The upper Pleistocene north of the Mahakam delta, offshore East Kalimantan has been studied from the inner shelf to basin floor, using 3D seismic data. Three cycles deposited between ~18 and ~370 ka are defined on the shelf by progradational packages separated by parallel reflectors with carbonate buildups. The prograding packages were apparently deposited during highstands and gradually falling eustatic sea level. Parallel reflectors and carbonate buildups are interpreted as transgressive deposits. During the lowest of the three cycles (~270-370 ka), a lowstand delta has prograded over the underlying shelf margin, and sand-rich sediment has spilled downslope, feeding a slope-channel complex and basin-floor fan. This slope-channel complex has: (1) a lower part consisting of an incised channel filled with high-amplitude reflectors that are inferred to be sand-rich, and (2) an upper part dominated by a low-amplitude channel-levee complex inferred to be shale-rich. The slope-channel complex passes downdip into a basin-floor fan. The basin-floor fan contains a lower part, which contains high-amplitude, continuous reflectors (interpreted as sand-rich fan lobes), and an upper part that represents an aggrading channel-levee complex. These levees also are inferred to be shale-dominated, whereas some channel-fills are apparently sand-rich. The channel-levee complex has prograded over the lower fan and fed additional unconfined high-amplitude (sand-rich?) lobes. These Pleistocene cycles are significantly different from sequences and systems tracts defined in the late 1980s. Lowstand systems tracts can not be identified by onlap of the slope. Rather, lowstand strata on the slope and basin floor are generally parallel reflectors that have local variations caused by channels and fans. Lowstand strata extend down the slope from distal clinoforms of the prograding lowstand delta, and hence cannot be consistently separated from prograding highstand and falling stage systems tracts. Cycles are best separated at the tops of prograding packages (transgressive surfaces).

Abstract In August 1998 Unocal Indonesia drilled the discovery well for the West Seno Field located in the Kutei Basin offshore East Kalimantan, Indonesia. Following an accelerated delineation program, a Plan of Development was submitted and approved in September, 1999, which served to commercialize the Makassar Straits Production Sharing Contract (PSC) area. In the West Seno area, hydrocarbon accumulations occur where upper and middle Miocene sandstones are fault and stratigraphically trapped in an updip position. The lateral continuity of seismic reflectors, biostratigraphic analysis, and well log data in the West Seno area suggest the sand-prone intervals display significant lateral distribution. These sandstones are interpreted to be amalgamated, turbidite channel sands associated with interbedded, levee-overbank sand/shale sequences deposited in a mid-slope position. Porosity in the reservoir sandstones ranges from 22 to 32 % and permeability ranges from 150 to 1500 mD. Productive sandstones are characterized by resistivity readings ranging from 4 to 20 ohms. Core data show that low-resistive pay intervals, with 4 to 5 ohms resistivity, are characterized by reservoir bed thickness ranging from millimeter to centimeter scale, interbedded with shale and carbonaceous laminae of similar bed thickness. Geochemical analyses of the Miocene oils and gases demonstrate they are most likely derived from predominantly terrestrial plant organic material. The hydrocarbons found in West Seno are interpreted to have migrated vertically along faults from the source area to the upper Miocene reservoirs. The oils are all good quality crude having an API gravity range between 35 to 46 degrees. Data gained from conventional cores, special petro-physical logging tools, and drill stem tests were used to construct a petrophysical model to calculate reservoir properties used in the quantification of the West Seno Field reserves. The West Seno Field is a “fast tracked” project; only 14 months from time of first discovery through to Plan of Development approval. This will be the first deepwater development for Indonesia and for Unocal; expected first production will commence in 2002.