The Muruk Gas Field: Insights into Structural Style in The Western Foldbelt of Papua New Guinea

Abstract

Muruk 1 was drilled in 2016/2017 by the PPL402 joint venture (Oil Search operated) and intersected two separate gas accumulations (‘Muruk A’ and ‘Muruk B’ thrust sheets) in the Toro Formation. It represents the first test of a prospect where the reservoir was interpreted to be decoupled from the overlying surface structure, and not only provides constraints on the geometry of the Muruk structure, but has important implications for the interpretation of structural style in the frontal part of the Western Foldbelt. The presence of at least one major detachment above reservoir level facilitates the decoupling of surface and deep structures, and allows prospective structures with subtle or no direct surface expression to be interpreted and modelled. As a result, exploration success at Muruk improves prospectivity in areas that have previously been considered non-prospective. Integration of multiple datasets, consideration of multiple structural models, and planning for operational flexibility proved essential to a successful Muruk 1 operation. Data used to constrain the pre-drill interpretation included surface geological mapping and dip measurements, poor-quality 2D seismic, LIDAR, potential fields (airborne gravity/magnetics, magnetotellurics), surface age data (Sr-isotope stratigraphy and biostratigraphy) and regional wells. The Muruk 1 well results validated the predicted structural style, and demonstrated additional complexity not modelled pre-drill, including the major Muruk A thrust fault separating the two discoveries, and additional folding/faulting on the backlimb of the greater Muruk anticline. High-resolution dip data interpreted in real time and while-drilling biostratigraphic analysis, along with joint venture engagement and alignment, were instrumental to the success of the Muruk 1 well, allowing operational decisions to be made quickly and efficiently. Muruk 2 was planned as a vertical well to appraise the Muruk B gas resource ~11.7km north-west and along strike of the original discovery well. The well was drilled in 2018/2019 in PDL9 (ExxonMobil operated) and incorporated a similar evaluation program to Muruk 1, with the addition of an extended well test for in-place resource estimation. The well penetrated gas-bearing Toro Formation close to the fold axis, down plunge of the interpreted crest, and within 200m of pre-drill prognosis. Due to significant mud invasion of the reservoir, most of the pressure data acquired was non-definitive with operations hindered by drilling-induced reservoir damage and adverse wellbore conditions. A single wireline gas pressure acquired at the top of the reservoir lay on the same gas gradient as that defined for the Muruk B thrust sheet at Muruk 1ST3, suggesting pressure connectivity between the two locations. An extended well test was conducted across the Toro A Sandstone producing a total of ~146MMscf gas and confirmed a reservoir pressure in line with the Muruk 1ST3 gas gradient. The well was plugged and abandoned with downhole gauges for long-term pressure monitoring. Build-up analysis is ongoing to define potential resource size and assist in building the structural framework for the field. The Muruk field opens an exciting new structural play in the Western Foldbelt that may help realize the exploration potential of the region through application to new and existing leads and prospects along the frontal foldbelt trend.

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