Diagenetic Alterations and Reservoir Quality of the Mahatta Humaid Group (Amin and Miqrat Formations), Sultanate of Oman

Abstract

Amin and Miqrat formations of Early to Middle Cambrian age act as the most important subsurface reservoirs in the north of Oman. This study attempts to delineate and document the dominant diagenetic alterations and their subsequent control on reservoir quality of Amin and Miqrat formations. A combination of textural petrographic and compositional analysis along with geochemical analysis including SEM, EDS and XRD and stable O- and C-isotopes of carbonate cements revealed that Huqf outcrop-based and Saiwan East Field core-based samples of Amin and Miqrat formations are medium silt-sized to coarse sand-sized grains and are dominantly litharenite to sublitharenite in composition and have undergone various diagenetic alterations during eo-, meso- and possibly telogenetic regimes. However, outcrop-based samples of Amin Formation are very fine to very coarse sand-sized grains (0.06 mm-0.87 mm) and are litharenite to sublitharenite (Q55.8%F2.5%RF13.8%) in composition whereas core-based samples of Amin Formation are silt- to coarse sand-sized grains (0.04 mm-0.85 mm) and sublitharenite in composition (Q81%F5.1%RF13.8%). On the other hand, outcrop-based samples of Miqrat Formation are coarse silt-sized to coarse sand-sized grains are subfeldsarenite to sublitharenite (0.04 mm-0.59 mm) and rarely quartzarenite (Q82%F7.4%RF8.6%) in composition and core-based samples of Miqrat Formation are medium silt-sized to medium sand-sized grains (0.02 mm-0.3 mm) and are subfeldsarenite to lithic arkose (Q71.7%F16%RF12.4%) in composition. Eodiagenetic alterations for Amin Formation; both outcrop- and core-based samples are dominantly mechanical compaction as evidenced by point-to-point contacts between detrital quartz and chert grains, dissolution of detrital chert and cementation by kaolinite, calcite type I and siderite type I. In contrast, mesodiagenetic alterations include chemical compaction as evidenced by concave-convex and suture contacts between detrital quartz and chert grains and cementation by quartz overgrowths and calcite type II and siderite type II. However, cementation by calcite type II and siderite type II, which are postdating quartz overgrowths, may also account for telodiagenetic regime, especially for the outcrop-based samples. The bulk oxygen and carbon isotopes of siderite cement in the studied sandstones of Amin Formation reveal a range of δ18OV-PDB between -7.54 and +1.88‰ and δ13CV-PDB between −3.98 and +1.94‰ and of calcite cement reveal a range of δ18OV-PDB between -8.41 and +0.92‰ and δ13CV-PDB between −4.05 and +1.61‰. The negative values of δ18OV-PDB for both siderite and calcite cement support their formation during eodiagenesis regime and the positive values for mesodiagenetic regime. Similarly, eodiagenetic alterations for Miqrat Formation; both outcrop- and core-based samples are mechanical compaction as evidenced by point-to-point contacts between detrital quartz and chert grains, dissolution of detrital chert and feldspar grains and cementation by kaolinite, calcite type I and siderite type I whereas mesodiagenetic alterations include cementation by quartz overgrowths, illite and dolomite and calcite type II. Cementation by calcite type II, which is in some cases postdating quartz overgrowths, may account for telodiagenetic regime. The bulk oxygen and carbon isotopes of siderite cement in the studied sandstone samples of Miqrat Formation reveal a rangesδ18OV-PDB is -0.44‰ and δ13CV-PDB is +1.08‰ and of calcite cement reveal a range of δ18OV-PDB is -0.58‰ and δ13CV-PDB is +1.58‰. The negative values of δ18OV-PDB for both siderite and calcite cement support their formation during eodiagenetic regime and the positive values for mesodiagenetic regime. It has been noted that the studied outcrop and subsurface sandstone samples from Amin and Miqrat formations share similar diagenetic alterations which revealed that the burial history of the outcrops is similar to that of the subsurface. The results of this study revealed that compaction (mechanical and chemical) and cementation by calcite and siderite have played a major role in porosity reduction and thus deterioration of reservoir quality for both Amin and Miqrat formations during eo- and mesodiagenetic regimes and possibly telodiagenetic regime. On the other hand, dissolution of detrital chert and feldspar grains resulted in porosity creation and thus reservoir quality enhancement. Keywords: Sandstone diagenesis, Reservoir Quality, Amin Formation, Miqrat Formation, Mahatta Humaid Group, Cambrian, Huqf, Oman

AAPG Datapages/Search and Discovery Article #90360 © 2019 AAPG Middle East Region Geoscience Technology Workshop, 3rd Edition of the Siliciclastic Reservoirs of the Middle East, Muscat, Oman, October 28-30, 2019