Microscopic pore characteristics of andesite and implication from mineral content:A case study in Huoshiling Formation of Changling fault depression,Songliao Basin

NIU Penghui ,SHAN Xuanlong* ,REN Xianjun ,YI Jian ,LIU Chaoyang and XING Jian

1.College of Earth Sciences,Jilin University,Changchun 130061,China;

2.Exploration and Development Research Institute of SINOPEC Northeast Oil and Gas Company,Changchun 130062,China

Abstract:In order to study the microscopic pore characteristics of andesite reservoir and the effect of mineral content on the andesite pore,this study takes the andesite of the Huoshiling Formation in Longfengshan of Changling fault depression,Songliao Basin as a subject.The andesite reservoir space was discerned through the observation of cores and casting thin sections.Besides,the pore size distribution of andesites and their mineral contents were quantitatively characterized by high-pressure mercury injection,nitrogen adsorption and XRD,respectively.The results show that:(1) There are various types of reservoir space in andesites,including vesicles,amygdule,intergranular pores,matrix dissolution pores and dissolution pores of amygdala,and three types of fractures including dissolution,structural and explosion fractures.(2) The pore size distribution of andesite is complex.The main pore size of andesite is mid-pore (10-20 nm) with some large-pores (＞50 nm).Mid-pore and large-pore provide the main specific surface area,which are the main space for gas storage.(3) The andesite reservoir space in the study area is mainly controlled by dissolution,as supported by the relationship between the change of mineral content and porosity evolution.The porosity of andesites decreases with the increase of quartz and chlorite content,but increases with the increase of soluble mineral,e.g.,feldspar content.

Keywords:andesite;microscopic pore;high pressure mercury injection;nitrogen adsorption;XRD

Introduction

As an unconventional oil and gas reservoir,volcanic rocks have been studied for more than one hundred years (Tanget al.,2020).In recent years,great achievements have been made in volcanic oil and gas exploration in Songliao Basin,Bohai Bay,Erlian and Junggar (Liuet al.,2014;Maet al.,2017).According to statistics,the favorable area for the exploration of volcanic gas reservoir is more than 20 000 km2,and the geological reserves of the gas reservoir are more than 3 trillion cubic meters (Zhou &Tang,2007;Yuanet al.,2007;Leiet al.,2008).Volcanic oil and gas reservoirs have become an important field of oil and gas exploration in China.

Many studies have indicated that the formation of volcanic reservoirs is controlled by many factors,e.g.,the variation of magma properties,volcanic eruption types,lithology and lithofacies,as well as the late transformation (Hou,2003;Panget al.,2007).The reservoir quality mainly depends on the microscopic pore in the volcanic reservoir.

Recently,many volcanic reservoirs have been found in the Lower Cretaceous Huoshiling-and Yingcheng formations in the fault depression of southern Songliao Basin.The discovered volcanic gas reservoirs are characterized by large reservoir thickness and large-scale reserves.However,the exploration and research on oil and gas reservoirs in the fault depression of southern Songliao Basin is relatively insufficient,since most of work has been focusing on the genesis and distribution of lithology and lithofacies (Liuet al.,2020),reservoir characteristics and formation mechanism (Miaoet al.,2020),and reservoir evaluation (Wang,2020),but lacking the research on microscopic pore characteristics.This study aims to explore the microscopic pore types and characteristics of andesitic reservoir of Longfengshan Huoshiling Formation in Changling fault depression of Songliao Basin and provide guidance for future volcanic oil and gas development.

1 Geological setting

The Longfengshan area is geographically located in Qianqi Town,Changling County,Jilin Province,and structurally located in the south of Changling fault depression in the central Songliao Basin (Zhanget al.,2017).Fig.1a shows buried volcanic reservoirs in Songliao Basin (Tanget al.,2012;Wanget al.,2015b;Canget al.,2021).The study area can be roughly divided into the northern depression zone,the western denudation zone and the southeastern slope zone(Fig.1b).The structure of the study area is a large nose structure controlled by detachment faults,which is low in the northeast and high in the southwest (Zhanget al.,2016;Qinet al.,2016).

