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Petroleum Geology Petroleum geology comprises those geological disciplines which are of greatest significance for the finding and recovery of oil and gas. Since most of the obvious and “easy to find” petroleum already has been discoveredit is necessary to use sophisticated methods in the exploration of sedimentary basins. These include advanced geophysical techniques and basin modelling. There is also much more emphasis now on enhanced recovery from the producing fields. Petroleum technology has made great progress and many new tools and modelling programs have been developed, both in exploration and production. It is however important to understand the geological processes which determine the distribution of different sedimentary rocks and their physical properties. This knowledge is fundamental to being able to successfully apply the methods now available. Pantelis Soupios, February, 2017
Petroleum Geology The field of geology that is of mostimportance to the oil industry issedimentology, for itis in certain sedimentary environments that hydrocarbonsare formed. Precise and detailedstudy of the composition, texture and structure of therocks, the color of the constituents, and identificationof any traces of animal and plant organisms. The geologistshould: identify the physical,chemical and biological conditions atthe time of deposition; and to describe the changesthat the sedimentary series has undergonesince deposition. The organizationof different strata into series, and theirpossible deformation by faulting, folding, and so on should be studied. To predict the location of different facies and from that possible source, reservoir and cap rocks. Pantelis Soupios, February, 2017
Petroleum Geology Methods applied The biostratigraphic correlation of strata encountered in exploration wells is achieved by micropalaeontology(including palynology), a field developed by the oil industry. The sedimentary environments (sedimentary facies) determine the distribution of reservoir rocks and their primary composition. Sediments do, however, alter their properties with increasing overburden due to diagenesis during burial. Diagenetic processes determine the porosity, permeability and other physical properties such as velocity, in both sandstone and limestone reservoirs. Chemical processes controlling mineral reactions are important. Organic geochemistry, which includes the study of organic matter in sediments and its transformation into hydrocarbons. Pantelis Soupios, February, 2017
Petroleum Geology Methods applied Tectonics and structural geologyprovide an understandingof the subsidence, folding and uplift responsible for the creation and dynamic history of a basin. The timing of the folding and faulting that forms structural traps is very important in relation to the migration of hydrocarbons. Seismic methodshave become the main tool for mapping sedimentary facies, stratigraphy, sequence stratigraphy and tectonic development. Other geophysical measurementsmay include gravimetry and magnetometry; electromagnetic and well-logging methods. Pantelis Soupios, February, 2017
Petroleum Geology Methods applied Well-logging methods have developed equally rapidly, from simple electric and radioactive logs to highly advanced logging tools which provide detailed information about the sequence penetrated by the well. Logs provide a continuity of information about the rock properties which one can seldom obtain from exposures or core samples. This information makes it possible to interpret not only the lithological composition of the rocks and the variation of porosity and permeability, but also the depositional environment. Image logs make it possible also to detect bedding and fractures inside the wells. A good background in basic chemistry, physics, mathematicsand computing is also required. Pantelis Soupios, February, 2017
Petroleum Geology Reservoir (source) rocks are mostly sandstones and carbonates which are sufficiently porous to hold significant amounts of petroleum. The composition and properties of other rock types such as shales and salt are also important. There are now relatively few sedimentary basins in the world that are unexploredand it is getting increasingly difficult to find new giant fields. There is now increasing interest in heavy oil, tar sand and oil shale. Oil shale is a source rock exposed near the surface. Hydrocarbons cannot be produced in the same way as from sandstone or carbonate reservoirs. The hydrocarbons can only be obtained by breaking and crushing the shale and heating. Shales can however contain gas. Gas shale is expected to be an important source of petroleum in the years to come, particularly in the US. Pantelis Soupios, February, 2017
Petroleum Geology OIL SHALE: Although very large quantities of petroleum can be produced from oil shale, production costs are too high compared to conventional oil. There are also serious environmental problems involved in production from oil shale, and the process requires very large quantities of water, a resource which is not always plentiful. The oil reserves in such deposits exceed conventional oil reserves, but the expense and environmental issues involved with production from these types of reservoirs clearly limit their exploitation. This is particularly true of production from oil shale. Pantelis Soupios, February, 2017
