Hydrocarbon potential and accumulation model in Chepaizi uplift, Junggar Basin

Transcription

1 CHINA PETROLEUM EXPLORATION Volume 21, Issue 3, May 2016 Hydrocarbon potential and accumulation model in Chepaizi uplift, Junggar Basin Song Mingshui 1, Lü Mingjiu 2, Zhao Leqiang 1, Sui Fenggui 1 1. Sinopec Shengli Oilfield Company; 2. Sinopec Henan Oilfield Company Abstract: Sinopec s licensed block in the western margin of the Junggar Basin is predominantly located in the Chepaizi uplift. Since it is far from oil sources and has complicated geological conditions, the block is far less explored with a protracted exploration cycle. To clarify the hydrocarbon potential in the block and thereby provide effective guidance for future exploration operations, in-depth research was conducted based on the distribution of formations and sedimentary facies within the Chepaizi uplift. The priority of the research was to systematically analyze oil sources, reservoir-cap rock assemblages, and other reservoir conditions. Research results show that the study area has favorable petroleum geological conditions for hydrocarbon accumulation. As the final destination for the long-term migration of hydrocarbons, the block has great hydrocarbon potential. Through discovering correlations between oil sources and investigating accumulation periods, the fault-blanket sandstone conductivity was determined, and the hydrocarbon accumulation model characterized by hydrocarbons supplied by lateral sources, with faults providing vertical conductivity and blanket sandstones providing horizontal conductivity, and accumulations occurring at blanket margins was established. In accordance with these conclusions, drilling operations were successfully deployed, and a series of discoveries with commercial value were obtained. Key words: hydrocarbon potential, fault-blanket sandstone conductivity, accumulation model, Junggar Basin, Chepaizi uplift Sinopec s licensed block in the western margin of the Junggar Basin ( the West Junggar Margin Block ) is predominantly distributed in the Chepaizi uplift, covering an area of 3560 km 2. The oil and gas exploration in the Chepaizi uplift began in the 1950s. The block is far from oil sources and has complicated geological conditions, making it an area far less explored with a protracted exploration cycle. The exploration in this block is restricted by two key factors. Firstly, the basic geological conditions in this block have not been thoroughly analyzed, including the structures, strata, sedimentation, oil sources, and reservoir-cap rock assemblage. Therefore, it is necessary to evaluate if this block has great discovery potential in order to determine if its exploration is worth investing in. And secondly, it is far away from the hydrocarbon kitchen, and the external source accumulation mechanisms and enrichment laws associated with situations where traps and sources are far apart are not well understood, so it is necessary to identify oil and gas enrichment zones and confirm favorable targets. Since 2010, further analysis has been made on basic geology and accumulation conditions based on various types of exploration data. Guided by the new insights, drilling operations were successfully deployed, and the Chunfeng oilfield, an oil field with ten million tons of resources, was discovered. Thus, the oil-bearing area of the Chunguang oilfield has been extended. After the hydrocarbon potential in the West Junggar Margin Block was reviewed, the hydrocarbon accumulation process was analyzed, and a hydrocarbon accumulation model was established for the areas in the basin margin that are far away from the hydrocarbon source so as to provide additional reference for exploration in similar blocks. 1. Overview The Chepaizi uplift is an inherited uplift that is developed above the Carboniferous volcanic basement. It is adjacent to the Zaire Mountain in the northwest, bounded by the Sikeshu sag in the south, and connected to the Changji sag through the Hongche faulted belt in the east (Fig.1). On the whole, it is shaped like a triangle, with its main part striking NW-SE. The main part of the Chepaizi uplift has been in an arching state since the Late Hercynian, and it is currently a wide and gentle slope. During the formation of the uplift, the structure was configurationally simpler even though it suffered multiphase tectonic actions during the Late Hercynian, Indosinian, Yanshan, and Himalayan. Its current structural pattern is still the same appearance that it had Received date: 01 Feb. 2016; Revised date: 26 Mar Corresponding author. songmingshui.slyt@sinopec.com Foundation item: China National Major S&T Project Forming Rule and Exploration Orientation of Large/Medium Oil and Gas Fields in Clastic Rock Reservoirs in the Junggar Basin (2011ZX05002). Copyright 2016, Petroleum Industry Press, PetroChina. All rights reserved.

