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Fluid inclusion constraints on temperatures of petroleum migration from authigenic quartz in bitumen veins
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ELSEVJER
ISOTOPE GEOSCIENCE
Chemical Geology 129 (1996) 217-226
Fluid inclusion constraints on temperatures of petroleum migration from authigenic quartz in bitumen veins J. Parnell, P.F. Carey, B. Monson School of Geosciences, Queen’s University, Belfast Bi7 INN, UK
Received 2 May 1995; accepted 23 August 1995
Abstract Solid bitumens in fracture systems commonly contain microscopic crystals of authigenic quartz which were precipitated coevally with the bitumen early in the paragenesis of the fracture-fillings. The quartz contains primary fluid inclusions (both aqueous and hydrocarbon), yielding homogenization temperatures particularly in the range 95-13O”C, indicating minimum temperatures of entrapment during petroleum migration. Volumes of up to 50% entrained quartz, and a predominance of aqueous inclusions, indicate that the petroleum fluid had a substantial aqueous component.
1. Introduction The secondary migration of petroleum through carrier and reservoir beds inevitably involves mixing of petroleum and aqueous fluids. Models for petroleum migration admit only very limited miscibility of the two fluids and migration into reservoirs is largely by buoyancy (England et al., 1987). The coexistence of the two fluids is evinced by the occurrence of hydro’carbon fluid inclusions in mineral cements including quartz (N.B. we follow common practice by reference to hydrocarbon inclusions, but recognise that the contents are petroleum-related and contain compounds additional to true hydrocarbons). The solubility of ions for quartz precipitation is low (10’s of ppm), and requires large volumes of water for transport (Bjorlykke and Egeberg, 1993). In some cases inclusions in cements contain immiscible mixtures of oil
petroleum residues (bitumens) indicate a significant aqueous component to the parent fluid (Parnell et al., 1994a). Our studies of solid bitumens from fracture systems have recorded that crystals of authigenic quartz are a component of bitumen veins in many basins. This quartz is a potential source of information about the composition of the petroleum-rich fluids which precipitated the bitumens. Fluid inclusions within the quartz crystals provide data on the temperature and broad-scale chemistry of the fluids. Quartz has a high potential to yield such data because it commonly contains fluid inclusions which are susceptible to examination and measurement. The quartz crystals contain almost exclusively primary inclusions, as recorded in other studies of quartz euhedra in sedimentary and vein environments (Murray, 1957; Touray and Barlier, 1975). A further advantage of quartz is that in comparison to other low-temperature minerals such as calcite it is less likely to be stretched during heating above the entrapment temperature
0009-2541/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SUN 0009.2541(95)0’Dl41-7
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129 (1996) 217-226
(Robinson et al., 19921, although some workers (e.g., Osborne and Haszeldine, 1993) believe that inclusions in quartz are susceptible to resetting. We report here data from authigenic quartz crystals extracted from fracture-bound bitumens in basins in North America and Europe.
2. Methodology Many of the quartz crystals are entirely suspended within a matrix of bitumen: they are perfect euhedra which are not attached to vein walls and have no identifiable nucleus. Some bitumen veins contain both suspended crystals and crystals nucleated at the vein margins. Where possible, crystals used for fluid inclusion studies were extracted from the bitumen matrix. Euhedral quartz crystals were extracted, cleaned and examined for fluid inclusions. The perfect form of many of the crystals makes them equivalent to polished wafers, and they can be entered directly into a heating-freezing stage with assured good thermal conductivity. Less perfect crystals were prepared as polished wafers. Homogenization temperatures were determined using a Linkam@ TH-600 stage, heated at lO”C/min, and measuring at O.l”C with the aid of a video screen coupled to a Nikon” OPTIPHOT microscope. Ice melting temperatures and eutectic temperatures were recorded from two samples. Inclusions were examined under ultraviolet light to detect fluorescence characteristic of hydrocarbon fluids.
3. Sample material Quartz crystals (Fig. 1) were extracted from bitumen from nine localities; the quartz contents of bitumen are recorded in Table 1. The samples were collected from seven successions: (1) Bitumen-bearing centimetre-scale vugs/cavities in upper Cambrian-Lower Ordovician dolomitic limestones in Montgomery and Herkimer Counties, New York, U.S.A. The vugs contain successive infillings of bitumen, quartz and calcite, but there appears to be some overlap in the stages. Some bitumen infillings are incomplete and geopetal. The quartz crystals (up to 1 cm, rarely larger) include the
Fig. 1. Scanning electron micrographs, in backscattered mode, showing authigenic quartz crystals (Q> suspended within solid bitumen: (A) Berlin, Connecticut; (B) Indian Canyon, Utah (bitumen appears granular due to weathering); and (C) Mountrich, Scotland. Field widths: (A), 230 pm: (B), 1.3 mm; (0, 600 km.
