Study On Coupling Mechanism Of Diagenesis And Accumulation Of Tight Sandstone In The Benxi Formation,Ordos Basin

The Ordos Basin is one of the important oil and gas producing reservoirs in China andits tight gas in Upper Paleozoic formation is one of the most essential parts of the current national natural gas production.The Shihezi Formation,the Shanxi Formation,and the Taiyuan Formation have been well explorated in the oil and gas production zones that have been discovered in the Upper Paleozoic.Their reservoir characteristics and enrichment mechanism have been wildly studied.However,the Benxi Formation in Carboniferous has not attracted much attentions due to its deeper burial depth and lower exploration level.In recent years the Benxi Formation has gradually attracted attentions with the continuous improvement of the degree of natural gas exploration in the Upper Paleozoic.The unblocked flow rate of 4 gas test wells drilled in the Benxi Formation is as high as 50×10⁴m³/d and another 165 wells have obtained natural gas shows which demonstrating good exploration potential and becoming a successor formation for natural gas production enhancement in the Ordos Basin.However,the reservoir characteristics,diagenesis,and natural gas accumulation mechanism of natural gas in the Benxi Formation poorly understood which led to generate bad strategy in targeting the high production formations.Combined with the forward modeling of geological process,core observation,petrographic microscopy,scanning electron microscope,overburden pressure permeability measurements,and high pressure mercury injection,The coupling relationship between diagenetic period and natural gas accumulation process of tight reservoirs in the Benxi Formation is investigated based on the quantitative porosity evolution modeling and homogenization temperature of fluid inclusions analyses.On this basis,combined with physical and numerical simulation experiments,the gas charging conditions and discrimination criteria of tight sandstone reservoirs are studied.The main controlling factors of natural gas enrichment are determined with the characteristics of gas and water wells distribution and the criterion of critical natural gas charging.The multi-element superposition method is used to predict favorable tight gas exploration areas in the Benxi Formation.The results show as follows:quartz sandstone and lithic quartz sandstone are the primary lithology types of the Benxi Formation.The average porosity and permeability of the Benxi Formation are 5.8%and 0.764×10^-3μm²,respectively.The pore types are dominated by secondary pores,followed by primary pores and microfractures.The reservoirs are divided into three types based on the data from high-pressure mercury injection.The physical properties of Type I and Type II reservoirs are better in porosity and permeability.The tight reservoirs of Benxi Formation have mainly experienced diagenesis such as compaction,cementation and dissolution.The diagenesis in the northeast（depth<3000 m）is in the stage B of middle diagenesis,and the southern region（depth>3000 m）is in the late diagenetic period.Porosity evolution model was established using back stripping method.It is calculated that the porosity losing caused by compaction is 25.0%～35.0%,the porosity losing caused by cementation is 4.0%～15.0%,and the porosity increased by dissolution is 2.0%～4.0%in the Benxi Formation.The homogenization temperature of fluid inclusions determines that two stages of gas charging occurred in Benxi Formation.The reservoir is first densified and then natural gas started to accumulation.Using physical and numerical simulation,the relationship between critical conditions for gas charging and depth in tight reservoirs of the Benxi Formation are determined.The hydrocarbon expulsion intensity is between 1.5×10⁸～6.0×10⁸ m³/km²,and the lower limit of the critical porosity that can be filled is 4.0%～10.2%when the buried depth is less than 3000 m.The hydrocarbon expulsion intensity is between3.0×10⁸～6.0×10⁸m³/km²,and the lower limit of the critical porosity that can be filled is4.2%～5.2%when the buried depth is greater than 3000 m.Integrating the four main controlling factors of natural gas charging capacity,sand body thickness,critical porosity,and overpressure sealing,the stacking method is used to determine the next favorable tight sandstone exploration area in the Carboniferous Benxi Formation of the Upper Paleozoic in the Ordos Basin.Type I favorable areas are mainly in Yulin,Zizhou,Yanchang and Yan’an:neritic sand bar and marine-continental transitional facies;Type II located in Shenmu and Wuqi:neritic sand bar and distal delta front facies;Type III are developed in Sulige and Shilou:distal delta front facies,followed by neritic sand bar facies near the eastern stucco flat.