Basin modeling is a powerful tool for integrating geological and geophysical data over a range of scales and provides a means to test and refine concepts related to basin evolution. The term “basin modeling” is used here for dynamic forward modeling of geological and physical processes taking place in sedimentary basins over a certain geological time span [1
]. Modeling incorporates deposition, subsidence and compaction, heat flow analysis, as well as hydrocarbon generation, expulsion and accumulation. Numerical forward modeling of petroleum systems is common practice in exploration, but areas with magmatic activity are often perceived as a high risk target for hydrocarbon exploration [2
]. There are, however, multiple examples of petroleum systems associated with igneous intrusions all over the globe, some of which contain significant hydrocarbon accumulations (e.g., references [3
Volcanic activity includes multiple surface and subsurface processes that have an extensive impact on the thermal regime in the surrounding region. Subsurface volcanic processes such as sills/dikes emplacement and magma-chamber formation are of particular interest because the complex interaction between host rock and magmatic intrusion may have both positive and negative effects on all elements of a petroleum system. High heat flow resulting from magmatic activity, for example, may accelerate the thermal maturation of petroleum source rocks, whereas it may also lead to “overcooking” of organic matter and the loss of generated hydrocarbons [5
]. It is, therefore, important to develop a good understanding of the architecture of a magmatic system, its age, and its geometric relationship to intruded strata, as well as associated heat flow variation through time when assessing a petroleum system. Interaction of magmatic intrusions (sills and dikes) with surrounding organic material may play a significant role in the timing of the hydrocarbon expulsion and accumulation, whereas the timing of emplacement and the size of intrusive body defines the volume of influence [7
]. Hydrocarbon generation associated with strata rapidly heated by magmatic intrusion has been studied numerically for several regions [2
]. Modeling of magmatic intrusions is, however, rarely conducted in 3D petroleum systems models as their effects are generally considered to be local rather than regional in scale.
The offshore northern Taranaki Basin, New Zealand is an ideal location to study the influence of magmatism on a petroleum system due to a proven hydrocarbon presence within a well-preserved buried submarine volcanic center. To complement these data, exploration boreholes around the Kora volcano provide valuable information for petroleum system model calibration. The results of this study are relevant to other petroleum regions with magmatic activity.
This study focuses on detailed 1D and 3D numerical models of a buried Miocene arc volcano (Kora) to examine the impact of magmatism on source rock maturation and burial history in the northern Taranaki Basin. The geometries of the stratigraphic layers were interpreted from 3D seismic data. Recent studies have focused on the geomorphology, structure, and geometry of sills and dikes associated with the Kora volcano [12
]. Previous numerical studies have focused on 1D thermal modeling and heat anomalies associated with shallow and deep seated intrusions of Kora and their impact on hydrocarbon generation [15
]. However, a study that provides a full 3D perspective of magmatic thermal influence on maturation of organic matter is lacking.
The modeling results are used to answer the following scientific questions:
How does the magmatic intrusion affect the thermal maturation of the surrounding source rock?
What is the sensitivity of the results to the shape and distribution of volcanic intrusion?
What are the effects of a deep seated magmatic intrusion (e.g., magma chamber) in comparison to dikes and sills intruding the sedimentary sequence?
These questions, although widely discussed in the last decades based on 1D and 2D studies, remain not fully answered when the third dimension is added. Recent studies on the thermal effects of magmatic intrusion on hydrocarbon maturity show a strong relation between sill thickness and temperature, as well as maturation of the surrounding organic matter [16
Industry 3D seismic reflection data tied to exploration wells were used to map the major stratigraphic units around Kora to define the geometry and shape of this sub-sea stratovolcano and underlying dikes and sill complexes. We use 3D numerical simulation to assess the influence of magmatic intrusion on the thermal maturation of organic matter and the petroleum system. The effects of deep seated intrusion, sill emplacement, and a system of dikes building a volcanic center are analyzed in this paper. We discuss local lateral changes in heat flow associated with different scenarios and its effect on hydrocarbon generation. To construct petroleum systems models we integrated previously published research data, core and well data, well completion reports, and geochemistry reports available online in the New Zealand Petroleum and Minerals Exploration Database, as well as further geological data from New Zealand Petroleum Exploration Data Pack 2018 provided by New Zealand Petroleum and Minerals.