J. Parnell et al. / Chemical Geology 129 (19%) 217-226
famous “Herkimer diamonds” (Dunn and Fisher, 1954) which are currently sought in commercial collecting grounds. Most Herkimer diamonds are not associated with visible bitumen. (2) Large-scale (up to metres width) bitumen veins in the thrusted terrane of the Ouachita Mountains, Oklahoma, U.S.A., to the south of the Arkoma Basin in rocks of Carboniferous age. Some veins are bedding-parallel and may have been formed during thrusting. The veins were large enough to have been mined for bitumen (Ham, 1956). Quartz crystals up to 1 mm are suspended in bitumen and occur at the margins of veins at Page and Kiamichi (Monson, 1992). (3) Bitumen-rich veins (up to centimetres width) cutting vertically through the organic-rich East Berlin Formation and overlying Hampden Basalt in the Jurassic Hartford Basin, Connecticut, U.S.A. Quartz crystals up to 5 mm in bitumen at different localities are known to contain either hydrocarbon or aqueous inclusions. The localities sampled here at Berlin and New Britain yield hydrocarbon and aqueous inclusions, respectively &rnell and Monson, 1995). Some veins also contain dolomite, barite and base-metal sulphides. (4) Veins of the bitumen known as wurtzilite (up to metres width) in the Eocene rocks of the western Uinta Basin, Utah, U.S.A. Quartz crystals up to 2 Table 1 Fluid inclusion Locality
data from individual Host age
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mm occur in some samples from a former wurtzilite mine in Indian Canyon (Monson and Parnell, 1992). The host vein cuts perpendicular to bedding through carbonate-rich mudrocks. The wurtzilite and quartz also occur in evaporite dissolution vugs in some beds near to the veins. (5) Subvertical millimetre-width bitumen-bearing calcite veins cutting an unconformity between Silurian limestones and Proterozoic basement at Dolyhir, Wales, U.K. (Pamell and Eakin, 1989). Quartz crystals up to 3 mm occur sparsely within the bitumen. (6) Subvertical bitumen veins (up to 5-cm width) cutting Devonian conglomerates in Easter Ross, Scotland, U.K. (Pamell and Eakin, 1987). Quartz crystals up to 0.5 mm occur near/at the vein margins, and particularly in sub-millimetre-width bitumen veinlets which cut clasts of Moinian (Proterozoic) gneiss-quartzite in the conglomerate. The bitumen also contains traces of orthoclase. Some veins occupy fault planes, and the vein wallrocks are locally brecciated, suggesting vigorous bitumen injection. (7) Bitumen-bearing calcite concretions (up to 5 cm diameter) in mineralized Carboniferous limestone at Baltic Quarry, Merthyr Tydfil, Wales (Parnell and Eakin, 1989). Quartz crystals up to 5 mm occur in bitumen at the centre of many centimetre-scale con-
localities Quartz content in bitumen
Inclusiion composition
n
(o/o) Flat Creek
Ordovician
Page
Tu range
Tu mean
(“Cl
(“Cl
20
aqueous
15
105-111
109
Carbaniferous
5
aqueous
29
70-90 140-155
83 147
Kiamichi
Carb~aniferous
5
aqueous
25
102-157
126
Berlin
Jurassic
30
hydrocarbon
18
124-128
126
New Britain
Jurassic
40
aqueous
16
109-127
117
Indian Canyon
Eocene
20
aqueous
49
92-109
98
Dolyhir
Siltman
10
aqueous hydrocarbon
17 27
103-119 143-152
112 148
Mountrich
Devonian
50
aqueous
25
99-l 17
113
Merthyr
Carboniferous
20
hydrocarbon
30
121-128
123
n = number of samples;
l;r = homogenization
temperature.
220
J. Parnell et al. /Chemical
cretions and are clearly post-concretion development. The concretions also contain dolomite, barite, calcite, fluorite and pyrite.
4. Petrography The crystals exhibit abundant two-phase (liquid, vapour) inclusions (Fig. 2), which are distributed in zones parallel to crystal faces. None of the crystals examined exhibit trails of cross-cutting (secondary) inclusions. However, quartz from Flat Creek, New York, includes irregular trails of inclusions. The inclusions are in the range from a few micrometres to tens of micrometres size, and up to hundreds of micrometres in the quartz from Dolyhir and Merthyr. Within individual crystals, vapour/liquid ratios are markedly consistent, except in the Flat Creek quartz, where variable ratios and
Geology 129 (1996) 217-226
inclusion sizes within individual inclusion trails suggest that they have experienced necking down. The majority of inclusions contain clear (aqueous) fluids, but several contain amber/brown fluids which fluoresce under ultraviolet light (Fig. 2C) and are therefore hydrocarbon-bearing (McLimans, 1987). The atypical New York quartz is also exceptional in containing a variety of inclusion types, including aqueous, oil-bearing, mixed oil-water-gas, and carbon dioxide-rich fluids (Roedder, 1963). Fluid inclusions parallel to crystal growth faces (i.e. primary inclusions) indicate that quartz growth occurred in several stages. Solid black bituminous coatings between successive stages in some quartz crystals from the Ordovician of New York (Dunn and Fisher, 1954) show that the stages may have been separated by pauses in growth. Trapping of fluid inclusions may occur during continuous growth, but films of solid bitumen suggest a break in silica
Fig, 2. Photomicrographs of quartz showing fluid inclusions examined in this study: (A) large hydrocarbon inclusions, Berlin; (B) hydrocarbon inclusions, Dolyhir; (C), as (B) but under ultraviolet light: and (D) trails of aqueous inclusions, Flat Creek. Field widths: (A), 2 mm; (B), (C),4 mm; CD), 0.5 mm.
J. Fame11 et al. /Chemical
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Geology 129 (1996) 217-226
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temperatures
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222
1. Parnell et al. / Chemical Geology 129 (1996) 217-226
precipitation. Possibly there were periods when the ambient fluid was relatively rich in petroleum, and widespread oil wetting of the quartz surfaces inhibited crystal growth. In two cases, at Page and Dolyhir, two distinct populations of inclusions were recognised in crystals. In each case the inclusions are divisible into an inner zone at the core of a crystal, and an outer zone nearer the margin. With the exception of the Dolyhir quartz, all crystals examined yielded solely aqueous or solely hydrocarbon inclusions.
