Pore types, genesis, and evolution model of lacustrine oil-prone shale: a case study of the Cretaceous Qingshankou Formation, Songliao Basin, NE China…

Effect of diagenesis on pores

By integrating the organic geochemical data (e.g. Ro), transformation of clay mineral composition and diagenesis, a comprehensive division scheme was proposed for the diagenetic stages for the K2qn1 shales of the Gulong sag (Fig.7). The shale in the Qingshankou Formation of the Qijia-Gulong sag was in the middle diagenetic stage.

Diagenetic evolution of the K2qn1 shale in the Gulong sag.

During diagenesis, clay minerals, biogenic silica, organic matter, and carbonate were transformed. When the temperature was above 70C, the conversion of clay mineral composition was an important driving factor for the change in shale material, wherein the transformation of smectite or I/S into illite was predominant than that into I/S with high illite content at temperatures between 70 and 100C20. Furthermore, burial diagenesis results in the existence of silica as microcrystal quartz in the clay matrix that has undergone illitization. The transformation of biogenic silica during burial diagenesis is common in siliceous biogenic shales. It changes from opal-A to opal-CT and then becomes quartz21. Additionally, our study reveals that the crystal morphology of carbonate minerals changes during burial diagenesis, wherein the thermal evolution of organic matter greatly influences the crystal size and morphology of carbonate minerals22.

Thus, using observations from the backscattered electron (BSE) mode of FE-SEM, the diagenetic evolution sequence of the K2qn1 shale in the Gulong sag can be summarised as follows (Fig.7): pyrite/siderite Icalcite Ichlorite Idissolution Iauthigenic quartz I/kaolinite Iillitesmectite mixed layer Idissolution IIchlorite IIillite I/authigenic quartz IIcalcite II. These observations correspond with a recent report stating that the transformation of swell clays to illite at the middle diagenetic stage makes the Gulong lacustrine shale more vulnerable to hydraulic fracturing17.

Similar to other clastic rocks, argillaceous sediments were unconsolidated after deposition in a soft mud state, thereby developing primary pores and free water. As the sediments in this stage were not separated from the overlying water, the pore water retained the properties of the bottom water of the sedimentary lake basin, which was rich in metal cations, such as Fe2+, Mg2+, Ca2+ and Na+. In this anoxic environment, self-shaped microcrystalline siderite, framboidal pyrite aggregates, and a small amount of micritic microcrystal calcite were formed. The development of primary pores in this stage provided sufficient space for the development of siderite and pyrite cements (Fig.4). Thus, the degree of self-shape was high, crystals were large, scattered distribution of self-shape single pyrite crystal reached 0.01m, and the long axis of pyrite aggregate and siderite crystals reached 0.05m23.

With the gravity load effect of overlying water and deposits, the enriched free water in the primary porosity continuously decrease; the primary porosity sharply decreases, and the slime sediments gradually change from unconsolidated sediments to weakly consolidationsemi-consolidation24. At this stage, small amounts of pyrite and siderite continued to form. Furthermore, in the Fe2+- and Mg2+-rich alkaline diagenetic environment, chlorite cement began to form, and chlorite films were formed along the surface of clastic particles in argillaceous rocks with a high silt content25. At this stage, with increasing burial depth, temperature and pressure, as a result of continuous cementation and significant compaction, plastic clay was continuously deformed, broken and rearranged, and rocks were almost consolidated. The diagenetic fluid environment gradually changed from alkaline to acidic, where unstable feldspar, carbonate and other easily soluble minerals were corroded forming secondary dissolution pores (Fig.5) because CO2 and organic acids entered the pore fluid and generated hydrocarbons with thermal evolution. The K+, Ca2+, Al3+ and Si4+ contents in the pore fluid continuously increased, forming authigenic quartz and kaolinite cement that filled the intergranular and feldspar dissolution pores after the dissolution of feldspar. During this stage, with temperatures ranging from 35 to 70C, abundant smectite in argillaceous sedimentary rocks gradually began transforming to illite, thereby forming an intermediate product, namely, the illitesmectite mixed layer20.

