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The density normal compaction trend is the mandatory parameter in order to calculate contribution of disequilibrium compaction to overpressure, in the presence of unloading to total overpressure. In this paper, mudrock compaction behaviour is studied in the Peciko Field. The density normal compaction trend was constructed in a normally pressured section, where the temperature is <120°C, within the transformation of smectite-illite zone. The compaction trend includes not only vertical effective stress, but also diagenesis, especially smectite-illite transformation, as dependant factor for compaction, both termed as β. This compaction trend is also known as Skempton-Duta compaction model. Quardros-Linares fifth-order kinetic reaction was found that could model smectite-illite transformation in the studied area fairly well. The plot between β and smectite content shows a very good match, confirming that Skempton-Dutta compaction model is valid for the entire range of smectite-illite transformation. In the area where the density log is unavailable, the density normal compaction can be constructed from temperature history acting as a proxy for smectite-illite transformation, with some local calibration. [ABSTRACT FROM AUTHOR]

Large-scale deformation on a cut slope composed of mudstone from the Neogene was observed over time during excavation of the powerhouse area at the Rajamandala Hydroelectric Power Plant in Indonesia. This paper discusses the characteristics of the mudstone and the mechanisms of slope behavior. As a part of detailed research into the mudstone, X-ray Computed Tomography scanning (CT scanning) and X-ray diffraction (XRD) analysis were applied to core samples. The purpose of CT scanning was to visually observe inside the core samples in three dimensions. The CT scanning image, which was a set of numbered CT scans, exhibited a color tone corresponding to the soundness of the core samples and this can be an indicator of weathering grade. The extent of weathering of each core sample was classified quantitatively by the CT scanning image and CT number. The extent of weathering maintains objectivity in the engineering classification of rock mass, which is generally assessed by core sample observation and hammer impacts. XRD analysis is commonly applied to dried samples in order to clarify their mineral components, especially the content of swelling clay minerals such as smectite. This paper discusses, in addition, XRD analysis for field-wet samples, because it is considered that a number of interlayer water molecules in smectite may have changed while preparing dried samples. Smectite, because of the relatively high quantity of interlayer water molecules, could likely cause swelling and slaking. [ABSTRACT FROM AUTHOR]

The Peciko Field contains gas in multiple stacked reservoirs within a Miocene deltaic sequence. In the deeper reservoirs, gas is trapped hydrodynamically by high lateral overpressure gradients. We have analysed overpressure and compaction in this field by using wireline log, pressure, temperature and vitrinite reflectance data. The top of the overpressure is located below 3 km burial depth, below the depth range for transformation of discrete smectite to mixed-layer illite/smectite. Density-sonic and density-resistivity crossplots for mudrocks show reversals within the transition zone into hard overpressure below 3.5 km depth. Vitrinite reflectance measurements indicate that the start of unloading coincides with the onset of gas generation. Moreover, mudrock density continues to increase with depth in the overpressured section to values above 2.6 g cmâ–“3. We conclude that gas generation and chemical compaction are responsible for overpressure generation, contradicting previous interpretations that disequilibrium compaction is the principal mechanism for generating overpressure in the Lower Kutai Basin. The particular circumstances which make our radical interpretation plausible are that it is a warm basin with lateral reservoir drainage, so the overpressured mudrocks are probably overcompacted as a result of diagenesis. [ABSTRACT FROM AUTHOR]

The Pongkor gold–silver mine is situated at the northeastern flank of the Bayah dome, which is a product of volcanism in the Sunda–Banda Arc. The hydrothermal alteration minerals in the Ciurug–Cikoret area are typical of those formed from acid to near-neutral pH thermal waters. On the surface, illite/smectite mixed layer mineral (I/Sm), smectite and kaolinite, and spotting illite, I/Sm and K-feldspar alteration occur at the top of the mineralized zone. Silicification, K-feldspar and I/Sm zones are commonly formed in the wall rock, and gradually grade outwards into a propylitic zone. The mineralization of precious metal ore zone is constrained by fluid temperatures between 180 and 220°C, and with low salinity (<0.2 wt% NaCl equivalent) and boiling condition. The minimum depth of vein formation below the paleo-water table is approximately 90–130 m for the hydrostatic column. Hydrogen and oxygen isotope data for quartz and calcite show relatively homogeneous fluid composition (−53 to −68‰δD and −5.7 to +0.3‰δ18O H2O). There is no specific trend in the data with respect to the mineralization stages and elevation, which suggests that the ore-forming fluids did not significantly change spatially during the vein formation. The stable isotope data indicate mixing between the hydrothermal fluids and meteoric water and interaction between the hydrothermal fluids and the host rock. [ABSTRACT FROM AUTHOR]