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PVTX history of the North Sea's Judy oilfield
Journal of Geochemical Exploration 69–70 (2000) 641–644 www.elsevier.nl/locate/jgeoexp
PVTX history of the North Sea’s Judy oilfield A.C. Aplin a,*, S.R. Larter a, M.A. Bigge a, G. Macleod a, R.E. Swarbrick b, D. Grunberger b a
NRG, Drummond Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK b Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK
Abstract Volumetric liquid:vapour ratios generated using a Confocal Laser Scanning Microscope are used along with PVT modelling software to estimate the composition, PT phase envelope and isochore of single petroleum inclusions in the North Sea’s Judy field. The data are used to quantify pressure histories and to determine the compositional evolution of petroleum in the field. Important constraints are also placed on the permeability structure and history of the overlying chalk. Key conclusions are: (1) quartz cementation and the emplacement of low Gas–Oil Ratio oil occurred 1–3 Ma ago in a significantly overpressured regime; (2) the present gas condensate was formed by the injection of gas-rich petroleum into a specific part of the field. Analysis of other North Sea fields suggests that many gas condensates form by the engulfment of oils by later gas. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Petroleum; fluid inclusion; palaeopressure; confocal laser scanning microscopy
1. Introduction The determination of geological pressure has proved a difficult problem. One approach is to use fluid inclusions in diagenetic minerals, which preserve the pressure at which they were trapped. Although the pressure cannot be directly measured, minimum trapping pressures of both aqueous and petroleum inclusions can be determined from the PT phase diagram of the fluid and, as shown by Roedder and Bodnar (1980), true trapping pressures can in principle be determined where petroleum and aqueous fluid inclusions coexist (Fig. 1). The PT phase diagram and isochore of an included fluid can be constructed using appropriate Equations of State if the composition of the fluid is known. * Corresponding author. Tel.: ⫹ 44-191-222-2605; fax: ⫹ 44191-222-5431. E-mail address: [email protected] (A.C. Aplin).
However, quantitative analysis of a single diagenetic inclusion is impossible, and micro-spectroscopic data are insufficiently detailed to generate accurate PVT models. We have recently reported a completely new, non-destructive approach to approximate the composition of petroleum in individual fluid inclusions (Macleod et al., 1996; Aplin et al., 1999). We use Confocal Laser Scanning Microscopy to generate pseudo-3D images of individual petroleum fluid inclusions. Under the laser, liquid petroleum fluoresces, distinguishing the liquid and vapour and allowing calculation of the volumetric ratio of liquid to vapour. Using commercial PVT modelling software (we used PVTSIM and its sister program, VTFLINC; Pedersen et al., 1989), the liquid:vapour ratio is used along with the homogenisation temperature and an initial estimate of the petroleum composition, derived from fluids elsewhere in the basin, to estimate the bulk composition, phase envelope, isochore and minimum saturation pressure of the included petroleum (Fig. 1). The true
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00066-2
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A.C. Aplin et al. / Journal of Geochemical Exploration 69–70 (2000) 641–644
Fig. 1. Phase envelope of a petroleum, showing the two phase line, the critical point (Tc), the isochore of the petroleum and a vertical line representing the homogenisation temperature of a coeval aqueous inclusion. The true trapping pressure and temperature occur where the petroleum isochore interesects the homogenisation temperature of the aqueous inclusion.
trapping temperature and pressure can be determined if an aqueous inclusion formed coevally with a petroleum inclusion (Roedder and Bodnar, 1980). The petroleum is then assumed to be saturated with water and the water assumed to be saturated with petroleum. In this circumstance, the temperature at which water homogenises into petroleum and the temperature at which petroleum homogenises into water give the true trapping temperatures of the inclusions. In this paper we show an example of the kinds of data and information that the CLSM technique generates. By combining fluid inclusion data with modelled time–temperature histories, we construct the pressure and petroleum compositional history of the North Sea’s Judy oilfield. 2. Geological setting Judy is located on a horst in the North Sea’s Central
Fig. 2. Time–temperature and pressure–temperature histories of Judy Field. The fluid inclusions record the evolution of pressure over the rapid phase of burial between 3 and 1 Ma. Solid line in the upper figure is fluid pressure modelled using Petromod using fluid inclusion pressure data as constraints on the permeability structure and evolution of the sedimentary sequences.
