Allochthonous salt sheets advance in four ways: (1) extrusive advance, (2) open-toed advance, (3) thrust advance, and (4) salt-wing intrusion. These mechanisms are determined primarily by the geometry and thickness of the roof that overlies the advancing sheet. An extrusive sheet spreads without a roof or with a roof of negligible mechanical strength. An open-toed sheet is partially covered by a mechanically significant roof but has an extrusive toe. An overthrusting sheet advances along a thrust fault at its leading edge, carrying its roof with it. A salt wing intrudes from the flank of a diapir into a shallower salt layer.Extrusive, open-toed, and overthrusting salt sheets are found in both passive margins and orogens. A sheet typically evolves through two or more of these mechanisms. Three lineages, or evolutionary paths, are common. Plug-fed extrusions emanate from the top of a salt dome or salt wall. Plug-fed thrusts form the base of the hanging wall of thrust faults that are rooted in the tops of salt domes or salt walls. Finally, source-fed thrusts initiate as thrust faults rooted in the autochthonous salt layer. Source-fed thrusts form the largest individual salt sheets, some covering thousands of square kilometers. Salt-wing intrusions form only under special circumstances, so they are not part of the three major lineages. These intrusions are restricted to compressionally inverted basins containing multiple salt layers and are known only in the Zechstein salt basin.

Advance of allochthonous salt sheets in passive margins and orogens

Allochthonous salt structures and associated primary and secondary minibasins are exposed in Neoproterozoic
strata of the eastern Willouran Ranges, South Australia. Detailed geologic mapping using high-quality airborne hyperspectral remote-sensing data and satellite imagery, combined with a qualitative structural restoration, are used to elucidate the evolution of this complex, long-lived (>250 Myr) salt system. Field observations and interpretations at a resolution unobtainable from seismic or well data provide a means to test published models of allochthonous salt emplacement and associated salt-sediment interaction derived from subsurface data in the northern Gulf of Mexico. Salt diapirs and sheets are represented by megabreccias of nonevaporite lithologies that were originally interbedded with evaporites that have been dissolved and/or altered. Passive diapirism began shortly after deposition of the Callanna Group layered evaporite sequence. A primary basin containing an expulsion-rollover structure and megaflap is flanked by two vertical diapirs. Salt flowed laterally from the diapirs to form a complex, multi-level canopy, now partly welded, containing an encapsulated minibasin and capped by suprasalt basins. Salt and minibasin geometries were modified during the Late Cambrian–Ordovician Delamerian Orogeny (ca. 500 Ma). Small-scale structures such as subsalt shear zones, fractured or mixed ‘rubble zones’ and thrust imbricates are absent
beneath allochthonous salt and welds in the eastern Willouran Ranges. Instead, either undeformed strata or halokinetic drape folds that include preserved diapir roof strata are found directly below the transition from steep diapirs to salt sheets. Allochthonous salt first broke through the diapir roofs and then flowed laterally, resulting in variable preservation of the subsalt drape folds. Lateral salt emplacement was presumably on roof-edge thrusts or, because of the shallow depositional environment, via open-toed advance or extrusive advance, but without associated subsalt deformation.

Geology and tectonics of Neoproterozoic salt diapirs and salt sheets in the eastern Willouran Ranges, South Australia

Pore pressure prediction using geophysical methods in carbonate reservoirs: Current status, challenges and way ahead

Parts of two third-order Neoproterozoic (Marinoan) depositional sequences are documented in the Wilpena Group (Wonoka Formation and Bonney Sandstone) at Patawarta diapir, located in the central Flinders Ranges, South Australia. These sequences represent an overall regressive succession transitioning upwards from outer to middle wave-dominated shelf deposits to a tidally dominated barrier bar to coastal plain. The lower, middle, upper limestone and green mudstone informal members of the Wonoka Formation comprise the Highstand Systems Tract of the lower sequence. The Sequence Boundary is at the top of the Wonoka green mudstone member and is overlain by the Lowstand Systems Tract of the upper sequence, which includes the lower dolomite, sandstone and upper dolomite beds of the Patsy Hill Member of the Bonney Sandstone. The upper sequence Transgressive Systems Tract comprises theBonneySandstone.Theseunitscompriseonecompletetaperedcompositehalokineticsequence (CHS). The lower halokinetic-sequence boundary is associated with the Maximum Flooding Surface of the lower depositional sequence and the upper halokinetic-sequence boundary is interpreted as the Transgressive Surface of the overlying depositional sequence where an angular truncation of up to 908is documented. The majority of our knowledge of salt tectonic processes is derived from the analysis of subsurface datasetsfromoffshorepetroleum-bearingsaltbasins such as the Gulf of Mexico, Brazil, West Africa and

