Study on Reservoir Characteristics and Physical Property Lower

Limit of Fengcheng Formation Tight Reservoir in South Slope of

Mahu Sag

Zongbin Zhang

, Jun Qin

, Mengyun Han

, Zhongchen Ba

, Xinyu Chen

, and Jiang He

3,*

Research Institute of Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay, Xinjiang 834000,

China

Fengcheng Oilfield Operation District, Xinjiang Oilfield Company, PetroChina, Karamay, Xinjiang 834000, China

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu,

610500, China

Keywords: Junggar Basin, Fengcheng Formation, Tight oil, Effective reservoir, Lower limit of physical properties

Abstract: Fan deltaic depositional system is developed in the Permian Fengcheng Formation of Kebai area on the south

slope of Mahu sag, the reservoir with ultra-low porosity, permeability and strong heterogeneity. The

characteristics and lower limit of reservoir were analyzed using core and thin section observation, pore

structure test. The reservoirs are mainly developed in intergranular pores of sandy conglomerate, intergranular

solution pores and unfilled half-filled pores of basalt, matrix and bainite solution pores. On the one hand, the

reservoir properties are affected by sedimentation, on the other hand, it is caused by the destruction of pore

space by illite, illite / Montmorillonite Mixed Minerals and other clay minerals. The lower limits of reservoir

porosity of P

, P

and P

are 5%, 3.5% and 3.8%, respectively and the lower limits of permeability are

0.013 mD, 0.02 mD and 0.02 mD, respectively. This study discussed the controlling factors of the lower limit

of reservoir properties, which provides support for the exploration of conglomerate reservoir.

1 INTRODUCTION

In recent years, unconventional resources exploration

has become the focus of the industry, especially the

tight sandy conglomerate oil, which has become a

new highlight after shale gas. Junggar basin is a large

superimposed petroliferous basin. Mahu sag in the

northwest margin of Junggar basin is a well-known

hydrocarbon rich sag in the world (Cao et al., 2005;

Lei et al., 2017; Tao et al., 2016). The latest

exploration shows that the Permian Fengcheng

Formation in Kebai area of the south slope of Mahu

sag has developed large-scale reservoirs of volcanic

rock, sandy conglomerate, and dolomite with good

exploration prospects (Zhi et al., 2019).

Reservoir lithology is complex, reservoir space

type and development degree are affected by many

factors, and reservoir heterogeneity is strong. At

present, there is little understanding about the

Permian reservoir characteristics and effective

reservoirs. Effective reservoir means that the

reservoir has storage and seepage capacity, and can

produce liquid production with industrial value under

the existing technological conditions (Gao et al., 2011;

Lu, 2016). The accurate identification of effective

reservoirs is meaningful for the exploration and

development. At present, many methods are used to

calculate the lower limit of reservoir properties,

including empirical statistics method, oil bearing

occurrence method, testing method, relative

permeability curve combination method, physical

property test method and test data constraint method

(Liu et al., 2014; Xiao et al., 2004).

2 STUDY AREA

The study area is one of the most oil-gas enriched

areas in Karamay Oilfield. Oil is widely distributed in

Permian, Triassic, and Jurassic. Structurally, it is

located on the Karamay Baikouquan fault zone, with

Zaire Mountain in the West and Mahu depression in

the East. There are Carboniferous, Permian

Quaternary strata from bottom to top, which are very

504

Zhang, Z., Qin, J., Han, M., Ba, Z., Chen, X. and He, J.

Study on Reservoir Characteristics and Physical Property Lower Limit of Fengcheng Formation Tight Reservoir in South Slope of Mahu Sag.

In Proceedings of the 7th International Conference on Water Resource and Environment (WRE 2021), pages 504-512

ISBN: 978-989-758-560-9; ISSN: 1755-1315

complete. Among them, Permian, Triassic and

Carboniferous are in unconformity contact. The

Permian can be divided into Jiamuhe (P

j), Fengcheng

f), Xiazijie (P

X), lower Wuerhe (P

W) and upper

Wuerhe formation (P

W). The early and middle

Hercynian movement resulted in the development of

unconformities of different scales among the Permian

groups. The Fengcheng Formation of Permian can be

divided into the first member (P

), the second

member (P

) and the third member (P

) from

bottom to top. The second member can be further

divided into P

and P

. Grain size of the

sediments in P

and P

is coarse, and the lithology

is mainly conglomerate and gravelly sandstone of fan

delta origin. The P

is mainly medium sandstone and

fine sandstone, and a set of relatively stable overflow

facies volcanic rocks are developed at the top.