Petroleum Geology Accumulation of Organic Matter (1) It is well documented that oil accumulations are of organic origin and formed from organic matter in sediments. Most of the organic materials which occur in source rocks for petroleum are algae, formed by photosynthesis. The zooplankton and higher organisms that are indirectly dependent on photosynthesis too. The energy which we release when burning petroleum is therefore stored solar energy. Since petroleum is derived from organic matter, it is important to understand how and wheresediments with a high content of organic matter are deposited. Nutrients for this organic production are supplied by erosion of rocks on land and transported into the ocean. Pantelis Soupios, February, 2017
Petroleum Geology Accumulation of Organic Matter (2) The supply of nutrients is therefore greatest in coastal areas, particularly where sediment-laden rivers discharge into the sea. Plant debris is also supplied directly from the land in coastal areas. Biological production is greatest in the uppermost 20–30 m of the ocean and most of the phytoplankton growth takes place in this zone. In clear water, sunlight penetrates much deeper than in turbid water, but in clear water there is usually little nutrient supply. At about 100–150 m depth, sunlight is too weak for photosynthesis even in very clear water. Pantelis Soupios, February, 2017
Petroleum Geology Accumulation of Organic Matter (3) Basins with restricted water circulation will preserve more organic matter and produce good source rocks which may mature to generate oil and gas (A) Depositional environments for potential source and reservoir rocks. Depressions on the sea floor with little water circulation provide the best setting for organic matter to be accumulated before it is oxidized. (B) Migration of petroleum from source rocks into reservoir rocks after burial and maturation. The carbonate trap (e.g. a reef) is a stratigraphic trap, while the sandstone forms a structural trap bounded by a fault. Pantelis Soupios, February, 2017
Petroleum Geology Accumulation of Organic Matter (4) In polar regions, cold dense water sinks to great depths and flows along the bottom of the deep oceans towards lower latitudes. This is the thermal conveyor belt transporting heat to higher latitudes and it keeps the deep ocean water oxidizing. In areas near the equator where the prevailing winds are from the east the surface water is driven away from the western coast of the continents. This generates a strong upwelling of nutrient-rich water from the bottom of the sea which sustains especially high levels of primary organic production. The best examples of this are the coast of Chile and off West Africa. Pantelis Soupios, February, 2017
Petroleum Geology Accumulation of Organic Matter (5) Energy stored by photosynthesis can be used directly by organisms for respiration. This is the opposite process, breaking carbohydrates down into carbon dioxide and water again, so that the organisms gain energy. This occurs in organisms at night when there is no light to drive photosynthesis. Also when we burn hydrocarbons, e.g. while driving a car, energy is obtained by oxidation, again essentially reversing the photosynthesis equation quoted above. Planktonic algae are the main contributors to the organic matter which gives rise to petroleum. Among the most important are diatoms, which have amorphous silica (opal A) shells. Diatoms are most abundant in the higher latitudes and are also found in brackish and fresh water. Blue–green algae (cyanobacteria) which live on the bottom in shallow areas, also contribute to the organic material in sediments. Pantelis Soupios, February, 2017
Petroleum Geology Formation of Source Rocks All marine organic material is formed near the surface of the ocean, in the photic zone, through photosynthesis. For the most part this is algae. Some phytoplankton are broken down chemically and oxidized and some are eaten by zooplankton. Both types of plankton are eaten by higher organisms which concentrate the indigestible part of the organic matter into fecal pellets which may be incorporated into sediments. Limited water circulation in semi-enclosed marine basins due to restricted outflow over a shallow threshold is a common cause of stagnant water bodies. The Black Sea is a good example. Pantelis Soupios, February, 2017
Petroleum Geology Migration of Petroleum (1) Petroleum migrates from low permeability to high permeability reservoir rocks. The main driving force for petroleum migration is the density difference (oil-water). The forces acting against migration are the capillary forces and the resistance to flow though rocks with low permeabilities. Migration of oil and gas will therefore nearly always have an upwards component. Pantelis Soupios, February, 2017
Petroleum Geology Migration of Petroleum (2) We distinguish between primary migration, which is the flow of petroleum out of the source rock and secondary migration, which is the continued flow from the source rock to the reservoir rock or up to the surface. Oil and gas may also migrate (leak) from the reservoir to a higher trap or to the surface. Hydrocarbons are relatively insoluble in water and will therefore migrate as a separate phase. However, solubility (oil and gas) increases markedly with pressure. Pantelis Soupios, February, 2017
