Well Pengtan 1 is a risk exploration well located in the Deyang-Anyue rift in the Sichuan Basin, with the Sinian Dengying Formation as the main target layer. Its objective is to explore the reservoirs and gas-bearing properties of the Dengying Formation in the main hydrocarbon kitchen area in the rift. Based on drilling data from Well Pengtan 1, combined with seismic data, this paper analyzes the stratigraphic and sedimentary sequence in Well Pengtan 1, discusses the controlling factors of the distribution of platform margin mound-beach facies in the 2nd member of Dengying Formation (hereinafter Deng-2 member) in the rift, and indicates the most favorable area for the next stage of exploration. The research results show that: ① The stratigraphic sequence of the Dengying Formation in the northern part of the Deyang-Anyue rift is characterized by the fact that the 4th member of Dengying Formation (hereinafter Deng-4 member) is missing in that area, and by the disconformity between the 3rd member of Dengying Formation (hereinafter Deng-3 member) and the Maidiping Formation of the Lower Cambrian. ② In the Deyang-Anyue rift, mound-beach bodies controlled by synsedimentary faults are developed in the Deng-2 member, as well as microbial carbonate rocks. ③ The lithology of reservoir in the Deng-2 member is mainly algal dolarenite and algal thrombolite dolomite. Dissolution vugs are well developed and the reservoir physical properties are good. ④ The gas-bearing properties of the Deng-2 member are good. According to well logging interpretation, the thickness of the gas layer is 119.26 m, and gas-water contact is at 5550 m. According to research, several rows of platform margin mound-beach belts controlled by fault terrace are developed in the Deng-2 member in the rift, which is in direct contact with the hydrocarbon source rocks of the Cambrian Qiongzhusi Formation. The conditions for near-source hydrocarbon accumulation are favorable. It is therefore an important field for large-scale exploration. On May 4, 2020, natural gas of 121.98×104 m3/d was tested in Deng-2 member from well Pengtan 1, which shows great exploration potential.

Well Manshen 1, located in the Northern depression of the Tarim Basin, has achieved a great breakthrough in the Ordovician Yijianfang Formation. Based on in-depth studies of hydrocarbon accumulation conditions, reservoir characteristics, and enrichment patterns of the well, the hydrocarbon accumulation model and enrichment patterns of high-production wells in fault-controlled karst areas in the Northern depression of the Tarim Basin have been summarized. There are four types of marine carbonate rocks in the Tarim Basin: buried hill karst, interlayer karst, reef-shoal karst and fault-controlled karst. Well Manshen 1 penetrated fault-controlled karst. Well Manshen 1 is a discovery well of fault-controlled karst reservoir with the deepest burial depth and the highest single well production so far. In the Yijianfang Formation of the Ordovician, with a buried depth of 7535 m, tested oil production is 624 m3/d and gas production is 37.13×104 m3/d. Three enrichment patterns of fault-controlled karst reservoirs are established: linear strike-slip up-dip high position, transtensional lateral high position, and compressive-shear local high position. The discovery of Well Manshen 1 is a successful practical application of the exploration idea of fault-controlled karst carbonate reservoirs with faults as the core. It has achieved a breakthrough in oil and gas discovery in the saddle position between the Tabei uplift and the Tazhong bulge, which is of great significance for deep and ultra-deep oil and gas exploration in the Tarim Basin as a whole.

