To investigate the hydrocarbon generation and pore evolution mechanisms of marine organic-rich shales at different maturity stages, representative shale samples from different regions in China were analyzed. Semi-closed system thermal simulation experiments on low-maturity samples identified hydrocarbon products at different evolutionary stages. Pore structures were characterised using focused ion beam-scanning electron microscopy (FIB-SEM) imaging, CO2 and N-2 adsorption, and high-pressure mercury intrusion, while spontaneous imbibition tests assessed connectivity and wettability. At the early maturity stage (Ro < 1.3 %), diagenesis primarily controls pore development, with interparticle and intraparticle pores dominating (78.98 %), mainly exhibiting slit-like morphologies. High-viscosity liquid hydrocarbons clog pores, reducing pore number, surface porosity, pore volume, and specific surface area, resulting in water-wet characteristics. As thermal evolution progresses (Ro: 1.3-3.3 %), hydrocarbon generation governs pore evolution. Kerogen pyrolysis (Ro < 1.6 %) and cracking gas from retained hydrocarbons/asphaltenes (Ro: 1.6-3.3 %) form a relay-type gas supply model. Organic matter pores become the primary storage space (86.17 %), and ink-bottle-shaped pores enhance connectivity and storage capacity, leading to oil-wet characteristics. At Ro > 3.3 %, organic carbonisation and compaction dominate. Pores return to slit-like morphologies with reduced connectivity. Further cracking of residual hydrocarbons occurs, while organic matter pores remain primary storage space, and the shale reverts to water-wet. These findings provide insights into high-maturity shale reservoir formation mechanisms and offer guidance for favorable exploration targets.

深层页岩气是非常规天然气勘探的重要目标.以川南威荣页岩气田五峰—龙马溪组为研究对象,利用地球化学测试、X线衍射矿物分析、场发射扫描电镜、气体吸附实验及含气性测试等方法,分析深层页岩气储层特征及含气性,探讨含气性主控因素.结果表明:深层页岩气储层具有高 TOC、高孔隙度、高含气量和高脆性矿物质量分数的特征;深层五峰—龙马溪组页岩主要孔隙类型为迁移有机质孔,存在黏土矿物层间孔,中孔和微孔约占孔体积的 88%.纵向含气量具有差异性,呈底部含气量高、顶部含气量低的特征,页岩含气性受多种地质因素控制,T OC质量分数是控制含气性主要因素之一,与含气性呈正相关关系;低矿物质量分数对含气量有利,石英对含气量具有积极作用,黏土和碳酸盐矿物对含气量具有双重作用;岩相对含气量影响明显,富碳硅质页岩含气量最好,为优质岩相类型;高角度裂缝对含气量不利,水平裂缝发育区含气量较高;地层压力越大,页岩含气量越高.该结果对深层页岩气勘探开发具有指导意义.

Clay minerals and organic matter are commonly developed in shale sediments, and have an important influence on the pore structure and pore complexity of shale reservoirs. However, due to the complexity of shale pore structure and the diversity of influencing factors, the influence of organic matter, clay minerals and organo-clay composites on shale pore structure is still unclear. In this study, the effects of organic matter, clay minerals and organo-clay composites on pore structure and pore complexity of marine Longmaxi shale (LMX shale) and continental Ziliujing shale (ZLJ shale) were investigated qualitatively and quantitatively by field emission scanning electron microscopy (FE-SEM), N2 adsorption experiment with Frenkel-Halsey-Hill (FHH) model. The results comprehensively reveal the differential effect of organic matter and clay minerals, the most important pore-forming materials in shale, on the pore development of shale reservoir. Overall, the maceral type, maturity, illite, kaolinite and organo-clay composites are the main controlling factors for shale pore development and complexity. More specifically, maceral type and maturity have great influence on shale pore structure and pore complexity of all shale samples, while the effect of organic matter abundance could be ignored in continental shale. Solid bitumen is prone to develop secondary organic pores, while other macerals such as vitrinite and inertinite do not develop secondary pores with the increase of maturity. In the high-over mature marine LMX shale, a large number of secondary organic pores are generated after solid bitumen cracking and gas generation, which promotes the pore complexity. In the low-mature continental ZLJ shale, the solid bitumen has not reached the maturity of cracking gas, and the original bioclastic pores do not develop, leading to the weak development of organic pores and small contribution to pore complexity. Different clay mineral types have different effects on shale pore structure and pore complexity. The shale pore volume (PV) and specific surface area (SSA) increase with the increase of illite content, and the existence of illite enhances pore surface roughness and pore heterogeneity. However, the effect of kaolinite content on shale pore structure and pore complexity is completely opposite to that of illite. The formation of organo-clay composites protects the primary pores, and promotes the hydrocarbon generation and pore development of organic matter, which greatly increases the development of shale pores. The outcomes of this studies would provide better understanding on the pore formation mechanism of shale reservoir, which is of great significance for the accurate assessment of shale gas resources.

