Polygonal faults in the Sanzhao sag of the Songliao basin: their significance in hydrocarbon accumulation

M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 20 (2010) 0300–0305

www.elsevier.com/locate/jcumt

Polygonal faults in the Sanzhao sag of the Songliao basin: their significance in hydrocarbon accumulation HE Chunbo1,2,*, TANG Liangjie1,2, HUANG Deli3, SHI Shangming4 1

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China 2 Basin and Reservoir Research Center, Faculty of Natural Resources & Information Technology, China University of Petroleum, Beijing 102249, China 3 Oil Recovery Plant No.8, Daqing Oilfield Corp. Ltd., Daqing 163514, China 4 Faculty of Earth Science, Daqing Petroleum Institute, Daqing 163318, China

Abstract: Polygonal faults, generally distributed in fine-grained sediments, are layer-bound faults and are important in hydrocarbon accumulation. Using 3D seismic data, we analyzed the plane and profile features of faults developed in the Qingshankou formation of the Sanzhao sag. We identified these faults as having typical features of polygonal faults: 1) layer-bound; 2) normal faults; 3) slight fault displacements and steep in dip angles; 4) multi-directional in strike and 5) a single fault has a short horizontal extension. In addition, these faults intersect each other and form polygons. These polygonal faults are the result from the combined action of compaction, volume contraction and episodic hydraulic fracturing, conditions favorable for oil/gas accumulation. They are the dominant channels for migration of fluids in the Qingshankou mudstone, forming a large number of fault-lithologic oil traps. Polygonal faults improve reservoirs. Keywords: polygnal faults; geometric characteristics; fluid expulsion; compartment; Sanzhao sag

1

Introduction

Polygonal faults are dominant channels for oil/gas migration[1-5]. As well, they are quite important for analyzing the diagenesis and sealing features of argillaceous cover[6]. Since the discovery of a polygonal fault in the North Sea basin in 1994 by Cartwright, using 3D seismic data, geologists have proved the existence of polygonal faults in more than 50 basins all over the world[4,7]. Polygonal faults or layer-bound faults have mainly developed in fine-grained sedimentary strata of marine facies and are also seen in continental strata[8-10]. The interpretation of 3D seismic data has proved to be an useful method for recognizing polygonal faults and many scientists in the world have investigated these geometric features and analyzed the mechanism of their formation[11-12], as well as the relation between polygonal faults and sand bodies[7,13-22]. These studies enriched the theory of structural geology. However, given the literature on the subject, it must be stated that few of these achievements are by Chinese scientists[6,23-26]. Received 10 September 2009; accepted 15 November 2009 *Corresponding author. Tel: 86 10 89733768 E-mail address: [email protected] doi: 10.1016/S1674-5264(09)60202-7

As an important sag for hydrocarbon generation and oil enrichment, the Sanzhao sag is a secondary structural unit of the central down warping region of the Songliao basin. As for the faults developed on the top of the Qingshankou formation (T11), the early pioneers held that these faults were typical polygonal faults and proposed a density inversion genesis mechanism[23]. However, density inversion usually forms “wave-like structures” which have not been found in seismic profiles up to now[15]. So the genesis mechanism of the faults on the top of the Qingshankou formation needs to be thoroughly discussed. In addition, there is a controversy about the property of the faults developed at the bottom of the Qingshankou formation (T2)[23-28]. Based on seismic data, our objective in tackling these issues was to study the properties of faults developed at both the top and bottom of the Qingshankou formation in order to make clear whether these faults are polygonal faults or not. We also hoped to establish a formation model for these faults and to analyze their significance in oil/gas geology.

