Introduction
A series of natural gas exploration breakthroughs have been got within the argillaceous limestone of the first member of Middle Permian Maokou Formation (referred to as the Mao-1 Member) in the Sichuan Basin during the past five years, with a daily production up to 1.67×104, 3.10×104 and 31.00×104 m3 from Mao-1 Member in wells JS1, YH1 and TT1, respectively [1]. It is estimated that the resource reserve of the Mao-1 Member reaches 5 000×108 m3 [2], exhibiting a very good prospect for natural gas exploration. The previous researches suggest that the Mao-1 Member is a mudstone-argillaceous limestone reservoir of sepiolite-bearing strata with the talc in majority and the sepiolite in the second place in clay mineral composition [3⇓−5]. It is mostly deposition-dominated and hydrothermal replacement in a little volume for the sepiolite genesis [3,5]. The collapse of the layer chain structure of sepiolite in the burial diagenetic stage results in the talc formation [3,6]. However, there are still some academic confusions on the other clay mineral types except sepiolite and talc, the paragenetic association and evolution sequence of the authigenic clay minerals as well as their material sources, evolution phases, by-products and the hydrocarbon geological significance in the Mao-1 Member.
Based on the measurement of several outcrop sections and the core description of typical coring wells, the types, occurrence and composition of the authigenic clay minerals of sepiolite-bearing strata within the Mao-1 Member in eastern Sichuan Basin were discussed through thin section identification, argon ion polishing, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), in order to reveal the evolution sequence of the authigenic clay minerals, establish the sedimentary-diagenetic evolution model and clarify its hydrocarbon geological significances, which are certainly helpful to further researches on the paleocean, paleoenvironment and the transformation process from diagenesis to hydrocarbon generation in the Mao-1 Member.
1. Regional geological settings
Under the impacts of the inherited development of Caledonian paleouplift, the transgression from both the western and eastern margins into the central area of the Sichuan Basin during the sedimentation of the Middle Permian Qixia Formation resulted in the formation of a carbonate ramp-type platform in the Sichuan Basin [7]. Besides, the west-east trending intracratonic sag along Tongjiang, Yanting and Changshou led to the sedimentary pattern of “two highs and one low” due to the weak extension inside the basin in this period [5,8⇓⇓ −11].
The Maokou Formation is subdivided into four members, that is, the Mao-1, Mao-2, Mao-3 and Mao-4 members in the upward direction. Inheriting the deposition pattern of the Qixia Formation, the Mao-1 Member is a carbonate ramp deposit, which is further divided into inner ramp, middle ramp and outer ramp [12⇓⇓−15]. And the outer ramp facies covered the eastern Sichuan Basin at that time with mudstone, micritic limestone, micritic bioclastic limestone, bioclastic micrite limestone and argillaceous limestone in lithology [1]. During the sedimentation period of the Mao-2 Member, the relative sea level fell, with middle ramp deposits. During the sedimentation of the Mao-3 Member, the water body continued to be shallower, and gradually transformed into inner ramp deposit [16]. Influenced by the main episode of Dongwu Movement, the top of the Maokou Formation exposed to the surface and suffered denudation during the sedimentation period of the Mao-4 Member [2,17]. During the sedimentation of the Maokou Formation, the Tongjiang-Changshou sag was further stretched by the upwelling of the E’mei mantle plume in the southwestern margin and the subduction of the Mian-Lue Ocean in the north margin of the basin, leading to the distinct depositional pattern of “two highs and one low” (Fig. 1a ) [5].
