Introduction
The Wensu salient is located in northwest Tarim Basin, covering an area of about 4 500 km2. Its three sides are surrounded by hydrocarbon-generating sags[1,2,3], and it is situated to the south of the Kuqa depression (constituted by Wushi, Baicheng and Yangxia sags) and to the north of the Awati sag (Fig. 1). The Kuqa depression has been proved to have abundant oil and gas resources. With the breakthroughs of mountain seismic prospecting, high-steep structure seismic imaging and drilling in much thick gypsum strata, etc., the Kela 2 giant gas field has produced 1 000×108 m3 of natural gas since its discovery in 1998 to July, 2017, which has guaranteed the implementation of "West-to-East Gas Transmission Project"[4,5,6]. Exploration of the Awati sag represents that the lower Cambrian Yuertus Formation and the upper Ordovician Yingan and Saergan Formations are good source rocks[7,8,9]. Hence, it was generally believed that the Wensu paleo-salient adjacent to hydrocarbon source sags might capture oil and gas for a long time and form industrial oil and gas reservoirs, but no exploration breakthrough had been achieved in this area untilearly 2017. The predecessors emphatically analyzed the macro-structural pattern and fault characteristics of the Wensu salient area[7], and explored the target of deep carbonate rocks in combination with its contact relationship with hydrocarbon-generating sags[6]. They did not pay enough attention to the shallow Neogene, thus restricting the exploration of shallow oil and gas. In fact, although the caprock conditions of shallow reservoirs may not be as good as those of deep reservoirs, they usually have good reservoir conditions. If they have the related reservoir-forming conditions, such as sufficient hydrocarbon sources, good traps and unobstructed migration channels, shallow strata can also form large-scale oil and gas reservoirs[6]. For example, the Trenton buried hill oil and gas reservoir in the Cincinnati uplift area in the central United States[10], the Neogene oil and gas reservoir in Chengdao Oilfield in Bohai Bay Basin, China (its buried depth is about 400-2 300 m)[11,12], and the oil and gas reservoir in Neogene Shawan Formation in Chepaizi uplift of the Junggar Basin (its buried depth is about 225-700 m)[13]. Both the XWD1 and XWD2 wells drilled in 2017 in the Wensu salient have achieved high-yield industrial oil flows, achieving the first major breakthrough in exploration of this area in more than 50 years. Based on the comprehensive research on drilling, oil testing, geochemistry of the two wells, in combination with the field surveys, 2D seismic data processing and interpretation as well as sedimentation and accumulation history, we confirm the source conditions, migration channels, reservoir-cap conditions and trap types in the Wensu salient, and subsequently construct a reservoir-forming pattern.
Fig. 1.
Fig. 1.
Tectonic location map of Wensu salient.
1. Overview of geologic conditions
The Wensu salient is located in piedmont zone of the southern Tianshan Mountains in northwest Tarim Basin. It belongs to the southern slope zone of the Wushi sag and is adjacent to the Keping fault-uplift area in the west. The eastern Wensu salient is separated from the Qiulitag structural zone by the Karayuergun fault, the northern part is connected with the Wushi sag by the Gumubez fault, and the southern part is separated from the Awati sag by the Shajingzi fault[14,15,16,17,18,19,20]. As a whole, it is surrounded by Wushi, Baicheng and Awati hydrocarbon-generating sags, and spreads in NE direction (Fig. 1).
According to field outcrop and actual drilling data (Fig. 2), the Wensu salient develops strata from upper to lower sections are: Cenozoic Quaternary (Q), Neogene Pliocene Kuqa Formation (N2k), Pliocene-Miocene Kangcun Formation (N1-2k) and Miocene Jidike Formation (N1j), Sinian (Z) and Mesoproterozoic Aksu Group (Pt2ak). The Kangcun Formation is dominated by lacustrine sediments and develops thick mudstones as a set of regional caprock. The Kuqa Formation above the Kangcun Formation is alluvial fan deposit, with major lithology of silt-medium sandstone interbedded with mudstone, being a positive cycle of upward fining as a whole. The Jidike Formation underlying the Kangcun Formation is shallow lake-delta sediment, dominated by siltstone and mudstone, with a reverse cycle of upward coarsening. The upper Sinian is thick to medium-thick layered light-grey mudstone and limy dolomite, while the lower Sinian is thin to medium-thick layered purple-red limy mudstone, mudstone and dolomitic mudstone. Mesoproterozoic Aksu Group is a thick layered grey-green chlorite schist and grey quartz schist.
Fig. 2.
Fig. 2.
Stratigraphic composite column graph of Well WC1 in Wensu salient (drilled in 2005). GR—gamma; Rt—resistivity; Δt—interval transit time.
Oil and gas exploration in this area began in 1965. By 2016, petroleum enterprises have deployed and implemented several 2D seismic lines in this area; drilled four exploration wells (Well A1, Well WC1, Well WS1 and Well AS2), and have achieved certain geological knowledge, but exploration has not made a breakthrough, thus they withdrawn from oil and gas exploration rights. We believe that the main reason why the Wensu salient has not achieved oil and gas breakthrough for a long time is that Neogene has not been studied as a key layer, and the following two key geological problems have not been clarified: (1) Although the Wensu salient itself does not develop hydrocarbon source rocks, whether the oil-generating sags in adjacent areas could provide sufficient oil source for it or not? (2) What types of traps are developed and how are favorable traps distributed in this area?
