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CENTRE FOR ENERGY, PETROLEUM AND MINERAL LAW AND POLICY CEPMLP, DUNDEE UNIVERSITY

INDUCTION PROGRAMME

THE PRODUCTION OF PETROLEUM AND MINERALS

“ The Upstream Aspects”

Source: http://energy.er.usgs.gov/products/papers/World_oil/oil/index.htm

By Dr. Arthur J. Warden

e-mail a.warden@dundee.ac.uk 2005

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INDUCTION PROGRAMME

THE PRODUCTION OF PETROLEUM AND MINERALS (The Upstream Aspects)

PART TWO : PETROLEUM

1. INTRODUCTION ......................................................................................3

1.1. Genesis of Petroleum Compounds.....................................................3 1.2. Primary Oil Migration ..........................................................................4 1.3. Secondary Migration...........................................................................4 1.4. Relationship Between Petroleum and Other Hydrocarbons ...............5 1.5. Nature of Crude Oil and Refined Products .........................................5

2. OILFIELD CHARACTERISTICS – THE BROAD PICTURE ....................8 2.1. Favourable Environments in Basinal Settings and Continental Margins8

3. OIL EXPLORATION...............................................................................10 3.1. Background (This sub-section is for information only and therefore is

non-examinable). ................................................................................10 3.2. Exploration Methods (Examinable text resumes) ...............................11

3.2.1. Steps in an exploration programme – The Investigative Stage.....11 3.2.3. Economic Phase ..........................................................................12 3.2.4. Consolidation Phase.....................................................................13 3. 2.5. Ongoing Tasks9...........................................................................13

4. FROM OIL EXPLORATION TO RESERVE DEFINITION ......................15 4.1. Trapping Mechanisms .....................................................................15 4.3. Delineation of Reservoirs ................................................................18 4.4. Role of Production Well Geologist’s Inputs.......................................19 4.5. Delineation of Petroleum Reserves ..................................................20

5. PRODUCTION GEOLOGY.....................................................................23

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1. INTRODUCTION The present induction course represents a fundamentally restructured version of the original geology course. The emphasis shifts to a more ‘hands on’ approach focusing on the steps involved in exploration up to the production stage. Col Roberts will cover the downstream aspects leading through to marketing and distribution. In order to complete the coverage of the initial aspects a brief mention in made of the organic matter (OM) comprising the starting point of oil formation and the process it undergoes during conversion to crude oil leading to primary migration and subsequent entrapment. This summary is given for reference purposes only and will not be included in your exam.

1.1. Genesis of Petroleum Compounds

The organic material from which crude oil is derived, like the related hydrocarbons such as coal and oil shale is a product of the fixation of atmospheric carbon dioxide (CO2) due to photosynthesis catalysed by solar energy. In the case of oil the starting point is the OM occurring in the surface layers of the ocean within the zone of sunlight penetration i.e. it is largely derived from plankton (phytoplankton i.e. of plant origin). Oil is a relatively unstable form of carbon and represents only a minute proportion of the carbon fixed from atmospheric sources during the carbon cycle. Much of the fossil carbon is fixed in more stable forms such as coal, graphite and organic and inorganic carbonates. Under favourable conditions this OM accumulates on the sea floor and is buried in areas of rapid sedimentation. As burial continues the oxygen supply is cut off and decomposition ceases. From this point onwards other processes take over as burial continues, marked mainly by an increase in pressure and temperature. Within a critical zone where temperature and pressure reach optimum levels the process of oil formation accelerates, This is known as the petroleum window and the oil formation is termed a thermo-catalytic reaction. The nature and quantity of the OM and the time factor are also important parameters in the oil forming process. Rising temperature during thermal maturation of the oil is beneficial as undesirable tarry substances are expelled from the liquid. Moreover the formation of the more valuable lighter oils with < 15 carbon atoms is favoured at the expense of the higher density forms with > 15 carbon atoms, resulting in the heavy liquids becoming more paraffinic. Rising pressure with increasing depth forces the gas released earlier back into solution, exerting a cleansing influence as this process helps to remove sulphur and helps to precipitate asphaltenes enhacing the quality of the oil. These processes which peak during oil formation are termed ‘the pressure cooker effect’ With deep burial > 5 km however retrograde processes take over beyond the floor of oil formation which are marked by the formation of asphaltenes and dry gas.

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Conversely to the generation of valuable oil compounds outlined above oil is vulnerable to degradation by natural agencies; leading to a process termed water washing which causes biodegradation. Recharge with fresh water washes away the lighter hydrocarbons in amounts approximately proportional to their solubility. The most vulnerable is the valuable gasoline fraction of crude oil comprising mainly aromatic ring compounds, including benzene. Freshwater recharge may bring in new becteria or re-activate dormant ones already present, due possibly to a reduction of salinity and an increase in nutrients. Normal paraffins and other compounds containing up to 34 carbon atoms are depleted by biodegradation as the bacteria continue to attack compounds with an increasing number of carbon atoms. Also counter-productive is the introduction of nitrogen, sulphur and oxygen compounds, which like the asphaltenes are created by bacterial metabolism. The combined action of washing and biodegradation may result in the production of only processable tar.

