Control of hydrocarbon accumulation by Lower Paleozoic cap rocks in the Tazhong Low Rise, Central Uplift, Tarim Basin, West China

Petroleum Science, Jan 2014

Despite the absence of regional cap rocks in the Lower Paleozoic for the entire Tazhong Low Rise, several sets of effective local cap rocks are well preserved on the Northern Slope. Of these the best is the Ordovician mudstone of the Sangtamu Formation; the second is the Silurian Red Mudstone Member of the Tatairtag Formation and the marl of the Ordovician Lianglitag Formation; and the third is the gray mudstone of the Silurian Kepingtag Formation. The dense limestone of the Ordovician Yingshan Formation and the gypsum of the Middle Cambrian have shown initial sealing capacity. These effective cap rocks are closely related to the distribution of Lower Palaeozoic hydrocarbons in the Tazhong Low Rise. With well-preserved Sangtamu Formation mudstone and its location close to migration pathways, rich Lower Paleozoic hydrocarbon accumulation can be found on the Northern Slope. Vertically, most of the reserves are distributed below the Sangtamu Formation mudstone; areally, hydrocarbons are mainly found in the areas with well-developed Sangtamu Formation mudstone and Lianglitag Formation marl. Burial history and hydrocarbon charging history show that the evolution of Lower Palaeozoic cap rocks controlled the accumulation of hydrocarbon in the Tazhong Low Rise. Take the Red Mudstone Member of the Tatairtag Formation and Sangtamu Formation mudstone for examples: 1) In the hydrocarbon charging time of the Late Caledonian — Early Hercynian, with top surfaces at burial depths of over 1,100 m, the cap rocks were able to seal oil and gas; 2) During the intense uplifting of the Devonian, the cap rocks with top surfaces at burial depths of 200–800 m and 500–1,100 m respectively were denuded in local areas, thus hydrocarbons trapped in earlier time were degraded to widespread bitumen; 3) In the hydrocarbon charging time of the Late Hercynian and Himalayan, the top surfaces of the cap rocks were at burial depths of over 2,000 m without intense uplifting and denudation thereafter, so trapped hydrocarbons were preserved. Based on cap rocks, the Ordovician Penglaiba Formation and Lower Cambrian dolomite could be potential targets for exploration on the Tazhong Northern Slope, and combined with hydrocarbon migration, less risk would be involved.

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Control of hydrocarbon accumulation by Lower Paleozoic cap rocks in the Tazhong Low Rise, Central Uplift, Tarim Basin, West China

Pet.Sci. Control of hydrocarbon accumulation by Lower Paleozoic cap rocks in the Tazhong Low Rise, Central Uplift, Tarim Basin, West China Zhang Yanping 0 1 Lü Xiuxiang 0 1 Yang Haijun Han Jianfa Lan Xiaodong 0 1 Zhao Yue Zhang Jinhui 2 0 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum , Beijing 102249 , China 1 College of Geosciences, China University of Petroleum , Beijing 102249 , China 2 CNOOC Tianjin Branch Company , Tianjin 300452 , China Despite the absence of regional cap rocks in the Lower Paleozoic for the entire Tazhong Low Rise, several sets of effective local cap rocks are well preserved on the Northern Slope. Of these the best is the Ordovician mudstone of the Sangtamu Formation; the second is the Silurian Red Mudstone Member of the Tatairtag Formation and the marl of the Ordovician Lianglitag Formation; and the third is the gray mudstone of the Silurian Kepingtag Formation. The dense limestone of the Ordovician Yingshan Formation and the gypsum of the Middle Cambrian have shown initial sealing capacity. These effective cap rocks are closely related to the distribution of Lower Palaeozoic hydrocarbons in the Tazhong Low Rise. With well-preserved Sangtamu Formation mudstone and its location close to migration pathways, rich Lower Paleozoic hydrocarbon accumulation can be found on the Northern Slope. Vertically, most of the reserves are distributed below the Sangtamu Formation mudstone; areally, hydrocarbons are mainly found in the areas with well-developed Sangtamu Formation mudstone and Lianglitag Formation marl. Burial history and hydrocarbon charging history show that the evolution of Lower Palaeozoic cap rocks controlled the accumulation of hydrocarbon in the Tazhong Low Rise. Take the Red Mudstone Member of the Tatairtag Formation and Sangtamu Formation mudstone for examples: 1) In the hydrocarbon charging cap rocks were able to seal oil and gas; 2) During the intense uplifting of the Devonian, the cap rocks with top surfaces at burial depths of 200-800 m and 500-1,100 m respectively were denuded in local areas, thus hydrocarbons trapped in earlier time were degraded to widespread bitumen; 3) In the hydrocarbon charging time of the Late Hercynian and Himalayan, the top surfaces of the cap rocks were at burial depths of over 2,000 m without intense uplifting and denudation thereafter, so trapped hydrocarbons were preserved. Based on cap rocks, the Ordovician Penglaiba Formation and Lower Cambrian dolomite could be potential targets for exploration on the Tazhong Northern Slope, and combined with hydrocarbon migration, less risk would be involved. Cap rock; hydrocarbon accumulation; hydrocarbon destruction; Lower Paleozoic; Tazhong Low Rise - Cap rocks play an important role in oil and gas preservation in hydrocarbon-bearing areas with strong tectonic activities. For instance, although the Papuan Fold & Thrust Belt of the Papua