Environmental control on the petrological characteristics of fluvial sandstones- A Triassic example.

The Triassic red beds successions that crop out in the Tabular Cover of the Iberian Meseta, SE Spain
(TIBEM), are an excellent example of sandstone bodies deposited in different environmental contexts,
particularly those of braidplain, channel fill (meandering, straight, braided and anastomosed) and overbank
(crevasse splay lobes and sheet floods). In all cases, the sandstone bodies are laterally and vertically well sealed
by overbank fines, and resemble the productive beds of the TAGI (Algeria). However, a preliminary petrological
study and a first approach to the characteristics of the porous system, using optical microscopy and mercury
injection porosimetry techniques, have revealed the main microscale differences between these types of
deposits.
The primary mineralogy is dominated, in all cases, by quartz and potassium feldspar, with minor amounts of
plagioclase. Grain-coating Fe oxides (hematite), clays (illite, kaolinite and chlorite), mica (biotite), tourmaline and
other opaque detrital minerals have been identified as accessory minerals. Kaolinite and sericite are also present
as alteration products of feldspars. These data allow to classify the deposits as arkosic-subarkosic arenites.
The microtexture of the studied samples is characterized by well sorted grains, with quite spherical, subangular
to subrounded morphology. The grain size varies from fine to medium (125 to 200 'm), being finer in overbank
and channel fill deposits than in the braidplain ones. The fabric is mostly grain-supported, with preferred
orientation of grains particularly evident in overbank and channel fill deposits.
In the sampled braidplain sediments, the low mechanically infiltrated clay content and the grade of
cementation stand out. In this group the materials with the highest values of primary porosity (more than 25%),
as well as with the largest pore radius are included, showing an unimodal distribution of sizes at around 30 'm.
In the overbank and channel fill deposits, the occurrence of carbonate, silica and/or gypsum cements and the
presence of intersticial clay considerably reduces the primary porosity (less than 17%). Moreover, the pore
radius tends to be lesser with a bimodal distribution of sizes at around 1 and 0.1 'm.
This preliminary study reveals that the environmental context directly affects the petrological characteristics
and the petrophysical properties, which in turn control other features of the deposits such as their potential as
reservoir rocks.