2

Geological setting The west side of the Sanzhao sag is adjacent to the

Polygonal faults in the Sanzhao sag of the Songliao basin …

6HULHV

)RUPDWLRQ 6HFWLRQ 1HQ 1HQ 1HQ 1HQ
6WUDWD WKLFNQHVV P

/LWKRORJLF SURILOH



 

4LQJ 4LQJ



4LQJ 4XDQ 4XDQ

 

4XDQ 4XDQ 'HQJ 'HQJ





'HQJ

'DTLQJ SODFDQWLFOLQH

QJ H LOL OLQ 6X QWLF D

 

Daqing placanticline, the east to the Chaoyanggou terrace and the north is adjacent to the Mingshui terrace, forming a triangular region with an area of about 5575 km2 (Fig. 1, from the geological team of the eighth oil extraction plant of Daqing limited company). Strata of Mesozoic and Cenozoic are, from low to high: the Huoshiling-, Shahezi-, Yingcheng-, Denglouku-, Quantou-, Qingshankou-, Yaojia-, Nenjiang-, Sifangtai-, Mingshui-, Yi’an-, Da’anand Taikang formations and the Quaternary deposits[29]. The first section (the Putaohua oil layer) of the Yaojia formation (upper Cretaceous) belongs to the upper oil/gas combination of the Sanzhao sag, whose lithologies are gray-green mudstone, argillaceous siltstone, with a typical “sand-in-mud” feature; the Fuyu- and Yangdachengzi oil layers of the third and fourth sections of the Quantou formation (Quan-3 and Quan-4) are low oil/gas combinations of the Sanzhao sag, whose lithologies are thin-bedded green mudstone and argillaceous siltstone (Fig. 2). During the deposition period of the Quan-3 and Quan-4 sections, the Sanzhao sag was a catchment lake of the paleo-Songhua river system, with a typical “sand-inmud” feature[30-31]. The Qingshankou formation is a thick shale deposition of the deep lake or semi-deep lake facies, with mudstone accounting for 90%~ 100% of its total thickness, which is a major hydrocarbon stratum in the Sanzhao sag[23]. In the seismic profile, the top interface of the Qingshankou formation is a T11 reflection profile and the bottom interface a T2 interface.

301

 

HE Chunbo et al

Fig. 1

Location of study area and faults on the top of the Qingshankou formation

'HVFULSWLRQRIOLWKRORJ\

2LOOD\HU

*UH\JUHHQPXGVWRQHZLWK SXUSOHUHGPXGVWRQHJUH\ JUHHQILQHVDQGVWRQHJUH\ JUHHQPXGVWRQHRLOVKDOH *UH\JUHHQPXGVWRQH $UJLOODFHRXVVLOWVWRQHJUH\ JUHHQPXGVWRQH *UH\JUHHQPXGVWRQHZLWK SXUSOHUHGPXGVWRQHRLOV KDOHJUH\EODFNPXGVWRQH DEXQGDQWZLWKRVWUDFRGD 3XUSOHUHGDQGJUH\JUHHQ PXGVWRQHDUJLOODFHRXV VLOWVWRQH *UH\JUHHQPXGVWRQHDUJLOOD FHRXVVLOWVWRQHJUH\JUHHQ VDQG\PXGVWRQHZLWKSXUSOH UHGVDQG\PXGVWRQH

3XWDRKXD

)X\X
3XUSOHUHGVLOW\PXGVWRQH SXUSOHUHGPXGVWRQHZLWK VLOWVWRQHVLOWVWRQHZLWKVLQH VDQGVWRQH

 Fig. 2 Strata columnar section of Sanzhao sag

3 Geometric features of faults As the basis for our study of polygonal faults, we used 3D seismic profiles and time slices in analyzing the geometric features of polygonal faults in the

Sanzhao sag. According to the interpretation of seismic profiles, three types of faults are identified in the Sanzhao sag. The first type of faults penetrates the entire Qingshankou formation, terminating upward in the Nenji-

302

Mining Science and Technology

ang formation and cutting downward into the basement. The hanging wall of the Qingshankou formation is thicker than the footwall, showing a feature of syn-depositional faults. They are not well developed but extend horizontally over a long distance (>1 km) in a NS direction (Figs. 3~4, where TWT: Two Way Time; TFF: Tectonically Formed Fault; PF: Polygonal Faults). They belong to the faults of tectonic genesis. The second type can only be found on the top of the Qingshankou formation; they are all normal faults, characterized by small fault displacement (20~50 m), steep dip angles (45°~70°) and small vertical extensions. In addition, they are limited by sedimentary strata, with clear layer-bound features (Fig. 3). Based on coherence slices from the top of the Qingshankou formation (Fig. 4a), mini-faults are densely developed in different directions, interweaving to show a net

