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CHAPTER 05 GENERATION AND MIGRATION OF HYDROCARBON 1- GENERATION OF HYDROCARBON 1.1-Petroleum Source Material 1.1.1-Formation and Preservation of Organic Matter • In the nineteenth century, it was widely believed that petroleum had a magmatic origin and that it migrated from great depths along subcrustal faults • But the overwhelming evidence now suggests that the original source material of petroleum is organic matter formed at the earth's surface • The process begins with photosynthesis, in which plants, in the presence of sunlight, convert water and carbon dioxide into glucose, water and oxygen: 6CO2 + 12H2O 6O2 C6H12O6 + 6H2O + • Photosynthesis is part of the larger-scale carbon cycle (Fig 01) Ordinarily, most of the organic matter produced by photosynthesis gets recycled back to the atmosphere as carbon dioxide This can occur through plant and animal respiration, or through oxidation and bacterial decay when organisms die Fig 01-CARBON CYCLE 1.1.2-Preservation and Organic Productivity • All organic matter in the ocean is originally formed through photosynthesis The main producers are phytoplankton, which are microscopic floating plants such as diatoms, dinoflagellates and the blue-green algae Bottom-dwelling algae are also major contributors in shallow water, shelf environments 1.1.3-Preservation and Organic Destruction • Areas of high productivity are not necessarily those best suited for preservation Destruction of organic matter must also be prevented Complete biological recycling of organic carbon is retarded by anything that limits the supply of elemental oxygen • This occurs most favorably in either one of two settings: rapid rate of deposition; and stratified, oxygen-poor water bodies with anoxic bottoms • First, rapid deposition may be necessary to keep the organic material from being destroyed • Preservation is also favored by density stratification, which produces oxygen-poor bottom waters • Water stratification and oxygen depletion are well known in the modern Black Sea, • The Eocene-age lakes of Utah, Colorado and Wyoming, in which the Green River oil shale formation was deposited, have been interpreted as seasonally stratified water bodies which at a later stage become permanently stratified (Fig 02) Fig 02 • In the present-day world's oceans, there is a zone of maximum oxygen depletion at a depth of about 200 meters, with oxygen more abundant in the shallow surface waters and again at deeper levels (Figure 03) SOLUTION IN GAS The third mechanism, expulsion of oil dissolved in gas, requires that there be a separate gas phase Such a phase could only exist where the amount of gas far exceeded the amount of liquid hydrocarbon Therefore, it would be expected only in the late stage of catagenesis or in source rock capable of generating mainly gas B-DISTANCE AND DIRECTION IN PRIMARY MIGRATION In most cases the distances of primary migration are short (probably between 10cm and 100m) Primary migration ends whenever a permeable pathway is reached Because the source rock is overpressure expulsion can be lateral, upward, or downward depending upon the carrier bed characteristics of the surrounding rocks Thus a source rock lying between to sands will expel hydrocarbon into both carrier beds 2.3.2 – SECONDARY MIGRATION • Secondary migration is much better understood than primary migration In secondary migration, petroleum occurs mainly as discrete oil droplets that migrate through a porous, permeable, water-wet conduit Because pore diameters are large, even relatively large oil droplets can be accommodated A-FACTORS CONTROLLING MIGRATION: Buoyancy Capillary pressure Hydrodynamic flow SECONDARY Buoyancy • With buoyancy, oil droplets move upward through the carrier beds with a force dependent mainly on the density difference between the petroleum and the formation water • The process will continue until the droplet reaches a pore space that is smaller than its diameter Capillary pressure • Further motion can occur only by deforming the droplet and squeezing it through the pore space The force required to this is called capillary pressure Capillary pressure becomes higher as pore diameter decreases, until capillary pressure becomes so high that buoyancy forces cannot overcome it, and entrapment of the oil droplet takes place Hydrodynamic flow Secondary migration will also be affected when the flow of subsurface waters creates hydrodynamic gradients • Upward hydrodynamic gradients assist flow by buoyancy (Figure 23) • Downward gradients oppose flow by buoyancy and can create hydrodynamic barriers to migration (Figure 24) • In some cases, these hydrodynamic barriers, either themselves or in combination with other factors, may produce traps Figure 23 UPWARD HYDRODYNAMIC GRADIENTS ASSIST FLOW BUOYANCE Figure 24 DOWNWARD GRADIENTS OPPOSE FLOW BUOYANCE AND CAN CREATE HYDRODYNAMIC BARRIERS TO MIGRATION B-DISTANCE AND DIRECTION IN SECONDARY MIGRATION Secondary migration generally occurs along the layering of the carrier beds, and therefore lateral migration can take place over a wide range of distances Short range migration is common where the reservoir is in close proximity to its source beds, for example in reefs on the flank of a deep, muddy basin or in shoestring sandstone bodies enclosed by their source shales B-DISTANCE AND DIRECTION IN SECONDARY MIGRATION (Cont.) Movement within a confined carrier bed will be updip perpendicular to structural contours whenever possible Migration may have to proceed at an oblique angle to structural contours where the faulting for facies changes create impassable barrier Within a massive sandstone secondary migration will occur both laterally and vertically Exp.Hassi Meesaud Res.Algeria Migration over long sometimes be proven distances can Oil is in Cambrian Sandstone reservoir immediately under an unconformity The Chem.of oil shows it to have come from Silurial shale Late Paleozic uplift and erosion removed all but except the Cambrian sandstone from the area and any oil would have been lost ° The nearest subcrop of Silurian shale is 40 km away The oil in Reservoir must have migrated long distance along the unconformity surface after the Mesozoic burial Figure 25 GEOLOGIC CROSS SECTION IN THE HASSI MESSAUD FIELD Exercise ... begins and only methane is produced Figure 09 GENERATION OF PET RELATION TO AVERAGE, MAX., MIN OF BURIAL DEPTH •The correlation of petroleum generation to depth is primarily a function of the... Temperature and Time in Petroleum Formation • The generation of hydrocarbons can be related to burial depths of source rocks, since temperature increases with increased depth The actual generation. .. 1.2-Hydrocarbons and Kerogen •Type The macerals and amorphous particles in kerogen affect its ability to generate hydrocarbons • Oil-prone kerogens generally are made of more than 65% exinite and
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