Inter-Well Scale Sedimentological Heterogeneities And Facies Architecture Of Upper Jurassic Carbonate Reservoir And Anhydrite Seals: Lessons Learned Using Outcrop Analogues

Abstract

Most regional studies of reservoir and seals focus on large-scale sequences and their architecture. However, understanding flow and geochemical processes happening in the subsurface requires a multiscale approach from the pore-size to the field-size. Here we focus on an intermediate, 'inter-well' scale loosely defined as 100-1000 meter scale. This inter-well scale is crucial as it represents the minimum distance between an injector and a producing well. Hence, constraints on the heterogeneities in the reservoir or within a sealing lithology can guide the production geologist during EOR or CCS operations. In this paper, we present a review of existing outcrop analogues for the subsurface of Qatar that were considered for our study. Out of the different possibilities, we selected outcrops from Wadi Naqab (Ras-Al-Khaima, UAE) as an analogue for the subsurface reservoirs in Qatar, and we focused on mine outcrops in the U.K. as potential analogues to derive the 3D geometry of evaporite-carbonate sequences analoguous to the Hith Formation. The Jurassic limestones at Wadi Naqab appear layer cake at the scale of observation, but we demonstrate here that even though beds rarely pinch out, the facies within bed changes markedly at a scale of Ͻ200 meters. For subsurface applications, this could be significant if the change in facies correlates with a change in petrophysical properties. The anhydrite layers at Brightling Mine are mostly composed of nodular, sabkha-type layers, inter-bedded with shallow-water algal limestones and shales. These lithologies are continuous at a 500 meters scale, but scouring impacts on the thickness of the deposits. Again, this could have important mechanical implications during CCS operations, because the interface of different lithologies and their thicknesses control fracture propagation within reservoirs and seals. We conclude that more geometrical heterogeneities exist in both reservoir and seals than previously thought, and that inter-well scale data are needed to inform subsurface reservoir models. We also caution that one of the limitations of using outcrop analogues is that porosity and permeabilities are not preserved during uplift, and thus petrophysical properties within numerial reservoir modelling need to be populated using subsurface data.