2. Exploration breakthrough
The Oil and Gas Survey Center of China Geological Survey deployed two prospecting wells in the No.1 structure of the Wensu salient in 2017. Among them, the completion depth of Well XWD1 is 1 058.00 m, and the completion horizon is Mesoproterozoic Aksu Group, while that of Well XWD2 is 998.77 m, and the completion horizon is Neogene Jidike Formation. The drilled strata from top to bottom in Well XWD1 are Cenozoic Quaternary, Neogene Pliocene Kuqa Formation, Pliocene-Miocene Kangcun Formation, Miocene Jidike Formation and Mesoproterozoic Aksu Group. Core samples from the Neogene Jidike Formation (the main production interval) show oil-saturated and oil-rich levels of 7.72 m (Fig. 3). Its sand bodies have porosity of 22%-31%, permeability of (60-322)×10-3 μm2, belonging to moderate-high porosity and moderate permeability reservoir bed. According to well logging interpretation result of Well XWD1, it has 21.3 m/13 oil-bearing layers, including 3.8 m/3 oil layers, 3.8 m/3 poor oil layers, 3.4 m/1 oil-water layer, and 10.3 m/6 oil-bearing water layers (Fig. 4). We selected the oil layer at 833.5-835.0 m depth in the Jidike Formation in Well XWD1 to test oil by pumping with 73 mm oil tube, and obtained 42.74 m3 of daily oil production. Moreover, Well XWD1 obtained 3.85 m oil patch and oil trace shows in well interval of 997.4-1 017.0 m in the bedrock weathering crust of Mesoproterozoic Aksu Group, with gas logging whole-hydrocarbon peak value of 2.88%. Oil and gas shows are active in fractured section of bedrock, and 14.3 m in thickness of reservoir bed was interpreted by comprehensive well logging. The open hole section 939.44-1 058.00 m in Well XWD1 was tested by pumping with 73 mm oil tube, it has totally produced oil of 2.19 m3.
Fig. 3.
Fig. 3.
Oil-bearing core photos of Jidike Formation in Well XWD1.
Fig. 4.
Fig. 4.
Relation charts of four properties of HC-bearing intervals in Well XWD1.
The drilled strata from top to bottom in Well XWD2 are Cenozoic Quaternary, Neogene Pliocene Kuqa Formation, Pliocene-Miocene Kangcun Formation, and Miocene Jidike Formation. Core samples from the Neogene Jidike Formation (the main production interval) show oil-saturated and oil-rich levels of 7.72 m. Its sand bodies have porosity of 20%-27%, permeability of (59-280)×10-3 μm2, belonging to moderate- high porosity and moderate permeability reservoir bed. According to well logging interpretation result of Well XWD2, it has 9.7 m/12 oil-bearing layers, including 7 m/8 oil layers, 1.7 m/3 poor oil layers, and 1 m/1 oil-water layer. We selected the oil layers at 842.0-859.0 m depth and 872.0-884.8 m in the Jidike Formation in Well XWD2 to test oil by pumping with 73 mm oil tube, and obtained 22.26 m3 of daily oil production, and total oil production of 51.7 m3.
3. Geochemical characteristics and oil source analysis of crude oil
Both Well XWD1 and Well XWD2 have obtained high- yield commercial oil flows, with basically consistent crude oil quality in Neogene Jidike Formation. The crude oil in Well XWD1 has density of 0.913 7 g/cm3, viscosity of 70.73 mPa·s (50 °C), wax content of 5.67%, freezing point of -30.00 °C. The crude oil in Well XWD2 has density of 0.9076 g/cm3, viscosity of 42.69 mPa·s (50 °C), wax content of 4.48%, freezing point of -30.01 °C. The crude oils in the two wells both belong to conventional thin oil (medium quality oil), with low viscosity, low wax content and low freezing point (Table 1).
Table 1 Analysis data of crude oil samples from Jidike Formation in Well XWD1 and Well XWD2.
| Well | Depth/ m | Density/ (g/cm3) | Viscosity at 50 °C/ (mPa•s) | Paraffin content/% | Freezing point/°C |
|---|---|---|---|---|---|
| XWD1 | 833.50-835.00 | 0.913 7 | 70.73 | 5.67 | -30.00 |
| XWD2 | 842.00-859.00 | 0.907 6 | 42.69 | 4.48 | -30.01 |
The family compositions of crude oil in the Jidike Formation in Well XWD1 and Well XWD2 are dominated by saturated hydrocarbons (57.1%-58.3%), followed by aromatic hydrocarbons (20.9%-21.8%) and nonhydrocarbons (15.8%- 15.9%), with lower bituminous content (5.1%-5.3%). They are featured with low ratio of saturated hydrocarbons/aromatic hydrocarbons (2.6-2.8), and higher ratio of nonhydrocarbons/bitumen (3.0-3.1). The terrestrial crude oil in the adjacent Quele-Yudong tectonic belt has ratio of saturated hydrocarbons/aromatic hydrocarbons between 12.31-38.13, and the ratio of nonhydrocarbons/bitumen <1.0. The total ion current graph of gas chromatography/mass spectrometry of saturated hydrocarbons in crude oil (Fig. 5) shows that most paraffin in crude oil in Well XWD1 and Well XWD2 has vanished, and the base peak shows apparent bulges. This indicates that crude oil has suffered moderate biodegradation and water washing, which is caused by poor preservation conditions of oil and gas reservoirs at high structural positions. The crude oil in Well XWD1 and Well XWD2 has certain pristine predominance (Pr/Ph = 1.68-2.04), which is higher than that of the marine origin crude oil in Tabei area (generally <1.2) and lower than that of the terrestrial crude oil Well WC1 in the Wushi sag (3.2-3.3), indicating that the primary sedimentary environment was weak oxidation-weak reduction for source rocks.
Fig. 5.
Fig. 5.
Total ion current graph of saturated hydrocarbons in part crude oil in Wensu salient and its periphery.
The regular sterane in crude oil in Well XWD1 and Well XWD2 is distributed in V-shaped, with much developed rearranged sterane, lower relative abundance of pregnane and tricyclic terpene with low molecular weight, and showing successively decreasing relative abundance tends of C19-C22 tricyclic terpene, which is different from the normal distribution of the marine crude oil in the Awati sag in Well SN1 etc. (Fig. 6). There are less gammacerane, but there are more C29 new norhopane and C30 rearranged hopane (Fig. 6). The above characteristics of crude oil in the Wensu salient are similar to those of continental crude oil in adjacent Quele-Yudong and Wushi sags (Fig. 6), indicating that the crude oil in the Wensu salient is of continental origin. The C29 hopane ααα20S/(S+R) and C29 hopane αββ/(ααα+αββ) hopane values of the crude oil in Well XWD1 and Well XWD2 are 0.45 and 0.42-0.43, respectively, being the crude oil with normal maturity. Based on the analysis of biomarkers and carbon isotope compositions, it is considered that the crude oil in Well XWD1 and Well XWD2 mainly migrated from the source rocks in the northeast Baicheng Sag.
