The pore-throat structures play a dominant role in the evaluation of properties of tight sandstone, but it remains difficult to determine the related parameters and understand their impact on reservoir quality. Hence, toward this end, we analyze the experimental data that are indicative of the pore-throat system, then we investigate the effect of fractal dimensions of pore-throat structures on petrologic and physical properties, and finally, the optical observations, fractal theory, and prediction model were integrated to explore the qualities of various reservoir types in tight sandstones. The results show that the fractal dimensions of the mercury intrusion curve correspond to three pore-throat types and those of the mercury extrusion curve could correspond to two pore-throat types. Five types of reservoirs were identified, the best reservoir type has a high percentage of interparticle and dissolution pores but a low proportion of clay-related pores, and the differences in pore-throat connectivity of various types affect storage capacity significantly. The storage ability prediction models of various reservoir types are raised by integrated experimental data. This work employed a comprehensive fractal theory based on capillary pressure curves and helps to explore how pore-throat systems influence reservoir quality in tight sandstones.
Tight sandstones, as typical unconventional oil and gas resources, have a complex pore-throat network and strong heterogeneity due to complicated diagenetic alterations, and the characteristics of the pore size distribution (PSD) and pore structure have significant impacts on the behavior of reservoir quality [
The HPMIP test is proposed to confirm features of pore-throat systems on a broad range of PSD, and this technique is generally accepted for its briefness, celerity, and authenticity in core experiments [
The Ordos Basin is a potentially promising tight sandstone play for hydrocarbon exploration and development, which is located in the eastern part of northwestern China and covers an area of about
(a) Location of the Ordos Basin concerning first-class tectonic elements of the Ordos Basin. (b) Stratigraphic column from Upper Triassic Yanchang Formation in Ordos Basin.
In this research, the helium-based porosity and nitrogen-based permeability tests were conducted by FYK-Iapparatus, manufactured by Nantong Feiyu Petroleum Technology Co., Ltd., under a confining pressure of 20 MPa. Red epoxy resin-impregnated thin sections were made for studying the petrology and pore systems were point-counted with at least 300 counts, using the Zeiss Axioskop II microscope. The spatial distribution of microspatial structures was analyzed using FEI Quanta 400 FEG scanning electron microscope (SEM). Besides, stub samples that were cut from the core plugs with the length of around 0.5 cm were analyzed using an X’Pert PRO energy-dispersive X-ray spectrometer to quantify the clay minerals. Finally, the samples were immersed into mercury for HPMIP tests by a Micromeritic Autopore IV 9420 Instrument, and the maximum injection pressure reached 200 MPa, corresponding to 3.675 nm according to Washburn’s equation [
The pore-throat systems in tight sandstones have a fractal property (
Thus, for the differential equation, we assumed that all the pores and throats are in cylindrical shapes; hence, the cumulative pore volume with a radius over
The following formula could be obtained by substituting Equation (
Then, the total pore volume could be acquired:
The mercury intrusion saturation, which corresponds to the saturation with capillary pressure derived from HPMIP tests, could be calculated using the following formula:
The minimum value can be omitted compared with the maximum radius due to the huge difference; Equation (
Based on the Washburn equation [
Equation (
The selected 16 tight sandstone samples varied from arkose to lithic arkose with quartz, feldspar, and rock fragments (Supplementary Table
Petrographic characteristics of tight sandstone reservoirs in Ordos Basin. (a) Classification of tight sandstones on a basis of Folk’s methods [
The physical property results of 16 tight sandstone samples show that the porosity ranges from 4.33% to 12.39%, with an average of 9.43%, and the permeability is mainly between 0.040 and 1.244 mD, with an average of 0.463 mD (Supplementary Table
Porosity versus permeability of tight sandstones in the Ordos Basin.
Basically, the pore-throat system could be characterized by the pore-throat structure and PSD, which were represented by some parameters, such as threshold pressure, median pressure, maximum mercury saturation, sorting coefficient, and mercury withdrawal efficiency. The entry pressure, which was determined by the point on the capillary curve at which the nonwetting phase (mercury) intrudes into the pores of the samples originally, is in a range of 0.07-7.39 MPa, with an average of 1.28 MPa. The point on the capillary curve where mercury saturation is at 50% is the median pressure, with values which range from 0.68 to 97.00 MPa (av. 15.22 MPa) in Yanchang Formation tight sandstones. The maximum mercury saturation is the point corresponding to 200 MPa capillary pressure in this case, and it ranges from 57.03% to 97.01% with a mean value of 85.44%. Sorting coefficient is the indicator of pore-throat heterogeneity, and the values which are in a range of 1.90-4.45, with an average of 2.56, reveal that the sorting of Yanchang Formation tight sandstones is poor. Furthermore, the mercury withdrawal efficiency is calculated as 18.09%-41.68%, with a mean value of 28.36%. Skewness varies from 0.63 to 2.01, with an average of 1.62. Meanwhile, mercury extrusion-derived threshold pressure, corresponding to the initial inflection point of the extrusion capillary pressure curves, ranged from 54.71 to 184.83 MPa (av. 128.80 MPa). The results are all expressly presented in Supplementary Table
Representative parameters and fractal dimensions from
Fractal dimensions of 16 tight sandstone specimens from the research area were obtained from the
The spaces in tight sandstones with self-similarity are characterized by similar structures at various pore-throat scales and exhibited a representative linear relationship on a
TS observations and multitype model (solid line represents mercury filling the voids, dashed line represents wetting phase filling the voids, and line thickness represents the diameters of pore throats).
