Pet.Sci.(2013)10:171-182 171 DOI 10.1007/s12182-013-0264-7
Geological characteristics and accumulation mechanisms of the “continuous” tight gas reservoirs of the Xu2 Member in the middle- south transition region, Sichuan Basin, China
Zou Caineng1, 2, Gong Yanjie1, 2 , Tao Shizhen1 and Liu Shaobo1, 2
1 5HVHDUFK,QVWLWXWHRI3HWUROHXP([SORUDWLRQ 'HYHORSPHQW3HWUR&KLQD%HLMLQJ&KLQD 2 State Key Laboratory of Enhanced Oil Recovery, Beijing 100083, China
© China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2013
Abstract: “Continuous” tight gas reservoirs are those reservoirs which develop in widespread tight sandstones with a continuous distribution of natural gas. In this paper, we summarize the geological IHDWXUHVRIWKHVRXUFHURFNVDQG³FRQWLQXRXV´WLJKWJDVUHVHUYRLUVLQWKH;XMLDKH)RUPDWLRQRIWKHPLGGOH VRXWKWUDQVLWLRQUHJLRQ6LFKXDQ%DVLQ7KHVRXUFHURFNVRIWKH;X0HPEHUDQGUHVHUYRLUURFNVRIWKH ;X0HPEHUDUHWKLFN ;X0HPEHU P;X0HPEHU P DQGDUHGLVWULEXWHGFRQWLQXRXVO\LQ WKLVVWXG\DUHD7KHUHVXOWVRIGULOOHGZHOOVVKRZWKDWWKHZLGHVSUHDGVDQGVWRQHUHVHUYRLUVRIWKH;X 0HPEHUDUHFKDUJHGZLWKQDWXUDOJDV7KHUHIRUHWKHQDWXUDOJDVUHVHUYRLUVRIWKH;X0HPEHULQWKH middle-south transition region are “continuous” tight gas reservoirs. The accumulation of “continuous” tight gas reservoirs is controlled by an adequate driving force of the pressure differences between source rocks and reservoirs, which is demonstrated by a “one-dimensional” physical simulation experiment. In this simulation, the natural gas of “continuous” tight gas reservoirs moves forward with no preferential SHWUROHXPPLJUDWLRQSDWKZD\V 3303 DQGWKHQDWXUDOJDVVDWXUDWLRQRI³FRQWLQXRXV´WLJKWJDVUHVHUYRLUV is higher than that of conventional reservoirs.
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1 Introduction reservoirs commonly consist of large volumes of sandstones SHUYDVLYHO\FKDUJHGZLWKJDV WKHVRXUFHVURFNVDUHFORVH “Continuous” tight gas reservoirs are those reservoirs to sandstones and most of these reservoirs are not directly which develop in widespread tight sandstones with a dependent on the buoyancy of gas in water for their existence. FRQWLQXRXVGLVWULEXWLRQRIQDWXUDOJDV *DXWLHUHWDO 7KHUHDUHULFKQDWXUDOJDVUHVRXUFHVLQWKH;X0HPEHU 6FKPRNHU 7KHJOREDOUHVRXUFHRI in the middle-south transition region, Sichuan Basin, China. “continuous” tight gas reservoirs is great, meaning that 6HYHUDOELJQDWXUDOJDV¿HOGVVXFKDV+HFKXDQ7RQJQDQDQG WKH\KDYHKXJHH[SORUDWLRQSRWHQWLDO *DXWLHUHWDO Anyue whose reserves are all over 0.1×1012 m3 have been 6FKPRNHU .OHWWDQG&KDUSHQWLHU found in recent years. The exploration results show that each .OHWWDQG6FKPRNHU &RRN &URYHOOL 7KH GULOOHGZHOOLQWKH;X0HPEHULQWKHVWXG\DUHDLVFDSDEOHRI American production of “continuous” tight gas reservoirs producing at least some gas, but the production characteristics 12 3 in 2007 reaches 0.5×10 m , accounting for 1/6 of the of the drilled wells can vary significantly ((0.001-200)×103 overall natural gas production (Pollastro, 2007). Therefore, m3G 0DQ\JHRORJLVWVKDYHFDUULHGRXWWKHLUVWXGLHVRIWKH it is important to enhance the research into exploration and ;XMLDKH)RUPDWLRQLQWKLVDUHD =KDQJDQG=KDQJ =RX development of “continuous” tight gas reservoirs. These HWDO There are still problems: Can the natural gas reservoirs IDXOWV /XRDQG7RQJ :DQJHWDO ,WFDQEH RIWKH;X0HPEHUEHFODVVLILHGDV³FRQWLQXRXV´WLJKWJDV divided into six tectonic units (Fig. 1): foreland depression reservoirs? What are the geological characteristics of the area in the west (I), flat fold area in the middle (II), thrust QDWXUDOJDVUHVHUYRLUVLQWKH;X0HPEHU":KDWLVWKH fold area in the north (III), steep structure area in the east driving force for the accumulation of “continuous” tight gas (IV), uplift in the west and south (V), less steep fold area in UHVHUYRLUV EXR\DQF\RUSUHVVXUH" the south (VI). Our study area is the middle-south transition In this paper, we studied comprehensively the geological area with