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Altered rhyolites as oil and gas reservoirs in the Minaminagaoka gas field (Niigata Prefecture, Japan)
Chemical Geology, 49 (1985) 363--370
363
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
ALTERED RHYOLITES AS OIL AND GAS RESERVOIRS IN THE MINAMINAGAOKA GAS FIELD (NIIGATA PREFECTURE, JAPAN) M. SHIMAZU Department of Geology and Mineralogy, Faculty of Science, Niigata University, Niigata (Japan) (Accepted for publication July 16, 1984)
Abstract Shimazu, M., 1985. Altered rhyolites as oil and gas reservoirs in the Minaminagaoka gas field (Niigata Prefecture, Japan); In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 363--370. Subsurface geology and petrography of volcanic rocks of the recently discovered Minaminagaoka gas field show that good reservoirs in the field are altered rhyolites of the mid-Miocene Nanatani Formation. The Nanatani F o r m a t i o n is composed of mudstone, lava and pyroclastic rock with a composition of rhyolite, andesite and basalt. Rhyolites are divided into rhyolites A, B, B', C and D according to petrographical features. Rhyolite A is a hyaloclastite and pumiceous tuff of glassy rhyolite. Rhyolites B, B' and D are aphyric rhyolite lavas characterized by needle-like plagioclase in groundmass. Rhyolite C is perlite lava. Rhyolites B, D and C are suitable for reservoirs, whose pores are not primary, but are secondary macroscopic drnses and micropores formed b y hydrothermal alteration. Micropores are openings among crystal grains of quartz and albite and are several tens of micrometers in size. Alteration of rhyolitic rocks is of hydrothermal type and is characterized by a smectite (or smectite/ chlorite mixed-layer mineral)--sericite--albite--quartz--carbonate mineral assemblage in the upper zone, but clay minerals in the lower zone are chlorite and sericite. Alteration of andesite and basalt is similar to that of rhyolite in general, but in the Kitafukazawa bore hole, andesite and basalt below 4640-m depth have chlorite--laumonite--prehnite--(pumpellyite--epidote)assemblage which is correlated to the assemblage of the higher part of the laumonite subfacies of zeolite facies. F o r m a t i o n temperature of secondary quartz, estimated by the measurement of the filling temperature of fluid inclusions, is ~ 150°C and is nearly the same as the present temperature at the bottom of the hole. From the geological environment, characteristics of alteration and paleogeotherm, formation of rhyolite reservoirs and migration of oil and gas can be summarized as follows: (1) formation of volcanic piles by submarine volcanism of rhyolite, andesite and basalt, and deposition of mudstone around the piles; (2) generation of hydrocarbon in mudstone along with increasing temperature related to volcanism; and (3) formation of rhyolite reservoirs by hydrothermal alteration, followed by lateral migration of oil and gas into secondary pores.
1. Introduction Oil and gas fields in Japan, which are productive or are being prepared for exploration at present, are distributed in t h e Niigata and the Akita sedimentary basins and the 0009-2541/85/$03.30
Joban'oki basin. The Aga'oki field of the Niigata basin and Joban'oki field are offshore whereas the others are onshore. Most of the fields except the J o b a n ' o k i field are distributed in the inner belt of northeast Honshu, which is a main part of the Green T u f f Re-
© 1985 Elsevier Science Publishers B.V.
364
gion where distinct tectonic m o v e m e n t and igneous activity t o o k place in Miocene times (Fig. 1). Reservoirs in m a n y oil and gas fields explored before 1960 were found in Pliocene sandstones and/or andesitic rocks. However, reservoirs of the Mitsuke field (discovered in 1958) and those of the Yoshii--Higashikashiwazaki gas field (discovered in 1968) are found to be in Miocene volcanic rocks of the Niigata basin (Kujiraoka, 1965). Pores of the reservoirs of these fields mainly consist of primary fractures in acid volcanic rocks (Katahira, 1974; Katahira and Ukai, 1976). Deep drilling prospecting for oil and gas fields with volcanic reservoirs has been accelerated in surrounding areas b y the discovery o f the large gas field. The Minaminagaoka gas field located 10 km southwest of Nagaoka City was discovered in 1976. The depth of the volcanic reservoir in the field is ~ 4 5 0 0 m, and it is larger than
those of the Mitsuke and Yoshii-Higashikashiwazaki fields. The reservoirs in this field are in altered rhyolites of middle Miocene age. Pores of the reservoirs are not primary fractures, b u t secondary ones formed by hydrothermal alteration. This paper is concerned with volcanic reservoirs in the Minaminagaoka gas field, which is one of the largest fields in Japan.
