Journal of Earth Science  2019, Vol. 30 Issue (2): 367-375   PDF    
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Deformation Mechanism and Vertical Sealing Capacity of Fault in the Mudstone Caprock
Fu Xiaofei 1,2,3, Yan Lingyu 1,2,3, Meng Lingdong 1,2,3, Liu Xiaobo 4,5     
1. Laboratory of CNPC Fault-Controlling Reservoir, Northeast Petroleum University, Daqing 163318, China;
2. Science and Technology Innovation Team in Heilongjiang Province "Fault Deformation, Sealing and Fluid Migration" Northeast Petroleum University, Daqing 163318, China;
3. State Key Laboratory Base of Unconventional Oil and Gas Accumulation and Exploitation, Northeast Petroleum University, Daqing 163318, China;
4. School of Energy Resource, China University of Geosciences, Beijing 100083, China;
5. Daqing Yushulin Oilfield Development Co. Ltd., Daqing 163453, China
ABSTRACT: The petrophysical property of mudstone often transforms from ductile to brittle in the process of burial-uplift. The deformation mechanism of fault in brittle and ductile mudstone caprock is different, which leads to the formation of different types of fault zone structure. Different methods are required to evaluate the sealing mechanism of those fault zones. Based on the caprock deformation mechanism, fault sealing mechanism, quantitative evaluation method of vertical fault sealing capacity is put forward in this study. Clay smear is formed in the process of plastic deformation and its continuity controls the sealing capacity of fault. The outcrop and oil field data have confirmed that when sealing parameter SSF is less than 4-7, the clay smear becomes discontinuous and then oil and gas go through the caprock and migrate vertically. Quantities of fractures are formed in mudstone in the process of brittle deformation. The fracture density increases with the increase of the fault displacement. When the fractures are connected, oil and gas go through the caprock and migrate vertically. The connectivity of fault depends on the displacement and the thickness of caprock. On the basis of the above, a method is put forward to quantify the connectivity of fault with the juxtaposition thickness of caprock after faulting. The research on the juxtaposition thickness of caprock after faulting of the member Ⅱ of Dongying Formation in Nanpu depression and the distribution of oil and gas indicates when the juxtaposition thickness of caprock is less than 96.2 m, the fault becomes leaking vertically. In the lifting stage, with the releasing and unloading of the stress, the caprock becomes brittle generally and then forms through going fault which will lead to a large quantity of oil and gas migrate vertically.
KEY WORDS: mudstone    fault deformation    brittle-ductile    shale smear    CJT    quantitative evaluation    
0 INTRODUCTION

With very low permeability and high capillary pressure, mudstone has effective sealing capacity for hydrocarbon (Watts, 1987; Schowalter, 1981). The vast majority of caprock of the oil and gas fields in the world is mudstone (Fu et al., 2013, 2012b, 2002; Ingram and Urai, 1999; Grunau, 1987). There are mainly three ways for oil and gas to migrate vertically: (1) when the hydrocarbon buoyancy in trap exceeds the minimum displacement pressure of caprock, oil and gas break through the caprock (Watts, 1987). The distribution of mudstone in the lacustrine basins is extensive and continuous, and the displacement pressure of this mudstone is high, the sealing capacity of this caprock is very good (Fu et al., 2009) and micro leakage will not result in the effusion of a large quantity of hydrocarbon. (2) When the hydrocarbon pressure in trap is equal to the minimum principal stress and tensile strength, hydrofractures will be formed in the caprock (Caillet et al., 1997; Roberts, 1996; Roberts and Gawthorpe, 1995), and then the trap leaks. Generally, the overpressure of reservoir in the basins of eastern China is not large enough to cause the leakage. (3) The integrity of the caprock is broken by faults, through which hydrocarbon leaks and migrates vertically (Ingram and Urai, 1999). This is the main way of hydrocarbon leakage and migration adjustment in eastern lacustrine basin in China. Although it is hard to quantatively evaluate the critical depth of mudstone on the transition from brittle to ductile in oil and gas bearing basin. The mudstone will generally transform from brittle to brittle-ductile or ductile with the increase of depth (Runar et al., 2006). Triaxial compression test shows that mudstone bearing with oil will transform from brittle to brittle-ductile if the confining pressure exceeds 12 MPa (Nygård et al., 2006). While in the uplifting process, the ductile mudstone will gradually retransform to brittle. The deformation mechanism of fault in different diagenetic stage of mudstone is different. With the outcrop and oil field anatomy, and based on the analysis of fault deformation mechanism of mudstone in brittle, ductile and uplifting stages, a quantitative evaluation method for fault sealing and leaking is put forward, which will be an effective guidance for the determination of primary reservoir boundary and the exploration of secondary reservoir.

