Petroleum Occurrence & Migration (Lecture Notes) PDF

Parameters Controlling Petroleum Occurrence: 1) Source Rocks: The non-hydrocarbon organic substance "kerogen" is the major source of oil and gas in source rocks. The generation of hydrocarbons from the source material depends primarily on the temperature to which the organic material is subje...

Parameters Controlling Petroleum Occurrence: 1) Source Rocks: The non-hydrocarbon organic substance "kerogen" is the major source of oil and gas in source rocks. The generation of hydrocarbons from the source material depends primarily on the temperature to which the organic material is subjected. Hydrocarbon generation appears to be negligible at temperatures less than 150oF (65oC) in the subsurface and reaches a maximum within the range of 225oF to 350oF (107oC and 176oC), the “hydrocarbon window”. Increasing temperatures convert the heavy hydrocarbons into lighter ones and ultimately to gas. At temperatures above 500oF (260oC), the organic material is carbonized and destroyed as a source material. So, if source beds become too deeply buried no hydrocarbons will be produced. 2) Carriers Rocks: The carriers rock is porous and permeable rocks, it is joint between the source rock and the reservoir rocks. 3) Reservoir Rocks: The reservoir rock is porous and permeable, and the structure is bounded by impermeable barriers which trap the hydrocarbons. Sandstones and carbonate rocks form the majority of reservoirs rock. 4) Cap Rocks: Rocks don't have porosity or permeability which prevent vertical oil movement, for example anhydrite rocks. 5) Traps: The trap consists of an impermeable layer of rocks to prevent incline and horizontal oil movement. Trap is formation of rocks suitable for containing hydrocarbons and closed by impermeable formation through which hydrocarbons will not migrate. The part of the trap that contains hydrocarbons is called a reservoir. A reservoir is that portion of a trap, which contains oil and/or gas.  Reservoir is: ✓porous so that stores oil ✓permeable so that allows flow of oil ✓has a mechanism to prevent escape of oil (trapping mechanism) Classification of Traps 1. Structural Traps It is related to tectonic structures. A. Domes and Anticlines Trap Formed by the tectonic uplift and/or folding of sedimentary rocks. Dome is circular in shape, and anticline is an elongate fold. Dome Anticline B. Fault Trap In which closure is controlled by the presence of at least one fault surface. 2. Salt Domes Traps Salt domes traps are caused when salt mass is forced upwards through layers Salt Dome Trap 3. Stratigraphic Traps It is the trap created by the limits of reservoir rock itself, without any structural control. It is formed by changes in rock type (lithofacies change) or pinch-outs, unconformities, or sedimentary features such as reefs. A. Pinch-Out Trap Oil and gas may accumulate in traps formed by the bodies of porous lithofacies (rock types) embedded in impermeable lithofacies, or by the pinch-outs of porous lithofacies within impermeable ones.  Trap formed by lithofacies change (Sandstone pinch-out). B. Unconformity Trap Trap whose closure is controlled by the presence of an unconformity. Tilted permeable rocks erosionally truncated and covered by younger impermeable deposits. Oil and gas trapped below an unconformity. C. Reef Trap Sedimentary rock, most commonly produced by organisms that secrete shells such as corals. Because the rocks that surround reefs can differ in composition and permeability, porous reefs can form stratigraphic traps for hydrocarbons. 4. Combination Traps It is a combination trap is where two (or more) trapping mechanisms come together to create the trap. Remarks: 1- Up to 75% of world petroleum accumulation are associated with structural traps (major; anticline, minor; fault). 2- 25% of oil traps are stratigraphic nature or combination traps (Unconformity, reef). Petroleum Migration & Accumulation: Oil and gas move out of the source beds and accumulate in the reservoir rocks. The transfer from source rocks to reservoir rocks is called primary migration. Movement of petroleum within the porous and permeable reservoirs beds is known as secondary migration. Oil, gas and water slowly migrate through permeable rocks, when they meet an impermeable barrier, they can go no further, so oil and gas accumulate. This barrier is generally referred to as a trap.