Fluid transport properties and estimation of overpressure at the Lusi mud volcano, East Java Basin

Generation and maintenance of overpressure can prevent sediments from compaction and weaken sedimentary rocks in deep basins. Excess fluid pressure is one of the key factors to explain the disastrous mud eruption that took place in Sidoarjo, East Java, on 29 May 2006, though the mechanism by which it developed is not well known. We measured permeability and specific storage at a confining pressure of 100 MPa in outcrop samples from the East Java Basin. Both permeability and specific storage in our samples showed large stratigraphic variations. The mudstone of the Upper Kalibeng Formation that is thought to be the source of mud at Lusi had the lowest permeability of our samples at around 10− 19–10− 20 m2, and the permeability of the Upper Kujung Formation limestone was 10− 16 m2, which is two orders of magnitude larger than that of the Lower Kujung Formation limestone. In addition, the permeability and porosity of cemented sedimentary rocks showed low sensitivity to effective pressure. From numerical basin analysis of the Lusi site together with laboratory data, we evaluated the evolution of pore pressure and porosity histories and their present distributions. Our results show that high overpressure was generated below the mudstone of the Upper Kalibeng Formation and almost reached lithostatic levels. The modeled fluid pressure variation is consistent with the observed data. The long-lived overpressure at depth is mainly caused by the existence of thick low-permeability sediments and a high sedimentation rate. Undercompaction of the Upper Kalibeng Formation because of overpressurization may have caused the mud to lose strength and cause liquefaction (and hydrofracturing) as a result of small stress fluctuations induced by the Yogyakarta earthquake, which may have ended up causing the mud eruption.