1.2. Primary Oil Migration Important criteria for oil migration include : a variable, non uniform rock sequence, expulsion, release and the ability to flow as a continuum down the pressure gradient. Oil can travel through, and under appropriate conditions become trapped within a porous medium, provided that the minimum saturation factors are exceeded. These are: Gas flow > 8% of the available volume Oil Flow > 22% of the available volume. To be capable of flowage the oil must be linked through the pores (or fracture network) of the medium through which it migrates. Oil will migrate after water and before gas. It should be noted that where oil is capable of emulsifying it could migrate in very small amounts within the emulsion. Traces of oil may be left along the migration route. This is an important consideration when exploring a sedimentary basin with possible oil potential when attempting to trace a migration path from source rocks to a potential oil trap environment. From this point on, commencing with a review of secondary migration the text comprises examinable material.

1.3. Secondary Migration Reservoir rocks have a much greater porosity and permeability than source rocks. Migration can, however, be halted by traps which allow the oil to pool when it cannot migrate any further. As noted the movement takes place in the direction of least hydrostatic/lithostatic pressure. Oil is capable of migrating vertically over 100’s of metres and over thousands of metres. In the case of

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the Russian platform oil has migrated in excess of a hundred kms in some instances.

1.4. Relationship Between Petroleum and Other Hydrocarbons A close relationship is evident between oil and other members of the hydrocarbon family such as coal, oil shale and tar sands which are capable of conversion into oil. Both oil and coal are formed from types of plant material. In the case of coal, it is commonly derived from swampy forests occurring in deltaic settings. Other hydrocarbons such as tar sands representing bituminous residues, and oil shales are also competitive sources for fuel. In fact the first oil was originally distilled from oil shale mines in the Lothians of Scotland before it was discovered in the natural liquid form in Pennsylvania. It has been demonstrated that coal can be converted into oil by the sasoil process ( for example in Germany during WW 2 and South Africa during the period the country was subjected to embargoes). Oil can also be distilled on a commercial basis from the richer oil shales ( for example in Estonia and the Green river shales of the Southwestern US). The relationship between these hydrocarbons is shown in the accompanying figure 2. The fears expressed in recent years that ‘oil may soon run out and what then?’ now seem groundless considering that not only is this unlikely while industry is continually adding to their reserves, and there is also undoubted potential left on land and below the deep sea floor, but there are also immense reserves in the tar sands of Alberta and the Orinoco valley in Venezuela. Moreover research in Canada has already demonstrated that the tar sands can be profitably converted into oil. Despite vigorous initiatives towards the reduction of greenhouse gases (GHG’s) for example under the Kyoto Protocol coal remains the most economic type of fossil fuel. Obviously countries such as the US and Australia will depend on this energy source for some time to come and new rising industrial powers such as China and India will probably also increase their coal consumption as they continue to industrialise. Therefore it may be some time before universal agreement is reached on the need to limit emissions of greenhouse gases. However this course will be confined to petroleum products which will probably continue to be the dominant form of fuel that powers our industries and transport for the foreseeable future. To place the importance of this industry in context it is worth five times the total value of the entire mining industry.

1.5. Nature of Crude Oil and Refined Products Crude oils can be classified into three groups based on the nature of their contained hydrocarbons.

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1. Paraffin based crudes These contain paraffin wax (i.e. high molecular weight paraffins solid at room temperature) and little or no bituminous matter. They consist mainly of paraffinic hydrocarbons and provide good yields of paraffin wax and high grade lubricating oils.

2. Asphaltic based crude oils These contain little or no paraffin wax and asphaltic matter, which is commonly present in large amounts. Consisting mainly of napthenes they yield lubricating oils with viscosities more sensitive to temperature than those derived from paraffin based crudes but can be converted to the latter by special refining techniques. These crudes tend to be designated napthene based crude oils.

3. Mixed base crude oils These contain substantial proportions of both the above i.e. paraffins and napthenes together with some aromatic hydrocarbons.

Another important aspect is the sulphur content. Low S values of < 1.0% are favoured over high S contents up to as much as 3% which makes the crude oil highly corrosive to handle. Most crudes contain small amounts of other hydrocarbons including long chained compounds, paraffins derived from higher plants, steroids from both plants and animals and some compounds derived from chlorophyll of possible terrestrial origin. Although simple in terms of their main component elements (they consist essentially of carbon, hydrogen and oxygen), petroleum compounds, which exist in all three phases, range systematically, with an increasing number of carbon atoms, from gases with a comparatively simple structure through liquids into solids with increasingly complex structures. Bitumens and tarry residues at the top end of the scale have the highest number of carbon atoms (up to C70). Figure 3 shows the compositional range of the principal petroleum compounds. Their molecular structure conforms to two main configurations. With the addition of ‘building block’ units they can form increasingly complex molecules. In the case of the aliphatic chain compounds this takes the form of adding - CH2 units to form new chain compounds. The simplest chain compound is methane (CH4) which is formed in nature from rotting vegetation. This gas also constitutes one of the main hazards in coal mining. Ring compounds need three carbon atoms or more and start with cyclopropane (C3H8). This has the same formula as the chain compound propane but possesses a different molecular structure. Compounds forming such variants are termed isomers. The most valuable compounds are the lighter oils including gasoline, kerosene and diesel which are found within the 5- 20 carbon atom range. The solid hydrocarbons are those with the highest number of carbon atoms. Asphalt with C 70 represents the top of this range,