A.C. Aplin et al. / Journal of Geochemical Exploration 69–70 (2000) 641–644
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Ranges of homogenisation temperatures of petroleum inclusions were 95–111⬚C (Well A), 57–89⬚C (B), 102–121⬚C (C) and 97–106⬚C (D). All the inclusions homogenised into a single phase liquid and were therefore trapped as oil. The homogenisation temperatures of coexisting aqueous inclusions were 15–20⬚C greater than those of the petroleum inclusions. Temperatures were converted into time using a temperature history modelled using Platte River’s BasinMod 1D thermal modelling software, constrained using the surface temperature history and the basement heat flow history (Fig. 2). 4. Evolution of fluid pressure and composition Fig. 3. Calculated Gas–Oil Ratio GOR C1–5 =C6⫹ of present-day and palaeopetroleum in Judy, plotted against temperature. Fluid inclusion homogenisation temperature data have been corrected by ⫹20⬚C to estimate true trapping temperatures.
Graben. Its burial history (Fig. 2) is typical of that for the Central North Sea and is characterised by very rapid subsidence over the last 3 Ma. The present reservoir temperature is around 145⬚C and the surrounding depositional lows contain mature to overmature Kimmeridge Clay source rocks. Above the Triassic reservoir section a thin layer of Kimmeridge Clay is overlain by ⬃700 m of chalk, followed by a 3 km, mud-rich Tertiary section. The present distribution of pressure in the field suggests that most of the field is supported by a common aquifer with an overpressure of around 24 MPa. Variations in oil–water contacts across the field indicate that some faults are currently acting as partial barriers to fluid flow. However, although GORs across the field vary greatly, geochemical data indicate that the composition of the oil’s C6⫹ fraction does not correlate with gas content.
3. Fluid inclusions Petroleum fluid inclusions were found in each of four wells across the field, mainly as secondary inclusions in healed fractures in detrital quartz grains and quartz cement. Aqueous inclusions were less common and only occasionally could we confirm the occurrence of coeval aqueous and petroleum inclusions.
Trapping temperatures for approximately 30 petroleum inclusions range between 115 and 136⬚C, recording a pressure and fluid composition history between 3 and 1 Ma. The palaeopressure data are plotted against temperature in Fig. 2, with lithostatic and hydrostatic pressure histories for context. The present day and palaeopressure data show that the reservoir unit has become increasingly overpressured over the last 12 Ma. Both the emplacement of petroleum and the formation of quartz cement occurred mainly or entirely within an overpressured regime. The geologically recent increase in overpressure is consistent with rapid burial, under which circumstance fluid pressures increase as a result of disequilibrium compaction. The inclusions also record the presence of low GOR petroleum throughout the reservoir between 3 and 1 Ma (Fig. 3). The compositionally distinct petroleum columns seen in the reservoir today are therefore the result of a spatially variable injection of gas-rich petroleum during the last million years. Low GOR petroleum has been retained in the southwestern part of the field, implying that the late charge of light petroleum has been injected from the east and north. Rapid mixing of fluids in Judy is inhibited by the presence of low permeability faults which crosscut the field. 5. Conclusions Palaeopressure and GOR data derived from fluid inclusions show that in the Judy Field:
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A.C. Aplin et al. / Journal of Geochemical Exploration 69–70 (2000) 641–644
1. Quartz cementation occurred within an overpressured environment. 2. Low GOR oil was present throughout the reservoir between 3 and 1 Ma. 3. The pre-existing oil was converted, in parts of the field, to gas condensate or volatile oil as a result of the injection of very gas-rich petroleum. This happened within the last 1 million years.
References Aplin, A.C., Macleod, G., Larter, S.R., Pedersen, K.S., Sorensen, H., Booth, T., 1999. Combined use of Confocal Laser Scanning
Microscopy and PVT simulation for estimating the composition and physical properties of petroleum in fluid inclusions. Mar. Petrol. Geol. 16, 97–110. Macleod, G., Larter, S.R., Aplin, A.C., Pedersen, K.S., Booth, T.A., 1996. Determination of the effective composition of single petroleum inclusions using Confocal Scanning Laser Microscopy and PVT simulation. In: Brown, P.E., Hagemann, S.G. (Eds.), Biennial Pan-American Conference on Research on Fluid Inclusions (PACROFI VI). Madison, WI, pp. 81–82. Pedersen, K.S., Fredenslund, Aa., Thomassen, P., 1989. Properties of Oils and Natural Gases, Petroleum Geology and Engineering, vol. 5, Gulf Publishing Company (No. 5, 252pp.). Roedder, E., Bodnar, R.J., 1980. Geologic pressure determinations from fluid inclusion studies. Ann. Rev. Earth. Planet. Sci. 8, 263–301.
