Lithologic characteristics and diagenesis of the Upper Triassic Xujiahe Formation, Yuanba area, northeastern Sichuan Basin

Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Contents lists available at ScienceDirect

Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse

Lithologic characteristics and diagenesis of the Upper Triassic Xujiahe Formation, Yuanba area, northeastern Sichuan Basin Li Zhang a, Xusheng Guo b, Fang Hao a, c, Huayao Zou a, *, Pingping Li a a

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Changping, Beijing 102249, China Sinopec Exploration Company, Chendu, Sichuan 610041, China c Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 February 2016 Received in revised form 16 August 2016 Accepted 28 September 2016 Available online 29 September 2016

The Upper Triassic Xujiahe sandstone in the Yuanba area, northeastern Sichuan Basin, is a typical coalbearing tight gas reservoir. Whether or not the commercial production of gas from these tight sandstones can be achieved largely depends on the quality of the reservoirs. In this study, a petrographic analysis using a thin section and a scanning electron microscope (SEM), combined with reservoir property data, was carried out to discuss the effects of primary mineral compositions on diagenetic processes and reservoir quality. The Xujiahe sandstones are mainly litharenite and feldspathic litharenite with minor sublitharenite, quartzarenite and calcarenaceous sandstone. Reservoir quality is poor, with porosity ranging mainly from 1% to 4% and permeability mainly from 0.01 md to 0.1 md, among which the relatively high values are limited to litharenite and feldspathic litharenite. The anomalously poor quality of sandstones is attributed to the high compaction and calcite cementation during the early diagenetic stage while the relatively high quality is associated with the precipitation of chlorite rims. These diagenetic processes are functions of the sandstones composition and the primary sandstones composition is intrinsic to the quality of the Xujiahe sandstones. The sandstones with high contents of matrix and ductile rock fragments (DRFs) and/or carbonate rock fragments (CRFs) commonly experience intense compaction or cementation, which are key factors for the poor quality of reservoirs. Although the contents of matrix, DRFs and CRFs are low in quartzarenite, intense compaction also occurs due to the finer grain size. Only litharenite and feldspathic litharenite with minor contents of matrix, DRFs, CRFs and volcanic rock fragments (VRFs) commonly experience the precipitation of chlorite rims, and are therefore recommended to be the favourable tight gas reservoirs in the Yuanba area. © 2016 Elsevier B.V. All rights reserved.

Keywords: Sandstone composition Diagenetic process Reservoir quality Xujiahe Formation Yuanba area Sichuan Basin

1. Introduction The Upper Triassic Xujiahe Formation in the Sichuan Basin, with proven gas reserves of more than 1000
108 m3, has become a key target for tight sand gas exploration in China during the past decade (Zhang et al., 2009; Ma et al., 2010; Li et al., 2012a, 2012b; Dai et al., 2014). Previous studies have shown that the gasgenerating capacity of source rocks within the Xujiahe Formation is more than 100
108 m3/km2, suggesting an excellent gas source and the potential formation of a medium-giant gas field in the Yuanba area (Dai et al., 1997; Liu et al., 2011; Pan et al., 2011; Guo,

* Corresponding author. E-mail address: [email protected] (H. Zou). http://dx.doi.org/10.1016/j.jngse.2016.09.067 1875-5100/© 2016 Elsevier B.V. All rights reserved.

2013). However, whether commercial production of natural gas can be explored from the tight sandstone is largely dependent upon the effective porosity zones (Shanley et al., 2004; Cai, 2010; Li et al., 2010b; Guo, 2013). Although wells with a high-tested gas flow from the Xujiahe sandstone were drilled in the Yuanba area, few wells could maintain stable gas production. Therefore, it is important to understand the controlling factors for porosity destruction and preservation, which is still uncertain in the Yuanba area. The prediction of reservoir quality, based only on the reconstruction of a diagenetic history, is not always useful unless the diagenetic processes are understood (Bjørlykke and Jahren, 2012). In an early diagenetic stage, reactions are very sensitive to climate, which controls the flow of meteoric water and may result in a geochemically open system. However, during burial diagenetic environments, sediments are geochemically closed and mass

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

transport is also limited (Taylor et al., 2010; Bjørlykke and Jahren, 2012). Rock properties should be predicted based on the mineral composition of the sediments (Bjørlykke and Jahren, 2012). The Xujiahe sandstone has experienced a long burial stage (Li et al., 2010a; Zhang et al., 2015; Yang et al., 2016) and the flow ability of pore fluids is limited, therefore, the quality of the Xujiahe sandstone should take into consideration the effect of the primary mineral composition on diagenetic processes. The quality of the Xujiahe sandstones in Sichuan Basin has been reported in the literature. Most of the studies provide insight into the characteristics of mineralogical composition and diagenetic minerals (Feng et al., 2009; Lv and Liu, 2009; Pan et al., 2011; Zhou et al., 2011; Chen et al., 2012a; Jia et al., 2014; Xiao et al., 2014; Lai et al., 2015), the process of the porosity evolution (Chen et al., 2012b; Luo et al., 2012; Wang, 2012; Xiao et al., 2012; Chen et al., 2014), and the classification of the diagenetic facies (Du et al., 2006; Dai et al., 2011; Lai et al., 2014). However, few studies have discussed the effects of mineral compositions on the diagenetic processes and the impact of these compositional differences on sandstones quality. In deep buried sandstones, mineral compositions affect diagenetic process and reservoir quality obviously (Bjørlykke and Jahren, 2012; Dutton et al., 2012). Therefore, in order to accurately forecast the reservoirs quality, detailed information of sandstone compositions should be investigated. In this study, available data have been used from core samples of Xujiahe sandstones to determine the compositions of the sandstones and the purpose of this article is to delineate the differences in diagenetic processes of different lithologies, and determine the effects of primary mineral compositions on the diagenetic evolution of sandstones and their quality, in order to provide important references for favourable reservoir prediction in deep buried sandstones.

1321

2. Geological setting The Sichuan Basin, with an area of approximately 180,000 km2, is a large petroliferous basin located in the southwest part of China (Fig. 1). As a superimposed basin, it has generally experienced two tectonic deposition cycles namely, the Siniane-Middle Triassic passive continental margin tectonic evolution and the Late TriassicEocene foreland basin and depression evolution (Zhai, 1989; Tong, 1992). From the Early to Middle Triassic, the cratonic margins of the Sichuan Basin were raised because of the compression from the Tethys and the Pacific. In the Late Triassic, the Sichuan Basin evolved to a foreland basin with the uplifting of Longmen Mountain at the western margin (Tao et al., 2014). During the Precambrian to middle Triassic, the deposits were dominated by marine sedimentation, with a cumulative thickness from 6000 m to 7000 m; the overburden rocks (Upper Triassic to Quaternary) were deposited in a continental environment with a cumulative thickness of 2000e5000 m. Tectonically bounded by the Longmen Mountain Fold Belt in the northwest, the Micang Mountain Uplift in the north and the Daba Mountain Fold Belt in the northeast (Fig. 1), the Yuanba area is located in the northern part of the Sichuan Basin. The Upper Triassic Xujiahe Formation (T3x) in the Yuanba area was deposited in a braided channel delta to lacustrine environment, with a cumulative thickness varying from several hundreds of meters to 1000 m. Based on the depositional cycle, the Upper Triassic Xujiahe Formation can be subdivided into five members (T3x1eT3x5) from bottom up (Fig. 2). The T3x1, T3x3 and T3x5 are mainly black shale, mudstone interbedded with siltstone, sandstone and coal seams, and they are regional petroleum source-rocks and cap units of the Xujiahe Formation; while T3x2 and T3x4 are dominantly grey sandstones with a common occurrence of thin coal seams and form

Fig. 1. Location map showing Yuanba area in the NE Sichuan Basin. LM Mt., MC Mt. and DB Mt. around Sichuan Basin represent Longmen Mountain thrust faulting belt, Micang Mountain uplift and Daba Mountain uplift, respectively.

