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Distribution and formation of Mesozoic low permeability underpressured oil reservoirs in the Ordos Basin, China
Journal Pre-proof Distribution and formation of Mesozoic low permeability underpressured oil reservoirs in the Ordos Basin, China Yi Duan, Yingzhong Wu PII:
S0920-4105(19)31174-X
DOI:
https://doi.org/10.1016/j.petrol.2019.106755
Reference:
PETROL 106755
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 26 June 2019 Revised Date:
27 November 2019
Accepted Date: 28 November 2019
Please cite this article as: Duan, Y., Wu, Y., Distribution and formation of Mesozoic low permeability underpressured oil reservoirs in the Ordos Basin, China, Journal of Petroleum Science and Engineering (2019), doi: https://doi.org/10.1016/j.petrol.2019.106755. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Distribution and formation of Mesozoic low permeability
2
underpressured oil reservoirs in the Ordos Basin, China
3
Yi Duana,b,*, Yingzhong Wuc,
4
a
5
Provincial Key Laboratory of Petroleum Resources, Lanzhou 730000, China
6
b
7
Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences and Gansu
School of Earth Science & Resources, Chang’an University, Xi’an 710054, China c
Shaanxi Center of Mineral Geological Survey, Xi'an 710068, China
8
* Corresponding author. Fax: 0086-0931-8278667. E-mail: [email protected] (Yi Duan)
9
ABSTRACT: The stratum pressure characteristics of Mesozoic reservoirs in the Ordos Basin
10
were first studied. It was found that the Mesozoic reservoirs were mainly ultra-underpressured
11
reservoirs with an average stratum pressure coefficient of 0.63 to 0.86 and the differences in the
12
abnormal underpressure between the different regions and layers were distinct. The results showed
13
that with increases in the eroded stratum thickness and temperature decrease in the reservoirs, the
14
stratum pressure coefficients showed a decreasing trend. The pore water volume contraction in the
15
Yanchang Formation was from 0.82% to 1.94% after tectonic uplift and stratum temperature
16
reduction. It was proposed that because of the strong uplift of the basin for a long time at the end
17
of the Cretaceous, the function of stratum erosion and paleotemperature reduction resulted in the
18
formation of underpressured reservoirs. It is considered that this underpressured closed system of
19
the Mesozoic in the Ordos Basin was advantageous for reservoir preservation and might have
20
played an important role in adjustment, re-enrichment of hydrocarbons by migration, and oil and
21
water distribution in reservoirs. The formation of underpressured anhydrous sand lens reservoirs
22
in the Chang 7 subsection could be related to the distribution of such an underpressured closed
23
system.
24
Key words: Ordos Basin; Mesozoic; underpressured reservoirs; formation
25
1. Introduction
26
When the pore fluid pressure is significantly below or above the normal hydrostatic pressure 1
27
for the appointed depth, the pressure is considered to be abnormal. Abnormal pressures can be
28
divided into abnormal overpressure and abnormal underpressure in terms of the ratio between the
29
stratum pressure and hydrostatic pressure. Many studies regarding reservoir abnormal
30
overpressure have been performed (e.g., Dickison, 1953; Barker, 1972; Chen and Tang, 1983; Luo
31
and Vasseur, 1992; Osbome and Swarbrick, 1997; Duan et al., 2008) and the results indicate that
32
stratum abnormal overpressure plays an important role in oil migration. However, increasingly
33
more underpressured oil and gas reservoirs have been found in many basins, such as San Juan,
34
Denver, Alberta, Powder River, Songliao, Qaidam, and Santanghu basins (Hitchon, 1969; Belitz
35
and Bredehoeft, 1988; Bachu, 1995; Serebryakov and Chilingar, 1994; Xie et al., 2003; Zhang,
36
2007). Only a few studies have been conducted regarding the distributional characteristics and
37
formation of abnormal underpressure (Senger et al., 1987; Belitz and Bredehoeft, 1988; Allan and
38
Creaney, 1991; Corbet and Bethke, 1992; Bachu and Underschultz, 1995; Du et al., 1995).
39
Abnormal underpressure should have a significant effect on hydrocarbon accumulation.
40
Therefore, the researches regarding the distribution and formation of stratum underpressure are
41
helpful in understanding reservoir genesis. Previous studied results showed that stratum
42
underpressure in a sedimentary basin results from many geological factors, such as tectonic uplift
43
and resulting erosion rebound, stratum temperature reduction, rapid leakage of gas from gas pools,
44
the role of groundwater flow, chemical penetration and original pressure preservation at increasing
45
the burial depth and so on (Russell, 1972; Neuzil and Pollock, 1983; Neuzil, 1993; Gurevich, et al.
46
1987; Dobrynin and Serebryakov, 1989; Serebryakov et al., 1994; Hunt, 1995; Warbrick and
47
Osborne, 1998; Xia and Song, 2001). Hunt (1995) analyzed that the abnormal underpressure of all
48
gas fields in North America and the results indicated that it was related to tectonic uplift. Dobrynin
49
and Serebryakov (1989) completed a quantitative analysis of stratum abnormal underpressure on
50
the Siberian Platform. The results showed that the abnormal underpressure was derived from the
51
temperature reduction in the geological section. However, comprehensive research in a specific
52
basin is limited to the distribution and formation of stratum underpressure which is significantly
53
different in different basins.
54
The Mesozoic reservoirs in the Ordos Basin are characterized by low porosity and
55
permeability. Specifically, low permeability tight lithologic oil reservoirs are rich in the Triassic
56
Yanchang Formation. To date, the proven geological reserves of tight oil in the formation, with the 2
57
air permeability less than 2×10−3 µm2, reaches approximately two billion tons. Previous studies
58
have mainly focused on the relationship between the accumulation time and the densification
59
process of the Mesozoic reservoirs in the Ordos Basin. However, the distribution and formation of
60
underpressure and its effect on oil and gas accumulation have not yet been studied yet. We
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collected the data of the measured stratum pressure in the Mesozoic strata and found that
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underpressure was widely distributed. Our objectives in this study were to (1) understand the
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distributional features of underpressure in the Mesozoic of the basin, (2) determine the
64
relationship between the underpressure and some of geological factores, and (3) understand the
65
formation process of the underpressure.
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2. Geological setting
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The Ordos Basin is a large inland sedimentary basin in western China. The deformation of
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the internal structure in the Ordos Basin is very weak except along the edge of the basin, only
69
existing as a stratigraphic discontinuity or erosion (Zhao et al., 1990; Yang, 2002). The Ordos
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Basin contains six structural units (Fig. 1a), and the discovered Mesozoic reservoirs in the Ordos
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Basin was mainly located in the Yishan slope structural unit. During recent years, several major
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oilfields, such as the Wuqi, Jiyuan, Maling, Huaqing, Xifeng, and Northern Shaanxi oilfields, have
73
been discovered in the structural unit. The Ordos Basin was a huge inland freshwater lacustrine
74
basin in the Late Triassic. From the Late Triassic to the Early Cretaceous the Basin essentially
75
kept on subsiding process, and was filled with Mesozoic fluvial lacustrine series (Yang, 2002),
76
which formed Upper Triassic Yanchang Formation and Lower Jurassic Yan’an Formation. At the
77
end of the Early Cretaceous, due to the influence of the Late Yanshanian (the Late Old Alpine)
78
movement, the Ordos Basin suffered extensive uplift and erosion (Yang, 2002). The oil-bearing
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series mainly consist of the Upper Triassic Yanchang Formation and Lower Jurassic Yan’an
80
Formation. Based on lithology, from bottom to top, the Upper Triassic Yanchang Formation can
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be further divided into ten subsections (Chang 10 to Chang 1). The Chang 8 and Chang 6
82
subsections are the main sandstone reservoirs and the Chang-7 subsection contains the best
83
hydrocarbon source rocks (Duan et al., 2008). The Lower Jurassic Yan'an Formation is also
84
divided into ten subsections (Yan 10 to Yan 1) from bottom to top. The Yan'an Formation is
85
composed mainly of coal-bearing clastic rocks (Fig. 1b). The Mesozoic reservoirs are widely 3
86
distributed and are characterized by low porosity and permeability.
