PETROLEUM GEOLOGY/Overview 247 into a new carrier system much more rapidly than does a maturing source rock See Also Petroleum Geology: The Petroleum System; Exploration; Production Sedimentary Environments: Depositional Systems and Facies; Carbonate Shorelines and Shelves Sedimentary Processes: Karst and Palaeokarst Sedimentary Rocks: Chalk; Dolomites; Sandstones, Diagenesis and Porosity Evolution; Limestones Tectonics: Faults; Fractures (Including Joints) Further Reading Figure 21 Petroleum migration along high permeability sand stone beds within a stacked sequence of turbidite sandstones and siltstones The migration route was exposed during the excav ation of a road cutting in Ecuador Photograph by M Heffernan Reproduced from England WA, Mackenzie A, Mann D, and Quigley T (1987) The movement and entrapment of petroleum fluids in the subsurface J Geol Soc., vol 144, p 327 London Tertiary Migration Tertiary migration includes leakage, seepage, dissipation, and alteration of petroleum as it reaches the Earth’s surface The products of seepage may be gas chimneys in the shallow sediment, gas hydrate layers and mounds, cemented pock marks and mud volcanoes, effects on vegetation, and live oil and gas seepage at the surface The physical processes that drive tertiary migration are the same as those that operate during secondary migration Buoyancy drives the petroleum to the surface This may be helped or hindered by overpressure gradients or hydrodynamics The only major difference that can be used to separate tertiary migration from secondary migration is the rate of petroleum supply Trap failure, through capillary leakage, hydraulic fracture, or tectonism, supplies petroleum Abbots IL (1991) United Kingdom Oil and Gas Fields, 25 Years Commemorative Volume, Geological Society Memoir No 14 London: Geological Society Allen PA and Allen JR (1990) Basin Analysis, Principles and Applications Oxford: Blackwell Science Archer JS and Wall PG (1986) Petroleum Engineering, Principles and Practice London: Graham & Trotman Bathurst RGC (1976) Carbonate Sediments and Their Dia genesis, Developments in Sedimentology 12 Oxford: Elsevier Dickson JAD and Saller AH (1995) Identification of sub aerial exposure surfaces and porosity preservation in Pennsylvanian and Lower Permian shelf limestones, east ern central Basin Platform, Texas In: Budd DA, Saller AH, and Harris PM (eds.) Unconformities and Porosity in Carbonate Strata, American Association of Petroleum Geologists Memoir 63, pp 239 258 Tulsa, OK: Ameri can Association of Petroleum Geologists England WA and Fleet AJ (1991) Petroleum Migration, Special Publication 59 London: Geological Society Foster PT and Rattey PR (1993) The evolution of a frac tured chalk reservoir: Machar Oilfield, UK North Sea In: Parker JR (ed.) Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference, pp 1445 1452 London: Geological Society Glennie KW (1998) Petroleum Geology of the North Sea, Basic Concepts and Recent Advances, 4th edn Oxford: Blackwell Science Gluyas JG and Hichens HM (2003) United Kingdom Oil and Gas Fields Commemorative Millennium Volume, Memoir 20 London: Geological Society Gluyas JG and Swarbrick RE (2003) Petroleum Geoscience Oxford: Blackwell Science McBride BC, Weimer P, and Rowan MG (1998) The effect of allochthonous salt on the petroleum systems of the Northern Green Canyon and Ewing Bank (Offshore Lou isiana), Northern Gulf of Mexico American Association of Petroleum Geologists Bulletin 82: 1083 1112 Reynolds AD (1994) Sequence stratigraphy and the dimen sions of paralic sandstone bodies In: Johnson SD (ed.) High Resolution Sequence Stratigraphy: Innovations and Applications, pp 69 72 Liverpool: Liverpool University Selley RC (1996) Elements of Petroleum Geology, 2nd edn San Diego: Academic Press