a total area of 17.5×103 km2ZKLFKLVÀDWO\IROGHG characteristics of the source rocks and sandstones of )LJ DQGFRQWDLQV6XLQLQJ0R[L/RQJQVL$Q\XH ³FRQWLQXRXV´WLJKWJDVUHVHUYRLUVLQWKH;X0HPEHUDQGWKH +HFKXDQ7RQJQDQDQG+HEDRFKDQJQDWXUDOJDV¿HOGV,QWKH physical simulation of “continuous” tight gas reservoirs was geologic history, this area went through the following tectonic carried out in the Key Laboratory of Petroleum Accumulation evolution: Caledonian and ancient Indosinian uplift, late of CNPC to analyze the accumulation mechanism. Indosinian and Yanshan foreland slope, and the Himalayan WHFWRQLFXSOLIW *XRHWDO 'HQJ 2 Geological setting 7KH8SSHU7ULDVVLF;XMLDKH)RUPDWLRQZDVIRUPHGDIWHU The Sichuan Basin is the fourth largest basin in China, the early Indosinian movement (Fig. 2). It is primarily a set locating in the eastern part of Sichuan Province near of continental foreland basin system with coal. It can be Chongqing City. It is a very important petroliferous basin GLYLGHGLQWRVL[PHPEHUVIURPERWWRPWRWRS ;X;X;X /RQJHWDO =KDQJHWDO ,WLVERUGHUHGE\WKH ;X;XDQG;X 7KH;X;XDQG;XPHPEHUVDUH 0LFDQJ0RXQWDLQVDQG'DED0RXQWDLQVWRWKHQRUWKWKH SULPDULO\GDUNPXGVWRQHVDQGFRDOV7KH;X;XDQG;X 'DOLDQJ0RXQWDLQVWRWKHVRXWKWKH/RQJPHQ0RXQWDLQVDQG members are primarily sublitharenite, feldspathic litharenite 4LRQJODL0RXQWDLQVWRWKHZHVWDQGWKH4L\DR0RXQWDLQVWR DQGOLWKDUHQLWH =KDRHWDO =KDQJHWDO :DQJ the east, with a total area of about 180×103 km2. The Sichuan HWDO 7KH/RZHU-XUDVVLF=KHQ]KXFKRQJ)RUPDWLRQ Basin is a large sedimentary basin surrounded by folds and RYHUOLHVWKH;XMLDKH)RUPDWLRQZLWKDSDUDOOHOXQFRQIRUPLW\ o o o 70 E 100 130 o 106 33'E N o 40N Beijing The Sichuan Basin Guangyuan Sichuan Ę o Chongqing 20 Mianyang 0 1000 km Zhongba A Laoguanmiao ė Dachuan Bajiaochang Dujiangyan Chengdu Nanchong Wanxian Chongxi Ė Suining Guang’an Longnüsi B Dianjiang Yaan Moxi Anyue ę TongnanHechuan Fuling Leshan Hebaochang Chongqing o 2935'N 100 km Ě Yibin ě Fault Tectonic units Xu2 boundaries gas reservoir Zhongba Laoguanmiao Bajiaochang Chongxi Guang’an A B 1000 2000 Xu5 Xu6 Xu4 50 km Xu3 3000 Xu2 Xu1 Gas reservoir 4000 (m) Fig. 1 Location map showing the sub-units of the Sichuan Basin. The area in the black frame is the middle-south transition region and WKHFURVVVHFWLRQ$%VKRZVWKHORFDWLRQRIWKH;X0HPEHULQWKH;XMLDKH)RUPDWLRQ Pet.Sci.(2013)10:171-182 173 ZKLOHWKH;XMLDKH)RUPDWLRQRYHUOLHVWKH0LGGOH7ULDVVLF QDWXUDOJDV¿HOGVLQWKH;X0HPEHUKDYHEHHQGLVFRYHUHG Leikoupo Formation also with a parallel unconformity (Fig. 2). since 1950s (Fig. 2). Reserves of Hechuan, Tongnan and The Sichuan Basin is rich in natural gas resources. Several $Q\XHQDWXUDOJDV¿HOGVDUHDOORYHUî12 m3. Thickness FormationMember Lithology Source rocks Reservoir rocks Seal rocks System Series Formation m Quaternary Quaternary Lushan Tertiary Lower Mingshan Xu6 70-200 Guankou Upper Cretaceous Jiaguan Lower Jiading Penglaizhen First Suining Jurassic Second Shaximiao Third Lianggaoshan Ziliujing Xu5 100-150 First Xujiahe Leikoupo Triassic Second Jialingjiang Third Feixianguan Changxing Upper Longtan Permian Maokou Xujiahe Lower Qixia Liangshan Upper Huanglong Xu4 78-140 Carboniferous Lower Hezhou First Huixingshao Second Hanjiadian Silurian Xiaoheba Third Longmaxi Wufeng First Linxiang Baota Second Xu3 40-80 Ordovician Shizipu Dawan Third Honghuayuan Tongzi First Maotian Houba Pingjing Second Maoping Xu2 75-160 Cambrian Gaotai Longwangmiao Third Lianglangpu Shizhusi Dengying Upper Labagang Sinian Xu1 30-60 Lower Nantuo Pengtuo SandstoneSiltstone Silty mudstone mudstone Unconformity Fig. 2 Stratigraphic column in the Sichuan Basin 3 Distribution characteristics of AB (17 drilled wells across the Anyue, Tongnan and Hechuan “continuous” tight gas reservoirs natural gas fields) of natural gas saturation in “continuous” WLJKWJDVUHVHUYRLUVRIWKH;X0HPEHU7KHUHVXOWVVKRZWKDW We selected a two-dimensional seismic exploration area the