Fig. 1. Depositional facies map of the Mao-1 Member (a) and stratigraphic composite column of the Maokou Formation (b) in the Sichuan Basin (modified from Reference [1]). GR—Gamma ray; Δt—acoustic time difference; ρ—density; ϕCNL—neutron porosity. |
The special eyeball shaped structures developed in the Mao-1 Member, with three submembers recognized as Mao-1-a, Mao-1-b and Mao-1-c in top-down order [4,12]. The deep gray mudstone, argillaceous limestone, micritic bioclastic limestone and siliceous rock with interbedded micritic limestone in a small volume constitute the Mao-1-c submember, with jagged variations in the logging curves such as gamma ray (GR), acoustic time (AC) and compensated neutron logging (CNL) (Fig. 1b ). However, the deep gray nodular limestone and micritic bioclastic limestone developed in the Mao-1-b submember, in which the slightly smaller values in jagged variations appear on the GR curves with a sharp point at the top surface but the box-shaped smaller values in both AC and CNL curves. In the Mao-1-a submember, dark gray mudstone, micritic bioclastic limestone and argillaceous limestone are dominant with interbedded thin siliceous rocks and shale, containing brachiopod, coral and sponge. The logging curves of GR and AC within the Mao-1-a submember are mostly with jagged variation in much larger values and reduction in GR value at the top surface while the CNL curve is zigzagged with several sharp points.
2. Sample collection and experimental methods
We selected the Mao-1 Member argillaceous limestone from the outcrop sections from Huayingshan, Tieqiaocun, Shizhu and the wells S6, Z8, HS4, XT1, LJ1, HB1 and C20 for analysis (Fig. 1a ). The total 160 pieces of samples in this study, were grouped into bulk samples for lithofacies analysis and morphology observation (including thin section observation, argon ion polishing, SEM, EDS and LA-ICP-MS, and powder samples ground into screen mesh size less than 0.075 mm for XRD analysis. The thin sections were examined under a polarized microscope Nikon E600 Pol+. The argon ion polishing and SEM analyses were carried out on FESEM Quanta 250 FEG, and the EDS tests were done on the energy dispersive spectrometer Oxford Inca X-Max20. The above lab work was all completed at State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology). The in situ element mapping on LA-ICP-MS was completed by Wuhan Sample Solution Analytical Technology Co., Ltd., China, with the laser ablation system GeoLas HD GeoLas Pro and ICP-MS Agilent 8900. The X-ray diffraction analysis was carried out on the X-ray diffractometer Bruker D2 Advance in the Chengdu Survey Center of China Geological Survey.
3. Type, paragenetic association and evolution sequence of the authigenic clay minerals of the Middle Permian Mao-1 Member
3.1. Authigenic clay mineral types in the Mao-1 Member
The clay mineral-rich intervals of the Mao-1 Member are mostly located in both Mao-1-a and Mao-1-c submembers with layered or eyelid-shaped occurrence. The black clay-bearing intervals are pure in composition and interbedded in layered limestone with 20-50 cm in total thickness and millimeters in single layer thickness (Fig. 2a , 2b ). And the eyelid-shaped ones are composed of deep gray to grayish black argillaceous bioclastic limestones irregularly lining the eyeball-shaped limestones (micritic bioclastic limestone) in thickness of 3-10 cm (Fig. 2c ). The XRD analysis on the argillaceous limestone samples shows that the authigenic clay minerals in the Mao-1 Member are composed of talc, sepiolite, stevensite, disordered talc and a small amount of montmorillonite, chlorite and illite, with talc and sepiolite as the dominant in proportion of 50.2%-100.0% and 5%-10% respectively within the total clay mineral content.
Fig. 2. Outcrop occurrence and SEM characteristics of main clay minerals within the Mao-1 Member in eastern Sichuan Basin. (a) Huayingshan section, Mao-1-b submember, limestone interbedded with clay; (b) Shizhu section, Mao-1-a submember, limestone interbedded with clay; (c) Tieqiaocun section, Mao-1-a submember, eyelid-shaped limestone; (d) Huayingshan section, Mao-1-a submember, argillaceous micritic bioclastic limestone, fibrous sepiolite, SEM; (e) Well XT1, 4 445.56 m, argillaceous bioclastic limestone, flake talc, SEM; (f) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, feather-like stevensite, SEM; (g) Well XT1, 4 415.03 m, argillaceous bioclastic limestone, flake-shaped disordered talc, SEM; (h) Huayingshan section, Mao-1-a submember, argillaceous micritic bioclastic limestone, scaly montmorillonite, SEM; (i) Huayingshan section, Mao-1-c submember, argillaceous micritic bioclastic limestone, scaly montmorillonite, SEM; (j) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, laminar chlorite single crystal precipitates as the pore liner, SEM; (k) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, tiny plate-shaped chlorite aggregates lining the pores, SEM; (l) Huayingshan section, the Mao-1-a submember, argillaceous bioclastic limestone, honeycomb-like illite, SEM. |
The sepiolite of the Mao-1 Member in eastern Sichuan Basin is mainly in both argillaceous limestone and mudstone, with single fibrous crystal in length of 15-30 μm under SEM (Fig. 2d ).