1 DEFORMATION MECHANISM AND QUANTITATIVE EVALUATION OF FAULT IN BRITTLE MUDSTONE 1.1 Deformation Characteristics of Fault in Brittle Mudstone

Although it is very difficult to define the threshold of consolidation and over-consolidation diagenetic stage of mudstone from the deformation mechanism of faulting, outcrop obviously shows that the shale smear is not well developed and there are a lot of fractures in the over-consolidated diagenetic stage, and then the mudstone becomes brittle and begins to form extension fractures, with the development of soft gouge, the sealing capacity of fault becomes better and better (Holland, 2006). With the increase of strain both the displacement of those throughgoing faults and the density of slip fractures are increased. When those throughgoing faults are interconnected and form networks, the permeability increases greatly, and the hydrocarbon will migrate through the caprock (Ingram and Urai, 1999; Bolton and Maltman, 1998; Anderson, 1994). The vertical permeability of fault depends on two key factors: the fault displacement and the thickness of caprock. The bigger the displacement is, the stronger the deformation is, then the more fractures are developed. The thinner the caprock is, the easier the fractures will be connected. It is put forward that the parameter of the juxtaposition thickness of the caprock after faulting (Fig. 1a) (CJT, that is the difference between the thickness of caprock and the fault displacement, the unit is meter) can quantitatively evaluate the vertical connectivity of fractures, that is, the higher the value is, the lower the vertical permeability will be.

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Figure 1. Different internal structure of fault in mudstone and quantitative evaluation index. (a) Throughgoing fault and CJT concept model (modified after Lü et al., 2007); (b) shale smear and SSF concept model (modified after Lindsay et al., 1993).
1.2 Typical Case—The Mudstone Caprock of Member Ⅱ of Dongying Formation in Nanpu Depression

There are three tectonic evolution stages of Nanpu

depression: rifting (E3s2+3), rift-depression transition (E3s2+3-E3d) and depression (N1g-Qp). There are three strongly active periods of faulting: the sedimentary period of the second Shahejie-third Shahejie member, the sedimentary period of the first Dongying member, sedimentary period of the upper part of Mingshui member to Quaternary sedimentary period. There are three reservoir-caprock assemblage in Nanpu depression vertically: (1) Es3 (reservoir)-Es2 (caprock); (2) Es1 (reservoir)-Ed2 (caprock); (3) Ed1 (reservoir)-Nms (caprock) (Sun et al., 2013) (Fig. 2).

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Figure 2. Schematic diagram of strata, structure and hydrocarbon distribution in Nanpu depression.

The mudstone caprock of the member Ⅱ of Dongying Formation is the regional caprock which separates the deep and shallow hydrocarbon system. The faults connecting the source rock and the reservoir and being active in the accumulation period have both broken the caprock of the member Ⅱ of Dongying Formation and led to part of hydrocarbon migrate up to the reservoir above Dongying Formation. Comparison of the hydrocarbon distribution above and down the caprock indicates that two cases exist (Fig. 3): one is that there is hydrocarbon in both above and under the caprock, showing vertical migration of hydrocarbon through the caprock; the second is that hydrocarbon only accumulates under the caprock, showing that the fault in caprock is sealed and there is no vertical migration of hydrocarbon. Statistics about the juxtaposition thickness of caprock after faulting in these two kinds of faults indicates that the threshold of juxtaposition thickness is about 96.2 m (Fig. 3), and it means that when the juxtaposition thickness is less than 96.2 m, the fault becomes leaking vertically.

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Figure 3. Determination of the threshold of juxtaposition thickness of caprock in the member Ⅱ of Dongying Formation in Nanpu depression.
2 DEFORMATION MECHANISM AND QUANTITATIVE EVALUATION OF FAULTS IN BRITTLE-DUCTILE MUDSTONE 2.1 Formation Mechanism and the Type of Clay Smear