1322

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Fig. 2. Generalized stratigraphic column of the Yuanba field, NE Sichuan Basin. Num. ¼ number, Sym. ¼ symbol.

the main reservoir intervals in Xujiahe Formation (Fig. 2, Zhang et al., 2006). The burial and thermal history show that the maximum burial depth of T3x in the Yuanba area reached 5800e6300 m during the period of Middle Crataceous (at approximately 100 Ma), and the maximum geotemperature was 180e220  C with a corresponding vitrinite reflectance (Ro %) 2.0% (Fig. 3; Li et al., 2010a; Zhang et al., 2015; Yang et al., 2016). The Sichuan Basin experienced a long period of continuous subsidence and deposition, and the natural gas in T3x was mainly generated from the Late Jurassic to the Early Cretaceous until the Middle-Late Cretaceous, when the Late

Yanshanian-Early Himalayan tectonic movement activated. Affected by this regional uplift and compression, a significant volume of overburden sediments was eroded, with the erosion depths of approximately 1500 m in the Yuanba area. Due to the continuous decrease of temperature, the hydrocarbon generation in T3x gradually terminated since Middle Cretaceous. 3. Methods In this study, a total of 927 core plugs were drilled from 19 different wells for sandstone composition and quality analysis.

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

1323

Fig. 3. Burial and thermal history of the Xujiahe Formation in the Yuanba area, NE Sichuan Basin (from Li et al., 2010a).

Locations of these 19 wells are shown in Fig. 1 and they are distributed in various tectonic units in the study area. This set of samples can be adequate to acquire the general sandstone composition. All these samples were taken for thin section observation, and 525 samples were selected for porosity and permeability measurements. Scanning electron microscopy (SEM) observation was taken from 42 representative samples for accurate studies of sandstone composition and paragenetic relationship. Table 1 presents detailed information of the samples, including the well name, the interval from which they were sampled and the number of the samples in each test. Thin sections were prepared by vacuum impregnation with blue or pink epoxy resin before cutting and grinding to a standard 30 mm thickness. The contents of framework grains, matrix, authigenic minerals and porosities were quantitatively assessed using the Adobe Photoshop quantification method as described by Zhang et al. (2014). Ten microscopic images with a
5 objective lens were commonly captured per thin section in order to cover 1/8 of the whole section area, which normally included 200 grains. Grain size was determined by point counts of 100 grains per thin section. In order to measure porosity and permeability, samples were cut from one side of thin section plugs and were over-dried to a constant weight (±1 mg). Porosity and permeability were determined in Chengdu Petroleum Engineering Laboratory under ambient condition with room temperature and pressure. Porosity was measured using an UltraPore 300 helium porosimeter; permeability was measured on an UltraPerm 400 gas permeameter. Scanning electron microscopy was employed to characterize authigenic clays, intergranular cements and micropore structure. Freshly broken rock fragments for each of the samples were goldcoated and identified using Quanta2000 SEM combined with an EDAX energy dispersive spectrometer. Samples were examined at magnifications ranging from 100
to 30,000
.

4. Results 4.1. Sandstone composition Most Xujiahe sandstones are texturally immature with fine-to medium-grain size and poor to moderate sorting. Matrix is abundant, with a maximum content of 40% and averaging 7.1%. The distribution of detrital grains is quite variable with the average framework composition of Q60.4F5.5R34.1. Quartz grains are mostly monocrystalline, and the content ranges from 12% to 89%. Feldspar is limited, with the average value of 5.7%. Most of the feldspars are K-feldspars, and plagioclase is rare, with contents less than 3%. Rock fragments are common and usually constitute approximately 10%e 50% of the whole rock volume; the fragments consist predominantly of sedimentary and low-grade metamorphic rocks (Fig. 4A). The analysis of detrital composition shows that the Xujiahe sandstones are predominantly litharenite and feldspathic litharenite with a minor amount of sublitharenite and quartzarenite (Fig. 4A). In addition, based on the content of carbonate rock fragments (Folk, 1974), litharenite can be subdivided into ordinary litharenite and calcarenaceous sandstone (carbonate rock fragments of greater than 50% of the whole rock volume). Each of these five lithologies has significant differences in textural and detrital composition. In litharenite, feldspathic litharenite and sublitharenite, the grain sizes are mostly medium-grained, while most of quartzarenite and calcarenaceous sandstone are very fine to fine-grained (Table 2). The sorting of the litharenite, feldspathic litharenite and sublitharenite are mainly moderate, with an average matrix content of 7.5%, 4.3% and 8.4%, respectively, whereas in quartzarenite and calcarenaceous sandstone are well sorted, and the content of matrix are <1% (Table 2, Fig. 5C). Based on different effects from diagenetic processes, rock fragments can be further divided into three categories: carbonate rock fragments, ductile rock fragments and rigid rock fragments. The

1324

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Table 1 Detailed information of the samples. Well

Depth (m)

Member

Number of thin sections

B10 B102 B104

4924.4e4931.3 4716.8e4721.5 4591.3e4597.3 4267.1e4271.1 4174.5e4176.4 4544.5e4551.6 4621.0e4646.1 4689.3e4696.8 4270.0e4403.2 4086.7e4104.9 4776.0e4777.6 4772.0e4780.0 4440.9e4685.2 4266.0e4302.2 4458.0e4474.7 4526.5e4539.5 4539.5e4547.1 4482.2e4918.6 4436.7e4460.5 4458.0e4499.0 3515.3e3520.1 3312.3e3314.9 4074.5e4081.6

T3x2 T3x2 T3x2 T3x4 T3x4 T3x4 T3x4 T3x2 T3x2 T3x3 T3x2 T3x2 T3x2 T3x3 T3x4 T3x2 T3x4 T3x2 T3x3 T3x2 T3x2 T3x3 T3x2

2 4 20 9 4 15 58 33 138 18 5 8 43 7 99 15 23 91 36 241 22 7 29 927

B122 B123 B16 B222 B271 B4 B5 B6 L1 L10 L17 L2 L4 L6 L7 L8 Total

Number of properties

Number of SEMs

3 18 8 4 15 53 33 105 17 5

1 3 2 1 2

12 5 51 13 23 24 19 102 3

4 1 1 1 3 1 2 4 2 4 4 3 2

12 525

1 42

Fig. 4. Ternary diagram illustrating (A) the composition of the Xujiahe sandstones in the Yuanba area (1. Quartzarenite; 2. Subarkose; 3. Sublitharenite; 4. Arkose; 5. Lithic Arkose; 6. Feldspathic Litharenite; and 7. Litharenite); (B) the rock fragments composition of the Xujiahe sandstones in the Yuanba area (after Folk, 1974); RF ¼ rock fragments, SRF ¼ sedimentary rock fragments, MRF ¼ metamorphic rock fragment, VRF ¼ volcanic rock fragment.

Table 2 Petrographic composition and textural data of the Xujiahe sandstone, Yuanba area. Lithology

Litharenite Feldspathic Litharenite Sublitharenite Quartzarenite Calcarenaceous Sandstone Total

Texture

Quartz (%)

Ductile RF (%)

Rigid RF (%)

Carbonate RF (%)

Matrix (%)

Size

Sorting

Min

Max

Mean

Feldspar (%) Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

Min

Max

Mean

M M M VF-F F F-M

M M M W W M

17 17 49 74 12 12

71 69 84 89 42 89

53.4 42.8 62.6 83.1 22.4 51.5

0 8 0 0 0 0

16 21 11 0.5 4 21

4.4 12.4 3.3 0.1 0.4 5.7

0 4 2 0 0 0

40 27 14 1.5 8 40

11.8 12.3 7.2 0.5 0.9 10.6

0 2 3 3 0 0

20 17 17 8 18 35

9.5 8.5 11 4 2 9.2

0 0 0 0 50 0

48 20 6 1.5 77 77

6 3.9 0.4 0.5 57.3 7.2

0 0 5 0 0 0

40 22 16 3 4 43

7.5 4.3 8.4 0.4 0.1 7.1

Size: M ¼ medium-grained size, VF ¼ very fine-grained size, F ¼ fine-grained size. Sorting: M ¼ moderate sorting; W ¼ well sorting.

contents of these three types vary significantly in different lithologies (Fig. 4B). In litharenite and feldspathic litharenite, both carbonate and ductile rock fragments are well developed. The average content of ductile and carbonate rock fragments in litharenite is

11.8% and 6%, respectively, while these values in feldspathic litharenite are 12.3% and 3.9% (Table 2, Fig. 5AeB). However, in sublitharenite, the carbonate rock fragments are rare, and the rigid rock and ductile rock fragments dominate the entire rock volume

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

1325

Fig. 5. Detrital components, interstitial materials and thin section porosity of the Xujiahe sandstones; L. ¼ Litharenite, F.L. ¼ Feldspathic Litharenite, SubL. ¼ Sublitharenite, Q. ¼ Quartzarenite, C.S. ¼ Calcarenaceous Sandstone.

