Formation mechanism, geological characteristics and development strategy of nonmarine shale oil in China

ABSTRACT: Fuxing continental shale is characterized by poor physical property, strong heterogeneity, low brittleness index, and poor compressibility. Based on the concept of integrated geological and engineering, differential staged fracturing technology of horizontal well was proposed to unlock the production potential and enable commercial productivity. This paper takes well X as an example to perform fracturing design and field application. Firstly, the basic parameters of well X, such as porosity, gas-bearing property, mineral composition, rock mechanic, crustal stress and so on were introduced. Then, a series of numerical simulation was conduct to investigate the effect of pre-fluid, injection rate, natural fractures by using unconventional reservoir model (UFM). Finally, the field application of differential staged fracturing was performed and post-fracturing effect was evaluated by using micro-seismic monitoring technology. Laboratory experiments and logging interpretation show that the porosity of the target layer is 2.85∼6.66%, Young's modulus is 22-34 GPa, Poisson's ratio is 0.2∼0.3, and the horizontal stress difference is 6-10 MPa. Numerical simulation results show that the preferred single fracturing section length is 42-72m, the number of clusters is 6-8, the clusters spacing is 7-9m, the viscosity of pre-flush fluid is 50-100mpa.s, and the discharge rate is 18-20m3/min. Micro-seismic monitoring shows that fracture size is basically formed when the fluid strength exceeds 35 m3/m. This study provides a theoretical basis and reference for the efficient stimulation of continental shale oil. 1. INTRODUCTION China's continental shale oil resources are abundant. Sixteen sets of shale formations have developed in basins such as Songliao, Ordos, Junggar, Bohai Bay, Sichuan, and Qaidam(Li et al., 2022; Sun et al., 2023). The rejuvenation area of the Sichuan Basin is an important area for exploration, increase, and production of shale oil and gas in China. The Fuling, Changning, Weirong, and Yongchuan shale oil and gas fields have been successively discovered. The proved oil and gas reserves exceed 2×1012m3. Continental shale oil differs from marine shale oil. It is characterized by low total organic carbon(TOC), low formation pressure, high clay content, strong plasticity, and low brittleness(Wei et al., 2022).There are significant lateral variations in sedimentary facies, and strong heterogeneity exists. These characteristics determine that the experience of transforming marine shale in North America cannot be simply replicated for continental shale oil. Typically, this type of reservoir utilizes horizontal well closely spaced multi-stage fracturing technology to remodel the target reservoir, aiming to expand the scale of fracture control and achieve the goal of increasing and stabilizing production(Huang et al., 2022; Zhang, 2021). Conventionally, geometric well completions are employed, which evenly distribute stage clusters along the horizontal well section. Statistical results from multi-stage fracturing of horizontal wells in the United States indicate that conventional geometric well completions result in preferential fracturing of clusters corresponding to low stress within the section, with over half of the clusters failing to achieve successful fracturing, leading to insufficient transformation of the target formation and consequently reducing production and increasing construction costs(Waters et al., 2006). 70% of oil and gas production comes from perforation clusters corresponding to low stress in each fracturing stage (Miller et al., 2011). Tang et al. (Tang et al., 2023; Wang et al., 2022; Chen et al.) conducted fracturing design on continental shale oil reservoirs in various regions and found that the main construction parameters affecting reservoir transformation were cluster number, cluster spacing, the viscosity of pre-flush fluid, and the discharge rate.

Development potential and technical strategy of continental shale oil in China

Types and resource potential of continental shale oil in China and its boundary with tight oil

