Olga Sivalneva

The article presents lithological studies of reservoir rocks of terrigenous deposits of the Lower Cretaceous of the Krasnoleninsky arch of Western Siberia. A distinctive feature of these deposits is the high variability of lithotypes, which is expressed in a wide range of textural features of thedeposits that determine the nature of oil saturation. According to the results of textural analysis, it was revealed that the studied sediments were formed in various zones of underwater channels of the delta complex and lagoons. To assess the total number of oil-saturated reservoir rocks and the degree oftheir stratification, 10 models were selected that allow us to characterize the distribution of oil-saturated reservoir rocks in different facies complexes. The methodology developed at Gubkin University at the Department of Lithology made it possible to quantify the heterogeneity of the distribution of oil saturation in the multifacial deposits of the Lower Cretaceous of the Krasnoleninsky arch.

Among the large ring structures considered meteorite (impact) craters, the Siljan structure is one of the best-known in central Sweden. Samples of basement rocks were collected and studied from four sections penetrated by exploration boreholes along the periphery of the Siljan crater. Based on the results of laboratory studies, detailed petrographic characteristics of the rocks were obtained, which included a description of their structure and texture, mineral composition, and secondary transformations. Significant differences in the composition of the rocks in the sections located in the eastern and western parts of the structure were revealed. In the western part, boreholes exposed volcanic and metamorphosed volcanic-sedimentary rocks, while the section in the eastern part of the structure is composed of igneous rocks of predominantly acidic composition. This is probably due to structural shifts as a result of the influence of intense tectonic processes.

This paper presents the results of lithological and geophysical studies of the Vikulov Formation sediments, as well as cyclostratigraphic and facies analyses. The geophysical and seismic data have been linked to identify and trace the main isochronal levels of the Vikulov Formation sediments in different facies zones of the West Siberian palaeobasin. The detailed alignment of the reflecting boundaries with the boundaries of sedimentation cyclites made it possible to conduct facies modeling of five sedimentation units of the Vikulov Formation. The modelling results showed that the sediments of the Formation were formed in the conditions of the underwater part of the avandelta of a semi-isolated shallow marine basin. The change in the source area location during the sediments formation was reflected in the whimsical structure and distribution of sediments in the sedimentation basin. The lithological heterogeneity of the Vikulov formation deposits determines the differentiation of filtration and reservoir properties and oil saturation.

The ring structure of Siljan, located in the central part of Sweden, is considered by many researchers to be a meteorite (impact) crater. Impact craters are among the most complex geological objects on the Earth. The origin and formation of these structures still raises many questions. To find answers to these questions we need reliable geological information about the structure of the crater and the composition of the rocks. Information about the thickness and geological structure of the Siljan Ring area sedimentary cover will help to understand the process of the Siljan Ring structure’s formation as well as other similar geological formations on the Earth. Here, we present the results of laboratory studies of sedimentary rock samples taken from four exploration wells drilled in the vicinity of the Siljan Ring crater, which made it possible to compile their detailed lithological description. The laboratory studies included a structural analysis of the samples, and a texture and mineralogical analysis in thin sections. A structure analysis was carried out visually, while structural and mineralogical analyses were carried out on thin sections using a polarizing microscope and a scanning electron microscope. The main components of the rocks (minerals and fragments), along with their ratio and secondary transformations, were determined. The results of the structural analysis of the samples, and the textural and mineralogical analysis of the rocks in thin sections, showed that the sediments’ composition in the sedimentary cover near the Siljan Ring structure changes in different areas in accordance with their facies and stratigraphic characteristics. Furthermore, a change in the thickness of the sections and the succession sequences of rock units was established. A change of this nature is presumably caused by tectonic disturbances of an endogenous or impact source.

The article describes the results of lithological and petrophysical investigations that would be a base for characterization of reservoir rocks in Upper Cretaceous deposits. These investigations include thin sections description, SEM and NMR analysis. As found that three main factors have constrained final quality of reservoir rocks: 1) depositional settings favorable for coccoliths and chalk sedimentation; 2) late diagenesis changes – compaction and recrystallization degree; 3) fracture intensity.

The results from the geological and geophysical investigations of the Siljan Ring impact structure (central Sweden) have shown that the Paleozoic sedimentary succession and the Precambrian basement were strongly affected by complex deformational processes. Studies of a new drill core from the C-C-1 well provide valuable additional information necessary for the reconstruction of the geological setting in the southwestern part of the Siljan Ring. It was found that the contact between the basement and the sedimentary cover is tectonic, not normal sedimentary, in origin. The basement interval comprises Precambrian metavolcanic and metasedimentary rocks with a single mafic intrusion (gabbro-dolerite) in the upper part. The rocks have only been partially metamorphosed. The intercalation of calcareous mudstones, skeletal wackstones, and black shales in the sedimentary cover interval is not consistent with the regional lithostratigraphy scheme. Thus, more likely that the sedimentary sequence is not complete as a result of tectonic displacements, and a significant part of the Lower and Middle Ordovician succession is missing. The Post-Proterozoic tectonic reactivation and impact event also caused the formation of four types of fracture. The third type of fracture is accompanied by cataclastic zones and probably have an impact-related nature. In the highly fractured basement rocks, a dissolution along the second type of fracture has resulted in the development of open vugs. Open vugs and microporosity in cataclastic zones have been considered to be an effective storage space for hydrocarbons.

Results of geological and geophysical investigations of the Siljan Ring impact structure (central Sweden) revealed complicated relationships between Paleozoic sedimentary succession and the Precambrian basement. Tectonic and depositional evolution caused complex geology. Studies of a new drill core from the C-C-1 well provide information necessary for the reconstruction of the geological setting in the southwestern part of Siljan Ring. The whole interval of the core section is from 32.60 to 634.90 m with almost no breaks. The sedimentary cover is 373.55 m thick in total. The sedimentary sequences are predominantly composed of wackestones, mudstones, and shales. In the lower part of the sedimentary section, limestone layers intercalate with black shales. In a result of the investigations, it has been suggested that sedimentary layers represent Late Ordovician and Silurian deposits and have disturbed stratigraphic relations. The basement section is composed of Precambrian meta-volcanic and meta-sedimentary rocks. The contact between the basement and the sedimentary cover is tectonic, not normal sedimentary, in origin. Tectonic processes caused intensive rock fracturing. Four generations of fractures were identified with analysis of fracture relations and mineralization sequence. Only two of them occur in sedimentary rocks that probably belong to the latest stages of tectonic activity. Highly fractured basement rocks in some cases contain open vugs developed along the fractures. Rock matrix is tight either in sedimentary and basement rocks and only micro-porosity space is recognized in cataclastic zones. Single evidence of bituminous filling of micro-porosity zone and partly cemented vug is established in limestone from the lower part of the sedimentary section. These findings are particularly valuable for stratigraphy refinement and tectonic setting reconstructions as well as oil and gas reservoir forecasts.