87
3. Results and discussion
88
3.1. Underpressure distributional characteristics
89
3.1.1. Underpressure classification
90
In general, if the fluid pressure in the stratum pore space is lower than the hydrostatic
91
pressure, i.e. the stratum pressure coefficient is less than 1, the stratum pressure is considered to be
92
underpressure or negative pressure. Many researchers have proposed different classification
93
criteria for stratum abnormal pressures. Researchers in the former Soviet Union defines the
94
stratum pressure coefficient of 1.0–1.05 as normal pressure, 0.8–1.0 as underpressure and less
95
than 0.8 as ultra underpressure (Du et al., 1995). However, researchers for EXXON company of
96
the United States define a stratum pressure coefficient of less than 0.96 as underpressure and less
97
than 0.8 as ultra underpressure (Du et al., 1995). Based on the statistical data of 260 oil and gas
98
fields in different countries, Chinese scholars define a stratum pressure coefficient of 0.9–1.1 as
99
normal pressure, 0.9–0.75 as underpressure, and less than 0.75 as ultra underpressure (Du et al.,
100
1995). Previous data show that underpressured oil and gas fields contain a high proportion of the
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oil and gas fields in the world. For example, underpressured oil and gas fields account for 11.7%
102
of the 160 worldwide oil and gas fields. Underpressured gas fields occupy 15.7% of the 210
103
medium and large gas fields around the world (Du et al., 1995).
104
3.1.2. Underpressure distribution
105
The measured stratum pressure coefficients in the Mesozoic reservoirs of the Ordos Basin are
106
listed in Table 1. The average stratum pressure coefficient in the Yan’an Formation is 0.80,
107
showing that the Yan’an Formation reservoirs are underpressured. The average stratum pressure
108
coefficients in the Chang 1–Chang 10 subsections of the Yanchang Formation range from 0.63 to
109
0.86 among different regions; they are mostly in the range of ultra underpressure. This shows the
110
Chang 1–Chang 10 subsections of the Yanchang Formation are underpressured reservoirs. The
111
average stratum pressure coefficients in the Yanchang Formation are similar among different
112
regions, ranging from 0.72 to 0.76. However, the average stratum pressure coefficients show a
113
decreasing trend from Chang 1 to Chang 7 and an increasing trend from Chang 8 to Chang 10. 4
114
Although the plane distribution of the stratum pressure coefficient is controlled by many
115
factors, the stratum pressure coefficients in the same layer of the same region should have a
116
certain comparability. There are more measured stratum pressure coefficients data in the Chang 8
117
subsection in the Xifeng region of the Ordos Basin than other regions of the Basin. The horizontal
118
distribution of the data shows the stratum pressure coefficients in Chang 8 subsection have an
119
increasing trend from southeast to northwest (Fig.2a).
120
3.2. Underpressure formation
121
In general, underpressured reservoirs can originate from many geological functions, such as
122
tectonic uplift and erosional unloading, decreasing ground temperature, groundwater flow,
123
hydrocarbon dissipation, chemiosmosis, and original pressure preservation at increasing depth.
124
The Mesozoic underpressured reservoirs in the Ordos Basin may have formed mainly via tectonic
125
uplift and erosional unloading and decreasing ground temperature.
126
3.2.1. Tectonic uplift and erosion effect
127
In general, geological tectonic uplifting can lead to serious stratum erosion and an erosional
128
unloading phenomenon, which can cause the rebound and expansion of the rock in the strata. If
129
the stratum seal is good and the strata are not invaded by extraneous fluid, the underpressure is
130
preserved in the strata (Neuzil and Pollock, 1983; Neuzil, 1993; Toth and Corbet, 1986; Corbet
131
and Bethke, 1992). As shown in Fig. 2, there is an increasing trend in the stratum pressure
132
coefficient of Chang 8 (Fig. 2a) and a decreasing trend in the erosional thickness of its overlying
133
strata from southeast to northwest (Fig. 2b). The reason for this phenomenon is that the decrease
134
of the vertical stress caused by the increase of the erosional thickness leads to the rebound of the
135
rock skeleton, which stimulates the expansion of the pore volume of the rock, resulting in a
136
decrease in fluid pressure. The result shows that stratum erosion of great thickness is a main factor
137
in underpressure formation of in the Mesozoic of the Ordos Basin.
138
Meanwhile, in order to examine the significance of the effects of erosional unloading, a
139
dimensionless quantity m※ was proposed for "tight" rocks in various geologic environments
140
(Neuzil and Pollock, 1983; Toth and Corbet, 1986; Parks and Toth, 1995) as follows:
141
m※=Ssvl2/hK 5
142
where m※ is a dimensionless quantity, Ss is the water storage coefficient, v is the erosional
143
rate, h is the original thickness, l is the thickness after erosion, and K is the hydraulic conductivity.
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The Mesozoic rocks in the Ordos Basin have low permeability and porosity. For example, the
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mean porosity of the Chang 6 and Chang 8 subsections of the Yanchang Formation is 8.3%, and
146
their air permeability is largely less than 1×10−3 µm2. Therefore, the Mesozoic rocks in the Ordos
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Basin are mostly low permeability tight rocks. The stratum water salinity in the Yan’an and
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Yanchang formations of the Ordos Basin is high. For example, the salinity ranges from 34.2 to
149
112.0 g/L in the Northern Shaanxi, Jiyuan, and Southern Tianhuan depression regions and the
150
water type is mainly CaCl2 (Fig. 3). This shows that the Yan’an and Yanchang formations
151
generally have a good sealing condition. However, the geological structure of the Mesozoic in the
152
Ordos Basin is stable and shows a westward inclined gentle sloping massive monocline with a
153
stratigraphic dip of less than 1°. At the same time, the Yanchang Formation is a lacustrine-delta
154
deposit, the Chang 7 subsection of the Yanchang Formation contains very thick mudstone of a
155
deep-water lacustrine deposit and mudstone is widely distributed in the other subsections of the
156
Yanchang Formation. For example, the total thickness of the mudstone in the Yanchang Formation
157
reaches approximately 700 m (Duan et al., 2008). The Yan’an Formation mainly consists of fluvial
158
and lacustrine deposits and the top of each of its subsections has developed mudstones and coal
159
seams (Duan et al., 2008). These mudstones and coal seams have resulted in a good sealing
160
condition for fluid formation. These data show that the Mesozoic rocks in the Ordos Basin are
161
suitable for the calculation of the dimensionless quantity m※.
162
The calculated m※values for the Mesozoic in the Ordos Basin are listed in Table 2. The m※
163
values at the maximum erosion of the Mesozoic range from 3.4×10-2 to 5.7×10-2. In general, when
164
the m※ value is greater than 2×10-2 to 8×10-2, the underpressure in low permeability rocks forms
165
via erosional unloading (Neuzil and Pollock, 1983). The m※ values for the Mesozoic show that
166
erosion of the overlying strata caused the underpressure in the Yan’an and Yanchang formations,
167
which has remained because of good sealing conditions.