Basin experienced strong compression and deformation with plenty of seeps, over two billion barrels of recoverable oil equivalent were discovered; the biggest Hides field had over 1,800 m gas column height, and the key is the sealing and plastic adjustment from Cretaceous mudstone cap rocks with around 1,000 m thickness (Hill, 1991; Hill et al, 2004) . The Maracaibo Basin was located on the edge of plate convergence and experienced complex tectonic activities, with many oil seeps surrounding the basin. Still 44 billion barrels of recoverable oils were preserved, mainly owing to multiple sets of mudstone cap most productive in China, is located in the highly deformed thrust belt of the Tarim Basin, and its formation is mainly due to high-quality Eogene evaporite regional cap rocks (Jia and Li, 2008) . The Tarim Basin, a typical superimposed basin with long-term evolution in west China, experienced multi-phase tectonic movements, multi-phase hydrocarbon expu lsion (Gong et al, 2007 ; Zhang et al, 2011c; Tian et al, 2012), multiphase oil & gas accumulation (Li et al, 1996; Jin and Wang, 2004; Meng et al, 2008; Pang et al, 2010) , and multiple adjustment and destruction of hydrocarbon reservoirs ( Lü et al, 1997 ). The Tazhong Low Rise is one of the petroliferous areas in the Tarim Basin, and experienced complex processes of hydrocarbon accumulation, adjustment and destruction (Jiang et al, 2008; Pang et al, 2013) , thus research on cap rocks and preservation of hydrocarbons is important. However, the previous research on petroleum geology in the Tazhong Low Rise focused on source rocks and reservoir beds (Zhang et al, 2000; Li et al, 2009; Ding et al, 2012) , and the Lower Palaeozoic cap rocks were less studied. In addition, the oil and gas discovered in the Tazhong Low Rise were mostly located in the Lower Palaeozoic, thus the relationship of hydrocarbon accumulation with the Lower Palaeozoic cap rocks can be instructive to exploration in the Tazhong Low Rise. ZG1T6a3zZhG-5o010n05g TNZ4o7-5000 ZZ-55GG00Z1T23G1Z3152ZG11 CentraNl-o40-.302500-04-45000 Tazhong F00aultFault -3500 Be-l3t500 ZG17 TZ86 TZ451 ZG16 TZ45 ZG14 2 Geologic setting 2.1 Geologic structures in the Tazhong Low Rise The Tazhong Low Rise with an area of 27,500 km2 is located in the heartland of the Central Uplift of the Tarim Basin, and is adjacent to the Manjiaer Sag in the northeast with the Tazhong No.1 Fault as its border, the Awati Sag in the northwest, the Bachu Salient in the west with the Tumuxiuke Fault as borders, the Tangguzibasi Sag in the south and the Tadong Low Rise in the east (Fig. 1). Structural research shows that the Tazhong Low Rise with the eastern structure higher than the western one at present is a large complete NWW trending anticline structure with several secondary structural zones. From north to south, several fault belts were developed, namely the Tazhong No.1 Fault Belt, Tazhong No.10 Fault Belt, Tazhong No.2 Fault Belt, and the Tazhong Southern Fault Belt. In the plane, the fault systems spread to the west and converge to the east near the TZ5 well block (Fig. 1). The Tazhong Low Rise is a pre-Carboniferous uplifted 0 100 200km Akesu Kuqa Depression Korla Tabei Uplift Kashi Southwest BDaecphCureHesensSttiioraaaAnnlilweNnatotirSthaegTranzLThoaownTngZgR4gi5suezDiTbeZa8TpMs2TZri2aae6SnnsajasiganiUoernpSlifatgTDaUdepoplnirfegtsLsoiownRise Southeast N Boundary of tectonic unit Study area -5500 -6000 -6500 0 10 20km N TZ82 TZ83 TZ721 TZ726 -4500 TZ722 TZ724 TZ62 TZ16T2Z16 B’ TZ75 -3000 -3500 TZ-4204-452-5000 TC1 00 00 30 -50 -3000 -3500 TZ103 T-3Z50-4200040 -300 TZ8 TZ1 -3000 0 -4000 Tazho-30n00g1-8 Faul-3t500 Belt -5000 -4Z500honZgh3ongw3ell Fault TBZTe3Zl5Tt2azhong-335-3-04-00005000 Fau-l4t500Belt -2500 -5000 -4-0405000 TZ5 -3000 TZ26 Zhong4 ZG102 ZG5 ZG6 TZ82 TZ83TZ721TZ722SEB’ structure in the Paleozoic craton basin of the Tarim Basin. Tectonic activities on the southern basin margin played an important role in controlling the formation and development o f Ta z h o n g s t r u c t u r e s . A c c o r d i n g t o s t r a t i g r a p h i c development, structural deformation, unconformities and regional geology, the structural evolution in the Tazhong Low Rise can be divided into four stages (Jia, 1997) : 1) Formation of rudimental structure (Sinian-Ordovician); 2) Thrust, strike-slip structural deformation and structural finalization (Silurian-Devonian); 3) Development of giant nose-shaped uplift (Carboniferous-Permian); 4) Stable subsidence and uplifting (Triassic-Quaternary). 2.2 Reservoir bed–seal combination in the Lower Paleozoic The Silurian, Ordovician and Cambrian are developed in the Lower Paleozoic in the Tazhong Low Rise (Fig. 2). In the Silurian, sandstones and mudstones were deposited in the Tatairtag Formation (S2t) and Kepingtag Formation (S1k). The lower member of S2t is called Red Mudstone Member (S2t-rmm) and is a premium cap rock for the Silurian oil and gas. The upper member of S1k is further divided into 1st, 2nd and 3rd sub-members from top to bottom, the upper 2nd sub-member consists of gray mudstones and is called Gray Mudstone Sub-member (S1k-gms) while the upper 1st and 3rd sub-members are both mainly composed of sandstone, thus the S2t-rmm can seal the upper 1st sub-member of S1k while the S1k-gms can seal the upper 3rd sub-member of S1k. In the Ordovician, the Sangtamu Formation (O3s) mainly consists of mudstone, while the underlying Lianglitag (O3l), Yingshan (O1y) and Penglaiba Formations (O1p) mainly consist of carbonate rocks. O3s is a premium cap rock for Ordovician hydrocarbons and direct cap rock for the hydrocarbons in O l 3 limestone in the 3rd to 5th members of O3l could be cap rock for the underlying weathering crust karst reservoir beds along the top part of O1y; the thick dense limestone without weathering and karstification in the O1y is cap rock for underlying O1p hydrocarbons. In the Cambrian ( ) with upper dolomite member, gypsum member and lower dolomite member, the gypsum member can seal hydrocarbon in the lower dolomite member. In the structural highs of the Tazhong Low Rise, the Carboniferous directly overlay the Ordovician (such as in the TZ2 well), and even the Cambrian (such as in the TZ1 well) as the result of strong denudation. 