The central Sichuan paleo-uplift is a giant uplift that has the earliest formation time, the largest size, the longest duration, the largest denudation amplitude, and the widest coverage in the Sichuan Basin. Its formation and evolution largely controlled the Sinian to Lower Paleozoic hydrocarbon accumulation. Abundant exploration and development achievements have been made in the Sinian to Lower Paleozoic of the uplift. The PetroChina Southwest Oil & Gasfield Company has built a super-giant gas field with the annual production capacity of 150×10⁸ m³, but there is still a problem of uneven exploration in the Sichuan Basin. Most discoveries of gas are in the Gaoshiti-Moxi area (referred to as Gaomo area) in central Sichuan Basin, and in the Sinian Dengying For-mation and Cambrian Longwangmiao Formation^[1,2,3]. It is very urgent to search for new exploration prospects and targets.

The Gaomo area is located at the present structural high position on the top surface of the Cambrian. The Dengying Formation is a structural trap gas reservoir with a relatively uniform gas-water interface. The area to its north, that is, from the north of Suining to the Cangxi area, is a north-dipping monoclinic structure (hereinafter referred to as "North Slope"). Although this slope is currently at the lower part of the structure and below the gas-water interface on the whole, the study shows that this slope had structural-sedimentary setting similar to the Gaomo area in the Sinian to Early Paleozoic, and so this area may have mound shoal deposits at the trough margin in Sinian and intraplatform grain shoal deposits controlled by Cambrian paleo-uplift, and thus has the possibility to form lithological gas reservoirs. Since 2018, China National Petroleum Cooperation (CNPC) has implemented a 3D exploration aiming at multiple targets in the platform margin of the Dengying Formation at the North Slope^[4].

In 2020, two significant breakthroughs were made in risk exploration of the North Slope. Well PT1 there tested a gas production of 121.89×10⁴ m³/d in the Dengying 2 Member, and later Well JT1 tested a gas production of 51.62×10⁴ m³/d in the Lower Member of Lower Cambrian Canglangpu Formation on October 16, 2020. These breakthroughs further confirm that besides the Gaomo gas area, the Sinian to Lower Paleozoic in the North Slope of the central Sichuan paleo-uplift also have broad exploration prospects. In particular, the breakthrough of JT1 well has far-reaching significance for evaluating exploration domain and establishing exploration thoughts of the Cambrian Canglangpu Formation in the Sichuan Basin.

Although before this breakthrough, some literatures had reported that the Canglangpu Formation had dolomite reservoirs, little attention was paid to the Canglangpu Formation as gas testing in several wells in Gaomo area had gas shows but no commercial discovery for a long time. The Canglangpu Formation was primarily considered as a suite of blank strata, neither as reservoirs nor as source rocks between the Cambrian Qiongzhusi source rocks and the payzone of Longwangmiao Formation. In addition, on the development of high quality reservoirs in the Canglangpu Formation, there were disputes over whether the sandstone reservoirs in the Upper Member of Canglangpu Formation or dolomite reservoirs in the Lower Member of Canglangpu Formation took dominance, making the exploration idea of this formation unclear. Prior to the breakthrough of Well JT1, only three papers reported the distribution law, sedimentary characteristics, and sequence stratigraphy of Canglangpu Formation dolomite^[5,6,7], seriously restricting further study and exploration prospects evaluation of the Canglangpu Formation.

Up to now, 148 wells in the Sichuan Basin have encountered the Canglangpu Formation, of them 52 had fairly good oil and gas shows. The Canglangpu Formation, a new domain and layer series, has very weak basic geologic study. With the breakthrough of Well JT1, both the gas-bearing property of this formation and dolomite reservoirs in the Lower Member of Canglangpu Formation have been confirmed. On the basis of previous achievements and the latest exploration progress, the authors study sedimentary facies distribution and genesis of the dolomite reservoirs, source-reservoir assemblage, caprocks, and paleo-uplift geometry in the Lower Canglangpu Member of the basin, prepare a batch of basin-scale industrial maps, and conduct many experiments and obtained large amounts of data. All these will help promote the evaluation of and favorable blocks selection in the Lower Member of Canglangpu Formation in the central Sichuan paleo-uplift.

