--> Overpressure Distribution And Its Pattern On Onshore Part, East Java Basin

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

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Overpressure Distribution And Its Pattern On Onshore Part, East Java Basin

Abstract

The East Java Basin covers onshore and offshore area of the northwestern most of Java Island. This basin is an overpressured basin, proven by the presence of series of mud volcanoes, as well as several pressure-related drilling problems such as kicks, blow-out, and high gas content. However, to date, there is no regional synthesis available related to overpressuring in basinal scale in this basin. In this study, we are trying to make such regional synthesis aiming at understanding its distribution and characteristics on the onshore part of the basin. Our study shows that in the area experiencing severe erosion, the pressure regime is hydrostatic down to the basement. Overpressure is present in the area where the erosion is less and sedimentation rate has been relatively high. Our analysis reveals that overpressure pattern and generating mechanism on the onshore part of the East Java Basin can be well explained by geological condition in this area. In this study, we use pressure data measurement, drilling report related to the presence of overpressure (e.g kicks, gas-cut mud, drilling break, and connection gas), and wireline log data to infer pore pressure condition. We found that top of overpressure in the study area varies within the range of 630 – 1600 m below sea level. With respect to shale overpressure pattern, overpressured wells in the study area could be divided into two general patterns, i.e. lithostatic-parallel shale pressure profile (Shale Pressure Pattern-1/ SPP-1) and less than lithostatic-parallel shale pressure profile (Shale Pattern-2/ SPP-2). The SPP-1 is distributed almost everywhere in the region, while the SPP-2 is only limited in the southwestern part of the area. The lithologic columns of SPP-1 wells indicate that the wells are dominated with shale with relatively thin sandstone and limestone interbeds. The sandstone and limestone interbeds may facilitate pathway for shale pressure bleed-off providing that the sandstone and the limestone are widely distributed and cropped-out. However, since the sandstone and the limestone are quite thin, the shale pressure bleed-off is quite limited, thus resulting in lithostatic-parallel shale pressure profile. In contrary with SPP-1, in SPP-2, although the lithology is still dominated by shale, but sandstone and limestone are relatively thicker. The shale pressure bleed-off is thought to be greater compared to SPP-1 wells. Therefore, the shale pressure in SPP-2 is lower than the SPP-1 wells. The comprehensive analysis of overpressure generating mechanisms shows that the main cause of overpressure in the study area, until depth of investigation (TD of the wells) are disequilibrium compaction caused by rapid sedimentation. Clay diagenesis may start to contribute to overpressure at the depth where the smectite start to convert to illite, as indicated from XRD data. The depth range of the conversion in the study area is 1270 - 2120 m. In the deeper section, hydrocarbon generation may also contribute to overpressure magnitude. The result of XRD analysis also reveals that we have two compaction lines in the study area, i.e. mechanical compaction/smectite compaction trend (MC), and chemically-enhanced mechanical compaction/illite compaction trend (CEMC). As mentioned above, the depth range of CEMC is 1250 - 1750 m. The temperature ranges where the conversion takes place is 72.6 – 94.570C.