In the deformation stage of brittle-ductile, the mudstone has obvious competence contrast with other lithology. The typical clay smear generally forms in the process of fault deformation (Cuisiat and Skurtveit, 2010; Schmatz et al., 2010; Eichhubl et al., 2005; Doughty, 2003; Koledoye et al., 2003; Takahashi, 2003; Aydin and Eyal, 2002; Clausen and Gabrielsen, 2002; Sperrevik et al., 2000; Burhannudinnur and Morley, 1997; Lehner and Pilaar, 1997; Weber et al., 1978). Peacock et al. (2000) summarized the researches of precursors (Knott, 1994; Lindsay et al., 1993) and concluded that surrounding rock material, usually clay-rich, spreads along a fault surface and changes into smear. The early description of the clay smear is mainly about growth fault and the depth of fault deformation is less than 50 m (Weber, 1997; Smith, 1980; Weber et al., 1978). Lindsay et al. (1993) mainly focus on the study of the shale smear resulted by the deformation of fault after diagenesis and confirms that the clay smear can form in unconsolidated, half consolidated and consolidated sand and mudstone succession. There are three types of shale smear: abrasion, shearing and injection (Lindsay et al., 1993). The vertical sealing capacity of fault mainly relies on shearing smear and its formation is related with normal drag led by faulting (Cuisiat and Skurtveit, 2010; Takahashi, 2003; Lindsay et al., 1993; Smith, 1980; Weber et al., 1978). In the layer of the plastic shearing zone with lower ratio of sand to shale, the mudstone flows into fault zones and this is the typical formation mechanism of shearing smear. Shearing smear is also the most common type, which was certified again and again, with the method of physical simulation experiment (Cuisiat and Skurtveit, 2010; Schmatz et al., 2010; Takahashi, 2003; Clausen and Gabrielsen, 2002; Sperrevik et al., 2000), numerical simulation (Egholm, 2008; Gudehus and Karcher, 2007) and outcrop observation (Eichhubl et al., 2005; Doughty, 2003; Koledoye et al., 2003; Aydin and Eyal, 2002; Lehner and Pilaar, 1997) and drilling data interpretation (Faerseth, 2006).

The widely accepted viewpoint of clay smear is the linkage and propagation of fault segments model. As there is competence contrast between mudstone and sandstone, the plastic shale material will lead to fault segment and form extensional overlaps. With further activity of fault, the plastic mudstone will be dragged into fault zone and then shearing smear is formed (Fig. 4) (Davatzes and Aydin, 2005; Doughty, 2003; Koledoye et al., 2003, 2000; Aydin and Eyal, 2002; Lehner and Pilaar, 1997; Weber et al., 1978). When there are many sets of plastic mudstone material, the shearing smear will overlap vertically and form a complex shale smear zone (Aydin and Eyal, 2002).

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Figure 4. Process of the linkage of fault segment and the development of shale smear (modified after Fu et al., 2013).
2.2 Continuity of Clay Smear and Quantitative Evaluation

Most scholars believe that the continuity of clay smear is controlled by the ratio of the displacement of fault and the thickness of mudstone (SSF value, Fig. 1b) (Childs et al., 2007; Faerseth, 2006; Eichhubl et al., 2005; Doughty, 2003; Kim et al., 2003; Takahashi, 2003; Yielding, 2002; Younes and Aydin, 2001; Sperrevik et al., 2000; Yielding et al., 1997; Gibson, 1994; Lindsay et al., 1993). Faerseth (2006) suggests that the smear of small faults whose displacement are less than 15 m (i.e., subseismic faults) keeps continuous when SSF ranges from 1 to 50, with the thickness of mudstone usually a few millimeters to 10 cm, and the displacement about a few decimeters to 3-4 cm (Dewhurst et al., 2002; Sperrevik et al., 2002; Fisher and Knipe, 2001; Gibson, 1998; Hesthammer and Fossen, 1998; Lindsay et al., 1993; Speksnijder, 1987). For those large faults (with a displacement of more than 15 m), the critical value of SSF on smear continuity is small, it is usually 4-8 (Fig. 5) (Faerseth, 2006; Doughty, 2003; Kim et al., 2003; Yielding, 2002; Aydin et al., 1998; Yielding et al., 1997; Gibson, 1994). Physical simulations in high temperature and pressure show that when effective normal stress is 30 MPa the critical SSF is greater than 4.9 in siltstone, but when the effective stress is increased to 40 MPa the critical SSF is 6.6 (Takahashi, 2003). Therefore, with the increase of the depth in the same mudstone, the clay smear is developed better and better, and it is easier to maintain its continuity.

