--> Sandstone Diagenesis of the Second Member of Xu Jiahe Formation in Yuanba Area, Northeast Sichuan Basin, China

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Sandstone Diagenesis of the Second Member of Xu Jiahe Formation in Yuanba Area, Northeast Sichuan Basin, China


The second member of Xu Jiahe Formation (T3x2) of Upper Triassic, in Yuanba area, Northeast Sichuan Basin, is tight sandstone gas interval. The sandstone, distributed in the whole research area with an average thickness of 250 meters, is dominated by braided stream delta front. Much oil and gas have been extracted, indicating great resource, however why the sandstone is so tight still remain unclear. This study is focused on sandstone diagenesis of T3x2 and discussions on the effects on poroperm characteristics. Fourty-five sample from eight wells are analyzed via petrographic study including core observation, optical microscopy, SEM technology, electron microprobe analysis, X-ray diffraction. The results are summarized to four points:(1)The lithology of T3x2 is dominated by fine to medium-grained lithic sandstone and the porosity and permeability values range from 0.48 to 15.6%, 0.01mD to 8.3mD, with an average 3.6%, 0.38mD, respectively;(2)Six types of pores, including intergranular pore, intragranular dissolved pore, moldic pore, intercrystal micropore, grain-edge fractures, structure fractures, are identified; (3)Studies have showed that the sandstone exist five types of diagenesis, including compaction, cementation, metasomatism, dissolution and fracturing and each one has a significant influence on the evolution of the pore in the sandstone;(4)The effect of the main diagenesis on reservoir quality is as follows: the initial porosity was rapidly decreased (from 35% to 10%) by compaction and cementation and then increased by selective dissolution of unstable grains and cement. Denudation, including acidic denudation and alkalic denudation, lead to increased porosity by 3%. The influence of metasomatism on reservoir physical property is not obvious. Fracturing or fault that connects the favorable source bed is another major factor especially the conductive facture. It can be inferred that reservoir quality of T3x2 is mainly damaged by the process of compaction and cementation, and improves slightly by denudation.