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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