5. Data Most of the data sets fall into the range 95- 130°C and show only limited spreads (Fig. 3). The main exceptions are the samples from Oklahoma: the two populations from Page are above and below the main range, and the Kiamichi data show a wide spread. However, the Oklahoma samples may have experienced some re-equilibration after the entrapment (see below). The data sets from the two populations of inclusions at Page and Dolyhir show lower temperatures from the inner zone; the inner and outer zones yield clearly distinct sets of values. It was possible to determine freezing data for samples from Page (Oklahoma) and New Britain (Connecticut). Quartz from these two localities yielded mean ice melting temperatures (T,) of 3.8” and 5.6”C, respectively, and eutectic temperatures (T,) of 39.3” and 37.2”C, respectively. The melting temperatures are equivalent to salinities of N 6 and - 9 eq wt% NaCl.
this study. Most of the data distributions are unimodal and of low skewness, which precludes the occurrence of lower-temperature inclusions of the same generation. In crystals which contain hydrocarbon inclusions, fluorescence under ultraviolet light does not reveal unnoticed monophase inclusions. The temperature range is commensurate with successions buried to depths of a minimum of 2-3 km, given normal geothermal gradients in the range 25 35”C/km. Although the values are therefore not surprising, they do indicate that the quartz was precipitated during the deeper levels of burial rather than during shallow burial. The distribution of temperatures is similar to that recorded from inclusions in quartz overgrowths in petroleum reservoirs. The available database for overgrowths has been collated by Walderhaug (1994) along with an extensive new data set for Jurassic
rrrrr-r-rrrrr
T. (‘Cl
B
6. Discussion It is important to assess whether the range of data is biased by the method of observation. Most homogenization temperatures reported from quartz overgrowths are made on two-phase (vapour, liquid) inclusions. Monophase (liquid) inclusions are probably unnoticed or ignored by many workers. This could mean that inclusions trapped below - 70°C go unrecorded. We believe that this is not the case in
Fig. 4. Homogenization temperatures for (A) authigenic quartz occurrences (this study), compared to (B) quartz overgrowths (mean values collated by Walderhaug, 1994).
J. Pamell et al. /Chemical
Geldogy 129 (1996) 217-226
sandstones from the Norwegian Sea and North Sea; these are compared with that from the bitumen-hosted quartz in Fig. 4. All data sets are for combinations of aqueous and hydrocarbon inclusions. Data for sandstone quartz overgrowths and quartz euhedra are both measurements of the temperatures at which petroleum occurs in the carrier-reservoir system. It is therefore not surprising that the data sets occur over the same range. Because the samples used are from geologically complex regions or from sections where the burial history is poorly known, the assessment of pressure corrections to the h’omogenization temperatures is extremely difficult. The detailed histories and data on gas saturation determinable in most oilfields are not available to us. However, electron microscope studies of the bitumens from Oklahoma and Utah (Monson and Pamell, 1992) show that they contain very substantial quantities of gas cavities, and were probably gas-saturated and do not need significant
223
pressure corrections. In the Hartford Basin, Pratt and Burruss (1988) similarly interpreted a range of gas/liquid ratios to indicate that inclusions in calcite were trapped from a heterogeneous, gas-saturated fluid, and that pressure corrections would not be significant. For the other localities we suggest that as dissolved gases are usually present in pore waters in the vicinity of petroleum (Hanor, 1980) the pressure corrections will be minimal, as also assumed by Walderhaug ( 1990). The temperature ranges from quartz crystals are fairly consistent between tbe different localities, despite the wide variation in the geological settings. Fluid flow through brittle fractures occurred consistently at temperatures from 95” to 130°C. In each case, petrographic evidence suggests that petroleum migration occurred early in the fracture history and was not preceded by an extensive set of paragenetic stages. Examples of paragenetic sequences are shown in Fig. 5. In the case of large-scale (metres width)
Merthyr dolomite bttwnen was bartts -
calcite fluorite pyrite
New Briteln dolomite
bitumen qua* galena sphalerite chakxpytite chalcocie badts calcite
Mountrich
bttumen calcite
Time Fig. 5. Paragenetic
sequences for three occurrences
of mixed quartz-bitumen
occurrences.
