West Seno Field Discovery, Makassar Strait, East Kalimantan, Indonesia

Readhead, Richard, Eko Lumadyo, Art Saller, Jessie Noah, Trevor Brown, Yusri, Yusak, Josaay Inaray, T.A. Ma, and Ronald May; Unocal Indonesia Co.

Abstract

In 1998, Unocal Indonesia drilled the discovery well for West Seno Field. Following initial delineation, the project was "fast tracked" in order to prove up economic development. This project is a prime example of Unocal Indonesia's "SX" philosophy for e xploration and delineation in the East Kalimantan area.

West Seno is located on the Makassar Straits PSC, 50 km east northeast of the giant Attaka Field in 2400 ft to 3200 ft of water and is located 20 km to the northeast of the deepwater Merah Besar Field discovered by Unocal Indonesia i n 1996. The Seno structure is an elongate, four-way dipping anticlinal feature, which covers an area of approximately 70 km². It is composed of two separate structural domains; a large, eastward dipping tilted fault block (East Seno) and a gent le rollover anticline on the western down-thrown side of this fault block (West Seno).

At Merah Besar the structure is a rollover anticline and formed as a result of shelf edge listric-normal faulting. Down-slope, these listric faults sole-out into a basal detachment; farther down slope, ramp-thrusts rise from the deta chment surface and form large fault-propagation folds. The first test of one of these fault-propagation folds has been the Seno structure.

In West Seno, hydrocarbon accumulations occur where upper and middle Miocene sandstones are fault and stratigraphically trapped in an updip position. The lateral continuity of the seismic reflectors, biostratigraphic analysis and well log data in the West Seno area suggests the sand-prone intervals have significant lateral distribution, greater than that encountered in the Merah Besar Field area. These sandstones are interpreted to be amalgamated turbidite channels; interbedded overbank-levee sand/shale sequences are deposited in a more distal slope position relative to the Merah Besar Field. The sands are capped by low resistivity, pressure sealing, hemipelagic shales. These shales can be correlated from well to well using the seismic data and by their distinctive log character, which can be differentiated from terrigenous shales present in the wells. The majority of the hydrocarbon bearing sandstones are located in a depth range of -7000' to -9500' TVD and extend over a minimum are a of 4000 acres.

In West Seno, porosity in the sandstones is normally in the 22% to 32% range and permeabilities are in the 150 to 1500 mD range. The sandstones are quartzose and dominantly fine-grained; however sandstones ranging from very fine- to med ium-grained have been encountered at different levels across the fields.

Productive sandstones are characterized by resistivity readings ranging from 4 to 20 ohms. Cores show that low resistive pay intervals, 4 to 5 ohms, are characterized by reservoir bed thickness ranging from millimeter to centimeter scale, interbedded with shale and carbonaceous laminae of similar bed thickness. The reservoir beds are beyond the resolution of conventional wireline logging tools, which are measuring rock volume containing reservoir quality sandstone, carbonaceous material and shale. Thinly bedded reservoir sandstones display similar rock properties to thicker bedded sandstones. In thin-bedded intervals, therefore, variations of resistivity from 4 to 20 ohms represent variations in net to gross of pay sandstone and not variations in reservoir properties.

West Seno Miocene DST's achieve flow rates of 2,375 BOPD plus 2.5 MMCFGPD in low resistivity, thin bedded intervals, while in thicker bedded, higher resistivity zones, rates range between 8,250 BOPD plus 8.8 MMCFGPD to 10,069 BOPD plus 10 MMCFGPD.

Geochemical analyses of the Miocene oils and gases demonstrate they were most likely derived from similar source facies comprised of predominantly land plant organic material. Of particular interest is the association of this organic ma terial as carbonaceous laminae, often highly concentrated as fine tails within the interbedded turbiditic sands and shales. Though the stratigraphic section drilled to date is thermally immature, this same mechanism of deposition indicates that the deeper underlying middle Miocene section could contain significant volumes of source/reservoir rocks present within the active oil generation window. The hydrocarbons found in West Seno are interpreted to have migrated vertically along normal faults from this "kitchen" area to the upper Miocene reservoirs. Migration fractionation of the hydrocarbons into dry gas shallow and oil deeper probably occurred in association with minor rejuvenation of the faults some time after initial hydrocarbon migration. The oils a re all good quality crude and have an API gravity range between 35 to 46 degrees.

The wealth of data gained from the core, special petrophysical logging tools and the DST information was critical in building a petrophysical model used to calculate reservoir properties used in the quantification of the West Seno Field reserves.

The West Seno Field has been "fast tracked" with 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 with expected first production to commence in the first quarter of 2002.