There exist highly steep structures with complex forming mechanisms in eastern Sichuan Basin, making the exploration challenging. Thus, the evolution was discussed for the highly steep structural belt in an effort to unlock main controls on hydrocarbon preservation conditions in the Upper Permian Changxing Formation under the complexity. Then, the planar and sectional structural interpretation was performed, and the formation-water salinity in specific wells, formation denudation, and regional caprock extension were analyzed in combination with facies distribution and oil/gas testing of relevant wells. A system was established to evaluate the preservation conditions in such complex belt. At last, the conditions were comprehensively analyzed and classified by means of this system. Results show that (i) the preservation in the study area is principally constrained by structure, and its conditions mainly by faults and fault-induced fluid response characteristics; (ii) despite in this belt where experienced multiple-episode tectonism, the bulk of caprock is still conserved, bioreef-shoal reservoirs are massively extended, and structural traps are also developed, indicating great exploration potential; and (iii) some cores in the highly steep anticline possess very ordinary preservation conditions due to serious denudation and plentiful faults, while good conditions in the wide and gentle synclinal area among various structural belts, standing for a significant exploration prospect when the other favorable reservoir-forming assembly is available.

As an essential task for well demonstration and reserve submission, reservoir classification and evaluation are indicative significance to size the reservoir quality and type. Conditioned by karstification, fracture growth, and acid fracturing, most carbonate reservoirs exhibit strong heterogeneity. However, there is no relationship among these factors which are unclear in their priority and impact, bringing about more complex in quantitative evaluation and uncertainty in this evaluation. It is essential for quantitative reservior evaluation under main controls. Thus, the carbonate reservoirs in the fourth member of the Upper Sinian Dengying Formation (Dengying 4 Member) in the intra-platform zone of Anyue gasfield were taken as examples to identify main controls on reservoir development using static and dynamic data such as core, well logging, tracer, and testing. Furthermore, six parameters, including reservoir thickness, shoal-to-formation ratio, acoustic wave, porosity, fracture density, and acid intensity, were selected as reservoir characterization. And each weight was calculated through grey relation analysis to determine a comprehensive evaluation indicator. Results show that, (i) in the light of this indicator (I_REI), the reservoirs can be divided into four types, such as type I (I_REI≥36), II (30≤I_REI<36), III (24≤I_REI<30), and IV (I_REI<24), among them both type I and II reservoirs account for 57.1%, as high-quality ones, while type III of 28.6%, respectively; and (ii) the proposed quantitative evaluation method has been applied to verifying this classification in a single well, achieving a coincidence of above 90% with the measured production from each reservoir. The evaluation is believed reliable to a certain extent, and can provide a basis for subsequent exploration and development decision-making.

Exploration practices have revealed abundant oil and gas resources in the Neoproterozoic Upper Sinian Dengying Formation, Sichuan Basin. To ascertain paleostructure controls on reservoir and its forming together with sedimentation, and to theoretically support the subsequent exploration in this formation, it is essential to study the tectono-sedimentary response during Dengying period. Thus, the western boundary of craton was discussed according to gravity and magnetic, tectonic and sedimentary data to make clear both distribution and genesis of paleo-rift. Moreover, sedimentary patterns were explored for the Upper Yangtze Craton during the Late Sinian Dengying period in line with the train of thought on structure-controlled depositional architecture. Results show that (i) for the Yangtze Craton, its west side extends westward to Jinshajiang-Honghe fault zone from Longmenshan-Anninghe fault zone, creating the so-called Pan-Yangtze Craton where is developed with three intra-cratonic rifts; (ii) the western Pan-Yangtze Craton is divided into two sub-cratonic basins of the east and the west through these rifts; (iii) the east sub-cratonic basin is dominated by dolomite in relatively restricted depositional setting, while the west of thicker limestone in relatively open setting; and (iv) with lateral connection, a superior source-reservoir assemblage which serves as the key exploration target in the Dengying Formation is built due to sedimentary buildup in the rifts or platform marginal zones on both sides of the craton. It is worth to note Wanyuan-Dazhou rift as an exploration prospect.

