The Cambrian sub-salt (Upper Sinian-Middle Cambrian) dolomites are important strategic areas for deep to ultra-deep hydrocarbon exploration in the Tarim Basin. After the discoveries of the Qiaogu and Yakela buried-hill reservoirs in the Sinian System, northern Tarim^[1], a great breakthrough has been made in the exploration of some tectonic units, including the Tazhong Uplift^[2], Lunnan low salient in northern Tarim^[3], and Kaping faulted uplift^[4]. However, it is very difficult to evaluate favorable zones because some major exploratory wells, including Xinhe1 and Yulong 6, were failure due to poor under-standing about the tectono-lithofacies paleogeography and inaccurate prediction of reservoirs and source rocks before well drilling.

Several rounds of research have been carried out on the tectonic and lithofacies paleogeography of Cambrian sub-salt dolomites in the Tarim Basin. Based on geotectonic studies, single-factor analysis and multi-factor mapping, sequence and lithofacies paleogeographic study, Jia, Feng, Zhao et al.^[5,6,7] presented that in the Cambrian Period, the Tarim Plate was characterized by east-west differentiation framework, with a basin lying between two platforms, high in the south and low in the north, with ramps and strongly rimmed platforms. Wu, Guan, Zhu et al.^[8,9,10,11] investigated the internal geologic framework of the Western Tarim platform and its control on the depositional differentiation of the overlying carbonate platforms. They confirmed the occurrence of the Precambrian Awat-Manjar rift and a framework with three uplifts and two depressions in the Early Cambrian. These conclusions are important to the understanding of the deep geologic framework, tectono-lithofacies paleogeography, and hydrocarbon exploration and assessment in the Western Tarim platform. But there are three issues to be addressed, i.e. (1) the existence of the Precambrian rifts in southwestern part of Tarim and its control on the Lower Cambrian source rocks and reservoir rocks; (2) the Early Cambrian tectono-paleogeography and the distribution of favorable reservoir facies in Tabei area; (3) the depositional differentiation and evolution in the Western Tarim platform during the Late Sinian to the Middle Cambrian, and the types and distribution of source-reservoir-seal assemblages on paleohighs nowadays. The seismic interpretation results of more than 50 newly acquired/processed seismic lines, 3D seismic data have been used in the study together with data from 22 wells and outcrop data. Starting from the deep geologic framework and key tectonic unit evolution in southwestern Tarim, we have conducted tectono-lithofacies paleogeography reconstruction and evolution for each formation, and prediction and evaluation of large-scale source rocks and reservoir rocks. Combined with the structural evolution of paleohighs, the favorable zones have been evaluated in order to provide a theoretical basis for the next hydrocarbon exploration deployment.

The Tarim Basin is a large superimposed complex basin formed on a Precambrian craton, with several internal structural units (Fig. 1a). The Neoproterozoic-Lower Paleozoic carbonate rocks are the first structural sequence on the crystalline basement of the Tarim Plate^[3], and the sedimentation and evolution were closely related to the breakup of Rodinia supercontinent^[12,13,14]. In the surrounding Nanhuan-Sinian outcrops, there are direct evidence of tectothermal events related to the breakup of supercontinent, e.g. dual-peak volcanic rocks, A-type granites, basic dike swarms, and quickly deposited clastic rock associations thinning upward^[14,15,16]. The age data of the Lower Nanhuan System and Lower Sinian System, which are comparable within the error range, are 760 Ma and 615 Ma, corresponding to initial breakup (740-820 Ma) and final Rodinia breakup (600-650 Ma), respectively^[12]. Intra-plate paleo-rifts, including the Awat-Manjar rift system and southwestern Tarim rift system^[17,18], were identified based on rift features in surrounding outcrops and gravimetric/magnetic/electric/seismic data. The Awat-Manjar paleo-rift in the northern part of the basin has been confirmed, which was controlled by extensional normal faults, extending in a nearly east-west direction, and 100-200 km in width. The maximum thickness is over 2000 m, and the area accounts for 30% of the whole basin nowadays^[17,18,19].

