Lunnan-Tahe oilfield is a "complete" oilfield with late accumulation.

Lunnan-Tahe Oilfield is a "complete" heterogeneous reservoir (O 1), which is located in the giant unconformity trap of Caledonian-Hercynian foreland basin in Tarim Basin.

1. The Lower Ordovician and the Middle-Upper Ordovician in the basin respectively represent two basin prototypes or sequences that stretched and aggregated in different periods.

The subsidence of Tarim Basin began in the Early Sinian (Z 1) of 740Ma. From early Sinian to early Ordovician (z1-o1), the nature or prototype of the basin is similar to that of the Yangtze River and the western margin of Ordos, and it belongs to the extensional basin of passive continental margin (Figure 5- 16 and Figure 5- 17), not aulacogen. In the middle of the basin, this sequence, which is equivalent to Sauk(590 ~ 476Ma) in North America, takes Bachu-type carbonate platform as the core, and passes through platform-platform front (characterized by random reflection of hills) and front slope (characterized by parallel progradation reflection) on the east, north and south sides (Figure 5- 18). The latter represents a deep-sea environment with little biological interference below the wave surface, and is the most important oil source rock in the history of the basin rich in kerogen type I. This is the Jinghai Basin (Xu Jinghua, 1993). Based on this, Xu Jinghua estimated the resources of the Tarim Basin to be 50 billion tons. ..

Figure 5- 16 Sequence Stratigraphic Profile of Andiel Korla-Ganlan SN520 Line

Phanerozoic subsidence of the basin began in Early Sinian ()

The above situation is almost the same as that seen in the eastern margin of Sichuan Basin and the western margin of Ordos Basin. Sequence correlation of Paleozoic foreland basins in three basins (Table 5- 1).

Table 5- 1 Comparison Table of Sequence Division of Middle-Upper Ordovician-Devonian Foreland Basins in Ordos, Tarim and Yangzi Areas

① D3 () This is the original bottom of Donghetang sandstone.

Fig. 5- 17 Sedimentary Facies Paleogeographic Map of Early Ordovician (Lianghekou-Early Guniutan) in Northern Tarim Basin

(According to Zhou Dikang et al., 2000)

It shows the passive continental margin composed of three facies zones and eight facies zones.

▲ Location and serial number of outcrop profile: 1- Nanyashan; 2- Yuanbaoshan; 3-chaer Keke； 4- Hindi Tagg; 5- Ulige Zitag; 6— Penglai Dam; 7- Shi Si factory; 8- Dawangou; 9- Eagle Mountain; 10—Shoerbrak； 11-Sugath Braque; 12-Ruins of Weitouzhou City in Tang Dynasty; 13— Taipan Tage; Ⅰ—Kuruktag Trough Basin in South Tianshan: Ⅰ1—South Kuruktag Trough Basin, Ⅰ 2—North Kuruktag Platform Slope Facies, Ⅰ 3—South Tianshan Trough Basin Facies; Ⅱ-Maingard semi-deep sea basin facies belt: Ⅱ1-Manxi slope facies belt, Ⅱ 2-Zhong Man basin-basin margin facies belt; Ⅲ—Central shallow platform facies belt: Ⅲ 1—Lunnantai marginal facies belt, Ⅲ 2—Aksu-Manxi1Well open platform facies belt, Ⅲ 3—Limited platform facies belt (Er) denudation belt.

The Ordovician in the basin is bounded by the upper unconformity seen in the middle-upper Ordovician and the lower Ordovician in Maingard sag, which should belong to two different sequences (Figure 5- 19), in which the middle-upper Ordovician is equivalent to Tippe Canoe Ⅰ (441~ 475 Ma) divided in North America. In the eastern half of Tarim basin, the middle-upper Ordovician represents an active margin or pre-arc sedimentary sequence, which has been confirmed by several wells (Qunke 1 well and Tadong 1 well). This is a complete set of deep-sea graptolites of Ma Bao sequence, which is shallowly filled with terrigenous sandstone and mudstone flysch and has the highest sedimentation rate in the early Paleozoic (140m/Ma). The sedimentary type is equivalent to or similar to the extra-thick flysch (2500m) (Zhou Zhiyi, 1990) of Lanwein-Ashe period located on the ground in Qierkeshan and Yuanbaoshan areas.

Fig. 5- 18 Ordovician seismic facies model profile in northeast Tarim basin

TBB-88-E78 shows the passive margin of Sauk, which is composed of platform, platform front, front slope and basin facies, and has a giant sequence (z1-o1). Pay attention to the relationship between O2-3 (Tipi canoe) and Sauk, and the wedge shape (representing the residual sea and flysch accretionary wedge produced by B-type subduction) which is thick in the east and thin in the west and pinches out at the "bulge" of Akkule.

