Reservoir Architecture and Channel Belt Mapping of Fluvial Systems: A Case Study from the Middle and Upper Members of the G Formation in the B Field, Central Oman

Abstract The G Formation is one of the most prolific hydrocarbon reservoirs in the Sultanate of Oman and is the target for extensive exploration and production campaigns. The study seeks to understand the reservoir architecture, depositional environments, and reservoir quality of the upper and middle members of the G Formation. In particular, Middle G 2 (MG2), Middle G 1 (MG1), Upper G 4 (UG4), and Upper G 3 (UG3) in the B field. Well correlation was conducted on 53 wells in the B field using the Petrel (2020) software for all G units. Paleogeographic (channel fairway) maps of G subunits were created with the aid of log signature analysis, petrophysical facies logs (reservoir rock typing), and paleocurrent analysis. Integrated reservoir descriptions "plumbing diagrams" scenarios were created and linked to the paleogeographic maps. A conceptual depositional model of Middle and Upper G was created based on this analysis. The reservoir quality of Middle G was assessed using log data and limited core data and linked with the previous findings of the study. Well correlation results indicate that MG3 was deposited in a shoreface environment. In contrast, MG2 and MG1 were deposited in a fluvial environment dominated by ephemeral channels and sheetflood deposits in an arid to semi-arid climate. UG4 was deposited above an erosional surface, resulting in compensational stacking in a meandering fluvial environment, while UG3 was deposited in a classical high-sinuosity meandering fluvial system (humid/tropical climate). MG2 and MG1 paleogeographic maps indicate two channel belt orientations trending NW-SE and NE-SW, with relatively wide channel belts. UG4 and UG3 show an overall channel belt orientation of NE-SW, matching the regional channel belt orientations. MG1 in the B field is Arkose to subarkose in composition, while MG2 is dominantly litharenitic to sublitharenitic. The reservoir quality of MG1 is highly impacted by clay mineral infiltration directly below an erosional surface. Conversely, the reservoir quality of MG2 is controlled by the rock fabric and compaction. The distribution of the reservoir properties in the B field was found to closely resemble the paleogeographic maps of this unit. This work presents a novel approach to mapping the distribution and deposition of complex and heterogenous fluvial reservoir systems. The use of a combination of log signature analysis (qualitative), reservoir rock typing (quantitative), and paleocurrent analysis is seldom seen in these types of reservoir systems. This study also outlines methods of predicting the reservoir quality of fluvial reservoirs when core data is unavailable or limited.