Strata in the study mainly consist of the Cretaceous Huoshiling-(K1h),Shahezi-(K1sh),Yingcheng-(K1y),Denglouku-(K1d),Quantou-(K2q),Qingshankou-(K2qn),Yaojia-(K2y),Nenjiang-(K2n),Sifangtai-(K2s) and Mingshui formations (K2m) from bottom to top,which can be divided into three stages,i.e.,fault depression stage,fault-depression transition stage and depression stage (Fig.1c;Wanget al.,2016).Huoshiling Formation was dominated by volcanic rocks.Shahezi Formation,is known as the source rock horizon,mainly includes mudstone or sandstone,whereas Yingcheng Formation contains reservoirs with good physical properties and is the main oil-bearing unit (Wanget al.,2015a).The target of this study is the gas-bearing andesite of Huoshiling Formation.

Fig.1 Geological background of the study area (after Cang et al.,2021;Qin et al.,2016)

2 Experimental samples and methods

2.1 The samples

In this study,4 andesite samples were collected from 2 wells in Longfengshan area (Table 1,Fig.2).The samples are located in Huoshiling Formation,with a depth range of 3 218.75-3 391.04 m.

Fig.2 Photographs of andesite core samples

Table 1 List of four andesite samples

2.2 Instruments and analytical methods

In this study,the micro pores of andesite are studied using casting thin sections,XRD,highpressure mercury injection method and nitrogen adsorption method.

Casting thin section is the simplest method to study the pores of reservoir rocks.Its principle is to flil the pores of rocks with colored liquid glue,and then ground them into thin sections after solidification,so that the pores can be clearly seen under a microscope.Therefore the types and quantities of microscopic pores in rocks can be distinguished by casting thin sections.

XRD can be used to determine the content of each mineral component in rocks.Its theoretical basis is that the volume or weight of material involved in diffraction is directly proportional to the diffraction intensity.Therefore,the volume fraction or weight fraction of a phase involved in diffraction in the mixture can be calculated by the size of the diffraction intensity,so as to determine the content of a phase in the mixture (Jianget al.,2011).

Micromeritics AutoPoreIV9500 automatic mercury injection instrument was used for the high pressure mercury injection experiment.The maximum mercury injection pressure of the instrument is 227 MPa,the aperture measurement range is 0.003-1 000 μm,and the accuracy of mercury intrusion or extrusion volume is better than 0.1 μL.The experimental samples were dried for 24 hours,and the highest mercury injection pressure is 227 MPa.

Nitrogen adsorption experiment adopts the latest product developed by American Anton Pacanta Instrument Company--AutoSORB-IQ2-MP,which can accurately determine the specifci surface area,pore size distribution and pore volume of microscopic pores.The instrument can measure pore size in the range of 0.35-500 nm and specifci surface area ＞0.000 5 m2/g.

3 Casting thin section and XRD results

The reservoir space types of andesite reservoir in this study refer to the classification of volcanic reservoir space (Wanget al.,2003).

According to the observation results of cores and cast thin sections,abundant reservoir space were identified in the andesites in the study area,generally classified as primary pores,secondary pores and fractures.The primary pores mainly include vesicles(Fig.3a),amygdule (Baiet al.,2021) and intergranular pores (Fig.3b).The secondary pores include matrix dissolution pores (Fig.3c) and dissolution pores of amygdala.Fractures include structural fractures (Fig.3d),dissolution fractures (Fig.3e) and explosion fractures(Fig.3f).

Amygdules are formed when the pores are not filled with minerals,belong to the primary pores.Similarly,dissolution pores of amygdala are formed by dissolution after the filling of the primary pores by carbonate minerals and other later minerals,belonging to the secondary pores.The intergranular pores in the samples are mainly the intergranular pores of plagioclase in the matrix and are mostly filled with dark minerals.Matrix dissolution pores are formed by devitrification or microcrystalline feldspar dissolution in matrix.Structural fractures are relatively straight fractures formed by late tectonic stress.Dissolution fractures are formed by the dissolution on the previous fractures and most of them are reticular formation.Explosion fractures are formed by fliling with late rock fluid.

