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Tuesday, 1 September 2009

The Influence of Surface Temperatures on Source Rock Maturity to Aid Hydrocarbon Discovery

S. Nelskamp, T. Donders, J.-D. van Wees, O. Abbink

In September 2004, the first-ever drilling of the Lomonosov Ridge (Arctic Coring Expedition, ACEX, or IODP Expedition 302, Fig. 1a) recovered unprecedented sedimentary records of the central Arctic Ocean spanning the past - 56 Ma. With paleontological and geochemical techniques it has been possible to document the long-term development of the central Arctic for the first time. The environmental setting and paleo-climatic evolution turn out very different from that expected prior to the drilling operation. With the help of the new paleo-climatic evolution a tectonic paleo-heat flow prediction of the Kara Sea area was performed to show the influence of paleo-temperatures on the temperature and maturity history of that region.

The West Siberian Basin is one of the largest intra-cratonic basins of the world and is also the richest petroleum province of Russia. It covers an area of approximately 2.2 million km2 and is situated between latitude 55 and 75. In order to study the effect of surface temperatures on the maturity of the source rock, a synthetic well in the northern part of the basin, the Kara Sea (Fig. 1a) was created and modeled with 5 different surface temperature curves from different sources. Surface temperature evolution coupled with basin evolution processes determines the geothermal and associated maturity evolution.

Methods PetroProb
To predict geothermal and maturity evolution, a recently developed coupled lithosphere and basin thermal model has been used (PetroProb, Van Wees et al., 2009). PetroProb is capable of calculating tectonic basement heat flows, incorporating a variety of tectonic scenarios (including rifting, underplating, mantle upwelling), and capable of including feedback effects of sedimentation and surface temperature variation on basement heat flow and basin temperatures. The model inverts burial histories, calibrated to temperature and maturity data. Calibration and sensitivity analysis are done through Monte Carlo sampling analysis using an experimental design technique for computational efficiency.

Palynological analyses were used for deriving sediment ages, especially the remains of dinoflagellate cysts (dinocysts) and diatoms were keys in providing a stratigraphic framework. Both dinocysts and terrestrial plants remains (mainly pollen and spores) provide important information on the paleo-climatic evolution of the basin. These analyses are complemented by organic geochemical data that provide origin and (isotopic) composition of organic matter in the sediments. We further employed the newly developed paleo-thermometer TEX86', which is based on the relative distribution of crenarchaeotal membrane lipids (Schouten et al., 2002). Calibration of the TEX86' is based on 104 marine surface sediments and found to correlate very well with annual mean SST: TEX86' = 0.016 x SST + 0.20 with R2 = 0.93. This equation was used to convert TEX'86 into SST.

In petroleum systems modeling the calculation of the maturity of a source rock is mainly dependent on the basal heat flow, the sediment water interface temperature (SWIT), the thermal conductivity and the radiogenic heat production of the rocks in the system. The latter two parameters are usually defined by the used lithologies while the first two are considered user input. With our setup we want to stress the importance of good constraints on these values.

A detailed analysis of palynological proxies leads to a detailed surface temperature curve which can be used as input data for the SWIT curve in petroleum systems modeling. The analysis of the newly acquired data from the arctic at latitude 85 have revealed the successful recovery of the Paleocene - Eocene transition, with the occurrence of an Apectodinium augustum acme and a prominent, 6‰ drop in stable carbon isotopes of bulk organic carbon (d13C TOC) at the Paleocene Eocene Thermal Maximum (PETM) some 55.5 Ma ago (Fig. 1c). This finding contrasts predictions, which had placed the base of the sediment column, above Cretaceous basement, at 50 Ma. During the PETM our dinocyst and TEX86 paleo-thermometer records show combined increased runoff and sea level rise and a subtropical Arctic Ocean, with sea surface temperatures of - 23ºC (Sluijs et al., 2006).