natural gas saturation is above 35%, which indicates that (nearly 3,000 km2) which covers the Tongnan, Hechuan, WKHZLGHVSUHDGVDQGVWRQHVRIWKH;X0HPEHUDUHFKDUJHG north of Anyue, east of Hebaochang fields and contains 79 with natural gas (Fig. 4). drilled wells, compiled a natural gas saturation map using the logging data of drilled wells in this area, and corrected 4 Source rocks this map with the thickness of sandstone reservoir obtained by two-dimensional seismic data. The results show that the 4.1 Continuous distribution ZLGHVSUHDGVDQGVWRQHVRIWKH;X0HPEHUDUHFKDUJHGZLWK 7KHGDUNPXGVWRQHVDQGFRDOVRIWKH;X0HPEHU natural gas. The natural gas production is closed to the natural DQG;X0HPEHUDUHVRXUFHURFNVIRU³FRQWLQXRXV´WLJKW gas saturation in the drilled wells. The natural gas saturation JDVUHVHUYRLUVLQWKH;X0HPEHU7KHVRXUFHURFNVDUH of drilled wells in Hechuan and Tongnan areas is up to 70% distributed continuously in this study area. The thickness of (Fig. 3). WKHPXGVWRQHVLQWKH;X0HPEHULQFUHDVHVIURPVRXWKHDVW We also obtained the natural gas saturation of drilled wells to northwest, and the average thickness is about 40 m. The from the well log interpretation, and compiled the well section thickness in the Suining, Lujiaba, and Hebaochang areas is 174 Pet.Sci.(2013)10:171-182 o 40 km 0 1020 30 km 106 15'E Suining Longnüsi HH101 Moxi N110 N1 H101 Tongnan Anyue N107 Hechuan N301 Hc110 H102 Bishan H111 H2 Hebaochang H104 H3 H121 H113 N103 H112 H109 T5 H1 3 H122 H1 H6 H120 H108 M205 T10 Tongnan T4 H117 L1 H123 H107 H103 H5 T3 T1 H118 T6 H4 H11 6 T11 4 T108 T106 H114 T101 T107 H7 T6 H1 T102 H115 Y3 H106 H124 H5 T109 T2 TT111 T105 H105 T104 H110 T11 2 Y10 T11 3 T2 o T11 0 30 02'N WJ1 Hechuan S g, % Y8 T5 A9 T116 68 66 Legend YA2 64 62 60 H110 Well and its YA3 58 daily gas production YA5 56 Y9 54 T1 Well and its Z5 YA1 52 daily water production 3300 50 48 Dazu 46 Place name 44 Tongliang 42 Fig. 3 'LVWULEXWLRQPDSRIWKHQDWXUDOJDVVDWXUDWLRQRI³FRQWLQXRXV´WLJKWJDVUHVHUYRLUVLQWKH;X0HPEHULQWKH$Q\XH+HFKXDQDUHD H1 GR AC H107 H109 B 01020 km GR AC L1 H6 GR AC GR AC T6 H103 GR AC H120 T102 GR AC T105 2110 Ga1 GR AC H115 GR AC 2110 GR AC GR AC T109 GR AC GR AC 2170 A H7 2120 GR AC 2120 2120 T1 Y3 GR AC 2100 2240 2170 2170 2180 2230 GR AC 2280 Y6 CAL R1 2170 2130 2130 2130 AC 2230 2110 GR 2170 2190 2250 2240 2180 2180 2300 2150 2290 2140 2180 2140 2250 2240 2140 2180 2120 2260 2250 2190 2190 2200 2340 2310 2160 2300 2150 2190 2150 2260 2250 2150 2190 2130 2270 2260 2200 2200 2210 2350 2320 2170 2200 2310 2160 2270 2160 2160 2260 2140 2200 2220 2280 2270 2210 2210 2360 2330 2180 2320 2210 2170 2280 2170 2170 2270 2150 2210 2230 2290 2280 2220 2220 2370 2340 2190 2330 2180 2220 2180 2290 2280 2180 2220 2160 2300 2290 2230 2230 2240 2380 2350 2200 2340 2230 2190 2300 2190 2190 2290 2170 2310 2230 2240 2240 2250 2390 2360 2300 2210 2350 2240 2200 2310 2200 2200 2300 2180 2320 2240 2260 2310 2250 2250 2400 2370 2220 2360 2250 2320 2210 2210 2310 2190 2330 2250 2260 2260 2270 2410 2380 2320 2230 2260 2370 2330 2220 2220 2320 2200 2340 2270 2270 2280 2420 2390 2330 2240 2270 2380 2340 2230 2230 2330 2210 2340 2280 2280 2290 2430 2400 2250 2390 2280 2240 2440 2410 2260 18520000 18560000 18600000 18640000 2450 N 2420 Legend (S : saturation of gas) 50 km g A 2460 H109 3340000 2430 H1 H6 B Y6 Tongnan H120 2470 2440 T1 H107 T109 T6 H103 L1 2480 Mudstone 35% Fig. 46HFWLRQ$%RIWKHQDWXUDOJDVVDWXUDWLRQRI³FRQWLQXRXV´WLJKWJDVUHVHUYRLUVLQWKH;X0HPEHU7KHUHGOD\HUV¶ natural gas saturations are greater than 55% and the red layers can produce industrial natural gas the largest and the average thickness is more than 50 m. In in the Hechuan, Tongnan and Anyue areas is the highest and WKH0R[LDUHDWKHDYHUDJHWKLFNQHVVLVDERXWPDQGWKHUH the average thickness is more than 25 m (Fig. 7). is no mudstone in Hechuan, Tongnan areas because erosion occurred in the Late Cretaceous (Fig. 