Evolution of the Taranaki Basin - Hydrocarbon ...

Evolution of the Taranaki Basin - Hydrocarbon Maturation and Migration with Time RA Wood1, RH Funnell1, PR King1, ER Matthews2, GP Thrasher3, SD Killops1 and PG Scadden1 1 2 3

Institute of Geological and Nuclear Sciences Limited, PO Box 30-368, Lower Hutt New Zealand Oil and Gas Limited, 76 Berry Street, North Sydney, Australia GeoSphere Exploration Services Limited, PO Box 44-285, Lower Hutt

Abstract The Taranaki Basin has undergone several phases of deformation, with multiple phases of deposition and uplift. This complex history has led to a complex temporal and spatial pattern of hydrocarbon maturation and migration. Integration of structural and paleogeographic basin mapping with analysis of regional petrophysical, geothermal and geochemical data has allowed us to model the generation and migration of hydrocarbons. One of the primary inputs for the project is the set of structural and isopach maps of the basin. Mapped sedimentary units are Rakopi (Late Cretaceous), North Cape (Late Cretaceous), Paleocene to Oligocene, Miocene, and Pliocene to Pleistocene. Isopach data are gridded on a 1 km grid using a Geographic Information System (GIS), and decompacted according to estimated lithology. In addition, paleogeographic maps are used to predict the distribution of source rocks, reservoirs and paleo-bathymetry. Erosional events in the Late Miocene and Pliocene are identified and the amount of missing section mapped. Hydrocarbon generation from Late Cretaceous terrestrial source rocks is determined using a kinetic approach applied to each 1 km2 cell using 1D basin simulation software. Using studies of source rock distribution and organic content, quantitative estimates of the volume of hydrocarbons generated with time are mapped across the basin. Paleo-water depth is estimated from well and seismic data and combined with the decompacted unit thicknesses to predict paleo-structure. These maps are used to predict paleo-migration paths.

Introduction New Zealand’s position astride a plate boundary has meant that the evolution of its sedimentary basins has been complex, often involving radical changes in tectonic and depositional style. The Institute of Geological and Nuclear Sciences Limited (IGNS) has completed regional studies of most of New Zealand’s basins, and has recently begun more detailed projects to investigate the relationships between basin evolution and the formation and migration of hydrocarbons. This paper discusses the formulation of techniques and includes illustrations of their application to parts of the Taranaki Basin. Hydrocarbon maturation and expulsion is a complex function of source rock type and burial and thermal history. Standard 1D maturation modelling requires knowledge of thermal regime, tectonic history, sediment types, rock properties, source rock geochemistry and sedimentation rate to predict the timing and volumes of

hydrocarbon generation and expulsion. These parameters are usually reasonably well known for a well site, but the results are sensitive to changes in input parameters which can change relatively rapidly laterally, (for example sedimentation rate). Understanding the hydrocarbon potential of a basin therefore requires the extrapolation of model input parameters to areas beyond the well locations. Techniques to do this on a basin scale are discussed in this paper. Major structural changes, such as the tectonic inversion of half-grabens seen in the Taranaki Basin, have a significant impact on the timing of source rock maturity and predicted hydrocarbon migration paths. This paper also discusses techniques to estimate paleo-migration pathways.

1998 New Zealand Petroleum Conference • 30 March-1 April 1998

Structure, Isopach and Paleogeographic Data

Source Rock Characteristics, Thermal Regime and Kinetics

Prediction of the timing and distribution of hydrocarbon maturation, and of the timing and pattern of hydrocarbon migration, is based on basin-wide structure, paleogeography, heat flow and geochemical data, most of which have been compiled as part of IGNS’s Cretaceous-Cenozoic Project (King and Thrasher 1996). The structure and paleogeography used for the illustrations in this talk are taken from the Taranaki Atlas map series (Thrasher et al 1995).

Primary hydrocarbon source rocks in Taranaki Basin are non-marine, perhydrous coals, with a minor contribution from organic matter of marginal marine origin (Killops et al 1994, King and Thrasher 1996). Several coal-bearing formations are present in the Taranaki Basin, including the Late Cretaceous Rakopi and North Cape formations, the Paleocene Farewell Formation, and the Eocene Kaimiro and Mangahewa formations. To date in this study, only Late Cretaceous Rakopi Formation is suitably mapped across the basin and is the only source rock considered.

