Variability and Main Controlling Factors of Hydrocarbon Migration and Accumulation in the Lower Paleozoic Carbonate Rocks of the Tazhong Uplift, the Tarim Basin, Northwest China

Hydrocarbon migration patterns and pathways were studied on the basis of three-dimensional seismic interpretation, drilling, geochemistry, production performance, and other data. Using these ﬁ ndings, the main factors controlling hydrocarbon migration and accumulation in the Lower Paleozoic carbonate rocks of the Tazhong Uplift were discussed. The spatiotemporal relationship between the hydrocarbon kitchens and pathway systems of the Tazhong Uplift and the spatial pattern of pathway systems were considered the main factors causing di ﬀ erences in hydrocarbon enrichment. Results also revealed that the Lower Paleozoic carbonates of the Tazhong Uplift have two hydrocarbon accumulation systems (inside and outside the source rocks). For the accumulation system within the source rocks, hydrocarbon migration and enrichment are vertically di ﬀ erentiated. Middle Cambrian gypsum salt rocks serve as the boundary, above which thrust and strike-slip faults mainly allow vertical transport of hydrocarbons. A multistage superposition pattern of strike-slip faults controls the di ﬀ erences in hydrocarbon enrichment on the periphery of the fault zone. Beneath the gypsum-salt rocks, hydrocarbon migration and enrichment is controlled by the topography of paleostructures and paleogeomorphology. For the hydrocarbon accumulation system outside the source rocks, hydrocarbon migration and enrichment are restricted by the layered pathway system, and the topography of the paleostructures and paleogeomorphology is the key factor controlling hydrocarbon enrichment. The Tazhong No. 1 Fault is the main vertical pathway system in the area underlain by no source rocks, and hydrocarbons are enriched at the periphery of the Middle-Lower Cambrian and No. 1 Fault Zone.


Introduction
The ultradeep ancient carbonate rocks of the Tarim Basin, with an abundance of hydrocarbon resources, are an important field with the best exploration prospects [1,2]. Discovery of a series of large carbonate oil and gas fields, such as the Tazhong No. 1 Gas Field and Tazhong Block III Oil Field demonstrated that the Lower Paleozoic carbonate reservoirs of the Tazhong Uplift contain rich oil and gas resources. However, the complexity of its hydrocarbon distribution severely limits exploration and development progress. Here, the deep carbonate rocks present physical properties clearly different from those of clastic rocks. The average matrix porosity is less than 2%, permeability is less than 5 × 10 −3 μm 2 [3,4], and the compactness and high heterogeneity greatly impair the free flow of oil and gas. In these rocks, pathway elements occur as faults, fractures, and unconformity surfaces. Secondary dissolved pores participate in modifying the rock properties, enhancing reservoir porosity and permeability, and providing the necessary conditions for hydrocarbon migration, adjustment and accumulation in carbonate rocks [5,6].
Research on the main controlling factors of hydrocarbon accumulation in the carbonate rocks of the Tazhong Uplift has gradually shifted attention away from the initial search for favorable reservoirs and towards hydrocarbon pathway systems. By analyzing pathway systems such as faults and unconformity surfaces in the carbonate rocks of the Tazhong Uplift, past studies have summarized the structural style of the pathway framework and its control on hydrocarbons. In particular, they demonstrated the important role of faults on hydrocarbon accumulation in carbonate rocks, which helped guide oil field exploration in the early stages [7][8][9][10][11]. With advances in exploration techniques, focusing solely on the structural controls of a single pathway system on hydrocarbon flow direction and rate will impair decision-making and deployment of oil and gas exploration.
By analyzing the temporal-spatial coupling between different pathway systems and the main hydrocarbon source rocks in the Tazhong Uplift, this paper explores the hydrocarbon migration and enrichment characteristics of carbonate rocks under the control of various pathway systems and identifies favorable zones for hydrocarbon accumulation. This study will provide a reference for oil and gas exploration, and for development of deep carbonate oil and gas reservoirs in superimposed basins, enabling in-depth investigation of the patterns of hydrocarbon migration in carbonate rocks.