Whether exploration practices of marine shale can be applied to transitional shale remain unclear. To address this issue, it is first necessary to clarify similarities and differences in dominant pore types between the two shales. In this paper, samples were selected from Lower Silurian marine shale and Upper Permian transitional shale, in China. Methods utilized include organic geochemistry, X-ray diffraction, physical property tests and scanning electron microscopy. Results showed that marine shale and transitional shale were similar in OM abundances and thermal maturities, but were different in OM types, mineral components and pore properties. Further, facilitated by image statistics, OM-hosted pores (OMPs) and mineral-hosted pores (MPs) were separated and quantified between the two shales. In particular, more than 90% of pores were OMPs in OM-rich/medium lithofacies of marine shale, whereas above 90% of pores were supplied by MPs in transitional shale. Initially, differences in OM origins and OMP genetic characteristics may trigger the porosity differences. Thermal-genetic OMP, mainly developing in marine shale, features wide distributions, complex surfaces, various sizes, and extensive contributions to pore volume (PV)/pore surface area (PSA). Inheritance OMP, mainly developing in transitional shales, features isolated distributions, smooth surfaces, large diameters, and limited contributions to PV/PSA. Likewise, differences in mineral compositions and MP preservation mechanisms exert influence on the porosity differences. In marine shale, MP is rare and contributes little to PV/PSA due to fierce compaction, cementation and OM occupation. In transitional shale, MP (almost associated with clay flakes) can be protected from cementation and OM occupation, and contributes considerable PV but minimal PSA. It implies that different pore types may impact the reservoir characteristics. Consequently, the exploration targets in marine shales should be OM-rich siliceous lithofacies, whereas targets in transitional shale should be mainly silt-clay lithofacies.

深入分析不同沉积背景页岩储层物质基础、探究不同层系页岩储层发育主控因素是预测页岩气藏地质甜点的核心工作.中国南方海相页岩气已实现大规模商业开发,而海-陆过渡相和陆相页岩气勘探鲜有突破.选取川西南海相页岩、湘中海-陆过渡相页岩和川东北陆相页岩为研究对象,在沉积背景、地化特征、岩石学特征以及孔隙结构特征研究的基础上,通过对比多层系页岩储层特征,明确了不同层系页岩储集能力主控因素.研究表明:海相页岩发育Ⅰ型有机质具迁移和产孔能力,以有机质孔隙为主,优势岩相为富有机质硅质页岩,具有高孔体积(平均值0.026 cm3/g)和高比表面积(平均值28.99 m2/g)特征,有机质丰度是储集能力好坏的决定性因素;海-陆过渡相发育Ⅲ型有机质呈惰性,以粘土矿物孔隙为主,优势岩相为富含有机质泥质页岩,具有高孔体积(平均值0.023 cm3/g)和低比表面积(平均值6.33 m2/g)特征,合理的矿物组构匹配(硅泥比2/3)是储集能力好坏的决定性因素;陆相有机质显微组分混杂,以粘土矿物孔隙和有机质孔隙为主,优势岩相为富有机质泥质页岩和富有机质混合质页岩,孔体积(平均值0.017 cm3/g)和比表面积(平均值11.90 m2/g)适中,高腐泥质含量(大于60％)和合理的矿物组构匹配是储集能力优越的有利条件.不同沉积背景页岩物质基础在差异成岩改造作用下呈现出迥异的储集性能,勘探目标应依据特定页岩层系差别对待.

Shale gas has become one of the most important natural gas resources. The Sichuan Basin in China is one of the main shale gas production areas and has entered large-scale commercial development. The mechanism of shale gas accumulation is an important scientific problem in shale gas exploration. This work systematically summarizes the understanding of shale gas accumulation in terms of shale gas generation mechanisms, pore evolution mechanisms, gas migration mechanisms, and gas accumulation models. This work reviews the formation process of methane in shale and clarifies that the causes of methane formation vary according to the evolutionary stage. The formation and evolution of organic pores and mineral pores in shale are analyzed using a thermal simulation; the organic pore content is found to vary significantly with evolutionary stage. In addition, this work focuses on an analysis of the migration mechanism of shale gas, discussing the impacts of shale anisotropy, self-sealing, porosity and permeability, various forces, and pathway availability on shale gas migration. In addition to the actual gas content of the shale gas reservoir, shale gas accumulation is analyzed using mathematical and evolutionary models, and the accumulation process is revealed. This work provides important guidelines for shale gas exploration in the Sichuan Basin, which has undergone complex tectonic evolution.

