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1
TIGHT SANDSTONE GAS RESERVOIRS (EXPLORATION) IN UPPER PALEOZOIC OF
ORDOS BASIN
YANG Hua FU JinHua WEI XinShan REN JunFeng
(PetroChina Changqing Oilfield Company, Xi’an, 710018, China)
Key wordsOrdos basinUpper Paleozoic, Tight sand, Reservoir
1Introduction
Over the past decade, in the Upper Paleozoic of
Ordos basin, PetroChina Changqing Oilfield
Company has been eye-catching for a large
100-billion-cubic- meters-level gasfield found every
two years averagely. Especially in recent years, a
situation of progressive incremental of 500 billion
cubic meters natural gas reserves submitted
annually has been formed in the Sulige exploration
area. The rapid growth of proven gas reserves
makes Sulige region become the first unitary
large-scale natural gas district with trillion cubic
meters in China [1-4]
. The annual gas production from
tight sandstone reservoirs reaches more than 10
billion cubic meters.
For tight sandstone gas reservoirs in Ordos basin,
a number of scholars have been carried out a lot of
discussion in the relevant literatures [5-34]
. But most of
them concentrated in the discussion of common
features such as low porosity, low permeability, low
abundance, low-pressure, no bottom-water, no
edge-water etc., and less attention to the
characteristics of their personality. At present, nearly
1000 wells are drilled through the Upper Paleozoic of
the basin. Yulin, Zizhou and Sulige gas fields have
been put into large-scale development one after
another. In-depth exploration and development of
Upper Palaeozoic tight sandstone gas reservoirs
provides favorable conditions for revealing the
characteristics and controlling factors of
accumulation. Based on natural gas exploration and
development practice and comprehensive
geological study results of the basin’s Upper
Paleozoic in recent years, the authors mainly
discuss the geological features and unique reservoir
controlling factors of this type of gas reservoirs in the
paper, looking forward to improve constantly the
geological theory of tight sandstone gas
accumulation and to promote continuous deepening
of exploring for such gas reservoirs.
2, Geological background
Located in the middle part of China’s mainland, the
Ordos basin is the country's second largest
sedimentary basin covering a total area of 37 × 104
Figure.1 Location of the Ordos basin, structural units
and cross section
A
B
A
B
2
km2. The basin has experienced five stages of
evolution, that is, the Mid-Late Proterozoic
aulacogen, Early Paleozoic shallow foreshore
platform, Late Paleozoic coastal plain, Mesozoic
inland basin, and Cenozoic faulting depressions
surrounding the basin, as a large-scale natural
gas-rich polycyclic superimposed basin. According
to the characteristics of Mesozoic structures the
basin can be divided into six first-level tectonic units
of the Ih Ju Meng uplift, Ih Ju Meng - Shaanxi slope,
Weibei uplift, western margin of thrust belt, Tianhuan
depression, and Western Shanxi flexible fold belts,
of which Ih Ju Meng - Shaanxi slope is a
west-inclined gentle slope, with a formation dip of
less than 1°, undeveloped structures. The slope is
the main structural unit natural gas accumulated in
the basin (Figure 1).
Proterozoic, Paleozoic, Mesozoic and Cenozoic
sedimentary strata were developed in the basin from
bottom to top. The average thickness of sedimentary
rocks is 6000m, of which the Upper Paleozoic
average thickness is 800 ~ 1000m, mainly including
the gas-bearing strata intervals of Carboniferous
Benxi Formation, Permian Taiyuan Formation,
Shanxi Formation, Shihezi, and Shiqianfeng
Formations (Figure 2).
3, Gas reservoir characteristics of Upper
Paleozoic tight sandstone
3.1 Gas reservoirs are distributed in
large-scale, with multiple gas-bearing series of
strata.
The tight sandstone gas reservoirs of Upper
Paleozoic are distributed in a large linked-up area.
