Triassic Red Beds in SE Spain: Evaluation as Potential Reservoir Rocks Based on a Preliminary Petrological Study

Henares, Saturnina ¹; Viseras, César ¹; Fernández, Juan ¹; Pla, Sila ¹; Cultrone, Giuseppe ²
(1) Stratigraphy and Paleontology. Sedimentary Reservoirs Workgroup (SEDREGROUP), University of Granada, Granada, Spain. (2) Minerology and Petrology, University of Granada, Granada, Spain.

The outstanding outcrops of Triassic red beds successions in the Tabular Cover of the Iberian Meseta, SE Spain (TIBEM), are of great interest as outcropping analogues for active hydrocarbon reservoirs such as the TAGI (Algeria). The studied samples correspond to sandstones deposited under different environmental contexts: braidplain, overbank (crevasse splay lobes and sheet floods) and channel fill (meandering, straight and anastomosed). In all cases, the sandstone bodies are laterally and vertically well sealed by overbank fines, and they are similar to the productive beds of the TAGI. In this work, a preliminary petrological study has been performed to characterize the mineralogy, the texture and the relative porosity of the samples. These are properties which characterize sandstone reservoirs and result from both primary depositional environment and diagenetic alteration.

The primary mineralogy is the typical one of arkosic-subarkosic arenites in all deposits: quartz and potassium feldspar as major constituents, and 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. Many of the quartz and feldspar grains present sintaxial overgrowths of both natures. Kaolinite and sericite are also present as alteration products of feldspars.

In the braidplain samples, the clay content is lower than in the overbank and channel fill deposits. This feature combined with the occurrence of carbonate, silica and gypsum cements reduces the primary intergranular porosity in these materials in comparison with those of the braidplain. In the 3 types of deposits there is secondary porosity due to the dissolution of labile grains.

In general, all the studied samples are well sorted, with quite spherical, subangular to subrounded morphology of grains. The grain-size varies from fine to medium (125-200 micras), being finer in overbank and channel fill deposits than in the braidplain. The fabric is mostly grain-supported, with punctual to lineal contact between grains. Preferred orientation of grains is particularly evident in overbank and channel fill deposits, where the mica and the Fe oxide cutans highlight the internal structure.

This first approach to the petrology reveals that the braidplain sediments are those with a greater potential as a reservoir rock.

Acknowledgments: Project CGL2009-07830/BTE (MICINN-FEDER) and Group RNM369.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.