fields produce by natural drive mechanisms, and even these natural drive mechanisms do not recover all the oil. Artificial methods are used to produce oil from fields lacking natural drive, and enhanced recovery methods increase the recoverable reserves. TRAP : TRAP A trap is one of the five essential requisites for a commercial accumulation of oil or gas.
Levorsen (1967) gave a concise definition of a trap as "the place where oil and gas are barred from further movement."
Explorationists in general and geophysicists in particular search for hydrocarbon traps. Perhaps it would be more accurate to say that they search for potential traps. Slide 124: Only after drilling and testing it is known whether the trap contains oil or gas. In other words a trap is still a trap whether it is barren or productive. Traps and Sealing Rock : Traps and Sealing Rock Hydrocarbon migrate through drains before accumulating in the reservoir, where they will accumulate and concentrate only if there is a trap.
If a trap is to form, there must first of all be a sealing rock, which is an impermeable envelope on the top of the reservoir, preventing further migration towards the surface. Slide 126: In this rock, the pore entrance pressure has to be greater than the buoyancy pressure on the hydrocarbons, circulating in the reservoir.
The most common seals are fine-grianed rock (clay and silty clay), evaporites (halite and anhydrite) and more rarely carbonates (lime mudstone).
The effectiveness of the seal varies with the type of rock. It is very good as their plasticity limits the fracturing effects. Slide 127: Types of Traps
Two major genetic groups of traps are generally agreed on: structural and stratigraphic. A third group, combination traps, is caused by a combination of processes. STRUCTURAL TRAPS : STRUCTURAL TRAPS As previously stated, the geometry of structural traps is formed by postdepositional tectonic modification of the reservoir.
There are two classification of structural traps: 1) those caused by folding and 2) those caused by faulting. Anticlinal Traps : Anticlinal Traps Anticlinal, or fold, traps may be subdivided into two classes: compressional anticlines (caused by crustal shortening) and compactional anticlines (developed in response to crustal tension). Compressional Anticlines : Compressional Anticlines Anticlinal traps caused by compression are most likely to be found in, or adjacent to, subductive troughs, where there is a net shortening of the earth's crust.
Thus fields in such traps are found within, and adjacent to, mountain chains in many parts of the world. Slide 131: One of the best known oil provinces with production from compressional anticlines occurs in Iran. Here, in the foothills of the Zagros Mountains, many such fields occur.
Sixteen of these fields are in the "giant" category, with reserves of more than 500 million barrels of recoverable oil or 3.5 trillion cubic feet of recoverable gas. Compactional Anticlines : Compactional Anticlines A second major group of anticlinal traps is formed not by compression but by crustal tension.
Where crustal tension causes a sedimentary basin to form, the floor is commonly split into a mosaic of basement horsts and grabens.
The initial phase of deposition infills this irregular topography. Slide 133: Throughout the history of the basin, the initial structural architecture usually persists, controlling subsequent sedimentation.
Thus anticlines may occur in the sediment cover above deep-seated horsts. Closure may be enhanced both by compaction and sedimentation.
Good examples of oil fields trapped in compactional anticlines occur in the North Sea. Here, Paleocene deep-sea sands are draped over Mesozoic horsts Fault and Fault-Related Traps : Fault and Fault-Related Traps Faulting plays an indirect but essential role in the entrapment of many fields.
Relatively few discovered fields are caused solely by faulting.
A very important question in both exploration and development is whether a fault acts as a barrier to fluid movement (not only hydrocarbons but also water, which may be necessary to drive production) or whether it is permeable. Slide 135: The problem is that some faults seal, others do not.
A few guidelines are available, but they are by no means foolproof.
Where the throw of the fault is less than the thickness of the reservoir, it is unlikely to seal.
. Faults in brittle rocks are less likely to seal than those in plastic rocks. Slide 136: In lithified rocks faults may be accompanied by extensive fracturing, which may be permeable; indeed, some fields are caused solely by fracture porosity adjacent to a fault.
Sometimes, however, fractures may have undergone later cementation. DIAPIRIC TRAPS : DIAPIRIC TRAPS Diapiric traps are produced by the upward movement of sediments that are less dense than those overlying them.
In this situation the sediments tend to move upward diapirically and, in so doing, may form diverse hydrocarbon traps.
Such traps cannot be regarded as true structural traps, since tectonic forces are not required to initiate them (although in some cases they may do so). Slide 138: Similarly, diapirically related traps are not initiated by stratigraphic processes, although in some cases they may be caused by depositional changes across the structure.