The key factors restricting oil and gas discovery in the Laibei low bulge in the Bohai Bay Basin are unclear understandings of the structural settings and the hydrocarbon accumulation model. Since 2019, by transforming exploration ideas, research on faults and structures, favorable objective strata, and oil-gas accumulation rules in this area has been strengthened. It is considered that: (1) Oil and gas accumulation in the Neogene is controlled by the development position of the “convergence ridge” of the top unconformity of the Mesozoic; (2) micro paleo-geomorphology controls the development position of favorable lithologic traps in the lower part of the Minghuazhen Formation in the Laibei low bulge; and (3) oil and gas are mainly enriched at the top of the V oil layer in the lower part of the Minghuazhen Formation. Quantitative description of sand bodies and fluid identification technology based on lithologic-constrained geo-statistical inversion have been established. Guided by new understandings and new technologies, the exploration direction has been successfully changed and a new model of superimposed and contiguous hydrocarbon accumulation in Neogene lithologic reservoirs has been established. This has led to the first commercial discovery in the Laibei low bulge, after more than 40 years of exploration. The KL6-1 oilfield is a large-scale high-quality integral Neogene lithologic oil reservoir, with the major oil layer – of large-scale superimposed and contiguous sand bodies – situated at the top of the V oil layer in the lower Minghuazhen Formation. The burial depth of the oil reservoir is generally 1200–1550 m, with oil layer thickness generally more than 8m. The porosity of the oil-bearing reservoir is 14.9%–39.8%, with an average of 31.2%. Permeability is 7.9–19721.3 mD, with an average of 2205.52 mD. DST test results show that maximum daily production of the major oil layer could exceed 180 m3.

Volcanic oil and gas reservoirs are one of the key areas for oil and gas exploration in the Liaohe depression. Multiple middle-shallow volcanic oil reservoirs represented by trachyte have been discovered, although no breakthroughs were made in fields of deep volcanic rocks during natural gas exploration. In order to make large-scale resource discoveries, research on deep volcanic rocks was carried out. Through core observation, single well analysis, reservoir description, source rock evaluation, and other research, combined with wide-broadband, wide azimuth and high-density seismic data and time-frequency electromagnetic technology, multiple volcanic rock targets were identified. Risk exploration Well JT1 was successfully deployed and drilled, obtaining high-production industrial gas flow. Studies show that the deep source rocks of the third member of the Shahejie Formation in the eastern sag have the potential for large-scale gas generation, providing a rich material basis for large-scale gas accumulation. Compared with sedimentary rocks, the physical properties of volcanic rocks are less affected by burial depth, particularly the basaltic breccia, which has good reservoir conditions. The volcanic rocks in the source rock formations are wrapped by mudstones, which act as both gas source rocks and cap rocks. Accumulation conditions are very favorable. The success of Well JT1 represents an important breakthrough in gas exploration in the eastern sag. This is of great instructive significance for the exploration of deep volcanic rocks in the Liaohe depression and even in the Bohai Bay Basin.

The Sichuan Basin experienced a long period of continuous marine carbonate deposition. The basin has rich natural gas resources of 14.33?1012 m3, which ranks first in China. It is an important exploration field for discoveries, with large-scale and high-quality reserves. In recent years, PetroChina Southwest Oil & Gasfield Company has concentrated its main exploration efforts on discovering large-scale and high-quality reserves and looking for large and medium-sized gas fields. By focusing on the identification and evaluation of large-scale paleo-geological units controlling hydrocarbon accumulation, the company has continuously deepened understandings of geological theory, strengthened research in areas such as fine characterization of distribution of large-scale and high-quality reservoirs, fine structural modeling of piedmont complex structural zones, and hydrocarbon accumulation characteristic research of large-scale structural slope areas and low areas. Relying on the multi-disciplinary joint technical progress of geology, geophysics and engineering, exploration and evaluation efforts in new areas and new fields have been increased, and important new exploration achievements and new understandings have been obtained. These include: the fine exploration theory of inner-platform in the 4th member of the Dengying Formation in the Sinian system in the Anyue gas field in the central Sichuan Basin, the hydrocarbon accumulation theory of the Upper Paleozoic deep marine carbonate reservoir in the western Sichuan area, and the stereoscopic hydrocarbon accumulation and stereoscopic exploration of multiple marine formations in the northern areas of the central Sichuan Basin. These achievements have promoted the stable and rapid growth of gas reserves in marine carbonate rocks in the Sichuan Basin. Based on these new exploration achievements and understandings, potential exploration zones in other blocks of the Sichuan Basin have been effectively evaluated. According to comprehensive analysis, the Sinian System in the periphery of the Deyang-Anyue paleorift, the Cambrian System in the paleo-uplift around the central Sichuan Basin, and the multi-stage platform margin of the Upper Paleozoic in the northern area of west Sichuan are strategic areas for future large-scale discoveries. The Feixianguan Formation around the trough (platform sag) and the Leikoupo Formation in the western-central Sichuan areas are important areas for continuously deepening exploration.