2 2 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 after the Himalayan movement [1 2]. It is shown in the near EW geological structure section that the region from the Chepaizi uplift to the center of the Junggar Basin is divided into three zones, i.e., the overlap & denudation zone, the step fault zone, and the sag zone (Fig.2). The Chepaizi uplift is buried at a depth of m. From the bottom to the top, the Jurassic, Cretaceous, Palaeogene, and Neogene are developed with the Carboniferous as the basement, but the Permian and Triassic are absent [3 5]. The strata of each period are thinner and pinch out to the northwest. The Neogene is subdivided into the Shawan Formation and Taxihe Formation. The Shawan Formation is further divided into three members (Table 1). 2. Hydrocarbon accumulation conditions and hydrocarbon potential of the West Junggar Margin Block 2.1. Stratigraphic distribution Fig. 1 Schematic diagram of structural units in the western margin of the Junggar Basin The structural evolution that has occurred in the northwestern margin of the Junggar Basin since the Late Carboniferous is divided into three stages, and it controls the stratigraphic distribution [6 7]. Fig. 2 Structural profile of the Chepaizi uplift-changji sag in the east-west direction (1) Strong uplifting stage of the Late Carboniferous-Jurassic. The orogenesis along the northwest of the Junggar Basin during the Late Carboniferous led to the continuous uplifting of the West Junggar orogenetic fold belt and then its overthrusting to the southeast. Due to the existence of the barrier of the North Tianshan belt, the southwest part of Chepaizi uplifted violently and resulted in the formation of the Chepaizi uplift. In the meantime, the Hongche fault zone formed on the east of the Chepaizi uplift due to the eastward compressive thrusting. The Hongche fault zone had a steep dip and a short overthrust distance, and it generated the faulted uplift at the basement of the Chepaizi uplift with vertical uplifting denudation as its dominant characteristic. During the Permian-Triassic movement, the Late Hercynian movement, the Indosinian movement, and the Early-Middle Yanshan movement, the Chepaizi uplift underwent a continuous violent uplifting, and its main part was exposed above the water for a long period. As a result, the Permian, the Triassic, and most Jurassic strata are absent in its main part. During the Indosinian

3 Song Mingshui et al., Hydrocarbon potential and accumulation model in Chepaizi uplift, Junggar Basin 3 Table 1 Formations of the Chepaizi uplift, the Junggar Basin Horizon System Series Formation Member Neogene Horizon symbol Average thickness/m Lithology Pliocene Dushanzi N 2 d Grey mudstones with interbedded light grey argillaceous siltstones Miocene Taxihe N 1 t 380 Red and grey mudstones alternating with argillaceous siltstones Shawan 3 N 1 s N 1 s N 1 s Paleogene Palaeocene E 60 Cretaceous Lower Tugulu Group K 1 tg 130 Jurassic Lower J 60 Carboniferous Upper C Red argillaceous rocks with interbedded red argillaceous siltstones, grey fine sandstones, and sandy conglomerates Red argillaceous rocks with interbedded red argillaceous siltstones, grey fine sandstones, and pebbled fine sandstones Grey glutenites, pebbled sandstones, and medium-fine sandstones with interbedded grey argillaceous rocks Grey pebbled fine sandstones, argillaceous siltstones with interbedded grey pebbled mudstones Nonuniform interbeds of grey green and brown red mudstones and sandstones, with grey green conglomerates at the bottom Grey brown glutenites and grey conglomeratic sandstones with interbedded thin grey black medium conglomerates (Unpenetrated) Interbeds of grey black tuffs and metamorphics with different thickness and Yanshan movements, faulting activities were stronger in the Chepaizi uplift and generated several small faulted sags, and the deposited Triassic and Jurassic strata were thinner. (2) Slow subsiding stage of