Abstract

The Ngimbang Carbonate (Middle to Upper Eocene) in the western portion of the Kangean PSC was first encountered in 1988 by the West Kangean-1 well at nearly 7000 ft. The tight, fractured carbonate flowed gas at 12 MMCFPD. In the West Kangean-2 well nearly 250 feet of core was obtained from the 456 ft-thick carbonate section, which also tested significant volumes of gas (21 MMCFPD). Porosity and permeability of the rock matrix are low; measured flow rates could only be accounted for by the presence of an effective natural fracture system. Examination of that fracture system was carried out on the core material being displayed at the IPA Carbonate Core Workshop.

Based on log character the Ngimbang Carbonate appears homogeneous, except for local shale and muddy coal units. Petrographic analyses revealed a low-porosity sequence dominated by moderately-recrystallized carbonate mud and cut by a complex of fractures and stylolites. The carbonate mud-dominated rock matrix, which was generated by depositional and diagenetic processes, now controls reservoir porosity. The fracture/stylolite system, created by burial and tectonic processes, controls reservoir permeability.

Deposition of the Ngimbang Carbonate occurred on a laterally-extensive, shallow, and partly-restricted, marine platform. Carbonate exposure surfaces present in the core are indicative of local to laterally extensive subaerial exposure. Swampy conditions also were present across portions of the platform. The platform subsided and was covered progressively by deep-water deposits. After burial to as much as 12,000 ft, the carbonate sequence was uplifted, beginning about 15 million years ago, to its present depth of around 7,000 ft. Post-depositional modifications resulted in recrystallization of both carbonate mud matrix and skeletal elements during burial as well as the extensive development of fractures and stylolites during both burial and the subsequent tectonic inversion. Reservoir bitumen, a hydrocarbon residue which is present throughout the carbonate sequence, is evidence of various times of hydrocarbon migration, emplacement and modification.

The Ngimbang Carbonate sequence has an average matrix porosity of only about 2% and median matrix permeability of less than 0.1 md. Several stages of fracturing are present. Fracture frequency averages nearly 20/foot and ranges up to 60/foot. Over half of the fractures are near vertical (> 60°). A late-stage fracture system, probably developed during tectonic inversion, is dominated by near-vertical fractures that locally are partly open. Fractures provide an average of 0.68 porosity units, which represents as much as 35-40% of the whole rock porosity. Fractures also contribute at least 0.2 md of permeability, or 70% of the gross permeability.

Reservoir potential of the Ngimbang Carbonate depends on two main factors: bulk pore volume and deliverability, or access to pore volume. Evaluation of that potential will include drilling deviated wells to increase the probability of encountering fractures, cutting oriented cores and long term production tests.