Characterization of the Gas-Bearing Tight Paleozoic Sandstone Reservoirs of the Risha Field, Jordan: Inferences on Reservoir Quality and Productivity

This study presents the petrographical and petrophysical characteristics of the Cambro-Ordovician clastic reservoirs from the Risha field, northeastern Jordan. Routine core analysis, wireline logs, petrographic thin sections, scanning electron microscopy, and X-ray diffraction were integrated to characterize the gas reservoirs of the Risha, Dubeidib, and Umm Sahm formations (the equivalent of Sarah, Qasim, and Upper Saq formations of northern Saudi Arabia). These reservoirs are variably micro- and mesoporous, with permeability < 1 mD and dominantly < 6% porosity. Wireline log-based assessment exhibits low shale volume (< 30%) and high hydrocarbon saturation (45-95%) in these tight reservoirs. Petrographic investigation reveals that these reservoirs are fine-grained sandstones, moderately sorted with high mineralogical maturity. The Risha and Dubeidib reservoirs are subarkose, while the Umm Sahm reservoir is composed of quartz arenite. The late diagenetic silica cementation is inferred as reservoir quality-reducing diagenetic factor, with quartz overgrowth of > 10% corresponding to < 4% porosity. SEM images exhibit the presence of grain-coating, pore-filling, and pore-lining chlorite, and illite phases which hinder quartz overgrowth and had a positive effect in retaining the primary porosity. The sandstones with > 20% clay-coating coverage corresponds to a lower quartz overgrowth (< 5%) and therefore higher intergranular porosity (> 5%). Locally sutured grain contacts and stylolites are observed which indicate intense chemical compaction. The feldspar grains are observed to be partially dissolved, which generated minor secondary porosity. Micropore-dominated pore systems and rare secondary macroporosity are typically isolated by abundant cement and/or pore throats choked by clay minerals.