Petroleum Geology Migration of Petroleum (3) Lets assume that oil is mostly transported as a separate phase. Oil is lighter than water, and oil droplets would be able to move through the pores in the rocks but the capillary resistance is high for separate oil drops in a water-wet rock. In order to pass through the narrow passage between pores (pore throat), the oil droplets must overcome the capillary forces. When the pores are sufficiently small (fine-grained sediment), these forces will act as a barrier to further migration of oil. Oil can therefore not migrate as small discrete droplets, but moves as a continuous string of oil where most of the pores are filled with oil rather than water (highly oil-saturated). The pressure in the oil phase at the top is then a function of the height of the oil saturated column (string) and the density difference between oil and water. The rate of migration is a function of the rate of petroleum generation in the source rocks. This is a function of the temperature integrated over time. Pantelis Soupios, February, 2017
Petroleum Geology Hydrocarbon Traps (1) Trapsconsist of porous reservoir rocks overlain by tight (low permeability) rockswhich do not allow oil or gas to pass. These must form structures closed at the top such that they collect oil and gas, which is lighter than water. We can think of an oil trap as a barrel or bucket upside down which can then be filled with petroleum which rises through the water until it is full. The point where the petroleum can leak from this structure is called the spill point. The closure is the maximum oil column that the structure can hold before leaking through the spill point. Cap rocks are usually not totally impermeable with respect to water, but may be impermeable to oil and gas due to capillary resistance in the small pores. Pantelis Soupios, February, 2017
Petroleum Geology Hydrocarbon Traps (2) Traps can be classified in three categories, (1) Structural traps that are formed by structural deformation (folding, doming or faulting) of rocks. Pantelis Soupios, February, 2017
Petroleum Geology Hydrocarbon Traps (3) (2) Stratigraphic traps which are related to primary features in the sedimentary sequences Pantelis Soupios, February, 2017
Petroleum Geology Hydrocarbon Traps (4) (3) Other types of oil traps. If pore water flow in a sedimentary basin is strong enough, the oil-water contact may deviate from the horizontal because of the hydrodynamic shear stress that is set up. In some cases, oil may accumulate without closure. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) When we speak of a log in the oil industry we mean “a recording against depth of any of the characteristics of the rock formations traversed by a measuring apparatus in the well-bore.” The logs sometimes referred to as “wireline logs” or “well-logs”, are obtained by means of measuring equipment (logging tools) lowered on cable (wireline) into the well. Measurements are transmitted up the cable (which contains one or several conductors) to a surface laboratory or computer unit. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) The geologist depends on rock samples for defining the physical, chemical and biological conditions prevalent at the time of deposition. On the surface, these are cut from rock outcrops (sample of any desired size can be taken, or repeated). Sampling from the subsurface is rather more problematic. Rock samples are obtained as cores or cuttings.Cores obtained while drilling (using a core-barrel),by virtue of their size and continuous nature, permita thorough geological analysis over a chosen interval. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) “Sidewall-cores”, extracted with a core-gun, sample-taker or core-cutter from the wall of the hole after drilling, present fewer practical difficulties. They are smaller samples, and, being taken at discrete depths, they do not provide continuous information. Unfortunately, reconstruction of a lithological sequence in terms of thickness and composition, from cuttings that have undergone mixing, leaching, and general contamination, during their transportation by the drilling-mud to the surface, cannot always be performed with confidence. Due to the limitation or bad quality samples, the geologists are unable to answer with any confidence the questions fundamental to oil exploration: (a) Has a potential reservoir structure been located? (b) If so, is it hydrocarbon-bearing? (c) Can we infer the presence of a nearby reservoir? Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) An alternative, and very effective, approach to this problem is to take in situ measurements, by running well-logs. In this way, parameters related to porosity, lithology, hydrocarbons, and other rock propertiesof interest to the geologist, can be obtained. The first well-log, a measurement of electrical resistivity, devised by Marcel and Conrad Schlumberger, was run in September 1927 in France. They called this, with great foresight, “electrical coring”. