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Local Stratigraphy and Mud Gas Correlations of Wells in North-West Eastern Papuan Basin, Papua New Guinea

Abstract

Extraction of formation cuttings and fluid samples during drilling results in stratigraphic sequence profile. Mud-gas analysis is an important process for establishing stratigraphic sequence profiles including hydrocarbon characterization studies in a petroleum basin. Grain size and texture of cuttings provide evidence used in depicting the vertical and lateral variation of lithology in a formation. Gas ratio curves, gas composition diagrams, Pixler and gas cross plots analyse the processed mud-gas results - methane to pentane, and providing information on the quality of the hydrocarbon and its saturation area within a formation. Bobcat-1 has been the focus among wells situated along northwest Eastern Papuan Basin. The well is at the confluence of the south-eastern end of the Papuan fold belt, the Aure deformation zone and the north-western tip of the Eastern fold belt. The Elk-Antelope Gas field is south-east of Bobcat-1, where studies revealed fractured deep marine carbonates as reservoirs. Besides oil shows from cuttings in Bobcat-1, gas ratio curves also have revealed oil saturation zones in Antelope-2. This paper aspires to provide an independent perspective to the subsurface lithostratigraphy of the north-west segment of eastern Papuan Basin in an attempt to establish the local stratigraphic sequence and reservoir type, and identify formation fluids and their secondary migratory trends. Approach involved two methods with differing objectives: (a) examining grain size and texture of hand-held cuttings samples and those described on Bobcat-1 formation evaluation logs. Next, relating identifiable sequences to known stratigraphic sequences in the Elk-Antelope Gas field to name and demarcate into sequences and subsequences. Then, conformable formation tops from adjacent wells in the Elk-Antelope gas fields were picked and correlated against the reference well, Bobcat-1. Information gained has been used to generate a chart using Microsoft Office Paint software showing Bobcat-1 stratigraphy profile correlated against seven other wells. (b) Using gas ratio trend analysis, the raw compositional gas data, C1 to C5 was processed to display ‘hydrocarbon saturation intervals’ and ‘fluid contact points’ on the ratio trends. Other Gas Analysis Cross Plots were constructed to verify and assess the credibility of the identified ‘hydrocarbon saturation intervals’. Pixler and Triangular plots were used to distinguish fluids as either dry gas, condensate, oil, or water, and whether a hydrocarbon saturation zone was productive or otherwise. The lithostratigraphy sequence comprises conformable and unconformable sequences. Earlier hosts Kapau limestone, preceded by Orubadi formation, and the Era beds. Occasionally, Orbulina marl interfinger between Kapau and Orubadi with Era beds non-existent at instances. Unconformity occurred ensuing mid Tertiary differential uplifting and weathering. Facies variation is largely affected by a subsiding tectonic activity following early Tertiary uplift, and sea level fluctuations. On the other hand, a highly deformed or fractured reservoir hosting hydrocarbons and formation water is present. Strong presence of conventional hydrocarbons overshadow unconventional deposits especially gas and gas condensate. Residual oil presence is substantial and was flushed via highly permeable pores. As a result, the local paleo-depositional setting is an open marine-sand shoal-slope environment. Most of the hydrocarbons identified are hosted in the slope facies zone within the Kapau limestone unit. Apparently, northwest eastern Papuan basin hosts highly deformed reservoirs which are an extension of the reservoirs in the Elk-Antelope gas fields that are overlaid by a thick mudstone. Thus, facies variation depicts a paleo-depositional setting typical of platform interior to platform margin and down to the toe of the slopes. Economical hydrocarbons are hosted in the slope facies where a dominating formation water system is flushing most of the oil in a south-easterly direction.