the Cretaceous-Paleogene. The Chepaizi uplift began to subside slowly during the Early Cretaceous, which resulted in the creation of its characteristics of differential subsidence. Subsidence occurred at a low amplitude and rate in the east and the southeast of the Chepaizi uplift, and the middle and lower parts of the Lower Cretaceous were deposited with a lesser thickness. In the northwest, however, the Chepaizi uplift was still uplifting, so the Lower Cretaceous was not deposited. During the middle and late stages of the Early Cretaceous-Late Cretaceous, the subsidence amplitude was much smaller in the Chepaizi uplift, so the upper Lower Cretaceous and the Upper Cretaceous were absent in most areas of the Donggou Formation, but they were deposited with less thickness near the Hongche fault zone. After the Paleogene, the Chepaizi uplift was still dominated by a slow subsiding and the differential subsidence was more obvious. It was only in the southeast of the Chepaizi uplift that subsidence happened in a small area and the Paleogene Ziniquanzi and Anjihaihe Formations were deposited with a lesser thickness. (3) Rapid subsiding stage of the Neogene-Quaternary. Due to the Himalayan movement after the Neogene, the North Tianshan uplifted strongly and its northward thrusting resulted in a sharp flexture subsidence in the southern Junggar Basin. Consequently, the basin tilted to the south, and the North Tianshan piedmont foreland basin formed. The Chepaizi uplift, as a part of this foreland basin, subsided rapidly with a larger subsidence range and amplitude. The characteristics of differential subsidence were exemplified during the sedimentation of Shawan Formation. The Shawan Formation was absent at the piedmont zones in the northern and western Chepaizi uplift. After the sedimentation of the Shawan Formation, the subsidence ranges and amplitude of the Chepaizi uplift got larger until the uplift was wholly covered with water, and the Taxihe Formation, the Dushanzi Formation, and the Quaternary were deposited with a larger thickness Sedimentary facies distribution The Mesozoic-Cenozoic formations in the Chepaizi uplift present typical characteristics of overlap & denudation zones with multiple unconformities and sequences. The Jurassic-Neogene strata in this area are divided into 4 second-order sequences and 8 third-order sequences (Fig.3). Each sequence is overlapped and denudated layer by layer to the basin margin, and LST (lowstand system tract) and TST (transgressive system tract) overlaps and HST (highstand system tract) denudation are obvious within the sequence. Under the joint combination action of system tracts and stratigraphic overlaps, a sedimentary cycle that is coarse at the lower part and fine at the upper part is generated The LST is mainly composed of fan delta and braided delta glutenites. The TST is mainly composed of lacustrine mudstones with interbedded delta and beach bar sandstones. At the upper part of the HST, the coarse clastics are mostly denudated, and shore-shallow lacustrine mudstones and sandstones are preserved (Fig.3). In the sequences above larger unconformities, coarse clastics are developed, with N 1 SQ 1 present as the most developed one. The LSL coarse clastics are thick, extensive, and distributed in the shape of blanket. The TST-HST coarse clastics are thin and distributed in a limited range (Fig.4) Reservoir-cap rock assemblage During the Jurassic-Cretaceous and the Cenozoic, the Chepaizi uplift was characterized by its basin margins, shallow water, gentle slope, multiphase stratigraphic tilting,

4 4 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 Fig. 3 Stratigraphic column of the Mesozoic-Cenozoic formations in the Chepaizi uplift and residual distribution. It underwent the structural and sedimentary evolution of multiphase transgressive overlap-uplifting denudation and presented the sedimentary evolution features of multi-cycle positive superimposition.