(Fig. 4B). Calcarenaceous sandstone is dominated by carbonate rock fragments with maximum and average values of 77% and 57.3%, respectively (Table 2, Fig. 5B). The rock fragments are limited in quartzarenite and are mainly composed of rigid rocks (Fig. 4B). 4.2. Diagenesis The main diagenetic processes identified in Xujiahe sandstone include mechanical and chemical compaction and the precipitation of calcite, quartz and chlorite. Mechanical and chemical compaction is widely developed. The type and content of the authigenic minerals are significantly distinct in various lithologies. 4.2.1. Litharenite and feldspathic litharenite Compaction is the most important diagenetic process in

litharenite and feldspathic litharenite. Due to the high compaction, four events can be observed: (1) detrital grains present as concaveconvex contact and stylolite (Fig. 6AeB); (2) mica bends and even cracks (Fig. 6C); (3) the matrix deforms as an irregular shape filling the intergranular pores (Fig. 6B); and (4) sand-sized argillaceous clasts deform into a clay pseudomatrix (Fig. 6D). Because of the high compaction and the great volume of matrix, the content of the authigenic minerals in litharenite and feldspathic litharenite is small, usually constituting 0e10% and averaging 6.6%. The major authigenic components in litharenite and feldspathic litharenite are calcite, with a content ranging from 1% to 36% (Fig. 5E). Two generations of calcite cements are recognized: poikilotopic calcite and pore-filling calcite. Poikilotopic calcite, which occupies a great volume of primary pores and contacts directly with detrital grains, is formed early in the diagenetic sequence (Fig. 6E).

1326

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Fig. 6. Thin section and SEM images showing diagenetic characteristics of litharenite and feldspathic litharenite in the Xujiahe Formation, Yuanba area. CaC ¼ Calcite cement, QzO ¼ Quartz-overgrowth, K-F ¼ Potassium feldspar, P ¼ Plagioclase. (A) a great volume of matrix occupy intergranular pores and detrital grains present as line contact or concaveconvex contact [litharenite, 4629.18 m, T3x4, Well B16]. (B) The contact of detrital quartzes present as stylolite contact and clay matrix deformed as irregular shape [litharenite, 4390.23 m, T3x2, Well B271]. (C) The mica deformed and even cracked through the highly compaction [litharenite, 4485.98 m, T3x2, Well L6]. (D) Mudstone has deformed into irregular shape, and the deformation degree depends on the compaction of the surrounding grains [litharenite, 4524.83 m, T3x2, Well B6]. (E) Poikilotopic calcite fills pore spaces and replaces aluminosilicate grains [litharenite, 4460.82 m, T3x2, Well L6]. (F) Pore-filling calcite cement overlies the stage-II quartz overgrowth [feldspathic litharenite, 4627.00 m, T3x4, Well B16]. (G) Stage-I quartz overgrowth underlie the chlorite rim [feldspathic litharenite, 4474.84 m, T3x2, Well L6]. (H) SEM images of chlorite rim and pore-filling quartz. Chlorite occurs as rim around framework grains and pore-filling quartz overlie chlorite rim [litharenite, 4464.80 m, T3x2, Well L6]. (I) Pore-lining illite and pore-bridge illite overlie the chlorite rim (blue arrow), but underlie the pore-filling quartz [feldspathic litharenite, 4487.13 m, T3x2, Well L6]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

However, pore-filling calcite often grows over chlorite rim and quartz overgrowth and thus formed later in the diagenetic sequence (Fig. 6F). Other authigenic minerals include quartz, chlorite, illite and dolomite. The distribution of the authigenic chlorite is variable, and the content ranges from 0 to 3.5% (Fig. 5D). It generally occurs as a rim around detrital grains and is absent from the line contact portions of the grain surfaces (Fig. 6GeH). Before the generation of chlorite rims, the stage-I quartz overgrowth has already formed around part of the quartz grain (Fig. 6G). Quartz cement is rare, varying from 0 to 10% (Fig. 5F). It commonly occurs as an overgrowth on detrital quartz (Fig. 6FeG) or as a pore-filling cement overlying grain-coat chlorite (Fig. 6H). Furthermore, the quartz overgrowth can be subdivided into two stages. Stage-I quartz overgrowth, with a thickness commonly less than 2 mm, is precipitated earlier than grain-coat chlorite, and only a minor quartz grain can be observed to develop the imperfect Stage-I quartz overgrowth (Fig. 6G). Stage-II quartz overgrowth, with a thickness ranging from 2 mm to 50 mm, usually underlies the pore-filling calcite (Fig. 6F) and was probably formed concurrent with pore-filling quartz cements. Authigenic illite and dolomite are minor cements in litharenite

and feldspathic litharenite. Though volumetrically unimportant, authigenic illite, which is represented as pore lining and pore bridge (Fig. 6I), usually underlies the pore-filling quartz and effectively occludes the feldspar dissolution pore or pore throat on local scales. 4.2.2. Sublitharenite The main diagenetic processes that have affected the quality of sublitharenite include mechanical and chemical compaction, as well as the precipitation of quartz and calcite. Compaction is the most important process leading the sublitharenite to induration, as indicated by the long to concave-convex contacts of rigid grains such as quartz and quartzose lithic grains (Fig. 7AeB). The content of the authigenic minerals in sublitharenite is also rare, usually constituting 0e8% and averaging 3.25%. Varying from 0% to 5% and averaging 0.74% (Fig. 5F), quartz cement is not widespread and occurs as overgrowth on detrital quartz. Two stages of quartz overgrowth can be easily identified from the existence of dust rims (Fig. 7B). Calcite cement varies in abundance from 0 to 12% and averages 2.27% (Fig. 5E). Calcite cement grows over and thus postdates quartz cement and tends to fill the residual intergranular pores (Fig. 7A).

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

1327

Fig. 7. Thin section micrographs showing diagenetic characteristics of the sublitharenite, quartzarenite and calcarenaceous sandstone in the Xujiahe Formation, Yuanba area. Ca RF ¼ Calcite rock fragment. (A) Pore-filling calcite cement overlies the stage-II quartz overgrowth [sublitharenite, 4459.00 m, T3x4, Well L1]. (B) Before authigenic quartz precipitated, the detrital grains have already presented as line contact [sublitharenite, 4626.00 m, T3x4, Well B16]. (C) Stage-II quartz overgrowth and pore-filling calcite cement overlies the stage-II quartz overgrowth [quartzarenite, 4560.00 m, T3x2, Well L4]. (D) Sparry calcite cements interlock the micritic carbonate rock fragments [calcarenaceous sandstone, 4344.72 m, T3x3, Well L1].

