Characteristics and identification of weathering crust of Pennsylvanian volcanic rocks in Shixi area, Junggar Basin

HAN Junwei, SHAN Xuanlong , YIMING Ablimiti, BIAN Baoli, LIU Hailei, LI Ang and YI Jian

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

2. Research Institute of Exploration and Development, Xinjiang Oilfield Company, PetroChina, Karamay 834000, China

Abstract: Based on drill core and thin section observation, major geochemical element analysis and conventional well log analysis, this study summarizes the characteristics and thickness of weathering crust of Pennsylvanian volcanic rocks (Carboniferous) in Shixi area, Junggar Basin.The weathering crust is identified and divided into three types according to the petrological characteristics and well log interpretations in Shixi area, and the isopach of weathering crust is mapped.The results show that: (1) With the increase of depth, the weathering weakens, and the rocks become less fractured with decreased porosity; (2) the weathering crust of the Upper Carboniferous volcanic rocks can be divided into strongly weathered and mildly weathered layers in Shixi area; (3) the weathering crust is relatively thicker in Dinan uplift and Shixi uplift.This study provides research basis for further evaluation of Upper Carboniferous volcanic reservoir, and will benefit for well location deployment and potential oilfield development in the Shixi area.

Keywords: Junggar Basin; Pennsylvanian; weathering crust; identification and division; volcanic rocks

Introduction

In recent years, with the increasing oil and gas fields development and production in volcanic reservoirs, volcanic reservoirs have gradually become a research hotspot (Fanet al., 2020; Zhouet al., 2016;Li, 2019).After eruption, the magma cools down and forms volcanic rocks.The volatile substances in the magma escape during the cooling process and are retained in the volcanic rocks forming favorable primary storage spaces such as pores, or dissolve with the participation of later fluids, forming favorable secondary reservoirs (Yaoet al., 2021).At present,most of the volcanic rock oil and gas reservoirs in eastern part of China are mainly primary volcanic rock reservoirs (Wanget al., 2003; Wanget al.,2006; Wanget al., 2013), while the western part is dominated by secondary volcanic rock reservoirs related to weathering crusts (Chenet al., 2016).Zouet al.divided the weathered profile of Carboniferous volcanic rocks in the northern Junggar Basin into soil layers, hydrolysis zones, dissolution zones and disintegration zones (Zouet al., 2011).Hou Lianhuaet al.subdivided the disintegration zone into disintegration zone I and disintegration zone II (Houet al., 2013).Lu(2017) divided the Carboniferous volcanic weathering crust reservoir structure of Shixi Oilfield into weathered clay layer, strongly weathered layer, weakly weathered layer and un-weathered bedrock.The maximum thickness of weathered crust is more than 200 meters.

The Luliang uplift, where the study area is located, is a typical area with relatively completed, well developed and preserved Pennsylvanian volcanic rocks in the Junggar Basin (Zouet al., 2011b), which has great potential for oil and gas resources.Considerable oil and gas production has been confirmed in the wells drilled into the Pennsylvanian in the study area (Chenet al., 2016).Combining with the previous research results, it is found that the volcanic rock reservoirs in this area are mainly developed by weathering and leaching.The identification of weathering crust features and the division of structures is key for the deployment of oil and gas exploration.With the ongoing exploration and development of volcanic oil and gas reservoirs in the Shixi area, it is necessary to further clarify the identification features and structure division of weathering crusts.In this study, under the situation that only a few wells outcropped the Pennsylvanian rocks and the whole section of Pennsylvanian was not drilled through, the authors propose the classification of the identification features of the weathering crust of the Pennsylvanian volcanic rocks in the study area and the division method of the weathering crust structure.

