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Deformation Mechanism and Vertical Sealing Capacity of Fault in the Mudstone Caprock

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Deformation Mechanism and Vertical Sealing Capacity of Fault in the Mudstone Caprock Journal of Earth Science, Vol. 30, No. 2, p. 367–375, April 2019 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-018-0998-7 Deformation Mechanism and Vertical Sealing Capacity of Fault in the Mudstone Caprock Xiaofei Fu 1, 2, 3, Lingyu Yan1, 2, 3, Lingdong Meng *1, 2, 3, Xiaobo Liu *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 Xiaofei Fu: https://orcid.org/0000-0003-3079-7033; Lingdong Meng: https://orcid.org/0000-0002-3712-9153; Xiaobo Liu: https://orcid.org/0000-0002-8613-9323 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 mech- anism, 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 con- trols the sealing capacity of fault. The outcrop and oil field data have confirmed that when sealing pa- rameter 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 frac- tures 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 For- mation 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 basins is extensive and continuous, and the displacement pressure With very low permeability and high capillary pressure, of this mudstone is high, the sealing capacity of this caprock is mudstone has effective sealing capacity for hydrocarbon (Watts, very good (Fu et al., 2009) and micro leakage will not result in 1987; Schowalter, 1981). The vast majority of caprock of the oil the effusion of a large quantity of hydrocarbon. (2) When the and gas fields in the world is mudstone (Fu et al., 2013, 2012b, hydrocarbon pressure in trap is equal to the minimum principal 2002; Ingram and Urai, 1999; Grunau, 1987). There are mainly stress and tensile strength, hydrofractures will be formed in the three ways for oil and gas to migrate vertically: (1) when the caprock (Caillet et al., 1997; Roberts, 1996; Roberts and Gaw- hydrocarbon buoyancy in trap exceeds the minimum displace- thorpe, 1995), and then the trap leaks. Generally, the overpres- ment pressure of caprock, oil and gas break through the caprock sure of reservoir in the basins of eastern China is not large (Watts, 1987). The distribution of mudstone in the lacustrine enough to cause the leakage. (3) The integrity of the caprock is broken by faults, through which hydrocarbon leaks and migrates *Corresponding author: [email protected]; vertically (Ingram and Urai, 1999). This is the main way of hy- [email protected] drocarbon leakage and migration adjustment in eastern lacus- © China University of Geosciences (Wuhan) and Springer-Verlag trine basin in China. Although it is hard to quantatively evaluate GmbH Germany, Part of Springer Nature 2019 the critical depth of mudstone on the transition from brittle to ductile in oil and gas bearing basin. The mudstone will generally Manuscript received March 25, 2013. transform from brittle to brittle-ductile or ductile with the in- Manuscript accepted January 12, 2015. crease of depth (Runar et al., 2006). Triaxial compression test Fu, X. F., Yan, L. Y., Meng, L. D., et al., 2019. Deformation Mechanism and Vertical Sealing Ability of Fault in the Mudstone Caprock. Journal of Earth Science, 30(2): 367–375. https://doi.org/10.1007/s12583-018-0998-7. http://en.earth-science.net 368 Xiaofei Fu, Lingyu Yan, Lingdong Meng and Xiaobo Liu shows that mudstone bearing with oil will transform from brittle and depression (N1g–Qp). There are three strongly active periods to brittle-ductile if the confining pressure exceeds 12 MPa (Nygård of faulting: the sedimentary period of the second Shahejie-third et al., 2006). While in the uplifting process, the ductile mudstone Shahejie member, the sedimentary period of the first Dongying will gradually retransform to brittle. The deformation mechanism member, sedimentary period of the upper part of Mingshui mem- of fault in different diagenetic stage of mudstone is different. With ber to Quaternary sedimentary period. There are three reservoir- the outcrop and oil field anatomy, and based on the analysis of fault caprock assemblage in Nanpu depression vertically: (1) Es3 (reser- deformation