Figure 1: Sedimentary facies distribution of P

(a), P

(b) and P

(b)

member in Ke204 area.

3 RESERVOIR

CHARACTERISTICS

3.1 Characteristics of Sedimentary

Facies

The orogenic systems formed under the thrusting and

napping during the development period of the

foreland basin of Permian sedimentation is the

material source. Through the denudation and

transportation of water flow, it is dominated by

terrigenous pyroclastic rocks. Through the canyon

and Intermountain River, it is deposited in the fan

delta near the mountain pass. Reservoir lithology is

mainly sandy conglomerate, conglomerate,

arenaceous small conglomerate and gravel bearing

coarse sandstone of alluvial fan facies and fan delta

facies (Wang et al., 2018; You, 1986).

The top of P

formation of Permian is a set of

grey and light grey lacustrine mudstone, and the lower

part is fan delta front deposit with sand mud

Study on Reservoir Characteristics and Physical Property Lower Limit of Fengcheng Formation Tight Reservoir in South Slope of Mahu Sag

505

interbedding. The deep-water environment of slope

area is developed with dolomitic sandstone.

There are two kinds of sedimentary microfacies,

underwater distributary channel and inter channel, in

the fan delta front of P

member. The distributary

channel is composed of small conglomerate and

medium coarse sandstone. The logging curve shows

low GR box curve characteristics, and the SP curve

shows low value characteristics, indicating high-

energy sedimentary environment. The distributary

channel is composed of brown or gray mudstone or

silty mudstone, and the GR curve is toothed, indicating

a relatively low-energy sedimentary environment. The

characteristics of base level cycle show that the

regional base level is low, and the accommodation

space is small during the whole sedimentary period.

Sand body scale of main channel is controlled by the

dual factors of spin cycle and other cycle. the sand

content is less in the slope area, the lateral continuity

of sand body is poor, and the conglomerates at the

bottom are superposed (Figure 1a).

The P

member is a set of volcanic rock deposits.

The lithology is volcanic rock extrusive deposits

dominated by light gray, gray basalt or tuff. The

logging curve is characterized by a large set of box

curve. The main reservoir is basalt, and the reservoir

is in the effusion facies. The P

member is a set of

interbedded argillaceous sandstones and mudstones.

The developed dolomitic sandstones are fan delta front

deposits. On the logging curve, it shows the

characteristics of toothed box or bell. The reservoir

lithology is mainly medium fine sandstone, and the

reservoir is located in the outer front of fan delta

(Figure 1b, c).

The sedimentary facies profile of K88 well to M39

well shows that the sand body structure of P

and

member gradually changes from massive to

interbedded from plain facies to front facies along the

provenance direction, and the sand body gradually

becomes thinner and thinner away from the

provenance direction; overflow volcanic basalt is

distributed stably in P

member (Figure 1d).

Figure 2: Lithofacies characteristics of the Fengcheng Formation. a. grey sandy conglomerate, 4393.78 ~ 4393.94m, P

member, well JL 51; b. Dark gray basalt, 4383.00 ~ 4383.15m, P

member, well K204; c. grey medium fine sandstone,

4929.6 ~ 4929.8m, P

member, well M39; d. grey medium fine sandstone, 4839.50 ~ 4839.65m, P

member, well M28;

e. Grey sandy conglomerate, 3711.06 ~ 3711.20m, P

member, well JL17; f. Grey basalt, 4423.90 ~ 4423.98m, P

member,

well M16.

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3.2 Lithologic Characteristics of

Reservoir

Vertically, the lithology of each member is

significantly different, especially the reservoir rock

type. The lithology of P

member is sandy

conglomerate, gravel bearing fine sandstone, gravel

bearing argillaceous fine sandstone and fine

sandstone. The size of gravels varies from 0.125 cm

to 2 cm, mostly in sub angular to sub circular shape;

the composition of gravel is complex, mainly tuff,

accounting for 30% - 75%, followed by felsite and

andesite, accounting for 3% - 12%; the main sandy

component is tuff, accounting for 22% - 86%,

followed by quartz, feldspar, andesite, rhyolite and

basalt, accounting for 7% - 10%; the interstitial

materials are mainly calcite, analcite, dolomite, etc.,

with the content of about 2% - 5%; the impurity bases

are mainly argillaceous, micritic calcite, volcanic dust,

etc., with the content of about 1% - 6%; Porous

cementation is common; the rock structure is mainly

composed of sand gravel structure and unequal grain

sand structure, which are supported by grains; The

contact between particles is mainly point-line contact;

rock particles is mostly sub circular, and the sorting is

poor (Figure 2 a, e).

