Abstract

There are important hydrocarbon reserves in East Java in thick sedimentary sequences deposited to the north of the currently active and an older volcanic arc. The 'East Java basins' began to develop in the Eocene but they are not typical extensional or subductionrelated basins. Subduction has been continuous to the south of Java since at least the early Tertiary, but surprisingly arc activity has not. A volcanic arc was active in southern Java from the Middle Eocene, much earlier than previously suggested, until the Early Miocene. There was a lull in the Middle Miocene and arc activity resumed and shifted about 50 km northwards in the late Miocene. There are significant volumes of quartz in Eocene and Miocene sands and conglomerates that are important hydrocarbon reservoirs. Previous studies have suggested that the quartz was derived primarily from Sundaland and/or from the basement. However, there were major palaeogeographical barriers between Sundaland sources and there is no significant quartz in basement rocks. New field observations suggest that most of the quartz has a volcanic origin and sequences described as primarily sedimentary contain much volcanic material. The volcanic contribution has previously been underestimated, partly because of the type of material. Volcanic products such as the 'Old Andesites' are easy to recognise but there are also large volumes of ash reworked in marine environments, which are the products of dacitic Plinian-type explosive eruptions. We suggest that most of the Eocene to Miocene sands and muds onland in East Java have a high volcanogenic content.

Factors traditionally used to assess the maturity of sediment such as grain size distribution and grain morphology may not be applicable.

There are a variety of grain shapes, and sorting is a reflection not of maturity but volcanic processes. These new findings have important implications for provenance, reservoir properties and basin history in East Java.