3 Features of Lower Paleozoic cap rocks 3.1 Red Mudstone Member in the Silurian (S2t-rmm) The S2t-rmm mudstone is a set of cap rocks widely distributed in the Tazhong Low Rise, Manjiaer Sag and the southwest area of the Tabei Uplift. The S2t-rmm in the Tazhong Low Rise mainly consists of tidal flat facies brown mudstone, with an average total thickness of 70 m (29-109 m) and single layer thickness of 15-80 m. Areally, the S2t-rmm mudstone gradually becomes thicker from southeast to northwest, but was completely denuded at the east end of the Tazhong Low Rise and the Central Fault Horst Belt because of uplifting (Fig. 3). It is in sub-stage A of the middle diagenetic stage, because authigenetic clay minerals in the TZ23 well at 4,774 m near the bottom of S2t-rmm are dominated by 75% illite-smectite (I/ S), 15% illite and 10% kaolinite, and 20% smectite in I/S (S%) R o) is 0.9%-1.3%. Its breakthrough pressure ranges from 15.1 MPa to 25.1 MPa in four testing samples saturated with water (Wang et al, 2004) . 3.2 Gray Mudstone Sub-member in the Silurian (S1k-gms) S1k-gms in the Tazhong Low Rise is mainly composed of tidal flat facies gray mudstone, with an average total thickness of 18 m (8-35 m) and single layer thickness of 1-10 m, and thinner than the S2t-rmm. Areally, S1k-gms mudstone also gradually becomes thicker from south to north, but completely eroded at the east end of the Tazhong Low Rise and the Central Fault Horst Belt due to uplifting (Fig. 4). It is also in sub-stage A of the middle diagenetic stage, because authigenetic clay minerals of the TZ37 well at 4,679.93 m near the bottom of S1k-gms are dominated by 78% illite–smectite, 15% illite, 4% kaolinite and 3% chlorite, and 25% smectite in I/S (S%) (Zhang et al, 2011c) . The breakthrough pressures are more than 15.1 MPa in all four testing samples saturated with water (Wang et al, 2004) , thus it is able to seal the 3rd sub-member of S1k (Lü et al, 2007). 3.3 Mudstone of Ordovician Sangtamu Formation (O3s) Previous research shows that the Lower Mudstone Member of the Carboniferous is a good regional cap rock in the Tarim Basin, which covers a giant hydrocarbon accumulation system. However, exploration progress shows that around 80% of oil and gas reserves accumulate below O s mudstone, thus it actually plays the most important role 3 in sealing oil and gas in the Tazhong Low Rise. During the deposition of O3s with subsidence of the entire Tazhong area, the thick mudstone of shelf slope and deep water basin facies was deposited with an average total thickness of 570 m (100-1,100 m) and single layer thickness of 50-130 m. Areally, O3s mudstone gradually becomes thicker from the Central Fault Horst Belt to both sides with broad lateral distribution, but completely denuded in the Central Fault Horst Belt and the east end of the Tazhong Low Rise due to uplifting (Fig. 5). The displacement pressures of O s mudstone range from 6.7 MPa to 37.1 MPa with an 3 average of 15.3 MPa (Zhu et al, 2003) . According to statistics from 114 wells in the Tazhong Low Rise, the bottom depth of O3s ranges from 3,825 m to 6,830 m with an average of 5,200 m; the 3,682-4,500 m of O3s in the TZ28 well has Ro values of 1.03%-1.32%; the 5,415-5,420 m of O3s in the TZ35 well has the Tmax of 445-455 °C, while the 4,917-5,101 m of TZ10 well has the Tmax of 428-443 °C. The clay minerals of Zhong13 well at 5,219 m in the O3s are dominated by 78% illite, 5% kaolinite, 15% chlorite and 2% illite–smectite (I/S), and 15% (S%) in I/S (Qian et al, 2012) . Formation system Sys- Series Formation tem Member Devonian-Carboniferous S3 Yimugantawu Upper mud member S O S2 S1 O 3 O 1 3 2 1 Sangtamu Fm. (O3s) Tatairtag Kepingtag Lianglitag (O3l) Yingshan (O1y) Sandy mudstone member Red mud member (S2t-rmm) Upper 1st sub-member Upper 2nd sub-member (S1k-gms) Upper 3rd sub-member 1st member 2nd member 3.4 Marl of Ordovician Lianglitag Formation (O l) 3 O s mudstone is the direct cap rock for oil and gas in the 3 underlying O3l. In addition, hydrocarbon accumulation in reservoir beds of weathering crust karst was discovered in O y below O3l. However, O y in most parts of the Tazhong 1 1 Low Rise is far below O3s mudstone at more than a few rd hundred meters, thus dense marl in the 3 to 5 th members of O l directly seals the hydrocarbons in O y. 3 1 rd The dense marl in the 3 to 5 th members of O l is a set 3 of reef-flat complex sediments on a carbonate platform– shelf margin with an average total thickness of 220 m (100400 m) and single layer thickness of 5-80 m. Areally, it gradually becomes thicker from the Central Fault Horst Belt to both sides with wide lateral distribution, but thinner in the th ZG15-ZG24 well blocks with the 4 and 5 