Figure 4 shows the range of compounds that are derived from crude oil by fractional distillation. The most volatile are the C1 – C4 gases comprising the first fraction to separate at 20 degrees this is an important type of heating/

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cooking fuel which is marketed either in the gaseous of liquefied form. With increasing environmental concern gas is gaining popularity over oil because it is regarded as a cleaner (though less profitable form of fuel. Increasing temperature in the distillation column ( within the range 70 – 270 degrees) yields the main three types of liquid fuels noted above. At yet higher temperatures lubricating oils, waxes and polishes are produced in the C20 – 50 range followed by heavy fuel oil (C20 – 70) of the type used in ships, factories and for central heating. The ultimate product containing > C70 is a bituminous residue used for surfacing roads and for roofing. Note: the heavier fractions are separated afterwards by vacuum distillation.

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2. OILFIELD CHARACTERISTICS – THE BROAD PICTURE

2.1. Favourable Environments in Basinal Settings and Continental Margins Figures 5 & 6 put into perspective in terms of geographical distribution and size the world’s giant oilfields. The first map shows the distribution of the major oilfields in order of decreasing magnitude. The following map shows the distribution of the major oilfields relative to age and tectonic plate boundaries. There is a relationship between basin type and hydrocarbon richness. The two main types of basins with oil potential are those with a high carbonate content and those consisting predominately of sandstone. The former tend to contain a higher proportion of valuable fuel oils with a lower number of carbon atoms. The carbonate basins can be further divided into those developed in compressional and non-compressional environments.

Some 700 major basins were identified. Out of 400 of those explored, 150 have become commercially productive. Some 90% of the world’s known gas / oil resources (excluding tar sands and shales) occur in over 366 basins. However 75% of the known oil and 66% of the known gas are concentrated in only 4 key regions:

• Persian Gulf Basin • On the N and W side of the Gulf of Mexico and the Caribbean • Along both flanks of the Ouchita and Marathon mountains in the

US • The Urals in Russia

The richness in terms of hydrocarbon per volume unit area of sedimentary rock for conventional and converted energy equivalent is expressed in barrels/ cubic mile of sedimentary rocks. Average values expressed in this way fall within the range 30 – 60,000 Bls/ mi3 of sedimentary rock. Considering hydrocarbon richness relative to plates and plate margins, the following conclusions can be drawn (figure 6)

• Nearly all the Palaeozoic oil reserves (570 – 250 m.y) are intracratonic

(in other words they occur within the stable plates). • Cenozoic (Tertiary + Quaternary) i.e. < 65 m.y. oil reserves are found

close to plate boundaries. • Mesozoic ( 250 – 65 m.y.) oil is equally divided between platforms and

plate boundaries. • Nearly all gas reserves of all ages occur within intra-cratonic basins

An Example of a ‘Within Continent’ (intra-continental) Oil Basin.

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Some sedimentary basins lie entirely within major continental land masses. Some of these apparently owe their origin to incipient rifting and failed rifts while in other cases the role of rifting is uncertain. Commonly long lived for up to 500 years, they have been little affected by subsequent folding and faulting. The Williston Basin in the central northern US (Figure 7 )is an example of a productive basin although others are commonly less so or barren. The difference, at least in part may depend on whether or not the sea penetrated the basin – but this is a speculative issue. The most striking feature of the oilfields developed within the 300 x 350 km Williston basin (contoured in 1,000 nds of feet below sea level) is the concentration of major oil and gas fields within very gently flexed anticlines. Some oil / gasfields however conform to distinctive NW and northerly trends suggestive of strong directional controls.

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3. OIL EXPLORATION

3.1. Background (This sub-section is for information only and therefore is non-examinable).

Initially, during the latter half of the nineteenth century finding oil was largely a matter of following hunches ( so-called wildcat drilling) and particularly by focussing on areas of seepages where oil oozed to the surface Apart from the US the earliest producing locations were situated in the Caucases, Romania, Eastern Borneo and Peru and in all instances the oil came from Tertiary sandstones. It wasn’t until 1901 that the anticlinal theory gained momentum and began to influence exploration philosophy. A second pertinent observation concerned the presence of oil in hand dug pits, in Burma, China and Peru a situation that had been exploited for years without knowing why the oil reached the surface. However it was not until as late as 1966 than the cause for this phenomenon was clearly understood, namely that such leakages were due to the truncation of reservoir rocks by recent surface erosion. The anticlinal theory dominated until 1932 and worked extremely well in the early years. However the period 1925 – 45 saw the introduction of some major innovations such as gravity meters which accelerated the discovery of salt dome traps in the 1930’s and 40’s particularly on the US Gulf coast. Also reflection seismology enjoyed outstanding success in the 1930’s in helping to map unseen structures. Moreover electric well logs were also introduced in the 1930’s revolutionising fluid evaluation in wells and correlation between them. Rotary drilling displaced cable drilling extending the depth reach dramatically up to 5km and proved vital in developing a better understanding of basinal structural features and sequences. The thirties and forties also saw the start of petroleum geochemistry in North America and the Soviet Union. Micropalaeontology also took off greatly facilitating correlation between drill holes. The following the post WW 2 years saw a dramatic acceleration in the amount of drilling undertaken and a quantum leap in the sub surface data produced which facilitated better volumetric estimations for oil and gas that might be found in new prospects.. By the 1940’s drilling in shallow water regions such as inland seas and the shelf areas of many countries was well established. Concurrently there occurred a dramatic surge in seismic exploration and a better understanding was developed of sedimentalogical processes. The year 1960 marked a watershed in petroleum exploration which corresponded with the development of a large surplus and a shifting of the centre of gravity for oil production to the Middle East including Libya and discoveries in Nigeria between 1957 and 1967 There were also huge discoveries in Western Siberia and of natural gas also in Siberia in 1967-77 which made the USSR the world leader. The 1970’s also witnessed the