The six major gasfields of Yulin, Uxin Qi, Sulige,
Zizhou, Mizhi, and Shenmu discovered in the Upper
Paleozoic by Changqing Oilfield Company have a
total gas-bearing area of 30 000 km2, accounting for
1/4 of the area of Ih Ju Meng - Shaanxi slope. The
Er at hem Syst em Ser i es For mat i on Symbol Segment
Mai n
gas
l ayer s
Q1
Q2
Q3
Q4
Q5
H1
H2
H3
H4
H5
H6
H7
H8
S1
1
2
3
Da1
Da2
B1
B2
S2
P1t
C2b
50
60
The
Upper
Pal eozoi
c
Car bon
i f er ou
The
Upper
Benxi
For mat i on
Per mi a
n
Shanxi
For mat i on
The
Upper
The
Mi ddl e
Shi hezi
For mat i on
Shi qi anf en
For mayi on
45 55
The
Lower
Tai yuan
For mat i on
P1s
30 45
25 35
25 35
P2h
20 35
20 30
Thi ckness
( m)
P3q 40 50
40 50
60 80
20 30
0 30
35 45
30 40
0 30
25 35
30 40
50 70
40 50
Figure.2 The sketch form of Formation in the upper
Palaeozoic in Ordos basin
Figure.3 Lithological character of sandstones
in upper Paleozoic
Table 1 Characteristics for Upper Paleozoic Gas Reservoir in Ordos Basin
Gas field Series of strata Gas layer thickness/
m
Porosity/%
Permea- bility
Reserves abundanc
e Pressure coefficient
Burial depth of gas layer
Natural productivity before
acidiztion/fracturing
Sulige Permian Shan-1 Member,
He-8 Member 8.2 9.8 0.774 1.10
Lower than 1.00, average 0.87
3250–3390 Industrial gas flow is hardly
seen
Wushenqi Permian He-8 Member 8.0 10.4 0.629 0.60-1.20 Lower than 1, average
0.85 3000–3300
Industrial gas flow is hardly seen
Yulin Permian Shan-2 Member 8.9 6.5 2.050 0.60-1.20 0.93–0.98 2600–3100 0–20000
Zizhou Permian Shan-2 Member 8.5 6.4 1.780 0.81 Lower than 0.10,
average 0.95 2100–2600
Industrial gas flow is hardly seen
Shenmu Permian Taiyun Formation 8.8 7.8 0.643 0.93 average 0.93 2500–2800 Industrial gas flow is hardly
seen
3
gas-bearing area of the single gasfield exceeds
1000 km2. Vertically, each porous thin bed of the
entire Paleozoic profiles under the regional cap rock
(Upper Shihezi Formation) contains gas. There were
totally 11 gas bed sets developed from the Upper
Carboniferous Benxi Formation of the lower part to
the Upper Permian Shiqianfeng Formation.
Therefore, the Upper Paleozoic of the northern basin
is a huge and continuous gas-bearing aggregation,
with vertical and horizontal characteristics
performance of both large-scale distribution and
multiple gas-bearing series of strata.
3.2 The reservoirs are mainly of low porosity
and low permeability. Mid- coarse-grained quartz
sandstone reservoirs have good capability of
oil/gas reserving.
Upper Paleozoic gas-bearing sandstone takes
quartz sandstone and lithic quartz sandstone as the
dominated, locally arkose or feldspar sandstone
(Figure 3). The average reservoir porosity is less
than 8% in general, with the average permeability of
less than 0.5 × 10-3
µm2, but in the local reservoirs,
the porosity usually ranges from 8% ~ 14% and the
permeability 1 ~ 10 × 10-3
µm2 (Table 1). It shows
that the Upper Paleozoic reservoirs belong as a
whole to a set of low porosity and low permeability
reservoirs, but with relatively hypertonic local.
Exploration and further researches show that such a
relatively hypertonic area is generally located in the
mid- coarse-grained quartz sandstone developed
zones, namely, the type of mid-coarse-grained
quartz sandstone is with good reservoir quality.