The talc of the Mao-1 Member in eastern Sichuan Basin is mostly developed in the black and grayish black micritic bioclastic limestone, argillaceous limestone and mudstone, with sheet-shape texture under SEM (Fig. 2e ) and in occurrence of layered, lens-like and spotted authigenic minerals and bioclasts from replacement under a microscope [1].
The stevensite is a type of trioctahedral layered silicate minerals in the montmorillonite group [18]. The feather-like, tiny flake, or honeycomb-like textures are regarded as the aggregate of the fiber single crystal under SEM (Fig. 2f ), which indicates the transitional products from sepiolite to talc.
The disordered talc is the talc in unstable state, which is thought to be a transitional product from stevenite to talc. But the characteristics of a 50 °C lower heat absorption valley in the differential thermal analysis, much larger spacing between silicate layers, more disordered internal stacking style and much lower bonding strength between layers are presented in comparison with the standard talc [6,19], with tiny irregular flake textures under SEM (Fig. 2g ).
The montmorillonites are usually aggregated in scaly shape wrapping around or adhering to the calcite or pyrite crystals with irregular laminar single crystal in size of 2-3 μm under SEM (Fig. 2h , 2i ).
The chlorites are mostly in occurrence of pore liner (Fig. 2j ), precipitating perpendicular to the pore center (Fig. 2k ), with laminar single crystal and tiny plate texture in aggregation under SEM.
The illites fill the intergranular pores in curved sheet shape with honeycomb-like structures in the aggregates (Fig. 2l ).
3.2. Paragenetic association and evolution sequences of authigenic clay minerals in the Mao-1 Member
The paragenetic associations of sepiolite-stevensite- disordered talc-talc, sepiolite-montmorillonite-chlorite/ illite and sepiolite-talc-chert have been identified in the Mao-1 Member in the eastern Sichuan Basin. Two diagenetic evolution sequences of the authigenic clay minerals have been found out from the combined results of X-ray diffraction and Mg/Si ratio analysis, that is, from sepiolite, stevensite, disordered talc to talc, and from sepiolite, montmorillonite to chlorite/illite, with the former one in advantage.
3.2.1. The evolution sequence of sepiolite-stevensite-disordered talc-talc
The sepiolite within the argillaceous limestone of the Mao-1 Member in eastern Sichuan Basin is mostly fibrous and distributed between calcites in shape of cross-bridge (Fig. 3a ). And the feather-like and flaky transitional products are recognized under SEM with the same element components as sepiolite and talc of 0.675 to 0.691 and 0.701 to 0.742 in Mg/Si ratio by EDS, which have been approved to be stevensite and disordered talc respectively. The stevensite is fibrous for single crystal and feather-like for the aggregate (Fig. 3a ). However, the disordered talc is of tiny chaotic flakes in morphology (Fig. 3b ) while the talc is in shape of much larger flakes with oriented arrangement (Fig. 3c ). And the flake-shaped talc is much more evident in size of 25-40 μm, with the curvy shrinkage fractures in width of 0.5-1.0 μm and extension length of 25 μm around the talc under argon ion polishing and SEM (Fig. 3d-3f ).
