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Distribution and formation of tight oil in Qijia area, Songliao Basin, NE China
PETROLEUM EXPLORATION AND DEVELOPMENT Volume 42, Issue 1, February 2015 Online English edition of the Chinese language journal Cite this article as: PETROL. EXPLOR. DEVELOP., 2015, 42(1): 48–55.
RESEARCH PAPER
Distribution and formation of tight oil in Qijia area, Songliao Basin, NE China SHI Lizhi1,2,*, WANG Zhuozhuo1,2, ZHANG Ge2, ZHANG Yongsheng1, XING Enyuan1 1. MLR Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources, Chinese Academy of Geology Sciences, Beijing 100037, China; 2. Exploration and Development Research Institute of Daqing Oilfield Company Ltd., Daqing 163712, China
Abstract: By using geologic, geochemical, well test, well drilling and well logging data, the formation conditions and distribution pattern of tight oil in Gao 3 and Gao 4 oil layers of Gaotaizi layer, Upper Cretaceous Qingshankou Formation, Qijia area, Songliao Basin were examined systematically. There are four favorable conditions for the formation of tight oil: high quality source rocks and reservoirs, good source and reservoir combination, and formation pressure. There are three series of source rocks in this area, K2qn1, K2qn2+3 and K2n1, which provided material basis for the tight oil reservoirs; reservoirs including a variety of sands like distributaries channel, mouth bar and sheet sand are large in total thickness, wide in lateral distribution and good in continuity, providing accumulation space for tight oil; reservoir rocks of delta front in direct contact with the hydrocarbon source rock or pinching out in hydrocarbon source rock like fingers or interbedded with hydrocarbon source rock, constituted good source-reservoir combinations; abnormal overpressure in tight oil reservoirs with the pressure factor of 1.2-1.5, provided drive force for tight oil migration and charging. The study shows tight oil of Gao 3 and Gao 4 oil layers in Qijia area is mainly distributed in the center of, and slope and terrace around the sag. Key words: Songliao Basin; Qijia area; unconventional hydrocarbon; tight oil; formation condition; distribution pattern
Introduction As the exploration in northern Songliao Basin progresses, exploration targets gradually change from the conventional to the unconventional oil and gas reservoirs. In recent years, huge potential tight oil resource was found in Gaotaizi 3 and Gaotaizi 4 (Gao 3 and Gao 4) oil layers of the Upper Cretaceous Qingshankou Formation in Qijia area, Songliao Basin, a new breakthrough in tight oil exploration in China. By using geologic, geochemical, oil drilling, oil logging data, the formation conditions and distribution pattern of tight oil in Gao 3 and Gao 4 oil groups of Qijia area are systematically examined, so as to provide technical support for tight oil exploration in this area.
1
Overview of the study area
Located in the central depression in northern Songliao Basin, Qijia area includes the north main body of two secondclass structural units, the Qijia-gulong sag, Longhupao-da’an terrace, with an exploration area of about 3 500 km2 (Fig. 1). The Cenozoic-Mesozoic strata of Qijia area from bottom to top are Cretaceous, Paleogene, Neogene and Quaternary, where the oil and gas assemblies in the middle and lower part are the
main targets, including five oil bearing layers, Saertu, Putaohua, Gaotaizi, Fuyu and Yangdachengzi[1−4] (Fig. 1). The thick mudstones deposited in two major water transgression during the deposition period of Upper Cretaceous Qingshankou Formation and Nenjiang Formation, act as the main hydrocarbon source rock and seal in this area, controlling the source-reservoir-cap combination and oil and gas distribution[1−4]. In recent years, tight oil resource of huge potential has been found in Gao 3 and Gao 4 oil layers in Qijia area, Songliao Basin. Comparison with Bakken tight oil reservoir in North Dakota, the United State, shows there are some similar geological features between them[5−9] (Table 1): the tight oil is distributed continuously in large area on plane, lacks of obvious trap boundary, unified pressure system and oil-water boundary; both of them have good oil source conditions, good source-reservoir combination, and all-in-one source and reservoir; the reservoirs are poor in petrophysical property, the reservoirs in Qijia area have an average porosity of 8.5%, average air permeability of 0.40×10−3 μm2; the reservoirs have abnormal overpressure, the formation pressure coefficient in Qijia area ranges from 1.20 to 1.50; the reservoirs have high oil saturation, little movable water, and the oil is light.
