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There
are 3 million km2 of offshore area where oil exploration
began in 1960's. In the initial years, exploration was
focused in shallow-water area such as Bohai Sea; after
1979, in cooperation with foreign oil companies, exploration
was expanded to most offshore basins. Oil exploration
has now been carried out in each of the offshore basins.
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In
the past twenty years, a series of sedimentary basins
rich in hydrocarbon potential have been discovered in
Offshore China areas. They cover a total area of nearly
1 million km2. These sedimentary basins, from north to
south, are Bohai Basin, North Yellow Sea Basin, South
Yellow Sea Basin, East China Sea Basin, Okinawa Trough
Basin, West Taiwan Basin, Southwest Taiwan Basin, East
Taiwan Basin, Pearl River Mouth Basin, Beibu Bay Basin,
Yinggehai-Qiongdongnan Basin and basins in the south part
of South China Sea, etc.
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Oil
and gas exploration in offshore China has mainly focused
on Bohai Bay, Yellow Sea, East China Sea and the northern
continental shelf of South China Sea, where the predicted
hydrocarbon resource potential reaches 27.53 billion tons
of oil and 10.6 BCM of natural gas, while discovery rate
there is only 18.5% and 9.2% for oil and gas respectively. |
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Sedimentary
Basins |
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Junggar
Basin |
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The
Junggar Basin, which is situated in the hinterland of
Asia and far from the sea, is one of the major oil- and
gas-bearing basins in western China and covers an area
of 140,000 km2. Commercial oil flows of varying amounts
have been found in Late Paleozoic, Mesozoic and Cenozoic
strata. |
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Sedimentary
formations, including Carboniferous, Permian and Meso-Cenozoic,
appear only in the basin's marginal areas. A vast area
inside the basin is covered by the Quaternary. The Upper
Carboniferous and the Lower Permian are transitional sediments
of marine to continental facies. The Upper Permian is
a grayish-black, burnable organic rock formation. The
bottom of the Mesozoic (P2-T1) is a 200-1200m thick molasse
red-bed, its middle part (T2-J2) is coal-bearing sediments,
250-4000m thick, and the upper part (J3-K1) is a formation
of red-beds and varicolored sandstone and mudstone, 210-2,500
m thick. The Cenozoic consists mainly of red-beds of clastic
rocks. |
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The
most important source rock is Upper Paleozoic. The coal
series of the Mesozoic is confined only to the central-southern
part of the basin and has possible gas potential. Significant
Cenozoic source beds only occur in the southwestern part
of the basin. |
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Reservoirs
are mainly terrestrial lacustrine/fluvial facies sandstone
and conglomerate. |
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The
traps in this basin are characterized by their variety
in form and type. But rich hydrocarbon accumulations are
controlled by the main thrust fault. Different kinds of
traps and various types of reservoirs occur along both
sides of the fault, such as the fault-barriered oil pools,
fault block-anticlinal oil pools, buried mountain-unconformity
stratigraphic-trap oil pools, basement oil pools, stratigraphic-trap
oil pools, bitumen-sealed trap oil pools, anticlinal oil
pools, etc. |
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Tarim
Basin |
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The
Tarim Basin located between the Tianshan and Kunlun ranges
covers 560,000 km2 and is the largest in China. Its size
and shape is comparable to the Sea of Japan, a modern
back-arc basin. Commercial oil and gas flows of varying
amounts have been found in Jurassic and Tertiary sandstone
and Cambrian-Ordovician carbonates.
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The
crystalline basement of the Tarim Platform is composed
of Archeozoic and Proterozoic rocks. The marine sedimentary
cover formed in the Late Proterozoic and Paleozoic. During
the Mesozoic and Cenozoic, a continental basin structural
regime formed. The source rocks are Cambrian-Ordovician,
Carboniferous-Permian, Triassic-Jurassic, Upper Cretaceous-Lower
Tertiary and Upper Tertiary.
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Various
kinds of traps make different kinds of petroleum reservoirs
possible, including anticlinal and faulted reservoirs,
lithologic-structure reservoirs and the basement reservoirs
related to unconformities. |
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Qaidam
Basin |
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The
Qaidam basin located in NW China and situated in the northeastern
Qinghai-Tibetan Plateau, is a typical Meso-Cenozoic nonmarine
petroliferous basin. The area of Meso-Cenozoic deposits
is about 96,000 km2, with a maximum sediment thickness
of 16 km. |
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The
basement of this basin is lower Paleozoic and pre-Paleozoic
crystalline rock and granite. The Mesozoic (Jurassic and
Cretaceous) sedimentary rocks are found only in the northern
part of the basin. The overlying sedimentary strata start
from Paleogene. |
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The
Lulehe formation of Paleocene age, to the lower part of
lower Ganchaigou formation of Oligocene age, consists
of brownish-red mudstone interbedded with conglomerate
and sandstone. The 400-1900 m. thick formation underlay
the entire basin. The upper part of lower Ganchaigou formation
to upper Ganchaigou formation of Miocene age consisting
of dark gray mudstone intercalated with marl and siltstone,
are wildly distributed in the western part of the basin.