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Figure 5. Critical SSF values on smear continuity (modified after Faerseth, 2006; Doughty, 2003; Kim et al., 2003; Yielding, 2002; Aydin et al., 1998; Yielding et al., 1997; Gibson, 1994).
2.3 Typical Case: the South Depression of Hailar-Tamuchage Basin

There are four stages of the tectonic evolution in Hailar-Tamuchage Basin: rifting, rift-depression transition, inversion and depression (Fig. 6) (Fu et al., 2012a). And there are four main intensive deformation periods of the faults (Fu et al., 2012a): the sedimentary period of the late and middle part of member Ⅰ of Nantun Formation; the sedimentary period of the upper of members Ⅰ and Ⅱ of Nantun Formation; the sedimentary period of the late, and the end of the Yinmin Formation. There are two periods in Tanan depression. Primary oil and gas in the early stage was accumulated in the middle to the upper Yimin Formation (Dong, 2011) and the top of Yimin Formation. The reactivation of faults made adjustment of oil and gas to the Damoguaihe Formation and then formed the secondary reservoir (Fu et al., 2012a). There are three reservoir-caprock assemblage vertically: (1) K1nx 1+K1t (reservoir)-K1n1 (caprock); (2) K1n1 (reservoir)-K1d1 (caprock); (3) K1d (reservoir)-K1d2s (caprock).

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Figure 6. Schematic diagram of strata, structure and hydrocarbon distribution in the south part of Tamuchage depression.

Most reservoirs in south part of Tamuchage depression are mainly controlled by antithetic faults which also act as faults form sealing. And most hydrocarbon accumulated in the lower and middle part of the Nantun Formation. Reservoirs are found only in the traps which are controlled by F1, F9 and F19 in the member Ⅱ of Damoguaihe Formation (Fu et al., 2013).

The upper part and the member Ⅱ of Nantun Formation and the member Ⅰ of Damoguaihe Formation consist of a set of thick caprock. Because of the competence contrast between Nantun and Damoguaihe formations the fault segments propagated in the caprock of Damoguaihe Formation and then formed shearing smear (Ferrill and Morris, 2008). Calculation about the SSF value of each fault in Tamuchage depression by Fu et al. (2013) indicates that the SSF values of those faults which control the reservoir in the caprock of member Ⅰ of the Nantun Formation are generally less than 5; while those in member Ⅱ of Damoguaihe Formation are bigger than 5 but they are generally less than 6; therefore, when SSF is greater than 5 the clay smear will become discontinuous. In the inversion stage of the basin, with the increase of displacement, the clay smear becomes discontinuous and then the oil and gas migrate vertically (Fu et al., 2013).

3 DEFORMATION MECHANISM AND QUANTITATIVE EVALUATION OF FAULTS IN THE LIFTING STAGE OF MUDSTONE 3.1 Formation of Normal Throughgoing Faults and Vertical Migration of Hydrocarbon in the Lifting Stage of the Basin

As a result of the stress releasing and unloading, the plasticity of shale became weaker (Fossen et al., 2010), shearing smear of those faults which formed in the process of uplifting of the basin is not developed and then the faults often cut through the caprock. The fault in the outcrop of coal mine in Hailaer Basin is a representative, the tensional normal growth fault of upper part of this coal mine in Hailaer Basin has cut through the layer of interbedded black coal and gray silty mudstone (Fig. 7a). The displacement of this fault ranges from 40 to 50 m and the width of fault zone ranges from 5 to 25 cm (Fig. 7b) (Fu et al., 2013). The cohesionless filling material of the fault zone is mainly the mixture of coal and mudstone except those places which is developed with structural lenticles shown in Fig. 7b. The filling material is mainly from the mixture of wall rock. In macroscopic, the filling material of fault zone is mainly consists of coal with little clay overall. This fault is the representative which is formed in the uplifting stage of basin, although it is developed in plastic coal bed it did not form shearing smear, but formed a fault with platy fault rocks (Fig. 7c).

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Figure 7. The faults developed in coal bed and silty mudstone layer of the Yimin Formation in Lingquan coal mine of Hailar Basin. (a) Photograph showing a tensional normal fault (F1) developed in lacustrine coal strata interbedded with fluvial silty mudstone which is in the upper of Yimin Formation in Lingquan coal mine of Hailaer Basin, displacement of the fault is approximately 40-50 m. (b) The color of fault damage zone is dark gray, between black coal strata and grey-white silty mudstone, indicating the internal material in the fault is the mixture of the two strata. The fault zone with a width of about 5-25 cm, locally developed tectonic lens, and associated positive traction phenomenon in fault walls. (c) The color of the damage zone filling material is dark black and both walls are grey-white silty mudstone, indicating the coal of the hanging wall partly involved in the fault zone when slipping over the point. Between the damage zone and surrounding rocks apart with a smooth slip surface and palty fault rock, the fault plane feels quite smooth (modified from Fu et al., 2013).
3.2 Typical Case—The Wuerxun Depression in Hailar Basin

