Reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin and its periphery and inspirations to petroleum exploration, SW China
Research Institute of Exploration and Development, Southwest Branch Company, Sinopec, Chengdu 610041, China
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Received: 2021-02-7 Revised: 2021-07-15
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Based on a large number of field outcrops and cores taken systematically from boreholes, by microscopic observation, physical property analysis, mineralogy analysis, geochemical analysis etc., reservoir characteristics of the first member of Middle Permian Maokou Formation in Sichuan Basin ("Mao 1 Member" for short) are analyzed. (1) Rhythmic limestone-marl reservoirs of this member mostly exist in marl layers are a set of tight carbonate fracture-pore type reservoir with low porosity and low permeability, with multiple types of storage space, mainly secondary dissolution pores and fissures of clay minerals. (2) The clay minerals are mainly diagenetic clay minerals, such as sepiolite, talc and their intermediate products, aliettite, with hardly terrigenous clay minerals, and the reservoir in different regions have significant differences in the types of clay minerals. (3) The formation of high quality tight carbonate reservoir with limestone-marl interbeds is related to the differential diagenesis in the early seawater burial stage and the exposure karstification in the early diagenetic stage. It is inferred through the study that the inner ramp of southwestern Sichuan Basin is more likely to have sweet spots with high production, while the outer ramp in eastern Sichuan Basin is more likely to have large scale contiguous reservoir with low production.
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Cite this article
SU Chengpeng, LI Rong, SHI Guoshan, JIA Huofu, SONG Xiaobo.
Introduction
Limestone-marl alternations (LMAs) are a common type of carbonate rocks with vertical sequence structure in geological history. It has been widely concerned by researchers due to its regular combination of lithologic change[1,2,3,4,5,6]. In the rhythmic stratigraphic sequence, limestone and marl are only with descriptive meaning, but cannot represent their specific CaCO3 content[7,8,9,10,11]. According to the morphological characteristics, the LMAs can be divided into stratified, continuously beaded along the beds, discontinuously beaded along the beds and disorderly types, among which the discontinuously beaded along the beds and disorderly LMAs are called eyeball-shaped limestone or nodular limestone[12,13,14,15,16,17,18]. LMAs of the above four morphologies are all developed in the first Member of Middle Permian Maokou Formation (hereinafter “Mao 1 Member” for short) in Sichuan Basin and its periphery, and they are common in the discontinuous beaded along the beds and disorderly pattern. The difference in composition between limestone and marl reflects the characteristics of original sediment types and environmental change caused by climate change[19]. The continuous rhythmic sedimentary record is helpful for the study of orbital driven high-precision cycle stratigraphy[20,21]. However, due to the complexity of carbonate sedimentary and diagenetic processes, LMAs can also be developed by diagenesis[5, 22-23], such as the Middle Permian limestone-marl alternations in South China, which are developed mainly by diagenesis[9-10, 17].
For a long time, the marl in LMAs of the Mao 1 Member in Sichuan Basin has been regarded as a set of carbonate source rocks, and there are few studies on reservoirs. Therefore, it has become a blind spot for exploration. In recent years, based on shale gas exploration, commercial gas flow has been obtained from the Mao1 Member in Wells JS1, YH1, TT1, DS1 and JH1 in Sichuan Basin, indicating that the LMAs of the Mao 1 Member are not only source rocks, but also effective reservoirs, with large scale in distribution. The exploration potential is great. At present, the study on reservoir characteristics and genetic mechanism of LMAs of Mao 1 Member is rather weak. Based on the analysis of different samples, Su et al.[24] believed that matrix dissolved pores in LMAs were the main reservoir space, while Hu et al.[25] believed that talc shrinkage fractures were the main reservoir space. There were different understandings about the genetic mechanism. In addition, in different regions of Sichuan Basin, there are significant differences in gas production intervals and vertical well production from LMAs of Mao 1 Member. It is necessary to carry out a comparative analysis on Mao 1 Member in combination with the latest research results, so as to lay a foundation for the acceleration of exploration and development of the tight carbonate gas reservoirs in Mao 1 Member of Sichuan Basin.