Fig. 6.
Fig. 6.
Proton chromatogram of m/z 217 and m/z 191 of saturated hydrocarbons in part crude oil in Wensu salient and its adjacent regions.
4. Reservoir-forming conditions
At present, the comprehensive geological research and exploration of the Wensu salient are generally lower, and drilling data are scarce. There are only a few 2D seismic data in the study area, and the characteristics of underground strata and faults are difficult to describe because of the great difference in age, different quality and poor imaging effect. After being processed by the techniques of continuous static correction, combined denoising and prestack depth migration etc., the quality of reprocessed seismic data in 2017 has been greatly improved. Based on such seismic data, combined with drilling data of 17 wells in the Wensu salient area and its adjacent area, and field profile measured data, we emphatically studied the hydrocarbon source condition, oil and gas migration pathway, reservoir and cap conditions, trap types and distribution, reservoir-forming pattern in the Wensu salient area, then put forward favorable drilling targets to guide oil and gas exploration.
4.1. Hydrocarbon source conditions and migration
By integrating drilling, seismic and field outcrop data, in combination with former research results[14,15,16,17], we conducted well-seismic joint calibration, traced and interpreted the major tectonic horizons (such as the basement and the Neogene Jidike Formation) in seismic data, and have established the geological structure and sequence distribution profile of this area. The Wensu salient is a paleo-uplift sandwiched between the Wushi sag and the Awati sag as a whole. It is a nose-like uplift structure being higher in the west and lower in the east (Fig. 7). Its tectonic evolution has mainly undergone four development stages, in which the Wensu salient stopped uplifting and received deposits during the deposit period of the Jidike Formation, and the west Wensu salient was locally uplifted eastward and eroded during the deposition of the Kuqa Formation[18].
Fig. 7.
Fig. 7.
Typical seismic sections in the study area (their locations are shown in
Drilling and field outcrop data confirm that there is no strata as source rock in the Wensu salient, but both the continental petroleum system in the Kuqa depression on the northern margin and the marine petroleum system in the Awati sag on the southern margin have confirmed source rocks[4,5,6,7,8,9].
The marine source rocks in Cambrian-Ordovician in the southern Awati sag have been proved by Well SN1, Well SN2, oil seepages in Qingsong stone pit and Wuluqiao areas, and have been observed on field profiles in Shorebrack and Shierik etc[7,8,9]. However, the major hydrocarbon expulsion period of the Cambrian-Ordovician source rocks might be earlier than the forming period of Neogene traps in the Wensu salient. The hydrocarbon generation and expulsion time of the Jurassic Chukmark Formation in northern Baicheng sag was about 2-5 Ma age ago, and reached oil and gas generation peak during Neogene[21,22,23]. Oil expulsion period was well coupled with the formation time of traps in the Wensu salient, so oil can accumulate in Wensu area along the northern Karayulgun and other faults[20,24-25].
Oil and gas generated in the Baicheng sag mainly migrated transversely along bedrock weathering crust and Neogene delta facies skeleton sandstone, and were adjusted longitudinally along active oil-source faults during reservoir formation. There are three main evidences: (1) Faults and joints are widely developed in outcrops of weathering crust bedrock, and caverns being formed by differential weathering are common. Well XWD1 drilled 40 m thick zone with well-developed bedrock fractures, with 3.85 m of oil patch and oil trace shows (Fig. 8). According to well logging interpretation, there is 14.3 m of reservoir bed. (2) The delta sand body in the Jidike Formation is well developed, with good physical properties and large scales, being a set of effective hydrocarbon drainage layer. Both Well XWD1 and Well XWD2 drilled with good oil and gas shows in this sand body. (3) Oil-source comparison and analysis after drilling Well XWD1 has proved that its source rock is in continental facies, and mainly came from the northeastern Baicheng sag. Moreover, the Wensu salient uplifted quickly during reservoir-forming period, thus several faults of different grades around and in the main part of this salient were formed, which provided favorable conditions for hydrocarbon migration and vertical adjustment from sag to salient.
Fig. 8.
Fig. 8.
Typical outcrops and core photos of Aksu Group in the study area. (a) Angle disconformity between Aksu Group and Sinian; (b) Aeolian spheroidal weathering in Aksu Group; (c) many joint fissures in quartz schist in Aksu Group; (d) Well XWD1, 997.92 m, grey-green oil-bearing core photo of chlorite schist with oil patch.
4.2. Reservoir-cap conditions and assemblages
The Kuqa depression entered the stage of regeneration foreland basin during Neogene. The lake formed during Paleogene began to shrink to the south, and the lake water was generally shallow. The shoreline of the lake varied frequently with climate and tectonic activities[5,9]. Based on the field geological survey of Neogene profiles in Wensu Gumubez anticline, Awati, Yanshuigou in western Baicheng and other areas, combined with the study of sedimentary characteristics of the Kuqa depression, it is considered that small alluvial fans, deltas and shore-shallow lacustrine bar facies deposits were developed from north to south during the sedimentary period of the Jidike Formation (Fig. 9). Hereinto, the major Shenmu well field is located in delta plain sub-facies, Well WC1 is situated in delta front sub-facies, and some wells (SN1, SN2, etc.) in northern Awati sag are in shore-shallow lacustrine facies (Fig. 9). During the sedimentary period of the Kangcun Formation, the lake basin area was enlarged, its water body became deeper, and the lithology was mainly composed of thick mudstone and thin sandstone interbeds. During the depositional period of the Kuqa Formation, the lake in this area gradually became shallower and disappeared, with main facies of fluvial, delta and alluvial fan.
