Abstract

Pagerungan Field, in the East Java Sea Basin, north of Bali, produces gas and condensate from the Ngimbang Clastics formation. This sequence of alternating sandstones, shales, and coals, of middle to late Eocene age, occurs at a depth of about 6000 feet (1830 m.) and is about 300 ft. (90 m.) thick. In the Pagerungan-5 well, 220 feet (67 m.) of continuous, oriented core through the reservoir interval were recovered. This core and other formation evaluation information make this well suitable to illustrate most aspects of the Ngimbang Clastics formation for the field and the area of the Kangean Block. The Ngimbang Clastics should be elevated to formation status, and its four subdivisions recognized as members: 1) Upper Coal/Shale member; 2) Upper Sandstone member; 3) Lower Coal/Shale member; and 4) Lower Sandstone member.

These stratigraphic units were deposited during an overall transgression. They lie unconformably on the Pre-Ngimbang Formation, and are overlain by the Ngimbang Carbonate formation. The terrigenous Ngimbang Clastic sequence was deposited in fluvial environments, ranging from braided channels, upward through freshwater swamps to coastal floodplains with meandering streams and brackish swamps.

The contrasting origins of the Upper and Lower Sandstone members resulted in gas reservoirs with contrasting geometries and reservoir properties. In most wells, as in Pagerungan-5, sands of the Upper member are thinner, more discontinuous, and mostly finer grained than those of the Lower member, which are thicker, vertically and laterally more continuous, and much coarser in texture. Interparticle porosity is greater in the Lower member, and permeability is much higher in these coarse, pebbly sandstones. Interstitial clay and carbonaceous fragments in the finer grained, thin bedded sandstones of the Upper member reduce porosity and permeability. In some other wells, where thicker channel sandstones are present in the Upper member, porosity and permeability of these coarser, more well sorted units are comparable to those of the Lower member. Capillarity is also more favorable in the lower unit.

Profile permeametry and image analysis of photographs of the cores has been used to investigate the causes of heterogeneity in reservoir properties. Diagenesis has not greatly altered original patterns of porosity distribution, except where compaction of shale lithoclasts and lithic grains has reduced porosity. Consequently, natural textural variability of the sandstones and their sedimentary structures, such as cross bedding and lamination of sand and shale, are the main controls on variations in porosity and permeability, directional permeability, and thickness of pay sandstones.