The basalts of the P

member are of almond like

structure and glass crystal interwoven structure, in

which the matrix part accounts for 30% - 97%, and

the almond part accounts for 3% - 30%; the primary

minerals are plagioclase, accounting for 33% - 90%,

followed by pyroxene, accounting for 2% - 12%,

vitrinite, accounting for 8% - 15%, and magnetite,

accounting for 2% - 3%; the secondary minerals are

chlorite and calcite, accounting for 12% - 54%, 3% -

25%, respectively, followed by albite, accounting for

1% - 2%, calcite, accounting for 2% - 3%, and

ankerite, accounting for 2% - 5% (Figure 2b, f).

Figure 3: Typical cements habits and microstructural characteristics. a. honeycomb like illite / montmorillonite and

intergranular filiform illite, 4225.42m,P

member, well Jin35; b. honeycomb like illite / montmorillonite and intergranular

filiform illite, 4225.42m, P

member, well Jin35; c. honeycomb like illite / montmorillonite mixed layer mineral,

4844.94m,P

member, well M28; d. pyrite intergranular pore, 4226.62m, P

member, well Jin35; e. siliceous intergranular

cement, 4933.42m, P

member, well M28; f. dissolution pores in feldspar, 4378.82m, P

member, well K204.

The mineral composition of the P

member is

mainly tuff, feldspar and quartz, in which tuff

accounts for 54% - 73%, feldspar 10% - 15% and

quartz 5% - 8%; the tuff materials in the rocks are

mainly feldspar crystal chips, tuff rock chips and

volcanic dust, with a small amount of glass chips; the

content of intergranular argillaceous matrix is 1% ～

4%, and the main cements are calcite and ankerite,

which are distributed in a porphyritic and uneven

manner, accounting for 2% ～ 5%; the cementation

type is pore cementation, and the rock structure is

mainly fine-grained sandy structure, supported by

point-line contact particles; the particle size is

Study on Reservoir Characteristics and Physical Property Lower Limit of Fengcheng Formation Tight Reservoir in South Slope of Mahu Sag

507

0.0625mm-0.1250mm and the roundness is sub edge

to sub circle, and sorting is medium (Figure 2c, d).

Compared with sandstone, the interstitial material

of conglomerate is coarser and more complex. In the

framework composed of gravel particles, it is often

partially or completely filled with sand particles, and

in the framework composed of gravel and sand

particles, it is also filled with miscellaneous base or

chemical sediment (He et al., 2020). The interstitial

material is divided into miscellaneous base and

cement. The matrix is the material transported from

the parent rock, mainly clay minerals. Cements are

authigenic minerals formed by chemical precipitation

during diagenesis. Cements mainly includes

carbonate, zeolite, siliceous and clay minerals (Figure

3). The clay minerals are mainly illite and illite /

montmorillonite. The clay filled with honeycomb or

cotton wadding in the pores between the particles

reduces the pore space of the reservoir, deforms the

roar channel and even blocks it, which greatly reduces

the permeability (Wang et al., 2017; Wang et al.,

2019).

3.3 Characteristics of Reservoir Space

Pore space is an important part of reservoir rock.

According to the core, thin section and physical

property analysis data, the intergranular pores and

dissolution pores are the main pore types in the P

member of the wind tunnel, and a small amount of

analcite dissolution pores and crushing fractures

(Figure 4 a, b); The main pore types of the P

member are unfilled semi filled pores, matrix and

bainite solution pores, micro fractures, etc. (Figure

4c); The pore type of the P

member mainly

intragranular solution pore and intergranular pore,

followed by micro fracture (Figure 4d, e, f). The

fluorescence thin section shows that oil and gas

mainly occur in the intergranular pores, intragranular

dissolved pores and microfractures.

Figure 4: Microscopic pore structure of different lithofacies sample. a. the intergranular pores and dissolved pores, 4491.00m,

member, well Jin 35; b. residual intergranular pores and intergranular solution pores, 3709.46m, P

member, well Jin

17; c. dolomitic fine sandstone with intragranular solution pore, 4932.07m, P

member, well M 28; d. Semi filled pores in

basalt, 4380.73m, P

member, well K 204;e. Semi filled pores in basalt, 4445.79m, P

member, well Jin 51; f. Matrix

dissolved pores in basalt, 4449.56m, P

member, well Jin 51.

3.4 Characteristics of Pore Structure

Among the mercury injection test of 27 rock samples

in the P

member, there are 16 samples with porosity

greater than 4%, and the displacement pressure ranges

from 0 MPa to 2.26 MPa. The median pressure is

between 0 MPa and 18.14MPa. The maximum pore

throat radius is 0-1.17 μm, most of them are less than

1.0 μm; the median radius of pore throat is 0-0.04 μm;

the average of mercury removal efficiency is 16.11%.