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opening up of new fields in the North Sea and SE Mexico. With the discovery of supergiant oilfields confined to only a handful of sedimentary basins, mostly in the developing world or communist countries, most of the obvious trap structures had already been found. This resulted in a change of strategy in the mature regions which focused on subtle and obscure traps and the focus was on sedimentology with particular attention paid to the carbonate rocks. Also computerisation accelerated data processing. In turn this led to a better understanding of subtle and complex trapping mechanisms and thereafter large oil accumulations were discovered at an earlier stage in the exploration programme which was a bonus in frontier and offshore areas. Plate tectonics and satellite imagery in the 60’s and 70’s provided a new perspective on sedimentary basins particularly in offshore settings.

3.2. Exploration Methods (Examinable text resumes)

3.2.1. Steps in an exploration programme – The Investigative Stage. Step 1 Primary objective : Identification of a prospective sedimentary basins. Travellers records, geological records, topo base maps, satellite image and aerial photography interpretation. Previous company records. Step 2. Identify basin type. Adjacent to a mountain belt? Strata dipping off a landmass extending under water? Is it a fault trap? Or possibly an intra-cratonic graben? Are the contents deformed ? Different basin types carry different ratings for their hydrocarbon potential. Step 3 Ascertain the stratigraphy - a key consideration for petroliferous basins. Also check the palaeontology – a critical aspect for deciphering the former environment. Ascertain thickness and aerial extent of the various rock types.- Varied sequences are preferable to monotonous ones. Surface mapping may give direct evidence of depositional environments favourable for oil accumulation though older rocks may be obscured by younger sediments. Marine shales lacking large shelly fossils or an interfingering of fine and coarse grained sediments could be promising. Even if unpromising, the area should not be written off on surface evidence alone. Step 4 Check surface evidence for oil seeps, mud volcanoes, burnt clays, sulphur or gypsite in soils – These may show as tonal anomalies on the air photographs. Step 5 Check palaeontological records: fossil records often go back further than others. Ages and types of fossils can prove useful. Even their absence in shales can be significant. Step 6 - Structural Setting Establish the spatial relationship between the main outcrop or belt and the basin, especially if the belt arises from structural deformation. Lineaments are potentially significant as they could reflect favourable alignments for exploration targets. Check relations between

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structures and target strata. Eroded structures may be more promising than intact ones, though if the structures are still growing they may be promising. Step 7 Logistics Ascertain the situation regarding: taxes, royalties, labour land tenure etc.and assess the situation in the light of comparable basins elsewhere. Give conclusions on access. Establish the cost of pre-drilling evaluation. If the potential seems good but the logistics are unfavourable don’t give up immediately – miracles can sometimes be achieved by skilful negotiation! 3.2.2.Reconnaissance Phase Step 1 Delineate favourable stratigraphic trends. Detailed photogeological interpretation and reconnaissance. Step 2 A more detailed evaluation of the stratigraphy. Structural mapping supported by aerial gravity and magnetic surveys. Reconnaissance seismic profiles to locate sub-surface features like unconformities and high and low areas, depth to basement and palaeotopography. Step 3 Establish structural characteristics : extensional and compressional manifestations and fold amplitudes. Try to establish trends and onlap and offlap. Analyse drainage patterns for lineaments. Step 4 Cross check initial structural and stratigraphic interpretations by test holes in low cost locations to identify potential source and reservoir horizons. Cross check drill logs with any earlier old well logs. Check temperature recordings from test holes to establish gradients. Step 5 Using test well samples, cores and logs, focus on all horizons with source or reservoir potential plus dateable materials. Organic chemist to check for hydrocarbon traces and maturation characteristics of potential source beds. Combine with data on basin shape to evaluate fluid migratory directions ? Could help to highlight most favourable areas for hydrocarbon accumulation.

3.2.3. Economic Phase – Identification of Drillable Structures Based on the following criteria:-

• Results of intensified seismic investigations over anomalous areas targeting : highs, linear trends, unconformable truncations and gravity/magnetic discontinuities.

• Target features such as culminations, and intersections using dips, unconformities and linear trends.

• Choose target horizons and drill depths, calculate costs based on drill hole measurements including : logging, testing, temperature, pressure and velocities to tie in with the seismic data. Plan step out

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appraisal wells. More land/sea bed may be required to encompass a promising structure.