Organic-rich shales of Cretaceous age are widely developed in the Qian'an area of the Songliao Basin and represent an important exploration target for continental shale oil in China. However, due to complex depositional environments, diverse lithological assemblages, and strong lamination-related heterogeneity, the characteristics of reservoir space and their controlling factors among different lithofacies remain poorly constrained. This study aims to clarify the differences in reservoir space among various laminated shale lithofacies and to elucidate their formation mechanisms, thereby providing a geological basis for identifying favorable lithofacies in continental shale oil systems. Shales of the Qingshankou Formation in the Qian'an area were investigated through detailed core observations combined with thin-section petrography, scanning electron microscopy (SEM), X-ray diffraction (XRD), Rock-Eval pyrolysis, high-pressure mercury intrusion, low-temperature nitrogen adsorption, and laser confocal microscopy. Based on these integrated data sets, laminated shales were systematically classified into lithofacies, and the pore structure characteristics and controlling factors of different lithofacies were comparatively analyzed. The results indicate that the Qingshankou Formation shales are lithologically heterogeneous and well laminated and are generally characterized by low porosity and low permeability, with pronounced variations in reservoir properties under different depositional settings. Using an integrated hierarchical classification scheme based on lithology/mineral composition, sedimentary structure, and total organic carbon (TOC) content, eight lithofacies were identified, including clay-rich shale with high-density lamination and high TOC, felsic-rich shale with high-density lamination and high TOC, felsic-rich shale with low-density lamination and moderate TOC, felsic-rich shale with low-density lamination and low TOC, massive mud shale with low TOC, bedded siltstone with low TOC, bedded bioclastic limestone with moderate TOC, and bedded dolostone with moderate TOC. Significant differences in pore types, pore structure, and oil-bearing properties are observed among the identified lithofacies. Among them, the low-TOC bedded siltstone lithofacies exhibit the most favorable reservoir performance, characterized by abundant interparticle pores, dissolution pores and fractures, well-developed pore connectivity, and relatively high oil saturation. The formation and evolution of shale reservoir space are jointly controlled by mineral composition, organic matter abundance, and lamination development. TOC exerts a persistent control on micropore development throughout thermal evolution, whereas rigid detrital grains such as quartz and feldspar within silt-rich laminae form grain-supported frameworks that effectively preserve interparticle pores during compaction, thereby increasing the proportion of meso- to macropores and promoting shale oil enrichment. These results demonstrate that the enrichment potential of laminated shale oil systems is controlled by a multiscale coupling process linking lamina attributes, lithofacies architecture, and formation-scale source-reservoir configuration, rather than by organic matter abundance alone. This insight provides valuable implications for the identification of favorable lithofacies and shale oil exploration and development in the Qingshankou Formation and other continental shale oil basins.

In addition to marine and marine-continental transitional strata, the continental ones are also widely distributed in various oil and gas-bearing basins in China. The continental shale generally provides favorable material bases for hydrocarbon generation, such as wide distribution, large thickness, multiple series of strata, high TOC content, nice organic matter type, and moderate thermal evolution. Part of such shale contains shale oil, but the pore space characteristics for the occurrence of this oil are not thoroughly studied. In order to accurately and quantitatively characterize the pore space where the continental shale oil in different types of lithofacies occurs, we sampled the rock cores from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Area. The TOC content and mineral composition were analyzed, and we also carried out experiments on CO2 and N2 adsorptions, high-pressure mercury injection, and wash oil. Results show significant differences in pore space characteristics for the occurrence of shale oil in different types of lithofacies. In organic-rich mixed and clayey mudstones with the highest TOC content, the free shale oil, occupying the largest reservoir space, mainly occurs in macropores and mesopores, and the adsorbed shale oil, occupying the largest reservoir space, mainly occurs in mesopores. In the organic-bearing clayey mudstone, which has a higher TOC content, the free shale oil takes a larger reservoir space and mainly occurs in macropores, followed by mesopores, and the absorbed one, occupying a larger reservoir space, mostly occurs in micropores and then the mesopores. The organic-bearing mixed mudstone has a moderate TOC content, in which the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, followed by macropores, and the absorbed one, which takes a larger reservoir space, all occurs in mesopores. In the fine sandstone, the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, while the absorbed one occupies a smaller reservoir space and all occurs in mesopores.