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J. Parnell et al. / Chemical Geology I29 (1996) 217-226
bitumen veins, we believe that the veins and their host fractures may have formed due to substantial petroleum generation from very rich source rocks, with consequent overpressuring, fracturing to release the pressure, and concomitant petroleum migration into the fractures. The fractures would have remained open due to the high petroleum fluid pressure. This is particularly likely in the cases of Oklahoma and Utah (Comer and Hinch, 1987; Monson and Pamell, 1992). If petroleum injection is an intrinsic aspect of the fracturing, inevitably the paragenetic sequence commences with bitumen. In both case where two populations of inclusions were distinguished, the outer zone of inclusions yielded higher homogenization temperatures. In a crystal from Dolyhir, the inner inclusions are hydrocarbon-bearing and the outer inclusions are aqueous, therefore the temperature difference is difficult to interpret and may not actually represent a real difference in trapping temperature. Hydrocarbon-bearing inclusions tend to yield lower temperatures than coeval aqueous inclusions due to the greater compressibility of organic fluids (Kvenvolden and Roedder, 1971; Burruss, 1987, Karlsen et al., 1993). The two sets of inclusions in quartz from Page are both aqueous and exhibit a higher temperature difference than at Dolyhir. This trend of increasing temperature towards crystal margins possibly represents the effects of a re-equilibration process in the outer parts (see Goldstein and Reynolds, 1994); it is notable that samples from the other Oklahoma locality (Kiamichi) exhibit a much greater spread of data than the other quartz crystals and may also have experienced reequilibration. The salinities derived from melting temperatures at Page and New Britain represent moderately saline brines, but are significantly less than values recorded from Mississippi Valley-type deposits, in which fracture-bound bitumens commonly occur. This shows that the fracture-hosted bitumens examined in this study are distinct from these hydrothermal deposits from basinal brines. The salinities are, however, comparable to many salinities determined from inclusions in quartz overgrowths in North Sea oilfield sandstones (Aplin et al., 1992). This further indicates that the overgrowth and euhedra data both reflect fluid compositions from the petroleum carrier-reservoir system. The eutectic temperatures are typical
of sodium chloride-magnesium chloride systems (Davis et al., 1990). The bitumen-quartz at New Britain is very closely associated with abundant dolomite, consistent with a magnesium-rich brine. In most cases, the very simple mineralogy of the bitumen-bearing fractures means that there are no other minerals present which exhibit measurable fluid inclusions for comparison with the data from the bitumen-hosted quartz. An exception is at Merthyr, where fluorite contains small two-phase aqueous inclusions. The fluorite yields homogenization temperatures of 120-122°C comparing with 121-128°C from the quartz. As the fluorite probably just postdated the bitumen, the similarity is unremarkable. The growth of quartz crystals up to several millimetres length in bitumen indicates that in fact the ambient fluid was a petroleum-aqueous fluid mixture. Calculations to assess the water flow through sandstones necessary to precipitate quartz overgrowths from external sources suggest that a few per water, cent quartz volume requires lo8 cm3/cm2 which could only be attained by extreme focussing of fluid flow (Bjorlykke and Egeberg, 1993; Bjorlykke, 1994). The volume percentages of authigenie quartz (Table 1) in the bitumen are an order of magnitude greater, requiring correspondingly more water flow. Such large volumes of water may have been achieved by focussing through fracture systems, as in the cases described here. It does not seem possible for a purely organic fluid to transport the necessary silicon ions for such extensive silica precipitation. Although organo-silicon compounds, especially siloxanes, have been identified in geological environments (Novgorodova et al., 1991), there is no evidence for their occurrence on a substantial scale. The composition of the inclusions (Table 1) shows that in some cases the petroleum component of the parent fluid was ,trapped, but in other cases it was the aqueous component which is recorded. This variation suggests that the wetting characteristics of the petroleum varied from case to case. Quartz surfaces are normally water-wet (Heald and Larese, 1974; Anderson, 19861, but abnormally viscous petroleum might readily wet quartz, and evolving compositions of oil may cause changes in oil wettability (Denekas et al., 1959). Petrographic details of bitumen veins, such as suspension of entrained rock fragments, and indeed suspension of the quartz crystals being stud-
J. Pamell et al. / Chemical Geology 129 (1996) 217-226
ied, indicate that the bitumen was in a viscous state (Monson and Parnell, 1992; Parnell et al., 1994b). The alteration from viscous bitumen to solid bitumen could have been influenced by the various processes of degradation (biodegradation, deasphalting, water washing). As the bitumen experienced lower temperatures during emplacement, cooling below the pour point would have caused a marked increase in viscosity. A release of gas (predominantly methane) from the bitumen could also have engendered solidification by condensation to form heavier molecules. Studies of quartz overgrowth precipitation in petroleum reservoirs assume that petroleum emplacement in sandstone pores inhibits silica availability but that limited short-range diffusion of silica/silicon ions is possible along water-wet quartz grain surfaces (Walderhaug, 1990, Walderhaug, 1994). The quantities of quartz encountered in some bitumen veins indicate that silica availability was not limited by petroleum emplacement. Furthermore, where the veins occur in carbonate host rocks (including Utah and New York sequences), the silica clearly migrated on a scale of metres or more through the host fracture systems. These observations indicate that the fluids passing through the fractures had an aqueous component. The lack of immediate source of silica in several cases makes a diffusion model for silica transport into the bitumen very unlikely. Rather, we believe that quartz precipitation was relatively fast, i.e. transported by mixed aqueous-petroleum fluids which flowed through the fractures. In the wurtzilite vein at Indian Canyon quartz is concentrated at the vein margin, indicating that aqueous flow may have been focussed there after bitumen solidification commenced.
7. Conclusions The widespread occurrences of authigenic quartz crystals in veins of solid bitumen yield important information about the nature of the parent petroleum fluid: (1) the petroleum fluids must have had a substantial aqueous component to provide the abundant silica;
225
(2) the petroleum and aqueous components of the fluids were immiscible, evinced by the lack of twoliquid inclusions and the predominance of aqueous inclusions in bitumen-hosted quartz. (3) minimum temperatures of entrapment for fluid inclusions in the quartz are mostly in the range 95- 130°C indicating minimum temperatures for migration of accompanying petroleum. (4) the temperature range deduced for petroleum migration is comparable with that published for petroleum emplacement in reservoirs. (5) the two aqueous fluids measured have a moderate salinity (6 and 9 eq wt% NaCl).
Acknowledgements
Acknowledgement is made to the Donors of The Petroleum Research Fund (grant No. 25024-AC2), administered by the American Chemical Society, for support of this research. Technical support was provided by the QUB Electron Microscopy Unit. The manuscript benefitted from constructive criticism by G. Macleod, G.S. Saigal and 0. Walderhaug. (RA)
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ELSEVJER
ISOTOPE GEOSCIENCE
Chemical Geology 129 (1996) 217-226
Fluid inclusion constraints on temperatures of petroleum migration from authigenic quartz in bitumen veins J. Parnell, P.F. Carey, B. Monson School of Geosciences, Queen’s University, Belfast Bi7 INN, UK
Received 2 May 1995; accepted 23 August 1995
Abstract Solid bitumens in fracture systems commonly contain microscopic crystals of authigenic quartz which were precipitated coevally with the bitumen early in the paragenesis of the fracture-fillings. The quartz contains primary fluid inclusions (both aqueous and hydrocarbon), yielding homogenization temperatures particularly in the range 95-13O”C, indicating minimum temperatures of entrapment during petroleum migration. Volumes of up to 50% entrained quartz, and a predominance of aqueous inclusions, indicate that the petroleum fluid had a substantial aqueous component.
