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Influence of tectonic uplift-erosion on formation pressure

Influence of tectonic uplift-erosion on formation pressure a close relationship with tectonic uplift and the consequent erosion. In order to understand abnormally low-pressure reservoirs and to provide a scientifi c basis for exploration and development, we established, through numerical simulation and theoretical analysis, a set of equations for the formation pressure in a closed system infl uenced by uplift-erosion, discussed the relationship between the genesis of abnormal pressure and uplift-erosion, and put forward the concept of balance pressure (P ). The results showed that abnormally high pressure coefficient may form when the current formation pressure was higher than P , and abnormally low pressure may form when the current formation pressure was lower than P . b b In the Santanghu Basin, the current formation pressure of abnormally low pressure reservoirs is lower than P , so tectonic uplift-erosion leads to the decrease of the pressure coefficient. There is a positive correlation between the pressure drop caused by the decrease of fluid temperature and the rebound of rock porosity and strata erosion. Calculation results indicated that the reservoir pressure of Jurassic strata in the Santanghu Basin was decreased by 11.6-17.1 MPa due to tectonic uplift-erosion during the Late Yanshanian period. Uplift-erosion, formation pressure, temperature decrease, porosity rebound, abnormally Key words: low pressure, Santanghu Basin temperature (Xia et al, 2001; Wu et al, 2006; Zhang et al, 1 Introduction 2004), causing a decrease of formation pressure. However, Research into the formation and evolution history of the ratio of formation pressure to hydrostatic pressure, namely reservoir pressure has always been a focus of petroleum pressure coeffi cient, is usually used to defi ne the abnormality geology. In recent years, with the discovery of more and of formation pressure. Therefore, we think that several more abnormally low-pressure reservoirs in China and other problems exist. Will the pressure coeffi cient surely decrease, countries (Bachu and Underschultz, 1995; He et al, 2000; since tectonic uplift-erosion can cause a decrease in reservoir Michael and Bachu, 2001; Zeng et al, 2002; Sorenson, 2005; pressure? Will it defi nitely lead to abnormally low pressure? Dai et al, 2003; Yuan and Liu, 2005; Zhang, 2007; Zhang et Can it produce relatively high pressure? What conditions al, 2009), the change in formation pressure caused by tectonic should be met before it generates abnormally low pressure? uplift-erosion and its contribution to petroleum accumulation In this paper, numerical simulation and case analysis are used have aroused wide concern (Luo and Vasseur, 1992; Parks to discuss the action mechanism of tectonic uplift-erosion on and Toth, 1995; Xie et al, 2003; Jiang et al, 2004; Tian et formation pressure in depth. al, 2007; Xu et al, 2009). Previous research indicates that tectonic uplift-erosion will result in an increase in porosity 2 Basic theories as the confining pressure on the rock is relaxed, causing a The formation pressure in different systems is controlled rebound of porosity towards the levels before the rock is by different factors, so the infl uence of tectonic uplift-erosion compressed under higher confining pressure (Neuzil, 1993; on formation pressure is discussed for closed and open Peterson, 1958; Jiang et al, 2007) and decrease of fluid systems. The following different sets of circumstances can exist before and after strata uplift-erosion: (1) It is a closed *Corresponding author. email: xuhao600@163.com system before uplifting and an open system after that; (2) It Received August 11, 2009 is a closed system before and after uplifting; (3) It is an open 478 Pet.Sci.(2010)7:477-484 system before uplifting and a closed system after that; (4) It ED 11§·  Q rf ' pgU ' h 'T is an open system before and after uplifting. Among them, ¨¸ r (6) 31 QE E E E ©¹ fr f r for circumstances (1) and (4), the current formation pressure is controlled by hydrostatic pressure (Xu et al, 2008). For where, Δp represents the pressure change caused by strata circumstance (2), current formation pressure is controlled by uplift-erosion (Pa). The first item stands for pore pressure the rebound of rock porosity and decrease in temperature. change (Δp ) caused by the change in the load of overlying For circumstance (3), though current formation pressure is strata. 11 Q E §· controlled by the rebound of rock porosity and decrease in ' pgU 'h (7) 1r ¨¸ temperature, the buried depth of reservoir when an open 31 QE E ©¹ fr system is converted into a closed system should be viewed The second item represents the pore pressure change (Δp ) as the starting point for calculating the actual uplifting caused by temperature variation during the uplift-erosion. height, and it follows the action of the phreatic water table on reservoir pressure before the conversion. Therefore, this ' pT' (8) EE paper will center on the change in formation pressure during fr the uplift-erosion in a closed system (circumstance (2)). If the geothermal gradient stays unchanged and the Firstly, it is assumed that formation fluids are all in the decrease of temperature is caused by tectonic uplifting, liquid state and the classic equation of liquid state applies: 100' T , where G is geothermal gradient (°C/100m). (1) ' h VV  [1DE (T T ) p] 0f 0 f Consequently, Eq. (8) can be converted into: -1 where, α is the liquid expansion coefficient (K ); β is f f -1 the liquid compression coefficient (Pa ); V and T are D G 0 0 ' ph' (9) respectively the volume and temperature of liquid under 100(EE ) fr initial condition; and V, T and p are respectively liquid states From Eq. (6), two basic factors which make the fluid after the change and their units are m , K and Pa respectively. pressure of strata drop are decreases in the overburden The following equation can be derived from Eq. (1): load and temperature. From Eqs. (7) and (8), the decrease E p  D () TT (V V)/V (2) ff 0 0 0 of formation pressure has a positive correlation with the decrease of temperature and the intensity of tectonic uplift- where, T−T is temperature variation and is represented by erosion. However, can the two factors cause the decrease in ∆T (°C) and (V−V )/V is the rate of liquid volume change. 