Page 184 - Fundamentals of Gas Shale Reservoirs

164 PORE PRESSuRE PREdIcTIOn FOR ShAlE FORmATIOnS uSInG WEll lOG dATA

that hinge around the northampton high. The second and Kockatea Shale. The present‐day pressure of the upper sec-
more extensive phase of oblique extension occurred in the tion of Kockatea Shales exhibits normal pressure gradients.
late Jurassic to Early cretaceous periods. The center of the Overpressure in this section may have been developed and
second uplifting phase is near the coastal town of Jurien in then released due to high permeability of the overlaying
Beagle Ridge, where up to 8 km of sections were removed. Woodada sandstone sections, which have not allowed over-
normal pore pressures were observed mainly in wells where pressures to be preserved within the shales (Fig. 7.16, left).
Kockatea Shale was intersected at a shallower depth in the Additionally, this stratigraphical sequence suggests that
localities of Beagle Ridge and cadda Terrace. Overpressure overpressure has been generated internally (clay transforma-
in these areas may have developed and been due to the tion). The system may have been pressurized and then re‐
fractures which acted as conduits, overpressures were equilibrated back to the normal conditions (Ahmad et al.,
released through. The removal of significant parts of the 2014). Incomplete clay transformations could be a possible
formation may have facilitated a re‐equilibration of the pore explanation for having a normal pore pressure in the upper
pressure back to the normal condition. section of Kockatea Shale.
On the other hand, overpressures were observed in the In fact, clay transformation from smectite to illite is not to
Kockatea mainly in regions where there was less intense tec- be considered a significant mechanism that produces a high
tonic activity, particularly in wells where Kockatea Shale magnitude of overpressure (Osborne and Swarbrick, 1997).
was intersected at deeper depth. Regions with less intensity This is due to the fact that the maximum volume changes
in tectonic activity showed a progressive increase in pressure between interlayer and intergranular water only increase the
gradients away from the center of uplift (Fig. 7.23). The volume of intergranular water by an insignificant amount.
areas where overpressures were observed include Since the overpressure observed in the Kockatea Shale is
dandaragan Trough and adjacent terraces that have similar noticeably in high magnitude as noticed from the reversals
structures. The phenomenon of overpressure has been of the vertical effective stress, the clay transformation mech-
observed by the diversion of the effective stress‐dependent anism could be combining with other overpressure‐gener-
parameters from their normal trends. The top of the over- ating mechanisms.
pressure zones are the depths where the diversion occurred The most likely mechanism to be associated with the clay
and overpressures were confirmed by cross‐checking the transformation is the lateral tectonics compression. The main
available data such as drilling reports, mud log data. reasons for this deduction are (i) the forces induced by the
principal stress (S hmax ) which act in a horizontal plane EW per-
pendicular to the main north–south and northwest–southeast
7.5.4 Overpressure-Generating Mechanisms
faults trends (Fig. 7.15) and (ii) the positions and the trends of
It is noted that there were combination mechanisms that con- the main faults and the main transfer faults providing efficient
tributed to overpressure development, driven by the compli- lateral seals for the overpressure developed in the Kockatea
cated basin geology. Fluid expansion and later tectonic Shale. The lateral tectonics explains the high magnitude of
loading have contributed to different extents to overpressure overpressure observed as the diagenesis effects cannot pro-
development. duce high overpressure. lateral tectonics compression would
Among fluid expansion mechanisms, clay diagenesis is have caused the vertical effective stress to be reversed and
the mechanism most likely to have contributed to overpres- density log to change slightly or remain at a reasonably
sure development in the lower section of the Kockatea Shale. constant value because compaction is not reversible.
This deduction was reached from the results that were Additionally, lateral tectonics causes an increase in neutron
obtained from the analysis of wireline log responses, sonic‐ porosity (Fig. 7.24). more investigations are needed to ascer-
density cross‐plots, XRd, and nGS logs. The shale in tain whether clay transformation or lateral compression was
Figure 7.25 is of a depth and temperature where it is likely the primary mechanism for generating overpressure.
for the smectite to be mostly transformed to illite. This figure
shows points with a higher difference between neutron 7.5.5 Overpressure Results Verifications
porosity and density porosity falling farther from the smec-
tite‐rich trend. It has been noticed that as depth increases, the mud logging data including mud weight, drilling ROP,
difference between neutron porosity and density porosity kicks, and so on were used to validate the results and replace
increases while the ratio of smectite to illite decreases, and the absence of direct pressure measurement (e.g., RFT,
therefore, the pore pressure gradients increase (unloading dST). In locations where there were diversions of the
increases). It should be noted that while there are a few data porosity‐dependent parameters from their normal trends,
points where the difference of neutron porosity–density mud logging data in these sections showed every indication
porosity approaches the smectite rich trend (the ratio of of Kockatea Shale being overpressured. Overpressure‐
smectite to illite increases), these data points are not consid- related parameters obtained from mud logging data include
ered as being representative of the whole shale interval other a continuous increase in each of the ROP, the mud weight,
than in the inter‐bedded sandstone sections within the and the gas shows within these intervals. It is noted there