Shale pore evolution has a dramatic influence on gas occurrence and is of great significance for the evaluation of gas shale reservoirs. To better understand the nanopore evolution during the whole hydrocarbon generation process, thermal simulation experiment, gas adsorption (N-2 and CO2), and organic geochemistry experiments were carried out. The results show that total pore volume (PV) and specific surface area (SSA) exist in two favorable development periods during the whole hydrocarbon generation process with corresponding vitrinite reflectance (R-o) values of similar to 1.5-1.8% and similar to 2.5-3.2%, respectively, which are two peak periods of porosity development. In the two hydrocarbon generation intervals, primary organic matter (OM) and secondary OM crack to gases accompanied by the formation of OM-hosted pores in large quantities, resulting in the increase of porosity. Shale pore formation and evolution are dominantly controlled by hydrocarbon generation as well as diagenesis. Compaction and cementation exert destructive influences on mineral associated pores, especially in the relatively early diagenesis process, resulting in a great decrease of total porosity. Infilling of secondary OM to inorganic interparticle pores also results in the destruction of porosity. OM-hosted pores are documented as a function of thermal maturity, which is significantly related to kerogen and secondary OM cracking to hydrocarbon.

Based on the exploration and development practice of marine shale gas in Fuling, Weiyuan, Changning, Luzhou and Southeast Chongqing in southern China, combined with experiments and analysis, six factors controlling differential enrichment of marine shale gas are summarized as follows: (1) The more appropriate thermal evolution and the higher the abundance of organic matter, the higher the adsorption and total gas content of shale will be. (2) Kerogen pyrolysis and liquid hydrocarbon cracking provide most of the marine shale gas. (3) The specific surface area and pore volume of organic matter rich shale increased first and then decreased with the increase of thermal evolution degree of organic shale. At R-o between 2.23% and 3.33%, the shale reservoirs are mainly oil-wet, which is conducive to the enrichment of shale gas. (4) The thicker the roof and floor, the higher the shale gas content. The longer the last tectonic uplift time and the greater the uplift amplitude, the greater the loss of shale gas will be. (5) The buried depth and dip angle of the stratum have different controlling and coupling effects on shale gas in different tectonic positions, resulting in two differential enrichment models of shale gas. (6) The effective and comprehensive matching of source, reservoir and preservation conditions determines the quality of shale gas accumulation. Good match of effective gas generating amount and time, moderate pore evolution and good preservation conditions in space and time is essential for the enrichment of shale gas.

海相页岩和陆相页岩的孔隙结构差异明显.基于海相和陆相页岩在岩石组分、成熟度以及有机显微组分的差异开展的成因机理分析表明,页岩中不同岩石组分孔隙的孔径分布差异大,有机质孔隙在小孔径范围内占比多,黏土矿物孔隙与有机质孔隙的孔径分布特征相似,但亦发育中孔和宏孔.海相页岩的小孔径孔隙多由有机质提供,而陆相页岩的小孔径孔隙多来自黏土矿物.无论是海相页岩还是陆相页岩,其孔隙均以无机矿物孔隙为主,其次为有机质孔隙,所不同的是海相页岩中有机质孔隙的贡献率高于陆相页岩,而陆相页岩中黏土矿物孔隙的贡献率高于海相页岩.有机质孔隙在不同演化阶段的孔隙构成不同,未成熟阶段以发育镜质体和惰质体的原始胞腔孔隙为主,成熟阶段有机质孔隙的发育程度最差,高成熟—过成熟阶段以发育次生固体沥青孔隙为主.高成熟—过成熟海相页岩的有机质以腐泥组和固体沥青为主,其有机质孔隙发育程度高.在以松辽盆地沙河子组为代表的高成熟陆相页岩中,其有机质以镜质体和惰质体为主,原始胞腔孔隙损失大,含有少量固体沥青孔隙;在以鄂尔多斯盆地延长组为代表的低成熟陆相页岩中,由于腐泥组和固体沥青充填孔隙,使得页岩整体的孔隙发育程度变差.

