Trace Metal Variations through the Tartan and Waipawa Formations: Implications for the Environment of Deposition

<p>An inorganic geochemical study of the Late Paleocene organic matter-rich Waipawa and Tartan formations was undertaken in order to investigate the depositional environment. The formation varies in thickness between 2 and 50 metres and is distributed across many of New Zealand’s Cenozoic basins, where it forms an important potential hydrocarbon source rock. This study measured major and trace elements which can be loosely grouped into redox sensitive, biologically influenced, terrestrially sourced, and rare earth elements (REE). The study focused on three sections through the Waipawa and Tartan formations: Angora Quarry in the East Coast Basin, and the Great South Basin hydrocarbon exploration wells Kawau-1A and Pakaha-1. At Angora Quarry, x-ray fluorescence (XRF) was used to measure the major constituents Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, SO₃, K₂O, CaO, TiO₂, MnO and Fe₂O₃. inductively-coupled plasma mass spectrometry (ICP-MS) was used to measure Li, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Ba, REE, Hf, Tl, Pb, Th and U. For Pakaha-1 and Kawau-1A side wall core samples, ICP-MS was used to measure Ti, V, Cr, Mn, Co, Ni, Cu, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Hf, Ta, W, Tl, Pb, Bi, Th and U. Insufficient sample was available for XRF on these samples. No major changes in oxygen concentration during deposition were recorded by redox-sensitive elements from Angora Quarry and Pakaha-1 sediments; however samples from Kawau-1A and from a section 1 km upstream from Angora Quarry were deposited under somewhat oxygen-depleted conditions. As the anoxic and suboxic indicators show significantly lower variations than under present day anoxic environments, and in Angora Quarry CaO and SO₃ are significantly depleted with higher aluminosilicates a rapid deposition is required to explain the preservation of the organic matter. In the Great South Basin wells, the clay content correlates directly with increased gamma ray levels measured by well logs. Increased influx of terrestrial clays has been linked to marine transgressions in many New Zealand sediments and is been taken to mean the same for the Waipawa and Tartan formations. The oxygen depletion indicates that water depths during deposition exceeded 50 metres. The depositional model proposed here, therefore, is that of a major marine transgression that flooded and eroded near-shore swamps, re-depositing the terrestrial organic matter offshore. The increased nutrients released by this would have stimulated bioproductivity and locally, where conditions were suitable, depleted the oxygen content of the water column. This study also suggests ternary diagrams are valuable for calculating the enrichment of elements affected by two processes, such as Sr, which is related to both detrital Al and related to biological Ca. Ga, Ba and Al content are also related on a ternary diagram indicating the similar terrestrial and biological relationships for Ba and Ga. W was found to behave in a similar way to Bi. Enrichment factors proved less useful than absolute enrichment for Kawau-1A, where detrital input varied greatly and was found to be significantly different in composition to average shale as defined by Wedephol (1971).</p>

Trace Metal Variations through the Tartan and Waipawa Formations: Implications for the Environment of Deposition

<p>An inorganic geochemical study of the Late Paleocene organic matter-rich Waipawa and Tartan formations was undertaken in order to investigate the depositional environment. The formation varies in thickness between 2 and 50 metres and is distributed across many of New Zealand’s Cenozoic basins, where it forms an important potential hydrocarbon source rock. This study measured major and trace elements which can be loosely grouped into redox sensitive, biologically influenced, terrestrially sourced, and rare earth elements (REE). The study focused on three sections through the Waipawa and Tartan formations: Angora Quarry in the East Coast Basin, and the Great South Basin hydrocarbon exploration wells Kawau-1A and Pakaha-1. At Angora Quarry, x-ray fluorescence (XRF) was used to measure the major constituents Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, SO₃, K₂O, CaO, TiO₂, MnO and Fe₂O₃. inductively-coupled plasma mass spectrometry (ICP-MS) was used to measure Li, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Ba, REE, Hf, Tl, Pb, Th and U. For Pakaha-1 and Kawau-1A side wall core samples, ICP-MS was used to measure Ti, V, Cr, Mn, Co, Ni, Cu, As, Se, Rb, Sr, Y, Zr, Nb, Mo, Cd, Sn, Sb, Hf, Ta, W, Tl, Pb, Bi, Th and U. Insufficient sample was available for XRF on these samples. No major changes in oxygen concentration during deposition were recorded by redox-sensitive elements from Angora Quarry and Pakaha-1 sediments; however samples from Kawau-1A and from a section 1 km upstream from Angora Quarry were deposited under somewhat oxygen-depleted conditions. As the anoxic and suboxic indicators show significantly lower variations than under present day anoxic environments, and in Angora Quarry CaO and SO₃ are significantly depleted with higher aluminosilicates a rapid deposition is required to explain the preservation of the organic matter. In the Great South Basin wells, the clay content correlates directly with increased gamma ray levels measured by well logs. Increased influx of terrestrial clays has been linked to marine transgressions in many New Zealand sediments and is been taken to mean the same for the Waipawa and Tartan formations. The oxygen depletion indicates that water depths during deposition exceeded 50 metres. The depositional model proposed here, therefore, is that of a major marine transgression that flooded and eroded near-shore swamps, re-depositing the terrestrial organic matter offshore. The increased nutrients released by this would have stimulated bioproductivity and locally, where conditions were suitable, depleted the oxygen content of the water column. This study also suggests ternary diagrams are valuable for calculating the enrichment of elements affected by two processes, such as Sr, which is related to both detrital Al and related to biological Ca. Ga, Ba and Al content are also related on a ternary diagram indicating the similar terrestrial and biological relationships for Ba and Ga. W was found to behave in a similar way to Bi. Enrichment factors proved less useful than absolute enrichment for Kawau-1A, where detrital input varied greatly and was found to be significantly different in composition to average shale as defined by Wedephol (1971).</p>