Fig. 3. The characteristics of sepiolite-stevensite-disordered talc-talc paragenetic association within the Mao-1 Member in eastern Sichuan Basin. (a) Well XT1, 4 440.52 m, argillaceous micritic bioclastic limestone, in sepiolite-stevensite paragenetic association, SEM; (b) Well XT1, 4 431.31 m, argillaceous bioclastic limestone, in stevensite-disordered talc paragenetic association, SEM; (c) Well XT1, 4 440.52 m, argillaceous micritic bioclastic limestone, in disordered talc-talc paragenetic association, SEM; (d) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, shrinkage fractures developing within and around flaky talc, argon ion polishing-SEM; (e) Well S6, 4 164.82 m, argillaceous bioclastic limestone, shrinkage fractures developing within and surrounding flaky talc, argon ion polishing-SEM; (f) Well HS4, 2 679.79 m, argillaceous micritic bioclastic limestone, shrinkage fractures developing within and around flaky talc, argon ion polishing-SEM. |
Furthermore, the paragenetic associations of the authigenic clay minerals in the Mao-1 Member such as the sepiolite-stevensite, stevensite-disordered talc, disordered talc-talc have all been found out in large quantities but seldom for the sepiolite-disordered talc and stevensite-talc. And it is fibrous, feather-like, tiny flake to flake-shaped in the mineral occurrence from sepiolite, stevensite, disordered talc to talc under SEM.
It is statistical that the value ranges of the Mg/Si ratio from sepiolite, stevensite, disordered talc to talc are in a continuous surging trend of the Mao-1 Member in eastern Sichuan Basin. The sepiolite is in volume of 33.33% and 0.607 to 0.670 in Mg/Si ratio while the stevensite occupies 13.33% in proportion and 0.675 to 0.691 in Mg/Si ratio. And then, it is 24.44% of the total authigenic clay minerals for the disordered talc, with 0.701 to 0.742 in Mg/Si ratio. The talc covers the volume of 28.89% and 0.758 to 0.787 in Mg/Si ratio (Fig. 4 ).
Fig. 4. The distribution frequency of clay mineral types (a) and corresponding Mg/Si ratio (b) in the Mao-1 Member in eastern Sichuan Basin. |
The diffraction spectral features of the diagenetic evolution from sepiolite to talc in the Mao-1 Member in eastern Sichuan Basin have been figured out through the XRD clay mineral analysis (Fig. 5 ), which is similar to the reported ones of the Shangsi outcrop section in northwestern Sichuan Basin [1]. With the gradual increasing of the evolution degree from sepiolite to talc, the reflection intensity of the sepiolite peak (110) gradually decreases while the talc peak (001) moves towards the lower diffraction angle in a trend of much larger intensity and more symmetric peak in the XRD spectrum. And the talc peak is finally in completely symmetrical shape when the reflection intensity of the sepiolite peak disappears.
Fig. 5. X-ray diffraction pattern of the sepiolite and talc in the Mao-1 Member in eastern Sichuan Basin. |
In summary, the evolution sequence of the authigenic clay minerals from sepiolite, stevensite, disordered talc to talc, has been verified in dominance in the Mao-1 Member, which results from the sepiolite crystal collapse from layer chain into layered structure with the Si-O bond breaking and instable lattice under the conditions of increasing burial depth, temperature and pressure, leading to the formation of talc or stevensite [20]. Sepiolite begins to convert into stevensite when it is higher than 204 °C, which is the maximum temperature limit for sepiolite hydrothermal stability, and mostly finishes the evolution at 316 °C [21]. Moreover, the stevensite would dissolve into disordered talc in the alkaline magnesium-rich solution in hydrothermal and dissolution experiments [22].
3.2.2. The evolution sequence of sepiolite-montmorillonite-chlorite/illite
Meanwhile, the paragenetic associations of the sepiolite-montmorillonite, montmorillonite-chlorite and montmorillonite-illite have been recognized under SEM in the Mao-1 Member (Fig. 6a-6c ), but been unusual for the sepiolite-illite and sepiolite-chlorite associations. And the microscopic occurrences under SEM are different for them. The montmorillonite often adhering to the calcite crystal surface is in scaly shape with the fibrous sepiolite on the edge (Fig. 6a ), while the needle-shaped chlorite surrounding the montmorillonite is lining the pores (Fig. 6b ), and the illite lying on the montmorillonite margins is in extremely tiny feather-like morphology (Fig. 6c ).