PVTX history of the North Sea’s Judy oilfield A.C. Aplin a,*, S.R. Larter a, M.A. Bigge a, G. Macleod a, R.E. Swarbrick b, D. Grunberger b a
NRG, Drummond Building, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK b Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK
Abstract Volumetric liquid:vapour ratios generated using a Confocal Laser Scanning Microscope are used along with PVT modelling software to estimate the composition, PT phase envelope and isochore of single petroleum inclusions in the North Sea’s Judy field. The data are used to quantify pressure histories and to determine the compositional evolution of petroleum in the field. Important constraints are also placed on the permeability structure and history of the overlying chalk. Key conclusions are: (1) quartz cementation and the emplacement of low Gas–Oil Ratio oil occurred 1–3 Ma ago in a significantly overpressured regime; (2) the present gas condensate was formed by the injection of gas-rich petroleum into a specific part of the field. Analysis of other North Sea fields suggests that many gas condensates form by the engulfment of oils by later gas. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Petroleum; fluid inclusion; palaeopressure; confocal laser scanning microscopy
1. Introduction The determination of geological pressure has proved a difficult problem. One approach is to use fluid inclusions in diagenetic minerals, which preserve the pressure at which they were trapped. Although the pressure cannot be directly measured, minimum trapping pressures of both aqueous and petroleum inclusions can be determined from the PT phase diagram of the fluid and, as shown by Roedder and Bodnar (1980), true trapping pressures can in principle be determined where petroleum and aqueous fluid inclusions coexist (Fig. 1). The PT phase diagram and isochore of an included fluid can be constructed using appropriate Equations of State if the composition of the fluid is known. * Corresponding author. Tel.: ⫹ 44-191-222-2605; fax: ⫹ 44191-222-5431. E-mail address: [email protected] (A.C. Aplin).
However, quantitative analysis of a single diagenetic inclusion is impossible, and micro-spectroscopic data are insufficiently detailed to generate accurate PVT models. We have recently reported a completely new, non-destructive approach to approximate the composition of petroleum in individual fluid inclusions (Macleod et al., 1996; Aplin et al., 1999). We use Confocal Laser Scanning Microscopy to generate pseudo-3D images of individual petroleum fluid inclusions. Under the laser, liquid petroleum fluoresces, distinguishing the liquid and vapour and allowing calculation of the volumetric ratio of liquid to vapour. Using commercial PVT modelling software (we used PVTSIM and its sister program, VTFLINC; Pedersen et al., 1989), the liquid:vapour ratio is used along with the homogenisation temperature and an initial estimate of the petroleum composition, derived from fluids elsewhere in the basin, to estimate the bulk composition, phase envelope, isochore and minimum saturation pressure of the included petroleum (Fig. 1). The true
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00066-2
642
A.C. Aplin et al. / Journal of Geochemical Exploration 69–70 (2000) 641–644
Fig. 1. Phase envelope of a petroleum, showing the two phase line, the critical point (Tc), the isochore of the petroleum and a vertical line representing the homogenisation temperature of a coeval aqueous inclusion. The true trapping pressure and temperature occur where the petroleum isochore interesects the homogenisation temperature of the aqueous inclusion.
trapping temperature and pressure can be determined if an aqueous inclusion formed coevally with a petroleum inclusion (Roedder and Bodnar, 1980). The petroleum is then assumed to be saturated with water and the water assumed to be saturated with petroleum. In this circumstance, the temperature at which water homogenises into petroleum and the temperature at which petroleum homogenises into water give the true trapping temperatures of the inclusions. In this paper we show an example of the kinds of data and information that the CLSM technique generates. By combining fluid inclusion data with modelled time–temperature histories, we construct the pressure and petroleum compositional history of the North Sea’s Judy oilfield. 2. Geological setting Judy is located on a horst in the North Sea’s Central
Fig. 2. Time–temperature and pressure–temperature histories of Judy Field. The fluid inclusions record the evolution of pressure over the rapid phase of burial between 3 and 1 Ma. Solid line in the upper figure is fluid pressure modelled using Petromod using fluid inclusion pressure data as constraints on the permeability structure and evolution of the sedimentary sequences.