1. Introduction The secondary migration of petroleum through carrier and reservoir beds inevitably involves mixing of petroleum and aqueous fluids. Models for petroleum migration admit only very limited miscibility of the two fluids and migration into reservoirs is largely by buoyancy (England et al., 1987). The coexistence of the two fluids is evinced by the occurrence of hydro’carbon fluid inclusions in mineral cements including quartz (N.B. we follow common practice by reference to hydrocarbon inclusions, but recognise that the contents are petroleum-related and contain compounds additional to true hydrocarbons). The solubility of ions for quartz precipitation is low (10’s of ppm), and requires large volumes of water for transport (Bjorlykke and Egeberg, 1993). In some cases inclusions in cements contain immiscible mixtures of oil
petroleum residues (bitumens) indicate a significant aqueous component to the parent fluid (Parnell et al., 1994a). Our studies of solid bitumens from fracture systems have recorded that crystals of authigenic quartz are a component of bitumen veins in many basins. This quartz is a potential source of information about the composition of the petroleum-rich fluids which precipitated the bitumens. Fluid inclusions within the quartz crystals provide data on the temperature and broad-scale chemistry of the fluids. Quartz has a high potential to yield such data because it commonly contains fluid inclusions which are susceptible to examination and measurement. The quartz crystals contain almost exclusively primary inclusions, as recorded in other studies of quartz euhedra in sedimentary and vein environments (Murray, 1957; Touray and Barlier, 1975). A further advantage of quartz is that in comparison to other low-temperature minerals such as calcite it is less likely to be stretched during heating above the entrapment temperature
0009-2541/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SUN 0009.2541(95)0’Dl41-7
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et al. /Chemical
Geology
129 (1996) 217-226
(Robinson et al., 19921, although some workers (e.g., Osborne and Haszeldine, 1993) believe that inclusions in quartz are susceptible to resetting. We report here data from authigenic quartz crystals extracted from fracture-bound bitumens in basins in North America and Europe.
2. Methodology Many of the quartz crystals are entirely suspended within a matrix of bitumen: they are perfect euhedra which are not attached to vein walls and have no identifiable nucleus. Some bitumen veins contain both suspended crystals and crystals nucleated at the vein margins. Where possible, crystals used for fluid inclusion studies were extracted from the bitumen matrix. Euhedral quartz crystals were extracted, cleaned and examined for fluid inclusions. The perfect form of many of the crystals makes them equivalent to polished wafers, and they can be entered directly into a heating-freezing stage with assured good thermal conductivity. Less perfect crystals were prepared as polished wafers. Homogenization temperatures were determined using a Linkam@ TH-600 stage, heated at lO”C/min, and measuring at O.l”C with the aid of a video screen coupled to a Nikon” OPTIPHOT microscope. Ice melting temperatures and eutectic temperatures were recorded from two samples. Inclusions were examined under ultraviolet light to detect fluorescence characteristic of hydrocarbon fluids.
3. Sample material Quartz crystals (Fig. 1) were extracted from bitumen from nine localities; the quartz contents of bitumen are recorded in Table 1. The samples were collected from seven successions: (1) Bitumen-bearing centimetre-scale vugs/cavities in upper Cambrian-Lower Ordovician dolomitic limestones in Montgomery and Herkimer Counties, New York, U.S.A. The vugs contain successive infillings of bitumen, quartz and calcite, but there appears to be some overlap in the stages. Some bitumen infillings are incomplete and geopetal. The quartz crystals (up to 1 cm, rarely larger) include the
Fig. 1. Scanning electron micrographs, in backscattered mode, showing authigenic quartz crystals (Q> suspended within solid bitumen: (A) Berlin, Connecticut; (B) Indian Canyon, Utah (bitumen appears granular due to weathering); and (C) Mountrich, Scotland. Field widths: (A), 230 pm: (B), 1.3 mm; (0, 600 km.