0 0 pressure coeffi cient? Namely, will it produce abnormally low The rate of fluid volume change under closed conditions pressure? This will be analyzed in depth. is equal to the rate of pore volume change (Chilingar et al, 2002), then (V −V)/V = β (δ−p), where δ is the overburden 0 0 r -1 3 Analysis of the influence of pore rebound load pressure (Pa) and β is pore volume compressibility (Pa ). Consequently, Eq. (2) can be represented by: on formation pressure D E When overlying strata are eroded and vertical stress fr pT ' G (3) decreases, the rock matrix will rebound like an elastic E EE E fr fr solid, which will increase the pore volume (and hence the porosity) and result in a decrease of fl uid pressure (Zou et al, The average positive pressure of horizontal strata is 2003). Through Eq. (7), it is ascertained that the decrease in 2 1 , where δ represents the horizontal pressure G G  G x overburden load will lead to pressure drop. x z 3 3 To study the influence of pore rebound caused by strata component (δ = δ ) and δ is the vertical pressure component. x y z erosion on abnormally low pressure, we start with a defi nition of abnormally low pressure, study the change in pressure The horizontal bearing stress is and then G GG x yz 1 Q coeffi cient caused by the decrease in overburden load when the total overburden load pressure is equal to: the temperature variation is not considered, and analyze the characteristics of the change in pressure coeffi cient after the 11 Q §· (4) G G strata are eroded, namely the contribution of pressure drop ¨¸ z 31 Q ©¹ simply caused by erosional rebound of the formation. where, v is Poisson’s ratio and δ is the change in the vertical By definition, it is assumed that the current pressure component of stress during the erosion. With the change in coefficient C is equal to p/ρ gh and then the pressure sedimentary loading, δ can be represented by: coeffi cient before tectonic uplifting is: pp ' 11 Q §· 1 (10) (5) ' GUg ' h ¨¸ r U g() hh ' 31 Q ©¹ w where, Δh is thickness of strata eroded during tectonic where, ρ is the density of formation water, Δh is erosion uplifting (m) and ρ is average density of overlying strata thickness and Δp is the decrease of pressure caused by the r 1 (kg/m ). uplift-erosion. Through merging Eq. (5) into Eq. (3), the following Then, the change in pressure coeffi cient before and after equation can be obtained: uplift-erosion is: Pet.Sci.(2010)7:477-484 479 ph() ' h 1.0 CC / (11) 10°C hp() ' p 20°C When C/C is more than 1, the uplift-erosion tends to 30°C produce high pressure coefficient. When C/C is less than 40°C 0.8 50°C 1, the uplift-erosion tends to generate abnormally low pressure. When C/C is equal to 1, the pressure coefficient 0.6 stays unchanged before and after uplift-erosion, and the formation pressure at that time is defi ned as balance pressure 11 Q E §· and has a linear relationship with ph 1b ¨¸ 0.4 31 Q EE ©¹ fr burial depth of the strata. 0.2 4 Analysis of the influence of temperature drop on formation pressure During tectonic uplifting, the temperature factor is non- 0 0 5 10 15 20 25 negligible (Swarbrick and Osborne, 1998). As the temperature P, MPa of strata decreases, the fl uid will shrink and its volume will decrease, resulting in fluid pressure drop. The influence Fig. 1 Diagram of the relation between temperature decrease and the change of pressure coeffi cient of temperature decrease on formation pressure can be determined with Eq. (9). To study the infl uence of temperature uplifting is: decrease on the pressure coefficient, we undertook further ph() ' h analysis and found that according to Eq. (8), the change in CC / hp() ' p pressure coeffi cient before and after temperature decrease can be represented as: ph() ' h ph() ' h ph() ' h ­½ 11 Q ED G §· rf (12) CC / hp[] gU ' h o ®¾ ¨¸ r hp() ' p f 3 1 QE E 10E 0(E ) ©¹ 2 ¯¿ fr f r hp() ' T EE (15) fr When C/C is more than 1, the uplift-erosion tends to It is assumed that pores in a stratum are filled with o produce high pressure coeffi cient. When C/C is less than 1, the formation water, the density of rock (ρ ) is 2,300 kg/m , the o -4 -1 uplift-erosion tends to generate abnormally low pressure. When average pore volume compressibility (β ) is 26×10 MPa , -4 -1 C/C is equal to 1, the pressure coeffi cient stays unchanged average compressibility of formation water (β ) is 5×10 MPa , o before and after the uplift-erosion and the balance pressure and the average coefficient of thermal expansion (α ) is -4 -1 can be represented as: 5×10 K . When only temperature is taken into consideration, it is assumed that the depth stays unchanged before and after 11 Q ED G §· rf pg [] U h (16) temperature variation, and then Eq. (12) can be converted into: ¨¸ r 3 1 QE E 10E 0(E ) ©¹ fr f r CC / (13) It is assumed that geothermal gradient is 3°C/100m, and pT ' 0.16 if G is substituted by the assumed parameter, Eq. (15) can be changed into: From Eq. (13), the temperature decrease will cause the decreases of formation pressure and pressure coefficient ph() ' h CC / o (17) simultaneously. The greater the temperature reduction, the hp' 0.0157 h larger will be the decrease in formation pressure coeffi cient Based on Eq. (17), the relationship between the pressure (see Fig. 1). It indicates that temperature decrease can easily coeffi cients of strata with different burial depths and erosion produce abnormally low pressure. thickness can be derived (see Fig. 2). From Fig. 2, although tectonic uplift-erosion may make the pressure drop a little, 5 Analysis of the infl uence of tectonic uplift- the pressure coefficient has two tendencies, namely either erosion on formation pressure to increase or decrease, and all change curves of pressure In the above paragraphs, the influence of pore rebound coefficient intersect at one point. This point indicates that and temperature decrease on the formation pressure is when the formation pressure meets a specifi c condition, the discussed. However, in real geological process, the two pressure coeffi cient stays unchanged regardless the intensity factors often influence the formation pressure together and of tectonic uplift-erosion, and we defi ne it as balance pressure can be represented as follows: (P ) during tectonic uplift-erosion. When the current formation pressure is equal to the 11 Q ED G §· rf (14) balance pressure, the pressure coefficient stays unchanged ' pg[] U  'h ¨¸ 3 1 QE E 100E (E ) ©¹ although the formation pressure will drop after tectonic uplift- fr fr The change in pressure coefficient before and after the erosion. When the current formation pressure is lower than C/C 0 480 Pet.Sci.