5 Song Mingshui et al., Hydrocarbon potential and accumulation model in Chepaizi uplift, Junggar Basin 5 In the early stages of LST-TST sedimentation, a suite of stable floor sandstones (glutenites) that were regionally distributed was developed in the setting of a wide, gentle, and flat slope, and then it was covered with overlying transgressive meandering or shore-shallow lacustrine mudstones. As a result, multiple reservoir-cap rock assemblages were formed [8]. In particular, the Neogene Shawan Formation and the Cretaceous assemblages were the best rock assemblages that were developed during this time. In the Neogene Shawan Formation assemblage, the reservoir is mainly composed of pebbled coarse sandstone, medium-fine sandstones, and glutenites in the distributary channel, mouth bar, and beach bar of the fan delta front subfacies in the middle and lower parts of the Shawan Formation. The sandstones are medium-well and are characterized by their high compositional maturity, weak diagenetic compaction, and unconsolidated-loose cementation. It is a good reservoir with a high porosity and permeability, and it is also thick and has a stable lateral distribution. The mudstones in the middle and upper of the Shawan Formation and the argillaceous rocks of the Taxihe and Dushanzi Formations act as regional cap rocks. In the Cretaceous assemblage, the reservoir is mainly composed of fine sandstones and siltstones of the shore-shallow lacustrine beach bar and the underwater channel sand of the delta front subfacies. As a result, the reservoir is a medium-high porosity and permeability reservoir. The cap rock is composed of the lacustrine mudstones in the middle and upper parts of the Cretaceous. On the whole, the Cretaceous reservoirs in the Chepaizi uplift are small, and they are mainly composed of lacustrine mudstones or thin interbeds of sandstones and mudstones In the Paleogene and Jurassic strata, coarse clastics are developed and mudstones are regionally developed. The Paleogene and Jurassic reservoir-cap rock assemblages are poorer than those of Neogene and Cretaceous. In the Chepaizi uplift, stratigraphic downtruncation and onlap occur and multiphase unconformities are vertically superimposed. It is favorable for the formation of subtle traps, such as multi-strata distributed stratigraphic onlap and truncation, lithologic, and stratigraphic-structural traps Hydrocarbon source rock Fig. 4 Distribution of sedimentary facies and sand bodies in the N 1 SQ 1 system domains, the Chepaizi uplift The principal source rocks were evaluated again as part of a process to reinforce geological research results and support new exploration data. Their spatial distribution and hydrocarbon supply potential were further confirmed. In general, the Chepaizi uplift is adjacent to the Changji sag in the east and bounded by the Sikeshu sag in the south, so it is the preferable long-term target of two hydrocarbon generation centers, which makes it so that the Chepaizi uplift is sourced by a bidirectional hydrocarbon supply. In the

6 6 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 Changji sag, two sets of source rocks are developed, i.e., the Lower Wuerhe Formation of the Middle Permian and the Badaowan Formation of the Lower Jurassic. In the Sikeshu sag, the source rocks of the Lower Jurassic Badaowan Formation are developed. The source rocks in both sags are at a hydrocarbon generation threshold. In the Changji sag, shallow-semi-deep lacustrine dark mudstones m thick are developed in the Lower Wuerhe Formation in the Middle Permian (Fig.5). The maturity of the Lower Wuerhe Formation is the highest in the center of Changji sag, and its vitrinite reflectance is over 2% and declines gradually when moving to the north, east, and west. It is mostly at mature or highly mature stage. The Jurassic is a suite of limnetic coal bearing sedimentary formations, and currently, its mature zone is distributed in the southern Junggar Basin. The dark mudstones of the Lower Jurassic Badaowan Formation are widely distributed in the Junggar Basin. The central Junggar is cut by the Mosuowan uplift and two thick source rock zones are formed. The source rocks are more than 400 m thick in the Well Sha 1 and Well Zhuang 2 areas of Block Zhong 1, but they get thinner to the west in the Sikeshu sag. They are 102 m thick in Well Sican 1. In the Sikeshu sag, the Badaowan Formation dark mudstones are distributed in a smaller area with a thickness of m. Based on the source rock distribution