The Wuerxun depression and the south of Tamuchage depresion in Hailaer Basin are in the same geological background. And their structural evolution and reservoir and caprock assemblage are similar (Fu et al., 2013). The field observation of normal fault in inverse anticline in the east of Wuerxun depression is the same as our theory. Most of these normal faults are developed in the limb of inverse anticline in the late stage. As a result of uplifting, the stress is released and confining pressure is decreased (Sperrevik et al., 2000), and then it formed throughgoing fault. Based on the characteristic of homogenization temperature of fluid inclusions in Well Wu20 and the burial history it is concluded that there were two accumulation periods in Wuerxun depression. The early hydrocarbon accumulation of Nantun Formation was the middle part of Yimin Formation (Gao and Lin, 2007; Hou et al., 2004) while the hydrocarbon accumulation of Damoguaihe Formation was the middle part of Yimin Formation to the late of Qingyuangang Formation. The accumulation period of oil in Damoguaihe Formation was late and was the typical secondary reservoir adjusted by fault reactivation (Dong, 2011). Eadington et al. (1996) put forward the parameter of GOI (grains containing oil inclusion) to reflect the oil saturation of reservoirs: the critical GOI value of oil layer, migration pathway and water layer are respectively greater than 5%, 1%-5% and less than 1%. The GOI of petroleum inclusion of Nantun Formation (K1n2, 2 076-2 088.6 m) and Nantun Formation (K1n1, 2 199-2 104.7 m) in Well Wu20 are 9% and 6%, respectively (see details in Fu et al., 2013). But the test results show that they are all water layers.

So there was paleo-reservoir in Nantun Formation and its accumulation period was earlier than Damoguaihe Formation. Based on distribution characteristic of hydrocarbon, accumulation period and the genetic mechanism of the hydrocarbon in Damoguaihe Formation in the south part of Wuerxun depression, the migration and accumulation of hydrocarbon are classified into two stages (Fig. 8). One is in the middle of Yimin sedimentation there is no obvious activity of faults and then the hydrocarbon made lateral migration. And then as the result of barrier faults, the hydrocarbon accumulated in fault slope break zone. The reservoir in the east of Wuerxun depression is the representative. The other one is during the end of Yimin sedimentation to Qingyuangang sedimentation the basin was reversed and then the south of Wuerxun sub-depression inversed. Those newly formed faults made adjustment of the hydrocarbon to Damoguaihe Formation. And then the secondary hydrocarbon accumulation zone of Wu20-Wu16-Wu32 in Damoguaihe Formation was formed. As there is no obvious reversion in reservoir in east of Wuerxun depression, the hydrocarbon in it was preserved.

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Figure 8. Model of migration-accumulation and formation of oil in southern Wuerxun depression. (a) Lateral migration of oil and gas in Yimin Formation and then formed the Nantun Formation primary reservoir; (b) inversion faults adjust the early reservoirs and then formed the secondary reservoir in Damoguaihe Formation.
4 CONCLUSIONS

(1) The deformation of fault in the brittle mudstone caprock begins with rupturing and then forms a large number of fractures. As the connectivity of fractures increases the vertical permeability of fault becomes better. So, it is concluded that the vertical sealing capacity is controlled by the thickness of caprock and the fault displacement. The parameter of juxtaposition thickness of caprock after faulting is put forward to quantitatively evaluate the vertical sealing capacity of fault in brittle mudstone caprock. Statistics about the juxtaposition thickness of caprock after faulting of the member Ⅱ of Dongying Formation in Nanpu depression and its relationship with the distribution of oil and gas indicates that when the juxtaposition thickness of caprock after faulting is less than 96.2 m the fault becomes leaking vertically.

(2) The deformation of faults in brittle-ductile mudstone caprock mainly forms typical clay smear. And the continuity of clay smear controls the vertical sealing capacity of the fault. Outcrop, theoretical calculation and data from oilfield show that when the SSF value is less than 4-7, the clay smear keeps continuous and the fault is sealed vertically.

(3) In the lifting stage of basin, as a result of the stress releasing and unloading the caprock usually becomes brittle and then forms throughgoing fractures. And this will lead to the vertical migration of quantities of oil and gas through the caprock.

(4) The caprock has transformed from ductile to brittle in the process of burial-uplifting. Further study is necessary to quantitatively evaluate the brittle or ductile capacity of caprock based on the burial history and reasonable parameters.

ACKNOWLEDGMENTS

This study was financially supported by the National Natural Science Foundation of China (Nos. U1562214, 41702156, 41272151), the National Science and Technology Major Project (No. 2016ZX05003-002). The authors gratefully acknowledge the Exploration and Development Research Institute of Daqing Oil Field Company Ltd. for providing the original data. This paper benefited considerably from the reviewers and editors. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0998-7.


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