2. Geological setting The Genozoic formations from the middle Miocene to middle Pleistocene are distributed in the Nagaoka area located in the southern part of the Niigata basin. These formations are the Nanatani, Teradomari, Shiiya, Nishiyama--Haizume and U o n u m a formations, in ascending order (Fig. 2). The total thickness of these formations reaches ~ 6 0 0 0 m. Most of the formations were laid d o w n in a marine environment. The Teradomari and Shiiya N
Kyus
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Fig. 1. Oil and gas fields in Japan.
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365
formations are composed of mudstone, sandstone and andesitic rocks of middle to late Miocene age. The Nishiyama--Haizume Formation is composed of sandstone, siltstone and andesitic rock of Pliocene to early Pleistocene age. The U o n u m a Formation consists of sandstone, siltstone and conglomerate deposited in marine or lacustrine environment in the early to middle Pleistocene. The Nanatani Formation of middle Miocene age is composed of mudstone in the upper portion, and rhyolite, andesite and basalt in the lower portion. The Cenozoic formations form five anticlinal structures running parallel in the Nagaoka area. Oil and gas entrapped in the Pliocene sandstones of the upper part of the anticlines have been produced since 1880. 3. Volcanic rocks in the Nanatani Formation Rhyolites in the Nanatani Formation are lava, hyaloclastite and pyroclastic rock. From Asahibara
rn
Asahibara
petrographical features, these are divided into rhyolites A, B, B', C and D. Rhyolite A is hyaloclastite and pumiceous tuff of glassy rhyolite showing a bluish-green color and is composed of small amounts of plagioclase crystals and sericitized glass or pumice shards. Rhyolite B is compact aphyric rhyolite lava showing a greyish-white or pale greyish-brown color and is composed of small amounts of plagioclase and/or quartz phenocrysts and devitrified groundmass. Rhyolite B' is slightly more basic than rhyolite B. Rhyolite D is a variety of rhyolite B and has spherulitic texture. Rhyolite B, B' and D are characterized by needle-like plagioclase phenocrysts in the groundmass. Rhyolite C is perlite lava showing a pale-green or pale pinkish-white color and is composed of small amounts of plagioclase phenocrysts and devitrified groundmass which shows a perlitic texture. Phenocrysts of mafic minerals are not clear owing to later alteration. The andesite is aphyric lava and consists Shinoyasawa
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366
rhyolite eruption is from rhyolite C through rhyolite B to rhyolite A.
of plagioclase phenocrysts and lath-shaped plagioclase. The basalt is blackish or greenishbrown colored lava and hyaloclastite, and shows partly variolitic texture. It consists of plagioclase phenocrysts and lath-shaped plagioclase and mafic minerals replaced by smectite or chlorite and dusty opaque minerals. The distribution of these volcanic rocks is shown in columnar sections of seven wells (Fig. 2). The deepest well is the Kitafukazawa drilled to 4,890 m deep. The north--south stratigraphical profile is illustrated in Fig. 3. This profile was drawn by assuming the t o p of the Nanatani Formation as a datum horizon, and it shows the existence of a synchronous high formed b y volcanic rocks and also the mutual relation among volcanic rocks of the formation at the time when these rocks were deposited. Most of the basalts and andesites occurs in the lower part, but three basalt layers are intercalated with rhyolites in the upper part and sometimes occur as pillow lavas. The sequence of
4. Alteration o f volcanic
rocks
Volcanic rocks o f the Nanatani Formation in the Minaminagaoka gas field have been heavily altered. Most of the rhyolites show a mineral assemblage of sericite--albite--quartz with dolomite; however, in the lower part of the Kitafukazawa well, rhyolite, andesite and basalt show a mineral assemblage of laumontite--prehnite--chlorite with epidote and pumpellyite. Rhyolite in the upper part of the well has the same assemblage as that of other wells. The assemblage of chlorite, sericite, sericite/smectite mixed-layer mineral, quartz, albite and dolomite in the Kitafukazawa well is transitional in nature between the above t w o assemblages. The t w o types of alteration f o u n d in these mineral assemblages are mainly due to differences in chemical potential of H20 and CO2, namely the laumontite--prehnite--
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Fig. 3. Stratigraphic profile o f t h e Minaminagaoka gas field. S y m b o l s are the same as those of Fig. 2. Chain lines w i t h " / ÷ iv" and w i t h " c h " are t h e upper limit of assemblage o f l a u m o n t i t e - - p r e h n i t e and of o c c u r r e n c e of chlorite, respectively.