The observation of andesite cores,ordinary thin sections and cast thin sections in the study area shows that there are many secondary pores and fractures in andesite,but only a few primary pores,most of which are fliled by carbonate minerals.Therefore,natural gas are mainly accumulated in the secondary pores,and the fractures in andesite provide migration pathway for natural gas.

XRD experimental analysis of four andesite samples can be seen in Table 2.The andesites contain quartz,K-feldspar,plagioclase,dolomite,biotite,clay minerals and amorphous etc.S-1 and S-2 contain more quartz and clay minerals,while S-3 and S-4 contain more K-feldspar and plagioclase (Quartz veins are developed in andesite in the study area,so there is a certain amount of quartz in andesites).

Table 2 XRD results of andesite composition %

4 High pressure mercury injection results

High-pressure mercury injection experiment is very important to study the microscopic pores characteristics of reservoir rocks (Panget al.,2007).The maximum mercury intrusion pressure of conventional mercury injection instruments is relatively small,which is not able to break through the capillary pressure in the the nanoscale pores of andesite,and thus cannot accurately describe the microscopic pore structure of andesite.Therefore,high pressure mercury injection was developed to study the microscopic pores.Mercury intrusion and extrusion curves (Fig.4) and pore size distribution curves (Fig.5) of andesite in this study were obtained.The maximum mercury injection pressure was up to 227 MPa and the mini-mum pore size was described as 3.238 nm.

Fig.4 Mercury intrusion and extrusion curves of andesites

Fig.5 Pore size distribution of andesites from mercury intrusion

It can be seen from Fig.4 that the displacement pressure of andesite is extremely high.The displacement pressure of the four samples is 8.53 MPa,19.93 MPa,13.77 MPa and 16.87 MPa respectively,with an average of 14.78 MPa.The displacement pressure of andesite is larger than that of conventional reservoirs(Panget al.,2007;Wanget al.,2009),reflecting a poor pore structure.None of them had high mercury saturation,and only Sample 3 exceeded 50%.The capillary pressure curves of andesite tend to the upper left as a whole.When the mercury intrusion pressure is 30-100 MPa,the mercury intrusion curve is nearly horizontal,and the amount of mercury intrusion is large,accounting for more than 80% of the total mercury intrusion,indicating that the pores of andesite are mainly concentrated between 7-25 nm.

As can be seen from Fig.5,the pore size distribution curves of the four samples are single-peak or dual-peak type,and the main pore size distribution range is 4-100 nm,with a wide distribution range and poor sorting.The average pore diameter of the four samples is 26.50 nm,21.80 nm,28.61 nm and 25.04nm,respectively,with an average of 25.49 nm (Table 3).The sorting coefficient is an important parameter to describe the degree of concentration of pore distribution.The sorting coefficient of the four samples is 5.4,5.8,4.32 and 5.87,respectively,with an average of 5.35.The sorting coefficient is large but showing poor sorting,indicating that there are not only a large number of micro-pores in andesite,but also some large pores,which is conducive to the storage and migration of natural gas.In addition to the sorting coefficient,the mercury extrusion efficiency is also an important parameter refelcting the pore structure.Their mercury extrusion efficiency is 16.40%,7.47%,27.59% and 25.82%,respectively,with an average of 19.32%.The mercury extrusion efficiency is low,and large number of nanoscale pores in andesite make mercury easy to lose continuity and cannot exit.Moreover,clay minerals in andesite may also lead to reduced mercury extrusion efficiency (Tang &Zeng,1994).

Table 3 Pore parameters of andesites from mercury intrusion

5 Nitrogen adsorption results

Volcanic rock reservoirs have strong heterogeneity and thus their pore structure is very complex.High-pressure mercury injection method can only accurately describe the microscopic pores larger than 3 nm,while nitrogen adsorption method can be used to distinguish the pores smaller than 3 nm.Fig.6 shows the nitrogen adsorption and desorption isotherms of four samples.