The Dengying Formation of the Sinian system in the Sichuan Basin has significant consequences for exploration and great potential. Analysis of the petrological and geochemical characteristics of outcrops and cores in the southwestern Sichuan Basin was carried out in combination with seismic data. Comprehensive study shows that the Sinian Dengying Formation in Well Hanshen 1, which is located in the Hanwangchang structure in southwestern Sichuan, is characterized by poor algae content, rich silicon content, relatively low Ca and Mg contents, high Si content, and low δ13C and δ18O contents. This is basically consistent with the 4th member of Dengying Formation (hereinafter Deng-4 member) in terms of lithologic characteristics, major elements and isotopic content, and it is therefore considered that the penetrated stratum is the Deng-4 member of Dengying Formation. According to the prediction of well-seismic combination, the Deng-4 member in the southwestern Sichuan Basin is mainly distributed in the west of the Chengdu-Qianwei area, with a thickness of 200?300 m and a distribution area of 1.5×104 km2 in the basin, extending southward to the Xichang and Yunnan areas. The Deng-4 member gradually thins towards the Deyang-Anyue rift, which is characterized by gentle slope type platform margin. The reservoirs in the Deng-4 member are well developed in the southwestern Sichuan area, which is close to Cambrian high-quality source rocks in the rift. In the oil generation stage, the reservoirs were located at the high position of the paleo-uplift, so that paleo oil reservoirs were formed. In the later stage, after several tectonic movements, oil cracking gas was re-adjusted and re-accumulated. This provided the geological conditions to form gas reservoirs of structural type, structural-lithologic type and structural-stratigraphic type. The areas of Qionglai-Daxingchang-Meishan are favorable zones for petroleum exploration.

This paper presents a set of deep to ultra-deep vuggy carbonate reservoirs of penecontemporaneous dolomitization genesis superposed with early-stage exposed karstification. Based on outcrops and drilling data from the study area, research has been carried out on petrology, reservoir geology and geochemistry. Two types of shallowing-upward sequences have been identified in the Qixia Formation of the Middle Permian in the Sichuan Basin. The first is the coarsening-upward type; from micrite to bioclastic micrite to micrite-sparry grain limestone. The second is the fining-upward type; from sparry grain dolomite to medium- crystalline dolomite to fine-powder crystalline algal dolomite. The early high-frequency exposed karstification of both types has the following characteristics: (1) There are exposed unconformities at the tops of the cycles, and in each cycle the initial flooding sediment of the next cycle filled in the earlier karst system and overlapped the micro paleo-geomorphic highs. (2) The porous bedrocks in the upper part of the cycle were cut by dominant karst channels and formed near in-situ breccias. (3) Patch-like karst systems are developed at the top of some cycles, which have been filled with terrigenous clay, silty clasts and some breccias. (4) Karst cave breccias are developed in some cycles, and the inter-breccia caves were filled with carbonate vadose silts. (5) The stable carbon and oxygen isotopes exhibit a certain negative bias under the exposure surface. Further study on reservoir physical properties shows that, when karstification is relatively strong, high-quality reservoirs are developed in the lower part of the cycle, and the reservoir space is primarily small vugs formed by dolomitization and dissolution expansion of the intergranular pores preserved in bedrocks. When karstification is relatively weak, high-quality reservoirs are developed in the middle and upper part of the cycle, and the reservoir space is similar to that produced by strong karstification. When the intensity of karstification is moderate, high-quality reservoirs are developed in the middle of the cycle, and the reservoir space is generally residual vugs from collapse of inter-breccias caves. The vugs formed by the early exposed karstification superimposed penecontemporaneous dolomitization of the dolomite bedrocks are preserved today as the effective reservoir space. Therefore, early high-frequency exposed karstification with different intensities is considered to be the primary factor for the formation of reservoir space in the Qixia Formation.