Fig. 3

Vol.20

No.2

pattern and irregular polygons. The number of faults joined at the end point is 2~4 and their horizontal extensions are usually shorter than 1 km. The third type of faults is found at the bottom of the Qingshankou formation. All are normal faults, with fault displacements and dip angles slightly greater than those of the second type (30~100 m and 50°~80°). In addition, the vertical extensions are limited by sedimentary strata, with clear layer-bound features (Fig. 3). Fig. 4c shows that faults have short horizontal extensions with interweaving patterns. Without taking into consideration the NS tectonic genesis faults, we can still see multi-directional features from the rose diagram (Fig. 4c). From the seismic profile and the coherence slices, the first type consists of well developed faults, while the second and the third types are under-developed (Figs. 3, 4b).

3D seismic profile showing faults in the Qingshankou formation (see Fig. 1 for the line location)

(a) Top

(b) Middle

(c) Bottom

TFF=Tectonically Formed Fault; PF=Polygonal Faults

Fig. 4

Coherence slices and strike-rose diagram of the faults on the top, middle, and bottom of the Qingshankou formation (see Fig. 1 for its location)

Generally faults, developed on both the top and bottom of the Qingshankou formation, are normal faults, with features of small fault displacements, steep dip angles and are layer-bound. Horizontally, they are not well extended, characterized by multi-

directions and developed in fine mudstone (Fig. 2). They are in good correspondence with the standards proposed by Cartwright in 1998 for identifying polygonal faults[9]. Therefore, all the small normal faults developed on both the top and bottom of the Qing-

HE Chunbo et al

Polygonal faults in the Sanzhao sag of the Songliao basin …

shankou formation are polygonal faults. The intensity of the polygonal faults is closely related to the density of sand bodies. The material for deposition of the Yao-1 section came from the north of the Sanzhao sag[29]. From Fig. 1 we can see that the south-east area is far from the source, leading to a small number of sand bodies, poor physical properties and well developed polygonal faults. The northern area, however, is close to its provenance, leading to a large number of sand bodies, good physical properties and sparsely developed polygonal faults. All these features indicate that the overlying sand bodies clearly affect polygonal faults, possibly because the overlying sand bodies cause the fluids within the mudstone to be easily discharged, considerably slowing down the speed in forming a super pressure compartment, which is unfavorable for densely developing polygonal faults. The sandstone of the Quan-3 and Quan-4 sections is similar to that of the Yao-1 section with good physical properties affecting the development and strike of polygonal faults. This conclusion has been proven by physical simulation[22].

4 Model generation

bends to form a wave-like structure, similar to the “salt density inversion”. However, this phenomenon cannot be seen in seismic profiles of the Sanzhao sag, nor can the density inversion mechanism explain the polygonal faults developed at the bottom of the Qingshankou formation. So the density inversion is not a suitable mechanism for forming polygonal faults. Referring to the model of episodic hydro fracturing, we build a new model of forming polygonal faults developed on both the top and bottom of the Qingshankou formation and propose that the development of these faults followed three stages[7]. 4.1

Formation of abnormally high pressure compartment

The time of the Qingshankou formation was a flourishing period in forming the Songliao basin, with only a few tectonic genesis NS faults being formed. As an inherited sag, Sanzhao accepted large amounts of argillaceous sediments. Under the action of compaction, the fluid near the margin of the thick mudstone was first compressed and discharged, forcing the pressure in the capillaries near the margin to be greater than that of the internal area to form a sealing layer. The fluid of the mudstone on the bottom of the Qingshankou formation was discharged downward into the highly permeable sand bodies, forming a bottom sealing layer. Similarly, the fluid of the mudstone on top of the Qingshankou formation was discharged upward into overlying strata, forming a top sealing layer. Laterally, the fluid migrated along strata boundaries, forming a lateral sealing layer[7]. In this way, an abnormally high pressure fluid compartment was formed (Fig. 5a).

