Publications

The North Belut Field, located in the offshore Indonesian West Natuna Basin, contains significant Tertiary clastic reservoirs of the Arang (Miocene), Gabus and Udang Formations (late Oligocene - early Miocene). These reservoirs contain a complex stratigraphic history and an equally important diagenetic overprint which has created a complex permeability envelope. Structurally the North Belut Field represents a unique form of inversion in the Natuna Basin. Instead of the normal inverted basin, the North Belut Structure was originally a paleostructure which was flooded, filled and then inverted. The paleo-structure is interpreted to play a critical role in the history of the diagenesis, hydrocarbon migration, and hydrocarbon fill of the structure. Deeper gabus sections onlap to the basement and in general The North Belut Field was part of a transitional succession that evolved from isolated rift basins to widespread shallow marine conditions (since Eocene time). The reservoir section was deposited as fluvial, deltaic, and estuarine environments. The clastic systems of the Udang and Gabus formations are dominated near shore deposits in fluvial deltaic to distal deltaic processes. A number of flooding surfaces readily correlate throughout the field. These demonstrate a high sequence frequency in an overall regressive system. These units are punctuated by sub-regional flooding surfaces. Clastic units of 1 to 25 m thickness are common in the 1200’ sedimentary column. Clastic units of the Gabus thicken off structure to the north along the back of the paleo-structure. The North Belut Field contains a complex diagenesis history which appears to have created a permeability window along the crest of the structure. The history is difficult to decipher due to the complexities of bioturbated and diagenetically clay filled sands. Clean sands, however, are the key to understanding the diagenetic history of the field. These sands represent a wide variety of diagenetic conditions in the field from high quality sands to extremely tight sands. Study of diagenesis in cores from the North Belut Field shows expected features of quartz and feldspar dissolution and precipitation, clay mineral precipitation, and carbonate dissolution and precipitation. Quartz overgrowths dominate the secondary cement within the clastics. Early diagenesis included clay coating of grains followed by siderite precipitation. Downdip sands are dominated by tight sands with quartz overgrowths, clay, and siderite choked pores. Updip sands along the crest of the field contain medium to excellent quality reservoir sands characterized by leached rock fragments. The best reservoir facies have a high percentage of leached rock fragments often resulting in a honeycomb textured clastic comprised of fused quartz grains and clay rims. Sidertie cement is rarely seen in the better reservoirs. The working hypothesis at North Belut is a crestal porosity and permeability enhanced envelop created by fluid flushing and leaching reservoir sands along the crest of the structure. The conduits for the fluids are interpreted to be the main inversion fault and associated faults in the crestal area