367 chlorite assemblage m i g h t be f o r m e d in conditions o f l o w c h e m i c a l p o t e n t i a l o f CO2 and high c h e m i c a l p o t e n t i a l o f H20. A l t e r e d r o c k s having t h e l a u m o n t i t e - p r e h n i t e - - c h l o r i t e assemblage were f o r m e d at the metamorphic condition between the l a u m o n t i t e subfacies o f t h e zeolite facies and p r e h n i t e - - p u m p e U y i t e facies. T h e assemblage o f s e r i c i t e - - a l b i t e - - q u a r t z m i g h t have been f o r m e d at n e a r l y t h e same o r l o w e r t e m p e r a tures. T h e chain line w i t h " c h " in Fig. 3 d e n o t e s t h e u p p e r limit o f o c c u r r e n c e o f chlorite. Formation temperature of secondary quartz
in r h y o l i t e C was e s t i m a t e d b y t h e measurem e n t o f t h e filling t e m p e r a t u r e o f fluid inclusions in q u a r t z . Filling t e m p e r a t u r e , w h i c h was m e a s u r e d b y h e a t i n g stage (rate o f heating: 20°C/rain., e r r o r o f m e a s u r e m e n t : + 2°C), is 1 3 8 - - 1 6 7 ° C a n d is 150°C o n average. T h e p r e s e n t t e m p e r a t u r e at b o t t o m o f t h e well is 1 7 0 ° C at m a x i m u m . 5. Characteristics
r h y o l i t e zeservoiz
of the
Chemical c o m p o s i t i o n s o f r h y o l i t e s are s h o w n in Table I. Silica c o n t e n t s o f r h y o l i t e s B and C range f r o m 61 t o 75 wt.%. K 2 0 con-
TABLE I Chemical composition of rhyolites B and C (1) SiO 2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P~Os
(2)
(3)
(4)
73.85 0.90 12.43
68.26 1.86 11.80
61.40 0.29 14.14
73.08 0.36 11.84
H20-
3.54 0.09 0.41 0.72 6.29 0.16 0.24 0.16 1.21
6.38 0.14 1.43 0.76 3.93 1.20 0.29 0.19 3.76
7.94 0.24 1.74 0.30 3.83 2.69 0.05 0.18 7.17
5.21 0.13 0.86 0.34 5.61 0.16 0.05 0.17 2.21
Total
100.00
100.00
100.00
34.11 1.18 0.94 53.23
36.13 3.34 7.09 33.26
22.04 4.51 15.89 32.41 1.14 . 4.33 8.15 3.45 0.55 0.12
H20 ÷
Q
C Or Ab An Wo En
1.99
Ap
. 1.02 2.35 1.54 1.71 0.56
An (%)
4
Fs Mt II
1.88
.
. 3.56 3.82 2.77 3.53 0.67 5
3
(5)
(6)
75.15 0.32 10.36 0.33 2.32 ~ 0.59 2.00 5.03 0.44 -2.88
68.49 0.31 15.24 1.14 2.76 -0.95 1.65 6.75 0.82
100.02
99.42
100.39
35.67 1.95 0.94 47.47 1.34
38.66 2.60 42.56 4.39 2.31 1.47 3.46 0.47 0.61 --
19.49 0.25 4.85 57.12 0.19
2.14 5.07 2.26 0.68 0.12 3
9
--
2.28
--
2.37 3.62 1.65 0.59 -0
Samples: I = rhyolite B; 2 = rhyolite B; 3 = rhyolite C; 4 = rhyolite C; 5 = rhyolite B; 6 = rhyolite C. Locations: I, Kitafukazawa; 2--4 and 6, Asahibara; 5, Shinoyasawa. Normative minerals: Ab = a l b i t e ; A n = a n o r t h i t e ; A p = a p a t i t e ; C = corundum; E n = e n s t a t i t e ; F s = f e r r o s i l i t e ; I1 = i l m e n i t e ; M t = m a g n e t i t e ; Or = orthoclase; Q ffi quartz; Wo = wollastonite.
