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The first commercial oil production in the Philippines is from the Nido “B” field located about 30 miles northwest of Palawan. Oil is being produced from a Lower Miocene atoll-like carbonate buildup at a depth of approximately 6,800 feet.

During development drilling, a 30-foot core was taken in the Nido B-3A well drilled near the margin of the buildup. Rocks in the core are intraclast packstones and skeletal packstones/grainstones. Intraclasts are poorly sorted, moderately well-rounded, and surrounded by a shaly matrix containing abundant planktic foraminifers. They were apparently derived from a marine-cemented reef rich in red algae and corals. Fragments of red algae, corals, larger benthic foraminifers and molluscs are the dominant grains in the skeletal pack-stones and grainstones. All rocks in the core were deposited as proximal fore-reef talus.

Essentially all primary porosity in these limestones was filled by marine cement now recrystallized to calcite. A small amount of equant calcite filled the remaining primary pore space. Secondary vuggy porosity is present locally, but most porosity in the core is in a complex set of anastamosing hairline fractures that divides the rock into irregular polygons a few centimeters in diameter. Good permeability in these fractures allowed the well to produce more than 10,000 BOPD after treatment with acid during initial testing.

Study of the Nido B-3A core is significant because it shows that (1) fore-reef talus may be a good reservoir facies in Miocene buildups in the Philippines, (2) hairline fractures can apparently form a significant reservoii in the subsurface even though they are barely visible in polished slabs, (3) fore-reef talus, like the reef framework, is susceptible to extensive marir cementation, (4) micritization is a minor process in fore-reef talus, and (5) difficulty would exist in trying to distinguish fore-reef talus from other facies of a reef complex in small samples, particularly cuttings and sidewall cores.

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