th members of O l 3 missing. In addition, it was also completely eroded in the Central Fault Horst Belt and the east end of the Tazhong Low Rise due to uplifting (Fig. 6). According to testing of dense marl cap rocks in 13 wells, no breakthrough occurred in 85% 40km N of samples after 48 hours under 14 MPa applied maximum pressure, while the two remaining samples respectively had the breakthrough pressure of 7 MPa and 10 MPa (Table 1). According to statistics from 38 wells in the Tazhong Low Rise, the bottom depth of O3l ranges from 4,033 m to 7,102 m with an average of 5,474 m. The homogenization temperatures of hydrocarbon inclusions range from 67 °C to 125 °C in the 3,976-6,099 m of O3l from the TZ6, TZ12, TZ45 wells (Chen et al, 2010) . According to PVT reports of 28 wells in the Tazhong Low Rise, the current formation temperatures of the 4,387-6,500 m in O3l range from 126 °C equivalent (VRE) values of O3l range from 0.8% to 1.3% in the Zhong12, Zhong13, TZ10, TZ12 wells (Wang et al, 2001) . O3l in most areas of the Tazhong Low Rise is in sub-stage A2 of the middle diagenetic stage, while in a few areas with deeper burial it entered sub-stage B of the middle diagenetic stage. Thus the dense marl from the 3rd to 5th members of O3l can seal the underlying hydrocarbons of O1y. 3 . 5 D e n s e l i m e s t o n e o f O r d o v i c i a n Yi n g s h a n Formation (O1y) A large amount of drilling and seismic reservoir prediction ZG2 ZG5 TZ61 0 10 20 30 40km ZG1Z0G102ZG501 ZG2 ZG5ZG930Z0G1 TZ822 ZG6 TZ828 TZ82 300ZG7 TZ825 ZG42 TZ80 TZ82310 0 0 0 TZ721 400 TZ7T22ZT7Z26621 TZ61 TZ115TZ162T20TZ0Z4462 data in the Tazhong Low Rise show that good carbonate reservoir beds of weathering crust karst in O1y mainly developed below the top surface of O y within 120 m, with a 1 depth of 180 m in local areas (Ji et al, 2012) . Thus the thick dense limestone below the karst reservoir bed can seal the underlying hydrocarbons of O1p. Due to uplifting and denudation before the deposition of O3l, most areas of the Tazhong Low Rise lost the 1st and 2nd members of O1y and the 3rd and 4th members of O1y remained, and only platform margin and slope areas had relatively wellpreserved O1y. O1y is partially preserved and becomes thicker from Tazhong No.2 Fault to Tazhong No.1 Fault (Fig. 7). For instance, in the TZ162 well near the Tazhong No.1 Fault O y 1 has a thickness of 703 m, while in the TZ75 well near the Tazhong No.2 Fault O y thins to 202 m. Thus the northeast 1 part of the Tazhong Northern Slope has thicker limestone in O1y without weathering and karstification, where the hydrocarbons of O p could be better sealed. 1 O y consists of limestone, micrite and dolomite of open 1 platform facies, but the relevant characteristics of the dense limestone cap rock cannot be evaluated currently, because there are few wells penetrating through O1y. Notes: Above testing was completed by the laboratory of Langfang Branch, Research Institute of Petroleum Exploration and Development, CNPC TZ201 ZG9 ZG1 NE Tazhong No.1 Fault The 1st and 2nd sections The 3rd section The 4th section Penglaiba Fm. Yingshan Fm. Fault Unconformity Fig. 7 Distribution of the Middle-Lower Ordovician on the Tazhong Northern Slope (see Fig. 6 for location) 3.6 Gypsum member of the Cambrian The Middle Cambrian gypsum member acts as a good regional cap rock in the central-west area of the Tarim Basin, and was penetrated in the Tazhong Low Rise and Bachu Salient. In the Bachu Salient, gypsum-salt rocks of evaporite lagoon facies with a thickness of over 350 m developed at 3,822-4,379 m of the Fang-1 well and at 5,104-5,792 m of the He-4 well. In the Tazhong Low Rise, only the TC1 well penetrated through the Cambrian in the east of the Tazhong Low Rise, and the Middle Cambrian at 6,958-7,058 m is mainly composed of gypseous dolomite, dolomitic gypsum, dolomite and thin bedded gypsum, with no salt rock. Seismic data (Tang et al, 2012) and the TC1 well data indicate that the Middle Cambrian gypsum member is preserved in the Central Fault Horst Belt and the east end of the Tazhong Low Rise where the Lower Palaeozoic was denuded, thus the gypsum member might be present in the entire Tazhong Low Rise and becomes better cap rock from east to west according to sedimentary characteristics (Zhang et al, 2012) . 3.7 Evaluation of cap rocks in the Tazhong Low Rise The macroscopic and microscopic evaluation criteria of cap rocks in the Tazhong Low Rise are summed up according to previous research (Table 2). Based on characteristics of Lower Palaeozoic cap rocks in the Tazhong Low Rise, the best cap rock is the O s mudstone; secondly, the mudstone of 3 S2t-rmm and the marl in the 3rd to 5th members of O3l; lastly, the S1k-gms mudstone. The dense limestone of O y and the 1 gypsum member of Middle Cambrian ( 2) preliminarily show sealing capacity and their regional distribution is waiting for further study (Table 3). 