&RPSDFWLRQLQFUHDVLQJ

&RPSDFWLRQ

Scientists have built a correspondent formation model for polygonal faults and proposed several mechanisms based on many studies, such as episodic hydrofracturing, syneresis, volumetric contraction, density inversion and gravity sliding or collapse[7,9,19-20,22]. Pioneers having built an evolutionary model for polygonal faults developed on the top of the Qingshankou formation[23]. When this model is in a density inversion, the overlying high density layer

303

Fig. 5

Generation model of polygonal faults in the Qinghshankou formation[7]

4.2 Breaking of pressure compartment Towards the end of the Nenjiang formation, a tectonic inversion took place, reviving part of the tec-

tonic genesis faults in the Sanzhao sag[23]. With an increase in compaction, the revival of basement faults dehydrated the clay minerals in mudstone considera-

304

Mining Science and Technology

bly, causing the mudstone to contract and breaking the pressure compartment, forming polygonal faults on both the top and bottom of the Qingshankou formation[9]. Fluid of the mudstone from the Qingshankou formation migrated upwards through the top polygonal faults into the overlying strata and through the bottom polygonal faults, downwards into the underlying Quan-3 and Quan-4 sedimentary sand bodies (Fig. 5b). In Fig. 5, PF=Polygonal Faults; TFF=Tectonically Formed Fault; PD=Provenance Direction; and SPL=Super-Pressured Layer. 4.3 Stage of fault healing After the fluid had been discharged from the high-pressure compartment, the pressure within the mudstone was released, reducing the pressure to a level close to normal pressure of the strata, with a tendency for polygonal faults to become healed. With an increase in depth, the mudstone became a high-pressure area again, resulting in the formation of new faults, starting a new circle.

5

Significance in hydrocarbon accumulation

As mentioned earlier, polygonal faults are of great importance in the formation of oil/gas pools. Mudstone of the Qingshankou formation is several hundred meters thick and the polygonal faults actively promote the discharge of fluids (hydrocarbon expulsion) inside the mudstone, resulting in an increase of effective hydrocarbon expulsion. With increasing depth and temperature, the degree of maturity of organic matter increases, causing the mudstone to become a super high-pressure area again. When the pressure of the fluid in a sealed compartment becomes greater than the pressure of capillaries in the sealing layer, the polygonal faults and their associated fractures revive, playing an important dredging role. The oil and gas generated from hydrocarbon source rocks go upward into the Putaohua oil layer through the polygonal faults on the top of the Qingshankou formation to form oil pools. At the same time, the oil and gas descend into the Fuyang oil layer through polygonal faults at the bottom of the Qingshankou formation. This has been proved by oil source comparison[32]. Laterally, the polygonal faults also have sealing properties, which, in combination with sand bodies, form fault-lithologic traps, such as the Mofantun and Zhaozhou oil fields. Modified by polygonal faults and their associated fractures, mudstone can also form a fractured oil/gas pool. Therefore, attention should be paid to the fractured oil/gas pools in mudstone of the Sanzhao sag. Up to now, scientists have proved that the polygonal faults can, to a certain extent, modify sand bodies for they can cut through the underlying sedimentary sand bodies[22]. The polygonal faults at the bottom of

Vol.20

No.2

the Qingshankou formation are almost in an EW direction while the sedimentary sand bodies of the Quan-3 and Quan-4 sections are nearly all in a NS direction, quite similar to the direction of the tectonic genesis faults, indicating that these sand bodies were quite possibly cut by polygonal faults. Hence, we can say that polygonal faults have played a role in modifying these sand bodies. In addition, polygonal faults can enhance the performance of reservoirs, because they can cause the reservoirs to generate more fractures, which are of vital importance for low permeability oil/gas pools. Drilling data show that fractures in the Putaohua and Fuyang oil layers are well developed and their reservoir performance greatly improved. The interweaved net-like polygonal faults and sand bodies form a dominant migration pathway for secondary migration and accumulation of oil and gas.

