Case Study: How a Conventional Core Led to a Material Uplift in the Usano Oilfield

Abstract

The Papua New Guinea Usano oil field is part of the Kutubu complex of fields with the main producing intervals being the sub-units within the Toro Formation (Toro A, Toro B and Toro C). There are currently 13 producer wells and two injector wells. One of the petrophysical challenges within the Usano field is interpretation of the basal Toro C. The Toro C is predominantly a clean Upper Shoreface sand sequence with increasing bioturbation towards the base. Overall, the Toro C is shallowing/cleaning upwards. From recorded log responses, an apparent, log-derived oil-water contact can be inferred from petrophysical interpretation. However, an alternative view is that the log responses represent a lithological change due to increasing presence of structural clays towards the base of the Toro C, resulting in a low-resistivity contrast, hydrocarbon-bearing, reservoir unit. In the absence of core, the first approach to address this issue was through formation pressure tests. However, pressure data across the basal Toro C can be problematic due to low mobilities, supercharging effects and reservoir depletion. A comprehensive core acquisition and associated analysis program was planned for the UDT 13 well bore, with one of the primary objectives to confirm basal Toro C as a viable reservoir. In early 2011, a total of 100m of oriented, 4” full-diameter core was recovered across the Toro and Imburu and transferred to the core laboratory for the analytic program. A total of 116 core plugs were taken for routine and special core analysis experiments. The slabbed section was photographed and described. UV light core photographs confirmed the presence of hydrocarbons across the basal Toro C. Core description also confirmed viable reservoir within the basal Toro C which consisted of Upper Offshore to Lower Shoreface sands, with low to moderate porosity and permeability. The increasing bioturbation observed towards the base of the Toro C also assisted in the understanding of historically poor formation pressure test results across this interval. Integration of core program results confirmed that log responses were being affected by lithology, resulting in an extension of interpreted reservoir. Core calibration was a major contributor to a 40% uplift of field in-place volumes. Whilst the cost of acquiring the core was both costly and labor intensive, the incremental increase of in place volumes was far more significant.

AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020