Natural gas exploration in SINOPEC exploratory areas in the Sichuan Basin has achieved rapid development. Since the discovery of Xinchang gas field, exploration domain of natural gas has been expanded gradually, with both reserves and production growing rapidly. Deepening of theory of natural gas geology and progress of key technology was major driving forces of great development of natural gas industry. Considering the rich natural gas resources, various types of favorable targets, exploration degree of 16% of conventional natural gas, and the initial stage of shale gas exploration, it’s predicted by HCZ model that the exploration of natural gas including shale gas in SINOPEC exploratory areas in the Sichuan Basin will remain rapid development with proved reserve increasing by (1 600-2 000)×108 m3/a. The development strategy in the future is to vigorously develop shale gas, steadily promote marine conventional natural gas, and continually research of continental tight sandstone gas. Aiming at the efficient exploration of the three resource types, the following tasks need to be strengthened: Prediction of deep and ultra-deep reef-shoal reservoirs, weathered crust reservoirs, and technological breakthrough of efficient drilling and fracturing test of ultra-deep wells; technological breakthrough of geological-logging-seismic integrated sweet spot prediction of tight sandstones and low-cost multi-layer fracturing of vertical wells; technological breakthrough of increasing-yield and reducing-cost drilling, completion and fracturing of deep marine shale gas; resources potential evaluation and adaptive exploration & development technologies preparation of continental and transitional shale gas.
Natural gas exploration in the Middle Triassic Leikoupo Formation (T2l) in the Sichuan Basin has achieved great breakthrough in recent years and thus attracts wide attention, however, there is no consensus on the gas source. The genetic identification and gas-source correlation based on the geochemical characteristics of natural gas indicate that, natural gas in the Leikoupo Formation is mainly from the underlying Upper Permian Longtan/Wujiaping Formation (P3l/P3w) source rocks (Moxi), with assistance (Zhongba) or hydrocarbon supply condition (Chuanxi) by the source rocks in the 1st Member of the Upper Triassic Xujiahe Formation (T3x), or mainly from the T3x source rocks with certain contribution by the T2l source rocks (Yuanba and Longgang). The T2l gas pools display three types of source-reservoir assemblages, and they suggest different exploration directions. The P3l source rocks as main source rocks can constitute the lower-generation and upper-accumulation pattern, and the positive structures around the deep faults connecting the P3l and T2l are the most favorable areas to explore large-scale T2l gas pools. The source rocks at the bottom of T3x and the reservoirs at the top of the T2l can constitute the side-generation and side-accumulation pattern, which is favorably developed in the karst residual hills or the local tectonic high position of the karst slope. The T2l can constitute self-generation and self-accumulation pattern, which tends to participate assistantly in the formation of gas pools.
Fluvial facies is a significant type of hydrocarbon reservoirs in Mesozoic and Cenozoic continental basins in China, while the strata division of fluvial facies has always been a difficult theme. In Chuanzhong-Chuanxi area of Sichuan Basin, Shaximiao Formation developed fluvial facies. Based on the theory of continental high resolution sequence stratigraphy, combined with the principle of point-line-plane, this paper attempted to carry out sequence stratigraphy research on Shaximiao Formation. The results showed that: (1) Shaximiao Formation can be divided into three third-order base-level cycles and five fourth-order base-level cycles. The five fourth-order base-level cycles respectively corresponded to the first member, section 1 in second member, section 2 in second member, section 3 in second member, section 4 in second member of the Shaximiao Formation. (2)The base-level cycle affected the development and superimposed pattern of the source channel. When the base-level was high, the accommodation space was large, the strata got deposited quickly, and the channel was relatively spatial independent with small scale, the mudstone content was large. Differently, when the base-level was low, the strata got deposited slowly, the channel migrated frequently and incised each other, resulting in the formation of large scale channel, and the content of mudstone was relatively low. The sequence stratigraphic classification of Shaximiao Formation not only provided a high resolution isochronous stratigraphic framework, but also has a function of reservoir prediction, which is of great significance for gas exploration and development of the Shaximiao Formation.