1. Regional geological setting
During the Permian period (252-299 Ma), the South China Craton was located near the equator, separating the Paleo-Tethyan Ocean from the Panthalassa, with Paleo-Tethyan Ocean to the west and Panthalassa to the east[26], consisting of two colliding plates (Yangtze Plate and Cathaysia Plate) and two deep-water basins (Jiangnan Basin and Youjiang Basin)[27]. The Yangtze Plate was a tectonic stable area, covered by widely distributed shallow-water carbonate deposits. The statistical results show that the LMAs of the Middle Permian in South China were developed under shallow to deep water environments, mainly developed in carbonate platform (ramp) with water depth of 0-50 m and shelf with water depth of 50-200 m, but rarely developed in deep water basin with water depth more than 200 m[17].
According to the characteristics of lithology, electrical property and sedimentary cycle etc., Maokou Formation can be subdivided into the first, second, third and fourth members from bottom to top. The Mao 1 Member is mainly composed of LMAs, with a thickness of 85-130 m[25] or even 190 m[28], having obvious eye-ball structure, local black chert bands and masses[29,30]. The second Member of Maokou Formation is gray medium-thick stratified limestone, with a small amount of argillaceous. The third Member of Maokou Formation is mainly composed of light gray and grayish white massive limestone, with high grain content. Dark gray micritic limestone and bioclastic limestone are the main rocks in the fourth Member of Maokou Formation. Based on the regional sedimentary background of the South China, combined with the sedimentary characteristics of the non-platform margin reef-shoal in the depositional period of Mao 1 Member in Sichuan Basin, the sedimentary model of Mao 1 Member in Sichuan Basin is defined as carbonate ramp[31], and the sedimentary facies of inner ramp, middle ramp, outer ramp and shelf are developed successively from southwest to northeast (Fig. 1).
Fig. 1.
Fig. 1.
Sedimentary facies map of the Mao 1 Member in Sichuan Basin and its periphery.
2. Common characteristics of reservoir
2.1. Petrologic characteristics
In the Mao 1 Member of Sichuan Basin and its periphery, the limestone layers of LMAs are light gray to gray in color, with a thickness of 10-30 cm, and are mainly layered and nodular. The marl layers are dark gray to gray black in color, and the thickness changes greatly, with the thinnest less than 5 cm and the thickest more than 1 m (Fig. 2). The contact relationship between limestone layer and adjacent marl layer is usually in gradual change. The thickness of the gradual transition zone is millimeter, and there are obvious spatial changes in color, biological preservation state, mineralogy, etc. Black talc masses of millimeter-centimeter size are common in cores (Fig. 2). Bitumen is visible in the core section. The bubbles are dense in the water entry test of the core. Most of the bubbles are concentrated in the marl layer, and a few bubbles can be seen in the limestone layer. The bubble overflow in the marl layer seems to be a lack of regularity, and the main parts of the bubble overflow in the limestone layer are bioclastic cavity and fissures. The logging curves of LMAs show the characteristics of low resistivity, low natural gamma ray and low interval transit time. In the LMAs intervals, the content of Si and Mg is high, and oil and gas show active during drilling (Fig. 2).
Fig. 2.
Fig. 2.
Comprehensive histogram and polishing surface petrologic characteristics of LMAs of the Mao 1 Member in Well JH1. GR—natural gamma ray; ϕL—porosity of logging interpretation; ϕC—porosity of core analysis; KL—permeability of logging interpretation; KC—permeability of core analysis; RLLD—deep lateral logging resistivity; RLLS—shallow lateral logging resistivity; Δt—interval transit time; ρ—density; The yellow circles mark the bubbles overflow position in the water entry test of the core.