Geological Setting
The Tazhong Uplift is located in the center of the Tarim Basin. It is sandwiched between the Manjiar Sag and the Tangguzibasi Sag, on the west of the Tadong Low Uplift and to the east of the Bachu Fault (Block)-Uplift ( Figure 1). As a typical inherited paleouplift structure that has experienced multicycle tectonic sedimentary evolution, the Tazhong Uplift has undergone tectonic evolution stages such as the Middle-Late Caledonian period, the Late Hercynian-Indosinian period, and the Himalayan period [12,13]. These multiple events have had significant influences on hydrocarbon migration, accumulation, adjustment, preservation, and spatial distribution [14]. During the Middle-Late Caledonian period, NW thrust fault zones formed due to the compression of the active continental margins of the basin to the south and north. These include the No. 1 Fault Zone, the Central Horst Zone, and No. 10 Fault Zone. Some of the thrust fault zones are absent from the Middle-Upper Ordovician Series and the Silurian-Devonian Systems, or from the large-scale unconformity developing at the top of the Middle-Lower Ordovician. During the Late Hercynian-Indosinian period, the Tarim Basin evolved towards a retroarc foreland basin, and the NE-striking strike-slip fault zones of the Tazhong Uplift further developed. During the Himalayan period, the basin was affected by the collision of the Indian and Eurasian continental plates and the Altyn Tagh strike-slip faulting; therefore, parts of the early-formed strike-slip faults of the Tazhong Uplift were reactivated.
The Cambrian-Ordovician System of the Tazhong Uplift is a carbonate rock deposit (Figure 1(b) It has been suggested that a general exploration of the strike-slip fault zone is required [15]. The Tazhong Uplift has developed two strike-slip fault groups in different directions. One group is NE-SW, which is the main strike-slip fault of the Tazhong Uplift; the other group is in the NEE-SWW direction, containing several strike-slip faults confined to the central region of the Tazhong Uplift.
Taking the strike-slip faults on the top of the Ordovician carbonate rocks as an example, the strike-slip faults have a north-south segmented tectonic style and are divided into linear, braided, diagonal, and ponytail segments ( Figure 2). Generally speaking, the strike-slip faults in the south are mainly linear strike-slip faults, and the faults are upright in profile; the strike-slip faults in the north are divergent in the stress release areas such as the thrust tectonic belt and No. 1 Fault Zone, featuring diagonal or ponytail patterns. Their overall planar distribution is characterized by southnorth zoning. The southern fault zone is strongly compressed, forming a straight and closed linear strike-slip segment, which gradually changes to tensile stress towards the north. The tectonic style of the fault plane changes to diagonal, braided, or ponytail patterns, and the tectonic style of the profile changes from plate-like to a positive or negative flower-like pattern. The stress transition zone of the strike-slip fault tends to form an extensional or compressive bending belt. The strata within the bending belt are strongly fragmented, greatly increasing the fluid transport capacity in both horizontal and vertical directions [16].
The actions of compressive and tensile stresses in the strike-slip faults, and differences in the degree of fragmentation of fault zones, have caused significant differences between individual fault zone segments, in terms of the width and intensity of damage. In a straight and tightly closed linear strike-slip segment, the activity of the strikeslip fault zone gradually declines from deep to shallow [17]. Meanwhile, the braided, diagonal, and horsetail strike-slip segments and other extensional or compressive bending faults show more damaged fault zones and increasing activity from deep to shallow. According to the statistics of emptying and lost circulation obtained during drilling and imaging logging, the proportion of dissolved cavities (above 0.1 m) around the fault zone is more than 75% [18].   A braided segment that forms a "flower-like" bulge below the carbonate rock top and has a strongly fragmented fault zone; when further compressive or extensional faulting develops, the original fault still has capacity for hydrocarbon transport. (d) A diagonal segment which is a product of tensile and torsional stresses, with a highly fragmented fault zone and strong vertical transport capacity. (e) A horsetail segment, which is a product of tensile and torsional stress, with a high degree of fragmentation in the stress release area.

Geofluids
The fault displacement and the fault width of its above salt fault are generally smaller than those of the No. 1 Fault Zone.