3.2.4. Consolidation Phase Critical information is assembled on the basin/continental marginal shelf target then summarised and tabulated. The following items are included.

1. Name, location and type of basin 2. Nature of Basin’s boundaries 3. Age of inception and termination of basin (correlatable deformation

events) 4. Intervening deformations (marked by significant unconformities) 5. Age and nature of basement where economic. 6. Thickness of sedimentary rocks a) in basin centre and b) along

margins 7. Estimated volume of sedimentary rocks 8. Maximum depth of deposition and nature of related formation. 9. Geothermal gradients 10. Most favoured source rocks and maturation values 11. Most favoured reservoir rocks and depths 12. Most favoured trap types and timing of their creation. 13. Probable directions and times of migration 14. Periods of maximum pooling 15. Hydrodynamic characteristics 16. Probable distribution of Oil fields 17. Any additional observations and references.

3. 2. 5. Ongoing Tasks

• Continual updating and evaluation of all available data taking account of it’s variable reliability

• Extrapolation of data from well documented models to lesser known areas can bring success where a good fit is achievable. The tools required here are well logs, maps, cross sections and seismic records. 3-D computer modelling is essential.

• From these data structure contour maps are developed on every correlatable stratigraphic horizon, plus isopach contours to generate volumes for reservoir calculations

• All potentially valuable information from well logs (formation tops, thicknesses, unconformities and gaps) must be added to the database

• Cross sections can be combined into fence diagrams which can be extrapolated into 3-D models. Configurations of formation contacts, unconformities and sub-outcrops, plus decompositional and erosional features and wedge outs must be included.

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• Time stratigraphic correlations are needed to distinguish facies changes and diachronous (time-transgressive) units which may be crucial for indicating trapping mechanisms.

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4. FROM OIL EXPLORATION TO RESERVE DEFINITION This section takes a look at some typical oil trap exploration targets which could be discovered by the exploration strategies outlined in the preceding sections. It follows with a summary of the type of instrumentation used in the context of the drilling programme to provide further data essential to ascertain the commercial viability of the reservoir. The ongoing programme will include laying out the depths of prospective horizons to be cored, logged or tested and other levels which require sampling for lithological, micropalaeontological or geochemical data. One of the key requirements is the evaluation of mud logs, drill cuttings etc. Finally the data are collated and processed to establish the quantity and quality of the oil reserves.

4.1. Trapping Mechanisms (Figures 8 & 9)

Eventually migrating oil and gas begin to accumulate in potentially economic quantities when they can move no further because they are blocked by an impermeable barrier which can be caused in several different ways, such as differences in lithology and/or particular structural configurations. Some basic examples illustrated in figures 8-10 are cited below:

• Anticlines as noted, these were the initial target in the early days once

a more systematic approach was adopted to oil exploration. They are produced by crustal shortening when strata are folded and the oil is impounded by an impermeable roof consisting, for example, of shale or evaporates. Such folds are still a prime target but are more difficult to find nowadays, as they are commonly concealed by unconformities that are covered by flat lying beds. In many cases, however folds may be asymmetric (figure 9) which makes it more difficult to delineate the fold axis and position the drill effectively.

• Fault traps The two examples in figure 8 show the entrapment of oil beneath an impermeable bed. Also the fault must be ‘locked’ by impermeable material above otherwise the oil would migrate along it to the surface to form an oil seep.

• Stratigraphic traps The first examples show a feathering out of a porous bed with oil trapped at the pinch out where the two impervious enclosing beds converge. The other example shows a porous sand bar sealed by an impermeable sediment and also a reef consisting of organic material, parts of which are fragmental. Once capped by impermeable material this is an ideal trap. Not only do carbonate trap rocks yield the valuable lower density oils but with solution cavities and spaces in rubbly fans they offer good storage space. This is sometimes further enhanced by conversion to dolomite/magnesite i.e. Mg rich carbonate which is accompanied by a diminution in volume.

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• The two lower examples in figure 8 are related to structural breaks in the sequence (unconformities) and might well be strictly classified as structural stratigraphic traps. In both cases there is an angular break in the sequence and the inclined porous bed in the first case is truncated and sealed in beneath an unconformity. In the second example the sea has overlapped onto a land surface and the oil is sealed in the porous lowest unit in the overlying sequence.

• Figure 9 in the lower part shows a drape compression anticline over an upfaulted horst block Note the overlying beds are gently arched and the convexity is soon lost in the overlying beds. This Figure 10 is a combination structural-stratigraphic trap structure which exhibits a variety of features. Note the upward migration of the plastic salt diaper due to their contrasted lighter density than the surrounding rocks. This makes them ideal oil traps.in which the oil migrates to the upturned edges of the porous beds and also where these beds abut against tensional faults dipping off the dome as well as the cap rock itself. Common along the Red Sea and Persian gulf coasts they are easily targeted on account of their contrasted density.

• Intra-continental graben and rift traps Tensional forces caused by stretching within continental land masses open grabens which can also serve as oil traps. This is the starting point for ocean formation. For example the Red Sea is an embryonic ocean in the making. In the North Sea most of the oilfields are concentrated within a branching system of grabens shown in Figure 12 This is also how the Atlantic started . 60 m.y. ago and it was instrumental in creating the environments in which the continental margin oilfields of Nigeria developed.