Both the central Sichuan paleo-uplift and Deyang-Anyue rift are the two most important geological units controlling hydrocarbon accumulation in the study area. The former took an embryonic shape at the end of Sinian, was evolved to a synsedimentary paleo-uplift from the Cambrian to Ordovician, basically fixed in shape after experiencing the Yunnan Movement at the end of Ordovician, went through the Guangxi Movement at the end of Silurian, and finalized in shape at last^[8]. Its geometry is very clear in paleo-geological map prior to the Permian deposition (Fig. 1). Statistics considering the erosion area of Silurian show it covers an area of 6.25×10⁴ km^2[9].

Development of the Deyang-Anyue rift started in the early deposition period of the Sinian Dengying Formation and ended in the middle to late deposition period of the Lower Cambrian Qiongzhusi Formation. The Deyang-Anyue rift is in N-S strike and 6×10⁴ km² in area. In this rift are deepwater-facies mudstone and shale and argillaceous dolomite of the Lower Cambrian, of which source rocks are 100-350 m thick and 1.95% in average TOC^[10], making the rift the most important hydrocarbon-generating and hydrocarbon-supplying center for Sinian to Cambrian. High quality mound shoal body reservoirs of the Dengying Formation occur around two sides of the rift.

Under the joint effect of structural and depositional setting of the syn-sedimentary paleo-uplift and paleo-rift, multiple sets of scale Sinian and Cambrian dolomite reservoirs of mound shoal facies developed around the rift margin and at the higher position of the paleo-uplift.

In the Sichuan Basin, the Cambrian from the bottom to top is divided into the Lower Cambrian Maidiping Formation (—C₁m), Qiongzhusi Formation (—C₁q), Canglangpu Formation (—C₁c), Longwangmiao Formation (—C₁l), Middle Cambrian Gaotai Formation (—C₂g), and Middle to Upper Cambrian Xixiangchi Formation (—C_2—3x). The Canglangpu Formation is subdivided into lower and upper members. The Lower Member is mainly composed of carbonate rocks, including limestone, dolomite and some clastic rock interbeds; whereas the Upper Member is dominated by clastic rocks, including mudstone, sandstone, and siltstone, with dolomitic siltstone interbeds in local parts (Fig. 2).

Outcrop, and drilling and seismic data show that the distribution of the Lower Canglangpu Member is affected by both the rise of the ancient lands around basin margin and evolution of Deyang-Anyue rift. Affected by regional tension, a rift extending toward craton basin hinterland from west Sichuan sea basin developed at the west margin of Upper Yangtze Craton during the deposition of the Sinian Dengying Formation, that is the Deyang-Anyue rift. The early deposition period from the Early Cambrian Maidiping Formation to the Qiongzhusi Formation was the period of rift development. At this time, 500-1000 m thick mudstone of deepwater shelf facies deposited inside the rift, which would become high quality source rock^{[2, 11-13]}. The period from the middle-late deposition of the Qiongzhusi Formation to the deposition of the Canglangpu Formation was the key transformation period of Middle to Upper Yangtze Craton, when the early tensional structure changed to compressional one due to the effect of Caledonian Movement. The Longmenshan-Micangshan mountains at the west margin of Upper Yangtze Craton started uplifting to form ancient lands^{[10, 14-15]}, including Motianling, Kangdian, and Hannan ancient lands. Among them, Kangdian ancient land was broader, extending from Longmenshan Mountains all the way to Honghe area in Yunnan Province, and made up of Xuelongbao, Pengguan, and Kangding complex rock masses, forming the paleo- geomorphologic framework high in the west and low in the east. Consequently, large amounts of terrigenous clast was transported to the sea in the east, and Kangdian and Motianling ancient lands were the main provenances^[16,17].

It can be seen from the thickness map of the Lower Member of Canglangpu Formation (Fig. 2) except the region west of Meishan city (where this member is absent due to erosion), the Lower Member of Canglangpu Formation is continuous in distribution, and about 50-300 m thick in the other areas. It is thin in the west and thick in the east on the whole, and thinner closer to the erosion line and near the three ancient lands, showing the ancient lands have obvious effect on this member, which thins and overlaps towards the ancient lands.

