In the past two decades, numerical forward modeling of petroleum systems has been extensively used in exploration geology. However, modeling of petroleum systems influenced by magmatic activity has ...not been a common practice, because it is often associated with additional uncertainties and thus is a high risk associated with exploration. Subsurface processes associated with volcanic activity extensively influence all the elements of petroleum systems and may have positive and negative effects on hydrocarbon formation and accumulation. This study integrates 3D seismic data, geochemical and well data to build detailed 1D and 3D models of the Kora Volcano—a buried Miocene arc volcano in the northern Taranaki Basin, New Zealand. It examines the impact of magmatism on the source rock maturation and burial history in the northern Taranaki Basin. The Kora field contains a sub-commercial oil accumulation in volcanoclastic rocks that has been encountered by a well drilled on the flank of the volcano. By comparing the results of distinct models, we concluded that magmatic activity had a local effect on the thermal regime in the study area and resulted in rapid thermal maturation of the surrounding organic matter-rich sediments. Scenarios of the magmatic activity age (18, 11 and 8 Ma) show that the re-equilibration of the temperature after intrusion takes longer (up to 5 Ma) in the scenarios with a younger emplacement age (8 Ma) due to an added insulation effect of the thicker overburden. Results of the modeling also suggest that most hydrocarbons expelled from the source rock during this magmatic event escaped to the surface due to the absence of a proper seal rock at that time.
The mass of organic carbon in sedimentary basins amounts to a staggering 10
16
t, dwarfing the mass contained in coal, oil, gas and all living systems by ten thousand-fold. The evolution of this ...giant mass during subsidence and uplift, via chemical, physical and biological processes, not only controls fossil energy resource occurrence worldwide, but also has the capacity for driving global climate: only a tiny change in the degree of leakage, particularly if focused through the hydrate cycle, can result in globally significant greenhouse gas emissions.
To date, neither climate models nor atmospheric CO
2 budget estimates have quantitatively included methane from thermal or microbial cracking of sedimentary organic matter deep in sedimentary basins. Recent estimates of average low latitude Eocene surface temperatures beyond 30
°C require extreme levels of atmospheric CO
2. Methane degassing from sedimentary basins may be a mechanism to explain increases of atmospheric CO
2 to values as much as 20 times higher than pre-industrial values. Increased natural gas emission could have been set in motion either by global tectonic processes such as pulses of activity in the global alpine fold belt, leading to increased basin subsidence and maturation rates in the prolific Jurassic and Cretaceous organic-rich sediments, or by increased magmatic activity such as observed in the northern Atlantic around the Paleocene–Eocene boundary. Increased natural gas emission would have led to global warming that was accentuated by long lasting positive feedback effects through temperature transfer from the surface into sedimentary basins. Massive gas hydrate dissociation may have been an additional positive feedback factor during hyperthermals superimposed on long term warming, such as the Paleocene–Eocene Thermal Maximum (PETM). As geologic sources may have contributed over one third of global atmospheric methane in pre-industrial time, variability in methane flux from sedimentary basins may have driven global climate not only at time scales of millions of years, but also over geologically short periods of time. Earth system models linking atmospheric, ocean and earth surface processes at different timescales with the sedimentary organic carbon cycle are the tools that need to be developed in order to investigate the role of methane from sedimentary basins in earth's climate.
► Mobilization of a fraction of the 15,000,000 Gt of organic carbon in sedimentary basins could drive global climate. ► A large part of methane leakage to the atmosphere is related to thermal cracking of buried organic matter. ► Late Paleocene/Early Eocene warm climate was potentially related to global increase in burial rates of rich Jurassic -Cretaceous source rocks. ► Integration of variable time scale models is required to investigate the impact of methane leakage from sedimentary basins.
Earth surface temperatures, including in the deep sea increased by 5–10°C from the late Paleocene ca. 58 Myr ago to the Early Eocene Climatic Optimum (EECO) centered at about 51 Myr ago. A large ...(∼2.5‰) drop in δ13C of carbonate spans much of this interval. This suggests a long‐term increase in the net flux of13C‐depleted carbon to the ocean and atmosphere that is difficult to explain by changes in surficial carbon cycling alone. We reveal a relationship between surface temperature increase and increased petroleum generation in sedimentary basins operating on 100 kyr to Myr time scales. We propose that early Eocene warming has led to a synchronization of periods of maximum petroleum generation and enhanced generation in otherwise unproductive basins through extension of the volume of source rock within the oil and gas window across hundreds of sedimentary basins globally. Modelling the thermal evolution of four sedimentary basins in the southwest Pacific predicted an up to 50% increase in petroleum generation that would have significantly increased leakage of light hydrocarbons and oil degeneration products into the atmosphere. Extrapolating our modelling results to hundreds of sedimentary basins worldwide suggests that globally increased leakage could have caused a climate feedback effect, driving or enhancing early Eocene climate warming.