168
3.2.2. Decreasing ground temperature
169
The reduction in temperature in sediment and its fluid is an important factor for the formation
170
of underpressured reservoirs. This is because water contracts more than the sediment framework 6
171
that contains it, such that the pressure on the pore fluid decreases with cooling (Corbet and Bethke,
172
1992).
173
As shown in Fig. 2b and 4, the tectonic movement at the end of the early Cretaceous in the
174
Ordos Basin resulted in the overlying strata of the Yanchang Formation uplifting and eroding. This
175
may have led to a reduction in the volume of pore fluid in the sediments because of the decreasing
176
temperature of the Yanchang Formation. Previous results have shown that the change in volume of
177
the formation brine is 44 times greater than those of the rock pores while the ground temperature
178
decreased 1oC (the rock expansion coefficient is 9×10-6 oC -1 while the formation brine expansion
179
coefficient is 400×10-6 oC -1) (Hodgman, 1957), which would generate abnormal underpressure.
180
We analyzed the homogenization temperature of hydrocarbon inclusions in the Yanchang
181
Formation. The results showed that the mean homogenization temperature was 110 oC in the
182
Gufengzhuang, Jiyuan and Baibao areas and 100 oC in the Xifeng and Luochuan areas (Table 3).
183
The homogenization temperature of hydrocarbon inclusions represents the hydrocarbon
184
accumulation temperature. Meanwhile, we collected average ground temperature data at present in
185
each subsections of the Yanchang Formation. The statistical data showed that a significant
186
difference in the ground temperature of the Yanchang Formation occurs among different regions
187
but the difference was small among different subsections in the same region and had a decreasing
188
trend with increasing depth (Table 4). Therefore, the average temperature of different subsections
189
in the same region can represent the reservoir temperature at present in this area. As shown in
190
Table 4, the average reservoir temperatures in the Jiyuan, Huaqing, Xifeng, Wuqi, and Northern
191
Shannxi areas are 64.8, 58.3, 66.1, 52.6, and 40.1 oC, respectively. The difference (∆T) between
192
the hydrocarbon accumulation temperature and the reservoir temperature at present should
193
represent the value of the ground temperature reduction after tectonic uplift and stratum erosion.
194
This value shows that reservoir temperatures in the Jiyuan, Huaqing, Xifeng, and Northern
195
Shaanxi areas decreased on average by 45.2, 51.7, 33.9, and 59.9 oC, respectively. This
196
temperature reduction should have played a certain role in the formation of underpressured
197
reservoirs. Figure 5 is a correlation diagram between the value of reservoir temperature reduction
198
after tectonic uplift and the stratum pressure coefficient for the Chang 8 subsection. It is observed
199
that the stratum pressure coefficient decreases with increasing reservoir temperature reduction. 7
200
This indicates that the underpressure formation is also closely related to the reservoir temperature
201
reduction.
202
A formula of V=nV0[1+αf (T-T0)-βf (P-P0)] for estimating the contraction value of stratum
203
pore water volume after tectonic uplift and a stratum temperature decrease was proposed by Xia
204
and Song (2001) (αf is the stratum water expansion coefficient equal to 4×10-4K-1, βf is the stratum
205
water compressibility equal to 3×10-10Pa-1, V is the volume, T is the temperature (°K), and P is the
206
pressure (Pa)). As previously described, the Yanchang Formation has good closed condition, such
207
that this formula is suitable for the Yanchang Formation reservoirs in the Ordos Basin. According
208
to the previously studied results of reservoir inclusion temperature and reservoir temperature at
209
present as well as the trapping pressure of reservoir fluid inclusions and the reservoir pressure at
210
present, we calculated the contraction value (V0/V) of the pore water volume in the Yanchang
211
Formation reservoirs after tectonic uplift and stratum temperature reduction. The calculated results
212
are listed in Table 5. The contraction value ranged from 0.82% to 1.94%, showing that the
213
reservoir pressure was much lower than the hydrostatic pressure at the current depth with the pore
214
water volume contracting. As shown Table 5, a significant difference in the contraction value of
215
the pore water volume occurred among subsections of the Yanchang Formation in different
216
regions.
217
3.3. Formation process of underpressured reservoirs
218
The aforementioned stratum abnormal pressure data showed that the Mesozoic reservoirs in
219
the Ordos Basin are mainly abnormal underpressured reservoirs at present (Table 1) ); the erosion
220
of the overlying strata and the the paleotemperature reduction caused by tectonic uplift have
221
resulted in the formation of these underpressured reservoirs. The Mesozoic crude oils in the Ordos
222
Basin were derived mainly from Chang 7 source rock. During the end of the early Cretaceous,
223
Ordos Basin subsidence reached a maximum (Fig. 4) and the oil generation of the Chang 7 source
224
rock also achieved to a maximum. The Mesozoic reservoirs in the Ordos Basin formed during this
225
period and the restoration of evolutionary history of the fluid pressure and the calculation of the
226
balance depth from the Mesozoic mudstones indicates that the Mesozoic reservoirs were
227
abnormally high-pressure reservoirs (Fig. 6). For example, the ancient fluid residual pressure of
228
the Chang 8 reservoirs in the Huaqing area was between 6.2 and 13.6 MPa duiring the end of the 8
229
early Cretaceous (Yao et al., 2015). Following the late Cretaceous, the Ordos Basin underwent
230
extensive tectonic uplift and stratum erosion over a long time, resulting in stratum decompression
231
and formation of underpressured reservoirs (Fig. 6).
232
The underpressured closed system caused by tectonic uplift and stratum erosion in the Ordos
233
Basin should have played an important role in the preservation of the early oil reservoirs.
234
Meanwhile, the underpressured closed system may have had a significant influence on reservoir
235
formation, such as reservoir adjustment and re-enrichment of hydrocarbon driven by local fluid
236
backflow caused by the underpressure, and oil and water distribution caused by the different
237
rebound of the sandstone and the mudstone. An example is the sand lens reservoirs in the Chang 7
238
subsection of the Yanchang Formation, which are underpressured anhydrous reservoirs caused by
239
tectonic uplift and stratum erosion (Figure. 7). The Chang 7 subsection was deposited in a deep
240
lacustrine environment and is thought to be the excellent source rock in the basin (Duan et al.,
241
2008). Lake delta front and deep to semi-deep lake turbidite sand bodies are widely developed in
242
the Chang 7 subsection. The lens turbidite sandstone, a good reservoir, is surrounded by Chang 7
243
source rock. The oil and gas generated from the Chang 7 source migrated into the lens sandstone
244
driven by abnormal fluid high pressure, forming the overpressured lens reservoirs. The tectonic
245
uplift and stratum erosion in the Ordos Basin during the late Cretaceous caused the rock
246
framework rebound and pore volume expansion of the Chang 7 subsection, resulting in
247
underpressured reservoirs. However, because of the difference in the pore expansion rate between
248
the sandstone and mudstone (Fatt, 1958; McLatchie et al., 1958), the lens reservoirs in the Chang
249
7 subsection are underpressured anhydrous reservoirs.