Previous study showed that the Deyang-Anyue rift went extinct during the late deposition of the Early Cambrian Qiongzhusi Formation^[10], but the thicker parts of the Lower Member of Canglangpu Formation are mainly in two regions (Figs. 2 and 3): (1) in the east and northeast margins of Sichuan Basin, namely east of Chengkou profile in Dabashan Mountains, Sanxia profile in Hubei Province, Dayan profile in Xishui city, where this member is more than 200 m thick in general; and (2) the stripe along Foushan profile, ZY-N2 well block, and N2 well block, where the member is generally more than 150 m thick and thins towards both wings; this thick stripe is very similar to the Deyang-Anyue rift in shape. It can be seen from Fig. 3 this member changes significantly in thickness in Z5-CS17-HT1 well block. It is 134 m thick in Well CS17, and thins towards both wings rapidly, and 63 m thick in Well Z5, and 77 m thick in Well HT1. It is stable in thickness from the east of Well HT1 to Well Li1 (77-103 m), and suddenly thickens to 283 m from Well Li1 to Well EC1. The Upper Member of Canglangpu Formation shows certain inheritance in thickness, it is thicker (122 m) in Well GS 17, but decreases in thickness difference, (70 m and 93 m thick in Well Z5 and HT1 respectively). In addition, this member increases gradually in thickness from Well HT1 to Well EC1.

From the thickness distribution of the Lower Member of Canglangpu Formation, it can be seen that after the deposition of the Qiongzhusi Formation, the Deyang-Anyue rift wasn’t filled up and still controlled the sedimentary filling of the Lower Member of Canglangpu Formation. The Lower Member of Canglangpu Formation continued to fill up in the rift, forming thicker areas. The feature of onlap can be seen in Fig. 4. The Upper Member of Canglangpu Formation is still thicker near Well GS17, but decreases in thickness difference with the surrounding areas. To the deposition of the Longwangmiao Formation, the formation was basically uniform in thickness (90-100 m thick)^[18,19]. Hence, it’s concluded that the Deyang-Anyue rift was finally filled up during the middle to late deposition of the Canglangpu Formation.

On the basis of analysis of sedimentary facies revealed by outcrops and drilling data, stratigraphic thickness framework, seismic-facies data, and previous achievements^[20], we prepared lithofacies paleogeographic map of the Lower Member of Canglangpu Formation (Fig. 5). This member appears high in the west and low in the east on the whole, and has sedimentary facies of ancient land, shoreland, detrital shallow-water shelf, turbidite shallow-water shelf, intrashelf sag, fresh-water shallow-water shelf, and deep-water shelf from west to east (Fig. 5b).

Bounded by the intrashelf sag, the Lower Member of Canglangpu Formation on two sides of the sag differ noticeably in thickness and lithology. The member in the west side of the sag, with sufficient sediment supply from nearby ancient lands, is primarily clastic sediment of shoreland and detrital shallow-water shelf facies. The sediment of shoreland facies is mainly light grey to greyish yellow block fine sandstone, whereas the sediment of detrital shallow-water shelf facies is composed of light grey and greyish green silitstone and argillaceous siltstone. In the intrashelf sag, this member shows sedimentary differentiation. In the lower sag area, this member with bigger thickness is mainly dark grey and greyish black argillaceous siltstone interbedded with dark grey micritic limestone and marl, reflecting low-energy sedimentary characteristics. Near the sag, there is a transitional zone between carbonate and clastics, and mixed shelf facies occurs along the sag edge, with carbonate and clastics appearing alternately.

In the east of the sag is fresh-water shallow-water shelf facies dominated by carbonate rocks, indicating the rift not filled up could effectively stop sediment from the ancient lands feeding into the basin, resulting in a clean- water setting in the east of the sag. Additionally, 9 m thick gympum was encountered in the Lower Member of Canglangpu Formation in Well JS1 of eastern Sichuan Basin. Based on seismic-facies features around the well, there developed a gypsum dolomitic lagoon in E-N strike near the well. The widespread grain shoal in the shallow-water shelf is the key reservoir development zone of the Lower Member of Canglangpu Formation. The shoal bodies primarily occur in three belts, east margin of the sag in the shelf, shallow-water shelf margin, and around gypsum dolomitic lagoon. The shoal bodies are made up of sparry oolitic dolomite and limestone, and crystalline dolomite. The limestone shoal mainly turns up around the shelf margin, whereas dolomite shoal occurs just around Gaomo- North Slope-Well WT1 in the eastern basin. Well JT1 is located in this facies belt, its drilling reveals that this member is mainly composed of thick sandy limestone, oolitic dolomite, and powder-fine dolomite from bottom to top (Fig. 6).

