--> Overpressuring In The Jambi Merang Area, South Sumatra Basin, Indonesia: Distribution, Characteristics And Implication To Petroleum System

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

Datapages, Inc.Print this page

Overpressuring In The Jambi Merang Area, South Sumatra Basin, Indonesia: Distribution, Characteristics And Implication To Petroleum System

Abstract

The South Sumatra Basin is a back-arc basin located in the southern part of Sumatra Island. Physiographically, the basin is divided into three sub-basin, i.e. the Central Palembang Sub-basin, the Jambi Sub-basin, and the South Palembang Sub-basin based on major depocenters located in each sub-basin. This paper discuss pore pressure regime in Jambi Sub-basin, which is located on Jambi Merang area. The understanding of pore pressure regime in this area is expected to aid drilling optimization in this area, as well as to help analyzing petroleum system. The well and seismic data is used to analyse pore pressure condition in this area. The well data comprises pressure-data measurements, mud weight used during drilling, in collaboration with drilling-events related to the presence of overpressure such as kicks, gas while drilling, and cutting shapes. The seismic data consists of 2D and 3D PSDM velocity data. The well data is used to construct 1D pressure model, while the seismic data is used to analyse the lateral distribution of overpressure in the study area. The result of the analysis shows that overpressure is presence if depth to basement is greater than 5000 ft. If the basement is shallower than 5000 ft, the pressure regime is hydrostatic all the way down to the basement. The top of overpressure is consistently located at the depth 5300–5900 ft below sea level. Geologically, top of overpressure is located within Gumai Formation, which is a massive shale sequence. The seismic data shows that the presence of overpressure is limited only in basin depocenter area. Geochemical data indicates top of overpressure coincides with top of hydrocarbon maturation. Therefore, it is instructive to interpret that hydrocarbon generation contributes to the presence of overpressure. However, disequilibrium compaction may also contribute to overpressure, since the sedimentation rate in the depocenter area is in exceed of 100 m/mY, considered to be sufficient to generate overpressure. The post-well analysis also reveals that several old wells in this area might be drilled in overbalance condition with respect to the magnitude of overpressure. The result of this study can be used to optimize mud weight to be used in the next well to be drilled, as well as to located casing depth points. One of the exploration targets in this area is fractured basement reservoir. Geometrically, the basement could be located either below the overpressure zone or beside the overpressure zone. As discussed above, in terms of petroleum system, the overpressure zone is mature source rock zone. Therefore, hydrocarbon from the overpressured section could be migrated into the fractured-basement reservoir, resulting new exploration play in the area. As summary, overpressure in the Jambi Merang Area is strictly located in basin depocenter area. The cause of overpressure in the area is hydrocarbon generation in combination with disequilibrium compaction mechanism. The analysis shows that some previous wells in this area were drilled in overbalance condition. This study provides a general framework for overpressure estimation to be applied in the next well to be drilled in this area, so that the drilling program could be conducted more safely and efficiently. This study also demonstrates that understanding of overpressure could aid in analyzing petroleum system, which in this case is hydrocarbon migration and trapping.