Key Points
Southwest Pacific petroleum systems reacted to climate warming
Models predict an up to 50% increase in oil and gas generation
Leakage from sedimentary basins may have driven early‐Eocene climate
Miocene strata in the southern Taranaki Basin (STB), up to 3 km thick, provide a distal record of erosion associated with plate boundary deformation in New Zealand. 2D and 3D seismic reflection data ...tied to drillhole stratigraphy have been used to constrain four main phases of basin development. These are: (a) Early Miocene (22–19 Ma) subsidence, dominantly bathyal water depths and deposition of minor submarine fans along the eastern basin margin. (b) Middle Miocene (19–14 Ma) widespread submarine fan deposition on a bathyal basin floor in the central STB. (c) Rapid Middle–Late Miocene (14–7 Ma) progradation of the shelf break northwards across the STB. (d) Widespread uplift and erosion of the STB during the latest Miocene–Pliocene (7–4.5 Ma). Bathyal water depths and fan deposition in the Early Miocene were influenced by vertical motions on major reverse faults and regional subsidence produced by subduction of the Pacific plate beneath northern New Zealand. Subsequent submarine fan deposition and northward shelf‐break progradation reflect increasing input of terrigenous material, primarily eroded from an uplifting region to the south of the STB. Sedimentation patterns in the STB are consistent with the age and locations of conglomerates deposited in onshore West Coast basins, related to this uplift and erosion. Sediment transport in the West Coast region was mainly parallel to NNE trending active reverse faults, and in the STB was perpendicular to the NE‐SW orientated shelf break, especially from ca. 14–7 Ma, when sedimentation rates exceeded fault‐displacement rates. Increases in sedimentation rates in the STB coincide with regional increases in the rates of shortening that appear to reflect plate boundary‐wide events and have been attributed to, or correlated with, increases in the plate convergence rate. Miocene sedimentation patterns in the STB thus reflect both intra‐basinal deformation and tectonic signals from the wider developing New Zealand plate boundary.
We present a three‐dimensional gas hydrate systems model of the southern Hikurangi subduction margin in eastern New Zealand. The model integrates thermal and microbial gas generation, migration, and ...hydrate formation. Modeling these processes has improved the understanding of factors controlling hydrate distribution. Three spatial trends of concentrated hydrate occurrence are predicted. The first trend (I) is aligned with the principal deformation front in the overriding Australian plate. Concentrated hydrate deposits are predicted at or near the apexes of anticlines and to be mainly sourced from focused migration and recycling of microbial gas generated beneath the hydrate stability zone. A second predicted trend (II) is related to deformation in the subducting Pacific plate associated with former Mesozoic subduction beneath Gondwana and the modern Pacific‐Australian plate boundary. This trend is enhanced by increased advection of thermogenic gas through permeable layers in the subducting plate and focused migration into the Neogene basin fill above Cretaceous‐Paleogene structures. The third trend (III) follows the northern margin of the Hikurangi Channel and is related to the presence of buried strata of the Hikurangi Channel system. The predicted trends are consistent with pronounced seismic reflection anomalies related to free gas in the pore space and strength of the bottom‐simulating reflection. However, only trend I is also associated with clear and widespread seismic indications of concentrated gas hydrate. Total predicted hydrate masses at the southern Hikurangi Margin are between 52,800 and 69,800 Mt. This equates to 3.4–4.5 Mt hydrate/km2, containing 6.33 × 108–8.38 × 108 m3/km2 of methane.
Plain Language Summary
Gas hydrates are ice‐like substances of natural gas enclosed in a lattice of water molecules. They are stable under pressure and temperature conditions found beneath the sea‐floor offshore beyond continental shelfs. Gas hydrates house a significant part of the natural gas methane contained in the geosphere and hence are a potential energy resource. However, if methane is released into surface systems through decomposition of gas hydrates, for instance, due to an increase in ocean bottom temperatures, it will contribute to ocean acidification, and may acerbate climate warming. Hence, quantification and a better understanding of controls on formation and distribution of gas hydrates is important. Here we present a basin‐wide study predicting hydrate formation offshore eastern New Zealand, where the Pacific plate is subducted beneath the Australian plate. We explore the implications of deep gas generation and migration patterns in this setting for gas hydrate formation and distribution.
