SYNOPSIS: This paper summarizes the geomechanical data - necessarily incomplete - collected from the studies of the oil shale deposit of Puertollano (Spain). The geological features of this deposit and also the basic physico-chemical properties are being described. Special interest has been devoted to the analysis of the shear strength and the deformability of the oil shale samples tested, in order to establish the basic geotechnical data for mining studies.
RESUME: Dans cette communication on decrit l'information geotechnique obtenue pendant les etudes du bassin aux schistes bitumineux de Puertollano. Les caracteristiques geologiques de ce bassin ont ete decrites ainsi que les proprietes physiques et chemiques des schistes. Une attention particulière est prêtee aux proprietes de la resistance à l'effort tangentiel et à la deformabilite des echantillons de roche soumis aux essais, pour etablir une information geotechnique fondamentale pour les etudes d'exploitation.
ZUSAMMENFASSUNG: Die Arbeit beschreibt die Felsmechanik-Forschungsarbeiten der Ölschiefer von Puertollano (Spanien). Die geologischen Kennzeichen und physika1isch-chemischen Gesteinsparameter werden dargelegt. Unter anderem wurden folgende Versuche durchgefuehrt: Messungen der einaxia1en Druckfestigkeit und des Elastizitatsmoduls parallel und normal zur Schichtung. Die fundamentalen geotechnischen Eigenschaften werden dargelegt im Hinblick auf die Abbauverfahren.
1. INTRODUCTION The increasing cost of oil in the last decade has raised the need for evaluating any potential source of energy. Within this frame, oil shale processing is knowing a rapid expansion all over the world. In Spain, renewed attention is being paid to the oil shale deposit of Puertollano (in Central Spain), already developed in the 50's. Preliminary research has included a feasibility study comprising technical and economical aspects, as well as the geomechanical problems related to open cast or underground mining. This paper summarized the geomechanical data, necessarily incomplete, collected in said studies, in the aim to increase the scarce information available on the subject.
2. GEOLOGICAL FEATURES The Puerto llano mining bassin is located 250 Km southwards from Madrid. It is 3,5 Km wide and 12–13 Km long. The ancient coal mine workings have been abandoned and now only open pit coal mining is in operation. The overall morphology corresponds to a flat valley with geological structure of synclinorium. The Paleozoic materials, folden and diagenized, dip very gentle as a consequence of the Hercinian orogeny. After the Miocene, the Alpidic orogeny developed in a block tectonics, with vertical faults and the injection of volcanic materials through the longitudinal fractures. The plan of the deposit appears in fig.1, along with a typical N-S profil, including the following materials: a) Quaternary: sub-horizontal deposits of quartzitic and schistose gravels. b) Miocene: marls and fissured clay marls. c) Carboniferous, including up to 5 coal seams and the oil shales, as well as layers of schists, sandstones and graywackes. d) Devorian-Silurian and Cambrian, the sound rock of the Deronian bassin (schists, sandstones and quartrites). As shown in fig.1, oil shales appear at a depth between 15 and 350 m, in three seams. The upper (A) and lower (C) levels have a mean thickness of 1,5 m, whereas the middle one (B) is 3,5 to 6,0 m thick. The 40% of the deposit dips below 10°, with a gentle increase up to 25–28° at the boundaries of the trough. Site investigation has comprised a boring campaign, sampling from the a old mine workings and an experimental inclined shaft, 60 m long, for direct appraisal of geomechanical behaviour.
3. PHYSICO-CHEMICAL PROPERTIES Some 500 samples have been tested in order to assess their mineralogy, bulk density, caloritic power and oil content as established by the Fisher assay. The main constitutive minerals are mica (40–45%), Kaolinite (20–25) and quartz (15–20%), with minor quantities (0–10%) of calcite, siderite, pyrite, ankerite-dolomite and chlorite. The oil content Vo, usually taken as a fundamental parameter in oil shale extraction, can be related to the bulk density and calorific power (Fig. 2) without excessive scattering. Oil contents up to 320 m /mT have been obtained, corresponding to very low bulk densities, said 1400 Kg/m. The mean oil content of the deposit (~ 185 m /mT) corresponds to a mean bulk density of 1750 Kg/m, i.e. a calorific power of 1750 Kcal/Kg. If is believed that said values are sufficiently reliable for estimating the actual oil yields without further testing.
4. GEOMECHANICAL PROPERTIES 4.1. Strength The following types of tests have been carried out:Unconfined compressive strength.
Point load test
Indirect tensile strength (Brazilian test)
Deformability
Joint shear strength.
Most of the samples have been trimmed out from blocks taken out of the old mine workings or the new inclined shaft. The blocks were mainly plate-shaped cut following the joint system, parallel to the bedding. Many samples disintegrated when trimmed, due to opening of minor cracks. A preliminary classification of oil shales was attempted following the proposal of Beniawski (1973), and taking into account the oil content.