Abstract

Volcaniclastics are generally regarded as very poor reservoirs by most explorationists, however, recent studies suggest that the reservoir potential of these rocks within Indonesia and elsewhere may be underrated.

The paper describes the reservoir quality of selected Plio-Pleistocene Indonesian volcaniclastic sandstones and compares these with examples of volcaniclastic reservoirs and oil/gas fields productive from volcaniclastic horizons worldwide.

The studied rocks are from the Porong-1 and WD-8 wells in East Java as well as various East Java outcrops and were deposited in a wide variety of depositional settings from non-marine (lacustrine, fluvial) to transitional marine (deltaic) and deep marine (turbidite). The deposits consist of mainly sand and gravel sized volcanic sediments that have been extensively reworked by marine or fluviatile sedimentary processes. The sandstones are composed of predominantly plagioclase feldspar and andesitic rock fragments with subordinate heavy minerals and bioclasts. Quartz is almost completely absent and clays are dominated by sodium smectite.

The volcaniclastics have good to excellent reservoir properties, with total porosity greater than 30% at shallow depths, decreasing to approximately 20% at a depth of 7,500 feet. Permeability ranges between 3–550md in the Porong-1 well, averaging 100md at shallow depths and decreasing to 20md at 5,000 feet. Outcrop samples have generally higher permeability, in some cases up to 20,000md.

The main macroporosity types are primary intergranular porosity and secondary porosity created by leaching of feldspars, volcanic rock fragments and heavy minerals. Porosity shows a distinct change with depth and age. At depths shallower than 4,500 feet within Pleistocene horizons, intergranular porosity is greater than secondary. Deeper than 4,500 feet within Late Pliocene horizons, secondary porosity is greater than primary intergranular porosity. Brine production of up to 20,000 barrels of water per day from volcaniclastics in the WD-8 area illustrates the potential of these horizons as high quality reservoirs.