Deformation Mechanisms in Porous Sandstones: Implications for Development of Fault Seal and Migration Paths in the Recôncavo Basin, Brazil
P. Magnavita Luciano, 2000. "Deformation Mechanisms in Porous Sandstones: Implications for Development of Fault Seal and Migration Paths in the Recôncavo Basin, Brazil", Petroleum Systems of South Atlantic Margins, Marcio Rocha Mello, Barry J. Katz
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Fault zones in porous sandstones consist of a series of light-colored, more resistant strands of comminuted or pulverized rock, to which various terminology has been applied. Resulting fault geometry is variable but can be predicted under certain fractal distributions, thus a multifractal approach is able to discriminate different fault sets. Texturally deformed sandstones along individual microfaults are characterized as breccia and gouge zones. Fault evolution involves a fracture processes, probably followed by diffusion mass transfer mechanisms, which make possible a balance in the silica budget. Analysis of failure criteria suggests that, initially, deformation occurs as a dilation phase through strain-softening processes, which change to strain-hardening because of reduction in grain size and porosity enhanced by healing of opening microcracks. The drastic reduction in porosity and permeability results in a membrane seal, the develpment of which depends on such factors as cataclasis, shale smear, diagenesis, and hydrocarbon migration. Along this membrane seal, displacement pressures are higher than in the undeformed sandstone, so the fault zone acts as a potential barrier capable of focusing oil migration. This is constrained by mercury injection curves indicating reductions in pore throat width of at least two orders of magnitude between the intact sandstones and the deformed rock.
These observations are exemplified by the Mata–Catu transfer zone, the most prolific trend in the Recôncavo Basin. Isotopic analyses of gases along and across the trend corroborate that the trend is in a key positon to allow long-distance oil migration. Thus, some oil accumulations in fields at great distances from the oil kitchen can be explained.
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Petroleum Systems of South Atlantic Margins
“Giant oil and gas fields have been discovered through recent exploration in the deep and ultra-deep water regions of Angola, Brazil, and Nigeria. This volume includes 1. Descriptions of the petroleum geology of more than 12 basins along the two margins 2. Discussion of various components of the petroleum systems within these basins 3. Discussion on the use of the petroleum systems concept in the South Atlantic marginal basins to classify and characterize the diversity of the systems 4. Methodologies used in the assessment of petroleum system components 5. Overviews of the different components of the region's petroleum systems. Petroleum explorationists and researchers working in the South Atlantic will welcome this addition to their libraries.
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