ON THE CHARACTERIZATION OF THE TEXTURAL AND STRUCTURAL FEATURES OF JURASSIC SIDERITES AS POSSIBLE INDICATORS OF THE DEGREE OF DIAGENESIS

(BASED ON OIL- AND GAS-BEARING DEPOSITS OF THE TOMSK REGION)

L. V. Peshekhonov

(Presented by the Department of Historical Geology)

Siderites in the section of Jurassic deposits of the West Siberian Plate are very limited in distribution; therefore, in the published journal and monographic geological literature reflecting the results of lithological and petrographic study of oil- and gas-bearing formations, information on siderites is either limited or entirely absent. A number of researchers have noted the enrichment of Lower and Middle Jurassic deposits with authigenic siderite \[1\]; in addition to lenses and interbeds (up to 0.6 m thick) of authigenic siderite, the presence was noted of secondary siderite, developed in the form of a pelitomorphic variety along micas and plant remains, or distributed as cement in sandstones and siltstones \[3\].

As a result of the lithological and mineralogical study of Jurassic deposits in several oil- and gas-bearing areas of the Tomsk Region, we have accumulated factual material indicating the occurrence of oolitic, spherulitic, pelitomorphic, and granular structural-textural forms of siderites, and the confinement of some of them to a definite depth in the section.

Oolitic siderites are the most rarely encountered structural-textural form. Previously they were described in the section of Novo-Vasyugan well 1-R, in the interval 2775.5—2782.5 m \[4\]. We found them in the core of well 2-R (2991—2997 m) of the Cheremshanskaya area, associated with the elevation of the same name, identified in the northeastern part of the Kaimysov Arch. Here the oolitic siderites occur at the base of the Tyumen Formation (Lower—Middle Jurassic).

These are usually dense, lenticular-bedded siderites of dark-brown color, with a large number of carbonized plant fragments and a weakly expressed micro-oolitic structure. Under the microscope, in thin section, it is established that the sizes of the oolites vary from 0.1 to 0.9 mm. Oolites of isometrically rounded shape predominate, but oolites of elliptical shape are present in appreciable quantity. The oolites are characterized by a clearly expressed concentric-zonal structure. In the inner part of some oolites there are rounded and subrounded fine-silt fragments of microquartzites, quartz, and sometimes small fragments of carbonized plant material with relics of cellular structure, etc. In such oolites one dark-brown concentric zone of sideritic composition is clearly expressed.

Other oolites are multilayered, concentric-zonal, and usually of purely sideritic composition. The zonality is caused by the observed

with a change in color to brown tones and with varying degrees of recrystallization of the siderite material from the periphery toward the center of the ooids. The central part of the ooids is the most recrystallized. The cementing mass fills the spaces between the ooids; the sizes of these spaces range from 0.1 to 0.5 mm. The cementing mass is characterized by a siderite–kaolinite composition with hydromica, and by a pelitomorphic or fine-grained fabric. These oolitic siderites are typical sedimentary formations of lacustrine or floodplain facies. Their occurrence in one or another part of the Jurassic section is determined by favorable facies, and not by stratigraphic confinement, and does not depend on the degree of diagenesis of the sediments, expressed in the recrystallization of the siderite material of the ooids and the cementing mass with the formation of fine-grained aggregates.