Based on seismic sections calibrated by log data of Well JT1 and CS1 etc, there is bright spot reflection under the strong lithology axis of the top of Lower Member of Canglangpu Formation when there is shoal body developing; whereas there is no obvious reflection or only strong wave trough below the strong lithology axis when there is either very thin or no grain shoal developing. Accordingly, planar distribution of shoal bodies in the Lower Member of Canglangpu Formation in Gaomo- North Slope area is delineated to define the shoal-body distribution in this area (Fig. 7). The dolomite shoal body is mainly distributed along the sag margin in N-S strike, around Well HT1-PT1-JT1-CS1. From the seismic depiction, this dolomite shoal body covers about 8900 km², and the abnormal reflection body around Well PT1 and JT1 has the largest area of 3100 km². The shoal bodies in other areas are delineated based on drilling and outcrop data or the correlation between sedimentary topography and microfacies (for example the restricted setting of lagoon).

The deepwater shelf facies belt mostly occurs in the east of Wanyuan-Wuxi-Pengshui-Renhuai area, where the Lower Member of Canglangpu Formation is more than 200 m thick generally, for instance 283-m-thick in Well EC1, eastern Sichuan Basin (Fig. 3). This member shows characteristics of over compensation, with sediments in dark color and fine grain size.

2.2.1. Reservoir characteristics

On the basis of outcrop survey, core observation, and thin-section identification, combined with well-logging interpretation, it’s found that the rocks in the Lower Member of Canglangpu Formation with reservoir capacity are granular and crystalline dolomites. Moreover, the granular dolomite can be subdivided into residual oolitic dolomite, arenaceous dolomite, limy ooltic dolomite, and a little powder-crystalline dolomite. The major pays in Well JT1 are primarily oolitic dolomite, secondarily powder-crystalline and fine-crystalline dolomite (Figs. 6 and 8). The reservoirs are largely pore type, with intergranular and intercrystalline dissolution pores, and intercrystalline pore as main storage space, and some fractures seen in core or under SEM.

There is little core information on the Lower Member of Canglangpu Formation in central Sichuan paleo-uplift available. Logging interpretation results of eighteen reservoirs in Lower Member of Canglangpu Formation in 10 wells of central basin (including JT1, CS1, MX8, and GS16) (Fig. 9) show that reservoirs in individual wells are 2-26 m thick, 2%-5% (3.7% on average) in porosity, and (0.004-0.120)×10^-3 μm² (0.053×10^-3 μm² on average) in permeability, representing low to medium porosity and low permeability ones. The reservoirs in Well JT1, the breakthrough well, have a cumulative thickness of 25.9 m, a porosity of 3.8%-8.1% (4.1% on average), and a permeability of (0.022-0.092)×10^-3 μm² (0.053×10^-3 μm² on average).

Reservoir correlation of several wells shows that the reservoirs in Lower Member of Canglangpu Formation are heterogeneous laterally, there are three suites of reservoirs vertically, laterally, and the reservoir at the top of this member is more developed in Moxi area and the North Slope (Fig. 10). In Well JT1 and CS1 in the North Slope, the middle and lower reservoirs are more developed than in the Moxi area. They are 17.1 m and 25.9 m thick in Well CS1 and JT1 respectively, but only 9.0 m, 3.1 m, and 8.1 m respectively in Well MX8, MX123, and MX107.

2.2.2. Major factors affecting reservoirs

The formation and evolution of carbonate reservoirs are jointly influenced by three geological factors, deposition, diagenesis, and structure. Affected by rigid basement of central Sichuan Basin, central Sichuan paleo-uplift developed successively over a long period, had weaker structural deformation and mainly microfractures. The formation of reservoirs in this uplift is mainly controlled by deposition and diagenesis. Our study shows that penecontemporaneous dolomitization and karstification are the two major factors affecting development of high-quality reservoirs there.