Two categories of quartz sandstone and lithic
quartz sandstone are mainly developed in the
mid-coarse-grained quartz sandstone reservoirs of
the Upper Paleozoic of the basin. The quartz
sandstone includes two types. One was those
formed in the marine-facies deltaic sedimentary
systems and developed in the Shan2 Member of the
Shanxi Formation of Yulin and Zizhou gasfields, with
mainly intergranular pores type, characterized by
electrical properties of low natural gamma (≤ 35API),
low transitional time (192 ~ 210 ms/m), low
compensated neutron (≤ 6%), low Pe (≤ 1.8 b/e),
high resistivity (100 ~ 1000 Ω.m), and the best
reservoir properties; The other was those formed in
the continental facies fluvial-lake shallow water
deltaic depositional system and developed in He8
Member of Lower Shihezi Formation of Sulige, Uxin
Qi gasfields, with mainly reservoir pore space of
intergranular pores and dissolution pores, and the
main electrical features of lower natural gamma (≤
35 API), low-mid resistivity (40 ~ 80 Ω.m), mid-high
interval transit time (220 ~ 240 µs/m), and reservoir
properties the second place. Also the lithic quartz
sandstone was formed respectively in marine deltaic
depositional system and fluvial-lake shallow water
deltaic depositional system, and developed in He8
Member of Taiyuan and Lower Shihezi Formations of
Shenmu gasfield and Eastern District of Sulige
gasfield, with dissolution pores as the main pore type,
and the main electrical features of mid-high natural
gamma (<55API), low-mid acoustic wave slowness
(210 ~ 230 µs/m), low-mid compensated neutron
(8~12 %), low-mid resistivity (30 ~ 80 Ωm), and
relatively poor reservoir properties (Table 2, Figure 4,
Figure 5).
33 The vertical superimposition relationship
of gas reservoirs and source rocks
Table.2 characteristic of three types of quartz sandstones of upper Paleozoic in Ordos basin
Quartz
(%)
Pore
Structure
Logging
characters
Sedimentary
facies
Permea-
bility
Gas
Fields
1 >90% Mainly of intergranular pores
High resistivity
Low interval transit time
Low gamma ray
Marine-facies shallow
deltaicbest
Yulin
Zizhou
2 >90%
Mainly of dissolved
pores,some of intergranular
pores
Low resistivity
Hih interval transit time
Low gamma ray
fluvial delta facies
shallow deltaicbetter
Sulige
Wushenq
i
Lithic
quartz
sandstone >75% Mainly of dissolved pores
Low resistivity
Hih interval transit time
High gamma ray
Marine-facies shallow
deltaic,fluvial delta
facies shallow deltaic
bad
Shenmu
Estern
Sulige
Lithology
Quartz
sandstone
4
According to source rocks and gas reservoirs
distribution relationship, gas migration and
accumulation of Upper Paleozoic tight sand gas
reservoirs can be further divided into three types:
within source rocks, above source rocks, and far
from source rocks(Figure 6).
To the type within source rocks, the relationship
of gas reservoirs and source rocks is interbedded
superimposition, with competent gas injection and
sufficient hydrocarbon supply, so gas saturation is
high, generally over 70%, such as Benxi Formation,
Taiyuan Formation in Shenmu gas field, and S2 of
Shanxi Formation in Shenmu gas field. To the type
above source rocks, gas reservoirs are on the top of
coal source rocks, and vertical distance is less than
55m, so that the vertical distance of gas migration
and accumulation is close, hydrocarbon supply
relatively is sufficient, and gas saturation relatively is
1.Intergranular pore2. Dissolved pore
Quartzose sandstone with Intergranular por, Quartzose sandstone with intergranular and dissolved pores, lithic quartz sandstone with dissolved pores
Figure.4 Types of quartz sandstones of upper Paleozoic in Ordos basin
Quartzose sandstone with Intergranular por, Quartzose sandstone with intergranular and dissolved pores, lithic quartz sandstone with dissolved pores
Figure.5 Logging characteristic of three types of quartz sandstones of upper Paleozoic in Ordos basin
5
high, generally 65 70 ,
such as H8 of Shanxi
Formation and S1 of Shanxi
Formation in Sulige and
Shenmu gas field. To the type
far from source rocks, gas
reservoirs are on source rocks,
with relatively long vertical
distance. This kind of gas
reservoirs generally develop
within the regional cover, with
small scale and low gas
saturation, such as Shihezi Formation top, and
Shiqianfeng Formation.