368
tents are 0.16--2.69 wt.% and are smaller than those of fresh rhyolites containing the same silica contents, as generally found near the Earth's surface; therefore, leaching o f K20 t o o k place during hydrothermal alteration. Although the mineral assemblage of rhyolites is nearly the same, petrographical features of altered rhyolites are various. Poorness of clay minerals such as sericite and celadonite in rhyolites B and C is due to a small K20 content, and large amounts of recrystallized quartz in rhyolite C reflect perlitic texture and large glass contents. Quartz in the rhyolites replaced glass, or occurs as small lenses and veinlets. Pores in the rhyolites are not primary cracks or joints formed by rapid cooling of lava or tectonic fracturing. Primary fractures have been lost by compaction and recrystaUization. Effective pores as reservoirs are mainly macroscopic druse or "vug" and micropores formed by recrystallization. Druses are several millimeters in diameter and are coated b y secondary quartz and/or albite. Micropores are openings among crystal grains of quartz or albite and are several tens of micrometers in size (Plate I). Glasses of rhyolite C have been extensively replaced by quartz grains, and hence rhyolite C has a large amount of
druses and micropores, and contributes to accumulation of hydrocarbons as a good reservoir. Much of rhyolite B is also a suitable reservoir, whereas rhyolite A is not, because it contains a large amount of clay minerals which filled up the pores. The quality of the reservoir depends on these features o f altered rhyolites (Shimazu, 1982). 6. Formation o f rhyolite reservoirs and migration of oil and gas The hydrothermal alteration forming the pores mentioned above might have taken place in earlier times after rhyolite volcanism as it is not found in the mudstones of the Nanatani and Teradomari formations or andesitic rocks of the Teradomari Formation. Galena and pyrite crystals occur in some druses; therefore it seems that the hydrothermal solution was similar to that of hydrothermal ore deposits. Ascending hydrothermal solution and the geotherm may be brought on from rhyolite magma. Burial depth was small, however, the hydrocarbons may have been generated in a stage of increasing geotherm around or in volcanic piles and migrated into rhyolite reservoirs along primary fractures and some secondary pores in an earlier stage.
PLATE I
Scanning electron micrographs of rhyolite C (length of bars: a, 1000 ~rn; b, 100 ~m; and c, 10 ~m).
369
® _
__
:<-
....
t
t
"f
~-
---<-
t
Heat
CH z
I
-C
'r "'~/
----------m
1"
t"
t
_3
t Fig. 4. Schematic representation of formation of rhyolite reservoirs and migration 'of hydrocarbons inl~o them: (a) rhyolite A; (b) rhyolite B; and (c) rhyolite C (CH z = hydrocarbons). Dashed lines show presumed isothermal lines (~ 150°C).
Although many problems remain unsolved, relations between formation of rhyolite reservoirs and migration of oil and gas are inferred as follows (Fig. 4); (a) Formation of volcanic piles as a result o f submarine eruption of rhyolite, andesite and basalt, and deposition of mudstone (source rock) around piles. (b) Generation of hydrocarbons by compaction of mudstone during abnormal hightemperature conditions (~150°C) related to rhyolite volcanism and hydrothermal alteration of volcanic rocks.
(c) Lateral migration of oil and gas into secondary pores of rhyolites and enclosure of them under impermeable cap rocks (shale and argillized tuff). Acknowledgements The author wishes to express his sincere thanks to the Teikoku Oil Company for giving facilities for observing samples from wells and for permission to present this paper, and Dr. I, Kobayashi for taking scanning electron micrographs and Dr. M. Nambu for
370
measurement of filling temperature of fluid inclusions. References Katahira, T., 1974. Hydrocarbon deposits found in the Green Tuff in the Niigata sedimentary basin. J. Jpn. Assoc. Pet. Technol., 3 9 : 3 3 7 - - 3 5 6 (in Japanese). Katahira, T. and Ukai, M., 1976. Petroleum fields of Japan with volcanic-rock r e s e r v o i r s - Summary.
Circum-Pac. Energy Miner. Resour., Mere., No. 25, pp. 276--279. Kujiraoka, A., 1965. Volcanic activity and its influence on the migration and accumulation of oil and gas in the Nagaoka plain, Japan. ECAFE (Econ. Comm. Asia--Far East) 3rd Pet. Symp., Tokyo, pp. 1--13. Shimazu, M., 1982. Geological and petrological problems on the so-called Green Tuff reservoirs. J. Jpn. Assoc. Pet. Technol., 4 7 : 2 7 7 - - 2 8 7 (in Japanese).