Fig. 6. The characteristics of sepiolite-montmorillonite-chlorite/illite and sepiolite-talc-chert mineral combinations within the Mao-1 Member in eastern Sichuan Basin. (a) Well XT1, 4 452.63 m, argillaceous bioclastic limestone, sepiolite and montmorillonite, SEM; (b) Huayingshan section, Mao-1-a submember, argillaceous micritic bioclastic limestone, montmorillonite and chlorite, SEM; (c) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, montmorillonite and illite, SEM; (d) Xingwen section, Mao-1-a submember, sepiolite (talc)-rich layers associated with siliceous concretions, SEM; (e) Well XT1, 4 395.10 m, argillaceous bioclastic limestone, assemblage of calcite-talc-dolomite-silica, plane-polarized light; (f) Well S6, 4 170.53 m, bioclast-bearing argillaceous limestone, assemblage of sepiolite-talc-chert, argon ion polishing-SEM. |
The other evolution sequence from sepiolite, montmorillonite to chlorite/illite has also been figured out in the Mao-1 Member. It is concluded that the montmorillonite is able to transform towards chlorite through solid- state evolution and dissolution recrystallization in Mg2+- Fe3+-rich alkaline diagenetic environment [23⇓⇓−26]. In addition, the montmorillonite may also turn into illite [27⇓−29], during which the necessary Al3+ could be sourced from the terrestrial inputs and upwelling current inputs [30]. But this evolution sequence is thought to be in the second place because of the very limited Al3+ content in the deep-water depositional zone of the Mao-1 Member in Middle Permian of the eastern Sichuan Basin.
3.2.3. The evolution sequence of sepiolite-talc-chert
The mineral assemblage of sepiolite, talc and chert has been observed in the Mao-1 Member of the eastern Sichuan Basin (Fig. 6d-6f ). In the Xingwen outcrop section, the sepiolite (talc)-rich layers are associated with the siliceous concretion and bands (Fig. 6d ). And the assemblage of calcite, talc, dolomite and chert has been presented in the cores of Well XT1 under a microscope (Fig. 6e ). Moreover, the mineral combination of sepiolite, flaky talc, flaky chert and dolomite has occurred in the cores of Well S6 through argon ion polishing (Fig. 6f ). The chert concretion or bands surrounding the sepiolite-talc are thought to result from the SiO2 releasing from the prior dissolution during the evolution process from sepiolite to talc [5].
4. Diagenetic evolution model of the authigenic clay minerals in the Mao-1 Member
The authigenic sepiolite would undergo thermal-driven mineral phase transformation in different degrees during the diagenetic process, leading to the formation of talc and stevensite [20,31]. Statistically, the exinites have been reserved in the coal beds associated with the typical sepiolite ore sites locating along both the north and south margins of “Jiangnan land”, with low to medium coal metamorphism degree (stages I to III) and long flame coal, gas coal and fat coal in industrial grade [32]. Thus, the formation temperature of the fat coal in the third coal metamorphism stage (130 °C) has been regarded as the critical temperature for the sepiolite phase transformation in the previous researches [33], which has also been calibrated to be 120 °C according to the vitrinite reflectance data (Ro) of the Middle Permian coal samples in South China area (less than 1.0%, 1.0%-1.2%, greater than 1.2%) [31,33]. Therefore, it is held that the critical temperature for sepiolite transforming to stevensite is 120-130 °C under natural strata conditions, which is much lower than the experiment limit of over 310 °C, indicating that the diagenetic evolution of sepiolite is affected by formation pressure and burial duration besides temperature [34].
It is concluded in the previous outcomes that the montmorillonite can convert into chlorite of pore-lining type in the Mg2+-Fe3+-rich pore fluids through the dissolution and recrystallization mechanism, which ceases at 65 °C in burial temperature [26,35]. The montmorillonite keeps stable when it reaches 65-70 °C. However, it would start to interact with K+ in the alkaline solution and transform into illite/smectite mixed layer with the hydrogen bond breakage of the interlayer water and dehydration of free water. The illite/smectite mixed layer maintains in a stable state in the temperature range from 95 °C to 130 °C and finally evolves into illite when illite crystal layer content is more than 90% with interlayer water discharging under the condition of 130-180 °C [35⇓−37].