3 4 Gas reservoirs are mainly large-scale
lithologic trap, with indistinct trap boundaries
and continuous gas reservoirs characteristics
Upper Paleozoic tight sandstone gas reservoirs
are distributed in a large area with continuous gas
reservoirs characteristics[35,36]
. First of all,
gas-bearing sand body is interconnected, and a
number of gas reservoirs exist in this region. This
has been confirmed by gas reservoirs development,
with 800m development well spacing. But in the
current technical conditions, this kind of gas
reservoirs boundary is difficult to determine, and the
submitted reserve is generally determined by
artificial-calculated boundary, which shows that
continuous oil and gas reservoirs have no distinct
trap boundary. In addition, in gas bearing area,
specific well producing water indicates part of the
region contains water, but it has no clear gas-water
interface. While, it has “water-in-gas” gas-water
horizontal distribution pattern..
35 Atmospheric pressure, low pressure, high
pressure gas coexistence
By computationally analyzing the measured data
of formation hydrostatic pressure, formation
temperature, and other parameters, the pressure
coefficient 0.95 should be the lower limit of Upper
Paleozoic normal pressure. Therefore, in Ordos
Basin, the Upper Paleozoic pressure coefficient 0.95
~ 1.05 is the normal pressure, below 0.95 for low
pressure, and up 1.05 for high pressure. For 33 wells
in Sulige gas field, the main gas bearing formation
pressure coefficients are from 0. 69 to 0. 98, with an
average of 0. 87, belonging to low pressure
reservoirs. For 24 wells in Yulin gas field, the main
gas bearing formation pressure coefficients are from
0. 95 to 1. 02, with an average of 0.99, belonging to
atmosphere pressure reservoirs. While for other 9
wells, such as Yu4 of Mizhi gas field, the main gas
bearing formation pressure coefficients are from 1.
05 to 1. 16, showing some gas reservoirs have high
pressure characteristics[37-40]
(Table 1).
4 The formation of tight sandstone gas reservoirs
4 1 Large-scale hydrocarbon generation of
coal-measure source rocks, with vertical
migration and accumulation
4.1.1 Large-scale distribution and hydrocarbon
generation of coal bed and carbon-bearing
mudstone
Coal was steadily generated with large scale in
the special paleogeographic background of
epicontinental marine basin in Upper Paleozoic [41-42]
.
Drilling and seismic data reveal that coal develops in
92% area(major in Benxi, Taiyuan and Shanxi
Formation) of present-day basin with 10 30m
totalized thickness, and the organic carbon content
as high as 70.8% ~ 83.2%. Dark mudstone is also
widely distributed in the whole basin, with thickness
of about 50m, and the organic carbon content of
2.0% ~ 3.0%. Cover depth of coal strata source
rocks reached the maximum in the Early
Cretaceous(Figure 7), and large-scale source rocks
were in the evolution phase form maturation to
post-maturation, under the co-action of abnormal
thermal events. Rof mature source rocks is greater
than 1.2%, with an area of 18×108m
3/km
2, occupying
Figure.6 Model of gas migration and accumulation the upper Paleozoic
in Ordos basin
6
72% of present-day basin. The performance of
hydrocarbon source rocks with large-scale
distribution, large-sized mature, and large-sized
hydrocarbon generation (Figure 8), laid the material
foundation of Upper Paleozoic gas reservoirs
formation.
4.1.2 Migration and accumulation of natural gas are
mainly vertical in close distance.
Studies show that formation process of Upper
Paleozoic tight sandstone gas reservoirs, may not
be a large-scale lateral migration, but vertical
migration and accumulation in short distance[43-52]
(Figure 6). The main bases are followed below. First,
in the Upper Paleozoic 16 formations, gas bearing
formation and gas-bearing bed are discovered at the
moment. At the same level of evolution of source
rock area, methane content increases along the
vertical migratory direction, and methane
isotopeδ13
C1 becomes light. Secondly, natural gas
composition in the gas bearing formation is mainly
affected by the maturity of near source rocks. In gas
bearing formation, for some south region in which Ro
is over 2.0, its methane content will reach 99.5.
