Abstract
Clay minerals in structurally complex settings influence fault zone behavior and characteristics such as permeability and frictional properties. This work aims to understand the role of fault zones on clay authigenesis in arkosic, high-porosity sandstones of the Cretaceous Rio do Peixe basin, northeast Brazil. We integrated field, petrographic and scanning electron microscopy (SEM) observations with X-ray diffraction data (bulk and clay-size fractions). Fault zones in the field are characterized by low-porosity deformation bands, typical secondary structures developed in high-porosity sandstones. Laboratory results indicate that in the host rock far from faults, smectite, illite and subordinately kaolinite, are present within the pores of the Rio do Peixe sandstones. Such clay minerals formed after sediment deposition, most likely during shallow diagenetic processes (feldspar dissolution) associated with meteoric water circulation. Surprisingly, within fault zones the same clay minerals are absent or are present in amounts which are significantly lower than those in the undeformed sandstone. This occurs because fault activity obliterates porosity and reduces permeability by cataclasis, thus: (1) destroying the space in which clay minerals can form; and (2) providing a generally impermeable tight fabric in which external meteoric fluid flow is inhibited. We conclude that the development of fault zones in high-porosity arkosic sandstones, contrary to other low-porosity lithologies, inhibits clay mineral authigenesis.
Highlights
Clay minerals have important economic applications in industry—e.g., [1]
The goal of this study is to investigate how fault zones in arkosic sandstones modify grain-scale fabric and control clay mineral authigenesis at shallow burial depths
We studied clay mineral assemblages in faulted, high-porosity arkosic sandstone of the Rio do
Summary
Clay minerals have important economic applications in industry—e.g., [1]. in most geological settings clay minerals can occur in faults, influencing their permeability, frictional properties [2,3,4,5,6,7,8] and subsurface fluid flow [4,9,10,11,12,13,14]. Faults in high-porosity sandstones are generally considered as barriers to fluid flow, due to the combined effect of grain size and porosity reduction within fault cores and associated deformation bands in damage zones [15,16,17,18,19,20,21,22]. In this context, clay minerals are commonly described as mechanically weak minerals, and because of this weakness their presence in faults commonly contributes to stable sliding failures [23,24]. The origin and distribution of clays in sandstone are important in oil industry, because these minerals contribute to increases in the sealing potential of faults and can determine reservoir compartmentalization [3,6,8]
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