Isopach maps available for the Taranaki Basin are Rakopi Formation (Late Cretaceous), North Cape Formation (Late Cretaceous), Paleogene, Oligocene-Miocene, and PlioPleistocene. For the modelling project, the digital contours are converted to surfaces on a 1 km2 grid and imported into a Geographic Information System (GIS). To accurately model burial history for each 1 km2 cell, grids are also created describing the age and lithologic composition of each layer. Lateral variations in sediment type are based on paleogeographic maps and chronostratigraphic diagrams (King and Thrasher 1996). The age and distribution of erosional events and thickness of eroded strata are input as grids in exactly the same way as the units mapped in the Atlas. The amount of erosion is estimated from porosity-depth relationships observed in wells, apatite fission track results, sediment maturity indicators, and seismic reflection interpretations. We interpret two significant erosion events, one in the late-Miocene and one in the late-Plio-Pleistocene. Miocene sediments are modelled to be originally deposited in a simple eastwards-thickening pattern reflecting development of a foreland basin. The map of Miocene erosion is determined by removing the effect of deposition of PlioPleistocene sediments (decompaction) on the present-day distribution of Miocene sediments, and subtracting the result from the assumed pre-erosion pattern. The late-Plio-Pleistocene erosion event is modelled as regional northeast-southwest tilting across the Taranaki peninsula and is determined primarily from porosity-depth trends in wells. This event merges with the Miocene event in the northern part of the basin, making their discrimination difficult. Structure maps generated from isopachs and paleo-water depths are used to reconstruct the evolving basin through time. Each time-stratigraphic interval is backstripped and decompacted to produce structural maps at specific times in the basin’s history. The present decompaction modelling is based on general porosity-depth relationships typical of Taranaki sediments (Funnell et al 1996). Paleobathymetry, though not important for maturation calculations, is a critical component of the paleo-structure analysis to predict migration pathways. Values for water depths were derived from the paleogeographic maps, assuming the shelf edge to be 200 m in depth, upper bathyal to be 200-600 m, mid bathyal to be 600-1000 m, lower bathyal to be 1000-2000 m, and abyssal to be >2000 m.

The hydrocarbon potential of these kerogens is thought to be represented by the hydrogen index (HI=S2/TOC) determined from Rock-Eval analysis. HI values for coals, when plotted against maturity such as Rank(S), exhibit a maximum of ca. 300 kg/tCorg at a Rank(S) of about 12-13, as shown in Figure 1 (King and Thrasher 1996; Suggate and Boudou, 1993; Killops et al, this volume). The hydrocarbon potential of Rakopi coals, such as in Maui-4, could be as low as 100 kg/tCorg if the Rock-Eval S2 parameter overestimates the potential at maturities approaching the onset of expulsion, or as high as 300 kg/tCorg if the maximum of HI is the true measure of hydrocarbon potential. The lower value is unlikely as these coals are known to have generated oil in Taranaki Basin, and modelling indicates that at 100 kg/tCorg little or no expulsion of hydrocarbons would occur. Stoichiometric estimates of the amount of hydrocarbons generated from an average New Zealand coal during catagenesis fall in the range 200-290 kg/tCorg (Killops et al 1996), which is consistent with the maximum value HI attained on Figure 1. Thus the value for HI of 300 kg/tCorg is applied as the total hydrocarbon potential yield for Rakopi Formation source rock. Our approach is to assume expulsion is controlled by the ability of kerogen to adsorb and absorb hydrocarbons (Cooles et al 1986; Mackenzie and Quigley, 1988; Pepper and Corvi 1995). Hydrocarbons are retained within coals until their concentrations exceed the sorbtive capacity of the residual organic carbon. The onset of significant oil expulsion is thought to occur at maturity levels roughly equivalent to the maximum recorded values of S1/TOC, given that the Rock-Eval S1 parameter measures the amount of oil generated but still retained in the source kerogen matrix. Studies of New Zealand coals indicate that hydrocarbon expulsion occurs once a saturation of about 40 kg/tCorg is reached (Figure 1), with initial bitumen levels at about 10 kg/tCorg (Killops et al this volume). Until more detailed studies are completed on petroleum potential and expulsion threshold variations across the basin, these constant values are used in the modelling. Geographic variations in TOC are able to be included in the modelling by contouring TOC as an extra grid associated with each source rock interval, but has not yet been incorporated within the modelling.


