Shale reservoir characterization and control factors on gas accumulation of the Lower Cambrian Niutitang shale, Sichuan Basin, South China

Abstract

The Lower Cambrian Niutitang shale is reported to be the oldest strata for large‐scale shale gas reserves. Based on outcrop observations and laboratory analyses, this study used a comprehensive approach that integrated sedimentology, mineralogy, petrology, organic geochemistry, and reservoir geology to estimate the prospects for exploring for shale gas in the Lower Cambrian Niutitang Formation, southern China. Whole rock and clay mineral X‐ray diffraction, scanning electron microscope (SEM) observations, and petro‐physical property measurements indicate that the Niutitang shale is dominated by quartz and clay minerals with minor amounts of plagioclase, potassium feldspar, calcite, dolomite, and pyrite. The organic matter in the Niutitang shale is dominated by type‐1 kerogen, with TOC levels ranging from 1.35% to 14.3% (average 4.46%) and Ro values ranging from 1.86% to 4.18% (average 2.79%). The three primary types of pores in the shale are organic pores, microfractures, and micropores. The micropores are primarily found in the clay mineral crystals and pyrite, and the microfractures are found along the boundaries of quartz grains. Most of the pore diameters are less than 20 nm, mainly concentrate at about 10 nm, which is consistent with that the organic pores act as the main storage space. The pore volume and gas content are strongly controlled by the mineral composition and lithofacies, and TOC content. The factors favouring shale gas accumulation of the Niutitang shale in the study area include (1) an effective thickness and widespread distribution, (2) a high organic carbon content and appropriate organic matter maturity, and (3) a favourable depositional environment.

Lower Cambrian Organic-Rich Shales in Southern China: A Review of Gas-Bearing Property, Pore Structure, and Their Controlling Factors

“…The Lower Cambrian shales are important marine source rocks in southern China and are also key targets for shale gas exploration [12][13][14][15][16][17][18]. Such a shale succession is widely distributed, with greater thickness and total area up to ð30 − 50Þ × 10 4 km 2 , and the predicted geological resource of shale gas is as high as 35:16 × 10 12 m 3 (< 4500 m) [1,19,20].…”

Pore Evolution and Formation Mechanism of Organic-Rich Shales in the Whole Process of Hydrocarbon Generation: Study of Artificial and Natural Shale Samples

“…Pore network is one of the most important factors controlling the storage capacity as well as the percolation ability of shales. − Investigating the type, morphology, quantity, and distribution of pores is of critical importance to the evaluation of shale reservoir quality. − Organic matter in shales was known to merely determine the hydrocarbon generation potential. Jarvie et al suggested that microporosity can be created by organic matter cracking to oil and gas.…”

Study on the Mechanism of Organic Matter Enrichment in Early Cambrian Marine Shales in the Lower Yangtze Area, South China: An Example Using Well JXY1

“…Because of its wide distribution area, it has different names in different regions. It is called the Wangyinpu Formation to the west of the Lower Yangtze region and the Hetang Formation to the east of the Lower Yangtze region [26][27][28][29][30].…”