According to the observation of microplariscope, the bioclast content of rhythmic layers in different intervals of the Mao 1 Member in the study area varies greatly. Micrite (Fig. 3a), bioclastic wackstone (Fig. 3b-3c) and bioclastic packstone (Fig. 3d-3e) are all developed. In local non-rhythmic strata, bioclastic grainstone is developed (Fig. 3f). In addition, marl layers usually have higher bioclastic content than adjacent limestone layers (Fig. 3).
Fig. 3.
Fig. 3.
Typical microscopic petrological characteristics of LMAs of the Mao 1 Member in the Sichuan Basin under microplariscope. (a) Well YY9, 1775.43 m, limestone layer of Mao 1 Member, micrite; (b) Well YY9, 1776.77 m, limestone layer of Mao 1 Member, bioclastic wackstone; (c) Well JH1, 2892.14 m, limestone layer of Mao 1 Member, bioclastic wackstone; (d) Well JH1, 2891.61 m, limestone layer of Mao 1 Member, bioclastic packstone; (e) Well JH1, 2892.34 m, marl layer of Mao 1 Member, bioclastic packstone; (f) Well YY9, 1777.75 m, limestone layer of Mao 1 Member, bioclastic grainstone; (g) Well JH1, 2891.76 m, marl layer of Mao 1 Member, argillaceous bioclastic packstone, adjacent to the limestone layer of
There are obvious differences in fossils and diagenesis between the limestone and marl layers in LMAs. In terms of biofossils, a large number of green algae, brachiopods, sponges, ostracoda, echinodermata, bryozoans and foraminifera can be found in the adjacent limestone and marl layers, but there are significant differences. For example, gastropods and bivalves of aragonitic mollusks are only well preserved in the limestone layers. Most bioclasts in limestone layers are well preserved without any signs of compaction (Fig. 3b, 3d and 3e). Compared with limestone layer, in marl layer, except some calcite fossils such as brachiopods and foraminifera are well preserved, other bioclasts are usually incomplete preserved. Pairwise ostracoda shells are rarely found. Most of the bioclasts are too small to identify the species and genus. Besides, due to the strong compaction, the bioclastic arrangement often has a certain orientation (Fig. 3g-3i). In diagenesis, limestone layer has obvious recrystallization compared with marl layer[6, 22]. Microsparry calcite is mainly filled between grains of limestone layers (Fig. 3a-3e), while mud is mostly filled between grains, and insoluble matters such as clay and bitumen are mostly enriched in marl layers (Fig. 3g-3i). In marl layers, intense clay metasomatism, siliceous metasomatism, siliceous cementation, and dolomitization also occurred (Fig. 4).
Fig. 4.
Fig. 4.
Reservoir space types of marl layers in LMAs of the Mao 1 Member in Sichuan Basin. (a) Well JH1, 2906.77 m, dissolved pores of matrix, plane-polarized light; (b) Well YY9, 1779.64 m, intragranular dissolved pores of bioclast and dissolved pores of matrix, cross-polarized light; (c) Well JH1, 2895.58 m, intragranular dissolved pores of bioclast and dissolved pores of matrix, plane-polarized light; (d) Well YY9, 1782.30 m, intragranular dissolved pores of dolomites, plane-polarized light; (e) Well JH1, 2896.40 m, intragranular dissolved pores of dolomite and intergranular dissolved pores, plane-polarized light; (f) Well JH1, 2893.80 m, microfracture, casting thin section; (g) Well JH1, 2906.88 m, fractures of grain margin and diagenetic pores and fractures of clay mineral, scanning electron microscopy (SEM) by argon ion polishing; (h) Well JH1, 2907.78 m, dissolution pores and cracks in grain of calcite, SEM by argon ion polishing; (i) Well JH1, 2901.92 m, pores within organic matter, SEM by argon ion polishing.
