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Abstract: Merah Besar and West Seno Field Discoveries, Makassar Strait, East Kalimantan, Indonesia

In 1996 and 1998 the first two significant hydrocarbon accumulations in Indonesia's deep water were discovered by Unocal Indonesia. These were Merah Besar and West Seno, respectively. Merah Besar is located 25 kms ENE of the giant Attaka Field in 1700' to 2700' of water, while West Seno is located 25 kms to the NE of Merah Besar in 2400' to 3200' of water. The Merah Besar structure is a rollover anticline encompassing 40 kms2 and formed as a result of shelf edge listric-normal faulting. Down slope, these listric faults sole-out into a basal detachment; further down slope, ramp-thrusts rise from the detachment surface and form large fault-propagation folds. The first test of one of these fault-propagation folds was the Seno structure. The Seno structure is an elongate, four-way dipping anticlinal feature, which covers an area of approximately 70 kms2. It is composed of two separate structural domains; a large, eastward dipping tilted fault block (East Seno) and a gentle rollover anticline on the western down-thrown side of this fault block (West Seno).

Seismic and wire-line log data, along with conventional core and biostratigraphic data analyzed from the Merah Besar wells indicate the productive sequences are Pliocene and Upper Miocene, turbidite channel-levee sandstones, deposited in an upper to mid-slope position. Trap in the Pliocene and Miocene is dominantly structural with some associated stratigraphic trapping. The Miocene hydrocarbon accumulation is located on the highly faulted southwest part of the anticlinal structure over an area of approximately 4000 acres. The Pliocene traps extend further north and occur over a 16,000 acre area. The Miocene traps are three way dip closures bounded by normal faults forming the up-dip seal. Lateral pinchout out of channel and levee deposits adds a stratigraphic trapping component to the Pliocene and Miocene accumulations. The majority of hydrocarbon bearing sandstones are located in a depth range of -4000 to -9500 ft.TVD.

In West Seno, hydrocarbon accumulations occur where Upper and Mid Miocene sandstones are faulted 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 those encountered to date in the Merah Besar area. These sandstones are interpreted to be amalgamated turbidite channels capped by correlative, low resistivity, pressure sealing, hemipelagic shales and deposited in a more distal position relative to Merah Besar. The majority of the hydrocarbon bearing sandstones are located in a depth range of -7000' to -9500'TVD and extend over a minimum area of 4000 acres.

In Merah Besar and West Seno, porosity in the sandstones is normally in the 24 to 32 percent range with permeabilities in the 150 to 1500 md range. The sandstones are quartzose and dominantly fine grained however sandstones ranging from very fine to coarse 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 with similar bed thicknesses. The reservoir beds are beyond the resolution of conventional wireline, which are measuring a rock volume containing both reservoir sandstone and shale. Thin reservoir sandstones display similar rock properties to thicker 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.

Geochemical analyses of the Miocene oils and Pliocene and Miocene gases demonstrate they were most likely derived from similar source facies comprised of predominantly land plant organic material. The hydrocarbons are interpreted to have migrated vertically along normal faults from the "kitchen" area, deeper in the section, to the Upper Miocene and Pliocene 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 are all good quality crude with an API gravity range between 35 to 46 degrees.

Miocene DST's achieved flow rates of 2375 BOPD plus 2.5 MMCFGPD in low resistivity, thin bedded intervals. In thicker bedded, higher resistivity zones, the rates ranged between 8250 BOPD plus 8.8 MMCFGPD to 10069 BOPD plus 10 MMCFGPD. Pliocene DST flow rates ranged between 18 and 21 MMCFGPD.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England