Depositional and halokinetic-sequence stratigraphy of the Neoproterozoic Wonoka Formation adjacent to Patawarta allochthonous salt sheet, Central Flinders Ranges, South Australia

As seismic data quality improves, time-lapse seismic data is increasingly being called upon to interpret and predict changes during reservoir development and production. Since pressure change is a major component of reservoir change during production, a thorough understanding of the influence of pore pressure on seismic velocity is critical. Laboratory measurements show that differential pressure (overburden minus fluid pressure) does not adequately determine the actual reservoir conditions. Changes in fluid pressure are found to have an additional effect on the physical properties of rocks. The effective-stress coefficient n is used to quantify the effect of pore pressure compared to confining pressure on rock properties. However, the current practice in time-lapse feasibility studies, reservoir-pressure inversion and pore-pressure prediction is to assume that n = 1. Laboratory measurements, reported in both this and previous research show that n can be significantly less than unity for low-porosity rocks and that it varies with porosity, rock texture and wave type. We report the results of ultrasonic experiments to estimate n for low-porosity sandstones with and without microcracks. Our results show that, for P-waves, n is as low as 0.4 at a differential pressure of 20 MPa (about 3000 psi) for a low-porosity sandstone. Thus, in pore-pressure inversion, an assumption of n = 1 would lead to a 150% underestimation of the pore pressure. Comparison of the effective-stress coefficient for fractured and unfractured samples suggests that the presence of microfractures increases the sensitivity of P-wave velocity to pore pressure, and therefore the effective-stress coefficient. Our results show that the effective-stress coefficient decreases with the differential pressure, with a higher differential pressure resulting in a lower effective-stress coefficient. While the effective-stress coefficient for P-wave velocity can be significantly less than unity, it is close to one for S-waves.

/pdf/influence-of-pore-pressure-on-velocity-in-low-porosity-311xmvqtxk.pdf

Influence of pore pressure on velocity in low-porosity sandstone: Implications for time-lapse feasibility and pore-pressure study

Pore Pressure Prediction From S-wave, C-wave, And P-wave Velocities

Base of Salt Structure and Stratigraphy—Data and Models from Pompano Field, VK 989/990, Gulf of Mexico

Mapping overpressure prior to drilling is critical for safe and successful hydrocarbon exploration (Caricone and Helle, 2002). Lack of predictability of abnormal pressures results in cautious drilling, and this leads to slow penetration rates, excessive bit wear and increased well cost and risk while drilling (Mukerji et al., 2002). The excess drilling cost in regions with abnormal pressure can amount to over $20 million per well (Mukerji et al., 2002) or $1.08 billion per year worldwide (Dutta et al. 2002).

Pore pressure prediction using an Eaton's approach for PS‐waves

Summary VSP techniques have long held out the promise of ahead-of-the-bit pressure predictions, but a variety of data acquisition and interpretation issues have limited this potentially important application. In particular, P-wave velocities ahead of the bit, if determined from inversion of upcoming reflected amplitudes, contain the assumption that the up-hole velocity trend can be simply extrapolated downward. This assumption is inappropriate in a case where a deeper pressure seal causes a reversal of the trend. However, a simple interpretation technique (Thomsen, 1992), requiring only the picking and registration of upward traveling (i.e., reflected) P-waves and mode-converted S-waves allows straightforward determination of Vp/Vs ratios ahead of the bit using only zero-offset VSP data. The Vp/Vs ratios are derived without explicit knowledge of either the interval P-wave velocities or the interval S-wave velocities. Vp/Vs ratios are lower in the presence of higher effective stress (lower pore pressure) (Mueller et al., 1991). Vp/Vs ratios can be interpreted for pore pressure in several ways, including comparison against local Vp/Vs compaction trends (as measured by dipole sonics). Vp/Vs ratios appear to be more sensitive than P-wave interval velocities to effective stress variations, especially at low effective stress (hard over-pressure) Eberhart-Phillips et al. (1989).

/pdf/vsp-derived-vp-vs-ratios-for-pressure-prediction-ahead-of-5b0dmf2to1.pdf

VSP-Derived Vp/Vs Ratios for Pressure Prediction ahead of the Bit

Effective Stress And Minimum Velocity Trends

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