5). The thickness of the 4.2 High natural gas generation intensity FRDOVLQWKH;X0HPEHULQFUHDVHVIURPFHQWHUWRWKHHGJHV Peak gas generation is occurring now because the and the average thickness is about 2.5 m. The thickness in maturity of the source rocks ranges from 1.0% to 1.5% (Huang Suining, Sichuan, Anyue and Hebaochang areas is the largest et al, 2004) which means the organic matter can produce a and the average thickness is more than 3 m. In the Hechuan large amount of natural gas. The high natural gas generation and Tongnan areas, the average thickness is about 1-2 m intensity (average 1,000×106 m3/km2) (Fig. 8) guarantees the )LJ $VHWRIPXGVWRQHVLQWKH;X0HPEHUZKRVH IRUPDWLRQRI³FRQWLQXRXV´WLJKWJDVUHVHUYRLUVRIWKH;X sedimentary facies are underwater distributary interchannel of 0HPEHULQWKHVWXG\DUHD braided river delta front, develop in the whole study area, and The source rocks primarily contain of type III kerogen the average thickness is about 15 m. The mudstone thickness :DQJHWDO DQGDUHGLVWULEXWHGFRQWLQXRXVO\LQ Pet.Sci.(2013)10:171-182 175 o 106 2'E o Suining Longnüsi 106 2'E Longnüsi Lujiaba Suining Lujiaba Moxi Moxi Tongnan Tongnan 30o03'N Anyue Anyue Hechuan Hechuan 30o03'N Hydrocarbon-generating Thickness, m density, 108m3/km2 50 27 45 25 40 23 Dazu Bishan 35 Dazu Bishan 21 30 19 17 Hebaochang Hebaochang 15 0 20 km 13 0 20 km 11 9 Fig. 5 0DSRIWKHWKLFNQHVVRIPXGVWRQHV Fig. 80DSRIWKHJDVLQWHQVLW\RIVRXUFHURFNVLQWKHVWXG\DUHD7KH RIWKH;X0HPEHULQWKHVWXG\DUHD 6 3 2 overall average intensity is 1,000×10 m /km WKHDYHUDJHLQWHQVLW\LQ 6 3 2 o 6XLQLQJ0R[LDQG/RQJQVLQDWXUDOJDV¿HOGVLVDERYHî m /km 106 2'E Longnüsi 6 Suining Lujiaba the average intensity in Hechuan-Tongnan region is also above 1,200×10 m3/km2 Moxi WKLVVWXG\DUHD7KHVRXUFHURFNVRIWKH;X0HPEHUKDYHD Tongnan widespread distribution, high thickness, and high natural gas 30o03'N generation intensity. Therefore, they can provide adequate Anyue natural gas supply for the formation of “continuous” tight gas Hechuan reservoirs in the whole study area. Thickness, m 5 Reservoir rocks 6 4.5 3 1.5 5.1 Low porosity and permeability and volcanic Dazu Bishan 0 debris dissolution pores form the main pore space Hebaochang 7KHVHGLPHQWVRIWKH;X0HPEHULQWKHVWXG\ 0 20 km area belong to the braided delta front in a shallow lake si HQYLURQPHQW =KXHWDO /LHWDO 7KHPLFUR Fig. 60DSRIWKHWKLFNQHVVRIFRDOV sedimentary facies are primarily underwater distributary RIWKH;X0HPEHULQWKHVWXG\DUHD channel, mouth bar and underwater distributary interchannel. o 7KHUHVHUYRLUURFNVDUHODUJHVHWVRIPLGGOH¿QHVDQGVWRQHV 106 2'E Longnüsi Suining Lujiaba and some siltstones which are primarily sublitharenite, feldspathic litharenite and litharenite (Fig. 9). The average porosity of reservoir rocks is 5% to 6%, the logging Darcy Moxi permeability is less than 0.1 mD, and most of the pore Tongnan GLDPHWHUVDUHOHVVWKDQȝP 7DEOH 7KHFRQWHQWRIYROFDQLFGHEULVDQGRWKHUÀH[LEOHGXFWLOH Anyue particles in the sandstone reservoirs is high but early o Hechuan 30 03'N carbonate cements are not observed (Table 2). The diagenesis is strong, including compaction and silica, calcite and Thickness, m illite cementation. Strong compaction caused close contact 35 of sandstone particles, deformation of ductile particles 30 25 and breakdown of rigid particles (Fig. 10(a), (b)). Silica Dazu Bishan 20 15 cementation, calcite cementation and illite cementation (Fig. 10 10(c), (d)) were also key factors for the low porosity and Hebaochang 5 0 permeability of reservoirs. Silica cement is always in the form 0 20 km of authigenic quartz and the content is generally 0.5% to 3.5% Fig. 70DSRIWKHWKLFNQHVVRIPXGVWRQHV or higher, up to 16%. Occurrences of silica cement can be RIWKH;X0HPEHULQWKHVWXG\DUHD the overgrowth of quartz grains and microcrystalline quartz. 176 Pet.Sci.(2013)10:171-182 1 Q 0 Calcite cement is formed in the middle-late A2 diagenesis and 0 0 later than the formation of the membrane of chlorite. 