1 Geological setting

Pennsylvanian, also known as Upper Carboniferous or Late Carboniferous, lasted from roughly 323.2 Ma to 298.9 Ma.The Shixi area is located in the south of the Luliang Uplift in the Junggar Basin,Xinjiang, and in the northeastern margin of the Well Pen-1 West Sag (Fig.1).For the Junggar Basin, the Carboniferous was an important transition period from marine to continental deposits (Maoet al., 2010; Suet al., 2012; Liet al., 2015).The Pennsylvanian Luliang Uplift was in an island arc environment under the background of subduction.The Mississippian slab retreated,and the Karamaili Ocean was basically closed, forming an intracontinental depression (Li, 2016).Active volcanic activity developed during Pennsylvanian in Shixi area, and wells that have encountered the Pennsylvanian reveal the lithological characteristics, which are dominated by volcanic lava and pyroclastic rocks.The Meso-Cenozoic strata with a thickness of 4 000 m to 4 500 m overlying the Carboniferous volcanic rocks, from old to new are Permian (Jiamuhe Formation, Fengcheng Formation, Xiazijie Formation, Wuerhe Formation), (Triassic Baikou-quan Formation,Karamay Formation, Baijiantan Formation), Jurassic,Cretaceous, Tertiary and Quaternary.There is a widely distributed unconformity at the top of the Upper Pennsylvanian Batamayineshan Formation in the Shixi area(Genget al., 2011; Heet al., 2013; Guanet al., 2014).Influenced by the middle-late Hercynian movement,the Shixi area not only experienced a long-term uplift,but also a long-term weathering denudation (Panet al.,2020), which has resulted in the lack of the uppermost of Pennsylvanian and Cisuralian strata in the research area and the development of a relatively complete and widely distributed weathering crust, which provides the basis for the development of weathered crust related volcanic rock reservoirs.

Fig.1 Structural units of Junggar Basin (a) and regional overview diagram of Shixi area (b) (after Chen et al., 2016)

2 Methods

Based on the core data and logging data of 16 wells in the study area that encountered the Pennsylvanian strata, the identification features and structural division of the Pennsylvanian volcanic weathering crust in the study area are studied.By observing and analyzing the petrological characteristics of the weathered crust of drilling cores and cast thin sections, which includes 311 m of core observation and description of 93 cast thin sections, the thickness of the weathering crust in the study area is predicted.Based on the chemical alteration index and plagioclase alteration index calculated from the whole-rock major geochemical elements analysis, and conventional well log characteristics, the weathering crusts are identified, and a typical cross-well profile and the the isopach of weathering crust is mapped.

3 Characteristics of weathering crust

3.1 Petrological and physical characteristics

The volcanic weathering crust is formed under the control of weathering and leaching.Weathering and leaching gradually deepens with time, resulting in more weathered fractures and deeper fractures in the top volcanic rocks than the underlying volcanic rocks(Houet al., 2012), while the fluid in the weathering subsequently fill the fractures with secondary minerals or argillaceous components (Wanget al., 2008).Through the observation of the sampled cores, it was found that sufficient fractures developed in the cores of volcanic rocks with strong weathering (Fig.2e), i.e.,the rocks are highly fractured.Under the microscope,the fractures could be seen cutting through pyroclastic particles or mineral crystals, and are filled with calcite or clay, and the characteristics of the erosion and the fil-ling can be seen (Fig.2a).With the increase of the depth from the top of the Carboniferous, the fragmentation of rocks weakens, the dissolution cracks and weathering cracks formed by weathering and leaching gradually decrease (Fig.2b, f), and the dissolution effect gradually weakens(Fig.2c), indicating a weakening of the weathering.Slightly weathered or un-weathered volcanic rocks are compacted (Fig.2d, h).Under the microscope, it can be observed that the pore space is dominated by intergranular pores, and the fractures consist of structural fractures or diagenetic explosion fractures (Fig.2g).

Fig.2 Petrological characteristics of volcanic weathering crusts in Pennsylvanian

Fig.3 Relationships between CIA, PIA and depth from top of Carboniferous unconformity (a) and porosity of volcanic rocks and depth (b)

At the same time, under the influence of weathering, the originally solid volcanic rock gradually became loose, and the porosity increased.Due to the vertical variation of weathering intensity, the porosity showed a regular change.Through the establishment of the relationship between porosity and depth (Fig.3b), we can understand the vertical variation characteristics of the weathering crust.For the weathered crust of Pennsylvanian volcanic rocks in Shixi area, the variation law of porosity with depth is that the porosity between 0–100 m increases with depth, whereas the porosity between 100–200 m decreases with the increase of depth.A high porosity can be seen at 250 m.The observation under the microscope shows that the rock has well-developed vesicular pores, which leads to an increase in porosity, rather than by weathering and leaching.