The Shunbei oilfield in the Tarim Basin is an ultra-deep carbonate oilfield controlled by strike-slip faults. Influenced by the plane segmentation and vertical stratification of strike-slip faults, there are obvious differences in oil and gas accumulation. In this study, the main controlling factors of oil and gas differential accumulation in the Shunbei area are analyzed, taking Shunbei No.1 and No.5 strike-slip fault zones as examples, combined with the relationship between the characteristics of strike-slip faults and oil-gas dynamic productivity. The results show that: the main controlling factors of oil and gas differential accumulation are the connectivity to source rocks, vertical transporting, and segmentation of the strike-slip faults. The stronger the connectivity of strike-slip faults to source rocks, the higher the degree of hydrocarbon accumulation. The larger the dip angle of the strike-slip fault in the Cambrian gypsum formation, the more favorable the vertical hydrocarbon migration into the Ordovician reservoir. The activity intensity and structural pattern of strike-slip faults determine the scale and connectivity of reservoirs. The scale and connectivity of the reservoirs in transtensional segments are better than those in translational and compression-uplift segments, and so the degree of hydrocarbon accumulation is higher.

The Cambrian-Ordovician carbonate rocks in the Gucheng area of Tadong are key exploration fields in the Tarim Basin. The paleogeographic pattern of "two platforms clamping one basin" determines the superior source-reservoir assemblage and hydrocarbon migration direction in the study area. However, in recent years, exploration activities have not achieved the expected results and the exploration potential of the area has again become the focus of attention. Based on a large number of cores, thin sections, CT scanning data, geochemical data, well logging, and seismic data, this paper examines the types and evolution processes of carbonate platforms in different geological periods of the Cambrian-Ordovician in the Tadong area, analyzes their controlling effects on sedimentary reservoirs, and evaluates the exploration potential of Cambrian-Ordovician carbonate rocks in the area. The results show that: Under the influence of tectonic movements and sea level change, the evolution characteristics of Cambrian-Ordovician platform types in the Gucheng area of Tadong is from ramp platform to rimmed platform to weak-rimmed platform to ramp platform to weak-rimmed open platform. This has resulted in the large-scale development of five sets of reservoirs: ramp microbial mound-shoal in the Cambrian subsalt Xiaoerbulake-Wusonggeer Formations, platform-margin mound-shoal in the Middle-Upper Cambrian, slope gravity flow in the Middle-Upper Cambrian and Middle-Lower Ordovician, ramp dolomitic shoal in the Lower Yingshan Formation, and grain limestone shoal in the Upper Yingshan-Yijianfang Formations. Based on the platform types, evolution processes, and favorable facies belts of reservoir, with prediction and evaluation of reservoir distribution as the core objective, it is indicated that ramp dolomitic shoal in the Lower Yingshan Formation and platform margin mound-shoal in the Middle-Upper Cambrian are the key exploration fields for reserves and production increase in the Tadong area in the near future. There are also three risk exploration fields worth exploring, which are the grain limestone shoal in the Upper Yingshan-Yijianfang Formations, the ramp microbial mound-shoal in the Cambrian subsalt Xiaoerbulake-Wusonggeer Formations, and the slope gravity flow in the Middle-Upper Cambrian and Middle-Lower Ordovician.

There are three principal types of reservoirs in the Ordovician objective strata in the Shunbei area: cave type, fracture type and vug type. Reservoir is characterized by strong heterogeneity and reservoir scale is controlled by strike-slip fault systems with multi-stage activities. The seismic response characteristics of fault-karst reservoirs penetrated by wells vary widely, which makes them difficult to predict and describe. Different from karst fracture-cavity trap, fault-karst reservoir trap is a kind of special carbonate fracture-cavity trap formed under complex conditions. It is characterized by large vertical range and strong horizontal heterogeneity. It is not controlled by local structural morphology and has no unified oil-water contact. Top sealing is provided by regional overlying mudstone, and lateral sealing can be provided by tight carbonate rocks. On the basis of previous research and understandings, seismic identification modes of strike-slip faults and fault-controlled fracture-cavity reservoirs have been established by the use of forward modeling, well-seismic calibration and seismic reflection characteristics analysis. Reservoir identification and description technologies have been formed using gradient structure tensor to determine the shapes of fault-karst reservoirs, using amplitude change rate and chaotic phase attributes to classify and predict interior reservoirs in fault zones, and using multi-attribute fusion to characterize and describe fault-karst reservoirs. Technologies for fault-karst reservoir identification and trap description have been established. The application of these technologies has guided the deployment of several wells in the Shunbei area, which have achieved major breakthroughs in oil and gas discovery. The penetration rate of reservoirs is 84%, which proves the validity and applicability of the technologies. This provides a solution and technical reference for identification and description of fault-karst reservoirs in the Shunbei Oil and Gasfield, as well as other areas, and is of great significance for the exploration and development of fault-karst reservoirs in ultra-deep formations in the Shunbei Oilfield.