Unconformity Surface and Interlayer Karst Fracture
Cavity Pathway Systems. The Yingshan Formation, together with the Lianglitage Formation, represents the largest angular unconformity surface of the carbonate rocks in the eastern and central regions of the Tazhong Uplift. The unconformity surface shows high levels of exposure, weathering, and denudation and has formed extensive weathering crust karst reservoirs. Similar to the unconformity surface, the interlayer karst fracture cavities developed on each carbonate rock bed boundary from the Upper Cambrian Series to the Ordovician system. Under the background of multistage tectonic activity, surface meteoric water or underground hydrothermal fluid infiltrated into each carbonate rock bed boundary through fractures, causing selective dissolution or metasomatism. Generally, a highly heterogeneous fracture cavity system formed within 200 m below the top bed boundary ( Figure 4). Regardless of the unconformity surface or interlayer karst fracture cavity, at a sufficient scale and connectivity, this structure acts as a horizontal hydrocarbon migration pathway. 5 Geofluids ( Figure 6) shows that CH 4 13 δC ranges from -55‰ to -50‰ in the source rock area of the western region, from -48‰ to -42‰ in the central region of Tazhong, and from -42‰ to -35‰ in the east. When combined with past studies of the types and accumulation periods of oil and gas reservoirs in Ordovician reservoirs in the Tazhong Uplift [23,24], these data indicate that the Upper Ordovician gas in the western region of the Tazhong Uplift has the lowest maturity and earliest formation time; the Upper Ordovician gas maturity in the eastern region has the highest maturity and latest accumulation time.
The oil and gas reservoir distribution of the Tazhong Uplift varies greatly across the plane and differs significantly along the vertical direction. The middle and lower reservoir cap assemblages are dominated by gas reservoirs, while conventional oil and condensate oil occur in the upper reservoir cap. The hydrocarbon distribution is generally characterized by having oil at the top and gas at the bottom, with density increasing upwards. In Well Z162 in the central region, three reservoirs of the Lianglitage, Yingshan, and Penglaiba Formations were drilled and revealed at the same time. In these three reservoirs, three different types of hydrocarbon inclusions were discovered, with blue-white/blue/yellow-white fluorescence under ultraviolet light (UV), and the homogenization temperatures of the associated brine inclusions changed from 120°C-130°C to 80°C-95°C. This indicated that hydrocarbons in the reservoirs changed from highly mature to mature with decreasing burial depth.
-3000   -55  G17  G16  Z63C  G14  G13  G11  G8  G45  G2  G502  G513  Z828  Z80  Z83  Z622  Z15  Z44  Z16  Z242  Z6  Z24  Z53 -53  According to the determination standards described above, the Middle Cambrian crude oil and Lower Cambrian natural gas of ZS1 were attributed to the Middle-Upper Ordovician and the Lower Cambrian source rock [31]. However, the Middle-Upper Ordovician Series does not have the geological conditions required for downward migration to deeper Middle-Lower Cambrian reservoirs. In addition, none of the wells of the platform area in the Tarim Basin have yielded source rocks presenting TOC > 0:5%. Therefore, it is speculated that even if there were source rocks with higher TOC in the Middle-Upper Ordovician in the Manjiar Sag, they would not be sufficient to be the main source rocks in the platform area, because of their small scale. The consistency of the Ordovician crude oil biomarkers and geochemical indicators with those of the Middle-Lower Cambrian crude oil from Well ZS1 suggests that the Ordovician crude oil is mainly derived from the Cambrian source rocks. Based on the surface outcrop in the north of the Tarim Basin and the core drilling of wells LT1 and XH1, the Yurtusi Formation of the Lower Cambrian Series has the characteristics of high TOC (1%-16%) and stable distribution [32,33]. Its hydrocarbon generation potential is much higher than the current predicted geological reserves, which demonstrates the favorable geological conditions and material basis to serve as main source rocks.