4.2.3. Quartzarenite Mechanical compaction is also the most important diagenetic processes in quartzarenite, and the grains mostly present as long contact or concavo-convex contact (Fig. 7C). Authigenic minerals are dominated by quartz, with minor amounts of calcite (Fig. 5). Quartz cement as overgrowth is widespread but volumetrically limited, usually constituting 4%e12% of the whole rock volume and averaging 7.5% (Fig. 5F). The content of calcite cement is very small, varying from 0 to 7% and averaging 0.34%. It commonly grows over quartz cement and occurs as a patchy to fill residual intergranular pores (Fig. 7C). 4.2.4. Calcarenaceous sandstone Calcite cement and mechanical compaction are the main diagenetic processes identified in the calcarenaceous sandstone. Calcite cement is the most important diagenetic processes, with a maximum content of 27% and averaging 20.1% (Fig. 5E). It usually occurs as microsparry or sparry and fills large intergranular pores prior to being highly compacted (Fig. 7D). Detrital grains are commonly present as point contact (Fig. 5E), indicating the poor mechanical compaction in calcarenaceous sandstone. 4.3. Pore types In litharenite, feldspathic litharenite and sublitharenite, the intergranular volume is largely filled with a detrital clay matrix, and the porosity is dominated by microporosity with a diameter less

than 2 mm (Fig. 8AeB). The residual primary pores (Figs. 5G and 8C) and secondary pores (Figs. 5H, 8 EeF) can only be observed in a few samples, mainly feldspathic litharenite and litharenite. The residual primary pores, varying from 30 mm to 100 mm in diameter, show an intimate association with chlorite rims (Fig. 8C). Secondary pores can be subdivided into intragranular and intergranular types. Due to the poor dissolution, intragranular secondary pores often retain the morphology of the pre-existing grain (Fig. 8D and F), while intergranular secondary pores mainly occur at the expense of unstable lithic and feldspar grains (Fig. 8EeF). However, in quartzarenite and calcarenaceous sandstone, the intergranular volume was entirely occluded by authigenic cements (quartz or calcite), and no visual pores can be observed even by SEM (Fig. 8GeI).

4.4. Porosity and permeability Based on the 525 core measured rock property data, the values of the core-porosity vary from 0.48% to 15.6%, and over 80% of samples fall in the zone (peak zone) between 1% and 4% (Fig. 9). Specifically, porosity values of different lithologies are obviously distinct. The values in feldspathic litharenite are the largest, and the majority of values range from 4.2% to 7.3%, with an average value of 5.8%, followed by litharenite and sublitharenite, with average values of 3.5% and 4.3%, respectively. Porosities from quartzarenite and calcarenaceous sandstone show the smallest values, with average values of 2.7% and 1.1%, respectively. The values of core measured permeability (include fracture

1328

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Fig. 8. Thin section and SEM images showing the pore types of the Xujiahe sandstones, Yuanba area. Qz ¼ Quartz. (A) Intercrystalline pores of the I/S mixed layer clay (blue arrows) [feldspathic litharenite, 4458.70 m, T3x2, Well L6]. (B) Intercrystalline pores of the authigenic chlorite (blue arrows) [litharenite, 4464.80 m, T3x2, Well L6]. (C) residual primary pores (black arrows, thin section impregnated with blue epoxy) [litharenite, 4464.44 m, T3x2, Well L6]. (D) Feldspathic dissolution pore [feldspathic litharenite, 4074.64 m, T3x2, Well L8]. (E) Lithic dissolution pores (thin section impregnated with red epoxy) [litharenite, 4464.15 m, T3x2, Well L6]. (F) Lithic dissolution pore and Feldspathic dissolution pore (thin section impregnated with blue epoxy) [sublitharenite, 4465.10 m, T3x4, Well L1]. (G) No visual pores even in scanning electron microscopic image [calcarenaceous sandstone, 4456.00 m, T3x3, Well L4]. (H) No obviously pores in thin section image (thin section impregnated with blue epoxy) [quartzarenite, 4560.00 m, T3x2, Well L4]. (I) Authigenic quartz and clay minerals fill the intergranular pore [quartzarenite, 4309.20 m, T3x2, Well B271]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

permeability) range from 0.001 md to 16.48 md, and most values are between 0.01 md and 0.1 md (Fig. 9). Similar to porosities, the values of permeability are also various between different lithologies (Fig. 10). The crossplot of porosity and permeability for samples of various lithologies shows that the permeability from feldspathic litharenite and litharenite are the highest values, whereas permeability from quartzarenite and calcarenaceous sandstones shows the lowest values. 5. Discussion 5.1. Relative timing of diagenetic process Diagenetic processes that occur in the early stages of a burial cycle are collectively known as ‘‘eodiagenesis’’, while processes that occur after eodiagenesis are collectively called ‘‘mesodiagenesis’’. Eodiagenesis is defined as sediments being buried to less than approximately 2000 m, which is usually equivalent to less than approximately 70  C (Morad et al., 2000). In this study, the boundary between eodiagenesis and mesodiagenesis is considered to be the vitrinite reflectance (Ro) equivalent to 0.5% according to the National Standards of Oil and Gas Industry in China (SY/T54772003). The vitrinite reflectance value of the Xujiahe Formation is approximately 2.0%, suggesting that the diagenetic processes have

already reached the mesodiagenetic stage. The relative timing of the major diagenetic sequences of various lithologies is determined based on the textural relationship that was observed by thin sections and SEM (Figs. 6, 7 and 11). The interpretation of the relative timing of these events is presented in Fig. 11. Due to the difference in diagenetic processes between sandstones with different composition, the diagenetic evaluation sequences of different lithologies are separately discussed in this study. Evaluation of petrographic observations indicates that litharenite and feldspathic litharenite experienced mechanical compaction, stage-I quartz overgrowths, grain-coating chlorite and crystalline calcite cementation in the eodiagenetic stage and dissolution, chemical compaction, stage-II quartz overgrowths, and calcite pore-filling cementation in the mesodiagenetic stage (Fig. 6). Chlorite rims provide a useful marker for distinguishing early cements that inhibit and therefore predate the chlorite coatings from later cements that engulf and therefore postdate chlorite rims. Worden and Morad (2003) considered chlorite to have a number of origins but to typically develop at temperatures greater than approximately 60e70  C. The observed presence of abundant chlorites surrounding grain dissolution pores suggests that chlorite precipitation occurs prior to grain dissolution. A previous study of Surdam et al. (1989) showed that the primary temperatures for organic acid generation are approximately 80e120  C in burial environments. Thus, it is speculated that the temperatures

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

1329

in Fig. 11B and C, respectively. The diagenetic processes under the microscope from calcarenaceous sandstone are dominated by mechanical-chemical compaction and sparry poikilotopic calcite cementation (Fig. 7D). Based on the study of fluid inclusions in sparry calcite cements from Xujiahe sandstones in the Sichuan Basin, Zhu et al. (2008) and Liu et al. (2014) found that only limited inclusions could be observed, and almost all inclusions are single-phase. They then concluded that the calcite cement precipitated relatively earlier, likely in the eodiagenesis stage. In addition, the detrital grains are mainly present as point contacts, which also suggest that the sparry calcite cements formed before a highly mechanical compaction. In addition, calcite cements, which occupy a great volume of the pores, would prevent the formation of intensive compaction. Although the mechanical compaction is poor, the morphology at the calcarenaceous grain-grain interface is obscured, suggesting the participation of chemical compaction in the eodiagenesis stage. Thus, it can be speculated that calcarenaceous sandstone experienced a mechanical-chemical compaction and calcite cementation during the eodiagenesis stage and no more diagenetic phenomena appear in the mesodiagenesis stage. The diagenetic evolution of sequences of calcarenaceous sandstone is shown in Fig. 11D. 5.2. Factors controlling diagenetic processes

Fig. 9. Porosity and permeability of the Xujiahe sandstones, Yuanba area; peak zone means that over 80% of samples fall in this zone.

