Petrographic Composition, Facies and Thermal Maturity of the Coals from the Semirara Formation, Maniguin Area

Monina T. Forbes¹ and Francis E. Casareo²

1PNOC-EC, Fort Bonifacio, Taguig, M.M., ²Sydney, Australia

Earlier assessment on the three exploratory wells drilled in the Maniguin area, Central Philippines, suggests that the potential source rocks of hydrocarbons in the area are the claystone and coal of the Lower Claystone Member of the Semirara Formation. In recognition of the important role played by the maceral compositions of the coal on its potential in generating petroleum, coal petrography including coal facies analysis was employed in this study. This was done in an attempt to determine possible explanations on the varying hydrocarbon yields in the area as exhibited by the three Maniguin Exploratory wells.

Results of the petrographic analysis reveal that the overall maceral compositions of the coals from the three wells are similar with variations in the individual maceral concentrations. The coals are all vitrinite-rich, designated as lignite to sub-bituminous in rank; and are in immature to early stage of oil generation. Significant differences in the amount of detrovitrinite were observed in the three wells. Maniguin-2, the only well among the three to yield significant amounts of oil, was found to contain the largest amount of detrovitrinite. Coal facies analyses confirm that the significant difference in the amount of detrovitrinite within the three wells is a function of lateral variation in the paleodepositional environment of the coal. Facies analysis suggests that the Maniguin coals are formed in a paralic setting and that the large amount of detrovitrinite in Maniguin-2 is related to a relatively higher marine-influenced environment, which is more suitable for the formation of an oil-prone source rock.