analysis, it can be concluded that there are hydrocarbon enriched sags with two source kitchens and multiple hydrocarbon generation strata in the periphery of the Chepaizi uplift. Some new insights were obtained. In particular, it was discovered that the thickness and the maturity of the Jurassic source rocks in the Sikeshu sag were actually higher than previously believed, and so, its estimated level of oil and gas resources were increased significantly. As a result, the hydrocarbon supply conditions in the Chepaizi uplift were greatly improved Hydrocarbon potential Hydrocarbon generation dynamic parameters for various types of source rocks were confirmed by using the latest source rock geochemical data and thermal simulation experiments. Then, 3D simulations were performed on the oil and gas conductivity of the Chepaizi uplift and its peripheral sags under the constraint of the current thermal evolution (Fig.6). The result shows that the charging direction of the Permian source rocks is from the southeast to the northwest, so the favorable exploration zones are located in the area where Wells Pai 2-Pai 60 are located. The Jurassic source rocks have two charging directions, so exploration for favorable oil and gas sources now focuses on Wells Ka 6-Pai 60. Therefore, the Chepaizi uplift is the most favorable hydrocarbon migration and accumulation area due to its characteristic of double-source convergence. The West Junggar Margin Block has the resource basis for great discoveries with its calculated oil and gas resource amount of t. 3. Hydrocarbon migration and accumulation process and accumulation model 3.1. Oil and gas source Fig. 5 Distribution of dark mudstones in different layers of the Junggar Basin Based on the comparative analysis of crude oil and source rock geochemistry, the oil and gas in the Chepaizi uplift is mainly sourced from the Permian of the Changji sag and the Jurassic of the Sikeshu sag. The oil and gas maturity and migration index indicates long-distance oil and gas

7 Song Mingshui et al., Hydrocarbon potential and accumulation model in Chepaizi uplift, Junggar Basin 7 Fig. 6 Pattern of hydrocarbon migration and accumulation in the Chepaizi uplift migration. The αββ20r/ααα20r, which reflects the migration distance, increases with the increasing maturity, and its maximum value is about 2.8. Oil and gas charges prefer traps with a short trap-source distance. As maturity increases, the oil and gas migration distance also increases, and oil and gas start to present an obvious chromatographic effect Hydrocarbon accumulation phase Based on source rock evolution analysis, the source rocks of the Middle Permian Lower Wuerhe Formation in the Changji sag reached the maturity threshold during the end of the Triassic-Early Jurassic. It stopped uplifting during the Middle-Late Jurassic. During the Cretaceous-Paleogene, it entered into a high-maturity stage and reached a hydrocarbon generation peak. Nowadays, it is at an over-maturity stage [9 12]. After the Paleogene, the Jurassic source rocks (maturity 0.6%) began to wholly generate hydrocarbon so that currently, it is at a mature hydrocarbon generation peak stage. Corresponding to the three hydrocarbon generation stages, the Mesozoic-Cenozoic fluid inclusion in the whole Chepaizi uplift also records three hydrocarbon-bearing fluid activities, i.e., C, C and C. Specifically, the principal exploration layer, the Shawan Formation, was deposited at a late period and was charged much later, predominantly during the Quaternary Oil and gas conductivity system The West Junggar Margin Block is far away from the source rocks, so a conductivity system is quite critical for the formation of large-scale industrial accumulations in this block. There are many sandstone-mudstone positive cycles and angular conformities in the Chepaizi uplift, but previous research showed that the clastics angular conformities are unfavorable for long-distance lateral migration and that unconformities are not the primary migration pathways [13]. Therefore, the long-distance oil and gas migration in this area is mainly facilitated via faults and framework sand bodies. The result shows that the oil sources between the sags and the uplifts suffer fracturing for a long period of time, so as a result, the deep source rocks communicate with the upper isolated sand bodies via the secondary minor fractures which are developed at a later stage. LST-TST sand bodies are developed in the setting of wide, gentle, and