Oil and gas phase and accumulation models in ultra-deep fields are hot topics in exploration and research. Taking reservoir inclusions in the Middle-Lower Ordovician in the Shuntuoguole area of the Tarim Basin as the research object, detailed and systematic analyses have been carried out, such as inclusions petrography, restoration of paleo temperature and pressure during oil and gas charging, fluid composition, Raman spectrum, and so on. Combined with the geological conditions of oil and gas reservoirs, the preservation mechanism and influencing factors of ultra-deep oil reservoirs in the Shuntuoguole area are discussed. There are 3 types of inclusions in the Ordovician reservoirs: solid asphalt bearing hydrocarbon inclusions, gas liquid hydrocarbon inclusions, and dry gas inclusions. Solid bitumen bearing hydrocarbon inclusions and gas liquid hydrocarbon inclusions are developed in the Yuejin-Shunbei area, and solid bitumen bearing hydrocarbon inclusions and dry gas inclusions are developed in the Shunnan area. Research on paleo temperature and pressure restoration of inclusions shows that there are at least 2 stages (early and late) of oil and gas charging in the Shuntuoguole area. The early-charged crude oil transformed into light oil through thermal evolution in the reservoirs. In the late stage, light oil was charged in the reservoirs. These two processes combined together to determine the preservation of ultra-deep reservoirs in the Shunbei area. The short duration of the maximum paleo temperature (greater than 150 °C) in this area is the main control factor for ultra-deep reservoir preservation. The medium environment of the reservoirs has a certain inhibiting effect on the thermal evolution of crude oil, which is also conducive to the preservation of liquid hydrocarbons.

A stratigraphic combination of carbonate rocks and gypsum rocks is developed in the Ordovician Majiagou Formation in the central-eastern Ordos Basin, with a thickness of nearly 1000 m and relatively large burial depth. The key issue restricting exploration is whether there is effective hydrocarbon supply and large-scale hydrocarbon accumulation in the Ordovician pre-salt strata, which is far away from the weathering crust. Based on research on the tectonic evolution of the Ordovician in the later sedimentary period, and the allocation relation between the Ordovician strata and the coal-bearing source rocks in the upper Paleozoic, it is considered that there is a hydrocarbon supply window for the Ordovician pre-salt strata in the areas adjacent to the paleo-uplift, which are in direct contact with the coal-bearing source rocks of the upper Paleozoic and are distributed on a large scale. The window zone was in a downdip structural position during the peak of hydrocarbon generation and expulsion. Favorable factors such as hydrocarbon generation and pressurization have provided migration force, which is conducive to the migration and accumulation of natural gas towards high positions. A good reservoir-cap assemblage is formed by continuous and stable lateral distribution of gypsum cap rocks and dolomite reservoirs. There is potential for large-scale hydrocarbon accumulation in the Ordovician pre-salt formations in the central-eastern part of the basin. The Wushenqi-Jingbian-Yan'an area is favorable for exploration and should receive more attention in the future.

In order to study the development characteristics of karst paleogeomorphology and the spatial distribution pattern of reservoir in the southeastern Ordos Basin, based on data from cores, well logging, outcrop, seismic data, and laboratory testing, several methods, such as paleo-geological mapping, residual thickness, and impression and geophysical attributes, are used for comprehensive analysis. Results show that the karst paleogeomorphology of the Ordovician in the southeastern Ordos Basin is as follows: “The west and the southwest areas are highlands, where saddles and platforms are developed. Water funnels into sags between platforms and flows eastward along the slope. Mounds and platforms are formed in the slope, between which there are several grooves. Water in the grooves finally funnels into the basin. There are sags and mounds in the basin.” The study shows that, in residual mounds, karst highlands, and transitional parts of karst platforms in karst slopes, as well as in the slope zones of the up-dip parts of paleo grooves, karstification is strong and dissolution pores are well developed. Due to the influence of later diagenesis such as compaction, cementation, metasomatism and dissolution, primary pores are rarely preserved. The reservoir space is mainly composed of nanometer-scale intercrystalline pores, followed by micro-scale dissolution pores. According to the distribution patterns of karst paleogeomorphology and paleo groove networks, and the spatial distribution characteristics of reservoir space, the slope zones and residual mound areas controlled by paleo grooves and the geomorphic transition belts in karst highlands are the key areas for hydrocarbon exploration and development.

