AAPG Middle East Region Geoscience Technology Workshop

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Controls on Reservoir Quality Of Late Ordovician Glaciogenic Deposits: Surface and Subsurface Equivalents

Abstract

Exploration and exploitation of tight sandstones reservoirs as one of the unconventional hydrocarbon resources have recently increased worldwide. Outcrop analogues are commonly studied to understand the reservoir characteristics of deeply buried reservoir equivalents. This study is intended to compare the effects of depositional facies and diagenetic imprints on outcrops and subsurface equivalents of the Late Ordovician tight sandstones in Saudi Arabia. We utilized facies analysis, μXRF, XRD, thin-section petrography, QEMSCAN, and measured porosity and permeability data to build and validate the database of this study. The results revealed that the depositional facies are more of glaciofluvial in the outcrop while heterogenous periglacial and proglacial facies were identified in the subsurface. At the outcrops, the sandstones are dominantly comprised of quartz arenites and are characterized by varied textural attributes. On the other hand, quartz arenite, subarkose and sublithic arenite with a wide range of grain sizes are observed in the subsurface. Porosity and permeability are mainly ≥13% and ≥20 mD in the outcrop and <13% and <10 mD in those from the subsurface samples. The reservoir quality (RQ) controls in the subsurface depend on the depositional settings and burial depth of the facies. Highly compacted grains, and pore-filling siderite, anhydrite, barite and calcite cement, occasionally reduced the RQ in the fluvial, low-matrix sandstones of glaciolacustrine delta and glaciofluvial facies. Detrital matrix content (mainly illite clay mineral) has also a negative impact on the RQ of the subglacial and high-matrix sandstones of the glaciolacustrine delta facies. Grain compaction is lower in the subsurface than in the outcrop samples and clay mineral (kaolinite) is only encountered in the outcrop samples. The RQ is occasionally enhanced by primary porosity and dissolution in the subsurface facies, while the porosity is well-preserved in the outcrop in addition to trace feldspar dissolutions. Comparison between outcrop and the subsurface appears to be a challenge where the RQ controls of the subsurface facies are different from those of the outcrop. However, integration of outcrop based detailed observation with subsurface equivalent will help to reconstruct reliable models that might help to reveal complexity and provide better understanding and prediction of facies, paleoenvironments and reservoir heterogeneity, quality and architecture. Such refined models might be utilized to identify tight gas exploration and development targets in the subsurface.