Unlocking the Nahr Umr Play Potential in North Oman from Regional Evaluation of the Limestone Marker Unit

Abstract

Abstract The Nahr Umr Limestone Marker Unit (NULMU) is a relatively thin layer encased in the regional Albian Nahr Umr shale over much of the Arabian Peninsula. In Oman, Nahr Umr Formation regionally seals Shuaiba Formation and underlies Natih Formation; both are prominent carbonate reservoir in northern Oman. The NULMU often presents hydrocarbon shows in the subsurface but has never been targeted as a reservoir or studied in detail. This study focused on understanding the lithology, facies, depositional environment, regional distribution, and reservoir properties of the NUMLU to identify potential prospective areas. The study was based on a detailed sedimentological and Gamma Ray logging of outcrops (Jebel Madar and Wadi Bani Kharus), core analysis from 2 wells, petrographic description of 17 thin sections from outcrops and the subsurface, and well log analysis of 34 well penetrations. Subsurface data allowed well correlations across the basin and creation of regional depth and isopach maps. The NULMU in Jebel Madar is ca. 4 m thick and comprises three distinct facies in ascending order: Orbitolina wackestone (Facies A), bioclastic mudstone (Facies B) and Orbitolina wackestone to packstone (Facies C). The facies represent a proximal, subtidal ramp depositional setting. Facies A is the most likely facies to act as a reservoir due to its coarser grain size and relatively higher visual porosity (up to 15%). However, in Wadi Bani Kharus the NULMU is ca. 9 m thick and dominated by mudstones (facies D) representing deeper marine, distal ramp depositional settings. The carbonate unit presents a shallowing upwards trend with a cycle boundary towards its top followed by rapid deepening into subtidal shales of the Nahr Umr Formation. In the subsurface, the NUMLU is present at an average depth of 1600 m and is ca. 4 m thick, except around the Lekhwair High, where it was eroded at the base Tertiary unconformity. The facies vary from Facies A to Facies B as described from cores and thin sections with average log porosity is 18% and permeability of 2 mD. The high porosity values are in areas where the erosional contact between the Shuaiba and Nahr Umr formations is more pronounced. Thus, indicating that on regional highs (Lekhwair, East Lekhwair, and Dhulaima highs), with long lived exposure the NUMLU has greater chance of reservoir properties enhancement through secondary dissolution. The results indicate that the main control on reservoir properties is related to original depositional setting with potential enhancement by later erosional exposure. This study helps to define on a regional scale, potential prospective areas that could be further explored to open a new play potential in Oman.