Two Precambrian rift systems in the Tarim Plate and their vertical evolution had a great influence on the deposition of overlying carbonate structural sequences. The Tarim Plate was divided into the southwestern Tarim rift zone, central basement paleohigh, Awat-Manjar rift zone, and Tabei basement paleohigh from south to north, forming a paleo-tectonic framework with two uplifts and two depressions, which laid the foundation for the paleogeographic differentiation of Cambrian sub-salt (Upper Sinian-Middle Cambrian) dolomites. The depositional hiatus, which were the product of the uplift caused by the Keping movement at the end of the Sinian Period, did not greatly change the macroscopic tectonic-paleogeographic framework with uplift and depressions. In the Cambrian Period, ramps and rimmed carbonate platforms developed in the Tarim Basin, the paleogeographic framework changed from two uplifts and two depressions into an east-west differentiation framework. The Nanhuan rift was characterized vertically by a binary structure of rifts and depressions^[16]. The rift period, rift-depression transform period, and post-rift subsidence and craton period controlled the vertical lithologic filling sequences (Fig. 1b, 1c), i.e. clastic rocks in Nanhuan- Sinian rifts and depressions, the Upper Sinian-Lower Cambrian carbonate rocks, and the Middle Cambrian evaporites. The last two sequences constitute a typical evaporite-carbonate association, which is the target to be investigated.

2.1.1. Luonan-Yubei paleo-rifts in southwestern Tarim

There are two completely different viewpoints about the Precambrian rifts in southwestern Tarim^{[14, 19]}. (1) The Makit slope mainly occurred at the Western Tarim uplift from the Nanhuan Period to the Early Cambrian. (2) The Makit slope was in an underwater slope-shelf environment in the Early Cambrian, which is unfavorable for hydrocarbon potential assessment. In accordance with Nanhuan-Sinian lithofacies associations in the Xinjiang-Tibet Highway section, representative tectonic-sedimentary sequences from continental facies to marine facies and from fault depression to depression are developed, and the age is 783-802 Ma for basic dikes and dual-peak magmatic rocks, which indicate that the extensional structural setting in southwestern Tarim is compatible with Rodinia breakup. This is the basis of the first viewpoint which seems more plausible than the second one. The isostatic effect in the rift period caused southwestern Tarim to uplift as a whole. But whether there are also paleo-rifts at the Western Tarim uplift has become the focus of controversy. The studies based on basement aeromagnetic anomalies and seismic wedge-like reflections suffered from large uncertainties due to the influence of the Permian basic dikes and seismic multiples^[14]. After multiple suppression processing and joint interpretation of 2D/3D seismic data, the location, geometric parameters, and vertical structures of Nanhuan rifts in southwestern Tarim were established for the first time^[19] (Fig. 2a, 2b). The southwestern Tarim rift lies on the south of Mazartag fault zone, and is composed of a NW-SE piedmont rift and two SW-NE branch rifts (Fig. 2a), which were entitled the Luonan rift in the west and Yubei rift in the east, respectively. The Luonan rift falls into two small secondary branch rifts in a NW-SE direction in Qungu1 and Hetian2 well blocks. The Western Tarim rift covers an area of 5.37×10⁴ km². The Luonan rift is 180 km in length in the axial direction, and 59-172 km in width, with an area of 1.9×10⁴ km². The Yubei rift is 152 km in length in the axial direction, and 21-39 km in width, with an area of 0.9×10⁴ km². The area of the piedmont rift is 2.57×10⁴ km². There are multi-phase normal faults at the border of the Luonan-Yubei rifts, which have graben-horst structures and a typical binary structure with rifts and depressions in the vertical direction. There were some inherited features after the Late Sinian (Fig. 2c). The Lower Cambrian formation thickness also indicates the feature of a depression lying between two uplifts. Formation thickness decreases in Qungu1 and Hetian2-Yubei1 well blocks and increases in Batan5-Luosi2 well block with the maximum thickness over 240 m.

2.1.2. Tectonic-sedimentary evolution

Bounded by the Keping movement at the end of the Sinian Period, tectonic-sedimentary evolution can be divided into three phases, i.e. rift-depression transform in the Late Sinian, uplift and denudation at the end of the Sinian Period, and post-rift subsidence and craton in the Early and Middle Cambrian^{[18,19,20,21]}.

By Late Sinian, the Tarim Basin had changed from rifting phase (Fig. 3a) into the paleo rift-depression transform phase (Fig. 3b), with topographically high uplifts in south and topographically low platforms in north, mainly experienced thermal precipitation except for local syndepositional rifting. Due to the Early Sinian deposition, the Luonan-Yubei and Awat-Manjar rift zones were filled and levelled up, forming relatively low-gradient intra-platform depressions with deep water, in which thick dark argillaceous sediments occurred from the area on the north of the Southeast uplift nowadays to Kuruktagh. The original Tabei basement paleohigh was completely underwater, and thus rich in medium- to high-energy microbial mounds and beaches. Another high-energy facies belt in the Upper Sinian Qigbulak Formation was the mound-shoal facies at the northern slope of the central basement paleohigh.