2.2. Reservoir space
Limestone layers are usually relatively dense, having no secondary dissolution pores, and with a few tension fissures and stylolites. Most of the tension fissures are full filled with calcite, while a few are half filled. The intercrystalline pores of calcite are filled with residual bitumen, while the stylolites are often filled with bitumen. It can also be seen that unfilled tension fissures are vertically distributed in limestone layer, and terminate in adjacent marl layer in some outcrop sections (e.g. Shangsi section). The marl layer is relatively loose and porous, with not only tension fissures and stylolites, but also a large number of secondary dissolution pores (Fig. 4). Among them, matrix dissolved pores are the most developed (Fig. 4a-4c), with uneven pore boundaries, irregular morphology, wide pore size distribution range (0.01-1.00 mm), and mostly semi-filled to fully filled by clay minerals and organic matter. The intragranular dissolved pores (Fig. 4b-4e) and dolomite intercrystalline dissolved pores (Fig. 4e) are relatively well developed in marl layers. The intragranular dissolved pores mainly include bioclasts (Fig. 4b-4c) and dolomites (Fig. 4d, 4e) intragranular dissolved pores, and the mold pores of bioclasts are occasionally seen locally.
SEM observation of marl layers showed that the clay minerals are porous and basal cemented with calcite. The pores of marl layers are mainly nanoscale with developed grain margin fractures. A large number of clay diagenetic pores and fractures are visible (Fig. 4g), with the pore sizes range from 6.18 nm to 22.87 nm, and the fracture width ranges from 150.3 nm to 374.0 nm, which are effective reservoir spaces till now. The diagenetic shrinkage fractures of clay are mainly used as reservoir spaces rather than migration channels due to their limited range of extension.
There are also a small amount of three types of reservoir spaces in LMAs of Mao 1 Member: (1) Dissolution micropores (with a diameter of 14.85-540.50 nm) and micro-fractures in calcite crystals (Fig. 4h); (2) Organic micropores with pore sizes of 122.8-137.1 nm (Fig. 4i); (3) Micro fractures, with a width of 0.1-5.0 mm, are mostly full to half-filled, and the fractures under casting thin sections and SEM are mostly unfilled (Fig. 4f).
2.3. Petrophysical properties
The petrophysical properties of 48 marl samples and 60 limestone samples from the core in southwest Sichuan Basin were analyzed. The results show that there is a great difference between them. The porosity of marl samples ranges from 0.47% to 4.70%, with an average of 2.10%. The samples with porosity greater than 2% account for 58% (Fig. 5a). The marl layers are mostly Ⅲ type reservoirs, with a small number of Ⅱ type reservoirs. The permeability is (0.003-14.000)×10-3 μm2, with an average of 0.920×10-3 μm2 (Fig. 5b). It is a set of low porosity and low permeability fracture-pore type tight carbonate reservoir, with good correlation between porosity and permeability (Fig. 5c). The porosity of limestone samples ranges from 0.20% to 1.62%, with an average of 0.81%. No sample has the porosity greater than 2% (Fig. 5a). The permeability is (0.003-9.280)×10-3 μm2, with an average of 0.379×10-3 μm2 (Fig. 5b). The poor porosity-permeability correlation (Fig. 5c) is consistent with the main fracture development observed in the core and microscopic thin sections.
Fig. 5.
Fig. 5.
Petrophysical properties of core and outcrop samples of LMAs of the Mao 1 Member in Sichuan Basin. (a) Porosity frequency distribution of core samples; (b) Permeability frequency distribution of core samples; (c) Correlation between porosity and permeability of core samples; (d) Porosity frequency distribution of outcrop samples; (e) Permeability frequency distribution of outcrop samples; (f) Correlation between porosity and permeability of outcrop samples.
The petrophysical properties of 34 LMAs samples (including 17 limestone samples and 17 matching marl samples) from 4 outcrop sections (Hongyuanxiang, Lengshuixi, Laohuangqian, Jiangkou) in eastern Sichuan Basin were analyzed (Fig. 5d, 5e). The petrophysical properties of limestone and marl layers are generally poor, and the reservoir property of marl layer is better than that of limestone layer. The cross plot of porosity and permeability (Fig. 5f) shows that the limestone and marl layers are characterized by ultra-low properties and low properties respectively. The correlation between porosity and permeability of limestone layer is poor, which shows that permeability is not affected by porosity. It has the characteristics of fractured reservoirs. The porosity and permeability of marl are well correlated, showing that the permeability increases with the increase of porosity, and it has the characteristics of porous reservoirs.
