Reservoir architecture modelling of alluvial fan deposits: Cornudella Formation, Tremp-Graus Basin, Spain

Knowledge of the complex subsurface is becoming increasingly important: for hydrocarbon exploration, possible storage of CO2, and the production of water for geothermal heating and energy production purposes. An example of a complex reservoir is a reservoir formed by alluvial fan sediments. Although research into alluvial fan deposits has been conducted, the geometry and sedimentary environment of alluvial fan deposits is still not completely understood at this time. 3D modelling of these sediments will contribute to the knowledge of alluvial fan reservoirs and give predictions of inter-well connectivity, hence improving production. To gain a better understanding in the reservoir architecture of alluvial fan deposits, an outcrop study in the Sis-palaeovalley, in the Tremp-Graus Basin in the Southern Pyrenees was conducted. An outcrop wall with a studied height of approximately 220 meters, in an area with a length of 1200 m and a width of 950 m, is investigated and mapped by using lithostratigraphic logs, photo panels and correlation observations. By analysing the gained data, a 3D reservoir architecture computer model is built giving a representation of an alluvial fan reservoir. Four lithofacies are encountered in the field: the conglomerate-, sandstone-, siltstone-, and limestone lithofacies. In the lower part of the outcrop the stacking of the sediment layers is farther apart compared to the upper part of the outcrop. This stacking pattern can be explained by a combination of climatic and tectonic events because of their mutual influence on the accommodation space, relative base level, and sediment supply. The built model gives insight in the alluvial fan outcrop and can form an analogue for alluvial fan reservoirs. With the improved knowledge and understanding of the reservoir architecture of alluvial fan deposits an increased production of water or hydrocarbons from alluvial fan reservoirs can be achieved, with a more sustainable well planning.