Overall uplift and denudation occurred at the end of the Sinian Period due to the Keping movement, and regional large-scale unconformity formed at the top of the Qigbulak Formation^[21] (Fig. 3b). The Keping movement affected the tectono-paleogeography of Tarim Basin in the following two aspects. (1) In Keping-Wensu area in the west of the original Tabei basement paleohigh, the am-plitude of uplift was greater than that of Lunnan area, laying the foundation for the paleotopography, being high in the west and low in the east at the depositional stage of the Cambrian System. (2) The Early Cambrian paleohigh region was further expanded due to extensive denudation around the central basement paleohigh.

From the Early Cambrian to the Middle Cambrian, post-rift subsidence and craton occurred under an extensional background in the Tarim Basin (Fig. 3c). In addition to topographic high in the south and topographic low in the north, the basin also exhibited topographic high in the west and low in the east. In the Early Cambrian, the Yurtusi, Xiaoerbulak, and Wusonger Formations were deposited and overlapped from paleo rift zone to adjacent paleohighs in succession. A new framework with three uplifts and two depressions occurred at the depositional stage of the Xiaoerbulak Formation^[10]. The Tanan uplift, Uqia uplift, and Wensu low uplift jointly controlled the distribution of the high-energy mound-shoal in inner-middle ramps. Due to the hot dry paleoclimate in the Middle Cambrian, strongly rimmed platform margin and large evaporative lagoons were developed, with large scale thick gypseous salt rocks. In the middle and late periods of the Middle Cambrian, the platforms showed more craton features, and finally not affected by the original uplift-depression framework.

2.2.1. The Upper Sinian Qigbulak Formation

At the depositional stage of the Qigbulak Formation, the Tarim Plate went through relatively stable thermal precipitation^[20], where the ramp-type platform was developed with dominant microbialites. Due to intrabasin differentiation, the uplifts existed in the south, and platforms existed in the north (Fig. 4a). The central basement paleohigh extended in a NW-SE direction in Uqia-Bachu- Tazhong. Due to regional denudation caused by the Keping movement, the area of the uplifts at the depositional stage, which accounted for 1/3 of the basin, was predicted to be slightly smaller than that on the residual thickness map. Inner-ramp shore-lagoon, inner-ramp microbial mound-shoal, middle-outer ramp, and basin occurred in succession from the uplift toward north. Inner-ramp microbial mound-shoal zone, 15-80 km in width and 420 km in length in the east-west direction, with an area of 4.26×10⁴ km², distributed on the north side of the central basement high and the original basement paleohigh in Tabei. The drilling cores from the Xiaoxigou section in Keping and Tabei indicate that there are two types of high-frequency lithofacies cycles in inner ramps, which is composed of layered thrombolite and spongiostromata stromatolites, slightly undulated stromatolites and oolites, algal debris, and thrombolite^[19], with a single cycle of 2-11 m in thickness. Inner shoals were further differentiated due to the influence of secondary faults. Five small grooves were identified in Lunnan recently, and weak moundy seismic facies turned up in groove margin. The outer ramp and basin with deep water, where the original Awat-Manjar rift located, were deposited with dark argillaceous dolomites and argillaceous rocks. The Luonan-Yubei paleo-rifts in southwestern Tarim subsided constantly in the Late Sinian, and mound-shoal turned up locally. The ancient land, inner-ramp shore and lagoon, and inner-ramp mound-shoal occurred in succession from the ancient land toward south. Hybrid granular rocks could be observed in the Xinjiang-Tibet Highway section.

2.2.2. The Lower Cambrian Yurtusi Formation

Yurtusi Formation has been encountered in six exploratory wells, i.e. Xinghuo1, Luntan1, Xinhe1, Qitan1, Luntan3 and Tashen5, and has been found in 12 section points in Keping area^[22], which all distributed in Tabei area. This set of formation is missing in nearly 20 exploratory wells in Bachu to Tazhong areas, and has not been encountered in southwestern Tarim. Through comprehensive analysis, it is believed that during the depositional stage of the Lower Cambrian Yurtusi Formation, the argillaceous ramps were controlled by the sustained subsidence of the Precambrian rift-depression system (Fig. 4b). There were 3 facies regions on the north side of the central basement uplift, i.e. (1) inner-ramp clastic rocks and hybrid tide flats which can be observed in the Laozhuanchang section and wells Fang1 and Shutan1; (2) the middle-ramp argillaceous (nodular) limestone, greyish black shale and argillaceous dolomite associations, with middle to small thickness, which mainly occur in Keping outcrops, around the Keping-Wensu uplifts and in Lunnan in addition to the north side of the central paleohigh; (3) Outer ramp and basin facies with black shale, siliceous rocks, and siliceous mudstones, which were developed in wells Tadong1 and Yingdong1 in eastern Tarim. The Yurtusi Formation in Keping outcrops was vertically divided into 3 submembers, i.e. lower source rocks with rich siliceous rocks, upper source rocks alternated frequently with thin limestones/dolomites, and top dolomites, with thickness ranging from 30 m to 50 m. The cumulative thickness of black shale is 10-15 m, indicating frequent sea level fluctuation at the depositional stage and overall subsidence at the middle and late stages of deposition.