Both the burial history and thermal evolution history of the sepiolite-bearing strata of the Middle Permian Mao-1 Member in Well XT1 have been reconstructed by the BasinMod software (Fig. 7a ). It is shown that the simulation results of the Ro are basically consistent with the previously measured values of the sepiolite-bearing strata. The burial temperature of 130 °C is corresponding to the Ro equal to 1.0% of the sepiolite samples, which is qualified for the sepiolite diagenetic transformation under strata conditions [31]. The diagenetic evolution process of the authigenic clay minerals in the Mao-1 Member is subdivided into eogenetic (0−85 °C), mesogenetic (85−175 °C) and telogenetic (175−200 °C) stages according to the diagenetic stage standards and the critical temperatures of the mineral phase transformation in the main sequence, with A (0−65 °C) and B (65−85 °C) periods in the eogenetic stage and A1 (85−130 °C), A2 (130−135 °C), B1 (135−145 °C) and B2 (145−175 °C) periods in the mesogenetic stage. On this basis, four diagenetic evolution phases have been proposed for the transformation of the authigenic clay minerals, that is, from A period of eogenetic stage to A1 period of mesogenetic stage, A2 period of mesogenetic stage, B1 period of mesogenetic stage and from B2 period of mesogenetic stage to telogenetic stage.
Fig. 7. Tectonic-thermal history of eastern Sichuan Basin (a) and the types of authigenic clay minerals assemblages and their diagenetic evolution models of the Mao-1 Member (b). K—Cretaceous; J1l—Lianggaoshan Formation; J1z—Ziliujing Formation; T3x—Xujiahe Formation; T2l—Leikoupo Formation; T1j—Jialingjiang Formation; T1f—Feixianguan Formation; P2c—Changxing Formation; P2l—Longtan Formation; P2m—Maokou Formation; P2q—Qixia Formation. |
4.1. The diagenetic evolution phases from A period of eogenetic stage to A1 period of mesogenetic stage
It is under the conditions of 168-280 Ma, 0-4.6 km in burial depth, burial temperature below 130 °C and Ro less than 1.0% in this phase. A large quantity of sepiolite associated with a little Al3+ from storm deposits in the Mg2+- and Si4+-rich seawater within the cool aragonite sea in eastern Sichuan Basin during the sedimentation of the Mao-1 Member of Middle Permian [1] (Fig. 7b ). A small amount of sepiolite converted towards montmorillonite in the Al3+-containing shallow burial environment during the A period of eogenetic stage, forming the paragenetic association of sepiolite-montmorillonite (Fig. 6a ). At the same time, a part of montmorillonite evolved into chlorite, and this evolution process ceased when the burial temperature reached 65 °C (Fig. 6b ). However, some montmorillonite began to transform towards illite/montmorillonite mixed layer when it rose to 70 °C during the B period of eogenetic stage, which would not stop until the temperature increased to 95 °C (Fig. 6c ). And the illite/montmorillonite mixed layer maintained stable till the end of the A1 period of mesogenetic stage. The volume and scale of the above two evolution process are smaller due to the limited contents of both the montmorillonite and the pore water. And most of the sepiolite still kept stable in this period because it didn’t reach the critical temperature for transformation from sepiolite to stevensite.
4.2. The diagenetic evolution phases of A2 period in the mesogenetic stage
It is characterized by 164-168 Ma in geological age, 4.6-4.8 km in burial depth, 130-135 °C in temperature and 1.0%-1.1% in Ro in this phase in Mao 1 Member of eastern Sichuan Basin. The sepiolite started to convert into stevensite (Fig. 7b ), forming the assemblage of sepiolite and stevensite (Fig. 3a ). The feather-shaped stevensite occurs under SEM, within which the residual sepiolite fibrous textures are found out (Fig. 2f ). In this evolution process, the chert concretion and bands were formed by the adjacent replacement of the Si4+ from the sepiolite layer chain structure collapse in the form of “sepiolite-chert” assemblage [4,32]. Concurrently, the illite/montmorillonite mixed layer in the second evolution sequence began to change into illite.
