Abstract
Abstract The physical property heterogeneity of tight sandstones was mainly caused by complex alteration of various diagenesis combinations during burial process. However, diagenetic evolution of different diagenesis combinations which generally result in the strong difference and heterogeneity of physical property and pore structure is rarely well understood. The Middle Permian lower Shihezi Formation is one of the most important tight gas sandstone reservoirs in the Hangjinqi area of Ordos Basin, China. The reservoir heterogeneity of lower Shihezi Formation, which was caused by the differential diagenesis combination, is crucial to efficient exploration and development. Evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight lower Shihezi Formation sandstone in the Hangjinqi area of Ordos Basin was investigated by means of thin-section description, cathodoluminescence (CL) imaging, X-ray diffraction (XRD), scanning electron microscopy (SEM), and homogenization temperature of fluid inclusions. The lower Shihezi Formation sandstones can be divided into four diagenesis combination types according to the reservoir characteristics and diagenetic relationship. The main diagenetic sequence was mechanical compaction-chlorite rim-early pore-filling calcite cementation-dissolution-authigenic kaolinite-quartz cementation-late calcite cementation. Differential diagenesis combination was mainly controlled by the petrological characteristics, microfacies, and fault. Low content of rock fragment and high content of detrital quartz were beneficial to the compaction resistance and cementation. The moderate content of pore-filling calcite was conducive to pore space protection and feldspar dissolution. The faults control dissolution and differential diagenesis combination by influencing the migration of acid fluids. Moderate compaction-moderate cementation-moderate dissolution type (BBB type) and weak compaction-moderate cementation-strong dissolution type (CBA type) were in favour of high-quality reservoir development.
Highlights
Reservoir quality is one of the crucial factors influencing on efficient petroleum and gas exploration of tight sandstones due to strong heterogeneity [1,2,3,4]
This study is aimed at understanding the evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight sandstone belonging to the Middle Permian lower Shihezi Formation, Hangjinqi area, Ordos Basin, China, in order that the reservoir quality can be forecast ahead of drilling in unexplored regions of the basin
The lower Shihezi Formation sandstones in the Dongshen gas field are predominantly litharenite according to Folk’s sandstone classification scheme [39], averaged as Q51.2F3.8R45 (Figure 3)
Summary
Reservoir quality is one of the crucial factors influencing on efficient petroleum and gas exploration of tight sandstones due to strong heterogeneity [1,2,3,4]. Tight sandstone reservoir prediction, which is generally related to diagenesis, has been a research hotpot of unconventional petroleum and gas exploration [2, 3, 5,6,7,8,9,10,11,12,13,14,15,16]. Tight sandstone reservoir generally experienced complex alteration of various diageneses, so that reservoir quality obviously varies in different diagenetic combinations [1, 3,4,5,6, 12, 16,17,18, 22, 23]. Understanding evolution mechanism of differential diagenesis combination and its effect on the reservoir quality in the tight sandstone is critical for reservoir quality prediction of the tight sandstones
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