The coefficient of homogeneity is 0-0.19; the

coefficient of variation ranged from 0.04 to 0.22, with

an average of 0.11. The separation coefficient ranges

from 0.51 to 2.53, with an average of 1.41 (Figure 5a).

The mercury injection experiment of 78 rock

samples in the P

member shows that there are 32

samples with porosity greater than 4%, and the

displacement pressure ranges from 0 to 2.95 MPa,

with an average of 0.57 MPa; the median pressure is

between 0 and 19.48 MPa, with an average of 7.40

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MPa. The maximum pore throat radius is 0-16.69 μm,

and the average was 1.9 μm. Most of them are more

than 1.0 μm, the median radius of pore throat is 0-

0.12μm, the average was 0.05 μm, the mercury

removal efficiency was 10.32% - 38.03%, with an

average of 24.92%. The homogenization coefficient

is between 0 and 0.19, with an average of 0.12. The

coefficient of variation ranged from 0.04 to 0.29, with

an average of 0.15. The separation coefficient ranges

from 0.51 to 3.18, with an average of 1.81 (Figure 5b).

The results of mercury injection experiments on

three rock samples in the P

member show that, two

samples with porosity greater than 4% were selected,

and the displacement pressure was between 2.67 MPa

and 2.95 MPa, with an average of 2.81 MPa. The

median pressure is 6.22-11.41 MPa, with an average

of 9.58 MPa. The maximum pore throat radius is 0.25

μm -0.28 μm. Most of them are less than 1.0 μm. The

median radius of pore throat ranged from 0.06 μm to

0.12 μm, 0.83 on average μm. The mercury removal

efficiency ranged from 19.76% to 21.77%, with an

average of 20.76%. The homogeneity coefficient is

0.16. The coefficient of variation was 0.07. The

separation coefficient ranges from 0.89 to 0.94, with

an average of 0.91 (Figure 5c).

Figure 5: Typical mercury-injection curves for each member. a. P

member; b. P

member; c. P

member.

4 RESERVOIR PHYSICAL

PROPERTY AND LOWER

LIMIT OF EFFECTIVE

RESERVOIR

4.1 Physical Properties of Reservoir

The physical property test of the core samples of the

member shows that the minimum porosity of the

core matrix is 1.7%, and the maximum porosity is

12.41%; The main distribution range of porosity is 4%

~ 7%, with an average of 5.23%; The porosity of

reservoir is 5.3% ~ 12.41%, with an average of 7.13%;

the minimum matrix permeability is 0.01 mD; the

main distribution range is 0.02 mD ~ 0.07 mD, and

the average is 0.06 mD; the reservoir analysis

permeability is 0.12 mD ~ 43.61 mD, and the average

is 0.74 mD.

The minimum matrix porosity of the P

member

is 1.7%, and the maximum porosity is 10.81%; the

main distribution range of porosity is 2 ~ 5%, with an

average of 4.56%; the porosity of reservoir analysis is

3.8% ~ 10.81%, with an average of 6.64%; the

minimum matrix permeability is 0.01 mD, the main

distribution range is 0.02 ~ 0.07 mD, the average is

0.09 mD; the reservoir analysis permeability is 0.13

mD~ 34.21 mD, the average is 2.73 mD.

The minimum matrix porosity of P

member is

1.9%, and the maximum is 8.81%; the main

distribution range of porosity is 2 ~ 5%, with an

average of 4.18%; the porosity of reservoir is ranges

3.0% ~ 8.81%; the minimum matrix permeability is

0.02 mD, the maximum is 0.47 mD, the main

distribution range is 0.02 ~ 0.07 mD, the average is

0.04 mD; the reservoir analysis permeability is 0.03

mD ~ 0.47 mD, the average is 0.07 mD.

4.2 Determination of Lower Limit of

Physical Properties of Effective

Reservoir

It is a complex work to determine the lower limit of

reservoir parameters, which is related to the

geological conditions. Different methods may lead to

different conclusions (Liu et al., 2014; Xiao et al.,

Study on Reservoir Characteristics and Physical Property Lower Limit of Fengcheng Formation Tight Reservoir in South Slope of Mahu Sag

509

2004). The reservoir characteristics of Fengcheng

Formation are obvious, and the reservoir

characteristics of different structures and different

layers are different. Study on the lower limit of

reservoir physical properties, then evaluate the

reservoir effectiveness.

4.2.1 Lower Limit of Porosity

The relationship between porosity and permeability in

different section is different.

