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A REVIEW OF THE GRAVITY DATA ON THE CENTRAL
VALLEY BASIN, LUZON, PHILIPPINES

ERNESTO P. SONIDO
Ph.D., M.S., B.S.E.M., Geophysicist
Geologist, and Mining Engineer

 

 

 

 

ABSTRACT



A reconnaissance gravity survey was conducted over the Central Valley Basin in the Island of Luzon, Republic of the Philippines in 1958 and 1959. The survey delineated a number of gravity highs that was interpreted to represent structural anticlines that could be an oil-trap. Several of the important looking gravity anomalies were verified to represent structural anticlines by pre-CDP reflection seismic works and drilled. Those which correlated with extinct
volcanoes, or volcanic flows and igneous intrussive plugs did not have to be verified.

Nine (9) wells, four (4) of which were shallow, were drilled for stratigraphic purposes while five (5) were deep structural test wells. Eight (8) of these wells were drilled in the western and northern part of the basin. No commercial oil or gas was discovered nor are there any reported oil seeps in the basin. Some gas seeps have been reported.

Since the discovery of oil and gas in reefal limestone and carbonate builds up, a new concept for exploration targets has resulted in evaluation of previous geological, geophysical and drilling results. The results of the previous works are presented as the Geological Map, Bouguer Anomaly Map, Residual Second Derivative Maps and Seismic Maps separately and as composite maps correlating geological-gravity and seismic reflection data. Inasmuch as the gravity leads associated with probable reservoir rocks consisting of mostly Lower and Middle Miocene Limestone occur mostly in the southeastern portion of the Basin and since this same area has not been tested by drilling it is the author's opinion that the gravity leads in this area merit detailed gravity and modern CDP seismic reflection works to determine the potential for oil.

The necessary parameters for oil potentials exist in the presence of source rocks in the Pre-Miocene Shale, good reservoirs as Miocene Limestone as possible pinnacle or hinge refs and tight sealing rocks of shale all of which are found in the area. However, the oil potentials can be positively established by additional geophysical surveys and drilling down to the reservoir targets.



 

 

 

 

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