To sum up, the petrophysical properties of the core samples are in good agreement with those of outcrop samples in the Mao 1 Member of Sichuan Basin. As a whole, it is a set of low porosity and low permeability fracture-pore type tight carbonate reservoirs, which are mostly Ⅲ type reservoirs, with a small number of Ⅱ type reservoirs.
3. Correlation of reservoir difference
3.1. Planar difference
In the northern and eastern Sichuan Basin, located at the outer ramp, the production of LMAs during production test was generally low. For example, the natural gas output from Well WJ1 in northern Sichuan Basin was 0.69×104 m3/d, and 1.67×104 m3/d, 3.06×104 m3/d and 5.4×104 m3/d from wells JS1, YH1 and DS1 in eastern Sichuan Basin respectively. In the middle to inner ramp of central and southwest Sichuan Basin, the production of LMAs was generally high during production test. For example, the natural gas output of Well TT1 in central Sichuan Basin was 31×104 m3/d, and 12.5×104 m3/d and 51.37×104 m3/d respectively from Well JH1 and Well B23 in southwest Sichuan Basin. This indicates that the reservoirs with LMAs and better petrophysical properties are well developed in the shallow middle to inner ramp.
3.2. Vertical differentiation
The gas producing intervals of Well DS1, Well JS1 and Well YH1 at the outer ramp of eastern Sichuan Basin are all located at the bottom to lower part of the Mao 1 Member. The gas producing interval of Well TT1 at the middle ramp of middle Sichuan Basin is located in the middle-lower to middle-upper parts of the Mao 1 Member. The gas producing interval of Well JH1 at the inner ramp of southwest Sichuan Basin is located in the middle-upper part of the Mao 1 Member.
The Middle Permian in Sichuan Basin has been analyzed as a whole. Based on the sequence stratigraphic analysis, the high natural gamma-ray segment at the bottom of the 6th layer of the Mao 1 Member is widely distributed in the whole basin (Fig. 6), which is easy to trace and compare, and can be used as the regional marker of the Maokou Formation. It is speculated that the bottom of the 6th layer may be the maximum sea flooding deposition or the tuff-rich vocanic event deposition of Maokou Formation. Based on this, the Middle Permian isochronous stratigraphic framework of the whole basin has been established. It is found that the bottom of the Mao 1 Member has the characteristics of overlap from east (outer ramp) to west (inner ramp). The first layer at the bottom of the Mao1 Member is fully developed in the outer ramp (Well JS1), partially missing in the middle ramp (Well YY1), and completely missing in the inner ramp (Well JH1) (Fig. 6). The first layer in the early transgressive stage, which is widely developed in the outer ramp of the eastern Sichuan Basin, is one of the main gas producing layers of the Mao 1 Member of wells DS1, JS1 and YH1.
Fig. 6.
Fig. 6.
Profile correlation of isochronal stratigraphic framework of the Mao 1 Member in Sichuan Basin (the profile position is shown in
3.3. Difference in types of clay minerals
The marl reservoirs of LMAs in Mao 1 Member are rich in clay minerals, which are mainly diagenetic clay minerals, such as sepiolite, talc and their intermediate product aliettite[6, 32-33], with almost no terrigenous clay minerals. The species of clay minerals in different regions are significantly different (Fig. 7). The LMAs of Mao 1 Member in Shangsi section, Guangyuan area, northwest Sichuan Basin is dominated by sepiolite (Fig. 7a), and talc peaks with weak X-ray diffraction intensity can be seen in some samples (Fig. 8). The LMAs of Mao 1 Member in the southwestern and eastern Sichuan Basin are dominated by aliettite, an intermediate product of sepiolite transforming to talc (Fig. 7b), and then by pure black talc (Fig. 7c). The abundance of black talc of Mao 1 Member in the eastern Sichuan Basin is significantly higher than that in the southwestern Sichuan Basin.