Key Points
Gas hydrate system modeling predicts three pronounced spatial trends in gas hydrate distribution along the southern Hikurangi Margin
Hydrate distribution is controlled by sources of microbial and thermogenic gas, basin fill architecture, and convergent margin deformation
Predicted volumes of gas stored in gas hydrate warrant further exploration as an energy resource in New Zealand
Rising ocean temperatures and falling sea level are commonly cited as mechanisms of marine gas hydrate destabilization. More recently, uplift—both isostatic and tectonic—has been invoked. However, ...the effect of tectonic shortening and uplift on gas hydrate stability zone extent has not been validated via integrated computational modeling. Here, modeling along the Hikurangi margin of New Zealand illustrates the mechanism of tectonic uplift as a driver of gas hydrate destabilization. We simulate how tectonic uplift and shortening affect the presence and decrease the extent of a gas hydrate stability zone. We suggest that resultant gas hydrate destabilization in the marine realm may impact the global carbon cycle and oceanic chemistry over geologic time.
Plain Language Summary
Gas hydrates form vast stores of carbon in marine deep water sediment. Large‐scale destabilization of gas hydrates therefore would impact the global carbon cycle and oceanic chemistry. Various causes for the destabilization of marine gas hydrate have been explored, primarily including increasing ocean temperature and falling sea level. Recent studies have also explored the impact of glacially induced isostatic rebound and of subsea mountain building—both of which comprise a form of uplift—on the destabilization of gas hydrate. Nonetheless, the impact of uplift—and in particular, tectonic uplift—has not been explored through quantitative, integrated computational modeling (i.e., basin modeling). We therefore use a forward modeling approach to illustrate the effect of tectonism on gas hydrate distribution. We show that tectonic uplift decreases the extent over which gas hydrate is stable, largely due to decreases in water depth. Our results suggest that tectonically mediated destabilization of gas hydrate should be considered as a driver of changes to the global carbon cycle over geologic timescales.
Key Points
We use a basin modeling approach to predict the impact of tectonic uplift on marine gas hydrate stability
We show that tectonic uplift decreases gas hydrate stability zone extent (i.e., destabilizes gas hydrate)
Tectonically mediated gas hydrate destabilization likely impacts the global carbon cycle over geologic timescales
We present an integrated 2D model of thermal and microbial generation of methane, migration into the gas hydrate stability zone (HSZ), and formation of methane hydrates. The model reconstructs the ...shallow (0–20 km) thermal structure of the subduction interface between the Australian plate and the subducting Pacific plate, and the trench basin (Pegasus Basin). Modelled temperatures of less than 110 °C within Pegasus Basin constrain the generation of oil and gas. Whilst a cool thermal regime is predicted to limit thermogenic generation of gas to a burial depth of >10 km, it extends the interval where prolific microbial gas generation occurs. The modelled rate of microbial generation of methane increases beneath the HSZ and peaks at ~1600 m below seafloor. Diffusive upward migration of microbially generated methane is interpreted to lead to widespread methane hydrate formation and the presence of a semi-continuous bottom simulating reflector (BSR). Predicted average hydrate saturation within the HSZ is 0.9% for a modelled sedimentary organic matter content of 0.5% and 1.6% for 1% organic matter in fine-grained Pegasus Basin sediments. Considerably higher concentrations of methane hydrate of up to 20–70% are predicted to occur where gas migration is focussed within the frontal anticline and proto-thrust zone southeast of the modern accretionary wedge and in channel and basin floor sandstones related to the Hikurangi Channel. The Hikurangi Channel sedimentary system transported coarse clastic sediments eroded from the rising Southern Alps along the eastern margin of the Pegasus Basin since the Miocene. It provides carrier beds specifically for transport of thermogenic gas generated close to the subduction interface. A buried Mesozoic accretionary wedge originating from subduction of the Pacific Plate beneath Gondwana further focusses the migration of gas. Focussed migration of thermogenic gas leads to the highest predicted hydrate concentrations in potential channel sand reservoirs.
•2D model of the Hikurangi subduction margin, New Zealand.•Thermal history modelling and prediction of thermal and microbial gas generation.•Coupled migration and quantitative methane hydrate formation modelling.•Microbial gas generation peaks at 1600 mbsf and relates to formation of BSR.•Concentrated hydrate deposits relate to focussed flow of thermogenic gas.