The reservoir of Dengying Formation of Sinian system is controlled by the combination of Tongwan II episodic karstification and dune beach facies in Anyue gas field, central Sichuan. Its lithology is complex and the reservoir is highly heterogeneous in vertical and horizontal directions, which results in great differences between the reservoir at the platform margin and in the platform, and the difference of single well productivity (the former is obviously better than the latter). In order to overcome the major problems and challenges in the process of fine evaluation and exploration, such as high-efficiency development of the platform edge zone, upgrading of reserves scale in the platform area, fine prediction of high-quality reservoir earthquakes, etc., this paper analyzes the reservoir commonness and differences of the four members of the Sinian Dengying Formation in Anyue gas field, central Sichuan Province, based on the observation of drilled cores and thin sections, combined with the analysis of laboratory, seismic and logging data and discuss the controlling factors. The results show that the reservoir space and reservoir types are basically the same in the platform margin area and in the platform area, but the development degree and physical properties of the reservoir dissolution holes in the platform margin area and in the platform area are weakening, the reservoir development degree, quality and development location are different, and the reservoir combination types are different. The main factors that cause the reservoir differences are the development degree of favorable mound beach facies and Tongtan facies according to the study of reservoir development characteristics and distribution law and the analysis of seismic reflection mode of high-yield wells, the high-quality reservoir development blocks are selected for exploration, and good application results are obtained. The high-efficiency development of the platform edge zone of Anyue Gas Field is realized, and the drilling rate and single well production of high-quality reservoir in the platform area are improved. The scale of reserves has been upgraded.
The Silurian Xiaoheba Formation in the southeastern Sichuan Basin has a good source-reservoir-seal assemblage and excellent geological conditions for hydrocarbon preservation. However, fundamental geochemical and sedimentological studies of Xiaoheba Formation are limited, and exploration activity is restricted. To solve this problem, 101 samples were collected from Huilongchang section in southeast Sichuan Basin, and palaeosedimentary environment and evolutionary characteristics were analyzed by using major and trace elements. The analytical results show that the Xiaoheba Formation was mainly formed in the shallow water delta within freshwater-brackish seawater, under a warm, humid, and weakly oxidized-weakly reduced condition. The sea level change of Xiaoheba Formation was divided into two transgression-regression cycles, which was divided into four sections X1, X2, X3 and X4 from bottom to top. X1 and X3 sections were transgressive system tracts (TST), with warm and humid climate, lower paleosalinity, strong water reducibility and upper paleo water depth. X2 and X4 sections were high system tracts (HST), with dry and hot climate, high paleosalinity, weak water reducibility and shallower paleo water depth.
The tight sandstone reservoir in 8th member of the Permian Shihezi Formation in the Ordos Basin has undergone intense dissolution and transformation, forming a wide range of secondary pores. However, the dissolution process and reservoir effect of feldspar minerals and calcite cement in sandstone under geological conditions have yet to be further studied. In this paper, the method that temperature and pressure co-controlled the dissolution simulation experiment under closed system was adopted, and the dissolution law of calcium-bearing litharenite and feldspathic litharenite in acetic acid solution was studied under different temperature and pressure conditions by means of test and analysis of inductively coupled plasma emission spectrometer, X-ray diffractometer, polarizing light microscope and scanning electron microscope. The experimental results show that the temperature increases the dissolution rate and the dissolution amount of feldspar, but the amount of Al3+ migration first increases and then decreases, so it is difficult for Al3+ to be migrated out of the deep sandstone. Calcite dissolves much faster than feldspar and contributes more to the pore formation. As the temperature increases, the calcite dissolution first increases and then decreases, and the pressure inhibits calcite dissolution and has a greater effect than temperature. Calcite and feldspar of He 8 Member dissolved in the Late Triassic to the early cretaceous, which was the main period of secondary pore formation. After the Middle Cretaceous, the reservoir uplifted and the temperature and pressure decreased continuously, and caused some calcite to dissolve away.
In order to improve the calculation method of the isosteric adsorption heat, clarify thermodynamic characteristics of CH4, the shale isotherm adsorption data with temperature range of 26.85-199.85 ℃ and pressure range of 0.08-14 MPa are selected to verify the rationality of the common calculation method of adsorption phase density, analyze the applicability of the adsorption isostere plotting method and the characteristic of isosteric adsorption heat of shale using absolute adsorption capacity. And the following research results were obtained. Firstly, Ozawa empirical formula is suitable for the correction of absolute adsorption capacity in a wide range of temperature and pressure. Secondly, in the temperature ranges from 149.85 ℃ to 199.85 ℃, and nab ranges from 0.103 8 mmol/g to 0.280 0 mmol/g, the curve of Lnp－nab has no linear characteristic, so the adsorption isostere plotting method is not suitable in the above temperature and nab ranges. The nab value of Lnp－1/T curve should be consistent with that of Lnp－nab curve, so that the isosteric adsorption heat curve can fully reflect the thermodynamic characteristics of the adsorption process. Finally, the adsorption heat of supercritical CH4 on shale increases at first and then decreases with the increase of adsorption capacity. It shows that in the early stage of adsorption, the influence of the interaction between molecules of CH4 on the adsorption heat of supercritical CH4 is dominant. When the adsorption capacity increases to a certain extent, the heterogeneity of the shale surface is dominant.
