Geophysical characteristics and structural composition of clay rocks in the terrigenous complex of the southeastern part of the West Siberian oil and gas bearing province

The basic attributes identifying the clay rock composition in the terrigenous complex of the southeastern part of the West-Siberian platform have been determined, based on the correlation of structural composition and geophysical data, including induced potential logging, apparent resistivity, induction logging, radioactive and caliper logging. According to the obtained results it is possible to identify above-mentioned rocks in well logs, even under conditions of limited core samples, their application in well log correlation and back stripping. Key words: geophysical well logging, the West-Siberian oil and gas bearing province, terrigenous complex, clay rocks, back stripping.


Introduction
In the southeastern part of the West Siberian platform the petroleum-bearing formations are composed of clastic, clay carbonaceous and carbonate rock alterations. However, siliceous, aluminous and ferrous rocks have also been revealed. Under the conditions of limited terrigenous profile core samples from Jurassic sedimentary formations and those emerging problems in log analysis interpretation [1], one of the major tasks in the development of an improved profile study of enclosing sedimentary rocks in oil-gas reservoirs is the integration of geophysical and lithological data. In this case one aspect of the above-mentioned problem is to assign rock structural composition to their geophysical field characteristics.

Subject and Research Methods
The study data included the following: core samples of clay and carbonaceous rocks and their produced thin sections; results of XD and quantitative spectrum analyses and geophysical well logging data package including induced potential logging (SP log), apparent resistivity logging, inductive electro-magnetic logging, radioactive and caliper logging, describing the drill logs from the southeastern area of the southeastern part of West Siberian oil and gas bearing province, Tomsk region: Sobolinaya (Sob.), Gerasimovskaya (G), East-Gerasimovskaya EG), Kalinovaya (K), Fobosskaya, Nizhne-Tabaganskaya (NT), South-Tabaganskaya (ST) fields.