363
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
ALTERED RHYOLITES AS OIL AND GAS RESERVOIRS IN THE MINAMINAGAOKA GAS FIELD (NIIGATA PREFECTURE, JAPAN) M. SHIMAZU Department of Geology and Mineralogy, Faculty of Science, Niigata University, Niigata (Japan) (Accepted for publication July 16, 1984)
Abstract Shimazu, M., 1985. Altered rhyolites as oil and gas reservoirs in the Minaminagaoka gas field (Niigata Prefecture, Japan); In: Y. Kitano (Guest-Editor), Water--Rock Interaction. Chem. Geol., 49: 363--370. Subsurface geology and petrography of volcanic rocks of the recently discovered Minaminagaoka gas field show that good reservoirs in the field are altered rhyolites of the mid-Miocene Nanatani Formation. The Nanatani F o r m a t i o n is composed of mudstone, lava and pyroclastic rock with a composition of rhyolite, andesite and basalt. Rhyolites are divided into rhyolites A, B, B', C and D according to petrographical features. Rhyolite A is a hyaloclastite and pumiceous tuff of glassy rhyolite. Rhyolites B, B' and D are aphyric rhyolite lavas characterized by needle-like plagioclase in groundmass. Rhyolite C is perlite lava. Rhyolites B, D and C are suitable for reservoirs, whose pores are not primary, but are secondary macroscopic drnses and micropores formed b y hydrothermal alteration. Micropores are openings among crystal grains of quartz and albite and are several tens of micrometers in size. Alteration of rhyolitic rocks is of hydrothermal type and is characterized by a smectite (or smectite/ chlorite mixed-layer mineral)--sericite--albite--quartz--carbonate mineral assemblage in the upper zone, but clay minerals in the lower zone are chlorite and sericite. Alteration of andesite and basalt is similar to that of rhyolite in general, but in the Kitafukazawa bore hole, andesite and basalt below 4640-m depth have chlorite--laumonite--prehnite--(pumpellyite--epidote)assemblage which is correlated to the assemblage of the higher part of the laumonite subfacies of zeolite facies. F o r m a t i o n temperature of secondary quartz, estimated by the measurement of the filling temperature of fluid inclusions, is ~ 150°C and is nearly the same as the present temperature at the bottom of the hole. From the geological environment, characteristics of alteration and paleogeotherm, formation of rhyolite reservoirs and migration of oil and gas can be summarized as follows: (1) formation of volcanic piles by submarine volcanism of rhyolite, andesite and basalt, and deposition of mudstone around the piles; (2) generation of hydrocarbon in mudstone along with increasing temperature related to volcanism; and (3) formation of rhyolite reservoirs by hydrothermal alteration, followed by lateral migration of oil and gas into secondary pores.
1. Introduction Oil and gas fields in Japan, which are productive or are being prepared for exploration at present, are distributed in t h e Niigata and the Akita sedimentary basins and the 0009-2541/85/$03.30
Joban'oki basin. The Aga'oki field of the Niigata basin and Joban'oki field are offshore whereas the others are onshore. Most of the fields except the J o b a n ' o k i field are distributed in the inner belt of northeast Honshu, which is a main part of the Green T u f f Re-
© 1985 Elsevier Science Publishers B.V.
364
gion where distinct tectonic m o v e m e n t and igneous activity t o o k place in Miocene times (Fig. 1). Reservoirs in m a n y oil and gas fields explored before 1960 were found in Pliocene sandstones and/or andesitic rocks. However, reservoirs of the Mitsuke field (discovered in 1958) and those of the Yoshii--Higashikashiwazaki gas field (discovered in 1968) are found to be in Miocene volcanic rocks of the Niigata basin (Kujiraoka, 1965). Pores of the reservoirs of these fields mainly consist of primary fractures in acid volcanic rocks (Katahira, 1974; Katahira and Ukai, 1976). Deep drilling prospecting for oil and gas fields with volcanic reservoirs has been accelerated in surrounding areas b y the discovery o f the large gas field. The Minaminagaoka gas field located 10 km southwest of Nagaoka City was discovered in 1976. The depth of the volcanic reservoir in the field is ~ 4 5 0 0 m, and it is larger than
those of the Mitsuke and Yoshii-Higashikashiwazaki fields. The reservoirs in this field are in altered rhyolites of middle Miocene age. Pores of the reservoirs are not primary fractures, b u t secondary ones formed by hydrothermal alteration. This paper is concerned with volcanic reservoirs in the Minaminagaoka gas field, which is one of the largest fields in Japan.
2. Geological setting The Genozoic formations from the middle Miocene to middle Pleistocene are distributed in the Nagaoka area located in the southern part of the Niigata basin. These formations are the Nanatani, Teradomari, Shiiya, Nishiyama--Haizume and U o n u m a formations, in ascending order (Fig. 2). The total thickness of these formations reaches ~ 6 0 0 0 m. Most of the formations were laid d o w n in a marine environment. The Teradomari and Shiiya N
Kyus
I, o
Fig. 1. Oil and gas fields in Japan.
