An Evaluation on Organic Thermal Maturity Based on Color Index of Halang Formation, Banyumas Basin, Central Java, Indonesia

Abstract

We analyzed palinomorfs and foraminifera fossils on two samples taken from the claystone layer of the Halang Formation. We conducted this analysis to evaluate the thermal maturity of the sample as well as to determine its age and depositional environment. Both samples contained poor amounts of pollen, spores, dinoflagellate, and diatom, but they contained abundant foraminifera. In these two samples, pollens, spores, dinoflagellates, and diatom were primarily black or blackish, gray to dark brown—the color of the foraminifera tests was also dark, principally gray. No index fossils were found in the pollen assemblages. However, the presence of Spinizonocolpites echinatus, Monoporites annulatus, and Pinuspollenites type indicates the samples are not older than the Paleocene. The foraminifera assemblage in both samples suggests that the age of the samples is Early Pliocene. The lack of pollen grains and spores and the abundance of foraminifera indicate that the depositional environment of the two samples was offshore. Melonis pompiliodes, Cibicidoides wuellerstorfi, and Bulimina affinis suggest these samples were deposited in a lower slope/lower bathyal environment. The color of the pollen, spore, dinoflagellate, diatom, and foraminifera fossils indicates the degree of thermal maturity of the samples have reached a mature or dry gas (barren) level. The high level of thermal maturity of the relatively young Halang Formation is supposed to be related to the presence of post-depositional volcanic activities