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.

An unexpected raise of hydrogen sulfide levels during development of several gas condensate fields in Southwestern Gissar, producing from naturally fractured carbonate reservoirs, observed within a year, lead to necessity of full scale comprehensive investigation. For planning of effective mitigation strategy important questions related to the reasons of hydrogen sulfide level growth and prediction of its further behavior have been addressed in the present study. The entire investigation process encompassed both theoretical and practical parts. Theoretical part covered evaluation of sour gas sources that was crucial in respect to selection of conceptual methodology for predictions. All possible contributing sources including primary and secondary have been investigated to discern the causes and consequences of hydrogen sulfide occurrence. Practical component of the study employed cut to edge technologies tested and implemented in reservoir simulation. Based on conceptual constraints with the use of existing field data, interpretation results and regional knowledge basin and 3D static models with fracture network have been developed. Obtained modeling results have been integrated into compositional model, allowing to predict with applied uncertainty analyses further H2S content change during field development.

The Bulak field is a unique oil field located in the South-Turgai basin. It consists of heterogeneous carbonates. Throughout the field, the reservoirs vary from highly fractured clean limestones with strong aquifer to very tight limy claystone with low amount of bed-bound fractures and no water production. Active infill drilling followed after the exploration stage revealed uncertainties related to the main issues of development, i.e., location of new wells, their trajectory, EOR techniques and most importantly, waterflooding. A comprehensive study was conducted based on all the petrophysical, seismic, geological, and reservoir engineering data. As a result, 3D geological and dynamic models were built. During the analysis, special focus was on geological structure and petrophysical characteristics as well as pressure transient analysis due to drastic changes in reservoir properties within small distances. In addition, the production history and the main stimulation, and EOR measurements, especially fracturing, acidizing and nitrogen huff ‘n’ puff results, were taken into account. Eventually, the waterflooding was simulated to identify its benefits and possible field application. Based on comprehensive study, the field is divided into several zones. The main criteria for dividing became a lithology that is represented by weathered crust, pure limestones, shaly limestones, limy claystones, and bioliths. Each part requires a different approach in development. If the north part is highly fractured and consists of limestones, water shut off is required due to active aquifer, middle part pressure support and the south part horizontal drilling or sidetracking with multistage high-tonnage fracturing. As a result, waterflooding was decided to be implemented in the part of the field characterized by ultralow permeability formation. Thus, recently, four producers were converted to water injection along with the fact that the injection is performed using low-salinity water. Surprisingly, injectors' rates are high enough, current voidage replacement ratio is sufficient and the results are unexpected. The reservoir is very complex where the lithology is drastically distributed within a small area. However, most of the territory is represented by very tight formation. Eventually, after several years of fields operation, many aspects became clear after the research work and development strategy was updated. So, the company is ready to develop such an unconventional reservoir that is one of the few fields in Kazakhstan.

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