(2010)7:477-484 Erosion thickness not is determined by the relationship between the formation 1.2 500m pressure and balance pressure. It is noteworthy that although 400m 300m balance pressure theoretically exists, the current formation 1.1 200m 100m pressure can be equal to the balance pressure only when many 1.0 factors are well coupled. Therefore, the current formation pressure is unlikely to equal the balance pressure and is 0.9 almost always higher or lower than the balance pressure. 0.8 6 An example from the Santanghu Basin 0.7 6.1 Geologic background Present burial depth is 1500m 0.6 The Santanghu Basin lies in the northeast of Xinjiang 0.5 10 20 30 40 50 60 Uyghur Autonomous Region, China and has an area of 4 2 P, MPa 2.3×10 km . The basin pattern is one of two uplifts and one depression and can be divided into three tectonic units, 1.2 namely the NE thrust fold belt, central depression and SW Erosion thickness 500m thrust fold belt. The central depression belt consists of four 1.1 400m 300m uplifts and five depressions (see Fig. 3). The basement of 200m 100m 1.0 the basin is Carboniferous and Permian, and the caprock mainly consists of middle Cenozoic. The sedimentary 0.9 rock is over 3,000 m thick (see Table 1). Over ten years of exploration, considerable petroleum reserves have been 0.8 discovered in Jurassic reservoirs. However, due to low 0.7 reservoir pressure, insufficient producing energy and high reservoir heterogeneity, oil and production tests have not been Present burial depth is 2000m 0.6 favorable and exploration and development of oilfi elds have b proved diffi cult. The widely-spread abnormally low pressure 0.5 8 1624 3240485664 in Jurassic reservoirs has a close relationship with tectonic P, MPa uplift-erosion, so in-depth research on the influence of tectonic uplift-erosion on the formation pressure of Jurassic 1.2 strata is significant for the understanding of the genesis of Erosion thickness 1.1 abnormally low-pressure oil reservoirs and for guiding the 500m 400m exploration and development of oil reservoirs. 300m 1.0 200m 100m The Santanghu Basin experienced a stage of Carboniferous to early Permian basement evolution, one of late Permian to 0.9 early Cretaceous basin evolution and a basin reconstruction 0.8 stage since the late Cretaceous. The basin formation and evolution stage can be divided into three parts, namely 0.7 extensional faulted depression basin after the orogenies of the late Permian, extension-compression at the end of Triassic, 0.6 and depression in the early and middle Jurassic and late Present burial depth is 3000m Jurassic to early Cretaceous compression. In terms of the 0.5 8 1624 32 4048 56 64 whole basin evolution, the two tectonic movements during P, MPa the late Hercynian and late Yanshanian periods are the largest Fig. 2 Relation diagram of erosion thickness and pressure and the one during late Yanshanian period has an important coeffi cient at different depths infl uence on the formation pressure of Jurassic strata (see Fig. 4). the balance pressure, both the formation pressure and pressure 6.2 Characteristics of pressure system coefficient will decrease after tectonic uplift-erosion. The larger the amplitude of tectonic uplift-erosion, the larger will During this research, drill stem test (DST) data from be the decrease in pressure coeffi cient. When the formation over 20 wells have been collected and the data on the pressure is higher than the balance pressure, the pressure formation pressure are mainly from the Jurassic Badaowan, coeffi cient will increase although the formation pressure will Xishanyao, Toutunhe and Qigu Formations. The analysis decrease after tectonic uplift-erosion. The higher the intensity shows that the formation pressure of the Jurassic reservoir of tectonic uplift-erosion, the higher will be the increase in increases with depth, and the pressure coefficient is 0.48- pressure coefficient. It indicates that tectonic uplift-erosion 0.92, falling into the abnormally low pressure classification does not always produce abnormally low pressure and (see Fig. 5). Hydrochemical characteristics can reveal the whether it will cause a decrease in pressure coefficient or evolution of flow systems and if the formation is closed or C/C C/C C/C 0 0 0 Malang Depression Hanshuiquan Depression Central depression belt Tiaohu Depression Southwest fold and thrust belt Northeast fold and thrust belt Pet.Sci.