Page 171 - Fundamentals of Gas Shale Reservoirs

ThE ROlE OF TEcTOnIc AcTIVITIES On PORE PRESSuRE In ShAlES 151
7.4 THE ROLE OF TECTONIC ACTIVITIES The dandaragan Trough is a major depocenter in the north
ON PORE PRESSURE IN SHALES Perth Basin that covers more than 5000 km . It is virtually
2
unfaulted syncline and has the thickest sediment accumulation
This section stresses the importance of tectonic activities on of approximately 12,000 m (crostella, 1995). The mountain
pore pressure distribution in shale formations. A pore Bridge Fault bounds the western margin of the northern
pressure study conducted on the Perth Basin has linked the dandaragan Trough. This fault showed a progressive down‐
variations in pore pressure gradients in the same intervals to dip decrease of dip angle from considerably high angles at the
the variations in tectonic intensity that took place within the top ranges of 65° to 20° at the base of the sedimentary cover
same regions. normal trends in pore pressure gradients were (Fig. 7.16, right). The mid‐southern section of the trough is
observed in regions that were situated in severely uplifted marked by coomallo Fault that declines in dip from 62° at the
sections within tectonically active areas. On the other hand, surface to 42° at a depth of 10 km (Song and cawood, 2000).
pore pressure gradients increased in the same intervals where The northern boundary of the dandaragan Trough is marked
there was less intense tectonic activity. Regions with a lower by the Allanooka Transfer Fault, which separates the trough
intensity of tectonic activity showed an increase in pore from the Allanooka high to the north. The eastern boundary
pressure gradients when moving away from the center of of the dandaragan Trough is bounded by urella Fault and
the uplifting. This study demonstrated that there was a darling Fault system, and the cervantes Transfer Fault consti-
combination of mechanisms contributing to overpressure tutes the southern boundary of the dandaragan Trough and
development driven by the complicated basin geology. separates it from the Beermullah Trough (Fig. 7.15).
Overpressure‐generating mechanisms for the shale forma- The structural history of the Perth Basin is dominated by
tions studied were attributed to fluid expansion and later two major phases of extension. An Early Permian phase cre-
tectonic loading. Both mechanisms have contributed to a ated half‐grabens that hinge around the northampton high.
different extent to overpressure development. A period of uplift and erosion in the late Permian termi-
This section starts with a geological summary and nated this period of basin development. The second, and
an overview of the stress field direction in the area studied. more extensive, phase of oblique extension occurred in the
In addition, observations of pore pressure profiles are late Jurassic to the Early cretaceous period during the sep-
presented both in tectonically active regions and reasonably aration of the Australian Plate from Greater India and Africa.
stable areas. Furthermore, a thorough approach is presented This phase caused extensive basin inversion and uplifting as
to identify with certainty the causes underlying the overpres- well as the development of transfer faults which influenced
sure development in this part of the Perth Basin, and that the geometry and divided the basin into compartmentalized
includes, firstly, the signatures of well log data being regions characterized as subbasins, ridges, and troughs of
analyzed and initial thoughts being established as to what similar structure that reflect the present form of geological
the potential overpressure‐generating mechanisms are. structure of the Perth Basin (Song and cawood, 2000). The
Secondly, additional methodologies used to identify the center of the uplift is near the coastal town of Jurien within
cause of overpressure are discussed, and these involve Beagle Ridge where up to 8 km of section has been removed.
analysis of sonic–density cross‐plots, analysis of X‐ray Extensive faulting systems were identified within the
diffraction (XRd) and analysis of the natural gamma ray Kockatea Shale in the Beagle Ridge and the adjacent cadda
spectrometry (nGS) logs. Terrace. In addition, the severe erosion and uplifting that
took place in these areas have removed considerable parts of
the Kockatea Shale in some localities. The tectonic divisions
7.4.1 Geology of the Study Area
and structural framework as well as maximum stress
The Perth Basin is a north‐northwest trending extension direction in the Perth Basin are illustrated in Figure 7.15.
located in the southwest of Western Australia. The basin covers The lower Triassic Kockatea Shale is one of the potential
more than 100,000 km along the western coast of Australia shale gas formations in the Perth Basin. The unit is made up
2
and extends from Geraldton to Augusta. The basin contains of dark shale or siltstone with minor thin sandstone and
sedimentary sequence that varies from Silurian to Pleistocene limestone beds (crostella and Backhouse, 2000). Well log
(mory et al., 2005). The eastern boundary of the basin is data were used to evaluate pore pressure regimes in the
defined by the darling Fault, while the western end margin Kockatea Shale. The log data that were used include sonic
extends offshore to the edge of the continental crust in water velocity, neutron porosity, density, and gamma ray. The
depths of up to 4500 m (Iasky and mory, 1994). northampton boreholes which intersect the candidate formation were
block forms the northern boundary of the basin and the divided into groups according to their geographic and geo-
southern boundaries extend to the edge of the continental shelf. logical locations. mud log data, for example, mud weight,
The focus of this study is on two different areas: (1) dandaragan well flows and kicks while drilling, and ROP, were also
Trough and its adjacent terrace of similar characterization and reviewed and correlated to the log data analysis. All data
(2) Beagle Ridge and its adjacent terraces. were combined and analyzed and the results revealed the