4.2. Geophysical Exploration Techniques

Geophysical techniques play a critical role in exploration, they enable us to measure quantitatively diagnostic physical qualities of concealed rocks and structures in depth. The interpretation of the data so obtained enable us to define the presence of oil and the configuration of the containing structure. The most important large scale methods in petroleum exploration are ‘potential field’ (gravity and magnetic) and seismic methods. Electrical, resistivity, neutron and gamma logging are also briefly mentioned in the context of wire logging..

• Gravity surveys measure small changes in gravity that reflect

variations in density of the rocks tested. Positive anomalies higher than the regional average suggest the presence of heavy rocks of the type found in the core of anticlines (figure 13) Negative anomalies indicate lighter rocks such as salt which commonly forms diapiric bodies which, as we have seen are good potential targets for various types of oil traps. Gravity surveys are at a premium during the exploration stage and can be conducted on land or at sea, from the air and even from satellites.

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• Magnetic surveys Rocks also vary in the extent to which they are

magnetised. Strongly magnetised rocks show small local effects and the intensity and direction of the earth’s magnetic field. As in the case of gravity variations these can be mapped to outline anomalies. The strength and sharpness of the anomalies is related to the depth below the surface particularly of igneous and metamorphic rocks which are characteristic of the crystalline basement. Therefore the prime role of a magnetic survey is to indicate the depth to basement, from which the total thickness of sediments in the sedimentary basin can be calculated and also the major structures within it.

• Seismic reflection surveys This is the dominant and most important technique in the exploration context. Basically it works on the principle of echo sounding the various layers in the basinal sequence. A noise is transmitted from the surface of the ground or sea, which as far as possible is directed downwards. When layers of different density are encountered a small amount of the energy is reflected back to the surface, although most carries on downwards and again some will be reflected when interfaces of contrasted density are encountered and so on. The times of arrival of the successive reflections after the initial shock can be recorded and if the velocity of travel through the overlying rocks is known we can calculate the depths to the reflecting horizons. The contours in a two way time profile (TWT) are actually a measure of time translated into distance. The receptors for the reflection surveys are termed geophones on land and hydrophones when used on water In practice a very large number of geophones are used to give more refined results – and up to 3000 hydrophones in the case or marine seismic surveys (figure 14) Rather than firing shots which are disturbing to life both on land and in the sea, trucks are used to pound the ground with a heavy plate on land to set up vibrations, and an air gun is used at sea. Figure 15 shows how well seismic reflection surveys can profile oil trap environments such as those related to unconformities and faults and ‘intrusive’diapiric salt bodies pointing directly to promising exploration targets.

• Seismic refraction surveys These are still used for reconnaissance

purposes, but less frequently than formerly. In this case the geophones are much further away from the shot which therefore needs a more powerful charge. The energy travels for some distance along the top of the hard bed giving information over a wide area. However it tends to iron out unevennesses in the structure instead of defining them. Consequently it is more difficult to interpret the results of such surveys in detail.

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4. 3. Delineation of Reservoirs With the oilfield outlined and a number of the individual reservoirs defined, exploration is succeeded by an intensive drilling programme and more detailed seismic surveys are undertaken with the aim of establishing an oil resource of adequate dimensions and quality to sustain a viable commercial operation. In this context a key role is played by wireline logging undertaken by specialised geophysical contractors using methods pioneered by the Franco American firm Schlumberger and termed Schlumberger logs. A device containing a variety of measuring instruments is lowered into the hole. As it is slowly withdrawn a variety of the physical properties of the rocks are measured and transmitted electrically to the surface. As most of the devices will not work through casing, several surveys have to be made as the hole deepens before the unstable segments are cased. The methods used for measuring the different physical properties and the information they give us are summarised below.

• Gamma ray log plots (figure 16) measure the natural radioactivity of the rocks. In particular the plots indicate the higher level of radioactivity found in shales In contrast the levels are appreciably lower in sandstones and carbonates.

• Electrical resistivity (Figures 17,18) : other devices measure the

electrical resistivity of the rocks. An electrical current is passed from the sonde into the rocks surrounding the well and is then picked up through another electrode and the resistivity is calculated. As resistivity varies according to rock type this enables us to differentiate between them. Moreover it is possible to distinguish between saline and fresh water contained within porous rocks as the current passes more readily through the saline water. This system can also distinguish between contained oil and gas as the current passes more easily through the former. Moreover resistivity logs can give an indication of porosity values.

• Sonic/acoustic logs Sound from an emitter at one end of the sonde is

passed through the borehole wall and picked up by a receiver at the other end and the time lag is recorded. This yields data not only on seismic velocities but also on the actual nature of the rocks including their porosity.

