Hydrocarbon kitchen evolution of the Lower Silurian Longmaxi Formation in the Sichuan Basin of China and its contribution to shale gas and natural gas accumulation

Major factors controlling the shale gas accumulations in Wufeng-Longmaxi Formation of the Pingqiao Shale Gas Field in Fuling Area, Sichuan Basin, China

Mechanisms of shale gas generation and accumulation in the Ordovician Wufeng-Longmaxi Formation, Sichuan Basin, SW China

URTeC 1619344 Very little attention were paid for the continental shale sequence compared to marine shale of the Sichuan Basin, although the continental shale owns good conditions for the shale gas accumulation. The further research of shale gas accumulation conditions is meaningful to realize the exploration and development prospects effectively, and to expand the field of exploration and development. Based on the research of the development characteristics, organic type of shale grade of maturity, abundance of organic matter, rock and ore characteristics, physical properties of reservoir, type of pore, and preservative conditions of the continental shale of the 5th sector of the Xujiahe Formation of Upper Triassic in Western Sichuan Depression, oil-bearing property and resource potential are comprehensive studied. The research shows that the thickness of shallow lake facies black mud shale in the 5th sector of the Xujiahe Formation is about 250 to 300 m, and the average of TOC is more than 2% in the mature to high mature stage. The micro-fractures, nano-micron level pores, clay mineral intergranular micro pores and slightly soluble holes are developed in shale, and the quantity of brittle minerals such as quartz, feldspar, carbonates is more than 60%. In spite of multiphase tectonic evolution, the shale’s preservation conditions of shale are excellent. The shale shows very well oil-bearing property. Natural gas was generally released while drilling, and the real quantity of shale gas of the in-situ sample ranges from 0.42 to 6.27 cubic meters per ton, with an average of 1.37 cubic meters per ton, and to absorbed gas mainly, which shows the better characteristics of gas content. The Xujiahe Formation shale has high gas resource potential and high abundance of gas resources. The quantity of the shale gas resource with depth less than 3500m is 0.01 Tcf, gas-bearing abundance is from 10.06×108 to 13.15×108 cubic meters per square kilometer. The quantity of brittle mineral of the continental shale reservoir in the Xujiahe Formation (quartz and carbonate) is high, which predicts that fracturing is easier to form network fractures, to achieve volume transformation and showing a good development prospects.

ABSTRACT: Continental shale oil resources are abundant in Sichuan Basin in China, according to multiple limestone interbeds and variable longitudinal stress characteristic, continental shale reservoir characteristic evaluation method including radar map, geological sweetness and fracability index, were established considering multi-thin interlayer, horizontal stress difference, etc. The continental shale in area A demonstrates the characteristics of high clay mineral content with strong plasticity and serious heterogeneous. The fracture geometry inverse analysis is established based on the results of microseismic monitoring and numerical simulation, the continental shale has simple fracture geometry and lower fracture length propagation rate. Further reducing cluster spacing and increasing fluid intensity are the technological directions to improve the SRV of fractured wells in area A. The study could provide guidance for the shale evaluation and fracturing program design of continental shale oil wells. 1. GEOLOGICAL CHARACTERISTICS OF CONTINENTAL SHALE There are abundant shale oil and gas resources in the Sichuan Basin in China. At present, the marine shale gas have developed commercially especially in Wufeng & Longmaxi formation, and the continental shale oil resources are important strategic replacement resources (Wei et al, 2022; Shu et al, 2023; Guo et al, 2023; Jin et al, 2023; Zhao et al, 2023; Yu et al, 2023; Wang et al, 2023). Compared with the marine shale oil and gas reservoirs, continental shale in area A is of strong heterogeneity and low oil & gas abundance, the formation is strong plastic with clay mineral content more than 50%, and multiple limestone interbeds exist resulted in variable longitudinal stress, the higher shale thermal evolution leads to complex fluid properties and aggravates the difficulty of multiphase flow (Jiang et al., 2023; Shu et al, 2023; Sun et al, 2023; Wang et al, 2023). Because the huge geological charactristic between marine shale and continental shale, bottleneck problems such as fracture height and fracture complexity limit emerge in conventional hydraulic fracturing technology, which greatly restricts the effective exploration and economic development of continental shale oil & gas (Jiang et al, 2023). Thus, the reservoir characteristic and fracturing effect evaluation method should be developed according to the field cases. 2. COMPREHENSIVE EVALUATION OF RESERVOIR PARAMETERS 2.1. Radar map of key geological parameters Key geological parameters such as clay minerals, interlayer thickness, natural fracture, etc., and rock mechanics parameters such as horizontal stress difference and Poisson's ratio, etc., were selected as evaluation data set to draw a non-dimensional rose map of the above 9 parameters to qualitatively show the differences in geological parameters among wells.

Pore structure and fractal characteristics of organic-rich shales: A case study of the lower Silurian Longmaxi shales in the Sichuan Basin, SW China

Geological conditions and exploration potential of shale gas reservoir in Wufeng and Longmaxi Formation of southeastern Sichuan Basin, China

Geological characteristics and high production control factors of shale gas reservoirs in Silurian Longmaxi Formation, southern Sichuan Basin, SW China

Assessing the feasibility and CO2 storage capacity of CO2 enhanced shale gas recovery using Triple-Porosity reservoir model