Thermal models are similar to those developed for the Taranaki Basin and described in Funnell et al (1996) and

Armstrong et al (1996). Following rifting, which is

Taranaki Basin Coals 500

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Rank(S) Figure 1. Rock-Eval hydrogen index (HI=S2/TOC) and bitumen index (BI=S1/TOC) maturity (Suggate Rank) trends for isolated coals from exploration wells in Taranaki Basin. The maximum observed in HI matches general New Zealand coal trends published by Suggate and Boudou (1993). described by a grid for crustal extension or b factor, the basal heat flow (at the base of the lithosphere) is kept constant at a value of 40 mW/m2. Areas of the basin experiencing a change in thermal regime during the Miocene, due to the effects of the subducted Pacific plate, have basal heat flow controlled by an additional grid. Sensitivity analyses indicate that the maturation results are relatively insensitive to small variations in thermal regime, and that sedimentation history and uplift are more critical factors affecting maturation. Kinetic parameters are used to describe the rate of reaction for conversion of kerogen to hydrocarbons, representing terrestrial type III kerogens. Results from bulk hydrocarbon laboratory pyrolysis experiments on Rakopi Formation coal samples from the Taranaki Basin (IGNS unpublished data) are used in calculating the amount of hydrocarbon generation with respect to burial depth and thermal history. These parameters lie within the range described by type III, organofacies DE and F kerogens published by Pepper and Corvi (1995) and predict earlier maturity than kinetic parameters for type III kerogens

published by Tissot et al (1987). Figure 2 shows predicted hydrocarbon generation, or transformation ratio, as a function of depth and temperature using different kinetic parameters. This example compares kinetic parameters for a site with constant sedimentation through time and a constant geothermal gradient of 30oC/km. Until full compositional kinetic parameters are available an assumed gas-oil ratio (GOR) of 0.67 is applied to hydrocarbon generation. For such a GOR and an inferred total potential (HI) of 300 kg/tCorg, the expulsion threshold corresponds to a transformation ratio (proportion of available kerogen converted) of 0.22, a vitrinite reflectance value of about 0.8% and Rank(S) of about 12. Further work is required to model the possible contribution from marine-influenced source rocks, and in determining full compositional kinetic parameters for all source rocks found in the basin.

Burial History and Maturity Modelling An interactive 1D finite element basin simulator, BASSIM (modified from Willett, 1988), described by Armstrong et


al (1996), is used to compute conductive heat transfer and

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Depth (m) Figure 2. A comparison of using different kinetic parameters on the maturity with depth or temperature profile. This example represents maturity for a site having undergone constant sedimentation through time with a constant geothermal gradient of 30oC/km and a surface temperature of 20oC. Kinetic parameters are for bulk hydrocarbons from a sample of Rakopi Formation (Maui-4 at 3304m; K Peters, pers comm), standard IFP type III data for oil and gas generation (Tissot et al 1987), and BP type III data for oil and gas generation from organofacies DE and F (Pepper and Corvi, 1995). the basin grid. In this way, a maturity (transformation ratio) profile is calculated through the source rock interval for each 1 km2 cell. Primary control on the maturity calculations comes from maturity data acquired from petroleum exploration wells. Figure 3 shows an example of the results of this modelling for one site. Using a 1D conductive heat flow program in a 3D sense to determine source rock maturity has some limitations, especially in basins where lateral heat transfer exists. Perturbations to the 1D conductive heat transfer model may occur due to fluid flow, refraction across thermal conductivity boundaries such as faults, and fault- or diapiric-related heat advection. In Taranaki Basin, such processes are assumed to have a minor effect on subsurface temperatures; however, further work is presently being undertaken to study such processes using 2D models. In order to estimate volumes of oil (cf gas) expelled from source rocks with a GOR of 0.67, a simplifying assumption is applied, based on bulk hydrocarbon kinetic parameters, whereby an upper limit of 0.6 is used for the transformation ratio for oil generation. The transformation ratio or maturity profile is simply combined with source rock potential and total rock volume in each cell, normalised to the bulk organic content of the formation, and converted to barrels of oil according to