Fig. 9.
Fig. 9.
Plane map of sedimentary facies in Jidike Formation in Wensu area, Tarim Basin.
Affected by sedimentary evolution, there are several sets of reservoir-cap assemblages in the Wensu salient area, of which three are the most favorable: (1) Reservoir-cap assemblage of thick mudstone in the third member of the Jidike Formation and the bedrock weathering crust in the Aksu Group (Fig. 10a). According to the actual drilling data in Well XWD1, the reservoir space of bedrock weathering crust is mainly fractures. It is presumed that its effective thickness is about 30 m away from the top surface, and it is extensively distributed along the unconformity surface. The mudstone in the lower Jidike Formation is a caprock with a thickness of 80-100 m. (2) Reservoir-cap assemblage formed by sandstone-mudstone interbeds in the middle Jidike Formation (Fig. 10b). Sandstone-mudstone interbeds are extensively developed in the second member of the Jidike Formation in the Wensu salient area, with lower sand/mud ratio, wide sand body distribution, total thickness of 200-500 m, thin single sand layer (0.5-10.0 m), being the best oil and gas producing layer revealed by Well XWD1 and Well XWD2. (3) Reservoir-cap assemblage of thick mudstone of the Kangcun Formation and sandstone of the first member of the Jidike Formation (Fig. 10c). Several sets of sand bodies are developed in the first member of the Jidike Formation, and the thickness of single layer (1-15 m) is larger than that of the second member. The thick mudstone caprock overlying the Kangcun Formation is 20-40 m thick. The sandstone in the first member of the Jidike Formation of Well XWD1 and Well XWD2 is too far from the main channel of oil and gas migration, and most of them are oil-bearing water layers or water layers.
Fig. 10.
Fig. 10.
Three sets of most favorable reservoir-cap assemblages in Well XWD1 in Wensu area.
On the whole, during the sedimentary period of the Miocene Jidike Formation, the Wensu salient had the characteristics of differential subsidence and low-amplitude uplift, which controlled the formation of low-amplitude broad paleogeomorphology in Wensu area (Seismic sections show that the interior of the Jidike Formation is characterized by parallel-subparallel reflections, and the stratum thickness has less change. See Fig. 7). This forms the sedimentary system of coexistence and intergrowth of delta and shore-shallow lacustrine beach bar, and sand bodies are widely distributed and overlapped. Affected by the later tilting process of the Wensu salient, the delta sandbodies from the north provenance can form southward updip pinch-out lithologic traps, while the beach bar sandbodies developed in the main part of the salient can form lenticular structural-lithologic traps. Affected by multi-period structure and long-term weathering process, fractured reservoir beds are well developed in the bedrock weathering crust, and stably distributed mudstone layers overly the Jidike Formation as regional cap, the reservoir-cap assemblages can be formed.
4.3. Reservoir-forming pattern
In conclusion, the large-scale extensive sand bodies in Neogene delta and beach bar facies and bedrock weathering crust in the Wensu salient provide channels for oil and gas transverse migration (“two transverses”. They form a highly efficient oil and gas drainage system together with the continuously active faults ("one vertical") (Fig. 11), and connect the continental hydrocarbon source rock in northern Baicheng sag. The oil and gas generated in the continental source rock in the Baicheng sag were extensively transported from northeast to southwest through a long distance. After entering the Wensu salient, they laterally migrated along the bedrock weathering crust at the Neogene bottom and the Neogene sand layers with moderate-high porosity (22%-31%) and moderate-high permeability ((60-322)×10-3 μm2), and adjusted and migrated along vertical faults to the target zones to form oil and gas reservoirs, then the oil and gas drainage system with “two transverses and one vertical” was formed. However, the Jurassic source rock in northern Baicheng sag has big thickness (250-600 m), high organic matter quality (mainly type II kerogen, average TOC of 1.69%, Ro of 1.0%-3.0%), strong hydrocarbon generation capacity, and sufficient hydrocarbon expulsion driving force (the pressure coefficient is generally more than 1.5, and maximum pressure coefficient is up to 2.15)[22,23]. The time coupling between hydrocarbon expulsion period and the formation of traps in the Wensu salient is good[26,27,28,29], then oil and gas can accumulate in large scale in traps widely developed in the Wensu salient. This area mainly develops two types of oil reservoirs: (1) Structural-lithologic oil reservoirs (lithologic control is dominant); (2) Buried hill oil reservoir (carbonate rocks weathering crust + metamorphic fractures) (Fig. 11).
Fig. 11.
Fig. 11.
Reservoir-forming pattern of Wensu salient.
Structural-lithologic oil reservoirs are mainly distributed in sandstone-mudstone interbeds of the upper-middle Neogene Jidike Formation, with medium-high porosity and permeability sandstone as reservoir bed and mudstone as caprock. Continental oil and gas from the northern Baicheng sag migrated transversely along bedrock weathering crust and Neogene beach bar sand bodies, and further accumulated to sandstone reservoir beds along continuously active longitudinal faults, forming structural-lithologic oil reservoirs. These oil reservoirs are characterized by orderly distribution in space and superimposed joint on plane, being the major type of oil reservoirs revealed by Well XWD1 and Well XWD2.
Buried hill oil reservoirs are mainly distributed in Sinian carbonate rocks and bedrock weathering crust. Their reservoir beds are Sinian carbonate rocks and bedrock respectively. The mudstone at the bottom of Neogene Jidike Formation is the regional caprock. Continental oil and gas from the Baicheng sag migrated transversely along bedrock weathering crust and unconformities, and further accumulated to favorable carbonate rocks and fractured reservoir beds in bedrock. The structural location of Well XWD1 on the northern slope of the Wensu salient is relatively low. The Sinian dolomite reservoir bed mainly produces water and a small amount of natural gas. It is speculated that there will be better oil and gas accumulation in the high part. Well XWD1 reveals the buried hill oil reservoir in bedrock weathering crust.
