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.

3D Deep Electrical Resistivity Tomography of the Lusi Eruption Site in East Java

“…Unfortunately, only less than 50 m of log (i.e., between 1,200‐1,300 feet depth) is available for comparison with our data. In addition, as broadly documented, the survey area was exposed to extensive fracturing, subsidence, and fluids circulation through systems of fractures and seepage sites that mutated their position during more than 15 years of vigorous eruption (Lupi et al., 2018; Mauri, Husein, Mazzini, Irawan, et al., 2018; Mazzini et al., 2009, 2012, 2021; Moscariello et al., 2018; Obermann et al., 2018; Osorio Riffo et al., 2021; Panzera et al., 2018; Sciarra et al., 2018; Zaputlyaeva et al., 2019). Therefore it would be impossible to compare the properties of this totally new environment with potential resistivity logs collected in pristine sedimentary deposits.…”

“…(2018) to speculate that the Watukosek may actually be a regional‐scale feature. Numerous scientific studies from different disciplines have shown that this eruption is connected and fed at depth by the neighboring volcanic complex (Fallahi et al., 2017; Inguaggiato et al., 2018; Zaputlyaeva et al., 2019). The migration of magmatic and hydrothermal fluids into the deep organic‐rich source rock deposits of the basin, triggered thermometamorphic reactions and fluids overpressure at depth (Malvoisin et al., 2018; Mazzini et al., 2012; Svensen et al., 2018; Zaputlyaeva et al., 2020).…”

“…This part of the island is also characterized by numerous diapirs and seismic profiles show that some of these have undergone eruption and subsidence (Istadi et al., 2009; Miller & Mazzini, 2018; Moscariello et al., 2018). Several geochemical (Inguaggiato et al., 2018; Mazzini et al., 2012, 2018; Sciarra et al., 2018; Zaputlyaeva et al., 2019), geological (Mazzini et al., 2021; Moscariello et al., 2018; Samankassou et al., 2018; Zaputlyaeva et al., 2020), numerical (Collignon, Schmid, et al., 2018; Lupi et al., 2013, 2014; Svensen et al., 2018) and geophysical (Karyono et al., 2017; Lupi et al., 2018; Mauri, Husein, Mazzini, Irawan, et al., 2018, Mauri, Husein, Mazzini, Karyono, et al., 2018; Panzera et al., 2018; Obermann et al., 2018; Osorio Riffo et al., 2021) studies contributed to the better understanding of the subsurface structure of Lusi. However, to date a clear picture of the Lusi shallow plumbing system is still missing.…”