While for some north region in which Ro is less than
1.2 , its methane content only has about 90 .
Methane isotope content distribution manifests
low-South (δ13
C1: -28%) and high-North (δ13
C1:
-32%) pattern. This illustrates the lateral migration
distance of natural gas is short, and
north-toward-south riverdeltaic deposit sand zone
develops in the Upper Paleozoic of the basin.
Mudstone or siltstone distributes between the zones,
becoming lateral barrier so that this phenomenon
inhibits the upward large-scale lateral migration of
Figure.7 Model of the diagenesis and gas acculumcation of the upper Paleozoic in Ordos basin
Figure.8 The bio-hydrocarbon intensify map of upper
Paleozoic in Ordos Basin
7
natural gas in the east-west direction.
42 Hydrocarbon-bearing intensity dominates
gas boundary.
4.2.1 Tight sand gas reservoirs development with
low hydrocarbon-bearing intensity
According to Chinese scholars’ study on
hydrocarbon-bearing intensity of
big-and-middle-scale gas fields, these fields are
ordinarily located in the regions where
hydrocarbon-bearing intensity is above 20×108
m3/km
2. But exploration in Ordos basin proves that
large-scale gas reservoirs are still distributed in the
regions where hydrocarbon-bearing intensity is 14
20×108 m
3/km
2. So tight sand gas reservoirs with low
permeability, low abundance and large scale can
develop with low hydrocarbon-bearing intensity, and
the boundary of hydrocarbon-bearing intensity
being in the range of 14×108 m
3/km
2. For the region
below 14×108 m
3/km
2, gas-water commixture region
forms in the neighborhood, while part of it forms
gas-in-water distribution pattern.
4.2.2 The distance between source rocks and gas
reservoir determines the degree of enrichment of
gas reservoirs
The vertical contrast analysis of the gas reservoir
showed that, due to directly being inside the
coal-bearing series of Carboniferous - Permian
source rock, the Carboniferous Benxi Formation -
Taiyuan Formation - Shanxi Formation in the
mid-lower parts of the Upper Paleozoic asume a
state of salternating beds with the source rock,
having sufficient gas sources. Additionally marine -
transitional facies quartzose sandstone reservoirs
were developed there, gas saturation of the strata
was evidently higher than that of the other strata;
while the gas saturation of upper Shihezi Formation
and Shiqianfeng Formation gas reservoirs on the top
of the source rock was relatively low. The
configuration relationship between gas reservoirs
and source rocks directly determines the degree of
gas enrichment (Figure 6).
4.3 The large-scaled linked-up reservoir
sandbody formed by large-scale shallow-water
delta deposits
Sedimentary characteristics analysis shows that
typical shallow-water delta deposits were developed
in the Upper Paleozoic, due to the flat basin
basement in Late Paleozoic and continuing uplift of
the source area in northern basin [53,54]
. The reservoir
sand bodies distributed in a large area are favorite
place to the development of tight sandstone gas
reservoirs (Figure 9, Figure 10).
4.3.1 The shallow-water delta sand body
distributed in large-area and multiple strata
The sedimentary evolution of Upper Paleozoic of
the basin is quite regular. From the Carboniferous
Benxi Formation to Permian Shiqianfeng Formation,
the sedimentary evolution experienced a course of
shallow continental shelf - barrier coast - Delta -
shallow lake and fluvial delta facies, developed
large-area distribution and reservoir sand bodies of
multi-type genesis.
And the large-area distribution of sand bodies has
been confirmed by drilling. Taking the He8 Member
of Lower Shihezi Formation as an example, based
on statistics from 234 exploratory wells, the
probability of penetration of sandbodies is 92%, and
probability of penetration of gas reservoirs is 65%.
Probability of penetration of sandbodies in Sulige
area is more than 95%, and so is the probability of
penetration of gas reservoirs.