Contrary to the low sedimentation rate of Cambrian-Lower Ordovician in Jinghai Basin (65,438+0 ~ 5 m/ma) and the stratum on complacent Garci slope thinning due to the deepening of seawater and lack of compensation, the middle-upper Ordovician showed wedge-shaped deposition of pre-arc and back-arc proliferation at the beginning of basin convergence stage, with the wedge-shaped structure of contemporaneous foreland basin with thick east and thin west and super-sharp west. Therefore, the interface between the Middle and Upper Ordovician and the Lower Ordovician, that is, the interface between Sauk and Tippecanoe (), should be an inversion surface, which represents the beginning of convergence and collision activities between the micro-landmasses located in the east of the basin and those contained in the East Tianshan Mountains and Altun Mountains after the Early Ordovician (Figure 5-20).

Synorogenic flysch, which began in the middle and late Ordovician, and moral red beds represented by Dongtanghe sandstone in the late Devonian (roughly equivalent to the second stage of Tipp canoe in North America of 438 ~ 350 Ma) are the manifestations of the Caledonian Qilian orogenic cycle in the basin. It is very important for the formation and deformation of the interior of the basin represented by Tazhong, Tadong (Oman sag) and Tabei, as well as the formation and destruction of the early reservoir-forming assemblage.

Traditionally, we call the "open", "eat", "touch" and "block" cycles (Zhu Xia, 1990) of lithospheric evolution, that is, Wilson cycle and Caledonian cycle. Sinian-Early Ordovician deposits in Tarim Basin were formed on the passive edge of "two oceans" in West Kunlun and South Tianshan, representing rift-drift); Controlled by extensional structure. The middle and late Ordovician-late Devonian represented a flexible stage of convergence and collision of micro-landmasses, which constituted a complete opening and closing cycle. Jinghai basin, which was formed in the detachment stage, is the most important source rock in Tarim geological history. The Middle-Upper Ordovician flysch-Silurian-Devonian molasse is a back-arc foreland basin (Zhou Dikang, 1996), which is the same as the western margin of Paleozoic Ordos and the eastern margin of Sichuan Basin (Figure 5-20).

Fig. 5- 19 Ordovician seismic sequence stratigraphic system in Tarim basin

(According to Yu Bingsong et al., 2005, supplement)

It shows the different sequences of O 1-2(O 1) and O2-3(O3) and the upwelling of O3-D uplift.

2. There are three unconformities (top O 1), (bottom D3) and (bottom P2-T) in Paleozoic strata in Akkule area, which divide Z-O 1, O2-3-D 1-2, D3-C(P) and Triassic in this area.

Regarding the relationship between sequences, there are the following characteristics.

The unconformity angle of the upper and lower interfaces reaches18 (apparent dip angle), with the upper strata facing north and the Lower Paleozoic facing south (Figure 5-2 1). In other words, due to the gradual disappearance or truncation of O2-3, S-D and D3-C 1 strata in Akkum (Lunnan) area, Triassic is directly adjacent to the erosion surface of O 1, and O2-3-D 1-2 sequences (Tippecanoe) and D3-0-2 sequences. Therefore, it is proved that the top of O 1 is a huge non-isochronous erosion surface, and the potential difference caused by the difference of sedimentary sequence between north and south is also obvious.

Fig. 5-20 Schematic Diagram of Sedimentary-Tectonic Evolution of Northern Margin of Tarim Basin (A) (according to Zhou Dikang, 1996) and Schematic Diagram of Phanerozoic Movement Pattern of Tarim Plate (B) (according to Fang Dajun, 1994).

Shows the opening and closing cycles and the location of the back-arc foreland basin.

Fig. 5-2 1t B- 85-n 136 geological interpretation profile.