Pore structure determines the gas occurrence and storage properties of gas shale and is a vital element for reservoir evaluation and shale gas resources assessment. Field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (HMIP), and low-pressure N2/CO2 adsorption were used to qualitatively and quantitatively characterize full-scale pore structure of Longmaxi (LM) shale from the southern Sichuan Basin. Fractal dimension and its controlling factors were also discussed in our study. Longmaxi shale mainly developed organic matter (OM) pores, interparticle pores, intraparticle pores, and microfracture, of which the OM pores dominated the pore system. The pore diameters are mainly distributed in the ranges of 0.4–0.7 nm, 2–20 nm and 40–200 μm. Micro-, meso- and macropores contribute 24%, 57% and 19% of the total pore volume (PV), respectively, and 64.5%, 34.6%, and 0.9% of the total specific surface area (SSA). Organic matter and clay minerals have a positive contribution to pore development. While high brittle mineral content can inhibit shale pore development. The fractal dimensions D1 and D2 which represents the roughness of the shale surface and irregularity of the space structure, respectively, are calculated based on N2 desorption data. The value of D1 is in the range of 2.6480–2.7334 (average of 2.6857), D2 is in the range of 2.8924–2.9439 (average of 2.9229), which indicates that Longmaxi shales have a rather irregular pore morphology as well as complex pore structure. Both PV and SSA positively correlated with fractal dimensions D1 and D2. The fractal dimension D1 decreases with increasing average pore diameter, while D2 is on the contrary. These results suggest that the small pores have a higher roughness surface, while the larger pores have a more complex spatial structure. The fractal dimensions of shale are jointly controlled by OM, clays and brittle minerals. The TOC content is the key factor which has a positive correlation with the fractal dimension. Clay minerals have a negative influence on fractal dimension D1, and positive influence D2, while brittle minerals show an opposite effect compared with clay minerals.

The success of shale gas exploration in North America as well as China has shown that there're abundant natural fractures in shales which must have an important effect on shale gas accumulation and develop-ment. However,it's still not clear how different fractures affect shale gas accumulation and development. A classifi-cation scheme of fractures was offersed based on the origin and mechanism of fractures and recognizes two major types of fractures,namely the tectonic and non-tectonic, which can be further divided into extensional fractures, shear fractures, tectonic stylolites, bedding-parallel fractures, diagenetic fractures and fluid-overpressure driven factures. The origin and characteristics of these types of fractures are summarized and their interaction with shale gas accumulation and development are explored using data in North America shale plays. The case studies suggest that microfractures might be of little importance in gas accumulation and larger, regional opening-mode fractures may be detrimental to shale gas accumulation. Fracture density has a positive correlation with gas production within a certain threshold and vice versa,if the seal hasn't been fractured and stays efficient. Large open-mode fractures may lead to fracturing fluid leakoff and geological hazards.

飞仙关组作为川东主力气层,陆续已经发现了普光、毛坝、东岳褰、渡口河等一系列大中型气藏,展示了极其重要的战略地位.目前常规的组成分析方法已经不能满足该层天然气类型及成因的判别要求.通过分析烷烃气碳同位素特征,判识天然气类型、分析其成因机制.实验分析显示部分样品C1 ～ C3烷烃气的碳同位素比值呈δ13C1 ＜δ13C2＜δ13C3分布(倒转),显示为多源、多期天然气混入.同时根据C1/C2+ C3与δ13C1及ln(C1/C2)与l1(C2/C3)的值域变化特征,最终判断川东大多气藏以原油裂解气为主,混入一定的煤型气.川东飞仙关组古油藏于晚侏罗世沉积期,其埋藏深度介于5 300～8 200m之间,油藏温度普遍超过150℃,油发生热裂解,生成烷烃气及沥青,持续时间大于30 Ma.

对标分析表明,轮古油田轮古7试验区与毗邻的塔河油田A—G区石油地质条件相似,原油地质储量丰度基本一致.两个油田均采用不规则布井、一套井网衰竭式开发.轮古7试验区目前总体钻井密度低于塔河油田A—G区,但局部位置钻井较密集.通过主要开发指标的对比,轮古7试验区在累计产油、井均产油、采油指数、中高产井比例、原油采收率等方面均偏低,但综合含水率偏高,开发成效远低于塔河油田A—G区同期的开发水平.分析认为,目前轮古7试验区原油剩余资源量多、开发潜力巨大.当前最为迫切的工作是做好采油井的稳油控水治理,下一步要在加强剩余油分布和油藏地质研究的基础上进行开发调整.