Characteristics of Facies Associations, Ichnofacies and Microfossils for Depositional Environment Interpretation of The Clastic Pulau Balang Formation, Samarinda

Abstract

The Kutai Basin contains prolific reserves of oil and gas. The study of depositional environments is one of the goals of oil and gas exploration. The location of this research is situated in the Tanah Merah area, Samarinda. The objective of this research was to analyse outcrops of the Pulau Balang Formation exposed in the Tanah Merah area to determine the depositional environment. Site specific studies were conducted at 3 localities in this area; TM1, TM2 and TM3. This study combines geological mapping, measured sections, facies analysis, petrography, ichnofacies analysis and microfossil analysis. Geological mapping was carried out to determine the distribution of rock units and geological structures. Measured sections were used for facies analysis and the identification of sedimentary structures and ichnofacies. Petrography was carried out to determine the mineral content of rocks and microfossil analysis for palaeobathymetric environmental analysis. The geological structure of the study area comprises a NE-SW trending anticline and syncline and a left lateral strike-slip fault with E-W direction. The study area is entirely within the Middle Miocene age Pulau Balang Formation and can be divided into three facies associations. The TM1 facies association comprises strata interpreted to be deposited in a supratidal marsh and intertidal flat environment. The TM2 facies association comprises strata interpreted to be deposited in a subtidal, intertidal, and supratidal environment. The TM3 facies association comprises strata interpreted to be deposited in a shoreface environment. The petrography of the study area indicates that rock units predominantly comprise quartz wacke and lithic wacke. Two ichnofacies were identified in the research area 2, namely the Skolithos ichnofacies and the Skolithos-Cruziana ichnofacies and contain ichnogenera namely Ophiomorpha, Skolithos, Planolites, Thalassinoides, Paaleophycus. Microfossil analysis found benthonic foraminifera species including Nodosaria lamellala, N. radicula, Vaginulinopsis tricarinata, Lagena costata, Striatissima vaginulina, Bulimina lappa, Planularia auris, Quinqueloculina seminulum, Bolivina punctata and Lahena laevis. Based on the presence of these microfossils and ichnofacies, it is interpreted that the research area was deposited in a neritic-bathyal environment.