0kin
/ °°
°°i .... i ......
f reo
365
formations are composed of mudstone, sandstone and andesitic rocks of middle to late Miocene age. The Nishiyama--Haizume Formation is composed of sandstone, siltstone and andesitic rock of Pliocene to early Pleistocene age. The U o n u m a Formation consists of sandstone, siltstone and conglomerate deposited in marine or lacustrine environment in the early to middle Pleistocene. The Nanatani Formation of middle Miocene age is composed of mudstone in the upper portion, and rhyolite, andesite and basalt in the lower portion. The Cenozoic formations form five anticlinal structures running parallel in the Nagaoka area. Oil and gas entrapped in the Pliocene sandstones of the upper part of the anticlines have been produced since 1880. 3. Volcanic rocks in the Nanatani Formation Rhyolites in the Nanatani Formation are lava, hyaloclastite and pyroclastic rock. From Asahibara
rn
Asahibara
petrographical features, these are divided into rhyolites A, B, B', C and D. Rhyolite A is hyaloclastite and pumiceous tuff of glassy rhyolite showing a bluish-green color and is composed of small amounts of plagioclase crystals and sericitized glass or pumice shards. Rhyolite B is compact aphyric rhyolite lava showing a greyish-white or pale greyish-brown color and is composed of small amounts of plagioclase and/or quartz phenocrysts and devitrified groundmass. Rhyolite B' is slightly more basic than rhyolite B. Rhyolite D is a variety of rhyolite B and has spherulitic texture. Rhyolite B, B' and D are characterized by needle-like plagioclase phenocrysts in the groundmass. Rhyolite C is perlite lava showing a pale-green or pale pinkish-white color and is composed of small amounts of plagioclase phenocrysts and devitrified groundmass which shows a perlitic texture. Phenocrysts of mafic minerals are not clear owing to later alteration. The andesite is aphyric lava and consists Shinoyasawa
Kitafukazawa
O. m
3500. 1oo0
E
8
E I
N ~n
2000
"I"
z:
4000-
I
7"
3000
LL~
0E "o
Z
4500-
I--
4000
[15 t,J
LUL Lt.t
P
,_,_c
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LL~ ~LL ~.LL L,~,L L.LL ~LL CL'~ CL'L. AAA
~ ~
, ,
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Z
5000 :
Fig. 2. S t r a t i g r a p h y o f t h e M i n a m i n a g a o k a gas field a n d c o l u m n a r s e c t i o n s o f t h e N a n a t a n i F o r m a t i o n in wells (1 = b a s a l t ; 2 = a n d e s i t e ; 3 = r h y o l i t e B ' ; 4 = r h y o l i t e B; 5 = r h y o l i t e C; 6 = r h y o l i t e A ; 7 = m u d s t o n e ; 8 = d o l e r i t e ) .
366
rhyolite eruption is from rhyolite C through rhyolite B to rhyolite A.
of plagioclase phenocrysts and lath-shaped plagioclase. The basalt is blackish or greenishbrown colored lava and hyaloclastite, and shows partly variolitic texture. It consists of plagioclase phenocrysts and lath-shaped plagioclase and mafic minerals replaced by smectite or chlorite and dusty opaque minerals. The distribution of these volcanic rocks is shown in columnar sections of seven wells (Fig. 2). The deepest well is the Kitafukazawa drilled to 4,890 m deep. The north--south stratigraphical profile is illustrated in Fig. 3. This profile was drawn by assuming the t o p of the Nanatani Formation as a datum horizon, and it shows the existence of a synchronous high formed b y volcanic rocks and also the mutual relation among volcanic rocks of the formation at the time when these rocks were deposited. Most of the basalts and andesites occurs in the lower part, but three basalt layers are intercalated with rhyolites in the upper part and sometimes occur as pillow lavas. The sequence of
4. Alteration o f volcanic
rocks
Volcanic rocks o f the Nanatani Formation in the Minaminagaoka gas field have been heavily altered. Most of the rhyolites show a mineral assemblage of sericite--albite--quartz with dolomite; however, in the lower part of the Kitafukazawa well, rhyolite, andesite and basalt show a mineral assemblage of laumontite--prehnite--chlorite with epidote and pumpellyite. Rhyolite in the upper part of the well has the same assemblage as that of other wells. The assemblage of chlorite, sericite, sericite/smectite mixed-layer mineral, quartz, albite and dolomite in the Kitafukazawa well is transitional in nature between the above t w o assemblages. The t w o types of alteration f o u n d in these mineral assemblages are mainly due to differences in chemical potential of H20 and CO2, namely the laumontite--prehnite--
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Fig. 3. Stratigraphic profile o f t h e Minaminagaoka gas field. S y m b o l s are the same as those of Fig. 2. Chain lines w i t h " / ÷ iv" and w i t h " c h " are t h e upper limit of assemblage o f l a u m o n t i t e - - p r e h n i t e and of o c c u r r e n c e of chlorite, respectively.
