The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry:
Maturing Through Exploration and Production

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Advance Mud Gas Evaluation in complex Clastic Reservoir while Drilling in Usano field of PNG

Abstract

Drilling in the Usano field is difficult due to the complex geology and extreme geomorphology associated with the Papuan fold and thrust belt region. The field is structurally complex, with an abundance of faulting, which creates a system of compartments. The tectonic history of the area results in significant geo-mechanical challenges that result in a variety of borehole problems which lead to evaluation difficulties. The discoveries made so far contain prolific Jurassic and Cretaceous sand reservoirs. PNG highlands reservoirs consist of siliciclastic sequences, predominantly fine to medium grained quartz rich sandstone grading to occasional coarse grained in places. Hydrocarbon bearing sequences are relatively low porosity and high permeability with the reservoir quality in producing intervals ranging between 10 to 20% porosity and associated permeability ranging in the hundreds up to few thousands of milliDarcy. The objective of the current study was to obtain early understanding of reservoir level, type of fluid in place and identification of potential vertical flow barriers. This information was to be used in real time, optimizing fluid sampling and removing uncertainty in reservoir characterization. This was achieved by performing an Advanced Mud Gas Analysis Technique utilizing high resolution chromatographic analysis of C1 to C8 components. In particular, the analysis of heavy components has enabled reservoir zonation and identification of potential vertical flow barriers in the Toro and Digimu formations. Specific proxies from the gas data were also used to confirm fluid type and identify oil/water contacts (OWC). A comparison between the gas composition acquired in real time and the PVT sample data obtained post-mortem has confirmed the reliability of the advanced gas detection system as an effective fluid character indicator. These wellsite mud gas analyses become more important where traditional log analysis often remains inconclusive. In some cases, they remain the only source of data for reservoir characterization, as these data are available while drilling without the risk of hole conditions or other drilling challenges ((HP-HT, H2S bearing zones and slim holes).