3D modelling of Paraíso Formation (Iberian Pyrite Belt), based on well logs data and surface geological mapping

Abstract

The Iberian Pyrite Belt - IPB of the South Portuguese Zone – SPZ is a 250 km long (along Portugal and Spain) and 25 to 70 km wide structure, which comprises late Devonian to middle Carboniferous units, locally covered by Cenozoic alluvial deposits. Establishing the lithostratigraphic succession for the IPB is extremely problematic due to lateral variation of facies and intense tectonic deformation. The IPB is classically divided into a southern parautochthonous (rooted) branch and a northern, essentially allochthonous branch. The stratigraphic successions are relatively well studied (locally) for the southern branch, but the correlation of these with the northern branch is difficult.

One of the IPB main units is the Volcano-Sedimentary (VS) sequence (also called Volcano-Siliceous Formation), dated as late Famennian to early late Visean (based on its fauna and microfauna), with thicknesses ranging between 100 and 900 m.

At the top of the VS sequence occurs a pelite-organic black shale and fine grained sandstone unit, known as the “Intermediate Series” in Spain, Paraíso Fm. in Aljustrel's mine zone, Brancanes Fm. in Neves Corvo, and several other names, depending on the mining area. This unit contains locally scattered beds of jasper and very rare limestone, interstratified with VA1 (a lowermost rhyolitic to intermediate sequence with fine- to coarse-grained pyroclastics and lava) to VA2 (a second rhyolitic sequence with pyroclastics and lava flows). These greasy black shales are virtually not deformed and generally leave some “oil” in the hand and emanate a clear oil aroma. Its potential as hydrocarbon source-rock is therefore a possibility to be explored. National Laboratory of Energy and Geology (LNEG) from Portugal, has hundreds of well data logs, largely obtained by mining companies that operate in IPB. Taking into account this information, this work aims to understand the geometry and structural behavior of Paraíso Fm. thus contributing to the assessment of the potential for unconventional resources of this unit. A 3D model of the top of Paraíso Fm. has been built, based on well log information and surface geological mapping (1:25000), using ArcGis 10.2 and Golden Software Surfer 12 through the krigging method. This model shows the structural complexity of the whole area and of this organic-rich unit in particular.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015