The exploration and development of tight gas for the Upper Triassic Xujiahe Formation in the Yuanba and Tongnanba areas of northeastern Sichuan Basin have achieved stable growth in production and reserves. However, there have been few important breakthroughs in the adjacent southern area, due to the low exploration level and lack of systematic and detailed research on sedimentary facies. Based on logging identification, logging lithology analysis, field outcrop investigation, and drilling core observation, a systematic study was conducted on the sedimentary characteristics and evolution of the Xujiahe Formation in northeastern Sichuan Basin. The results are shown as follows. (i) The Xujiahe Formation is a braided river delta plain subfacies, which can be further divided into distributary channel microfacies and distributary interchannel microfacies. The latter is dominated by swamp sediments with fine grains, which are mainly mudstones and shales. The former is chiefly composed of channel bar and channel filling sediments with coarse grains, which are mainly fine-medium sandstones. (ii) These sediments mainly came from the southeast provenance. And controlled by the provenance direction, the sedimentary facies zone shows a spindle or banded distribution along the NW-SE direction. (iii) In the northeastern Sichuan Basin, the Xu2 and Xu4 Members belong to the braided river delta with high material supply, which dominatly develops distributary channel microfacies with extensive channel bar sediments; the Xu3 and Xu5 Members belong to the braided river delta with low material supply, which mainly develops distributary interchannel microfacies.

Deep shale from the Upper Ordovician Wufeng Formation to the Lower Silurian Longmaxi Formation (Wufeng-Longmaxi formations), with burial depth ranging from 3,500 m to 4,500 m, has emerged as the main domain for marine shale gas exploration in Sichuan Basin. Dazu area, southern margin of central Sichuan uplift (central basin), is characterized by gentle structures without any faults. But shale reservoirs in this area exhibit strong heterogeneity both vertically and horizontally, leading to great difference between production performance and available reservoir evaluation. So, a fine stratigraphic correlation was performed on the deep shale based on data of well logging and core analysis together with paleogeomorphologic research. A method of reservoir evaluation was built up for massive and continuous sweet spots. Both vertical and horizontal distribution was made clear for the shale reservoirs. Results indicate that (i) the sedimentary thickness from Wufeng Formation to the first submember of the first member of Longmaxi Formation (Long₁¹ submember) is controlled by three paleogeomorphologic units, indicating thicker in the south than in the north. And the depocenter also from Wufeng Formation to Long₁¹ submember is located in the north of unit III; (ii) reservoir distribution differs from stratigraphic extension. And the total thickness of Class I reservoirs from Wufeng Formation to Long₁¹ submember within unit II is larger than that within units I and III; and (iii) unit II is the most favorable belt in Dazu area, while unit I with development potential as another belt of productivity replacement. The above results have enriched understandings on both organic-rich shale thickness and distribution of quality reservoirs in Dazu area from Late Ordovician to Early Silurian, and provide a new tool for reservoir evaluation and ideas to optimize exploration targets for the efficient shale-gas development in Wufeng-Longmaxi formations, southern margin of central Sichuan uplift.

The lower limit of physical properties is a key parameter for identifying effective reservoirs and determining their net pays, as well as a necessary subject in estimating gas in place (GIP). The lower limits of physical properties of effective reservoirs in the Middle Permian Maokou Formation, central Sichuan Basin, are often in analogy to those of carbonate reservoirs in the Lower Triassic Feixianguan and Upper Permian Changxing Formations, eastern Sichuan Basin. However, the Maokou Formation reservoirs are significantly different in regional geological conditions, sedimentary environments, diagenesis, and rock types, seriously impacting the accuracy of GIP estimation and restricting the exploration and development. The reservoir characteristics of the Maokou Formation were systematically analyzed based on drilling, logging, core, and analytic data. And then the lower limits of physical properties were determined by utilizing the methods of minimum flow pore-throat radius, productivity simulation, and single-layer gas testing. The results show that, (i) the Maokou Formation reservoirs are composed of fine-crystalline dolomites, as well as a small amount of psammitic dolomites, limy dolomites, and dolomitic limestones; (ii) the reservoir spaces in the study area are dominated by various dissolution pores/vugs, with fractures and throats acting as key flow pathways; such spaces are diverse in types and complex in geometry, with strong heterogeneity; (iii) considering the development of dissolution pores/vugs and microfractures, the experimental results of full-diameter samples were used to reflect the physical properties, showing generally low porosity and permeability and locally high porosity and permeability; and (iv) by using the above-mentioned three methods, the lower limits of porosity and permeability are determined as 1.97% and 0.009 0 mD, respectively. The study provides a reference for identifying effective reservoirs, determining net pays, and estimating GIP.