Received date: 18 Feb. 2014; Revised date: 29 Oct. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by China National Science and Technology Major Project (2011ZX05001-001-04). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 1
Overlap map of the Songliao Basin stratigraphic division, Qijia regional structure unit and the top structure of Gaotaizi reservoir Table 1
Comparison of tight oil reservoirs in Qijia area of Songliao Basin and Bakken[5−9] Source rocks
Tight hydro-
Horizon
carbon
Bakken Formation Gao 3+4 reservoirs
2 2.1
Depositional Buried environment
depth
Thick- TOC/ Ro/ ness/m
%
%
Upper De- Shallow sea fa2–
lower Car-
with shallow
18
boniferous
delta facies
3 203
ess/m sity/%
Permeability/ 10−3 μm2
Type of
pressure Kerogen coeffi-
Oil quality
cient
Crude
11.3
0.7– siltstone,
20–
1.0
50
silty
6.0
0.04
1.35– 1.58
Lacustrine fa3 300
delta facies
150
2.6
0.7– 1.4
Siltstone, argillaceous,
Formation of the tight oil Source rocks
The study area is located in the hydrocarbon generation center of Qijia-Gulong sag where there developed several sets of good widespread source rocks. The main hydrocarbon source rocks include the first member of Qingshankou Formation (Qing 1, K2qn1), the second and third members of Qingshankou Formation (Qing 2 + 3, K2qn2+3) and the first member of Nenjiang Formation (Nen 1, K2n1)[1−2, 10−12]. The hydrocarbon source rocks have high organic matter abundance,
Relationship
oil vis-
between
cosity
source and
(mPa·s)
reservoir Source and
I＋II
Light 0.3–0.4
reservoir alternate
dolomite
cies interbeded 2 500– 100–
Cretaceous with shallow
Rock type
Thickn Poro-
tion
Dolomitic
vonian— cies interbeded 2 593–
Upper
Forma-
Reservoir
Source and 8– 24
8.5
0.40
1.20– 1.50
I＋II
Medium
0.8–1.3
reservoir alternate or in close contact
good kerogen type, mainly typeⅠand type II, and maturity to high maturity stage. The source rocks are mainly thick dark mudstone and oil shale, the thicknesses of Qing 1, Qing 2 + 3, Nen 1 source rocks is 50–250 m, 100–450 m, and 60–200 m respectively. The source rocks in the sag are thicker in the south, and gradually thin to the North. The depositional stage of Qingshankou Formation was a major transgression period in Songliao Basin, when the whole area was inundated by water, a deep lake grey-black mudstone, sandstone, shale interbedded with shale oil and marlstone strip deposited in Member Qing 1; and shallow coastal lake
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SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
on the whole, tight oil is mainly distributed in various types of sands in delta front and the external front facies belt, such as distributary channel, mouth bar, and sheet sand etc. Fine description of sands in the tight oil development zone shows that the single distributary channel sand is 2–8 m thick and 200–500 m wide in general; single mouth bar sand is 1–5 m thick; single sand sheet is 0.8–2.0 m thick and 0.6–11.0 km long in general. Sands of various types are big in cumulative thickness, wide in distribution area on the plane and good in continuity. Sands, thicker in the north, gradually thin toward south, and sands are not developed in the lake facies. Sands of various types in Gao 3 reservoir group are 2.5–75.0 m thick cumulatively, thicker in the middle and North of the study area, generally 25–75 m thick, thinner in the South, generally less than 25 m. Sands in Gao 4 oil layers have a cumulative thickness of 5–70 m, and distribution features similar to Gao 3, thicker in the north and thinner in the South. Sands in Upper Gao 3 and Gao 4 reservoir are 10–35 m thick in the north part of the work area, thin to less than 10 m thick in the south part of the work area, sands are not developed in the shore and shallow lake areas (Fig. 3). Fig. 2
TOC distribution of Qing 1 source rocks in Qijia region
2.2.2 Characteristics of lithology, physical property and oiliness
grey-black grey-green mudstone with sandstone deposited in Member Qing 2 + III. The depositional stage of Nenjiang Formation was another major transgression period of Songliao Basin, depositing deep lake black mudstone interbedded with oil shale in Member Nen 1. With an average total organic carbon (TOC) of up to 2.13% (Fig. 2, and Table 2), average chloroform asphalt "A" content of 0.43%, average total hydrocarbon of 4 149×10−6, the hydrocarbon generation potential (S1＋S2) of 18.49 mg/g on average, Qing 1 in Qijia-Gulong sag source rock is premium source rock according to the evaluation standard for China continental source rock organic matter abundance[10] (Table 2). Qing 2 + 3 and Nen 1 source rocks are also up to the standard of good source rock. Among the 3 sets of source rocks, Qing 1 and Nen 1 source rocks with better oil generation indexes, are the best. 2.2
Tight oil reservoirs in Qijia area are mainly made up of siltstone, followed by argilliferous siltstone, calcareous siltstone, and ostracod-bearing siltstone. The reservoirs are poor in physical properties, with a porosity of 4%–12%, on average 8.5%, less than 12% for 80% of samples, and air permeability of (0.01–0.50)×10−3 μm2, 0.4×10−3 μm2 on average, less than 1×10−3 for 93% samples (Fig. 4). By using 823 data points in 37 wells of the study area, according to oil occurrence statistical method, tight oil lower porosity of Qijia area is estimated at 4%, while air permeability is 0.02×10−3 μm2 (Fig. 5). Oil-soaked sandstone is usually greater than 10% in porosity, sandstone with oil patches is usually greater than 8% in porosity, and siltstone with oil traces is usually greater than 3% in porosity. According to the features of tight oil reservoirs in Qijia area, the reservoirs can be divided into two classes: classⅠwith the porosity from 8% to12%, air permeability of (0.05–0.50)×10−3 μm2, mainly oil soaked and oil flecked siltstone; classⅡ with the porosity of 4%−8%, air permeability of (0.02–0.05)×10−3 μm2, mainly oil
Reservoir
2.2.1
Distribution of reservoir sand
The tight oil reservoir in Qijia area is delta facies deposits Table 2
Organic matter abundance of source rock Qijia-Gulong sag Organic carbon/%
Chloroform bitumen“A”/%
Total hydrocarbon/10-6
(S1+S2)/(mg·g 1) －
Formation
Min.