Fig. 7.
Fig. 7.
Scanning electron microscope characteristics of main clay minerals of LMAs in the Mao 1 Member at different areas of Sichuan Basin. (a) Shangsi section, fibrous sepiolite aggregate, modified according to reference [30]; (b) Well JH1, pinnate aliettite and granular calcite; (c) Jiangkou section, lamellar talc aggregate.
Fig. 8.
Fig. 8.
Whole rock X-ray diffraction pattern of marl layer of LMAs in the Mao 1 Member at Shangsi section in Guangyuan (θ represents the angle between incident light and atomic plane; numbers in the figure represent interplanar spacing, 10-10 m).
The aliettite of the Mao 1 Member of Well JH1 in southwestern Sichuan Basin is mainly at the initial stage of the transformation from sepiolite to talc, and its XRD pattern shows that the diffraction intensity of the sepiolite peak is significantly higher than that of the talc peak (Fig. 8). Energy dispersive spectroscopy (EDS) test results showed that the molar ratio of magnesium to silicon was close to the standard stoichiometric ratio of 2:3 for sepiolite[34]. The aliettite of the Mao 1 Member in eastern Sichuan Basin is mainly at the middle and late stages of the transformation from sepiolite to talc, and its XRD pattern shows that the diffraction intensity of the talc peak is significantly higher than that of the sepiolite peak[34]. Electron probe micro-analysis (EPMA) test results showed that the molar ratio of magnesium to silicon was close to the standard stoichiometric ratio of 3:4 for talc.
The origin of layered magnesium-silicate clay minerals (seiolite, aliettite and talc) in the Middle Permian marine sedimentary sequence in South China has been shown to be controlled by diagenesis[32, 34-35]. Sepiolite was formed during early diagenesis[6, 35], while aliettite and talc were derived from diagenetic transformation during deep burial of sepiolite[17, 32, 36]. According to the combination of vitrinite reflectance (Ro) and the spatial distribution difference of sepiolite, aliettite and talc, some scholars have proposed that when Ro is less than 1%, the clay mineral in LMAs is mainly sepiolite. When Ro is greater than 1%, the clay minerals in LMAs are mainly aliettite and talc. The greater the Ro, the higher the degree of transformation of sepiolite to talc[32]. The Ro of LMAs of the Mao 1 Member at Shangsi section in northwestern Sichuan Basin is 0.64%, and the clay mineral is mainly sepiolite. Ro value in the southwestern Sichuan Basin is 1.8%-2.0%[37], and Ro in the eastern Sichuan Basin is greater than 2.0%[33]. The clay minerals are mainly aliettite, but the abundance of talc in the eastern Sichuan Basin is significantly higher than that in the southwestern Sichuan Basin. The difference of clay minerals in LMAs of Mao 1 Member in different areas of the Sichuan Basin is controlled by diagenesis of varying degrees.
4. Reservoir genesis and enlightenment for hydrocarbon exploration
4.1. Reservoir genesis
When the Qixia Formation was deposited on the Yangtze Craton platform during Kungurian of Cisuralian, the Carboniferous-Permian great glaciation entered into the glacier-greenhouse effect transition period[38]. During the process of the gradual melting and disintegration of the Gondwana continental ice sheet in the Permian, a series of high-frequency sea-level fluctuations occurred in low-latitude regions (such as the South China Yangtze Platform)[39]. Due to the fluctuation of sea level, the LMAs of shallow water area was frequently exposed to the surface, and subjected to the meteoric water karstification at the early diagenesis. For example, the piebald karst system of limestone layer in the Laohuangqian section of eastern Sichuan Basin (Fig. 9a, 9b) is a typical identification mark of the eogenetic karstification[40,41,42]. Recent drilling core data also show that there are obvious exposed karst phenomena (Fig. 9c, 9d) in LMAs of Qixia and Maokou Formations of Well HB1 in southwestern Sichuan Basin, such as vertical karren of Maokou Formation (Fig. 9c) and near-in-situ breccias formed by the karst system cutting bedrock of Qixia Formation (Fig. 9d). The imaging logging data of Well ZT1 in southwestern Sichuan Basin show that there are several exposed surfaces in LMAs of the Mao 1 Member because of eogenetic karstification, with the influence depth within 10 m (Fig. 10). During the process of drilling, the phenomenon of lost circulation and drilling break in LMAs of Middle Permian in shallow water areas (such as southwestern Sichuan Basin) often occurred, which is a lateral evidence of the development of eogenetic karstification.
