THE SHUTDOWN OF DIAGENESIS IN THE EARLY PLIOCENE GLOBIGERINID LIMESTONES OF THE MUNDU FORMATION AND IMPLICATIONS FOR POROSITY AND PERMEABILITY LEVELS (EAST JAVA BASIN, INDONESIA)

Abstract

Globigerinid limestones, in the Madura Strait of Indonesia, as producing reservoirs are unique. This reservoir sediment was deposited and winnowed in a deepwater complex with substantial bottom current reworking. Although there is now more than 800 meters of overburden sediment and a strong tectonic overprint, as seen in seismic, this reservoir didn’t experience significant diagenetic alteration. Early hydrocarbon emplacement ended the diagenetic process and preserved pristine intraparticle porosity with interconnection across foraminiferal punctae, so allowing connection between intrafossil and extrafossil porosity. The best reservoir quality occurs in globigerinid sands on Early Pliocene paleohighs where winnowing processes (bottom-current reworking) were stronger. Winnowing driven by bottom current reworking can result >40% porosity and excellent permeability. Partial destruction of reservoir quality is caused by micritization and rupture/collapse of foraminifera shells. There is strong evidence that the Pliocene collision of Southeast Asia and Australia plates drove changes in oceanic circulation and bottom current intensities in the Madura Straits area, via restriction then closure of the Indonesian Seaway. In the Pliocene deep water setting that was the Madura Strait, the restriction (prior to complete closure) drove more intense westward bottom currents and this was likely the driving force creating enhanced contourite currents that winnowed portions of the Mundu Fm. Once closure occured, bottom currents in the vicinity of the Madura Island trend weakened and 800 m of muddy seal sediments were deposited from suspension to form a regional seal. The improved understanding of the controls on this unique reservoir will allow improvements in future field development and an improved ability to predict the positions of possible future drilling targets in the Madura Straits and further a field.