Fault seal due to juxtaposition or the generation of low-permeability fault rock has the potential to change through time with displacement accumulation. Temporal variations in cross-fault flow of ...hydrocarbons have been assessed for the Cape Egmont Fault (CEF), Taranaki Basin New Zealand, using displacement backstripping, juxtaposition and Shale Gouge Ratio (SGR) analysis. The timing of hydrocarbon migration and charge of the giant Maui Gas-condensate Field across the CEF have been assessed using seismic reflection lines (2D & 3D), coherency cubes, VShale curves from the Maui-2 well and PetroMod modelling. Displacement–backstripping analysis suggests that between the Late Miocene and early Pleistocene (5.5 and 2.1 Ma) sandstone reservoir units of the Maui Field (Mangahewa, Kaimiro and Farewell Formations) and underlying source rocks (Rakopi Formation) were partly juxtaposed across the CEF with low SGRs (< 0.2) present in the fault zone. Following 2.1 Ma SGRs increased to 0.2–0.55 adjacent to the Eocene–Palaeocene reservoir succession which was not in juxtaposed contact with source rocks. PetroMod modelling using these SGR values and juxtaposition relationships supports cross-fault flow prior to 2.1 Ma with later charge across the fault being less likely. Gas chimneys and the gas–water contact in the Eocene reservoir proximal to the fault suggest that despite limited cross-fault flow, upward leakage of hydrocarbons from the reservoir occurred after 2.1 Ma, possibly associated with active fault movement or fracturing related to faulting, and may account for the loss of an early oil phase.
The Hikurangi Margin off the east coast of the North Island (Te Ika-a-Māui) is a tectonically active subduction zone and the location of New Zealand’s largest gas hydrate province. Faults are ...internally complex volumetric zones that may play a significant role in the migration of fluids beneath the seafloor. The combined processes of deformation and fluid migration result in the formation of concentrated hydrate accumulations along accretionary ridges. It is not fully understood to what extent faults control fluid migration along the Hikurangi Margin, and whether deep-seated thrust faults provide a pathway for thermogenic gas to migrate up from sources at depth. Using 2D models based on seismic data from the region we investigated the role of thrust faults in facilitating fluid migration and contributing to the formation of concentrated gas hydrates. By altering permeability properties of the fault zones in these transient state models we can determine whether faults are required to act as fluid flow pathways. In this study we focus on two study sites offshore southern Wairarapa, using realistic yet simplified fault geometries derived from 2D seismic lines. The results of these models allow us to start to disentangle the complex relationship between fault zone structure, permeability, geometry, fluid migration and gas hydrate formation. Based on the model outputs we propose that faults act as primary pathways facilitating fluid migration and are critical in the formation of concentrated gas hydrate deposits.
We investigate gas hydrate system dynamics beneath a submarine canyon on New Zealand's Hikurangi subduction margin using seismic reflection data and petroleum systems modeling. High seismic ...velocities just above the base of gas hydrate stability (BGHS) indicate that concentrated gas hydrates exist beneath the canyon. Two‐dimensional gas hydrate formation modeling shows how the process of canyon incision at this location alters the distribution and concentration of gas hydrate. The key modeling result is that free gas is trapped beneath the gas hydrate layer and then “captured” into a concentrated gas hydrate deposit as a result of a downward‐shift in the BGHS driven by canyon incision. Our study thus provides new insight into the functioning of this process. From our data, we also conceptualize two other models to describe how canyons could significantly change gas hydrate distribution and concentration. One scenario is related to deflection of fluid flow pathways from over‐pressured regions at the BGHS toward the canyon, and the other is based on relationships between simultaneous seafloor uplift and canyon incision. The relationships and processes described are of global relevance because of considerations of gas hydrate as an energy resource and the influence of both submarine canyons and gas hydrate systems on seafloor biodiversity.
Plain Language Summary
Large areas of Earth's seafloor are traversed by submarine canyons that bear similarities to canyon and gully systems that are known on land. As submarine canyons erode down into the seafloor, they can significantly alter temperature and pressure in the sediments beneath the seafloor. In certain water depths, these environmental changes influence the formation of gas hydrates ‐ ice like compounds of natural gas. In this study, we show how concentrated deposits of gas hydrate can form at depth beneath the seafloor as a result of canyon formation. It is important to understand interactions between submarine canyons and gas hydrate systems because both play key roles in supporting diverse seafloor ecosystems. Additionally, natural gas from concentrated gas hydrate deposits represents a possible future energy resource in the transition to cleaner energy alternatives.
Key Points
High seismic velocities beneath a submarine canyon indicate concentrated gas hydrates directly above the base of gas hydrate stability
Canyon incision causes depression of the base of gas hydrate stability which captures free gas and converts it into gas hydrates
Focused fluid flow toward canyons, and relationships between seafloor uplift and canyon incision, can also influence gas hydrate formation
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