In the P

, porosity is positively correlated with

permeability in the middle and high porosity section,

which has the characteristics of porous reservoir. The

intersection points of porosity and permeability in low

porosity section are scattered. According to the

change of intersection curve when porosity is 5%, it

is considered that 5% is the lower limit of porosity,

and the corresponding permeability is 0.013 mD

(Figure 6 a, d).

In the P

, porosity and permeability are

exponential in medium and high porosity section, and

have the characteristics of pore type reservoir, and the

relationship between porosity and permeability is

poor power relationship in low porosity section.

According to the transition of intersection curve when

porosity is 3.5%, it is considered that 3.5% is the

lower porosity limit in the P

, and the

corresponding permeability lower limit is 0.02 mD

(Figure 6 b, e).

In the P

, the linear correlation between porosity

and permeability shows the characteristics of porous

reservoir. The intersection points of porosity and

permeability data in low porosity section are scattered,

and the relationship between porosity and

permeability is poor. According to the change of

intersection curve when the porosity is 3.8%, and the

corresponding lower limit of permeability is 0.02 mD

(Figure 6 c, f).

Figure 6: Correlation diagram of core porosity with permeability and mercury injection displacement pressure. a, d. P

member; b, e. P

member; c, f. P

member.

Figure 7. Correlation diagram of permeability with mercury injection displacement pressure. a. P

member; b. P

member;

c. P

member.

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4.2.2 Lower Limit of Porosity

The throat size controls the seepage ability, the critical

pore throat radius for the largest oil molecule in tight

oil reservoir is 54 nm. According to the relationship

between displacement pressure and permeability in the

experimental parameters of core mercury injection.

There is a good power relationship between the

permeability and displacement pressure of the P

member, and the correlation coefficient is high. With

the increase of displacement pressure, the permeability

decreases sharply. When the displacement pressure of

the P

member increases to 2 MPa, the core

permeability changes slowly, indicating that the flow

resistance of fluid in the micro pore throat increases,

The fluid flow state tends to be static. Therefore, the

permeability at this time is used as the lower limit to

judge whether the reservoir still has the ability of fluid

seepage, and the lower limit of permeability of the P

member is 0.013 mD (Figure 7a).

There is a good power relationship between

permeability and displacement pressure. After the

displacement pressure increases to 2 MPa, the core

permeability changes slowly, which indicates that the

flow resistance of fluid in the micro pore throat

increases, and the flow state of fluid tends to be static.

Therefore, the permeability at this time is used as the

lower limit to judge whether the reservoir still has the

ability of fluid seepage, and the lower limit of

permeability of the P

member is 0.02 mD (Figure

7b).

There is a good power relationship between

permeability and displacement pressure. When the

displacement pressure increases to 2 MPa, the core

permeability changes slowly. Therefore, the

permeability at this time is used as the lower limit to

judge whether the reservoir still has the ability of fluid

seepage, and the lower limit of permeability of the

member is 0.02 mD (Figure 7c).

5 CONCLUSION

1. The Permian fan delta deposits are developed. The

lithology of the fan delta front of P

and P

members is mainly sandy conglomerate, gravel

bearing fine sandstone, gravel bearing argillaceous

fine sandstone and fine sandstone. In P

member,

volcanic exhalative deposits and basalt are developed.

2. The pore types in sandy conglomerate are

mainly intergranular pores and dissolution pores, with

a small amount of analcite dissolution pores and

crushing fractures. The pore types in volcanic rocks

are mainly unfilled semi filled pores, matrix and

bainite dissolution pores, micro fractures, etc.

3. The structural characteristics of the original

deposition of sandy conglomerate leads to poor

preservation conditions of original pores. The

development of authigenic cements, especially illite,

illite / montmorillonite mixed minerals and other clay

minerals in the later stage, lead to the destruction of

pore space in the main reservoir section, the

deformation of roar channel, even plugging, and

greatly reduced permeability.

4. The lower limits of reservoir porosity of P

and P

member are 5%, 3.5% and 3.8%,

respectively. The lower limits of permeability are

0.013 mD, 0.02 mD and 0.02 mD, respectively.

ACKNOWLEDGMENT

This study was financially supported by the Science

and Technology Cooperation Project of the CNPC-

SWPU Innovation Alliance, Science and Technology

Agency of Sichuan province (No.18YYJC1120),

China Postdoctoral Science Foundation (No.

2017M623059) and the Open Fund of State Key

Laboratory of Oil and Gas Reservoir Geology and

Exploitation, Southwest Petroleum University (CN).

We would like to thank the Southwest Oil & Gas Field

Branch Company Ltd. PetroChina for providing shale

samples and data.

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