渝东南牛蹄塘组与龙马溪组高演化海相页岩全孔径孔隙结构特征对比研究

中国南方主要富有机质海相页岩寒武系牛蹄塘组和志留系龙马溪组是当前页岩气勘探的重要目标地层,但演化程度普遍偏高,孔隙结构复杂。为加深对这种复杂性的认识,通过场发射扫描电镜、高压压汞实验与低温低压吸附实验(CO_2与N_2),对页岩全孔径孔隙结构特征进行对比研究。研究发现,牛蹄塘组孔体积与龙马溪组页岩的孔体积分别介于0.0202~0.0402 m L/g与0.0255~0.0310 m L/g之间,介孔在两套页岩孔体积中所占比例最大。两套页岩比表面积分别介于13.74~41.26 m~2/g与21.42~27.82 m~2/g之间,微孔与介孔几乎提供了两套页岩全部的比表面。孔隙结构的差异主要表现为牛蹄塘组页岩内溶蚀孔和粒间孔较为发育,有机质孔隙发育不均匀;而龙马溪组页岩内粒内孔、粒间孔和有机质孔隙均较为发育;牛蹄塘组在微孔范围内的孔体积与比表面积占有优势,而介孔与宏孔范围内的孔体积与比表面积均小于龙马溪组。根据实验数据结果与文献的调研,两套页岩孔隙结构差异的主要原因主要是成熟度与埋深的不同影响而形成的。

A Comparative Study on Whole-aperture Pore Structure Characteristics between Niutitang and Longmaxi Formation of High-matruity Marine Shales in Southeastern Chongqingelt

The marine organic-rich shales of Niutitang and Longmaxi formation in south China became the key horizons for shale gas exploration. But the thermal evolution degree of the shales is very high and its pore structure is complicated. In order to deepen the understanding of this complexity, we analyzed the whole-aperture pore structure characteristics of two sets shales using a combination of field-emission environmental scanning electron microscope (FEI-SEM), high-pressure mercury injection and low-temperature gas (N2 and CO2) adsorption experiments. Results show that the pore volume of the Niutitang and Longmaxi formation were in the range of 0.0202~0.0402 mL/g and 0.0255~0.0310 mL/g, respectively. Mesopores had the largest proportion in two sets of shale pore volume. The average specific surface area of two sets shales were in the range of 13.74~41.26 m2/g and 21.42~27.82 m2/g, respectively. Micropores and mesopores amounted to almost 95% of the specific surface area in shale. The pore structure characteristics of two sets shales were very different. The dissolution pores and interparticle pores were developed in the Niutitang formations while organic-matter pores were unevenly distributed. And organic-matter pores, interparticle and intraparticle pores in the mineral matrix were well-developed in the Longmaxi formations. Micropores in the Niutitang formations were well-developed while mesopores and macropores in the Longmaxi formations were better developed. According to the result of the experiment data and past literature, the level of maturity and buried depth played a decisive role in the difference of pore structure characteristics.