Overpressure generating mechanisms in the Peciko Field, Lower Kutai Basin, Indonesia

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 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 cm-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

Polygonal fault networks in fine-grained sediments – an alternative to the syneresis mechanism

Polygonal fault networks have previously been identified in fine-grained sediments in several passively subsiding sedimentary basins worldwide. In the absence of tectonic activity, the lack of any preferential strike orientation clearly indicates that the faults developed due to compaction. Since faulting generally does not accompany passive subsidence, it has been proposed that some shrinkage process associated with the colloidal nature of these sediments must have been responsible for horizontal contraction of the bedding. A simpler explanation, consistent with the Mohr-Coulomb theory of shear failure and with laboratory measurements of residual shear strength, is that the coefficient of friction in the fine-grained sediments is exceptionally low

Reservoir stress path during depletion of Norwegian chalk oilfields

Pore pressure drawdown during reservoir depletion results in reduced horizontal principal stresses within a reservoir due to three distinct mechanisms: normal compaction, poroelastic behaviour and normal faulting. Established relationships, based on simplifying assumptions, give the ratio of the change in minimum horizontal stress,Sh, to the change in pore pressure, P, in terms of sediment properties for each mechanism. In spite of the approximations introduced by the assumptions, these relationships may be useful for discriminating between the mechanisms that control the reservoir stress path. For the Norwegian chalk oilfields, it is important to know whether normal faulting, in particular, is the governing mechanism because slip on active faults can shear well casings, and active faulting and fracturing can increase reservoir permeability. Previously reported field observations and laboratory measurements on chalk samples are compared to infer the mechanisms governing the reservoir stress path for the Ekofisk and Valhall fields. The amount of subsidence at the seabed observed at Ekofisk is evidence that the weaker horizons within the reservoirs are yielding plastically through pore collapse. Nevertheless, the reservoir stress path corresponds to that expected for poroelastic behaviour or normal faulting, and not that expected for plastic yielding

Emplacement mechanism of the Great Whin and Midland Valley dolerite sills

The Great Whin and Midland Valley dolerite sill complexes appear to have been emplaced laterally from the walls of feeder dykes. The overall thickness of each sill increases with depth, as would be expected if the magma finally reached a state of hydrostatic equilibrium. Variations in the thickness of the sills with estimated intrusion depth imply that the head of magma was about 100 m below the contemporary ground surface in the areas of the present-day outcrops at the end of the intrusive episodes. Before hydrostatic equilibrium was established, the magma pressure would probably have been somewhat greater, so it is likely that intrusion of the sills was accompanied by the extrusion of flood basalts. Step-and-stair transgressions of the bedding are commonly found within the sills, mostly stepping downwards in the direction of bedding dip. The reason for this directionality is that the weight of sediments floating on an intruding sill has a downdip component that applies a tensile stress to intersecting fractures below the sill when magma is moving downdip, and to intersecting fractures above the sill when magma is moving updip

Geomechanical behaviour of the overburden above compacting hydrocarbon reservoirs--what would we predict from coalmining experience?

Observations of subsidence due to longwall coalmining and a time-lapse seismic profile over a longwall working have several implications for the geomechanical behaviour of the overburden above compacting hydrocarbon reservoirs. Subsidence at the ground surface or seabed is expected to amount to ~90% of the vertical compaction of the reservoir. Stress arching is unlikely to occur to any great extent in the overburden above a compacting reservoir unless the overburden contains thick beds of massive, competent sedimentary rock. The sensitivity of seismic velocity to vertical extensional strains is likely to be high in time-lapse seismic surveys because subsidence causes an irreversible disruption of the rock fabric with the development of fresh cracks and microcracks. Other observations consistent with the inferred behaviour of the Coal Measures during mining subsidence are difficulties experienced with extended reach drilling at Valhall, and the large differences in sonic velocity between the highly overpressured and normally pressured intra-reservoir Jurassic shales of very similar bulk porosity on the Halten Terrace, offshore mid-Norway

Mechanical compaction behaviour of natural clays and implications for pore pressure estimation

In pore pressure estimation and in basin modelling programs it is often assumed that porosity in clay-rich sediments depends on the vertical effective stress. An alternative assumption is that porosity depends on the mean effective stress. Yet triaxial test data on natural clays have shown that porosity depends on both the mean effective stress and the differential stress. Triaxial test data for Winnipeg clay are re-plotted here to quantify the errors in estimated pore pressures that would result if it is assumed that porosity depends on either vertical or mean effective stress. The assumption that porosity depends on vertical effective stress may result in gross underestimates of pore pressures in compressional basins, where horizontal stresses in overpressured zones at depth are greater than the vertical stress. Sections through the yield surface of Winnipeg clay are consistent with a generalized yield locus for clays, normalized for composition as well as volume, based on data from several natural clays. Consequently, a refined equivalent depth method of pore pressure estimation that accounts for porosity dependence on both differential and mean effective stress could, in principle, be implemented. The method would require some knowledge of yield properties and of all three principal stresses in the subsurface

Geomechanics of polygonal fault systems: a review

Layer-bound systems of polygonal faults are found in sequences of very fine-grained sediments that have typically undergone passive subsidence and burial. In the absence of tectonic extension, the heave of the faults must be complemented by horizontal compaction of the sediments. Density inversion, syneresis and low coefficients of friction on fault planes have all been proposed as causal mechanisms for the development of polygonal fault systems, but most sequences which contain polygonal faults are not underlain by sediments of lower density and there is a lack of evidence to support the idea that syneresis is responsible. Laboratory measurements of clay properties and a recent field test based on well data strongly suggest that low coefficients of residual friction in fine-grained sediments are key to the growth of faults that eventually develop into polygonal systems. However, coefficients of residual friction only apply to faults after initial slip has taken place, so some other mechanism must be responsible for the initial nucleation of the faults. Various speculative suggestions have been made, but there is no evidence that nucleation of those faults which evolve into polygonal systems differs fundamentally from the processes involved in fault nucleation in other soft sediments

Mechanics of layer-bound polygonal faulting in fine-grained sediments

Abstract: Extensive polygonal networks of normal faults have reportedly been identified within layer-bound sequences in about 28 sedimentary basins worldwide. The gentle regional dips, passive tectonic settings and geometry of the fault networks have led to the conclusion that faulting must have resulted from gravity-driven mechanical compaction. The faulted sequences comprise very fine-grained sediments, with lithofacies that range from smectitic claystones to almost pure chalks. In most, if not all, cases it is clear that volumetric contraction has occurred with horizontal contraction of the sediments complementing the heave of the faults. One explanation which has previously been offered is that the fine-grained sediments have shrunk due to syneresis, a process that involves spontaneous contraction of the solid network with expulsion of the pore fluid. However, syneresis is an implausible mechanism because it does not explain the observed lithological variation in the sediments concerned, why the initiation of faulting occurs in the depth range 100–1000 m, and why faulting continues for millions of years. A much simpler explanation is that shear failure inevitably results from one-dimensional compaction if the coefficient of friction is sufficiently low; and there is some evidence from laboratory measurements that the coefficient of friction is likely to be exceptionally low in these fine-grained sediments. Qualitatively, low coefficients of friction also explain why these compaction faults preferentially dip towards the basin margin where the regional dip of the bedding is greater than 1. Furthermore, they help to explain the origin of a polygonal fault system in the Eromanga Basin, South Australia, where the situation is complicated by the presence of a low velocity, ductile layer at the base of the faulted sequence