J. Parnell et al. / Chemical Geology 129 (19%) 217-226
famous “Herkimer diamonds” (Dunn and Fisher, 1954) which are currently sought in commercial collecting grounds. Most Herkimer diamonds are not associated with visible bitumen. (2) Large-scale (up to metres width) bitumen veins in the thrusted terrane of the Ouachita Mountains, Oklahoma, U.S.A., to the south of the Arkoma Basin in rocks of Carboniferous age. Some veins are bedding-parallel and may have been formed during thrusting. The veins were large enough to have been mined for bitumen (Ham, 1956). Quartz crystals up to 1 mm are suspended in bitumen and occur at the margins of veins at Page and Kiamichi (Monson, 1992). (3) Bitumen-rich veins (up to centimetres width) cutting vertically through the organic-rich East Berlin Formation and overlying Hampden Basalt in the Jurassic Hartford Basin, Connecticut, U.S.A. Quartz crystals up to 5 mm in bitumen at different localities are known to contain either hydrocarbon or aqueous inclusions. The localities sampled here at Berlin and New Britain yield hydrocarbon and aqueous inclusions, respectively &rnell and Monson, 1995). Some veins also contain dolomite, barite and base-metal sulphides. (4) Veins of the bitumen known as wurtzilite (up to metres width) in the Eocene rocks of the western Uinta Basin, Utah, U.S.A. Quartz crystals up to 2 Table 1 Fluid inclusion Locality
data from individual Host age
219
mm occur in some samples from a former wurtzilite mine in Indian Canyon (Monson and Parnell, 1992). The host vein cuts perpendicular to bedding through carbonate-rich mudrocks. The wurtzilite and quartz also occur in evaporite dissolution vugs in some beds near to the veins. (5) Subvertical millimetre-width bitumen-bearing calcite veins cutting an unconformity between Silurian limestones and Proterozoic basement at Dolyhir, Wales, U.K. (Pamell and Eakin, 1989). Quartz crystals up to 3 mm occur sparsely within the bitumen. (6) Subvertical bitumen veins (up to 5-cm width) cutting Devonian conglomerates in Easter Ross, Scotland, U.K. (Pamell and Eakin, 1987). Quartz crystals up to 0.5 mm occur near/at the vein margins, and particularly in sub-millimetre-width bitumen veinlets which cut clasts of Moinian (Proterozoic) gneiss-quartzite in the conglomerate. The bitumen also contains traces of orthoclase. Some veins occupy fault planes, and the vein wallrocks are locally brecciated, suggesting vigorous bitumen injection. (7) Bitumen-bearing calcite concretions (up to 5 cm diameter) in mineralized Carboniferous limestone at Baltic Quarry, Merthyr Tydfil, Wales (Parnell and Eakin, 1989). Quartz crystals up to 5 mm occur in bitumen at the centre of many centimetre-scale con-
localities Quartz content in bitumen
Inclusiion composition
n
(o/o) Flat Creek
Ordovician
Page
Tu range
Tu mean
(“Cl
(“Cl
20
aqueous
15
105-111
109
Carbaniferous
5
aqueous
29
70-90 140-155
83 147
Kiamichi
Carb~aniferous
5
aqueous
25
102-157
126
Berlin
Jurassic
30
hydrocarbon
18
124-128
126
New Britain
Jurassic
40
aqueous
16
109-127
117
Indian Canyon
Eocene
20
aqueous
49
92-109
98
Dolyhir
Siltman
10
aqueous hydrocarbon
17 27
103-119 143-152
112 148
Mountrich
Devonian
50
aqueous
25
99-l 17
113
Merthyr
Carboniferous
20
hydrocarbon
30
121-128
123
n = number of samples;
l;r = homogenization
temperature.
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J. Parnell et al. /Chemical
cretions and are clearly post-concretion development. The concretions also contain dolomite, barite, calcite, fluorite and pyrite.
4. Petrography The crystals exhibit abundant two-phase (liquid, vapour) inclusions (Fig. 2), which are distributed in zones parallel to crystal faces. None of the crystals examined exhibit trails of cross-cutting (secondary) inclusions. However, quartz from Flat Creek, New York, includes irregular trails of inclusions. The inclusions are in the range from a few micrometres to tens of micrometres size, and up to hundreds of micrometres in the quartz from Dolyhir and Merthyr. Within individual crystals, vapour/liquid ratios are markedly consistent, except in the Flat Creek quartz, where variable ratios and
Geology 129 (1996) 217-226
inclusion sizes within individual inclusion trails suggest that they have experienced necking down. The majority of inclusions contain clear (aqueous) fluids, but several contain amber/brown fluids which fluoresce under ultraviolet light (Fig. 2C) and are therefore hydrocarbon-bearing (McLimans, 1987). The atypical New York quartz is also exceptional in containing a variety of inclusion types, including aqueous, oil-bearing, mixed oil-water-gas, and carbon dioxide-rich fluids (Roedder, 1963). Fluid inclusions parallel to crystal growth faces (i.e. primary inclusions) indicate that quartz growth occurred in several stages. Solid black bituminous coatings between successive stages in some quartz crystals from the Ordovician of New York (Dunn and Fisher, 1954) show that the stages may have been separated by pauses in growth. Trapping of fluid inclusions may occur during continuous growth, but films of solid bitumen suggest a break in silica
Fig, 2. Photomicrographs of quartz showing fluid inclusions examined in this study: (A) large hydrocarbon inclusions, Berlin; (B) hydrocarbon inclusions, Dolyhir; (C), as (B) but under ultraviolet light: and (D) trails of aqueous inclusions, Flat Creek. Field widths: (A), 2 mm; (B), (C),4 mm; CD), 0.5 mm.
J. Fame11 et al. /Chemical
221
Geology 129 (1996) 217-226
0 70
80
w
ml
HO
120
130
140
1x1
160
(70
180
130
2c.a
T.("C)
6
Beriir
6-
5
%4 3 It 2
2-
1
I-
O 100
105
110
,,?I 120
125
130
135
140
145
150
155
160
165
120
12,
(22
123
124
125
T. (“c)
6
5
Y 2 I
c
0 4 3
‘26
127
128
123
New Britain
l_EJ il:~lll~ 104
108
103
110
112
114
116
130
13,
132
1X
T. (‘C)
118
120
,22
124
126
128
Indian Canyon
,3C
T. 1°C)
T, (“C)
I
12 10
Dolyhir
12
IO
8"
F"
lb 4 2 c
0 8 6
4
2
0
80
85
,oo
105
1'0
115
120
125
130
135
140
145
150
155
9433
88
100
102
T.('C)
Fig. 3. Homogenization hydrocarbon inclusions).
temperatures
IO4
106
108
110
112
H4
116
(18
1x
T. (‘Cl
for fluid inclusions
in authigenic
quartz from individual
occurrences
(ruled = aqueous;
black =
222
1. Parnell et al. / Chemical Geology 129 (1996) 217-226
precipitation. Possibly there were periods when the ambient fluid was relatively rich in petroleum, and widespread oil wetting of the quartz surfaces inhibited crystal growth. In two cases, at Page and Dolyhir, two distinct populations of inclusions were recognised in crystals. In each case the inclusions are divisible into an inner zone at the core of a crystal, and an outer zone nearer the margin. With the exception of the Dolyhir quartz, all crystals examined yielded solely aqueous or solely hydrocarbon inclusions.