The Pliocene consists mainly of brownish red and grayish
green mudstone interbedded with conglomerate, granulite
and sandstone. In Quaternary, the landscape of Qaidam
basin became wide fluvial plain with several remnant salt
lakes in it. |
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In
the northern part of the basin, the Lower-Middle Jurassic
is the source rock and the Tertiary sandstone is the reservoir.
In the western part of the basin, the thick Oligocene
and Miocene mudstone is the major source rock and the
Tertiary sandstones below and above the source rock are
reservoirs. |
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There
are different types of oil and gas pools in the basin:
lower reservoir-upper source-rock type, lower source-rock-upper
reservoir type, oil and gas pools of laterally connected
source- and reservoir-rock assemblages, carbonate fissure-vug-type
oil and gas pools. |
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Ordos
basin |
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The
Ordos basin, with an area of 250,000 km2 is the oldest craton in China. The basement structure of the Ordos basin
is double-layered. One layer is of Archeozoic granulite
lithofacies. The other layer is dominated by Lower and
Middle Proterozoic greenschist facies. |
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The
development of the sedimentary cover can be divided into
five evolutionary stages as follows: aulacogen basin in
the Middle-Late Proterozoic continental rift stage; Early
Paleozoic marine carbonate platform stage with a geosyncline-platform
tectonic regime; the Late Paleozoic (C2) to Early Mesozoic
(T2) intermediate stage characterized by the change from
marine facies to inland basinal facies; the formation
and development of the Mesozoic polycyclic inland basin;
the uplift and disintegration of the basin and the developing
stage of the Cenozoic graben system. |
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There
are multiple sequences of source rock in the basin, such
as Early Paleozoic carbonate rocks, Late Paleozoic coal
measures and Mesozoic shales of the Yanchang and Yanan
formations. The total thickness of the source rocks exceeds
2000 m. |
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There
are different types of oil and gas pools in the basin:
lithologic pinchout oil and gas pools, sand lens oil and
gas pools, stratigraphic oil and gas pools, weathered
crust oil and gas pools, anticlinal oil and gas pools.
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Songliao
Basin |
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The
Songliao Basin of northeast China is a large Meso-Cenozoic
continental sedimentary basin, with the total area of
260,000 km2. The basement of the basin consists mainly
of Paleozoic and pre-Paleozoic metamorphic rocks and various
igneous rocks.
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The
Jurassic deposits are sandstone, shale and conglomerate
with coal intercalations. The climax in the development
of the basin was in Cretaceous time. The process includes
three stages, i.e., a rift-valley stage of taphrogeny,
a subsiding stage of a large sedimentary depression, and
shrinkage and uplift stage. Fluvial-lacustrine sediments
attain a total thickness of more than 5000 m, and comprise
the main source rocks and reservoir rocks. |
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The
three main oil source rocks of the Cretaceous system in
the basin are Qingshankou 1, 2-3, Nenjiang 1, which are
all abundant in organic matter. There are different types
of oil and gas pools in the basin: anticlinal oil and
gas pools, lithologic oil and gas pools, lithologic-structural
oil and gas pools, subtle oil and gas pools. |
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Bohai
Bay Basin |
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The
Bohai Bay Basin covers 200,000 km2 Bohai Bay and its coastal
areas. |
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The
Bohai Bay basin was formed and evolved on the basis of
the ancient North China Platform through the platform-development
period of the Middle-Upper Proterozoic and Paleozoic,
and the faulted-subsidence development period of Mesozoic
and Cenozoic.
The basement is composed of metamorphic rocks with ages
of 1.7-3.7Ga.
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The
depositional history of Paleogene Bohai Bay basin consists
of 4 cycles, namely the cycle of kongdian formation, cycle
of 4th and 3rd members of Shahejie formation, cycle of
2nd and 1st members of Shahejie formation and cycle of
Dongying formation. The Kongdian formation represents
deposits in the initial stage of rifting. The Shahejie
formation makes up the frame of the basin deposits with
expansion and contraction, deep faulting and uplifting.
The Dongying formation is the record of the late stage
of rift-faulting when contraction dominates with intercalations
of smaller scale. The middle part of Kongdian formation's
dark mudstone is the first source bed of the basin. The
3rd and 1st members of Shahejie formation are the two
major oil-generating beds. |
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The
reservoir rock bodies can be classified into five types,
i.e., alluvial-fan, delta, lake-floor-fan, clastic-limestone
or algal-reef, and lenticular sandbodies.
