Page 185 - Fundamentals of Gas Shale Reservoirs

nOmEnclATuRE 165
were also no changes in the other drilling parameters such as faults and the main transfer faults providing efficient
WOB, bit and surface rotations (RPm), mud flow rate, and lateral seals for the overpressure developed in the
mud pump pressure. Kockatea Shale, and (iii) the observed high magnitude
of overpressure, suggesting that lateral tectonics com-
pression has contributed significantly to overpressure
7.6 CONCLUSIONS development as the diagenesis effects cannot produce
such high overpressure.
• normal pore pressure profiles were observed in the • It is difficult to ascertain whether either the lateral tec-
Kockatea Shale in areas that have been rigorously tonic compression or the clay transformation is the
uplifted and where there was severe uplifting and major overpressure‐generating mechanism in the lower
erosion. uplifting and erosion due to tectonics compres- section of Kockatea Shale. however, both mechanisms
sion removed significant portions of the Kockatea Shale contributed to a different extent to overpressure
in some areas and induced fractures in some other areas. development.
The removal of significant parts of the formation and the • The severe tectonic activity accompanying the final
induced fractures have acted as communication channels breakup of the continents and seafloor spreading are
and facilitated a re‐equilibration of the pore pressure responsible for most of the major structural features of
back to the normal state or condition. the Perth Basin and responsible also for the distribution
• Overpressures were observed in the Kockatea mainly in of pore pressure in the basin.
areas where there was less intensity in tectonic activity
and where Kockatea Shales intersected at a greater
depth. Regions with less intensity of tectonic activity NOMENCLATURE
showed an increase in pressure gradients away from the
center of uplift. The depth to the top of the overpressure
zone is linearly related to the depth to the top of the Abbreviation Log name (unit)
Kockatea Shale. dTc compressional wave transit time log (µs/ft)
• The occurrence of overpressures in Kockatea Shale that ncT_Son normal compaction trend obtained from
sonic log (µs/ft)
were buried to deeper depths and the analysis of data GR Gamma ray (API)
suggest that overpressures could be generated by a VclGR Volume of shale from gamma ray log (%)
number of overpressure‐generating mechanisms. RhOB Bulk density log (g/cm )
3
• The overpressures in the lower section of Kockatea Shale RhoGard density calculated from Gardner’s method
were developed internally due to clay transformation (g/cm )
3
processes mainly by complete transformation of smec- PPG_Son Pore pressure gradient estimated from
tite clay to illite and mixed‐layer clay (smectite/illite). sonic log (psi/ft)
This conclusion was reached by analyzing well logs, PP_Son Pore pressure estimated from sonic log (psi)
clay compositional variations, and the stratigraphical OBPres Overburden pressure/stress (psi)
sequence. On the other hand, the upper section of dST drill stem pressure test in gradient (psi/ft)
Kockatea Shale showed a normal pressure profile as a ROP drilling rate of penetration (m/h)
result of either incomplete clay transformations or over- ZdEn Bulk density log (g/cm )
3
pressure that initially developed and then re‐ equilibrated g Estimated pore pressure gradient (psi/ft)
back to the normal conditions through overlaying high g p Overburden pressure gradient (psi/ft)
permeability Woodada sandstone. Woodada formation g ob normal pore pressure gradient (psi/ft)
n
would not allow overpressure to develop, and it main- dT Sonic transit time (µs/ft)
tained normal pressure in the upper section of Kockatea Δt normal sonic transit time (µs/ft)
Shale. This stratigraphical sequence also suggests that Δt n Observed sonic transit time (µs/ft)
o
overpressure has been generated internally. TVd True vertical depth (m)
• The principal direction of the stress and the compli- R normal resistivity (ohm)
n
cated structure of the northern Perth Basin indicate that lld Resistivity (ohm)
the lateral tectonics compression mechanism is the Sn Resistivity (ohm)
other mechanism that is associated with the clay trans- R Observed resistivity (ohm)
o
formation mechanism. The main reasons for this claim Phnd Porosity obtained from enhanced density (%)
are (i) the forces induced by the principal stress (S hmax ), nPOR neutron porosity (%)
which act in a horizontal plane EW perpendicular to the nPhI neutron porosity (%)
main north–south and northwest–southeast faults cnc Borehole size corrected compensated
trends, (ii) the positions and the trends of the main neutron porosity (%)