• Neutron log Neutrons are expelled into the well bore and react with

hydrogen in the rocks generating gamma rays which are measures in the sonde. As most of the hydrogen is present in the water or hydrocarbons it not only gives an indication of the porosity but helps to identify gas as opposed to oil or water. This implement is also useful with carbonates. There are further methods for logging and new ones are continually under development. A combination of several methods give a detailed picture of the strata penetrated. The overriding objective is to determine:

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• the rock types or lithologies; • the porosity of potential reservoir formations; • the nature of the fluids contained in the pore spaces

4. 4. Role of Production Well Geologist’s Inputs Mud Logging – The aim is to evaluate the oil and gas shows that circulate from the bit to the surface. The oil is detected through fluorescence under ultra violet light, and gas by ignition. Hydrogen sulphides and chlorides are also monitored. Drill cuttings – For these, rock type and texture should be monitored and forwarded to the laboratory for more detailed examination as necessary. Density measurements can be important as an indicator of well pressures. Sample descriptions are recorded on strip logs. Cores and larger samples - On entering a zone of abnormal fluid pressure suggestive of a favourable permeable reservoir rock the well may ‘kick’.The lithology, porosity, potential permeability and fluid content (by UV) are noted. The core is then sent on for further investigation. Formation tests - These may be requested by the production geologist. Though a costly process, when overlooked it could cost the production team a worthwhile discovery, as it has done on past occasions. Fluid recovery, hydrocarbons and formation water or drilling muds and their relative amounts are recorded. The nature of the oil or water is important, as is the formation pressure. Wire tests can also be run without unduly interrupting the drilling process. Wireline Logging - Key features include ascertaining the fluid contents of the porous zones. The complete range of logs should then be run to obtain maximum information; likewise for step out wells where the choice rests with the exploration and production departments. Standard logs measure electric, acoustic and radio-active properties. The geologist is also responsible for monitoring the geometry of the hole and the beds penetrated using a dipmeter logger. Borehole gravimeters and cameras may also be needed under special circumstances. Geophysicists may also need velocity surveys to tie in with the seismic records of important sections. Casing - Unstable/friable sections must be cased to prevent cave ins in weak segments of the hole. Laboratory Support - Detailed laboratory work may be required to provide more data on critical segments of the drill hole which may include slabbing, thin section preparation and the study of floras and faunas, particularly the micro fossil content which gives a valuable insight into the former ecology and environment of. potentially crucial horizons. Fine sedimentary material such

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as sand, clay and minerals can be electronically scanned to yield more in depth information on clay and minerals present indicative of environments, depth of deposition and diagenetic history. Porosity and the degree to which it is reduced by secondary processes is also critical. Geochemists will test for total ,organic carbon and for the degree of maturation. Pollen spores are also examined. They are valuable for correlation purposes. Drill holes are then correlated and tied in with others using stratigraphic markers to construct a fence and ultimately a 3-D model.

4.5. Delineation of Petroleum Reserves First let us remind ourselves of the 5 ‘magical’ requirements to produce an Oil accumulation:

• SOURCE ROCKS noted (but dimensions unknown) • MATURITY of organic matter- ok probably basinal model – but where

generated and in what quantities? • RESERVOIR - Easy to predict but initially difficult to understand host

rocks which accumulated in restrictied environment? . Lateral extent? Palaeogeography? Beach? Sandbar? Use wireline logs /seismic can help. Check drill cores –several BH needed..

• SEALS/CAP ROCK e.g shales – no problem • TRAPS – Use seismic, extrapolation, structural traps (small ones are

commonly missed) Stratigraphic traps more difficult, use Seismic 3-D Base drilling programme on knowledge of ‘magic 5’

NOTE : AS WITH MINERALS ONLY AFTER THE OIL IS PRODUCED IS THE QUANTITY AND QUALITY KNOWN PRECISELY !! The following definitions are important:

• OIL IN PLACE Total amount of oil present is measured in barrels present in underground accumulation

• STOOIP = Stock Tank Oil Originally In Place (i.e oil in place corrected to in situ conditions)

• RECOVERABLE OIL = The amount produced at the surface from a given accumulation The remainder requires artificial assistance for recovery therefore termed SECONDARY

• TERTIARY RESERVES are difficult to extract and have recourse to exotic techniques Confidence levels related to the degree of certainly for these categories are calculated statistically

RESERVE CATEGORIES (Table 1)

• PROVEN RESERVES proven to 80 – 90% confidence level.

• PROBABLE RESERVES Proven to 50% confidence level • (possibly only 10 – 15%)

Reserve categories are also shown in table 1. The system used is flexible and comparable to that used for minerals

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The category ‘Undiscovered’ is sometimes used. At best this is a guesstimate. Ultimate reserves are a grand total for a given basin and equal the amount extracted plus the amount remaining in the oil basin. DISCOVERED RESERVES are also an important category being that upon which feasibility calculations are based. This starts with the reservoir volume in the closural trap and then eliminates everything from the volume that is non oil. Recoverable reserves = BV x Fill x N/G x Φ (1-Sw) x RF x constant FVF BV = Volume of reservoir in trap closure above spill point. Governed by geometry, faulting and reservoir thickness. Fill = percentage of BV actually containing oil minus the volume of the gas cap and water bearing rock (latter discounted) adequacy of source rock to provide oil trap = charge. Cap rock quality and strength also important. N/G Net to gross ratio. Parts of the reservoir may not be porous and permeable and don’t contribute. Arbitrary cut off value is used. O = average porosity of net reservoir across entire accumulation (most reliable values come from core and wireline results). Sw = H20 saturation = amount of pore space occupied by water. Average value is applied to the field. FVF = the formation value factor. The oil occupied a larger volume in situ than at the surface because gas separates as the pressure is released. RF = the recovery factor which can be as much as 50-60% for sandstone but is much less for carbonates. Based on experience elsewhere. Constant = area in acres x feet. The figure is then multiplied by 7758 to convert to barrels (The latter calculation is not needed with the metric system) Even after complying with these constraints the estimates may be very uncertain. Statistical methods may then be applied to reduce the level of uncertainty. In addition to the foregoing one must be mindful of other important factors such as the RISK FACTOR which is the chance of finding no oil at all expressed as a percentage i.e. a gamble! Consider the magic five, if any one fails abandon the project! RISKED RESERVES, are the expected estimated reserves discounted by risk.