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There
are different types of oil and gas pools in the basin:
the faulted basement-block pools, folding structural pools,
stratigraphic-lithologic pools, draped structural pools,
reverse-drag structural and stratigraphic pools and heavy-oil
pools. |
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Sichuan
basin |
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There
is a long history of exploration and exploitation in the
Sichuan Basin. A complete system of drilling, recovery,
transportation and utilization had already been developed
in ancient times. The Sichuan basin is a large-sized oil-
and gas- bearing basin in the Yangze paleoplate. Its basement
is metamorphic rocks of Pre-Sinian. |
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In
the Early Paleozoic, the regional elevation and subsidence
resulted in large-sized uplift and depression structures,
and it was dominated by neritic platform deposits. In
Devonian and Carboniferous, the Upper Yangtze Craton was
gradually rising from west to east to form the Leshan-Longnusi
paleouplift, which later underwent denudation. The process
from the vast transgression in the Early Permian to the
salting and shrinking of the sea basin in the Middle Triassic
formed a complete and sequential sedimentary cycle. Since
the Late Triassic, the margin of the basin uplifted and
fluvial-lacustrine sedimentary system distributed in the
whole basin. At the end of Early Cretaceous, the basin
experienced folding and uplift. |
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The
sediments of the Early Paleozoic are mainly polycyclic,
neritic-platform facies in which there are many oil- and
gas-generating strata. The Triassic sediments developed
bay facies and transitional marine and continental facies
which intercalates with the lacustrine-bog facies. The
two types of deposit are superimposed upon each other
and can provide considerably abundant gas resources. |
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There
are different types of oil and gas pools in the basin:
the anticlinal pools and stratigraphic-lithologic pools. |
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Jianghan
Basin |
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The
Jianghan Basin is located in the middle part of Jianghan
plain, Hubei Province, with an area of 28,000 km2. Structurally,
the basin is located the middle part of Yangtze depression
in Yangtze paraplatform. It is a Mesozoic-Cenozoic extensional-fault
basin that developed after the Yanshan movement. |
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Because
of regional structural movement, basement differential
subsidence and block-faulting, a series of sags and depressions
were formed in the Jianghan basin. The basin is divided
into eleven sags and four uplifts |
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The
Cretaceous-Eogene sedimentary sequence is divided into
Yuyang, Shashi Xingouzi Jingsha, Qianjiang Jinghezhen
and Guanhua formations in the Jianghan basin. |
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There
are four source rocks in the basin. They are Yuyang formation,
upper member of Shashi formation, lower Xingouzui formation
and Qianjiang Formation. Of them, Q3 and Q4 members of
Qiangjiang formation and lower member of Xingouzui formation
are important source rocks. According to our research
result, there are six petroleum systems (Jingling, Mianyang,
Chengtuokou, North Qianjiang, South Qianjiang and Xiaoban)
and seventeen plays. |
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Subei
Basin |
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The
Subei basin covering 35,000 km2 is located in northeast
Yangtze continental mass and belongs to the offshore part
of Subei-South Yellow sea basin. It is a Mesozoic-Cenozoic
continental basin that developed after Yizhen movement
of Yanshan structural cycle. According to the distribution
and development of Eogene strata, the Subei basin is divided
into two depressions (Dongtai and Yanfu depression) and
one uplift (Jianhu uplift). |
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The
Mesozoic-Cenozoic sedimentary sequence with the thickness
of over 7,000 m is divided into Cretaceous Taizhou formation,
Eogene Funing Dainan and Sanduo formations, Neogene Yancheng
formation and Quaternary Dongtai formation in Subei basin. |
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In
Subei basin, there are six source rocks from Cretaceous
to Eogene. They are Tai-2 member of Taizhou formation,
Fu-1, Fu-2, Fu-3 and Fu-4 members of Funing formation
and Dai-1 member of Dainan formation. The source rocks
are widely distributed over these depressions with the
total thickness of over 400 m. |
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There
are three petroleum systems and eleven oil-bearing strata.
The petroleum systems are Taizhou-Funing system, Dainan-Sanduo
system and Mesozoic-Paleozoic system. |
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Nanxiang
Basin |
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Nanxiang
basin with an area of 17,000 km2 is located in the Qinling-Dabie
geosynclinal region, south of Henan Province, and is a
Mesozoic-Cenozoic intermountain rift basin formed at the
end period of the Yanshan Movement. Since the late Cretaceous,
the Nanxiang basin undergone two development stages (faulting
and depression), and the former stage is the main period
of the sags and uplifts formation. |
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The
Nanxiang basin includes three sags (Miyang, Nanyang and
Xiangyang) and three uplifts (Shigan, Sheqi and Xinye).
The sags of the basin are controlled by faults, which
include the pre-Mesozoic NW faults active chronically
and the new NE faults formed during the Yanshan movement
period, and they cut the basin into square figures by
which the sags and the uplifts are distributed alternately.