4 Relationship of hydrocarbon accumulation and Lower Paleozoic cap rocks have been discovered in the sandstone reservoir beds of S1k, Class 4 Fluvial facies, alluvial fan facies Argillaceous siltstone, argillaceous sandstone Late diagenetic stage <2.5 <50 <5 Macro: Class 2-3 Micro: Class 1 Sedimentary environment Lithology Diagenetic stage Single layer thickness, m Cumulative thickness, m Breakthrough pressure, MPa Class 1 facies, basinal facies, open sea shelf facies Gypsum-salt rock mudstone, calcareous mudstone Class 2 Platform facies lagoonal facies, inshore shallow lake facies, delta front sub-facies Mudstone containing sand, mudstone containing silt Sub-stage A of the middle diagenetic stage Sub-stage B of the early diagenetic stage >20 >300 >15 10-20 150-300 15-10 the reef-flat complex reservoir beds of O3l, the weathering crust karst reservoir beds of O1y and the carbonate karst reservoir beds of O1p. Around 80% of reserves in the Tazhong Low Rise are found in O3l and O y below O s mudstone cap 1 3 rocks. Previous research shows that there were at least three periods of large-scale hydrocarbon accumulation, including Himalayan (Zhang et al, 2011a; Pang et al, 2013) . 4.1 Hydrocarbon accumulation versus Silurian mudstone cap rock Dry bitumen, asphalt, heavy oil and light oil coexist in the upper 1st and 3rd sub-members of S1k below S2t-rmm, indicating that the sub-members experienced multiple phases of hydrocarbon charging and complex processes of Macroscopic feature of cap rocks Microscopic feature of cap rocks Evaluation and an average of 70 m, single layer thickness of 15-80 m, in sub-stage With breakthrough pressure of 15.1-25.1 MPa A of the middle diagenetic stage an average of 18 m, single layer thickness of 1-10 m, in sub-stage A of the middle diagenetic stage Mudstone of shelf slope facies and deep water basin facies, with a thickness of 100-1,100 m and an average of 570 m, single layer thickness of 50-130 m, in sub-stage A2 of the middle diagenetic stage With breakthrough pressure of more than 15.1 MPa Macro: Class 3 Micro: Class 1 With displacement pressure of 6.7-37.1 MPa and an average of 15.3 MPa Macro: Class 1 Micro: Class 1 accumulation and loss. Bitumen is widely distributed in the upper 1st and 3rd submembers of S1k in the Tazhong Low Rise (Fig. 3). It was Early Hercynian but degraded by Devonian uplifting (Zhang et al, 2004; 2011c) . Almost all the Silurian bituminous sandstones are distributed below the S2t-rmm cap rock, but there is no bitumen in the Silurian sandstone above the Red Mudstone Member cap rock except four wells including TZ10 well due to connection of faults (Zhang et al, 2004). This indicates that S2t-rmm cap rock possessed sealing capacity during the early period of hydrocarbon charging and controlled the vertical distribution of hydrocarbons. In the Silurian in the Tazhong Low Rise, there are five oil and gas reservoirs (Fig. 3), all in the upper 1st and 3rd submembers of S1k below S2t-rmm cap rock, while there is no hydrocarbon accumulation in Silurian sandstones above S2trmm cap rock (Fig. 8). This indicates that cap rock from S2t-rmm still controlled vertical distribution of Silurian hydrocarbons charged in late periods. In addition, Silurian movable oil is mainly found in the upper 3rd sub-member of S1k below the direct cap rock from S1k-gms, because the upper 3rd sub-member of S1k is the oil-bearing formation of st sub-member of S1k is the oil-bearing formation of only two reservoirs. The reservoir bed quality of the upper 3rd sub-member of S1k is better than the upper 1st sub-member of S1k. Meanwhile, the direct cap rock of the upper 3rd sub-member of S1k, that is S1k-gms possessed sealing capacity during the late period of hydrocarbon charging. 4.2 Hydrocarbon accumulation versus O3s mudstone cap rock Ordovician oil and gas exploration is mainly on the Tazhong Northern Slope. There is a large oil and gas accumulation in O3l with a superimposed petroliferous area of over 1,000 km2 including natural gas, condensate and normal crude oil, whose principal payzones are limestone reservoir beds of the 1st to 3rd members of O3l reef-flat complex of platform margin facies, while its direct cap rock is the O s 3 mudstone. Meanwhile, the O s mudstone also plays the role 3 of local cap rock in the Tazhong Low Rise, thus about 80% of Tazhong Low Rise’s reserves are concentrated below O3s. The locations of O3l hydrocarbon reservoirs correspond well with areas of thick O s 3 hydrocarbons accumulate in the area where the mudstone cap rocks have a thickness of more than 300 m (Fig. 5); secondly, the thinner the mudstone cap rocks from northeast to southwest, the more the low productivity wells (such as the TZ15 well) and hydrocarbon show wells (such as the TZ23 and the TZ42 well) were drilled. In addition, on the top and both sides of the Central Fault Horst Belt with missing mudstone cap rocks, a number of dry wells and a few wells with only hydrocarbon shows were drilled. Therefore, the abundance of O3l hydrocarbons correlates with the thickness of overlying mudstone, although source rock, structural configuration, migration and reservoir bed quality are also that the sealing of thick O3s mudstone controlled regional distribution of underlying oil and gas. 4.3 Hydrocarbon accumulation versus O3l marl cap rock In the Tazhong Low Rise, the weathering crust karst carbonate oil and gas reservoirs in O y have a superimposed 1 petroliferous area of 3,000 km2 (Fig. 6). The marl of the 1st to 3rd members of O3l acts as cap rock for underlying O1y, and also controls hydrocarbon accumulation in O1y. 45T84Z47 4437TZ10 417T5Z11 407T6Z12 407T3Z50 412T8Z15 397T8Z44 39T6Z4169 39T08Z161 38T8Z836 Strata 4783T.5Z45 u w a t n a g u m i Y 4895 r e b m e m g r ittraa ppeU a T re reb rmm) 5106 ow em ttL m (S2 5175 s r 4684 4820.5 4888 Bitumen and heavy oil are widespread in O1y in the Central Fault Horst Belt, Tazhong Low Rise, where nearly 20 wells were drilled targeting carbonate buried hills of O1y and encountered widespread bitumen and heavy oil in weathering crust