flat slopes that constitute blanket carrier beds for lateral migration. As a result, fault-blanket sandstone patterns act as a distal hydrocarbon migration framework Vertical conductivity of faults Faults are the primary and quickist migration pathways of fluids from the deep layers to the shallow layers [14 15]. The Hongche and West Aika faults that are developed in the periphery of the Chepaizi uplift are the primary oil source faults in this area. Due to the Himalayan movement, a large number of extensional or tenso-shear normal faults formed within these two major faulting systems after the Neogene, and these faults constitute the shallow faulting systems (Fig.7). The Hongche fault is the discordogenic fault that separates the Chepaizi uplift from the Changji sag. It is predominantly a reverse fault, with defining characteristics including a NS strike and a west dipping with a propagation distance of 120 km and a maximum throw of 3000 m. It was, to some extent, reversed at the late stage. Regional seismic interpretation and analysis indicate that the Hongche fault is not a single fault, but is rather composed of two faults (i.e., the East Hongche and West Hongche faults). The West Hongche fault cuts through the Carboniferous basement, Permian, Triassic, Jurassic, Cretaceous,

8 8 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 Fig. 7 Faulting system of the Neogene Shawan Formation in the western margin of the Junggar Basin Paleogene, and Neogene. The development period of the Hongche fault was long and its activity lasted after the Neogene, so the migration pathways were active for a long period of time and provided good vertical pathways for the oil and gas generated in the Changji sag to migrate to the Chepaizi uplift. The geochemistry of the fluids on both sides of the Hongche fault was previously investigated. It revealed that the closer the fluids were to the fault, the higher their organic inclusion abundance was. The frequency and abundance of hydrocarbon bearing fluids recorded at the lower wall was significantly higher than that recorded at the upper wall. The homogenization temperature and salinity variation range of the upper wall was greater than that of the lower wall. The Fe-Mg-Mn content of the calcite cement in the main zone is accordant with that of the upper wall. All these characteristics reveal that hydrocarbons definitely migrated into the reservoirs through the Hongche fault zone, with the preferred migration direction being an updipping direction. The West Aika faulting system is located in the western section of the Aika fault zone, and it is mainly comprised of the North Aika fault, No.1 West Aika fault, No.2 West Aika fault, and No.3 West Aika fault. It is the major fault that separates the Chepaizi uplift from the Changji sag. The North Aika fault along the south of the Chepaizi uplift is a normal fault of near EW striking and southern dipping, with a propagation distance of 26 km. It began to grow at the end of Indosinian epoch, and its activity reached a peak during Yanshan epoch and then weakened until the early Himalayan. No.1, No.2, and No.3 West Aika faults are all NW striking and NE dipping, and their areal propagation distances are 16 km, 13 km, and 25 km respectively. Their activity stage was during the Indosinian-Yanshan epoch. The No.1 West Aika fault was still active during the early Himalayan, but the No.2 and No.3 West Aika faults got less active during the middle Yanshan. Geochemical migration parameters confirm that the West Aika fault is the primary vertical conductivity pathway and that it plays an important role in the migration of oil and gas from the Sikeshu sag to the Chepaizi uplift Horizontal conductivity of blanket sandstones Sand bodies are the primary lateral oil and gas migration pathways, and their conductivity is controlled by their areal distribution, connectivity, and physical properties. In the Chepaizi uplift, the sandstones developed in the first member of the Neogene Shawan Formation and the Jurassic are good framework sand carrier layers, especially the sand bodies in the first member of the Neogene Shawan Formation, which are LST-TST sand bodies developed at the top of regional unconformities. The blanket sandstones are tens to hundreds of meters thick and are widely distributed with a good lateral connectivity. The sand bodies are characterized by a large thickness, a wide distribution, and a good porosity and permeability with a porosity of 9.1%-32%, that averages