In the middle diagenetic stage, the K2qn1 shales in the Gulong sag were completely consolidated, the formation temperature reached 85140C20, and a large amount of smectite transformed to illite. Under the catalytic activity of temperature and clay minerals, the organic matter evolution entered the thermal catalytic hydrocarbon generation stage, forming a large number of carboxylic acids and dissolving in water, making the diagenetic fluid weakly acidic. Thus, feldspar and carbonate minerals underwent continuous dissolution. However, the water rock reaction was not significant; thus, the pores formed by dissolution were also limited because the shale was subjected to continuous high compaction and cementation26,27. Owing to the continuous hydrocarbon generation and expulsion, most organic acids are constantly discharged from shale reservoirs28, and acidic substances in pore fluids are consumed during dissolution. The diagenetic fluid environment thus gradually changed from acidic to weakly alkaline owing to the decarboxylation of carboxylic acids. In an alkaline diagenetic environment, a small amount of hairy authigenic illite, amorphous microcrystalline quartz, leucine calcite and iron dolomite cement filled some residual intergranular pores and secondary dissolution pores. Detrital quartz particles underwent weak alkaline dissolution, forming a small amount of quartz dissolution pores (Fig.5).

Reservoirs at different diagenetic stages have different diagenetic strengths and physical properties during burial diagenesis29,30. The porosity of reservoir rocks is affected by both burial depth and time. The burial time has a continuous effect on the porosity, and the uplift of the stratum decreases the effect of depth31. The K2qn1 shale in the study area reached its maximum burial depth in the early Late Cretaceous (100Ma), followed by large-scale uplift. After the Palaeogene, slight subsidence occurred again; however the burial depth did not exceed the maximum burial depth during the early Late Cretaceous; therefore, the depth effect disappeared during the maximum burial depth period to the present. Combining the differences in diagenesis, thermal evolution, hydrocarbon generation, and tectonics during the different burial stages, the pore evolution of the K2qn1 shale can be divided into two sub-stages (Fig.8).

Shale pore evolution of the K2qn1 shale in the Gulong sag.

In the middle A1 stage, wherein the rate of porosity decrease was relatively low, the porosity decreased from 25 to 10% (Fig.8). The burial depth of this stage was 10001700m; the thermal evolution of organic matter began and gradually entered the peak of hydrocarbon generation, as shown by the increase in the Ro value with depth. Furthermore, at this stage, the porosity significantly decreased as the drainage of pore water became difficult and the intensity of compaction gradually weakened. With the progress of thermal evolution and hydrocarbon generation, organic matter gradually began to crack and formed pores. Simultaneously, large amounts of organic acids were produced during hydrocarbon generation, causing the corrosion of feldspar, carbonate and other susceptible minerals and the formation of secondary dissolution pores. Additionally, the mineral conversion of smectite and kaolinite to chlorite reduced the development of inter-crystal micropores in clay minerals. Owing to the conversion of smectite to illite, authigenic quartz cemented the micropores; the conversion of potash feldspar to hydronic feldspar inhibited the dissolution of potash feldspar and acidic plagioclase at this stage, thereby reducing the development of secondary dissolution pores.

In the middle A2 stage, the porosity slowly decreased until it became stable, and the porosity decreased from 10 to 5%. At this stage, the burial depth was>1700m, and the diagenetic fluid environment gradually changed from acidic to alkaline. Compaction during the diagenesis was not evident at this stage, and cementation was enhanced. In the early stage, as the smectite transformed to illite, a large amount of authigenic quartz had cemented the pores as the diagenetic environment changed from acidic to alkaline, and calcite and iron dolomite cement filled the pores. The dissolution was relatively weak at this stage, and only a portion of the clay minerals and quartz underwent dissolution to form a small number of secondary dissolution pores.

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Pore types, genesis, and evolution model of lacustrine oil-prone shale: a case study of the Cretaceous Qingshankou Formation, Songliao Basin, NE China...