reservoir beds (Table 4). The area experienced Devonian uplifting, which led to erosion of the entire Middle and Upper Ordovician as well as part of Lower Ordovician. From then on, this area did not subside and receive sediment until the Carboniferous with the absence of marl cap rock in the 3rd to 5th members of O3l (Fig. 6). Hence large-scale uplifting resulted in destruction of O1y hydrocarbons charged in the earlier period (Lü et al, 2004; Zhang et al, 2011b) . The discovered oil and gas in O1y is concentrated on the Tazhong Northern Slope. In addition to reservoir bed development and hydrocarbon migration, the O3l marl cap rock is also an important factor. According to updated exploration data, oil and gas of O1y are concentrated in the area with a marl thickness of more than 100 m, and the thicker the marl, the higher the gas/oil ratio (Fig. 6). Therefore, for the Tazhong Northern Slope with multiple phase hydrocarbon charging, earlier oil charging and later gas charging, the quality of cap rock is one of the factors determining gas/oil ratio in oil and gas bearing formations. There is a difference in the development characteristics of the marl cap rock in O3l between the west and east of the Tazhong Northern Slope. In the west of the Tazhong Northern Slope, there are no 4th and 5th members of O3l, thus cap rock quality of marl in the 3rd member of O3l controls the vertical distribution of underlying hydrocarbons (Fig. 9). That is to say, to the west of ZG15 well, hydrocarbons could not accumulate in O1y below the 3rd member of O3l but migrated upward to the 2nd and 3rd members of O3l, due to poor marl cap rock quality in the 3rd member of O3l with relatively low natural gamma, low shale content and low breakthrough pressure (there was a breakthrough in the sample of ZG19 well under the pressure of 7 MPa). While to the east of ZG15 well, hydrocarbons almost all accumulated in O1y below the 3rd member of O3l, due to good cap rock quality of marl in the 3rd member of O3l with relatively higher natural gamma, higher shale content and higher breakthrough pressure (there was no breakthrough after 2 days in the sample of ZG8 well under the maximum pressure of 14 MPa). In the east of the Tazhong Northern Slope, the marl cap rock in the 3rd to 5th members of O3l has a thickness of more than 200 m, meanwhile, there was no breakthrough after 2 days during breakthrough pressure testing of core samples from 8 wells under the maximum pressure of 14 MPa (Table 1). This indicates that marl cap rock in the 3rd to 5th members Fissures and holes developed, filled or half-filled with asphalt. Holes of O3l could seal the underlying hydrocarbons of O1y to form sizable O1y oil and gas accumulation. 4.4 Hydrocarbon accumulation versus O1y dense limestone cap rock There is thick bedded dolomite with a thickness of around 700 m in O p 1 obvious “string of beads” shaped seismic reflections along the top surface of O p 1 caves, vugs and fractures (Yuan et al, 2012) . The overlying dense limestones without weathering in O1y act as cap rock for O1p. The cap rock becomes thicker in the northeast part of the Tazhong Northern Slope near the Tazhong No.1 Fault, thus this area is favorable for exploration for O1p hydrocarbon. Taking the TZ162 well as an example, with a top unconformity depth of O1y at 4,900 m, the depth of the karst reservoir bed controlled by the unconformity is 5,120 m from log interpretation; meanwhile, the depth of O y bottom bed 1 boundary with underlying O1p is 5,603 m, thus the dense limestone section from 5,120 m to 5,603 m in O1y with a thickness of 483 m could act as cap rock for underlying O1p. In O1p, acidizing testing for 5,931-6,050 m with a 9 mm nozzle produced 183,880 m3 of gas. Therefore, the karst reservoir bed in O1p and the dense limestone section without weathering in O1y combination. 4.5 Hydrocarbon accumulation versus Cambrian gypsum cap rock There are high quality source rocks with a high abundance of organic matter in the Middle and Lower Cambrian around the Tazhong Low Rise. Meanwhile, the structures and dolomite reservoir beds below the Middle Cambrian gypsum cap rock are relatively well developed. Seismic data show that the Middle and Lower Cambrian salt-related structures are distributed in rows or belts along basement faults or faultblock belts in the Tazhong Low Rise (Tang et al, 2012) . The TC1 well penetrated the finely crystalline and siltsized crystalline dolomites in the 7,085-7,162 m of Lower Cambrian. In parts of the core, vugs and fractures are well developed and are mostly half-filled or filled with crystalclustered dolomite, bitumen or shale, while the largest vertical fracture is 0.33 m long and 2 mm wide and the largest vug is 35×12 mm. There was no oil smell when out of core barrel and some air bubbles emerged along the fractures. In the Tazhong Low Rise, only the TC1 well in the east area penetrated through the Middle Cambrian gypsum, but the cap rock quality of gypsum in the central-west area should be better than the east area according to sedimentary sizable oil and gas could have been accumulated in the Lower Cambrian dolomites below the Middle Cambrian gypsum member. 