The Jurassic continental shale oil (gas) is a favorable unconventional resource in the Sichuan Basin of China. In this paper, analysis methods such as core and outcrops observation, thin section identification, scanning electron microscopy (SEM), and X-ray diffraction (XRD) are used to describe the characteristics of lacustrine fine-grained rock reservoir in Da’anzhai Member of the Ziliujing Formation in Sichuan Basin, and it discussed the influence and control of lacustrine fine-grained rock diagenesis on the quality of the reservoir, in order to determine the shale reservoir control factors. The results show that there are three types of rocks in the Da’anzhai Member, which are mudstone, siltstone, and limestone, and the rock combination of shell shale intercalated with shell limestone is developed in the Da 2 submember. Fine-grained sedimentary rocks in this section have undergone compaction, cementation, dissolution, metasomatism, transformation of clay minerals, and hydrocarbon generation of organic matter, which are currently in the middle diagenetic stage A or B substage. Compaction and cementation are the main factors that control the physical use of shale and limestone, and strong cementation is the main reason for the tightness of limestone reservoirs in the Da’anzhai Member in the northern and eastern parts of Sichuan Basin. The difference between dissolution and cementation is the main control factor for the formation of limestone or argillaceous limestone reservoirs in the same section in central Sichuan Basin. The organic-rich shale and shell shale of the Yuanba and Fuling area are the most favorable reservoirs of the Ziliujing Formation in this region. The shell limestone that experienced favorable dissolution in the Da’anzhai Member in the central of the Sichuan Basin has become a limestone reservoir. Discussing the impact of continental fine-grained rock diagenesis on the reservoir can better explore and develop similar intervals, enrich unconventional shale oil and gas accumulation and storage theories, and provide basic theoretical support for finding favorable unconventional shale oil and gas reservoirs.

The Fuling gas field in the southeastern Sichuan Basin is the first and the largest shale gas play in China that has been producing primarily from the organic-rich shale in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation. Newly processed 3D seismic data along with well-completion data in the gas field reveal important structural, depositional, and reservoir details in the Lower Paleozoic sedimentary section. Lateral (along-stratal) variations in time structure and bed curvature demonstrate the diversity in faults that can be classified based on their orientation (regional and cross-regional), scale (small, intermediate, and large), and mode (contractional, extensional, and wrench). Vertical (cross-stratal) variations in time structure and bed curvature demonstrate that the deformational intensity increases from the Lower Cambrian to the Upper Ordovician, then decreases from the Upper Ordovician to the Silurian. Seismic isochron and facies analyses indicate that the structural deformation influenced the syntectonic deposition of turbidite sand in a channel complex above the reservoir. The pore pressure, porosity, and gas productivity of the reservoir are the highest in the central portion of the field, where small-scale faults dominate, but drop significantly at the edges of the field, where large-scale lineaments dominate. The relationships suggest that faults and fractures could either reduce or enhance pore pressure, porosity, and gas productivity, depending on their scale. Large-scale faults have the most negative impact on gas enrichment and pressure build-up, leading to reduced pressure, porosity, and productivity; whereas, small-scale ones have the least negative or even positive impact on gas enrichment and pressure build-up, leading to increased pressure, porosity, and productivity. These observations and interpretations offer new insight into the dynamic interplay among tectonic deformation, syn-tectonic sedimentation, and reservoir integrity during the Caledonian (Late Ordovician to Silurian) in the southeastern Sichuan Basin (China).

The rock physics experiments and fracture toughness tests of shales from the Lower Silurian Longmaxi Formation in the Sichuan Basin in China were carried out. Based on this, the calculation model of the fracture toughness was constructed, thus, the single well evaluation of the fracture toughness in shale formation would be obtained based on the well logging data, which can be used to summarize the spatial distribution characteristics of the fracture toughness in the shale formation. However, it is difficult to obtain transverse distribution characteristics of fracture toughness in shale formation based solely on the well logging data. Therefore, in order to investigate the spatial distribution of the fracture toughness, jointing well logging and seismic method could be adopted to quantitatively predict the fracture toughness in shale formation. The results show that fracture toughness of shales is sensitive to acoustic interval transit time and wave impedance. The prediction model of the fracture toughness of shales was constructed, which had a good prediction effect. The fracture toughness values of shales from the Upper Silurian Wufeng-Longmaxi Formation were larger, whereas those of shales from the Lower Silurian Wufeng-Longmaxi Formation were lower. The fracture toughness is mainly distributed in strips along the vertical direction while the distribution area is continuous in the lateral direction, indicating that it has obvious stratification characteristics.

This paper discusses the main factors affecting generation and accumulation of shale gas in the Qiongzhusi and Longmaxi formations in and around the Sichuan Basin, China, based on case studies and comparative analysis of sedimentary environments, lithological associations, basic parameters of shale gas, roof and floor conditions and carrier systems. Both the Qiongzhusi and Longmaxi Formations contain good-quality thick shale strata with high TOC, brittle minerals contents, and gas contents. The accumulation of shale gas is affected by the transport systems and the preservation conditions, especially the roof and floor lithology. The overlying strata of the Longmaxi Formation is tight and thick mudstone and muddy siltstone. The underlying strata of the Longmaxi Formation is tight and thick limestone. Both overlying and underlying strata all have low porosity and low permeability, which acts as a good roof and floor for shale gas preservation and subsequent mass fracturing reformation. Overlying strata of Qiongzhusi formation and Longmaxi is similar and served as a good roof. But the underlying stratum of the Qiongzhusi Formation is a regional reservoir that usually contains aquifers; theredore, it isn't a good floor for shale gas preservation. As a result of these conditions, the Longmaxi Formation has better shale gas accumulation conditions than that of the Qiongzhusi Formation.

Tectonic evolution and controls on natural gas generation and accumulation in the Ordovician system of the Ordos Basin, North China

A new application of atomic force microscopy in the characterization of pore structure and pore contribution in shale gas reservoirs

Comparison of marine, transitional, and lacustrine shales: A case study from the Sichuan Basin in China