4.3. The diagenetic evolution phases of B1 period in the mesogenetic stage
It is characterized by 157-164 Ma in geological age, 4.8-5.0 km in burial depth, 135-145 °C in temperature and 1.1%-1.2% in Ro in this phase. The stevensite started to transform towards disordered talc (Fig. 7b ) to form the combination of stevensite and disorder talc (Fig. 3b ). The disordered talc is in shape of irregular or tiny flake under SEM (Fig. 2g ), which is in relative smaller crystalline and much more rough on the mineral surface in contrast to the standard talc. Besides, a small amount of feather- shaped stevensite was reserved to attach with the calcite crystal surface. And the illite/montmorillonite mixed layer in the second evolution sequence still kept on changing into illite because it did not yet reach the critical temperature for the illite/montmorillonite mixed layer transforming to illite (180 °C).
4.4. The diagenetic evolution phases from B2 period of mesogenetic stage to telogenetic stage
It is 157 Ma to now, greater than 5.0 km in burial depth, higher than 145 °C in temperature and greater than 1.2% in Ro in this phase. The disordered talc turns into talc (Fig. 7b ) in the authigenic mineral assemblage with each other (Fig. 3c ). And the flaky talc in a larger quantity of filling pores or adhering to other mineral surface has been observed under SEM, but almost no fibrous authigenic clay minerals have been identified (Fig. 2e ). Furthermore, the illite/montmorillonite mixed layer in the second evolution sequence continued to convert into illite under the temperature of 145-175 °C. After entering the telogenetic stage, the illite/montmorillonite mixed layer converted into illite completely when it was over 180 °C.
5. Hydrocarbon geological significance of the authigenic clay mineral evolution of the Mao-1 Member
5.1. The clay minerals adsorbing organic matter is beneficial for the enhancement of the hydrocarbon generation potential of source rocks
The sepiolite has extremely strong adsorption capacity, and is initially white to light gray and then gradually turns to black or dark gray after absorbing a large amount of organic matter [38]. The dark organic matter has been recognized within the pores surrounding the fibrous or feather-shaped sepiolite under argon ion polishing-SEM (Fig. 8a ). Moreover, a positive correlation of the talc content with TOC has been revealed in the further research outcome [5,10 −11].
Fig. 8. The influence of the authigenic clay mineral evolution in the Middle Permian on the source and reservoir assemblage and the model diagram (section position of |
Influenced by the mega-monsoon during the Permian period, more and more iron flowed into the ocean, which would significantly promote the primary productivity and lead to the accumulation of large quantities of organic matter [5]. The Si4+-rich fluids flowed towards the low- lying zone driven by its own gravity and submarine traction currents, to mix with the Mg2+-rich seawater to form sepiolite. Then the quality and hydrocarbon generation potential of the source rocks of the Mao-1 Member can be effectively improved considering a large amount of organic matter being adsorbed on the sepiolite surface [5].
5.2. The evolution of the authigenic clay minerals has exerted constructive effects on enlarging reservoir spaces
The evolution process of authigenic clay minerals in the Mao-1 Member can create a large number of effective reservoir spaces. On the one hand, a large amount of organic matter pores were formed during the thermal evolution process of organic matter adsorbed by the authigenic sepiolite (Fig. 8b ) [39]. On the other hand, a large volume of clay shrinkage pores (fractures) was produced by layer chain structure collapse in the transformation process from sepiolite to talc (Fig. 3d-3f ) [13], which would release Mg2+-rich diagenetic fluids after the sepiolite dissolution, resulting in the dolomitization of the adjacent bioclastic limestone, with the combination of talc and dolomite under a microscope [15] (Fig. 8c ). The contents of elements Al, Mg, and Si within the talc are relatively larger and in coordinated variation pattern in the paragenetic association with each other under the element mapping analysis of the LA-ICP-MS (Fig. 8d-8f ). Element Mg in the dolomite is relatively larger, which further testifies the dolomitization process during the diagenetic transforming from sepiolite to talc, and a large number of inter-crystal pores are formed to dramatically improve the reservoir quality and provide high permeability channels for the fluid activities in the following stage [1].