Max.
Average
Min.
Max.
Average
Min.
Max.
Average
Min.
Max.
Average
Grade of source rock
K2qn1
0.22
6.67
2.13(87)
0.016
1.150
0.429(69)
108
10 771
4 149(105)
0.06
697.00
18.49(190)
Good
K2qn2+3
0.12
6.56
1.32(536)
0.006
0.904
0.164(146)
55
7 412
1 866(152)
0.01
719.51
10.41(427)
Good
K2n1
0.12
8.59
2.64(186)
0.024
1.351
0.492(38)
144
15 643
4 023(72)
0.04
420.00
25.15(143)
Good
Note: the values in parentheses are the number of samples.
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SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 3
Fig. 4
Sandstone thickness distribution of Upper Gao 3 and Upper Gao 4 reservoir in Qijia region
Frequency distribution histogram of properties of Upper Gao 3 and Upper Gao 4 reservoirs in Qijia region (643 samples)
Fig. 5 Reservoir classification evaluation of Upper Gao 3 and Upper Gao 4 oil layers in Qijia area
flecked and oil stained siltstone (Fig. 5). 2.2.3
Types and characteristics of reservoir space
According to the thin section analysis, the reservoir space of tight oil sands in the study area includes intergranular pores, intergranular dissolution pores and micro-cracks etc (Fig. 6), of 5–100 μm in size. Pore structure features small pore and fine throat, with pore throat radius of less than 0.1 μm in general, leading to low permeability, fluid flow difficulty, but various secondary pores, micro-cracks etc improve the physical properties of the tight reservoirs.
Fig. 6 Microscopic pore structure of tight oil reservoirs in Gaotaizi Formation, Qijia region
2.3
− 51 −
Source and reservoir configuration
Good source and reservoir configuration is one of the key
SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 7
Sedimentary section of Gaotaizi reservoir and the upper and lower adjacent layers through Well L28 - Well G702 in Qijia region.
conditions for tight oil accumulation[11−12]. Tight oil reservoirs in Qijia area distribute in Delta front facies, which inserts into source rock of lacustrine facies from North to South, resulting in direct contact of Delta front reservoir rocks with hydrocarbon source rock or pinch-out in lacustrine source rocks in finger shape or lamination with source rocks. The source rocks of Member Qing 2 + 3 not only provide oil source for tight reservoirs but also act as good cap for some reservoirs (Fig. 7). Meanwhile the bottom of the tight oil reservoir is in direct contact with widely distributed Qing 1 source rock, so oil and gas generated by Member Qing 1 can directly feed to the tight oil reservoir. 2.4
has premium oil generation conditions, developed reservoir sands, good formation pressure conditions, strong abnormal pressure, all indicating good reservoir forming conditions; the sands in the outer front of delta in the south of the work area insert (as Well L28-J47 in Fig. 9) directly into the source rock (Fig. 9), favourable for oil and gas accumulation preferably. From the tight oil area to the north, the structure gradually uplifts, and reservoir physical property gets better, oil and water begin to differentiate, and an oil-water transitional zone and water layer developed zone occur (Fig. 8).
Reservoir pressure
Gao 3+4 reservoirs in tight oil rich portion, Qijia region have a measured pressure coefficient of 1.20–1.50, indicating abnormal over pressure. This is mainly because the tight oil area is in the hydrocarbon generation center of the sag, where big burial depth, uneven compaction, tectonic activity, hydrocarbon generation action, and hydrothermal together lead to overpressure[13−15]. Since tight oil reservoirs are relatively poor in physical properties, a certain amount of overpressure is necessary to push the oil and gas migration and accumulation, overpressure serves as driving force for tight oil migration and accumulation.