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ULTIMATE RESERVES, include what has yet to be discovered The following steps apply.:-

1. Add risked reserve estimates for all prospects. 2. Consider the geochemical material balance, the volume of the mature

source rocks their richness then the volume of oil generated available for entrapment.

3. Check the known part of the basin. Calculate the average oil content per square mile and extrapolate to unexplored areas.

4. Use past statistics for each 1000 ft and extrapolate to future drill sites. 5. Consult experts and attempt to obtain a consensus. This is aptly

termed ‘the oracular approach’

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5. PRODUCTION GEOLOGY With a general idea of the oil and gas reserve base we are now in a position to move into the production phase. Up to this point geological considerations predominated. Although geological inputs are still essential from this point onwards the overriding responsibility rests with the reservoir engineer who is charged with the job of bringing the oil to the surface as safely and as economically as possible. The main stages are as follows:-

1. Understand the Distribution of the Petroleum in the sub surface

The amount of oil present in individual licence areas i.e. in a given block when the field is unitised. The cost and revenue are interpreted of their share of the whole.

2. Location , monitoring, and appraisal of production wells: The optimum number of wells and their best location has to be decided. A few more may be drilled with this aim in view- perhaps 4-5 for a big field. The oil/water and oil/gas interface will vary. Therefore the well must be positioned to catch the last of the remaining oil.

3. Production Control The rates of extraction and oil levels in the field must be controlled in line with the range of permeability. Preferential flow will occur from high permeability channels and coning may occur causing premature production of water and gas (figure 19) . Much oil may be by passed.

4. Enhanced Oil Recovery In this case artificial means are applied by the engineer to boost recovery

Modelling Procedure – The starting point will be a conceptual model of the entrapment reservoir.

1. Model external shape – Structure of the top and bottom and thickness changes

2. Porosity and permeability changes 3. How gas oil and water would move through the reservoir during

production for different well patterns and production roles. The reservoir is divided into blocks with assigned values for porosity, permeability, saturation, pressure etc. Movements are simulated through the field and computer modelled. The top of the reservoir is detected by seismic and the bottom by drill hole records.

4. Variations in oil quality pose the greatest challenge Data used are based on correlation of well logs and cores. The area is sub divided into layers/units and wireline logs are checked and supplemented by micropalaeontological evidence and the palaeogeography is deciphered layer by layer. This sheds evidence on: the depositional environments, extent and nature of the rock units e.g deltaic

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deposits, channel sands, beach and offshore sand bars. Carbonate reservoirs comprise another lithological entity comprising : ancient reefs, lagoons, fore reefs etc from these data porosity and permeability trends can be established. Figure 20 gives a good indication of the variability of different types of carbonate rocks.

LIST OF FIGURES AND TABLES

FIGURES

Figure 1 Upper part of the geological time scale covering oil forming Epochs Figure 2 This carbon-hydrogen- oxygen plot shows the inter-relationship between the main hydrocarbon fuels. Figure 3 Structures of some typical petroleum compounds Figure 4 Shows products derived from crude oil Figure 5 The world’s mega-petroleum provinces ranked in order of ultimate estimated oil and oil equivalent in gas. Figure 6 Shows basins containing giant oil and gas fields of different ages in reelation to plate boundaries. Figure 7 Shows the shape of the Williston basin in the north west of the U.S. Figure 8 Shows fold and fault traps and different types of stratigraphic traps, related to pinch outs, carbonate reefs and channel sands. Figure 9 Cross-sections of traps forming anticlines. Figure 10 Example of a structural stratigraphic trap Figure 11 Map showing relationship between North Sea oilfields. Figure 12 Gravimetric map between rock structure and vertical gravitational pull based on assumption that deeper rock horizons are denser than shallow ones. Figure 13 Shock waves produce diagnostic patterns as they are reflected back to hydrophones at the surface and a seismic profile from the Norwegian N.Sea. Figure 14 Part of a seismic profile from the Norwegian N.Sea. Length of

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The profile is 25 kms and the vertical scale is in seconds of two way time. Figure 16 Schematic example of an interpretation from an electric log and radio-activity from a gamma log. Figure 17 Comparison of a neutron log of porosity log with those determined by core analysis. Figure 18 Section of a basic electric log through reservoir sand Figure 19 Illustrating the effects of coning during oil production a) water coning and b) gas coning. Figure 20 Shows porosity types in limestone reservoirs TABLES Table 1 Diagrammatic representation of the petroleum resource classification used bythe USGS and the US Bureau of Mines

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