HC vol =

TOC [TR. HI o - S r ]. ρ rock .vol source / ρ HC 100

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where HC vol is the volume of expelled oil (bbl), TOC is the percentage of total organic carbon, HIo is the initial hydrogen index for the source rock, TR is the transformation ratio determined from thermal modelling using kinetic parameters for total hydrocarbons, Sr is the retained hydrocarbons in the coal (kg/t), vol source is the volume of source rock associated with a particular transformation ratio (m3), and rrock and rHC are the densities of source rock and hydrocarbons (kg/m3). A value of 800 kg/m3 is used as the density of oil. Although latest models allow for time-transgressive unconformities, initial models assumed that the missing Late Miocene sediments were deposited between 9 and 5 Ma, that the subsequent erosion event occurred between 5 and 3 Ma, and that missing Plio-Pleistocene sediments were deposited between 1 and 2 Ma and eroded between 0 and 1 Ma. Volumes of hydrocarbons expelled were determined by interpolating maturity (transformation ratio) profiles between available wells and a number of pseudowells sited at critical points. The parameters of these pseudo-wells were derived from the regional mapping and data form nearby wells.


Figure 4 shows some initial results of modelling an area south and west of the Maui Field. The predicted expulsion at time of maximum depth of burial and assuming a Rakopi Formation TOC of 9% is more than 600 million barrels per square kilometre in the Fresne half-graben.


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Figure 3. Results of maturation modelling at an arbitrary site on the grid showing burial history, and predicted temperature, vitrinite reflectance, Rank (S) and total hydrocarbons generated for the Rakopi Formation. Kerogen converted or transformation ratio refers to total hydrocarbons generated resulting from using two sets of kinetic parameters, IFP type III (Tissot et al 1987) and those derived from analysis of a sample of Rakopi Formation coal from Maui-4. Vitrinite reflectance is modelled based on the kinetics of Burnham and Sweeney (1989). The estimated total volume of oil expelled in this area is 189 billion barrels, 119 billion barrels expelled prior to maximum depth of burial (by the end of the Miocene), and 70 billion barrels since then. The results are quite sensitive, however, to the saturation threshold and kinetic parameters used, and to the contributing effect of differing burial histories. For example, an increase of 50% in the saturation threshold to 60 kg/tCorg would result in approximately a 25-35% decrease in expelled volumes, depending on burial history. Use of differing kinetic parameters, such as IFP type III

(Tissot et al 1987), rather than those derived from Rakopi Formation samples, would cause an 80% decrease in areas of early-mature source rocks and a 40% decrease in estimated expelled volume estimates for areas containing more mature source rocks.

Migration Modelling Initially, hydrographic modelling tools within our GIS (Arc/ Info) have been used to model hydrocarbon flow. The major limitation of these tools is that they do not allow onward migration once a trap is filled or along a


Figure 4. Predicted volumes of expelled hydrocarbons from Rakopi Formation source rocks at about 5 Ma, the time of maximum depth of burial. Transformation ratios for each grid cell were interpolated from models run at the wells and pseudo-wells (PW-1, PW-2, etc). porosity boundary, hence the results are a simplification of general flow directions. The migration vectors have been related to the predicted expulsion volumes to predict relative volumes of migrated hydrocarbons. Figure 5 shows a map of predicted direction and volume of hydrocarbon migration in the area west and south of the

Maui field. The modelling assumes that migration took place in the Eocene D Sand, that the D Sand structure controlling the migration direction was similar to the present-day structure of the base of Tikorangi Formation, and that hydrocarbons were sourced from the Rakopi Formation. The results show that some hydrocarbons could have flowed east into the Maui