1 Strong compaction and cementation caused the low 2 3 7 5 porosity and permeability of the reservoir rocks, while the 5 2 late dissolution, especially the dissolution occurring in volcanic debris effectively protected the pores and improved 5 0 0 the porosity and permeability of the reservoirs (Fig. 11), 5 which is helpful for the accumulation of “continuous” tight gas reservoirs. This secondary porosity in volcanic debris is 2 the main pore space in the reservoir rocks. 5 5 7 7KHUHVHUYRLUURFNVRIWKH;X0HPEHUKDYHORZ porosity, low permeability and low saturation of free water 4 56 7 0 0 0 (Table 3), which will result in poor potential energy exchange 1 between lower and upper natural gas volumes. The natural F L 0 25 50 75 100 gas reservoirs are thin (3-8 m), so the buoyancy is less than Fig. 9 Sandstone classification ternary diagram. There are 956 samples the capillary pressure (Table 4), meaning that the buoyancy RIWKH;X0HPEHULQWKHJUDSK 4XDUW]DUHQLWH 6XEDUNRVH 3=Sublitharenite, 4=Arkose, 5=Lithic arkose, 6=Feldspathic litharenite, cannot provide enough driving force for gas migration. 7=Litharenite Table 13RURVLW\DQGSHUPHDELOLW\RIUHVHUYRLUURFNVLQWKH;X0HPEHULQWKHVWXG\DUHD Porosity, % Permeability, mD 0HPEHU Average 0D[ 0LQ Number of samples Average 0D[ 0LQ Number of samples ;X 6.21 17.68 0.65 1310 0.07 1.8 0.002 462 Table 2'HEULVRIWKH;X0HPEHULQWKHVWXG\DUHD Rock debris, % Quartz, % Feldspar, % Volcanic 0HWDPRUSKLFURFNV Sedimentary rocks 0DWUL[ rocks Quartzite Phyllite Schist 0XGVWRQH Sandstone Chert 0LFD Content 24-86 0.5-18 5-56 1-35 0.5-1.5 0-1 0-6 0-8 0-5 0-2 0.5-5 Average 58.6 8.7 20.6 6.2 1.4 0.1 1.3 1.2 1.7 0.2 1.5 ȝP (a) ȝP (b) Secondary enlargement of quartz grains Calcite cement (d) ȝP (c) Fig. 106WURQJGLDJHQHVLVRIWKH;X0HPEHULQWKHVWXG\DUHD D + P;X¿QHPHGLXPJUDLQHGOLWKLFVDQGVWRQH E 7P;X PHGLXPFRDUVHJUDLQHGOLWKLFVDQGVWRQH F 7PPHGLXPJUDLQOLWKLFVDQGVWRQHHXKHGUDOTXDUW]¿OOLQJWKHGLVVROXWLRQSRUHVRIFKORULWH G + PFU\VWDOOLQHFDOFLWHFHPHQW¿OOLQJLQWKHIRUPRISODTXH;XPHGLXPJUDLQHGOLWKLFVDQGVWRQH Pet.Sci.(2013)10:171-182 177 100 ȝm (a)100 ȝm (b) 100 ȝm (c) 100 ȝm (d) Fig. 11'LVVROXWLRQSRUHVRIWKH;X0HPEHULQWKHVWXG\DUHD D +P;XPHGLXPJUDLQHG IHOGVSDWKLFOLWKDUHQLWH E +P;XFRDUVHJUDLQHGOLWKDUHQLWH F 7P;XFRDUVH JUDLQHGOLWKDUHQLWH G 7P;XPHGLXPJUDLQHGOLWKDUHQLWH Table 3 :DWHUW\SHVRIUHVHUYRLUURFNVLQWKH;X0HPEHULQWKHVWXG\DUHD Single well Porosity, % Proportion, % Permeability, mD Water type water production, m3 <8 69 >1.1 Adsorbed water<5 8-10 22 0.35-1.1 Bound water 5-15 >10 9 <0.35 Free water >15 5.2 Thick and continuous distribution 6 The relationship between source rocks and As mentioned above, the micro sedimentary facies of reservoir rocks WKH;X0HPEHUDUHSULPDULO\XQGHUZDWHUGLVWULEXWDU\ channel and mouth bar, and some underwater distributary 6.1 Sources rocks are close to reservoir rocks interchannel. Large trough cross bedding (Fig. 12(a)), scour filling structure (Fig. 12(b)) and large sets of thick coarse Braided delta front distributary channels develop generally grain sized sandstones with a bell-shaped log gamma curve LQWKH;X0HPEHULQWKHVWXG\DUHD&KDQQHOVFKDQJHG always develop in underwater distributary channel. Small frequently during the deposition, resulting in interspersed trough cross bedding (Fig. 12(c)), wormhole construction distribution of large sets of sandstones and mudstones (Fig. 12(d)), and small sized sandstones with a funnel-shaped (Fig. 16), which is also advantageous for the formation gamma curve and good sorting (Fig. 13) develop in delta RI³FRQWLQXRXV´WLJKWJDVUHVHUYRLUV2XWFURSVRIWKH;X mouth bars. 0HPEHULQ+XDQJVKLEDQDUHDRIWKH6LFKXDQ%DVLQVKRZWKDW Because the braided channels frequently changed during the large sets of sandstones (thicknesses are mostly 2 m) and deposition, sand lens formed by the multi-channel deposits coals (thicknesses are mostly 0.5 