Source rocks geometry

Under hydrostatic conditions the basin-wide secondary hydrocarbon migration patterns and consequently also the final distribution of the oil and gas accumulations in a sedimentary basin are closely linked to the stable basin geometry present during hydrocarbon expulsion from the source rocks. 

When the distribution of mature source rocks is known, the first step of the above-described method can be carried out in a comparable way. The pattern of lateral hydrocarbon migration is derived from the basin geometry at the level of the source rock. 

Stainforth presents new models for reservoir filling and mixing. He presents data to support the idea that, in many cases, petroleum does not mix at all during reservoir filling. As new petroleum enters the trap, it fills from the crest of the structure, forcing previously emplaced petroleum downwards. This is a result of the general decrease in fluid density with maturity. This model predicts that the shapes of saturation pressure versus depth curves are related to trap geometry (depth versus volume curves) as well as source rock kitchen parameters. Field data are presented to support this model. 

As we have seen, the controls on the GOR of the oil in the trap may be different from those of the API gravity. If the oil had a gas cap during part of the trap-filling period, the evolving PT conditions in the trap control the GOR, rather than the evolving maturity of the source rock kitchen. Thus, the shape of a GOR trend versus depth in a trap results mainly from the interaction between trap geometry ((2) above), and... 

Data for characterisation of faults in the subsurface are limited to two sources, seismics and wells. Seismic reflection data allow the displacement distribution over a fault surface to be mapped while well and core data may allow determination of fault rock types and deformation mechanisms at specific points, in addition to characterising the lithologies of the host sequence. It is evident from outcrop studies that the internal geometries of fault zones are usually complex, in terms of the numbers of individual slip surfaces, the partitioning of slip between them and in the distribution of different fault rocks, all of which vary over a fault surface. This 3-D complexity of fault zone structure may not be apparent from either seismic or core data but is nevertheless crucial to the bulk hydraulic properties of a fault. 

Nuclear magnetic resonance (NMR) provides a powerful method for the study of molecular motion. The techniques can distinguish molecular reorientation and translation and have proved particularly valuable for the study of self-diffusion in bulk liquids. The molecular motion of liquids in the confined geometry provided by their containment in porous materials has been of considerable interest for many years. It is of importance both as a fundamental scientific problem and because of its technological importance in such diverse systems as oil recovery from rocks and catalytic agents. The purpose of this paper is to question the reliability of many previous investigations and the validity of their interpretation. Potential sources of error are demonstrated by measurements on mobile liquids adsorbed into porous silicas with different geometrical characteristics. The principles illustrated are equally valid for other porous systems. Preliminary measurements of the diffusion coefficient of n-butane in silica as a fimction of temperature and the effect of pore dimensions are presented. 

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