Sugeng Sapto Surjono

A Comparison of Maceral and Microlithotype Indices for Interpretation of Coals in the Samarinda Area, Lower Kutai Basin, Indonesia

Strata of the Middle Miocene Balikpapan Formation from the Lower Kutai basin are well exposed in a section near the Samarinda city, East Kalimantan, Indonesia. The succession is characterized by thick sandstone bodies alternating with shales and coal beds. A 250u2009m thick composite section of exposed sediments (not including the soil interval) was measured, from which 25 coal samples were collected. Petrographic, microlithotype, and maceral analyses were performed in order to determine the depositional environment of the Samarinda coals. In order to assess the development of paleomires, coal facies diagrams were obtained from microlithotype and maceral composition. According to the organic petrologic results, the Samarinda coals represent a highly degraded humodetrinite-rich group deposited from terrestrial into telmatic condition of peat formation with vegetation characteristics of highly degraded woody forest type evolved under alternate oxic to anoxic moor conditions. These formed with intermittent moderate to high flooding as the paleopeat environment shifted from mesotrophic to ombrotrophic.

Triassic petroleum system as an alternative exploration concept in offshore western Timor Indonesia

Located in the north-western part of Bonaparte Basin, offshore western Timor presents the opportunity for a distinct exploration concept due to the discovery of the Abadi gas field. A classic Jurassic petroleum play did not develop as a proven reservoir and source rock in the study area due to severe erosion during the Valanginian event. Therefore, the Triassic interval requires assessment to reveal an alternative petroleum system in the study area. A petroleum system analysis utilising 2D seismic lines and three exploration wells was performed to construct a new exploration concept. The study showed that the Scythian Mount Goodwin interval could be considered a primary gas-prone source rock, with type II/III kerogen, a total organic content up to 2.09% and a hydrogen index that could reach 569xa0mg/g. Based on a study of a pseudo-well conducted here, the Mount Goodwin interval is found to have reached gas generation in the Early Cretaceous and expulsion in the Early Eocene. The potential reservoir rocks are the Carnian–Ladinian Challis and Anisian Pollard intervals. The Challis interval, the primary reservoir, has typical shoreline sandstones and is interbedded with claystones and limestone intercalations. Petrophysical and petrographic studies in this interval indicate good reservoir properties. However, the Pollard interval mainly consists of carbonates with minor claystones and has a low quality of reservoir properties. The intraformational shale of the Challis is considered as the potential regional seal, especially within areas that are not truncated by the Valanginian event. From the perspective of exploration, the Jurassic north-east–south-west-tilted fault blocks are still favourable for new discoveries in the offshore western Timor area.