250
4. Conclusions
251
The Mesozoic reservoirs in the Ordos Basin are mainly tight oil reservoirs. The average
252
stratum pressure coefficient ranges from 0.65 to 0.74; thus, they are mainly ultra underpressured
253
reservoirs. There is a significant difference in the stratum pressure coefficient among different
254
regions
255
underpressure-generating conditions, a significant negative correlation between the stratum
256
pressure coefficient and the eroded stratum thickness as well as the stratum temperature reduction
257
value, and a pore water volume contraction of 0.82%–1.94%, showing that the formation of
and
layers.
The
m※
value
distribution
9
range
reflects
the
abnormal
258
Mesozoic underpressured reservoirs resulted mainly from stratum erosion and temperature
259
reduction. The underpressured closed system formed via tectonic uplift and stratum erosion in the
260
Ordos Basin following the late Cretaceous would have played a significant role in the preservation
261
of the early oil reservoirs and the formation of the secondary reservoirs.
262
Acknowledgments
263
China (Grant No. 41972110 and 41772108). We thank an anonymous reviewer and Dr. Tahar Aïfa
264
for their valuable suggestions and critical comments.
265
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266
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322 323
Xia, X.Y., Song, Y., 2001. Temperature effects on geopressure during deposition and erosion. Petrol. Explor. Dev. 28(3), 8-13 (in Chinese with English abstract).
324
Xie, X.N., Jiao, J.J., Tang, Z.H., Zheng, C.M., 2003. Evolution of abnormally low pressure and its
325
implications for the hydrocarbon system in the southeast uplift zone of Songliao basin, China.
326
AAPG, 87(1), 99-119.
327 328
Yang, J.J, 2002. Tectonic evolution and oil-gas reservoirs distribution in Ordos Basin. Petroleum industry press, Beijing, pp.1-85.
329
Yao, J.L., Xu, L., Xing, L.T., Luo, A.X., Deng,X.Q., Duan, Y., Zhao, Y., Wu, Y.Z., 2015. Fluid
330
overpressure and oil migration in Chang 7 and Chang 8 subsections of Yanchang Formation
331
in Ordos Basin, China. Nat. Gas. Geosci. 26(12), 2219-2226 (in Chinese with English
332
abstract).
333
Zhao, C.Y., Liu, C.Y., 1990. The formation and evolution of the sedimentary basins and their
334
hydrocarbon occurrence in the North China Craton. Northwest University press, Xi'an, pp.
335
93-122.
336
Zhang, J.F., 2007. Geological characteristics and formation mechanism of Mesozoic abnormal
337
low-pressure oil reservoir in the Santanghu Basin. Research Institute of Petroleum
338
Exploration and Development , Beijing.
339 340 341 342 343 344 345 346 12
347
Figure captions:
348 349 350 351 352 353 354 355 356 357 358 359 360 361 362
Fig. 1. Tectonic units of the Ordos Basin (a) and stratigraphic column of the Yan’an and Yanchang formations (b). Fig. 2. Isopach maps of the stratum pressure coefficient of the Chang 8 reservoir rocks (a) and erosional thickness (m) of the stratum during the end of the early Cretaceous (b). Fig. 3. Histogram of the average total salinity of the formation water in the Yan’an and Yanchang formations. Fig. 4. Stratigraphic burial history of the Mesozoic strata in well Feng 7 showing the relationship between the stratum depth (m) and the geological age (Ma). Fig. 5. Cross plots of the stratum pressure coefficient of the Chang 8 reservoir rocks vs. difference in temperature (oC) before and after uplift of the oil reservoir. Fig. 6. Changes in stratum (m), porosity (%), hydrocarbon generation, and pressure coefficient in the Yanchang Formation over time. Fig. 7. Chang 7 oil reservoir profile in the Shangliyuan and Baibao regions (NE—Profile direction; GR—Natural gamma curve; AC—Interval transit time curve).
363 364 365 366 367 368 369 370 371
13
372
Table captions:
373 374
Table 1 Mean stratum pressure coefficients of the Mesozoic reservoir rocks in the Ordos Basin.
375
Table 2 Distribution of m
376
Table 3 Maximum paleotemperature of hydrocarbon generation for the Chang 7 source rocks and
377
※
values.
paleotemperature of the hydrocarbon accumulation in the Yanchang Formation reservoir.
378
Table 4 Average current temperature of the Yanchang Formation (oC).
379
Table 5 Contraction value of pore water volume in the Yanchang Formation (%).
380 381 382 383 384 385 386 387 388 389 390
14
Table 1 Mean stratum pressure coefficients of the Mesozoic reservoir rocks in the Ordos Basin Region Formation
Subsection
Yan'an
Maling
Jiyuan
Xifeng
Wuqi
Northern Shaanxi
0.80(4) Chang 1
0.70(1)
0.86(1)
Chang 2
0.75(7)
0.81(5)
Chang 4+5
0.67(7)
0.65(1)
0.74(3)
0.71(4)
0.77(14)
0.71(16)
0.74(2)
0.71(18)
0.69(2)
0.84(2)
0.72(11)
0.71(35)
0.70(48)
Chang 6
0.63(8)
Chang 7
0.83(2)
Chang 8
0.74(10)
Chang 9
0.83(3)
0.69(5)
0.70(1) 0.76(16)
0.75(26)
0.75(1)
0.75(1)
Chang 10 Mean
0.80(4)
Mean 0.80(4)
Chang 3
Yanchang
Huaqing
0.72(4)
0.75(44)
0.76(16)
0.76(8)
0.79(1)
0.79(1)
0.74(45)
0.74(127)
The figure in parentheses is the number of samples; The measured stratum pressure coefficients data are from Changqing Oilfield.
Table 2 Distribution of m※ values Region
Gufengzhuang
Jiyuan
Baibao
Xifeng
Luochuan
Maximum buried depth (m)
3015
3150
3300
3000
3015
Maximum erosion thickness (m)
630
600
1100
630
1500
Maximum residual thickness (m)
2385
2200
2370
Erosion rate (mm/s) m※ value at maximum erosion thickness
2.2×10
-10
0.034
2550 2.1×10
-10
0.044
3.9×10
-10
0.057
2.2×10
-10
0.042
1515 5.3×10-10 0.040
Table 3 Maximum paleotemperature of hydrocarbon generation in the Chang 7 source rocks and paleotempeature of the hydrocarbon accumulation in the Yanchang Formation reservoir Region Maximum buried depth (m) of the Chang 7 source rocks in the Early Cretaceous a
Gufengzhuang
Jiyuan
Baibao
Xifeng
Luochuan
2800
2850
2950
2300
2000
130
125
130
110
100
110
110
110
100
100
Maximum paleotemperature (oC) of
hydrocarbon generation in the Chang 7 source rocks b
Paleotemperature (oC) of the hydrocarbon
accumulation a
data from the stratigraphic burial history and thermal evolution history; binclusion homogenization temperature.
Table 4 Average current temperature of the Yanchang Formation (oC) Subsection
Region Jiyuan
Chang 1
61.6(1)
Chang 2
62.9(6)
Chang 3 Chang 4+5
Huaqing
Xifeng
Wuqi
a
Northern Shaanxi 38.6(3)
51.5(6)
47.6(2)
31.5(14)
57.0(12) 69.9(5)
60.9(2)
44.8(4)
Chang 6
62.7(5)
Chang 7
59.6(1)
Chang 8
63.5(8)
66.1(11)
64.8
58.3
66.1
45.2
51.7
57.5(7)
45.6(56)
52.6
40.1
Chang 10 Average ∆T
b
a
33.9
59.9
b
The figure in parentheses is the number of samples; ∆T is the difference between the hydrocarbon accumulation
temperature and the reservoirs temperature at present, which should represent the value of ground temperature reduce after tectonic uplift and stratum erosion.
