Nido Limestone (Primary Target) The Nido Limestone was intersected at the Coron-1 well 7 kilometres to the south of Coron North. At this location the Nido limestone consisted of a 200 metre interval of tight micritic limestones overlying a porous ?chalky? limestone. A repetition of this sequence would not be desirable at Coron North, as the upper section is likely to act as a thief zone for hydrocarbons. Seismic signatures at Coron North are distinctly different to those seen at Coron-1 (see figures 10A to 10F). Coron North is characterised by parallel reflectors with some shingling in the chalky section. The latter is associated with a cleaning upward signature on well logs. The shingled zone can be seen locally in the Nido IIX-1 to Coron-1 area, and can be mapped prograding northwards from Nido IIX-1 and eastwards from Coron-1. The top of this sequence is marked by a flooding surface which can mapped over a large area. In contrast, the Coron North Prospect shows a set of seismic characteristics related to reefal buildup. These include stratigraphic thickening and wedges, hummocky upper surface, onlap related to downward shifts of base level, small reefs, slumping, and drape. These features definitively point NIDO Petroleum Philippines Pty Ltd Coron North, SC 42, NW Palawan Basin Farmin Opportunity October 1999 9 towards reefal style buildups. Karst development is also considered likely. It is analogous to the Camago-Malampaya buildup, with the main difference being that the buildups persisted for a longer time at Camago-Malampaya before being killed off by clastic influx, resulting in a much higher buildup than at Coron North. Another feature shown on line 84-30A (figure 10D) clearly shows crestal pop-out at Coron North related to the post-depositional compressional structuring. Associated fracturing is expected to provide reservoir enhancement. In contrast to Coron-1, the reservoir risk at Coron North is not considered to be a concern. ?Pre-Nido? Limestone (Secondary Target) In the Coron-1 well, a section of interpreted outer shelfal limestones was encountered 400 metres below the top of the Nido Limestone. The limestones were described as recrystallised with remnant vuggy and mouldic porosity enhanced by fracturing, and porosity was described as fair. The limestones are variably carbonaceous, argillaceous and siliceous. A significant oil column (120 metres) is present at Coron-1, and an interpreted gas column is present in the Nido IIX-1 well in the pre-Nido Limestone. For commercial production, an improvement in reservoir quality is required. Faulting at Coron North may provide this through an increase in fracture porosity. This section offers a secondary target. No reserve estimates have been made for the Coron North Prospect. 5.3 Seal Seal to the Nido Limestone is provided by the Pagasa Shale. The Pagasa is a series of progradational shale dominated units, but in some locations (e.g. North Iloc) it is suspected that interbedded sands act as thief beds where they downlap/onlap structural culminations. This is not a concern at Coron North since the topographic relief at top Nido Limestone level up into the Pagasa Formation is low, and seismic sections do not show onlapping reflectors. As a result seal risk for the primary target is considered to be low. Adequate seal to the pre-Nido Limestone is demonstrated by the existence of hydrocarbon columns at Coron-1 and Nido IIX-1. 5.4 Source Rocks, Maturation, Migration Prospectivity at Coron North is buoyed by the excellent source potential. There are two lines of evidence to substantiate this. The first is from the Coron-1 well, where a 120 metre untested oil column is present within the pre-Nido Limestone at 3700 m. In this section resistivity logs, fluid inclusion data, mud gas and oil shows point conclusively to the presence of an oil column. The fluid inclusion work conducted by Nido Petroleum through Advance Hydrocarbon Stratigraphy in Tulsa is particularly precise in recognising the oil column (see figure 11). Fletcher Challenge in drilling the well in 1993, recognised the possibility but were not able to run a test because of downhole engineering problems (Fletcher Challenge, Coron-1 Well Completion Report 1993). NIDO Petroleum Philippines Pty Ltd Coron North, SC 42, NW Palawan Basin Farmin Opportunity October 1999 10 The second line of evidence is from regional observations and detailed oil to source studies conducted by Shell and Fletcher Challenge which identified potential source rocks within both the carbonate section and the syn-rift fill. Shell?s work suggests the majority of the hydrocarbons are generated from marine carbonates, marls or shales, with a strong land plant influence. Fletcher Challenge concluded that many oils have a playa lake or lacustrine source. Both companies biased their work towards the oils and it appears likely that the oil and the gas are sourced from separate intervals. Using the Chinese basins as an analogue, it is reasonable to expect that the syn-rift fill will contain terrigenously dominated source rocks capable of producing both gas and oil. In contrast, the Nido limestone source rocks will predominantly produce high sulphur oils. In terms of maturity, the syn-rift section regionally is for the most part within the gas window, and in the deeper areas (Camago-Malampaya) it has fully generated as demonstrated by the generation chart in figure 12A. In the shallower areas, e.g. in the vicinity of Coron North (figure 12B) only the deepest parts of the syn-rift fill have entered the gas window. Most of the syn-rift section and the pre-Nido and Nido Limestones are in the oil window. Thus it is likely that the dominant hydrocarbon phase at Coron North will be oil. The fluid inclusion work at Coron-1 corroborates this indicating the presence of light oil to possibly wet gas. The fetch area and migration directions for the pre-Nido and Nido Limestones at Coron North are shown on figure 13. Migration pathways are present into the Coron North structure from the western, northern and eastern flanks. Migration from the pre-Nido Limestone into the Nido Limestone will be facilitated at Coron North by the presence of faulting (figure 10D). Similar faults are absent at Coron-1.
NDO Price at posting:
15.0¢ Sentiment: Buy Disclosure: Held
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