Hydrocarbon accumulation, timing

Several conditions need to be satisfied for the existence of a hydrocarbon accumulation, as indicated in Figure 2.1. The first of these is an area in which a suitable sequence of rocks has accumulated over geologic time, the sedimentary basin. Within that sequence there needs to be a high content of organic matter, the source rock. Through elevated temperatures and pressures these rocks must have reached maturation, the condition at which hydrocarbons are expelled from the source rock. 

Whether a hydrocarbon accumulation that is no longer replenished from the source rock will persist through extended periods of geological time basically depends on the type of trap, the type of accumulated hydrocarbon and the geological and associated hydrodynamic evolution of the sedimentary basin since entrapment. 

A kinetic trap that is no longer charged with hydrocarbons will loose its accumulated hydrocarbons in time. In addition, when the geological and associated hydrodynamic conditions are stable, a hydrocarbon accumulation may be destroyed by diffusion or by removal of hydrocarbons in aqueous solution. Changes in the chemical composition of accumulated hydrocarbons may result from inorganic oxidation, biodegradation and water washing. 

The secondary hydrocarbon migration system influences both the distribution and the accumulation of hydrocarbons in a sedimentary basin (Table 6.1). Knowledge of the characteristics of a hydrocarbon migration pattern at a certain time during the basin s evolution, provides information on preferred paths of hydrocarbon migration and may indicate areas favourable or unfavourable for hydrocarbon accumulation during that time. In order to establish the amounts and nature of accumulated hydrocarbons and the exact location of the most favourable positions for hydrocarbon accumulation in a... 

The time required for diffusion to occur can sometimes be restrictive. Indeed the time required not only often exceeds the age of the hydrocarbon accumulation but also quite often exceeds the age of the host rock. If this were the dominant process for migration, then the appearance of soil-gas anomalies in the near subsurface would indicate only very shallow accumulations. If a non-steady state exists, where the hydrocarbon signal observed represents only 0.001 times the steady-state signal, then diffusion times could be reduced by a factor of 25 compared to that of the steady-state model. Table 5-V... 

High temperature quartz cementation and the timing of hydrocarbon accumulation in the Jurassic Norphlet sandstone, offshore Gulf of Mexico, USA... 

Even if all of the elements described so far have been present within a sedimentary basin an accumulation will not necessarily be encountered. One of the crucial questions in prospect evaluation is about the timing of events. The deformation of strata into a suitable trap has to precede the maturation and migration of petroleum. The reservoir seal must have been intact throughout geologic time. If a leak occurred sometime in the past, the exploration well will only encounter small amounts of residual hydrocarbons. Conversely, a seal such as a fault may have developed early on in the field s history and prevented the migration of hydrocarbons into the structure. 

When oxidation occurs for a long period of time, the hydrocarbon is consumed and this influences on the concentration of hydroperoxide and the rate of hydrocarbon oxidation. The exact solution for the description of hydrocarbon consumption during oxidation can be found by the common integration of two differential equations one for hydrocarbon consumption and another for hydroperoxide accumulation. The approximation for the time of oxidation t > tmax, where tmax is the moment when [ROOH] = [ROOH]max gives the following equation [3,56] ... 

Beneath the facility, LNAPL product occurs as three main pools (Pool No. I, II, and III) and two smaller pools (Pool No. IV and V) of localized occurrence. The known extent of Pool No. I, HI, IV, and V is entirely beneath the main portion of the refinery. Pool No. II is located beneath the southwestern portion of the site. LNAPL occurrence beneath the facility is shown in Figure 12.19. For purposes of this discussion, a pool is defined as an aerially continuous accumulation of LNAPL. The three main pools consist of individual accumulations of differing product character and are therefore referred to as coalesced pools. Individual product accumulations within these coalesced pools were delineated on the basis of physical and chemical properties characteristic of free hydrocarbon samples retrieved from the wells. Individual accumulations of relatively uniform product are referred to as subpools since it is inferred that they coalesced to form areally continuous occurrences or pools. These data suggest multiple sources over time for coalesced pools. The numbering of individual product accumulations is based on product type and... 

The ubiquitous occurrence, areal extent, and estimated volume of major LNAPL hydrocarbon pools beneath these facilities situated on the Los Angeles coastal plain are shown in Ligure 12.25. Lor purposes of this discussion, a pool is defined as an areally continuous accumulation of LNAPL. Two or more pools that have distinct differences in their respective physical and chemical properties are referred to as coalesced pools. Individual accumulations of relatively uniform product are referred to as subpools, since it is inferred that they have coalesced to form areally continuous occurrences. The occurrence of several pools and subpools at a particular site reflects releases from multiple sources at various times. The combined areal extent of these LNAPL pools is on the order of 1500 acres. The estimated cumulative minimum volume is on the order of approximately 1.5 million barrels an estimated cumulative maximum volume is on the order of 7.5 million barrels. The discrepancy in LNAPL hydrocarbon volume reflects varying methodologies involved in the estimates. 

Thomas et al. (43) showed that phenanthrene is actively metabolized in salmonTds and Lee et al. (41) have shown that benzo[a]pyrene is biodegraded in three species of marine fish. Varanasi et al. (VI ) demonstrated for the first time that the skin of fish exposed to aromatic hydrocarbons via either force-feeding, intraperitoneal injection, or in flowing seawater accumulate substantial concentrations of metabolic products. This is of particular interest since studies of mammalian systems have shown that some alkyl naphthalenes can be accelerators of skin carcinogenesis (32). Varanasi et al. (11) also demonstrated that the mucus of rainFow trout exposed to radiolabeled naphthalene... 

Chlorination of benzene gives an addition product that is a mixture of stereoisomers known collectively as hexachlorocyclohexane (HCH). At one time, this was incorrectly termed benzene hexachloride. The mixtnre has insecticidal activity, though activity was found to reside in only one isomer, the so-called gamma isomer, y-HCH. y-HCH, sometimes under its generic name lindane, has been a mainstay insecticide for many years, and is about the only example of the chlorinated hydrocarbons that has not been banned and is still available for general use. Although chlorinated hydrocarbons have proved very effective insecticides, they are not readily degraded in the environment, they accumulate and persist in animal tissues, and have proved toxic to many bird and animal species. 

Since the solvent is never completely removed at any time prior to final distillation of the product, the accumulation of higher boiling hydrocarbons results if petroleum ether is used. As a result the fore-run of the final distillation will be larger, and the yield of 2-bornene will be reduced. The pentane was distilled to assure the removal of any higher boiling impurities. 

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