3.2 Characteristics of major geochemical elements

During the formation of weathering crusts, volcanic rocks experience various chemical process, such as hydrolysis, hydration, oxidation, etc.(Wanget al.2011).During weathering, the most important process is the weathering of plagioclase and the formation of clay minerals, during which calcium, sodium and potassium migrate out of the system along with the weathering fluid, resulting in the relative increase of aluminum and alkaline components.Through the major geochemical elements analysis of the volcanic rocks in the study area, it is found that during the weathering process, it is characterized by the enrichment of iron and aluminum elements and the depletion of sodium and potassium elements.For example, the basalt sample at 7.2 m from the top of the Carboniferous has an Al2O3content of 14.3% and a Fe2O3content of 11.54%, and the basalt sample at 84.5 m from the Carboniferous top has an Al2O3content of 9.80% and Fe2O3content of 7.46%.

4 Identification of volcanic weathering crust

4.1 Identification from major geochemical elements

According to the changes in the content of major elements in volcanic rocks during the weathering process, the chemical alteration index (CIA) and the plagioclase alteration index (PIA) were proposed to describe the degree of weathering (Nesbitt & Young,1982).

Fig.4 Comprehensive column chart of typical wells with weathering crust structure

Sampling and major geochemical tests were carried out for the drill cores in the Shixi area to get a scatter plot of CIA and PIA with the depth from the top of the Carboniferous (Fig.3a).It can be seen that CIA and PIA decrease with the increase of depth from the top of the Carboniferous.The andesite at 240 m is not strongly affected by weathering and dissolution under the microscope, so it is predicted that about 200 m below the top of the Carboniferous is the average base boundary of weathering crust.Rock porosity, CIA and PIA values all have peaks at 250 m, which is near the depth of the interface during the volcanic eruption which is a good fluid conduction interface due to its wide lateral distribution, so that there are peaks of weathering intensity at this depth.

4.2 Identification from conventional well logs

Weathering causes volcanic rocks to be altered into clay and semi-weathered rocks, etc.With the change of weathering intensity vertically, volcanic rocks also undergo vertical phase transformation, which show different characteristics on logging curves.Due to the long-term weathering and leaching at the top of the weathering crust, the loosening of the rock reduces the density.The fluid filled in the pores increases the acoustic time difference, and the effect of the rock alteration superimposed on the pores and fractures increases the compensated neutron (Wanget al.,2015).The radioactive elements in the rock migrate horizontally or deposit downward with the fluid, which weakens the radioactivity of the rock at the top of the weathering crust and increases the radioactivity of the lower rock slightly(Zouet al., 2011).Based on previous results, the sections with strong weathering can be identified by the abnormal fluctuation characteristics of the well log curves – the natural gamma of the top of the weathering crust is much lower, whereas the natural gamma of the lower part is slightly higher.Rocks with strong weathering are indicated by higher acoustic time difference within the same lithological section, higher compensated neutron log, and generally speaking, lower density (Li, 2019).

According to the regular changes shown by gamma ray, neutron log, sonic log and density curve, the weathering crust structure in Shixi area is divided into strongly weathered layer and mildly weathered layer.Under the weathering crust is the hardly weathered bedrock layer.

The strongly weathered layer refers to the top part of the Pennsylvanian volcanic stratum that has been exposed on or above the surface and has been weathered for a long time.Within the same lithological section, the natural gamma curve, compensated neutron curve, sonic curve and density curve, from top to bottom, all show obvious funnel-shaped curve and box-shaped curve (Fig.4a).The top and base of the box-shaped part do not appear at the lithological interface, which proves that this box-shaped shape is not caused by lithological changes.

There is no secret to why our marriage has lasted while so many others have failed. There is no formula for success that I can offer, other than to express at the most important feature of our relationship is that it has never lost the sense of romance3 that bloomed when we first met. Too often marriage kills the romance that was born in the courtship of a relationship. To me, I have always felt that I am still courting Patricia, and therefore the romance has never died.