4.3.2 High-energy facies belts and constructive
diagenetic facies co-act to form "dessert" of relative
hypertonic districts
Figure.9 Sedimentary model of shallow delta
in upper Paleozoic
8
Comprehensive geologic study and analysis show
that, during the fluvial - delta deposition, rock
composition of different sedimentary facies belts has
significant differences, because of differences in
hydrodynamic conditions. And it further affects the
late diagenetic evolution and characteristics of
porosity development. A corresponding relationship
can be established between them. The main
distributary channel sand body was mainly
composed of mid-coarse-grained quartz sandstone,
with the strongest hydrodynamic conditions, and
good sorting of rock, high content of rigid particles of
quartz, and increased anti-compaction. This was
conducive to preservation of primary porosity, pore
fluid activities and the formation of secondary pores.
The dissolution diagenetic facies are mainly formed,
with main pore type of intergranular and dissolution
pores, and a good capability of reservoir. It generally
can form a local "dessert" of relative high
permeability district. Due to the weakening of
hydrodynamic conditions, the edge parts of the two
channels and the natural levee sites take generally
fine-grains as the dominant, with sorting deteriorated,
and increased content of soft components like debris.
In the course of the late diagenetic evolution, the
reservoirs are easily compacted and become dense,
with primary pores compacted out basically. Further
more, the tight reservoirs are not conducive to the
pore fluid activities and the formation of secondary
dissolution porosity. Diagenetic facies take alteration
and compaction as the dominant. Reservoir
properties were gradually deteriorated.
4.4 Excellent preservation conditions
4.4.1 Upper Shihezi Formation regional caprock
with good sealing conditions
The lacustrine mudstone of Upper Shihezi
Formation developed above the major gas layers of
Upper Paleozoic was the regional caprock of Upper
Paleozoic. This set of mudstone is distributed
continuously in the whole basin, with thickness of 80
m–110 m. The average permeability of mudstone or
sandy mudstone is between 10-10
and 10-8µm
2, the
breakthrough pressure is higher than 5 MPa, so the
regional cap rock provides good sealing condition for
conservation of large-scale lithologic gas reservoir.
4.4.2 The stability of Cratonic block guarantees
the late preservation of gas reservoirs
Destruction in the late stage was the prominent
problem of continental facies oil and gas reservoirs
in China [55-59]
. However, the stability of craton
during late evolution on Yi-Shaan slope of Ordos
Basin avoids such a problem effectively, which is
featured by weak magmation and steady structure
and dominated by overall uplift and subsidence.
Therefore, the present monocline structure with flat
formation and strata dip lower than 1° was formed,
providing good conservation condition for
large-scale lithologic gas reservoirs.
5 Conclusions
Upper Paleozoic of Ordos Basin develops
large-scale, multi-series strata distribution of the
tight sandstone gas reservoirs, with the
characteristics of continuity. Exploration and
geological research show that the control factors of
the Upper Paleozoic formation of tight sandstone
gas reservoirs include: large-scale distribution of the
coal-bearing source rocks in large areas of
hydrocarbon generation, and vertical migration and
accumulation: large-scale sandbody of
shallow-water delta deposits provided favorable
natural gas gathering places for tight sandstone gas
reservoirs; hydrocarbon generating intensity
Figure.10 Sedimentary evolution in later Paleozoic
in Ordos basin
9
controlled the enrichment of the tight gas reservoir; a
good seal of regional cap, and reservoir densification,
as well as the stability of cratonic basin ensured the
preservation of gas reservoir.
The exploration and development of tight
sandstone gas reservoirs are difficult, but after a
long-term exploration and development, Changqing
Oilfield Company has formed a set of tight
sandstone gas reservoirs exploration technology
and methods in Upper Paleozoic of Ordos Basin,
mainly including high-resolution the rate of
two-dimensional seismic prestack sandbody
prediction, seismic prestack gas detection
technology, logging of lithology recognition
technology as well as the multi-line multi-layered
fracturing technology for tight reservoirs. For this
type of gas reservoirs exploration, we should use "
the overall study, development, and evaluation, as
well as the implementation of step-by-step" to
construct Ordos Basin into a base of tight sandstone
gas reservoir exploration and development.
Acknowledgements:
We show our sincere thanks to Mr. LIU XinShe,
ZHAO HuiTao, ZHAO TaiPing and Ms. WANG Xin,
ZHAN Sha, for their helpful materials and graphics.
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