Preferential pathway model

Horizontal pathways deflect the surface location of the anomaly laterally away from its subsurface origin. Thus if an anomaly is associated with the surface expression of a porous formation, one should suspect a down-dip source (or down-groundwater gradient source). The same conclusions can be inferred for anomalies associated with unconformities, low angle faults and listric faults. 

Vertical pathways are dominated by the intersection of high angle faults and fractures with reservoir and carrier beds. In this case the surface expression of the source of the hydrocarbons will lie directly above, or only slightly displaced from the source. The 

The high permeability of fractures causes them to preferentially focus fluid flow. The effectiveness of fractures as mass transport systems for fluids is evident from a casual examination of mineralisation in fractured rocks and leakage of groundwater at fracture outcrops. Similarly, these fractures act as preferential hydrocarbon pathways, focusing their flow from source beds to surface. 

Relation between fracture intensity and gas leakage (A) plan showing lineament, fractures and gas sample sites (B) distribution of fracture intersections with distance from lineament (C) distribution of anomalous gas sample sites with distance from lineament (reproduced with permission of the American Association of Petroleum Geologists, whose permission is required for future use, from Richers et al 1986, AAPG Bull., vol. 70, no. 7, Fig. 13, p. 885, AAPG 1986). 

The following examples illustrate the means of interpreting what are often referred to as direct anomalies using preferential pathway models. These direct anomalies may be either vertically over their subsurface source, or laterally displaced by varying amounts (Sokolov, 1971b Pirson, 1969 Laubmeyer, 1933). What is generally not realised is that most areas contain microfractures to the extent that they allow gases to escape vertically. 

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The preferential pathway model summarises the movement of hydrocarbon fluids through the subsurface to their final destination as a surface seep, either directly or by way of an intermediate trap. It is certainly not definitive nor complete, but illustrates some of the ehallenges eonfronting the petroleum geologist in his quest for new resources. 

Numerical models are used to predict the performance and assist in the design of final cover systems. The availability of models used to conduct water balance analyses of ET cover systems is currently limited, and the results can be inconsistent. For example, models such as Hydrologic Evaluation of Landfill Performance (HELP) and Unsaturated Soil Water and Heat Flow (UNSAT-H) do not address all of the factors related to ET cover system performance. These models, for instance, do not consider percolation through preferential pathways may underestimate or overestimate percolation and have different levels of detail regarding weather, soil, and vegetation. In addition, HELP does not account for physical processes, such as matric potential, that generally govern unsaturated flow in ET covers.39 42 47... 

The Koongarra U deposit in the Northern Territory of Australia has been studied to evaluate the processes and mechanisms involved in the geochemical alteration of the primary ore zone, and to model the formation of the secondary U ore zone and dispersion fan (Duerden 1991 Duerden Airey 1994). Studies of the distribution of the U in the dispersion fan (Murakami et al. 1991) have provided data on the fixation of U leached from the primary ore deposit. Their work has shown that, for this system, fractures are not only preferential pathways for ground-water movement but also contain secondary minerals with high sorption capacity for elements such as U. Even in the monsoonal climate, in which this deposit is located, a significant proportion of the uranium has not been released from the vicinity of the primary ore body. 

There are also several experimental studies dedicated to the acetic acid + OH reaction [133-138]. According to the primary kinetic isotope effect (KIE) carried out by Singleton et al. [136], the preferential pathway in this case is also the H-atom abstraction from the carboxyl group. From the theoretical modeling on this reaction [137,138], it seems that the acidic H-abstraction, is greatly enhanced and largely controlled by the formation of very stable H-bonded reactant complexes. 

Soil structure, antecedent soil moisture and input flow rate control rapid flow along preferential pathways in well-structured soils. The amount of preferential flow may be significant for high input rates, mainly in the intermediate to high ranges of moisture. We use a three-dimensional lattice-gas model to simulate infiltration in a cracked porous medium as a function of rainfall intensity. We compute flow velocities and water contents during infiltration. The dispersion mechanisms of the rapid front in the crack are analyzed as a function of rainfall intensity. The numerical lattice-gas solutions for flow are compared with the analytical solution of the kinematic wave approach. The process is better described by the kinematic wave approach for high input flow intensities, but fails to adequately predict the front attenuation showed by the lattice-gas solution. 

In real porous media, mineral deposition or dissolution would create preferential pathways to solute migration by diffusion and by advective fluid flow. Simulation of the formation of preferential pathways is beyond the capabilities of the present, one-dimensional model. Future development will extend the present model to higher dimensionality in order to examine these interesting processes. 

For preferential pathway geometries other than the flat slab model, X has to be regarded as the characteristic dimension of the segregated regions corrected by a suitable shape factor. 

By changing from the simplest to larger aliphatic and cyclic ketones, structural factors may be introduced which favor alternative unimolecular primary photoprocesses or provide pathways to products not available to the simple model compound. In addition, both the increase in molecular size and irradiation in solution facilitate rapid vibrational relaxation of the electronically excited reactant as well as the primary products to thermally equilibrated species. In this way the course of primary and secondary reactions will also become increasingly structure-selective. In a,a -unsym-metrically substituted ketones, the more substituted bond undergoes a-cleavage preferentially. 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

The sense of asymmetric induction in the a-oxygenated aldehydes, which shows a strong kinetic preference for the formation of the syn diol, is consistent with the classic Gram model as in 27 (Scheme 32). Once the complexation occurs, the allyl group is transferred to the carbonyl carbon from the less hindered 7r-face opposite to that occupied by the R group. In 28, the chelation pathway is able to adopt a chair conformation which accommodates the favored formation of the syn-diol. For /3-chelate reactions, the factors which influence the product formation appear to be the same. When 29 forms, the intramolecular attack occurs syn to the hydroxyl group. This reaction trajectory leads preferentially to the anti-dio, provided that a chairlike transition states such as 30 is followed. 

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