2.2.2.1. Penecontemporaneous dolomitization

Many previous studies covered the effect of dolomitization on reservoir formation and development. It’s usually deemed that dolomitization can improve the petrophysical properties of reservoir^[21]. It can be seen from Fig. 6, Fig. 8, and Fig. 10 that reservoirs in the Lower Member of Canglangpu Formation are mainly in dolomite sections; whereas reservoirs are underdeveloped in limestone and dolomitic limestone sections, indicating dolomitization is an important constructive diagenesis to the reservoirs in this member. Order degree and C/O isotope composition analyses show that the dolomite of this member is mainly of penecontemporaneous dolomitization origin, and the dolomitization fluid is marine fluid.

(1) Order degree. Order degree is a key index to judge the evolution degree of dolomite. The slower the crystallization rate, the higher the order degree of dolomite is. Conversely, the faster the crystallization rate is, the lower the order degree of the dolomite is^[22].

Nine dolomite samples from the Canglangpu Formation were selected to do X-ray diffraction order degree test (Table 1). The results show they have order degrees from 0.3 to 0.6, and 0.47 on average, representing lower order degree, indicating the samples have poor crystallization degree due to insufficient time and space. It is inferred that the dolomite was formed in near-surface environment.

(2) C/O isotope composition. δ¹³C and δ¹⁸O values are sensitive to seawater salinity, temperature, fresh-water leaching, and biodegradation. Burial dolomite and hydrothermal dolomite often have lighter δ¹⁸O value than seawater^[23,24,25]. Considering this, Keith and Weber proposed an empirical formula in 1964 to distinguish paleo-salinity of marine and fresh-water facies carbonate rocks^[26]. The isotopic factor Z smaller than 120 indicates fresh-water diagenetic setting; whereas Z more than 120 indicates normal seawater diagenetic setting.

Based on the C/O isotope composition, Z values of 23 dolomite samples were calculated. The results show they all have Z higher than 120, and heavier O isotope composition of more than -8‰ (Fig. 11), suggesting the dolomitization fluid is marine nature. Combined with the near-surface feature indicated by low Z order degree, it’s inferred that the dolomite in the study area must be penecontemporaneous origin.

2.2.2.2. Hypergene karstification

At the end of the Caledonian Movement, central Sichuan paleo-uplift lifted and suffered erosion, forming the erosion window of the Canglangpu Formation. The paleo-uplift was a karst landform high in the west and low in the east (Fig. 1). The dolomite shoal body was just located at the window ramp, where bedding karstification might happen to the reservoirs formed earlier to further improve their properties.

Based on statistics on dolomite thickness from outcrop and drilling data, a thickness contour map of dolomite in the Lower Member of Canglangpu Formation, Sichuan Basin is prepared (Fig. 12). It can be seen from the figure that the dolomite is largely in the east of the sag in the shelf. With 30 m thickness contour as the dividing line, there are two areas with large dolomite thickness, MX107-CS1 well block in near N-S strike and GT2-WT1 well block in E-N strike. The thick dolomite is the result of two factors probably: (1) A restricted environment might be formed inside the clean-water shelf due to surrounding limestone shoal in both the intrashelf sag and clean-water shelf margin (Fig. 5). (2) The MX107-CS1 well block with thick dolomite is consistent with the central paleo-uplift margin in trend, this block was at the structural high of landform during the deposition of the Lower Member of Canglangpu Formation, which was favorable for penecontemporaneous dolomitization. Whereas the thick dolomite zone in WT1 well block is in similar trend with the Dazhou-Kaijiang Paleo-uplift in eastern Sichuan Basin. This paleo-uplift was an inherited one before Sinian to Middle Cambrian, and gradually evolved to a slope belt from Middle Cambrian to Ordovician^[27]. And the higher position of the paleo-uplift was probably the favorable zone for dolomitization of the Canglangpu Formation in eastern Sichuan Basin.

