This paper aims to gain new insights into clastic buried hill reservoirs. For this purpose, the Permian sandstone reservoirs in the Upper Shihezi Formation, Gubei low buried hill, Jiyang Depression, was taken as the object. Referring to the evolution histories of reservoirs and tectonics, the diagenesis of the reservoirs and its coupling with tectonic activity were investigated, with the aid of techniques like core observation, casting sheet observation, scanning electron microscopy (SEM), cathode luminescence, electron probe, back scattering, fluid inclusion, and pore permeability tests. The results show that the Permian sandstone reservoirs in the Upper Shihezi Formation, Gubei low buried hill, Jiyang Depression, are low-porosity, low-permeability reservoirs with complex diagenesis. The diagenetic evolution sequence can be summarized as early feldspar corrosion/kaolinite cemention/early pyrite cemention→carbonate cemention/secondary enlargement of quartz→quartz corrosion/corrosion of quartz and its secondary enlargement→late calcite cemention→late pyrite cemention/carbonate corrosion/late feldspar corrosion/corrosion of dissolvable miscellaneous matrix; compaction effect exists throughout the evolution process. The reservoirs went through (I) shallow burial epidiagenesis, (II) near-surface hydrothermal diagenesis, (III) deep burial alkaline diagenesis, and (IV) continuous burial acid diagenesis. The diagenetic evolution of these four stages is significantly affected by tectonic activities, and the article lists the evidence that diagenesis is affected by tectonic activity. The research results lay the basis for the prediction and evaluation of the Permian sandstone reservoirs in the Upper Shihezi Formation, Gubei low buried hill, Jiyang Depression, and shed new light on the exploration of tight sandstone reservoirs.
Multi-stage hydrocarbon generation and accumulation are very common in petroliferous basins. Also, determination of hydrocarbon accumulation stages and each related diagenetic feature is important for understanding the distribution of current oil and gas resources in one individual basin. However, in the case of conventional sandstone reservoirs, few studies have focused on these resulting reservoir features (e.g., physical property and wettability) that are caused by the first accumulation of hydrocarbons and their geological implications. In this study, taking the reservoir from the Eocene Dongying Depression as an example, we investigated the features of the early hydrocarbon accumulation and its effects on the late accumulation, by analyzing the early accumulation-related paragenesis, fluid inclusions, clay minerals content, reservoir physical property, and wettability in sandstones. Our results show that the early stage accumulation of the overpressured hydrocarbon fluid could support and offset some loads of overburden, weaken the normal compaction, and preserve some original pores. Also, the early hydrocarbon accumulation resulted in an alternated, diagenetic fluid environment from the initial acidic to subsequent alkalic conditions, with the overall dissolution intensity stronger than the cementation, and formed a large number of secondary pores. As a result, the early accumulation was beneficial to reservoir space development and provided more space for the late hydrocarbons. In addition, the early hydrocarbon accumulation could cause the alteration of rock wettability and gradually formed a lipophilic migration pathway from the source rock to the reservoir. Also, the late stage of hydrocarbons would preferentially choose this dominant pathway for their migration (i.e., inheritance of migration pathway).
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