The distribution range of Luonan-Yubei paleo-rifts has been newly confirmed, and the seismic reflection events in the sedimentary province are similar to those in Well Xinghuo1 in the north. According to this, it is pointed out that the Yurtusi Formation exists in the subsidence area of the southwestern Tarim paleo-rift area. The thickness is 10-50 m wtih the predicted area of 6×10⁴ km².

2.2.3. The Lower Cambrian Xiaoerbulak Formation

During the depositional period of the Xiaoerbulak Formation, a set of carbonate ramp deposits were developed (Fig. 4c), and internal differentiation was controlled by the paleogeographic framework with three paleohighs (Uqia, Tanan, and Keping-Wensu) and two depressions (southwestern Tarim Depression and North Depression). In accordance with the difference in the sediments around these three intra-platform paleohighs, we identified the grain banks in the north margin of the Tanan paleohigh as the major slope-flat ramp, mound-shoal complexes in the Keping-Wensu low paleohighs as a homoclinal ramp, and the Uqia paleohigh as a transitional ramp. The grain bank in the north margin of the Tanan paleohigh, which is the major slope-flat ramp, extend in Tazhong32 to Chutan1 well blocks. Hybrid flat, inner-ramp grain banks, middle- to outer-ramp dolomitic limy flat/limy argillaceous flat, and basin occur in succession on the north of the paleohigh. The inner-ramp intrafacies consists of algal dolarenite and oolitic dolomites with abundant terrigenous clastics. The shoal zone is 49-75 km in width and 25-71 m in thickness. The typical overlap, moundy, and foreset reflections can be observed. The Uqia paleohigh and Keping-Wensu low paleohighs are similar in facies distribution to the north margin of the Tanan paleohigh. Inner ramps around the Uqia paleohigh consist of interbedded algal mounds and grain banks, and the Keping-Wensu low paleohigh is characterized by small mounds and large shoals. Large scale inner-ramp mound-shoal facies, which are mainly transitional mound-shoal complexes, are predicted to occur in the low-gradient geomorphic between the Tanan paleohigh and Uqia paleohigh. In short, 3 intra-platform paleohighs controlled the distribution and scale of inner-ramp mound-shoal in the Xiaoerbulak Formation, and the area is 10.6×10⁴ km². The Xinhe-Lunnan area of the Tabei uplift is dominated by middle- to outer-ramp low-energy facies. The greyish black argillaceous (micritic) limestones with middle to large thickness and micrite mudstones have been confirmed by wells Xinhe1 and Luntan1.

2.2.4. The Lower Cambrian Wusonger Formation

In the late period of the Early Cambrian, 3 intra-platform paleohighs, i.e. Tanan, Uqia, and Keping-Wensu, still existed and controlled the intra-platform differentiation. However, middle- to high-energy facies began to occur in the Lunnan underwater low uplift due to the influence of sustained sea level drop and ocean current in northeast direction (Fig. 4d). According to the traditional viewpoint, middle- to high-energy facies may not turn up in the Wusonger Formation, which is composed of a set of shale-rich terrigenous clastics-rich sediments of tidal-flat facies^[4]. In accordance with the Keping outcrops, new well drilling results of Ketan1 (Beijing Energy Holding Co., Ltd.), Luntan1, Luntan3, and Zhonghai1, the lithology reexamination of old wells, and seismic facies, the platform margin of the Wusonger Formation was detected and delineated for the first time. During the depositional stage, Wusonger platform margin was a weakly rimmed carbonate platform. Wusonger platform margin is the first set of platform marginal buildup in the Cambrian Western Tarim platform (Fig. 5), which indicates the formation of the framework with the basin in the east and platform in the west in the Tarim Basin. The east platform margin, 15-30 km in width and 310 km in length, extends in NW-SE direction in Lunnan-Tazhong32 well block, with an area of 7080 km². The north platform margin has not been confirmed due to the existence of fractures and stratigraphic denudation. However, according to lithofacies change from outcrops to wells Xinhe1 and Luntan1, the platform margin was inferred to occur to the south of Well Xinhe1. There are unequally sized intra-platform shoals near the platform margin, and the area is estimated to be 3200 km². The inner platform, with typical features of hybrid deposition close to the ancient land, is rich in shales and terrigenous clastics. The tepee structures and edgewise breccia can often be observed. Thin low-energy shoals can be observed near the uplifted zone with the thickness of 0.5-1.2 m in the Xiate and Jianbizhenmutage sections etc.

