Integrated sedimentological and petrophysical characterization for clastic reservoirs: A case study from New Zealand

Abstract

The present study aims at characterizing the sedimentological and petrophysical properties of the Middle-Late Miocene Mt Messenger reservoir in Taranaki Basin, New Zealand. It is a detailed study based on availability of sedimentological (petrography, SEM and XRD), well log data (gamma ray, neutron, density, sonic, shallow and deep resistivity, PEF and RFT logs), in addition to conventional and special core analysis data for Mt Messenger reservoir.The Mt Messenger Formation is considered onshore fan units of low resistivity clastic rocks with high percentage of metamorphic lithic fragments. From the RFT logs, it is composed of two or three compartments partially separated from each other by some tight zones. The reservoir zones are represented by thin beds of well sorted very fine to silty-grained litharenite; sometimes is feldspathic, argillaceous and/or micaceous with very good intergranular porosity of average reaches to 25%. Petrographically and petrophysically four reservoir rock types (RRTs) or hydraulic flow units (HFUs) can be assigned, each with its diagnostic petrophysical properties and storage and flow capacities. The reservoir quality increases to the south at Ngatoro-2 well, where a total of gas- and oil-bearing 5 prospective thin beds are recognized, which are mostly composed of litharenite with very good porosity (21–27.8%) and permeability (63–368 md). Its reservoir quality index (RQI) and flow zone indicator (FZI) indicates fair to good reservoir quality (0.544 = RQI ≤ 1.151 μm, and 1.972 = FZI ≤ 3.049 μm). Presence of argillaceous and/or micaceous materials in addition to the silt-sized grains decreased the reservoir quality from RRT1 samples which are characterized by fair to good reservoir properties (homogeneous pore size distribution, 0.18 = R35 ≤ 1.32 μm) to the RRT4 samples which are considered slightly tight reservoir (slightly heterogeneous pore size distribution, 4.44 = R35 ≤ 9.97 μm).