(2010)7:477-484 481 0 40km Mongolia Shitoumei Uplift Chahaquan Uplift Fangfangliang Uplift Naomaohu Depression Legend Weibei Uplift Suluke National Basin Fault Secondary Depression boundary boundary structural belt Fig. 3 Division of tectonic units of the Santanghu Basin Table 1 Stratigraphic table of the Santanghu Basin Strata Thickness Description of lithology Erathem System Series Formation Symbol Quaternary Q 40-60 Yellow conglomeratic clay and gravel Cenozoic Non-isopachous interbeds of red brown mudstone and Tertiary R 35-161 gravel Chocolate brown mudstone, sandy mudstone with gray fi ne Cretaceous Lower K tg 736-1052 siltstone and dark gray conglomerate Upper Qigu Formation J q 176-274 Mudstone and interbedded fi ne sandstone and siltstone Grayish green tuffaceous conglomerate mixed with brown Toutunhe Formation J t 200-341 and chocolate brown tuffaceous conglomerate Mesozoic Jurassic Middle Its upper part consists of coal; the middle upper part is Xishanyao Formation J x 115-246 composed of gray mudstone; the middle lower part consists of sandstone; and the lower part is composed of mudstone. Sangonghe & Gray sandstone and siltstone mixed with dark gray Lower J 30-200 Badaowan Formations mudstone Triassic Middle Karamay Formation T k 43-230 Mudstone and interbedded siltstone and fi ne sandstone The upper part is composed of dark gray mudstone and Tiaohu Formation P t 0-772 the middle-lower part consists of gray andesite, basalt and grayish green diabase. Permian Upper Interbedded gray dolomite, dark gray tuffaceous mudstone Lucaogou Formation P l 0-508 and calcareous mudstone Gray amygdaloidal vesicular basalt mixed with andesite, Kalagang Formation C k 540-1027 gray and chocolate brown pyroclastic rock Upper Paleozoic The upper part consists of gray and grey black mudstone Upper Haerjiawu Formation C h 400-654 and tuffaceous sandstone and the lower part is composed of gray basalt and andesite. Carboniferous Batamayineishan Gray and grayish green basalt and andesite mixed with C b 1000-2150 Formation thin layers of gray sandstone and mudstone Grey black mudstone and interbedded dark gray and Lower Jiangbasitao Formation C j 600-1900 grayish green siltstone and sandstone Hydrostatic pressure 482 Pet.Sci.(2010)7:477-484 of strata (ρ ) is 2,320 kg/m , compressibility coefficient of PT J K N+Q -4 -1 N+Q volume (β ) of formation water is 5×10 MPa (Dobrynin and Serebryakov, 1989), and the average coefficient of thermal 40°C K tg -4 -1 expansion (α ) is 5×10 K . 1000 f J q The geothermal gradient in the Santanghu Basin after the J t Cretaceous is 2.4-2.8 °C/100m (Hao et al, 2006). Based on the J x 80°C J current temperature of the Jurassic reservoir, the temperature gradient is 2-2.4 C/100m, that is, since the late Yanshanian, the geothermal gradient in the basin has changed little and the basin has been slowly cooling down. In this research, we take P t 120°C the average value 2.4 C/100m as the geothermal gradient 4000 during tectonic uplift-erosion. The erosion thickness of strata is obtained using the acoustic time method and the decrease in temperature of the abnormally low-pressure reservoir is obtained through basin simulation. Based on Eq. (14), the 300 200 100 0 Time, Ma current pressure of the abnormally low-pressure reservoirs in the Santanghu Basin is lower than the balance pressure (see Fig. 4 Burial history curve of Well TC1 Table 2), indicating that the pressure coefficient generally decreases after tectonic uplifting. Therefore, Eq. (6) can open hydrogeologically. Forty-three water samples from be used to calculate the change of pore pressure caused by the Jurassic abnormally low-pressure reservoir are all of porosity rebound and temperature drop (see Table 3). CaCl , MgCl and NaHCO type, representing a retained 2 2 3 hydrogeological condition, which suggests that the Jurassic Table 2 Balance pressure of Jurassic abnormally low-pressure reservoir in abnormally low-pressure reservoir is closed and fl uids have the Santanghu Basin not fl owed into or out of the reservoir. Pressure Balance Name Pressure Pressure Horizon depth pressure Pressure, MPa of well coeffi cient MPa m MPa M1 Well J t 1150.47 0.68 8.39 17.72 M1 Well J t 1201 0.68 8.80 18.50 M1 Well J t 1200 0.65 8.39 18.48 N101 Well J t 1220 0.78 10.27 18.79 M1 Well J x 1543.5 0.52 8.60 23.77 M13 Well J x 1819 0.77 15.08 28.01 M1 Well J x 1540 0.7 11.64 23.72 M3 Well J x 1500 0.72 11.58 23.1 N101 Well J x 1495.2 0.73 11.75 23.03 N103 Well J x 1550 0.63 10.48 23.87 Well T3 J x 1162 0.66 8.25 17.89 Fig. 5 Longitudinal distribution of formation pressure of Jurassic strata in the Santanghu Basin T2 Well J x 1517 0.57 9.40 23.36 TC1 Well J b 2074.97 0.81 18.19 31.95 6.3 Infl uence of tectonic uplift-erosion on formation pressure in the Santanghu Basin TC1 Well J b 2103.63 0.73 16.47 32.40 Serebryakov and Chilingar (1994) thought that pore rebounding and fl uid temperature decrease caused by tectonic 6.4 Pressure evolution history uplift-erosion made the pressure of reservoirs in the Powder With the tectonic movement of the late Yanshanian period, River Basin decrease by 8.3-11.9 MPa and 11.2-15.7 MPa mature hydrocarbon in deep formations migrates vertically respectively. In this research, it is assumed that the fluid in along fault zones, and then flows laterally in reservoirs pores is formation water and the tests show that average under the power of superpressure. This stage is the main compressibility coeffi cient of volume (β ) of the rock sample -4 -1 hydrocarbon-filling period controlled by an overpressure- is 26×10 MPa , and Poisson’s ratio (v) of consolidated fault. At this stage, the pore fl uid pressure increases rapidly rock is 0.25. The analysis of continuous core samples from and gives rise to abnormally high pressure due to the great well M5 in Jurassic strata shows that the average density Depth, m Depth, m Pet.Sci.(2010)7:477-484 483 Table 3 Summary table of genesis of abnormally low pressure in the Santanghu Basin Pressure drop Current Current Current Uplift Temperature Pressure drop caused Overall Paleo- caused by pore Well Depth Horizon temperature pressure pressure height drop by temperature pressure drop pressure rebound o o C MPa coeffi cient m C MPa MPa coeffi cient MPa N101 1221.41 J t 38.9 10.26 0.78 801 32 6.15 8.49 14.64 1.2 M3 1484.56 J x 46 11.76 0.72 881 34 6.54 9.34 15.88 1.14 M1 1540 J x 46.8 11.64 0.7 738.5 34 6.54 7.83 14.37 1.11 TC1 2090.19 J b 63.4 18.27 0.78 608 27 5.19 6.44 11.63 1.08 T2 1521.2 J x 52.4 9.4 0.57 846 31 5.96 8.97 14.93 1 T3 1167.27 J t 40.7 8.25 0.66 939 37 7.12 9.95 17.07 1.17 prediction of abnormal formation pressures. 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Influence of tectonic uplift-erosion on formation pressure

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Springer Journals