Fig. 10. Cross plot of porosity and permeability for samples of various lithologies.

of chlorite generation are approximately 60e80  C and that the corresponding Ro% is close to 0.5%, indicative of the end of the early diagenetic stage. Based on the timing of the development of chlorite rims and the contact relationship between detrital grains and diagenetic minerals, the diagenetic paragenesis of litharenite and feldspathic litharenite are obtained (Fig. 11A). During the eodiagenesis stage, the main diagenetic processes that sublitharenite experienced include mechanical compaction and stage-I quartz overgrowths, followed by chemical compaction, dissolution, stage-II quartz overgrowths, calcium pore-filling cementation in the mesodiagenesis stage. The diagenetic sequence of quartzarenite in T3x is similar to that of sublitharenite, but dissolution is not common in quartzarenite due to the lack of soluble minerals, such as aluminosilicate minerals. The specific diagenetic sequences of sublitharenite and quartzarenite are shown

In burial diagenetic environments, diagenetic reactions must then be geochemically balanced, and primary texture and mineral composition will control on diagenesis and rock properties (Taylor et al., 2010; Bjørlykke and Jahren, 2012; Bjørlykke, 2014). The similarity of grain sorting and size of certain sandstone in T3x minimizes the influence of textural variation on the differences of diagenetic processes (Fig. 12A) and rock properties (Fig. 12B), so that the mineral composition should be a key factor that controls the diagenetic processes. The importance of variation in detrital mineralogy as a control on diagenesis and reservoir quality was also reported by Reed et al. (2005) and Dutton et al. (2012). Litharenite and feldspathic litharenite, according to the mineralogical composition, can be classified into the following three types: (1) sandstone containing a large volume of matrix and ductile rock fragments; (2) sandstone containing a high content of carbonate rock fragments; and (3) sandstone with minor contents of matrix, ductile rock fragments and carbonate rock fragments. When the sandstone contains a great volume of matrix and ductile rock fragments, both cements and pores are poorly developed (Fig. 13), and the porosity will be decreased predominately as a result of mechanical compaction. This phenomenon has also been observed by many researchers, where soft lithic grains that usually develop as a pseudomatrix by crushing at certain depths may significantly affect the sandstone porosity and permeability, especially where argillaceous clasts are abundant (Imam and Shaw, 1985; Pittman and Larese, 1991; Worden et al., 2000; Aminul Islam, 2009). Worden et al. (2000) concluded that ductile-lithic sand grains allowed the rock to undergo plastic deformation during burial and compression, thus reducing the porosity at a greater rate than by the compression of rigid quartzose grains. In addition, matrix that fills the intergranular volume easily blocks the pores and pore throats during compaction. Therefore, sandstones with high contents of matrix and ductile rock fragments tend to possess low porosity and permeability due to intense compaction. When litharenite and feldspathic litharenite contain a great volume of carbonate rock fragments, the content of the calcite cement increased obviously, accounting for the very low thin section porosities (Fig. 13). Calcite cementation in the sandstones is a function of the detrital composition of rocks and is enriched where the carbonate rock fragments are abundant in the study area

1330

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Fig. 11. Diagenetic sequence for (A) litharenite and feldspathic litharenite; (B) sublitharenite; (C) quartzarenite; and (D) calcarenaceous sandstone.

Fig. 12. Cross plot of cements content and core measured porosity versus average grain size.

(Fig. 14). Bjørkum and Walderhaug (1990) concluded that calcite precipitation is controlled by the degree of supersaturation of pore water, and the critical degree of supersaturation necessary for nucleation of calcite would be first achieved in layers rich in

carbonate rock fragments. Abundant calcite cements with favourable carbonate nucleation substrate was also reported by Wilkinson (1991). This phenomenon can also reasonably account for the observation that calcite cements are abundant in calcarenaceous

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

1331

Fig. 13. The Vertical distribution of the contents of matrix, ductile rock fragments, carbonate rock fragments, chlorite rims and thin section porosity in the litharenite and feldspathic litharenite; RF ¼ rock fragment.

Fig. 14. Cross plot of carbonate rock fragments and calcite cements for samples of various lithologies.

sandstone but absent in quartzarenite and sublitharenite in the Yuanba area. In addition, the porosities of litharenite and feldspathic litharenite show a significant decrease when the contents of calcite cements are greater than 10% (Fig. 15). Calcite cements, which create barriers to fluid flow, reduce the porosity and permeability significantly (Anjos et al., 2000; Dutton, 2008). Therefore, for those litharenite, feldspathic litharenite and calcarenaceous sandstones rich in carbonate rock fragments, the intense calcite cementation accounts for their poor reservoir quality. In the sandstones with minor matrix, ductile rock fragments and carbonate rock fragments, the chlorite rims were commonly present, and the porosity is relatively higher than in other sandstones (Fig. 13). Numerous studies have discussed the relationships between chlorite and porosity preservation, and concluded that

continuous chlorite rims can inhibit extensive quartz cementation and preserve the primary pores with an optimal thickness between 5 and 10 mm (Heald and Anderegg, 1960; Pittman and Lumsden, 1968; Heald and Larese, 1974; Taylor, 1978; Thomson, 1982; Pittman, 1992; Ehrenberg, 1993; Ryan and Reynolds, 1996; Anjos et al., 2003; Huang et al., 2004). However, in the study area, even though the continuous chlorite rims reached a thickness of 5e10 mm, there is still a great volume of quartz cements in the pore space, engulfing the chlorite rims (Figs. 6H and 8C). Previous studies have generally reported on areas with burial depths of less than 5000 m (Ehrenberg, 1993; Anjos et al., 2003), and only a few studies have focused on the sequence with burial depths over 5000 m. The Tuscaloosa sandstones in Louisiana have maximum burial depths over 6000 m (Thomson, 1982; Weedman et al., 1996); however, the diagenetic evolution is relatively low, and the maximum geotemperature reaches merely 170  C (Weedman et al., 1996). For Xujiahe sandstones, chlorite rims can retard the precipitation of quartz cements to some extent but fail to completely inhibit quartz cementation, especially in a much deeper burial depth/higher diagenetic stage. Even though, sandstones with chlorite rims still exhibit relatively higher porosities (Fig. 13). An alternative explanation proposed by other researchers is that the relatively high quality of sandstones with abundant chlorites is mainly associated with high textural and compositional maturity, which, to some extent, reflects the environmental characteristics (Baker et al., 2000; Yao et al., 2011). As documented by Baker et al. (2000) and Yao et al. (2011), if the sandstones in the study area contain a great volume of matrix, chlorite rims will not form, even though the volcanic rock fragments, which are a source for chlorite precipitation, are abundant (Fig. 16) (Anjos et al., 2003; Berger et al., 2009; Yao et al., 2011; Sun et al., 2012; Bahlis and De Ros, 2013). Thus, sandstones that contain chlorite rims commonly exhibit higher porosities can be attributed to two aspects: (1) chlorite rims can retard the precipitation of quartz cements, and (2) chlorites commonly precipitate in sandstones with high textural and compositional maturity.

1332

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

Fig. 15. Cross plot of porosity and calcite cements for samples of litharenite and feldspathic litharenite.

Fig. 16. The contents of the volcanic rock fragments versus the contents of the chlorite rims split by the quantity of matrix.

In sublitharenite and quartzarenite, the content of cements is low (Fig. 5CeF) and compaction also dominates the porosity reduction. From the preceding discussions, compaction is more intense in those sandstones that contain a large amount of ductile rock fragments. Although the content of ductile grains slightly decreases in sublitharenite, the matrix still occupies a great volume of the pore space (Fig. 5A and C). The matrix is often observed to be uniformly distributed in the sandstone, filling in the intergranular pore spaces, whereas ductile rock fragments are grains and exists heterogeneously between mineral grains. According to the distribution pattern, more primary pores and pore throats are blocked by matrix compared to ductile rock fragments under compaction. Thus, the content of the matrix is the ultimate controlling factor for porosity in sublitharenite. In quartzarenite, the volume of matrix and ductile rock fragments is relatively low. The high compaction may be associated the finer grain size. The grain size of the quartzarenite in Xujiahe sandstones is almost very fine to fine-grained (Table 2). For quartzose sandstone, finer-grained samples experienced more intense intergranular pressure solution than coarser-grained samples (Porter and James, 1986; Houseknecht and Hathon, 1987; Houseknecht, 1988). However, compared with other fine-grained sandstones, the cementation content in quartzarenite is lower (Fig. 12A), suggesting that the mineral composition still exerts a primary control on the diagenetic processes. 5.3. Fundamental control on reservoir quality 5.3.1. Diagenetic control on reservoir quality Diagenesis is commonly invoked to explain the variation in