Although the Lower Member of Canglangpu Formation in Well WT1 has 46.7 m thick dolomite, thicker than that in Well JT1 (25 m), the dolomite hasn’t evolved into reservoir, maybe because Well WT1 is far away from the erosion window and the dolomite has not subjected to karstification. Hence, with thicker dolomite and near the erosion window, the MX107-CS1 well block is the most favorable area for reservoir development. WT1-GT2 well block with thick dolomite but poor karstification might be the second favorable zone for reservoir development if there are fractures adding in the reconstruction.

The Sinian to Cambrian source rocks are mainly distributed along Deyang-Anyue rift, Hechuan-Dianjiang Sub-rift, and Chengkou-Exi Trough (Fig. 13). Among them, the source rocks inside Deyang-Anyue rift have the largest scope and thickness, and contribute 56% Sichuan basin 65% natural-gas resources in the Sinian to the Cambrian. The Dengying 3 Member, and the Maidiping and Qiongzhusi formations all have source rocks. Of them, the Qiongzhusi Formation is the major source rock formation, in which the source rocks are black and greyish black shale and carbonaceous mudstone. The source rocks are 150-350 m thick combined, 0.50% to 8.49% and 1.95% on average in TOC value, and (20-100)×10⁸ m³/km² in hydrocarbon-generating intensity, representing a set of high quality source rock^{[1, 10]}. Right above this set of high quality source rock is the grain shoal of the Lower Member of Canglangpu Formation, the first set of high quality reservoir, which has the favorable conditions such as direct gas charging and hydrocarbon capture in priority.

The PT1-CS1 well block is right next to hydrocarbon- generating center margin of the Qiongzhusi Formation in Deyang-Anyue rift, and has extensive granular dolomite, so, this block is the area with the most favorable source-reservoir assemblage. The dolomite shoal body in HT1-WT1 well block is farther away from the Deyang- Anyue rift, but is above or near the sub-hydrocarbon-generating center in Hechuan-Dianjiang Sub-rift, so this block also has favorable near source charging condition and is a second most favorable area for source-reservoir assemblage.

Above the Lower Member of Canglangpu Formation are the Upper Member of Canglangpu Formation and Longwangmiao Formation. The Longwangmiao Formation is one of the major pays of Cambrian in Sichuan Basin, which has proven gas reserves of 4400×10⁸ m³ discovered in Gaomo area^[9]. Whether the Upper Member of Canglangpu Formation is able to seal effectively is the key to the preservation of the gas reservoir in the Lower Member of Canglangpu Formation. Drilling reveals that the Upper Member of Canglangpu Formation is composed of grey and dark grey siltstone and mudstone, interbedded with thin sandstone, argillaceous siltstone, and silty mudstone (Fig. 3), and the mudstone layers are small in continuous thickness.

Based on statistics on mudstone (including silty mudstone) thickness revealed by drilling and outcrop, the thickness distribution map of the Upper Member of Canglangpu Formation in Sichuan Basin is prepared (Fig. 14). It can be seen from this figure that the mudstone thickness has an obvious zoning feature between Gaomo area and the North Slope. In Gaomo area, the mudstone is usually less than 50 m thick, largely from 10 m to 30 m, and thicker in the south and thinner in the north, 40 m in Well GS10 in the south, 30.4 m and 29.0 m respectively in Well MX8 and MX17 in the center, and only 9 m in Well BL1 in the north. In North Slope, the mudstone is more than 50 m thick in general, and thickens toward north on the whole, for example 52 m in Well JT1 and 68 m in Well CS1.

So far, a few wells around HT1-GT2 well block has got active gas shows at the shoal body of dolomite in the Lower Member of Canglangpu Formation. But no commercial discovery has been made yet. It’s inferred that there are two reasons for this: (1) Thin mudstone in the Upper Member of Canglangpu Formation may have poor sealing ability, resulting in gas escape; and (2) little attention was paid to the Canglangpu Formation before the breakthrough of Well JT1, just because of thin mudstone in this member. If commercial discovery is made from testing of existent wells in the future, it can be confirmed that the tight sandstone in the Upper Member of Canglangpu Formation also can seal the gas reservoir effectively. But now, the mudstone thickness is still the main index to evaluate the sealing ability of the Upper Member of Canglangpu Formation.

