Page 93 - Geology and Geochemistry of Oil and Gas

Hydrocarbon generation Conversion of organic material/kerogen to petroleum. Generation of oil and gas from kerogen (maturation) results in
a significant increase in volume ORIGIN
Decomposition of hydrocarbons Breakdown of hydrocarbons. About 2- to 3-fold volume increase caused by breakdown of hydrocarbon long-chained
(thermogenic) molecules into shorter-chained molecules. Such reactions generally occur at depths below 2–4 km and temperatures OF
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greater than 70–120 1C. Thermal cracking of organic molecules is initiated at temperatures of 120–140 C depending
upon the depth of rocks. At temperatures greater than 180 1C, almost all the hydrocarbons are converted in to
methane
Migration of fluids Gas migration. Upward migration of hydrocarbon gases from lower to upper horizons along faults. This can result in ABNORMAL
overpressuring of upper horizons
Changes in fluid pressure (hydraulic
head); movement of fluids
Osmosis Osmosis. Contrasts in the brine concentration of formation fluids can induce the transfer of fluids across a FORMATION
semipermeable membrane. On regional basis, e.g., San Juan Basin, New Mexico; Western Sedimentary basin,
Canada; San Joachim Valley, CA, USA, Gulf Coast; and Paradox Basin, IL, USA
Fluid pressure head Piezometric fluid level. Effect of regional potentiometric surface, e.g., artesian water system. Examples would include
the Artesian Basin, FL, USA, Great Artesian Basin, Australia; and North Dakota Basin, USA
Structure of permeable reservoir. Pressure transmission to a shallower part of reservoir. Large anticlines, steeply PRESSURES
dipping beds, etc
Oilfield production operations Repressuring of reservoir rocks. Can occur as a result of massive fluid injection or fluid influx into the formation, i.e.,
massive water injection program (secondary recovery). Pressure increase may occur across faults, or behind casing
Permafrost environment Formation of frost heaves (pingos). Permafrost environment: trapping of unfrozen zone in practically closed system.
Freeze-back pressures around shut-in arctic wells. Gas hydrate reservoirs (e.g., Mackenzie Delta, Canada)
Difference in specific weight Density difference. Difference between the weight of gas column and that of a fluid (oil or water) column 67