reservoir quality. Mechanical compaction and calcite cementation are the two dominant diagenetic processes controlling the evolution of porosity in Xujiahe sandstones. Porosity reduction by compaction and cementation can be evaluated through the methodology of Lundegard (1992); the plot of porosity reduction indicates that the pores in litharenite, feldspathic litharenite, sublitharenite and quartzarenite are mainly destroyed by compaction, whereas in calcarenaceous sandstone, the porosity is reduced predominantly by cementation (Fig. 17). Mechanical compaction is dominant in siliceous sediments to a burial depth below 2 km but has a limited influence above 3 km (Selley, 1978; Zheng and Wu, 1996; Bjørlykke, 1999; Paxton et al., 2002). Therefore, the destruction of most pore volumes in litharenite, feldspathic litharenite, sublitharenite and quartzarenite in the early stage of the diagenetic process (eodiagenesis) is attributed to compaction. Although porosity reduction of calcarenaceous sandstone is mainly controlled by calcite cementation, most of the pore volume had been destroyed during the stage of eodiagenesis based on the inferred diagenetic sequence (Fig. 11D). Surdam et al. (1989) considered that if early carbonate cements, compaction or other events extensively occlude the pore volume in sandstone, it is difficult, and in some cases impossible, to subsequently enhance porosity with later diagenetic processes. For the Xujiahe sandstones, a great volume of the pore spaces had been destroyed by the early carbonate cementation or mechanical compaction. In addition, the limited contribution of burial dissolution, together with further chemical compaction and burial cementation, resulted in high densification of sandstones. However, based on the core measured rock property data, there are still some favourable reservoirs in litharenite and feldspathic litharenite (Figs. 9 and 10). These favourable reservoirs usually show an intimate association with chlorite rims (Figs. 8C and 13). When sandstones contain chlorite rims over 1%, porosity of the sandstones usually better than others (Fig. 13). 5.3.2. Compositional control on reservoir quality As mineral compositions exert a primary control on the diagenetic processes, the rock properties of the Xujiahe sandstones should be fundamentally controlled by the mineral composition of the sediments. From the discussion above, high compaction and calcite cementation in the early diagenetic stage are the two dominant reasons for the ultra-low porosity and permeability, which are fundamentally controlled by the contents of matrix, DRFs and CRFs. When sandstones contain a great volume of matrix and DRFs, compaction dominates the porosity reduction and the quality of the sandstones is poor. When sandstones contain a large volume of CRFs, the content of the calcite cement increases obviously and the thin section pores totally absent. Figs. 14 and 15 show a significant decrease of porosities when the contents of CRFs greater than 15%. Only sandstones with minor CRFs (<15%), DRFs, VRFs

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

1333

Fig. 17. Diagrams for evaluating the importance of compactional processes and cementation on porosity decrease in the Xujiahe sandstone, Yuanba area. (A) Litharenite, feldspathic litharenite and sublitharenite with 35% original porosity. (B) Quartzarenite and calcarenaceous sandstone with 40% original porosity.

(2%e6%) and matrix (<5%) commonly possess chlorite rims and the porosity is relatively higher than in other sandstones (the content of each compositions is determined from Figs. 13e16). 6. Conclusions Based on the above systematic studies for T3x in Yuanba area, the following conclusions can be drawn: (1) Xujiahe sandstones have five lithologies, namely, litharenite, feldspathic litharenite, sublitharenite, quartzarenite and calcarenaceous sandstone, and each lithology has distinct diagenetic processes. A comparison of the relative importance of compaction and cementation on pore reduction indicates that litharenite, feldspathic litharenite, sublitharenite and quartzarenite are mainly affected by compaction, whereas calcarenaceous sandstone is mainly controlled by cementation. The types of authigenic minerals vary in different lithologies. In litharenite and feldspathic litharenite, the authigenic minerals mainly include stage-I quartz overgrowths, grain-coating chlorite, crystalline calcite cementation in eodiagenesis and stage-II quartz overgrowths, pore-filling quartz cementation and calcite cementation in mesodiagenesis. However, sublitharenite and quartzarenite mainly contain stage-I quartz overgrowth in eodiagenesis and stage-II quartz overgrowth in mesodiagenesis, and calcarenaceous sandstone only have crystalline calcite cementation in eodiagenesis. (2) The anomalously low porosity in Xujiahe sandstones is attributed to the high compaction and carbonate cementation in eodiagenesis, which are fundamentally controlled by the mineral composition of the sediments. Sandstones with high contents of matrix, ductile rock fragments and carbonate rock fragments, are more likely to be highly compacted and cemented, resulting in a significant decrease in pore spaces. Only litharenite and feldspathic litharenite with minor CRFs, DRFs, VRFs and matrix are potential effective hydrocarbon reservoirs. In that case, the favourable hydrocarbon reservoir prediction can be simplified and it merely needs to analyze the provenance and sedimentary facies of Xujiahe formation, which is much easier. Acknowledgements This study was supported by the National Natural Science

Foundation of China (No. 41472118). We acknowledge the collaboration and support provided by Tonglou Guo, Xiaoyue Fu, Renchun Huang and Jinbao Duan from the SINOPEC Exploration Company. We appreciate the enthusiastic support of Qinhong Hu (The University of Texas at Arlington, Arlington, TX) for providing language help. Our special thanks are extended to Editor-in-Chief David A. Wood, as well as four anonymous reviewers, for the critical and constructive comments.

References Aminul Islam, M., 2009. Diagenesis and reservoir quality of Bhuban sandstones (Neogene), Titas Gas Field, Bengal, Bangladesh. J. Asian Earth Sci. 35, 89e100. Anjos, S.M.C., De Ros, L.F., Silva, C.M.A., 2003. Chlorite authigenesis and porosity preservation in the Upper Cretaceous marine sandstones of the Santos Basin, offshore eastern Brazil. In: Worden, R.H., Morad, S. (Eds.), Clay mineral Cements in Sandstones, vol. 34. International Association of Sedimentologists Special Publication, pp. 291e316. Anjos, S.M.C., De Ros, L.F., Souza, R.S., Silva, C.M.A., Sombra, C.L., 2000. Depositional ^ncia and diagenetic controls on the reservoir quality of Lower Cretaceous Pende sandstones, Potiguar rift basin, Brazil. AAPG Bull. 84, 1719e1742. Bahlis, A.B., De Ros, L.F., 2013. Origin and impact of authigenic chlorite in the Upper Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil. Pet. Geosci. 19, 185e199. Baker, J.C., Havord, P.J., Martin, K.R., Ghori, K.A.R., 2000. Diagenesis and petrophysics of the early Permian Moogooloo sandstone, southern Carnarvon Basin, western Australia. AAPG Bull. 84, 250e265. Berger, A., Gier, S., Krois, P., 2009. Porosity-preserving chlorite cements in shallowmarine volcaniclastic sandstones: evidence from Cretaceous sandstones of the Sawan gas field, Pakistan. AAPG Bull. 93, 595e615. Bjørlykke, K., 1999. An overview of factors controlling rates of compaction, fluid generation and fluid flow in sedimentary basins. In: Jamtveit, B., Meakin, P. (Eds.), Growth Dissolutions and Pattern Formation in Geosystems, pp. 381e404. The Netherlands. Bjørlykke, K., 2014. Relationships between depositional environments, burial history and rock properties. Some principal aspects of diagenetic process in sedimentary basins. Sediment. Geol. 301, 1e14. Bjørlykke, K., Jahren, J., 2012. Open or closed geochemical systems during diagenesis in sedimentary basin: constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs. AAPG Bull. 96, 2193e2214. Bjørkum, P.A., Walderhaug, O., 1990. Geometrical arrangement of calcite cementation within shallow marine sandstones. Earth-Science Rev. 29, 145e161. Cai, X., 2010. Gas accumulation patterns and key exploration techniques of deep gas reservoirs in tight sandstone: an example from gas exploration in the Xujiahe Formation of the western Sichuan Depression, the Sichuan Basin (in Chinese with English abstract). Oil Gas Geol. 31, 707e714. Chen, G., Lin, L., Wang, W., Chen, H., Guo, T., 2014. Diagenesis and porosity evolution of sandstones in Xujiahe Formation, Yuanba area (in Chinese with English abstract). Pet. Geol. Exp. 36, 405e415. Chen, B., Shen, J., Hao, J., Du, W., 2012a. Characteristics of quartz sandstones and its reservoir significance of Xujiahe Formation in Yuanba area, northeastern Sichuan Basin (in Chinese with English abstract). Acta Sedimentol. Sin. 30, 92e100. Chen, D., Pang, X., Yang, K., Yang, Y., Ye, J., 2012b. Porosity evolution of tight gas sand