Pelitomorphic siderites are most widely distributed in deposits of the Tyumen Formation, but they also occur in the section of the Vasyugan, Naunak, and Georgievka formations. As a rule, these are homogeneous, brownish-gray, dense, indistinctly bedded rocks with thin cracks filled with calcite. Siderites occur in the form of lenses and interbeds with thicknesses from several centimeters, more rarely up to the first tens of centimeters. Most often, however, pelitomorphic siderites occur as difficult-to-recognize microinterbeds and microlenses, from several to fractions of a mm thick. Under the microscope in thin section these siderites are characterized by a pelitic or aleuritic-pelitic structure and a chaotic and finely laminated texture. Aleuritic material of quartz–feldspar composition is present in amounts of no more than 5–10%. The size of the clasts varies within 0.1–0.05 mm. The main mass is hydromicaceous-sideritic; it sometimes contains an admixture of calcium carbonate and gelified plant fragments. Diagenetic changes are expressed in the recrystallization of pelitomorphic siderite material with the formation of fine-grained structures distributed throughout the entire rock mass or in separate parts of it, as well as in the formation of rim cement around clasts of the terrigenous fraction. Such siderites were described by us in the deposits of the Georgievka Formation exposed by well 125-R (2571–2574 m) of the Olenya area, confined to the northeastern part of the Kaimysov arch, and by well 150-R (2422–2426 m) of the Olimpiiskaya area, located in the northeastern part of the Kazan swell. They are characterized by a conchoidal fracture and a homogeneous fine-crystalline structure. The grain size does not exceed 0.1–0.2 mm; they are colorless, with distinct rhombohedral cleavage and sharply expressed pseudoabsorption. In the intergranular space, thin films of yellowish-brown oxidized ferruginous material are sometimes noticeable. As individual grains in these siderites there occur glauconite, marcasite, and allothigenic clasts of quartz and feldspars. The wide distribution of pelitomorphic siderites in the deposits of the Tyumen Formation is explained by the favorable facies of that time. The distribution of altered varieties of pelitomorphic siderites throughout the entire Jurassic section does not make it possible to differentiate the diagenesis of the oil- and gas-bearing deposits by intensity.

Spherolitic siderites, which are the most interesting formations, are found in deposits of the Tyumen, and more rarely the Naunak (Vasyugan), formations. In the lower part of the Tyumen Formation, spherolitic siderites were described by us in core from well 337-R (3115–3116 m) of the Yuzhno-Cheremshanskaya area, located within the western margin of the Koltogor megatrough. These are interbeds up to 0.5 m thick of brownish-gray, dense, microlenticularly bedded rocks with rare fragments of black cherts, white quartz, and feldspars up to...

7–5 mm in cross section, as a result of which the siderites are breccia-like. Under the microscope a spherulitic structure is established in these rocks. The spherulites make up 45–60% of the rock volume; they are distributed unevenly. These are either spherulites isolated from one another, up to 0.5 mm in size, or closely intergrown spherulites. The spherulites are characterized by distinct outer contours, either oval-isometric in form or resembling a polyhedral form. In polarized light, a cross-shaped extinction figure is clearly expressed in the spherulites. Inside the spherulites there are numerous fine-aleuritic inclusions of quartz fragments, more rarely feldspars, which occur both in the core and in the peripheral part of the spherulites. The bulk of the spherulitic siderites described is characterized by a hydromicaceous composition with an admixture of kaolinite and siderite. It contains up to 2–3% fine-aleuritic and psammitic fragments of quartz and siliceous shales, and a large quantity of the finest scraps of carbonized plants.

At the base of the Tyumen Formation, spherulitic siderites are also known in the section of well 350 (2918–2919 m) of the Kiev-Egan area, confined to the northern part of the Kargasok arch. Here the siderites are dense, homogeneous, dark brown, even brownish black; in structure they resemble fine-grained sandstones. Under the microscope, a homogeneous spherulitic fabric of the siderites is established in thin section. The sizes of the spherulites do not exceed 0.15–0.18 mm in diameter. The form of the spherulites is rounded and is established only in polarized light by the cross-shaped extinction. From the close arrangement of the cross-shaped extinction figures, the tight intergrowth of the spherulites with one another is inferred, which is confirmed by the macrostructure of these siderites, resembling a “fish-roe stone.” In transmitted light the spherulitic fabric of the siderites is not noticeable, since in this case the siderites constitute a homogeneous mass.

In the upper part of the Tyumen Formation, spherulitic siderites occur more rarely and were described by us only in the core of well 190-R (2563–2570 m) of the Peredovaya area, located in the northern part of the Middle Vasyugan megaswell. Macroscopically, these are dense, homogeneous and indistinctly bedded brownish-gray rocks, in which there is up to 15% quartz–feldspar fine-aleuritic clastic material. Under the microscope, the bulk of the rock consists of pelitomorphic sideritic material, separate areas of which have a spherulitic structure. The spherulites are usually isolated from one another; more rarely they are grouped into chain-like intergrowths. In polarized light the spherulites are characterized by cross-shaped extinction and a weakly expressed concentric-zonal structure.