out at20.41% and a permeability of md that averages out at 61.4 md, all of which indicates their viability to act as lateral hydrocarbon migration pathways. The oil and gas shows in the first member of the Shawan Formation are abundant and occur at different degrees in nearly all of the exploration wells (Fig.8). Oil layers, dry layers, and hydrocarbon migration pathways can be quickly identified on the basis of the QGF index derived from quantitative particle inclusion hydrocarbon fluorescence analysis [16], which provides an important basis for determining effective conductivity. The QGF index is the highest at Well Pai 8 in the south and Well Pai 606 in the center, and the QGF indices of sampling intervals in both wells are higher than 5. The QGF indices of Wells Pai 9, Cheqian 1-7, and Pai 204 take second place, and the QGF indices of all testing intervals reach at least 4 and come close to 5. In Wells Pai 20, Pai 206, Pai 7, and Pai 19, only several intervals have a QGF index of 4. The QGF index of testing samples in the northern area is higher than that of the southern and central areas. The highest QGF index in the north occur at Wells Pai 60, Pai 613, and Pai 103, and their sampling interval QGF indices are all over 4. The QGF index of the

9 Song Mingshui et al., Hydrocarbon potential and accumulation model in Chepaizi uplift, Junggar Basin 9 Fig. 8 Distribution of QGF index in sand bodies of the Shawan Formation in the Chepaizi uplift samples (oil traces indicated by mud logging) acquired from the calibration well Pai 612 at a depth of m is 3.76, which is taken as the lower conductivity limit. Based on this value, the blanket sandstones in the first member of Shawan Formation are effective hydrocarbon migration pathways with multiple preferential migration directions Hydrocarbon accumulation positions Under the domination of stratigraphic lithofacies and unconformities, a large number of stratigraphic overlap and truncation traps are formed near the wedge out (blanket margin), such as the layers and sand bodies in overlap & denudation zones. Multiple oil and gas reservoirs in the Chepaizi uplift were analyzed in detail, and the results showed that hydrocarbon accumulates mainly at blanket margins through faults-blanket sandstones and that these sandstones produce spatial distribution patterns of Shawan Formation oil and gas in the Chepaizi uplift [17 18]. Oil source faults cut through horizons and control the vertical distribution of oil and gas. The Jurassic, Cretaceous, and Shawan Formation sand bodies and even the Carboniferous volcanic rocks in the Chepaizi uplift are directly in contact with the Hongche fault. As a result, 4 oil and gas show horizons are generated, especially the sand body at the bottom of Shawan Formation, which is thick, extensive, and laterally communicated. The sand bodies that are distributed in the shape of a blanket constitute lateral hydrocarbon migration pathways with micro predominance and their presence ensures that this layer is the primary hydrocarbon enrichment horizon in the Chepaizi uplift, with large lithologic-stratigraphic oil reservoirs Hydrocarbon accumulation model The Chepaizi uplift is always the preferential hydrocarbon migration target zone, and the hydrocarbon accumulation in this area is characterized by a lateral-source hydrocarbon supply, long-distance hydrocarbon migration, multiphase charging, late stage domination, vertical conductivity of faults, lateral conductivity of blanket sandstones, and lithologic-stratigraphic trap accumulations at blanket sandstone margin. During the hydrocarbon accumulation with the Permian source rocks as the dominant (E-Q), the faults (e.g. the Hongche fault) connected to the sources opened, and the framework sand bodies (e.g. the Neogene Shawan Formation) were shallow with good physical properties, a certain dip angle, and a high conductivity index. Additionally, the traps and source rocks were communicated because of the effective configuration of faults and blanket sandstones. Fault-blanket sandstone conductivity was overall very good. Oil and gas firstly migrated along blanket sandstones and then faults, and accumulated at blanket margins, forming the Neogene Shawan Formation lithologic-stratigraphic oil reservoir. At the late stage, however, the crude oil was degraded and thickened. During the hydrocarbon accumulation with the Jurassic source rocks as the dominant (Q), the faults (e.g. Hongche and West Aika faults) connected to the sources opened vertically for hydrocarbon migration, and the framework sand bodies of the Neogene Shawan Formation, Jurassic, and Cretaceous provided lateral conductivity, so hydrocarbons accumulated at blanket margins and the late thin oil reservoirs formed. To clarify and summarize, the external distal hydrocarbon accumulation model of the Chepaizi uplift was built up with the conductivity system as the core, i.e., hydrocarbons supplied by lateral sources, with faults providing vertical conductivity and blanket sandstones providing horizontal conductivity, and accumulation occurring at blanket margins (Fig.9). 