Petroleum Accumulation

The fluids contained within petroleum accumulations are mixtures of organic compounds, which are mostly hydrocarbons (molecules composed of hydrogen and carbon atoms), but may also include sulphur, nitrogen, oxygen and metal compounds. This section will concentrate on the hydrocarbons, but will explain the significance of the other compounds in the processing of the fluids. 

Lee, M.-K. and C. M. Bethke, 1994, Groundwater flow, late cementation, and petroleum accumulation in the Permian Lyons sandstone, Denver basin. American Association of Petroleum Geologists Bulletin 78,217-237. 

An important aspect of the formation of petroleum accumulations is timing. The reservoir must have been deposited prior to petroleum migrating from the source rock to the reservoir rock. The seal and trap must have been developed prior to petroleum accumulating in the reservoir, or else the petroleum would have migrated farther. The source rock must have been exposed to the appropriate temperature and pressure conditions over long periods of time to change the organic matter to petroleum. The necessary coincidence of several conditions is difficult to achieve in nature. 

Once the geologist or geophysicist has gathered evidence of potential for a petroleum accumulation, called a... 

Gases migrating from depth are also constituents of the soil air, their supply to the soil air varying according to proximity to sources and conduits. Sources of these gases include, but are not exclusively, mineral deposits and petroleum accumulations. 

Some gases are physically trapped in mineral deposits and petroleum accumulations at depth but escape in trace quantities and migrate to die surface. The emplacement of many hydrothermal mineral deposits is accompanied by the introduction of large quantities of CO2 into the surrounding host rocks. Much of this CO2 is either trapped in fluid inclusions or incorporated into carbonate minerals. Its detection may act as a guide to the presence of the mineral deposit with which its introduction was associated (Chapter 4). 

By their very provenance, these are not gases indicative of mineral deposits or petroleum accumulations. Rather a bubbles aets as a carrier for atoms of other elements which attach to the surface of the bubble. The atoms that attach to bubble surfaces include not only indicator and pathfinder gases such as Rn but also non-gaseous species such as metals. Streams of gas bubbles therefore have the capacity to deliver to the near surface minute geochemical sanples from considerable depth. 

Fenn, I.J., 1940. A hypothesis for the origin of certain geochemical anomalies associated with petroleum accumulation. In E.E. Rosaire (ed.). Symposium on Geochemical Prospecting for Petroleum. Amer. Assoc. Pet. Geol. Bull., 24 1400-1433. 

Horvitz, L., 1954. Near-surface hydrocarbons and petroleum accumulation at depth. Mining Eng., December, pp. 1205-1209. 

Saunders, D.F., Burson, K.R. and Thompson, C.K., 1991. Observed relation of soil magnetic susceptibility and soil gas hydrocarbon analyses to subsurface petroleum accumulations. Amer.Assoc. Pet. Geol. Bull., 75 389-404. 

Atwater, G.I, and Forman, M.J., 1959. Nature and growth of southern Louisiana salt domes and its effect on petroleum accumulation. Am. Assoc. Pet. Geol. Bull., 43 2592-26622. 

Di Primio Skeie use compositional data from closed system pyrolysis experiments to construct a compositional kinetic model of petroleum generation from the Draupne Formation. Although this, in itself, is not new, they developed the model in a format compatible with the data resolution used in PVT models that are familiar to reservoir engineers. Most importantly, they tuned their PVT models to the observed properties of actual petroleum accumulations observed in a well-defined portion of the North Sea. This represents a significant... 

Kuo, L.-C. 1997. Gas exsolution during fluid migration and its relation to overpressure and petroleum accumulation. Marine and Petroleum Geology, 14, 224-229. 

Karlsen et al. (1995) concluded, on the basis of a database of 33 petroleums from the Haltenbanken region, that the fractions of the oils and condensates of this region were all sourced from the Spekk Formation. These petroleums represented the majority of petroleum accumulations at that time, apart from Smorbukk. This interpretation was based on biomarkers, medium range alkanes and aromatic hydrocarbons as well as on individual molecular... 

Whilst certain aspects of the exact formation mechanism for these petroleum inclusions is unknown (cf. Roedder 1984 Nedkvitne et al. 1993), the data presented here, as well as unpublished work from several petroleum accumulations in Jurassic sands from the Viking Graben and the Haltenbanken area (Norwegian Shelf), suggest that diagenesis proceeds, utilizing residual water films, even under petroleum saturations corresponding to those found today in these petroleum-filled accumulations (up to 80% hydrocarbon saturation). 

Bhullar, A. G., Karlsen, D. A., Lacharpagne, J. C. Holm, K. 1999a. Reservoir screening using latroscan TLC-FID and identification of palaeo-oil zones, oil water contacts, tar-mats and residual oil saturations in the Froy and Rind petroleum accumulations. Journal of Petroleum Science and Engineering, 23, 41—63. 

Chung H. M., Walters C., Buck S. and Brinham G. (1998) Mixed signals of the source and thermal maturity for petroleum accumulations ftom light hydrocarbons an example of the Beryl field. Org. Geochem. 29, 381 -396. 

The vertical interval from the crest of the reservoir to the petrolenm water contact is termed the pay zone. Not ah of this interval may be prodnctive. It may also contain impermeable strata. Thns it is nsnal to differentiate between the gross pay and the net effective pay. The vertical interval from the crest of a reservoir to the lowest closing contour on a trap is termed the closnre. The lowest closing contonr is termed the spill plane. The nadir of the spiU plane is termed the spill point. Depending onthe amount of petroleum available a trap may or may not be full to the spill point. The term field is applied to a petroleum-productive area. An oil field may contain several separate pools. Apool is a petroleum accumulation with a single petroleum-water contact. 

Crovelli, R.A. and Barton, C.C. (1995). Fractals and the Pareto distribution applied to petroleum accumulation-size distributions. In Fractals in Petroleum Geology and Earth Processes, Barton, C.C. and La Pointe, P.R. (eds). Plenum Press, New York, p. 59-72. 

Significant oil and gas fields are mainly associated with large basins located on continental margins, on former continental margins, or in intercontinental basins where significant sediment accumulations have occurred. These accumulations may be currently located onshore or offshore. Areas devoid of petroleum accumulations include areas of igneous rocks or sedimentary rocks that have been subjected to very high levels of heat and pressure. 

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