In the Upper Jurassic deposits, spherulitic siderites are also very rare. We established them only in the deposits of the Naunak Formation in the core of well 350 (2687–2692 m) of the Kiev-Egan area. As in the Tyumen Formation, these are strong, homogeneous, indistinctly bedded, brownish-gray rocks with rare disseminations of marcasite. Their spherulitic structure is established only under the microscope in polarized light by the weakly expressed cross-shaped extinction figure. The spherulites are closely intergrown with one another; their outer boundaries are practically absent because of the imperfection of the spherulitic forms, as is also indicated by the weakly expressed extinction figures, the sizes of which are usually no more than 0.2–0.25 mm in cross section. In individual areas of the thin section, scattered spherulites are noticeable, the sizes of which vary from 0.15 to 0.005 mm. Some spherulites are so small that under the microscope individual ones are practically lost in the cementing pelitomorphic clayey-sideritic mass.

The characterization of spherulitic siderites presented here indicates that their structural and textural features depend on their position in the section of Jurassic deposits. This dependence is expressed in varying degrees of distinctness of the spherulitic structure and in different numbers of spherulites. Spherulitic siderites confined to the base of the Tyumen Formation (depth 3116 m) are characterized by distinctly expressed spherical and polygon-like outlines of individual spherulites. Sideritic material is absent from the interspherulitic space. The impression is created that sideritic material, initially dispersed throughout the entire mass of the rock, was concentrated during diagenesis into spherical concretions. Diagenesis also explains the radial-fibrous orientation of newly formed sideritic aggregates with spherulitic outlines.

With decreasing depth of occurrence (2918), a tendency toward imperfection of the external contours of individual spherulites appears in spherulitic siderites. In polarized light, the spherulites are characterized by the presence of extinction cross figures with distinct rays of equal length. In the interspherulitic space (if it is present), sideritic material is likewise absent.

The imperfection of the forms of the spherulites is still more pronounced in the siderites of the Naunak Formation (depth 2690 m). In addition to siderite participating in the structure of imperfect spherulites, pelitomorphic sideritic material, not yet involved in the process of spherulite formation, is present here in the interspherulitic space. The imperfection of the spherulites is expressed both in the absence of clear external boundaries and in the indistinctness of the observed extinction figure, manifested in different ray lengths of the cross-shaped figure even within a single spherulite.

In the siderites of the upper part of the Tyumen Formation (2570 m), the process of diagenetic transformation with the formation of spherulites was manifested still more weakly. This explains why, in these siderites, spherulites are isolated, while the main mass of the rock consists of an unrecrystallized pelitomorphic aggregate with incipient spherulites.

The factual material presented above indicates the existence of a definite dependence of the structural and textural features of spherulitic siderites on depth of occurrence. These features may be used to differentiate the degree of diagenesis of Jurassic oil- and gas-bearing deposits.

REFERENCES

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2. E. M. Galiev, V. M. Mazur. Relationship of the isotopic composition of carbon in siderites with the facies characteristics of deposits and the conditions of existence of fauna (based on Upper Jurassic and Lower Cretaceous rocks). Geology and Exploration, 1972, No. 10.

3. T. I. Gurova, V. P. Kazarinov. Lithology and Paleogeography of the West Siberian Lowland in Connection with Oil-and-Gas Potential. Moscow, Gostoptekhizdat, 1962.

4. S. G. Sarkisyan, T. I. Gurova, E. G. Sorokina, M. V. Korzh. Lithology of the Mesozoic and Paleogene of Oil-and-Gas-Bearing Regions of the West Siberian Lowland. Moscow, “Nauka” Publishing House, 1967.

5. E. V. Shumilova. Terrigenous Components of Mesozoic and Cenozoic Deposits of the West Siberian Lowland and Their Role in Paleogeographic Reconstructions. Novosibirsk, Publishing House of the Siberian Branch of the USSR Academy of Sciences, 1962.