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The Upside Potential of the Early Miocene Kujung-1 Carbonate, Offshore East Java, Indonesia

Abstract

Cenozoic carbonates host a majority of hydrocarbons in SE Asia. The Oligocene-Miocene carbonates of the Kujung Formation offshore East Java, Indonesia is a well-known reservoir with significant reserves. Despite its importance as producing reservoir, the entire Oligocene to Early Miocene succession (13.5 My, 2500-3000 feet thick) is only divided into three units based on the order of carbonate lithology penetrated during drilling operations (Kujung-1, Kujung-2, and Kujung-3). This study aims to develop a better understanding of the stratigraphic architecture of the Kujung Formation in order to support field development strategy. The Kujung-1 is Early Miocene in age and is the main producing reservoir in the region. Based on seismic and well data, we interpret the Kujung-1 as a carbonate buildup complex that can be divided into three area; core, flank, and inter-buildup. Hydrocarbons production has been mainly coming from the buildup core consists of mostly reefal carbonates facies with minor amount of shales. However, wells from KE-X3, KE-X4 and KE-X8 penetrated the buildup flank and inter-buildup consist of carbonates-shales alternation. We interpret this alternation as a product of episodic carbonate re-sedimentation during periods of decreasing accommodation outside the buildup core. In contrast, shales were deposited during periods of increasing accommodation in the flank and inter-buildup area under low-energy condition. Based on well data and petrophysical calculations, we interpret an interval of up to 120 feet thick near the top of this carbonates-shales alternation as an upside potential in KE-X8 area. This shows that the carbonate beds in the buildup flank and inter-buildup area can act as a reservoir that is trapped stratigraphically. The calculated porosity ranges from 15 – 21% with highest permeability measured from core analysis is 42.6 mD. We utilized Acoustic Impedance inversion to map the reservoir distribution. Preliminary reserves calculations show encouraging results for this interval to be developed.