Carbonate Reservoirs Within Abalak Сlay Rocks and Bazhenov Shales in the Central West Siberia

Abstract:

It has been established that main reservoirs within deposits of Abalak and Bazhenov formations are confined with carbonate rocks. Carbonate rocks are often fractured, irregularly distributed, genesis of carbonate material is not obvious, reservoir properties can strongly change at short distances. It complicates prediction of prospective carbonate rocks location in the sedimentary column and laterally. Main objective of the work was to reveal genesis of carbonate material and environmental factors influencing on reservoir properties of carbonate rocks.

Carbonate rocks of different types have been studied in details using petrographic, mineralogical, geochemical methods. To reveal genesis of carbonate material and environmental conditions during its precipitation analysis of stable carbon and oxygen isotopes distribution and analysis of gas-liquid inclusions in calcite were carried out.

As a result three groups of carbonates have been distinguished: methane-derived carbonates that precipitated as a result of microbial activity in the upper part of marine sediments; high-temperature calcite that precipitated from hydrothermal fluids, fills cracks within methane-derived carbonates of the first group; secondary limestones and dolomites that precipitated during katagenesis or also from hydrothermal fluids. Main reservoirs are associated with: 1. limestones at top of Abalak deposits with numerous cracks and caverns partially filled with high-temperature calcite – fractured cavernous reservoir; 2. dolomitized silicites within Bazhenov formation – fractured microporous reservoir. In both cases reservoir properties were formed as a result of secondary alterations of carbonate rocks, particularly induced by hydrothermal fluids migrating from underlying strata. For this reason presence of reservoirs in carbonate rocks is closely connected with the areas of excessive fissuring where fluids migration pathways exist.

The work has been carried out with support of RFBR (Russian Foundation for Basic Research) grant №14-05-31344.