Origin of sedimentary organic matter in Mahakam Delta, Borneo, Indonesia

The Mahakam Delta is located on the east coast of Kalimantan, the Indonesian portion of Borneo, about 50 kilometres south of the equator, and contains sediments of Miocene to Pliocene age. The Mahakam Delta has a regular lobate morphology typical of a mixed fluvial and tidal delta. Tidal and fluvial currents dominate sedimentation processes. The delta covers about 5,000 km², including 1,300 km² of swampy subaerial delta plain, 1,000 km² of subtidal platform, and 2,700 km² of prodelta deposits (Allen et al., 1979). Since the middle of the Miocene, the Mahakam Delta has been an important fluvio-deltaic depocenter. It is also a significant hydrocarbon province, and the majority of the oil and gas found there comes from humic sources. The purpose of this study is to unravel the source and origins of organic matter in sediments influenced by tropical fluvio-tidal currents. The studied section is primarily composed of organic-rich shales, coaly shale and coal.  TOC values of shale approximate 1.57 - 9.55 wt.%, coaly shale 6.59 - 19.66 wt.% and coal samples  27.42 – 50.99 wt.% indicating to be a source rock in the study area. According to the classification of Peters and Cassa (1994), the Mahakam delta classifies as good-excellent source rock with the TOC value range 1.57 – 50.99 wt.%. Geochemical analyses of organic-rich sediments from the Miocene indicate that the possible source rocks are delta plain-delta front coals and shales containing predominately type III kerogen organic. Delta front deposits are mostly black, organic-rich mud with plant fragments and silt laminae. Sulfur and siderite nodules indicate anoxic conditions in these sediments. Sediments from the Mahakam Delta plain are a mixture of muds and rare to abundant plant (mangrove swamp, Nypa swamp, and transitional forest). The upper delta plain rainforest may supply sedimentary organic matter. Sedimentary organic matter can be accounted for by the incorporation of produced leaves into waterways through direct leaf failure, slumping of channel banks, and sporadic tidal export. The plants in the lower delta plain make a dense, tangled web of dead Nypa palm petioles and leaves and aerial roots. Sedimentary organic matter in Mahakam deltaic environments comes mainly from delta plain vegetation. Organic matter from the delta plain, mostly degraded plant remains, is incorporated into the deltaic system by tidal channel erosion of Nypa and transitional forest. Vegetal debris like wood and leaves accumulated in situ, preserved by sedimentation, forming thick layer coaly shales and coal beds Geochemical analyses of organic-rich sediments from the Miocene indicate that the potential source rocks are delta plain-delta front coals and shales containing predominately type III kerogen organic matter as having good to excellent potential as gas source rocks.