SOUTH SUMATRA BASIN PROVINCEhere is some info on the drilling...

SOUTH SUMATRA BASIN PROVINCE
here is some info on the drilling site from the article you found

http://pubs.usgs.gov/of/1999/ofr-99-0050/OF99-50S/overburden.html


Talang Akar Formation
The late Oligocene lower Talang Akar Formation is also referred to as the Gritsand Member and the Oligocene to early Miocene upper Talang Akar Formation as the Transition Member (Sitompul and others, 1992; Tamtomo and others, 1997). The Talang Akar Formation is as much as 610 m thick (Hutchinson, 1996). It is a late synrift to post-rift formation that is thick where the underlying Lahat Formation is thickest (Fig. 3). The Talang Akar Formation unconformably overlies the Lahat Formation. It onlaps the Lahat and the basement, extending farther outside of the depositional basins than the depositional limits of the Lahat Formation (Hutchinson, 1996). This reservoir consists of quartzose sandstones, siltstones, and shales deposited in a delta plain setting that changed basinward, generally to the south and west, into marginal marine sandstones and shales (Adiwidjaja and de Coster, 1973; Hutchinson, 1996; Eko Widianto and Nanang Muksin, 1989). Specific depositional environments that have been identified include open marine, nearshore, delta plain, delta, distributary channel, fluvial, and beach (Hutapea, 1981). Talang Akar Formation sandstones, which were deposited during marine transgressions and regressions, form important stratigraphic traps (Tamtomo and others, 1997). These shoreline sands are generally aligned east to west, are supplied with sediment from the Sunda Shelf to the north and the Palembang High (Lampung High) to the east, can be laterally restricted, and thicken and thin in response to topography at the time of deposition (Adiwidjaja and de Coster, 1973; Hamilton, 1979; Hutapea, 1981; Sitompul and others, 1992). Other shoreline sandstones that surround basement highs are productive reservoirs for several fields (Tamtomo and others, 1997). Here the quality of the reservoir depends on the type of basement rock eroded to provide the clastics.

The Talang Akar Formation reservoir accounts for more than 75% of the cumulative oil production in South Sumatra (Tamtomo and others, 1997). Approximately 2 BBOE ultimate recoverable reserves have been found in Talang Akar reservoirs (Petroconsultants, 1996). Porosity of this reservoir rock ranges from 15—30 % and permeability is as much as 5 Darcies (Tamtomo and others, 1997; Petroconsultants, 1996). Porosity of the Gritsand Member is primarily secondary and averages 25% (Sitompul and others, 1992). Porosity of the Transitional Member is also primarily secondary and caused by the dissolution of grains and detrital clays. This cleaner and more mature sandstone has 25% average porosity (Sitompul and others, 1992). Clays in both members include smectite, illite, and abundant kaolinite (Sitompul and others, 1992).

Batu Raja Limestone
The early Miocene Batu Raja Limestone is also known as the Basal Telisa Limestone (Hutchinson, 1996). The formation consists of widespread platform carbonates, 20—75 m thick, with additional carbonate buildups and reefs, from 60—120 m thick (Hutchinson, 1996; Hartanto and others, 1991). The Basal Telisa is shale and calcareous shale deposited in deeper water as the carbonates were being developed on the platforms and highs (Courteney and others 1990). At outcrop the Batu Raja is 520 m thick in the Garba Mountains area of the Barisan Mountains (Fig. 2) (Hutchinson, 1996).

Discoveries in Batu Raja limestone and sandy limestone total over 1 BBOE ultimate recoverable reserves, with gas comprising just over half of that amount (Petroconsultants, 1996). Oil gravity ranges from 26—61° API (Petroconsultants, 1996). Reservoir porosity ranges from 18—38% and reservoir permeability is as much as 1 Darcy (Petroconsultants, 1996). Porosity has been enhanced in the upper parts of the formation due to subaerial exposure late in the early Miocene, at approximately 17.5 Ma, and also because of only partially cemented fractures (Courteney and others, 1990; Hartanto and others, 1991; Sitompul and others, 1992).

Gumai Formation
The Oligocene to middle Miocene Gumai Formation, also known as the Telisa Formation, is composed of fossiliferous marine shales with thin, glauconitic limestones that represent a rapid, widespread maximum transgression (Fig. 4) (Hartanto and others, 1991; Hutchinson, 1996). The transgression was toward the northeast, and water depths were shallow in the northeast and bathyal in the southwest (Hamilton, 1979). Fine-grained sandstones and siltstones occur on the basin margins (Hutchinson, 1996). The thickness of the Gumai Formation varies and is as much as 2,700 m thick in basins. The formation thins at basin margins and across highs (Hartanto and others, 1991; Hutchinson, 1996).

The Gumai Formation is the regional seal for the Batu Raja Limestone in South Sumatra but also contains some reservoir intervals. These carbonates contain 130 MMBOE ultimate recoverable reserves (Petroconsultants, 1996). These reserves average 33—52° API gravity and are found primarily in shoreline and shallow marine sandstones with 20% porosity, however, Hartanto and others (1991) have used well logs to identify turbidites and suggest that these sands could be exploration targets in the basins. Thee turbidites suggest that a rapid drop in sea level occurred at the end of Gumai deposition in middle Miocene time (Hartanto and others, 1991).