CHARACTERIZATION AND RESTORATION OF INVERTED FEATURES ACROSS THE SOUTHERN TARANAKI BASIN by Wesley Shayne Bucker A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulillment of the requirements for the degree of Master of Science (Geology). Golden, Colorado Date:_________________ Signed:_________________________ Wesley Bucker Signed:_________________________ Dr. Bruce Trudgill Thesis Advisor Golden, Colorado Date _________________ Signed:_________________________ Dr. M. Stephen Enders Professor and Department Head Department of Geology and Geological Engineering ii ABSTRACT The Southern Taranaki Basin is a predominantly offshore basin located between the two islands of New Zealand. The basin has a complex structural history due to its current location along an unconventionally shaped plate boundary and changes in a tectonic regime that have affected the basin over the last ~80 m.y. The focus of this study is the restoration and strain analysis of the Southern Taranaki Basin, which consists of the Central Graben and the Southern Inversion Zone. 2D and 3D seismic data sets are interpreted across the Southern Taranaki Basin using Schlumberger’s Petrel to create seven horizons that represent boundaries between signiicant structural events. The horizons and transecting fault surfaces are transferred to cross sections perpendicular to the major structures, which are each restored sequentially in 2D using Midland Valley’s Move. The restorations incorporate unfaulting, unfolding, and decompaction to measure the strain within each section and assess the overall distribution of strain within the Southern Taranaki Basin. Cretaceous extension associated with rifting and break-up of Eastern Gondwana shows a range of 8.1% - 0.8% extension along the cross sections. Localized basin extension in the Eocene (tied to the initiation of spreading between the Paciic and Australian plates in the southwest) resulted in a 1.9 - 0% extension. During the Oligocene-Miocene, Paciic Plate subduction beneath the North Island began to the northeast, resulting in 6.1 - 0.1% shortening. Within the last 7 m.y., as shortening continued in the south of the Southern Taranaki Basin, back-arc extension migrated from the north, resulting in up to 1.8% extension in the northern sections, up to 0.5% shortening across the southern end of the study area, and no strain in sections in the middle of the study area. Across the Southern Taranaki Basin, the net change along the lengths of individual sections range between 7.8% shortening and 2.7% extension. This study shows that the latitudinal transition of strain over time across Southern Taranaki Basin is not as uniform as previously assumed. iii TABLE OF CONTENTS ABSTRACT ................................................................. iii LIST OF FIGURES ...........................................................vii LIST OF TABLES ........................................................... xiv ACKNOWLEDGEMENTS .....................................................xv CHAPTER 1: INTRODUCTION ..................................................1 1.1 Introduction ..........................................................1 CHAPTER 2: BACKGROUND GEOLOGY .........................................7 2.1 Basin Overview .......................................................7 2.2 Early Basin History (>65 Ma). 8 2.3 Middle Basin History (65-30 Ma) ........................................10 2.4 Late Basin History (30 Ma - present) .....................................14 2.5 Petroleum Signiicance ................................................17 2.6 Brief History of Petroleum Interests and Early Seismic Surveys ................18 2.7 Petroleum System ....................................................18 CHAPTER 3: DATA SELECTION ...............................................20 3.1 Data ...............................................................20 3.2 Extent of Study Area ..................................................22 3.3 Cross Section Placement ...............................................24 CHAPTER 4: SEISMIC INTERPRETATION & MODELING ..........................25 4.1 Seismic Horizon Convention ...........................................25 4.2 Interpretation Methodology ............................................29 iv 4.3 2D Model Generation .................................................41 4.4 Depth Conversion ....................................................48 CHAPTER 5: RESTORATIONS .................................................53 5.1 Restoration Methodology ..............................................53 5.2 Compaction and Isostasy Variables. 