3 Pattern and controlling factors of tight oil distribution 3.1
Distribution of tight oil
In Qijia area, tight oil mainly occurs in the center of the sag and slopes and terraces around the sag on the plane (as shown in Fig. 8 and Fig. 9), vertically, tight oil is mainly in the Gao 3+4 oil layers, with small amount in Gao 1 +2 oil layers. The central and south parts of the study area are universally oil-bearing, featuring large oil-bearing area, and continuous reservoirs without distinct boundaries. Located in the center of the hydrocarbon generation sag, the center of the work area
Fig. 8 Planar distribution of oil and water of the lower Gao 3 oil layers in Qijia region
− 52 −
SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 9 Oil layer correlation profile of Gao 3, Gao 4 oil layers through Well Lin L28 – Well J51 in Qijia region
3.2 3.2.1
Controlling factors Reservoir lithology and physical property
The reservoir lithology in Qijia area has a strong effect on tight oil distribution; on the whole, sandstone is better than mudstone in oil-bearing property. In the tight oil area, siltstones generally contain oil, pelitic siltstone and calcareous siltstone with higher mud and calcium content, have worse oil-bearing property than pure siltstone. Sandy strips in silty mudstones are oil-bearing, but mudstones are not oil-bearing. Reservoir physical properties have a strong control over the distribution of tight oil. The reservoirs in the tight oil area have a main porosity range of 4%–12%, the better the reservoir physical properties, the better the oil-bearing property of the reservoir will be. Statistics show that tight oil reservoirs 8%–12% in porosity have the best oil-bearing property, followed by reservoirs 4%–8% in porosity, reservoirs less than 4% in porosity are poor in oil-bearing property (Fig. 5). 3.2.2
Sedimentary facies and sands
Affected by northern provenance, there developed large area of delta deposits in Qijia area, which include subfacies such as outer delta front, inner delta front and shore-shallow lake. The area, with reservoirs in long extension and wide distribution, sands stacking and overlapping stable in distribution, good in continuity, moderate sand to formation ratio, is the main area for tight oil distribution. It is the main distribution area of tight oil. Fig. 7 and Fig. 10 show that tight oil in the Upper Gao 4 oil layers mainly occurs in mouth bar and distributary channel sands in the inner delta front, and sheet sand in the outer delta front subfacies, shallow lake bars in shallow lake facies also contain some tight oil. Sedimentary facies and sands in Qijia area control the planar and spatial distribution of tight oil, providing large stretches of reservoir for tight oil. 3.2.3
Excessive pressure
According to mudstone sonic logging data of more than 150 wells in the study area, by using the equilibrium depth method[16], excessive pressure of the target layer in the key
Fig. 10 Planar sedimentary facies distribution and reservoir development area in Upper Gao 4 oil layers, Qijia region
period of reservoir formation (at the end of Mingshui Formation depositional stage) (Fig. 11) was restored, the equation calculating excessive pressure is: Δpz=rwHe + rbw(H − He) − rwH where, ΔPZ—excessive pressure, Pa; He—balance depth (a point on the normal compaction curve, if its interval transit time is the same with a calculated point in the abnormal section, the depthof this point is the balance depth), m; rw—hydrostatic pressure gradient, Pa/m; rbw—pressure gradient of the rock column from balance depth to target layer , Pa/m; H—target layer depth, m. Fig. 11 shows that tight oil is mainly distributed in sag center and slopes and terraces on the migration paths where the reservoir pressure is over 2.5 MPa. The areas have higher excessive pressure, favorable for the migration and accumulation of oil and gas, so overpressure controls the distribution scope of tight oil in the work area.
4
Assessment of tight oil potential
Based on the reservoir physical properties and changes of planar reservoir thickness, hydrocarbon source rock distribution, sedimentary and structural data, tight oil in Qijia region was evaluated (Fig. 12), class I area mainly distributes in the sag center and Longhupao-Da'an terrace, class II area mainly situates in slopes around the sag, and sag center. According to the volume method, Gao 3+4 tight oil resources are 3.0×108 t
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SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Table 3 Statistics on tight oil resources of Gao 3 and Gao 4 oil layers in Qijia region FavorArea/ Reservoir Poroable km2 thickness/m sity/% area
Permeability/ 10−3 μm2
Class I 1 130
5−10
8−12
0.05−1.00
1−5
2
Class II 1 120
<5
4−8
0.02−0.50
<2
1
5
Fig. 11 Overlap map of excessive pressure and oil well location in Gao 4 oil layers, Qijia region
Oil proReduction/ souces/ (t·d−1) 108 t
Conclusions
There developed tight oil resources of large potential in Gao 3+4 oil layers of Gaotaizi reservoir in Qijia area of Songliao Basin, thanks to the good hydrocarbon source rock conditions, reservoir conditions, source-reservoir configuration and formation pressure condition. There are three sets of premium source rocks, Qing 1, Qing 2 +3, and Nen 1 in this area, providing good material base for hydrocarbon generation; a variety types of sand, including distributary channel, mouth bar, and sheet sand, act as reservoirs, the various kinds of sands large in thickness, wide in planar distribution, good in continuity, provide storage space for tight oil; source-reservoir configuration is good, delta front reservoirs directly insert into hydrocarbon source rocks or pinch out in lacustrine source rocks in finger shape or interbed with source rocks, conducive to oil and gas feeding to the adjacent reservoir preferably; tight oil reservoirs have overpressure in common with a pressure coefficient of 1.20–1.50, providing driving force for reservoir formation; tight oil mainly distributes in sag center and slopes and terraces around the sag, vertically in Gao 3+4 oil layers, according to volume method, tight oil resources in Gao 3+4 oil layers are 3.0×108 t, including 2×108 t in classⅠarea and 1×108 t in class Ⅱarea, making this area an important relay area of future reserves.
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Fig. 12 Resource potential distribution of Gao 3 and Gao 4 oil layers in Qijia region
[5]
in total, including 2×108 t in class I area and 1×108 t in class II area, making this area an important relay area for future reserve submission (Table 3).