2.2.5. The Middle Cambrian

Till the Middle Cambrian, the intra-platform uplift-depression differentiation has been disappeared. The platform margin entered the rapid build-up stage, and forming typical barrel-shaped structures. Besides, hot dry paleoclimate facilitated the formation of strongly rimmed platforms, dominated by evaporative lagoons (Fig. 4e, 4f). Compared with the platform margin at the depositional stage of the Wusonger Formation, the platform margin in the Middle Cambrian was characterized by large distribution area and multi-phase superimposition. The south end extends from Tazhong 32 well block to the Luoxi platform^[23]. Gypseous dolomitic flat and intra-platform shoal, evaporative tide flat, and argillaceous dolomitic flat intrafacies (microfacies) occur in succession from the central gypseous salt lake to the external side. As shown by the lithofacies paleogeography in the Shayilik Formation (Fig. 4e) and Awatag Formation (Fig. 4f), the gypseous salt lake expanded, with area increasing by 1/3 from the early period to the late period of the Middle Cambrian, and was over 14.2×10⁴ km² at the depositional stage of the Awatag Formation. Gypseous salt rocks are 400-700 m in thickness, and surrounding gypseous dolomitic flats are 200-400 m in thickness, with an area of 5.1×10⁴ km². The gypseous salt lake mainly formed in the original Awat-Manjar paleo-rift, and gypseous dolomitic flats turned up in Tanan, Uqia, Kalpin- Wensu, and Lunnan.

3.1.1. Major Yurtusi source rocks

The distribution of the Yurtusi Formation is obviously controlled by the Precambrian tectonic framework, with uplifts alternated with depressions (Figs. 5 and 6a). The source rocks mainly occur in middle- to outer-ramp intrafacies. The Yurtusi Formation is distributed mainly in sustained subsidence area of Awat-Manjor paleo-rift zone and the original Tabei basement paleohigh, with an area of 26×10⁴ km², which has been confirmed by Keping outcrops and well drilling results of Luntan1 and Xinghuo1^[22]. The thickness of source rock is controlled by the ancient landform at the depositional stage. The thickness increases from west to east, locally thinning due to the occurrence of ancient topographic highs. Black shales are 10-14 m in thickness in the outcrop area, increase to 30 m in Xinghuo1 well block and 55-99 m in Tadong, and then decrease to 22 m in Lunnan due to the occurrence of the underwater low uplift. Source rock quality shows a similar trend. In the western Tarim platform, the TOC content is generally above 3%, and the maximum may reach 16.5%. In the eastern basin, the TOC content is only 1%-3%, which indicates the influence of sedimentary facies on source rock quality. At present, the Yurtusi Formation source rocks have not been directly discovered in southwestern Tarim in view of well drilling or outcrops, but the geochemical index of hydrocarbons in the discovered oil and gas reservoirs, such as Baxtopu, Hotan River, and Yubei indicated the oil and gas were sourced from the Lower Cambrian Yurtusi Formation^[24]. Combined with paleogeography mapping, it is predicted that source rocks are well developed in Manan of the Makit slope-Baxtopu areas, with an area of 6×10⁴ km², and 10-50 m in thickness.

3.1.2. Potential Sinian source rocks

Due to the occurrence of Precambrian paleorifts in the Tarim Plate and the ancient marine environment in the interglacial periods which were favorable for lower organisms to bloom, the Nanhuan-Sinian systems may be the potential hydrocarbon sources for the Cambrian sub-salt reservoirs. Dark mudstones in the Sinian System on both sides of the Awat-Manjar rift, i.e. Keping in the northwest margin and Kuruktagh in the northeast margin, have been detected to have some potential of hydrocarbon generation (Fig. 6b). Sinian dark mudstones in the Shairike section in Keping area are 60 m in thickness, with the TOC content of 0.3%-1.0%. Outcropped argillaceous rocks in the neighboring Xiaoxigou section is 50 m in thickness, which have been weathered, and exhibited khaki color, and with fractured surfaces in black color. The TOC content is < 0.5%. Sinian mudstones in southern Kuruktagh, northeastern Tarim are 60-80 m in thickness, with the TOC content of 0.5%-1.8%. For shallow-hole drilling samples, the TOC content may reach 1.8%-2.6%. With respect to a TOC standard of 0.5% as the effective source rocks, the Sinian source rocks are in good quality in southern Kuruktagh, northeastern Tarim. According to the location of the Awat-Manjar paleo-rift, it was roughly predicated that such source rocks mainly distribute in the Manjar paleo-rift in the east.

