Effects of sandstone mineralogy and diagenesis on thermal conductivity

Abstract

Feasibility studies of geothermal plants typically include modelling of the reservoir conditions, flow rates, thermal properties and expected water temperatures. Though a simplified geological model is used for the modelling; the original variation is known and can be used when performing a risk assessment. Thermal conductivity values applied in the models will typically be average values and the actual variations in the sediments are often not known. With our investigations we add to the knowledge of how the sandstone composition and diagenetic changes affect the thermal conductivity. Some of the most prominent clastic reservoir rocks for geothermal exploitation onshore Denmark are the Triassic Bunter Sandstone Formation and the Upper Triassic – Lower Jurassic Gassum Formation. The two formations are mineralogically very different. The Bunter Sandstone Formation has an immature mineralogical composition due to limited weathering in the sediment source areas and during deposition in the arid to semi-arid climate. The Gassum Formation is mineralogically more mature due to intensive weathering of the sediments source rocks under the humid climate. Intensive alteration resulting in extensive kaolinite formation also characterise the early diagenesis of the Gassum Formation, except where local early calcite or siderite cement prevail. The Bunter Sandstone Formation is instead characterised by early coatings of iron-oxide/ hydroxide and clay minerals, besides evaporative pore-filling cements as calcite and gypsum (later transformed into anhydrite). The early grain coatings in the Bunter Sandstone Formation inhibit or retard later feldspar and quartz overgrowths; whereas quartz diagenesis can be quite extensive in the Gassum Formation. Quartz abundance seems to have a direct influence on the rock thermal conductivity. High quartz content and abundant quartz cement typically result in high thermal conductivity, but other minerals, such as for example halite, glauconite and pyrite, may also increase the thermal conductivity. As quartz cement becomes more abundant during burial, there also seems to be a relation to burial depth. With our still ongoing investigations we add to the knowledge of how the sediment composition and diagenesis affect the thermal conductivity, which may improve the risk assessment of geothermal reservoirs.