Structure, Faulting and Gas Accumulation: Southeast Wanganui Basin, New Zealand

<p>There has been low interest in petroleum exploration in the Wanganui Basin as it lacks known hydrocarbon source rock of sufficient age or burial depth. However, the onshore Southeast Wanganui Basin has many occurrences of methane-rich biogenic gas found in shallow water wells. This project used three studies across the Horowhenua area to examine the faulting style in the Southeast Wanganui Basin where it is bounded by the Tararua range- front, and how this faulting relates to the accumulation of gas deposits in the shallow sedimentary section. South of Levin the Tararua range front steps laterally near Muhunoa East Road. A previous seismic reflection line identified a deep intra-basement arrival, which could have been either a low-angle thrust fault or side-swipe from a pull-apart basin at the step in the Tararua range front. Two seismic lines and a gravity survey found no sub-vertical drops in basement depth which would indicate the presence of a pull-apart basin or a favourable surface off which a laterally travelling seismic wave could reflect. The intra-basement arrival on the previous seismic line was therefore interpreted to be from an intra-basement low-angle thrust fault. Also two biogenic gas sites also were surveyed. A shallow gas reservoir east of Levin on Wallace Road, abutting the Tararua range front, had been discovered when a water well was drilled; and a potential reservoir southwest of Sanson was located when an aerial survey identified a domed structure with high resistivity. In both areas biogenic gas was thought to be trapped in buried sand dunes at a depth of approximately 20 m. Shallow seismic refraction and reflection methods and amplitude variation with offset analysis were used to map both reservoir bodies and confirm the presence of biogenic gas.</p>

Structure, Faulting and Gas Accumulation: Southeast Wanganui Basin, New Zealand

<p>There has been low interest in petroleum exploration in the Wanganui Basin as it lacks known hydrocarbon source rock of sufficient age or burial depth. However, the onshore Southeast Wanganui Basin has many occurrences of methane-rich biogenic gas found in shallow water wells. This project used three studies across the Horowhenua area to examine the faulting style in the Southeast Wanganui Basin where it is bounded by the Tararua range- front, and how this faulting relates to the accumulation of gas deposits in the shallow sedimentary section. South of Levin the Tararua range front steps laterally near Muhunoa East Road. A previous seismic reflection line identified a deep intra-basement arrival, which could have been either a low-angle thrust fault or side-swipe from a pull-apart basin at the step in the Tararua range front. Two seismic lines and a gravity survey found no sub-vertical drops in basement depth which would indicate the presence of a pull-apart basin or a favourable surface off which a laterally travelling seismic wave could reflect. The intra-basement arrival on the previous seismic line was therefore interpreted to be from an intra-basement low-angle thrust fault. Also two biogenic gas sites also were surveyed. A shallow gas reservoir east of Levin on Wallace Road, abutting the Tararua range front, had been discovered when a water well was drilled; and a potential reservoir southwest of Sanson was located when an aerial survey identified a domed structure with high resistivity. In both areas biogenic gas was thought to be trapped in buried sand dunes at a depth of approximately 20 m. Shallow seismic refraction and reflection methods and amplitude variation with offset analysis were used to map both reservoir bodies and confirm the presence of biogenic gas.</p>

The Origin of Silica of Marine Shale in the Upper Ordovician Wulalike Formation, Northwestern Ordos Basin, North China

The shale of the Wulalike Formation developed in the northwestern Ordos Basin is considered to be an effective marine hydrocarbon source rock. One of the key factors for successful shale gas exploration in the Wufeng–Longmaxi Formation in the Sichuan Basin is the high content of biogenic silica. However, few people have studied the siliceous origin of the Wulalike shale. In this study, we used petrographic observation and element geochemistry to analyze the origin of silica in the Wulalike shale. The results show that the siliceous minerals are not affected by hydrothermal silica and mainly consist of biogenic and detrital silica. A large number of siliceous organisms, such as sponge spicules, radiolarians, and algae, are found under the microscope. It has been demonstrated that total organic carbon has a positive correlation with biogenic silica and a negative correlation with detrital silica, and biogenic silica is one of the effective indicators of paleoproductivity. Therefore, the enrichment of organic matter may be related to paleoproductivity. Through the calculation of element logging data in well A, it is found that biogenic silica is mainly distributed in the bottom of the Wulalike Formation, and the content of biogenic silica decreases, while the content of detrital silica increases upward of the Wulalike Formation. Biogenic silica mainly exists in the form of microcrystalline quartz, which can form an interconnected rigid framework to improve the hardness and brittleness of shale. Meanwhile, biogenic microcrystalline quartz can protect organic pores from mechanical compaction. Therefore, it may be easier to fracture the shale gas at the bottom of the Wulalike Formation in well A.