Source beds

The hydrocarbons in some altered form migrate from the source beds through other more porous and permeable beds to eventually accumulate in a rock called the reservoir rock. The initially altered (i.e., within the source beds) organic material may continue to alter as the material migrates. The hydrocarbon movement is probably the result of hydrodynamic pressure and gravity forces. As the source beds are compacted by increased burial pressures, the water and altered organic material are expelled. Water movement carries the hydrocarbons from the source beds into the reservoir, where the hydrocarbon establishes a position of equilibrium for the hydrodynamic and structural conditions [26-29]. 

Demaison, G.J., and G.T. Moore. 1980. Anoxic environments and oil source bed genesis. American Association of Petroleum and Geology Bulletin 64(8) 1179-1209. 

Reed, K. J., "Environment of Deposition of Source Beds of High-Wax... 

Yang, S.X. and Blum, N. (1999a) A fossil hydrothermal system or a source -bed in the Madiyi formation near the Xiangxi Au-Sb-W deposit, NW Hunan, PR China Chemical Geology, 155(1-2), 151-69. 

The exact origin of petroleum is not known, but scienrifie opinion is in favor of origin from the transformation of plant, animal, and marine organisms after marine deposition within finely divided muds. Consensus holds that, after bring formed, petroleum was transferred from its source beds to the rock strata where it is now found. 

Demaison G. I. and Moore G. T. (1980) Anoxic marine environments and oil source bed genesis. Am. Assoc. Petrol. Geol. Bull 64, 1179-1209. 

The physical state of the hydrocarbons during transport is not well known see Matthews (1996a) and Matthews (1996b) for a full discussion. Nevertheless, most of the models proposed for the transport of these fluids from source to reservoir (aqueous transport, micellular, discrete oil-phase transport, gaseous transport, etc.) are applicable to the continued transport of hydrocarbons from these source beds and/or reservoirs to the near-surface environment. An additional constraint on land is that the last stage of transport is generally above the water table. The physics of transport can be subdivided into two categories, effusion and diffusion. 

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. 

An apical anomaly comprises either a continuous zone of elevated gas concentrations or an area with erratic elevated gas concentrations directly over the oil or gas field. An annular or halo anomaly has the form of a doughnut of continuous or discontinuous high gas concentrations surrounding a central zone of lower or background values, the latter overlying the surface projection of the oil or gas field. In a linear or belt anomaly, high gas concentrations are found continuously or intermittently along a line or confined in a belt, usually associated with faults, fracture zones, or matured source beds. 

Results and conclusions The fault zones at Frechen contain clay fillings of up to 1 m in thickness, derived from extremely plastic shale source beds and smeared out over distances as much as 70 m in dip direction. The generation of substantial smears requires slow fault displacement rates and sufficient shale ductility. When a thick shale source bed is traversed by a normal fault, it is first flexed and eventually disrupted by a pull-apart mechanism that creates room for the emplacement of thick clay smears. Simple theoretical considerations suggest that the source bed thickness to some power n + 1 > 2 may be a key parameter in the ranking of seal quality. The length of continuous smears increases with source bed thickness, but will ultimately be controlled by the smearing process. The latter remains to be investigated. 

Plate 4. (a) Max Rudolph fault (Frechen mine, throw approx. 70 m). Clay from different source beds on upthrown and downthrown blocks merges to form layered clay smear free of sandy or coal material, (b) Flexed shale bed on downthrown side of Max Rudolph fault (F). The clay smear (approx. 20 cm thick) and sand wedge to the left of the person resemble situation shown in Fig. 2. 

The clay smears found in the major fault zones form a continuous band which is gradually thinning away from the source bed (see Plates 1 and 3). The smears consists of remarkably pure clay material. At places these smears can reach a thickness of approximately 1 m, but thicknesses of the order of 10-20 cm are more common. Along portions of the Max Rudolph fault, a continuous smear exists over the... 

The overall picture emerging in the course of this field study was that clay smears are formed consistently on all scales in the Frechen exposures, where they represent a universal phenomenon. This would suggest that the conditions necessary for the emplacement of clay smears in minor shear bands are the same as those met along the major faults. Among the factors that are likely to determine the thickness, length and continuity of a smear, the thickness of the source bed and the fault throw are readily identified in the field. The requirement that the shales possess the necessary plasticity is also clearly met, i.e., the shale source bed material may be characterized as highly plastic, fat clay in accordance with the standard soil mechanics classification (Bowles, 1984). 

Extrusion of plastic clays from source beds... 

It is now evident that if the above stress distribution in a horizontally stratified sand/shale sequence were to persist even after the truncation of a particular shale bed by the fault, then this hydrostatically stressed source bed would be put in juxtaposition with a sand at a much lower horizontal stress. In con-... 

A continuous supply of clay material from the source bed to the shear zone can be maintained, given the necessary plastic properties of the material, if a driving horizontal stress difference between distant parts of the source bed and the immediate vicinity of the fault can be maintained. This requires some mechanism of horizontal stress relief to operate at the fault intersection of a source bed. Preferably, the same mechanism should be capable of resolving the space problem implied by the emplacement of massive clay smears. 

Such a mechanism was in fact conceived at some stage in the course of this study, in the first place as a way to overcome the space problem kinematically. Thus we invented the pull-apart mechanism shown in Fig. 3. On traversing a shale source bed, a normal fault only has to be offset in the direction of the downthrown block in order to make room for the emplacement of a clay smear. Evidently, this mechanism... 

Fig. 3. Kinematics of pull-apart of shale source bed during faulting.

Extrusion is consistent with the thinning and associated faulting of the source bed close to the fault (cf. also Figs. 1 and 2). 

The last relationship provides some information on parameters that are likely to control the quality of a clay smear, where quality is expressed simply in terms of clay smear thickness close to the source bed. Other things remaining equal, it suggests that the smear quality may be expected to improve with increasing source bed thickness and burial depth, but to deteriorate with increasing fault slip rates and clay viscosity. 

The negative effects of an increase in slip rate and clay viscosity are intuitively expected. However, the capability of relation (7) to capture these effects quantitatively should not be overrated in view of the strongly simplifying assumptions on which it is based. Indeed, the most useful conclusion to be drawn from Eq. (7) is that clay smear thickness may be expected to depend strongly on source bed thickness. Thus, for linear viscous behavior Eq. (7) predicts the proportionality w °= h. The equivalent result for a power-law secondary creep behavior of the clay would take the form w where n is the appro-... 

The proportionality hP- or wocM"" " does, however, provide a simple criterion for ranking the sealing quality of any two clay smears that originate from distinct source beds with thicknesses /i, and hi, respectively. In particular, in instances where two source beds form a layered clay smear within a given throw interval, the total contribution from these source beds to the smear would be ranked below the contribution from a single source bed of thickness h H- hi, simply because w (hi + hi ) in the former case while woe h +hi in the latter. 

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