To evaluate the effect of particle size on the determination of shale physical parameters and explore the suitable particle size range for such analysis, three samples with different TOC content from the Lower Cambrian Niutitang Formation in southeast Chongqing area were investigated. Combined with low pressure N2/CO2 adsorption experiments, organic petrology, laser Raman spectroscopy and XRD mineralogical results, the influence of crushing and sieving on the measurement of mineral composition, specific surface area and pore size distribution were discussed. The results are as follows:(1) Pores within organic matter and intergranular of clay minerals are the main types in shale samples, and the development of organic pores are heterogeneous;(2) The procedure of sieving can cause irregular differentiation to mineral composition of shale samples;(3) The results of low pressure N2 adsorption experiment indicate that when the particle size is less than 0.425 mm(>40 Mesh), smaller particle size can increase the specific surface area and obviously affect pore volume of mesopores and macropores; However, when the particle size is greater than 2 mm(<10 Mesh), further increase in particle size will significantly increase the experimental time; (4) Particle size has no distinct impact on the micropores in shales. Combining the experimental reliability and time efficiency as well as the heterogeneous nature of shale samples, particle sizes between 10-40 Mesh are recommended for the experimental analysis of shale physical parameters.
Under formation condition, the flow state of shale gas is affected by multi-scale effects, including viscous flow, diffusion flow and slippage flow, etc., gas production is the result of synergy of various mechanisms, and previous diffusion models can no longer accurately describe the diffusion behavior of shale gas in the shales. In order to clarify the influencing factors of shale gas diffusion ability, reveal the flow law of gas wells in the whole life cycle development process as well as the impact on production capacity, experiments on shale gas under the conditions of 0-1 MPa micro-pressure difference is carried out by using the self-developed experimental system with high temperature and high pressure resistance, and put forward a diffusion coefficient calculation method comprehensively considering permeability, temperature and pressure. It was successfully applied to the Wufeng-Longmaxi formations shale in the south of Sichuan, indicating that the critical pressure of the high-quality reservoir in this area is 4.5 MPa when diffusion flow occupy the main position. It is of great significance for the shale gas well productivity evaluation and the quantitative characterization of the diffusion capacity. The experimental results and theoretical analysis show that the diffusion will have a higher partition coefficient under high temperature, low permeability and low pressure level, the diffusion coefficient model considering the permeability of shales can be better applied in actual flow, and there will be a large error in productivity calculation if ignoring the effects of diffusion.
In order to analyze the carbon isotope characteristics of shale gas from the Wufeng-Longmaxi formations at different depths in more detail, and research the shale gas accumulation, three typical shale gas evaluation wells in southeastern Sichuan Basin were selected to carry out carbon isotope logging. The carbon isotope of mud gas was continuously sampled and measured with drilling, and the carbon isotope change during the gas release from cuttings was also measured. Based on data obtained from the isotope logging, the carbon isotope distribution and vertical variation, as well as the carbon isotope reversal and carbon isotope fractionation of shale gas are comprehensively analyzed, which reveals that the carbon isotope value of Wufeng-Longmaxi shale gas in the Sichuan Basin gradually decreases from the margin to the center of the basin, and this change is mainly controlled by the maturity of the organic matter. The carbon isotope characteristics of shale gas of these three wells have certain commonality, which indicates that shale gas accumulation rules were similar in this area. In sweet point, the carbon isotope fractionation of the top gas of the cuttings jar is much greater, and the amount of gas released from the cuttings is higher, reflecting this section has high initial pressure, large gas content, and more nano pores.
In order to deepen the geological understanding of the source rocks of the Upper Paleozoic coal measures in Jizhong Depression of Bohai Bay Basin, the organic geochemical experiment is used as the research method to analyze the geological characteristics of the source rocks of the Upper Paleozoic coal measures in Jizhong Depression, evaluate the hydrocarbon generating capacity, and discuss the hydrocarbon generating evolution process by using the burial history-thermal history analysis and regional geological background. The results show that the organic carbon abundance of Upper Paleozoic source rocks in Suqiao Wen'an area, Wuqing Depression and Dacheng Uplift of Baxian Depression is the highest, followed by Hexiwu structural belt in Langgu Depression, and Shenxian and Shulu depressions are relatively low; the content of Upper Paleozoic coal macerals in the northeast of Jizhong Depression is the highest in Suqiao area, with an average of 29.1%, and the content of crust is more than 20%. The number of samples accounts for more than 80% of the total number of samples; the thermal evolution of hydrocarbon generation has experienced low maturity stage, secondary maturity stage, high maturity condensate wet gas stage and over mature dry gas stage; the burial depth of source rock of coal measures at the end of Cretaceous was about 1 200-2 000 m, and RO was 0.5%-0.7%, reaching the evolution state of primary hydrocarbon generation stage; the maximum burial depth of source rock of coal measures in Paleogene was more than 7 000 m, exceeding the maximum burial depth or paleotemperature in Mesozoic, the secondary hydrocarbon generation begins, and it has superior resource potential.