The mildly weathered layer refers to the part of the stratum where weathering and leaching gradually weakens due to the increasing depth.The log curve in this part is generally finger-like or jagged.Taking the tuffsection from 4 792 m to the bottom of well SX3 as an example (Fig.4b), the average value of natural gamma of tuffin the study area is 108 API.The average value of acoustic time difference is 60.1 um/ft.The average value of density is 2.533g/m3.The average value of compensated neutron is 0.102.Within the example section, which is the section from 4792m to the bottom of well SX3, average natural gamma value of this section is 107–123 API.The average acoustic time difference is 54–59 um/ft.The average density is 2.557–2.592 g/m3.The average compensated neutron is 0.048–0.112.Compared with the average, every logging parameter shows that there was weathering but the weathering was weak, indicating that the section belongs to the mildly weathered layer.

The hardly weathered layer refers to the rock layers that is hardly affected by weathering or is weakly affected by weathering.The influence of weathering and leaching in this part on the formation of favorable reservoirs is far less than the original reservoir potential brought by the lithology itself.Since most of the wells in the study area did not drill through the entire weathered body structure, no bedrock was intersected in some wells.The bedrock layer is characterized by log curves with little variation, and within the same lithological section, the curves from top to bottom fluctuate slightly in a small range.Taking the section from 4 825 m to the bottom of well SX17 as an example (Fig.4c), the sonic, neutron, and density curves are micro-toothed,with no obvious dip characteristics.There are some abrupt changes in the natural gamma curve, but they are all located at the lithological inter-face, which is caused by abrupt changes in lithology.It shows a good box or micro-tooth shape inside each lithologic section.However, due to the influence of the interface and the fault zone during the volcanic eruption, there are horizontal dissolution layers in the parent rock layers of some wells, which are shown as vertical fingershaped curves sandwiched by abrupt funnel-shaped curves.As a result of the downward dissolution along the interfaces or faults, this type of dissolution layer is relatively thin, and only develops within 10–20 m below the lithological interface of the bedrock layer.Because it is weakly affected by extensive weathering,it is classified as the hardly weathered layer in this study.This type of dissolution layer is represented by the dacite section at 4 524–4 530 m from the parent rock section of well SX1 (Fig.4d), which is 263 m from the top of the Carboniferous.and the well log of the bottom of the overlying andesite volcanic breccia is bell-shaped or box-shaped.The characteristics of the curve show a relatively weak weathering.The lower dacite in the same lithological section also shows a box-shaped curve, and only this section shows a funnel-shaped curve, indicating a dissolution layer.

4.3 Planar distribution of weathering crust

According to the above weathering crust structure division method, this study divided the weathering crust structure of 16 wells in the Shixi area.Based on the analysis of each well, a cross-well profile was established.Characteristics of the volcanic weathering crust in the Shixi area were also studied.

4.3.1 Combined profile across wells SX3- SX2- SX1-SX16- SX18

This section is a north-south section of the,passing through the Shixi uplift and the Dinan uplift in the study area(Fig.5).From north to south, the thickness of the weathering crust first increases and then decreases.At the northern end of the profile,well SX3 encountered about 169 m of weathered crust, consisting of about 72 m of strongly weathered layer, and about 97 m of mildly weathered layer.SX1 encountered 231 m, including about 119m of strongly weathered layer and about 112m of mildly weathered layer.Continuing to the south, the weathering crust gradually gets thinner, and about 88m of weathered crust is developed at well SX18 in the southern end of the section, comprising about 42 m of strongly weathered layer and 46 m of mildly weathered layer.The Carboniferous in well SX1 is overlain by the Triassic Karamay Formation, with Permian-Triassic Baikouquan Formation missing, because this area has been uplifted for a long time due to the compressional tectonic movement in the Pennsylvanian, and has undergone long-term weathering.The paleo-landform is an elevated area, where it experienced long term weathering, so a thick weathering crust has developed.The Carboniferous at well SX18 is overlain by the Permian Fengcheng Formation.Here the Pennsylvanian paleomorphology is in the slope area, which experienced short weathering, so a thin weathering crust is developed.