As a synsedimentary structural uplift which started to develop after the deposition of the early Cambrian Qiongzhusi Formation, central Sichuan paleo-uplift has features of both synsedimentary and erosion paleo-uplift^[9]. Its evolution affected obviously the oil and gas accumulation in Cambrian.

Basically, most Cambrian gas reservoirs in this paleo- uplift went through three reservoir-forming periods^[1,9]. The Cambrian source rocks began to generate liquid hydrocarbon at the end of Ordovician, and reached the peak of hydrocarbon generation and expulsion in the Permian to Middle Tertiary, leading to the formation of the paleo oil reservoirs. From Late Jurassic to Cretaceous, these paleo oil reservoirs reached the gas-generating peak. To the end of Cretaceous, crude oil in these paleo oil reservoirs had roughly cracked to form gas reservoirs. The gas reservoirs were adjusted and finally fixed during Himalayan Movement. In this study, the paleo-structural characteristics of Canglangpu lithological gas reservoirs were analyzed indirectly from paleo-structural evolution of top Longwangmiao Formation near Canglangpu Formation.

As shown in the paleo-structure of top Longwangmiao Formation (Fig. 15), at the time of the main reservoir- forming periods (including oil-generating peak and oil reservoir cracking period), the dolomite shoal bodies in the Moxi area and north of the central Sichuan Basin were situated at the core and ramp of the central Sichuan paleo-uplift, which were the favorable direction of hydrocarbon migration.

In conclusion, the breakthrough of Well JT1 is related to its excellent reservoir-forming factors.

Just like wells in Gaomo area, Well JT1 is also close to Deyang-Anyue rift and the Canglangpu Formation erosion window, and is at the ancient structural high during the main reservoir forming period. Different from other wells in Gaomao area with thinner dolomite and mudstone in the Lower and Upper members of Canglangpu Formation, Well JT1 has thicker dolomite and mudstone in the Lower and Upper members of Canglangpu Formation. Like some wells in eastern Sichuan Basin, Well JT1 has thicker dolomite in the Lower Member of Canglangpu Formation; but the eastern Sichuan Basin is far from the Deyang-Anyue rift and the Canglangpu Formation erosion window and was at the slope or structural low in the main reservoir-forming period.

It is concluded through comprehensive analysis that out of the five factors, sedimentary facies, karstification, source-reservoir assemblage, caprock, and paleo-structure, the distance to the hydrocarbon generating center and position at the paleo-structure in main reservoir- forming period have less significant effect on the reservoir forming in the Lower Member of Canglangpu Formation, the first suite of reservoir above the Qiongzhusi Formation source rock. Oil and gas can charge into this member directly and in-situ, as long as there is reservoir, the member can receive oil and gas. Therefore, the reservoir forming in the Lower Member of Canglangpu Formation may have lower requirements on source rock and paleo-uplift. Located at dolomite shoal belt, closer to the erosion window, and mudstone more than 50 m thick are the main factors affecting gas accumulation in the Lower Member of Canglangpu reservoirs, while source rock and paleo-uplift are secondary factors affecting reservoir forming in this member. To expand the exploration in the whole Canglangpu Formation, seismic prediction technologies for reservoir must be researched, and seismic response features of high quality reservoirs in the Lower Member of Canglangpu Formation must be confirmed.

According to the main reservoir-forming conditions of source rock, reservoir, caprock, paleo-uplift, and their time-space configuration, the Lower Member of Canglangpu Formation is evaluated, and three classes of favorable zones in the member are sorted out (Fig. 16 and Table 2).

(1) PT1-CS1 well block: With an area of 1.6×10⁴ km², this block is situated in the dolomite shoal body development region of the Lower Member of Canglangpu Formation, with dolomite 25 m to 40 m thick. This block is 0-131 km away from the erosion window of the Canglangpu Formation, and is likely to be reconstructed by later karstification, so this block has more developed reservoirs. Meanwhile, it’s very close to the Cambrian hydrocarbon-generating center in Deyang-Anyue rift with Lower Cambrian source rocks of about 200 m thick. Moreover, the mudstone in the Upper Member of Canglangpu Formation is more than 50 m thick. The source rock, reservoir and caprock constitute a good assemblage. During the hydrocarbon expulsion period of the major source rock, the vast bulk shoal bodies were situated in the slope or the structural high of the paleo-uplift, favorable for oil and gas accumulation. The top of Canglangpu Formation is currently at the elevation between -8000 m to -4500 m. Exploration breakthrough has been made in Well JT1 of this block, confirming that this block has effective reservoir-forming conditions. Therefore, this block is evaluated as class I area.