367 chlorite assemblage m i g h t be f o r m e d in conditions o f l o w c h e m i c a l p o t e n t i a l o f CO2 and high c h e m i c a l p o t e n t i a l o f H20. A l t e r e d r o c k s having t h e l a u m o n t i t e - p r e h n i t e - - c h l o r i t e assemblage were f o r m e d at the metamorphic condition between the l a u m o n t i t e subfacies o f t h e zeolite facies and p r e h n i t e - - p u m p e U y i t e facies. T h e assemblage o f s e r i c i t e - - a l b i t e - - q u a r t z m i g h t have been f o r m e d at n e a r l y t h e same o r l o w e r t e m p e r a tures. T h e chain line w i t h " c h " in Fig. 3 d e n o t e s t h e u p p e r limit o f o c c u r r e n c e o f chlorite. Formation temperature of secondary quartz
in r h y o l i t e C was e s t i m a t e d b y t h e measurem e n t o f t h e filling t e m p e r a t u r e o f fluid inclusions in q u a r t z . Filling t e m p e r a t u r e , w h i c h was m e a s u r e d b y h e a t i n g stage (rate o f heating: 20°C/rain., e r r o r o f m e a s u r e m e n t : + 2°C), is 1 3 8 - - 1 6 7 ° C a n d is 150°C o n average. T h e p r e s e n t t e m p e r a t u r e at b o t t o m o f t h e well is 1 7 0 ° C at m a x i m u m . 5. Characteristics
r h y o l i t e zeservoiz
of the
Chemical c o m p o s i t i o n s o f r h y o l i t e s are s h o w n in Table I. Silica c o n t e n t s o f r h y o l i t e s B and C range f r o m 61 t o 75 wt.%. K 2 0 con-
TABLE I Chemical composition of rhyolites B and C (1) SiO 2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P~Os
(2)
(3)
(4)
73.85 0.90 12.43
68.26 1.86 11.80
61.40 0.29 14.14
73.08 0.36 11.84
H20-
3.54 0.09 0.41 0.72 6.29 0.16 0.24 0.16 1.21
6.38 0.14 1.43 0.76 3.93 1.20 0.29 0.19 3.76
7.94 0.24 1.74 0.30 3.83 2.69 0.05 0.18 7.17
5.21 0.13 0.86 0.34 5.61 0.16 0.05 0.17 2.21
Total
100.00
100.00
100.00
34.11 1.18 0.94 53.23
36.13 3.34 7.09 33.26
22.04 4.51 15.89 32.41 1.14 . 4.33 8.15 3.45 0.55 0.12
H20 ÷
Q
C Or Ab An Wo En
1.99
Ap
. 1.02 2.35 1.54 1.71 0.56
An (%)
4
Fs Mt II
1.88
.
. 3.56 3.82 2.77 3.53 0.67 5
3
(5)
(6)
75.15 0.32 10.36 0.33 2.32 ~ 0.59 2.00 5.03 0.44 -2.88
68.49 0.31 15.24 1.14 2.76 -0.95 1.65 6.75 0.82
100.02
99.42
100.39
35.67 1.95 0.94 47.47 1.34
38.66 2.60 42.56 4.39 2.31 1.47 3.46 0.47 0.61 --
19.49 0.25 4.85 57.12 0.19
2.14 5.07 2.26 0.68 0.12 3
9
--
2.28
--
2.37 3.62 1.65 0.59 -0
Samples: I = rhyolite B; 2 = rhyolite B; 3 = rhyolite C; 4 = rhyolite C; 5 = rhyolite B; 6 = rhyolite C. Locations: I, Kitafukazawa; 2--4 and 6, Asahibara; 5, Shinoyasawa. Normative minerals: Ab = a l b i t e ; A n = a n o r t h i t e ; A p = a p a t i t e ; C = corundum; E n = e n s t a t i t e ; F s = f e r r o s i l i t e ; I1 = i l m e n i t e ; M t = m a g n e t i t e ; Or = orthoclase; Q ffi quartz; Wo = wollastonite.