Fig.6 Adsorption-desorption isotherms in andesites

In general,the isothermal adsorption and desorption curves of the four samples all show reverse“S”-type (Fig.6),which are similar to IV-type of the classical adsorption-desorption curve (Xuet al.,2021) proposed by Sing (1985).The adsorption and desorption process can be divided into three stages:(1) at relatively lower pressure (P/P0＜0.05),that is,before point B,the adsorption curve is slightly convex upward.At this time,it is the monolayer adsorption stage of nitrogen on the pore surface of andesite.The adsorption capacity in the first stage is very small,generally less than 5 cm3/g;(2) with the increase of relative pressure,the adsorption capacity slowly increases,and there is a hysteresis loop at point C caused by the difference of evaporation pressure and condensation pressure in the process of nitrogen desorption,indicating that the pores are in an open state,and the adsorption capacity at this stage is generally less than 15 cm3/g;(3) with the further increase of pressure,nitrogen begins to liquefy.After point D,the isothermal adsorption curve rises almost vertically until it approaches the saturated vapor pressure,indicating the existence of a certain number of large pores in the sample.The hysteresis loops characteristics of desorption curve and adsorption curve at medium pressure are similar to H3-type proposed by Sing (1985) (Fig.7),indicating that the samples all contain fissure pores formed by the aggregation of plate-like particles,and the hysteresis loops in this form should be caused by the dissolution fractures and structural fractures developed in the samples,which improve the adsorption capacity and permeability of the samples.

Fig.7 Types of hysteresis loops (after Sing,1985)

Specifci surface area is an important parameter to describe pore characteristics through nitrogen adsorption method.The specific surface area was alculated using BET equations derived by Brunauer,Emmett and Teller (Yanget al.,2013).The specifci surface area of the 4 samples was 15.673 m2/g,10.777 m2/g,17.639 m2/g and 20.425 m2/g,the average value is 16.123 m2/g.

In addition,pore size distribution can be another important parameter to describe pore characteristics.This paper used BJH method to calculate pore size distribution,which is a method to analyze pore size distribution between 2-100 nm.Fig.8 shows the pore size distribution of andesites from nitrogen adsorption.The abscissa is the pore size,and the ordinate is the pore volume increment.As can be seen in the figure,the BJH adsorption curves of the four samples have the same morphology and are single-peak type.In the pore size range of 2-10 nm,the increment of pore volume is large,indicating more pore distribution below 10nm.According to IUPAC classifciation,pores with a pore diameter less than 2 nm are called micro-pores,pores with a pore diameter between 2-50 nm are called midpores,and pores with a pore diameter greater than 50 nm are called large-pores.Therefore,the main pore size of andesite is in the mid-pores range,with some large-pores.The average BJH pore sizes of the four samples are 26.28 nm,33.44 nm,28.45 nm and 25.96 nm,respectively,with an average of 28.53 nm.The pore structure parameters of nitrogen adsorption method are shown in Table 4.

Fig.8 Pore size distribution of andesites from nitrogen adsorption

Table 4 Pore parameters of andesites from nitrogen adsorption

6 Discussion

6.1 Pore size distribution of andesite

The high pressure mercury injection method and nitrogen adsorption method are used in the study of andesite pores.The two methods have different measuring ranges and applicable pore sizes (Wanget al.,2019).With the high pressure mercury injection experiment,the measurement of nano-scale pores is very inaccurate due to the limitation of the precision of experimental instruments and the upper limit of pressure.In the nitrogen adsorption experiment,the measurement range is 2-200 nm,which can be more accurate characterization of nano-scale pores,but the precise adsorption capacity of larger pores cannot be obtained at higher pressure (Yue,2021).Therefore,the two methods are combined to characterize the pore size distribution of andesite in the study area.