With the burial depth ranging from 5,600 m to 7,000 m and well-developed reservoirs, the fourth member of Sinian Dengying Formation in D1 well block, PL area, central Sichuan Basin, belongs to ultra-deep reservoirs. Seeking for the most favorable high-quality reservoirs in such ultra-deep strata even with a bit low seismic resolution has received great concerns in current exploration and development. Thus, based on such drilling data as reservoir thickness and extensional position, the forward modeling was conducted to identify reservoir response under geological conditions, and accordingly several seismic attributes containing absorption coefficient, waveform length, reflection intensity and instantaneous frequency were individually optimized to analyze the multi-attribute fusion, namely that the correlation of individual attribute to reservoir thickness was explored to fix on both number and weight coefficient in these attributes, and finally to clarify the fusion attributes in combination with the weighted sum. Results show that (i) the forward modeling may effectively guide attribute extraction and optimization, and avoid excessive extraction blindly; and (ii) compared with a single attribute, the fusion attributes have higher correlation with reservoir thickness, leading to an increasing accuracy in reservoir prediction. This process is time-saving in both extraction and screening of conventional attributes, and raises the accuracy to predict the attributes qualitatively.

The latest studies suggest strike-slip faults developed in northern slope of central Sichuan Basin. With steep plane and small throw, these faults are not apparent. It is hard to rapidly and accurately identify them running through targets of the deep-seated Lower Paleozoic due to low signal-to-noise ratio (SNR) of seismic data. So, an optimized AI technique has been used for fault identification. Firstly, background simulation is performed to obtain the residual error between local background energy data and original seismic data, thereby enhancing the signal manifest of minor faults beneath stratigraphic reflection. Secondly, the SNR is improved by means of structure-oriented filtering which makes both continuity of seismic event and fault features more pronounced. Finally, AI technique is used for this identification. Results demonstrate that this technique is not only greatly time saving in artificial interpretation, but exhibits strong noise resistance for redundant suppression with effect, improves the SNR, identifies the minor faults that are difficult to be detected using conventional attributes, and makes the prediction on strike-slip faults’ distribution and intersection even better.

This paper presents a new method to automatically identify strata based on masked autoencoder (MAE) neural network in an effort to address two problems of low accuracy in computerized automatic identification and poor efficiency in artificial identification. As characteristic variables, the spontaneous potential, natural gamma, acoustic slowness, resistivity, well coordinates, and kelly bushing are input into the trained simulation which is predicted by optimizing logging curves. Furthermore, all simulation performance is evaluated using loss function, accuracy and precision, as well as recall and F1 values. It is found that, this trained simulation makes the prediction accuracy up to 95.54%. Some experimental contrasts demonstrate that, with the increasing accuracy by 8.3% and 6.32%, respectively, the simulation is better in its performance and prediction accuracy than those of both convolutional neural network (CNN) and boundary-guided CNN. Without any stratigraphic disturbance, this new method enjoys an obvious cutting edge in stratigraphic identification. In addition, after its application to unknown wells, the accuracy achieved 98.07%, showing a better effect. In general, this method may provide theoretical support and helpful discussion for stratigraphic identification based on self-supervised neural network algorithms.

Dolomite reservoir occupies a critical position in global oil and gas exploration and development. As a type of calcium magnesium carbonate mineral, dolomite occurs in both marine and continental sedimentary environments, and also in diagenetic processes such as syngenesis, diagenesis, shallow burial, and deep burial. However, it is scarce in modern marine deposits. Therefore, the genesis of dolomite has been a hot point in researches at home and abroad on carbonate reservoir. Study results are as follows: (i) The genesis of dolomite can be divided into two types: the original and the secondary. Ultra-thick dolomite formations are mostly originated from the preexisting limestones permeated and reworked by Mg²⁺-containing diagenetic fluids under specific hydrodynamic conditions. (ii) According to the dolomite-forming environments, hydrodynamic conditions, and the ion concentration in diagenetic fluids, the genesis of dolomite can be classified as seven models, i.e., evaporation pump, infiltration reflux, mixed water, seawater, burial, hydrothermal, and biogenic models. (iii) The process of dolomitization impacts the development of reservoir space. Under the theoretical conditions, the ionic radius of Mg²⁺ is smaller than that of Ca²⁺, and the porosity often increases when limestone transforms into dolomite. However, under the influence of complex factors such as diagenetic environment, newly generated dolomite content, and crystal structure, the reservoir space of dolomite may not necessarily be better than that of limestone. Usually, the dolomitization process improves the compressive strength and permeability of reservoir particles. (iv) The genetic mechanism of high-quality dolomite reservoir indicates that the original sedimentary environment is the fundamentation, the dolomitization is a necessary condition, and the diagenetic transformation is a key factor.