5 Control on hydrocarbon accumulation by cap rocks T h e Ta r i m B a s i n e x p e r i e n c e d c o m p l e x t e c t o n i c movements and multi-phase hydrocarbon charging, and hydrocarbon accumulations were formed in multiple sets of of hydrocarbon accumulation by the Lower Palaeozoic cap rocks has to be combined with tectonic evolution, history of hydrocarbon generation and expulsion and hydrocarbon migration. There are two sets of effective source rocks in the Cambrian-Lower Ordovician and the Middle-Upper Ordovician (Cai et al, 2009; Zhang et al, 2012) . The Tazhong Low Rise experienced at least three periods of largescale hydrocarbon accumulation: Late Caledonian–Early Hercynian, Late Hercynian and Himalayan (Zhang et al, 2011c; Pang et al, 2013) . The 1st period of large-scale hydrocarbon accumulation of the Manjiaer Sag and Awati Sag adjacent to the north of the Tazhong Low Rise started large-scale hydrocarbon expulsion, and the Tazhong Low Rise was charged with hydrocarbon beginning from parts in the northwest and northeast (Zhang et al, 2011b) . During the 1st period of hydrocarbon accumulation, S2t-rmm cap rock developed with its top surface at a maximum burial depth of 1,150-1,350 m (Fig. 10(a)) and highest palaeo-geotemperature of around 60 °C (Fig. 10(c), 10(d)), at sub-stage A of the early diagenetic stage. The ages of the authigenic illite of the Silurian bituminous sandstones in the wells of TZ11 and TZ47 respectively are 364 Ma and 384 Ma, Early Hercynian (Zhang et al, 2011c) . This suggests that the widespread Silurian bitumen in the Tazhong Low Rise evolved from the hydrocarbon accumulated during the 1st period of hydrocarbon accumulation. Meanwhile, almost all the Silurian bituminous sandstones were distributed below the S2t-rmm cap rock, suggesting that S2t-rmm cap rock with sealing capacity controlled vertical distribution of Silurian hydrocarbons (Zhang et al, 2004) . In addition, the O s mudstone cap rock also had sealing capacity at a top 3 surface depth of 1,400-1,500 m (Fig. 10(b)). Intense Devonian uplifting destroyed the hydrocarbons charged in the Late Caledonian–Early Hercynian. In the Central Fault Horst Belt and the east end area of the Tazhong Low Rise, the cap rocks including the mudstone of S2t-rmm, the mudstone of O3s and the marl of O3l were completely denuded (Fig. 10(c), 10(d)). Meanwhile, in the remaining area of the Tazhong Low Rise, the cap rocks were preserved with S2t-rmm mudstone at top surface depth of 200-800 m and O s mudstone at top surface depth of 500-1,100 m (Fig. 3 10(a), 10(b)). When the burial depth of oil reservoirs was less than 1,500 m, crude oil was prone to suffering water washing oxidation and biodegradation (Larter et al, 2003) . Meanwhile, the main strike-slip fault activity of the Tazhong area was sustained in the Early Hercynian (Li et al, 2013) , thus tectonic uplifting and fault cutting resulted in the destruction of the Lower Paleozoic hydrocarbon reservoirs which were charged in earlier periods forming bitumen in sandstones and carbonates. The 2nd period of large-scale hydrocarbon accumulation happened in the Late Hercynian. Permian volcanic activity resulted in a geothermal gradient of the Tarim Basin of up to 34-38 °C/km, thus the effective source rocks of Middle-Upper Ordovician in the west of the Manjiaer Sag started to expel oil at a large-scale then the hydrocarbon migrated into structural high areas along karstic carrier beds and unconformities from north to south and from northwest to southeast (Zhang et al, 2011b) . During the 2nd period of hydrocarbon accumulation, S2t-rmm and O3s mudstone developed with a top surface burial depth of 2,100-2,500 m and 2,350-2,800 m respectively (Fig. 10(a), 10(b)), all more than 2,000 m. Thereafter, there was no cap rock destruction by uplifting and denudation (Fig. 10(c), 10(d)), thus the accumulated hydrocarbons were well preserved. In addition, Ordovician reservoir beds in the Tazhong Low Rise experienced long-term and widespread karstification with well-developed dissolution pores, thus providing good space for large-scale oil expulsion and charging. This is why the discovered commercial oils in the Tazhong Low Rise were mainly generated from the MiddleUpper Ordovician source rocks (Zhang et al, 2000; Li et al, 2010; Zhang et al, 2011b; Tian et al, 2012) . The 3rd period of large-scale hydrocarbon accumulation TZ16 well Evolution of top surface depth of the O3s TZ11 well TZ16 well Evolution of top surface depth of the S2 t-rmm ,mm TZ47 well TZ11 well t-rm2 0 S1000 e h t fo2000 h t p de3000 e c fra4000 u s p o T The first period of hydrocarbon accumulation Uplifting and denudation in Early Hercynian The second period of hydrocarbon accumulation The third period of hydrocarbon accumulation Evolution of burial history and thermal history of TZ11 well Caledonian Hercynian -sInindiaon Yanshanian -Hlaiymaan O S D C P T J K E-Q 35°C 50°C 65°C 0 1000 m t,h 2000 p e d l a i ru 3000 B 4000 5000 m TZ47 well ,s3 0 eO1000 h t fo 2000 h tpe 3000 d ce 4000 a f ru 5000 s p o T 0 1000 m , th2000 p e d l a i ru3000 B 4000 5000 (a) 80°C 95°C 110°C 125°C 300 250 200 Time, Ma (c) 140°C 500 450 400 350 150 100 50 0 Fig. 10 Top depth change diagram of S2t-rmm and O3s mudstone (attached with single well evolution diagrams of burial history and thermal history) Twheell wOa3ssofuflTlyZ16 denuded in Early Hercynian The first period of hydrocarbon accumulation Uplifting and denudation in Early Hercynian The second period of hydrocarbon accumulation The third period of hydrocarbon accumulation (b) Caledonian O S 30°C 45°C Evolution of burial history and thermal history of TZ47 well Hercynian -sInindiaon Yanshanian -Hlaiymaan D C P T J K E-Q 60°C 75°C 90°C 105°C 120°C 300 250 Time, Ma (d) happened in the Himalayan. The