m) are interspersed because overlapped vertically, developing sandstone layers of 10 m the braided rivers changed frequently. Abandoned river or more (Fig. 14). The sandstone layers are distributed sections formed oxbow lakes, in which were deposited a set of continuously in the study area, and are thick (Fig. 15). coal beds and a mudstone cap. After that, the channel changed 7KHUHVHUYRLUURFNVRIWKH;X0HPEHUDUHGLVWULEXWHG again and formed a braided river, eroded the mudstone cap, continuously in the study area, which is favorable for the and the channel sandstones covered the coal bed directly. In formation of “continuous” tight gas reservoirs. planar view, source rocks and reservoir rocks are distributed 178 Pet.Sci.(2013)10:171-182 Table 4&RPSDULVRQRIWKHFDSLOODU\SUHVVXUHDQGEXR\DQF\RIUHVHUYRLUURFNVLQWKH;X0HPEHULQWKHVWXG\DUHD Capillary pressure in mercury Buoyancy is less than the Well Top depth, m Gas reservoir thicknessm BuoyancyPa test03D capillary pressure H6 2195.3 8.1 81×103 0.29×106 Yes H6 2139 3 30×103 1.08×106 Yes H7 2191.2 6 60×103 0.19×106 Yes H7 2169.8 4.6 46×103 0.43×106 Yes T104 2205 8.6 86×103 0.31×106 Yes T107 2250 7.2 72×103 0.2×106 Yes T107 2261 7 70×103 0.23×106 Yes T108 2235 3 30×103 0.4×106 Yes T111 2225.5 8.5 85×103 0.42×106 Yes (a) (b) (c) (d) Fig. 126HGLPHQWDU\IDFLHVRIWKH;X0HPEHULQWKHVWXG\DUHD continuously in the study area. In section, the source rocks displacement pressure when the mercury content is 50% in contact closely with the reservoir rocks. Therefore, the the experiment corresponds to the expulsion pressure (Wang distribution relationship between source rocks and reservoir et al, 2000). According to the study of Southwest Oil and Gas rocks is also helpful for the formation of “continuous” tight Field Company of PetroChina, the average pressure value of gas reservoirs. mercury in the experiment in Hechuan and Tongnan areas is 2 03DZKLFKLQGLFDWHVWKDWWKHDFFXPXODWLRQRI³FRQWLQXRXV´ 6.2 Enough pressure in source rocks and reservoir rocks natural gas pools is controlled by an adequate driving force between the pressures in source rocks and reservoirs. The hydrocarbon generation and differential compaction GXULQJEXULDOSURFHVVRIWKH;XMLDKH)RUPDWLRQLQWKHPLGGOH 7 Physical simulation of “continuous” south transition region resulted in enough pressure in source rocks and reservoir rocks to control the accumulation of natural gas pool ³FRQWLQXRXV´WLJKWJDVUHVHUYRLUV =RXHWDO D E Two sets of “one-dimensional” physical simulation The main accumulation period of “continuous” tight gas experiments were designed in order to further explain that UHVHUYRLUVLQWKH;X0HPEHULV&UHWDFHRXV :DQJHWDO the accumulation of “continuous” tight gas reservoirs is 7KHSUHVVXUHVLQWKH;X0HPEHUDQG;X0HPEHU controlled by the adequate driving force between pressures )LJ LQWKHVWXG\DUHDDUHDOOJUHDWHUWKDQ03DDQG in source rocks and reservoirs rather than buoyancy which PRVWRISUHVVXUHVDUHJUHDWHUWKDQ03D7KHPHUFXU\ controls the accumulation of conventional natural gas pools. Pet.Sci.(2013)10:171-182 179 GR Porosity Rt Depth, m Lithology Porosity Permeability Saturation Production GR(API) AC(us/ft) 5' ȍÚm) 0 200 90 40 2 2000 Log Log Water Gas test 3 Sandstone 4 CAL(in) 14 DEN(g/cm ) 56 ȍ m) porosity permeability saturation production 2 3 2Ú 2000 Coal CN(m3/m3) 05. 0 Mudstone 0 0.18 001. 