5. Implications of oil and gas discovery in Wensu salient
The breakthrough of oil and gas exploration in the Wensu salient has brought the following important implications for oil and gas exploration in the Tarim Basin: (1) The areas with undeveloped source rocks and structural traps at the margin of the Tarim Basin etc. can also form large-scale oil and gas bearing zones. To find efficient oil and gas migration pathways and subtle traps (such as stratigraphic-lithologic traps) is an effective way and direction for oil and gas exploration. (2) Hydrocarbon migration and accumulation in the Wensu salient mainly occurred during Neogene-Quaternary, and the oil and gas reservoirs have obvious characteristics of late accumulation. Hence, late hydrocarbon accumulation should be the general feature of hydrocarbon accumulation in the piedmont zone of the Tarim Basin, and has great exploration potential. (3) Well XWC1 mainly produces water in Sinian carbonate reservoir beds and a small amount of natural gas, which is mainly because of its structure that is not conductive to hydrocarbon accumulation. However, the lower Paleozoic and Sinian carbonate rocks in the Wensu salient slope area can act as good reservoir beds, the lower Jidike Formation has better mudstone caprock condition, and they are close to the main pathways of oil and gas migration, thus the high structure locations have broad prospects for oil and gas exploration. (4) Well XWD1 has achieved low production in bedrock weathering crust of the Mesoproterozoic Aksu Group by oil testing. The main reason is the small borehole and open hole testing, thus this can not represent its real productivity. The weathering crust has a wide range and large area, thus it has broad prospects for oil and gas exploration, which deserves our attention.
6. Conclusions
During the sedimentary period of the Miocene Jidike Formation, the Wensu salient had the characteristics of differential subsidence and low-amplitude uplift, which controlled the formation of low-amplitude broad paleogeomorphology in Wensu area, and formed the sedimentary system of coexistence and intergrowth of Neogene delta and shore-shallow lacustrine beach bar, and also formed large-scale extensive lithologic traps. Though the Wensu salient had no source rocks, the northern hydrocarbon generation sags could provide sufficient oil source for the Wensu salient, and extensive large-scale bedrock weathering crust and delta sandbodies and persistent faults together constituted an efficient oil and gas drainage system. The Wensu salient developed two types of oil reservoirs (buried hill and structural-lithology). Preliminary evaluation shows that the resources of Neogene Jidike Formation in No.1 trap of the Wensu salient is up to 1.21×108 t and has a broad oil and gas exploration prospect.
Reference
Geometry and kinematics of Wensubei-Yeyungou fault and its implication for the genetic mechanism of North Tarim uplift
Petroleum geology characteristics of Kela2 gas field
Exploration and discovery of large and deep subsalt gas fields in Kuqa foreland thrust belt
After the discovery of Kela-2 gas field, exploration for oil and gas hit bottom in Kuqa Foreland Thrust Belt. Except for Kela-2 gas reservoir, no new boreable traps were discovered. To explore deep oil and gas fields was also faced with such difficulties as geologic complexity, difficult trapping, and lag in technology. To solve the difficulties above, first is to rediscover Kuqa Foreland Thrust Belt, be confident in exploration, and identify main exploration areas; second is to develop seismic and geologic integration technology such as wide line combination, 3D acquisition and processing, modeling of the structure relevant to salt, and phase velocity field building, to discover Keshen-Dabei zone, and to discover and fulfill some drillable traps; third is to intensify development of drilling technology and fulfill efficient and rapid drilling in subsalt ultra-deep, high-pressure and high-temperature gas reservoir. In the past ten years, the exploration depth of Kelasu structural belt was increased from 4000m to 8000m, several large gas fields including Keshen 2 and Bozi 1 gas fields were discovered, and one trillion cubic meters of natural gas resources were gradually identified in Keshen zone. Breakthroughs in exploration of large and deep subsalt gas fields here benefits from innovation of geologic recognition, improvement of exploration techniques, and perseverant exploration practice.
Oil and gas exploration prospect in Wushi sag and Wensu uplift of Tarim Basin
DOI:10.1016/S1872-1508(06)60066-1
URL
[Cited within: 4]
Oil and gas exploration prospect in Wushi sag and Wensu uplift of Tarim Basin was studied by the methods of geological investigation in the field and analysis of lithological sections and seismic information.The results of geologic investigation on the surface show that three series source rocks of Permian,Triassic,Jurassic and series of reservoir-caprock combinations developed in Wushi sag.The thermal evolution history of source rocks and the oil seeps proved that petroleum accumulation had happened in the area once.Compression anticlines formed during late Hymalaya tectonic movement were favorable site for petroleum accumulation in Wushi sag.Wensu uplift as part of Tabei uplift was an inheriting paleo-uplift during geologic epoch.It has been separated from Tabei uplift by the huge Kalayuergun fault since Paleocene epoch.Wensu uplift was hold by Wushi and Awati hydrocarbon-generating sag.There are poly exploratory horizons including two rock types of carbonates and sandstones at the same time.Therefore,Wushi sag and Wensu uplift are stratagem-superseding areas for oil and gas exploration with fine prospect in Tarim Basin.
Main hydrocarbon source rocks and contrasts for Awati Sag in Tarim Basin
With the help of the studies on the regional tectonics,main hydrocarbon source rock distribution,geochemical characteristics and so on,the key hydrocarbon source rocks developed in Awati Sag are made clear.The achievements show that there are three sets of the source rocks developed in the sag:Middle- Lower Cambrian series,Middle- Upper Ordovician series and Carboniferous-Permian system:the rocks in the former entered into the generation threshold-low mature stage in the later period of Caledonian orogeny,into the peak mature stage in the early period of Hercynian orogeny,into the peak-high mature stages in the later period of Hercynian orogeny,into the high-over mature stages in the period of Yanshan orogeny,into the over mature stage in the later period of Himalayan;the middle is mainly distributed in Northwest Awati Sag,the immature stage is distributed in the later period of Caledonian orogeny and early period of Hercynian orogeny,the low-over mature stages are in the later period of Hercynian orogeny,the peak mature stage is in the period of Yanshan orogeny,the high-over mature stages are in the later period of Himalayan orogeny.