1334

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335

of the second member of Xujiahe Formation of Upper Triassic, western Sichuan depression (in Chinese with English abstract). J. Jilin Univ. (Earth Sci. Ed. 42, 42e51. Dai, C., Zheng, R., Zhu, R., Li, F., Gao, Z., 2011. Diagenesis and diagenetic facies of Xujiahe Formation in the central west of Sichuan foreland basin (in Chinese with English abstract). J. Chengdu Univ. Technol. Sci. Technol. Ed. 38, 211e219. Dai, J., Ni, Y., Hu, G., Huang, S., Liao, F., Yu, C., Gong, D., Wu, W., 2014. Stable carbon and hydrogen isotopes of gases from the large tight gas fields in China. Sci. China Earth Sci. 57, 88e103. Dai, J., Song, Y., Zhang, H., 1997. The main factors controlling the foundation of medium-giant gas fields in China. Sci. ChinaSer.s D) 40, 1e10. Du, Y., Ji, H., Zhu, X., 2006. Research on the diagenetic facies of the Upper Triassic Xujiahe Formation in the western Sichuan foreland basin (in Chinese with English abstract). J. Jilin Univ. (Earth Sci. Ed. 36, 358e364. Dutton, S.P., 2008. Calcite cement in Permian deep-water sandstones, Delaware Basin, west Texas: origin, distribution, and effect on reservoir properties. AAPG Bull. 92, 765e787. Dutton, S.P., Loucks, R.G., Day-Stirrat, R.J., 2012. Impact of regional variation in detrital mineral composition on reservoir quality in deep to ultradeep lower Miocene sandstones, western Gulf of Mexico. Mar. Pet. Geol. 35, 139e153. Ehrenberg, S.N., 1993. Preservation of anomalously high porosity in deeply buried sandstones by grain-coating chlorite: examples from the Norwegian Continental Shelf. AAPG Bull. 77, 1260e1286. Feng, M., Liu, J., Meng, W., Wang, J., Zhang, X., Feng, Y., 2009. Characteristics and major controlling factors of reservoirs in the Xujiahe Formation of the central and western Sichuan Basin (in Chinese with English abstract). Oil Gas Geol. 30, 713e719. Folk, R.L., 1974. Petrology of Sedimentary Rocks. Hemphill Publishing Company, Austin, Texas. Guo, T., 2013. Key controls on accumulation and high production of large nonmarine gas fields in northern Sichuan Basin (in Chinese with English abstract). Pet. Explor. Dev. 40, 139e149. Heald, M.T., Anderegg, R.C., 1960. Differential cementation in the Tuscarora sandstone. J. Sediment. Petrol. 30, 568e577. Heald, M.T., Larese, R.E., 1974. Influence of coatings on quartz cementation. J. Sediment. Petrol. 44, 1269e1274. Houseknecht, D.W., Hathon, L.A., 1987. Petrographic constraints on models of intergranular pressure solution in quartzose sandstones. Appl. Geochem. 2, 507e521. Houseknecht, D.W., 1988. Intergranular pressure solution in four quartzose sandstones. J. Sediment. Petrol. 58, 228e246. Huang, S., Xie, L., Zhang, M., Wu, W., Shen, L., Liu, J., 2004. Formation mechanism of authigenic chlorite and relation to preservation of porosity in Nonmarine Triassic reservoir sandstones, Ordos Basin and Sichuan Basin, China. J. Chengdu Univ. Technol. Sci. Technol. Ed. 31, 273e281 (in Chinese with English abstract). Imam, M.B., Shaw, H.E., 1985. The diagenesis of Neogene clastic sediments from the Bengal Basin, Bangladesh. J. Sediment. Petrol. 55, 665e671. Jia, S., Li, H., Xiao, K., 2014. Reservoir characteristics and main controlling factors of high quality reservoir in the lower 2th member of Xujiahe formation in Yuanba area. J. Northeast Pet. Univ. 38, 15e22 (in Chinese with English abstract). Lai, J., Wang, G., Chen, Y., Huang, L., Zhang, L., Wang, D., Sun, Y., Li, M., 2014. Diagenetic facies and prediction of high quality reservoir of Member 2 of Xujiahe Formation in Penglai area, central Sichuan Basin. J. Jilin Univ. (Earth Sci. Ed. 44, 432e445 (in Chinese with English abstract). Lai, J., Wang, G., Ran, Y., Zhou, Z., 2015. Predictive distribution of high-quality reservoirs of tight gas sandstones by linking diagenesis to depositional facies: evidence from Xu-2 sandstones in the Penglai area of the central Sichuan basin, China. J. Nat. Gas Sci. Eng. 23, 97e111. Li, G., Li, N., Xie, J., Yang, J., Tang, D., 2012a. Basic features of large gas play fairways in the Upper Triassic Xujiahe Formation of the Sichuan foreland basin and evaluation of favorable exploration zones. Nat. Gas. Ind. 32, 15e21 (in Chinese with English abstract). Li, J., Guo, B., Zheng, M., Yang, T., 2012b. Main types, geological features and resource potential of tight sandstone gas in China. Nat. Gas. Geosci. 23, 607e615 (in Chinese with English abstract). Li, J., Guo, T., Zou, H., Zhang, G., Li, P., Zhang, Y., 2010a. Hydrocarbon generation history of coalemeasure source rocks in the Upper Triassic Xujiahe formation of the northern Sichuan Basin. Nat. Gas. Ind. 32, 25e28 (in Chinese with English abstract). Li, W., Zou, C., Yang, J., Wang, K., Yang, J., Wu, Y., Gao, X., 2010b. Types and controlling factors of accumulation and high productivity in the Upper Triassic Xujiahe formation gas reservoirs, Sichuan Basin. Acta Sedimentol. Sin. 28, 1037e1045 (in Chinese with English abstract). Liu, R., Guo, T., Shao, M., 2011. Sources and genetic types of gas in the middleshallow strata of the Yuanba area, northeastern Sichuan Basin. Nat. Gas. Ind. 31, 34e38 (in Chinese with English abstract). Liu, S., Huang, S., Shen, Z., Lv, Z., Song, C., 2014. Diagenetic fluid evolution and waterrock interaction model of carbonate cements in sandstone: an example from the reservoir sandstone of the Fourth Member of the Xujiahe Formation of the Xiaoquan-Fenggu area, Sichuan Province, China. Sci. China Earth Sci. 57, 1077e1092. Luo, W., Peng, J., Du, J., Du, L., Han, H., Liu, H., Li, J., Tang, Y., 2012. Diagenesis and porosity evolution of tight sand reservoirs in the 2nd member of Xujiahe Formation, western Sichuan Depression: an example from Dayi region. Oil Gas Geol. 33, 287e295 (in Chinese with English abstract).