5. Data Most of the data sets fall into the range 95- 130°C and show only limited spreads (Fig. 3). The main exceptions are the samples from Oklahoma: the two populations from Page are above and below the main range, and the Kiamichi data show a wide spread. However, the Oklahoma samples may have experienced some re-equilibration after the entrapment (see below). The data sets from the two populations of inclusions at Page and Dolyhir show lower temperatures from the inner zone; the inner and outer zones yield clearly distinct sets of values. It was possible to determine freezing data for samples from Page (Oklahoma) and New Britain (Connecticut). Quartz from these two localities yielded mean ice melting temperatures (T,) of 3.8” and 5.6”C, respectively, and eutectic temperatures (T,) of 39.3” and 37.2”C, respectively. The melting temperatures are equivalent to salinities of N 6 and - 9 eq wt% NaCl.
this study. Most of the data distributions are unimodal and of low skewness, which precludes the occurrence of lower-temperature inclusions of the same generation. In crystals which contain hydrocarbon inclusions, fluorescence under ultraviolet light does not reveal unnoticed monophase inclusions. The temperature range is commensurate with successions buried to depths of a minimum of 2-3 km, given normal geothermal gradients in the range 25 35”C/km. Although the values are therefore not surprising, they do indicate that the quartz was precipitated during the deeper levels of burial rather than during shallow burial. The distribution of temperatures is similar to that recorded from inclusions in quartz overgrowths in petroleum reservoirs. The available database for overgrowths has been collated by Walderhaug (1994) along with an extensive new data set for Jurassic
rrrrr-r-rrrrr
T. (‘Cl
B
6. Discussion It is important to assess whether the range of data is biased by the method of observation. Most homogenization temperatures reported from quartz overgrowths are made on two-phase (vapour, liquid) inclusions. Monophase (liquid) inclusions are probably unnoticed or ignored by many workers. This could mean that inclusions trapped below - 70°C go unrecorded. We believe that this is not the case in
Fig. 4. Homogenization temperatures for (A) authigenic quartz occurrences (this study), compared to (B) quartz overgrowths (mean values collated by Walderhaug, 1994).
J. Pamell et al. /Chemical
Geldogy 129 (1996) 217-226
sandstones from the Norwegian Sea and North Sea; these are compared with that from the bitumen-hosted quartz in Fig. 4. All data sets are for combinations of aqueous and hydrocarbon inclusions. Data for sandstone quartz overgrowths and quartz euhedra are both measurements of the temperatures at which petroleum occurs in the carrier-reservoir system. It is therefore not surprising that the data sets occur over the same range. Because the samples used are from geologically complex regions or from sections where the burial history is poorly known, the assessment of pressure corrections to the h’omogenization temperatures is extremely difficult. The detailed histories and data on gas saturation determinable in most oilfields are not available to us. However, electron microscope studies of the bitumens from Oklahoma and Utah (Monson and Pamell, 1992) show that they contain very substantial quantities of gas cavities, and were probably gas-saturated and do not need significant
223
pressure corrections. In the Hartford Basin, Pratt and Burruss (1988) similarly interpreted a range of gas/liquid ratios to indicate that inclusions in calcite were trapped from a heterogeneous, gas-saturated fluid, and that pressure corrections would not be significant. For the other localities we suggest that as dissolved gases are usually present in pore waters in the vicinity of petroleum (Hanor, 1980) the pressure corrections will be minimal, as also assumed by Walderhaug ( 1990). The temperature ranges from quartz crystals are fairly consistent between tbe different localities, despite the wide variation in the geological settings. Fluid flow through brittle fractures occurred consistently at temperatures from 95” to 130°C. In each case, petrographic evidence suggests that petroleum migration occurred early in the fracture history and was not preceded by an extensive set of paragenetic stages. Examples of paragenetic sequences are shown in Fig. 5. In the case of large-scale (metres width)
Merthyr dolomite bttwnen was bartts -
calcite fluorite pyrite
New Briteln dolomite
bitumen qua* galena sphalerite chakxpytite chalcocie badts calcite
Mountrich
bttumen calcite
Time Fig. 5. Paragenetic
sequences for three occurrences
of mixed quartz-bitumen
occurrences.