For the segmented subdivision cutting fracturing in horizontal wells in shale oil reservoirs, using the boundary element method, a horizontal wellbore fracturing model is established considering the stress interference of the horizontal well section and the flow distribution at the opening of multiple hydraulic fractures (HF). The simulation of subdivision cutting fracturing of horizontal wells in shale oil reservoirs was carried out. Combined with the orthogonal test method, the influencing factors of the fluid pressure of HFs, the effective volume of HFs, and the fracturing fluid efficiency were analyzed. The results show that when the spacing between the sections and perforation clusters in horizontal wells is small, HFs in the middle are “squeezed” by the stress induced by the outer HFs, resulting in a much smaller length than the outer HFs. During the segmented subdivision cutting fracturing in horizontal wells, the spacing of each section has the most significant impact on the effective volume of HFs and fracturing fluid efficiency, followed by the number of HFs within a section. The spacing of each perforation cluster has the most negligible effect. Introduction Shale oil is an unconventional oil and gas resource, which is a vital alternative resource to conventional oil and gas (Qian, 2015). According to the EIA assessment, the technically recoverable shale oil resources in the world are 473 × 108 t (Qianru et al., 2020). Horizontal well volume fracturing technology is an effective means of achieving efficient development of shale oil and gas. Currently, the staged fracturing of horizontal wells in shale oil reservoirs is no longer aimed at increasing the length of the hydraulic fracture (HF) but at maximizing the comprehensive stimulation of the near well zone (Minsheng et al., 2019). Sinopec has proved the feasibility of using horizontal well subdivision cutting and volume fracturing technology for continental medium and high-maturity shale oil in several development areas (Tingxue and Haitao, 2021). In 2020, after the Jimsar shale oil reservoir was stimulated with subdivision cutting volume fracturing technology, the average daily oil production in the first year reached 36 t (Chengmei et al., 2021). Continental shale oil reservoirs have significant heterogeneity, and the effect of fracturing operations is poor. The relationship between the cost of fracturing operations and the expected development benefits must be comprehensively considered (Chengmei et al., 2021, Shanshan et al., 2021, Daohan et al., 2022, Xiaodong et al., 2022).

Key exploration & development technologies and engineering practice of continental shale oil: A case study of Member 2 of Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin, East China

Staged fracturing technology of horizontal wells is the key to efficient development of shale oil. After more than a decade of research and practice, Sinopec has initially developed the staged fracturing technology for horizontal shale oil wells, which is characterized by ultra-dense fractures, temporary plugging and diverting, high concentration proppant adding and reservoir protection. Sinopec achieved a major breakthrough in shale oil exploration in several areas. However, a gap still exists when comparing technological parameters and the technical level of advanced staged fracturing technology for horizontal wells. Engineering geological characteristics of shale oil in China and abroad were compared, and the requirements for Sinopec’s shale oil fracturing technology and challenges were analyzed. In addition, based on the characteristics of continental shale oil reservoirs, development recommendations for the fracturing technology of shale oil with medium-high maturity and the in-situ upgrading technology for the shale oil with medium-low maturity were advanced with respect to the research and implementation of integration of geology and engineering as well as taking economy and field operational feasibility into consideration. The recommendations can provide guidance in accelerating the building of a technical system for continental shale oil development and achieving the goal of economic development of shale oil.

Shale oil is mainly extracted by fracturing. However, it is difficult to determine the optimum construction parameters to obtain maximum productivity. In this paper, a fuzzy comprehensive production evaluation model for fractured shale oil horizontal wells based on random forest algorithm and coordinated principal component analysis is proposed. The fracturing parameters of the target wells are optimized by combining this model with an orthogonal experimental design. The random forest algorithm was used to calculate the importance of data sample factors. The main controlling factors of the production of fractured horizontal wells in shale oil were obtained. To reduce the noise of the sample data, principal component analysis was used to reduce the dimensions of the main control factors. Furthermore, the random forest algorithm was used to determine the weight of the principal components after reducing the dimensionality. The membership function of the main control factors after reducing dimensionality was established by combining the fuzzy statistics and assignment methods. In addition, the membership matrix of the effect prediction of fractured horizontal wells in shale oil was determined. The fuzzy comprehensive evaluation method is used to score and evaluate the effect of fractured horizontal wells. Combined with the orthogonal experimental design method, the optimized parameter design of a fractured horizontal well considering the comprehensive action of multiple parameters is realized. After construction according to the optimized parameters, production following fracturing increases significantly. This verifies the rationality of the optimization method that is proposed in this paper.

