Internal structures within Yellow Bank Creek Injectite Complex (YBCC) Northwest Santa Cruz, California: Importance of outcrop analogues for subsurface modelling of Sand Injectites

Scott, Anthony Stephen John¹, Mario Vigorito¹, Andrew Hurst¹, Joe Cartwright² (1) Aberdeen University, Aberdeen, United Kingdom (2) Cardiff University, Cardiff, United Kingdom

The Yellow Bank Creek Injectite Complex (YBCC) is up to 15 m thick and consists of subhorizontal to subvertical banded sandstone. The YBCC overlays and underlies erosional surfaces cut into the Santa Cruz Mudstones (host rock, Late Miocene). It is possible to distinguish many types of internal structure on the basis of grain size, sorting, mineralogy including host rock content (mudstone fragments) and physical internal structures including banding, clay seams, host rock gouge and sand filled fractures. YBCC petrophysical properties are quantified using mercury porosimetry, image scan analysis and minipermeametry. The banded units have porosity >25 % and permeability > 2000 mD. Banding displays permeability anisotropy; permeability parallel-to-band can be an order of magnitude more relative to permeability perpendicular-to-band. This reflects the preferential alignment and packing of the grains in a direction parallel to banding. The injected sandstone in proximity to host rock has porosity <5% and permeability more than three orders of magnitude less than the banded sandstone. This is because of the increased clay content derived by erosion of the surrounding host rock. Fluid flow within sand injectites is a function of injection architecture and the petrophysical properties of internal structures. Quantitative analysis of sand injectite architecture and internal structure allows a more accurate prediction of fluid flow within sand injectite reservoirs. The complex nature of the YBCC demonstrates the importance of outcrop analogues in providing information that can aid 3D reservoir modeling of similar systems within the subsurface.