2. Geological Setting
Taranaki Basin, situated beneath and to the west of the western North Island is presently New Zealand’s only producing hydrocarbon province (Figure 1
). The study area is located offshore west of New Zealand’s North Island on the western margin of Neogene arc volcanism and intra-arc rifting [20
] (Figure 1
). The sedimentary and tectonic history of the northern Taranaki Basin can be subdivided into three major periods: an early syn-rift phase (Late Cretaceous to Paleocene), a post-rift phase (Eocene to Mid Miocene) and a young extensional and volcanic back-arc phase (Late Miocene to Recent) phase (Figure 2
Taranaki Basin developed in the Late Cretaceous as Zealandia separated from the continental landmass of Gondwana [20
]. The basin contains up to 11 km of Late Cretaceous to Recent strata that records continental breakup through to passive margin subsidence and the subsequent development of the modern plate boundary through New Zealand [20
In its early phase, Late Cretaceous rifting in the Taranaki Basin was characterized by widely distributed NW-NNW to NE trending half-grabens developed in response to the Gondwana break-up and the opening of the Tasman Sea [23
]. The predominantly Late Cretaceous extensional episodes developed large normal faults with thick terrestrial to marginal syn-rift sequences (2 to 3 km) [20
]. In subsequent deformation phases, these Late Cretaceous faults have, in some cases, reactivated multiple times [24
]. Extension waned in the Latest Cretaceous/Early Paleocene, resulting in a transition to a passive transgressive margin undergoing gradual thermal subsidence [20
The development of the modern Australia-Pacific plate boundary through New Zealand began in the middle Eocene resulting in contraction along the eastern margin of the Taranaki Basin [26
]. This contraction, characterized by reverse faulting and foreland basin development, resulted in uplift and erosion of Late Cretaceous to Paleocene strata along the eastern margins of the basin [20
]. Maximum transgression in the basin occurred during the Oligocene and was followed by regression in the Early Miocene contemporaneous with initiation/reactivation of subduction along the Hikurangi margin to the east [20
]. Arc volcanism along the Mohakatino Volcanic Centre developed in the northern Taranaki Basin from this time (ca. 18–20 Ma) and continued until ca. 5 Ma [22
Contraction along the eastern basin margin continued until the Mid Miocene (ca. 12 Ma), when oblique subduction and steepening/roll back of the subducting Pacific plate resulted in extension across the northern part of the basin [20
]. Arc volcanism and intra-arc rifting in the northern Taranaki Basin migrated southwards during the Late Miocene to Pliocene to its present location on the Taranaki peninsula (Mt. Taranaki) [21
] (Figure 1
C). From the Latest Miocene, uplift and exhumation of the central North Island and the southern Taranaki Basin resulted in high sedimentation rates, the rapid westward progradation of the shelf, and burial of stratovolcanoes of the northern Taranaki Basin [20
]. The stratigraphic and tectonic framework for the northern Taranaki Basin is shown in Figure 2
The Kora volcano belongs to a chain of buried arc-related stratovolcanoes and intrusions (Mohakatino Volcanic Centre) that developed in the northern Taranaki Basin from the Early Miocene [20
]. This north-northeast-trending volcanic arc was subsequently dissected during rifting associated with the development of the Northern Graben [20
]. Volcanic edifices of the Mohakatino Volcanic Centre cover about 20% of the Northern Graben area [20
]. Radiometric dating and seismic reflection mapping tied to nearby exploration wells indicate that Kora developed between 19.5 and 5.5 Ma [22
Petroleum exploration of the Mohakatino Volcanic Centre started in the 1960s with acquisition of 2D seismic data and drilling of the first well, targeting an off-shore volcanic center (Mangaa-1) in 1970 [12
]. The first well into the Kora volcanic structure (Kora-1) was drilled in 1987 by Arco Petroleum NZ Ltd. The target of this drilling operation was a structural high within Eocene sandstones (Tangaroa Formation). However, an oil productive interval was discovered within Miocene volcanoclastics [28
]. Therefore, based on the estimated hydrocarbon accumulations and reservoir rock properties, the Miocene volcanics became an attractive target for further exploration around the Kora volcano and a further three wells were drilled (Kora-2, Kora-3, and Kora-4).
