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Deformation Mechanisms in Porous Sandstones: Implications for Development of Fault Seal and Migration Paths in the Recôncavo Basin, Brazil

By
P. Magnavita Luciano
P. Magnavita Luciano
Petrobr´s/E & P–BA Salvador, Brazil
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Published:
January 01, 2000

Abstract

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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Contents

AAPG Memoir

Petroleum Systems of South Atlantic Margins

Marcio Rocha Mello
Marcio Rocha Mello
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Barry J. Katz
Barry J. Katz
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American Association of Petroleum Geologists
Volume
73
ISBN electronic:
9781629810706
Publication date:
January 01, 2000

GeoRef

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