368
tents are 0.16--2.69 wt.% and are smaller than those of fresh rhyolites containing the same silica contents, as generally found near the Earth's surface; therefore, leaching o f K20 t o o k place during hydrothermal alteration. Although the mineral assemblage of rhyolites is nearly the same, petrographical features of altered rhyolites are various. Poorness of clay minerals such as sericite and celadonite in rhyolites B and C is due to a small K20 content, and large amounts of recrystallized quartz in rhyolite C reflect perlitic texture and large glass contents. Quartz in the rhyolites replaced glass, or occurs as small lenses and veinlets. Pores in the rhyolites are not primary cracks or joints formed by rapid cooling of lava or tectonic fracturing. Primary fractures have been lost by compaction and recrystaUization. Effective pores as reservoirs are mainly macroscopic druse or "vug" and micropores formed by recrystallization. Druses are several millimeters in diameter and are coated b y secondary quartz and/or albite. Micropores are openings among crystal grains of quartz or albite and are several tens of micrometers in size (Plate I). Glasses of rhyolite C have been extensively replaced by quartz grains, and hence rhyolite C has a large amount of
druses and micropores, and contributes to accumulation of hydrocarbons as a good reservoir. Much of rhyolite B is also a suitable reservoir, whereas rhyolite A is not, because it contains a large amount of clay minerals which filled up the pores. The quality of the reservoir depends on these features o f altered rhyolites (Shimazu, 1982). 6. Formation o f rhyolite reservoirs and migration of oil and gas The hydrothermal alteration forming the pores mentioned above might have taken place in earlier times after rhyolite volcanism as it is not found in the mudstones of the Nanatani and Teradomari formations or andesitic rocks of the Teradomari Formation. Galena and pyrite crystals occur in some druses; therefore it seems that the hydrothermal solution was similar to that of hydrothermal ore deposits. Ascending hydrothermal solution and the geotherm may be brought on from rhyolite magma. Burial depth was small, however, the hydrocarbons may have been generated in a stage of increasing geotherm around or in volcanic piles and migrated into rhyolite reservoirs along primary fractures and some secondary pores in an earlier stage.
PLATE I
Scanning electron micrographs of rhyolite C (length of bars: a, 1000 ~rn; b, 100 ~m; and c, 10 ~m).
369
® _
__
:<-
....
t
t
"f
~-
---<-
t
Heat
CH z
I
-C
'r "'~/
----------m
1"
t"
t
_3
t Fig. 4. Schematic representation of formation of rhyolite reservoirs and migration 'of hydrocarbons inl~o them: (a) rhyolite A; (b) rhyolite B; and (c) rhyolite C (CH z = hydrocarbons). Dashed lines show presumed isothermal lines (~ 150°C).
Although many problems remain unsolved, relations between formation of rhyolite reservoirs and migration of oil and gas are inferred as follows (Fig. 4); (a) Formation of volcanic piles as a result o f submarine eruption of rhyolite, andesite and basalt, and deposition of mudstone (source rock) around piles. (b) Generation of hydrocarbons by compaction of mudstone during abnormal hightemperature conditions (~150°C) related to rhyolite volcanism and hydrothermal alteration of volcanic rocks.
(c) Lateral migration of oil and gas into secondary pores of rhyolites and enclosure of them under impermeable cap rocks (shale and argillized tuff). Acknowledgements The author wishes to express his sincere thanks to the Teikoku Oil Company for giving facilities for observing samples from wells and for permission to present this paper, and Dr. I, Kobayashi for taking scanning electron micrographs and Dr. M. Nambu for
370
measurement of filling temperature of fluid inclusions. References Katahira, T., 1974. Hydrocarbon deposits found in the Green Tuff in the Niigata sedimentary basin. J. Jpn. Assoc. Pet. Technol., 3 9 : 3 3 7 - - 3 5 6 (in Japanese). Katahira, T. and Ukai, M., 1976. Petroleum fields of Japan with volcanic-rock r e s e r v o i r s - Summary.
Circum-Pac. Energy Miner. Resour., Mere., No. 25, pp. 276--279. Kujiraoka, A., 1965. Volcanic activity and its influence on the migration and accumulation of oil and gas in the Nagaoka plain, Japan. ECAFE (Econ. Comm. Asia--Far East) 3rd Pet. Symp., Tokyo, pp. 1--13. Shimazu, M., 1982. Geological and petrological problems on the so-called Green Tuff reservoirs. J. Jpn. Assoc. Pet. Technol., 4 7 : 2 7 7 - - 2 8 7 (in Japanese).