Carbonate reservoirs are a major area for onshore oil and gas exploration in China. A number of large and medium- sized oil and gas fields have been discovered in carbonate reservoirs in the Tarim, Sichuan, Ordos and other basins. With continuous exploration, deep and ultra-deep carbonate reservoirs with strong heterogeneity will play an important role in large-scale exploration. However, there are some issues with the related seismic prediction technologies, such as weak theoretical methods and low prediction accuracy. During the 13th Five Year Plan period, important progress has been made in theoretical research and new technologies. Among them, innovative methods, such as a petrophysical model for forward and inversion modeling of complex wave fields of fracture-porous media, petrophysical analysis and reservoir pore structure identification by digital carbonate rock core data, have laid an important foundation for the development of new technologies for reservoir prediction. New technologies, such as small faults identification by seismic gradient structure tensor, quantitative prediction of fracture- cavity reservoirs by cloud transform stochastic simulation, and gas reservoir detection based on pre-stack elastic parameter inversion and frequency division attributes, have effectively improved the recognition accuracy of fault, reservoir and fluid. On this basis, combined with the research status of seismic prediction technologies for carbonate reservoirs with strong heterogeneity, some suggestions for technical development are proposed. According to the development trend of “deep fusion, refinement and intelligentization”, basic theoretical research is strengthened, such as petrophysical modeling methods based on fracture-porous media with heterogeneity of pore shape, fracture-induced anisotropy, and characteristics of dispersion and attenuation. New technologies should be focused on, such as refined seismic prediction technology for reservoir sensitive attributes based on two-phase medium frequency, wave dynamic characteristics such as dispersion and attenuation, seismic prediction technology for reservoir pore structure based on petrophysical analysis of digital core, and quantitative prediction and fluid detection of carbonate reservoirs by artificial intelligence.

There are various types of carbonate reservoirs, generally with low matrix porosity, well developed fractures and cavities, and complex pore structures. Pore structure and physical properties are the key factors for evaluation of reservoirs and their oil and gas-bearing properties. In order to study the pore types of core samples and the distribution laws of pore structure in reservoirs from multiple perspectives, a total of 45 core plugs, sampled from dolomite in the Triassic Leikoupo Formation and from tight limestone in the Lower Permian Maokou Formation in the Sichuan Basin, were tested for conventional porosity and permeability, and also subjected to rock-electrical experiments and Nuclear Magnetic Resonance testing. The results show that, for the dolomite reservoir of the Leikoupo Formation, the overall NMR T2 spectrum of the core samples shows obvious single-peak, double-peak and triple-peak characteristics, indicating that there are triple-porous media in the cores (matrix pores, fractures and dissolution vugs). The pore structure is of strong heterogeneity, and the overall physical properties are poor. NMR measurements of tight limestones show that the T2 spectrum presents a significant ‘double peak’ shape. The reservoir space is matrix intercrystalline pores and dissolution pores. The left peaks of the T2 spectra of different cores are basically stable at about 0.1ms, indicating that the pore size is smaller than that of dolomite in the Leikoupo Formation. The maximum absolute error between NMR permeability inverted by the SDR model and permeability measured by gas measurement is 0.31 mD, which shows good consistency. Meanwhile, the NMR results also reflect shale content to a certain extent. For cores with higher shale content, the front-end signal of the NMR T2 spectrum is strong, the distribution width is large, and NMR porosity is usually larger than gas porosity. The experimental measurement and analysis methods used in this paper are generally applicable to carbonate fractured-vuggy reservoirs throughout the Sichuan Basin.