A.C. Aplin et al. / Journal of Geochemical Exploration 69–70 (2000) 641–644
643
Ranges of homogenisation temperatures of petroleum inclusions were 95–111⬚C (Well A), 57–89⬚C (B), 102–121⬚C (C) and 97–106⬚C (D). All the inclusions homogenised into a single phase liquid and were therefore trapped as oil. The homogenisation temperatures of coexisting aqueous inclusions were 15–20⬚C greater than those of the petroleum inclusions. Temperatures were converted into time using a temperature history modelled using Platte River’s BasinMod 1D thermal modelling software, constrained using the surface temperature history and the basement heat flow history (Fig. 2). 4. Evolution of fluid pressure and composition Fig. 3. Calculated Gas–Oil Ratio GOR C1–5 =C6⫹ of present-day and palaeopetroleum in Judy, plotted against temperature. Fluid inclusion homogenisation temperature data have been corrected by ⫹20⬚C to estimate true trapping temperatures.
Graben. Its burial history (Fig. 2) is typical of that for the Central North Sea and is characterised by very rapid subsidence over the last 3 Ma. The present reservoir temperature is around 145⬚C and the surrounding depositional lows contain mature to overmature Kimmeridge Clay source rocks. Above the Triassic reservoir section a thin layer of Kimmeridge Clay is overlain by ⬃700 m of chalk, followed by a 3 km, mud-rich Tertiary section. The present distribution of pressure in the field suggests that most of the field is supported by a common aquifer with an overpressure of around 24 MPa. Variations in oil–water contacts across the field indicate that some faults are currently acting as partial barriers to fluid flow. However, although GORs across the field vary greatly, geochemical data indicate that the composition of the oil’s C6⫹ fraction does not correlate with gas content.
3. Fluid inclusions Petroleum fluid inclusions were found in each of four wells across the field, mainly as secondary inclusions in healed fractures in detrital quartz grains and quartz cement. Aqueous inclusions were less common and only occasionally could we confirm the occurrence of coeval aqueous and petroleum inclusions.
Trapping temperatures for approximately 30 petroleum inclusions range between 115 and 136⬚C, recording a pressure and fluid composition history between 3 and 1 Ma. The palaeopressure data are plotted against temperature in Fig. 2, with lithostatic and hydrostatic pressure histories for context. The present day and palaeopressure data show that the reservoir unit has become increasingly overpressured over the last 12 Ma. Both the emplacement of petroleum and the formation of quartz cement occurred mainly or entirely within an overpressured regime. The geologically recent increase in overpressure is consistent with rapid burial, under which circumstance fluid pressures increase as a result of disequilibrium compaction. The inclusions also record the presence of low GOR petroleum throughout the reservoir between 3 and 1 Ma (Fig. 3). The compositionally distinct petroleum columns seen in the reservoir today are therefore the result of a spatially variable injection of gas-rich petroleum during the last million years. Low GOR petroleum has been retained in the southwestern part of the field, implying that the late charge of light petroleum has been injected from the east and north. Rapid mixing of fluids in Judy is inhibited by the presence of low permeability faults which crosscut the field. 5. Conclusions Palaeopressure and GOR data derived from fluid inclusions show that in the Judy Field:
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A.C. Aplin et al. / Journal of Geochemical Exploration 69–70 (2000) 641–644
1. Quartz cementation occurred within an overpressured environment. 2. Low GOR oil was present throughout the reservoir between 3 and 1 Ma. 3. The pre-existing oil was converted, in parts of the field, to gas condensate or volatile oil as a result of the injection of very gas-rich petroleum. This happened within the last 1 million years.
References Aplin, A.C., Macleod, G., Larter, S.R., Pedersen, K.S., Sorensen, H., Booth, T., 1999. Combined use of Confocal Laser Scanning
Microscopy and PVT simulation for estimating the composition and physical properties of petroleum in fluid inclusions. Mar. Petrol. Geol. 16, 97–110. Macleod, G., Larter, S.R., Aplin, A.C., Pedersen, K.S., Booth, T.A., 1996. Determination of the effective composition of single petroleum inclusions using Confocal Scanning Laser Microscopy and PVT simulation. In: Brown, P.E., Hagemann, S.G. (Eds.), Biennial Pan-American Conference on Research on Fluid Inclusions (PACROFI VI). Madison, WI, pp. 81–82. Pedersen, K.S., Fredenslund, Aa., Thomassen, P., 1989. Properties of Oils and Natural Gases, Petroleum Geology and Engineering, vol. 5, Gulf Publishing Company (No. 5, 252pp.). Roedder, E., Bodnar, R.J., 1980. Geologic pressure determinations from fluid inclusion studies. Ann. Rev. Earth. Planet. Sci. 8, 263–301.