Shale oil, as an unconventional oil and gas resource with huge reserves, has become an important replacement resource and great significance to develop. Aiming at the problems of rapid decline oil production in the depletion development of shale oil reservoirs by "depleted horizontal well+volume fracturing", and the lower recovery factor predicted by the existing productivity evaluation methods in shale reservoir, we construct a productivity evaluation method for fractured horizontal wells based on naive Bayes algorithm. Taking the Jimsar shale reservoir in Xinjiang oilfield as the target reservoir, we establish the multi-classification Naive Bayes prior probability model and the conditional probability model containing five attributes of geological parameters and engineering parameters by taking the three-year cumulative production as the classification evaluation index. Then we obtain the posterior-probability model of the four types of production capacity based on Bayesian theory to achieve the capacity assessment of fractured horizontal wells in shale oil reservoirs. The results show that the proposed shale reservoir capacity classification method based on the Naive Bayes model is applicable. The accuracy of productivity prediction is 94% for Class Ⅰ wells, 71% for Class Ⅱ wells, 87% for Class Ⅲ wells, and 92% for Class Ⅳ wells, respectively. The productivity classification trend distribution map of fracturing horizontal wells in Jimsar shale reservoir is drawn, and the high-production potential area is mainly distributed in the southeast of the reservoir. The analysis of the constructed posterior probability model shows that the optimization of fracturing construction parameters can improve the probability of high well production rate. The study provides a guiding basis for the subsequent large-scale fracturing reconstruction of horizontal wells for shale oil development.

The reasonable production system for enhanced fracture network stimulation was studied to maximize the cumulative production in the full period of continental horizontal shale oil wells in the Dongying Sag. According to the complex storage and seepage mechanisms of the shale oil reservoir, a model was established to characterize the full period of fracturing, shut-in and oil production of two-phase flow in the dual media in shale oil reservoir. Production variation with the different production systems (different shut-in time and pressure drop rates in the flowing and pumping stages), and the method for production system optimization were preliminarily discussed by simulation. According to the simulation results, a reasonable production system for target wells was obtained. To be specific, the reasonable shut-in time was 60 days; the pressure drop rate was controlled to be 0.06–0.10 MPa/d at the early flowing stage and 0.02–0.04 MPa/d at the middle flowing stage; tapping was carried out at the last flowing stage to rapidly release the oil pressure to 0; the pressure drop rate was controlled to ensure continuous production of oil wells at the pumping stage, avoiding insufficient liquid supply from the formation matrix due to excessively fast pressure drop. The research results can provide a theoretical guidance for development optimization of continental shale oil in the Dongying Sag, and also provide references for the optimization of production systems for horizontal shale oil wells in other regions.

Formation, distribution and resource potential of the "sweet areas (sections)" of continental shale oil in China

Geological characteristics and key exploration technologies of continental shale oil sweet spots: A case study of Member 2 of Kongdian Formation in the Cangdong sag in the Huanghua depression, Bohai Bay Basin