4.3.2 Well section across SHI006-SHI003-SX1-SX101-SX17

Fig.5 Combined profile across wells SX3- SX2- SX1- SX16- SX18

This section is an east-west section in the Shixi area, passing through the Shixi uplift and the Dinan uplift (Fig.6).At the east end of the profile, well SHI006 encountered about 110 m of weathering crust,consisting of 65 m of strongly weathered layer, and about 45 m of mildly weathered layer.The weathering crust gradually thickened to the east of well SX101,where it encountered about 256 m of weathered crust,of which the strongly weathered layer is about 153 m,and the mildly weathered layer is about 103 m.Continuing to the east, the weathering crust is gradually thinner.The well SX17 at the eastern end of the profile encountered a weathering crust of about 50 m,of which the strongly weathered layer is about 26 m,and the mildly weathered layer is 24 m.The profile indicates that the weathering in the Shixi area from west to east shows a trend of increasing slightly at first and then weakening, with the strongest weathering in the Shixi bulge where well SX101 is located.The Carboniferous at well SX101 is overlain by the Triassic Baikouquan Formation.Compared with well SX1, the weathering time at well SX101 is slightly shorter.However,because the top basalt section is developed from the self-broken basalt belt, it forms a good fluid migration pathway.The thickness of its weathering crust is slightly thicker than that of well SX1.The Carboniferous at well SHI006 is overlain by the Triassic Baikouquan Formation, the Carboniferous at well SX17 is overlain by the Permian Wuerhe For-mation.These two places experienced weathering for a short period of time, with thin weathering crusts de-veloped.

4.3.3 Planar distribution of weathering crust in Shixi area

According to the weathering crust thickness data of each well in Shixi area, a isopach map of the Pennsylvanian weathering crust thickness in Shixi area is drawn(Fig.7).The weathering crust in the northwestern margin of Dinan uplift where the well SX18 are located is relatively thin, and the thickness of the weathering crust in the Shixi area as a whole gradually decreases from the northeast to the southwest.After the volcanic rock is solidified, it gets less affected by compaction.Although the Shixi area has undergone long-term weathering, its overall shape inherited paleo-landform.The thickness of the weathering crust has a nonlinear proportional relationship with the weathering time(Zouet al., 2011; Houet al., 2013).According to the thickness of the weathering crust, its paleo-geomorphic features can be roughly inferred.Combined with the Pennsylvanian layers revealed by drilling well, the overlying strata can indicate its ancient morphological features.Wells SX1, SX2, SX101, SHI005 and SX4 are located in the long-term uplift areas that have experienced long-term weathering.Along the southwest direction, the weathering crust is thinning to the slope area near well SX16 and SX18.

Fig.7 Isopach of Pennsylvanian weathering crust in Shixi area

5 Conclusions

(1) The top of the weathered crust of the Pennsylvanian volcanic rocks in the Shixi area is more fractured than the underlying volcanic rocks.The cracks of the volcanic rocks with strong weathering are filled with mud and calcite, and the dissolution of calcite can be seen.The dense volcanic rocks under weathering gradually become loose and the porosity increases and the intensity of weathering gradually weakens with the increase of depth from the top of the Carboniferous.

(2) According to the changes of Al3+, Ca2+, Na+,K+element contents in the volcanic rocks, it is found that the general thickness of the Pennsylvanian weathering crust in the Shixi area is about 200 m, and can reach 50 m in the area with weak weathering, and 256 m in the area with strong weathering.

(3) According to the value and shape of natural gamma, acoustic time difference, compensated neutron and density curves, the Pennsylvanian weathering crust in Shixi area can be divided into strongly weathered layers and mildly weathered layers.The strongly weathered layer is characterized by a funnel-shaped curve and the mildly weathered layer is characterized by box-shaped curves sandwiched with finger-shaped curves.The value of compensated neutron log within mildly weathered layers is slightly higher than the average value of the study area with same lithology,whereas the density is slightly lower.The hardly weathered layers are characterized by vertical microdentate curves.

(4) The weathering crust of the Pennsylvanian volcanic rocks around the wells SX1 and SX101 in the Shixi area is the thickest, and gradually decreases from northeast to southwest where the weathering crust is the thickest, and the northwestern edge of the Dinan uplift is relatively thinner.The Shixi area as a whole presents the characteristics of a weathering crust that is thick in the northeast and thin in the southwest.