No abstract

Using wireline logs to estimate pore pressure in mudstones at the chemical compaction stage is not straightforward because clay diagenesis proceeds independently of effective stress, and neither density nor velocity is uniquely related to the maximum effective stress experienced by the mudstones. We propose the Budge–Fudge method, in which we assume there is a unique trend on the sonic–density cross-plot for mudstones at the chemical compaction stage that have not been unloaded. In addition to the sonic–density chemical compaction trend, an initial guestimate of maximum effective stress previously experienced by the mudstones is required. Additional overpressure from unloading processes is then estimated from the sonic log, referenced to the density response. The initial guestimate of maximum effective stress may be adjusted to fit any available measured pressures or pressures estimated from geological knowledge. We have applied the Budge–Fudge method to Cretaceous mudstones at Haltenbanken, and find that estimated pressures match measured pressures and expected pressure–depth profiles. Furthermore, the analysis suggests that the lateral variations in mudstone porosity, previously reported, result from lateral variations in overpressure build-up immediately following rapid burial by glaciogenic sediments; subsequently, overpressures have increased through clay diagenesis and equilibrated laterally across the area.

Knowledge of compaction behaviour underpins basin modelling and pore-pressure estimation for drilling wells. Mudstones of the Cretaceous Lange and Kvitnos formations at Haltenbanken are diagenetically mature and overpressured with a pressure-depth profile that shows little lateral variation. From density logs, we made the unexpected discovery that porosities vary by a factor of two at depths of around 2700 m below seafloor, with greater porosities in the west, so we investigated possible causes for the variation. Exhumation cannot be the cause because the Cretaceous mudstones are presently at their maximum burial depths across most of the area. Nor are lateral variations in geothermal gradient high enough for diagenesis to be responsible for the lateral porosity differences. X-ray diffraction and grain-size analyses were conducted on cuttings but no significant lithological variations were found. We infer that the lateral differences in compaction trends developed because porewater escape was more inhibited in the west during recent rapid burial by glaciogenic sediments. Associated lateral variations in overpressure may subsequently have decayed. The novel finding in this study is that diagenetically mature mudstones at Haltenbanken display large lateral variations in porosity that cannot be attributed to lateral differences in overpressure, exhumation, temperature or lithology. Received

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACT:The Peciko Field contains gas in multiple stacked reservoirs within a Miocene 8 deltaic sequence. In the deeper reservoirs, gas is trapped hydrodynamically by high lateral 9 overpressure gradients. We have analysed overpressure and compaction in this field by using 10 wireline log, pressure, temperature, and vitrinite reflectance data. The top of overpressure is 11 located below 3 km burial depth, below the depth range for transformation of discrete 12 smectite to mixed-layer illite/smectite. Density-sonic and density-resistivity crossplots for 13 mudrocks show reversals within the transition zone into hard overpressure below 3.5 km 14 depth. Vitrinite reflectance measurements indicate that the start of unloading coincides with 15 the onset of gas generation. Moreover, mudrock density continues to increase with depth in 16 the overpressured section to values above 2.6 g cm -3 . We conclude that gas generation and 17 chemical compaction are responsible for overpressure generation, contradicting previous 18 interpretations that disequilibrium compaction is the principal mechanism for generating 19 overpressure in the Lower Kutai Basin. The particular circumstances which make our radical 20 interpretation plausible are that it is a warm basin with lateral reservoir drainage, so the 21 overpressured mudrocks are probably overcompacted as a result of diagenesis. 22 23

In sedimentary basins, compaction disequilibrium generates overpressure during rapid burial of fine-grained sediments in the mechanical compaction regime, at temperatures below ~70°C. Mudstones behave differently at greater depths in the chemical compaction regime, at temperatures above ~100°C, where evidence suggests that porosity reduction with increasing depth and temperature continues independently of effective stress up to high values of overpressure. We offer an explanation for this behaviour. The horizontal alignment of clay mineral grains is enhanced during clay diagenesis, creating sub-horizontal, flat pores. Because of their flexibility, the flat pores tend to close even under low values of normal effective stress acting across them. Thus, chemical compaction can proceed unless the net expulsion of pore water from the mudstones is inhibited sufficiently for the flat pores to be held open, which necessarily requires the pore pressure to approach the lithostatic stress. In the Lower Kutai Basin, density log reversals are encountered in mudstones in the chemical compaction regime at depths of 3–4 km, where the pore pressure is close to the lithostatic stress. We attribute these reversals to the inhibition of dewatering during clay diagenesis at shallower depths, when the pore pressure was already close to lithostatic stress. Porosity was preserved by the very high pore pressure holding the flat pores open while the mudstone matrix was being cemented by the products of clay diagenesis. We coin the term ‘chemical undercompaction’ for this process.