Table 5 Contraction value of pore water volume in the Yanchang Formation (%) Subsection Chang 2
Region Jiyuan
Huaqing
1.49
1.94
1.13
1.59
Xifeng
Northern Shaanxi 1.47
Chang 3 Chang 4+5 Chang 6
1.28
Chang 8
1.33
1.21 0.82
Highlights:
The Mesozoic reservoirs in the Ordos Basin are mainly ultra-underpressured ones. The stratum erosion and paleotemperature control the formation of this reservoir. The underpressure system is beneficial to the reservoir formation and preservation.
Duan and Wu jointly carried out data collection, drawing and writing.
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
S0920-4105(19)31174-X
DOI:
https://doi.org/10.1016/j.petrol.2019.106755
Reference:
PETROL 106755
To appear in:
Journal of Petroleum Science and Engineering
Received Date: 26 June 2019 Revised Date:
27 November 2019
Accepted Date: 28 November 2019
Please cite this article as: Duan, Y., Wu, Y., Distribution and formation of Mesozoic low permeability underpressured oil reservoirs in the Ordos Basin, China, Journal of Petroleum Science and Engineering (2019), doi: https://doi.org/10.1016/j.petrol.2019.106755. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
1
Distribution and formation of Mesozoic low permeability
2
underpressured oil reservoirs in the Ordos Basin, China
3
Yi Duana,b,*, Yingzhong Wuc,
4
a
5
Provincial Key Laboratory of Petroleum Resources, Lanzhou 730000, China
6
b
7
Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences and Gansu
School of Earth Science & Resources, Chang’an University, Xi’an 710054, China c
Shaanxi Center of Mineral Geological Survey, Xi'an 710068, China
8
* Corresponding author. Fax: 0086-0931-8278667. E-mail: [email protected] (Yi Duan)
9
ABSTRACT: The stratum pressure characteristics of Mesozoic reservoirs in the Ordos Basin
10
were first studied. It was found that the Mesozoic reservoirs were mainly ultra-underpressured
11
reservoirs with an average stratum pressure coefficient of 0.63 to 0.86 and the differences in the
12
abnormal underpressure between the different regions and layers were distinct. The results showed
13
that with increases in the eroded stratum thickness and temperature decrease in the reservoirs, the
14
stratum pressure coefficients showed a decreasing trend. The pore water volume contraction in the
15
Yanchang Formation was from 0.82% to 1.94% after tectonic uplift and stratum temperature
16
reduction. It was proposed that because of the strong uplift of the basin for a long time at the end
17
of the Cretaceous, the function of stratum erosion and paleotemperature reduction resulted in the
18
formation of underpressured reservoirs. It is considered that this underpressured closed system of
19
the Mesozoic in the Ordos Basin was advantageous for reservoir preservation and might have
20
played an important role in adjustment, re-enrichment of hydrocarbons by migration, and oil and
21
water distribution in reservoirs. The formation of underpressured anhydrous sand lens reservoirs
22
in the Chang 7 subsection could be related to the distribution of such an underpressured closed
23
system.
24
Key words: Ordos Basin; Mesozoic; underpressured reservoirs; formation
25
1. Introduction
26
When the pore fluid pressure is significantly below or above the normal hydrostatic pressure 1
27
for the appointed depth, the pressure is considered to be abnormal. Abnormal pressures can be
28
divided into abnormal overpressure and abnormal underpressure in terms of the ratio between the
29
stratum pressure and hydrostatic pressure. Many studies regarding reservoir abnormal
30
overpressure have been performed (e.g., Dickison, 1953; Barker, 1972; Chen and Tang, 1983; Luo
31
and Vasseur, 1992; Osbome and Swarbrick, 1997; Duan et al., 2008) and the results indicate that
32
stratum abnormal overpressure plays an important role in oil migration. However, increasingly
33
more underpressured oil and gas reservoirs have been found in many basins, such as San Juan,
34
Denver, Alberta, Powder River, Songliao, Qaidam, and Santanghu basins (Hitchon, 1969; Belitz
35
and Bredehoeft, 1988; Bachu, 1995; Serebryakov and Chilingar, 1994; Xie et al., 2003; Zhang,
36
2007). Only a few studies have been conducted regarding the distributional characteristics and
37
formation of abnormal underpressure (Senger et al., 1987; Belitz and Bredehoeft, 1988; Allan and
38
Creaney, 1991; Corbet and Bethke, 1992; Bachu and Underschultz, 1995; Du et al., 1995).
39
Abnormal underpressure should have a significant effect on hydrocarbon accumulation.
40
Therefore, the researches regarding the distribution and formation of stratum underpressure are
41
helpful in understanding reservoir genesis. Previous studied results showed that stratum
42
underpressure in a sedimentary basin results from many geological factors, such as tectonic uplift
43
and resulting erosion rebound, stratum temperature reduction, rapid leakage of gas from gas pools,
44
the role of groundwater flow, chemical penetration and original pressure preservation at increasing
45
the burial depth and so on (Russell, 1972; Neuzil and Pollock, 1983; Neuzil, 1993; Gurevich, et al.
46
1987; Dobrynin and Serebryakov, 1989; Serebryakov et al., 1994; Hunt, 1995; Warbrick and
47
Osborne, 1998; Xia and Song, 2001). Hunt (1995) analyzed that the abnormal underpressure of all
48
gas fields in North America and the results indicated that it was related to tectonic uplift. Dobrynin
49
and Serebryakov (1989) completed a quantitative analysis of stratum abnormal underpressure on
50
the Siberian Platform. The results showed that the abnormal underpressure was derived from the
51
temperature reduction in the geological section. However, comprehensive research in a specific
52
basin is limited to the distribution and formation of stratum underpressure which is significantly
53
different in different basins.
54
The Mesozoic reservoirs in the Ordos Basin are characterized by low porosity and
55
permeability. Specifically, low permeability tight lithologic oil reservoirs are rich in the Triassic
56
Yanchang Formation. To date, the proven geological reserves of tight oil in the formation, with the 2
57
air permeability less than 2×10−3 µm2, reaches approximately two billion tons. Previous studies
58
have mainly focused on the relationship between the accumulation time and the densification
59
process of the Mesozoic reservoirs in the Ordos Basin. However, the distribution and formation of
60
underpressure and its effect on oil and gas accumulation have not yet been studied yet. We
61
collected the data of the measured stratum pressure in the Mesozoic strata and found that
62
underpressure was widely distributed. Our objectives in this study were to (1) understand the
63
distributional features of underpressure in the Mesozoic of the basin, (2) determine the
64
relationship between the underpressure and some of geological factores, and (3) understand the
65
formation process of the underpressure.