5.3. New source and reservoir assemblages for Middle Permian sepiolite-bearing strata
Two new source and reservoir assemblages, i.e., self-generation and self-accumulation, and lower (side) source and upper (lateral) reservoir, have been formed for the Middle Permian sepiolite-bearing strata, which could significantly improve the hydrocarbon accumulation efficiency (Fig. 8g ). The sepiolite deposited inside the sag in a large volume adsorbing and accumulating a huge amount of organic matter, accounts for the formation of high-quality source rocks [1,5]. Moreover, the relative better reservoir spaces such as the organic matter pores and clay shrinkage pores (fractures) have been produced in a large volume within the source rocks during the transformation process from sepiolite to talc. Therefore, the self-generation and self-accumulation pattern has been formed within the sag. Besides, the Mg2+-rich diagenetic fluids from the sepiolite-bearing strata within the sag could lead to the dolomitization of the high-permeability limestone within or on the edge of the sag, forming the large-scale stratified or patchy dolomite reservoirs, and thus the assemblage of lower (side) source and upper (lateral) reservoir.
6. Conclusions
Rich authigenic clay minerals have been discovered in layered or eyelid-shaped occurrence within the argillaceous limestone of the sepiolite-bearing strata in the Mao-1 Member of Middle Permian in eastern Sichuan Basin, which are composed of sepiolite, talc, stevensite, disordered talc and a small amount of montmorillonite, chlorite and illite, with talc and sepiolite in majority. Three mineral paragenetic associations of sepiolite-stevensite-disordered talc-talc, sepiolite-montmorillonite- chlorite/illite and sepiolite-talc-chert have been identified. The two diagenetic evolution sequences of the authigenic clay minerals are developed, that is, from sepiolite, stevensite, disordered talc to talc, and from sepiolite, montmorillonite to chlorite/illite, with the former one in advantage.
The sepiolite deposited directly from the Mg2+-Si4+-rich seawater in the cool aragonite sea of the Mao-1 Member in eastern Sichuan Basin. In the diagenetic evolution phases from the A period of eogenetic stage to A1 period of mesogenetic stage, the sepiolite kept stable in the shallow burial environment widely lack of Al3+, and then began to transform into stevensite in the A2 period of mesogenetic stage. In the B1 period of mesogenetic stage, the feather-shaped stevensite evolved into the flaky disordered talc. And the disordered talc finally changed into talc in the B2 period of mesogenetic stage to telogenetic stage. As for the secondary diagenetic evolution sequence, with the participation of a little Al3+ inputted with storm or current, a small amount of sepiolite converted into the montmorillonite and a part of montmorillonite continued to turned into chlorite in the A period of eogenetic stage. From the B period of eogenetic stage to the A1 period of mesogenetic stage, a part of montmorillonite evolved toward illite/smectite mixed layer. The illite/smectite mixed layer transformed into illite in A2 to B2 periods of mesogenetic stage, and then totally developed into illite in the telogenetic stage.
The types and evolution of authigenic clay minerals in argillaceous limestone of sepiolite-bearing strata is of great significance for the hydrocarbon geological indication. On the one hand, the sepiolite can adsorb and accumulate a large amount of organic matter, which is beneficial for the effective enhancement of the quality and hydrocarbon generation potential of source rocks. On the other hand, the evolution process from sepiolite to talc could produce a large number of organic matter pores and clay shrinkage pores (fractures), leading to the formation of unconventional reservoirs inside the sag, while releasing Mg2+-rich diagenetic fluids and resulting in the dolomitization of the limestone within or on the edge of the sag and the formation of high-quality conventional dolomite reservoirs above the source rocks. Thus, the new assemblages of source and reservoir of self-generation and self-accumulation, lower (side) source and upper (lateral) reservoir have been formed for the Middle Permian sepiolite-bearing strata, which could significantly improve the hydrocarbon accumulation efficiency.