This paper present a method relating fluid flow evolution (direction of fluid percolation) to the bulk brittle network (microfracture system) for each stage in a sequence of deformational events. Fluids percolating through cracks are generally trapped as fluid inclusion trails (alignment of fluid inclusions). The principles of the method involve use of fluid inclusions not only as a tool for the establishment of the physicochemical conditions under which fluids were entrapped, but also as a microstructural marker of the geometry of the permeability. The study shows that through the determination of the paleopressures for each stage of fluid migration, the chronology of the cracks and P-T conditions prevailing during each crack formation can be established. Such a method can also be applicable to the reconstruction of the structural evolution of a petroleum basin through the characterization of the principal directions of migration of hydrocarbons fluids.
Within the last few years, much work has been done on microcracks in rocks. Propagation mechanisms, spatial distribution, relation with local stress axis have been carried out by numerous authors (see a review by Krantz, 1983). Experimental studies (e.g Brace and Bombolakis, 1963; Friedman and Logan, 1970; Peng and Johnson, 1972; Tapponier and Brace, 1976; Krantz, 1979 a, b, 1983) indicate that most cracks appear to be extensional fractures propagated roughly parallel to the local maximum stress axis (mode I cracks, Pollard et al, 1982; Seggal, 1984). Compared to the rather large quantity of experimental data, only a few papers have discussed the relationships between the preferred orientation of cracks, the regional framework of deformational events affecting the surrounding area and the nature of the fluids which are percolating in the rock. In a hydrothermal system, evidences of paleofluid movements in rocks are mostly recognized when fluid-rock interaction occur, yielding an alteration of previous mineral assemblages. In the case of superposition of 2 or 3 alteration stages in the same area, it may be difficult to decipher the space - timing relationships of the different stages of fluid percolation since no geometric markers (fractures on all scales) are available in the main fault zones. The best records of this process are the paleofluids trapped as fluid inclusions in healed microcracks (fluid Inclusion trails) of the rock forming minerals (figure 1). However, in most rocks, the repeated microfracturing and healing of the minerals yield complex superimposed patterns of healed microcracks. Such patterns are often difficult to interpret, due to the lack of suitable chronological criteria, and to relate to the open space geometry of the rock. Principles of the method consist in considering the fluid inclusions, not only as a tool for the reconstitution of the physico - chemical conditions under which fluids were entrapped, but also as a microstructural marker giving data on the geometry of the permeability. This study shows that through the determination of the paleopressures for each stage of fluid migration, the chronology of the cracks and P- T conditions prevailing during each crack formation can be established.