Distribution Range of Main Source
Rocks. According to the lithology logging response characteristics of the source rocks of the Yurtusi Formation encountered in Wells XH1 and LT1 [33], a logging-seismic calibration relation was established. The corresponding 3D seismic reflection characteristics represent a set of low-frequency continuous deep valley amplitude reflections with stable distribution [32]. Interpretation across the platform area, achieved by merging 2D and 3D seismic data, enabled interpretation and tracking of the source rock distribution of the Yurtusi Formation (Figure 7). This showed a close relationship with the Lower Cambrian depositional facies investigated by previous studies [34]. During deposition of the Lower Cambrian Series, the sedimentary environment of the Tazhong Uplift changed from shallow-water shelf slope to deep-water basin towards the Manjiar Sag. The Yurtusi Formation was characterized by obvious thickening, its depocenter should be located in the north of the Manjiar Sag (in a northeast direction), and the source rocks are thicker towards the center of the basin. The source rocks of the Yurtusi Formation in the Lower Cambrian Series are distributed across the entire Manjiar-Awati Sag in the north of the Tazhong Uplift but are only seen in parts of the central and western regions of the Tazhong Uplift. This is also consistent with the drilling results of Wells ZS1, ZS5, and ZH1. Source rocks failed to develop in the eastern region and Central Horst Zone of the Tazhong Uplift.

Main Factors Controlling Accumulation.
In the three reservoir cap assemblages (upper, middle, and lower) of the carbonate rocks in the Tazhong Uplift, karst fracture cavity reservoirs have generally developed; the superthick mudstone of the Sangtamu Formation, and the thick inner marl and the super-thick Middle Cambrian gypsum-salt rocks constitute the corresponding regional caprocks; and the conditions for hydrocarbon sealing and preservation are good. The hydrocarbon distribution in the abovementioned zones is subject to the source rock distribution and the source rock pathway configuration. The distribution of the source rocks determines the hydrocarbon charging pattern and the roles of various pathway systems. Therefore, this paper focuses on the pathway systems and the distribution of hydrocarbon reservoirs.

Pathway Systems inside and outside the Source Rocks.
In the carbonate rock hydrocarbon system, pathway systems not only serve as channels for hydrocarbon migration but also a space for hydrocarbon accumulation. There are four pathway systems for carbonate rocks in the Tazhong area: faults, unconformities, interlayer karst fracture cavities, and dolomite pathway systems [7]. According to the distribution of source rocks in the Tazhong area and the close relationship between the period of tectonic activity and the time of hydrocarbon accumulation, two types of source pathway configurations (inside and outside source rocks) are identified. These were chosen to study the differences in hydrocarbon supply and transport modes.
The pathway system inside the source rocks is characterized by hydrocarbon supply from local source rocks. The reservoir is located above the source rocks. After hydrocarbons are expelled from the source rocks, they are transported vertically via a connecting fault, or horizontally via the dolomite pathway layer, or diverted to supply other connected pathway systems. The pathway system outside the source rocks is characterized by hydrocarbon supply from nonlocal source rocks. In this case, the reservoir and the source rocks are not in the same projection range. The unconformity surface and the dolomite layers are the main 7 Geofluids pathways connecting source-distal hydrocarbons, and the hydrocarbon transport characteristics are thus strongly influenced by paleogeomorphology and paleostructural ridges. After the hydrocarbons are discharged from the source rock, two or more migration stages are required for hydrocarbon accumulation and reservoir formation. In the faulted area, hydrocarbons that are laterally transported will change their flow direction by the fault, eventually accumulating at the top of the reservoir.