100 10 2100 Mouth bar 2129-2132m 2138-2147m- 820 /820md m3/d 21402140 Underwater distributary channel r 21802180 e b Xu2 Member 2220 2260 Fig. 13/RJFXUYHVRI+P PRXWKEDUSRURVLW\SHUPHDELOLW\P' P underwater distributary channel, porosity 7%, permeability 0.15 mD The experimental samples were 80-100 mesh glass beads (on JODVVPRGHOZLWKZDWHUWKHQ¿OOHGLWZLWKWKHJODVVEHDGVDQG behalf of conventional natural gas pools) and 250-300 mesh ensured the water saturation was 100%. Second, we added the glass beads (on behalf of “continuous” tight gas reservoirs) simulated overburden pressure with the piston for 10 hours, (Table 5). and then recorded the experimental permeability and charge The physical simulation experiment was performed using pressure data. Third, we injected natural gas into bottom equipment in the Key Laboratory of Petroleum Accumulation of the model for 1 hour, measured the gas input and the of CNPC. The whole equipment includes the increase water output, and calculated the experimental gas saturation extrusion system, the injection system, the “one-dimensional” of the model (equivalent to the overall gas saturation of model system, and the measurement system. The increase the actual pool). The experimental data indicated that the extrusion system is used to add the overburden pressure overall gas saturation of the small beads sample (simulating WKURXJKWKHK\GUDXOLFSXPSRUDLUSXPS WKHLQMHFWLRQV\VWHP a “continuous” natural gas reservoir) was higher than that of LVXVHGWRFKDUJHJDVRLORUZDWHU WKHPRGHOV\VWHPLVXVHG the large beads sample (simulating a conventional natural gas to perform the experiments, including the invisible tube glass pool). PRGHODQGWKHYLVLEOHJODVVPRGHO WKHPHDVXUHPHQWV\VWHP In the second set of experiment, we used visible glass LVXVHGWRPHDVXUHWKHÀRZRIJDVRLODQGZDWHU )LJ model which cannot withstand the overburden pressure, but ,QWKH¿UVWVHWRIH[SHULPHQWZHXVHGWKHLQYLVLEOHWXEH can be observed. Different from the first set of experiment, glass model whose maximum overburden pressure is 40 we measured two samples (80-100 mesh glass beads and 250- 03D:HPHDVXUHGWKHPHVKJODVVEHDGVVDPSOH¿UVW 300 mesh glass beads) at the same time for easy comparison. and then the sample with the smaller 250-300 mesh beads. 7KHVSHFL¿FVWHSVZHUHDVIROORZV)LUVWZH¿OOHGWKHJODVV The specific steps were as follows. First, we filled the tube PRGHOZLWKUHGLQNWKHQ¿OOHGLWZLWKEHDGVDQGHQVXUHGWKH 180 Pet.Sci.(2013)10:171-182 5m 5m Fig. 14+XDQJVKLEDQ2XWFURSRIWKH;X0HPEHULQ:HL\XDQDUHDWKHPLGGOHVRXWKWUDQVLWLRQUHJLRQ 106o22'E Suining Lujiaba Longnüsi Sandstone Moxi Mudstone Tongnan 30o03'N Anyue Sandstone Hechuan Mudstone Thickness, m Sandstone Bishan 130 Dazu 120 110 1 m Hebaochang 100 0 20 km 90 80 Fig. 16+XDQJVKLEDQ2XWFURSRIWKH;X0HPEHULQ:HL\XDQ area, the middle-south transition region Fig. 150DSRIWKHWKLFNQHVVRIUHVHUYRLUURFNVRI 106o22'E WKH;X0HPEHULQWKHVWXG\DUHD Longnüsi Suining Lujiaba water saturation was 100%. Second, we packed the glass beads for 10 hours, injected gas into the bottom of the model for 1 hour, and then observed the color change and the gas Moxi migration within the glass model. One hour after the gas injection, the migration process was completed. We found that Tongnan o the natural gas of “continuous” tight gas reservoirs (simulated 30 03'N by 250-300 mesh glass beads) moved up with no preferential Anyue SHWUROHXPPLJUDWLRQSDWKZD\V 3303 )LJ D DWDUDWH Hechuan of 0.3 cm/min and the gas saturation was greater than 70%. Comparatively, the natural gas in the 80-100 mesh glass beads (simulating a conventional natural gas pool) moved up Pressure difference, ZLWKD¿[HG3303 FPZLGH LQWKHFHQWHURIWKHFROXPQ MPa (Fig. 19(b)) at a rate of 0.8 cm/min and the gas saturation was 6 Dazu Bishan 5 less than 30%. 4 Because the permeability of the “continuous” natural gas 3 Hebaochang 2 reservoir model was much less than that of the conventional 0 20 km natural gas pool model, the gas moved up through the column much more slowly. However, all the small beads inside the column were a natural gas-saturated pathway, and the pores Fig. 17 Pressure in source rocks and reservoir rocks in the study area between the beads could capture the gas, which resulted in high gas saturation (70%). Conversely, the accumulation was higher, the natural gas moved up much more quickly. of simulated conventional natural gas pool was controlled +RZHYHURQO\WKHSRUHVRIEHDGVLQWKH3303FRXOGFDSWXUH E\WKHEXR\DQF\2QFHWKH3303ZDVIRUPHGWKHUHVLGXDO the gas rather than the whole section, which resulted in a low natural gas changed the wettability of the beads’ surface. gas saturation (30%). :KDWLVPRUHWKH3303ZDVLQIDFWWKHPRVWFRQGXFLYHIRU PLJUDWLRQVRWKH3303UHPDLQHGVWDEOHGXULQJWKHHQWLUH 8 Conclusions migration experiment. 