Diapiric traps are generally caused by the upward movement of salt or, less frequently, overpressured clay. OIL RECOVERY : OIL RECOVERY Primary recovery, using only the natural
energy of reservoirs, typically recovers up to 50% of OOIP (average 19%).
Secondary recovery involves adding energy
to the natural system by injecting water to
maintain pressure and displace oil (also known
as waterflood). Typical recoveries are 25-45%
OIP after primary recovery (average 32%). Slide 140: Tertiary recovery includes all other methods used to increase the amount of oil recovered. Typical recoveries are 5-20% of OIP after primary and secondary recovery (average 13%).
Secondary and tertiary recovery are together referred to as enhanced oil recovery (EOR). Different Types of Reservoirs : Different Types of Reservoirs Reservoirs are usually classified in two broad categoties:
Terrigenous and calcareous.
Among terrigenous (sand and sandstone) reservoirs, reservoir quality will depend on the particle size grading and shaliness, which affect the permeability considerably. Slide 142: The sand-shale ratiois often used to define reservoir quality.
Geological history gives examples of reservoirs from Cambrian to Neogene and in different types of environments (continental, litoral and deep ocean).
Sand and sandstone account for about 60% of reservoir rock discories world wide.
The internal architecture of siliciclastic reservoirs is determined by the depositional system. Slide 143: Eolian and fluvial deposits can make very good reservoirs, however, the most attractive reservoirs are surely the delta and coastal sands.
Deep gravity deposits should also be considered as reservoirs.
Calcareous reservoirs are also extremely varied, with lacustrine limestone, platform deposits, chalk, dolomite and reef constructions. Slide 144: Their primary characteristics are very important, for example in reefs, but still diagenesis plays a considerable role in determining both the porosity and permeability.
Many calcareous reservoirs have good petrophysical characteristics only if they have sufficient fracturing. Oil Pools and Fields, Oil Zones : Oil Pools and Fields, Oil Zones When a trap is filled with hydrocarbon, it is known as a pool. After a pool is discovered, it becomes a field for development.
The idea of a field is generally larger, because it could include the production of several localized pools in different reservoir horizons.
A pool is defined in terms of a commercial accumulation. Slide 146: A petroleum accumulation is the initial quantity of hydrocarbon (oil and/or gas) in the site.
The reserves are the recoverable quantity of hydrocarbon, or the final cumulative production or recoverable reserves.
The recoverable coefficient is the reserve/accumulation ratio.
Once the height of the deposit is known, with the integrated zone and closure, a first estimate of the reserves can be made by delineation. Slide 147: An estimated average of 25% of the oils is recovered, and at least 75% of the gas.
Petroleum zones and systems can be defined on the basis of geographic and stratigraphic distribution of the basins.
A petroleum zone is defined a a vast region where many oil and gas fields have been found.
In many cases, oil fields are located in different reservoirs at distinct stratigraphic levels, depending on the geological history of the region. Global Guidelines For Exploration : Global Guidelines For Exploration Relation with the global geodynamics history (shelves and mobile belts), specially the concentration (69%) of reserves along the line of Mesogea (or Tethys).
Relations with paleo-oceanographic and climatic variations and events that regulate the formation of source rock and reservoirs (and specially the most effective cap rock of evaporites). Slide 149: This global approach has also led to thinking on how to define a petroleum system, from the initial organic source to the production of a series of fields. Critical Moment : Critical Moment Petroleum system is first defined by the existence of oil or gas in a basin. Whether in abundance or in traces.
Thus the presence of oil and gas is evidence of an ordered sequence of geological processes that have led to an accumulation.
The petroleum system idea is therefore to define the sequence of events precisely in their time and stratigraphic aspects, along with the geographical area concerned. Slide 151: A petroleum system is thus characyerised by:
Source rock (leading to the generation of thermal or biogenic gas, oil, or condensate, asphalt)
Burial, which defines the possibility and rate of maturation
Trap formation age, which depends on the structural evolution. Slide 152: The presumed age of expulsion-migration-accumulation
The preservation or retention time (during which alteration, degradation or even erosion may occur)
All of these aspects are recorded in a stratigraphic table to define the critical moment, which correspondent to the migration-accumulation period.
This moment is critical because, at this time, traps must exist in the basin to retain an accumulation. Slide 153: If there are no traps, no pool will form. The sequence of different events must thus be considered carefully.
The subsidence curves and burial history developed will be of capital importance for defining this critical moment.