(showing that S-D, O2-3 and D3-C 1 strata overlap and cut O 1 northward)

According to the drilling data, Zhou Xingxi divided D3-C 1-2 strata in Lunnan-Tahe area into C 1 -C7 rock segments, in which C1(Xiaohaizi Formation) has not been preserved in this area, and C2 sandstone mudstone segment-C7 (Donghetang sandstone segment) obviously overlaps from south to north. In other words, because (Donghetang sandstone) pinches out at the bulge, two thirds of the bulge area is covered by the relatively impermeable stratum of the lower mudstone section (C5)- sandstone mudstone section (C2), thus causing the above stratigraphic trap. At this time, Akkule, as a uplift after O2-3, continued to be high in the north and low in the south during D3-CP, with the arc-shaped plow fault in the north of Akkule as the highest point, and from south to north, along with the gradual disappearance of 5-6 rock segments of Bachu Formation (D3) and Caraccia Yi Formation (C1), D3-C 1 stratum was formed by. But after Akkule fault, that is, in the so-called platform area south of Lunnan fault, there is still C 1 stratum distribution. In other words, if Akkule Uplift is regarded as a narrow paleogeomorphic highland with O 1 as the core and Akkule (Lunnan Oilfield) as the high point, then the periphery of this highland is surrounded by caprocks with different properties in different lithologic sections of D3-C 1 stratum (Figure 5-22) (Sun, 1999)

3. The formation time and ground stress (power) of Akkule's "bulge" did not come from the Tianshan problem.

As shown in the line AE- 1-59 of the earthquake in the northern part of the basin in Figure 5-23, Akkule uplift was obviously formed before (D3). Many researchers often classify the deformation dynamics in the early Hercynian and Yakela fault uplift in the north (Tabei uplift in a narrow sense) as the north-south compression of Tianshan orogenic belt (Cheong Chow,1998,2000; Kang Yuzhu,1998,2001).

Figure 5-22 shows the layer-by-layer overlap of the erosion surface at the top of Lower Ordovician D3-C 1 from south to north.

(According to Zhou Xingxi 200 1, modified)

At present, the static marine basin with -O 1 passive edge distribution in Paleozoic in Maingard and the O2-3 residual marine basin (contemporaneous flysch foreland) and S-D 1 with deep water turbidity, sedimentary wedge and rock mineral characteristics (O2-3 sandstone contains 40% ~ 50% volcanic debris from island arc) have been reconsidered. That is, it controls the boundary faults in Tabei, Tazhong and Tarnum (or Beiminfeng) in Tarim Basin, as shown in Figure 5-24. Because the main boundary faults (Yan 'an, Tumuxiuke and Beiminfeng) that constitute or control the uplifts are a set of ductile shear zones that meet with the southward supporting faults (Lunnan and Mazatake) in a "Y" shape, according to the shallow metamorphism in Tadong 1 Well O2-3 area, it is speculated that there should be a convergence between Tarim block and Qaidam block along the present Peacock River slope or the eastern edge of Kurutake. According to this judgment, the O2-3-D 1-2 foreland basin with Akkule uplift in Maingard area should have the nature of back-arc foreland (the back-arc opinion in Figure 5-20 is understood by Comrade Zhou Dikang).

Here, I quote a passage from Zhou Dikang's article "Characteristics of Cambrian-Ordovician Reservoir in Northern Tarim Basin and Its Oil and Gas Prospect": "At the end of Early Ordovician, the Guttari wood plate drifted northward from south latitude 18 (see Fang Dajun, 1994 paleomagnetic data), and the northern margin of the ancient plate (equivalent to the northeast and southeast of Tarim today) changed from early. The tectonic setting has changed from extensional ... The southern part of the ancient plate (now the northwest and southwest margin of Tarim) may continue to maintain the passive continental margin ... "(Ye Desheng et al., 2000). Together with the author's previous views, this passage not only shows that Akkule uplift and its depressions on the east and west sides are the earliest NE-SW trending structures in the basin, but also shows that the compression power in the middle and early stage of the basin is from south to north (see Figures 5-20 and 5-24b).

4. Lunnan-Tahe Early Ordovician oil (gas) field is a unified and complete oil (gas) field problem.

Fig. 5-23 Sequence Stratigraphic Interpretation Profile of AE- 1-E59 Line in Northern Tarim Basin

(According to Fan Tailiang, 1994)

Pay attention to the upwelling and obvious wedge shape of the subsurface O2-3d stratum to Akkule uplift (forerise).

Within the scope of the bulge, from Akkumu in the north, through the platform area, and then through Akkule to Aisan area, there are 25 industrial oil and gas wells drilled in Ordovician, showing 24 wells in different degrees (the data comes from the Northwest Bureau 1988 exploration results map; The figures in the annual report of China Petroleum 1987 show that 32 wells have been discovered, and 14 wells have obtained industrial oil flow; The oil well at the end of Kang Yuzhu 1998 is 2 1). Therefore, the author very much agrees with Comrade Kang Yuzhu's view that the Ordovician oil-bearing scale or range is "probably the whole uplift" in the article "Ordovician is an important horizon and exploration suggestion for finding large oil and gas fields" (Kang Yuzhu, 1998). That is to say, the Ordovician top contour map (), that is, the uplift with an area of more than 4000km2 (see Figure 5-5), first proposed by the Geology and Mineral Resources System 10 years ago, is the basis for us to consider the mechanism and scope of oil and gas traps (Sun, 1999).