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Copyright © 2010 by China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
ISSN
1672-5107
eISSN
1995-8226
DOI
10.1007/s12182-010-0094-9
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Abstract

a close relationship with tectonic uplift and the consequent erosion. In order to understand abnormally low-pressure reservoirs and to provide a scientifi c basis for exploration and development, we established, through numerical simulation and theoretical analysis, a set of equations for the formation pressure in a closed system infl uenced by uplift-erosion, discussed the relationship between the genesis of abnormal pressure and uplift-erosion, and put forward the concept of balance pressure (P ). The results showed that abnormally high pressure coefficient may form when the current formation pressure was higher than P , and abnormally low pressure may form when the current formation pressure was lower than P . b b In the Santanghu Basin, the current formation pressure of abnormally low pressure reservoirs is lower than P , so tectonic uplift-erosion leads to the decrease of the pressure coefficient. There is a positive correlation between the pressure drop caused by the decrease of fluid temperature and the rebound of rock porosity and strata erosion. Calculation results indicated that the reservoir pressure of Jurassic strata in the Santanghu Basin was decreased by 11.6-17.1 MPa due to tectonic uplift-erosion during the Late Yanshanian period. Uplift-erosion, formation pressure, temperature decrease, porosity rebound, abnormally Key words: low pressure, Santanghu Basin temperature (Xia et al, 2001; Wu et al, 2006; Zhang et al, 1 Introduction 2004), causing a decrease of formation pressure. However, Research into the formation and evolution history of the ratio of formation pressure to hydrostatic pressure, namely reservoir pressure has always been a focus of petroleum pressure coeffi cient, is usually used to defi ne the abnormality geology. In recent years, with the discovery of more and of formation pressure. Therefore, we think that several more abnormally low-pressure reservoirs in China and other problems exist. Will the pressure coeffi cient surely decrease, countries (Bachu and Underschultz, 1995; He et al, 2000; since tectonic uplift-erosion can cause a decrease in reservoir Michael and Bachu, 2001; Zeng et al, 2002; Sorenson, 2005; pressure? Will it defi nitely lead to abnormally low pressure? Dai et al, 2003; Yuan and Liu, 2005; Zhang, 2007; Zhang et Can it produce relatively high pressure? What conditions al, 2009), the change in formation pressure caused by tectonic should be met before it generates abnormally low pressure? uplift-erosion and its contribution to petroleum accumulation In this paper, numerical simulation and case analysis are used have aroused wide concern (Luo and Vasseur, 1992; Parks to discuss the action mechanism of tectonic uplift-erosion on and Toth, 1995; Xie et al, 2003; Jiang et al, 2004; Tian et formation pressure in depth. al, 2007; Xu et al, 2009). Previous research indicates that tectonic uplift-erosion will result in an increase in porosity 2 Basic theories as the confining pressure on the rock is relaxed, causing a The formation pressure in different systems is controlled rebound of porosity towards the levels before the rock is by different factors, so the infl uence of tectonic uplift-erosion compressed under higher confining pressure (Neuzil, 1993; on formation pressure is discussed for closed and open Peterson, 1958; Jiang et al, 2007) and decrease of fluid systems. The following different sets of circumstances can exist before and after strata uplift-erosion: (1) It is a closed *Corresponding author. email: xuhao600@163.com system before uplifting and an open system after that; (2) It Received August 11, 2009 is a closed system before and after uplifting; (3) It is an open 478 Pet.Sci.(2010)7:477-484 system before uplifting and a closed system after that; (4) It ED 11§·  Q rf ' pgU ' h 'T is an open system before and after uplifting. Among them, ¨¸ r (6) 31 QE E E E ©¹ fr f r for circumstances (1) and (4), the current formation pressure is controlled by hydrostatic pressure (Xu et al, 2008). For where, Δp represents the pressure change caused by strata circumstance (2), current formation pressure is controlled by uplift-erosion (Pa). The first item stands for pore pressure the rebound of rock porosity and decrease in temperature. change (Δp ) caused by the change in the load of overlying For circumstance (3), though current formation pressure is strata. 11 Q E §· controlled by the rebound of rock porosity and decrease in ' pgU 'h (7) 1r ¨¸ temperature, the buried depth of reservoir when an open 31 QE E ©¹ fr system is converted into a closed system should be viewed The second item represents the pore pressure change (Δp ) as the starting point for calculating the actual uplifting caused by temperature variation during the uplift-erosion. height, and it follows the action of the phreatic water table on reservoir pressure before the conversion. Therefore, this ' pT' (8) EE paper will center on the change in formation pressure during fr the uplift-erosion in a closed system (circumstance (2)). If the geothermal gradient stays unchanged and the Firstly, it is assumed that formation fluids are all in the decrease of temperature is caused by tectonic uplifting, liquid state and the classic equation of liquid state applies: 100' T , where G is geothermal gradient (°C/100m). (1) ' h VV  [1DE (T T ) p] 0f 0 f Consequently, Eq. (8) can be converted into: -1 where, α is the liquid expansion coefficient (K ); β is f f -1 the liquid compression coefficient (Pa ); V and T are D G 0 0 ' ph' (9) respectively the volume and temperature of liquid under 100(EE ) fr initial condition; and V, T and p are respectively liquid states From Eq. (6), two basic factors which make the fluid after the change and their units are m , K and Pa respectively. pressure of strata drop are decreases in the overburden The following equation can be derived from Eq. (1): load and temperature. From Eqs. (7) and (8), the decrease E p  D () TT (V V)/V (2) ff 0 0 0 of formation pressure has a positive correlation with the decrease of temperature and the intensity of tectonic uplift- where, T−T is temperature variation and is represented by erosion. However, can the two factors cause the decrease in ∆T (°C) and (V−V )/V is the rate of liquid volume change. 