Characterization of Key Quality Factors of Highly Argillaceous and Naturally Fractured Paleozoic Carbonate Deposits with Complex Tectonic Setting South Turgai Basin, Kazakhstan

Abstract With decreasing potential of more common Mesozoic reservoirs, Paleozoic formations within the South Torgai basin are becoming more attractive explorational targets. However, complexity of the internal structure, variation of depositional environments and rock types of these formations makes the process of reservoir characterization extremely difficult for exploration, further well placement and field development planning. Natural fractures complicate the reservoir dynamic behavior even more, resulting in very diverse well performance and high watercut across the field. In this study, we try to reveal the reservoir nature, distribution and properties by integrating seismic, wellbore and regional geological data. The study is based on the data of Bulak oil field. Operated by PetroKazakhstan Kumkol Resources JSC, this is a unique oil field producing from Carboniferous reservoirs within the Aksay horst of the Aryskum trough. Considering well penetration intervals are limited to pay zones, the complex internal reservoir structure and properties were characterized by integration of methods such as seismic chromatic geology extraction, seismic inversion, volumetric petrophysical modeling, regional stress analysis, 3D static and dynamic modeling (DPDP) and others. As a result of the study, different conclusions were made regarding the structure, distribution and properties of the main reservoirs, development of different fracture types and resulting dynamic field behavior. Compressional regime in the Late Paleozoic - Early Mesozoic eras and associated structures were identified and mapped; two main trends defining the reservoir architecture were identified: uplift of the whole section towards the North and the tectonic forces that created and overturned fault blocks to the Southwest; general decrease of carbonate content and increase of shaliness from Early to Late Carboniferous were observed; abundance of near-vertical open fractures in the Northern part of the field (as opposed to lower dips and possible mineralization of fractures in the Southern part) was identified and modelled; main hydrocarbon bearing rock types were described and attempts to identify them across the field were made. Being one of the first fields commercially producing from Paleozoic rocks of the Torgai basin, Bulak revealed a lot of information that is being used for further exploration in the area. Different field feature trends can be extended to neighbor blocks and existing data can be used as analogues, especially based on seismic features where no wells are drilled yet.