The Role of Strike-Slip Faults in Oil and Gas Transport.
Previous studies of the faults of the Tazhong Uplift have usually analyzed the multistage superposed products as a whole [10,15]. However, the superposition of structural styles and the properties of multistage strike-slip faults differ from those of single-stage faults. The differences in their tectonic styles are superposed on the profile of the Tazhong Uplift, leading to vertical stratification ( Figure 2) and to different hydrocarbon transport capacities above and below the stress transition surfaces of different stages of strike-slip faults (Figure 8). Early faults present as linear compression segments-regardless of whether they are compressed or extended after superposition-and show an unchanged degree of fragmentation and poor hydrocarbon migration conditions; in contrast, if the early faults develop a flowerlike structure under compression or extension, then the fragmentation of the fault zone is conducive to vertical hydrocarbon migration. When the superimposed fault is a linear compression segment, hydrocarbons are preserved under the transition surface. When the superimposed fault has a flower-like structure, the fault zone is connected both upwards and downwards, and hydrocarbons will continue to move upward, charging favorable reservoirs along the way, and relatively enriching rocks lying under the top fault sealing surface (Figure 8). Statistics on the production capacity of hydrocarbon wells and their distance from the core of the fault zone show that the hydrocarbons are mainly enriched within a zone extending 1000 m from the strike- 8 Geofluids slip fault zone. At greater distances, the hydrocarbon charging intensity will decrease (Figure 9). The above interpretation of how multistage superposed strike-slip faults influence hydrocarbon transport is based on the premise that hydrocarbons come from underlying hydrocarbon sources. Under the condition of hydrocarbons coming migrating laterally, then instead the horizontal transport of the strike-slip fault is dominant. The strikeslip fault zone enables hydrocarbon redistribution via short-distance horizontal transport, similar to the horizontal hydrocarbon transport associated with tectonic ridges. Previous studies have shown that the unconformity surface of clastic rocks features high porosity and permeability and could be used as an effective pathway for long-distance hydrocarbon transport [35]. Nevertheless, limited by the high heterogeneity of carbonate rocks, the unconformity surface, and the interlayer karst fracture cavity aggregate are indeed inferior to those of clastic rocks in terms of their horizontal extent and continuity. The unconformity surface and the interlayer karst fracture cavities in the study area are not in contact with the source rocks, and the hydrocarbon supply capacity is limited. Therefore, they mainly act as pathways for local hydrocarbon adjustment.
In the source rock area, when hydrocarbons migrate from thrust faults or strike-slip faults to the karst fracture cavity area, and if the middle and upper reservoir cap assemblages present a sudden variation of rock porosity and permeability on both sides of the faults, then hydrocarbons migrate laterally into the reservoir and accumulate at the top of the envelope plane. When this karst fracture cavity system is relatively independent, hydrocarbons can become enriched and will accumulate; when the connectivity is good, hydrocarbons can migrate further, reaching higher storage spaces, to forming a pathway for hydrocarbon adjustment in the reservoir. In the area outside the source rocks, the hydrocarbons from the northern Manjiar Sag The hydrocarbon enrichment on the top of the carbonate rocks is then likely controlled by the paleostructure and paleogeomorphology. Hydrocarbons accumulate in tectonic high points, like ridges and peak clusters, while low positions of the paleogeomorphology represent aquifers or gas reservoirs with high water saturation.
As a layered pathway system, the Lower-Middle Cambrian dolomite's transport capacity depends strongly on the potential difference caused by the paleotectonic background. Tectonic ridges and low uplift both present favorable hydrocarbon migration pathways and accumulation areas. In the source rock areas in the west of the Tazhong Uplift, a large range of paleotectonic uplifts were maintained and/or inherited during the main tectonic activity periods, such as the Middle Caledonian, the Early Hercynian, the Late Hercynian, and the Himalayan, thus providing good conditions for hydrocarbon accumulation in the dolomite reservoirs under gypsum-salt rocks. At the same time, the faults of the superthick gypsum-salt rocks became pathways for upward adjustment of some of the hydrocarbons, leading to hydrocarbon enrichment and accumulation near the fault zone in the fracture cavity system of the Upper Cambrian-Ordovician series.
Taking wells ZS1 and ZS5 in the eastern region of the Tazhong   The hydrocarbon charging and distribution of carbonate rocks above the Middle Cambrian gypsum salt rocks are also controlled by the strike-slip fault zone and its related multistage activity. In the northern region, the strike-slip faults are mostly manifested as the superposition of extensional or compressive bending belts in the middle and upper reservoir cap assemblages. The crude oil of the Late Caledonian period or the Late Hercynian period migrated vertically to the reservoirs on both sides of the fault zone through the strike-slip faults. Sealed by the superthick mudstone of the Sangtamu Formation, the crude oil is most strongly enriched on the top of the limestone. Although the strike-slip faults maintained the hydrocarbon transport capacity during the late stage, this may have been limited by the sealing of the gypsum salt rocks, and insufficient natural gas to migrate through the gypsum salt rocks. Consequently, there are generally oil reservoirs on both sides of the strike-slip fault zone, the gas-oil ratio is low (65-1824 m 3 /m 3 ), the dryness coefficient of natural gas is 0.66-0.94, and the natural gas is characterized by the associated gas in the oil reservoir.
If there is a suitable reservoir cap assemblage on the periphery of the fault zone, then a multilayer superimposed compound oil and gas reservoir will probably form. In the southern region, the vertical fault displacement and strikeslip fault displacement of the strike-slip faults with linear compression formed during the Middle Ordovician period are small, and the level of fragmentation of these faults is low. Poor reservoirs generally formed near the fault zones, indicating that the strike-slip faults have weak hydrocarbon transport capacity in deep layers, and little crude oil can migrate upwards in the early stage. As the Late-middle Cambrian caprock became more strongly sealed, the late natural gas rarely charged into the Upper Cambrian-Upper Ordovician Series and remained preserved under the Middle Cambrian gypsum salt rocks.
The Cambrian source rocks in the Manjiar Sag, in the north of the Tazhong Uplift, are the main hydrocarbon sources in the current accumulation area. The hydrocarbons of the Manjiar Sag can accumulate in the remote uplift area only by lateral transport (Figure 10 In the control area of the pathway system outside the source rocks, hydrocarbon migration and enrichment under the gypsumsalt rock is restricted by the layered pathway system. The height of the paleostructure and paleogeomorphology is the key factor controlling hydrocarbon enrichment. In addition to the leading role of source rocks and pathway systems during carbonate hydrocarbon accumulation in the Tazhong Uplift, the influence of the Middle Cambrian gypsum salt caprock cannot be ignored. In the western region of the Tazhong Uplift, as the burial depth increases, the fluid plasticity of the gypsum-salt rock becomes more prominent [36], and its ability to seal hydrocarbons is also greatly enhanced. The superthick gypsum salt caprock seals late-stage natural gas under the gypsum-salt rock. The Lower Cambrian dolomite reservoir is rich in natural gas, while the upper and middle reservoir caps above the gypsum salt rock are rarely charged with natural gas from underlying source rocks; these upper or middle reservoir caps are characterized by heavy methane carbon isotopes and rich oil. Although there is no source rock in the eastern region of the Tazhong Uplift, the high tectonic positions of the uplift tend to be the targets of lateral hydrocarbon migration. The late-stage dry natural gases from the northern Manjiar Sag migrate through the dolomite layers or migrate towards  the Tazhong Uplift via tectonic ridges, thus charging the middle and upper reservoir cap assemblages through the No. 1 Fault Zone and forming reservoirs in the platform margin belt. The surplus natural gas laterally adjusts via the unconformity surface. Therefore, the eastern region is characterized by its light carbon isotopes in methane and high density (0.83 g/cm 3 ) of condensate oil.

