a b c d e SOURCE ROCKS & MATURATION HYDROCARBON MIGRATION CAP ROCKS / SEALS STRUCTURE / TRAP RESERVOIR ROCKS Petroleum System Processes • Generation - Burial of source rock to temperature and pressure regime sufficient to convert organic matter into hydrocarbon • Migration - Movement of hydrocarbon out of the source rock toward and into a trap • Accumulation - A volume of hydrocarbon migrating into a trap faster than the trap leaks resulting in an accumulation • Preservation - Hydrocarbon remains in reservoir and is not altered by biodegradation or “water-washing” • Timing - Trap forms before and during hydrocarbon migrating Petroleum System Processes Gas Cap Oil Entrapment Accumulation Water Seal Rock Reservoir Rock Migration 120° F Source Rock Generation 350° F 2480 SOURCE ROCKS • Hydrocarbon originates from minute organisms in seas and lakes When they die, they sink to the bottom where they form organic-rich "muds" in fine sediments (usually become gray– black shale) • These "muds" are in a reducing environment or "kitchen", which strips oxygen from the sediments leaving hydrogen and carbon • The sediments are compacted to form organic-rich rocks with very low permeability • The hydrocarbon can migrate very slowly to nearby porous rocks, displacing the original formation water The principal zone of oil formation during the thermal generation of petroleum hydrocarbons • If the temperature is too low, the organic material cannot transform into hydrocarbon • If the temperature is too high, the organic material and hydrocarbons are destroyed HYDROCARBON MIGRATION • Hydrocarbon migration takes place in two stages: – Primary migration - from the source rock to a porous rock This is a complex process and not fully understood It is probably limited to a few hundred metres – Secondary migration - along the porous rock to the trap This occurs by buoyancy, capillary pressure and hydrodynamics through a continuous water-filled pore system It can take place over large distances CAP ROCK • A reservoir needs a cap rock • Impermeable cap rock keeps the fluids trapped in the reservoir • It must have zero permeability • Some examples are: – Shales – Evaporites such as salt or anhyhdrite – Zero-porosity carbonates TRAPS GENERAL The reservoir form depends on the depositional environment and post depositional events such as foldings and faulting The criteria for a structure is that it must have: •Closure, i.e the fluids are unable to escape •Be large enough to be economical STRUCTURAL TRAPS Structural traps are formed where the space for petroleum is limited by a structural feature • Tilted fault-block traps are formed where the upward flow of the petroleum is prevented by impermeability along the fault plane and by an overlying cap or seal: common in the North Sea • Anticlinal traps are formed by folding in the rocks • Unconformity traps are generated where an erosional break in the stratigraphic succession is followed by impermeable strata SALT DOME TRAP • Salt Dome traps are caused when "plastic" salt is forced upwards • The salt dome pierces through layers and compresses rocks above This results in the formation of various traps: • In domes created by formations pushed up by the salt • Along the flanks and below the overhang in porous rock abutting on the impermeable salt itself POROSITY AND GRAIN SIZE • A rock can be made up of small grains or large grains but have the same porosity • Porosity depends on grain packing, not the grain size • Total porosity, φt = Total Pore Space Bulk Volume • Effective porosity, φe = Interconnected Pore Space Bulk Volume • Very clean sandstones : φt = φe • Poorly to moderately well -cemented intergranular materials: φt ≈ φe • Highly cemented materials and most carbonates: φe < φt DIAGENESIS • • The environment can also involve subsequent alterations of the rock such as: Chemical changes Diagenesis is the chemical alteration of a rock after burial An example is the replacement of some of the calcium atoms in limestone by magnesium to form dolomite • Mechanical changes - fracturing in a tectonically-active region • CARBONATE POROSITY TYPES • Carbonate porosity is very heterogeneous It is classified into a number of types: Interparticle porosity: Each grain is separated, giving a similar pore space arrangement as sandstone Intergranular porosity: Pore space is created inside the individual grains which are interconnected Intercrystalline porosity: Produced by spaces between carbonate crystals Mouldic porosity: Pores created by the dissolution of shells, etc CARBONATE POROSITY TYPES Fracture porosity: • Pore spacing created by the cracking of the rock fabric Channel porosity: • Similar to fracture porosity but larger Vuggy porosity: • Created by the dissolution of fragments, but unconnected CARBONATE POROSITY • Intergranular porosity is called "primary porosity" • Porosity created after deposition is called "secondary porosity" • The latter is in two forms: – Fractures – Vugs FRACTURES • Fractures are caused when a rigid rock is strained beyond its elastic limit - it cracks • The forces causing it to break are in a constant direction, hence all the fractures are also aligned • Fractures are an important source of permeability in low porosity carbonate reservoirs VUGS • Vugs are defined as non-connected pore space • They not contribute to the producible fluid total • Vugs are caused by the dissolution of soluble material such as shell fragments after the rock has been formed • They usually have irregular shapes PERMEABILITY • The rate of flow of a liquid through a formation depends on: – The pressure drop – The viscosity of the fluid – The permeability • The permeability is a measure of the ease at which a fluid can flow through a formation • The unit of measurement is the Darcy • Reservoir permeability is usually quoted in millidarcies, (md) DARCY LAW • • • • • • K = permeability, in Darcies L = length of the section of rock, in centimetres Q = flow rate in centimetres / sec P1, P2 = pressures in bars A = surface area, in cm2 µ = viscocity in centipoise PERMEABILITY AND ROCKS In formations with large grains, the permeability is high and the flow rate larger PERMEABILITY AND ROCKS • In a rock with small grains the permeability is less and the flow lower • Grain size has no bearing on porosity, but has a large effect on permeability ANISOTROPY Horizontal Permeability K K Vertical Permeability V H ≤ K K V ≤ h The permeability in the horizontal direction is controlled by the large grains The permeability in the vertical direction is controlled by the small grains CLASTIC RESERVOIRS • Sandstone usually has regular grains; and is referred to as a grainstone • Porosity : Determined mainly by the packing and mixing of grains • Permeability : Determined mainly by grain size and packing, connectivity and shale content • Fractures may be present CARBONATE RESERVOIRS • Carbonates normally have a very irregular structure • Porosity: Determined by the type of shells, etc and by depositional and postdepositional events (fracturing, leaching, etc.) LIMESTONES • Permeability: Determined by deposition and postdeposition events, fractures • Fractures can be very important in carbonate reservoirs DOLOMITES [...]...STRATIGRAPHIC TRAPS ̈Stratigraphic traps are traps created by the limits of the reservoir rock itself, without any structural control PETROLEUM RESERVOIR ROCKS DEFINITION • A body of porous and permeable rock containing oil and gas through which fluid may move toward recovery opening under the pressure existing or that may be applied ... Structural traps are formed where the space for petroleum is limited by a structural feature • Tilted fault-block traps are formed where the upward flow of the petroleum is prevented by impermeability... biodegradation or “water-washing” • Timing - Trap forms before and during hydrocarbon migrating Petroleum System Processes Gas Cap Oil Entrapment Accumulation Water Seal Rock Reservoir Rock Migration... original formation water The principal zone of oil formation during the thermal generation of petroleum hydrocarbons • If the temperature is too low, the organic material cannot transform into