Fig. 9.
Fig. 9.
Identification marks of exposure karstification in early diagenetic stage of the Mao 1 Member and Qi 1 Member in Sichuan Basin. (a)-(b) Laohuangqian section, Mao 1 Member, piebald karst system developed in limestone layers of LMAs; (c) Well HB1, 3579.28-3579.38 m, Mao 1 Member, vertical karren developed; (d) Well HB1, 3687.64-3687.89 m, Qi 1 Member, karst system cutting bedrock to form near in-situ breccia.
Fig. 10.
Fig. 10.
Imaging log in the Mao 1 Member of Well ZT1.
Due to differential diagenesis, stratified fabric difference was formed in the primary carbonate sediments at the stage of early seawater burial diagenesis. The marl layer was loose and porous due to the dissolution of aragonite and high-magnesium calcite (Fig. 11a), while the limestone layer was cementing and compact due to CaCO3 precipitation (Fig. 11b), which made the porosity and permeability of marl layer better than that of limestone layer, and laid a material foundation for the superposition and transformation of diagenetic fluids at the later stage. Influenced by eogenetic karstification, the petrophysical properties of marl layer with better porosity and permeability was further improved by meteoric freshwater (Fig. 11c, 11d). The karstification of marl was strongly developed, the content of insoluble argillaceous content was relatively high, and various secondary dissolution pores (such as matrix solution pores and intragranular solution pores) were developed, which further contributed to the differentiation of petrophysical properties between marl and limestone layers of LMAs. The efficient karst system channel in marl layer was favorable for the late oil and gas charging, which made the bitumen relatively enriched, and the corresponding organic acid dissolution pores were developed during the burial period. During the process of deep burial, affected by differential compaction, the bioclastic orientation and flow structure of organic matter in marl layer were further strengthened. Due to the gradual enhancement of differential compaction, the morphology of limestone layer showed continuously beaded along beds, discontinuously beaded along beds (Fig. 11e) and disorderly (Fig. 11f) in turn to present. In addition, pressolutional fractures were produced in limestone and marl layers to form the reservoirs with LMAs.
Fig. 11.
Fig. 11.
Genetic features of the reservoir with LMAs in shallow water exposed area. (a) The marl layer is loose and porous because of the dissolution of aragonite and high-magnesium calcite during early seawater burial diagenesis; (b)The limestone layer is cementing and compact due to CaCO3 precipitation during early seawater burial diagenesis; (c) The transformation of relatively dense limestone layer by meteoric freshwater is limited during the stage of eogenetic karstification; (d) The petrophysical properties of marl layer is further improved by meteoric freshwater during the stage of eogenetic karstification; (e) Continuously/discontinuously beaded limestone layer along beds occurred because of differential compaction during deep burial; (f) Disorderly limestone layer occurred because of differential compaction during deep burial.
The post-sedimentary karstification, including the dissolution of marl layers during early differential diagenesis and the subsequent eogenetic karstification, is the key to the formation of such reservoirs. The correlation between the quality of the reservoir with LMAs and the total organic carbon content (TOC) is not strong, which is different from the correlation between the quality of shale reservoir and the TOC content of Wufeng and Longmaxi Formations in Sichuan Basin[33]. It can also indicate that the quality of the reservoir with LMAs is significantly affected by the later diagenesis.
