Mantle-Derived Fluids in the East Java Sedimentary Basin, Indonesia

Abstract

The Tertiary back‐arc sedimentary basin in East Java (Indonesia) hosts a large variety of piercement structures and hydrocarbon fields. Some of the latter (Wunut, Tanggulangin, Carat, Watudakon) are located a few kilometers away from the Arjuno‐Welirang volcanic complex and neighboring Lusi, the largest active sediment‐hosted hydrothermal system on Earth. In order to investigate interactions between volcanic and sedimentary settings, we performed gas sampling on these four shallow (200‐ to 1,000‐m depth) petroleum fields. The fields around Lusi are dominated by thermogenic gas that was altered during biodegradation processes. The helium isotope ratios (3He/4He) are as high as 6.7 RA, which is remarkably similar to those measured at the fumaroles of the adjacent volcanic complex (R = 7.3 RA) and at the Lusi site (up to 6.5 RA). This highlights the pervasive outgassing of mantle‐derived fluids in the sedimentary basin. Despite these two systems sharing the same mantle‐derived helium source, their hydrocarbons have two different genetic histories: Lusi hydrocarbon gas has been more recently generated and is less molecularly and isotopically fractionated, while the gas trapped in the reservoirs is older and more altered. Unlike Lusi, the hydrocarbon fields contain small amounts of CO2 resulting from biodegradation processes. The Watukosek fault system, originating from the Arjuno‐Welirang volcanic complex and extending toward the northeast of Java, intersects Lusi and the hydrocarbon fields. This network of faults controls the migration of mantle‐derived fluids within the sedimentary basin, feeding the focused venting at the Lusi site and promoting the slower and pervasive migration in the reservoirs.