Overpressure mechanism prediction based on well log and mineralogy analysis from drill cuttings of well NSE-001 in the North Sumatra Basin area, Indonesia

Abstract

The well NSE-001 is an exploration well located in the North Sumatra Basin, Indonesia, which penetrates several formations with overpressure zones that have the potential as a drilling hazard in this area. The results of the pore pressure fracture gradient model show that the response of the wireline log data of well NSE-001 based on the normal compaction trend has two types of overpressure mechanisms, namely loading and unloading. Rapid sedimentation in the development of the Lower Baong Formation and Upper Baong Formation as thick and massive shale sequences in the North Sumatra Basin is the main factor in the generation of an overpressure zone with a loading mechanism. This correlates with the travel time in the sonic log which is relatively constant with response to the normal compaction trend at depth intervals of 620–1500 mMD (from the Middle Baong Sand Formation to the Top Gebang Sand Formation). The presence of sillimanite and kaliophilite minerals in bulk analysis from drill cuttings at depth intervals of 2200–3820 mMD contributed to the generation of hydrocarbon, while clay analysis showed that the mineral content of kaolinite + illite as alteration product (clay diagenesis) was higher than the mineral content of smectite. The results of the mineralogy analysis (bulk and clay) correlated with the response of the wireline log data of well NSE-001 to the normal compaction trend, which showed that at depth interval of 1500 m to 3200 mMD (from the Top Gebang Sand Formation to the Top Upper Belumai Formation) is an overpressure zone with an unloading mechanism, which was characterized by fluid expansion caused by clay diagenesis and possibly hydrocarbon generation.