The Sichuan Basin is rich in shale oil and gas resources, with favorable geological conditions that the other shale reservoirs in China cannot match. Thus, the basin is an ideal option for fully “exploring petroleum inside source kitchen” with respect to onshore shale oil and gas in China. This paper analyzes the characteristics of shale oil and gas resources in the United States and China, and points out that maturity plays an important role in controlling shale oil and gas composition. US shale oil and gas exhibit high proportions of light hydrocarbon and wet gas, whereas Chinese marine and transitional shale gas is mainly dry gas and continental shale oil is generally heavy. A comprehensive geological study of shale oil and gas in the Sichuan Basin reveals findings with respect to the following three aspects. First, there are multiple sets of organic-rich shale reservoirs of three types in the basin, such as the Cambrian Qiongzhusi Formation and Ordovician Wufeng Formation-Silurian Longmaxi Formation marine shale, Permian Longtan Formation transitional shale, Triassic Xujiahe Formation lake-swamp shale, and Jurassic lacustrine shale. Marine shale gas enrichment is mainly controlled by four elements: Deep-water shelf facies, moderate thermal evolution, calcium-rich and silicon-rich rock association, and closed roof/floor. Second, the “sweet section” is generally characterized by high total organic carbon, high gas content, large porosity, high brittle minerals content, high formation pressure, and the presence of lamellation/bedding and natural microfractures. Moreover, the “sweet area” is generally characterized by very thick organic-rich shale, moderate thermal evolution, good preservation conditions, and shallow burial depth, which are exemplified by the shale oil and gas in the Wufeng-Longmaxi Formation, Longtan Formation, and Daanzhai Member of the Ziliujing Formation. Third, the marine, transitional, and continental shale oil and gas resources in the Sichuan Basin account for 50%, 25%, and 30% of the respective types of shale oil and gas geological resources in China, with great potential to become the cradle of the shale oil and gas industrial revolution in China. Following the “Conventional Daqing-Oil” (i.e., the Daqing oilfield in the Songliao Basin) and the “Western Daqing-Oil & Gas” (i.e., the Changqing oilfield in the Ordos Basin), the Southwest oil and gas field in the Sichuan Basin is expected to be built into a “Sichuan-Chongqing Daqing-Gas” in China.

Hydraulic fracturing and horizontal well drilling technologies have enabled the oil and gas industry to safely unlock large reserves of oil and gas in unconventional resources, especially in shale gas and oil, and tight gas and tight oil reservoirs. However, there is an ongoing debate on whether "the best practice" is to drill a horizontal well in the direction of minimum horizontal stress, which would create transversely fractured well or to drill the well in the direction of maximum horizontal stress, which would create longitudinally fractured well. Additionally, little work has been done to understand the complex relationship that exist between principal stresses, well azimuth and/or lateral direction. This paper presents the results of a comprehensive multiphase flow study that investigated the relationship between the principal stresses and lateral direction in hydraulically fractured horizontal wells, and its impact on well performance. Secondly, the study also incorporated previous studies, where applicable, of a single phase flow study that was conducted by the co-authors of this paper. Both studies focused on transversely fractured wells versus longitudinally fractured wells, and how well azimuth affects productivity, reserves and economics of horizontal wells. The previous study primarily focused on wells that produced single phase fluids, and used single phase reservoir numerical models to study well performance. The study investigated the importance of lateral direction as a function of reservoir permeability, lateral length, fracture-half length, number of fracture stages, fracture conductivity, and well completion type (open-hole vs cased-hole). The Single phase study also included a number of actual field cases where the results were compared to actual wells in both oil and gas reservoirs that had transversely fractured or longitudinally fractured horizontal wells. This study would extend the findings of the single phase flow study by adding multiphase flow dimensions such as effects of relative permeability, non-Darcy flow, adsorption gas, stress dependent permeability on induced fractures and conductivity changes in the fracture from the tip to the wellbore. The study used black oil reservoir simulator to study two phase flow mechanisms such as gas-water (dry gas reservoir) and under-saturated oil reservoir (oil-water), and compositional reservoir simulator to model three-phase flow (oil-gas-water) to investigate each parameters' impact on well performance. This study is unique as it examines the permeability ranges from wells with 1 Nano-Darcy (0.000001 md) to 10.0 milli-Darcy in a multiphase flow reservoir simulation. Additionally, this paper presents the first multiphase flow study that thoroughly compared the performance of transversely fractured versus longitudinally fractured horizontal wells. Key features of the study that would benefit the petroleum industry are; Methodologies for modeling shale gas and shale oil wells with stress dependent permeability, adsorption gas and non-Darcy flow effect using black oil models and compositional reservoir simulators.Reservoir permeability based cut-off criterion that can be used as guide when selecting whether to drill transversely fractured vs longitudinally fractured horizontal wells.Integrating the reservoir objectives and geo-mechanical limitations into horizontal well completions and stimulation strategies.Incorporate the effect of reservoir fluid type and fluid properties such as oil composition and density (API) into the decision analysis when comparing transverse horizontal wells to longitudinal horizontal wells.Stimulation optimization strategies focused on well recovery, productivity and EUR as function of hydraulic fracture spacing (or number of fracture stages) and reservoir permeability