0 0 pressure coeffi cient? Namely, will it produce abnormally low The rate of fluid volume change under closed conditions pressure? This will be analyzed in depth. is equal to the rate of pore volume change (Chilingar et al, 2002), then (V −V)/V = β (δ−p), where δ is the overburden 0 0 r -1 3 Analysis of the influence of pore rebound load pressure (Pa) and β is pore volume compressibility (Pa ). Consequently, Eq. (2) can be represented by: on formation pressure D E When overlying strata are eroded and vertical stress fr pT ' G (3) decreases, the rock matrix will rebound like an elastic E EE E fr fr solid, which will increase the pore volume (and hence the porosity) and result in a decrease of fl uid pressure (Zou et al, The average positive pressure of horizontal strata is 2003). Through Eq. (7), it is ascertained that the decrease in 2 1 , where δ represents the horizontal pressure G G  G x overburden load will lead to pressure drop. x z 3 3 To study the influence of pore rebound caused by strata component (δ = δ ) and δ is the vertical pressure component. x y z erosion on abnormally low pressure, we start with a defi nition of abnormally low pressure, study the change in pressure The horizontal bearing stress is and then G GG x yz 1 Q coeffi cient caused by the decrease in overburden load when the total overburden load pressure is equal to: the temperature variation is not considered, and analyze the characteristics of the change in pressure coeffi cient after the 11 Q §· (4) G G strata are eroded, namely the contribution of pressure drop ¨¸ z 31 Q ©¹ simply caused by erosional rebound of the formation. where, v is Poisson’s ratio and δ is the change in the vertical By definition, it is assumed that the current pressure component of stress during the erosion. With the change in coefficient C is equal to p/ρ gh and then the pressure sedimentary loading, δ can be represented by: coeffi cient before tectonic uplifting is: pp ' 11 Q §· 1 (10) (5) ' GUg ' h ¨¸ r U g() hh ' 31 Q ©¹ w where, Δh is thickness of strata eroded during tectonic where, ρ is the density of formation water, Δh is erosion uplifting (m) and ρ is average density of overlying strata thickness and Δp is the decrease of pressure caused by the r 1 (kg/m ). uplift-erosion. Through merging Eq. (5) into Eq. (3), the following Then, the change in pressure coeffi cient before and after equation can be obtained: uplift-erosion is: Pet.Sci.(2010)7:477-484 479 ph() ' h 1.0 CC / (11) 10°C hp() ' p 20°C When C/C is more than 1, the uplift-erosion tends to 30°C produce high pressure coefficient. When C/C is less than 40°C 0.8 50°C 1, the uplift-erosion tends to generate abnormally low pressure. When C/C is equal to 1, the pressure coefficient 0.6 stays unchanged before and after uplift-erosion, and the formation pressure at that time is defi ned as balance pressure 11 Q E §· and has a linear relationship with ph 1b ¨¸ 0.4 31 Q EE ©¹ fr burial depth of the strata. 0.2 4 Analysis of the influence of temperature drop on formation pressure During tectonic uplifting, the temperature factor is non- 0 0 5 10 15 20 25 negligible (Swarbrick and Osborne, 1998). As the temperature P, MPa of strata decreases, the fl uid will shrink and its volume will decrease, resulting in fluid pressure drop. The influence Fig. 1 Diagram of the relation between temperature decrease and the change of pressure coeffi cient of temperature decrease on formation pressure can be determined with Eq. (9). To study the infl uence of temperature uplifting is: decrease on the pressure coefficient, we undertook further ph() ' h analysis and found that according to Eq. (8), the change in CC / hp() ' p pressure coeffi cient before and after temperature decrease can be represented as: ph() ' h ph() ' h ph() ' h ­½ 11 Q ED G §· rf (12) CC / hp[] gU ' h o ®¾ ¨¸ r hp() ' p f 3 1 QE E 10E 0(E ) ©¹ 2 ¯¿ fr f r hp() ' T EE (15) fr When C/C is more than 1, the uplift-erosion tends to It is assumed that pores in a stratum are filled with o produce high pressure coeffi cient. When C/C is less than 1, the formation water, the density of rock (ρ ) is 2,300 kg/m , the o -4 -1 uplift-erosion tends to generate abnormally low pressure. When average pore volume compressibility (β ) is 26×10 MPa , -4 -1 C/C is equal to 1, the pressure coeffi cient stays unchanged average compressibility of formation water (β ) is 5×10 MPa , o before and after the uplift-erosion and the balance pressure and the average coefficient of thermal expansion (α ) is -4 -1 can be represented as: 5×10 K . When only temperature is taken into consideration, it is assumed that the depth stays unchanged before and after 11 Q ED G §· rf pg [] U h (16) temperature variation, and then Eq. (12) can be converted into: ¨¸ r 3 1 QE E 10E 0(E ) ©¹ fr f r CC / (13) It is assumed that geothermal gradient is 3°C/100m, and pT ' 0.16 if G is substituted by the assumed parameter, Eq. (15) can be changed into: From Eq. (13), the temperature decrease will cause the decreases of formation pressure and pressure coefficient ph() ' h CC / o (17) simultaneously. The greater the temperature reduction, the hp' 0.0157 h larger will be the decrease in formation pressure coeffi cient Based on Eq. (17), the relationship between the pressure (see Fig. 1). It indicates that temperature decrease can easily coeffi cients of strata with different burial depths and erosion produce abnormally low pressure. thickness can be derived (see Fig. 2). From Fig. 2, although tectonic uplift-erosion may make the pressure drop a little, 5 Analysis of the infl uence of tectonic uplift- the pressure coefficient has two tendencies, namely either erosion on formation pressure