66
2. Geological setting
67
The Ordos Basin is a large inland sedimentary basin in western China. The deformation of
68
the internal structure in the Ordos Basin is very weak except along the edge of the basin, only
69
existing as a stratigraphic discontinuity or erosion (Zhao et al., 1990; Yang, 2002). The Ordos
70
Basin contains six structural units (Fig. 1a), and the discovered Mesozoic reservoirs in the Ordos
71
Basin was mainly located in the Yishan slope structural unit. During recent years, several major
72
oilfields, such as the Wuqi, Jiyuan, Maling, Huaqing, Xifeng, and Northern Shaanxi oilfields, have
73
been discovered in the structural unit. The Ordos Basin was a huge inland freshwater lacustrine
74
basin in the Late Triassic. From the Late Triassic to the Early Cretaceous the Basin essentially
75
kept on subsiding process, and was filled with Mesozoic fluvial lacustrine series (Yang, 2002),
76
which formed Upper Triassic Yanchang Formation and Lower Jurassic Yan’an Formation. At the
77
end of the Early Cretaceous, due to the influence of the Late Yanshanian (the Late Old Alpine)
78
movement, the Ordos Basin suffered extensive uplift and erosion (Yang, 2002). The oil-bearing
79
series mainly consist of the Upper Triassic Yanchang Formation and Lower Jurassic Yan’an
80
Formation. Based on lithology, from bottom to top, the Upper Triassic Yanchang Formation can
81
be further divided into ten subsections (Chang 10 to Chang 1). The Chang 8 and Chang 6
82
subsections are the main sandstone reservoirs and the Chang-7 subsection contains the best
83
hydrocarbon source rocks (Duan et al., 2008). The Lower Jurassic Yan'an Formation is also
84
divided into ten subsections (Yan 10 to Yan 1) from bottom to top. The Yan'an Formation is
85
composed mainly of coal-bearing clastic rocks (Fig. 1b). The Mesozoic reservoirs are widely 3
86
distributed and are characterized by low porosity and permeability.
87
3. Results and discussion
88
3.1. Underpressure distributional characteristics
89
3.1.1. Underpressure classification
90
In general, if the fluid pressure in the stratum pore space is lower than the hydrostatic
91
pressure, i.e. the stratum pressure coefficient is less than 1, the stratum pressure is considered to be
92
underpressure or negative pressure. Many researchers have proposed different classification
93
criteria for stratum abnormal pressures. Researchers in the former Soviet Union defines the
94
stratum pressure coefficient of 1.0–1.05 as normal pressure, 0.8–1.0 as underpressure and less
95
than 0.8 as ultra underpressure (Du et al., 1995). However, researchers for EXXON company of
96
the United States define a stratum pressure coefficient of less than 0.96 as underpressure and less
97
than 0.8 as ultra underpressure (Du et al., 1995). Based on the statistical data of 260 oil and gas
98
fields in different countries, Chinese scholars define a stratum pressure coefficient of 0.9–1.1 as
99
normal pressure, 0.9–0.75 as underpressure, and less than 0.75 as ultra underpressure (Du et al.,
100
1995). Previous data show that underpressured oil and gas fields contain a high proportion of the
101
oil and gas fields in the world. For example, underpressured oil and gas fields account for 11.7%
102
of the 160 worldwide oil and gas fields. Underpressured gas fields occupy 15.7% of the 210
103
medium and large gas fields around the world (Du et al., 1995).
104
3.1.2. Underpressure distribution
105
The measured stratum pressure coefficients in the Mesozoic reservoirs of the Ordos Basin are
106
listed in Table 1. The average stratum pressure coefficient in the Yan’an Formation is 0.80,
107
showing that the Yan’an Formation reservoirs are underpressured. The average stratum pressure
108
coefficients in the Chang 1–Chang 10 subsections of the Yanchang Formation range from 0.63 to
109
0.86 among different regions; they are mostly in the range of ultra underpressure. This shows the
110
Chang 1–Chang 10 subsections of the Yanchang Formation are underpressured reservoirs. The
111
average stratum pressure coefficients in the Yanchang Formation are similar among different
112
regions, ranging from 0.72 to 0.76. However, the average stratum pressure coefficients show a
113
decreasing trend from Chang 1 to Chang 7 and an increasing trend from Chang 8 to Chang 10. 4
114
Although the plane distribution of the stratum pressure coefficient is controlled by many
115
factors, the stratum pressure coefficients in the same layer of the same region should have a
116
certain comparability. There are more measured stratum pressure coefficients data in the Chang 8
117
subsection in the Xifeng region of the Ordos Basin than other regions of the Basin. The horizontal
118
distribution of the data shows the stratum pressure coefficients in Chang 8 subsection have an
119
increasing trend from southeast to northwest (Fig.2a).
120
3.2. Underpressure formation
121
In general, underpressured reservoirs can originate from many geological functions, such as
122
tectonic uplift and erosional unloading, decreasing ground temperature, groundwater flow,
123
hydrocarbon dissipation, chemiosmosis, and original pressure preservation at increasing depth.
124
The Mesozoic underpressured reservoirs in the Ordos Basin may have formed mainly via tectonic
125
uplift and erosional unloading and decreasing ground temperature.
126
3.2.1. Tectonic uplift and erosion effect
127
In general, geological tectonic uplifting can lead to serious stratum erosion and an erosional
128
unloading phenomenon, which can cause the rebound and expansion of the rock in the strata. If
129
the stratum seal is good and the strata are not invaded by extraneous fluid, the underpressure is
130
preserved in the strata (Neuzil and Pollock, 1983; Neuzil, 1993; Toth and Corbet, 1986; Corbet
131
and Bethke, 1992). As shown in Fig. 2, there is an increasing trend in the stratum pressure
132
coefficient of Chang 8 (Fig. 2a) and a decreasing trend in the erosional thickness of its overlying
133
strata from southeast to northwest (Fig. 2b). The reason for this phenomenon is that the decrease
134
of the vertical stress caused by the increase of the erosional thickness leads to the rebound of the
135
rock skeleton, which stimulates the expansion of the pore volume of the rock, resulting in a
136
decrease in fluid pressure. The result shows that stratum erosion of great thickness is a main factor
137
in underpressure formation of in the Mesozoic of the Ordos Basin.
138
Meanwhile, in order to examine the significance of the effects of erosional unloading, a
139
dimensionless quantity m※ was proposed for "tight" rocks in various geologic environments
140
(Neuzil and Pollock, 1983; Toth and Corbet, 1986; Parks and Toth, 1995) as follows:
141
m※=Ssvl2/hK 5
142
where m※ is a dimensionless quantity, Ss is the water storage coefficient, v is the erosional
143
rate, h is the original thickness, l is the thickness after erosion, and K is the hydraulic conductivity.
144
The Mesozoic rocks in the Ordos Basin have low permeability and porosity. For example, the
145
mean porosity of the Chang 6 and Chang 8 subsections of the Yanchang Formation is 8.3%, and
146
their air permeability is largely less than 1×10−3 µm2. Therefore, the Mesozoic rocks in the Ordos
147
Basin are mostly low permeability tight rocks. The stratum water salinity in the Yan’an and
148
Yanchang formations of the Ordos Basin is high. For example, the salinity ranges from 34.2 to
149
112.0 g/L in the Northern Shaanxi, Jiyuan, and Southern Tianhuan depression regions and the
150
water type is mainly CaCl2 (Fig. 3). This shows that the Yan’an and Yanchang formations
151
generally have a good sealing condition. However, the geological structure of the Mesozoic in the
152
Ordos Basin is stable and shows a westward inclined gentle sloping massive monocline with a
153
stratigraphic dip of less than 1°. At the same time, the Yanchang Formation is a lacustrine-delta
154
deposit, the Chang 7 subsection of the Yanchang Formation contains very thick mudstone of a
155
deep-water lacustrine deposit and mudstone is widely distributed in the other subsections of the
156
Yanchang Formation. For example, the total thickness of the mudstone in the Yanchang Formation
157
reaches approximately 700 m (Duan et al., 2008). The Yan’an Formation mainly consists of fluvial
158
and lacustrine deposits and the top of each of its subsections has developed mudstones and coal
159
seams (Duan et al., 2008). These mudstones and coal seams have resulted in a good sealing
160
condition for fluid formation. These data show that the Mesozoic rocks in the Ordos Basin are
161
suitable for the calculation of the dimensionless quantity m※.
