Upper Triassic sandstones in the Ordos Basin, northern‐central China, comprise tight oil reservoirs. Using a combination of thin sections, SEM, BSE, EDS, XRD, and fluid inclusion analyses, 24 core samples from 13 wells were collected to study the petrology, paragenesis, and diagenetic processes and implications for reservoir quality. Quartz cement usually occurs as overgrowths or euhedral quartz. Extensive dissolution and albitization of K‐feldspar can be observed. Five types of carbonate cements, ferrocalcite, ankerite, dolomite, calcite, and siderite, occur during different diagenetic stages. Two main types of illite and 5 main habits of chlorite are observed in this study. Kaolinite mainly occurs as booklets and vermicular aggregates. Diagenetic illite, chlorite, biotite, mixed‐layer illite/smectite (I/S), and other minor minerals are also observed. The diagenetic processes include compaction, alteration of volcanic materials and mica, clay mineral transformation, cementation (silica, aluminosilicate, and carbonate), and dissolution of feldspars and rock fragments. Compaction was a significant porosity‐reducing agent, and the presence of carbonate cement exerts a dominant impact on the reduction of porosity. Quartz cement and authigenic clays are less important; however, it is worth mentioning that pore‐lining clays are conducive to porosity preservation. In this study, most of the porosity variation is caused by a combination of compaction, carbonate cements, quartz cement, and authigenic clays. This study gives insights into diagenetic alterations within tight sandstones and has implications for reservoir quality prediction in similar settings.
Several fractal models have been widely used to evaluate the heterogeneity of tight sandstones based on mercury intrusion porosimetry (MIP). However, most of these models are pore fractal models rather than surface fractal models. Therefore, the suitability of existing surface fractal models for tight sandstones and the consistency of their results remain questionable. In the present work, a series of tight sandstones from the Upper Triassic Yanchang Formation, Ordos Basin, was studied. The relative pore surface heterogeneity degree was predicted using petrographic observations and petrophysical measurements. Three popular pore surface fractal models proposed by Neimark, Friesen and Mikula, and Zhang and Li were applied to calculate the surface fractal dimensions of these sandstones based on MIP data. The results of the fractal analyses from different models were not consistent. One or two regions could be identified by applying Neimark’s and Friesen and Mikula’s models. However, the results of the fractal analyses, obtained by using these two methods, were not consistent with the petrographic observations and petrophysical property measurements. In contrast, Zhang and Li’s model was applicable over the whole pore size range, and the results provided by their model were consistent with the petrographic observations and petrophysical property measurements: a negative correlation was observed between the fractal dimensions and porosity and permeability. Therefore, the fractal model proposed by Zhang and Li can be successfully used to characterize the pore surface roughness in this study.
With the success of Bakken tight oil (tight sandstone oil and shale oil) and Eagle Ford tight oil in North America, tight oil has become a research focus in petroleum geology. In China, tight oil reservoirs are predominantly distributed in lacustrine basins. The Triassic Chang 6 Member is the main production layer of tight oil in the Ordos Basin, in which the episodes, timing, and drive of tight oil charging have been analyzed through the petrography, fluorescence microspectrometry, microthermometry, and trapping pressure simulations of fluid inclusions in the reservoir beds. Several conclusions have been reached in this paper. First, aqueous inclusions with five peaks of homogenization temperatures and oil inclusions with three peaks of homogenization temperatures occurred in the Chang 6 reservoir beds. The oil inclusions are mostly distributed in fractures that cut across and occur within the quartz grains, in the quartz overgrowth and calcite cements, and the fractures that occur within the feldspar grains, with blue–green, green, and yellow–green fluorescence colours. Second, the peak wavelength, Q 650/500 , and Q F535 of the fluorescence microspectrometry indicate three charging episodes of tight oil with different oil maturities. The charging timings (141–136, 126–118, and 112–103 Ma) have been ascertained by projecting the homogenization temperatures of aqueous inclusions onto the geological time axis. Third, excess-pressure differences up to 10 MPa between the Chang 7 source rocks and the Chang 6 reservoir beds were the main driving mechanism supporting the process of nonbuoyancy migration.
Chlorite rims have been of interest to petroleum geologists over the past several decades, as a significant number of abnormal high-porosity sandstone reservoirs at great depths have been related to chlorite rims. To clarify the contribution of chlorite rims to porosity preservation in sandstones, the relationship between chlorite rims and porosity evolution in Chang 7 sandstones of the Upper Triassic Yanchang Formation in the Ordos Basin in north-central China was investigated using thin sections, scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction analyses. Based on the detrital composition and diagenetic evolution pathway analyses, it was concluded that chlorite rims inhibited the formation of quartz cementation. However, in addition to quartz cementation, compaction and other cements (commonly carbonates and clays) also control the porosity evolution of sandstones. In sandstones in which porosity reduction is largely controlled by compaction or other cements, chlorite rims may have a limited effect on porosity preservation. Thus, chlorite rims can play an important role in porosity preservation only in sandstones in which quartz cementation is the main process of porosity destruction. The results of this study indicate that the development of high-porosity sandstones is often controlled by many factors, and the effects of chlorite rims on porosity preservation are not always significant.
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