Distribution and sedimentary environments of Saergan and Yingshan shales of the Middle-Uppler Ordovician in western Tarim Basin
Identification and distribution of marine hydrocarbon source rocks in the Ordovician and Cambrian of the Tarim Basin
DOI:10.1016/s1876-3804(12)60044-5
URL
[Cited within: 4]
Dominant factors controlling hydrocarbon distribution are analyzed from three aspects: the types, structural units and structural belts of the foreland basins of central and western China. There are four types of foreland basins recognized in China, superimposed, reformed, presenile, and newly-generated foreland basins. Hydrocarbon distribution is different in the four types of basins and is controlled by their respective hydrocarbon accumulation conditions, characteristics and patterns. Thrust belts, foredeeps, slope belts, uplift belts, and other structural units are developed in foreland basins. The different controls of these structural units on source rock development and evolution, trap type, hydrocarbon accumulation process, and preservation condition, cause different characteristics of hydrocarbon distribution in different structural belts. The main hydrocarbon enriched structural units are foreland thrust belts, in which the structural styles, tectonic evolution and the preservation of regional cap-rock are the critical factors for hydrocarbon accumulation. The configuration of faults and cap rocks in thrust belts determines the features and enrichment regularity of hydrocarbon and indicates hydrocarbon enriched locations and favorable exploration targets in various structural belts.
Characteristics and hydrocarbon accumulation controlling factors of Neogene Shawan Formation reservoir in Chepaizi uplift, Junggar Basin
The characteristics of the Neogene Shawan Formation reservoirs in Chepaizi uplift are summarized and analyzed according to their layers, sizes, oil qualities, physical property, origins, and types. Lithological, fault-lithological and fault nose traps are the major traps where Shawan Formation was developed. Reservoir characteristics and sizes vary in different reservoirs. The distribution of oil character is very complex. Based on an analysis of the typical reservoirs, the authors hold that long-term inherited paleouplift, efficient blanket-like sand body, fault sealing properties, difference of litholoies and physical properties, and multi-source and multistage accumulation together with storage condition constitute the main controlling factors for the hydrocarbon accumulation. On such a basis, it is pointed out that the blanket sand's lateral pinch zone is the favorable oil accumulation zone. Drilling reveals that the area possesses rich oil resources and shows good prospect for exploration.
Hydrocarbon accumulation rules of Neogene in Xinchun area, Junggar Basin
The hydrocarbon accumulation conditions and rules of Neogene Shawan Formation(N_1s)in Xinchun area are unclear,resulting in the failure of several exploratory wells. The regional structure,oil-source correlation,migration system and reservoir accumulation characteristics were analyzed. The results show that the favorable reservoir accumulation conditions include structural evolution,source rock,reservoir,seal and trap. Hydrocarbon generated from Jurassic in Sikeshu Sag,migrated northward and enriched in the first member(N_1s_1)and second member(N_1s_1)of Shawan Formation. The sand group I of N_1s_1 developed thick sands with large single layer thickness,which plays an important role in hydrocarbon migration and accumulation. N_1s_2 and sand group of N_1s_1 mainly developed sand-shale interbed. Under the vertical communication of small Himalayan faults,the thin sandstone of N_1s_2 is finally accumulated. It is believed that two sets of thick layers of sandstone supply the lateral pathway for the hydrocarbon migration and enrichment,the coupling relationship of unconformity with pinchout sandstones controls the hydrocarbon enrichment of the sand group I of N_1s_1,and the coupling relationship of sandstones with small Himalayan faults developed controls the hydrocarbon accumulation of N_1s_2 and sand group of N_1s_1.
Migration performance of fluvial sand body in middle-shallow layers of Chengdao district
Fluvial sand body is the main carrier bed and reservoir in middle-shallow layers of Chengdao district. The results indicate whether the channel and source fault are connected is the premise of hydrocarbon migration in Fluvial sand body and the structural background influences the direction of hydrocarbon migration. The transporting capacity of sand body in river channel is the best, embankment is the secondary, floodplain sediment is the worst, the transporting capacity of fine-grained sandstone is the best, sandstone is the secondary and the siltstone was the third. The physical properties of the lower limit of migration sand decreases with the increase of depth, the better connectivity of sand body, the better migration performance of sand body. According to the basic types of sand body macro-connection, three types of sand connection patterns are established, i.e. thick sand body direct connection, micro fault connection, and thin or poor physical property sand body connection, and the migration performance evaluation plan of sand bodies in middle-shallow layers was provided.
Tectonic framework of the Tarim basinand its tectonic stress field analysis
Geodynamic evolutionand petroleum system cycle in Tarim Basin
This paper discussed the gdriyndric processes of Tarim basin. The basin had undergone two tectonic mega-cycles, Sinian to Triassic and Jurassic to Quaternary, and subjected to a full process from extension to convergence in such a mega-cycle. The geodynamic evolution Of the basin has been characterized by the process from strong extension to moderate contraction and once again from weak extension to strong convergence. Such geodynamic cycles had played a great role in the hydrocarbon generation, migration, accUmulation and dispersion, The paper sUggested the concept of Petroleum system cycle, and analyzed the Petroleum system cycles of Cambrian to Ordovician, Carboniferous to lower Permian and Jurassic etc. The paper also proposed the studies of the Pened of formation and redistribution of the Petroleum system cycle more practical.
