Carbonate Petrology of The Nido B-3A Core, Offshore Palawan, Philippines
Published:January 01, 1980
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.
Figures & Tables
Carbonate Reservoir Rocks
This core workshop is the product of an effort by the SEPM Carbonate Research Group to emphasize the value of careful core interpretation to hydrocarbon exploration and production programs. Initially conceived as a small conference the format was expanded in response to strong interest from the geologic community. The end result was a workshop including 14 core displays. The notes were intended as a guide but the thorough documentation of these cores will be of interest to many others.