Lv, Z., Liu, S., 2009. Ultra-tight sandstone diagenesis and mechanism for the formation of relatively high-quality reservoir of Xujiahe Group in western Sichuan. Acta Petrol. Sin. 25, 2373e2383 (in Chinese with English abstract). Lundegard, P.D., 1992. Sandstone porosity loss- a “big picture” view of the importance of compaction. J. Sediment. Petrol. 62, 250e260. Ma, Y., Cai, X., Zhao, P., Luo, Y., Zhang, X., 2010. Distribution and further exploration of the large-medium sized gas fields in Sichuan Basin. Acta Pet. Sin. 31, 347e354 (in Chinese with English abstract). Morad, S., Ketzer, J.M., DeRos, L.F., 2000. Spatial and temporal distribution of diagenetic alterations in siliciclastic rocks: implications for mass transfer in sedimentary basins. Sedimentology 47 (Suppl. 1), 95e120. Pan, C., Liu, S., Ma, Y., Hu, D., Huang, R., 2011. Reservoir characteristics and main controlling factors of the Xujiahe formation in northeastern Sichuan Basin. J. Southwest Pet. Univ. Sci. Technol. Ed. 33, 27e34 (in Chinese with English abstract). Paxton, S.T., Szabo, J.O., Ajdukiewicz, J.M., Klimentidis, R.E., 2002. Construction of an intergranular volume compaction curve for evaluating and predicting compaction and porosity loss in rigid-grain sandstone reservoirs. AAPG Bull. 86, 2047e2067. Pittman, E.D., 1992. Clay coats: occurrence and relevance to preservation of porosity in sandstones. SEPM Spec. Publ. 47, 241e255. Pittman, E.D., Larese, R.E., 1991. Compaction of lithic sands: experimental results and applications. AAPG Bull. 75, 1279e1299. Pittman, E.D., Lumsden, D.N., 1968. Relationship between chlorite coatings on quartz grains and porosity, Spiro Sand, Oklahoma. J. Sediment. Petrol. 38, 668e670. Porter, E.W., James, W.C., 1986. Influence of pressure, salinity, temperature and grain size on silica diagenesis in quartzose sandstones. Chem. Geol. 57, 359e369. Reed, J.S., Eriksson, K.A., Kowalewski, M., 2005. Climatic, depositional and burial controls on diagenesis of Appalachian Carboniferous sandstones: qualitative and quantitative methods. Sediment. Geol. 176, 225e246. Ryan, P.C., Reynolds Jr., R.C., 1996. The origin and diagenesis of grain-coating serpentine-chlorite in Tuscaloosa Formation sandstone, U.S. Gulf Coast. Am. Mineral. 81, 213e225. Selley, R.C., 1978. Porosity gradients in North Sea oil-bearing sandstones. J. Geol. Soc. Lond. 135, 119e132. Shanley, K.W., Cluff, R.M., Robinson, W., 2004. Factors controlling prolific gas production from low-permeability sandstone reservoirs: implications for resource assessment, prospect development, and risk analysis. AAPG Bull. 88, 1083e1121. Sun, Q., Sun, H., Jia, B., Yu, J., Luo, W., 2012. Genesis of chlorites and its relationship with high-quality reservoirs in the Xujiahe Formation tight sandstones, western Sichuan depression. Oil Gas Geol. 33, 751e757. Surdam, R.C., Crossey, L.J., Hagen, E.S., Heasler, H.P., 1989. Organic-inorganic interactions and sandstone diagenesis. AAPG Bull. 73, 1e23. Tao, S., Zou, C., Mi, J., Gao, X., Yang, C., Zhang, X., Fan, J., 2014. Geochemical comparison between gas in fluid inclusions and gas produced from the Upper Triassic Xujiahe Formation, Sichuan Basin, SW China. Org. Geochem. 74, 59e65. Taylor, J.C.M., 1978. Control of diagenesis by depositional environment within a fluvial sandstone sequence in the northern North Sea Basin. J. Geol. Soc. Lond. 135, 83e91. Taylor, T.R., Giles, M.R., Hathon, L.A., Diggs, T.N., Braunsdorf, N.R., Birbiglia, G.V., Kittridge, M.G., Macaulay, C.I., Espejo, I.S., 2010. Sandstone diagenesis and reservoir quality prediction: models, myths, and reality. AAPG Bull. 94, 1093e1132. Thomson, A., 1982. Preservation of porosity in the deep Woodbine/Tuscaloosa Trend, Louisiana. J. Pet. Technol. 34, 1156e1162. Tong, C., 1992. Structure Evolution and Petroleum Accumulation of Sichuan Basin (in Chinese). Geology Press, Beijing. Wang, W., 2012. Pore evolution and gas accumulation in tight sandstone reservoir of Member 2 of Xujiahe Formation in Yuanba area of Sichuan, China (in Chinese with English abstract). J. Chengdu Univ. Technol. Sci. Technol. Ed. 39, 151e157. Weedman, S.D., Brantley, S.L., Shiraki, R., Poulson, S.R., 1996. Diagenesis, compaction, and fluid chemistry modeling of a sandstone near a pressure seal: lower Tuscaloosa Formation, Gulf Coast. AAPG Bull. 80, 1045e1064. Wilkinson, M., 1991. The concretions of the Bearreraig sandstone formation: geometry and geochemistry,. Sedimentology 38, 899e912. Worden, R.H., Mayall, M., Evans, I.J., 2000. The effect of ductile-lithic sand grains and quartz cement on porosity and permeability in Oligocene and Lower Miocene clastics, South China Sea: prediction of reservoir quality. AAPG Bull. 84, 345e359. Worden, R.H., Morad, S., 2003. Clay minerals in sandstones: a review of the critical problems. In: Worden, R.H., Morad, S. (Eds.), Clay mineral Cement in Sandstones, vol. 29. International Association of Sedimentologists, Special Publication, pp. 1e20. Xiao, K., Li, H., Jia, S., 2014. Characteristics of calcarenaceous sandstone reservoirs and gas accumulation control factors of the 3rd Member of Xujiahe Formation in Yuanba area, Northeast Sichuan Basin (in Chinese with English abstract). Oil Gas Geol. 35, 654e660. Xiao, Y., Peng, J., Zhang, J., Luo, W., Liu, H., 2012. Reservoir spaces of the second section of Xujiahe Formation and evolution in middle part of west Sichuan foreland basin (in Chinese with English abstract). Nat. Gas. Geosci. 23, 501e507. Yang, R., He, S., Li, T., Yang, X., Hu, Q., 2016. Origin of over-pressure in clastic rocks in Yuanba area, northeast Sichuan Basin, China. J. Nat. Gas Sci. Eng. 30, 90e105. Yao, J., Wang, Q., Zhang, R., Li, S., 2011. Forming mechanism and their environmental implications of chlorite-coatings in Chang 6 sandstone (Upper Triassic) of Hua-

L. Zhang et al. / Journal of Natural Gas Science and Engineering 35 (2016) 1320e1335 Qing area, Ordos Basin (in Chinese with English abstract). Acta Sedimentol. Sin. 29, 72e79. Zhai, G., 1989. Petroleum Geology of China,, vol. 10. Petroleum Industry Press, Beijing (in Chinese). Zhang, J., Li, G., Xie, J., Qiu, J., Wei, X., Tang, D., 2006. Stratigraphic division and correlation of Upper Triassic in Sichuan Basin (in Chinese with English abstract). Nat. Gas. Ind. 26, 12e15. Zhang, J., Zou, H., Li, P., Fu, X., Wang, W., 2015. A new PVT simulation method for hydrocarbon-containing inclusions and its application to reconstructing paleopressure of gas reservoirs (in Chinese with English abstract). Pet. Geol. Exp. 37, 102e108. Zhang, S., Mi, J., Liu, L., Tao, S., 2009. Geological features and formation of coalformed tight sandstone gas pools in China: cases form Upper Paleozoic gas pools, Ordos Basin and Xujiahe Formation gas pools, Sichuan Basin (in Chinese with English abstract). Pet. Explor. Dev. 36, 320e330.

1335

Zhang, X., Liu, B., Wang, J., Zhang, Z., Shi, K., Wu, S., 2014. Adobe photoshop quantification (PSQ) rather than point-counting: a rapid and precise method for quantifying rock textural data and porosities. Comput. Geosci. 69, 62e71. Zheng, J., Wu, R., 1996. Diagenesis and pore zonation of sandstone reservoirs in Huanghua Depression (in Chinese with English abstract). Oil Gas Geol. 17, 268e275. Zhou, Y., Li, P., Tan, X., Quan, Y., He, Y., 2011. Study on diagenesis characteristics of sandstone reservoir in Xujiahe Formation, northeastern Sichuan Basin (in Chinese with English abstract). J. Southwest Pet. Univ. Sci. Technol. Ed. 33, 20e24. Zhu, R., Zou, C., Zhang, N., Wang, X., Cheng, R., Liu, L., Zhou, C., Song, L., 2008. Diagenetic fluids evolution and genetic mechanism of tight sandstone gas reservoirs in Upper Triassic Xujiahe Formation in Sichuan Basin, China. Sci. China Ser. D Earth Sci. 51, 1340e1353.