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Jing Dongsheng, Ding Feng, Yuan Jihua. The exploration and development situation, experience and revelation on U.S. tight oil. Land and Resources Information, 2012(1): 18–19.
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mudstone, Nagaoka Plain, Japan. AAPG Bulletin, 1968, 52: 2466–2501.
RESEARCH PAPER
Distribution and formation of tight oil in Qijia area, Songliao Basin, NE China SHI Lizhi1,2,*, WANG Zhuozhuo1,2, ZHANG Ge2, ZHANG Yongsheng1, XING Enyuan1 1. MLR Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources, Chinese Academy of Geology Sciences, Beijing 100037, China; 2. Exploration and Development Research Institute of Daqing Oilfield Company Ltd., Daqing 163712, China
Abstract: By using geologic, geochemical, well test, well drilling and well logging data, the formation conditions and distribution pattern of tight oil in Gao 3 and Gao 4 oil layers of Gaotaizi layer, Upper Cretaceous Qingshankou Formation, Qijia area, Songliao Basin were examined systematically. There are four favorable conditions for the formation of tight oil: high quality source rocks and reservoirs, good source and reservoir combination, and formation pressure. There are three series of source rocks in this area, K2qn1, K2qn2+3 and K2n1, which provided material basis for the tight oil reservoirs; reservoirs including a variety of sands like distributaries channel, mouth bar and sheet sand are large in total thickness, wide in lateral distribution and good in continuity, providing accumulation space for tight oil; reservoir rocks of delta front in direct contact with the hydrocarbon source rock or pinching out in hydrocarbon source rock like fingers or interbedded with hydrocarbon source rock, constituted good source-reservoir combinations; abnormal overpressure in tight oil reservoirs with the pressure factor of 1.2-1.5, provided drive force for tight oil migration and charging. The study shows tight oil of Gao 3 and Gao 4 oil layers in Qijia area is mainly distributed in the center of, and slope and terrace around the sag. Key words: Songliao Basin; Qijia area; unconventional hydrocarbon; tight oil; formation condition; distribution pattern
Introduction As the exploration in northern Songliao Basin progresses, exploration targets gradually change from the conventional to the unconventional oil and gas reservoirs. In recent years, huge potential tight oil resource was found in Gaotaizi 3 and Gaotaizi 4 (Gao 3 and Gao 4) oil layers of the Upper Cretaceous Qingshankou Formation in Qijia area, Songliao Basin, a new breakthrough in tight oil exploration in China. By using geologic, geochemical, oil drilling, oil logging data, the formation conditions and distribution pattern of tight oil in Gao 3 and Gao 4 oil groups of Qijia area are systematically examined, so as to provide technical support for tight oil exploration in this area.
1
Overview of the study area
Located in the central depression in northern Songliao Basin, Qijia area includes the north main body of two secondclass structural units, the Qijia-gulong sag, Longhupao-da’an terrace, with an exploration area of about 3 500 km2 (Fig. 1). The Cenozoic-Mesozoic strata of Qijia area from bottom to top are Cretaceous, Paleogene, Neogene and Quaternary, where the oil and gas assemblies in the middle and lower part are the
main targets, including five oil bearing layers, Saertu, Putaohua, Gaotaizi, Fuyu and Yangdachengzi[1−4] (Fig. 1). The thick mudstones deposited in two major water transgression during the deposition period of Upper Cretaceous Qingshankou Formation and Nenjiang Formation, act as the main hydrocarbon source rock and seal in this area, controlling the source-reservoir-cap combination and oil and gas distribution[1−4]. In recent years, tight oil resource of huge potential has been found in Gao 3 and Gao 4 oil layers in Qijia area, Songliao Basin. Comparison with Bakken tight oil reservoir in North Dakota, the United State, shows there are some similar geological features between them[5−9] (Table 1): the tight oil is distributed continuously in large area on plane, lacks of obvious trap boundary, unified pressure system and oil-water boundary; both of them have good oil source conditions, good source-reservoir combination, and all-in-one source and reservoir; the reservoirs are poor in petrophysical property, the reservoirs in Qijia area have an average porosity of 8.5%, average air permeability of 0.40×10−3 μm2; the reservoirs have abnormal overpressure, the formation pressure coefficient in Qijia area ranges from 1.20 to 1.50; the reservoirs have high oil saturation, little movable water, and the oil is light.
Received date: 18 Feb. 2014; Revised date: 29 Oct. 2014. * Corresponding author. E-mail: [email protected] Foundation item: Supported by China National Science and Technology Major Project (2011ZX05001-001-04). Copyright © 2015, Research Institute of Petroleum Exploration and Development, PetroChina. Published by Elsevier BV. All rights reserved.
SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 1
Overlap map of the Songliao Basin stratigraphic division, Qijia regional structure unit and the top structure of Gaotaizi reservoir Table 1
Comparison of tight oil reservoirs in Qijia area of Songliao Basin and Bakken[5−9] Source rocks
Tight hydro-
Horizon
carbon
Bakken Formation Gao 3+4 reservoirs
2 2.1
Depositional Buried environment
depth
Thick- TOC/ Ro/ ness/m
%
%
Upper De- Shallow sea fa2–
lower Car-
with shallow
18
boniferous
delta facies
3 203
ess/m sity/%
Permeability/ 10−3 μm2
Type of
pressure Kerogen coeffi-
Oil quality
cient
Crude
11.3
0.7– siltstone,
20–
1.0
50
silty
6.0
0.04
1.35– 1.58
Lacustrine fa3 300
delta facies
150
2.6
0.7– 1.4
Siltstone, argillaceous,
Formation of the tight oil Source rocks
The study area is located in the hydrocarbon generation center of Qijia-Gulong sag where there developed several sets of good widespread source rocks. The main hydrocarbon source rocks include the first member of Qingshankou Formation (Qing 1, K2qn1), the second and third members of Qingshankou Formation (Qing 2 + 3, K2qn2+3) and the first member of Nenjiang Formation (Nen 1, K2n1)[1−2, 10−12]. The hydrocarbon source rocks have high organic matter abundance,
Relationship
oil vis-
between
cosity
source and
(mPa·s)
reservoir Source and
I＋II
Light 0.3–0.4
reservoir alternate
dolomite
cies interbeded 2 500– 100–
Cretaceous with shallow
Rock type
Thickn Poro-
tion
Dolomitic
vonian— cies interbeded 2 593–
Upper
Forma-
Reservoir
Source and 8– 24
8.5
0.40
1.20– 1.50
I＋II
Medium
0.8–1.3
reservoir alternate or in close contact
good kerogen type, mainly typeⅠand type II, and maturity to high maturity stage. The source rocks are mainly thick dark mudstone and oil shale, the thicknesses of Qing 1, Qing 2 + 3, Nen 1 source rocks is 50–250 m, 100–450 m, and 60–200 m respectively. The source rocks in the sag are thicker in the south, and gradually thin to the North. The depositional stage of Qingshankou Formation was a major transgression period in Songliao Basin, when the whole area was inundated by water, a deep lake grey-black mudstone, sandstone, shale interbedded with shale oil and marlstone strip deposited in Member Qing 1; and shallow coastal lake
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on the whole, tight oil is mainly distributed in various types of sands in delta front and the external front facies belt, such as distributary channel, mouth bar, and sheet sand etc. Fine description of sands in the tight oil development zone shows that the single distributary channel sand is 2–8 m thick and 200–500 m wide in general; single mouth bar sand is 1–5 m thick; single sand sheet is 0.8–2.0 m thick and 0.6–11.0 km long in general. Sands of various types are big in cumulative thickness, wide in distribution area on the plane and good in continuity. Sands, thicker in the north, gradually thin toward south, and sands are not developed in the lake facies. Sands of various types in Gao 3 reservoir group are 2.5–75.0 m thick cumulatively, thicker in the middle and North of the study area, generally 25–75 m thick, thinner in the South, generally less than 25 m. Sands in Gao 4 oil layers have a cumulative thickness of 5–70 m, and distribution features similar to Gao 3, thicker in the north and thinner in the South. Sands in Upper Gao 3 and Gao 4 reservoir are 10–35 m thick in the north part of the work area, thin to less than 10 m thick in the south part of the work area, sands are not developed in the shore and shallow lake areas (Fig. 3). Fig. 2
TOC distribution of Qing 1 source rocks in Qijia region
2.2.2 Characteristics of lithology, physical property and oiliness
grey-black grey-green mudstone with sandstone deposited in Member Qing 2 + III. The depositional stage of Nenjiang Formation was another major transgression period of Songliao Basin, depositing deep lake black mudstone interbedded with oil shale in Member Nen 1. With an average total organic carbon (TOC) of up to 2.13% (Fig. 2, and Table 2), average chloroform asphalt "A" content of 0.43%, average total hydrocarbon of 4 149×10−6, the hydrocarbon generation potential (S1＋S2) of 18.49 mg/g on average, Qing 1 in Qijia-Gulong sag source rock is premium source rock according to the evaluation standard for China continental source rock organic matter abundance[10] (Table 2). Qing 2 + 3 and Nen 1 source rocks are also up to the standard of good source rock. Among the 3 sets of source rocks, Qing 1 and Nen 1 source rocks with better oil generation indexes, are the best. 2.2
Tight oil reservoirs in Qijia area are mainly made up of siltstone, followed by argilliferous siltstone, calcareous siltstone, and ostracod-bearing siltstone. The reservoirs are poor in physical properties, with a porosity of 4%–12%, on average 8.5%, less than 12% for 80% of samples, and air permeability of (0.01–0.50)×10−3 μm2, 0.4×10−3 μm2 on average, less than 1×10−3 for 93% samples (Fig. 4). By using 823 data points in 37 wells of the study area, according to oil occurrence statistical method, tight oil lower porosity of Qijia area is estimated at 4%, while air permeability is 0.02×10−3 μm2 (Fig. 5). Oil-soaked sandstone is usually greater than 10% in porosity, sandstone with oil patches is usually greater than 8% in porosity, and siltstone with oil traces is usually greater than 3% in porosity. According to the features of tight oil reservoirs in Qijia area, the reservoirs can be divided into two classes: classⅠwith the porosity from 8% to12%, air permeability of (0.05–0.50)×10−3 μm2, mainly oil soaked and oil flecked siltstone; classⅡ with the porosity of 4%−8%, air permeability of (0.02–0.05)×10−3 μm2, mainly oil
Reservoir
2.2.1
Distribution of reservoir sand
The tight oil reservoir in Qijia area is delta facies deposits Table 2
Organic matter abundance of source rock Qijia-Gulong sag Organic carbon/%
Chloroform bitumen“A”/%
Total hydrocarbon/10-6
(S1+S2)/(mg·g 1) －
Formation
Min.