3.2.1. Upper Sinian dolomites of microbial mound-shoal facies

Inner-ramp microbial mound-shoal facies is developed on the north side of the central basement paleohigh and the original Tabei basement paleohigh (Fig. 4a), which is the carriers of the large-scale Upper Sinian Qigbulak dolomite reservoirs of microbial mound-shoal facies. It has been proved by the geological modeling for the Xiaoxigou section that the reservoir rocks mainly consist of algal binding dolarenite, spongiostromata stromatolite dolomites, stromatolite dolomites, oolitic dolomites, and thrombolite dolomites; and brecciated algal thrombolite dolomites may occur at the top with hypergenic karstification. Microbial growth framework (dissolved) pores, intergranular (intragranular) dissolved pores, and dissolved pores and cavities are effective reservoir space. The maximum porosity is 19.6%, and the average porosity is 3.6%. The cumulative thickness of type I and type II reservoirs is 53.7 m, and reservoir-formation thickness ratio is 30.9%. On the basis of reservoir lithofacies and the difference in major controlling factors, 3 reservoir intervals were divided. The lower interval is mainly composed of moundy stromatolites, with middle to small thickness, intercalated with oolitic/algal dolomites with middle to small thickness. The content of terrigenous clastics is high, and the reservoir property is poor. The middle interval is mainly composed of slightly undulated stromatolites with middle to large thickness as well as oolitic/algal and thrombolite dolomites to form high-frequency inverse cycles. There are mainly type I and type II reservoirs with good quality, which were controlled by the original sedimentary facies and early dolomitization. The upper interval consists of thick bedded thrombolite and spongiostromata stromatolite dolomites with good reservoir properties, which were related to hypergenic karstification in addition to above two controlling factors for the middle interval. Three reservoir intervals are developed at different scales in Tabei area. The middle and upper intervals can be correlated in the whole region^[26] (Fig. 7).

3.2.2. Lower Cambrian Xiaoerbulak algal mound-shoal dolomites

The Lower Cambrian Xiaoerbulak algal mound-shoal dolomite reservoirs are developed in Tazhong-Badong grain banks, mound-shoal and algal banks in the Makit slope and Bachu, and mound-shoal complexes in Keping-Wensu. The total area is 10.6×10⁴ km². It has been proved by geological modeling on the outcrops around Keping-Wensu that the reservoir distribution, quality, and thickness are closely related to inner-ramp mound-shoal features. Reservoir thickness increases with the increasing in mound-shoal thickness, and reservoir rocks mainly occur in the middle and upper sections of the Xiaoerbulak Formation^[27]. In view of mound- shoal difference, two dolomite reservoir types, i.e. algal mound and grain bank, have been identified. Reservoir space is composed of algal growth framework pores, intergranular pores, intragranular pores and intercrystalline pores. The porosity of 139 samples ranges 1.11% to 10.80%. Type I and type II reservoirs are 16-65 m in thickness, and reservoir-formation thickness ratio ranges from 12.2% to 41.2%. Large unsheltered unconformities were absent inside and at the top of the Xiaoerbulak Formation. The development and preservation of large-scale effective reservoirs are controlled by three factors, i.e. microbial mound-shoal facies, penecontemporaneous karstification related to high-frequency sequence boundaries, and early dolomitization. Preexisting pores were mainly damaged in the burial period, rather than increased, they are inherited rather than reconstructed.

3.2.3. The Lower Cambrian Wusonger platform marginal mound-shoal

Platform marginal zones in Lunnan-Tazhong32 and post-margin intra-platform shoals are the major material basis for Wusonger reservoir, and the low-energy shoals in the tidal-flat facies around three paleohighs also make some contribution. The area of platform marginal zones is 7080 km², and the area of intra-platform shoals in the west is 3200 km². It has been revealed by well drilling that the platform marginal mound-shoal in the Wusonger Formation of Lunnan area is correlated with the reservoir. Wusonger platform margin was penetrated in Well Tashen1, with one layer of type II reservoirs with 14 m in thickness, and 4 layers of type III reservoirs with 76.7 m in thickness. Platform marginal backreef psammitic banks were penetrated in Well Luntan1, with 2 layers of type II reservoirs with 11 m in thickness. Reservoirs were identified to be fractured-porous-vuggy type with mainly residual granular dolomites, and the porosity is 3.1%-3.5%. The discovery of Wusonger Formation reservoirs has expanded the Cambrian subsalt exploration, and changed the traditional view that the Wusonger Formation cannot be reservoir rocks. Whether or not there is large-scale mound-shoal dolomite reservoirs inside the Wusonger Formation should be the focus of next evaluation.