162
The calculated m※values for the Mesozoic in the Ordos Basin are listed in Table 2. The m※
163
values at the maximum erosion of the Mesozoic range from 3.4×10-2 to 5.7×10-2. In general, when
164
the m※ value is greater than 2×10-2 to 8×10-2, the underpressure in low permeability rocks forms
165
via erosional unloading (Neuzil and Pollock, 1983). The m※ values for the Mesozoic show that
166
erosion of the overlying strata caused the underpressure in the Yan’an and Yanchang formations,
167
which has remained because of good sealing conditions.
168
3.2.2. Decreasing ground temperature
169
The reduction in temperature in sediment and its fluid is an important factor for the formation
170
of underpressured reservoirs. This is because water contracts more than the sediment framework 6
171
that contains it, such that the pressure on the pore fluid decreases with cooling (Corbet and Bethke,
172
1992).
173
As shown in Fig. 2b and 4, the tectonic movement at the end of the early Cretaceous in the
174
Ordos Basin resulted in the overlying strata of the Yanchang Formation uplifting and eroding. This
175
may have led to a reduction in the volume of pore fluid in the sediments because of the decreasing
176
temperature of the Yanchang Formation. Previous results have shown that the change in volume of
177
the formation brine is 44 times greater than those of the rock pores while the ground temperature
178
decreased 1oC (the rock expansion coefficient is 9×10-6 oC -1 while the formation brine expansion
179
coefficient is 400×10-6 oC -1) (Hodgman, 1957), which would generate abnormal underpressure.
180
We analyzed the homogenization temperature of hydrocarbon inclusions in the Yanchang
181
Formation. The results showed that the mean homogenization temperature was 110 oC in the
182
Gufengzhuang, Jiyuan and Baibao areas and 100 oC in the Xifeng and Luochuan areas (Table 3).
183
The homogenization temperature of hydrocarbon inclusions represents the hydrocarbon
184
accumulation temperature. Meanwhile, we collected average ground temperature data at present in
185
each subsections of the Yanchang Formation. The statistical data showed that a significant
186
difference in the ground temperature of the Yanchang Formation occurs among different regions
187
but the difference was small among different subsections in the same region and had a decreasing
188
trend with increasing depth (Table 4). Therefore, the average temperature of different subsections
189
in the same region can represent the reservoir temperature at present in this area. As shown in
190
Table 4, the average reservoir temperatures in the Jiyuan, Huaqing, Xifeng, Wuqi, and Northern
191
Shannxi areas are 64.8, 58.3, 66.1, 52.6, and 40.1 oC, respectively. The difference (∆T) between
192
the hydrocarbon accumulation temperature and the reservoir temperature at present should
193
represent the value of the ground temperature reduction after tectonic uplift and stratum erosion.
194
This value shows that reservoir temperatures in the Jiyuan, Huaqing, Xifeng, and Northern
195
Shaanxi areas decreased on average by 45.2, 51.7, 33.9, and 59.9 oC, respectively. This
196
temperature reduction should have played a certain role in the formation of underpressured
197
reservoirs. Figure 5 is a correlation diagram between the value of reservoir temperature reduction
198
after tectonic uplift and the stratum pressure coefficient for the Chang 8 subsection. It is observed
199
that the stratum pressure coefficient decreases with increasing reservoir temperature reduction. 7
200
This indicates that the underpressure formation is also closely related to the reservoir temperature
201
reduction.
202
A formula of V=nV0[1+αf (T-T0)-βf (P-P0)] for estimating the contraction value of stratum
203
pore water volume after tectonic uplift and a stratum temperature decrease was proposed by Xia
204
and Song (2001) (αf is the stratum water expansion coefficient equal to 4×10-4K-1, βf is the stratum
205
water compressibility equal to 3×10-10Pa-1, V is the volume, T is the temperature (°K), and P is the
206
pressure (Pa)). As previously described, the Yanchang Formation has good closed condition, such
207
that this formula is suitable for the Yanchang Formation reservoirs in the Ordos Basin. According
208
to the previously studied results of reservoir inclusion temperature and reservoir temperature at
209
present as well as the trapping pressure of reservoir fluid inclusions and the reservoir pressure at
210
present, we calculated the contraction value (V0/V) of the pore water volume in the Yanchang
211
Formation reservoirs after tectonic uplift and stratum temperature reduction. The calculated results
212
are listed in Table 5. The contraction value ranged from 0.82% to 1.94%, showing that the
213
reservoir pressure was much lower than the hydrostatic pressure at the current depth with the pore
214
water volume contracting. As shown Table 5, a significant difference in the contraction value of
215
the pore water volume occurred among subsections of the Yanchang Formation in different
216
regions.
217
3.3. Formation process of underpressured reservoirs
218
The aforementioned stratum abnormal pressure data showed that the Mesozoic reservoirs in
219
the Ordos Basin are mainly abnormal underpressured reservoirs at present (Table 1) ); the erosion
220
of the overlying strata and the the paleotemperature reduction caused by tectonic uplift have
221
resulted in the formation of these underpressured reservoirs. The Mesozoic crude oils in the Ordos
222
Basin were derived mainly from Chang 7 source rock. During the end of the early Cretaceous,
223
Ordos Basin subsidence reached a maximum (Fig. 4) and the oil generation of the Chang 7 source
224
rock also achieved to a maximum. The Mesozoic reservoirs in the Ordos Basin formed during this
225
period and the restoration of evolutionary history of the fluid pressure and the calculation of the
226
balance depth from the Mesozoic mudstones indicates that the Mesozoic reservoirs were
227
abnormally high-pressure reservoirs (Fig. 6). For example, the ancient fluid residual pressure of
228
the Chang 8 reservoirs in the Huaqing area was between 6.2 and 13.6 MPa duiring the end of the 8
229
early Cretaceous (Yao et al., 2015). Following the late Cretaceous, the Ordos Basin underwent
230
extensive tectonic uplift and stratum erosion over a long time, resulting in stratum decompression
231
and formation of underpressured reservoirs (Fig. 6).
232
The underpressured closed system caused by tectonic uplift and stratum erosion in the Ordos
233
Basin should have played an important role in the preservation of the early oil reservoirs.
234
Meanwhile, the underpressured closed system may have had a significant influence on reservoir
235
formation, such as reservoir adjustment and re-enrichment of hydrocarbon driven by local fluid
236
backflow caused by the underpressure, and oil and water distribution caused by the different
237
rebound of the sandstone and the mudstone. An example is the sand lens reservoirs in the Chang 7
238
subsection of the Yanchang Formation, which are underpressured anhydrous reservoirs caused by
239
tectonic uplift and stratum erosion (Figure. 7). The Chang 7 subsection was deposited in a deep
240
lacustrine environment and is thought to be the excellent source rock in the basin (Duan et al.,
241
2008). Lake delta front and deep to semi-deep lake turbidite sand bodies are widely developed in
242
the Chang 7 subsection. The lens turbidite sandstone, a good reservoir, is surrounded by Chang 7
243
source rock. The oil and gas generated from the Chang 7 source migrated into the lens sandstone
244
driven by abnormal fluid high pressure, forming the overpressured lens reservoirs. The tectonic
245
uplift and stratum erosion in the Ordos Basin during the late Cretaceous caused the rock
246
framework rebound and pore volume expansion of the Chang 7 subsection, resulting in
247
underpressured reservoirs. However, because of the difference in the pore expansion rate between
248
the sandstone and mudstone (Fatt, 1958; McLatchie et al., 1958), the lens reservoirs in the Chang
249
7 subsection are underpressured anhydrous reservoirs.