In order to ascertain the effect of iron-bearing minerals on gas generation from water-organics reactions, three groups of isothermal pyrolysis involving a high-mature kerogen (kerogen + water, kerogen + pyrite + water, kerogen + magnetite + water) were conducted in hydrothermal conditions by a gold-tube system. Through determination of the yields of gas products, it is indicated that the presence of pyrite and magnetite both led to a certain decrease in the yields of hydrocarbon gases. For instance, the methane yields at Easy%RO of 3.08% in hydrothermal experiments with pyrite and magnetite are about 8.5 and 13.3 mL/gTOC lower than that in pyrolysis with only water. The yield and carbon isotopic ratios of CO2 in pyrolysis involving water and pyrite are evidently higher than those in pyrolysis involving only water and involving water and magnetite. The H2S yields in hydrothermal experiments with Fe-bearing minerals (especially magnetite) are much lower compared with those without minerals. The analysis of gas compositions shows that the presence of pyrite apparently resulted in the increase of gas dryness and the decrease of relative content of isomeric hydrocarbons. The hydrogen isotope of the gas product of the water-magnetite system is relatively lower than that of the water-only system, indicating that the addition of magnetite promoted the formation of early H2 and hydrogenation occurred with the organic matter. These results indicate that the addition of iron-bearing minerals may inhibit the carbocation reaction, and the hydrogenation of water-organic matter may be mainly a radical reaction.
The reserve producing degree of sandstone reservoirs with different permeability under different water saturation conditions has been systematically studied by combining physical simulation experiment with mathe-matical evaluation method during well control range from 500 m to 100 m (the equivalent of well spacing from 1 000 m to 200 m). This paper reveals the effect of well pattern infilling on increasing reserve producing degree，and the chart for identifying the feasibility of well pattern infilling has been established based on the increase of the recovery degree by 5%-10% and more than 10%,which provides a reference for the well pattern disposetion and infilling scheme optimization of gas reservoirs. The core with conventional air permeability of 1.63×10-3 μm2，0.58×10-3 μm2，0.175×10-3 μm2 and 0.063×10-3 μm2 and the water saturation between 30.3% and 71.1% has been used in the experiments. The results show that the reservoir with permeability of 1.63×10-3 μm2 has a high degree of production. Except when the water saturation is as high as 69.9%, the production degree is related to the well control range, the production degree has little relation with well control range, and it can be developed with large well spacing. For reservoirs with permeability of 0.58×10-3 μm2, the degree of production is closely related to water saturation and well control range，and it increases with the decrease of water saturation and the densification of well control range. For reservoirs with permeability of 0.175×10-3 μm2，only when the water saturation is less than or equal to 52.3%, well pattern infilling optimization can improve the degree of reserve production, and when the water saturation is more than 52.3%，the degree of reserve production is low, usually less than or equal to 10%. Even if the well control range is encrypted to 100m，it is difficult to improve. For the reservoir with a permeability of 0.063×10-3 μm2, it has a very low degree of production as a whole，even if the water saturation is only 31.6% and the well control range is infilled to 100m，the highest degree of production is only 2.3%，therefore it is difficult to improve by well pattern infilling for this kind of reservoir.
In order to reveal the controlling factors of productivity difference between adjacent coalbed methane well groups and the same well group, based on the production dynamic characteristics of typical adjacent coalbed methane groups in SZB block, the differences of productivity types, average gas production and average water production are discussed, and the detailed analysis is made from geological controlling factors, engineering process controlling factors and drainage management factors. The resource conditions and development degree of high quality coal reservoir, well bore quality, well cementation quality, fracturing technology and different stages of drainage system control the coalbed methane productivity. The results show that under the condition of similar coalbed methane resources, the heterogeneity of coal reservoir and the development degree of favorable reservoir are the internal main geological factors that affect the production difference of adjacent well groups; on the basis of ensuring reasonable well bore structure and qualified cementing quality, the fracturing effect is the main engineering factor of the production difference of adjacent coalbed methane groups; the difference of production management in different production stages is the main management factor. This view not only provides a theoretical basis for the analysis of the controlling factors of coalbed methane productivity, but also has a reference value for the rapid production increase of coalbed methane.
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