Foreland thrust-fold belt features and gas accumulation in Midwest China
The 15 foreland thrust-fold belts developed in Midwest China are important gas enriching areas in the country. The foreland thrust-fold belts are of united tectonic background: ①The Mesozoic ones belong to the Northern Thetys basin-group, and the depositional basins, which developed under uniform tectonic settings and climatic paleo-zones, formed regional gas enriching Mz to Cz source-reservoir-caprock association. They also belong to the east part of Middle Asia coal related gas accumulation domains. The geological conditions are favorable for the formation of giant gas pools. ②The Cenozoic ones fall into the giant peripheral Qinghai-Tibet Plateau basin-mountain system, which resulted from the uplift of the Qinghai-Tibet Plateau and its northward compression due to the Indian/Eurasian collision. The foreland thrust-fold belts developed in the transitional zone between the mountain and basin. The movement of Indian Plate and Qinghai-Tibet Plateau during the Himalayan era controlled the thrust activities, which controlled the structural traps forming and the gas accumulation.
Episodes and geodynamic setting of Himalayan movement in China
DOI:10.11743/ogg20040201
URL
[Cited within: 2]
In 1945, Mr. Huang Jiqing suggested using the term Himalayan movement to describe the Cenozoic orogenic movement in China. This concept has widely been accepted by the geoscientists. However, disagreements still exist regarding the episodes and geodynamic setting of tectonic movements. Based on analysis of basic concepts suggested previously, and in combination with various data, including geology, geomorphology, magmatic activities and structural deformation, it is proposed that Himalayan movement can be divided into early, middle and late episodes, which are corresponding respectively to tectonic activities in late Eocene, between Paleogene and Neogene, and between Noegene and Quaternary. Collision of Indian and Eurasia plates and continuous compression led to strong compression, shortening and uplifting of Qinghai Tibet Plateau including its southeastern and northern margins. East ward compression of East Asia continent, uplifting of deep lithosphere and back arc spreading resulted in extensional rifting in eastern China and its peri Pacific zone.
Poly-phase differential fault movement and hydrocarbon accumulation of the Tarim Basin, NW China
Structural analysis and hydrocarbon formation history of Wushi-Wensu Area in the Mid-south Tianshan foreland thrust belt
The Wushi-Wensu area is an important transfer zone of the southern Tianshan foreland thrust belt, and has large oil and gas resources. Using outcrop, remote satellite pictures, seismic profiles and gravity data, we built a structural model of this area, restored it's evolutionary history, and further discussed it's gas and oil formation process. The results show that the structural style transforms laterally, and the area is comprised of two units. The northern part, Wushi sag, is a contractive fault depression, while the southern part, Wensu uplift, is a paleo-uplift controlled by basement-involved reverse faults. The main deformation occurred during late Oligocene to early Miocene and Pliocene to Quaternary, caused 13~21 km horizontal shortening with 14%~20% shortening rate and average shortening speeds of 0.56~0.9 mm/a. There are abundant traps, which are controlled by the evolutionary history of the Northern margin of the Tarim basin. The traps in the Wensu uplift formed earlier, and the traps in the Wushi sag formed later. The difference in trap evolution controlled the reservoir formation.
Structural pattern and its control on hydrocarbon accumulations in Wushi Sag, Kuche Depression, Tarim Basin
DOI:10.1016/S1876-3804(08)60092-0
URL
[Cited within: 2]
Affected by the South Tianshan orogenesis, the faulted structure patterns in the Wushi Sag are very complex. Aimed to find out the basic characteristics of the structures, this article studies many seismic profile sections, the regional geology, the structural patterns, and the formative stage of the structures. The Wushi Sag has three main fault patterns (face to face thrust, back to back thrust, sphenoid thrust), and many local fault structural patterns (fault-bend fold, fault-propagation fold, duplex structure, outburst structure, growth structure, etc.). There mainly develop four groups of reverse faults (NE, NEE, NW, EW) and one group of NNW trending strike-slip faults. According to the structural background, seismic profiles, and balanced cross sections, the main fault in the studied area experiences at least five evolving stages, which are, pre-Mesozoic, late Permian – early Tertiary, Jurassic, late Jurassic – early Cretaceous, and late Neogene. The structural styles have a close relationship with source rocks, hydrocarbon reservoirs, and the character and distribution of petroleum systems.
Characteristics of J2q source rocks in Kuqa Depression
Organic maturity and hydrocarbon generation history of the Mesozoic oil-prone source rocks in Kuqa Depression, Tarim Basin
Geochemical characteristics and origins of the oils in Wushi sag, Tarim Basin
DOI:10.11764/j.issn.1672-1926.2014.01.0062
URL
[Cited within: 1]
Great exploration potential in Wushi Sag has been revealed by the successive breakthroughs in Wells of Wucan1,Shenmu1and Shenmu2.By using gas chromatographs of saturated and light hydrocarbons,we comprehensively studied the geochemical characteristics of the oils in the Wushi Sag and compared them with typical terrestrial and lacustrine(marine)oils in other parts of the Tarim Basin.The pristane/ phytane ratio in the Shenmu2and Wucan1 Wells is 2.35and 3.53,respectively,obviously demonstrating that it is of coal-derived origin.The oils in the Wushi Sag has a relatively high content of aromatics(benzene and toluene),methylcyclohexane and cyclohexane while a low content of cyclopentane and acyclic hydrocarbons,indicating that a great contribution from the terringenous organic matters was made.The thermal maturity of the oils in the Wushi Sag is at the early period of the high-mature stage with the heptane index ranging from 25.0%to 27.6%and the isoheptane index ranging from 2.5%to 2.7%,respectively. Water washing and biodegradation of oils in the Wushi Sag were not observed.The oils in the study area have obviously experienced the evaporative fraction,resulting in the increase of aromatics and the decrease in n-alkanes.By means of Oil Correlation Star Diagram(OCSD)and the Triangular diagrams of C7 hydrocarbon,the oils in Wushi Sag was derived from humic organic matters,the same as those in the Dina2gas-condensates field,which were distinctively different from the lacustrine and marine oils in the Kelasu structure and Tazhong1Gasfield.From all geochemical features above,it is concluded that the oils in the Wushi Sag were derived from the Jurassic coal measures.
