Comparison of Marine and Continental Shale Gas Reservoirs and Their Gas-Bearing Properties in China: The Examples of the Longmaxi and Shahezi Shales

Abstract

Following the marine shale gas, the continental shale gas also presents great exploration potential in China. However, few studies focused on the high-over-mature stage (HOMS) continental shale gas in terms of mineral composition, pore structure, and gas-bearing properties, as well as their comparisons with successfully marine shale gas in the same mature stage, which restricts its better potential evaluation and “sweet spots” suggestion. In this paper, the lower Cretaceous Shahezi Formation in Xujiaweizi fault depression, and the lower Silurian Longmaxi Formation in Pengshui Area, were selected as the target areas for the study of continental and marine shale gas in the HOMS. By means of thin sections, scanning electron microscopy, nitrogen adsorption, high-pressure mercury injection, and methane isothermal adsorption, a systematic comparison was carried out to reveal the differences in reservoir characterizes and gas-bearing properties, and summarize their controlling factors and “sweet spots” distribution. The type and distribution of organic matter (OM) dominate the differences between marine and continental shale in the HOMS. Compared with Longmaxi marine shale with OM of type I–II1 kerogen, there are fewer biogenic siliceous, lower proportion of organic pores, and more inorganic pores (including dominant clay related pores, quartz intergranular pores, and dissolution pores) in Shahezi continental shale with OM of type III–II2 kerogen, corresponding to larger average pore size and smaller specific surface area (SSA). Quartz content, porosity, and SSA are mainly controlled by TOC for marine shale, while those are controlled by deposition and burial depth for continental shale. Plain marsh shale, adjacent to a coal seam, is favorable for the development of cemented quartz grains and the limited conversion of mixed illite/smectite (I/S) to illite, which can effectively restrain the impact of burial depth on the proportion of intergranular and dissolution pores, porosity, and pore size for continental shale. I/S, rather than OM, mainly contributes to the methane adsorption capacity in continental shale, of which the adsorbed gas content is obviously lower than that of marine shale. Due to the higher pressure coefficient (PC), porosity and total gas content of marine shale still increase with increasing the burial depth, while those of Shahezi continental shale decrease rapidly due to the slight increase of PC with depth. Some exploration potential in organic-rich thick shales should be expected in the Shahezi Formation, which are developed in plain marsh or delta front with burial depths in the range of 2600–3600 m.

Characteristics, Genesis, and Hydrocarbon Aspects of Lacustrine Siderite-Rich Marl (Early Jurassic): A Case Study in the Dachanggou Basin, Xinjiang, Northwest China

“…In the in situ development and utilization of shale hydrocarbon resources, it is important to evaluate the physical properties of reservoirs. − Porosity and fracturing determine the permeability of shale reservoirs during exploitation, and the mineral composition of shale allows the determination of these parameters. − Compared with marine shale, continental shale has a higher clay content and a lower brittle mineral content, and it is mainly composed of terrestrial clasts, such as quartz and feldspar, while marine shale is more likely to have a high content of brittle minerals, such as authigenic carbonate. − Therefore, the existence of lacustrine marls significantly improves the overall fracturing ability of shale reservoirs. In particular, in lake–delta sedimentary systems with shallow water depths, organic-rich shale is interbedded with thin layers of siderite-rich marl.…”

Application of mathematical statistics to shale gas-bearing property evaluation and main controlling factor analysis

“…In another work, Li et al 22 used the methane isothermal adsorption experiment and field gas desorption methods to obtain the adsorbed gas content and total gas content, respectively. Similarly, Xiao et al 23 calculated the adsorbed gas content and total gas content according to the Langmuir adsorption model and gas state equation and recovered the total gas content based on the AMOCO method, indicating that the calculated gas content suitably agreed with the recovered gas content.…”

Study on Mechanical Properties of Shale Under Different Loading Rates

“…Shale reservoir characteristics mainly depend on mineral composition, bedding structure, macro-/micro-pore structure, fracture/microfracture development degree, and other factors, especially on mineral composition and mesoand micro-view structures. Different reservoir characteristics, corresponding brittleness characteristics, bedding and microfracture development characteristics, and hydraulic characteristics will have great differences (Xiao et al, 2021), which will have a significant impact on the fracturing effect of shale gas wells. Therefore, classifying shale according to its mineral composition, microstructure, and mechanical properties has important engineering application significance.…”