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Well-log measurements have firmly established applications in the evaluation of the porosities and saturations of reservoir rocks, and for depth correlations. More recently, however, there has been an increasing appreciation of the value of log data as a source of more general geological information. Geologists realized that well-logs can be to the subsurface rock what the eyes and geological instruments are to the surface outcrop. Through logging we measure a number of physical parameters related to both the geological and petrophysical properties of the strata that have been penetrated. In addition, logs tell us about the fluids in the pores of the reservoir rocks. Log data constitute, therefore, a “signature” of the rock; the physical characteristics they represent are the consequences of physical, chemical and biological (particularly geographical and climatic...) conditions prevalent during deposition. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Determination of Rock Composition This is the geologist’s first task. Interpretation of the well-logs will reveal both the mineralogy and proportions of the solid constituents of the rock (i.e. grains, matrix and cement), and the nature and proportions (porosity, saturations) of the interstitial fluids. Log analysts distinguish only two categories of solid component in a rock - “matrix” and “shale”. This classification is based on the sharply contrasting effects they have, not only on the logs themselves, but on the petrophysical properties of reservoir rocks (permeability, saturation, etc.). Shale is in certain cases treated in terms of two constituents, “clay” and “silt”. We will discuss this log-analyst terminology in more detail. For a sedimentologist, matrix is “The smaller or finer-grained, continuous material enclosing, or filling the interstices between the larger grains or particles of a sediment or sedimentary rock. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Matrix For the log analyst, matrix encompasses all the solid constituents of the rock (grains, matrix, cement), excluding shale. A simple matrix lithology consists of single mineral (calcite or quartz, for example). A complex lithology contains a mixture of minerals. For a log analyst, matrix is “all the solid framework of rock which surrounds pore volume”. Shale – Silt and Clay A shaleis a fine-grained sedimentary rock formed by the consolidation of clay or silt (50% silt, 35% clay or fine mica and 15% minerals). It is characterized by a finely stratified structure. A siltis a rock fragment or detrital particle having high content of clay minerals. A clayis an extremely fine-grained natural sediment or soft rock consisting of very small particles of clay minerals and minor quantities of finely divided quartz. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Fluids The arrangement of the grains usually leaves spaces (pores and channels) which are filled with fluids: water, air, gas, oil, tar, etc. Just how much fluid is contained in a rock depends on the space, or porosity, available. With the exception of water, these pore-fluids have one important property in common, they are poor electrical conductors. Water, on the other hand, conducts electricity by virtue of dissolved salts. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) The electrical properties of a rock are therefore strongly influenced by the water it contains. The quantity of water in the rock is a function of the porosity, and the extent to which that porosity is filled with water (as opposed to hydrocarbons). This explains why the resistivity of a formation is such an important log measurement. From the resistivity we can determine the percentage of water in the rock (provided we know the resistivity of the water itself). If we also know the porosity, we may deduce the percentage of hydrocarbons present (the hydrocarbon saturation). Porosity Porosity is the fraction of the total volume of a rock that is not occupied by the solid constituents. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Porosity (2) Several kinds of porosity: Total porosity, φτ (φ1+φ2), consists of all the voidspaces (pores, channels, fissures, vugs) between thesolid components. Φ1is the primary porosity, which is intergranularorintercrystalline. It depends on the shape, size andarrangement of the solids, and is the type of porosityencountered in clastic rocks. Φ2is the secondaryporosity, made up of vugs caused by dissolution ofthe matrix, and fissures or cracks caused by mechanicalforces. It is a common feature of rocks of chemicalor organic (biochemical) origin. Interconnected porosity, Φc, is made uponly of those spaces which are in communication.This may be considerably less than the totalporosity. For example, pumice-stone has zero Φc, because each pore-space is isolated from the others: there are no interconnecting channels. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Porosity (3) Several kinds of porosity: Potential porosity, Φpot, is that part of the interconnected porosity in which the diameter of the connecting channels is large enough to permit fluid to flow (greater than 50 μm for oil, 5 μm for gas). Effective porosity, Φe, is a term used specifically in log analysis. It is the porosity that is accessible to free fluids, and excludes, therefore, non-connected porosity and the volume occupied by the clay-bound water or clay-hydration water (adsorbed water, hydration water of the exchange cations) surrounding the clay particles. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) The resistivity (R)of a substance is the measure of its opposition to the passage of electrical current. It is expressed in units of ohm.m. The electrical conductivity (C) is the measure of the material’s ability to conduct electricity. It is the inverse of the resistivity, and is usually expressed in units of millimhos/m (mmho/m) or mS/m (milli Siemens per metre). There are two types of conductivity: (a) Electronic conductivity is a property of solids such as graphite, metals (copper, silver, etc.), haematite, metal sulphides (pyrite, galena) etc. (b) Electrolytic conductivity is a property of, for instance, water containing dissolved salts. Dry rocks, with the exception of those mentioned above, have extremely high resistivities. The conductive properties of sedimentary rocks are of electrolytic origin, presence of water or mixtures of water and hydrocarbons in the pore space. The water phase must of course be continuous in order to contribute to the conductivity. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) The resistivity of a rock depends on: The resistivity of the water in the pores. This will vary with the nature and concentration of its dissolved salts. The quantity of water present; that is, the porosity and the saturation. Lithology, i.e. the nature and percentage of clays present, and traces of conductive minerals. The texture of the rock; i.e. distribution of pores, clays and conductive minerals. The temperature. Resistivity may be anisotropic (stratification/layering) in the rock, caused, for instance, by deposition of elongated or flat particles. oriented in the direction of a prevailing current. This creates preferential paths for current flow (and fluid movement), and electrical conductivity is not the same in all directions. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Relation between Resistivity and Salinity The resistivity of an electrolyte depends on the concentration and type of dissolved salts. Theresistivity decreases as concentration increases, up to a certain maximum beyond which undissolved, and therefore non-conducting, salts impede the passage of current-carrying ions. The salinity is a measure of the concentration of dissolved salts. Sodium chloride (NaCI) is the most commonsalt contained in formation waters and drilling muds. Relation between Resistivity and Temperature The resistivity of a solution decreases as the temperature increases. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Resistivity of Clays With the exception of pyrite, haematite,graphite and a few others, the dry minerals have infinite resistivity. Certain minerals do exist that appear to be solid conductors. Clay minerals are an example. According to Waxman and Smits (1967), a clayey sediment behaves like a clean conductor and appears to be more conductive than expected from its bulk salinity. Thus the conductivity of a clayey sediment is the sum of two terms: One associated with free water or the water-filled porosity (indeed this type of sediment has porosity as high as 80% at the time of deposition. Subsequently, as they become compacted, some of the free water is expelled. However, this porosity is never reduced to zero in sedimentary rocks which have not yet reached the metamorphic phase). The other associated with the CEC (Cation Exchange Capacity). Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Rock Texture and Structure The shape and size of the rock grains, degree of sorting, the manner in which they are cemented and the relative importance of the cement itself, have three important consequences. They determine the porosity; the size of the pores and connecting channels influences the permeability, and hence the saturation; and the distribution of the porositydecides the tortuosity. The internal structure of layers (homogeneity, heterogeneity, lamination, continuous gradation), the configuration of individual sedimentary structures, the thickness of the nature of their interfacing, their arrangement in sequences, and the boundaries and trends of these sequences provide valuable information about the depositional environment. This is why the study of the many different characteristics of sedimentary formations is of such interest to the geologist and reservoir engineers. Pantelis Soupios, February, 2017
WELL-LOG (Basic Concepts) Permeability A permeable rock must have connected porosity. The permeability of a rock is a measure of the ease with which fluid of a certain viscosity can flow through it, under a pressure gradient. The absolute permeability kdescribes the flow of a homogeneous fluid, having no chemical interaction with the rock through which it is flowing. Darcy’s law describes this flow as: Porosity – Permeability Can we have high porosity without any permeability? pumice-stone (where there is no interconnected porosity) and clays and shales (pores and channels are so fine that surface tension forces are strong enough to prevent fluid movement) Pantelis Soupios, February, 2017