Figure 5-24 Fault systems (a) and (b) controlling Paleozoic "Three Uplifts and Two Sags" in the basin.

Master-slave relationship and principal stress (σ 1) direction of double "Y" fault in Shi Yanan (Erbatian) and Luntai.

Figure 5-25 is the O 1 oil-bearing layer bitmap provided by the industrial department, which at least illustrates the following two problems:

Drilling proves that the O 1 oil-bearing horizon is closely located under the unconformity () and in the weathering crust at the top of O 1.

The oil-bearing horizon O 1 is a continuous whole from Akkumu (Lunnan) through the platform area to Akkule and the area including Tahe No.3 to No.4.. The poor oil and gas situation in some wells is due to the tight O 1 carbonate reservoir, which is caused by the extremely uneven physical properties of carbonate rocks.

5. Formation time of Lunnan-Tahe unconformity truncation trap

In the oil and gas exploration department, it is very popular to regard Akkule as "the lower Paleozoic is a nose uplift, the upper Paleozoic is a fault, the Mesozoic is covered, and the Cenozoic is a big trap because Kuqa leans northward" or "Akkule uplift was formed in the late Himalayan period". There are two problems that need to be clarified conceptually: one is the level of traps, because the whole raised area has different levels of properties and periods; Second, the trap formation time, of course, refers to the time when the top of O 1 is used as the reservoir to form unconformity trap.

Figure 5-25 shows the location map of O 1 oil layer in Tahe area of Lunnan.

Like Yakela oil and gas field, the trap of Ordovician gas-bearing layer is unconformity, while the trap of J-K sandstone gas-bearing layer is anticline. The Mesozoic and Early Paleozoic in Akkule Uplift, dominated by O 1, are also so-called anticlines or draped anticlines and unconformity double-layer structures.

If we discuss the formation time of O 1 top unconformity trap, we should consider two times: one is after the deposition of Lower Carboniferous (C 1) (or south of the pinch-out line in the northern part of C 1 formation and south of Lunnan 34 well); The other time was after Lunnan (Akkum) was covered by Triassic.

The key to O 1 trap cutting is whether the strata above the unconformity surface can play the role of caprock. According to the map (Zhou Xingxi, 1998) published by China Petroleum and Natural Gas Group Company, in the broad sense of Lunnan area, the Carboniferous valley south of Aishan Mountain is as thick as150m, and it becomes thinner to the north, and the thickness is zero. Six bottom-up rock segments belonging to Carboniferous are regularly pointed out from south to north in different uplift zones (see Figure 5-2 and Figure 5) ③ From the northern boundary of C4 (bimodal limestone) to Lunnan 12 well; ④C2+3 (upper mudstone and sandy mudstone section) pinchout line reaches Lunnan 34 well area in the southern margin of Akkum (Lunnan) Plateau.

Figure 5-26 Ordovician palaeokarst landform profile in Akkule area

(According to Zhou Xingxi, 2000)

It is the northward cusp extinction line of C 1 different horizons.

As can be seen from Figure 5-26, the gravel section of Bachu Formation (C6+7) was pinchouted near Tahe, and the upper and lower mudstone sections of the Early Carboniferous blocked the unconformity main body of the bulge, so a trap was formed after the deposition of C 1 with the truncated weathering crust of O 1 as the boundary. Due to the direct contact between O 1 and Triassic in Lunnan area, with the extinction of C 1 stratum, the mudstone layer at the bottom of Triassic in Lunnan area sealed the caprock. Therefore, the whole uplift unconformity trap, including Akkumu area, was formed after Triassic deposition.

6. Lunnan-Tahe O 1 oil (gas) reservoir formation time.

Up to now (including the author's article before 1999), the concept of "multi-source and multi-period" with the connotation of "four coexistence" seems to have become the consensus of the industry on the time of Tarim reservoir formation. However, the following two data, including the analysis of effective source rocks mentioned later, support that Lunnan-Tahe Oilfield is another example of "Late Tertiary-accumulation".

Table 5-2 is the data of 4 wells in Lunnan-Tahe area, including formation temperature, formation pressure, saturation pressure and lower limit of reservoir burial depth. The data of these four wells support that the oilfield was formed since the reservoir-forming period of Well Jiefang 128 (platform area) was Kuqa Formation ().