Results
The clay rocks are composed of 0.01 mm particles, where the basic rock-forming clay minerals are kaolinite, hydromica, montmorillonite, involving alternating layers, such as illite-montmorillonite.
Chlorite, ferrum and aluminium oxides and hydroxides and silica could also be important components. Two genetic types of clay rocks have been revealed-chemogenic and clastic. Chemogenic clay rocks within the formation are classified as residual products of weathering crust, i.e. structured clay residual rocks, according to B.B. Polinov [9] and V.P. Kazarinov [5]. The cross-sections show that there are several different structured clay residual rocks, the mineralogical composition of which was revealed by XD analysis data (18 samples). 1. Clays are light gray, practically white in color with sparse angular fragments of highly weathered siliceous-ferruginous rocks ( figure 1A). The thin sections show (figure 1B) that the basic rock body embraces kaolinite with fine-grained quartz and siderite as fine oolites, sometimes oxidized oolites. According to the XD analysis data the mineral percentage is the following: kaolinite (45-62 %), chlorite (8-10 %), hydromica (8-15 %), montmorillonite (4-5 %), siderite (7-15 %), quartz (10-13 %). The well log shows that clays are defined by positive SP curves, low (up to 5 Ohm) electrical resistance, high (from 350 millisiemens) conductivity, insignificant increase (3-4 cm) of the borehole diameter, increased radioactivity (up to 10 γ) and increased (2.0-2.2 pulse/min.) neutron gamma-ray curve values ( figure 1С). Based on the mineralogical composition and the evidence of intensely altered angular fragments and their view location in the profilein breccia roof, the described clays are neo-formed residual rocks. The latter (according to B.B. Polinov [9]) is a structured clay residual rock horizon formed during the Permo-Triassic period due to the decomposition of loose coarsegrained sediments.
2. Siliceous-clay rocks are white in color with horizontal and vertical cracks ( figure 2A).
In the thin sections (figure 2B), as in the previous ones, the basic rock body embraces kaolinite with fine-grained quartz and extra fine hydromica crystalloids. XD analysis data revealed the fact that this composition is as follows: kaolinite (40-45 %), chlorite (5-7 %), hydromica (30-35 %), montmorillonite (up to 5 %), quartz (25-30 %), siderite (often oxidizied) up to 10%. In single samples powder quartz contributed the white color to the rock. The rocks show the following characteristic features (figure 2C): positive SP anomalies, low (about 5 Ohm) electrical resistance, high (from 200 millisiemens) conductivity, insignificant increase (up to 22 cm, if rated value is 19 cm) of the borehole diameter, rather high radioactivity (up to 40γ) and increasingly intensive (1.8-2.0 pulse/min.) neutro gamma-ray curve values. The above-mentioned factorsevidence of powder quartz, mineral composition and location in the profiles, i.e. top of siliceous rocks (spongolites and quartzites) indicate that these clays could be classified as para-residual siliceous sedimentary rocks. 3. Chemogenic clay rocks include a limited number of chloritized clays underlying mafic effusiveclay ortho-residual rocks. According to the XD analysis data the mineral percentage is the following: kaolinite (15-20 %), chlorite (25-30 %), hydromica (30-35 %), montmorillonite (up to 5 %), siderite (often oxidizied) up to 10 %, quartz (up to 5%). Geophysical characteristics of lay rocks are similar to the ones described above. Clastic clays are the result of erosion and redeposition of the weathering crust and pre-existing sedimentary rocks. The mineralogical clay material alteration processes in the diagenetic period in the case of free water loss and compaction of sediments. Iron sulphides, carbonates, sulphates, iron oxide and iron hydroxide minerals are formed in clay rocks. During catagenesis clay rocks are affected by increasing temperatures and pressure involving interstital pores, resulting in compacted clays and argillites, plasticity loss and gradual mineral deswelling. Primary clay rock alteration occurs during metagenesis, whereas clay rocks become foliated and further form mud shales and argillaceous slates. According to the degree of composition, clay rocks include the following sequence: clays-compacted clays-argillites-mud shales-argillaceous slates, within which only clays and compacted clays exhibit plasticity and exchange properties. Due to these properties the transformation of clay minerals, alteration of their mineralogical composition and replacement of minerals under amenable conditions occur during the post-sedimentation period. In this respect, the composition of clay rocks in sedimentary thicknesses is rarely correlated to the sedimentation environment [6]. However, particular differences in the mineralogical composition of the diversely genetic clay rocks can be observed within the study area (100 sample results based on XD analysis).
The thin sections (figure 3B) show insignificant quartz impurities, fine-flaky hydromica and glauconite inclusions. The well log (figure 3C) shows that clays are defined by low (up to 5 Ohm) electrical resistance, relatively high conductivity (100-120 millisiemens), positve SP anomaly, increased radioactivity (14-16γ), the increasing of the borehole diameter from 24 to 28 cm, if rated value is 19 cm.

Conclusions
The basic characteristic features of clay and carbonaceous rocks and coals in the profiles of studied formation are the following: chemogenic clays are residual products of crust weathering formationstructured clay residual rocks in the profile are neo-formed residual rocks according to petrographic and mineralogical composition (during decomposition of loose coarse-clastic rocks) and para-residual (due to siliceous rocks) and ortho-residual rocks (during weathering of effusives); the mineralogical composition of chemogenic clays depends on the composition of the rocks, showing a significant abundance of kaolinite, chlorite and hydromica content; deswelling component (montmorillonite) is not more than 5 % and differently oxidied quartz and siderite could also be found in the samples; geophysical attributes of clay rocks of crust weathering does not depend on the mineralogical composition, in broader terms: low electrical resistance, significantly high (up to 400 millisiemens) conductivity, positive SP anomalies, insignificant increase of the well diameter, increased raadioactivity values and intensive gamma rays; clastic rocks in investigated formation is related to compacted clays, with kaolinite, chlorite and hydromica content and different alternating hydromicamontmorillonite sequences and dependable of marine montmorillonite-hydromica clays with chlorite; continental: hydromica-kaolinite, hydromicas, often sideritized with abundant flora remains; in geophysical logs homogeneous clays show low electrical resistivity, increased conductivity, high radioactivity, increasing well diameter, positive PS anomaly; terrigeneous impurities initiate negative PS curve deviations, while bituminous matter causes the increase of electrical resistivity and radioactivity; deswelling minerals decrease in agrillites which lessen the plastic property to further fracturing or fracture porosity, which, in its turn, reveals negative deviations on SP curves.