mechanism of mudstone in brittle, ductile and uplift- voir)–Es2 (caprock); (2) Es1 (reservoir)–Ed2 (caprock); (3) Ed1 ing stages, a quantitative evaluation method for fault sealing and (reservoir)–Nms (caprock) (Sun et al., 2013) (Fig. 2). leaking is put forward, which will be an effective guidance for the The mudstone caprock of the member II of Dongying For- determination of primary reservoir boundary and the exploration mation is the regional caprock which separates the deep and shal- of secondary reservoir. low hydrocarbon system. The faults connecting the source rock and the reservoir and being active in the accumulation period 1 DEFORMATION MECHANISM AND QUANTITATIVE have both broken the caprock of the member II of Dongying For- EVALUATION OF FAULT IN BRITTLE MUDSTONE mation and led to part of hydrocarbon migrate up to the reservoir 1.1 Deformation Characteristics of Fault in Brittle Mudstone above Dongying Formation. Comparison of the hydrocarbon Although it is very difficult to define the threshold of con- distribution above and down the caprock indicates that two cases solidation and over-consolidation diagenetic stage of mudstone exist (Fig. 3): one is that there is hydrocarbon in both above and from the deformation mechanism of faulting, outcrop obviously under the caprock, showing vertical migration of hydrocarbon shows that the shale smear is not well developed and there are a through the caprock; the second is that hydrocarbon only accu- lot of fractures in the over-consolidated diagenetic stage, and then mulates under the caprock, showing that the fault in caprock is the mudstone becomes brittle and begins to form extension frac- sealed and there is no vertical migration of hydrocarbon. Statis- tures, with the development of soft gouge, the sealing capacity of tics about the juxtaposition thickness of caprock after faulting in fault becomes better and better (Holland, 2006). With the increase these two kinds of faults indicates that the threshold of juxtapo- of strain both the displacement of those throughgoing faults and sition thickness is about 96.2 m (Fig. 3), and it means that when the density of slip fractures are increased. When those throughgo- the juxtaposition thickness is less than 96.2 m, the fault becomes ing faults are interconnected and form networks, the permeability leaking vertically. increases greatly, and the hydrocarbon will migrate through the caprock (Ingram and Urai, 1999; Bolton and Maltman, 1998; An- 2 DEFORMATION MECHANISM AND QUANTITA- derson, 1994). The vertical permeability of fault depends on two TIVE EVALUATION OF FAULTS IN BRITTLE-DUCTILE key factors: the fault displacement and the thickness of caprock. MUDSTONE The bigger the displacement is, the stronger the deformation is, 2.1 Formation Mechanism and the Type of Clay Smear then the more fractures are developed. The thinner the caprock is, In the deformation stage of brittle-ductile, the mudstone the easier the fractures will be connected. It is put forward that the has obvious competence contrast with other lithology. The typi- parameter of the juxtaposition thickness of the caprock after fault- cal clay smear generally forms in the process of fault defor- ing (Fig. 1a) (CJT, that is the difference between the thickness of mation (Cuisiat and Skurtveit, 2010; Schmatz et al., 2010; Eich- caprock and the fault displacement, the unit is meter) can quanti- hubl et al., 2005; Doughty, 2003; Koledoye et al., 2003; tatively evaluate the vertical connectivity of fractures, that is, the Takahashi, 2003; Aydin and Eyal, 2002; Clausen and Gabrielsen, higher the value is, the lower the vertical permeability will be. 2002; Sperrevik et al., 2000; Burhannudinnur and Morley, 1997; Lehner and Pilaar, 1997; Weber et al., 1978). Peacock et al. 1.2 Typical Case—The Mudstone Caprock of Member II of (2000) summarized the researches of precursors (Knott, 1994; Dongying Formation in Nanpu Depression Lindsay et al., 1993) and concluded that surrounding rock mate- There are three tectonic evolution stages of Nanpu rial, usually clay-rich, spreads along a fault surface and changes 2+3 2+3 depression: rifting (E3s ), rift-depression transition (E3s –E3d) into smear.
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