The main controlling factors of reservoir physical property and oiliness in the structural lithologic reservoirs in the east of Wushi sag, Kuqa foreland basin
DOI:10.11764/j.issn.1672-1926.2016.06.0994
URL
[Cited within: 1]
The Yilake condensate gas reservoir and Shenmu-1oil reservoir are structural-lithologic reservoirs in the east of Wushi Sag,Kuqa foreland basin.The sedimentary facies and physical properties of the Cretaceous Shushanhe Formation had a very fast change,and the distribution of oil,gas and water is very complex.The controlling factors of reservoir physical properties and oiliness are unclear,which were analyzed in this paper,based on conventional reservoir physical property analysis,micro pore throat analysis by Nitrogen Adsorption and Mercury Injection experiment,and the oiliness analysis by Quantitative Fluorescence Technique.The result shows that reservoir sedimentary facies and lithology are the key factors to control reservoir physical properties.Themouth bars,distributary channelsin fan delta front and braided delta front deposits are better than other sedimentary facies.The gritstone and medium-grained sandstone in fan delta front have the best physical properties.The study area has no native source rock,hydrocarbon migrates a long distance from the east Talake area to Wushen area through the unconformity-fault pathway combination,so there are two factors controlling the oiliness of reservoirs.One is the fault that controls oil and gas migration,hydrocarbon accumulate in the local high positions near fault;the other is the physical property of reservoir,in the same sand sets reservoir physical property controls the oiliness.The favorable structural-lithologic traps are the next exploration direction to the east of Wushi Sag,the direction to petroleum sources.Meticulous depiction of local structure map and sand body is the key for the exploration and development in the study area.
Types of hydrocarbon migration pathways and its controlling effects on hydrocarbon distribution in Tarim Basin
DOI:10.1016/j.clinph.2009.07.032
URL
[Cited within: 1]
Tarim Basin is the largest hydrocarbon-bearing basin in the west part of China and has the most abundant hydrocarbon resources.The commercial oil and gas flows were found from the Tertiary to the Cambrian,except the Permian.Oil and gas reservoirs were also found on the plane from Kuche Depression in the north part to the Southwest Depression in the south part of Tarim Basin.Broad and vertical distribution of hydrocarbon is closely related to the extensive hydrocarbon migration pathways.Many kinds of hydrocarbon migration pathways such as unconformity,fault,fracture,permeable carrier bed and volcanic plug systems were developed in Tarim Basin.Among them,the fault is the efficient pathway for vertical migration of oil and gas and controls the vertical distribution of reservoirs,while the unconformity is the pathway for long-distance lateral migration of oil and gas and controls the transverse distribution of reservoirs.The permeable bed is the pathway for long-distance lateral migration and is also the important reservoir of oil and gas.The fractures have the important roles in improving the physical properties of reservoir rock,enhancing the permeability and reducing the resistance to oil-gas migration.The paper summaried the control factors for the validity of the hydrocarbon migration pathways and its controlling effects on hydrocarbon distribution,which would provide an important reference for the better understanding of hydrocarbon distribution in Tarim Basin.
Adjustment and alteration of hydrocarbon reservoirs during the late Himalayan period, Tarim Basin, NW China
DOI:10.1016/S1876-3804(12)60096-2
URL
[Cited within: 1]
To figure out the oil and gas distribution pattern in the Tarim Basin, the adjustment and reformation of oil and gas reservoirs under the background of late Himalayan Orogeny are analyzed. Strongly affected by the tectonic movement, the oil and gas reservoirs in the Tarim Basin experienced secondary actions in physical adjustment and chemical alteration: on the one hand, during the course of physical adjustment, reversed strata caused the early-formed oil to seep vertically and to migrate laterally along the sandstone in large scale with long distance; on the other hand, in the process of chemical alteration, the deposition of massive strata accelerated the thermal evolution of organic matter, generating large amounts of cracking gas, which went into the pre-existing reservoirs and led to big change in oil and gas properties and the coexistence of heavy oil, light oil, waxy oil and condensate gas in the same area. The physical adjustment and chemical reformation of oil and gas reservoirs in late Himalayan period resulted in the lateral differential distribution of oil and gas in large area and multiple vertical oil-bearing layers with complex and diverse oil and gas properties.
History of hydrocarbon accumulations spanning important tectonic phases in marine sedimentary basins of China: Taking the Tarim Basin as an example
DOI:10.1016/S1876-3804(11)60010-4
URL
[Cited within: 1]
Subaerial marine sedimentary basins in China have undergone three phases of important tectonic changes at the end of the Early Paleozoic, Late Paleozoic-Early Mesozoic and Late Cenozoic. These tectonic changes have exerted strong impacts on hydrocarbon generation, migration and accumulation as well as on the occurrence and distribution of hydrocarbon reservoirs. The Tarim Basin is a typical example of multiphase hydrocarbon accumulations in a marine basin of China. The Early Paleozoic tectonic framework laid a geological basis for the marine setting of hydrocarbon formation in the Tarim Basin, but most accumulations were destroyed later due to frequent tectonic movements so that only bitumen is left at present. In the Late Paleozoic, the Ordovician source rock located in the western Manjar Sag entered peak generation stage and hydrocarbons successively accumulated at its southern and northern uplifted positions. Tectonic compressional deformation developed in the Early Mesozoic and the subsequent uplift and denudation allowed hydrocarbons accumulated at this phase to suffer from bacterial degradation. In the Late Cenozoic, the superimposition of basinal margin reconstruction and terrestrial molasse formation accelerated the formation of secondary cracking of hydrocarbons to gases in the platform of the basin, the influx of deeper natural gases dissolved in crude oils accumulated early and a unique large-scale condensate pool was then formed. The three tectonic changes controlled not only the formation of the marine basin, the sedimentation and evolution of source rocks, and the formation of reservoirs, but also the accumulation process and alteration of hydrocarbons in the marine sedimentary basin.
Important role of the formation of gas accumulations in the late stage in the formation of large gas fields
