On the nature of Paleozoic oil deposits and their exploratory "reflection" in the geophysical section of the Jurassic layers (southeast of Western Siberia)

Abstract

Commercial significance of the majority of Western Siberian oil fields is concerned with the Senomanian, Neocomian and, above all, Upper Jurassic horizons. For now, oil fields are at the late development stage and resource potential of the Jurassic horizon is strongly expired. Commercial potential of the pre-Jurassic (Paleozoic) rocks has been brought out throughout all territory of oil and gas province. Extensive work on estimation of the pre-Jurassic rocks oil and gas potential is performed in southeast, in the territory of Tomsk Region, within which 13 hydrocarbon deposits have been discovered in the Paleozoic. Original hypothesis of anomalousness of geophysical and petrophysical characteristics of the Jurassic layers — uniqueness of «indication» the Paleozoic deposits in geophysical parameters of overlaying Mezozoic-Cenozoic section was stated as a foundation of new prospecting criterion for the Paleozoic deposits. The Paleozoic formations are accepted as a complex with its own oil generating potential, which results in upward migration of hydrocarbon fluids. Additionally, downward direction of vertical interstratal hydrocarbon migration from the Jurassic source rocks into the pre-Jurassic complex is brought out. It was accepted as a conception that as in case of upward, so in case of downward fluid migration, processes of superposed epigenesis perform and lead to secondary epigenetic transformations of rocks of transit Jurassic layers, which result in their anomalous geophysical and petrophysical characteristics. This paper analyzes and compares geophysical and petrophysical characteristics of the Jurassic layers of different field types in Tomsk Region: without oil and gas potential in pre-Jurassic section, with commercial inflows from the pre-Jurassic complex and unknown type. Results of exploration electrical resistivity and carbonatization in the Jurassic layers of 200 wells and also spontaneous potential variation, electrical resistivity and natural radioactivity in Bazhenov suite confirm anomalousness of geophysical and petrophysical parameters of Jurassic rocks in case of pre-Jurassic deposits. This paper determines 6 geophysical and petrophysical characteristics of the Jurassic layers as predictive indicators for oil and gas potential estimation in pre-Jurassic section. Efficiency analysis of using predictive indicators for bringing out fields with and without deposits in the pre-Jurassic complex was performed for different prospecting cases in the research territory with account taken of possible complexing of indicators, their rank and actual availability. This paper states preference of indicators complexing. Application of a new prospecting criterion will improve efficiency of searching in new prioritized stratigraphic horizon — the Paleozoic, which contains unconventional oil.