Middle-Upper Ordovician effective source rocks in the slope area of the Tazhong Low Rise started to expel hydrocarbon, and the oil generated from the Cambrian-Lower Ordovician effective source rocks in earlier periods cracked into gas (Zhang et al, 2011b) . Thus the accumulated Ordovician carbonate oil reservoirs were gradually altered by gas invasion from deep reservoirs, while the intersection of the Tazhong No.1 Fault with main strike slip faults acted as hydrocarbon charging points (Pang et al, 2013) . On the Tazhong Northern Slope which was near the hydrocarbon charging points, the cap rocks of O s mudstone 3 and O3l marl were well preserved with relatively large thickness. In addition, during the 3rd period of hydrocarbon accumulation, S2t-rmm and O3s mudstone developed with top surface burial depths of 3,000-3,750 m and 3,500-4,000 m respectively (Fig. 10(a), 10(b)), thus sizable condensate reservoirs were formed in the Ordovician carbonate along the Tazhong Northern Slope. In structural high areas of the Central Fault Horst Belt and the east end of the Tazhong Low Rise, the Lower Paleozoic cap rocks were denuded by Devonian uplifting after the 1st period of large-scale hydrocarbon accumulation, resulting in the absence of regional sealing for hydrocarbons migrating in late periods. Therefore, there is no important discovery in the Lower Paleozoic after drilling 20 wells in the study area. In summary, oil and gas accumulated in the Lower P a l a e o z o i c d u r i n g t h e e a r l y p e r i o d o f h y d r o c a r b o n accumulation were destroyed or degraded due to shallow burial depth of cap rocks; while oil and gas accumulated during late periods of hydrocarbon accumulation have been preserved due to suitable burial depth of cap rocks. The Tazhong Northern Slope with relatively well-preserved Lower Palaeozoic cap rocks and its location near hydrocarbon migration pathways is rich in oil and gas; while the Central Fault Horst Belt and the east end of the Tazhong Low Rise suffered denudation of Silurian and Ordovician cap rocks, and no important discovery has been made in the Lower Palaeozoic. 6 Conclusions 1) Several sets of effective local cap rocks are well preserved on the Tazhong Northern Slope despite the lack of regional cap rock covering the entire Tazhong Low Rise. The best cap rock is the mudstone of O3s; the second is the mudstone of S2t-rmm and the marl of O3l; the third is the mudstone of S1k-gms. The dense limestone of O y and 1 the gypsum of 2 show sealing capacity, but their regional 2) The Lower Palaeozoic effective cap rocks are closely related to hydrocarbon accumulation in the Tazhong Low Rise. The Tazhong Northern Slope near migration pathways and with well-preserved cap rocks is rich in Lower Palaeozoic hydrocarbons. However, the Central Fault Horst Belt and the east end of the Tazhong Low Rise suffered denudation of multiple sets of cap rocks, thus no large-scale discovery of hydrocarbons in the Lower Palaeozoic has been made there. Areally, Ordovician hydrocarbons are distributed in the areas where cap rocks including O3s mudstone and O3l marl are well-developed; vertically, most of reserves in the Tazhong Low Rise are distributed below O s mudstone cap rock, and 3 almost all bituminous sandstones and movable oils from the Silurian are concentrated below S2t-rmm mudstone cap rock. 3) The Tazhong Low Rise experienced three periods of large-scale hydrocarbon accumulation and complex tectonic movements, and the evolution of Lower Paleozoic cap rocks controlled hydrocarbon accumulation. Taking cap rocks of S2t-rmm and O3s mudstone as examples, in the charging, the cap rocks with top surface burial depths of 1,150-1,500 m could seal oil and gas; thereafter, with intense Devonian uplifting, the cap rocks with the top surface burial depths of 200-800 m and 500-1,100 m respectively were denuded in local areas, thus the hydrocarbons accumulated in earlier periods were degraded to widespread bitumen. In the Late Hercynian period of oil expulsion, the top surfaces of the cap rocks were at burial depths of 2,100-2,800 m and without denudation by uplifting thereafter, thus commercial oil accumulations were preserved. In the Himalayan period large-scale gas invasion occurred, and top surfaces of O s 3 mudstone were at burial depths of 3,500-4,000 m and without denudation by uplifting thereafter, thus sizable condensate reservoirs of O3l were formed. 4) The potential exploration targets in the Lower Paleozoic in the Tazhong Low Rise can be determined based on the characteristics of cap rocks. The first target is O1p below O1y dense limestone cap rock, because good dolomite reservoir beds developed in the top part of O1p or along the by drilling. The second target is the Lower Dolomite Member below the 2 gypsum cap rock, because the cap rock could be widespread in the entire Tazhong Low Rise, and salt-related structures and dolomite reservoir beds were relatively well developed. The regional distribution of gypsum cap rock is awaiting further confirmation. Combined with hydrocarbon migration, less risk would be involved in exploration on the Tazhong Northern Slope. 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Yanping Zhang, Xiuxiang Lü, Haijun Yang, Jianfa Han, Xiaodong Lan, Yue Zhao, Jinhui Zhang. Control of hydrocarbon accumulation by Lower Paleozoic cap rocks in the Tazhong Low Rise, Central Uplift, Tarim Basin, West China, Petroleum Science, 2014, 67-80, DOI: 10.1007/s12182-014-0318-5