Taking the S-3 sample as an example,the ordinate of the pore size distribution curve obtained by the two experiments was converted into porosity.It can be seen that there is an intersection at 7 nm (Fig.9).When the pore size is less than 7 nm,the porosity obtained by mercury injection method is less than that obtained by adsorption method due to the limitation of nanoscale porosity measurement.However,the two curves show same trend.When the pore diameter is larger than 7 nm,the measured values are lower than those of mercury injection method due to the influence of nitrogen adsorption experiment on the measurement accuracy of large pore diameter.To sum up,nitrogen adsorption method is convincing when pore size is less than 7 nm,while high pressure mercury injection method is adopted when more than 7 nm.The pore size distribution curve of andesites is fnially obtained by splicing pore size distribution curves of other samples in the same way (Fig.10).

Fig.9 Pore size distribution curve of S-3

As indicated from Fig.10,andesite pores have strong heterogeneity and wide pore size distribution,ranging from nano-scale pores to micro ones.The main pore size of S-1 is between 50-100 nm,and that of S-2,S-3 and S-4 is between 10-20 nm.As to S-1,there is hydrothermal breccia in the core (Fig.2a),and most fractures are filled.Therefore,although there are many pores with a diameter of 50-100 nm,their porosity is not high.In addition,according to the nitrogen adsorption experiment results,the specific surface area distribution histogram was classified by IUPAC (Fig.11).The specific surface areas of micropores,mid-pores and large-pores account for 3.20%,82.95% and 13.85% of the total specifci surface area of andesite,respectively.In conclusion,the main pore sizes of andesite in the study area are between 10-20 nm (mid-pore) and contain a certain number of largepores,which provide the main storage space for gas.

Fig.10 Pore size distribution curves of andesites

Fig.11 Specific surface area distribution of andesites

6.2 Mineral implications for pore system in andesite

Andesite is an intermediate extrusive rock with a fast formation and condensation speed,and a large number of bubbles in the magma have not got chance to escape.Therefore,a large number of primary pores are developed in andesite.However,the primary pores of the andesite in the study area are mostly filled with carbonate minerals (Fig.3a).Andesite reservoir space formation in the study area is very complex.After the andesitic eruption,it has experienced structural movement,weathering leaching effect,alteration,dissolution and cementation,etc.(Liuet al.,2008;Liuet al.,2016;Chen,2013).These effects on reservoirs quality can be manifested by the minerals in the andesites.

Combining the XRD and porosity results of four andesite samples (Fig.12),it is found that the porosity of andesite decreases with the increase of quartz and chlorite content,and increases with the increase of feldspar content.In the process of alteration,plagioclase is altered into chlorite,which makes the mineral volume expand,blocks the pores formed earlier,reduces the porosity of andesite,and creates conditions for the later fluid dissolution.In the process of dissolution,acid fluid penetrates andesite along fractures,reacts with feldspar in andesite and forms dissolution pores.Quartz is stable compared with feldspar and cannot be dissolved easily by fluid.Therefore,the porosity of andesite increases with the increase of feldspar content,but decreases with the increase of quartz content.

Fig.12 Relationship between mineral content and porosity of andesites

7 Conclusions

(1) The reservoir space types of andesite in the study area are diverse,including primary pores,secondary pores and fractures.The primary pores mainly consist of vesicles,amygdule and intergranular pores.The secondary pores include matrix dissolution pores and dissolution pores of amygdala,whereas the fractures include dissolution fractures,structural fractures and explosion fractures.

(2) The andesite pores in the study area have strong heterogeneity and wide pore size distribution.The main pore size of the andesite is mid-pore between 10-20 nm,and also contains a certain amount of large pores.These mid-pores and large pores provide the main storage space for gas.

(3) Andesite in the study area is mainly controlled by dissolution.The reservoir spaces are mostly dissolution pores and dissolution fractures.Meanwhile,through the analysis of dissolution and alteration process,it is found that the porosity of andesite decreases with the increase of quartz and chlorite content,and increases with the increase of feldspar content.