Max.
Average
Min.
Max.
Average
Min.
Max.
Average
Min.
Max.
Average
Grade of source rock
K2qn1
0.22
6.67
2.13(87)
0.016
1.150
0.429(69)
108
10 771
4 149(105)
0.06
697.00
18.49(190)
Good
K2qn2+3
0.12
6.56
1.32(536)
0.006
0.904
0.164(146)
55
7 412
1 866(152)
0.01
719.51
10.41(427)
Good
K2n1
0.12
8.59
2.64(186)
0.024
1.351
0.492(38)
144
15 643
4 023(72)
0.04
420.00
25.15(143)
Good
Note: the values in parentheses are the number of samples.
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Fig. 3
Fig. 4
Sandstone thickness distribution of Upper Gao 3 and Upper Gao 4 reservoir in Qijia region
Frequency distribution histogram of properties of Upper Gao 3 and Upper Gao 4 reservoirs in Qijia region (643 samples)
Fig. 5 Reservoir classification evaluation of Upper Gao 3 and Upper Gao 4 oil layers in Qijia area
flecked and oil stained siltstone (Fig. 5). 2.2.3
Types and characteristics of reservoir space
According to the thin section analysis, the reservoir space of tight oil sands in the study area includes intergranular pores, intergranular dissolution pores and micro-cracks etc (Fig. 6), of 5–100 μm in size. Pore structure features small pore and fine throat, with pore throat radius of less than 0.1 μm in general, leading to low permeability, fluid flow difficulty, but various secondary pores, micro-cracks etc improve the physical properties of the tight reservoirs.
Fig. 6 Microscopic pore structure of tight oil reservoirs in Gaotaizi Formation, Qijia region
2.3
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Source and reservoir configuration
Good source and reservoir configuration is one of the key
SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 7
Sedimentary section of Gaotaizi reservoir and the upper and lower adjacent layers through Well L28 - Well G702 in Qijia region.
conditions for tight oil accumulation[11−12]. Tight oil reservoirs in Qijia area distribute in Delta front facies, which inserts into source rock of lacustrine facies from North to South, resulting in direct contact of Delta front reservoir rocks with hydrocarbon source rock or pinch-out in lacustrine source rocks in finger shape or lamination with source rocks. The source rocks of Member Qing 2 + 3 not only provide oil source for tight reservoirs but also act as good cap for some reservoirs (Fig. 7). Meanwhile the bottom of the tight oil reservoir is in direct contact with widely distributed Qing 1 source rock, so oil and gas generated by Member Qing 1 can directly feed to the tight oil reservoir. 2.4
has premium oil generation conditions, developed reservoir sands, good formation pressure conditions, strong abnormal pressure, all indicating good reservoir forming conditions; the sands in the outer front of delta in the south of the work area insert (as Well L28-J47 in Fig. 9) directly into the source rock (Fig. 9), favourable for oil and gas accumulation preferably. From the tight oil area to the north, the structure gradually uplifts, and reservoir physical property gets better, oil and water begin to differentiate, and an oil-water transitional zone and water layer developed zone occur (Fig. 8).
Reservoir pressure
Gao 3+4 reservoirs in tight oil rich portion, Qijia region have a measured pressure coefficient of 1.20–1.50, indicating abnormal over pressure. This is mainly because the tight oil area is in the hydrocarbon generation center of the sag, where big burial depth, uneven compaction, tectonic activity, hydrocarbon generation action, and hydrothermal together lead to overpressure[13−15]. Since tight oil reservoirs are relatively poor in physical properties, a certain amount of overpressure is necessary to push the oil and gas migration and accumulation, overpressure serves as driving force for tight oil migration and accumulation.