250
4. Conclusions
251
The Mesozoic reservoirs in the Ordos Basin are mainly tight oil reservoirs. The average
252
stratum pressure coefficient ranges from 0.65 to 0.74; thus, they are mainly ultra underpressured
253
reservoirs. There is a significant difference in the stratum pressure coefficient among different
254
regions
255
underpressure-generating conditions, a significant negative correlation between the stratum
256
pressure coefficient and the eroded stratum thickness as well as the stratum temperature reduction
257
value, and a pore water volume contraction of 0.82%–1.94%, showing that the formation of
and
layers.
The
m※
value
distribution
9
range
reflects
the
abnormal
258
Mesozoic underpressured reservoirs resulted mainly from stratum erosion and temperature
259
reduction. The underpressured closed system formed via tectonic uplift and stratum erosion in the
260
Ordos Basin following the late Cretaceous would have played a significant role in the preservation
261
of the early oil reservoirs and the formation of the secondary reservoirs.
262
Acknowledgments
263
China (Grant No. 41972110 and 41772108). We thank an anonymous reviewer and Dr. Tahar Aïfa
264
for their valuable suggestions and critical comments.
265
References
266
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332
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339 340 341 342 343 344 345 346 12
347
Figure captions:
348 349 350 351 352 353 354 355 356 357 358 359 360 361 362
Fig. 1. Tectonic units of the Ordos Basin (a) and stratigraphic column of the Yan’an and Yanchang formations (b). Fig. 2. Isopach maps of the stratum pressure coefficient of the Chang 8 reservoir rocks (a) and erosional thickness (m) of the stratum during the end of the early Cretaceous (b). Fig. 3. Histogram of the average total salinity of the formation water in the Yan’an and Yanchang formations. Fig. 4. Stratigraphic burial history of the Mesozoic strata in well Feng 7 showing the relationship between the stratum depth (m) and the geological age (Ma). Fig. 5. Cross plots of the stratum pressure coefficient of the Chang 8 reservoir rocks vs. difference in temperature (oC) before and after uplift of the oil reservoir. Fig. 6. Changes in stratum (m), porosity (%), hydrocarbon generation, and pressure coefficient in the Yanchang Formation over time. Fig. 7. Chang 7 oil reservoir profile in the Shangliyuan and Baibao regions (NE—Profile direction; GR—Natural gamma curve; AC—Interval transit time curve).
363 364 365 366 367 368 369 370 371
13
372
Table captions:
373 374
Table 1 Mean stratum pressure coefficients of the Mesozoic reservoir rocks in the Ordos Basin.
375
Table 2 Distribution of m
376
Table 3 Maximum paleotemperature of hydrocarbon generation for the Chang 7 source rocks and
377
※
values.
paleotemperature of the hydrocarbon accumulation in the Yanchang Formation reservoir.
378
Table 4 Average current temperature of the Yanchang Formation (oC).
379
Table 5 Contraction value of pore water volume in the Yanchang Formation (%).
380 381 382 383 384 385 386 387 388 389 390
14
Table 1 Mean stratum pressure coefficients of the Mesozoic reservoir rocks in the Ordos Basin Region Formation
Subsection
Yan'an
Maling
Jiyuan
Xifeng
Wuqi
Northern Shaanxi
0.80(4) Chang 1
0.70(1)
0.86(1)
Chang 2
0.75(7)
0.81(5)
Chang 4+5
0.67(7)
0.65(1)
0.74(3)
0.71(4)
0.77(14)
0.71(16)
0.74(2)
0.71(18)
0.69(2)
0.84(2)
0.72(11)
0.71(35)
0.70(48)
Chang 6
0.63(8)
Chang 7
0.83(2)
Chang 8
0.74(10)
Chang 9
0.83(3)
0.69(5)
0.70(1) 0.76(16)
0.75(26)
0.75(1)
0.75(1)
Chang 10 Mean
0.80(4)
Mean 0.80(4)
Chang 3
Yanchang
Huaqing
0.72(4)
0.75(44)
0.76(16)
0.76(8)
0.79(1)
0.79(1)
0.74(45)
0.74(127)
The figure in parentheses is the number of samples; The measured stratum pressure coefficients data are from Changqing Oilfield.
Table 2 Distribution of m※ values Region
Gufengzhuang
Jiyuan
Baibao
Xifeng
Luochuan
Maximum buried depth (m)
3015
3150
3300
3000
3015
Maximum erosion thickness (m)
630
600
1100
630
1500
Maximum residual thickness (m)
2385
2200
2370
Erosion rate (mm/s) m※ value at maximum erosion thickness
2.2×10
-10
0.034
2550 2.1×10
-10
0.044
3.9×10
-10
0.057
2.2×10
-10
0.042
1515 5.3×10-10 0.040
Table 3 Maximum paleotemperature of hydrocarbon generation in the Chang 7 source rocks and paleotempeature of the hydrocarbon accumulation in the Yanchang Formation reservoir Region Maximum buried depth (m) of the Chang 7 source rocks in the Early Cretaceous a
Gufengzhuang
Jiyuan
Baibao
Xifeng
Luochuan
2800
2850
2950
2300
2000
130
125
130
110
100
110
110
110
100
100
Maximum paleotemperature (oC) of
hydrocarbon generation in the Chang 7 source rocks b
Paleotemperature (oC) of the hydrocarbon
accumulation a
data from the stratigraphic burial history and thermal evolution history; binclusion homogenization temperature.
Table 4 Average current temperature of the Yanchang Formation (oC) Subsection
Region Jiyuan
Chang 1
61.6(1)
Chang 2
62.9(6)
Chang 3 Chang 4+5
Huaqing
Xifeng
Wuqi
a
Northern Shaanxi 38.6(3)
51.5(6)
47.6(2)
31.5(14)
57.0(12) 69.9(5)
60.9(2)
44.8(4)
Chang 6
62.7(5)
Chang 7
59.6(1)
Chang 8
63.5(8)
66.1(11)
64.8
58.3
66.1
45.2
51.7
57.5(7)
45.6(56)
52.6
40.1
Chang 10 Average ∆T
b
a
33.9
59.9
b
The figure in parentheses is the number of samples; ∆T is the difference between the hydrocarbon accumulation
temperature and the reservoirs temperature at present, which should represent the value of ground temperature reduce after tectonic uplift and stratum erosion.
Table 5 Contraction value of pore water volume in the Yanchang Formation (%) Subsection Chang 2
Region Jiyuan
Huaqing
1.49
1.94
1.13
1.59
Xifeng
Northern Shaanxi 1.47
Chang 3 Chang 4+5 Chang 6
1.28
Chang 8
1.33
1.21 0.82
Highlights:
The Mesozoic reservoirs in the Ordos Basin are mainly ultra-underpressured ones. The stratum erosion and paleotemperature control the formation of this reservoir. The underpressure system is beneficial to the reservoir formation and preservation.
Duan and Wu jointly carried out data collection, drawing and writing.
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.