From outcrop to functional reservoir model : using outcrop data to model the tidally dominated esdolomada sandstone, NE Spain
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The Esdolomada Sandstone member 2 crops out in the Tremp-Graus Basin of north-central Spain and forms the uppermost part of the Eocene Roda Formation. The second Sandstone unit within the Esdolomada member (ESD2) consists of bioturbated and shell-rich, very-fine sandstones as well as stacked sets of fine- to coarse-grained cross-stratified sandstones. The overall upward trend in the member is commonly upward thickening and coarsening of beds into and through the cross-stratified interval, though at some few locations there is no obvious trend or even upward thinning of beds. The internal architecture of the member is one in which groups of beds lie between master surfaces that dip highly obliquely to the migration direction of the individual cross strata. The ESD2 is interpreted to be a shelf tidal sand bar within the overall transgressive Esdolomada Sandstone member. It is likely that these bars migrated in a coast parallel fashion, as suggested by the cross-bed orientations, but also accreted laterally away from the coast along the seaward-dipping master surfaces. LIDAR (light detection and ranging) data collection for the Esdolomada member was attempted along the Isábena River near the village of Roda de Isábena, with a total lateral coverage of approximately 3 kilometers. Detailed outcrop measurements were made in accessible areas along the same transect. Outcrop analogs are the best source of data to understand reservoir heterogeneities and to build reservoir analogs for fluid flow simulations. Sand-rich, offshore tidal sandbodies are usually surrounded by marine mudstones, and are recognized from their very orderly stacking of cross-stratified sets (more orderly than in fluvial settings) , their complex internal architecture of master surfaces dipping obliquely to the direction of migration of the contained cross strata and their significant sandstone/mudstone heterogeneities. Tidal bar systems such as the ESD2 are appealing hydrocarbon prospects for several reasons. Primarily, they are relatively coarse grained, have a high degree of lateral continuity, and are relatively clean sands. In places where sand beds are stacked, they create enough thickness to offer good vertical permeability; however, mud-draped cross-beds can create heterogeneities in this type of system that buffer fluid flow. Due to a fairly unsuccessful attempt to obtain LIDAR coverage of the ESD2, in order to build an analog reservoir model, surfaces were instead based on measured sections and outcrop photomosaics. Using Schlumberger’s Petrel software, facies logs were created from measured section data, and then interpolated to make a facies and porosity model.