Abstract

Highly porous and permeable deep marine limestones composed of sand-sized planktonic forams have been encountered in many surface outcrops and exploration wells within Indonesia. The deposits have received a variety of names, but they are best described as planktonic foram packstones and grainstones. Field and lab studies conducted throughout 1993 in Eastern Java, and on cores from the MDA-1 and MDA-3 wells, offshore Madura Strait, illustrate both the high reservoir quality of these deep marine limestones and the overlooked hydrocarbon potential of foram sand reservoirs within Indonesia and elsewhere.

Reservoir quality and thickness are variable, however, the most porous intervals average from 30–45% porosity, 100–1000 md permeability and 30–40 meters thick (some up to 60m thick). Nearly all porosity is primary rather than secondary. The major porosity types are interparticle porosity, found in between forams, and intraparticle porosity, contained within foram chambers. Most of the foram intraparticle porosity is capable of producing hydrocarbons because of pore interconnections provided by the foram "aperture" and "punctae" within the skeletal wall. Globigerinid sand facies can produce oil, although they are probably more favorable gas and condensate reservoirs.

The globigerinid sand deposits of this study are Pliocene in age, however, they were not all deposited by the same processes. At least two distinct types of globigerinid sand deposits are documented: planktonic foram sand "drifts" deposited by bottom currents and less pervasive planktonic foram "turbidites" deposited as submarine channel-fills and fans.

The bottom current-deposited "foram drift" facies are far more common, possess the best reservoir characteristics and probably account for the majority of past hydrocarbon production and discoveries within globigerinid sands. The foram drift deposits within the East Java - Madura Strait area are all of near identical age, restricted to mostly the latest Early Pliocene. Development of the facies is believed related to a tectonic event, which partly coincides with the 3.8 Ma global sea level lowstand. Similar globigerinid-rich facies are also associated with the Late Pliocene Selorejo Formation of onshore Central and East Java. Deposition of Early Pliocene drift facies was widespread, extending from at least easternmost-Central Java to the Bali Sea, while the Late Pliocene examples appear restricted to the Rembang Zone of onshore Northeast-Northcentral Java.

Other Indonesian examples of planktonic foram-rich facies indicate the development of the facies is not restricted to only Pliocene horizons within the East Java - Madura Straits area.