Depositional Model and Reservoir Characterization of Buah Formation Carbonates (Neoproterozoic, Sultanate of Oman)

Abstract

The Buah Formation represents a Late Neoproterozoic (552–547 Ma) carbonate ramp, and was studied in Block 61 on the Makarem High between the Fahud and Ghaba salt basins of Oman. The inner ramp typically consists of tidally-influenced pack-grainstones, stromatolites and low-energy lagoonal-peritidal facies. The middle ramp is characterized by storm-dominated thrombolite-stromatolite bioherms and grainstones, which may form a distal shelf break. The outer ramp consists of organic-rich carbonate muds and re-sedimented carbonates. The Buah tight gas reservoir is considered a dual porosity system, and whilst flow is dominated by fractures, identifying areas of improved matrix reservoir quality is considered essential to improve flow rates. This combined seismic-petrographic study allowed construction of an improved seismic stratigraphic model, establishment of a diagenetic model, and definition of possible play fairways. Seismic analysis shows two stages of Buah deposition: (1) an older platform confined to basement highs, and (2) a younger platform with significant lateral progradation to the NW. The Buah was grouped into 9 mappable seismic facies. Deposition within the older platform was dominated by mid- and outer ramp carbonates, with limited distribution of bioherms/shoals. The younger platform shows much more widespread lagoonal/peritidal facies and thicker accumulations of high-energy and bioherm facies along the NW margin. This latter facies is yet to be appraised but the likelihood of high depositional porosities in mid-ramp bioherms makes this a good reservoir candidate. The main diagenetic processes include marine cementation, meteoric dissolution, early burial pervasive dolomitization and burial cementation, dissolution and fracturing. Supported by production data, three plays for an optimal exploitation of the Buah have been identified: (1) Platform margin facies, which can preserve relatively fair porosities (porosity 4–5%), thanks to the original large size of the pores, occasionally enhanced by meteoric dissolution. (2) A play related to an important dissolution phase, whose fluid seems to be linked to fault movement on the eastern side of the ramp. This created pervasive microporosity (porosity >6%), and in combination with fractures provides permeabilities of 1 mD. (3) The uppermost part of the Buah has particularly favorable reservoir properties and the highest production rates. It shows considerable karstification and fracturing in core.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015