Geological history and fracture network characterization in unconventional reservoirs of the McArthur Basin (NT Australia)

Master thesis (2018)

Authors

A.N. Pragt Civil Engineering & Geosciences

Contributors

G. Bertotti (supervisor 1)
J.C. Blom (supervisor 1)
P.B.R. Bruna (supervisor 1)
Faculty
Civil Engineering & Geosciences
Copyright: © 2018 Nasar Pragt
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Published Date

30-08-2018

Language

English

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Faculty

Civil Engineering & Geosciences

Abstract

The
geological history of the McArthur Basin (NT Australia) is poorly understood.
It consists of five onshore Paleo- to Mesoproterozoic packages with mainly
siliciclastic and carbonate rocks, with cumulative thicknesses up to 15km. The
basin contains the world’s oldest hydrocarbons, principally hosted in
unconventional reservoirs in the Wilton Package. Fluid flow in these reservoirs
is related to natural, reactivated or induced fractures. Characterizing the
fracture network is an important part of predicting fluid flow. This study
tries to link the geological history to the generation of fractures.

The
geological history needs to be better understood to characterize the fracture
network. In this study seismic, well, outcrop and geophysical data are
integrated to construct a cross section that links outcrops (Batten Fault Zone
and Broadmere Complex) with the subsurface (Beetaloo Sub-basin). The literature
in combination with the cross section is used to revise the geological history.

A fieldwork
is conducted to study fracture geometries on outcrops of the Wilton Package
that are analogues to subsurface fracture networks. A drone is used to image
fracture pavements at an order (102m) that is normally missed by
geologists (101m) and satellite images (103m). The
Tanumbirini-1 well, located in the sub-basin, provides a FMI log for
interpreting fractures in the subsurface. A key objective is to differentiate
fractures associated with fracture drivers like regional stress, folds and
faults.

This study
identified two unconformities in the seismic data, corresponding to two
deformation events. The Carpenteria Event between the Wilton Package and the
Inacumba Group is associated with a dominantly N-S oriented stress field and
the Borroloola Event within the Inacumba Group corresponds to a mainly E-W
oriented stress field. Both events created their own fracture sets and are
observed on outcrops and in the subsurface. The tectonic stress is σ1 at the surface but σH,max in the Beetaloo Sub-basin. Fracture generation
in the sub-basin happened at another stress regime than the surface outcrop
analogues, making any direct comparison less reliable. Hence this study gives a
prediction of the fracture density and permeability trends in the sub-basin. A
conceptual model of the subsurface permeability is proposed where the
permeability trend is mainly E-W oriented.