Abstract
In this study, it is aimed to characterize the Early pliocene sandstone (EP-SD) and the Late Miocene-Early Pliocene Mangaa sandstone reservoirs and the efficiency of their sealing cap rocks using the petrographical and petrophysical data of these sandstone zones in northern Taranaki basin, New Zealand. The prospective potential reservoirs were studied using impregnated thin sections, XRD data analysis, and well log data (self-potential, gamma-ray, sonic, density, neutron, shallow\\deep resistivity and PEF) to characterize the reservoir zones, in addition to Mercury intrusion capillary pressure data (MICP) to check the efficiency of some potential seals. The EP-SD and the Mangaa sandstone units are typically poorly consolidated very fine sandstone to siltstone, with porosities averaging 25%. The sands are composed of quartz (38.3–57.4%), with common feldspars (9.9–15.2% plagioclase, and 2.7–6.3% K-feldspars) and up to 31.8% mica. In Albacore-1 well to the north of the Taranaki Basin, the Mangaa formation includes three separate for each of the EP-SD zones (EP-SD1, EP-SD2, and EP-SD3), and the Mangaa sequence (Mangaa-0, Mangaa-1, and Mangaa-2). The thin section studies indicate that, the studied samples are grouped into greywackes, arenites and siltstone microfacies with much lithic fragments and feldspars, sometimes with glauconite pellets. From the XRD data, it is achieved that the mineral composition is dominated by quartz, mica/illite, feldspars, and chlorite. The petrophysical investigation revealed absence of pay zones in the EP-SD zones, and presence of thin pay zone with net thickness 5.79 m and hydrocarbon saturation of about 25.6%. The effective porosities vary between 23.6 and 27.7%, while the shale volume lies between 12.3 and 16.9%. Although the shale content is relatively low, the relatively high API (50–112 API of average 75 API) is contributed by the relatively high K-feldspar content and intercalations with thin siltstone and muddy siltstone beds. Sealing units include the intra-formational seals within the Mangaa sequence, mudstones and fine grained units overlying the Mangaa and further intra-formational mudstones, within the shallower EP-SD units. The efficiency of these seals indicates the capability to trap 16.4–40.6 m gas or 17.4–43.0 m oil which is relatively low in correlation with their efficiency in the central parts of the Taranaki Basin Overlying the primary seals, mudstones of the Giant Foresets Formation provide additional regional seal.
Highlights
The Great Taranaki Basin represents one of the largest sedimentary basins along the western coast of New Zealand
The present study aims at characterizing the Mangaa sandstone in Taranaki Basin by utilizing different data sets including petrographic, and petrophysical data
Due to the gas shows during the well testing processes during the exploration activities in north of the Taranaki Basin, the reservoir characterization of the Early Pliocene and the Mangaa sandstone has been achieved using the well log analysis and interpretation. It aims at getting an insight into the hydrocarbon potentiality of the Mangaa reservoir and the probable Early Pliocene reservoirs by evaluating its petrophysical characteristics
Summary
To examine the Early Pliocene and the Mangaa sandstone reservoir properties and to define the prospective flow units, net-pay thickness, lithology, porosity, shale volume, and the overall fluid types and saturation, a log-based petrophysical analysis has been carried out on the available well logging data of the Abacore-1 well. It is required to know a number of other variables, such as the water and hydrocarbon pressure gradients in the reservoir (Gw and GH, respectively), and the interfacial tension and contact angles at the reservoir and the laboratory conditions (Vavra et al, 1992) As these variables are not known with any confidence in this study, the input values for these variables were taken from another publication on the Mangaa sandstone sequence (Awatea-1 well; Murray and de Bock, 1996). 13.4 ρsh is the shale bulk density, ∅Nsh is the shale neutron porosity, Δtsh is the shale acoustic transit time, and Rsh is the shale deep resistivity values, GT is the gross thickness, NRT is the net reservoir thickness, NPT is the net-pay thickness, N/G is the net to gross thicknesses, ∅e is the effective porosity, Vsh is the shale volume, Sw is the water saturation and SHc is the hydrocarbon saturation
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