4. Application results The proposed hydrocarbon accumulation model of the

10 10 CHINA PETROLEUM EXPLORATION Vol. 21, No. 3, 2016 Fig. 9 Hydrocarbon accumulation model in the Chepaizi uplift West Junggar Margin Block plays an important role in guiding successful exploration practices. A giant oil field with billions of OOIP (original oil in place), the Chunfeng Oilfield was discovered and confirmed in the Shawan Formation along the eastern flank of the Chepaizi uplift, and its discovery expanded the oil-bearing area of the Chunguang oilfield. A high-yield reserve block was also discovered along the western flank of the Chepaizi uplift. While the Shawan Formation is prioritized as the primary exploration target, some significant oil and gas discoveries were obtained in the Carboniferous, Cretaceous, and Jurassic, and their proved, controlled, and predicted oil in place values were submitted to the authorities. Since 2010, the practical exploration results in the West Junggar Margin Block have been fruitful, with proven, probable, and possible oil reserves of t, t, and t, respectively, and an available productivity of over t. The successful exploration in the western margin of Junggar Basin serves as the embodiment of the forward-looking guidance of potential analysis and accumulation research on exploration and production. Solid analysis on basic geological data and accumulation laws provides the preconditions and insurance necessary for ensuring continuous exploration breakthroughs and expansion along with the high-efficiency development of this block. The hydrocarbon accumulation model of hydrocarbons supplied by lateral sources, with faults providing vertical conductivity and blanket sandstones providing horizontal conductivity, and accumulations occurring at blanket margins which was established by analyzing the hydrocarbon accumulations in the Chepaizi uplift, solves the problems related to distal hydrocarbon accumulation and distribution predictions. Furthermore, it provides theoretical guidance for searching distal oil and gas reservoirs, and it also acts as reference for exploration in similar domestic situations. References [1] Chen Yequan, Wang Weifeng. Structural evolution and pool-forming in Junggar Basin [J]. Journal of the University of Petroleum, 2004, 28(3): 4 8. [2] Wu Xiaozhi, Zhang Nianfu, Shi Xin, et al. Characteristics and reservoiring mode of Chepaizi-Mosuowan paleo-uplift in Junggar Basin [J]. China Petroleum Exploration, 2006, 11(1): [3] Jin Xin, Lu Yongchao, Lu Lin. Analysis of the Mesozoic-Cenozoic subsidence history in Chepaizi area, Junggar Basin [J]. Offshore Oil, 2007, 27(3): [4] Hong Taiyuan, Wang Lichi, Zhang Fushun, et al. Study on the stratigraphic and depositional features in Chepaizi uplift of western Junggar Basin [J]. West China Petroleum Geosciences, 2006, 2(2): [5] Zhuang Xinming. Petroleum geology features and prospecting targets of Chepaizi uplift, Junggar Basin [J]. Xinjiang Geology, 2009, 27(1): [6] He Dengfa, Yin Cheng, Du Shekuan, et al. Characteristics of structural segmentation of foreland thrust belts - a case study of the fault belts in the northwestern margin of Junggar Basin [J]. Earth Science Frontiers, 2004, 11(3), [7] Sui Fenggui. Tectonic evolution and its relationship with hydrocarbon accumulation in the northwest margin of Junggar Basin [J]. Acta Geologica Sinica, 2015, 89(4), [8] Liu Chuanhu, Wang Xuezhong. Sedimentary and hydrocarbon accumulation control factors about Shawan Formation of Chepaizi area in western Junggar Basin [J]. China Petroleum Exploration, 2012, 17(4): [9] Cao Jian, Hu Wenxuan, Zhang Yijie, et al. Geochemical analysis on petroleum fluid activity in the Hongshanzui-Chepaizi fault zone, the Junggar Basin [J]. Geological Review, 2005, 51(5): [10] Xu Xingyou. Study on pool-forming stages of oil in Chepaizi area of

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