At the early - middle Eocene transition (- 49 Ma) stunning concentrations of remains of the fresh water fern Azolla and freshwater tolerant dinocysts suggest that, at least episodically, completely fresh surface water settings characterized the Arctic Basin (Brinkhuis et al., 2006). During the middle Eocene, shifts in salinity and in ice-rafted debris follow a strong orbital driven cyclical pattern (Sangiorgi et al., 2008a). Moreover, dinocyst stratigraphy was instrumental in recognizing and assessing the - 26 Ma hiatus, which marks the transition from the greenhouse world to the icehouse world (Sangiorgi et al., 2008b). Sediment erosion and/or non-deposition that generated the hiatus were likely due to a progressive shoaling of the Lomonosov Ridge. Above the hiatus, a new Miocene dinocyst genus Arcticacysta (Sangiorgi et al., 2009) and higher than expected sea surface temperatures (15-19ºC) (Sangiorgi et al., 2008b) mark the recovery of sedimentation on the Lomonosov Ridge near the Miocene Climatic Optimum. The Neogene record has relatively low sedimentation rates and perennial glacial conditions starting from 14 Ma, after which the late Pliocene marked the start of continuous glaciation.

Figure 1: A) Arctic Ocean map (modified from International Bathymetric Chart of the Arctic Ocean, Jakobsson et al., 2000), with indication of the Arctic sub-basins and ridges: AR, Alpha Ridge; FS, Fram Strait; GR, Gakkel Ridge; KS, Kara Sea; LR, Lomonosov Ridge; MR, Mendeleev Ridge; MB, Makarov Basin; NB, Nansen Basin; AB, Amundsen Basin; CA, Canada Basin. Star indicates the location of IODP 302 drilling on the LR; B) Location of drilling within the early Eocene paleo-geographical reconstruction of the Arctic Ocean (Brinkhuis et al., 2006) TO, Tethyan Ocean; P-AO, Proto-Atlantic Ocean; NS, North Sea; C) ACEX age model (modified from Backman et al., 2008) with indication of the Lithologic Units (Lith. Unit) and sub-units (Expedition 302 Scientists, 2006). Pictures of the dinoflagellate cysts Apectodinium augustum (1), Phthanoperidinium clithridium (3), Arcticacysta backmanii (4), A. moraniae (5) and the remains of Azolla (2) used as biostratigraphical markers are also shown. The palynological events considered in building the age model in the early Cenozoic are: Last Occurrence (LO) of A. augustum (F), LO of Azolla (E), Last Abundant Occurrence of P. clithridium (D) and the mid point of the Burdigalian stage where A. backmanii and A. moraniae occur (C). The oldest identified paleomagnetic chron datum (top of magnetochron C25n, Chron C25n), (G) deepest Berillium-10 samples (B) and top of the section (A) on which the age model is based are also shown. TD: Terminal Depth. Depth scale in meters composite depth (mcd)

The new surface temperature curve from the Lomonosov Ridge was compared to surface temperatures generated from PetroMod® of IES/Schlumberger for the Eurasian arctic at latitude 72 (Hantschel and Kauerauf, 2009), to data extracted from PetroMod® from a constant latitude of 85 through time and to two surface temperature curves generated from the newly acquired but corrected for the shift of latitude of the study area through time with a factor of 0.2 and 0.4 per degree latitude (Fig. 2). The new surface temperatures show higher temperatures for the Cretaceous, lower surface temperatures during the Paleogene and drastically higher temperatures during the Miocene.

Fig. 2 Different paleo-surface temperature reconstructions. Donders 85, refers to the Lomonosov ridge at 85 Latitude. Adjusted 0.2 and 0.4 have been corrected from the Lomonosov ridge to Latitude 72 corresponding to the Kara Sea., adopting 0.2 and 0.4 C per degree latitude respectively. PetroMod® paleo-surface temperature using a plate reconstruction for the Kara Sea at 72 Latitude and a Latitude of 85 (Hantschel and Kauerauf, 2009)

These surface temperature curves can be directly imported into PetroProb and are automatically corrected for the water depth to generate the correct sediment water interface temperature (SWIT).