Figure 5. Predicted migration pathways in the D Sand for hydrocarbons expelled from Rakopi Formation source rocks in the area west of the Maui Field. The size of the points reflects the amount of predicted volumes of hydrocarbon expulsion. The shaded area indicates the extent of the D Sand. NZOG’s mapping of the structure and thickness of the Rakopi Formation, and the present-day structure of basement and base Tikorangi Limestone were used in this model. The extent of the Tikorangi D Sand was based on NZOG’s mapping. structures, and that large amounts could have flowed northeast into structures along the Maui trend. This model predicts that the largest volumes of expelled hydrocarbons would have flowed into structures southwest of Kiwa or into structures or stratigraphic traps along the northwest D Sand boundary. Figure 6 shows the predicted migration pattern using the top Eocene structure at 5 Ma, the time of maximum depth of burial, for the same source distribution as in Figure 5. The regional structure at that time was a broad syncline, plunging to the northeast, and the migration paths reflect

this gradient. A northeast-southwest striking ridge between Te Whatu-2 and Kiwa-1 blocks further migration to the southeast. The predicted flows shown in these figures are supported by the occurrence of hydrocarbons in the Maui wells, and absence of hydrocarbons in Kiwa-1.

Conclusions The availability of digital gridded surfaces for geologic horizons allows numerical modelling of basin evolution


Figure 6. Predicted migration pathways in the top Eocene for hydrocarbons expelled from Rakopi Formation source rocks in the area west of the Maui Field. The size of the points reflects the amount of predicted volumes of hydrocarbon expulsion. The shaded area indicates the extent of the D Sand. NZOG’s mapping of the structure and thickness of the Rakopi Formation, and IGNS’s mapping of the present-day structure of the top Eocene were used in this model. The extent of the Tikorangi D Sand was based on NZOG’s mapping. and of the generation of hydrocarbons. Integration of paleo-structure with the predicted history of hydrocarbon generation allows qualitative predictions of the timing and volume of hydrocarbon expulsion, and the direction and volume of hydrocarbon migration. The technique is limited by its use of a 1D conductive heat flow algorithm, although processes such as heat refraction and convective fluid flow at deep levels in Taranaki Basin are likely to have a relatively minor effect on subsurface temperatures.

Critical factors to predicting volumes of hydrocarbons expelled include knowledge of the timing and distribution of the depositional and erosional events, and use of appropriate kinetic parameters, generation potential, organic richness and saturation thresholds for source rocks. Burial history plays a significant role in determining the timing of hydrocarbon expulsion. Knowledge of erosional events (estimates of the amount of section removed and of the time represented by both the missing and eroded sections) is difficult to determine but is essential for predicting the burial history.


Migration paths are a function of structure and lithology/porosity. Present-day migration paths can be predicted from the mapped structures and paleogeography. Paleo-migration paths can be inferred by progressively backstripping and decompacting the sedimentary layers. Knowledge of the layer lithologies is important for both the decompaction and for the prediction of porosity/permeability, and can be interpreted from the paleogeographic maps. Estimates of paleo-bathymetry are critical for predicting paleo-structure in areas such as Taranaki where water depth varies across the area of interest and there have been large changes in water depth with time. Further developments of these techniques will incorporate improved seismic mapping and geochemical characterisation of source rocks, the determination of full compositional kinetic parameters, greater detail in the litho-stratigraphic units mapped, studies into the effects of 3D heat transfer processes, and more sophisticated migration modelling algorithms.

Acknowledgments The authors thank Dr Ken Peters (Mobil Oil) for pyrolysis data on kinetic parameters, and New Zealand Oil and Gas for their financial support and access to proprietary data. This research is funded by the New Zealand Foundation for Research , Science and Technology under contract C05608 and NSOF contract GNS-045.

References Armstrong, P.A., Chapman, D.S., Funnell, R.H., Allis, R.G. and Kamp, P.J.J. 1996. Thermal modelling and hydrocarbon generation in an active-margin basin: Taranaki Basin, New Zealand. American Association of Petroleum Geology Bulletin, 80: 1216-1241. Burnham, A.K. and Sweeney, J.J. 1989. A chemical kinetic model of vitrinite maturation and reflectance, Geochem. Cosmochem. Acta, 53: 2649-2657. Cooles, G.P., Mackenzie, A.S. and Quigley, T.M. 1986. Calculation of petroleum masses generated and expelled from source rocks. Advances in Organic geochemistry, 10: 235-245.