3 Pattern and controlling factors of tight oil distribution 3.1
Distribution of tight oil
In Qijia area, tight oil mainly occurs in the center of the sag and slopes and terraces around the sag on the plane (as shown in Fig. 8 and Fig. 9), vertically, tight oil is mainly in the Gao 3+4 oil layers, with small amount in Gao 1 +2 oil layers. The central and south parts of the study area are universally oil-bearing, featuring large oil-bearing area, and continuous reservoirs without distinct boundaries. Located in the center of the hydrocarbon generation sag, the center of the work area
Fig. 8 Planar distribution of oil and water of the lower Gao 3 oil layers in Qijia region
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SHI Lizhi et al. / Petroleum Exploration and Development, 2015, 42(1): 48–55
Fig. 9 Oil layer correlation profile of Gao 3, Gao 4 oil layers through Well Lin L28 – Well J51 in Qijia region
3.2 3.2.1
Controlling factors Reservoir lithology and physical property
The reservoir lithology in Qijia area has a strong effect on tight oil distribution; on the whole, sandstone is better than mudstone in oil-bearing property. In the tight oil area, siltstones generally contain oil, pelitic siltstone and calcareous siltstone with higher mud and calcium content, have worse oil-bearing property than pure siltstone. Sandy strips in silty mudstones are oil-bearing, but mudstones are not oil-bearing. Reservoir physical properties have a strong control over the distribution of tight oil. The reservoirs in the tight oil area have a main porosity range of 4%–12%, the better the reservoir physical properties, the better the oil-bearing property of the reservoir will be. Statistics show that tight oil reservoirs 8%–12% in porosity have the best oil-bearing property, followed by reservoirs 4%–8% in porosity, reservoirs less than 4% in porosity are poor in oil-bearing property (Fig. 5). 3.2.2
Sedimentary facies and sands
Affected by northern provenance, there developed large area of delta deposits in Qijia area, which include subfacies such as outer delta front, inner delta front and shore-shallow lake. The area, with reservoirs in long extension and wide distribution, sands stacking and overlapping stable in distribution, good in continuity, moderate sand to formation ratio, is the main area for tight oil distribution. It is the main distribution area of tight oil. Fig. 7 and Fig. 10 show that tight oil in the Upper Gao 4 oil layers mainly occurs in mouth bar and distributary channel sands in the inner delta front, and sheet sand in the outer delta front subfacies, shallow lake bars in shallow lake facies also contain some tight oil. Sedimentary facies and sands in Qijia area control the planar and spatial distribution of tight oil, providing large stretches of reservoir for tight oil. 3.2.3
Excessive pressure
According to mudstone sonic logging data of more than 150 wells in the study area, by using the equilibrium depth method[16], excessive pressure of the target layer in the key
Fig. 10 Planar sedimentary facies distribution and reservoir development area in Upper Gao 4 oil layers, Qijia region
period of reservoir formation (at the end of Mingshui Formation depositional stage) (Fig. 11) was restored, the equation calculating excessive pressure is: Δpz=rwHe + rbw(H − He) − rwH where, ΔPZ—excessive pressure, Pa; He—balance depth (a point on the normal compaction curve, if its interval transit time is the same with a calculated point in the abnormal section, the depthof this point is the balance depth), m; rw—hydrostatic pressure gradient, Pa/m; rbw—pressure gradient of the rock column from balance depth to target layer , Pa/m; H—target layer depth, m. Fig. 11 shows that tight oil is mainly distributed in sag center and slopes and terraces on the migration paths where the reservoir pressure is over 2.5 MPa. The areas have higher excessive pressure, favorable for the migration and accumulation of oil and gas, so overpressure controls the distribution scope of tight oil in the work area.
4
Assessment of tight oil potential
Based on the reservoir physical properties and changes of planar reservoir thickness, hydrocarbon source rock distribution, sedimentary and structural data, tight oil in Qijia region was evaluated (Fig. 12), class I area mainly distributes in the sag center and Longhupao-Da'an terrace, class II area mainly situates in slopes around the sag, and sag center. According to the volume method, Gao 3+4 tight oil resources are 3.0×108 t
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Table 3 Statistics on tight oil resources of Gao 3 and Gao 4 oil layers in Qijia region FavorArea/ Reservoir Poroable km2 thickness/m sity/% area
Permeability/ 10−3 μm2
Class I 1 130
5−10
8−12
0.05−1.00
1−5
2
Class II 1 120
<5
4−8
0.02−0.50
<2
1
5
Fig. 11 Overlap map of excessive pressure and oil well location in Gao 4 oil layers, Qijia region
Oil proReduction/ souces/ (t·d−1) 108 t
Conclusions
There developed tight oil resources of large potential in Gao 3+4 oil layers of Gaotaizi reservoir in Qijia area of Songliao Basin, thanks to the good hydrocarbon source rock conditions, reservoir conditions, source-reservoir configuration and formation pressure condition. There are three sets of premium source rocks, Qing 1, Qing 2 +3, and Nen 1 in this area, providing good material base for hydrocarbon generation; a variety types of sand, including distributary channel, mouth bar, and sheet sand, act as reservoirs, the various kinds of sands large in thickness, wide in planar distribution, good in continuity, provide storage space for tight oil; source-reservoir configuration is good, delta front reservoirs directly insert into hydrocarbon source rocks or pinch out in lacustrine source rocks in finger shape or interbed with source rocks, conducive to oil and gas feeding to the adjacent reservoir preferably; tight oil reservoirs have overpressure in common with a pressure coefficient of 1.20–1.50, providing driving force for reservoir formation; tight oil mainly distributes in sag center and slopes and terraces around the sag, vertically in Gao 3+4 oil layers, according to volume method, tight oil resources in Gao 3+4 oil layers are 3.0×108 t, including 2×108 t in classⅠarea and 1×108 t in class Ⅱarea, making this area an important relay area of future reserves.
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