6

Conclusions

1) Both the top and bottom of the Sanzhao sag have polygonal faults, characterized by densely developed, small fault displacements, steep dip angles and are obviously layer-bound. The faults are horizontally extended over short distances, interwoven and have different directions. 2) Polygonal faults have undergone three stages: a formation of abnormally high pressure compartments, the breaking of pressure compartments and faults healing. As well, they are characterized by episodic evolution. 3) Polygonal faults are dominant migration channels for both the primary and the secondary migration of oil and gas, which are favorable for reservoir improvement, forming a large amount of fault-lithologic oil/gas pools.

Acknowledgements We acknowledged the National Natural Science Foundation of China (No.40672143) and the National Basic Research Program of China (No.2005CB 4221007). Valuable guidance from associate professor Fu Xiaofei and excellent suggestions from Yu Yixin, Jiang Fujie, Jin Wenzheng, Nen Yuan, Cui Min, Zhang Lei, Wang Zhiqiang and Ning Fei are gratefully acknowledged.

References [1]

Gay A, Lopez M, Cochonat P, Levache D, Sermondadaz G, Seranne M. Evidences of early to late fluid migration from an upper Miocene turbiditic channel revealed by 3D seismic coupled to geochemical sampling within seafloor pockmarks, lower Congo basin. Marine and Petroleum Geology, 2006(23): 387-399.

HE Chunbo et al

[2]

[3]

[4]

[5]

[6] [7] [8] [9] [10]

[11]

[12]

[13] [14]

[15] [16] [17]

Polygonal faults in the Sanzhao sag of the Songliao basin …

Steinar H, JÜrgen M, Stefan B, Hervé N. High-resolution 3D-seismic data indicate focussed fluid migration pathways above polygonal fault systems of the midNorwegian margin. Marine Geology, 2007(245): 89-106. Hansen J P V, Cartwright J A, Huuse M, Clausen O R. 3D seismic expression of fluid migration and mud remobilization on the Gjallar Ridge, offshore mid-Norway. Basin Research, 2005(17): 123-139. Hansen M D. Development of a major polygonal fault system in upper Cretaceous chalk and Cenozoic mudrocks of the Sable Subbasin, Canadian Atlantic margin. Marine and Petroleum Geology, 2004(21): 1205-1219. BÜnz S, Mienert J, Berndt M. Geological controls on the Storegga gas-hydrate system of the mid-Norwegian continental margin. Earth and Planetary Science Letters, 2003(209): 291-307. Yu Y X, Tang L J, Song C L. Study on polygonal fault systems. Global Geology, 2005, 24(2): 149-153. (In Chinese) Cartwright J A. Episodic basin-wide fluid expulsion from geopressured shale sequences in the North Sea basin. Geology, 1994(22): 447-450. Cartwright J A, Lonergan L. Seismic expression of layer-bound fault systems of the Eromanga and North Sea basins. Explore Geophys, 1997(28): 323-331. Cartwright J A, Dewhurst D N. Layer-bound compaction faults in fine-grained sediments. Geological Society of America Bulletin, 1998, 110(10): 1242-1257. Cartwright J A, Wattrus N, Rausch D, Bolton A. Recognition of an early Holocene polygonal fault system in Lake Superior: implications for the compaction of fine-grained sediments. GSA Bulltin, 2004, 32(3): 253256. Cartwright J. The impact of 3D seismic data on the understanding of compaction, fluid flow and diagenesis in sedimentary basins. Journal of the Geological Socitety, London, 2007(164): 881-893. Davies R J, Stewart S A, Cartwright J A, Lappin M, Johnston R, Fraser S, Brown A. 3D seismic technology:are we realizing its full potential? Journal of the Geological Society, London, 2004(29): 1-9. Lonergan L, Cartwright J A. The geometry of polygonal fault systems in Tertiary mudrocks of the North Sea. Journal of Structural Geology, 1998, 20(5): 529-548. James D M D, Walsh J J, Watterson J, Nicol A, Nell P A R, Bretan P G. Discussion on geometry and origin of a polygonal fault system. Journal of the Geological Society, London, 2000(157): 1261-1264. Watterson J, Walsh J, Nicol A. Geometry and origin of a polygonal fault system. Journal of the Geological Society, London, 2000(157): 151-162. Goulty N R. Mechanics of layer-bound polygonal faulting in fine-grained sediments. Journal of the Geological Society, London, 2001(159): 239-246. James D M D, Goulty N R, Swarbrick R E. Discussion on development of polygonal fault systems: a test of hypotheses. Journal of the Geological Society, London, 2006(163): 221-223.