The tectonic heat flow model uses 1D wells or 3D depth maps as input for modeling of tectonic subsidence. Further input is water depth evolution of the study area, the sediment composition, lithospheric parameters such as initial thickness and surface temperature. The heat flow is calculated by matching a calculated tectonic subsidence curve to the observed curve from the input data using user defined rift phases in agreement with with tectonic interpretation. Based on the calculated tectonic subsidence curve a heat flow curve is calculated.

In our case study in the Kara Sea, we defined a two stage rift event influencing the tectonic evolution of the area. According to many studies (e.g. Nikishin et al., 2002; Saunders et al., 2005), a rifting event in the Late Permian to Triassic created the West Siberian Basin. The beginning and the maximum duration of rifting is still under discussion. According to Nikishin et al. (2002) the rift event was no longer than 10 Ma while Saunders et al. (2005) argue that the oldest sediments onlapping on the footwalls of the rift faults are around 165 Ma old; the rifting therefore could have lasted up to 85 Ma. Still, modeling the tectonic subsidence with one rift phase lasting from 250 to 165 Ma does not explain the increased subsidence rate after 165 Ma (Fig. 3). Saunders et al. (2005) therefore propose that the main rift phase, accompanied by a mantle plume, lasted only a short while but afterwards the tectonic subsidence due to thermal cooling was inhibited by the mantle plume until approximately 190 Ma. The results shown in figure 3 were achieved by adapting this assumption to the model.

Fig. 3 Observed and modeled tectonic subsidence curve, paleo-water depth curve and resulting paleo-heat flow curve. The two stage rifting event is marked by significant mantle upwelling, characterized by subcrustal stretching values (a) in excess of crustal stretching (b).

This tectonic model was then calculated with different surface temperature curves and the resulting maturity for the source rock was compared. The results from the models show that the Cenozoic surface temperature evolution has a big effect on the source rock maturity. During the Paleogene the new unadjusted surface temperature has maturities in the same range as the PetroMod® curve for latitude 85. The maturity increases drastically in the Early Miocene to show present-day maturities in the same range as the PetroMod® curve for latitude 72. The maturities of the adjusted surface temperature curves have even higher maturities compared to the PetroMod® curves. This difference can have a noticeable influence on the timing of generation and trapping of hydrocarbons.

Fig. 4 Maturity of the top of the source rock interval in the Kara Sea region for the different surface temperature reconstructions (see Fig. 2 for explanation)

The recent ACEX data complement earlier paleobotanical "snapshots" into Neogene development of the Arctic. Plant macrofossils have for years been the only source of inflormation on the paleo-climatic evolution available to researchers, but data have been very limited in terms of stratigraphic range. The new ACEX data as well as recent studies from the Norwegian Sea (Eldrett et al., 2009) and Alpha ridge (Jenkyns et al., 2004) now extend the paleo-climate record further back into the Paleogene and even upper Cretaceous, revealing a warm wet greenhouse world which extended even to the high Arctic. Only the last 14 million years show the persistent influence of glacial conditions.

The modeled differences between the surface temperatures extracted from PetroMod® to the newly acquired, result in lower maturities during the Paleogene but a drastic increase in maturity during the Miocene. Oil and gas generation will be influenced by this. Slow but steady generation during the Cenozoic prevails in the models with the PetroMod® surface temperature curve while rapid generation in the Miocene can be seen in the models with the new surface temperatures. Depending of the timing of the trap formation this can result in either more or less trapped hydrocarbons.

A detailed study of paleo-surface temperatures and tectonic paleo-heat flow can have a huge impact on the modeled source rock maturity and on the timing of generation. Especially in frontier areas where the quality of a source rock is not yet known, it is, therefore, crucial to get a good understanding of the paleo-surface temperature evolution. But, also, in well-studied basins, an analysis of the paleo-surface temperatures can lead to a reevaluation of regions previously considered under- or over-mature and, therefore, deemed unprospective.

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