Conclusions
Based on our evaluation of the temporal-spatial coupling between different pathway systems and the main hydrocarbon source rocks in the Tazhong Uplift, this paper discusses the hydrocarbon migration and accumulation characteristics of carbonate rocks under the control of various pathway systems. The following conclusions are drawn: (1) Our proposed source pathway transport characteristics of the carbonate rocks in the Tazhong Uplift provide a new basis for detailed investigation of hydrocarbon migration in carbonate rocks. After determining the sources of marine carbonate rock hydrocarbons in the Tazhong Uplift, this paper studied the distribution, activity, and classification of various pathway systems (namely, those inside and outside the source rocks). This was achieved through detailed analysis of the source rocks of the Yurtusi Formation in the Lower Cambrian Series in combination with well logging and seismic facies (2) For the hydrocarbon accumulation system within source rocks, hydrocarbon migration and enrichment are vertically differentiated. The Middle Cambrian gypsum salt rocks serve as the boundary, above which hydrocarbons are transported vertically by thrust and strike-slip faults, becoming enriched in multiple layers on the periphery of the thrust and strike-slip fault zones. In addition, the multistage superposition pattern of strike-slip faults controls the differences in hydrocarbon enrichment of the fault zones. Below the gypsum-salt rocks, the layer pathway system controls the hydrocarbon migration, while the topography of the paleostructure and paleogeomorphology controls the hydrocarbon accumulation (3) For the hydrocarbon accumulation system outside of the source rock, hydrocarbon migration and enrichment are restricted by the layer pathway system, and the height of the paleostructure and paleogeomorphology is the key factor controlling hydrocarbon enrichment. The Tazhong

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.