Two sets of large-scale source rocks, 3 sets of large- scale effective reservoirs, and Middle Cambrian gypseous salt cap rocks constitute two source-reservoir-seal assemblages in Cambrian subsalt series. The lower assemblage consists of the potential Sinian source rocks, the Upper Sinian microbial mound-shoal dolomite reservoirs, and the Lower Cambrian Yurtusi argillaceous cap rocks. The upper assemblage consists of the Lower Cambrian Yurtusi source rocks, the Lower Cambrian Xiaoerbulak and Wusonger reservoirs, and the Middle Cambrian gypseous salt cap rocks. Oil and gas have been discovered in these two assemblages. In accordance with tectonic stability of uplifts nowadays and new understanding of source rocks and reservoir rocks, 4 promising prospects in Cambrian subsalt series, i.e. the north slope in Tazhong, south slope in Tabei, area around Keping-Wensu, and upper Makit slope, were evaluated (Fig. 8).

The north slope in Tazhong lies in the south margin superimposed area of potential Sinian source rocks with major source rocks in the Lower Cambrian Yurtusi Formation, and it features dual settings of central basement paleohigh and inherited paleohigh. The upper and lower source-reservoir-seal assemblages had been the destination of hydrocarbon accumulation for a long period (Table 1). The lower assemblage mainly turns up in the west section of the north slope in Tazhong. The upper assemblage distributes throughout the region, but the Yurtusi Formation is absent in the east section, where there are only Xiaoerbulak grain-bank dolomite reservoirs and Middle Cambrian gypseous dolomite overburden. The complete upper assemblage appears in the middle and west sections. The favorable area with the buried depth < 8500 m nowadays covers an area of 10 069 km², 2764 km² for the middle and lower assemblages and 7305 km² for the upper assemblage. Hydrocarbon traps in Tazhong area formed in the early Caledonian and finalized in the Hercynian; the whole process coincided with the major period of hydrocarbon accumulation^[28]. Four exploratory wells had been completed at the north slope in Tazhong by 2019. Well Zhongshen1C yielded commercial gas flow of 15.8×10⁴ m³ from the Lower Cambrian Series in gas testing, and Well Zhongshen5 output low-yield oil and gas flow from the Middle Cambrian Series. As per the failure analysis for Well Zhonghan1, exploration deployment should first concentrate on the middle and west sections with more confirmed upper and lower assemblages and small damage by the strike-slip faults from the middle and late Caledonian to the early Hercynian. Well Zhonghan2 is being penetrated now.

The south slope in Tabei lies directly above major source kitchens in the Lower Cambrian Yurtusi Formation and the north side of potential Sinian source rocks. The discoveries of some extremely large oil and gas fields in the Ordovician System, e.g. Halahatang, Tahe and Shunbei etc., demonstrated the effectiveness and large scale of source kitchens (Table 1). The south slope in Tabei was dominated by the Tabei basement paleohigh and residual Tabei paleohigh, and thus both the upper and the lower reservoir assemblages occur in this region. The lower assemblage extends throughout the Tabei area. Large-scale Qigbulak microbial mound-shoal dolomites are overlaid with pervasive Yurtusi mudstone-shale, which are connected laterally with potential Sinian source rocks. The upper assemblage only occurs in Lunnan, where Wusonger platform marginal and backreef-beach dolomites are overlaid with Middle Cambrian gypseous dolomites. The favorable area with the buried depth < 8500 m nowadays covers an area of 12 112 km², 11 511 km² for the lower assemblage and 601 km² for the upper assemblage. The south slope in Tabei went through the major periods of hydrocarbon accumulation in the Caledonian and Hercynian. Abundant fractures formed in the Yanshanian and Himalayan due to intense tectonic movements. Recent hydrocarbon discoveries in the suprasalt Ordovician System were usually related to large strike-slip faults connected with Lower Cambrian source kitchens. Hydrocarbon evaluation should focus on large-scale near-source reservoir-seal assemblages which have not been seriously damaged by fractures.