Kora-1 well penetrated the Eocene to present day stratigraphic sequence represented by deep to shallow marine sediments and volcanoclastics, reaching a depth of 3421 m. Seismic reflection data and mapping show the Kora stratovolcano overlying deformed Early Miocene strata and onlapped by Pliocene-Recent strata (Figure 3
]). The oldest stratigraphic units encountered include several hundreds of meters of Eocene Turi Formation, which is predominated by shales with siltstones and sandstones. This unit is directly overlain by late Eocene-early Oligocene turbidite sandstones of the Tangaroa Formation submarine fan complex [32
]. The Oligocene-early Miocene section is represented by deep-water limestones of Tikorangi and Taimana Formations. Series of pyroclastic deposits of andesite composition and volcanoclastics related to Miocene volcanic activity represent the equivalent of Mohakatino Formation in the region and possess good reservoir rock properties in several wells drilled around the Kora volcano. This volcanoclastic interval is underlain by up to 1000 m of claystones of Manganui Formation and overlain by a thick layer of progradational Giant Foresets Formation [33
The main target of the drilling of the Kora-1/1A wells, the Eocene Tangaroa Formation was found to contain hydrocarbons [34
], but in a too low permeability reservoir rock to be commercially productive. However, in the Kora-1A well, petroleum was produced from the Miocene volcano-sedimentary section with an average flow rate of 668 barrels of oil per day for several days. The other wells either encountered oil shows in Miocene volcanics or were dry. During a drill stream test in well Kora-3, only formation water was produced and core saturation analysis of the well Kora-2 indicated the presence of only residual oil in the formation [34
Geochemical analyses indicate that the oils from the Kora discovery were expelled from the Late Paleocene Waipawa Formation [37
]. The Waipawa Formation is a widespread source rock in New Zealand basins, represented by about 2 to 80 m thick marine mudstone. Higher contributions of terrestrial (mainly woody) plant matter are indicated by higher land plant biomarkers and high phenol/naphthalene ratios [39
]. The presence of a Late Paleocene source rock in the study region is confirmed by Ariki-1 well, where the Waipawa Formation is represented by a 12 m thick organic-rich marine shale located at depth of 4122–4134 m with total organic carbon (TOC) averaging 3.6 wt% [40
]. A maximum TOC value of 11.5 wt% was derived for the samples from Ariki-1 well [41
]. The Waipawa Formation is mainly characterized by terrigenous organic matter [39
] but HI values are partly above 400 mg/g TOC falling into the field of type II and type II/III source rock. This is consistent with general marine nature of the Waipawa Formation, but with a major contribution of terrestrial – derived woody material [39
We present the results of a seismo-stratigraphic interpretation and 1D and 3D basin modeling, that is used for exploring the effect of magmatic events on the thermal history and petroleum generation around the Kora volcano. The modeling study represents the first investigation of potential interaction of the Waipawa Formation and magmatic intrusion in 3D in the northern Taranaki Basin, New Zealand. The thermal history of the study area was constrained using vitrinite reflectance data from four wells, with two of them intersecting the flank of the Kora structure, and two located within 15 km from it, as well as knowledge of the tectonic settings. Distinct scenarios of the intrusion emplacement and its timing provide information on the timing of hydrocarbon expulsion and illustrate the interaction of hydrocarbon generation and widespread magmatic activity during the Late Miocene time.
The modeling results show that the Waipawa Formation was thermally immature before the magmatic activity started in the late Miocene. Modeled magmatic intrusion in the Paleocene and Late Cretaceous sediments (P00 and K90) at 11 Ma caused rapid heating of the surrounding sediments and maturation of the regional source rock (Waipawa Formation). Depending on the proximity of the source rock to the intrusion, the Waipawa Formation became overmature or was in the gas window directly after the magmatic activity ceased. The scenario with magmatic intrusion placed in the basement (basal heat flow increase) shows similar results. A sensitivity study involving variations of the intrusion age (18 Ma, 11 Ma and 8 Ma) indicates that Waipawa Formation would experience higher temperatures if magmatic activity occurs at 8 Ma due to the coupling of two events strongly influencing the thermal history: magmatic activity, and rapid subsidence related due to deposition of Giant Foresets Formation. The same magmatic event modeled at 11 and 18 Ma has less effect on the thermal regime at the level of source rock because the system cooled down more rapidly.
3D modeling results show that the lateral effect of magmatic activity has a local effect on the maturation of the source rock. The thermal aureole caused by volcanic activity at the level of the source rock gradually decreases from its center influencing the area in a radius of 5 km, with a strong impact being restricted to 1–2 km.
The modeling results suggest that most of the hydrocarbons expelled from the Waipawa Formation due to local magmatic heat escaping to the surface, because at the time of expulsion there was no well-developed seal rock that could prevent hydrocarbon leakage (Figure 19