TECTONIC-PHASE-CHANGES WITHIN THE MANIGUIN-3 WELL AREA, OFFSHORE SOUTHERN MINDORO ISLAND, AS MANIFESTED BY VARIABLE MACERAL COMPOSITION OF COALS

Casareo, F. E. and Forbes, M.T.
U.P. National Institute of Geological Sciences, Diliman, Quezon City 1101

This research obtained financial assistance from the U.P. National Institute of Geological Sciences, the Philippine National Oil Company-Exploration Corp. , and the Dean’s Office-U.P. College of Science.

ABSTRACT

        The structural evolution of the offshore Mindoro-Panay area is characterised by a changeover from a passive to an active margin which occurred during the Tertiary. The sequence of tectonic events recognised is as follows: 1) Early Oligocene (or older) Rift; 2) Late Oligocene Drift; 3) Middle Miocene Collision; and 4) Recent Wrenching. This polyphase tectonic history is related to the rifting of the Eurasian margin and subsequent opening of the South China Sea followed by collision with the Philippine Island-Arc system. Structural data confirms this interpretation and validated by maceral analyses on coal particles isolated from well-cuttings recovered from the Maniguin-3 Well.

        Cuttings from the Maniguin-3 Well reveal that the passive-margin deposits are typical of lacustrine and mire environments, that is, coal-bearing claystone/mudstone-dominated sequences. Active-margin deposits on the otherhand are characterised by coarse-grained siliciclastic sequences with fewer coal seams. Maceral analyses of coal isolates reveal two distinct coal facies, a "wet-type" and a "dry-type" facies. The "wet-type" facies (WTF) indicate limno-telmatic to telmatic conditions and processes of mire formation that occur on or in deep water by free-floating or deeply-rooted plants, or at the water-table whereby plants grow under conditions of periodic flooding. The "dry-type" facies (DTF) indicate terrestric conditions and processes of mire formation that occur above the general water-table. The WTF is most likely associated with the rifting stage whereby the water-table is relatively high whereas the DTF is indicative of high oxidation and occurs generally in areas with relatively lower water-table levels. These conditions for the development of DTF coals can be associated with the collision stage.