305

[18] Goulty N R, Swarbrick R E. Development of polygonal fault systems: a test of hypotheses. Journal of the Geological Society, London, 2005(162): 587-590. [19] Cartwright J A, Lonergan L. Volumetric contraction during the compaction of mudrocks: a mechanism for the development of regional-scale polygonal fault systems. Basin Research, 1996(8): 183-193. [20] Dewhurst D N, Cartwright J A, Lonergan L. The development of polygonal fault systems by syneresis of colloidal sediments. Marine and Petroleum Geology, 1999(16): 793-810. [21] Lonergan L, Cartwright J A. Polygonal faults and their influence on deep-water sandstone reservoir geometries, Alba Field, United Kingdom Central North Sea. AAPG Bulletin, 1999, 83(3): 410-432. [22] Victor P, Moretti I. Polygonal fault systems and channel boudinage: 3D analysis of multidirectional extension in analogue sandbox experiments. Marine and Petroleum Geology, 2006(23): 777-789. [23] Fu X F, Song Y. Nontectonic mechanism of polygonal “T11” faults in the Sanzhao sag of Songliao basin. Acta Geologica Sinica, 2008, 82(6): 738-749. (In Chinese) [24] Hu C Z, Fu X F, Zeng Z X. An overview of polygonal fault. Geological Science and Technology Information, 2008, 27(3): 26-34. (In Chinese) [25] Wu S G, Sun Q L, Wu T Y, Yuan S Q, Ma Y B, Yao G S. Polygonal fault and oil-gas accumulation in deep-water area of Qiongdongnan basin. Acta Petrolei Sinica, 2009, 30(1): 22-26. (In Chinese) [26] Mei L F, Fei Q, Xu S H. “T2” hydrofracture faults system in Songliao basin. Earth Science—Journal of China University of Geosciences, 1996, 21(6): 632. (In Chinese) [27] Hu W S. “T2” faults system and early Qingshankou stretched rifiting in Songliao basin. Petroleum Exploration and Development, 1995, 22(2): 8-12. (In Chinese) [28] Liu D L, Chen F J, Wen X Q. Analysis on the origin of T2 post-rift formations in Songliao basin. Journal of Daqing Petroleum Institute, 1996, 20(1): 23-26. (In Chinese) [29] Liu Z B, Ma S Z Sun Y, Zhang J G, Lv Y F. High-resolution sequence stratigraphy division and depositional characteristics of Putaohua reservoir, Sanzhao depression. Acta Sedimentologica Sinica, 2008, 26(3): 399-406. (In Chinese) [30] Zhang Z L, Yang X N, Meng Q A, Cao L J, Zhang Y. Environmental significance of ostracod fossils from upper part of 3rd member of Quantou formation, Sanzhao area, Songliao basin. Acta Palaeontologica Sinica, 2005, 44(2): 260-260. (In Chinese) [31] Wang B Q, Xu W F, Liu Z L, Kong F Z, Chang Z Y, Wang C R. Diagenesis of reserviors in Fuyu and Yangdachengzi of Sanzhao region. Oil & Gas Geology, 2001, 22(1): 82-87. (In Chinese) [32] Chi Y L, Xiao D M, Yin J Y. The injection pattern of oil and gas migration and acuumulation in the Sanzhao area of Songliao basin. Acta Geologica Sinica, 2000, 74(4): 371-377. (In Chinese)