Dolomite reservoirs in the Middle Permian Qixia Formation of central Sichuan Basin are thin and strongly heterogeneous, with their development models unknown. It is needed to accurately understand the reservoir characteristics and development models for purpose of beneficial exploitation. By means of core observation, thin section identification, cathodoluminescence and geochemical analysis for wells of the Formation, the reservoir characteristics and development models were studied. The results are as follows: (i) The Qixia Formation dolomite reservoirs, belonging to beach facies, are dominated by fine- to medium-crystal residual psammitic dolomites. (ii) The dolomite reservoirs are thin layers, with an average thickness of only 5.8 m per well, featured by low porosity and low permeability. They can be divided into three types: the fractured-vuggy, the vuggy, and the porous, in which the fractured-vuggy and vuggy reservoirs are of high quality. (iii) Grainy beach is the material basis for the development of dolomite reservoirs. The reservoirs are formed by beach bodies subjected to penecontemporaneous karstification and penecontemporaneous dolomization. In the burial stage, the fault system further transformed the reservoirs to form high-quality dolomite reservoirs with developed fractures and vugs. In other words, the development of dolomite reservoirs shows a “three-control” mechanism, i.e., control of beach body on reservoir formation, control of dolomization on reservoir scale, and control of fault on reservoir quality. It is concluded that the high-quality dolomite reservoirs of Qixia Formation have two development models: high-energy beach facies + penecontemporaneous karstification and penecontemporaneous dolomization; and high-energy beach facies + penecontemporaneous karstification and penecontemporaneous dolomization + fault transformation.

With low matrix porosity as well as developed secondary dissolution pores and vugs, the very thick, kart-controlled Sinian dolomite reservoirs in the north slope of central Sichuan paleouplift, Sichuan Basin, exhibit complex gas-water relationship. Moreover, they are diverse in types. These performances lead to a non-Archie phenomenon existing in reservoir's rock electric relationship. In addition, a difference between heterogeneity and formation-water salinity in the reservoirs may conceal well-logging response characteristics of gas and water, making it pretty difficult to identify gas and water through well logging. Thus, parameters on the saturation model were optimized after the analysis of rock electric parameters, acoustic characteristics experimental data and formation-water data about various types of full-diameter cores. And dependent on acoustic and resistivity data, three logging methods to identify a gas zone have been established, such as the calculated resistivity from different saturation and real resistivity overlapping identification, matrix resistivity and actual resistivity overlapping identification, and the ratio of V_P/V_S computed by the theory and real ratio overlapping identification. These methods were applied to the Sinian Dengying reservoirs in the north slope of central Sichuan paleouplift, achieving the coincidence rate of 93.1%.

With poor quality of seismic data, S gasfield, Xihu depression, East China Sea, features complex geological conditions and strong vertical heterogeneity in reservoirs, which may make the characterization of sandbodies with different thickness challenging by using seismic data and lead to findings in great discrepancy between prestack inversion and actual drilling. Thus, integrated broadband amplitude-preserving processing into reservoir prediction, one technology of integrated quality control was adopted to conduct a fine test on parameters and scientific quality-control analysis for some key linkages, such as interference-wave together with ghost and multiple suppression from broadband amplitude-preserving processing, and gather optimization. Results show that (i) there exists numerous information on frequency in seismic-processing results; (ii) the signal-to-noise ratio is significantly improved; and (iii) information on remote trace gather is so natural and real, and AVO attributes of CRP gather are evident. It is concluded that the technology of broadband amplitude-preserving processing can effectively eliminate the influence of interference wave, protect low- and high-frequency signals of seismic data with effect, and increase an accuracy to predict reservoirs with various thickness.