(2) HT1-GT2 well block: With an area of 0.9×10⁴ km², it is also located in the dolomite shoal body development region of and has dolomite 5 m to 25 m thick in the Lower Member of Canglangpu Formation. It is 40-135 km away from the erosion window, so the Canglangpu Formation in the west of the block probably suffered some karstification. This block is between the Cambrian hydrocarbon-generating center in Deyang-Anyue rift and the sub-hydrocarbon-generating center in Hechuan- Dianjiang Sag, about 43-133 km away from the main hydrocarbon-generating center and closer to the Hechuan-Dianjiang sag, so it may receive oil and gas coming from source rocks on both sides. The mudstone in the Upper Member of Canglangpu Formation is thinner in the north of the block and thickens southward to 50 m. Moreover, this block was at structural high of the paleo-uplift in the main hydrocarbon expulsion period and is shallower in burial depth now, so, after the comprehensive evaluation, this block is classified as class II.

(3) Eastern GT2 well block: With an area of 0.8×10⁴ km², this block is also located in the dolomite shoal body development region of and has dolomite 20 m to 40 m thick in the Lower Member of Canglangpu Formation. It is 130-135 km away from the erosion window of Canglangpu Formation, and has suffered weak karstification. It is more than 160 m away from the hydrocarbon-generating center in Deyang-Anyue rift, so mainly depend on hydrocarbon supply from Hechuan-Dianjiang Sag. The mudstone in the Upper Member of Canglangpu Formation is thinner in the north of the block and increases southward to 50 m. It was in the slope of the paleo-uplift during the main hydrocarbon expulsion period, favorable for oil and gas accumulation. The top of Canglangpu Formation in this block is at the elevation between -7800 m to -6000 m currently. Compared with the class Ⅰ and Ⅱ zones, this block has poorer source rock, reservoir, and caprock conditions, but better location at the paleo-uplift and shallower burial depth. This block was evaluated as class Ⅲ.

There are two shoal bodies around Well WT1. They have poorer source rocks, caprocks, and paleo-uplift position. Besides, although Well WT1 encountered 46.7 m thick dolomite in the Lower Member of Canglangpu Formation, the dolomite has hardly any reservoir. This means this area, very far from the erosion window, may have no reservoir in Canglangpu Formation, so it isn’t a favorable exploration area.

For the Canglangpu Formation, its early deposition is controlled by the ancient uplifts at the basin margin and the sedimentary landform inside the basin. Bounded by the Deyang-Anyue rift, the Lower Member of Canglangpu Formation in west of the rift is mainly composed of shoreland and mixed shelf clastics, and have clean-water shelf carbonates in east of the rift, with shoal-facies dolomite well-developed.

Jointly affected by penecontemporaneous dolomitization and karstification, grain-shoal-facies dolomite in the Lower Member of Canglangpu Formation has well-developed reservoirs mainly in the north of central Sichuan Basin. Seismic depiction shows these reservoirs cover an area of 8900 km².

The dolomite shoal body in the Lower Member of Canglangpu Formation is the first set of large-scale reservoir above the Qiongzhusi source rocks, and thus has the advantage of direct gas charging. Also the reservoirs in Lower Member of Canglangpu Formation are next to Deyang-Anyue rift and Hechuan-Dianjiang Sub-rift, covered by the sandy mudstone in the Upper Member of Canglangpu Formation, giving them good hydrocarbon accumulation conditions.

According to six indexes, including source-rock thickness, thickness of dolomite in the Lower Member of Canglangpu Formation, reservoir origin, thickness of mudstone in the Upper Member of Canglangpu Formation, paleo-uplift, block coverage, and burial depth, the Lower Member of Canglangpu Formation is evaluated. PT1-CS1 well block 1.6×10⁴ km² is evaluated as class I, HT1-GT2 well block 0.9×10⁴ km² class II, and eastern GT2 well block 0.8×10⁴ km² class III.