Occurrence in space and time of the Globigerina-sands of eastern Java; their stratigraphy, and controls on reservoir quality

Abstract This review examines approximately 25 million years of deep water sedimentation in eastern Java, covering two distinct periods of deposition of plankton-micritic carbonates, rich in Globigerina planktonic foraminifera. Closer examination of seismic and well data reveals several important differences in the depositional setting of each “Globigerina-sand” lithofacies, linked to its stratigraphic history. These differences impact the quality of the lithofacies as a gas reservoir. The two periods are around the Oligo-Miocene boundary, a unit previously called the Prupuh Formation but shown here to have two very distinct parts, and also in the later Early Pliocene, a lithofacies called the Mundu Formation. Both these units have tested gas at commercial rates. The importance of analyses and sample observation is stressed, as many important characteristics are not visible on seismic data. Deep marine, tropical and Cenozoic sediments offer good age control from biostratigraphy. Rates of sedimentation are linked to lithofacies and are examined through age-depth plots and geohistory diagrams. In addition there are important lithological features such as the small amounts of allochthonous grains including lithoclasts, in what should be a clastic-starved deposit, as well as subtle seismic bedding styles, and evidence for winnowing. It can be shown that globigerine-rich lithofacies are not the most condensed stratigraphic units, being only moderately condensed and diluted by micrite derived from adjacent carbonate platforms. There is at least one period with neither micrite nor siliciclastic input to deep marine locations, and a prolonged hiatus in sedimentation, which is a geographically restricted phase of sediment starvation of importance in understanding the regional geological history. This, and a review of the timing and geographic distribution of the two Globigerina lithofacies, improves our understanding of a complex tectono-stratigraphic history.