With the increasing exploration of the Lower Paleozoic oil and gas, Ordos Basin, more and more evidences indicate that there exists epigenic karstification under the regional aquiclude in Ma5₅ submember of Majiagou Formation. However, both formation mechanisms and distribution laws have not been made clear yet for these karst reservoirs. So, based on drilling, seismic and test data, not only development mechanisms but controlling factors were studied for them in the middle assemblage of the Ordovician Majiagou Formation, northern Ordos Basin. Results show that (1) with the aquiclude of Ma5₅ submember as the boundary, the covered area can be divided into one unconfined-water karst above this submember and the other confined-water karst below; (2) numerous faults induced during the Caledonian may serve as discharge channels for dissolution products in confined water, and dissolution along these faults just form fault-controlled karst reservoirs; (3) this assemblage has some bedding karst characteristics in a certain zone with underdeveloped faults. With in-situ fractured or collapsed strata, the karst is largely affected by the original lithology in addition to confined-water properties, and limy and gypsum dolomites are prone to dissolution to generate reservoirs; and (4) a confined-water karst model has been established from three aspects, such as the freshwater supply in the bare area, the confined-water dissolution in the covered area and the discharge of dissolution products in the fault-developed area. And this model can be used to explain development laws for the karst in the middle assemblage of Majiagou Formation.

Although it is a common practice in carbonate gas reservoirs, Sichuan Basin, a qualitative evaluation on upgraded controllable and predicted reserves has some limitations. To optimize the prospect to upgrade both reserves, the key factor affecting this upgrade was found out. Furthermore, a set of quantitative evaluation methodology was developed, including an evaluation system and a upgraded coefficient. Results show that (1) for reserves, the upgrade is directly related to their size and quality, reservoir property, and exploration investment, particularly the size, stable rate of production per kilometer well depth, and reserve abundance; (2) the upgrade coefficient for each classification indicator is positively correlated with the reserve quality and size, reservoir property, and exploration investment. The higher the coefficient, the larger the upgraded possibility; and (3) upgraded criteria may vary with different strata in the same prospect due to different geologic accumulation condition. Thus, to evaluate the upgraded reserves should be performed separately for each stratum.

Net pay thickness is an important parameter for estimating petroleum reserves using the volumetric method because it has a great impact on the credibility of final reserves. As for some directional wells drilled at high steep structural zones in eastern Sichuan Basin, the vertical thickness interpreted by well logging differs greatly from true thickness owing to the influence of wellbore trajectory and dip angle. If this vertical thickness is directly plugged as the net pay thickness into the equation of geologic reserves in the volumetric method, there may be a large error between the estimated and actual data. So, in accordance with the principle of volumetric method and geology, a mathematical model was established. Moreover, some geophysical interpretation data was used to obtain the net pay thickness which could be directly applied to reserve estimation. Results show that (1) the relation between wellbore trajectory and reservoir occurrence may be simplified as two kinds of mathematical model, and which one is feasible to estimate the net pay thickness depends on actual drilling geology; (2) the influence of wellbore trajectory and dip angle on the estimation could be effectively alleviated using this simple method, and the computational efficiency and credibility could be greatly improved; and (3) the established mathematical model based on actual geology may be validated using reserve estimation for several gas reservoirs in high steep structural zones in eastern basin.