224
J. Parnell et al. / Chemical Geology I29 (1996) 217-226
bitumen veins, we believe that the veins and their host fractures may have formed due to substantial petroleum generation from very rich source rocks, with consequent overpressuring, fracturing to release the pressure, and concomitant petroleum migration into the fractures. The fractures would have remained open due to the high petroleum fluid pressure. This is particularly likely in the cases of Oklahoma and Utah (Comer and Hinch, 1987; Monson and Pamell, 1992). If petroleum injection is an intrinsic aspect of the fracturing, inevitably the paragenetic sequence commences with bitumen. In both case where two populations of inclusions were distinguished, the outer zone of inclusions yielded higher homogenization temperatures. In a crystal from Dolyhir, the inner inclusions are hydrocarbon-bearing and the outer inclusions are aqueous, therefore the temperature difference is difficult to interpret and may not actually represent a real difference in trapping temperature. Hydrocarbon-bearing inclusions tend to yield lower temperatures than coeval aqueous inclusions due to the greater compressibility of organic fluids (Kvenvolden and Roedder, 1971; Burruss, 1987, Karlsen et al., 1993). The two sets of inclusions in quartz from Page are both aqueous and exhibit a higher temperature difference than at Dolyhir. This trend of increasing temperature towards crystal margins possibly represents the effects of a re-equilibration process in the outer parts (see Goldstein and Reynolds, 1994); it is notable that samples from the other Oklahoma locality (Kiamichi) exhibit a much greater spread of data than the other quartz crystals and may also have experienced reequilibration. The salinities derived from melting temperatures at Page and New Britain represent moderately saline brines, but are significantly less than values recorded from Mississippi Valley-type deposits, in which fracture-bound bitumens commonly occur. This shows that the fracture-hosted bitumens examined in this study are distinct from these hydrothermal deposits from basinal brines. The salinities are, however, comparable to many salinities determined from inclusions in quartz overgrowths in North Sea oilfield sandstones (Aplin et al., 1992). This further indicates that the overgrowth and euhedra data both reflect fluid compositions from the petroleum carrier-reservoir system. The eutectic temperatures are typical
of sodium chloride-magnesium chloride systems (Davis et al., 1990). The bitumen-quartz at New Britain is very closely associated with abundant dolomite, consistent with a magnesium-rich brine. In most cases, the very simple mineralogy of the bitumen-bearing fractures means that there are no other minerals present which exhibit measurable fluid inclusions for comparison with the data from the bitumen-hosted quartz. An exception is at Merthyr, where fluorite contains small two-phase aqueous inclusions. The fluorite yields homogenization temperatures of 120-122°C comparing with 121-128°C from the quartz. As the fluorite probably just postdated the bitumen, the similarity is unremarkable. The growth of quartz crystals up to several millimetres length in bitumen indicates that in fact the ambient fluid was a petroleum-aqueous fluid mixture. Calculations to assess the water flow through sandstones necessary to precipitate quartz overgrowths from external sources suggest that a few per water, cent quartz volume requires lo8 cm3/cm2 which could only be attained by extreme focussing of fluid flow (Bjorlykke and Egeberg, 1993; Bjorlykke, 1994). The volume percentages of authigenie quartz (Table 1) in the bitumen are an order of magnitude greater, requiring correspondingly more water flow. Such large volumes of water may have been achieved by focussing through fracture systems, as in the cases described here. It does not seem possible for a purely organic fluid to transport the necessary silicon ions for such extensive silica precipitation. Although organo-silicon compounds, especially siloxanes, have been identified in geological environments (Novgorodova et al., 1991), there is no evidence for their occurrence on a substantial scale. The composition of the inclusions (Table 1) shows that in some cases the petroleum component of the parent fluid was ,trapped, but in other cases it was the aqueous component which is recorded. This variation suggests that the wetting characteristics of the petroleum varied from case to case. Quartz surfaces are normally water-wet (Heald and Larese, 1974; Anderson, 19861, but abnormally viscous petroleum might readily wet quartz, and evolving compositions of oil may cause changes in oil wettability (Denekas et al., 1959). Petrographic details of bitumen veins, such as suspension of entrained rock fragments, and indeed suspension of the quartz crystals being stud-
J. Pamell et al. / Chemical Geology 129 (1996) 217-226
ied, indicate that the bitumen was in a viscous state (Monson and Parnell, 1992; Parnell et al., 1994b). The alteration from viscous bitumen to solid bitumen could have been influenced by the various processes of degradation (biodegradation, deasphalting, water washing). As the bitumen experienced lower temperatures during emplacement, cooling below the pour point would have caused a marked increase in viscosity. A release of gas (predominantly methane) from the bitumen could also have engendered solidification by condensation to form heavier molecules. Studies of quartz overgrowth precipitation in petroleum reservoirs assume that petroleum emplacement in sandstone pores inhibits silica availability but that limited short-range diffusion of silica/silicon ions is possible along water-wet quartz grain surfaces (Walderhaug, 1990, Walderhaug, 1994). The quantities of quartz encountered in some bitumen veins indicate that silica availability was not limited by petroleum emplacement. Furthermore, where the veins occur in carbonate host rocks (including Utah and New York sequences), the silica clearly migrated on a scale of metres or more through the host fracture systems. These observations indicate that the fluids passing through the fractures had an aqueous component. The lack of immediate source of silica in several cases makes a diffusion model for silica transport into the bitumen very unlikely. Rather, we believe that quartz precipitation was relatively fast, i.e. transported by mixed aqueous-petroleum fluids which flowed through the fractures. In the wurtzilite vein at Indian Canyon quartz is concentrated at the vein margin, indicating that aqueous flow may have been focussed there after bitumen solidification commenced.
7. Conclusions The widespread occurrences of authigenic quartz crystals in veins of solid bitumen yield important information about the nature of the parent petroleum fluid: (1) the petroleum fluids must have had a substantial aqueous component to provide the abundant silica;
225
(2) the petroleum and aqueous components of the fluids were immiscible, evinced by the lack of twoliquid inclusions and the predominance of aqueous inclusions in bitumen-hosted quartz. (3) minimum temperatures of entrapment for fluid inclusions in the quartz are mostly in the range 95- 130°C indicating minimum temperatures for migration of accompanying petroleum. (4) the temperature range deduced for petroleum migration is comparable with that published for petroleum emplacement in reservoirs. (5) the two aqueous fluids measured have a moderate salinity (6 and 9 eq wt% NaCl).
Acknowledgements
Acknowledgement is made to the Donors of The Petroleum Research Fund (grant No. 25024-AC2), administered by the American Chemical Society, for support of this research. Technical support was provided by the QUB Electron Microscopy Unit. The manuscript benefitted from constructive criticism by G. Macleod, G.S. Saigal and 0. Walderhaug. (RA)
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