55 5.3 Restorations .........................................................57 5.4 Data Trend Analysis ..................................................69 5.5 Decompaction Control Group ...........................................78 CHAPTER 6: DISCUSSION ....................................................81 6.1 Review ............................................................81 6.2 Plate Margin Signiicance ..............................................81 6.3 Petroleum Signiicance ................................................85 6.4 Resolution of Smaller Faults ............................................88 6.5 Signiicance of Folding in Strain Measurements ............................88 6.6 Interpretation Discrepancies ............................................94 6.7 Further Compartmentalization of Study Packages ...........................95 6.8 Signiicance of Sub-Seismic Faulting .....................................97 6.9 Signiicance of Section Placement ......................................97 CHAPTER 7: CONCLUSIONS ..................................................99 7.1 Summary ...........................................................99 7.2 Summary of Southern Taranaki Basin Strain ..............................100 7.3 Future Work .......................................................101 v REFERENCES ..............................................................102 vi LIST OF FIGURES Figure 1.1: Map of New Zealand showing basins, rises, and plate boundaries relevant to this study. Base map is modiied from ArcGIS World Ocean Base. Alpine Fault location is simpliied from the New Zealand Active Faults Database available through GNS. Plate movement vectors are compared to a static Australian Plate and are taken from Douglas, et al., (2005). ................1 Figure 1.2: The Taranaki Basin split into sub-basins. Labels show the Western Stable Platform and the Eastern Mobile Belt, split into the North Graben, Central Graben, Tarata Thrust Belt, and Southern Inversion Zone. Pink represents offshore volcanism. Modiied from King and Thrasher, 1996. ..............2 Figure 1.3: Map of faults and wells within the Southern Taranaki Basin. The red polygon is the study boundary. Numbered dark blue lines are the locations of the restored cross sections. Light grey lines across the map show the location of available 2D seismic lines. S.I.F.1 and S.I.F.2 stand for Southern Inverted Fault 1 & 2 since Published papers show these faults without names, yet they are referenced in this study in some instances The six wells used to calibrate the depth conversion are illed with red. Modiied from Reilly et al. (2015) ...3 Figure 1.4: Petroleum ields across the entire Taranaki Basin, showing large faults and sediment Thickness. Reverse faults are shown with triangles instead of sticks for dip direction. The boundary for this study is shown in red. Modiied from MBIE, (2014) ................................................5 Figure 2.1: New Zealand 72 Ma. Edited from King & Thrasher (1996) ................9 Figure 2.2: Stratigraphic column for the Southern Taranaki Basin. The “Seismic Horizon” column shows the horizons that are interpreted and used to reconstruct the cross sections in this study. This column compares the age of the formations to the thickness of these formations commonly observed in seismic surveys within the STB. The columns on the right show the general depositional environment and structural regime over the life of the Taranaki Basin. Modiied from Reilly, et al. 2014. .............................. 11 Figure 2.3: Active faults throughout the life of the basin. Coastlines shown are present- day. Black lines represent normal faulting, red lines represent reverse faulting. The dashed line represents the boundary between compressional and extensional strain in the basin, which migrated south over time. Taken from Reilly et al. (2015) ...........................................13 Figure 2.4: Evolution of the proto-New Zealand island and subduction zone. For reference, the timing of the horizons shown in Figure 2.2 (described in Section 4.1) are, U_P50 ~30 Ma, N15 ~19 Ma, U_N50 ~7 Ma, and N82 ~2 Ma. Modiied from King and Thrasher, 1996. ..........................14 Figure 2.5: Producing oil and gas ields of the Southern Taranaki Basin and Taranaki Peninsula in relation to study cross sections. Field locations are from GNS’s New Zealand Exploration Map. Modiied from Reilly et al. (2015) .........17 vii Figure 3.1: Data prevalence compared to cross sections (dark blue) and study boundary (red). Available 2D lines shown in light grey, lines used to interpret surfaces in black. 3D volumes used in this study are shown as boxes. Pink areas are places within the project boundary where structures are not adequately imaged with present survey density. Major faults are included. Modiied from Reilly et al. (2015) ...........................................20