The difference of sedimentary thickness, distributing
area, hydrocarbon-generation condition and accumulation
degree is very distinctive in the three sags, of the three
sags, the hydrocarbon accumulation condition of the Miyang
sag is the best and the Nanyang is the second, and no
show has been discovered in the Xiangyang sags. |
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The
upper Cretaceous and Tertiary strata of the basin developed
on the bases of the Proterozoic metamorphic rocks. The
Paleogene period is the main stage of the basin formation
and the lower Tertiary is the thickest (over 6000 m).
From younger to older, it could be divided into Yuhuangding-Dacangfang
formation, Hetaoyuan formation and Liaozhuang formation.
The Hetaoyuan formation belonged to the stable sedimentation
of the late-Eocene lake, and it consists mainly of dark-gray
shale and sandstone deposit of deep lake environment.
This is as main exploration target formation. |
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Baise
Basin |
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Baise
basin is located in the west-south of Guangxi Chuang municipality.
It is the popedom of Baise, Tianyang and Tiandong counties.
It lies between east longitude 106°34'-107°21' and north
latitude 23°23'-23°47'. Its area is 830 km2. |
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Stratum:
Basin basin is a residual basin in Eogene system superimposed
on Triassic basement. From the lower to the upper: strata
are the Hongseyan formation in lower and middle Eocene
in Eogene system, Nadu formation in upper Eocene, Baigang
formation in Oligocene, Fuping formation in Miocene
of Neogene system, Jianduling formation in Miocene and
Pliocene, Changsheling formation in Pleistocene in Quaternary
system and Holocene series.
Structure:
Baise basin is a Mesozoic inland rifted-basin developed
on fold basement in middle Triassic, controlled by
north-west tectonic. From west to east, Baise basin
is divided into 5 secondary structural units: Baise
depression, Sitang swell, Tianyang depression, Nabai
swell and Tiandong depression. There is north-east
fault between units. Depressions in basin are all
asymmetric syncline which is flat in the south and
steep in the north. Its subsidence center is in the
north of the basin.
Oil
source bed, reservoir and caprock:
The oil source bed in Baise basin is mainly Nadu and
Baigang formations. Its main reservoir is middle Triassic
Nadu formation and Baigang formation in Eogene system.
Reservoir types are porous-type sandstone and fracture-type
carbonate.
Oilfields
and oil reservoir type:
Since Linpeng oil-bearing structure was known in 1959,
Shangfa, Zimou, Lunwei, Tangzhai, Huacha, Nakun and
Leigong, etc., oilfields have been found. The oil
and gas pool type is mainly structural, fracture,
stratigraphic unconformity and lithological
reservoirs.
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East
Sea Basin |
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East
sea basin is located in the east to Fujian and Zhejiang
provinces and in the north to Taiwan Island. Its area
is 260,000 km2. It is a large-scale sedimentary basin
with its main clastic sediment of Caenozoic era. Mesozoic
and Caenozoic strata were developed well in East Sea basin.
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Structures:
East sea basin is a Cenozoic back-arc spreading basin.
It has two-layer structure with half graben-type depression
in the upper. From south to north, this basin is divided
into Taixi, Taibei, Zhedong and Fujiang depressions. Every
depression is divided into secondary sags and swells. |
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Oil
and gas exploration work in East Sea basin is mainly concentrated
on Xihu sag and Taibei depression. The oil source bed
in Xihu sag is Pinghu formation of Eocene age, and its
reservoir is sandstone of Eocene and Oligocene age. The
oil and gas pool is anticline-type and stratigraphic-structural-type.
Up to now, Pinghu and Chunxiao oil and gas fields have
been found in East Sea basin.
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Resources
Potential |
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Although
there are complex geological settings, imbalanced resource
structure and main oilfields' decrement in China, it has
great potentials of oil-gas resources.
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- Totally, the extent of exploration has been relatively
low and uneven in most of the sedimentary basins all over
the China. Especially the oil-gas exploration and development
in the offshore regions has been at the initial stage.
Some big oil and gas fields have been discovered. For
example, in 1999-2000, huge oil-gas fields, such as No.
2 Kela and Sulige with the proven reserves of over 7 Tcf
and Penglai 19-3, an equipped huge oilfield with the proven
reserves of 4.4 billion barrels, have successively been
discovered.
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Lots of new areas still are under explored such as
basins in the south and other areas. It has been
predicted that these areas hold in store rich oil-gas
resources of 270 billion barrels; over 30% of total
oil-gas resources.
- There exist many fields and strata that have good resources
potentials and perspectives, for example, pre-tertiary
in the eastern regions of China, deep-water regions in
the offshore and thrust blocks in the both southern and
northern sides of Qinglin-Dabie Mountains.
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