to increase or decrease, and all change curves of pressure In the above paragraphs, the influence of pore rebound coefficient intersect at one point. This point indicates that and temperature decrease on the formation pressure is when the formation pressure meets a specifi c condition, the discussed. However, in real geological process, the two pressure coeffi cient stays unchanged regardless the intensity factors often influence the formation pressure together and of tectonic uplift-erosion, and we defi ne it as balance pressure can be represented as follows: (P ) during tectonic uplift-erosion. When the current formation pressure is equal to the 11 Q ED G §· rf (14) balance pressure, the pressure coefficient stays unchanged ' pg[] U  'h ¨¸ 3 1 QE E 100E (E ) ©¹ although the formation pressure will drop after tectonic uplift- fr fr The change in pressure coefficient before and after the erosion. When the current formation pressure is lower than C/C 0 480 Pet.Sci.(2010)7:477-484 Erosion thickness not is determined by the relationship between the formation 1.2 500m pressure and balance pressure. It is noteworthy that although 400m 300m balance pressure theoretically exists, the current formation 1.1 200m 100m pressure can be equal to the balance pressure only when many 1.0 factors are well coupled. Therefore, the current formation pressure is unlikely to equal the balance pressure and is 0.9 almost always higher or lower than the balance pressure. 0.8 6 An example from the Santanghu Basin 0.7 6.1 Geologic background Present burial depth is 1500m 0.6 The Santanghu Basin lies in the northeast of Xinjiang 0.5 10 20 30 40 50 60 Uyghur Autonomous Region, China and has an area of 4 2 P, MPa 2.3×10 km . The basin pattern is one of two uplifts and one depression and can be divided into three tectonic units, 1.2 namely the NE thrust fold belt, central depression and SW Erosion thickness 500m thrust fold belt. The central depression belt consists of four 1.1 400m 300m uplifts and five depressions (see Fig. 3). The basement of 200m 100m 1.0 the basin is Carboniferous and Permian, and the caprock mainly consists of middle Cenozoic. The sedimentary 0.9 rock is over 3,000 m thick (see Table 1). Over ten years of exploration, considerable petroleum reserves have been 0.8 discovered in Jurassic reservoirs. However, due to low 0.7 reservoir pressure, insufficient producing energy and high reservoir heterogeneity, oil and production tests have not been Present burial depth is 2000m 0.6 favorable and exploration and development of oilfi elds have b proved diffi cult. The widely-spread abnormally low pressure 0.5 8 1624 3240485664 in Jurassic reservoirs has a close relationship with tectonic P, MPa uplift-erosion, so in-depth research on the influence of tectonic uplift-erosion on the formation pressure of Jurassic 1.2 strata is significant for the understanding of the genesis of Erosion thickness 1.1 abnormally low-pressure oil reservoirs and for guiding the 500m 400m exploration and development of oil reservoirs. 300m 1.0 200m 100m The Santanghu Basin experienced a stage of Carboniferous to early Permian basement evolution, one of late Permian to 0.9 early Cretaceous basin evolution and a basin reconstruction 0.8 stage since the late Cretaceous. The basin formation and evolution stage can be divided into three parts, namely 0.7 extensional faulted depression basin after the orogenies of the late Permian, extension-compression at the end of Triassic, 0.6 and depression in the early and middle Jurassic and late Present burial depth is 3000m Jurassic to early Cretaceous compression. In terms of the 0.5 8 1624 32 4048 56 64 whole basin evolution, the two tectonic movements during P, MPa the late Hercynian and late Yanshanian periods are the largest Fig. 2 Relation diagram of erosion thickness and pressure and the one during late Yanshanian period has an important coeffi cient at different depths infl uence on the formation pressure of Jurassic strata (see Fig. 4). the balance pressure, both the formation pressure and pressure 6.2 Characteristics of pressure system coefficient will decrease after tectonic uplift-erosion. The larger the amplitude of tectonic uplift-erosion, the larger will During this research, drill stem test (DST) data from be the decrease in pressure coeffi cient. When the formation over 20 wells have been collected and the data on the pressure is higher than the balance pressure, the pressure formation pressure are mainly from the Jurassic Badaowan, coeffi cient will increase although the formation pressure will Xishanyao, Toutunhe and Qigu Formations. The analysis decrease after tectonic uplift-erosion. The higher the intensity shows that the formation pressure of the Jurassic reservoir of tectonic uplift-erosion, the higher will be the increase in increases with depth, and the pressure coefficient is 0.48- pressure coefficient. It indicates that tectonic uplift-erosion 0.92, falling into the abnormally low pressure classification does not always produce abnormally low pressure and (see Fig. 5). Hydrochemical characteristics can reveal the whether it will cause a decrease in pressure coefficient or evolution of flow systems and if the formation is closed or C/C C/C C/C 0 0 0 Malang Depression Hanshuiquan Depression Central depression belt Tiaohu Depression Southwest fold and thrust belt Northeast fold and thrust belt Pet.Sci.