7KHUHVHUYRLUVRIWKH;X0HPEHULQWKHPLGGOHVRXWK Because the permeability of conventional gas pools transition region are typical “continuous” tight gas reservoirs. Pet.Sci.(2013)10:171-182 181 Table 5&RPSDULVRQRISDUDPHWHUVRIH[SHULPHQWDOVDQGDQGUHVHUYRLUURFNVLQWKH;X0HPEHU ǂ 80-100 mesh glass beads 250-300 mesh glass beads 5HVHUYRLUURFNVRI;X0HPEHU Densityg/cm3 1.6 1.6 1.02 Porosity, % 33.2 31.5 6.25 Permeability, mD 185.7 4.3 0.07 Critical injection pressure03D 0.01 0.03 3.5 z1 z2 z3 z4 z5 z6 Hydraulic pump Oil injection Water injection Water Other injection z1 2 z3 4 z5 6 Gas flow meter Injection system Fluid-adding system Piston Two- One- Extrusion-adding system dimensionalҼ㔤⁑ර dimensional model model Three-phase metering Air pump Manual pump Back pressure Camera equipment valve Model system Metering system Fig. 18([SHULPHQWDOÀRZFKDUW The high natural gas generation intensity (average 1,000×106 m3/km2) guarantees the formation of “continuous” natural gas UHVHUYRLUVRIWKH;X0HPEHULQWKLVDUHD 7KHUHVHUYRLUURFNVSULPDULO\ODUJHVHWVRIPHGLXP¿QH VDQGVWRQHV DYHUDJHWKLFNQHVVLVP RIWKH;X0HPEHU are also thick and are distributed continuously in this study DUHD7KHDYHUDJHSRURVLW\RIWKH;XUHVHUYRLUURFNVLV to 6%, and the logging Darcy permeability is less than 0.1 P'7KHVHGLPHQWDU\IDFLHVRIWKH;X0HPEHUDUHEUDLGHG (a) (b) delta fronts in shallow lakes. Channels changed frequently during the deposition, resulting in interspersed distribution of Fig. 19 Photos of the second set of experiment (one hour after gas large sets of sandstones and mudstones, which is helpful for LQMHFWLRQ D ³&RQWLQXRXV´WLJKWJDVUHVHUYRLUPHVKQR3303 forming “continuous” tight gas reservoirs. The low porosity, pressure difference between source rocks and reservoir rocks is the driving permeability and free water saturation of the reservoir rocks force for the migration. (b): Conventional natural gas reservoir, 80-100 mesh, VWDEOH3303WKHEXR\DQF\LVWKHGULYLQJIRUFHIRUWKHPLJUDWLRQ result in poor potential energy exchange between lower and upper natural gas layers, and the natural gas layers are thin, Source rocks are primarily coals (average thickness is 2.5 so the buoyancy is less than the capillary pressure, meaning P DQGPXGVWRQHV DYHUDJHWKLFNQHVVLVP RIWKH;X that the buoyancy cannot provide enough driving force for 0HPEHUDQGVRPHPXGVWRQHVRIWKH;X0HPEHU DYHUDJH gas migration. The gas accumulation of “continuous” tight thickness is 12 m). Source rocks primarily contain type III gas reservoirs is controlled by an adequate driving force kerogen, and are distributed continuously in this study area. between the pressures in source rocks and reservoirs, which Peak gas generation is occurring at the present day because is demonstrated by a “one-dimensional” physical simulation the maturity of the source rocks ranges from 1.0% to 1.5%. experiment of “continuous” tight gas reservoirs. 182 Pet.Sci.(2013)10:171-182 Acknowledgements 6FKPRNHU-:86*HRORJLFDOVXUYH\DVVHVVPHQWPRGHOIRUFRQWLQXRXV XQFRQYHQWLRQDO RLODQGJDVDFFXPXODWLRQV²WKH³)2563$1´ 7KHVWXG\ZDVVXSSRUWHGE\WKH1DWLRQDO0DMRU*UDQWRI model. U.S. Geological Survey Bulletin. 1999. 2168(9) “Accumulation Law, Key Technologies and Evaluations of the 6FKPRNHU-:5HVRXUFHDVVHVVPHQWSHUVSHFWLYHVIRUXQFRQYHQWLRQDO 6WUDWLJUDSKLF5HVHUYRLUV´ 1R=; IURPWKH gas systems. AAPG Bulletin. 2002. 86(11): 1993-2000 5HVHDUFK,QVWLWXWHRI3HWUROHXP([SORUDWLRQ 'HYHORSPHQW 6FKPRNHU-:86JHRORJLFDOVXUYH\DVVHVVPHQWFRQFHSWVIRU continuous petroleum accumulations. U.S. Geological Survey Digital 3HWUR&KLQD:HWKDQN55% 3340000 Ga1 H7 g g g Anyue Y3 T102 H115 T105 2490 Hechuan 18520000 18560000 18600000 18640000