4.2. Enlightenment for hydrocarbon exploration
The analyzing results of reservoir genesis shows that, the Mao 1 Member in the inner ramp with shallow water in southwestern Sichuan Basin is easier to form high- quality reservoirs than the outer ramp with deep water in eastern Sichuan Basin. This is consistent with the planar distribution pattern of natural gas output of vertical well from the Mao 1 Member in Sichuan Basin (Table 1). When the calcite of rigid mineral was replaced by plastic clay mineral in a large proportion, the original clay mineral pores and fractures were compacted because of the loss of support from calcite particles, resulting in poor petrophysical properties, such as pure sepiolite layer, nodular and banded talc.
Table 1 Natural gas output of LMAs intervals in vertical well of the Mao 1 Member in Sichuan Basin.
Well | Sedimentary environment | Natural gas output/(104 m3·d-1) |
---|---|---|
WJ1 | Outer ramp | 0.69 |
JS1 | Outer ramp | 1.67 |
YH1 | Outer ramp | 3.06 |
DS1 | Outer ramp | 5.40 |
W61 | Middle ramp | 23.15 |
TT1 | Middle ramp | 31.00 |
B23 | Inner ramp | 51.37 |
JH1 | Inner ramp | 12.50 |
The extremely high atmospheric oxygen content in the Permian[43] significantly increased the dissolved oxygen in the seawater, resulting in high organic matter deposition rate, which was the key factor for the enrichment of TOC in the Mao 1 Member under the condition of high primary productivity in the Middle Permian[17, 44]. It was easier to form the sweet spot regions of the Mao 1 Member in the depression with shallow water area of inner ramp in southwestern Sichuan Basin. In the outer ramp of eastern Sichuan Basin, the first sublayer of the Mao 1 Member was well developed, while this layer was generally missing in the inner ramp of southwestern Sichuan Basin. In addition, the outer ramp of eastern Sichuan Basin had a larger sedimentary space than the inner ramp of southwestern Sichuan Basin, which made the deposition thickness of the outer ramp thicker than that of inner ramp in the same time under the condition of high carbonate yield. As a result, the thickness of the Mao 1 Member in eastern Sichuan Basin is significantly thicker than that in southwestern Sichuan Basin.
In conclusion, the tight carbonate source rock gas reservoir of the Mao 1 Member in Sichuan Basin is more likely to form high producing sweet spots in the inner ramp of southwestern Sichuan Basin. In the outer ramp of eastern Sichuan Basin, it is easier to form contiguous low producing areas with large volume.
5. Conclusions
The reservoirs with LMAs of the Mao 1 Member in Sichuan Basin are mainly developed in marl layers. The clay minerals of LMAs are mainly diagenetic clay minerals, such as sepiolite, talc and their intermediate products, aliettite. There are almost no terrigenous clay minerals in LMAs. It is a set of tight carbonate fracture-pore reservoirs with low porosity and low permeability. The reservoir spaces are complex, with secondary dissolution pores and pores and fractures of clay minerals dominated. The pores and fractures of clay minerals are essentially the readjustment of the distribution pattern of the original dissolution pores by the diagenetic clay minerals at the early diagenesis.
In different regions, there is difference in the vertical distribution of gas producing intervals in LMAs of the Mao 1 Member, which is closely related to the regional geomorphology difference and the sedimentary filling process controlled by geomorphology during the depositional period of the Mao 1 Member. The differences of clay minerals in LMAs of the Mao 1 Member at different areas are controlled by diagenesis of varying degrees.
At the early seawater burial stage, due to the differential diagenesis, the stratified fabric of LMAs was different, which laid the material foundation for the superposition and transformation of diagenetic fluids at the later stage. During the eogenetic karstification period, the meteoric freshwater selectively acted on marl layers with better porosity and permeability, which further contributed to the differentiation of petrophysical properties between marl and limestone layers of LMAs.
In the inner ramp of southwestern Sichuan Basin, it is more likely to form high producing sweet spots. In the outer ramp of eastern Sichuan Basin, it is easier to form contiguous low producing areas with large volume.
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