In order to ascertain the effects of fractal dimensions on networks, the correlations between fractal dimensions and the typical parameters of pore-throat structures were studied using Microsoft Excel software. The correlation coefficients of the typical parameters related to pore-throat structures and fractal dimensions are listed in Supplementary Table
Correlation coefficients of the typical parameters related to pore-throat structures and fractal dimensions. Positive values mean positive relationships, while negative values represent a negative correlation. TR: threshold radius; MR: median radius; AR: average radius; MS: maximum mercury saturation; EW: efficiency of mercury withdrawal; SC: sorting coefficient; SK: skewness.
Fractal dimensions
Fractal dimensions
There are positive relationships between sorting coefficients and all the fractal dimensions, and the impact of
The main mineral compositions which may control the fractal dimensions of pore-throat systems in the tight sandstones are volcanic rock fragments, illite, and carbonate minerals (Supplementary Table
Correlation coefficients of the (a) skeleton particles and (b) interstitial minerals and fractal dimensions. Positive values mean positive relationships, while negative values represent a negative correlation. QU: quartz; FS: feldspar; IF: igneous rock fragments; MF: metamorphic rock fragments; SF: sedimentary rock fragments; MI: mica; IL: illite; CH: chlorite; I/S: I/S mixed layer; CA: carbonate; QO: quartz overgrowth.
Correlations between fractal dimensions and physical properties. ((a–e) represent porosity while (f–j) represent permeability. Figures from the left to the right represent
Microscopic observations are effective ways for pore-throat type determination. However, due to the resolution of these methods and the restricted spatial field of view, the space types recognized by optical observations were ambiguous and circumscribed to a certain degree. Therefore, it is vital to input the pore types to the total sandstone spaces and classify the reservoir types according to the percentage of different kinds of reservoir voids via optical observations associated with the HPMIP-derived curves. Based on these principles, five communities of reservoir types were classified, namely, interparticle pore-dominated, dissolution pore-dominated, throat-dominated, clay-related pore-dominated, and tight type.
For the interparticle pore-dominated reservoir type, micro reservoir spaces are mainly interparticle pores associated with some moldic pores (Figure
TS observation, SEM images, fractal dimensions from
Porosity versus permeability of different types of reservoirs. IPDR: interparticle pore-dominated reservoir; DPDR: dissolution pore-dominated reservoir; TDR: throat-dominated reservoir; CPDR: clay-related pore dominated reservoir; TR: tight reservoir.
Dissolution pores are quite common in a dissolution pore-dominated reservoir type, and hence, kaolinite which is derived from feldspar dissolution is easily visible within pore voids [
The primary interparticle pores were abundant in the throat-dominated reservoir type; however, the mica and pore-bridging clay minerals, especially pore-filling chlorite, resulted in the macroscale reduction of interparticle pores space, and the deterioration of reservoir quality was triggered by narrowed pores (porosity equal to 8.74% and permeability equal to 0.168 mD) (Figures
For the clay-related pore-dominated reservoir type, the interstitial minerals, especially dolomite, chlorite, and illite, intensively appeared (Figures
The tight type is the worst reservoir type in a tight sandstone reservoir with quite low physical properties (4.33% and 0.040 mD in typical sample) due to densely distributed carbonate, especially ferrocalcite (Figures
The fractal dimensions of tight sandstones from the Upper Triassic Yanchang Formation are segmented based on capillary pressure curves derived from HPMIP. The intrusion curve can be divided into three ranges including
All HPMIP-derived parameters show relatively notable negative correlations with fractal dimension
A high percentage of illite and carbonate minerals could deteriorate the pore-throat structure and enhance the heterogeneity of pore-throat systems. Vague correlations between fractal dimensions and detrital grain content which manifest as interstitial minerals are the decisive factor in PSD. With the increase of physical properties, pore-throat structures tend to be homogeneous.
According to petrographic observations and fractal characterization, five major reservoir types are defined, namely, interparticle pore-dominated, dissolution pore-dominated, throat-dominated, clay-related pore-dominated, and tight type, and the storage capacity decreases gradually. This work provides insights into determining the different reservoir types by more comprehensive ways, and it turns out that the fractal theory based on the multitype model was reliable, which favors the development of tight sandstones.
The experimental data used to support the findings of this study are included within the manuscript and the supplementary materials.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Conceptualization was handled by Dengke Liu and Zhaolin Gu; data curation by Dengke Liu and Ruixiang Liang; formal analysis by Dengke Liu and Ruixiang Liang; funding acquisition by Dazhong Ren, Chuanqing Huang, and Chao Yang; investigation by Dengke Liu and Junwei Su; methodology by Dengke Liu and Junwei Su; supervision by Zhaolin Gu; figure drawing by Bin Chen; writing of the original draft by Dengke Liu; and writing, review, and editing by Junwei Su.
This work was supported by the Open Fund of Shaanxi Key Laboratory of Petroleum Accumulation Geology (No. PAG-201901), the National Science and Technology Major Project (No. 2016ZX05047-003-005), the National Natural Science Foundation of China (Nos. 11872295 and 41702146), the Open Foundation of Shaanxi Key Laboratory of Lacustrine Shale Gas Accumulation and Exploitation (under planning) (No. YJSYZX18SKF0004), and the China National Petroleum Corporation’s Basic Advanced Reserve Technology (No. 2018A-0908).
Supplementary material contains Table S1 to Table S8, which show the parameters derived from physical properties, TS, XRD, and HPMIP.