Killops, S.D., Woolhouse, A.D., Weston, R.J. and Cook, R.A. 1994. A geochemical appraisal of oil generation in the Taranaki Basin, New Zealand. American Association of Petroleum Geology Bulletin, 78: 1560-1585. Killops, S., Allis, R.G. and Funnell, R.H. 1996. Carbon Dioxide generation from coals in Taranaki basin, New Zealand: Implications for petroleum migration in southeast Asian tertiary basins. American Association of Petroleum Geology Bulletin, 80: 545-566. Killops, S., Funnell, R.H., Suggate, P., Sykes, R., Cook, R.A., Peters, K.E., Walters, C., Woolhouse A.D., Weston R.J. and Boudou J.P. 1998. Assessing oil generation and expulsion from New Zealand coals. In 1998 New Zealand Petroleum Conference Proceedings, (this volume). Ministry of Commerce. King, P.R., Thrasher, G.P. 1996. Cretaceous-Cenozoic geology and petroleum systems of the Taranaki Basin, New Zealand. Institute of Geological and Nuclear Sciences Ltd monograph 13. Lower Hutt. Mackenzie, A. S. and Quigley, T.M. 1988. Principles of geochemical prospect appraisal, American Association of Petroleum Geology Bulletin, 72: 399-415. Pepper, A.S. and Corvi, P.J. 1995. Simple kinetic models of petroleum formation. Part 1: oil and gas generation from kerogen. Marine Petroleum Geology, 12: 291-319. Suggate R.P. and Boudou J.P. 1993. Coal rank and type variation in Rock-Eval assessment of New Zealand coals. Journal of Petroleum Geology, 16: 73-88. Thrasher, G.P., King, P.R., Cook, R.A. 1995. Taranaki Petroleum Atlas. Institute of Geological and Nuclear Sciences Ltd, Lower Hutt. Tissot, B.P., Pelet, R. and Ungerer, P. 1987. Thermal history of sedimentary basins, maturation indices, and kinetics of oil and gas generation, American Association of Petroleum Geologists Bulletin, 71: 1445-1466. Willett, S.D. 1988. Spatial variation of temperature and thermal history of the Uinta Basin, PhD Thesis, University of Utah, Salt Lake City.

Funnell, R., Chapman, D., Allis, R. and Armstrong, P. 1996. Thermal state of the Taranaki Basin, New Zealand. Journal of geophysical research 101, B11: 25197-25215.

Authors Ray Wood is a programme leader with the Institute of Geological and Nuclear Sciences Limited. His research interests are in basin modelling and tectonic analysis. He is also interested in the application of geographical information systems to these areas. Ray has a BA in geology from Dartmouth College and a MA in geology from the University of Texas, Austin.


Rob Funnell is a geophysicist with the Institute of Geological and Nuclear Sciences Limited, specialising in well log analysis, and modelling of thermal and pressure regimes, fluid flow, and petroleum generation in sedimentary basins. Rob has an MSc in Physics from the University of Waikato, and a MSc in Geophysics from the University of London. Peter King is a sedimentary basin analyst and stratigrapher with the Institute of Geological and Nuclear Sciences Limited. He is the senior author of two monographs on the Taranaki Basin, and is currently working on stratigraphic correlations and tectonic reconstructions nationwide. Peter has a MSc (Hons) in Earth Sciences from the University of Waikato. Eric Matthews PhD, MSc, BSc has been employed by NZOG since 1982 and has extensive experience in Australasia working primarily in the Taranaki and the Carnarvon basins. He is currently Group Exploration Manager, based in Sydney. Glenn Thrasher is a co-founder of GeoSphere Exploration Services Limited, and previously held a scientific position with the Institute of Geological and Nuclear Sciences Limited and its antecedents for several years. GeoSphere undertakes consulting projects as well as participating directly in exploration projects, and is operator for the Petroleum Exploration Permit 38464 Joint Venture in Taranaki Basin. Phil Scadden is GIS Data Manager/Programmer with Resource Group of the Institute of Geological and Nuclear Sciences Limited. Beside his role in managing research databases for the group, he is involved in programming and modelling support for many of the science projects. Phil has a BSc(Hon) from University of Otago.