Drilling practice in T₂l₄³ submember, Zitong-Pingchang area, Sichuan Basin, shows big difference in reservoir-forming geological conditions between eastern and western area. So, it's necessary to implement a comparative analysis on the difference to guide the next exploration. Through experimental analysis and drilling data, and seismic prediction to examine depositional evolution and feature in the Leikoupo 4 Member (T₂l₄), the reservoir distribution, source condition, and structural evolution affected by multiple factors were studied to make clear the reservoir-forming difference of T₂l₄³ in both eastern and western Zitong-Pingchang area. Results show that (1) the predominant sedimentary facies are intra-grain bank (intra-oolitic beach) and dolomitic flat in the east whereas dolomitic flat in the west; (2) dominated by the development of grain bank, karst, and fracture, reservoirs are composed of beach-facies and karstic dolomite and brecciated dolomite with good petrophysical property in layer a in the east whereas fine to crystal powder dolomite with poor petrophysical property in layer b in the west; (3) in the west, the source rock of Xujiahe Formation and the T₂l₄³ reservoirs form two sets of assembly by means of unconformity and small faults, containing “top source and bottom reservoir” as well as “one side source and the other reservoir” . In the east, the source rock of both Xujiahe Formation and Permian and the T₂l₄³ reservoirs generate three sets of assembly also by means of unconformity and small faults, such as “top source and bottom reservoir”, “bottom source and top reservoir”, and “one side source and the other reservoir” ; and (4) in the west, not only lithologic but also structural-lithologic reservoirs may occur against the background of large slope and pinchout. Whereas in the east, both structural-stratigraphic and structural reservoirs with complicated gas-water correlation, are developed at structural high or residual beach. In conclusion, the east area, which is superior to the west one in reservoir-forming conditions, is favorable to further development whereas preliminary prospecting is the focus in the west for some breakthroughs.

In a gasfield, both reservoir characteristics and lower property limits are crucial to reservoir identification and net pay thickness definition in reserve estimation, and they are the prerequisites to large-scale efficient field development. In Mizhi area, eastern Ordos Basin, the Middle Permian Shihezi 8 Member, Shanxi 1 and 2 members, and Taiyuan Formation are typical tight sandstone gas reservoirs. Shihezi 8 Member contributed 25% of total gas deliverability among these reservoirs. So, taking the Shihezi 8 tight sandstone reservoirs as objectives, their characteristics were studied based on experimental analysis, well-logging interpretation, and gas testing. Moreover, the lower property limits of effective reservoirs were determined by four methods of empirical statistics, testing, mercury injection, and shale content. Results show that (1) the Shihe 8 Member in Mizhi area is typical tight sandstone reservoirs with extremely low porosity and low to extremely low permeability. Lower limits of both porosity and permeability are 3.8% and 0.09 mD, respectively; (2) these limits agree with real data with the accuracy of 95%, which has been demonstrated by well testing; and (3) the lower limits are scientific and effective, which may provide reliable evidence for favorable zone prediction and resource evaluation on Shihezi Member in this area.

As low-energy stacked seismic source featured by a little environmental harm, expansible energy, and operational safety compared with dynamite source, spark source has not yet been utilized in VSP prospecting in Sichuan Basin. To check its feasibility in VSP acquisition, a comparative test on both spark and dynamite source was carried out in northern Luzhou area, Sichuan Basin. Results show that (1) spark source is inferior to dynamite source in total energy, resolution, and signal to noise ratio; (2) a spark wavelet may become unstable in the case of poor excitation condition; (3) spark excitation yields well-marked first break of Z-component and upgoing and downgoing wave in VSP data; and (4) the corridor stacked section agrees with synthetic seismogram and surface seismic section in the reflection of major geologic horizons. In conclusion, spark source is a supplement to dynamite source in some shallow and middle VSP survey in Sichuan Basin. For those zones of interest below 3 500 m, it is recommended to use dynamite source or high-power spark source.

The development mode of cluster well for HTHP (high temperature and high pressure) gasfields, B block, Yinggehai Basin, often encounters dense rabbets and large hole size, leading to serious wellbore collision. Moreover, characterized by high temperature (up to 190℃) and pressure coefficient (up to 2.0), the reservoirs of Huangliu Formation in this block challenge downhole rotary steering tools. With high density and solid, the drilling fluid for HTHP wells may result in high friction torque, slow ROP, and difficult trajectory control. So, some directional drilling technologies and supporting measures have been developed and made, including slim hole reaming, high-temperature rotary-steering drilling tools with torsional tools, optimizing well trajectory, and taking effective cooling measures. Field application shows that (1) at the Ø660.4 mm surface section, the slim hole reaming technology can reduce wellbore collision caused by dense rabbets and large hole size; (2) high temperature rotary steering drilling tools with torsional tools can eliminate drag pressure and increase ROP; and (3) by optimizing well trajectory and taking effective cooling measures, the friction torque is reduced, and the stability of rotary steering tools is enhanced. It is concluded that these technologies not only effectively reduce the risk of wellbore collision at the surface section, but also increase ROP to more than 30% in the HTHP gasfields.