Table 5-2 Determination of Formation Time of Ordovician Reservoir in Lunnan

(According to Liang Digang 1999)

If the oil in the lower Ordovician weathering crust of Akkumu (Lunnan) Triassic is regarded as a dynamic reservoir system, the oil and gas in the platform area, Akkule area and Santamu area after Triassic are obviously deposited in this unconformity trap. This is consistent with the view that the depth of reservoir formation in Lunnan 14 well is 2435 meters, which is equivalent to the late Jidike Formation (Liang Digang, 1999). According to PVT phase diagram calculation, Triassic-1 oil group and Santamu Carboniferous gas reservoir in Lunnan fault base were formed in the late sedimentary period of Kuqa Formation, which is also consistent with Chen Youcai's view of Southwest Petroleum Institute.

7. Source rocks in Lunnan-Tahe Oilfield

This problem has always been the object of discussion and research in many scientific and technological research projects. Perhaps the problems and fields involved can not be clearly explained by a word "source", so there are still different opinions until now.

According to the understanding of this problem, it can be divided into two stages with 1999 as the boundary. Before 1999, the main source rocks of Tarim basin, including Lunnan-Tahe, were concentrated in the Early Cambrian basin facies on the passive continental margin of Cambrian-Ordovician, and it seems that this problem has been solved by taking multi-source, multi-stage and "four coexistence" (Sun Caizhao, 1996b, c) as a unified concept. Among them, including Xu Jinghua's opinions expressed abroad, Tarim is the basis of 50 billion tons of prospective reserves, and it is also the "Jinghai basin facies" with subduction and condensation characteristics in the seismic reflection in the eastern part of Maingard Depression. However, since the second half of 1999, especially in Liang Digang's 96-11-03 report, after the results of biomarker compounds of a batch of middle and upper Ordovician source rocks and lower Ordovician to Carboniferous crude oil samples in Lunnan area, which were studied in cooperation with Stanford University, they were previously known with the hinterland of Maingard depression. Because of this, since 1999, there have been two different opinions that the source rocks in this area are -O 1 and O2-3. Coupled with the viewpoint that the O 1-C 1 oil source in Tazhong area comes from the west and Shen 2 well area (under salt), this problem, including the suspicion of multi-source, multi-phase and "four coexistence", further complicates the problem (it seems).

Figure 5-27 Value of 24- isopropyl cholesterol /24- n-propyl cholesterol in Paleozoic rocks and crude oil in Tarim Basin

(According to Liang Digang 1999)

It shows the distribution and oil source comparison of 24- isopropyl cholestane in different periods.

In the author's opinion, if the Middle-Upper Ordovician is a set of foreland flysch in the same orogenic period, which consists of the residual sea in the early convergence stage, it is not appropriate to define the so-called marl mound facies formed by O2-3 from east to west on the uplift edge (such as Lunnan area) as the "slope" of O2-3. As the author said in the Introduction to Foreland Basins, all wedge bodies in foreland basins have the problem of inconsistent settlement and sedimentary center (see Figure 2-23). In Mangar, when the thickness exceeds 4000 m, the organic matter abundance in the deposition center of O2-3 is low, and the TOC is 0.2%. However, in the front facies belt, although the thickness is less than 500m, the TOC reaches 0.85% on average, which is equivalent to Jones' C facies belt or the productivity model with ocean upwelling (Liang Digang, 1999). In this similar sedimentary model, we can see the narrow facies belt composed of Pingliang shale (including graptolite) and Wufeng shale inside the O2-3 deposit in the western margin of Ordos and the eastern margin of the Yangtze River.

Figure 5-28 Variation of relative content of 24- norcholestane in crude oil and rocks with geological age

(According to Liang Digang 1999)

The difference between CAMBRIAN and Ordovician

Figure 5-29 Relationship between the reserves of 24- norcholestane and triaryl cholestane in crude oil and rock extracts.

(According to Liang Digang 1999)

Therefore, the author supports the opinions of Liang Digang and Moldovan from the original (1996b and c) biomarkers, which is related to the above-mentioned connotation of late reservoir formation and the understanding of a kind of oil (gas) in recent years. Of course, because the phase state of oil and gas produced by the Lower Ordovician in this area changes greatly, there are heavy crude oil with density greater than 0.9, conventional oil, light oil or condensate oil; There are natural gas, condensate gas and even dry gas associated with conventional oil. Coupled with the nature of the structural site and the complexity of the internal structure, it is an obvious topic to continue to study and discuss this issue.

Figure 5-30 The ratio of triaryl sterane to 3- methyl sterane in crude oil and rock extracts changes with time.

(According to Liang Digang 1999)