(2010)7:477-484 481 0 40km Mongolia Shitoumei Uplift Chahaquan Uplift Fangfangliang Uplift Naomaohu Depression Legend Weibei Uplift Suluke National Basin Fault Secondary Depression boundary boundary structural belt Fig. 3 Division of tectonic units of the Santanghu Basin Table 1 Stratigraphic table of the Santanghu Basin Strata Thickness Description of lithology Erathem System Series Formation Symbol Quaternary Q 40-60 Yellow conglomeratic clay and gravel Cenozoic Non-isopachous interbeds of red brown mudstone and Tertiary R 35-161 gravel Chocolate brown mudstone, sandy mudstone with gray fi ne Cretaceous Lower K tg 736-1052 siltstone and dark gray conglomerate Upper Qigu Formation J q 176-274 Mudstone and interbedded fi ne sandstone and siltstone Grayish green tuffaceous conglomerate mixed with brown Toutunhe Formation J t 200-341 and chocolate brown tuffaceous conglomerate Mesozoic Jurassic Middle Its upper part consists of coal; the middle upper part is Xishanyao Formation J x 115-246 composed of gray mudstone; the middle lower part consists of sandstone; and the lower part is composed of mudstone. Sangonghe & Gray sandstone and siltstone mixed with dark gray Lower J 30-200 Badaowan Formations mudstone Triassic Middle Karamay Formation T k 43-230 Mudstone and interbedded siltstone and fi ne sandstone The upper part is composed of dark gray mudstone and Tiaohu Formation P t 0-772 the middle-lower part consists of gray andesite, basalt and grayish green diabase. Permian Upper Interbedded gray dolomite, dark gray tuffaceous mudstone Lucaogou Formation P l 0-508 and calcareous mudstone Gray amygdaloidal vesicular basalt mixed with andesite, Kalagang Formation C k 540-1027 gray and chocolate brown pyroclastic rock Upper Paleozoic The upper part consists of gray and grey black mudstone Upper Haerjiawu Formation C h 400-654 and tuffaceous sandstone and the lower part is composed of gray basalt and andesite. Carboniferous Batamayineishan Gray and grayish green basalt and andesite mixed with C b 1000-2150 Formation thin layers of gray sandstone and mudstone Grey black mudstone and interbedded dark gray and Lower Jiangbasitao Formation C j 600-1900 grayish green siltstone and sandstone Hydrostatic pressure 482 Pet.Sci.(2010)7:477-484 of strata (ρ ) is 2,320 kg/m , compressibility coefficient of PT J K N+Q -4 -1 N+Q volume (β ) of formation water is 5×10 MPa (Dobrynin and Serebryakov, 1989), and the average coefficient of thermal 40°C K tg -4 -1 expansion (α ) is 5×10 K . 1000 f J q The geothermal gradient in the Santanghu Basin after the J t Cretaceous is 2.4-2.8 °C/100m (Hao et al, 2006). Based on the J x 80°C J current temperature of the Jurassic reservoir, the temperature gradient is 2-2.4 C/100m, that is, since the late Yanshanian, the geothermal gradient in the basin has changed little and the basin has been slowly cooling down. In this research, we take P t 120°C the average value 2.4 C/100m as the geothermal gradient 4000 during tectonic uplift-erosion. The erosion thickness of strata is obtained using the acoustic time method and the decrease in temperature of the abnormally low-pressure reservoir is obtained through basin simulation. Based on Eq. (14), the 300 200 100 0 Time, Ma current pressure of the abnormally low-pressure reservoirs in the Santanghu Basin is lower than the balance pressure (see Fig. 4 Burial history curve of Well TC1 Table 2), indicating that the pressure coefficient generally decreases after tectonic uplifting. Therefore, Eq. (6) can open hydrogeologically. Forty-three water samples from be used to calculate the change of pore pressure caused by the Jurassic abnormally low-pressure reservoir are all of porosity rebound and temperature drop (see Table 3). CaCl , MgCl and NaHCO type, representing a retained 2 2 3 hydrogeological condition, which suggests that the Jurassic Table 2 Balance pressure of Jurassic abnormally low-pressure reservoir in abnormally low-pressure reservoir is closed and fl uids have the Santanghu Basin not fl owed into or out of the reservoir. Pressure Balance Name Pressure Pressure Horizon depth pressure Pressure, MPa of well coeffi cient MPa m MPa M1 Well J t 1150.47 0.68 8.39 17.72 M1 Well J t 1201 0.68 8.80 18.50 M1 Well J t 1200 0.65 8.39 18.48 N101 Well J t 1220 0.78 10.27 18.79 M1 Well J x 1543.5 0.52 8.60 23.77 M13 Well J x 1819 0.77 15.08 28.01 M1 Well J x 1540 0.7 11.64 23.72 M3 Well J x 1500 0.72 11.58 23.1 N101 Well J x 1495.2 0.73 11.75 23.03 N103 Well J x 1550 0.63 10.48 23.87 Well T3 J x 1162 0.66 8.25 17.89 Fig. 5 Longitudinal distribution of formation pressure of Jurassic strata in the Santanghu Basin T2 Well J x 1517 0.57 9.40 23.36 TC1 Well J b 2074.97 0.81 18.19 31.95 6.3 Infl uence of tectonic uplift-erosion on formation pressure in the Santanghu Basin TC1 Well J b 2103.63 0.73 16.47 32.40 Serebryakov and Chilingar (1994) thought that pore rebounding and fl uid temperature decrease caused by tectonic 6.4 Pressure evolution history uplift-erosion made the pressure of reservoirs in the Powder With the tectonic movement of the late Yanshanian period, River Basin decrease by 8.3-11.9 MPa and 11.2-15.7 MPa mature hydrocarbon in deep formations migrates vertically respectively. In this research, it is assumed that the fluid in along fault zones, and then flows laterally in reservoirs pores is formation water and the tests show that average under the power of superpressure. This stage is the main compressibility coeffi cient of volume (β ) of the rock sample -4 -1 hydrocarbon-filling period controlled by an overpressure- is 26×10 MPa , and Poisson’s ratio (v) of consolidated fault. At this stage, the pore fl uid pressure increases rapidly rock is 0.25. The analysis of continuous core samples from and gives rise to abnormally high pressure due to the great well M5 in Jurassic strata shows that the average density Depth, m Depth, m Pet.Sci.(2010)7:477-484 483 Table 3 Summary table of genesis of abnormally low pressure in the Santanghu Basin Pressure drop Current Current Current Uplift Temperature Pressure drop caused Overall Paleo- caused by pore Well Depth Horizon temperature pressure pressure height drop by temperature pressure drop pressure rebound o o C MPa coeffi cient m C MPa MPa coeffi cient MPa N101 1221.41 J t 38.9 10.26 0.78 801 32 6.15 8.49 14.64 1.2 M3 1484.56 J x 46 11.76 0.72 881 34 6.54 9.34 15.88 1.14 M1 1540 J x 46.8 11.64 0.7 738.5 34 6.54 7.83 14.37 1.11 TC1 2090.19 J b 63.4 18.27 0.78 608 27 5.19 6.44 11.63 1.08 T2 1521.2 J x 52.4 9.4 0.57 846 31 5.96 8.97 14.93 1 T3 1167.27 J t 40.7 8.25 0.66 939 37 7.12 9.95 17.07 1.17 prediction of abnormal formation pressures. 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Published: Nov 10, 2010

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