The region around Keping-Wensu lies above potential Sinian source rocks superimposed with major source rocks in the Lower Cambrian Yurtusi Formation, and oil and gas tended to migrate vertically in-situ (Table 1). The Wensu low uplift in the Early Cambrian has paleohigh settings in the Late Hercynian coinciding with the major period of hydrocarbon accumulation^[28] dominated the upper and lower reservoir assemblages and hydrocarbon accumulation. The lower assemblage mainly occurs around the Wensu low uplift and pinches out toward Keping. Qigbulak microbial mound-shoal dolomite reservoirs feature large thickness and good properties. The upper assemblage extends throughout the region; Yurtusi source rocks are absent in the core area close to Keping and the Wensu low uplift. The favorable area with the buried depth < 8500 m nowadays covers an area of 27262 km², 682 km² for the lower assemblage and 26 580 km² for the upper assemblage. Due to dual paleohigh settings, the region around Keping-Wensu exhibits large potential of hydrocarbon exploration. On the other hand, intense tectonic reworking in the Himalayan led to Cambrian subsalt hydrocarbon redistribution and reaccumulation. This idea has been confirmed by the high- yield commercial gas flow of 28×10⁴ m³ from the Wusonger Formation in the upper assemblage in Well Ketan1 penetrated by Beijing Energy Holding Co., Ltd. in 2019. The breakthrough in hydrocarbon discoveries is dependent on the exploration of tectonically stable regions.

In accordance with the rift-depression framework in the Luonan-Yubei paleo-rifts confirmed for the first time, evolutionary model of passive continental margin and seismic facies, the Lower Cambrian Yurtusi source rocks were comfirmed to exist in the Makit slope, southwestern Tarim. This conclusion lays a solid resource foundation for hydrocarbon exploration (Table 1). The Makit slope in the updip direction lies in the key tectonically stable region of southwestern Tarim paleohigh migration and reversal area, and thus it may be favorable for the exploration of Cambrian subsalt primary reservoirs. The favorable area with the buried depth < 8500 m nowadays covers an area of 32 820 km², all of which are related to the upper assemblage. There are 3 phases of fracture formation in the Early Caledonian, Late Caledonian-Early Hercynian and Late Hercynian-Himalayan in the Makit slope^[24]. Late Hercynian-Himalayan strike-slip faults, which did not penetrate the Middle Cambrian overburden composed of gypseous salt rocks, dominated Cambrian subsalt traps and hydrocarbon accumulation. Eleven traps covering an area of 437.8 km² has been confirmed, where it is expected to make a breakthrough in Cambrian subsalt hydrocarbon discoveries in southwestern Tarim.

The paleo-tectonic framework with two uplifts and two depressions forming at the beginning of the Nanhuan Period was adjusted through three tectonic evolution stages from the Late Sinian to the Middle Cambrian, including the rift-depression transform in the Late Sinian, uplifting and denudation at the end of the Sinian Period, and post-rift subsidence and craton in the Early and Middle Cambrian. The tectonic-sedimentary evolution controlled the transition in the Tarim Basin from north-south differentiation to a framework with the basin in the east and platform in the west, the evolution from ramp to strongly rimmed platform, and promising intra-platform facies. The carbonate ramps occurring in the Late Sinian, shale-rich ramps at the depositional stage of the Early Cambrian Yurtusi Formation, carbonate ramps at the depositional stage of the Xiaoerbulak Formation, weakly rimmed platforms at the depositional stage of the Wusonger Formation, and strongly rimmed evaporative platforms in the Middle Cambrian Series were well developed.

The Luonan-Yubei paleo-rifts in southwestern Tarim and its rift-depression framework have been confirmed. On the basis of the model of passive continental margin and seismic facies, the Lower Cambrian Yurtusi source rocks have been predicted to occur in the paleo-rift depressed zone, with new effective source-rock area of nearly 6×10⁴ km². This depressed zone and the north depression constitute a major Cambrian subsalt source region of 32×10⁴ km².

There are 3 sets of large-scale Cambrian subsalt reservoirs, i.e. the Upper Sinian Qigbulak microbial mound- shoal dolomites, the Lower Cambrian Xiaoerbulak algal- mound and algal-beach dolomites, and Wusonger platform marginal mound-shoal dolomites in Lunnan-Gucheng, extending around or above intra-platform paleohighs of the Late Sinian to the Early Cambrian. Original facies, early dolomitization, and unsheltered unconformities of different levels controlled the reservoir distribution and properties. Reservoir properties in the late burial period were inherited rather than reconstructed.

According to the tectonic stability of paleohighs nowadays and lithofacies paleogeography mapping, we have evaluated four favorable zones, i.e. the north slope in Tazhong, the south slope in Tabei, the area around Keping-Wensu, and the upper Makit slope. It is most likely to make a breakthrough of hydrocarbon discoveries in the Cambrian sub-salt in the north slope, Tazhong area. The south slope in the Tabei is favorable for the exploration of the Upper Sinian weathering-crust dolomites. The region around Keping-Wensu and the upper Makit slope exhibit large potential of hydrocarbon exploration. Efforts should focus on these prospects.